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Fertilization of an oligotrophic coastal montane lake using high N:P ratio fertilizers : effects on… Werring, John 1986

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F E R T I L I Z A T I O N OF AN OLIGOTROPHIC COASTAL MONTANE LAKE USING HIGH N:P RATIO F E R T I L I Z E R S : EFFECTS ON PHYTOPLANKTON AND ZOOPLANKTON COMMUNITY STRUCTURE B Y JOHN WERRING B S c , U n i v e r s i t y Of B r i t i s h C o l u m b i a , 1980 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Z o o l o g y ) We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1986 @ (c) J o hn W e r r i n g , 1986 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f /JooAo^1-^ - The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e V ancouver, Canada V6T 1W5 n r . ft ( 9 /7Q i i i ABSTRACT C o m p a r i s o n s of t h e r e s p o n s e of p h y t o p l a n k t o n c o m m u n i t i e s t o d i f f e r e n t methods of n u t r i e n t a d d i t i o n u s e d i n p a s t w h o l e - l a k e f e r t i l i z a t i o n s t u d i e s , s u g g e s t e d t h a t i t m ight be p o s s i b l e t o g e n e r a t e p r e d i c t a b l e c h a n g e s i n t h e p h y t o p l a n k t o n and z o o p l a n k t o n c o m m u n i t i e s o f l a k e s by c o n t r o l l i n g t h e f r e q u e n c y , m a g n i t u d e and N:P r a t i o o f t h e a u t o c h t h o n o u s n u t r i e n t l o a d . R e s u l t s o b t a i n e d by s e v e r a l w o r k e r s i n s e p a r a t e , but r e l a t e d , l a b o r a t o r y and s m a l l s c a l e e n c l o s u r e e x p e r i m e n t s s u p p o r t e d t h i s b e l i e f . The e v i d e n c e s t r o n g l y s u g g e s t e d t h a t N:P r a t i o s a r e i m p o r t a n t i n d e t e r m i n i n g t h e outcome o f c o m p e t i t i v e i n t e r a c t i o n s between a l g a l t a x a , w i t h g r e e n a l g a e o u t c o m p e t i n g b l u e - g r e e n a l g a e a t h i g h N:P r a t i o s , w h i l e t h e f r e q u e n c y o f n u t r i e n t s u p p l y i s i m p o r t a n t i n d e t e r m i n i n g t h e s i z e s t r u c t u r e of an a l g a l community. T h e r e i s a l s o e v i d e n c e t h a t s u g g e s t s t h a t c o m p e t i t i v e r e l a t i o n s h i p s between z o o p l a n k t o n s p e c i e s a r e s e n s i t i v e t o c h a n g e s i n t h e s p e c i e s and s i z e c o m p o s i t i o n o f t h e p h y t o p l a n k t o n community. L a r g e c l a d o c e r a n s p e r f o r m p o o r l y when l a r g e a l g a e a r e p r e s e n t i n a b u ndance. T h i s t h e s i s e x a m ines t h e e f f e c t s of s m a l l , f r e q u e n t d o s e s of h i g h N:P r a t i o f e r t i l i z e r s on t h e l i t t o r a l and l i m n e t i c p h y t o p l a n k t o n and z o o p l a n k t o n c o m m u n i t i e s i n a s m a l l o l i g o t r o p h i c c o a s t a l montane l a k e i n t h e C o a s t a l Range M o u n t a i n s of B r i t i s h C o l u m b i a . The e x p e r i m e n t s were c a r r i e d o u t i n - s i t u i n e i g h t l a r g e p o l y e t h y l e n e e n c l o s u r e s , e a c h m e a s u r i n g 150 m 2 i n a r e a . The e n c l o s u r e s were f e r t i l i z e d t h r e e t i m e s e a c h week from June t o O c t o b e r , 1982 w i t h s m a l l d o s e s of NH 4N0 3 and (NH f t) 2HPO a a t an N:P a t o m i c r a t i o of 40:1. P h o s p h o r o u s was t h o u g h t t o be t h e n u t r i e n t l i m i t i n g p h y t o p l a n k t o n g r o w t h i n t h i s l a k e but t h e r e s u l t s s u g g e s t t h a t n i t r o g e n may be more i m p o r t a n t i n t h i s r e g a r d . The r o l e of Sphagnum mats s u r r o u n d i n g t h e l a k e i n t h e c o n t r o l of n i t r o g e n s u p p l y t o t h e l a k e i s d i s c u s s e d . In g e n e r a l , e f f e c t s o f f e r t i l i z a t i o n were more p r o n o u n c e d i n t h e l i m n e t i c t h a n t h e l i t t o r a l z o n e . P h y t o p l a n k t o n s t a n d i n g c r o p i n c r e a s e d 4 - f o l d and 7 - f o l d i n l i t t o r a l and l i m n e t i c f e r t i l i z e d e n c l o s u r e s , r e s p e c t i v e l y , o v e r c o n t r o l s . C y a n o p h y c e a e and C h r y s o p h y c e a e , w h i c h d o m i n a t e d t h e a l g a l b i o m a s s i n t h e l a k e and c o n t r o l s were a l m o s t c o m p l e t e l y e l i m i n a t e d from a l l f e r t i l i z e d e n c l o s u r e s by mid-summer and r e p l a c e d by C h l o r o p h y c e a e . More t h a n 80 % of t h e a l g a l b i o m a s s i n f e r t i l i z e d e n c l o s u r e s c o n s i s t e d o f a l g a e l e s s t h a n 20 um i n s i z e , whereas l e s s t h a n 20 % o f t h e b i o m a s s i n c o n t r o l s and t h e l a k e f e l l i n t o t h i s s i z e r a n g e . Changes i n t h e a l g a l community l e d t o s i g n i f i c a n t c h a n g e s i n t h e s p e c i e s c o m p o s i t i o n , s t r u c t u r e and b i o m a s s o f t h e z o o p l a n k t o n community. W i t h t h e e x c e p t i o n s o f D a p h n i a r o s e a and H o l o p e d i u m g i b b e r u m a l l h e r b i v o r o u s z o o p l a n k t o n i n a l l e n c l o s u r e s f o l l o w e d t h e p a t t e r n o f s p e c i e s s u c c e s s i o n t h a t o c c u r r e d i n t h e l a k e b u t a t h i g h e r d e n s i t i e s . F e c u n d i t y and body s i z e o f a l l h e r b i v o r o u s z o o p l a n k t o n i n c r e a s e d f o l l o w i n g f e r t i l i z a t i o n , b u t D a p h n i a r o s e a c a p i t a l i z e d on t h e e n h a n c e d a l g a l b i o m a s s and became t h e community d o m i n a n t o v e r t h e whole summer i n f e r t i l i z e d e n c l o s u r e s w h i l e i n c o n t r o l s and t h e l a k e , i t c e a s e d t o be d o m i n a n t by l a t e J u l y . H o l o p e d i u m d i s a p p e a r e d i v from a l l e n c l o s u r e s by t h e end o f J u l y . F i s h p r e d a t i o n and c o m p e t i t i v e i n t e r a c t i o n s w i t h D a p h n i a p r o b a b l y l e d t o t h e i r d e c l i n e . F i s h p r e d a t i o n was g e n e r a l l y s e l e c t i v e f o r H o i o p e d i u m and D. r o s e a i n c o n t r o l and f e r t i l i z e d e n c l o s u r e s and i t s r o l e i n t h e d i s a p p e a r a n c e of H o l o p e d i u m and i n s t r u c t u r i n g t h e z o o p l a n k t o n community i n b o t h , i s d i s c u s s e d . A d u l t c u t t h r o a t t r o u t (Salmo c l a r k i c l a r k i ) i n f e r t i l i z e d e n c l o s u r e s a l t e r e d t h e i r d i e t i n f a v o u r of D a p h n i a • F i s h i n c o n t r o l e n c l o s u r e s and t h e l a k e f e d a l m o s t e x c l u s i v e l y on t e r r e s t r i a l i n s e c t s . P l a n t e d j u v e n i l e ( f r y ) c u t t h r o a t t r o u t d i d n o t s u r v i v e i n any o f t h e e n c l o s u r e s . A v i a n p r e d a t i o n a p p e a r e d t o be r e s p o n s i b l e f o r t h e i r d i s a p p e a r a n c e s u g g e s t i n g t h a t p r e d a t i o n i s more i m p o r t a n t t h a n f o o d t o t h e s u r v i v a l o f j u v e n i l e f i s h i n l a k e s . My r e s u l t s s u g g e s t t h a t t h e p h y t o p l a n k t o n and z o o p l a n k t o n c o m m u n i t i e s i n f r e s h w a t e r s c an be m a n i p u l a t e d i n a p r e d i c t a b l e manner b a s e d on a c o n t r o l o f t h e p a t t e r n o f n u t r i e n t s u p p l y . T h i s knowledge can be o f g r e a t b e n e f i t t o t h e management l a k e s f o r b o t h f i s h p r o d u c t i o n and a m e l i o r a t i n g t h e e f f e c t s o f e u t r o p h i c a t i o n . V TABLE OF CONTENTS ABSTRACT i i L I S T OF TABLES i x L I S T OF FIGURES x i ACKNOWLEDGEMENTS x i v 1 .0 INTRODUCTION 1 1.1 H i s t o r i c a l and T h e o r e t i c a l B a c k g r o u n d 1 1.2 R e s e a r c h O b j e c t i v e s 14 2.0 MATERIALS AND METHODS 17 2. 1 S t u d y S i t e 17 2.1.1 C l i m a t e 17 2.1.2 Lake B a s i n M o r p h o l o g y and Morphometry 21 2.1.3 C h e m i c a l F e a t u r e s 27 2.1.4 F l o r a ' 30 2.1.5 Fauna 31 2.2 E x p e r i m e n t a l Methods 34 2.2.1 L i m n o c o r r a l D e s i g n and I n s t a l l a t i o n 34 2.2.2 E x p e r i m e n t a l T r e a t m e n t s 38 2.2.3 S a m p l i n g Methods 41 ( 1 ) Z o o p l a n k t o n 42 (2) P h y t o p l a n k t o n 43 (3) P l a n t P i g m e n t s 45 (4) Water C h e m i s t r y 46 3.0 RESULTS 47 3.1 P h y s i c a l P a r a m e t e r s 47 v i 3.1.1 T e m p e r a t u r e 47 3.1.2 D i s s o l v e d Oxygen 50 3.1.3 T r a n s p a r e n c y 53 3.1.4 Summary of P h y s i c a l R e s u l t s 53 3.2 C h e m i c a l P a r a m e t e r s 56 3.2.1 C o n d i t i o n s i n Lake W a t e r s 56 3.2.2 E f f e c t s o f E n c l o s u r e 74 3.2.3 E f f e c t s o f F e r t i l i z a t i o n 74 ( 1 ) P h o s p h o r u s 74 (2) N i t r o g e n 76 (3) H y d r o g e n Ion C o n c e n t r a t i o n 77 3.2.4 Summary of C h e m i c a l R e s u l t s 78 3.3 PHYTOPLANKTON STANDING CROP, SPECIES AND SIZE COMPOSITION 79 3.*3.1 S t a n d i n g c r o p 79 (1) E f f e c t s o f E n c l o s u r e 79 (2) E f f e c t s of F e r t i l i z a t i o n 94 3.3.2 S p e c i e s C o m p o s i t i o n 99 ( 1 ) In The Lake 99 (2) E f f e c t s o f E n c l o s u r e 103 (3) E f f e c t s of F e r t i l i z a t i o n 104 3.3.3 S i z e c o m p o s i t i o n 107 (1) E f f e c t s of E n c l o s u r e 107 (2) E f f e c t s of F e r t i l i z a t i o n 114 3.3.4 Summary of P h y t o p l a n k t o n R e s u l t s 118 3.4 Z o o p l a n k t o n S t a n d i n g C r o p , S p e c i e s C o m p o s i t i o n and Community S t r u c t u r e 119 v i i 3.4.1 S t a n d i n g C r o p and S p e c i e s C o m p o s i t i o n 119 (1 ) In The Lake 119 (2) E f f e c t s o f E n c l o s u r e 122 (3) E f f e c t s of F e r t i l i z a t i o n 131 3.4.2. Z o o p l a n k t o n Community S t r u c t u r e 132 ( 1 ) In The Lake 132 (2) E f f e c t s of E n c l o s u r e 152 (3) E f f e c t s o f F e r t i l i z a t i o n 171 3.4.3 Summary o f Z o o p l a n k t o n R e s u l t s ..179 3.5 EFFECTS OF FE R T I L I Z A T I O N ON CUTTHROAT TROUT DIETS ..180 4.0 DISCUSSION 186 4.1 E f f e c t s of E n c l o s u r e 186 4.1.1 On P h y s i c a l C o n d i t i o n s and Water C h e m i s t r y ....187 4.1.2 On P h y t o p l a n k t o n 192 4.1.3 On Z o o p l a n k t o n 193 4.1.4 On I n t e n s i t y of T r o u t P r e d a t i o n on Z o o p l a n k t o n 196 4.2 E f f e c t s of F e r t i l i z a t i o n 200 4.2.1 On Water C h e m i s t r y 200 (1) P h o s p h o r o u s 200 (2) N i t r o g e n and P o s s i b l e N i t r o g e n L i m i t a t i o n s on P h y t o p l a n k t o n Growth 209 (3) pH change and Ammonium Nitrate-Ammonium P h o s p h a t e A d d i t i o n s 213 4.2.2 On P h y t o p l a n k t o n ...216 (1) S h i f t s i n S p e c i e s C o m p o s i t i o n 217 (2) D e l a y e d B i o m a s s I n c r e a s e 219 (3) S i z e F r a c t i o n of E n h a n c e d A l g a e 222 v i i i 4.2.3 On Z o o p l a n k t o n Community S t r u c t u r e 226 ( 1 ) H o l o p e d i u m g i b b e r u m 227 (2) D a p h n i a r o s e a 229 (3) Diaphanosoma b r a c h y u r u m 238 (4) Bosmina l o n q i r o s t r i s 240 (5) C e r i o d a p h n i a q u a d r a n q u l a 245 (6) Diaptomus o r e q o n e n s i s 248 (7) D i a c y c l o p s t h o m a s i 250 5.0 PLACID LAKE CUTTHROAT TROUT 253 5.1 L i f e H i s t o r y T r a i t s 253 5.2 F e r t i l i z a t i o n and The D i e t o f t h e T r o u t 257 6.0 CONCLUSIONS 261 REFERENCES CITED 265 APPENDIX 1 283 APPENDIX 2 291 APPENDIX 3 294 i x L I S T OF TABLES T a b l e 1. M o r p h o m e t r i c p a r a m e t e r s of P l a c i d Lake 20 T a b l e 2. P h y s i c a l and c h e m i c a l p r o p e r t i e s o f P l a c i d Lake . 28 T a b l e 3. A q u a t i c v e r t e b r a t e s and c r u s t a c e a n z o o p l a n k t o n community o f P l a c i d L a k e 33 T a b l e 4. E x p e r i m e n t a l t r e a t m e n t s 40 T a b l e 5. R e s u l t s o f one way a n a l y s i s of v a r i a n c e on t o t a l b i o m a s s of p h y t o p l a n k t o n ( as d e t e r m i n e d by c e l l v o l umes) i n t h e 0-2 m d e p t h s t r a t u m o f t h e l a k e and a l l e n c l o s u r e s 82 T a b l e 6. Mean summer c o n c e n t r a t i o n s of c h l o r o p h y l l a, p h a e o p h y t i n a and t o t a l p i g m e n t s a t a l l d e p t h s i n t h e l a k e , and c o n t r o l and t r e a t m e n t e n c l o s u r e s 85 T a b l e 7. R e s u l t s o f a n a l y s i s o f v a r i a n c e on 1 / s q r t p l a n t p igment measurements 86 T a b l e 8. R e s u l t s o f a n a l y s i s of v a r i a n c e on measurements of t h e s e a s o n a l mean p r o p o r t i o n o f l i v e c h l o r o p h y l l a i n a l l e n c l o s u r e s and t h e l a k e 97 T a b l e 9. R e s u l t s o f one way a n a l y s i s o f v a r i a n c e on t o t a l b i o m a s s of z o o p l a n k t o n i n t h e 0-2 m d e p t h s t r a t u m of t h e l a k e and a l l e n c l o s u r e s 124 T a b l e 10. Mean d e n s i t i e s of t h e dominant h e r b i v o r o u s z o o p l a n k t o n i n t h e l a k e , and c o n t r o l and t r e a t m e n t e n c l o s u r e s 161 T a b l e 11. R e s u l t s o f a n a l y s i s of v a r i a n c e on t h e d e n s i t i e s X of t h e dom i n a n t h e r b i v o r o u s z o o p l a n k t o n 162 T a b l e 12. R e s u l t s of n o n - p a r a m e t r i c one way a n a l y s i s of v a r i a n c e on t h e mean b r o o d s i z e s o f t h e h e r b i v o r o u s z o o p l a n k t o n 163 T a b l e 14. A v e r a g e f o r k l e n g t h (mm) and w e i g h t (grams) and numbers o f c u t t h r o a t t r o u t f r y p l a n t e d i n e a c h e n c l o s u r e 181 T a b l e 15. Mean numbers and t y p e s of p r e y f o u n d i n t h e stomachs of c u t t h r o a t t r o u t c a p t u r e d i n t h e l a k e and a l l e n c l o s u r e s 184 T a b l e A1. L i s t of common p h y t o p l a n k t o n s p e c i e s i n t h e l i m n o p l a n k t o n of P l a c i d Lake i n 1982 284 x i L I S T OF FIGURES F i g u r e 1. L o c a t i o n of s t u d y s i t e 18 F i g u r e 2. R a i n f a l l and t e m p e r a t u r e d a t a f o r 1982 22 F i g u r e 3. B a t h y m e t r i c map of P l a c i d Lake showing l o c a t i o n s of t h e e n c l o s u r e s and t h e v e g e t a t i o n 25 F i g u r e 4. L i m n o c o r r a l d e s i g n 35 F i g u r e 5. P l a c i d Lake w a t e r t e m p e r a t u r e s ( d e g r e e s C e l s i u s ) f o r summer 1982 48 F i g u r e 6. D e p t h - t i m e i s o p l e t h s of d i s s o l v e d oxygen 51 F i g u r e 7. S e c c h i d i s c d e p t h s 54 F i g u r e 8. D e p t h - t i m e i s o p l e t h s of o r t h o - p h o s p h a t e 57 F i g u r e 9. D e p t h - t i m e i s o p l e t h s o f t o t a l d i s s o l v e d p h o s p h o r o u s 60 F i g u r e 10. D e p t h - t i m e i s o p l e t h s of p a r t i c u l a t e p h o s p h o r o u s 62 F i g u r e 11. D e p t h - t i m e i s o p l e t h s of n i t r a t e n i t r o g e n 64 F i g u r e 12. D e p t h - t i m e i s o p l e t h s of ammonia n i t r o g e n 66 F i g u r e 13. D e p t h - t i m e i s o p l e t h s of t o t a l K j e l d a h l n i t r o g e n 68 F i g u r e 14. D e p t h - t i m e i s o p l e t h s of o r g a n i c n i t r o g e n 70 F i g u r e 15. D e p t h - t i m e i s o p l e t h s of pH 72 F i g u r e 16. A l g a l b i o m a s s a s d e t e r m i n e d by c e l l volume 80 F i g u r e 17. D e p t h - t i m e i s o p l e t h s of C h l o r o p h y l l a 83 F i g u r e 18. Cummulative c o n c e n t r a t i o n s o f c h l o r o p h y l l a and p h a e o p h y t i n a i n t h e 0-2 m d e p t h s t r a t u m 87 F i g u r e 19. Cummulative c o n c e n t r a t i o n s of c h l o r o p h y l l a and p h a e o p h y t i n a i n t h e 2-4 m d e p t h s t r a t u m 89 x i i F i g u r e 20. Cummulative c o n c e n t r a t i o n s of c h l o r o p h y l l a and p h a e o p h y t i n a i n t h e 4-6 m d e p t h s t r a t u m 91 F i g u r e 21. P r o p o r t i o n s of l i v e c h l o r o p h y l l a i n t h e 0-2, 2-4 and 4-6 meter d e p t h s t r a t a 95 F i g u r e 22. P h y l e t i c c o m p o s i t i o n o f a l g a l b i o m a s s 101 F i g u r e 23. Cummulative d e n s i t i e s of a l g a e i n s e l e c t e d s i z e c l a s s 108 F i g u r e 24. P e r c e n t c o m p o s i t i o n of a l g a l b i o m a s s by d e t a i l e d s i z e c l a s s e s 112 F i g u r e 25. Z o o p l a n k t o n b i o m a s s 120 F i g u r e 26. P e r c e n t c o m p o s i t i o n o f z o o p l a n k t o n b i o m a s s by s p e c i e s i n t h e 0-2 meter d e p t h s t r a t u m 125 F i g u r e 27. P e r c e n t c o m p o s i t i o n of z o o p l a n k t o n b i o m a s s by s p e c i e s i n t h e 2-4 meter d e p t h s t r a t u m 127 F i g u r e 28. P e r c e n t c o m p o s i t i o n of z o o p l a n k t o n b i o m a s s by s p e c i e s i n t h e 4-6 meter d e p t h s t r a t u m 129 F i g u r e 29. Cummulative d e n s i t i e s of t h e dominant h e r b i v o r o u s z o o p l a n k t o n s p e c i e s i n t h e 0-2 meter d e p t h s t r a t u m 133 F i g u r e 30. D e n s i t i e s o f H o l o p e d i u m g i b b e r u m 136 F i g u r e 31. D e n s i t i e s of D a p h n i a r o s e a 138 F i g u r e 32. D e n s i t i e s of Diaphanosoma b r a c h y u r u m 140 F i g u r e 33. D e n s i t i e s of D i a p t o m u s o r e g o n e n s i s 142 F i g u r e 34. D e n s i t i e s of C e r i o d a p h n i a q u a d r a n g u l a 144 F i g u r e 35. D e n s i t i e s of Bosmina l o n q i r o s t r i s 146 F i g u r e 36. D e n s i t i e s o f D i a c y c l o p s t h o m a s i ...148 F i g u r e 37. Mean body s i z e s o f a d u l t , g r a v i d f e m a l e and x i i i j u v e n i l e D a p h n i a r o s e a 153 F i g u r e 38. Mean b r o o d s i z e s of D a p h n i a r o s e a 155 F i g u r e 39. Mean body s i z e s of a d u l t Diaphanosoma b r a c h y u r u m 1 57 F i g u r e 40. Mean body s i z e s of a d u l t , g r a v i d f e m a l e and j u v e n i l e D i aptomus o r e q o n e n s i s 159 F i g u r e 41. D e n s i t i e s of c a l a n o i d n a u p l i i 168 F i g u r e 42. P r o p o r t i o n o f a d u l t D a p h n i a r o s e a i n t h e l a k e , and c o n t r o l and f e r t i l i z e d e n c l o s u r e s 177 F i g u r e A1. D e n s i t i e s o f a l g a e i n s e l e c t e d s i z e c l a s s e s i n a l l of t h e e n c l o s u r e s and t h e l a k e 295 x i v ACKNOWLEDGEMENTS I would l i k e t o t h ank D r . C a r l W a l t e r s , my a c a d e m i c s u p e r v i s o r who, w i t h o u t b a t t i n g an e y e , watched s e v e r a l o f h i s NSERC d o l l a r s go i n t h i s work. I would a l s o l i k e t o t h ank him f o r l e t t i n g me have t h e o p p o r t u n i t y t o say " T h i s i s my baby, a l l t h e way". Many t h a n k s go t o a l l of t h o s e s t u d e n t s who I c o u l d e n t i c e i n t o t h e f i e l d , w i t h o f f e r s of f r e e b e e r , t o h e l p me w i t h t h e i n s t a l l a t i o n o f t h e e n c l o s u r e s , but who I c o u l d n o t g e t back f o r a s e c o n d t i m e a f t e r t h e y saw how much work was i n v o l v e d : D e n n i s L a s s e a u , G r a n g e r J u n g , K e v i n L o f t u s , Max L e d b e t t e r , J o y c e Andrew, A l i s t a i r B l a c h f o r d , P a u l B e n t z e n , Mark Ridgway, Ga r y B i r c h , J a y H e s b e c k . A few words of a d v i c e : Don't f e e d them b e e r  u n t i 1 t h e work i s done. A s p e c i a l t h a n k s go t o t h o s e who made r e t u r n t r i p s : Don R o b i n s o n , H a r o l d Waldock and B a r b F r a s e r . A v e r y s p e c i a l t h a n k s go t o t h e p e o p l e who h e l p e d make t h e i n s t a l l a t i o n of my e n c l o s u r e s c o m p l e t e by coming back e v e r y day f o r a week: my l o v e l y w i f e CAROLYN, and t h e boys from F i s h and W i l d l i f e , who had no c h o i c e b e c a u s e t h e i r b o s s s e n t them, E r i c P a r k i n s o n , K a n j i Tsumura, and A n g e l o F a c c h i n . I r e s e r v e my g r e a t e s t t h a n k s f o r B i l l N e i l l f o r h i s a d v i c e and c r i t i c a l r e v i e w o f t h i s m a n u s c r i p t and f o r J.W. The guy who made t h i s a l l p o s s i b l e . J.W s p e n t many h o u r s , d a y s and months c o u n t i n g b e a s t i e s under a m i c r o s c o p e f o r me. He r a n my c h l o r o p h y l l a s a m p l e s , a s s i s t e d me i n t h e f i e l d , a n a l y z e d my d a t a and t h e n w r o t e my t h e s i s . X V THANK'S J.W. T h i s work was s u p p o r t e d i n p a r t by a GREAT award from t h e B.C. S c i e n c e C o u n c i l , D r . C a r l W a l t e r ' s NSERC g r a n t No. 67-5869 and U n i v e r s i t y T e a c h i n g A s s i s t a n t s h i p s . I 1.0 INTRODUCTION J_.J_ H i s t o r i c a l and T h e o r e t i c a l B a c k g r o u n d The p r a c t i c e of f e r t i l i z i n g n a t u r a l l a k e s i n N o r t h A m e r i c a d e v e l o p e d i n r e s p o n s e t o r e p o r t s of r e m a r k a b l e i n c r e a s e s i n f i s h p r o d u c t i o n f o l l o w i n g t h e a d d i t i o n o f i n o r g a n i c f e r t i l i z e r s t o f i s h c u l t u r e ponds i n t h e s o u t h e r n U n i t e d S t a t e s ( S w i n g l e , 1947). I t was f e l t t h a t t h i s t e c h n o l o g y m i g h t a l s o be u s e d t o i n c r e a s e f i s h p r o d u c t i o n i n n a t u r a l l a k e s , however, l i t t l e was known a b o u t t h e r o l e t h a t n u t r i e n t s p l a y e d i n t h e s t r u c t u r e and f u n c t i o n i n g of a q u a t i c e c o s y s t e m s . T h e r e f o r e , i n v e s t i g a t i o n s were l a u n c h e d i n an a t t e m p t t o e l u c i d a t e t h e e f f e c t s of n u t r i e n t a d d i t i o n s on w h o l e - l a k e e c o s y s t e m s . The f i r s t a t t e m p t t o a p p r o a c h l a k e f e r t i l i z a t i o n on a s c i e n t i f i c b a s i s i n N o r t h A m e r i c a was made by J u d a y and S c h l o e m e r ( 1 9 3 8 ) . T h i s work was l a t e r f o l l o w e d by t h e works of B a l l ( 1 9 4 8 ) , S m i t h (1945, 1948, 1955), L a n g f o r d ( 1 9 4 8 ) , N e l s o n and Edmondson ( 1 9 5 5 ) , B a x t e r ( 1 9 5 9 ) , Rabe and Dyer ( 1 9 6 4 ) , D o n a l d s o n e t a l . ( 1 9 7 1 ) , P a r s o n s e t a l . ( 1 9 7 2 ) , S c h i n d l e r et. a l . (1971, 1973), S t o c k n e r e t a l . (1980) and S t o c k n e r ( 1 9 8 1 ) . As w e l l , l a k e f e r t i l i z a t i o n s t u d i e s were b e i n g c a r r i e d out i n S c o t l a n d ( G r o s s e t a l . , 1944; H o l d e n , 1954; Munro, 1961), A u s t r a l i a ( W e a t h e r l y and N i c h o l l s , 1955), P o l a n d ( M a r c i a k , 1975; W e g l e n s k a e t a l . , 1975; Zdanowski e_t a_l. , 1975) and Sweden ( J a n s s o n , 1978; L u n d g r u n , 1978; S o l a n d e r , 1978; P e r s s o n , 1978). However, p r i o r t o t h e s t u d i e s by S c h i n d l e r e t a l . (1971, 1973) and P a r s o n s e t a l . ( 1 9 7 2 ) , v e r y l i t t l e c o n s i d e r a t i o n had been 2 g i v e n t o s t u d y i n g t h e e f f e c t s of n u t r i e n t e n r i c h m e n t on t h e p h y s i c a l , c h e m i c a l o r . b i o l o g i c a l d y n a m i c s , or t h e i r i n t e r a c t i o n s , i n t h e r e c e i v i n g w a t e r s . Nor was any heed g i v e n t o t h e methods of f e r t i l i z e r a p p l i c a t i o n ( i . e . r a t e of a d d i t i o n , amount of n u t r i e n t added, n u t r i e n t r a t i o s ) and t h e i r p o t e n t i a l f o r i n d u c i n g e i t h e r f a v o u r a b l e or d e l e t e r i o u s r e s p o n s e s i n t h e t a r g e t e d t r o p h i c r e l a t i o n s h i p s i n t h e r e c i p i e n t l a k e s . The main o b j e c t i v e was u s u a l l y t o d e t e r m i n e whether t h e r e was an i n c r e a s e i n f i s h p r o d u c t i o n f o l l o w i n g f e r t i l i z a t i o n . V e r y few of t h e p u b l i c a t i o n s p r o v i d e good q u a l i t a t i v e o r q u a n t i t a t i v e d a t a d e a l i n g w i t h a s p e c t s o f t h e l a k e e c o s y s t e m o t h e r t h a n f i s h , s u c h as p h y t o p l a n k t o n o r z o o p l a n k t o n d y n a m i c s . Some r e p o r t s d e a l w i t h t h i s s u p e r f i c i a l l y ( D o n a l d s o n e_t a l . , 1971; S t o c k n e r e t a l . , 1971) w h i l e o t h e r s e m p h a s i z e c h a n g e s i n t h e p h y t o p l a n k t o n w h i l e i g n o r i n g c o r r e s p o n d i n g c h a n g e s i n t h e z o o p l a n k t o n ( S c h i n d l e r e t a l . , 1971, 1973). Thus, w h i l e t h e r e has been a c o n s i d e r a b l e amount of work done on l a k e e n r i c h m e n t , ou r knowledge o f i t s e f f e c t i v e n e s s as a l a k e management t o o l i s s t i l l v e r y l i m i t e d . My r e s e a r c h c o n c e r n s t h e e f f e c t s o f t h e a d d i t i o n o f i n o r g a n i c n i t r o g e n and p h o s p h o r o u s , i n f r e q u e n t , s m a l l d o s e s and a t h i g h N:P . r a t i o s , on t h e w a t e r c h e m i s t r y , p h y t o p l a n k t o n c o m p o s i t i o n and z o o p l a n k t o n community s t r u c t u r e i n a s m a l l o l i g o t r o p h i c l a k e . I t a l s o a d d r e s s e s t h e e f f e c t s o f c h a n g e s i n z o o p l a n k t o n community s t r u c t u r e on t h e s u r v i v a l o f p l a n t e d j u v e n i l e c u t t h r o a t t r o u t (Salmo c l a r k i c l a r k i ) and on t h e d i e t of a d u l t t r o u t of t h i s s p e c i e s i n d i g e n o u s t o t h e l a k e . I t i s my 3 i n t e n t i o n t o show t h a t t h e f r e q u e n c y , m a g n i t u d e and N:P r a t i o o f n u t r i e n t s u p p l y t o f r e s h w a t e r s can p r o d u c e p r e d i c t a b l e c h a n g e s i n t h e p h y t o p l a n k t o n c o m m u n i t i e s o f l a k e s and t h a t t h e s e c h a n g e s can be u s e d t o a c h i e v e v a r i o u s management g o a l s l i k e i n c r e a s i n g f i s h p r o d u c t i o n o r a m e l i o r a t i n g t h e water q u a l i t y o f c u l t u r a l l y e u t r o p h i e d l a k e s . In o r d e r t o s e t t h e s t a g e f o r t h e d i r e c t i o n I have t a k e n i n t h i s r e s e a r c h , I w i l l r e v i e w l a k e f e r t i l i z a t i o n a s i t h as been p r a c t i c e d t o d a t e . L a k e f e r t i l i z a t i o n , w i t h t h e o b j e c t i v e of i n c r e a s i n g t h e p r o d u c t i o n o f f i s h , u s u a l l y i n v o l v e s t h e a d d i t i o n of a c o m b i n a t i o n o f n i t r o g e n and p h o s p h o r o u s i n o r g a n i c f e r t i l i z e r s , a l t h o u g h , i n some c a s e s , o r g a n i c f e r t i l i z e r s have a l s o been u s e d ( s e e M a c i o l e k , 1954, and M o r t i m e r , 1954, f o r a r e v i e w ; Juday and S c h l o e m e r , 1938). E a r l y a t t e m p t s a t l a k e f e r t i l i z a t i o n ( p r i o r t o 1960) employed l a r g e d o s e s o f low N:P r a t i o f e r t i l i z e r s w h i c h were added t o t h e r e c i p i e n t l a k e v e r y i n f r e q u e n t l y . N u t r i e n t a d d i t i o n s o f t e n r a i s e d i n - s i t u i n o r g a n i c p h o s p h a t e l e v e l s t o as h i g h as 100 u g / l i t e r ( S m i t h , 1948; H o l d e n , 1954; B a x t e r , 1959), 300 u g / l i t e r ( B a l l , 1948), o r even i n e x c e s s o f 500 u g / l i t e r ( L a n g f o r d , 1948). The r a t i o o f n i t r o g e n t o p h o s p h o r o u s of t h e a d d e d f e r t i l i z e r s r a n g e d between 0 and 5:1. N u t r i e n t s were u s u a l l y a p p l i e d i n one y e a r l y dose ( S m i t h , 1945, 1948; H o l d e n , 1954; W e a t h e r l e y and N i c h o l l s , 1955; B a x t e r , 1959) o r i n m o n t h l y i n t e r v a l s ( B a l l , 1948, Morgan, 1961). Most of t h e p u b l i c a t i o n s a r i s i n g f r o m t h e s e e a r l y s t u d i e s document a t l e a s t some t e m p o r a l c h a n g e s i n t h e c h e m i s t r y and b i o l o g y o f t h e f e r t i l i z e d l a k e s ( L a n g f o r d , 1948; H o l d e n , 1954; N e l s o n and Edmondson, 1955; 4 S m i t h , 1969). T h e s e d a t a show t h a t , r e l a t i v e t o p e r - e n r i c h m e n t o l i g o t r o p h i c c o n d i t i o n s , f e r t i l i z e d w a t e r s c o n s i s t e n t l y e x h i b i t e d t h e f o l l o w i n g c h a r a c t e r i s t i c s : 1) S t r i k i n g i n c r e a s e s i n p h y t o p l a n k t o n a b u n d a n c e , but a l m o s t t o t a l dominance by f i l a m e n t o u s and t o x i c f o r m s o f b l u e - g r e e n a l g a e ( B a l l , 1948; Rabe and G a u p h i n , 1964; Rabe and D y e r , 1964; S m i t h , 1969). 2) I n c r e a s e d z o o p l a n k t o n a b u n d a n c e s but fewer s p e c i e s and d i f f e r e n t community d o m i n a n t s , e s p e c i a l l y w i t h d i s a p p e a r a n c e s of l a r g e f i l t e r - f e e d i n g c l a d o c e r a n s , l i k e D a p h n i a ( B a l l , 1948; D o n a l d s o n e t a l . , 1971; S m i t h , 1969) and r e p l a c e m e n t by s m a l l , r a p t o r i a l f e e d e r s , s u c h as r o t i f e r s and c o p e p o d s ( L a n g f o r d , 1948; W e a t h e r l e y and N i c h o l l s , 1955; N e l s o n and Edmondson, 1955; Rabe and D y e r , 1964; S m i t h , 1969). 3) S h o r t - t e r m i n c r e a s e s i n t h e a b u n d a n c e s o f b o t h p h y t o p l a n k t o n and z o o p l a n k t o n (2 weeks - 1 month) ( L a n g f o r d , 1948; W e a t h e r l e y and N i c h o l l s , 1955; S m i t h , 1969). 4) I n c r e a s e d f i s h k i l l s c a u s e d by r e d u c e d l e v e l s o f d i s s o l v e d oxygen f o l l o w i n g f e r t i l i z a t i o n ( B a l l , 1948; T a n n e r , 1960) o r t o x i n s p r o d u c e d by b l u e - g r e e n a l g a e ( r e v i e w e d by Gorham, 1962). 5 F o l l o w i n g t h e s t u d i e s of V o l l e n w e i d e r (1968) w h i c h c l e a r l y i l l u s t r a t e d t h a t t h e d e g r e e of e u t r o p h i c a t i o n i n f r e s h w a t e r s was a f u n c t i o n of t h e a n n u a l r a t e o f n u t r i e n t i n p u t and of l a k e morphometry, l a k e managers m o d i f i e d t h e i r f e r t i l i z a t i o n p r ograms i n an e f f o r t t o r e d u c e t h e l e v e l o f e u t r o p h i c a t i o n t h a t was a s s o c i a t e d w i t h e a r l i e r f e r t i l i z a t i o n schemes. The amount of i n o r g a n i c p h o s p h o r o u s p e r a d d i t i o n was g r e a t l y r e d u c e d t o r e f l e c t V o l l e n w e i d e r ' s (1976) a c c e p t a b l e l o a d i n g r a t e s f o r m e s o t r o p h i c l a k e s ( c a . 3-8 gP/m 2/year, see S t o c k n e r e t a l . , 1980). A d d i t i o n s were made much more f r e q u e n t l y ( w e e k l y ) t o p r e v e n t a l g a e from d e p l e t i n g n u t r i e n t s t o o r a p i d l y between l o a d i n g e v e n t s . A l s o , N:P r a t i o s were i n c r e a s e d t o a r o u n d 10-15:1 t o b e t t e r r e f l e c t t h e a v e r a g e r a t i o o f t h e s e n u t r i e n t s i n h e a l t h y p h y t o p l a n k t o n p o p u l a t i o n s ( R e d f i e l d , 1958). As a r e s u l t o f t h e s e c h a n g e s , t h e f o l l o w i n g p a t t e r n s of c h ange i n t h e b i o t a o f r e c i p i e n t l a k e s , r e l a t i v e t o e a r l i e r methods o f e n r i c h m e n t , were n o t e d : 1) an i n c r e a s e i n o v e r a l l l a k e p r i m a r y p r o d u c t i o n w i t h o u t c a u s i n g a c o n d i t i o n o f e u t r o p h i c a t i o n ( P a r s o n s e_t a l . , 1972; S t o c k n e r et al., 1980); 2) enhancement o f a much b r o a d e r r a n g e o f p h y t o p l a n k t o n s p e c i e s , a l t h o u g h , t h i s may be more a r e f l e c t i o n of more r e f i n e d s a m p l i n g and i d e n t i f i c a t i o n t e c h n i q u e s r a t h e r t h a n a change i n t h e p a t t e r n of f e r t i l i z a t i o n ; 6 3) no v i s i b l e blooms o f b l u e - g r e e n a l g a e ; 4) a p r e d o m i n a n c e of s m a l l p h y t o p l a n k t o n s p e c i e s r e l a t i v e t o t h o s e i n t h e u n f e r t i l i z e d l a k e e n v i r o n m e n t ( S c h i n d l e r , 1971, 1973; L u n d g r u n , 1978); 5) p r o l o n g e d i n c r e a s e s (3 t o 7 months) i n a l g a l b i o m a s s ( P a r s o n s e t a l . 1972; S c h i n d l e r e t a l . , 1973; L u n d g r u n , 1978; S t o c k n e r e t a l . , 1980). T h e s e c h a n g e s i n t h e o v e r a l l p a t t e r n of r e s p o n s e of p h y t o p l a n k t o n i n f r e s h w a t e r l a k e s t o two d i f f e r e n t methods of f e r t i l i z e r a p p l i c a t i o n s u g g e s t e d t h e p o s s i b l i t y of r e g u l a t i n g s p e c i e s c o m p o s i t i o n s of p h y t o p l a n k t o n and u l t i m a t e l y z o o p l a n k t o n by c o n t r o l l i n g t h e r a t e of a d d i t i o n and N:P r a t i o o f t h e n u t r i e n t l o a d t o a w ater body. The m e c h a n i s t i c r e l a t i o n s h i p s between n u t r i e n t s , p h y t o p l a n k t o n and z o o p l a n k t o n i n a q u a t i c e c o s y s t e m s have a l l t o o o f t e n been v i e w e d i n a r e l a t i v e l y s i m p l i s t i c way, i . e . i n c r e a s e d n u t r i e n t l o a d i n g on a w a t e r body u s u a l l y i n c r e a s e s i t s c a p a c i t y t o s u p p o r t g r e a t e r p r o d u c t i o n and m a i n t a i n l a r g e r s t a n d i n g c r o p s of p h y t o p l a n k t o n and z o o p l a n k t o n . In r e a l i t y , t h e i n t e r r e l a t i o n s h i p s a r e e x t r e m e l y complex. T h a t v a r i a t i o n s i n t h e c h e m i c a l c o m p o s i t i o n o f n a t u r a l w a t e r s may be i m p o r t a n t i n r e g u l a t i n g t h e abundance, c o m p o s i t i o n and t h e g e o g r a p h i c and p e r i o d i c d i s t r i b u t i o n o f p h y t o p l a n k t o n has been r e c o g n i z e d s i n c e t h e s t u d i e s o f P e a r s a l l ( 1 9 3 0 , 1 9 3 2 ) . S i n c e t h e n , a g r e a t d e a l o f r e s e a r c h on t h e l i n k between p h y t o p l a n k t o n and water 7 c h e m i s t r y has been c o n d u c t e d ( f o r r e v i e w s see Lund, 1965; H e a l e y , 1973; Fogg, 1975; M o r r i s , 1980). T h e r e have a l s o been e x t e n s i v e i n v e s t i g a t i o n s c o n c e r n i n g t h e e f f e c t s o f c h a n g e s i n p h y t o p l a n k t o n s p e c i e s c o m p o s i t i o n on t h e p o p u l a t i o n d y n a m i c s of i n d i v i d u a l z o o p l a n k t o n s p e c i e s ( e g . A r n o l d , 1971; P o r t e r , 1977; P o r t e r and O r c u t t , 1980) and on c h a n g e s i n z o o p l a n k t o n community s t r u c t u r e . ( e g . G l i w i c z , 1980; K e r f o o t and DeMott, 1980; R i c h a r d s o n and Dodson, 1983). From t h e e x t e n s i v e l i t e r a t u r e d e t a i l i n g r e l a t i o n s h i p s between n u t r i e n t s , p h y t o p l a n k t o n and z o o p l a n k t o n , I i s o l a t e d t h r e e p r o p e r t i e s of f r e s h w a t e r a q u a t i c e c o s y s t e m s w h i c h a r e r e l e v a n t t o t h i s s t u d y . 1) N:P r e s o u r c e r a t i o s and p h y t o p l a n k t o n s p e c i e s c o m p o s i t i o n . S c h i n d l e r (1977) s u g g e s t e d t h a t N:P r a t i o s on t h e o r d e r o f 15:1 of n u t r i e n t s added t o n a t u r a l l a k e s s h o u l d n o t f a v o u r t h e d e v e l o p m e n t o f "water bloom" b l u e g r e e n a l g a e . The r a t i o n a l e f o r t h i s s u g g e s t i o n f o l l o w e d e x p e r i m e n t s ( S c h i n d l e r , 1977) w h i c h showed t h a t n u t r i e n t s added a t low N:P r a t i o s (5:1) d u r i n g w h o l e - l a k e f e r t i l i z a t i o n e x p e r i m e n t s , p r o m o t e d blooms of n i t r o g e n - f i x i n g b l u e - g r e e n a l g a l f o r m s w h i l e h i g h e r N:P r a t i o s (15:1) d i d n o t . However, t h e a d d i t i o n o f n u t r i e n t s a t t h e h i g h e r N:P r a t i o s s t i l l r e s u l t e d i n an o c c a s i o n a l dominance of t h e a l g a l community by n o n - n i t r o g e n f i x i n g , f i l a m e n t o u s b l u e -g r e e n a l g a l forms s u c h a s O s c i l l a t o r i a ( S c h i n d l e r e t a_l . , 1971, 1973). W h i l e t h e s e b l u e - g r e e n a l g a e d i d not f o r m v i s i b l e f l o a t i n g s u r f a c e scums, t h e i r d o minance o f t h e a l g a l b i o m a s s 8 c o u l d p o s e a c o n s i d e r a b l e t h r e a t t o t h e g r o w t h and s u r v i v a l of l a r g e c l a d o c e r a n z o o p l a n k t o n ( A r n o l d , 1971; P o r t e r and O r c u t t , 1980; I n f a n t e and L i t t , 1985) and u l t i m a t e l y change the s t r u c t u r e o f t h e z o o p l a n k t o n community (Richman and Dodson, 1983) i n a way t h a t m i g h t compromise g r o w t h and s u r v i v a l i n f i s h . S i m i l a r d o m i n a n c e s of b l u e - g r e e n a l g a e have been r e p o r t e d i n o t h e r s t u d i e s where n u t r i e n t s were added t o l a k e s i n N:P r a t i o s r a n g i n g from 10 ( P a r s o n s e t a l . , 1972) t o 15 ( S t o c k n e r e t a l . , 1980). In t h e l a t t e r s t u d y , f l o a t i n g mats o f Anabaena o c c u r r e d i n one of 17 f e r t i l i z e d l a k e s ( C l a y o q u o t Arm, Kennedy L a k e ) , most o f t h e o t h e r l a k e s d e v e l o p e d were d o m i n a t e d by t h e b l u e - g r e e n M e r i s m o p e d i a . In t h i s r e g a r d , Rhee (1978) s u g g e s t e d t h a t d i f f e r i n g optimum N:P r a t i o s f o r g r o w t h i n g r e e n a l g a e and b l u e - g r e e n a l g a e m i g h t i n f l u e n c e t h e i r m u t u a l c o m p e t i t i o n a l o n g d i f f e r i n g N:P g r a d i e n t s . L a t e r work (Rhee and Gotham, 1980) showed t h a t t h e s p e c i f i c o p t i m a l r a t i o s o f c e l l N and P q u o t a s d u r i n g n ormal g r o w t h , d i f f e r e d f o r e i g h t s p e c i e s o f a l g a e , r a n g i n g f r o m lows of 7 f o r t h e d i a t o m M e l o s i r a b i n d e r a n a and 9 f o r t h e b l u e - g r e e n M i c r o c y s t i s , t o as h i g h a s 20-30 f o r t h r e e s p e c i e s of c h l o r o c o c c o i d a l g a e . T h e s e d a t a s u g g e s t , a l t h o u g h not c o n c l u s i v e l y , t h a t g r e e n a l g a e m i g h t be a b l e t o o u t c o m p e t e b l u e -g r e e n a l g a e i n a medium w h i c h c o n t a i n s much h i g h e r N:P r a t i o s t h a n 15:1, a s s u g g e s t e d by S c h i n d l e r ( 1 9 7 7 ) . T h e r e i s e v i d e n c e t o s u p p o r t t h i s a r g ument. Edmondson (1972) g i v e s d a t a w h i c h c l e a r l y show t h a t t h e p r o p o r t i o n of b l u e - g r e e n a l g a e i n t h e p h y t o p l a n k t o n of Lake W a s h i n g t o n 9 d e c r e a s e d s i g n i f i c a n t l y f o l l o w i n g d i v e r s i o n o f sewage i n p u t s t o t h e l a k e and a s u b s e q u e n t i n c r e a s e i n i n - s i t u N:P r a t i o s t o as h i g h a s 50:1. B a r i c a et. a_l. (1980) f o u n d t h a t c y a n o p h y t e s w h i c h n o r m a l l y d o m i n a t e d t h e p h y t o p l a n k t o n i n p r a i r i e p o t h o l e l a k e s c o u l d be c o m p l e t e l y e l i m i n a t e d by e l e v a t i n g i n - s i t u N:P r a t i o s f r o m 5:1 ( t h e n a t u r a l l a k e l e v e l s ) t o more t h a n 50:1 by a d d i n g i n o r g a n i c n i t r o g e n . The r e s u l t i n g a l g a l a s s e m b l a g e c o n s i s t e d p r e d o m i n a n t l y o f s m a l l c h l o r o p h y t e s and c r y p t o m o n a d s . More r e c e n t l y , S m i t h (1983) f o u n d t h a t b l u e - g r e e n a l g a e ( b o t h n i t r o g e n - f i x e r s and n o n - n i t r o g e n - f i x e r s ) f o r m e d t h e dominant component o f t h e a l g a l a s s e m b l a g e s f o r most of t h e y e a r i n h u n d r e d s of n o r t h t e m p e r a t e l a k e s whose a v e r a g e summer e p i l i m n e t i c N:P r a t i o s f e l l below 29:1. Hence, i t a p p e a r s t h a t b l u e - g r e e n c a n s t i l l p r e d o m i n a t e i n i n l a k e s y s t e m s where i n - s i t u N:P r a t i o s a r e g r e a t e r t h a n t h o s e p r e v i o u s l y s u g g e s t e d ( 1 5 : 1 ) , w h i l e l e s s o b j e c t i o n a b l e forms ( f r o m a l a k e manager's s t a n d p o i n t ) d o m i n a t e a t much h i g h e r N:P r a t i o s . However, d a t a t o s u p p o r t t h i s c o n c l u s i o n a r e few. 2) S m a l l a l g a e grow f a s t e r t h a n l a r g e a l g a e when e n v i r o n m e n t a l  c o n d i t i o n s f o r g r o w t h a r e i d e n t i c a l ( B a n s e , 1976; T u r p i n and H a r r i s o n , 1980). The s e a s o n a l p a t t e r n o f a l g a l s u c c e s s i o n i n t e m p e r a t e f r e s h w a t e r s s u g g e s t s t h a t t h e f r e q u e n c y of n u t r i e n t s u p p l y t o l a k e s may be i m p o r t a n t d e t e r m i n i n g t h e s i z e s t r u c t u r e of a l g a l c o m m u n i t i e s . In t e m p e r a t e f r e s h w a t e r s , s m a l l a l g a e t e n d t o d o m i n a t e t h e p h y t o p l a n k t o n d u r i n g l a t e s p r i n g - e a r l y summer. 10 T h i s i s t h e t i m e when n u t r i e n t s a r e a v a i l a b l e i n s u f f i c i e n t c o n c e n t r a t i o n s t o s u p p o r t r a p i d g r o w t h o f s m a l l a l g a l forms t o t h e e x t e n t t h a t t h e y can o u t p a c e l o s s e s due t o g r a z i n g and s i n k i n g ( W e t z e l , 1975; P o r t e r , 1977). A l s o , b e c a u s e t h e w ater column i s t u r n i n g o v e r and snow m e l t and s e a s o n a l r a i n f a l l r e s u l t i n h i g h e r d i s c h a r g e s i n t o l a k e s , n u t r i e n t s a r e r e p l e n i s h e d f r e q u e n t l y . As summer p r o g r e s s e s , t h e a u t o c h t h o n o u s n u t r i e n t s u p p l y becomes p a t c h y a s r a i n f a l l e v e n t s o c c u r l e s s o f t e n . E p i l i m n e t i c n u t r i e n t c o n c e n t r a t i o n s become d e p l e t e d as s t r a t i f i c a t i o n s e t s i n and a r e o f t e n t o o low t o a l l o w s m a l l a l g a e t o m a i n t a i n t h e g r o w t h r a t e s needed t o o u t p a c e p r e d a t i o n ( P o r t e r , 1977). Thus t h e y a r e r e p l a c e d by t h e l a r g e r u n g r a z e d a l g a l forms w h i c h no l o n g e r have t o compete w i t h them f o r n u t r i e n t s . I f i t i s t r u e t h a t s m a l l a l g a l forms grow f a s t e r t h a n l a r g e r a l g a e u nder i d e n t i c a l c o n d i t i o n s , as s u g g e s t e d by Banse ( 1 9 7 6 ) , t h e n i t i s r e a s o n a b l e t o assume t h a t p r o v i d i n g a c o n t i n u o u s s u p p l y of t h e r i g h t c o m b i n a t i o n of n u t r i e n t s o v e r t h e summer s h o u l d f a v o u r t h e g r o w t h o f s m a l l a l g a l f orms o v e r l a r g e r a l g a e . T h i s has been shown f o r m a r i n e w a t e r s . T u r p i n and H a r r i s o n (1980) f o u n d t h a t t h e mean c e l l d i a m e t e r of n a t u r a l p h y t o p l a n k t o n c o m m u n i t i e s was r e l a t e d t o t h e t i m e between l i m i t i n g n u t r i e n t a d d i t i o n s . Mean c e l l d i a m e t e r d e c r e a s e d as f r e q u e n c y o f n u t r i e n t a d d i t i o n s i n c r e a s e d . I n r e c e n t l a k e f e r t i l i z a t i o n e x e r c i s e s , n u t r i e n t s were b e i n g a d d e d t o l a k e s on a w e e k l y b a s i s . T h i s r a t e o f a d d i t i o n does a p p e a r t o s t i m u l a t e t h e g r o w t h of s m a l l a l g a l forms i n some c a s e s ( S c h i n d l e r et. a l . , 11 1971, 1973; L u n d g r u n , 1978), however, S c h i n d l e r e t a l . (1971,1973) and P a r s o n s e t a l _ . (1972) b o t h r e p o r t s e a s o n a l d o m i n a n c e s by l a r g e a l g a e , s u c h as S t a u r a s t r u m , D i n o b r y o n , O s c i l l a t o r i a and v a r i o u s e u g l e n o i d s , f o l l o w i n g w e e k l y a d d i t i o n s . G i v e n t h a t s m a l l a l g a e (2-50 um i n s i z e ) a r e p r e f e r e n t i a l l y g r a z e d by z o o p l a n k t o n o v e r l a r g e r a l g a l forms ( B u r n s , 1968a; G l i w i c z and H i l l b r i c h t - I l k o w s k a , 1972; G l i w i c z , 1980), one o b j e c t i v e of l a k e managers w i s h i n g t o i n c r e a s e f i s h p r o d u c t i o n s h o u l d be t o c o n v e r t added n u t r i e n t s i n t o t h o s e a l g a l p a r t i c l e s on w h i c h z o o p l a n k t o n f e e d . I n c r e a s i n g l a r g e n e t p l a n k t o n a b u n d a n c e s would o n l y s e r v e t o i n h i b i t t h e p r o d u c t i o n of l a r g e c l a d o c e r a n s ( B u r n s , 1968b; P o r t e r and O r c u t t , 1980) upon w h i c h t h e f i s h w i l l s e l e c t i v e l y f e e d ( Z a r e t , 1980). 3) The r o l e of p h y t o p l a n k t o n c o m p o s i t i o n i n d e t e r m i n i n g  z o o p l a n k t o n community s t r u c t u r e . ( G l i w i c z , 1980; Richman and Dodson, 1983) G r a z e r z o o p l a n k t o n s p e c i e s change as t h e s p e c i e s c o m p o s i t i o n o f p h y t o p l a n k t o n c h a n g e s . In t e m p e r a t e l a k e s , n e t p h y t o p l a n k t o n t e n d t o w a r d p r e d o m i n a n c e d u r i n g t h e mid-summer months ( H u t c h i n s o n , 1967; P o r t e r , 1977). The g e n e r a l p a t t e r n o f change i n t h e z o o p l a n k t o n i s t h a t l a r g e c l a d o c e r a n s s u c h as D a p h n i a and H o l o p e d i u m , w h i c h d o m i n a t e t h e z o o p l a n k t o n d u r i n g e a r l y summer, d i s a p p e a r o r become l e s s a b u n d a n t , and a r e r e p l a c e d as community d o m i n a n t s by s m a l l e r c l a d o c e r a n s a n d / o r c o p e p o d s . I t i s b e l i e v e d t h a t s m a l l e r c l a d o c e r a n s a r e more e f f i c i e n t f i l t e r - f e e d e r s i n media c o n t a i n i n g l a r g e amounts o f 12 ne t p h y t o p l a n k t o n , so t h e y o u t c o m p e t e t h e l a r g e r z o o p l a n k t e r s f o r t h e a v a i l a b l e g r a z a b l e p h y t o p l a n k t o n ( G l i w i c z , 1980). L a r g e c l a d o c e r a n s become abundant a g a i n i n t h e f a l l , o r when n u t r i e n t i n p u t s p r omote blooms of t h e r a p i d l y g r o w i n g s m a l l a l g a l s p e c i e s ( P o r t e r , 1977; G l i w i c z , 1980; Richman and Dodson, 1983). S e v e r a l l a b o r a t o r y s t u d i e s have shown t h a t c o l o n i a l and f i l a m e n t o u s a l g a e c l o g t h e f i l t e r i n g a p p a r a t u s of l a r g e c l a d o c e r a n s l i k e D a p h n i a ( B u r n s , 1968b; P o r t e r , 1975; Web s t e r and P e t e r s , 1978; G l i w i c z and S e i d l a r , 1980). C l a d o c e r a n p o p u l a t i o n s have been known t o d e c l i n e o r d i s a p p e a r c o m p l e t e l y when f i l a m e n t o u s a l g a l forms p r e d o m i n a t e ( B u r n s , 1968b). L o s s e s o f l a r g e z o o p l a n k t e r s would mean l e s s f o o d f o r f i s h i n l a k e s and p r o d u c t i o n c o u l d be compromised i f t h e f i s h do n o t t u r n t o a n o t h e r f o o d r e s o u r c e . C o n s e q u e n t l y , f r o m a f i s h e r i e s s t a n d p o i n t , i t would be d e s i r a b l e t o m a i n t a i n t h e e x i s t e n c e o f l a r g e c l a d o c e r a n s f o r p e r i o d s l o n g e r t h a n n o r m a l p a t t e r n s o f z o o p l a n k t o n s u c c e s s i o n might d i c t a t e . T h i s c o u l d be made p o s s i b l e by r e d u c i n g o r e l i m i n a t i n g t h e n e t p h y t o p l a n k t o n i n t h e s y s t e m and e n h a n c i n g t h e s m a l l , r a p i d l y g r o w i n g a l g a e on w h i c h t h e l a r g e c l a d o c e r a n s f e e d . The e x t e n t t o w h i c h t h e r e s e a r c h d i s c u s s e d above c a n be a p p l i e d t o whole l a k e e c o s y s t e m s h a s y e t t o be f u l l y r e s o l v e d , s i n c e t h e many of t h e s e s t u d i e s have been p e r f o r m e d i n c u l t u r e f l a s k s i n t h e l a b o r a t o r y o r i n s m a l l s c a l e e n c l o s u r e e x p e r i m e n t s . However, t h i s r e s e a r c h does pose some i n t e r e s t i n g q u e s t i o n s o f e c o l o g i c a l i m p o r t a n c e . 1 3 1) Does the a d d i t i o n of combined n i t r o g e n and phosphorous i n o r g a n i c f e r t i l i z e r s at high N:P r a t i o s to a water body r e s u l t in a r e d u c t i o n i n or an e l i m i n a t i o n of the b l u e -green algae i n a system where they form a s i g n i f i c a n t p r o p o r t i o n of the a l g a l biomass, such as i n c u l t u r a l l y e u t r o p h i c l akes? To my knowledge, t h i s has not been t e s t e d e x p e r i m e n t a l l y , however, B a r i c a et a l . ( 1 9 8 1 ) d i d attempt to a l t e r N:P r a t i o s by adding only n i t r o g e n . 2 ) Can the frequency of n u t r i e n t a d d i t i o n to freshwaters be manipulated to produce phytoplankton communities dominated by small a l g a l c e l l s , thereby i n c r e a s i n g the e f f i c i e n c y of food chain energy t r a n s f e r ? T h i s has not been shown f o r freshwater a l g a l communites. 3 ) I f ( 1 ) and ( 2 ) occur, can these changes a f f e c t the outcome of c o m p e t i t i o n between herbivorous zooplankton and thus a l t e r the zooplankton community s t r u c t u r e ? 4) Can the a d d i t i o n of n u t r i e n t to l a k e s be manipulated to produce zooplankton communities which are dominated over the e n t i r e summer by l a r g e c l a d o c e r a n s , thereby enhancing the food f o r f i s h ? I chose to address these q u e s t i o n s by e x p e r i m e n t a l l y f e r t i l i z i n g l a r g e e n c l o s u r e s i n a s m a l l , o l i g o t r o p h i c c o a s t a l montane l a k e . The lake e x h i b i t e d a t y p i c a l north temperate seasonal p a t t e r n of a l g a l s u c c e s s i o n . In e a r l y s p r i n g the 1 4 dom i n a n t a l g a l s p e c i e s were s m a l l g r e e n s . B l u e g r e e n a l g a e were r a r e . The summer and autumn a s s o c i a t i o n s were d o m i n a t e d by l a r g e u n g r a z a b l e c h r y s o p h y t e s and c o l o n i a l b l u e - g r e e n a l g a e , r e s p e c t i v e l y . Z o o p l a n k t o n d o m i n a n t s d u r i n g t h e e a r l y summer c o n s i s t e d of l a r g e c l a d o c e r a n s , g i v i n g way t o s m a l l c l a d o c e r a n s and c o p e p o d s as summer p r o g r e s s e d . S i n c e N:P r a t i o s i n t h e l a k e a t s p r i n g o v e r t u r n , when s m a l l g r e e n a l g a e were community d o m i n a n t s , were h i g h ( 4 0 : 1 ) , t h i s was t h e r a t i o o f n u t r i e n t s I c h o s e t o u s e . J_.2_ R e s e a r c h O b j e c t i v e s My g o a l s i n t h i s r e s e a r c h f o c u s s e d on t e s t i n g t h e o r e t i c a l p r e d i c t i o n s p e r t a i n i n g t o t h e r e s p o n s e of a q u a t i c p l a n k t o n s y s t e m s i n an o l i g o t r o p h i c montane l a k e t o f r e q u e n t ( t h r i c e w e e k l y ) , s m a l l (3.3 u g / l i t e r P) d o s e s of h i g h N:P r a t i o (40:1) i n o r g a n i c f e r t i l i z e r s , and on a p p l y i n g t h e r e s u l t s of t h e s e p r e d i c t i o n s t o 1) l a k e f e r t i l i z a t i o n and t h e management of t r o u t p r o d u c t i o n i n l a k e s and 2) t h e c o n t r o l of "water blooms" i n l a k e s s u b j e c t t o c u l t u r a l e u t r o p h i c a t i o n . A s e c o n d a r y g o a l was t o improve upon e x i s t i n g t e c h n o l o g y f o r d o i n g i n - s i t u e n c l o s u r e e x p e r i m e n t s so as t o p r o d u c e r e s u l t s t h a t a r e more r e p r e s e n t a t i v e o f t h e r e s e a r c h l a k e a s a whole. T h i s was done t h r o u g h t h e use o f l i m n o c o r r a l s w h i c h were l a r g e enough t o s i m u l a t e t h e l a k e e n v i r o n m e n t , w i t h some of t h e c o r r a l s e n c l o s i n g p a r t s of t h e l i t t o r a l zone and o t h e r s e n c l o s i n g p a r t s of t h e l i m n e t i c zone o f t h e l a k e . To a c h i e v e t h e s e g o a l s , I f o r m u l a t e d t h e f o l l o w i n g r e s e a r c h o b j e c t i v e s . 1 5 1) To e v a l u a t e t h e use o f l a r g e 1 i m n o c o r r a l s (150 m 2 ) , c o n t a i n i n g s e v e r a l h u n d r e d t h o u s a n d l i t e r s of l a k e w a t e r , i n s i m u l a t i n g t h e p h y s i c a l , c h e m i c a l and b i o l o g i c a l d y n a m i c s o f t h e p a r e n t l a k e e n v i r o n m e n t . 2) To d e t e r m i n e t h e e f f e c t s o f f e r t i l i z a t i o n on t h e p h y s i c a l , c h e m i c a l and b i o l o g i c a l d y n a m i c s of t h e l a k e w a t e r s by c o m p a r i n g t h e c h a n g e s i n t h e s e p a r a m e t e r s i n t h e l i t t o r a l and t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s w i t h t h e i r r e s p e c t i v e u n t r e a t e d c o n t r o l s . 3) To d e t e r m i n e whether f e r t i l i z a t i o n a t h i g h N:P a t o m i c r a t i o s (> 15:1) w i l l r e s u l t i n a s i g n i f i c a n t r e d u c t i o n i n , o r a c o m p l e t e e l i m i n a t i o n o f , t h e b l u e - g r e e n a l g a e t h a t form one o f t h e m a j o r a l g a l d o m i n a n t s i n t h i s l a k e ' s p h y t o p l a n k t o n a s s e m b l a g e o v e r t h e e n t i r e summer. 4) To d e t e r m i n e whether f e r t i l i z a t i o n u s i n g s m a l l , f r e q u e n t d o s e s of n i t r o g e n and p h o s p h o r o u s f e r t i l i z e r s c a n promote a s h i f t i n a p h y t o p l a n k t o n a s s e m b l a g e t h a t i s d o m i n a t e d t h r o u g h o u t t h e summer by l a r g e u n g r a z e d c h r y s o p h y t e s and b l u e - g r e e n s t o w a r d s one t h a t f a v o u r s dominance by s m a l l e r a l g a l f orms t h a t z o o p l a n k t o n c a n a s s i m i l a t e much e a s i e r . 5) I f (4) o c c u r s , t o d e t e r m i n e i f , and t h e e x t e n t t o w h i c h , f e r t i l i z a t i o n w i t h h i g h N:P r a t i o s w ould l e a d t o an 16 i n c r e a s e i n t h e s t a n d i n g s t o c k of a v a i l a b l e e d i b l e p h y t o p l a n k t o n and t o compare t h e s e c h a n g e s i n t h e l i t t o r a l v s t h e l i m n e t i c z o n e s of t h e l a k e . 6) To a s s e s s t h e e f f e c t s o f c h a n g e s a t t h e p r i m a r y t r o p h i c l e v e l on z o o p l a n k t o n b i o m a s s and community s t r u c t u r e . 7) To a s s e s s t h e e f f e c t s of any c h a n g e s i n t h e z o o p l a n k t o n s t a n d i n g s t o c k o r community s t r u c t u r e a r i s i n g f r o m e x p e r i m e n t a l n u t r i e n t a d d i t i o n on t h e f e e d i n g p a t t e r n s o f c u t t h r o a t t r o u t i n d i g e n o u s t o t h e l a k e . 1 7 2 . 0 MATERIALS AND METHODS 2 . J _ S t u d y S i t e P l a c i d Lake i s a s m a l l ( 1 . 8 7 h a . ) , s h a l l o w (zmax = 6 . 5 m), d i m i c t i c , c o a s t a l montane bog l a k e s i t u a t e d on t h e s o u t h e r n s l o p e s o f t h e C o a s t M o u n t a i n r a n g e i n s o u t h w e s t e r n B r i t i s h C o l u m b i a ( F i g u r e 1 , T a b l e 1 ) . T h i s l a k e p r o v e d t o be a s u i t a b l e s t u d y s i t e b e c a u s e o f i t s s h a l l o w d e p t h and a c c e s s i b i l i t y . I t s s h a l l o w d e p t h h e l p e d t o keep t h e c o s t of m a t e r i a l s and t h e l o g i s t i c s o f l i m n o c o r r a l c o n s t r u c t i o n w i t h i n r e a s o n , and i t s a c c e s s i b i l i t y m i n i m i z e d t h e l o g i s t i c s o f l i m n o c o r r a l i n s t a l l a t i o n . I t i s a l s o p h y s i c a l l y , c h e m i c a l l y and b i o l o g i c a l l y s i m i l a r t o h u n d r e d s o f o t h e r c o a s t a l m o u n t a i n l a k e s i n t h e r e g i o n . I t i s l o c a t e d w i t h i n t h e bounds of t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a R e s e a r c h F o r e s t were l a k e s a r e u s e d f o r r e s e a r c h p u r p o s e s o n l y . The l a k e has a s i n g l e i n d i g e n o u s , f i s h s p e c i e s , c u t t h r o a t t r o u t (Salmo c l a r k i c l a r k i ) . 2 _ . J _ . J _ C l i m a t e The c l i m a t e f o r t h e r e g i o n i s d e s c r i b e d a s m a r i n e warm t e m p e r a t e r a i n y ( m e s o t h e r m a l ) w i t h no d i s t i n c t d r y s e a s o n ( F e l l e r , 1 9 7 5 ) . The d r i e s t months a r e J u l y and A u g u s t w h i c h r e c e i v e d a mean r a i n f a l l o f 5 7 and 7 0 mm r e s p e c t i v e l y f o r t h e y e a r s 1 9 7 7 t o 1 9 8 2 ( K r a u s e , 1 9 8 4 ) . The a v e r a g e m o n t h l y p r e c i p i t a t i o n was 1 9 4 mm f o r t h e same s i x y e a r p e r i o d . The y e a r l o n g m o n t h l y a v e r a g e f o r p r e c i p i t a t i o n i n 1 9 8 2 , a p a r t i c u l a r l y 18 F i g u r e 1. L o c a t i o n of s t u d y s i t e . From N e i l l ( 1 9 7 8 ) . 19 Table 1. Morphometric parameters of P l a c i d Lake. * Drainage area (ha) 173.70 * E l e v a t i o n (m) 510 S h o r e l i n e (m) 936 Surface area (ha) 1.87 Maximum len g t h (m) 195 Maximum width (m) 195 Volume (cubic meters) 63,501 Volume development 1 .55 Maximum depth (m) 6.50 Mean depth (m) 3.40 R e l a t i v e depth (%) 4 S h o r e l i n e development 2.13 * Data from Shepherd (1973) Area and Volume by Depth S t r a t a Depth (m) 0-1 1-2 2-3 3-4 4-5 5-6 6+ Area (m2) 15360 12843 11150 7989 5469 2545 101 Volume (m3) 16874 13983 11985 9525 6689 3915 529 % T o t a l Volume 27 22 19 15 10 6 1 21 wet y e a r , was 235 mm w h i c h f e l l m a i n l y as r a i n i n O c t o b e r t h r o u g h t o A p r i l . (Snow o c c a s i o n a l l y a c c u m u l a t e s t o d e p t h s o f 30 cm i n t h i s r e g i o n but i t r a r e l y r e m a i n s f o r more t h a n a few w e e k s ) . P l a c i d Lake i s n o r m a l l y c o v e r e d w i t h an i n t e r m i t t e n t , t h i n l a y e r of i c e , sometimes o v e r l a i n w i t h snow, from a b o u t mid-December u n t i l A p r i l . Summers a r e c o o l and w i n t e r s a r e m i l d w i t h an a v e r a g e d a i l y mean t e m p e r a t u r e f o r t h e warmest month of a b o u t 17 °C. The a v e r a g e d a i l y mean t e m p e r a t u r e f o r t h e c o l d e s t month i s c l o s e t o 1 °C ( F e l l e r , 1977). T h e s e t e m p e r a t u r e s were r e c o r d e d a t a w e a t h e r s t a t i o n a t an e l e v a t i o n a p p r o x i m a t e l y 500 m l o w e r t h a n P l a c i d Lake and t h e r e f o r e may n o t a c c u r a t e l y r e p r e s e n t t h e c o n d i t i o n s t h e l a k e ' s e l e v a t i o n . T o t a l d a i l y r a i n f a l l (mm) and mean d a i l y t e m p e r a t u r e s f o r June - O c t o b e r , 1982 a r e shown i n F i g u r e 2. 2.]_.2 Lake B a s i n M o r p h o l o g y and Morphometry The g e o l o g y and s o i l s of t h e r e g i o n a r e d e s c r i b e d i n d e t a i l i n i n E f f o r d (1967) and F e l l e r ( 1 9 7 5 , 1 9 7 7 ) . The most common r o c k t y p e i n t h e a r e a i s q u a r t z d i o r i t e d i s p l a y e d i n l a r g e , smooth s u p e r f i c i a l l y w e a t h e r e d s u r f a c e s r e l a t i v e l y f r e e o f open j o i n t s s u g g e s t i n g t h a t t h e b e d r o c k i s i m p ermeable and n o t e a s i l y e r o d e d . S o i l s i n t h e a r e a a r e p r e d o m i n a n t l y h u m o - f e r r i c p o d z o l s w i t h t e x t u r e s p r e d o m i n a n t l y sandy loam and loamy sand, w i t h s o i l s a d j a c e n t t o s t r e a m s and i n some v a l l e y s b e i n g d e s c r i b e d as d eep (> 1 m) and s l i g h t l y m o i s t . S o i l s e l s e w h e r e a r e s h a l l o w and d r y t o s l i g h t l y m o i s t . 22 F i g u r e 2. R a i n f a l l and t e m p e r a t u r e d a t a f o r 1982 Figure 2. R i i n f a l l and Temperature Data f o r the U n i v e r s i t y Research Forest f o r June - October. 1982 J U N J L Y A U G S E P OCT NOV data K 3 24 P l a c i d L a k e ' s w a t e r s h e d i s bounded on t h e s o u t h e a s t , e a s t and n o r t h e a s t s i d e s by s t e e p , t r e e - c o v e r e d m o u n t a i n s r i s i n g t o o v e r 1000 m e l e v a t i o n . To t h e n o r t h and n o r t h w e s t i s a g e n t l y r i s i n g t r e e - c o v e r e d v a l l e y w h i c h e v e n t u a l l y d r o p s v e r y s l i g h t l y t o two o t h e r l a k e b a s i n s , E u n i c e L a k e t o t h e n o r t h - n o r t h e a s t and G w e n d o l i n e Lake t o t h e n o r t h w e s t . The west t o s o u t h w e s t i s bounded by a l o n g r i d g e a t a b o u t 700 m e l e v a t i o n w h i c h i s not a c t u a l l y p a r t of t h e d r a i n a g e b a s i n but a c t s a s a b a r r i e r t o win d f l o w . To t h e s o u t h i s a f a i r l y s t e e p , h e a v i l y t r e e - c o v e r e d v a l l e y s l o p e w h i c h d e s c e n d s i m m e d i a t e l y s o u t h w a r d s and e v e n t u a l l y opens o n t o a g e n t l y r o l l i n g g l a c i a l outwash p l a i n . P r e v a i l i n g winds i n t h e r e g i o n a r e p r e d o m i n a n t l y from t h e s o u t h t o n o r t h w est. The l a k e i s p r o t e c t e d on t h e s o u t h by d e n s e f o r e s t and on t h e west and n o r t h w e s t s i d e s by s t e e p s l o p e s . Hence, i t c a n be s h a r p l y s t r a t i f i e d d u r i n g t h e summer. D u r i n g p e r i o d s o f hea v y summer r a i n f a l l , l a k e w a t e r s c an r i s e as much as 1.5 m i n l e s s t h a n 24 h o u r s . T h i s sometimes r e s u l t s i n d i s r u p t i o n of s t r a t i f i c a t i o n . I t c a n , a t . t i m e s , b r e a k down c o m p l e t e l y a l l o w i n g c o m p l e t e m i x i n g o f e p i l i m n e t i c and h y p o l i m n e t i c w a t e r s f o r a b r i e f p e r i o d . I f m i x i n g o c c u r s , s t r a t i f i c a t i o n i s u s u a l l y q u i c k l y r e s t o r e d i f t h e heavy r a i n f a l l d o e s n o t c o n t i n u e . S i n c e t h e l a k e b a s i n i s s i t u a t e d on b a s a l g l a c i a l t i l l i n a v a l l e y s c o u r e d by P l e i s t o c e n e g l a c i e r movement ( F e l l e r 1975,1977) and s i n c e t h e b a s i n i s s h a l l o w and i r r e g u l a r i n shape ( F i g u r e 3 ) , i t i s p r o b a b l y a k e t t l e l a k e a s d e f i n e d by W e t z e l ( 1 9 7 5 ) . The l a k e i t s e l f i s d e v e l o p i n g i n t o a q u a k i n g bog l a k e 25 Figure 3 . Bathymetric map of Placid Lake showing locations the enclosures and the vegetation. From Shepherd ( 1 9 7 3 ) . 27 w i t h t h i c k mats of Sphagnum s p . a r o u n d t w o - t h i r d s of t h e s h o r e l i n e and e x t e n d i n g as much as 5 m o u t i n t o t h e open water ( F i g u r e 3 ) . Lagg z o n e s ( W e t z e l 1975) e x i s t a r o u n d t h e p e r i p h e r y o f t h e l a k e . The l a k e has what a p p e a r s t o be a f a l s e b o t t om, a l o o s e l y a g g r e g a t e d , f l o c c u l e n t s e d i m e n t s e v e r a l m e t e r s t h i c k (> 1 m, W.E. N e i l l , p e r s o n a l c o m m u n i c a t i o n ) . T h i s i s p r o b a b l y a r e s u l t o f t h e p r e s e n c e of t h e Sphagnum mat and i t s o n g o i n g r e l e a s e o f much p a r t i c u l a t e o r g a n i c m a t t e r i n t o t h e open w a t e r , In s e v e r a l p l a c e s a r o u n d t h e l a k e , l o g s imbedded i n t h e Sphagnum mat p r o t r u d e out i n t o t h e open water a n d / o r down i n t o t h e s e d i m e n t s . T h e r e i s one d i s t i n c t i n l e t s t r e a m on t h e n o r t h s i d e of t h e l a k e . A minor r u n o f f c h a n n e l t o t h e west c a r r i e s w a ter i n t o t h e l a k e o n l y d u r i n g p e r i o d s of h i g h r a i n f a l l . The i n l e t s t r e a m ' s f l o w i s p r o b a b l y l e s s t h a n 900 c u b i c cm/sec d u r i n g summer d r y s p e l l s ( p e r s o n a l o b s e r v a t i o n ) . The o u t l e t s t r e a m i s t o t h e s o u t h . 2._.3 C h e m i c a l F e a t u r e s Most montane c o a s t a l l a k e w a t e r s o f B r i t i s h C o l u m b i a a r e c h a r a c t e r i z e d by a low d i s s o l v e d s o l i d s c o n t e n t a v e r a g i n g o f 46 ppm ( N o r t h c o t e and L a r k i n , 1956). Many a r e as low a s 25 ppm ( N o r t h c o t e and L a r k i n , 1963). They a r e m o s t l y s m a l l , c o m p a r a t i v e l y deep, m i l d l y a c i d i c and u n p r o d u c t i v e ( N e i l l , 1981) . The c h e m i c a l c o m p o s i t i o n o f P l a c i d Lake w a t e r ( T a b l e 2) r e f l e c t s t h e n a t u r e o f t h e h a r d , e r o s i o n r e s i s t a n t b e d r o c k , t h e h i g h a n n u a l r a i n f a l l o f t h e west c o a s t c l i m a t i c r e g i o n and t h e 28 Table 2. Mean summer water chemistry values f o r P l a c i d Lake (July-August, 1982). A l l values i n u g / l i t e r unless otherwise s t a t e d . * T o t a l d i s s o l v e d s o l i d s (ppm) 24.4 pH 6.2 5 T o t a l phosphorous 6.20 T o t a l d i s s o l v e d phosphorous 5.0 S o l u b l e r e a c t i v e P (ortho-P) <3.0 (range 1-15) T o t a l K j e l d a h l n i t r o g e n 220 N i t r a t e n i t r o g e n 40 (range <20-140) N i t r i t e n i t r o g e n <5.0 Ammonia n i t r o g e n 36 (range 6-130) Organic n i t r o g e n 164 S p e c i f i c c o n d u c t i v i t y (umhos/cm @ 25 C) 16.2 Co l o r (Pt u n i t s ) 25 Secchi d i s c depth (m) 6.0 * T o t a l a l k a l i n i t y (ppm CaC03) 6.3 * Free C02 (ppm CaC03) 24.5 Hypolimnetic 02 ( m g / l i t e r @ 5m) 5.1 * Data from Shepherd (1973) f o r August 3 1972 29 s h a l l o w s o i l s of t h e a r e a . The t o t a l d i s s o l v e d s o l i d c o n t e n t of t h e mid-summer e p i l i m n i o n (upper 4 m ) a v e r a g e s 24 ppm. N u t r i e n t c o n c e n t r a t i o n s a r e p a r t i c u l a r l y low, e s p e c i a l l y s o l u b l e r e a c t i v e p h o s p h o r o u s (PO<,-P). O n l y on one o c c a s i o n was PO f l-P m e a s u r a b l e i n l a r g e q u a n t i t i e s i n t h e w a t e r s o f t h e l a k e (15 u g / l i t e r ) and t h a t was i n t h e upper 2 m d u r i n g a heavy r a i n f a l l . F e l l e r (1977) g i v e s measurements of P O„-P i n p r e c i p i t a t i o n (as r a i n ) as h i g h a s 40 u g / l i t e r and o r t h o - P v a l u e s much h i g h e r i n s u r f a c e r u n o f f i n u n l o g g e d w a t e r s h e d s . I s u s p e c t t h a t much of t h i s n u t r i e n t i n t h e r a i n i s a r e s u l t o f p r e v a i l i n g winds p i c k i n g up d r y , n u t r i e n t r i c h d u s t and f e r t i l i z e r r e s i d u e s f r o m t h e f a r m l a n d of t h e F r a s e r R i v e r v a l l e y t o t h e s o u t h . S i n c e p r e c i s e i n f o r m a t i o n a b o u t t h e s i z e o f t r i b u t a r y w a t e r s h e d s , s t r e a m d i s c h a r g e , s u r f a c e r u n o f f and g r o u n d w a t e r s u p p l y was n o t a v a i l a b l e , I d i d not a t t e m p t t o c a l c u l a t e a n u t r i e n t b u d g e t f o r t h e l a k e . However, c o n s i d e r i n g F e l l e r ' s d a t a , i t i s l i k e l y t h a t i n f r e q u e n t h i g h n u t r i e n t i n p u t e v e n t s m i g h t o c c u r d u r i n g p e r i o d s o f r a i n f a l l i n t h e d r y e r months of t h e y e a r . K r a u s e (1984) g i v e s a v e r y r o u g h e s t i m a t e o f n u t r i e n t i n p u t t o t h e l a k e , and h e r d a t a s u g g e s t t h a t n u t r i e n t s u p p l y t o t h e l a k e i s g e n e r a l l y v e r y low ( e g . , a mean of 0.47 u g / l i t e r o f p h o s p h o r o u s f o r t h e month o f A u g u s t i n 1982). N i t r o g e n a p p e a r s t o be i n good s u p p l y r e l a t i v e t o p h o s p h o r o u s , w i t h t h e n i t r o g e n s p e c i e s most u t i l i z e d by p h y t o p l a n k t o n (NH f l + and N0 3") o c c u r r i n g a t a r a t i o o f a b o u t 40:1 n i t r o g e n t o p h o s p h o r o u s (as P 0 4 - P ) a t s p r i n g o v e r t u r n . However, v e r y s h o r t l y a f t e r s p r i n g o v e r t u r n ( w i t h i n 1-2 w e e k s ) , i n o r g a n i c 30 n i t r o g e n c o n c e n t r a t i o n s a r e s h a r p l y r e d u c e d . The water i s o n l y l i g h t l y c o l o r e d and v e r y s l i g h t l y a c i d i c . The mean pH i n 1982 (6.25) was somewhat h i g h e r t h a n t h a t q u o t e d by S h e p h e r d (1973) f o r A u g u s t 1973, ( 5 . 7 5 ) , but i s i n l i n e w i t h o t h e r l a k e s i n t h e a r e a ( E f f o r d , 1963, Marmorek, 1983). I t i s p o s s i b l e t h a t S h e p h e r d ' s pH v a l u e s were t a k e n n e a r t h e edge of t h e Sphagnum mats where pH a r o u n d t h e p l a n t s may be as low as 4.5 (Clymo, 1966). S e c c h i d i s c d e p t h i s a l w a y s a t o r n e a r t h e maximum d e p t h of t h e l a k e . F r e e C 0 2 i s t h e d o m i n a n t form of a v a i l a b l e i n o r g a n i c c a r b o n . A l k a l i n i t y i s low (0.13 m e q / l i t e r ) . 2._.4 F l o r a The w a t e r s h e d l i e s w i t h i n t h e d r y subzone of t h e C o a s t a l W e s t e r n Hemlock b i o g e o c l i m a t i c zone of B r i t i s h C o l u m b i a ( E f f o r d , 1967). The a r e a was l o g g e d i n t h e 1950's and i s now c o v e r e d m o s t l y by s e c o n d g r o w t h W e s t e r n Hemlock, D o u g l a s - f i r , r e d a l d e r and some w h i t e b i r c h . F e l l e r (1977) g i v e s a d e s c r i p t i o n of common u n d e r s t o r y s p e c i e s i n t h e s e f o r e s t s . The dominant v e g e t a t i o n a r o u n d and i n t h e l a k e a r e s p e c i e s t y p i c a l l y a s s o c i a t e d w i t h Sphagnum mats. Bog c r a n b e r r y ( V a c c i n i u m o x y c o c c u s ) and L a b r a d o r t e a (Ledum g r o e n l a n d i c u m ) were t h e most common p l a n t f o u n d g r o w i n g on t h e Sphagnum. In t h e l a k e , submersed m a c r o p h y t e s i n c l u d e Potomoqeton n a t a n s , S c i r p u s s u b t e r m i n a l i s , and D e p a n o c l a d u s e x a n n u l a t u s ( S h e p h e r d , 1973, F i g u r e 3 ) . Water l i l i e s , Nuphar p o l y s e p a l u m , can be f o u n d i n s m a l l s t a n d s a l l a r o u n d t h e s h o r e l i n e . The p h y t o p l a n k t o n s p e c i e s number i n t h e h u n d r e d s but o n l y 31 t h o s e t h a t were e n c o u n t e r e d i n water column s a m p l e s were r e c o r d e d ( A p p e n d i x 1 ) . A p p e n d i x 1 may not be a c o m p l e t e a c c o u n t of t h e l i m n e t i c p l a n k t o n s p e c i e s but t h e majo r g e n e r a a r e r e p r e s e n t e d . The p h y t o p l a n k t o n a s s e m b l a g e i s c h a r a c t e r i s t i c of o l i g o t r o p h i c l a k e s ( H u t c h i n s o n , 1967; W e t z e l , 1975) and i n c l u d e s s u c h i n d i c a t o r o r g a n i s m s as t h e c h r y s o p h y c e a n s ( D i n o b r y o n s p . , M a l l o m o n a s , S y n u r a s p . , Oochromonas s p . ) , c h l o r o p h y c e a n s ( B o t r y o c o c c u s , S p h a e r o c y s t i s and G l o e o c y s t i s ) . N u m e r i c a l l y , t h e d o m i n a n t p h y t o p l a n k t e r s a r e s m a l l g r e e n f l a g e l l a t e s of t h e genus Chlamydomonas and s m a l l c h l o r o c o c c o i d s . Mid-summer d o m i n a n t s w h i c h make up more t h a n 30 % of t h e p h y t o p l a n k t o n b i o m a s s , i n c l u d e s e v e r a l s p e c i e s o f c h r y s o p h y t e s and c y a n o p h y t e s . S e a s o n a l l y , t h e n u m e r i c a l l y abundant c h r y s o p h y t e s i n c l u d e D i n o b r y o n ( s p r i n g , summer and f a l l ) , S y n u r a ( l a t e summer), C h r y s o p h a e l l a l o n q i s p i n a (mid summer) and Oochromonas ( a l l summer). C y a n o p h y t e s become more d o m i n a n t l a t e i n summer and e a r l y f a l l when l a k e N:P r a t i o s d r o p c o n s i d e r a b l y (< 15:1, N : P ) . They a r e a l w a y s m o d e r a t e l y a b u n d a n t t h r o u g h o u t t h e summer. Fauna O n l y f a u n a d i r e c t l y l i n k e d t o t h i s s t u d y a r e m e n t i o n e d b e l o w : a v i a n p r e d a t o r s on f i s h , t h e f i s h , o t h e r a q u a t i c v e r t e b r a t e s and z o o p l a n k t o n . B e n t h i c f a u n a were not s t u d i e d . A l t h o u g h nymphs and l a r v a l s t a g e s o f s e v e r a l major g r o u p s of a q u a t i c i n s e c t s o c c u r i n t h e l a k e , t h e y were r a r e l y e n c o u n t e r e d i n s a m p l e s t a k e n f r o m t h e water c o l u m n , so t h e y t o o a r e not 32 c o n s i d e r e d . A v i a n p r e d a t o r s on t h e f i s h c o n s i s t e d o f two s p e c i e s , t h e B e l t e d K i n g f i s h e r ( M e q a c e r y l e a l c y o n ) and t h e G r e a t B l u e Heron ( A d r e a h e r o d i a s ) . A s i n g l e p a i r o f k i n g f i s h e r s a p p e a r e d t o be r e s i d e n t s a t t h e l a k e , and a p p e a r e d t o r e g u l a r l y p a t r o l i t s s u r f a c e s e v e r a l t i m e s e a c h d a y . O c c a s i o n a l l y , a h e r o n would a p p e a r on t h e l a k e d u r i n g t h e e a r l y m o r n i n g s and i n t h e l a t e a f t e r n o o n s . B o t h s p e c i e s were o b s e r v e d c a p t u r i n g a d u l t and j u v e n i l e f i s h f r o m t h e l i t t o r a l zone of t h e l a k e . No p i s c i v o r o u s mammals were o b s e r v e d i n t h e w a t e r s h e d , but e v i d e n c e of l o n g abandoned " o t t e r s l i d e s " s u g g e s t s t h e y were once p r e s e n t . The a q u a t i c f a u n a a r e l i s t e d i n T a b l e 3. The s o l e f i s h s p e c i e s i s c u t t h r o a t t r o u t (Salmo c l a r k i c l a r k i ) . S h e p h e r d (1973) made S c h n a b e l e s t i m a t e s by t r a p n e t t i n g and d i v i n g and e s t i m a t e d t h e p o p u l a t i o n a t a r o u n d 300, c o n s i s t i n g m o s t l y o f 2 and 3 y e a r o l d i n d i v i d u a l s . The f i s h a r e s m a l l (3+ f i s h a t t a i n a mean f o r k l e n g t h of 23 cm), and s l e n d e r (mean body d e p t h a t f r o n t o f d o r s a l f i n , 4,2 cm.) s u g g e s t i n g t h a t g r o w t h i s f o o d l i m i t e d . S a l a m a n d e r s o f t h e s p e c i e s T a r i c h a g r a n u l o s a a r e common i n t h e l a k e . S a l a m a n d e r s do not n o r m a l l y p e r s i s t i n water b o d i e s c o n t a i n i n g f i s h ( Z a r e t , 1980), t h o u g h S p r u l e s (1974) f o u n d Ambystoma and f i s h i n s e v e r a l B r i t i s h C o l u m b i a l a k e s and p o n d s . N e i s h (1970) f o u n d b o t h Ambystoma g r a c i l e and T a r i c h a g r a n u l o s a i n M a r i o n L a k e , w h i c h a l s o c o n t a i n s f i s h . T h e s e s a l a m a n d e r s a r e n o c t u r n a l f e e d e r s , a s t r a t e g y p r o b a b l y d e v e l o p e d t o a v o i d f i s h 33 T a b l e 3. Fauna1 c h a r a c t e r i s t i c s of P l a c i d L a k e . C r u s t a c e a n z o o p l a n k t o n d e n s i t i e s a r e mean d e n s i t i e s f o r June • t o O c t o b e r 1982 f o r t h e e n t i r e w a ter c o l u m n . F i s h Salmo c l a r k i c l a r k i O t h e r s S a l a m a n d e r T a r i c h a g r a n u l o s a C r u s t a c e a n z o o p l a n k t o n D a p h n i a r o s e a  H o l o p e d i u m g i b b e r u m  Diaphanosoma b r a c h y u r u m  Polyphemus p e d i c u l u s  C e r i o d a p h n i a q u a d r a n g u l a  Bosmina l o n q i r o s t r i s  S c a p h a l o b e r i s k i n g i i  C h y d o r u s spp. D i a p t o m u s k e n a i  D i a c y c l o p s t h o m a s i  T r o p o c y c l o p s p r a s i n u s P o p u l a t i o n s i z e 300 ( S h e p h e r d , 1973) 350 ( a p p r o x i m a t e l y ) D e n s i t y N o . / l i t e r 2.5 2.3 2.4 0.08 0.16 0.50 r a r e 0.01 0.01 3.9 r a r e 34 p r e d a t i o n . T a r i c h a g r a n u l o s a has a s k i n t o x i n w h i c h may make p r o t e c t i t from p r e d a t i o n by t h e c u t t h r o a t t r o u t i n P l a c i d L a k e . The z o o p l a n k t o n i n P l a c i d Lake ( T a b l e 3) a r e t h e same a s s e m b l a g e t h a t o c c u r s t h r o u g h o u t t h e r e g i o n ( N o r t h c o t e and C l a r o t t o , 1975) i n d e p e n d e n t o f f i s h c o m p o s i t i o n or f i s h a b undance ( N e i l l , 1981). T h i s a s s e m b l a g e d i f f e r s from l a k e t o l a k e o n l y i n t h e c o - e x i s t i n g c o p e p o d s p e c i e s . The z o o p l a n k t o n b i o m a s s i s g e n e r a l l y d o m i n a t e d by t h e f i l t e r - f e e d i n g c l a d o c e r a n s H o l o p e d i u m g i b b e r u m and D a p h n i a r o s e a . 2.2 E x p e r i m e n t a l Methods __.2.1_ L i m n o c o r r a l D e s i g n and I n s t a l l a t i o n E i g h t l a r g e l i m n o c o r r a l s were i n s t a l l e d i n P l a c i d Lake i n e a r l y May, 1982. The p u r p o s e o f t h e s e e n c l o s u r e s was t o i s o l a t e s e c t i o n s o f t h e n e a r s h o r e ( f r o m h e r e on r e f e r r e d t o a s " l i t t o r a l " ) and o f f s h o r e ( f r o m h e r e on r e f e r r e d t o as " l i m n e t i c " ) z o n e s o f t h e l a k e f r o m t h e main body o f t h e l a k e . I t was hoped t h a t t h e same p h y s i c a l , c h e m i c a l and b i o l o g i c a l p r o c e s s e s o c c u r r i n g i n t h e l a k e would m a n i f e s t t h e m s e l v e s w i t h i n t h e e n c l o s e d a r e a s . The e n c l o s u r e s were c o n s t r u c t e d o f t r a n s l u c e n t , 10 m i l p o l y e t h y l e n e s h e e t i n g s u s p e n d e d from t u b u l a r f l o a t s made from p . v . c . p l a s t i c i r r i g a t i o n p i p e s , 3m l o n g and 15 cm i n d i a m e t e r p l a c e d end on end ( F i g u r e 4 a ) . The p i p e s were " s e a l e d " by g l u i n g 15 cm d i a m e t e r p . v . c . p l a s t i c d i s c s o v e r t h e open ends making them w a t e r t i g h t and b u o y a n t . The l e n g t h s o f p i p e were 35 F i g u r e 4 . L i m n o c o r r a l d e s i g n 36 Figure 4. Design of the enclosure frames and method of a t t a c h i n g one enclosure to the other. {a) Enclosure Frame Wooden Braces 2m X .05m X .10m Jl—ll II T I L - i l p.v.c. p l a s t i c pipes 3m X 0.15m 6 mm Nylon Lashing Double Wrap Around 2 per s i d e T I — r r J l II f o (b) A t t a c h i n g one enclosure to the other. Enclosure 2 Enclosure 1 l a i n f l a t 0.5 m overlay 37 t h e n e n c l o s e d i n a p r e f a b r i c a t e d " c o l l a r " c o n s t r u c t e d by f o l d i n g o v e r t h e t o p 1m of p l a s t i c s h e e t i n g and s e a l i n g i t t o t h e main e n c l o s u r e body w i t h a h o t p r e s s i n g i r o n . The p i p e s were l a s h e d t o g e t h e r u s i n g 180 cm wooden two by f o u r s t o l e n d an e l e m e n t o f r i g i d i t y t o t h e s t r u c t u r e . Elbow j o i n t s made of 15 cm d i a p . v . c . p l a s t i c were u s e d a t c o r n e r j o i n t s . The e n c l o s u r e s measured 10 m wide by 15 m l o n g by 9 m deep (3 m d e e p e r t h a n t h e l a k e ) and were s e a l e d i n t o t h e l a k e s e d i m e n t s so as t o l e a v e t h e a r e a (m 2) of e x p o s e d s e d i m e n t s e q u a l t o t h e s u r f a c e a r e a of t h e e n c l o s u r e s . The b o t t o m s o f t h e e n c l o s u r e w a l l s were s e a l e d i n t o t h e s e d i m e n t s and a n c h o r e d by p l a c i n g g r a n i t e b o u l d e r s ( w e i g h i n g 10 - 15 kg e ach) e v e r y 0.5 m a l o n g t h e b o t t o m of t h e e n c l o s u r e s i d e s . S i n c e t h e b o t t o m s e d i m e n t s were a f l o c c u l e n t g y t t a , t h e w e i g h t o f t h e b o u l d e r s c a u s e d t h e p l a s t i c t o s i n k a b o u t 0.5 m i n t o t h e mud, e f f e c t i n g a c o m p l e t e s e a l . R o u g h l y 2m o f e x c e s s s h e e t i n g was l e f t a s s l a c k i n t h e s i d e s t o a l l o w t h e e n c l o s u r e s t o r i s e and f a l l w i t h f l u c t u a t i n g l a k e l e v e l s ( r e c a l l t h a t t h e l a k e l e v e l c o u l d r i s e as much as 1.5 m i n l e s s t h a n 24 h o u r s d u r i n g a heavy r a i n ) . The e n c l o s u r e s were p o s i t i o n e d i n t h e l a k e so a s t o make them as m o r p h o m e t r i c a l l y as s i m i l a r a s p o s s i b l e ( s e e F i g u r e 3) g i v e n t h e p r o b l e m s of l o g s p r o t r u d i n g o u t i n t o t h e w a t e r c o l u m n . The l i t t o r a l e n c l o s u r e s were a l s o p l a c e d i n a r e a s where t h e Sphagnum mat a r o u n d t h e l a k e was n o t u n d e r c u t . The l i t t o r a l e n c l o s u r e s were s e a l e d t o t h e f a c e o f t h e Sphagnum mat by d r i v i n g l a r g e wooden t e n t s t a k e s t h r o u g h t h e p l a s t i c and i n t o t h e mat. The " l i m n e t i c " e n c l o s u r e s were h e a t s e a l e d , on l a n d , 38 t o t h e o u t e r m a r g i n s o f t h e " l i t t o r a l " e n c l o s u r e s ( F i g u r e 4 b ) . The e n c l o s u r e s were a s s e m b l e d a t a wooden dock a t t h e e a s t end of t h e l a k e . Upon i n s t a l l a t i o n , t h e s i d e s of t h e e n c l o s u r e s were l a s h e d t o t h e f l o t a t i o n frame and s m a l l r o c k s were t i e d t o t h a t p a r t o f t h e p l a s t i c s h e e t t h a t was t o be t h e b o t t o m o f t h e w a l l s . The e n c l o s u r e s were t h e n towed i n t o p l a c e by b o a t and s e c u r e d t o s h o r e . The " s k i r t " w e i g h t s were t h e n f r e e d so t h a t t h e y would s i n k t o t h e b o t t o m , t h u s e n c l o s i n g t h e e x i s t i n g water c o l u m n . D i v e r s t h e n s e a l e d t h e b o t t o m edges i n t o t h e s e d i m e n t s . S u b s e q u e n t s a m p l i n g f o r p h y t o p l a n k t o n and z o o p l a n k t o n showed t h a t a l l s p e c i e s e x i s t i n g i n t h e open l a k e w ater were p r o p o r t i o n a l l y and n u m e r i c a l l y r e p r e s e n t e d i n a l l o f t h e e n c l o s u r e s . 2.2.2^ E x p e r i m e n t a l T r e a t m e n t s The e i g h t e n c l o s u r e s were a r r a n g e d i n t o two l o c a t i o n b l o c k s ( l i t t o r a l v s l i m n e t i c , 4 e a c h ) , and two of t h e e n c l o s u r e s w i t h i n e a c h b l o c k were t r e a t e d w i t h f e r t i l i z e r . The o t h e r f o u r e n c l o s u r e s s e r v e d as c o n t r o l s ( T a b l e 4, F i g u r e 3 ) . H a t c h e r y r e a r e d c u t t h r o a t t r o u t f r y were added t o a l l e i g h t e n c l o s u r e s i n o r d e r t o p r o v i d e a s t a n d a r d i z e d s e t of f i s h f o r g r o w t h and s u r v i v a l e s t i m a t i o n s ; however t h o s e f i s h q u i c k l y d i s a p p e a r e d f r o m a l l o f t h e e n c l o s u r e s . The f e r t i l i z e r s u s e d were c o m m e r c i a l g r a d e s o l u b l e diammonium h y d r o g e n p h o s p h a t e , ( N H „ ) 2 H P O „ , and r e a g e n t g r a d e s o l u b l e ammonium n i t r a t e , N H „ N 0 3 . A l l f e r t i l i z e d e n c l o s u r e s r e c e i v e d w e e k l y d o s e s of 10 u g / l i t e r o f P and 400 u g / l i t e r N 39 ( T a b l e 4 ) . The f e r t i l i z e r s were d i s s o l v e d i n l a k e water and a p p l i e d e v e n l y o v e r t h e s u r f a c e a r e a of t h e e n c l o s u r e s u s i n g a p r e s s u r i z e d g a r d e n i n s e c t i c i d e d i s p e n s e r . C o n s i d e r i n g t h e a r r a n g e m e n t of t h e e n c l o s u r e s ( e a c h l i t t o r a l p l u s l i m n e t i c u n i t d i v i d e d by a t h i n p l a s t i c s h e e t ) , I f e a r e d p o s s i b l e c o n t a m i n a t i o n of an u n f e r t i l i z e d e n c l o s u r e s h o u l d i t be i n j u x t a p o s i t i o n w i t h a f e r t i l i z e d e n c l o s u r e . Hence, I s a c r i f i c e d t h i s e l e m e n t o f r a n d o m i z a t i o n i n a s s i g n i n g t r e a t m e n t s t o e n c l o s u r e s . A l s o , b e c a u s e one s e t of f o u r e n c l o s u r e s was on t h e s o u t h s i d e o f t h e l a k e and t h e o t h e r s e t was on t h e n o r t h w e s t s i d e ( F i g u r e 3 ) , I f e l t t h a t e a c h l o c a t i o n s h o u l d have i t s own s e r i e s o f t r e a t m e n t s and c o n t r o l s s h o u l d t h i s f a c t o r p r o v e t o be a s i g n i f i c a n t s o u r c e of v a r i a t i o n i n s u b s e q u e n t s t a t i s t i c a l a n a l y s e s . The f i s h a dded t o e a c h e n c l o s u r e were p r o g e n y of a d u l t t r o u t c a p t u r e d f r o m t h e l a k e by t r a p n e t i n O c t o b e r o f 1981. The young were r e a r e d a t t h e A b b o t s f o r d T r o u t H a t c h e r y ( B r i t i s h C o l u m b i a F i s h and W i l d l i f e B r a n c h ) n e a r A b b o t s f o r d , B.C. Based on an e s t i m a t e d f e c u n d i t y of 120 eggs p e r f e m a l e , 30 f e m a l e and 10 male f i s h were t a k e n as b r o o d s t o c k . I i n t e n d e d t o s t o c k e a c h e n c l o s u r e w i t h 250 j u v e n i l e t r o u t and have 1500+ f i s h f o r emergency p u r p o s e s . Of t h e 30 f e m a l e s t a k e n i n l a t e O c t o b e r 1981, 12 f i s h had a l r e a d y spawned, 5 f i s h were n e a r i n g spawning c o n d i t i o n and t h e r e s t showed no e v i d e n c e of b e i n g r e a d y t o spawn u n t i l t h e f o l l o w i n g J a n u a r y . Due t o a d u l t m o r t a l i t i e s , d i s c o r d a n t t i m e s of s p a w n ing, and j u v e n i l e m o r t a l i t i e s , I f i n a l l y had 427 c u t t h r o a t t r o u t f r y , h a l f o f w h i c h had been Table 4. Experimental treatments a b c Treatment E n c l o s u r e Approximate Maximum Amount of Numbers of J u v e n i l e number Volume Depth F e r t i l i z e r C u t t h r o a t added (mX) (m) L i t t o r a l C o n t r o l 468 332 5.0 no a d d i t i o n 58 5.0 " 53 L imnet i c C o n t r o l 785 6.3 53 683 5.0 53 L 1 t t o r a l F e r t i 1 i zed 549 5.0 10 u g / 1 i t e r P 53 400 ug / 1 i t e r N 435 5.0 53 L i mnet i c F e r t i 1 i zed 840 6.5 53 698 5.0 53 a) determined by t r a n s e c t depth sounding and c a l c u l a t i o n . b) amount of n u t r i e n t added weekly from dune 1 to September 30 1982. T o t a l weekly a d d i t i o n s were d i v i d e d i n t o t h r e e and added to e n c l o s u r e s on Monday Wednesday and F r i d a y . c) r e l e a s e d i n t o e n c l o s u r e s on June 14, 1982. 41 spawned i n November 1981 and r a i s e d a t c o l d t e m p e r a t u r e s ( c a . 4 °C) t o slow down g r o w t h , and h a l f of w h i c h had been spawned i n F e b r u a r y 1982 and r a i s e d a t h i g h t e m p e r a t u r e ( c a . 25 °C) so t h e y c o u l d a c h i e v e s i z e s s i m i l a r t o t h e i r e a r l i e r spawned b r e t h r e n . 2.2..3 S a m p l i n g Methods Z o o p l a n k t o n , p h y t o p l a n k t o n , p l a n t pigment and water c h e m i s t r y s a m p l e s were a l l drawn from t h e e n c l o s u r e s by means of a 10 m l o n g , 2.5 cm d i a m e t e r c l e a r p l a s t i c hose c o n n e c t e d t o a b a t t e r y o p e r a t e d b i l g e pump t h a t d e l i v e r e d 25 l i t e r s p e r m i n u t e . Z o o p l a n k t o n , p h y t o p l a n k t o n and p l a n t pigment s a m p l e s were t a k e n w e e k l y . Water c h e m i s t r y measurements were t a k e n e v e r y two weeks. A l l samples were t a k e n on a T u e s d a y between 11 a.m. and 2 p.m. F i f t y l i t e r (2 m i n u t e ) s a m p l e s were t a k e n i n a d e p t h s t r a t i f i e d r e g i m e n by pumping from t h e t o p , m i d d l e and bot t o m ( r e s p e c t i v e l y ) of e a c h of t h e f o l l o w i n g d e p t h s t r a t a : 0-2, 2-4 and 4-6 m. In t h e c a s e of an e n c l o s u r e b e i n g o n l y 5 m deep, a one m i n u t e sample was t a k e n from t h e 4-5 m d e p t h . The hos e and b i l g e pump method of s a m p l i n g has been shown t o have e x c e l l e n t r e p l i c a b i l i t y and s m a l l s a m p l i n g b i a s w i t h r e s p e c t t o t h e z o o p l a n k t o n i n t h e r e s e a r c h f o r e s t l a k e s ( P e a c o c k , 1981; N e i l l , 1981; Marmorek, 1983). 42 (1) Z o o p l a n k t o n E a c h f i f t y l i t e r sample o f water was pumped t h r o u g h a 90 um n y t e x mesh n e t f i t t e d w i t h a c o d - e n d . Samples were p r e s e r v e d i n a s u c r o s e - f o r m a l d e h y d e m i x t u r e (Haney and H a l l , 1975) and e n u m e r a t e d by s p e c i e s , s i z e , sex, r e p r o d u c t i v e c o n d i t i o n and number of eggs p e r f e m a l e . Body s i z e was e s t i m a t e d f o r t h e f i r s t t w e n t y s p e c i m e n s of e a c h s p e c i e s c o u n t e d . C l a d o c e r a n s were measured from t h e a n t e r i o r t i p o f t h e c a r a p a c e ( e x c l u d i n g h e l m e t s o r p r o t u b e r a n c e s ) t o t h e most d i s t a l p a r t of t h e c a r a p a c e ( e x c l u d i n g t a i l s p i n e s ) . Copepods were measured from t h e a n t e r i o r t i p of t h e c e p h a l o t h o r a x t o t h e most d i s t a l p a r t o f t h e urosome ( e x c l u d i n g c a u d a l r a m i ) . Samples w i t h h i g h z o o p l a n k t o n d e n s i t i e s were s u b s a m p l e d u s i n g t h e s p l i t t i n g d e v i c e d e s c r i b e d i n N o r t h c o t e and C l a r o t t o ( 1 9 7 5 ) . S u c c e s s i v e s u b s a m p l e s were c o u n t e d u n t i l a t l e a s t 200 i n d i v i d u a l s of e a c h s p e c i e s were r e c o r d e d . A l l samples were c o u n t e d a t 25X m a g n i f i c a t i o n w i t h a W i l d M5, b i n o c u l a r d i s s e c t i n g s c o p e u s i n g a 10X o c u l a r f i t t e d w i t h a m i c r o m e t e r . Z o o p l a n k t o n b i o m a s s was e s t i m a t e d f r o m t h e c o u n t s and measurements u s i n g a l e n g t h - w e i g h t r e g r e s s i o n f o r m u l a d e v e l o p e d i n our l a b by D. R o b i n s o n and C . J W a l t e r s ( u n p u b l i s h e d ) . Normal p r o b a b i l i t y p l o t s and t h e g - s t a t i s t i c s of t h e s e d a t a i n d i c a t e d r e j e c t i o n o f t h e h y p o t h e s i s t h a t t h e s e d a t a came from a p o p u l a t i o n whose members f o l l o w a n o r m a l d i s t r i b u t i o n , t h e r e f o r e , t o meet t h e a s s u m p t i o n s o f a n a l y s i s o f v a r i a n c e , t h e e s t i m a t e s were s u b j e c t e d t o a l o g - t r a n s f o r m a t i o n t o c o r r e c t f o r n o n - n o r m a l i t y . The d a t a were t h e n a n a l y z e d u s i n g t h r e e - w a y 43 ( t i m e x t r e a t m e n t x l o c a t i o n ) , two-way ( t i m e x t r e a t m e n t ) and one-way ( t i m e ) a n a l y s e s o f v a r i a n c e . The o v e r a l l mean l e n g t h s f o r e a c h of t h e z o o p l a n k t o n c a t e g o r i e s D a p h n i a r o s e a ( a d u l t s , j u v e n i l e s and f e m a l e s w i t h e g g s ) , Diaphanosoma b r a c h y u r u m (combined a d u l t s and j u v e n i l e s ) and D i a p t o m u s o r e g o n e n s i s ( a d u l t s , f e m a l e s and j u v e n i l e s ) were computed and t h e u n t r a n s f o r m e d d a t a s u b j e c t e d t o a two f a c t o r ( l i t t o r a l - l i m n e t i c o r f e r t i l i z e d - u n f e r t i l i z e d ) K r u s k a l l - W a l l i s n o n - p a r a m e t r i c one way a n a l y s i s o f v a r i a n c e . The o v e r a l l mean b r o o d s i z e s o f D. r o s e a , D. b r a c h y u r u m , H. g i b b e r u m , C. q u a d r a n q u l a , B. l o n g i r o s t r i s , D i a ptomus  o r e g o n e n s i s , and D i a c y c l o p s t h o m a s i were computed and t h e u n t r a n s f o r m e d d a t a s u b j e c t e d t o t h e same t e s t . (2) P h y t o p l a n k t o n P h y t o p l a n k t o n s a m p l e s were t a k e n from e a c h o f t h e t h r e e d e p t h s t r a t a m e n t i o n e d a b o v e . Water f r o m e a c h s t r a t u m was pumped t h r o u g h a 90 um n y t e x n e t ( t o remove l a r g e z o o p l a n k t e r s and d e b r i s ) i n t o a 10 l i t e r p l a s t i c b u c k e t and s w i r l e d t o e n s u r e a d e q u a t e m i x i n g . A 100 ml g l a s s j a r was i n v e r t e d i n t o t h e b u c k e t a n d a sample drawn and p r e s e r v e d i n L u g o l ' s s o l u t i o n . In t h e l a b , a 60 ml subsample was l e f t t o s t a n d i n a 100 ml g r a d u a t e d c y l i n d e r t o a l l o w s e t t l i n g o f t h e p l a n k t o n f o r 36 h o u r s , a f t e r w h i c h t h e t o p 40 ml was s i p h o n e d away. The r e m a i n i n g 20 ml were t r a n s f e r r e d t o a 25 ml c y l i n d r i c a l , p l e x i g l a s s c o u n t i n g chamber (5cm h i g h , 2.5 cm d i a . , w i t h a s l i d e c o v e r s l i p f o r t h e b o t t o m ) , a l l o w e d t o s t a n d f o r a f u r t h e r 36 44 h o u r s and s u b s e q u e n t l y c o u n t e d u s i n g a W i l d Model M-40 i n v e r t e d m i c r o s c o p e a t 400X. The scope was e q u i p p e d w i t h a 10X o c u l a r u n i t w i t h a 100 um 2 g r i d i n t h e e y e p i e c e . C e l l s were enum e r a t e d and i d e n t i f i e d t o genus and, where p o s s i b l e , t o s p e c i e s . Edmondson ( 1 9 5 6 ) , P r e s c o t t (1962) and S t e i n (1975) were us e d f o r t a x o n o m i c i d e n t i f i c a t i o n . Up t o twenty c e l l s of e a c h t y p e were measured f o r maximum l e n g t h and maximum w i d t h . F o r t y g r i d s ( e a c h 100 um 2) were c o u n t e d f o r e a c h sample. K r a u s e (1984) u n d e r t o o k i n t e n s i v e r e p l i c a t e c o u n t i n g u s i n g t h i s method t o d e t e r m i n e t h e minimum number o f f i e l d s and c e l l s needed f o r q u a n t i t a t i v e a c c u r a c y . She f o u n d t h a t 20 t o 40 f i e l d s were r e q u i r e d t o a c h i e v e a ± 5 % a c c u r a c y ( w i t h 95% c o n f i d e n c e ) d e p e n d i n g on c e l l s i z e d i m e n s i o n s , w i t h t h e l a r g e r c e l l s r e q u i r i n g more f i e l d s . K r a u s e s e t a s t a n d a r d of 30 f i e l d s . I have c h o s e n 40 f i e l d s . I f a s p e c i e s or c a t e g o r y numbered 200 c e l l s p r i o r t o 40 g r i d s b e i n g c o u n t e d , t h e n f u r t h e r e n u m e r a t i o n o f t h a t s p e c i e s or c a t e g o r y was t e r m i n a t e d . Once 40 g r i d s had been c o u n t e d , t h e e n t i r e chamber was s c a n n e d a t 200X f o r r a r e a n d / o r l a r g e f o r m s . C e l l v o l u m e s were computed u s i n g measured maximum d i m e n s i o n s and t h e g e o m e t r i c shape ( c y l i n d e r , s p h e r e , f u s i f o r m , e t c . . . . ) t h a t most r e s e m b l e d e a c h c e l l s h a p e . L e n g t h s o f f i l a m e n t s were t a k e n t o t h e l i m i t o f t h e g r i d d i m e n s i o n s . L a r g e d i a t o m s were measured i n t h e i r e n t i r e t y . D i n o b r y o n c o l o n y volumes were computed as t h e sum o f t h e volumes o f i n d i v i d u a l l o r i c a s . C o l o n i e s of c e l l s were c o u n t e d and measured as s i n g l e c e l l s . A l g a l c e l l s were a l s o c l a s s i f i e d a c c o r d i n g t o t h e i r 45 maximum d i m e n s i o n i n t o t h e f o l l o w i n g s i z e c l a s s e s : <2, 2-6, 6-10, 10-14, 14-20, 20-30, 30-40, 40-80, >80 um. T o t a l a l g a l v olumes were c o n v e r t e d t o b i o m a s s a s s u m i n g a l l c e l l s had a s p e c i f i c g r a v i t y o f 1.0. A l g a l b i o m a s s e s t i m a t e s were l o g - t r a n s f o r m e d t o c o r r e c t f o r n o n - n o r m a l i t y and s u b j e c t e d t o two-way ( t i m e x l o c a t i o n ) and one-way ( t i m e ) a n a l y s e s o f v a r i a n c e . (3) P l a n t P i g m e n t s R e p l i c a t e d (x2) 250 ml s u b s a m p l e s were t a k e n from e a c h pumped water sample, and f i l t e r e d i n t h e f i e l d u s i n g a s m a l l hand pump, 4.25 Whatman GF/C g l a s s f i b e r f i l t e r s and MgC0 3. The f i l t e r s were f o l d e d , s t o r e d i n t h e d a r k and f r o z e n w i t h i n 2 h o u r s . The f i l t e r s were l a t e r g r o u n d i n 90 % a c e t o n e w i t h t h e a i d o f a t e f l o n T e r M e u l i n t i s s u e g r i n d e r and l e f t t o s i t r e f r i g e r a t e d i n t h e d a r k f o r 20-25 h o u r s . P i g m e n t s were t h e n m e a s u r e d f l o u r o m e t r i c a l l y u s i n g a T u r n e r Model 111 F l o u r o m e t e r . The f l o u r o m e t e r was s t a n d a r d i z e d w i t h a c e t o n e e x t r a c t s o f c o m m e r c i a l l y grown s p i n a c h l e a v e s i n w h i c h p l a n t p i g m e n t s had been d e t e r m i n e d s p e c t r o p h o t o m e t r i c a l l y . The r e s u l t s a r e e x p r e s s e d a s t o t a l p i g m e n t , c h l o r o p h y l l a and p h a e o p h y t i n . A l l p l a n t p i g m e n t d a t a were t r a n s f o r m e d ( 1 / s q u a r e r o o t ) t o c o r r e c t f o r n o n - n o r m a l i t y and s u b j e c t e d t o t h r e e - w a y and two-way a n a l y s e s o f v a r i a n c e . 46 (4) Water C h e m i s t r y F o r w ater c h e m i s t r y , a 1 l i t e r w a t er sample was t a k e n i n a s t e r i l i z e d , opaque p l a s t i c b o t t l e f r o m e a c h d e p t h s t r a t u m . The s a m p l e s were p l a c e d on i c e i n a 50 l i t e r p l a s t i c g a r b a g e p a i l l i n e d w i t h b l a c k p l a s t i c and t a k e n d i r e c t l y t o t h e B r i t i s h C o l u m b i a M i n i s t r y of t h e E n v i r o n m e n t ' s e n v i r o n m e n t a l l a b o r a t o r y a t B.C. R e s e a r c h on t h e U n i v e r s i t y of B.C.'s campus. T h e r e , t h e s a m p l e s were s u b j e c t e d t o c h e m i c a l a n a l y s e s w i t h i n 24 h o u r s . T r u e c o l o r ( P t u n i t s ) , s p e c i f i c c o n d u c t a n c e (umhos/cm) and pH were a l l measured a c c o r d i n g t o S t a n d a r d Methods ( 1 9 7 1 ) . T o t a l p h o s p h o r o u s and t o t a l d i s s o l v e d p h o s p h o r o u s and o r t h o -p h o s p h a t e were d e t e r m i n e d u s i n g t h e a u t o m a t e d c o l o r i m e t r i c : a s c o r b i c a c i d r e d u c t i o n method a c c o r d i n g t o S t a n d a r d Methods w i t h some m o d i f i c a t i o n ( B r y n j o l f s o n , 1973). T o t a l K j e l d a h l n i t r o g e n was q u a n t i f i e d by t h e N e s s l e r i z a t i o n method. D e t e r m i n a t i o n of ammonia n i t r o g e n was by t h e method of t h e a u t o m a t e d B e r t h e l o t r e a c t i o n ( T e c h n i c o n I n s t r u m e n t s C o r p o r a t i o n , I n d u s t r i a l Method No. 154-71W). N i t r a t e n i t r o g e n p l u s n i t r i t e n i t r o g e n q u a n t i f i c a t i o n i n v o l v e d t h e cadmium r e d u c t i o n method p l u s d i a z o t i z a t i o n whereas s i n g u l a r N i t r i t e n i t r o g e n was d e t a i l e d by d i a z o t i z a t i o n a l o n e . N i t r a t e n i t r o g e n was i s o l a t e d by c a l c u l a t i o n (mg/1 N ( N 0 3 ) = mg/1 N(N0 2+N0 3) - mg/1 N ( N 0 2 ) ) . T e m p e r a t u r e (°C) and o xygen (mg/1) were o b t a i n e d w e e k l y u s i n g a Y e l l o w S p r i n g s I n s t r u m e n t s M o d e l 56 O x y g e n - T e m p e r a t u r e -S a l i n i t y m e t e r . The oxygen p r o b e was a i r c a l i b r a t e d and c o r r e c t e d f o r e l e v a t i o n . D a t a were t a k e n s t a r t i n g a t t h e s u r f a c e and w o r k i n g down a t s u c c e s s i v e 0.5 m i n t e r v a l s . 47 3.0 RESULTS 3.]_ P h y s i c a l P a r a m e t e r s 3.J_.j_ T e m p e r a t u r e Summer t e m p e r a t u r e p r o f i l e s f o r P l a c i d Lake i n 1982 d e m o n s t r a t e a weak, u n s t a b l e p a t t e r n o f s t r a t i f i c a t i o n ( F i g u r e 5 ) . Warming of s u r f a c e w a t e r s began i n e a r l y J une r e s u l t i n g i n th e o n s e t of s t r a t i f i c a t i o n . A weak t h e r m o c l i n e was e s t a b l i s h e d a t a b o u t 3.5 m by m i d - J u n e . A two week p e r i o d (June 30 - J u l y 15) of c o o l , c l e a r w eather r e s u l t e d i n c o o l i n g o f t h e e p i l i m n e t i c w a t e r s from 20 °C t o 16 °C e l i m i n a t i n g s t r a t i f i c a t i o n . The p e r i o d f o l l o w i n g was c h a r a c t e r i z e d by a l t e r n a t i n g p h a s e s o f (1) c a l m c o n d i t i o n s , d u r i n g w h i c h m i c r o s t r a t i f i c a t i o n d e v e l o p e d w i t h i n s u p e r f i c i a l l a y e r s , and (2) more d i s t u r b e d p h a s e s , when t h e e p i l i m n i o n became more homogeneously m i x e d . A r o u n d A u g u s t 3-5 a heavy r a i n c a u s e d t h e newly s t r a t i f i e d l a y e r t o d e s t a b i l i z e , a l l o w i n g whole o r p a r t i a l m i x i n g o f t h e l a k e w ater w h i l e r e d u c i n g e p i l i m n e t i c t e m p e r a t u r e s once a g a i n . Towards l a t e A u g u s t , m i c r o s t r a t i f i c a t i o n was a g a i n e s t a b l i s h e d a t a b o u t 2m. Then a n o t h e r p e r i o d of c o o l w e a t h e r and heavy r a i n (September 8-12) r e s u l t e d i n d e c r e a s e d e p i l i m n e t i c t e m p e r a t u r e s w h i c h p e r s i s t e d u n t i l a d e f i n i t e a u t u m n a l c i r c u l a t i o n o c c u r r e d i n l a t e September. T h e r e a p p e a r e d t o be a s t a b l e w a t e r mass a t a b o u t 4m w h i c h would p r o b a b l y r e p r e s e n t t h e upper l i m i t s of t h e m e t a l i m n i o n i n a d e e p e r l a k e , so f o r l a c k o f a b e t t e r d i s t i n c t i o n , w a t e r s above 4 m were 48 F i g u r e 5. P l a c i d Lake water t e m p e r a t u r e s ( d e g r e e s C e l s i u s ) f o r summer 1982 (m) i p d a a 50 d e s i g n a t e d t h e e p i l i m n i o n and w a t e r s below 4m, t h e h y p o l i m n i o n . T h i s s e a s o n a l t e m p e r a t u r e p a t t e r n was i d e n t i c a l f o r a l l e n c l o s e d w a t e r s , t h u s o n l y one t e m p e r a t u r e p r o f i l e i s p r e s e n t e d . _ 3 D i s s o l v e d Oxygen S e a s o n a l d i s s o l v e d oxygen (DO) p r o f i l e s were s i m i l a r i n a l l c o n t r o l e n c l o s u r e s and t h e l a k e ( F i g u r e 6 ) . The w a t e r s were w e l l o x y g e n a t e d a t a l l d e p t h s t h r o u g h o u t t h e s e a s o n e x c e p t d u r i n g l a t e September a t 6m ( j u s t above t h e mud-water i n t e r f a c e ) i n t h e l a k e and i n c o n t r o l e n c l o s u r e 2, when a s h o r t p e r i o d of n e a r a n o x i a o c c u r r e d (0.6 m g / l i t e r , 0.6% s a t u r a t i o n ) . The DO c o n c e n t r a t i o n t h r o u g h o u t t h e summer i n t h e c o n t r o l e n c l o s u r e s and t h e l a k e r a n g e d from 8-12 m g / l i t e r (55-100 % s a t u r a t i o n ) t o a d e p t h o f 3 m, t o 2-4 m g / l i t e r (20-40 % s a t u r a t i o n ) below 4 m. The DO p r o f i l e s f o r t h e f e r t i l i z e d e n c l o s u r e s ( F i g u r e 6) d i f f e r e d f r o m t h e c o n t r o l e n c l o s u r e s i n two ways. F i r s t , d i s s o l v e d oxygen c o n c e n t r a t i o n s i n t h e upper f o u r m e t e r s were c o n s i d e r a b l y h i g h e r t h a n i n c o n t r o l s . S e c o n d , a l l f e r t i l i z e d e n c l o s u r e s e x h i b i t e d n e a r a n o x i a w i t h i n 0.5 m o f t h e bottom i n l a t e S eptember. The l i t t o r a l f e r t i l i z e d e n c l o s u r e s d i f f e r e d l i t t l e f r o m t h e c o n t r o l s w i t h r e s p e c t t o e p i l i m n e t i c DO c o n c e n t r a t i o n s . E n c l o s u r e 3 e x h i b i t e d an e x t e n d e d e p i l i m n e t i c maximum o f 11 m g / l i t e r (120 % s a t u r a t i o n ) t h r o u g h l a t e J u l y and a l l o f A u g u s t . E n c l o s u r e 7 a p p r o x i m a t e d t h e c o n t r o l s e r i e s . The l i m n e t i c f e r t i l i z e d e n c l o s u r e s had s i g n i f i c a n t l y h i g h e r e p i l i m n e t i c DO c o n c e n t r a t i o n s t h a n c o n t r o l s (mean 12 m g / l i t e r , 125 % s a t u r a t i o n ) w i t h v a l u e s r e a c h i n g a s h i g h a s 14 m g / l i t e r 51 F i g u r e 6. D e p t h - t i m e i s o p l e t h s of d i s s o l v e d o x y g e n . V a l u e s a r e m g / l i t e r . 52 LAKE Enclosure 2 Enclosure 4 (limnetic control) (limnetic fertilized) 53 ( 1 5 0 % s a t u r a t i o n ) . D i s t i n c t DO maxima were p r e s e n t i n e n c l o s u r e s 3 and 8 a t a b o u t 3 - 4 m i n l a t e A u g u s t . _3.J_._3 T r a n s p a r e n c y C o n t r o l e n c l o s u r e s and t h e l a k e d i d not d i f f e r m a r k e d l y i n t h e i r t r a n s p a r e n c y . The S e c c h i d i s c was u s u a l l y v i s i b l e on t h e b o t t o m o f t h e l a k e but d i s c v i s i b i l i t y was s l i g h t l y r e d u c e d i n t h e l a k e and l i m n e t i c c o n t r o l e n c l o s u r e 2 i n e a r l y A u g u s t ( F i g u r e 7 ) . S e c c h i d i s c v i s i b i l i t y i n t h e f e r t i l i z e d e n c l o s u r e s c h a n g e d d r a m a t i c a l l y compared t o c o n t r o l s f o l l o w i n g f e r t i l i z a t i o n . S e c c h i d i s c d e p t h s began t o d e c r e a s e i n A u g u s t , r e a c h i n g 3 . 0 m i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s and 1 . 5 - 2 . 0 m i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . _3.j_._4 Summary of P h y s i c a l R e s u l t s In summary, t h e r e were no major d i f f e r e n c e s i n t e m p e r a t u r e , d i s s o l v e d oxygen and S e c c h i d i s c v i s i b i l i t y p r o f i l e s between t h e c o n t r o l e n c l o s u r e s and t h e l a k e . N ote t o o , t h a t t h e s e p a r a m e t e r p r o f i l e s were s i m i l a r f o r b o t h l i t t o r a l and l i m n e t i c c o n t r o l e n c l o s u r e s . The s i m i l a r i t i e s i n t e m p e r a t u r e and DO s u g g e s t t h a t wind m e d i a t e d m i x i n g and d i f f u s i o n t h a t o c c u r r e d i n t h e l a k e a l s o o c c u r r e d i n t h e e n c l o s u r e s . S u p e r - s a t u r a t e d oxygen c o n c e n t r a t i o n s were r e c o r d e d i n t h e e p i l i m n i a o f t h e f e r t i l i z e d e n c l o s u r e s . A l s o , h y p o l i m n e t i c oxygen d e p l e t i o n e x t e n d e d f u r t h e r i n t o t h e w ater column i n t h e f e r t i l i z e d e n c l o s u r e s t h a n i n c o n t r o l s . S e c c h i . d i s c v i s i b i l i t y began t o change as a r e s u l t 54 F i g u r e 7. Secchi d i s c depths 55 Secchi depth (m) E n c l o s u r e 1 ( l i t t o r a l c o n t r o l ) J U B * J»ij **» 0 c t E n c l o a u r e 2 ( l i m n e t i c c o n t r o l ) E n c l o a u r e 5 ( l i t t o r a l c o n t r o l ) J u l , • • » ° " L A K E i 5. J u a . ' J u l ) E n c l o a u r e 3 ( l i t t o r a l f e r t i l i z e d ) Ju»« J u l y Au« • O c t E n c l o a u r e 4 ( l i m n e t i c f e r t i l i z e d ) TT i i — i — * • • % 1 • _ J u n a J « l y Aug ««» Oct E n c l o s u r e 7 ( l i t t o r a l f e r t i l i z e d ) Ju«« Juljf ••• E n c l o a u r e 6 ( l i m n e t i c c o n t r o l ) E n c l o s u r e B ( l i m n e t i c f e r t i l i z e d ) 56 of t h e f e r t i l i z a t i o n i n m i d - A u g u s t . Changes i n d i s c v i s i b i l i t y were more p r o n o u n c e d i n t h e l i m n e t i c t h a n i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s . 3.2 C h e m i c a l P a r a m e t e r s C o n d i t i o n s i n Lake W a t e r s C o n c e n t r a t i o n s o f d i s s o l v e d i n o r g a n i c and d i s s o l v e d and p a r t i c u l a t e o r g a n i c n u t r i e n t s i n P l a c i d Lake were low i n 1982, i n t h e ra n g e f o r u l t r a - o l i g o t r o p h i c l a k e s p r o p o s e d by V o l l e n w e i d e r ( 1 9 6 8 ) . S o l u b l e r e a c t i v e p h o s p h o r o u s (SRP) was u n d e t e c t a b l e (<3 u g / l i t e r ) i n t h e water column on a l m o s t a l l o c c a s i o n s ( F i g u r e 8 ) . Low l e v e l o r t h o p h o s p h a t e a n a l y s i s p e r f o r m e d on June 22 samples r e v e a l e d t h a t , on t h a t d a t e , <3 u g / l i t e r was i n f a c t <1 u g / l i t e r . M e a s u r a b l e q u a n t i t i e s o f SRP were d e t e c t e d o n l y i n t h e e a r l y p a r t o f t h e summer, and i n t h e e p i l i m n i o n d u r i n g two s e p a r a t e s t o r m e v e n t s i n A u g u s t and September. (The i n a b i l i t y t o d e t e c t t h i s n u t r i e n t made i t d i f f i c u l t t o e v a l u a t e v e r t i c a l and c h r o n o l o g i c a l c h a n g e s i n c o n c e n t r a t i o n o v e r t h e summer, however, when t h e n u t r i e n t d i d o c c u r i n d e t e c t a b l e q u a n t i t i e s , i t s c o n c e n t r a t i o n was u n i f o r m o v e r a l l d e p t h s . The o b s e r v a t i o n t h a t c o n c e n t r a t i o n s o f t h e i n o r g a n i c and o r g a n i c f r a c t i o n s of o t h e r , d e t e c t a b l e , n u t r i e n t s d i d n o t change w i t h d e p t h s u g g e s t s t h a t u n i f o r m i t y o v e r d e p t h a l s o o c c u r r e d f o r SRP o v e r t h e e n t i r e s e a s o n . ) T o t a l d i s s o l v e d p h o s p h o r o u s (TDP) ( F i g u r e 9) and g u r e 8. D e p t h - t i m e i s o p l e t h s of o r t h o - p h o s p h a t e . V a l u e s a r e u g / l i t e r . O r t h o - p h o s p h a t e u g / L Enclosure 1 (l i t t o r a l control) ft Enclosure 2 (limnetic control) o i l <' Enclosure 5 ( l i t t o r a l control) •••3 1 T 1 1 J y i « J y i y lug • Occ LAKE (I II n «> A i„, l.f ">«• Enclosure 3 ( l i t t o r a l fertilized) I I I t O U <> o j u a * J«*iy * - * S * » Oct Enclosure 4 (limnetic fertilized) • 11! \\ * / / J (0 I r / i Enclosure 7 (l i t t o r a l fertilized) <1 1 » > ' J u l , ' i«l ' «•»°" Enclosure 6 (limnetic control) Enclosure B (limnetic fertilized) ~ i i < i > < i JLLfi-~ J . « « 1 J - l F * « » » « » ° " j . a « J u l / »«r ° G t 59 p a r t i c u l a t e p h o s p h o r o u s (PP) ( F i g u r e 10) c o n c e n t r a t i o n s i n t h e l a k e r a n g e d from 3-8 and 1-5 u g / l i t e r , r e s p e c t i v e l y . H i g h e s t c o n c e n t r a t i o n s of t h e s e n u t r i e n t s o c c u r r e d i n t h e e p i l i m n i o n d u r i n g r a i n e v e n t s . V a l u e s of PP were a l s o h i g h i n t h e h y p o l i m n i o n , w h i c h may be i n d i c a t i v e e i t h e r o f s e s t o n s e d i m e n t a t i o n or o f d e e p - w a t e r p h y t o p l a n k t o n p r o d u c t i o n . N i t r a t e n i t r o g e n (N0 3-N) ( F i g u r e 11) was u n d e t e c t a b l e (<20 u g / l i t e r ) on a l m o s t a l l o c c a s i o n s . N i t r i t e n i t r o g e n (N0 2~N) n e v e r e x c e e d e d 5 u g / l i t e r , t h e a n a l y t i c a l l i m i t o f d e t e c t i o n . S e a s o n a l l y , ammonia (NH 3-N) c o n c e n t r a t i o n s ( F i g u r e 12) r a n g e d from 5-20 u g / l i t e r . C o n c e n t r a t i o n s of NH 3-N were lo w e r i n l a t e summer t h a n i n e a r l y summer, p r o b a b l y a s a r e s u l t of d e p l e t i o n by mid t o l a t e summer blooms o f a l g a e . T h e r e was a l s o e v i d e n c e o f h y p o l i m n e t i c a c c u m u l a t i o n of NH 3-N d u r i n g a s h o r t p e r i o d o f low oxygen c o n c e n t r a t i o n s n e a r t h e bot t o m i n t h e l a t t e r p a r t o f summer. T o t a l n i t r o g e n (TKN) ( F i g u r e 13) and t o t a l o r g a n i c n i t r o g e n (TON) ( F i g u r e 14) c o n c e n t r a t i o n s were a l w a y s < 200 u g / l i t e r . The h i g h e s t c o n c e n t r a t i o n s of TON ( a v e r a g e 190 u g / l i t e r ) o c c u r r e d i n t h e h y p o l i m n i o n f r o m A u g u s t t o September. The pH of t h e wa t e r column ( F i g u r e 15) was c i r c u m n e u t r a l (6.0 - 7.0) and was n e a r l y c o n s t a n t o v e r a l l d e p t h s and o v e r t i m e . I t i n c r e a s e d s l i g h t l y d u r i n g mid-summer a s compared t o e a r l y summer. 6 0 F i g u r e 9. D e p t h - t i m e i s o p l e t h s of t o t a l d i s s o l v e d p h o s p h o r o u s . V a l u e s a r e u g / l i t e r . Total Dissolved P ug/L Enclosure 1 ( l i t t o r a l control) Enclosure 2 ( l imnetic control) « 1 1 ft 4 \ 1 July Au| Oct Enclosure 5 ( l i t t o r a l control) _ 4 AC/ IS 1 JUB« July ' Aug ' fa* * Oct Enclosure 6 ( l imnet ic control) 61 LAKE a 1 , l I • \ v ~ J • 1 • t 1 \ i 4 Z • -s . 1 ,M , Juo* ' July ' Oct Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) O - i 1 ] ' 3 1 0 • \ 1 V 1°) \ \ • 1 1 r- i Juna Jul* Am f*P Oct Encloaure 4 ( l imnetic f e r t i l i z e d ) Encloaure 7 ( l i t t o r a l f e r t i l i z e d . \ 1 11 1 0 i k June 1 July * ' Aug * !•» * Occ Encloaure B ( l imnetic f e r t i l i z e d ) 1 : 1 W • 1 ! I / Juaa Jul* A M I lap Oct 62 F i g u r e 1 0 . D e p t h - t i m e i s o p l e t h s of p a r t i c u l a t e p h o s p h o r o u s . V a l u e s a r e u g / l i t e r . 63 L A K E P a r t i c u l a t e P ug/L E n c l o a u r e 1 ( l i t t o r a l c o n t r o l ) 1 10 \ IV •A 11 J u » * J u l y Aug tsp Occ E n c l o s u r e 2 ( l i m n e t i c c o n t r o l ) j . u J a l r »-l °<> E n c l o s u r e 3 ( l i t t o r a l f e r t i l i z e d ) Jua« J u l y Aug *•> Oct E n c l o s u r e 4 ( l i m n e t i c f e r t i l i z e d ) JMI« J H l y Aug • 0> j u s * J u l y Aug 0 e l E n c l o s u r e 5 ( l i t t o r a l c o n t r o l ) 10 ly » ? ? JUB» 1 J u l , 1 *«• " « M 1 Oct E n c l o s u r e 7 ( l i t t o r a l f e r t i l i z e d ) 11 Ul1 1 V > 1 \ I/" / I ' JUB* ' J u l y i u i f a r Oct E n c l o s u r e 6 ( l i m n e t i c c o n t r o l ) : 1 - i i i i E n c l o s u r e B ( l i m n e t i c f e r t i l i z e d ) •>"•• J u l * »«» • Oc I A / 1 - 1 —1 " — • • ' I 1 " June J u l y Aug Oct 64 F i g u r e 1 1 . Depth-time i s o p l e t h s of n i t r a t e n i t r o g e n . Values are u g / l i t e r . 65 N i t r a t e Nitrogen ug/L Enclosure 1 ( l i t t o r a l control) 1 1 1 1 J w a « J u l * A u g l«f Occ Enclosure 2 ( l imnetic control) » - w 420 4 0 1/ 2 0 0 *; 0 0 < 0 JO 4 0 V Lotf 1 * T " J u a a J u l y *"I **» Enclosure 5 ( l i t t o r a l control) JO 2 0 <2 )) 1 1 i <10 \ J ( T 1 1 1 1 J U B U J u l * 1UI l«B Oct Encloaure 6 ( l imnetic control) 1 0 1 \ 1 4 1 0 4 1 0 II 11 ii 1 2 0 II \ \ 4 0 2 / 0 <10 <2 0 , , , , J w a a J u l y A u g l a p Oct LAKE J u u u J u l f A u i ff Oct Encloaure 3 ( l i t t o r a l f e r t i l i z e d ) 3 A M i > J u n a ' J u l y ' A u g l a p ' Oct Encloaure 4 ( l imnetic f e r t i l i z e d ) J u a a J u l y A u g * • » Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) 1 \ o >^/A J" 'f J 1 1 1 I — — — I 1 — I J u o a J u l y A u g l a p Oct Encloaure B (l imnetic f e r t i l i z e d ) 1 I 1 1] 2 0 0 3 0 0 4 0 d II Ud 1 | 4 0 0 1 J /ip ««oYW / - ••• -r •" i 1 • J u n a J u l * Awa • • • Oct 66 F i g u r e 1 2 . D e p t h - t i m e i s o p l e t h s o f ammonia n i t r o g e n . V a l u e s a r e u g / l i t e r . 67 LAKE Ammonia Nitrogen ug/L Enclosure 1 (littoral control) J . B . J u l y A i t | l.p O c t Enclosure 2 (limnetic control) / 30 10 J u u « J u l y A u g !«• Oct Enclosure 5 (littoral control) Enclosure 3 (littoral fertilized) J u t * J u l y A u g lay Oct Enclosure 4 (limnetic fertilized) J u t t « J u l y A u g g « p Occ Enclosure 7 (littoral fertilized) J u n e J u l y A u g t a p Oct 1 0 / / - - \ \ / \V 40 \ \ J u o a 1 J u l y • l y , ' g „ 1 OCC Enclosure 6 (limnetic control) Enclosure B (limnetic fertilized) 68 F i g u r e 13. Depth-time i s o p l e t h s of t o t a l K j e l d a h l n i t r o g e n . Values are u g / l i t e r . L A K E Total Kjeldahl Nitrogen ug/L E n c l o s u r e 1 ( l i t t o r a l c o n t r o l ) J u a a J u l y Aug l a p O c t E n c l o s u r e 2 ( l i m n e t i c c o n t r o l ) 1)0 30, 100 "110 J u a a J u l y Aug gap O c t E n c l o s u r e 3 ( l i t t o r a l f e r t i l i z e d ) // 11 V 1 - — - ' ' 1— J u B * J u l y Awi t a p O c t E n c l o s u r e 4 ( l i m n e t i c f e r t i l i z e d ) J u a a J u l y Aug lap O a t J u a a J u l y Aug • • » 0 " E n c l o s u r e 5 ( l i t t o r a l c o n t r o l ) J-L E n c l o s u r e 7 ( l i t t o r a l f e r t i l i z e d ) ~7~ 110 100 100 100 110 ( I 1 ) / J u a a J u l y Aug l a p O c c E n c l o s u r e 6 ( l i m n e t i c c o n t r o l ) 1 J u a a J u l y Aug l a » O c t E n c l o s u r e 8 ( l i m n e t i c f e r t i l i z e d ) 70 F i g u r e 14. D e p t h - t i m e i s o p l e t h s o f o r g a n i c n i t r o g e n . V a l u e s a r e u g / l i t e r . 71 L A K E Organic Nitrogen ug/L E n c l o s u r e 1 ( l i t t o r a l c o n t r o l ) E n c l o s u r e 2 ( l i m n e t i c c o n t r o l ) J u a c J u l y Aug l a p O c t E n c l o s u r e 5 ( l i t t o r a l c o n t r o l ) J u a a J u l y Aug l a p O c c E n c l o a u r e 6 ( l i m n e t i c c o n t r o l ) 1.0 \ J 1 SIC 1 0 LOO D \ J u a a J u l y A u | l a p O c t E n c l o s u r e 3 ( l i t t o r a l f e r t i l i z e d ) J u o * J u l y Aug • •# O c c E n c l o s u r e 4 ( l i m n e t i c f e r t i l i z e d ) J u a a J u l y Aug l a p Oct E n c l o s u r e 7 ( l i t t o r a l f e r t i l i z e d ) Ju»a J u l y Aug I c y O c t E n c l o s u r e B ( l i m n e t i c f e r t i l i z e d ) June J u l y Aug l a p O c t 72 F i g u r e 15. D e p t h - t i m e i s o p l e t h s o f pH. Note t h e change i n s c a l e o f t h e Y - a x i s on t h e p l o t s f o r t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . L A K E Hydrogen ion Concentration E n c l o s u r e 1 ( l i t t o r a l c o n t r o l ) s. ' . I » . > r r C?T f E n c l o s u r e 2 ( l i m n e t i c c o n t r o l ) J u a a J u l y A u g l a p O c t E n c l o s u r e 5 ( l i t t o r a l c o n t r o l ) i 4 . n IT • . I \ i J u a a J u l y A u g • • • Oa I J u a a J u l y A u g l a y 0c I E n c l o s u r e 3 ( l i t t o r a l f e r t i l i z e d ) J » . « J » l > I,. d e l E n c l o s u r e 4 ( l i m n e t i c f e r t i l i z e d ) E n c l o a u r e 7 ( l i t t o r a l f e r t i l i z e d ) • ».> / , / t . l / 1 \ , J " J u a a J u l y . A u g l a y O c t . E n c l o a u r e 6 ( l i m n e t i c c o n t r o l ) • 1 t . l «•• Irv U t . t i 1 r 1 1 Jul J » I > • • • • • • ° c > E n c l o a u r e B ( l i m n e t i c f e r t i l i z e d ) 74 3.2.2 E f f e c t s of E n c l o s u r e T h e r e were no d e t e c t a b l e c h a n g e s i n any o f t h e c h e m i c a l c h a r a c t e r i s t i c s o f t h e l a k e w ater as a r e s u l t of e n c l o s u r e and i s o l a t i o n f r o m t h e main l a k e . C h e m i c a l p r o f i l e s f o r a l l n u t r i e n t s and pH i n t h e u n f e r t i l i z e d c o n t r o l e n c l o s u r e s m i r r o r e d t h o s e p r e s e n t e d f o r t h e l a k e o v e r t h e e n t i r e s e a s o n ( F i g u r e s 8 -15) . 3.2.3 E f f e c t s of F e r t i l i z a t i o n (1) P h o s p h o r u s A d d i t i o n o f a p p r o x i m a t e l y 10 ug P O „ - P / l i t e r ( o f e p i l i m n i o n w a t e r ) p e r week, d i v i d e d i n t o t h r e e e q u a l d o s e s , r e s u l t e d i n d e t e c t a b l e l e v e l s of SRP i n a l l o f t h e f e r t i l i z e d e n c l o s u r e s ( F i g u r e 8 ) . E a r l y summer e p i l i m n e t i c SRP c o n c e n t r a t i o n s were s t r i k i n g l y c l o s e t o t h e amounts I was a d d i n g on a w e e k l y b a s i s (10 u g / l i t e r ) . In J u l y , m i c r o - s t r a t i f i c a t i o n of t h e e p i l i m n i o n r e s u l t e d i n a s h a r p d e n s i t y g r a d i e n t a t a b o u t 2 m ( F i g u r e 5 ) . T h i s may e x p l a i n why t h e i n s i t u c o n c e n t r a t i o n s of SRP i n t h e 0-2 m r a n g e d o u b l e d t o 20 u g / l i t e r i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s ( F i g u r e 8 ) . T h i s i n c r e a s e s u g g e s t s t h a t t h e added p h o s p h a t e was n o t i m m e d i a t e l y i n c o r p o r a t e d i n t o t h e s e s t o n . C o n c e n t r a t i o n s of SRP became u n d e t e c t a b l e i n t h e f e r t i l i z e d e n c l o s u r e s from m i d - A u g u s t t o t h e end of t h e s e a s o n even t h o u g h f e r t i l i z e r was s t i l l b e i n g a d d e d a t t h e same r a t e and c o n c e n t r a t i o n s as b e f o r e . 75 The two l i t t o r a l f e r t i l i z e d e n c l o s u r e s e x h i b i t e d l i t t l e v a r i a t i o n i n TDP o r SRP c o n t e n t w i t h d e p t h ( F i g u r e 8 and 9 ) . E a r l y summer TDP (8-10 u g / l i t e r ) v a l u e s , however, were h i g h e r t h a n i n c o n t r o l s i n p r o p o r t i o n t o i n c r e a s e s i n l e v e l s of d e t e c t a b l e SRP. L a t e summer TDP and SRP c o n c e n t r a t i o n s were l o w e r (5-8 u g / l i t e r and <3 u g / l i t e r , r e s p e c t i v e l y ) and s i m i l a r t o t h o s e f o u n d i n t h e c o n t r o l s and l a k e f o r t h e same p e r i o d . The s e a s o n a l p r o f i l e s of TDP and SRP c o n c e n t r a t i o n s i n t h e two l i m n e t i c f e r t i l i z e d e n c l o s u r e s were i d e n t i c a l t o t h o s e i n t h e l i t t o r a l e n c l o s u r e s . However, e a r l y summer TDP (20-30 u g / l i t e r ) c o n c e n t r a t i o n s were 7.5 t i m e s c o n t r o l v a l u e s but d e c r e a s e d (8-9 u g / l i t e r ) t o o n l y d o u b l e l a k e v a l u e s i n l a t e summer. E a r l y summer SRP v a l u e s (20 u g / l i t e r ) d e c r e a s e d from o v e r 20 t i m e s c o n t r o l v a l u e s t o e q u a l l a t e summer c o n t r o l v a l u e s (<3 u g / l i t e r ) . -P a r t i c u l a t e p h o s p h o r o u s f r a c t i o n s ( F i g u r e 10) i n c r e a s e d i n a l l f e r t i l i z e d e n c l o s u r e s d u r i n g l a t e summer. Th e s e i n c r e a s e s were p a r a l l e l e d by t h e d i s a p p e a r a n c e s o f a v a i l a b l e P (SRP, TDP). The a v e r a g e i n c r e a s e i n PP of t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s was up t o 3 t i m e s (6-9 u g / l i t e r ) h i g h e r t h a n i n t h e two l i t t o r a l c o n t r o l s . I n c r e a s e s i n t h e PP c o n c e n t r a t i o n s i n t h e two l i m n e t i c f e r t i l i z e d e n c l o s u r e s were up t o 10 t i m e s (25-30 u g / l i t e r ) h i g h e r t h a n i n t h e two l i m n e t i c c o n t r o l s . 76 (2) N i t r o g e n T o t a l n i t r o g e n (TKN) c o n c e n t r a t i o n s i n a l l of t h e f e r t i l i z e d e n c l o s u r e s were h i g h e r t h a n i n t h e c o n t r o l s o v e r t h e e n t i r e s e a s o n ( F i g u r e 1 4 ) . Maximum e p i l i m n e t i c c o n c e n t r a t i o n s of TKN o c c u r r e d i n t h e l a t t e r p a r t o f t h e summer. C o n c e n t r a t i o n s i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s r e a c h e d 300 u g / l i t e r , a l m o s t d o u b l e c o n t r o l v a l u e s . E p i l i m n e t i c v a l u e s i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s r e a c h e d 600 - 650 u g / l i t e r , f o u r t i m e s h i g h e r t h a n c o n t r o l v a l u e s and t w i c e a s h i g h a s i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s . T h e s e l a t e summer i n c r e a s e s i n TKN were c h i e f l y due t o l a r g e i n c r e a s e s i n t h e o r g a n i c f r a c t i o n s o f n i t r o g e n , and were c o i n c i d e n t w i t h i n c r e a s e d p h y t o p l a n k t o n p r o d u c t i o n . The u n i f o r m i t y o f n u t r i e n t c o n c e n t r a t i o n s w i t h d e p t h i n e a r l y A u g u s t c o n f i r m s my e a r l i e r s u g g e s t i o n t h a t a l m o s t c o m p l e t e m i x i n g of t h e wa t e r column c a n o c c u r i n t h i s l a k e d u r i n g summer s t o r m e v e n t s . C o n c e n t r a t i o n s of N0 3-N i n t h e f e r t i l i z e d e n c l o s u r e s i n c r e a s e d d r a m a t i c a l l y f o l l o w i n g f e r t i l i z a t i o n ( F i g u r e 11). E p i l i m n e t i c N0 3-N c o n c e n t r a t i o n s r a n g e d from 100 - 200 u g / l i t e r and 300 - 500 u g / l i t e r i n t h e l i t t o r a l f e r t i l i z e d and l i m n e t i c f e r t i l i z e d e n c l o s u r e s , r e s p e c t i v e l y . N0 3-N a c c u m u l a t e d more i n t h e l i m n e t i c t h a n i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s a s t h e s e a s o n p r o g r e s s e d (100 - 500 u g / l i t e r ) , b u t r a p i d l y d e c l i n e d t o n o r m a l l a k e l e v e l s a t f a l l o v e r t u r n ( e a r l y O c t o b e r ) . S i n c e 180 u g / l i t e r o f N0 3-N were ad d e d w e e k l y , o n l y a s m a l l p e r c e n t a g e r e m a i n e d i n s o l u t i o n ; however, b u i l d up o f N0 3-N was s t i l l p r o g r e s s i v e o v e r t h e summer. 77 The NH,, -N added (as N H „ N 0 3 and ( N H „ ) 2 H P O „ ) t o t h e f e r t i l i z e d e n c l o s u r e s r e s u l t e d i n h i g h e r e p i l i m n e t i c l e v e l s of N H „ -N b e g i n n i n g i m m e d i a t e l y a f t e r i n i t i a t i o n o f f e r t i l i z a t i o n on June 1 s t , 1982 ( F i g u r e 1 2 ) . 220 u g / l i t e r of NH a were added w e e k l y , so, as was t h e c a s e f o r N0 3, o n l y a s m a l l p e r c e n t a g e r e m a i n e d i n s o l u t i o n . L e v e l s o f N H „-N i n t h e l i t t o r a l and l i m n e t i c f e r t i l i z e d e n c l o s u r e s were, r e s p e c t i v e l y , 3-5 and 20-25 t i m e s h i g h e r t h a n i n c o n t r o l s d u r i n g t h e e a r l y summer. Though t h e r e was some a c c u m u l a t i o n of t h e n u t r i e n t i n e p i l i m n e t i c w a t e r s i n June and J u l y , t h e n u t r i e n t d i s a p p e a r e d r a p i d l y f r o m t h e s y s t e m a f t e r a d d i t i o n , much more r a p i d l y i n t h e l i t t o r a l t h a n i n t h e l i m n e t i c e n c l o s u r e s . S e a s o n a l t o t a l o r g a n i c n i t r o g e n (TON) c o n c e n t r a t i o n s i n t h e f e r t i l i z e d e n c l o s u r e s d i f f e r e d s u b s t a n t i a l l y f r o m c o n t r o l v a l u e s ( F i g u r e 1 3 ) . The e p i l i m n e t i c c o n c e n t r a t i o n s of TON i n l a t e summer r e a c h e d 200-250 u g / l i t e r i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s and 400-500 u g / l i t e r i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s , 2-2.5 and 4-5 t i m e s h i g h e r t h a n i n t h e i r r e s p e c t i v e c o n t r o l s . (3) H y d r o g e n Ion C o n c e n t r a t i o n H y d r o g e n i o n c o n c e n t r a t i o n s (pH) i n b o t h t h e l i t t o r a l and l i m n e t i c f e r t i l i z e d e n c l o s u r e s d i d not c h a n g e, compared t o c o n t r o l s , f o l l o w i n g n u t r i e n t a d d i t i o n ( F i g u r e 1 5 ) . 78 3.2.4 Summary of C h e m i c a l R e s u l t s E n c l o s u r e had no d e t e c t a b l e e f f e c t on t h e d i s t r i b u t i o n s or c o n c e n t r a t i o n s of t h e major n i t r o g e n and p h o s p h o r o u s f r a c t i o n s i n t h e water c o l u m n . However, c h e m i c a l c h a n g e s r e s u l t i n g from n u t r i e n t a d d i t i o n were e v i d e n t i n a l l f e r t i l i z e d e n c l o s u r e s , and were g r e a t e r i n t h e l i m n e t i c t h a n i n t h e l i t t o r a l e n c l o s u r e s . M a j o r r e s p o n s e s t o f e r t i l i z a t i o n o c c u r r e d i n t h e p a r t i c u l a t e n i t r o g e n and p h o s p h o r o u s f r a c t i o n s , and i n t h e c o n s p i c u o u s a c c u m u l a t i o n s o f ammonium and n i t r a t e n i t r o g e n i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . A d d i t i o n s o f 10 u g P / l i t e r of e p i l i m n i o n / w e e k , made i n 3 e q u a l d o s e s w e e k l y , r e s u l t e d i n d e t e c t a b l e l e v e l s of SRP i n t h e wa t e r column u n t i l m i d - A u g u s t when p h y t o p l a n k t o n demands a p p a r e n t l y e x c e e d e d s u p p l y . Most o f t h e added p h o s p h a t e ended up i n t h e p a r t i c u l a t e f r a c t i o n , a l t h o u g h s l i g h t i n c r e a s e s o c c u r r e d i n t h e t o t a l d i s s o l v e d P f r a c t i o n i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . C o n c e n t r a t i o n s of added n i t r a t e i n c r e a s e d d r a m a t i c a l l y soon a f t e r f e r t i l i z a t i o n , o f t e n e x c e e d i n g 100 and 300 u g / l i t e r i n t h e l i t t o r a l and l i m n e t i c f e r t i l i z e d e n c l o s u r e s , r e s p e c t i v e l y . C o n c e n t r a t i o n s o f n i t r a t e r e m a i n e d h i g h u n t i l f a l l o v e r t u r n i n m i d - O c t o b e r . S i m i l a r l y , e p i l i m n e t i c c o n c e n t r a t i o n s o f added ammonia i n c r e a s e d soon a f t e r f e r t i l i z a t i o n , e x c e e d i n g 50 u g / l i t e r and 200 u g / l i t e r i n t h e l i t t o r a l and l i m n e t i c e n c l o s u r e s . However, c o n c e n t r a t i o n s o f t h i s n u t r i e n t had d e c r e a s e d s i g n i f i c a n t l y by t h e end of A u g u s t . H y d r o g e n i o n c o n c e n t r a t i o n i n t h e e p i l i m n i o n d i d n o t change 79 as a r e s u l t o f e i t h e r e n c l o s u r e or f e r t i l i z a t i o n . 3.3 PHYTOPLANKTON STANDING CROP, SPECIES AND S I Z E COMPOSITION _3.3_.J_ S t a n d i n g c r o p (1) E f f e c t s of E n c l o s u r e T o t a l a l g a l b i o m a s s a s d e t e r m i n e d by c e l l v o l u m e s d i d not e x c e e d 400 u g / l i t e r ( F i g u r e 16) and d i d not d i f f e r s i g n i f i c a n t l y between l a k e and t h e c o n t r o l e n c l o s u r e s ( T a b l e 5 ) . Mid-summer maximum c h l o r o p h y l l a c o n c e n t r a t i o n s i n b o t h t h e l a k e and c o n t r o l s d i d n o t e x c e e d 0.6 - 0.8 u g / l i t e r ( F i g u r e 17 and T a b l e 6 ) . C o n c e n t r a t i o n s o f c h l o r o p h y l l a i n c r e a s e d w i t h i n c r e a s i n g d e p t h ( T a b l e 6) ( s i g n i f i c a n t a t p < 0.05). S e a s o n a l mean s t a n d i n g c r o p s a t a l l d e p t h s i n t h e c o n t r o l s were n o t s i g n i f i c a n t l y d i f f e r e n t f r o m t h o s e i n t h e l a k e o r between l i t t o r a l and l i m n e t i c c o n t r o l s ( T a b l e 7 ) . S e a s o n a l f l u c t u a t i o n s i n c h l o r o p h y l l a i n e a c h of 3 d e p t h s t r a t a i n t h e c o n t r o l s were i n p hase w i t h t h o s e o c c u r r i n g a t a l l d e p t h s i n t h e l a k e ( F i g u r e s 18 - 2 0 ) , d i f f e r i n g o n l y s l i g h t l y i n t h e m a g n i t u d e o f c h a n g e . Mean summer s t a n d i n g c r o p s i n 1982 (0.44 ± 0.04 u g / l i t e r and 0.40 ± 0.02 u g / l i t e r f o r t h e l a k e and c o n t r o l s , r e s p e c t i v e l y ) were s i m i l a r t o t h o s e i n 1980 and 1981 i n n e a r b y E u n i c e L a k e (0.39 ± 0.05 u g / l i t e r , Marmorek, 1983). S i g n i f i c a n t d i f f e r e n c e s i n t h e c o n c e n t r a t i o n s of p h a e o p h y t i n a and t o t a l p i g m e n t i n t h e 2-4 m s t r a t u m were n o t e d i n t h e c o n t r o l v s l a k e c o m p a r i s o n ( T a b l e 7 ) . T h i s d i f f e r e n c e 8 0 F i g u r e 16. A l g a l b i o m a s s as d e t e r m i n e d by c e l l v olume. 81 Lake Enclosure 5 Enclosure 7 ( l i t t o r a l u n f e r t i l i z e d ) ( l i t t o r a l f e r t i l i z e d ) J U N J L Y A U G S E P OCT NOV J U N J L Y A U G S E P O C T NOV E n c l o s u r e 6 Enclosure B ( l i m n e t i c u n f e r t i l i z e d ) ( l i m n e t i c f e r t i l i z e d ) J U N J L Y A U G S E P OCT NOV J U N J L Y A U G S E P OCT NOV T a b l e 5 : R e s u l t s o f o n e w a y a n a l y s i s o f v a r i a n c e o n t o t a l b i o m a s s o f p h y t o p l a n k t o n ( n o t e x c l u d i n g d e t r i t u s ) i n t h e 0 - 2 m d e p t h s t r a t u m a s d e t e r m i n e d b y c e l l v o l u m e s . N u m b e r s I n b r a c k e t s a r e p r o b a b i l i t i e s o f n o s i g n i f i c a n t t r e a t m e n t e f f e c t ( F i t i ? f o r l a k e v s c o n t r o l m e a n s , F i , i o o f o r c o n t r o l v s f e r t i l i z e d m e a n s , a n d F I , . I . D a t a f o r t h e L i t t o r a l f e r t i l i z e d v s L i m n e t i c f e r t i l i z e d c o m p a r i s o n w e r e n o t t r a n s f o r m e d b e c a u s e t h e l a r g e s t a n d a r d d e v i a t i o n ( 1 1 4 7 8 ) a s s o c i a t e d w i t h t h e m e a n f o r t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e p r e c l u d e d a n a l y s i s b y t r a n s f o r m a t i o n . T r e a t m e n t a r i t h m e t i c m e a n s ± S E ( u g / l 1 t e r ) V a r i a b l e L a k e L i t t o r a l C o n t r o l s L i m n e t i c C o n t r o l s L i t t o r a l F e r t i 1 i z e d L i m n e t i c F e r t i 1 i z e d T o t a l B i o m a s s 9 7 2 ± 1 . 3 0 8 9 1 + 1 . 2 0 7 0 5 ± 1 . 2 2 1 9 1 4 ± 1 . 2 1 3 0 2 7 ± 1 . 3 6 T r e a t m e n t C o m p a r i s o n L a k e v s C o n t r o l C o n t r o l v s F e r t i 1 i z e d L i t t o r a l C o n t r o 1 v s L i m n e t 1 c C o n t r o l L 1 t t o r a l F e r t 1 1 i z e d L i m n e t i c F e r t i 1 i z e d L i t t o r a l C o n t r o l L i t t o r a l F e r t i 1 i z e d L i m n e t i c C o n t r o l L 1 m n e t i c F e r t i 1 i z e d n s ( 0 . 3 3 1 ) F ( 0 . 0 0 0 ) n s ( 0 . 3 3 3 ) L i m ( 0 . 0 0 4 ) F ( 0 . 0 0 0 ) F ( 0 . 0 0 0 ) n s - n o t s i g n i f i c a n t a t a • 0 . 0 5 F , L i t t . L i m , o r C = i n t e r a c t i o n s i g n i f i c a n t a t a = 0 . 0 1 8 3 F i g u r e 17. D e p t h - t i m e i s o p l e t h s of C h l o r o p h y l l a. V a l u e s a r e u g / l i t e r . LAKE Chlorophyll a ug/L Enclosure 1 ( l i t t o r a l control) J u a a J u l y Aug l a g O a t Enclosure 2 (limnetic control) J - i f • III O i l Enclosure 5 ( l i t t o r a l control) Juaa J - t y A u g Enclosure 3 (li t t o r a l fertilized) J m i Jwly A u g O t l Enclosure 4 (limnetic fertilized) Enclosure 7 (littoral fertilized) 0 0-1. 1. • ft « J u a a J u l y A » g . l a p Oat Enclosure 6 (limnetic control) Enclosure B (limnetic fertilized) 1 . J u a a J u l y Aug l a g O a l J u a a J u l y Aug l a g O a l T a b l e 6 . M e a n s e a s o n a l c o n c e n t r a t i o n s o f p l a n t p i g m e n t s a t a l l d e p t h s a n d i n a l l t r e a t m e n t s ± S E ( u g / l i t e r ) . T r e a t m e n t a r i t h m e t i c m e a n s ± S E ( u g / l 1 t e r ) V a r i a b l e D e p t h L a k e L i t t o r a l L i m n e t i c L i t t o r a l L i m n e t i c ( m ) C o n t r o l s C o n t r o l s F e r t i l i z e d F e r t i l i z e d 0 - 2 0 . 3 6 ± 0 . 0 3 2 0 . 4 3 ± 0 . 0 2 3 C h l o r o p h l l a 2 - 4 0 . 4 0 ± 0 . 0 3 8 0 . 4 1 + 0 . 0 2 3 4 - 6 0 . 5 6 ± 0 . 1 4 1 0 . 4 6 + 0 . 0 2 8 0 . 3 3 + 0 . 0 1 8 1 . 4 3 ± 0 . 1 5 2 6 . 1 4 ± 1 . 1 2 5 0 . 5 1 + 0 . 0 2 4 0 . 7 6 + 0 . 1 3 5 2 . 8 4 ± 0 . 6 5 5 0 . 5 4 ± 0 . 0 5 3 0 . 7 5 ± 0 . 0 9 9 0 . 8 6 ± 0 . 0 7 4 A L L 0 . 4 4 ± 0 . 0 3 9 0 . 4 0 ± 0 . 0 1 5 0 . 4 6 ± 0 . 0 2 2 0 . 9 8 ± 0 . 0 8 1 3 . 2 8 ± 0 . 4 8 5 0 - 2 0 . 4 9 ± 0 . 0 1 2 0 . 5 5 ± 0 . 0 2 4 0 . 4 9 + 0 . 0 6 0 1 . 8 1 + 0 . 1 7 3 7 . 2 8 ± 1 . 1 4 7 P h a e o p h y t i n a 2 - 4 0 . 7 1 + 0 . 0 4 1 1 . 0 4 + 0 . 0 7 5 0 . 8 1 + 0 . 0 6 1 1 . 2 8 + 0 . 0 8 1 5 . 0 7 1 1 . 1 3 1 4 - 6 1 . 2 0 + 0 . 1 6 9 1 . 3 8 + 0 . 1 4 6 1 . 6 0 + 0 . 2 9 1 1 . 8 2 ± 0 . 2 5 3 2 . 4 0 ± 0 . 1 4 9 A L L 0 . 8 0 + 0 . 0 7 2 0 . 9 9 + 0 . 0 6 5 0 . 9 7 ± 0 . 1 1 1 1 . 6 4 ± 0 . 1 0 8 4 . 9 2 ± 0 . 5 7 2 0 - 2 0 . 8 5 + 0 . 0 3 2 0 . 8 9 ± 0 . 0 4 3 0 . 8 3 + 0 . 0 6 5 3 . 2 4 ± 0 . 2 9 7 1 3 . 4 2 ± 2 . 2 5 6 T o t a l P i g m e n t 2 - 4 1 . 1 0 ± 0 . 0 6 5 1 . 5 0 ± 0 . 0 9 2 1 . 3 2 + 0 . 0 6 6 2 . 0 4 + 0 . 2 9 7 7 . 9 0 ± 1 . 7 7 6 4 - 6 1 . 7 6 ± 0 . 2 0 0 1 . 7 9 ± 0 . 1 6 9 2 . 1 4 + 0 . 3 3 5 2 . 5 7 ± 0 . 3 1 3 3 . 2 6 ± 0 . 2 1 0 A L L 1 . 2 4 + 0 . 1 0 6 1 . 3 9 + 0 . 0 7 6 1 . 4 3 ± 0 . 1 2 7 2 . 6 2 ± 0 . 1 6 2 8 . 1 9 ± 1 . 0 4 1 0 0 T a b l e 7 . R e s u l t s o f a n a l y s i s o f v a r i a n c e o n 1 / s q r t o f t r e a t m e n t m e a n s s h o w n 1 n T a b l e 6 . N u m b e r s I n b r a c k e t s a r e p r o b a b i l i t i e s o f n o s i g n i f i c a n t t r e a t m e n t e f f e c t ( F i , n i f o r l a k e v s c o n t r o l g r a n d m e a n s . F i . ' i f o r l a k e v s c o n t r o l d e p t h m e a n s , F i , m f o r c o n t r o l v s f e r t i l i z e d g r a n d m e a n s , FI.II . f o r c o n t r o l v s f e r t i l i z e d d e p t h m e a n s , FI,I.O f o r a l l o t h e r g r a n d m e a n s , a n d F i , « t f o r a l l o t h e r d e p t h c o m p a r i s o n s ) . V a r i a b l e D e p t h T r e a t m e n t C o m p a r i s o n L a k e v s C o n t r o l v s L i t t o r a l C o n t r o l L i t t o r a l F e r t 1 1 I z e d L I t t o r a l C o n t r o l L i m n e t t c C o n t r o l ( m ) C o n t r o l F e r t i 1 I z e d L I m n e t I c C o n t r o l L i m n e t 1 c F e r t 1 1 1 z e d L i t t o r a l F e r t i 1 I z e d 0 - 2 n s ( 0 3 5 0 ) F ( 0 . 0 0 0 ) n s ( 0 6 4 0 ) L l m ( 0 0 0 2 ) F ( 0 0 0 0 ) F ( 0 0 0 0 ) 1 / s q r t C h i a 2 - 4 n s ( 0 0 6 0 ) F ( 0 . 0 0 0 ) n s ( 0 0 7 0 ) L 1 m ( 0 0 0 0 ) F ( 0 0 0 0 ) F ( 0 0 0 0 ) 4 - 6 n s ( 0 8 6 0 ) F ( 0 . 0 0 0 ) L l m ( 0 0 2 0 ) L l m ( 0 0 3 0 ) n s ( 0 3 1 0 ) F ( 0 0 0 0 ) A L L n s ( 0 5 8 0 ) F ( 0 . 0 0 0 ) n s ( 0 0 5 8 ) L 1 m ( 0 O O O ) F ( 0 0 0 0 ) F ( 0 O O O ) 0 - 2 h s ( 0 6 9 0 ) F ( 0 . 0 0 0 ) L 1 t t ( 0 0 0 5 ) L l m ( 0 O O O ) F ( 0 0 0 0 ) F ( 0 0 0 0 ) 1 / s q r t P h a e a 2 - 4 C ( 0 0 0 6 ) F ( 0 . 0 0 0 ) L 1 t t ( 0 0 0 8 ) L i m ( 0 0 0 0 ) F ( 0 0 4 0 ) F ( 0 0 0 0 ) 4 - 6 n s ( 0 5 7 0 ) F ( 0 . 0 0 0 ) n s ( 0 6 6 0 ) L l m ( 0 0 0 0 ) n s ( 0 2 9 0 ) F ( 0 0 0 0 ) A L L n s ( 0 4 0 0 ) F ( O . O O O ) L 1 t t ( 0 0 2 0 ) L l m ( 0 ooo) F ( 0 0 0 0 ) F ( 0 0 0 0 ) 0 - 2 n s ( 0 4 7 0 ) F ( 0 . 0 0 0 ) n s ( 0 1 6 0 ) L l m ( 0 0 0 0 ) F ( 0 0 0 0 ) F ( 0 0 0 0 ) 1 / s q r t T o t a l 2 - 4 C ( 0 0 0 5 ) F ( 0 . 0 0 0 ) n s ( 0 2 2 0 ) L l m ( 0 0 0 0 ) F ( 0 0 3 0 ) F ( 0 0 0 0 ) P 1 g m e n t 4 - 6 n s ( 0 6 0 0 ) F ( 0 . 0 0 0 ) n s ( 0 7 9 0 ) L l m ( 0 0 C 1 ) n s ( 0 1 2 0 ) F ( 0 0 0 0 ) A L L n s ( 0 3 8 0 ) F ( 0 . 0 0 0 ) n s ( 0 3 3 0 ) L l m ( 0 0 0 0 ) F ( 0 000) F (0 000) L i m n e t i c e r t 1 1 1 z e d n s = n o t s i g n i f i c a n t a t a = 0 . 0 5 F , L 1 t t , L l m , o r C = i n t e r a c t i o n s i g n ! l e a n t a t a = 0 . 0 1 87 F i g u r e 18. Cummulative c o n c e n t r a t i o n s o f c h l o r o p h y l l a and p h a e o p h y t i n a i n t h e 0-2 m d e p t h s t r a t u m . Note t h e change i n s c a l e of t h e Y - a x i s on t h e p l o t s f o r t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . LEGEND Lake C h l o r o p h y l l a Phaeoplgments in E a c a E a> c a a J L Y AUG SEP J L Y E n c l o s u r e 1 ( l i t t o r a l c o n t r o l ) AUG SEP E n c l o s u r e 3 ( l i t t o r a l f e r t i l i z e d ) OB 10 CV a J L Y AUG SEP J L Y E n c l o s u r e 2 ( l i m n e t i c c o n t r o l ) AUG SEP E n c l o s u r e A ( l i m n e t i c f e r t i l i z e d ) o o o tv J L Y AUG SEP E n c l o s u r e 5 ( l i t t o r a l c o n t r o l ) J L Y AUG SEP E n c l o s u r e 7 ( l i t t o r a l f e r t i l i z e d ) CD 10 W O J L Y AUG SEP E n c l o s u r e 6 ( l i m n e t i c c o n t r o l ) E n c l o s u r e B ( l i m n e t i c f e r t i l i z e d ) o o cv JLY AUG SEP J L Y A U G SEP 89 F i g u r e 19. C u m m u l a t i v e c o n c e n t r a t i o n s of c h l o r o p h y l l a and p h a e o p h y t i n a i n t h e 2-4 m d e p t h s t r a t u m . Note t h e change i n s c a l e of t h e Y - a x i s on t h e p l o t s f o r t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . LEGEND L a k e 90 C h l o r o p h y l l a P h a e o p l g m s n t a co e a B JLY AUG S E P i n cu JLY E n c l o s u r e 1 ( l i t t o r a l c o n t r o l ) AUG S E P E n c l o s u r e 3 ( l i t t o r a l f e r t i l i z e d ) i n cu JLY AUG S E P cn cu JLY E n c l o s u r e 2 ( l i m n e t i c c o n t r o l ) AUG SEP E n c l o s u r e 4 ( l i m n e t i c f e r t i l i z e d ) a 10 cu a JLY AUG S E P m co cu JLY E n c l o s u r e 5 ( l i t t o r a l c o n t r o l ) AUG S E P E n c l o s u r e 7 ( l i t t o r a l f e r t i l i z e d ) CO cu JLY AUG S E P CO cu E n c l o s u r e 6 ( l i m n e t i c c o n t r o l ) E n c l o s u r e B ( l i m n e t i c f e r t i l i z e d ) o cn o cu JLY AUG S E P JLY AUG S E P 91 F i g u r e 2 0 . Cummulative c o n c e n t r a t i o n s o f c h l o r o p h y l l a and p h a e o p h y t i n a i n t h e 4 - 6 m d e p t h s t r a t u m LEGEND Lake C h l o r o p h y l l • P h B s o p l c j B a n t a a a B a »• c o °- JLY AUO S E P cn cu JLY E n c l o s u r e 1 ( l i t t o r a l c o n t r o l ) AUO S E P in •<» cn cu Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) A / \ / \ J L Y AUG S E P tn cu JLY E n c l o s u r e 2 ( l i m n e t i c c o n t r o l ) » / V / \ / \ I \ I \ I \ AUG S E P Enclosure 4 ( l i m n e t i c f e r t i l i z e d ) cn cu JLY AUG S E P cn i JLY E n c l o s u r e 5 ( l i t t o r a l c o n t r o l ) / s " Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) in cn cu AUG S E P JLY AUG S E P E n c l o s u r e 6 ( l i m n e t i c c o n t r o l ) in •» cn cu Enclosure 8 ( l i m n e t i c f e r t i l i z e d ) JLY AUG SEP JLY AUG S E P 93 can be e n t i r e l y a t t r i b u t e d t o h i g h e r p h a e o p h y t i n v a l u e s w h i c h a p p e a r e d a t t h a t d e p t h i n t h e c o n t r o l s ( T a b l e 6, F i g u r e 1 9 ) . T h i s may have been c a u s e d by s e s t o n s e t t l i n g o ut of t h e e p i l i m n i o n and d e c o m p o s i n g on t h e t h e r m a l d i s c o n t i n u i t y b a r r i e r a t 3.5-4 m. C o n c e n t r a t i o n s of p h a e o p h y t i n a were s i g n i f i c a n t l y h i g h e r o v e r a l l i n t h e l i t t o r a l compared t o t h e l i m n e t i c c o n t r o l e n c l o s u r e s ( T a b l e s 6 and 7 ) . T h i s d i f f e r e n c e was p a r t i c u l a r l y due t o h i g h e r mean p h a e o p h y t i n a c o n c e n t r a t i o n s a t 0-2 and 2-4 m i n t h e l i t t o r a l e n c l o s u r e s ( F i g u r e 18 and 1 9 ) . The mean v a l u e f o r t h e s e e n c l o s u r e s i s f o r c e d upward by t h e s l i g h t l y h i g h e r c o n c e n t r a t i o n s of p h a e o p h y t i n a i n l i t t o r a l e n c l o s u r e 5 compared t o l i t t o r a l e n c l o s u r e 1 a t b o t h of t h e s e d e p t h s . T h e s e h i g h e r c o n c e n t r a t i o n s were l i k e l y c a u s e d by wind g e n e r a t e d r e s u s p e n s i o n of l i t t o r a l s e d i m e n t s i n c o n t r o l e n c l o s u r e 5. E n c l o s u r e 5 had a g e n t l y s l o p i n g b o t t o m w i t h a n e a r s h o r e d e p t h of o n l y 0.25 m compared t o i t s c o u n t e r p a r t , l i t t o r a l c o n t r o l e n c l o s u r e 1, w h i c h had a 1 m d r o p t o t h e s e d i m e n t s i n t h e nea r s h o r e a r e a . Thus, a g r e a t e r a r e a of l i t t o r a l s e d i m e n t i n e n c l o s u r e 5 was e x p o s e d t o t u r b u l e n t m i x i n g by wind g e n e r a t e d c u r r e n t s , t h u s r e s u l t i n g i n h i g h e r c o n c e n t r a t i o n s o f s u s p e n d e d p a r t i c u l a t e m a t t e r . The e x c e p t i o n a l l y h i g h v a l u e o f p h a e o p h y t i n a a t 4-6 m e t e r s on A u g u s t 17th i n e n c l o s u r e 6 ( F i g u r e 20) was a r e s u l t o f c o n t a m i n a t i o n o f t h e sample w i t h b o t t o m s e d i m e n t s t h a t were s u c k e d up by t h e pump. V a r i a t i o n s i n t h e l i v e c h l o r o p h y l l a as a p e r c e n t a g e of t h e t o t a l p l a n t pigment c o n c e n t r a t i o n may be an i n d e x o f v i a b i l i t y 94 f o r t h e a l g a l p o p u l a t i o n . T e m p o r a l o r s p a t i a l r e d u c t i o n s i n t h e p e r c e n t a g e o f l i v e c h l o r o p h y l l c an be an i n d i c a t i o n t h a t a g r e a t e r p r o p o r t i o n of t h e a l g a l p o p u l a t i o n i s d y i n g o f f a t some t i m e s or i n some p l a c e s . T e m p o r a l f l u c t u a t i o n s i n t h e p e r c e n t a g e o f l i v e c h l o r o p h y l l a i n t h e c o n t r o l s d i d not d i f f e r f r o m t h o s e o c c u r r i n g i n t h e l a k e ( F i g u r e 2 1 ) . S e a s o n a l means were n o t s i g n i f i c a n t l y d i f f e r e n t between l i t t o r a l and l i m n e t i c c o n t r o l s or between c o n t r o l s and t h e l a k e ( T a b l e 8 ) . However, t h e mean p e r c e n t a g e of l i v e c h l o r o p h y l l a d e c r e a s e d s i g n i f i c a n t l y a t 4-6 m b o t h i n t h e c o n t r o l s (39.0 ± 1.9% a t 0-2 m, 37.0 ± 2.6% a t 2-4 m, and 28.0 ± 1.8% a t 4-6 m) and i n t h e l a k e (38.8 % a t 0-2 m, 35.0 % a t 2-4 ni, and 29.5 % a t 4-6 m) . T h i s p a t t e r n s u g g e s t s t h a t w h i l e t h e bi o m a s s of p h y t o p l a n k t o n a s mea s u r e d by c h l o r o p h y l l a was g r e a t e s t i n t h e h y p o l i m n i o n ( T a b l e 6 ) , a g r e a t e r p e r c e n t a g e of t h a t b i o m a s s may have e x i s t e d a s n o n f u n c t i o n a l pigment i n t h e form o f d e c o m p o s i n g d e t r i t u s . Many of t h e c e l l s i n t h e h y p o l i m n i o n may have been dead o r d y i n g . (2) E f f e c t s o f F e r t i l i z a t i o n P h y t o p l a n k t o n s t a n d i n g c r o p s i n c r e a s e d s e v e r a l f o l d a f t e r f e r t i l i z a t i o n ( F i g u r e s 16 and 1 7 ) . An e p i l i m n e t i c maximum c o n c e n t r a t i o n of 15 ug C h i a / l i t e r was r e a c h e d i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s i n l a t e A u g u s t . Maximum e p i l i m n e t i c c o n c e n t r a t i o n s o f c h l o r o p h y l l a i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s a v e r a g e d o n l y 2.5 ug C h i a / l i t e r w i t h e n c l o s u r e 3 v a l u e s a l m o s t d o u b l e t h o s e f o r l i t t o r a l e n c l o s u r e 7. However, t h e t o t a l p i gment c o n c e n t r a t i o n s i n b o t h 95 F i g u r e 21. P r o p o r t i o a s of l i v e c h l o r o p h y l l a i n the 0-2, 2-4 and 4-6 meter depth s t r a t a LEGEND Lake 96 0 - 2 a f t e r s 2 - 4 n e t o r a 4 - B a e t a r a C > . o c a *> a a Q. r-i-a c a L u a. 5° J L Y AUG SEP E n c l o s u r e 1 ( l i t t o r a l c o n t r o l ) CO o J L Y AUG SEP E n c l o s u r e 3 ( l i t t o r a l f e r t i l i z e d ) CD a a • • •— J L Y AUG SEP E n c l o s u r e 2 E n c l o s u r e 4 ( l i m n e t i c c o n t r o l ) ( l i m n e t i c f e r t i l i z e d ) J L Y AUG SEP J L Y AUG SEP E n c l o s u r e 5 E n c l o s u r e 7 ( l i t t o r a l c o n t r o l ) ( l i t t o r a l f e r t i l i z e d ) J L Y AUS SEP J L Y AUG SEP T a b l e 8 . R e s u l t s o f o n e w a y a n a l y s i s o f v a r i a n c e o n m e a s u r e m e n t s o f t h e s e a s o n a l m e a n p r o p o r t i o n o f l i v e C h l o r o p h y l l a I n a l l o f t h e e n c l o s u r e s a n d t h e l a k e . N u m b e r s I n b r a c k e t s a r e p r o b a b i l i t i e s o f n o s i g n i f i c a n t t r e a t m e n t e f f e c t ( F i , u f o r l a k e v s c o n t r o l m e a n s , F i , n f o r ' c o n t r o l v s f e r t i l i z e d m e a n s , a n d F i , n f o r a l l o t h e r c o m p a r i s o n s . V a r i a b l e T r e a t m e n t a r i t h m e t i c m e a n s ± S E ( u g / l i t e r ) L a k e L i t t o r a l C o n t r o l s L i m n e t i c C o n t r o l s L i t t o r a l F e r t i 1 i z e d L i m n e t i c F e r t i 1 i z e d P r o p o r t i o n L i v e C h i a 0 . 3 4 9 ± 0 . 0 1 9 0 . 3 2 4 ± 0 . 0 3 0 0 . 3 7 3 ± 0 . 0 2 1 0 . 3 4 1 + 0 . 0 3 0 0 . 3 4 4 ± O . 0 3 0 T r e a t m e n t C o m p a r i s o n L a k e v s C o n t r o l v s L i t t o r a l C o n t r o l L i t t o r a l F e r t i 1 i z e d L i t t o r a l C o n t r o l L 1 m n e t i c C o n t r o l C o n t r o l F e r t i 1 i z e d L i m n e t 1 c C o n t r o l L i m n e t i c F e r t 1 1 i z e d L i t t o r a l F e r t i 1 i z e d L i m n e t i c F e r t 1 1 < z e d n s ( 1 . 0 0 0 ) n s ( . 8 6 4 ) n s ( 0 . 2 2 0 ) n s ( 1 . 0 0 ) n s ( 0 . 6 8 9 ) n s ( 0 . 4 7 5 ) n s = n o t s i g n i f i c a n t a t a = 0 . 0 5 98 e n c l o s u r e s a t 0-2 m were a b o u t t h e same ( F i g u r e 1 8 ) . The low r a t i o of c h l o r o p h y l l a t o p h a e o p h y t i n a i n e n c l o s u r e 7 d u r i n g t h e l a t t e r p a r t of t h e summer was due t o d o m i n a t i o n o f t h e a l g a l community a t t h a t t i m e by c h r y s o p h y t e s , w h i c h have a p r e p o n d e r a n c e o f c a r o t e n o i d pigment ( R e y n o l d s , 1984) and c o n t a i n v e r y l i t t l e c h l o r o p h y l l . E p i l i m n e t i c c o n c e n t r a t i o n s o f c h l o r o p h y l l a i n a l l of t h e f e r t i l i z e d e n c l o s u r e s were s i g n i f i c a n t l y h i g h e r t h a n i n c o n t r o l s ( T a b l e 6 and 7, F i g u r e s 17 and 18-20). C o n c e n t r a t i o n s i n t h e l i t t o r a l e n c l o s u r e s were n o t s i g n i f i c a n t l y d i f f e r e n t from l i t t o r a l c o n t r o l s a t 4-6 m, s i n c e e n c l o s u r e 7 had o n l y a modest r e s p o n s e t o f e r t i l i z a t i o n . C h l o r o p h y l l a v a l u e s i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s were s i g n i f i c a n t l y l o w e r t h a n i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s a t a l l d e p t h s ( T a b l e s 6 and 7 ) . A l s o , c h l o r o p h y l l a i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s d i d not peak a s s h a r p l y a s i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s d u r i n g l a t e A u g u s t ( F i g u r e s 18 -20) e x c e p t a t 4~6m ( F i g u r e 2 0 ) . T o t a l a l g a l b i o m a s s i n l i m n e t i c f e r t i l i z e d e n c l o s u r e 8 began t o i n c r e a s e e x p o n e n t i a l l y i n t h e 0-2 m s t r a t u m , a p p a r e n t l y i n r e s p o n s e t o n u t r i e n t a d d i t i o n , a b o u t 1 week e a r l i e r t h a n i n a l l o t h e r f e r t i l i z e d e n c l o s u r e s ( F i g u r e 16 and 1 8 ) . The sudden d e c l i n e i n bi o m a s s t h a t o c c u r r e d i n e a r l y September was p a r a l l e l e d by a d r a m a t i c i n c r e a s e i n t h e b i o m a s s and abundance o f p h y t o p l a n k t o n a t 2-4 m, ( F i g u r e 19) w h i c h s u g g e s t s t h a t t h e a l g a e s e t t l e d o ut o f t h e 0-2 m s t r a t u m . An e x c e p t i o n a l l y h i g h v a l u e o f 39.4 ug C h i a / l i t e r was r e c o r d e d a t 2-4 m a t t h a t t i m e . T e m p o r a l f l u c t u a t i o n s i n t h e p e r c e n t a g e o f l i v e c h l o r o p h y l l 99 a i n t h e f e r t i l i z e d e n c l o s u r e s were n o t s i g n i f i c a n t l y d i f f e r e n t f r o m t h o s e o c c u r r i n g i n t h e c o n t r o l s and l a k e ( F i g u r e 2 1 ) . However, a much h i g h e r p e r c e n t a g e o f l i v e c h l o r o p h y l l a o c c u r r e d i n l i t t o r a l f e r t i l i z e d e n c l o s u r e 3 i n l a t e A u g u s t and l i m n e t i c f e r t i l i z e d e n c l o s u r e 8 f o r t h e 3 weeks i n A u g u s t a t 2-4 m e t e r s when t h e m e t a l i m n e t i c b i o m a s s maxima ( m e n t i o n e d above) o c c u r r e d . S e a s o n a l mean p e r c e n t a g e s of l i v e c h l o r o p h y l l a i n t h e f e r t i l i z e d e n c l o s u r e s were not s i g n i f i c a n t l y d i f f e r e n t between l i t t o r a l o r l i m n e t i c l o c a t i o n s , o r between t h e c o n t r o l s and t h e l a k e ( T a b l e 8 ) . As i n t h e c o n t r o l - l a k e s e r i e s , p e r c e n t a g e s a l s o d e c r e a s e d s i g n i f i c a n t l y w i t h d e p t h (42.8 ± 1.6 % a t 0-2 m, 31.4 ± 1.7 % a t 2-4 m and 28.6 ± 1.3 % a t 4-6 m) . However, r e c a l l t h a t t h e major p o r t i o n of t h e b i o m a s s i n t h e f e r t i l i z e d e n c l o s u r e s o c c u r r e d i n t h e u p p e r 4 m ( T a b l e 6) where t h e p e r c e n t a g e o f l i v e c h l o r o p h y l l a was t h e g r e a t e s t . T h i s p a t t e r n s u g g e s t s t h a t a g r e a t e r p r o p o r t i o n o f t h e t o t a l b i o m a s s o f t h e p h y t o p l a n k t o n i n t h e f e r t i l i z e d e n c l o s u r e s was l i v i n g , v i a b l e p l a n t m a t e r i a l compared w i t h c o n t r o l s and t h e l a k e . 3.3.2 S p e c i e s C o m p o s i t i o n ( 1 ) In The L a k e The p h y t o p l a n k t o n s p e c i e s p r e s e n t i n P l a c i d Lake seem t y p i c a l o f t h o s e f o u n d i n bog l a k e s (Welch, 1952) and of o t h e r s m a l l c o a s t a l l a k e s i n t h e a r e a . A l l o f t h e g e n e r a and many of t h e s p e c i e s i d e n t i f i e d i n t h i s s t u d y ( A p p e n d i x 1) have been r e p o r t e d t o a p p e a r i n n e a r b y M a r i o n Lake (Dickman, 1968). 1 0 0 A l t h o u g h most of t h e f o l l o w i n g a n a l y s i s d e a l s p r i m a r i l y w i t h d a t a f r o m t h e 0 - 2 m d e p t h s t r a t u m , t h e r e was no e v i d e n c e t h a t t h e s p e c i e s c o m p o s i t i o n c h a n g e d w i t h d e p t h i n t h i s s h a l l o w l a k e . The number of s p e c i e s of l i m n e t i c p h y t o p l a n k t o n i s s m a l l ( c a . 1 2 0 ) w i t h few s p e c i e s i n e a c h g e n e r a . Q u a l i t a t i v e l y , t h e C h l o r o p h y c e a e p r e d o m i n a t e i n t h e number o f s p e c i e s f o l l o w e d by C y a n o p h y c e a e , C h r y s o p h y c e a e and f i n a l l y C r y p t o p h y c e a e . B a c i l l a r i o p h y c e a e p l a y an i n s i g n i f i c a n t r o l e i n t h e c o m p o s i t i o n of summer p l a n k t o n a s s o c i a t i o n s but a r e p r o b a b l y p r e s e n t i n l a r g e numbers d u r i n g t h e s p r i n g bloom. Most o f t h e t a x a i n t h e l a k e , w i t h t h e e x c e p t i o n of t h e C r y p t o p h y c e a e , had s e a s o n a l c y c l e s o f peak abundance ( F i g u r e 2 2 ) . S m a l l C h l o r o p h y c e a n s , s u c h as Chlamydomonas spp., O o c y s t i s s p . , N a n n o c h l o r i s , C h l o r e l l a e l l i p s o i d a ( G e r n e c k ) , p a l m e l l a r s t a g e s of Chlamydomonas, S p h a e r o c y s t i s s c h r o e t e r i ( C h o d a t ) and G l o e o c y s t i s p l a n k t o n i c a (West) d o m i n a t e d t h e b i o m a s s i n J une t h r o u g h l a t e J u l y i n t h e l a k e , but c o n t r i b u t e d l i t t l e t o t h e s t a n d i n g s t o c k f o r t h e r e s t of t h e summer. C o i n c i d e n t w i t h t h e c o l l a p s e o f t h e c h l o r o p h y t e s , came an i n c r e a s e i n t h e c h r y s o p h y t e s . T h i s mid-summer bloom began w i t h t h e a p p e a r a n c e of Ochromonas ( W y s s o z t s k i ) , S y n u r a u v e l l a ( E h r e n b e r g ) and S. s p h a g n i c o l a ( K o r s h n i k o v ) and p r o g r e s s e d i n t i m e t o i n c l u d e C h r y s o p h a e l l a l o n g i s p i n a ( L a u t e r b o r n ) , D i n o b r y o n  s e r t u l a r i a ( E h r e n b e r g ) , and D. s o c i a l e ( E h r e n . ) . D i n o b r y o n s p p . were t h e o n l y p e r e n n i a l c h r y s o p h y t e s i n t h e l a k e , o c c u r r i n g a t a l l t i m e s i n t h e p l a n k t o n a t m o derate d e n s i t i e s ( 1 0 3 t o 1 0 " c e l l s / l i t e r ) . A l l o f t h e o t h e r s p e c i e s o f c h r y s o p h y t e s bloomed 101 F i g u r e 22. P h y l e t i c c o m p o s i t i o n o f a l g a l b i omass LEGEND J iiiiiiiimii lrgt's Chlorophytaa Cryptophytaa Cyanophyeaa Chryaophytaa Unclaaalflad 102 LAKE o • o JUN Enclosure 1 ( l i t t o r a l control) JLY AUB S E P OCT NOV Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) JUN JLY AUB S E P OCT NOV Enclosure 2 ( l imnetic control) JLY AUB 8 E P OCT NOV Enclosure 4 (l imnetic f e r t i l i z e d ) JUN JLY AUB S E P OCT NOV 8 8 Enclosure 5 ( l i t t o r a l control) JLY AUB SEP OCT NOV Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) JUN JLY AUB 8 E P OCT NOV JUN Enclosure 6 ( l imnetic control) JLY AUB S E P OCT NOV Enclosure 8 ( l imnetic f e r t i l i z e d ) o to a a M JUN JLY AUB 8 E P OCT NOV 1 03 and t h e n d i s a p p e a r e d . In l a t e A u g u s t when n i t r o g e n t o p h o s p h o r o u s r a t i o s ( t o t a l i n o r g a n i c N : t o t a l d i s s o l v e d P) i n t h e e p i l i m n i o n d e c r e a s e d s i g n i f i c a n t l y (N:P = 6:1) from mid summer v a l u e s (N:P = 2 7 : 1 ) , th e a l g a l a s s o c i a t i o n became d o m i n a t e d by t h e c o l o n i a l c y a n o p h y t e s C h r o o c o c c u s l i m n e t i c u s (Lemm.), C. P r e s c o t t i i ( D r o u e t and D a i l y ) and M e r i s m o p e d i a e l e g a n s (A. B r a u n ) . Even b e f o r e t h i s "bloom" (by P l a c i d L a k e s t a n d a r d s ) , t h e s e s p e c i e s f ormed a l a r g e component of t h e summer p h y t o p l a n k t o n a s s e m b l a g e . (2) E f f e c t s of E n c l o s u r e S e a s o n a l p h y l e t i c c o m p o s i t i o n s of t h e t o t a l a l g a l v o l u m e s i n t h e c o n t r o l e n c l o s u r e s were e s s e n t i a l l y t h e same as i n t h e l a k e ( F i g u r e 2 2 ) . An e x c e p t i o n was t h a t t h e c o l l a p s e of t h e e a r l y summer C h l o r o p h y c e a n bloom i n e n c l o s u r e 2 was f o l l o w e d i n J u l y w i t h a bloom of t h e c y a n o p h y t e M e r i s m o p e d i a e l e g a n s . No c h r y s o p h y t e bloom d e v e l o p e d i n t h i s e n c l o s u r e . The l i t t o r a l c o n t r o l s e x h i b i t e d a dominance of d i a t o m s r a t h e r t h a n g r e e n s i n t h e June 16th sample. T h e s e may have been remnants o f a s p r i n g bloom of d i a t o m s ( f o r w h i c h I have no d i r e c t e v i d e n c e ) but were most l i k e l y s t i r r e d up from l i t t o r a l s e d i m e n t s by SCUBA d i v e r s who, a day b e f o r e t h e samples were t a k e n , had been w o r k i n g i n s i d e t h e e n c l o s u r e s i n s p e c t i n g f o r a p r o p e r b o t t o m s e a l between t h e l i t t o r a l and l i m n e t i c e n c l o s u r e s . 104 (3) E f f e c t s of F e r t i l i z a t i o n S e a s o n a l c h a n g e s i n t h e p h y l e t i c c o m p o s i t i o n of t h e a l g a l b i o m a s s i n t h e two l i t t o r a l f e r t i l i z e d e n c l o s u r e s were b a s i c a l l y s i m i l a r t o t h o s e i n t h e c o n t r o l s and t h e l a k e ( F i g u r e 22) e x c e p t f o r f o u r m a j o r d i f f e r e n c e s . 1) The mid-June dominance of C h l o r o p h y t e s was c o n s i d e r a b l y s t r o n g e r t h a n i n t h e c o n t r o l s , making up a b o u t 80 % of t h e t o t a l b i o m a s s v e r s u s 40 % i n t h e c o n t r o l s and t h e l a k e . T h i s was l a r g e l y due t o a m a s s i v e i n c r e a s e i n t h e u l t r a p l a n k t o n (<10 um i n t h e i r maximum l i n e a r d i m e n s i o n , MLD) and a s i g n i f i c a n t i n c r e a s e i n t h e p r e s e n c e o f s m a l l , g e l a t i n o u s , c o l o n i a l g r e e n a l g a l forms l i k e S. s c h r o e t e r i , G. p l a n k t o n i c a and P a l m e l l a mucosa ( K e u t z i n g ) . The f i l a m e n t o u s g r e e n U l o t h r i x c y l i n d r i c u m ( P r e s c o t t ) , a l t h o u g h n o t p r e s e n t i n l a r g e numbers, ( 1 0 3 f i l a m e n t s / l i t e r ) , c o n t r i b u t e d s i g n i f i c a n t l y t o t h e e a r l y J u n e t o t a l b i o m a s s i n e n c l o s u r e 7. 2) U n l i k e i n t h e c o n t r o l s and t h e l a k e , t h e c r y p t o p h y c e a n a l g a e underwent an o b s e r v a b l e i n c r e a s e i n numbers i n b o t h l i t t o r a l f e r t i l i z e d e n c l o s u r e s , b e g i n n i n g i n mid J u l y and e n d i n g i n l a t e A u g u s t . The i n c r e a s e was l a r g e l y due t o a bloom of t h e s p e c i e s C r y ptomonas e r o s a ( E h r e n . ) and C r y p t o m o n a s o v a t a ( E h r e n . ) . D e n s i t i e s r e a c h e d as h i g h a s 10 6 c e l l s / l i t e r ( v s . 1 0 * c e l l s / l i t e r i n t h e c o n t r o l s ) . 1 0 5 3) The l a t e summer dominance o f C y a n o p h y t e s w h i c h o c c u r r e d i n b o t h t h e c o n t r o l s and t h e l a k e d i d not m a n i f e s t i t s e l f i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s . I n s t e a d , t h e r e was a dominance of s m a l l g r e e n a l g a e r e s u l t i n g f r o m a bloom o f A n k i s t r o d e s m u s f a l c a t u s ( C o r d a ) . S e c o n d a r y d o m i n a n t s i n c l u d e d S. s c h r o e t e r i , G. p l a n k t o n i c a , and some s m a l l c o c c o i d g r e e n s ( p r o b a b l y f r a c t u r e d c o l o n i e s ) . 4) B l u e - g r e e n a l g a e were g e n e r a l l y much l o w e r i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s when compared t o c o n t r o l s and t h e l a k e , o v e r t h e e n t i r e summer. In t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s , a l l o f t h e g e n e r a o f a l g a e i n d i g e n o u s t o t h e l a k e and common t o t h e c o n t r o l e n c l o s u r e s and t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s , were p r e s e n t a t a l l t i m e s , a l b e i t sometimes i n v e r y low numbers ( e s p e c i a l l y l a t e i n t h e summer). No new s p e c i e s a p p e a r e d i n t h e a l g a l a s s e m b l a g e i n t h e s e e n c l o s u r e s , however, some o f t h e forms t h a t were r a r e i n t h e l a k e , s u c h a s , B o t r y o c o c c u s s u d e t i c u s (Lemmerman) and G l o e o c y s t i s m a j o r ( G e r n e c k ) a p p e a r e d t o b e n e f i t g r e a t l y f r o m t h e add e d n u t r i e n t s (See A p p e n d i x 2 ) . The l a t e summer s p e c i e s c o m p o s i t i o n i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s ( F i g u r e 22) was not o v e r w h e l m i n g l y d i f f e r e n t f r o m t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s , however, t h e mid summer s p e c i e s c o m p o s i t i o n i n t h e s e e n c l o s u r e s was q u i t e d i f f e r e n t f r o m t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s , t h e c o n t r o l s and t h e l a k e . 106 E x c e p t f o r a s h o r t p e r i o d i n m i d - J u l y i n l i m n e t i c f e r t i l i z e d e n c l o s u r e 4, s m a l l c h l o r o p h y t e s were t h e dominant a l g a e o v e r t h e e n t i r e summer i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . More t h a n e i g h t y p e r c e n t of t h e e a r l y summer bio m a s s c o n s i s t e d of t h e same d o m i n a n t s and s u b d o m i n a n t s t h a t o c c u r r e d i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s and t h e c o n t r o l s . As i n t h e c o n t r o l s , t h e e a r l y summer c h l o r o p h y t e dominance began t o wane by m i d - J u l y i n l i m n e t i c f e r t i l i z e d e n c l o s u r e 4. L a r g e r c h l o r o p h y t e s s u c h as B. s u d e t i c u s and G. m a j o r , w h i c h were p r e v i o u s l y v e r y low i n abundance, r e a c h e d g r e a t e r p r o p o r t i o n s i n t h e b i o m a s s i n t h i s e n c l o s u r e . S p h a e r o c y s t i s  s c h r o e t e r i a l s o i n c r e a s e d i n abundance i n b o t h l i m n e t i c f e r t i l i z e d e n c l o s u r e s a t t h i s t i m e ( t o 10 5 c e l l s / l i t e r ) . W i t h i n 2 weeks o f t h e i r a p p e a r a n c e ( m i d - J u l y , F i g u r e 2 2 ) , t h e l a r g e c o l o n i a l g e l a t i n o u s g r e e n s had v i r t u a l l y d i s a p p e a r e d from t h e a l g a l a s s e m b l a g e i n b o t h l i m n e t i c e n c l o s u r e s and t h e g r a z e a b l e f r a c t i o n r e b o u n d e d d r a m a t i c a l l y , y i e l d i n g a p o p u l a t i o n d o m i n a t e d a l m o s t e x c l u s i v e l y by t h e s m a l l g r e e n a l g a e An k i s t r o d e smus f a l c a t u s ( C o r d a ) and A._f. v a r . t u m i d u s (West and W e s t ) . The s m a l l e r forms o f g e l a t i n o u s g r e e n s were a l s o g r e a t l y r e d u c e d i n number ( 1 0 3 c e l l s / l i t e r ) . S m a l l c h l o r o p h y t e s were a l w a y s t h e dom i n a n t a l g a e i n e n c l o s u r e 8. T h i s e n c l o s u r e d e v e l o p e d a bloom o f A n k i s t r o d e s m u s a p p r o x i m a t e l y 2 weeks e a r l i e r t h a n t h e one t h a t o c c u r r e d i n e n c l o s u r e 4. A n k i s t r o d e s m u s s p p . were p r e s e n t i n t h e c o n t r o l s and t h e l a k e i n f a i r l y low numbers ( 1 0 3 c e l l s / l i t e r ) b e g i n n i n g i n e a r l y J u l y , but t h e y n e v e r became a s i g n i f i c a n t component o f 1 07 t h e a l g a l b i o m a s s . D e n s i t i e s i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s , however, r e a c h e d 10 B c e l l s / l i t e r d u r i n g peak abundance ( F i g u r e 2 3 ) . B l u e - g r e e n a l g a e were v i r t u a l l y n o n - e x i s t e n t i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s i n t h e l a t t e r p a r t o f t h e summer. They were p r e s e n t , however, i n s m a l l amounts d u r i n g e a r l y summer. _3._3._3 S i z e c o m p o s i t i o n (1) E f f e c t s o f E n c l o s u r e The summer a l g a l a s s e m b l a g e s i n P l a c i d Lake and t h e c o n t r o l e n c l o s u r e s i n 1982 were n u m e r i c a l l y d o m i n a t e d by t h e u l t r a p l a n k t o n ( F i g u r e 2 3 ) , a l g a e <10 um i n t h e i r maximum l i n e a r d i m e n s i o n ( W e t z e l , 1975). A p p e n d i x 3 c o n t a i n s f i g u r e s w h i c h summarize t h e s e a s o n a l d e n s i t y p r o f i l e s of a l l of t h e v a r i o u s s i z e c l a s s e s of a l g a e d i s c u s s e d b e l o w . D e n s i t i e s o f a l g a e <2 um r a n g e d from mid-summer ( J u l y A u g u s t ) lows of a r o u n d 5.0 X 10 6 c e l l s / l i t e r t o e a r l y ( J une) and l a t e summer (September - O c t o b e r ) h i g h s of 50-80 X 10 6 c e l l s / l i t e r ( n o t shown). A l g a e i n t h e 2-6 um ra n g e were t h e s m a l l chlamydomonads and c h l o r o c o c c o i d s . They e x h i b i t e d t h e same s e a s o n a l abundance p a t t e r n s o f mid summer lows and e a r l y and l a t e summer h i g h s r a n g i n g i n d e n s i t y from 1-3 X 10 6 t o 5-10 X 10 6 c e l l s / l i t e r , r e s p e c t i v e l y ( A p p e n d i x 3 ) . Abundances of a l g a e i n t h e 6 - 10 um s i z e r a n g e were more e v e n l y d i s t r i b u t e d o v e r t h e summer w i t h d e n s i t i e s r a n g i n g between 2 X 10 6 and 10 X 1 0 8 Figure 23. Cummulative densities of algae in selected size c l a s s . Note the change in scale of the Y-axis on the plots for the limnetic f e r t i l i z e d enclosures Lake J U N J L Y AUfl SEP OCT NOV 0»te 110 Lake E n c l o s u r e 5 ( L i t t o r a l c o n t r o l ) J U N J L Y AUS S E P OCT NOV o (VI o . E n c l o s u r e 7 ( L i t t o r a l F e r t i l i z e d ) / * i .'» . i v i i JUN J L Y AU6 SEP OCT NOV E n c l o s u r e 6 ( L i t t o r a l c o n t r o l ) • I I — i — i — — i JUN J L Y AUB SEP OCT NOV E n c l o s u r e 8 (Limnetic F e r t i l i z e d ) e o o . o . o (VI o J — I 1 I 1 1 JUN J L Y AUG SEP OCT NOV 111 10 6 c e l l s / l i t e r . D e n s i t i e s of t h e n a n n o f l a g e l l a t e s and o t h e r s m a l l a l g a e i n t h e s i z e r a n g e most h e a v i l y g r a z e d by P l a c i d L a k e z o o p l a n k t o n ( c a . 10-20 um MLD; K r a u s e , 1984), were q u i t e low compared t o t h e u l t r a p l a n k t o n , b e i n g on t h e o r d e r of 5-30 X 10" c e l l s / l i t e r . D e n s i t i e s of t h e s e c e l l s d i d n o t show p r o n o u n c e d s e a s o n a l v a r i a t i o n , a l t h o u g h p e r i o d i c maxima were r e c o r d e d d u r i n g mid summer when p o p u l a t i o n s o f t h e l a r g e r c l a d o c e r a n s r e a c h e d t h e i r l o w e s t l e v e l s . Of t h e 10-20 um a l g a e , d e n s i t i e s of t h o s e i n t h e 10-14 um s i z e c l a s s were t h e l o w e s t , on t h e o r d e r o f 4-10 X 10 3 c e l l s / l i t e r . The b u l k o f t h e 10-20 um s i z e r a n g e was made up of c e l l s i n t h e 14-20 um s i z e c l a s s . Maximum d e n s i t i e s o f t h e l a r g e r a l g a e i n t h e s y s t e m (> 20 um MLD), w h i c h i n c l u d e d t h e " n e t " p l a n k t o n (50-100 um MLD), o c c u r r e d i n mid summer and were on t h e o r d e r o f 1.0 X 10 5 c e l l s / l i t e r . Of t h e s e l a r g e r a l g a e , c o l o n i a l c h r y s o p h y t e s s u c h as C h r y s o p h a e l l a l o n g i s p i n a , S y n u r a u v e l l a , S. s p h a g n i c o l a and D i n o b r y o n s p p . c o n s t i t u t e d t h e " n e t " p l a n k t o n and d o m i n a t e d t h e bi o m a s s i n J u l y and A u g u s t . L a r g e (20-50 um MLD) c o l o n i a l c y a n o p h y t e s , s u c h a s M e r i s m o p e d i a s p p . and C h r o o c o c c u s  p r e s c o t t i i d o m i n a t e d t h e l a t e summer b i o m a s s w i t h d e n s i t i e s r e a c h i n g 5-8 x 10" c e l l s / l i t e r . Though t h e n u m e r i c a l dominance o f t h e u l t r a p l a n k t o n p e r s i s t e d t h r o u g h o u t t h e summer i n t h e c o n t r o l s and l a k e ( F i g u r e 2 3 ) , t h i s s i z e f r a c t i o n was d o m i n a n t (> 30 %, H u t c h i n s o n , 1967) i n t h e t o t a l b i omass of t h e a l g a e o n l y i n t h e e a r l y p a r t o f t h e < summer ( F i g u r e 2 4 ) , becoming i n s i g n i f i c a n t by l a t e J u l y . By 1 1 2 F i g u r e 24. P e r c e n t c o m p o s i t i o n o f a l g a l b i o m a s s by d e t a i l e d s i z e c l a s s e s LEGEND j j B - 10 U l j I 10 - SO ua r w w i 8 0 - 3 0 ua > 3 0 ua illinium Lake ; fr-JUN J L Y AUB SEP OCT NOV Enclosure 1 ( l i t t o r a l control) J U N J L Y AUS 8 E P OCT NOV Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) J U N J L Y AUB SEP OCT NOV Enclosure 2 Enclosure 4 (limnetic control) (limnetic f e r t i l i z e d ) J U N J L Y AUB 8 E P OCT NOV J U N J L Y AUB SEP OCT NOV , Enclosure 5 Enclosure 7 ( l i t t o r a l control) ( l i t t o r a l f e r t i l i z e d ) J U N J L Y AUB 8 E P OCT NOV JUN J L Y AUB 8EP OCT NOV Enclosure 6 (limnetic control) J U N J L Y AUB S E P OCT NOV Enclosure 8 (limnetic f e r t i l i z e d ) J U N J L Y AUB S E P OCT NOV 1 1 4 t h e n , a t l e a s t 80 % o f t h e t o t a l p h y t o p l a n k t o n b i o m a s s was made up of c e l l s > 20 um i n t h e i r maximum l i n e a r d i m e n s i o n , and a t l e a s t 60 % o f t h a t was " n e t " p l a n k t o n (50-100 um). L e s s t h a n 20 % o f t h e t o t a l a l g a l b i o m a s s was made up of t h o s e c e l l s t h a t a r e s e l e c t i v e l y f e d upon (6-20 um MLD) by t h e domi n a n t h e r b i v o r o u s z o o p l a n k t o n i n t h i s s y s t e m . The c o n t r o l s had low e r t o t a l z o o p l a n k t o n b i o m a s s l e v e l s t h a n t h e l a k e i n l a t e summer, and fewer l a r g e c l a d o c e r a n s . As a r e s u l t 20 t o 30 % o f t h e l a t e summer b i o m a s s was made up of c e l l s i n t h e 6-20 um s i z e r a n g e , compared t o o n l y 10-20 % i n t h e l a k e . The s i z e c o m p o s i t i o n of t h e a l g a l a s s e m b l a g e i n l i m n e t i c e n c l o s u r e 2 d i f f e r e d from t h a t i n a l l of t h e o t h e r c o n t r o l s and th e l a k e . The mid summer p h y t o p l a n k t o n o f t h i s e n c l o s u r e was d o m i n a t e d by a l g a e i n t h e 10-20 um s i z e r a n g e , however, 50 - 80 % of t h e b i o m a s s d u r i n g t h a t t i m e was made up o f t h e i n e d i b l e c y a n o p h y t e M e r i s m o p e d i a ( F i g u r e 22) ( A r n o l d , 1971; F e r r a n t e , 1975; K r a u s e , 1984). (2) E f f e c t s o f F e r t i l i z a t i o n The u l t r a p l a n k t o n d i d n o t n u m e r i c a l l y d o m i n a t e t h e a l g a l a s s e m b l a g e s o f t h e f e r t i l i z e d e n c l o s u r e s f o r t h e e n t i r e summer as t h e y d i d i n t h e c o n t r o l s . F e r t i l i z a t i o n had g r e a t l y e n h a n c e d t h e a l g a e i n t h e 10 - 20 um s i z e f r a c t i o n by t h e m i d d l e o f A u g u s t i n most of t h e f e r t i l i z e d e n c l o s u r e s ( F i g u r e 2 3 ) . The i n i t i a l r e s p o n s e o f t h e p h y t o p l a n k t o n t o n u t r i e n t a d d i t i o n i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s was a r a p i d i n c r e a s e i n t h e u l t r a p l a n k t o n , t o c o n c e n t r a t i o n s 3-4 t i m e s 1 15 h i g h e r t h a n i n c o n t r o l s and t h e l a k e , r e a c h i n g d e n s i t i e s of a b o u t 2 X 10 7 c e l l s / l i t e r . T h i s i n c r e a s e was s h o r t i n d u r a t i o n , w i t h c o n c e n t r a t i o n s r e t u r n i n g t o l e v e l s s i m i l a r t o t h o s e i n c o n t r o l s and t h e l a k e w i t h i n one week, and r e m a i n i n g a t t h o s e l e v e l s f o r t h e d u r a t i o n of t h e s t u d y . S i g n i f i c a n t i n c r e a s e s i n n u m e r i c a l a b u n d a n c e s of p a r t of t h e g r a z a b l e f r a c t i o n of t h e a l g a e were f i r s t e v i d e n t i n l i t t o r a l f e r t i l i z e d e n c l o s u r e 3 i n l a t e J u l y . T h e r e was a s h o r t - t e r m bloom o f t h e c r y p t o m o n a d Cryptomonas o v a t a , w h i c h was a p p a r e n t l y r a p i d l y s u p p r e s s e d by g r a z e r s . G r e a t e r i n c r e a s e s of l o n g e r d u r a t i o n o c c u r r e d i n l a t e A u g u s t i n b o t h l i t t o r a l f e r t i l i z e d e n c l o s u r e s . T h e s e l a t t e r i n c r e a s e s were t h e r e s u l t of blooms o f t h e g r e e n a l g a e A n k i s t r o d e s m u s f a l c a t u s , an a l g a l f o r m w h i c h i n d u c e s e x c e l l e n t g r o w t h and s u r v i v a l r a t e s i n D X t ( A r n o l d , 1971). C o n c e n t r a t i o n s of t h e s e c e l l s r e a c h e d a maximum o f 2 X 10 7 c e l l s / l i t e r , "2-3 o r d e r s o f m a g n i t u d e h i g h e r t h a n t h e c o n c e n t r a t i o n s of a l l o f t h e a l g a e i n t h i s s i z e r a n g e (10-20 um MLD) i n t h e c o n t r o l s and t h e l a k e a t t h a t t i m e ( A p p e n d i x 3) . D e n s i t i e s of t h e l a r g e r , l e s s g r a z a b l e , a l g a e (> 20 um MLD) i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s were l a r g e l y unchanged f r o m d e n s i t i e s i n t h e c o n t r o l s and t h e l a k e . Maximum c o n c e n t r a t i o n s of 1-2 X 10 5 c e l l s / l i t e r o c c u r r e d i n e a r l y A u g u s t ( A p p e n d i x 3 ) . E a r l y and l a t e summer d e n s i t i e s of u l t r a p l a n k t o n i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s c l o s e l y a p p r o x i m a t e d t h o s e f o u n d i n t h e c o n t r o l s and t h e l a k e , on t h e o r d e r o f 5-10 X 10 6 1 1 6 c e l l s / l i t e r . U n l i k e i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s , t h e r e were no immediate l a r g e i n c r e a s e s i n t h e abundance of t h e s e c e l l s , however, blooms of u l t r a p l a n k t o n d i d o c c u r i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s i n A u g u s t ( F i g u r e 2 3 ) . The bloom i n l i m n e t i c f e r t i l i z e d e n c l o s u r e 8 (2 x 10 7 c e l l s / l i t e r , compared t o 1-3 X 10 6 c e l l s / l i t e r i n c o n t r o l s ) s u b s i d e d w i t h i n one week, but t h e bloom i n l i m n e t i c f e r t i l i z e d e n c l o s u r e 4 l a s t e d a b o u t 5 weeks and r e a c h e d maximum c o n c e n t r a t i o n s of 8.5 X 10 7 c e l l s / l i t e r . F e r t i l i z a t i o n s t r o n g l y f a v o u r e d t h e mid t o l a t e summer p r o d u c t i o n of a l g a e i n t h e 10 - 20 um s i z e r a n g e s ( F i g u r e 2 3 ) . D e n s i t i e s of t h e g r a z a b l e a l g a e r e a c h e d as h i g h a s 1 X 10 B c e l l s / l i t e r and c o n s i s t e d a l m o s t e x c l u s i v e l y o f t h e h i g h l y n u t r i t i o u s ( A r n o l d , 1971) a l g a A n k i s t r o d e s m u s f a l c a t u s . F e r t i l i z a t i o n d i d n o t r a d i c a l l y a l t e r t h e e a r l y summer s i z e c o m p o s i t i o n of t h e t o t a l a l g a l b i o m a s s i n e i t h e r t h e l i t t o r a l o r l i m n e t i c f e r t i l i z e d e n c l o s u r e s ( F i g u r e 24) when compared w i t h c o n t r o l s . Mid-summer a l g a l s i z e c o m p o s i t i o n s i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s , a s i n t h e c o n t r o l s , were d o m i n a t e d by a l g a e > 20 um and " n e t " p l a n k t o n ( F i g u r e 2 4 ) . The p r e d o m i n a n t a l g a l f o r m s i n t h i s s i z e c a t e g o r y were t h e same as i n t h e c o n t r o l s , namely, l a r g e c h r y s o p h y t e s . O c c u r r e n c e s of " n e t " p l a n k t o n i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s ( F i g u r e s 24) were, by c o n t r a s t , l e s s f r e q u e n t and c o n s i s t e d p r i m a r i l y o f l a r g e , c o l o n i a l , g e l a t i n o u s g r e e n s . T h e s e f o r m s t e n d t o become a b u n d a n t i n s y s t e m s t h a t a r e h e a v i l y g r a z e d by z o o p l a n k t o n ( P o r t e r , 1973). The l a r g e c h r y s o p h y t e s 1 1 7 t h a t made up t h e " n e t " p l a n k t o n i n a l l of t h e o t h e r e n c l o s u r e s and t h e l a k e , c o m p r i s e d a r e l a t i v e l y i n s i g n i f i c a n t p r o p o r t i o n of t h e t o t a l b i o m a s s o f t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . The p r o p o r t i o n of a l g a l b i o m a s s i n t h e 20 - 30 um s i z e r a n g e i n t h e f e r t i l i z e d e n c l o s u r e s d u r i n g mid-summer was g r e a t e r t h a n i n t h e c o n t r o l s . In t h e c o n t r o l s t h i s s i z e f r a c t i o n was d o m i n a t e d by t h e i n e d i b l e b l u e - g r e e n a l g a e M e r i s m o p e d i a and C h r o o c o c c u s . In t h e f e r t i l i z e d e n c l o s u r e s , i t was d o m i n a t e d a l m o s t e x c l u s i v e l y by t h e e d i b l e a l g a e Cryptomonas e r o s a and C. O v a t a (McQueen, 1969). T h i s s u g g e s t s t h a t a g r e a t e r p r o p o r t i o n of t h e mid summer a l g a l b i o m a s s i n t h e f e r t i l i z e d e n c l o s u r e s t h o u g h q u i t e l a r g e , was p a l a t a b l e and o f a manageable s i z e f o r some P l a c i d L a k e z o o p l a n k t o n . As i n t h e c o n t r o l s , t h e b i o m a s s of a l g a e i n t h e 6-20 um s i z e r a n g e i n t h e f e r t i l i z e d e n c l o s u r e s d r o p p e d below 20 % o f t h e t o t a l b i o m a s s d u r i n g mid-summer. However, r a t h e r t h a n r e m a i n i n g low f o r t h e b a l a n c e o f t h e summer as o c c u r r e d i n t h e c o n t r o l s , by l a t e A u g u s t , 60 - 80 % of t h e t o t a l a l g a l b i o m a s s i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s and a l m o s t 100 % of t h e b i o m a s s i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s was made up o f a l g a e <20 um i n s i z e , t h e b u l k of w h i c h was i n t h e 10 - 20 um s i z e f r a c t i o n . T h i s s i z e c o m p o s i t i o n was m a i n t a i n e d t h r o u g h t o f a l l o v e r t u r n i n O c t o b e r . 1 1 8 3.3.4 Summary of P h y t o p l a n k t o n R e s u l t s P h y t o p l a n k t o n s t a n d i n g c r o p i n t h e c o n t r o l e n c l o s u r e s was no t s i g n i f i c a n t l y d i f f e r e n t from t h a t i n t h e l a k e . P h y t o p l a n k t o n s t a n d i n g c r o p i n c r e a s e d 3 - f o l d and 7 - f o l d o v e r c o n t r o l v a l u e s i n t h e l i t t o r a l and l i m n e t i c f e r t i l i z e d e n c l o s u r e s , r e s p e c t i v e l y , f o l l o w i n g n u t r i e n t a d d i t i o n . Changes i n t h e p e r c e n t a g e of l i v e c h l o r o p h y l l a w i t h d e p t h show t h a t , i n t h e c o n t r o l e n c l o s u r e s and t h e l a k e , t h e h i g h e s t c o n c e n t r a t i o n of a l g a l b i o m a s s ( a t 4 - 6 m) c o n t a i n e d t h e l o w e s t p e r c e n t a g e s o f l i v e c h l o r o p h y l l a. The h i g h e s t p e r c e n t a g e s of l i v e c h l o r o p h y l l a i n t h e f e r t i l i z e d e n c l o s u r e s o c c u r r e d i n t h e 0 - 2 m d e p t h s t r a t u m where t h e a l g a l b i o m a s s was g r e a t e s t . S e a s o n a l c h a n g e s i n t h e p h y l e t i c c o m p o s i t i o n o f t h e a l g a e were s i m i l a r i n t h e c o n t r o l e n c l o s u r e s and t h e l a k e . The s p e c i e s c o m p o s i t i o n of a l g a e i n e n c l o s u r e 2 was n o t i c e a b l y d i f f e r e n t , but b e c a u s e a l l of t h e measured p h y s i c a l and c h e m i c a l r e g i m e s i n e n c l o s u r e 2 were s i m i l a r t o a l l o t h e r c o n t r o l s and t h e l a k e , i t i s d i f f i c u l t t o i s o l a t e t h e c a u s e o f t h i s c h a n g e . F r e q u e n t d o s e s of h i g h N:P r a t i o f e r t i l i z e r r e s u l t e d i n a l a r g e r p r o p o r t i o n o f t h e e a r l y summer a l g a l community b e i n g composed o f C h l o r o p h y c e a e . The l i t t o r a l f e r t i l i z e d c o m m u n i t i e s were s i m i l a r t o t h e c o n t r o l s i n t h a t t h e y a l s o e x h i b i t e d a mid-summer c h r y s o p h y c e a n bloom. The l a t e summer c y a n o p h y t e community, d i s t i n c t i v e of t h e l a k e and a l l c o n t r o l s , was r e p l a c e d by a c h l o r o p h y c e a n bloom composed a l m o s t e x c l u s i v e l y of t h e h i g h l y n u t r i t i o u s g r e e n a l g a A n k i s t r o d e s m u s . C h l o r o p h y t e s p e c i e s d o m i n a t e d t h e a l g a l community a l l summer l o n g i n t h e 1 1 9 l i m n e t i c f e r t i l i z e d e n c l o s u r e s . C h r y s o p h y c e a e were e s s e n t i a l l y n o n - e x i s t e n t . A s h o r t , (2 week) p e r i o d o c c u r r e d d u r i n g w h i c h a d o m i n a n t component o f t h e a l g a l community c o n s i s t e d o f l a r g e , g e l a t i n o u s g r e e n a l g a e , w h i c h were n o r m a l l y q u i t e r a r e i n t h e l a k e and d i d not o c c u r i n t h e c o n t r o l s . T h i s e p i s o d e was f o l l o w e d by a m a s s i v e i n c r e a s e i n t h e abundance and b i o m a s s of A n k i s t r o d e s m u s . O n l y 20-30 % and 10-20 % of t h e m i d - and l a t e summer bi o m a s s i n t h e c o n t r o l s and t h e l a k e , r e s p e c t i v e l y , was made up o f a l g a l c e l l s i n t h e g r a z a b l e 6-20 um s i z e f r a c t i o n . C o n v e r s e l y , 60-80 % and v i r t u a l l y 100 % of t h e mid t o l a t e summer a l g a l b i o m a s s i n t h e l i t t o r a l and l i m n e t i c f e r t i l i z e d e n c l o s u r e s f e l l w i t h i n t h i s s i z e f r a c t i o n . C y a n o p h y c e a e were g r e a t l y r e d u c e d i n abundance i n a l l of t h e f e r t i l i z e d e n c l o s u r e s . They were v i r t u a l l y e l i m i n a t e d f r o m t h e p l a n k t o n i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . 3.4 Z o o p l a n k t o n S t a n d i n g C r o p , S p e c i e s C o m p o s i t i o n and Community S t r u c t u r e 3._4._1_ S t a n d i n g C r o p and Spec i e s C o m p o s i t i o n (1) In The Lake Under n a t u r a l c o n d i t i o n s , t h e b i o m a s s of z o o p l a n k t o n i n P l a c i d L a k e , and a l l t h e l a k e s o f t h e U.B.C. R e s e a r c h f o r e s t , was v e r y low ( c a . 50-100 u g / l i t e r d r y w e i g h t ) ( F i g u r e 2 5 ) . T a b l e 3 summarizes t h e s p e c i e s o f z o o p l a n k t o n w h i c h a r e commonly 1 20 F i g u r e 25. Z o o p l a n k t o n b i o m a s s . Note t h e l o g - s c a l e . 121 LAKE Cv Zoo plankton Biomass o o n 8 JUN JLY AUS SEP OCT NOV o 8 8 Enclosure 1 ( L i t t o r a l Control) JUN JLY ALIO SEP OCT NOV Enclosure 3 ( L i t t o r a l F e r t i l i z e d ) o n 8 JUN JLY 8 E P OCT NOV 8 Enclosure 2 (Limnetic Control) JUN JLY AUO SEP OCT NOV Enclosure 4 (Limnetic F e r t i l i z e d ) o o n 8 JUN JLY AUO SEP OCT NOV 8 Enclosure 5 ( L i t t o r a l Control) JUN JLY AUB SEP OCT NOV Enclosure 7 ( L i t t o r a l F e r t i l i z e d ) S n 8 JUN JLY AUB 8 E P OCT NOV a a e Enclosure 6 (Limnetic Control) Enclosure B (Limnetic F e r t i l i z e d ) o o K) o CO JUN JLY AUB SEP OCT NOV JUN JLY AUB SEP OCT NOV 1 22 f o u n d i n t h e l a k e . N e a r l y a l l o f t h e z o o p l a n k t o n s p e c i e s were c o n c e n t r a t e d i n t h e upper 4 m o f t h e l a k e , but t h e r e i s some e v i d e n c e of d i f f e r e n t s p e c i e s b e i n g d e p t h s e g r e g a t e d . B o t h D a p h n i a r o s e a and Diaphanosoma b r a c h y u r u m a t t a i n e d maximum d e n s i t i e s i n t h e upper 4m o f t h e water column d u r i n g June and J u l y , but d u r i n g A u g u s t and September, D a p h n i a were most abundant i n t h e 2-4 m d e p t h zone, w h i l e Diaphanosoma m a i n t a i n e d maximum d e n s i t i e s i n t h e 0-2 m s t r a t u m . H o l o p e d i u m g i b b e r u m was f o u n d t h r o u g h o u t t h e wa t e r column (0-6 m) d u r i n g l a t e June but moved i n t o d e e p e r water (4-6 m) a s t h e summer p r o g r e s s e d . Polyphemus and Bosmina, w h i c h a r e b o t h r a r e i n t h e l a k e , a r e n e a r s u r f a c e f o r m s . C e r i o d a p h n i a i n h a b i t e d o n l y deep water (4-6 m). Di a p t o m u s  o r e g o n e n s i s o c c u p i e d t h e upper f o u r m e t e r s f o r most o f t h e summer, but moved t o d e e p e r water (4-6 m) a r o u n d t h e m i d d l e of September. D i a c y c l o p s t h o m a s i , t h e most abundant c o p e p o d , a t t a i n e d h i g h e s t d e n s i t i e s i n t h e mid t o deep water zone (4-6 m). F i n a l l y , D i a p t o m u s k e n a i , w h i c h was p r e s e n t i n t h e water column o n l y i n s p r i n g and e a r l y summer, was f o u n d near s u r f a c e and n e a r b o t t o m , r e s p e c t i v e l y i n t h o s e s e a s o n s . (2) E f f e c t s o f E n c l o s u r e S e a s o n a l f l u c t u a t i o n s i n t o t a l z o o p l a n k t o n b i o m a s s i n t h e c o n t r o l s were s i m i l a r t o t h o s e o c c u r r i n g i n t h e l a k e t o t h e d e g r e e t h a t maximum b i o m a s s l e v e l s were r e a c h e d i n mid t o l a t e J u l y and d e c r e a s e d s t e a d i l y a s t h e summer p r o g r e s s e d ( F i g u r e 2 5 ) . However, t h r e e major d i f f e r e n c e s between t h e c o n t r o l 123 e n c l o s u r e s and t h e l a k e a r e i m m e d i a t e l y n o t i c e a b l e . F i r s t , b i o m a s s l e v e l s i n t h e e n c l o s u r e s were l o w e r a t t h e commencement of s a m p l i n g on June 16. However, t h e y q u i c k l y r e c o v e r e d t o meet o r e x c e e d l a k e v a l u e s w i t h i n one week. The most l i k e l y r e a s o n f o r l o w e r s t a r t i n g b i o m a s s l e v e l s i n t h e c o n t r o l e n c l o s u r e s i s t h a t t h e l a k e z o o p l a n k t o n a v o i d e d t h e a r e a i n and a r o u n d th e e n c l o s u r e s w h i l e t h e y were b e i n g i n s t a l l e d and i n s u f f i c i e n t t i m e was p r o v i d e d t o a l l o w f u l l r e c r u i t m e n t o f t h e a r e a b e f o r e t h e e n c l o s u r e s i d e s were d r o p p e d and f u l l e n c l o s u r e e f f e c t e d . The s e c o n d major d i f f e r e n c e between t h e e n c l o s u r e s and t h e l a k e i s t h a t b i o m a s s l e v e l s i n t h e c o n t r o l s d e c l i n e d much more r a p i d l y a f t e r peak l e v e l s were r e a c h e d t h a n t h e y d i d i n t h e l a k e . I a t t r i b u t e t h i s t o f i s h p r e d a t i o n b e i n g more i n t e n s e i n s i d e t h e e n c l o s u r e s t h a n i n t h e l a k e . T h i s w i l l be d i s c u s s e d l a t e r . F i n a l l y , t h e l a t e summer z o o p l a n k t o n b i o m a s s l e v e l s i n t h e c o n t r o l s were m a i n t a i n e d a t l e v e l s s u b s t a n t i a l l y l o w e r t h a n l a k e v a l u e s . ( T h i s may have been m e d i a t e d by t h e i n t e n s e l e v e l o f f i s h p r e d a t i o n p o s t u l a t e d a b o v e ) . O v e r a l l mean z o o p l a n k t o n b i o m a s s l e v e l s i n t h e c o n t r o l e n c l o s u r e s were a c c o r d i n g l y l o w e r t h a n i n t h e l a k e ( T a b l e 9 ) . The m a j o r e f f e c t s o f e n c l o s u r e on t h e s p e c i e s c o m p o s i t i o n of z o o p l a n k t o n , d e t e r m i n e d by c o m p a r i n g e n c l o s u r e z o o p l a n k t o n p o p u l a t i o n s ( F i g u r e s 26-28) w i t h l a k e p o p u l a t i o n s were; (1) a r a p i d d e c l i n e i n t h e numbers o f H o l o p e d i u m g i b b e r u m i n a l l e n c l o s u r e s , (2) a s i g n i f i c a n t r e d u c t i o n i n t h e d e n s i t i e s o f T a b l e 9 . R e s u l t s o f o n e w a y a n a l y s i s o f v a r i a n c e o n t o t a l b i o m a s s o f z o o p l a n k t o n ( ± S E u g / 1 1 t e r ) a t a l l d e p t h s a v e r a g e d o v e r t h e e n t i r e e x p e r i m e n t . N u m b e r s I n b r a c k e t s a r e . p r o b a b l 1 1 1 1 e s o f n o s i g n i f i c a n t t r e a t m e n t e f f e c t ( F i t » . i f o r l a k e v s c o n t r o l g r a n d m e a n c o m p a r i s o n s . F i , n f o r l a k e v s c o n t r o l m e a n s , F i , M I f o r c o n t r o l v s f e r t i l i z e d g r a n d m e a n c o m p a r i s o n s , F i , n i f o r c o n t r o l v s f e r t i l i z e d d e p t h m e a n s , FI.III f o r a l l o t h e r g r a n d m e a n c o m p a r i s o n s , a n d F i > 1 5 f o r a l l o t h e r d e p t h m e a n c o m p a r i s o n s . V a r i a b l e T r e a t m e n t a r i t h m e t i c m e a n s ± S E ( u g / l i t e r ) D e p t h ( m ) L a k e L i t t o r a l C o n t r o l s L I m n e t I c C o n t r o l s L I t t o r a l F e r t 1 1 i z e d L I m n e t 1 c F e r t i 1 I z e d 0 - 2 6 7 . 8 5 + 5 . 6 2 6 0 . 8 3 + 9 . 9 7 3 5 . . 0 5 + 4 . 8 8 3 0 5 . . 0 0 + 4 1 . 3 8 4 7 8 . . 3 8 + 1 3 0 . . 1 5 Z o o p l a n k t o n B 1 o m a s s 2 - 4 5 3 . 1 0 + 5 . 2 7 4 7 . 0 4 + 4 . 5 0 4 4 , . 7 6 + 3 . 0 0 1 2 6 , . 5 2 + 1 8 . 9 5 2 1 8 . . 3 9 + 2 5 . . 2 5 ( u g / 1 I t e r ) 4 - 6 2 4 . 0 2 + 4 . 7 6 1 8 . . 5 1 2 . 6 1 3 4 . . 3 7 + 4 . 3 4 3 4 . 5 7 + 5 . 8 7 8 9 . 5 5 + 4 2 . 5 6 A L L 4 9 . 3 8 + 4 . 0 0 4 2 . 4 2 + 4 . 1 2 3 8 . 17 2 . 4 1 1 5 9 . 4 8 + 19 . 5 8 2 6 5 . . 5 6 + 4 9 . . 3 6 T r e a t m e n t C o m p a r i s o n D e p t h L a k e v s C o n t r o l v s L 1 t t o r a l C o n t r o l L i t t o r a l F e r t 1 1 1 z e d L 1 t t o r a l C o n t r o l L i m n e t i c C o n t r o l C o n t r o l F e r t l l 1 z e d L I m n e t 1 c C o n t r o l L 1 m n e t 1 c F e r t 1 1 i z e d L 1 t t o r a l F e r t i 1 1 z e d L 1 m n e t 1 c F e r t 1 1 i z e d 0 - 2 L ( 0 , , 0 0 3 ) F ( 0 . 0 0 0 ) L 1 t t ( 0 . 0 0 6 ) n s ( 0 . 2 2 2 ) * * F ( 0 . 0 0 0 ) F ( 0 . . 0 0 0 ) l o g l O Z o o p l a n k t o n 2 - 4 n s ( 0 , . 1 9 5 ) F ( 0 . 0 0 0 ) n s ( 0 . . 6 9 0 ) L i m ( 0 , . 0 0 2 ) F ( 0 . 0 0 0 ) F <°. . 0 0 0 ) B 1 o m a s s 4 - 6 n s ( 0 , . 9 2 1 ) F ( 0 . 0 0 0 ) L 1 m ( 0 . . 0 0 0 ) L 1 m ( 0 . . 0 3 8 ) F ( 0 . . 0 1 3 ) n s ( 0 . . 1 0 8 ) A L L L ( 0 . 0 2 6 ) F ( 0 . 0 0 0 ) n s ( 0 . 5 1 3 ) L i m ( 0 . . 0 1 1 ) F ( 0 . 0 0 0 ) F ( 0 . 0 0 0 ) n s = n o t s i g n i f i c a n t a t a = 0 . 0 5 F , L 1 t t , L 1 m , C o r L = I n t e r a c t i o n s i g n i f i c a n t a t a - 0 . 0 5 . * * = v a r i a n c e h e t e r o g e n e o u s a t a = 0 . 0 5 1 25 F i g u r e 26. P e r c e n t c o m p o s i t i o n o f z o o p l a n k t o n b i o m a s s by s p e c i e s i n t h e 0-2 meter d e p t h s t r a t u m 126 lllllllllllll A l l o t h a r a B o a a l n a D l a p h a n o a o a a D a p h n i a H o l o p a d l u u C y c l o p * C a r l o d a p h n l a D. o r e g o n a n a l i Lake 0 - 2 meters o 10 Q O (U Enclosure 1 ( l i t t o r a l control) Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) a to a a cu JUN J L Y AUO SEP OCT NOV JUN J L Y AUO SEP OCT NOV a co o to a Enclosure 2 (limnetic control) Enclosure 4 (limnetic f e r t i l i z e d ) a to a v a cu JUN J L Y AUG SEP OCT NOV JUN J L Y AUG SEP OCT NOV o 10 a cu Enclosure 5 ( l i t t o r a l control) Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) a n a to o cu J U N J L Y AUG SEP OCT NOV JUN J L Y AUG SEP OCT NOV JUN Enclosure 6 (limnetic control) J L Y AUG SEP OCT NOV Enclosure B (limnetic f e r t i l i z e d ) o CO o o cu T P w JUN J L Y AUB SEP OCT NOV 1 27 F i g u r e 27. P e r c e n t c o m p o s i t i o n o f z o o p l a n k t o n b i o m a s s by s p e c i e s i n t h e 2-4 meter d e p t h s t r a t u m Illllllllllll V///////A  IWAI L" A l l o t h a r a B o a a l n a Q l a p h a n o a o a a D a p h n i a H o l o p a d l u a C y c l o p a C a r l o d a p h n l a 0 . o r a g o n a n a l i Lake 2 - 4 meters 128 JUN J L Y AUO SEP OCT Enclosure 1 ( l i t t o r a l control) 1 — o o CVJ JUN J L Y AUO SEP OCT NOV Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) o to o (0 o (U J U N J L Y AUO SEP OCT NOV Enclosure 2 (limnetic control) o 10 a M Enclosure 4 (limnetic f e r t i l i z e d ) o CO a to o a OJ J U N J L Y AUG SEP OCT NOV JUN J L Y AUG SEP OCT NOV Enclosure 5 ( l i t t o r a l control) JUN J L Y AUB SEP OCT NOV Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) 3 a JUN J L Y AUG SEP OCT NOV Enclosure 6 Enclosure 8 (limnetic control) (limnetic f e r t i l i z e d ) JUN J L Y AUG SEP OCT NOV JUN J L Y AUB SEP OCT NOV 129 F i g u r e 28. P e r c e n t c o m p o s i t i o n of z o o p l a n k t o n b i o m a s s by s p e c i e s i n t h e 4-6 meter d e p t h s t r a t u m 130 illinium. Y///////A L" A l l o t h a r a B o a a l n a Q l a p h a n o a o a a D a p h n i a H o l o p a d l u a C y c l o p a C a r l o d a p h n l a 0 . o r a g o n a n a l a Lake 4 - 6 meters JUN J L Y AUB SEP OCT Enclosure 1 ( l i t t o r a l control) mm .*:*%**U%u...»****»m JUN J L Y AUB SEP OCT NOV Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) 10 o a N JUN J L Y AUB SEP OCT NOV o 10 o o 01 J U N Enclosure 2 (l imnetic control) J L Y AUG S E P OCT NOV Enclosure 4 (l imnetic f e r t i l i z e d ) J U N J L Y AUS SEP OCT NOV Enclosure 5 ( l i t t o r a l control) Q CO o 10 o cu Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) o 10 o M JUN J L Y AUB SEP OCT NOV JUN J L Y AUB SEP OCT NOV Enclosure 6 (l imnetic control) o (VI Enclosure B (l imnetic f e r t i l i z e d ) JUN J L Y AUB SEP OCT NOV JUN J L Y AUB S E P OCT NOV 131 Diaphanosoma b r a c h y u r u m i n t h e l i m n e t i c e n c l o s u r e s , and (3) a s i g n i f i c a n t i n c r e a s e i n t h e d e n s i t i e s of Diaptomus o r e g o n e n s i s i n t h e l i m n e t i c e n c l o s u r e s compared t o t h e l i t t o r a l e n c l o s u r e s and t h e l a k e . (3) E f f e c t s o f F e r t i l i z a t i o n F e r t i l i z a t i o n g r e a t l y e n h a n c e d t h e b i o m a s s of z o o p l a n k t o n i n a l l of t h e f e r t i l i z e d e n c l o s u r e s ( F i g u r e 25, T a b l e 9 ) . Mean z o o p l a n k t o n b i o m a s s o v e r t h e e n t i r e summer was s i g n i f i c a n t l y h i g h e r i n t h e f e r t i l i z e d e n c l o s u r e s t h a n i n t h e c o n t r o l s ( T a b l e 9 ) . B i o m a s s l e v e l s i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s i n c r e a s e d a p p r o x i m a t e l y 3-4 f o l d o v e r l i t t o r a l c o n t r o l v a l u e s , whereas b i o m a s s l e v e l s i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s i n c r e a s e d a p p r o x i m a t e l y 7 - f o l d o v e r l i m n e t i c c o n t r o l v a l u e s . The m a j o r e f f e c t o f f e r t i l i z a t i o n on t h e s p e c i e s c o m p o s i t i o n of t h e z o o p l a n k t o n ( F i g u r e 26-28) was a d r a m a t i c i n c r e a s e i n t h e d e n s i t i e s and b i o m a s s of C l a d o c e r a , e s p e c i a l l y D a p h n i a r o s e a and Diaphanosoma b r a c h y u r u m . D e n s i t i e s of Cer i o d a p h n i a q u a d r a n g u l a and D i a c y c l o p s t h o m a s i i n c r e a s e d s i g n i f i c a n t l y i n t h e deep water zone (4-6 m) f o l l o w i n g n u t r i e n t e n r i c h m e n t , whereas i n t h e l a k e l a r g e c l a d o c e r a n s l i k e D a p h n i a and H o l o p e d i u m d o m i n a t e d t h e b i o m a s s i n t h i s d e p t h s t r a t u m . However, t h e i r a b s o l u t e d e n s i t i e s were l o w e r t h e r e t h a n i n t h e f e r t i l i z e d e n c l o s u r e s . 1 32 _3._4._2. Z o o p l a n k t o n Community S t r u c t u r e D e n s i t i e s o f t h e m a j o r h e r b i v o r o u s z o o p l a n k t o n i n t h e l a k e , t h e c o n t r o l s and t h e f e r t i l i z e d e n c l o s u r e s were h i g h e s t i n t h e 0-2 m d e p t h s t r a t u m f o r most o f t h e summer ( F i g u r e 2 9 ) . D e n s i t i e s of a l l o f t h e s p e c i e s c o n s i d e r e d i n t h i s s t u d y , were o f t e n c o n s i d e r a b l y h i g h e r i n t h i s s t r a t u m t h a n t h e i r a v e r a g e d e n s i t i e s f o r t h e e n t i r e w a ter column ( F i g u r e s 3 0 - 3 6 ) . A l s o , t h e d i s t r i b u t i o n s o f z o o p l a n k t e r s , by s p e c i e s , a p p e a r e d t o be r e p r e s e n t a t i v e o f t h e w a t e r column as a w h o l e . T h e r e f o r e , i n o r d e r t o be c o n s i s t e n t w i t h t h e a n a l y s i s a p p l i e d t o t h e p h y t o p l a n k t o n d a t a , t h e r e s u l t s of t h e s t a t i s t i c a l a n a l y s e s t h a t f o l l o w were d e r i v e d a l m o s t e x c l u s i v e l y from t h e d a t a from t h e 0-2 m s t r a t u m w i t h t h e a s s u m p t i o n t h a t t h e o r g a n i s m s i n t h i s d e p t h s t r a t u m were r e p r e s e n t a t i v e of a l l o r g a n i s m s o f t h a t t y p e i n t h e l a k e . ( 1 ) In The Lake When s a m p l i n g began i n J u n e , t h e s p e c i e s o f h e r b i v o r o u s z o o p l a n k t o n p r e s e n t i n t h e l a k e were, i n d e c r e a s i n g o r d e r o f abu n d a n c e : H o l o p e d i u m g i b b e r u m , Diaptomus o r e g o n e n s i s , D a p h n i a  r o s e a , Diaphanosoma b r a c h y u r u m , Cer i o d a p h n i a q u a d r a n g u l a , Bosmina l o n g i r o s t r i s ( F i g u r e s 30-36) and Di a p t o m u s k e n a i ( n o t shown). D. k e n a i a r e o n l y p r e s e n t i n s i g n i f i c a n t numbers i n t h e w a t e r column i n t h e s p r i n g . They a r e o n l y r a r e l y f o u n d i n t h e summer. The p r e d a t o r y c o p e p o d , D i a c y c l o p s t h o m a s i ( F i g u r e 3 6 ) , was a b u n d a n t o n l y i n J u l y and l a r g e l y c o n c e n t r a t e d i n t h e 4-6 1 3 3 F i g u r e 29. Cummulative d e n s i t i e s of t h e dominant h e r b i v o r o u s z o o p l a n k t o n s p e c i e s i n t h e 0-2 meter d e p t h s t r a t u m 134 LEGEND Lake B o a a l n a O a p h n l a r o a a a D. o r a g o n a n a l D l a p h a n o a o a a H o l o p a d l u a a a I <u c > a 4 i AJ ** • a a B KI • c a u T J bl a 3 a > U a • a a T J rt C 3 B B a a 3 SO 3 o CJ r AJ JUN J L Y AUO SEP D a t a OCT NOV Enclosure 2 (limnetic control) a M O KI I 1 1 1 1 r — JUN J L Y AUO SEP OCT NOV 8 8 8 Enclosure 4 (limnetic f e r t i l i z e d ) 4 948 i i . 1 1 • • J U N J L Y AUB SEP OCT NOV 135 LEGEND L a k e Boaalna Daphnia roaaa 0 . oragonanal Dlaphanoaoaa Holopadlua o a Z N >> ca *t +> -* ma a a in c a u i l • 3 D > u a »< V . T t 4 i • • T l •1 c l i s 4 3 O U C I i 1 1 r 1— JUN J L Y AUG SEP OCT NOV Data E n c l o s u r e 5 ( l i t t o r a l c o n t r o l ) o o M a n a to JUN — i — J L Y AUG — i — SEP OCT — i — NOV E n c l o s u r e 7 ( l i t t o r a l f e r t i l i z e d ) a a i M a in o a • a in JUN J L Y AUG — i — SEP — i — OCT NOV E n c l o s u r e 6 ( l i m n e t i c c o n t r o l ) a N O n o . • 1 1 r-JUN J L Y AUG SEP OCT NOV Enclosure B ( l i m n e t i c f e r t i l i z e d ) a a cu a a a I 1—; 1 1 « r-j-JUN J L Y AUG SEP OCT NOV 1 36 i F i g u r e 30. D e n s i t i e s o f H o l o p e d i u m g i b b e r u m 137 Holopedium gibberum LAKE A d u l t s J U V f l f l l l M J U N J L Y AUB 8 E P OCT NOV Enclosure 1 ( l i t t o r a l control) J L Y AUB S E P OCT NOV Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) J U N J L Y OCT NOV Enclosure 2 ( l imnetic control) J U N J L Y S E P OCT NOV Enclosure 4 ( l imnetic f e r t i l i z e d ) J U N J L Y S E P OCT NOV OJ a Enclosure 5 ( l i t t o r a l control) 4* t \ J U N J L Y AUB S E P OCT NOV Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) A J U N J L Y S E P OCT NOV Enclosure 6 ( l imnetic control) \ r Enclosure 8 (l imnetic f e r t i l i z e d ) J U N J L Y AUB S E P OCT NOV J U N J L Y AUB S E P OCT NOV 1 38 F i g u r e 31. D e n s i t i e s of D a p h n i a r o s e a . Note t h e l o g - s c a l e . 139 Daphnia rosea LAKE 3 JUN JLY AUB BEP OCT NOV o a n Enclosure 1 ( l i t t o r a l control) Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) o. n JUN JLY AUB BEP OCT NOV JUN JLY AUB BEP OCT NOV S Enclosure 2 ( l imnetic control) JUN JLY AUB SEP OCT NOV Enclosure 4 (l imnetic f e r t i l i z e d ) 8 JUN JLY AUB SEP OCT NOV Enclosure 5 ( l i t t o r a l control) o n - li JUN JLY AUB SEP OCT NOV Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) JUN JLY AUB 8 E P OCT NOV Enclosure B ( l imnetic control) JUN JLY AUB BEP OCT NOV Enclosure B (l imnetic f e r t i l i z e d ) a in JUN JLY AUB SEP OCT NOV 1 40 F i g u r e 32. D e n s i t i e s of Diaphanosoma b r a c h y u r u m . Note t h e l o g - s c a l e . 141 Diaphanosoma brachyurum LAKE o c « v. 1 » fl 2 „ » / \ / \ JUN JLY AUS SEP OCT NOV Enclosure 1 ( l i t t o r a l control) JUN JLY AUB SEP OCT NOV Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) o tfl JUN JLY AUB 8EP OCT NOV Enclosure 2 ( l imnetic control) • ^ JUN JLY AUB SEP OCT NOV Enclosure 5 ( l i t t o r a l control) JUN JLY AUB 8EP OCT NOV Enclosure 6 ( l imnetic control) o 10 JUN JLY AUB 8 BP OCT NOV Enclosure 4 (l imnetic f e r t i l i z e d ) o n \ J-JUN JLY AUB SEP OCT NOV Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) o n JUN JLY AUB SEP OCT NOV Enclosure 8 (l imnetic f e r t i l i z e d ) o n JUN JLY AUB BEP OCT NOV 1 4 2 F i g u r e 33. D e n s i t i e s of D i a p t o m u s o r e g o n e n s i s . Note t h a t t h e j u v e n i l e c a t e g o r y c o n s i s t s o n l y of c o p e p o d i t e s and d o e s n o t i n c l u d e n a u p l i i . D i a p t o m u s o r e g o r t e n s i s LAKE 143 o (. OJ 5 «i ^ O f. • S » * O J U N J L Y AU8 SEP OCT NOV E n c l o s u r e 1 ( l i t t o r a l c o n t r o l ) E n c l o s u r e 3 ( l i t t o r a l f e r t i l i z e d ) o cu KI o J U N J L Y AUS 8EP OCT NOV JUN J L Y AUB 8 E P OCT NOV E n c l o s u r e 2 ( l i m n e t i c c o n t r o l ) 5 n KJ O E n c l o s u r e 4 ( l i m n e t i c f e r t i l i z e d ) t 47 JUN J L Y AUB S E P OCT NOV JUN J L Y AUB S E P OCT NOV E n c l o s u r e 5 ( l i t t o r a l c o n t r o l ) J U N J L Y AUB 8EP OCT NOV E n c l o s u r e 7 ( l i t t o r a l f e r t i l i z e d ) o OJ KI a KI O JUN J L Y AUB 8EP OCT NOV E n c l o s u r e 6 ( l i m n e t i c c o n t r o l ) 8 KI o KI E n c l o s u r e 8 ( l i m n e t i c f e r t i l i z e d ) /AC/ J U N J L Y AUB 8 BP OCT NOV J U N J L Y AUB SEP OCT NOV 1 44 F i g u r e 34. D e n s i t i e s of C e r i o d a p h n i a q u a d r a n g u l a 145 C a r l o d a p h n l a q u a d r a n g u l a LAKE 1 -JUN J L Y AUB 8 E P OCT NOV M a E n c l o s u r e 1 ( l i t t o r a l c o n t r o l ) J U N J L Y AUB 8 E P OCT NOV E n c l o s u r e 3 ( l i t t o r a l f e r t i l i z e d ) 0 3 JUN J L Y AUB 8 E P OCT NOV E n c l o s u r e 2 ( l i m n e t i c c o n t r o l ) JUN J L Y AUB S E P OCT NOV E n c l o s u r e 4 ( l i m n e t i c f e r t i l i z e d ) JUN J L Y AUB 8 E P OCT NOV o E n c l o s u r e 5 ( l i t t o r a l c o n t r o l ) J U N J L Y AUB 8 E P OCT NOV E n c l o s u r e 7 ( l i t t o r a l f e r t i l i z e d ) 01 JUN J L Y AUB 8 E P OCT NOV M E n c l o s u r e 6 ( l i m n e t i c c o n t r o l ) E n c l o s u r e 8 ( l i m n e t i c f e r t i l i z e d ) co 01 o JUN J L Y AUB S E P OCT NOV JUN J L Y AUB SEP OCT NOV 1 4 6 F i g u r e 35. D e n s i t i e s of Bosmina l o n g i r o s t r i s . Note t h e l o g -s c a l e . 147 LAKE o a i • ~> O B o s m i n a l o n g l r o s t r i s JUN J L Y AUB BEP Enclosure 1 Enclosure 3 ( l i t t o r a l control) ( l i t t o r a l f e r t i l i z e d ) J U N J L Y AUB BEP OCT JUN J L Y AUB SEP OCT Enclosure 2 Enclosure 4 (limnetic control) (limnetic f e r t i l i z e d ) O J U N J L Y AUB S E P OCT J U N J L Y AUB S E P OCT Enclosure 5 Enclosure 7 ( l i t t o r a l control) ( l i t t o r a l f e r t i l i z e d ) 8 JUN J L Y AUB S E P OCT JUN J L Y AUB S E P OCT o o n Enclosure 6 (limnetic control) o J U N J L Y AUB SEP OCT S n Enclosure 8 (limnetic f e r t i l i z e d ) JUN J L Y AUB 8EP OCT 1 4 8 F i g u r e 36. D e n s i t i e s of D i a c y c l o p s t h o m a s i 149 D i a c y c l o p s thomasi LAKE • fl 3 JUN J L Y AUB SEP OCT NOV Enclosure 1 ( l i t t o r a l control) JUN J L Y AUB BEP OCT NOV Enclosure 2 (limnetic control) JUN J L Y AUB SEP OCT NOV Enclosure 5 ( l i t t o r a l control) JUN J L Y AUB BEP OCT NOV Enclosure 6 (limnetic control) Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) J U N J L Y AUB S E P OCT NOV Enclosure 4 (limnetic f e r t i l i z e d ) JUN J L Y AUB SEP OCT NOV Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) JUN J L Y AUB SEP OCT NOV Enclosure B (limnetic f e r t i l i z e d ) J U N J L Y AUB S E P OCT NOV J U N J L Y AUB SEP OCT NOV 150 meter d e p t h s t r a t u m . As a r e s u l t , t h e s e two s p e c i e s a r e not c o n s i d e r e d i n s t a t i s t i c a l a n a l y s e s c o m p a r i n g d i f f e r e n t t r e a t m e n t s . O t h e r c l a d o c e r a n z o o p l a n k t o n s p e c i e s were l e s s commonly f o u n d i n t h e l a k e and i n t h e e x p e r i m e n t a l e n c l o s u r e s . T h e i r numbers were so low t h a t t h e y c o n t r i b u t e d l i t t l e t o t h e o v e r a l l b i o m a s s . T h e s e i n c l u d e d S c a p h a l o b e r i s k i n g i i , Polyphemus  p e d i c u l u s and C h y d o r i s s p h a e r i c u s . The p r i m a r i l y l i t t o r a l c l a d o c e r a n A c a n t h o l e b e r i s c u r v i r o s t r i s was e n c o u n t e r e d , but o n l y v e r y r a r e l y i n a l l of t h e l i t t o r a l e n c l o s u r e s and t h e l a k e . A l a r g e s p e c i e s o f c y c l o p o i d c o p e p o d , p o s s i b l y M e s o c y c l o p s edax, ( P e a c o c k , 1981) o c c u r r e d i n two l i t t o r a l l a k e s a m p l e s t a k e n d u r i n g a p r e - e x p e r i m e n t s i t e s u r v e y i n 1981, but was n e v e r f o u n d i n 1982 s a m p l e s . A l l o f t h e h e r b i v o r o u s z o o p l a n k t e r s , w i t h t h e e x c e p t i o n s o f C e r i o d a p h n i a ( F i g u r e 34) and Bosmina ( F i g u r e 35) ( b o t h o f w h i c h a p p e a r e d i n mid-September i n moderate numbers but were o t h e r w i s e r a r e i n t h e l a k e ) d o m i n a t e d t h e z o o p l a n k t o n community, i n terms o f a bundance a n d / o r b i o m a s s , a t some t i m e d u r i n g t h e summer. The s e q u e n c e o f s h i f t s i n s p e c i e s dominance was a s f o l l o w s : When t h e i c e l e f t t h e l a k e i n l a t e A p r i l , t h e c o p e p o d s Diaptomus  k e n a i and Diaptomus o r e g o n e n s i s d o m i n a t e d t h e z o o p l a n k t o n community ( C . J . W a l t e r s , u n p u b l . d a t a ; K r a u s e , 1984), w h i l e c l a d o c e r a n s were r a r e . By e a r l y J u n e , D. k e n a i had a l m o s t d i s a p p e a r e d from t h e z o o p l a n k t o n b u t D. o r e g o n e n s i s ( F i g u r e 33) r e m a i n e d unchanged i n a b u n d a n c e . H o l o p e d i u m ( F i g u r e 30) t h e n became n u m e r i c a l l y d o m i n a n t and r e m a i n e d so u n t i l J u l y , a t w h i c h 151 t i m e t h e numbers o f b o t h D a p h n i a ( F i g u r e 31) and Diaphanosoma ( F i g u r e 32) began t o i n c r e a s e w h i l e t h e p o p u l a t i o n of H o l o p e d i u m d e c r e a s e d . The p o p u l a t i o n o f D. o r e g o n e n s i s ( F i g u r e 33) d i d not d e c l i n e a s D a p h n i a and Diaphanosoma became a b u n d a n t , b ut r e m a i n e d a p p r o x i m a t e l y t h e same. D a p h n i a r e a c h e d maximum d e n s i t i e s (8-10 i n d i v i d u a l s / l i t e r i n t h e 0-2 m s t r a t u m ) i n t h e l a s t week o f J u l y , t h e n d e c l i n e d r a p i d l y w h i l e Diaphanosoma ( F i g u r e 32) i n c r e a s e d t o t h e i r summer maximum (10 - 12 i n d i v i d u a l s / l i t e r i n t h e 0-2 m s t r a t u m ) , a c h i e v i n g t h i s l e v e l by e a r l y A u g u s t . Diaphanosoma c o n t i n u e d t o d o m i n a t e n u m e r i c a l l y u n t i l l a t e A u g u s t when t h e j u v e n i l e s t a g e s of D. o r e g o n e n s i s m a t u r e d and t h e a d u l t s t a g e of t h a t s p e c i e s ( F i g u r e 33) became t h e l a t e summer dominant ( 6 - 7 i n d i v i d u a l s / l i t e r i n t h e 0-2 m s t r a t u m ) . The mean body s i z e o f a d u l t H o l o p e d i u m i n t h e l a k e d r o p p e d from a May - e a r l y June h i g h o f 1.4 mm. (max = 1.63 mm.) t o a mid-June low of 1.2 mm. (min = 0.78 mm), and r e m a i n e d a t t h i s l e v e l f o r t h e d u r a t i o n o f t h e s t u d y , i n c r e a s i n g s l i g h t l y i n l a t e O c t o b e r . V e r y few e g g - b e a r i n g H o l o p e d i u m were c a p t u r e d i n t h e l a k e , and t h o s e t o o showed a s i m i l a r t r e n d t o w a r d s a d e c r e a s e i n body s i z e . The mean b r o o d s i z e o f e g g - b e a r i n g a d u l t s c h a n g e d v e r y l i t t l e o v e r t h e c o u r s e o f t h e summer, b e i n g a r o u n d 1.0 e g g s / f e m a l e . Body s i z e s of j u v e n i l e H o l o p e d i u m r e m a i n e d c o n s t a n t o v e r t h e e n t i r e c o u r s e o f t h i s s t u d y , a t a mean s i z e o f 0.6 mm. Mean body s i z e s o f g r a v i d and n o n - g r a v i d a d u l t D a p h n i a d e c r e a s e d f r o m an a v e r a g e s i z e of 1.3 mm. (max = 1.6mm.) a t t h e 1 52 b e g i n n i n g of s a m p l i n g i n June, t o a b o u t 1.0 mm. (min = 1.0 mm.) i n mid-September ( F i g u r e 3 7 ) . The mean b r o o d s i z e of g r a v i d D a p h n i a a l s o d e c r e a s e d from a s p r i n g - e a r l y summer h i g h of a b o u t 1.6 e g g s / f e m a l e t o a mid-summer low of 1.0 e g g s / f e m a l e ( F i g u r e 3 8 ) . J u v e n i l e D a p h n i a i n c r e a s e d i n s i z e t o a maximum of 1.0 mm. d u r i n g mid summer ( F i g u r e 3 7 ) . The body s i z e s of Diaphanosoma ( F i g u r e 39) and D. o r e g o n e n s i s ( F i g u r e 40) d i d n o t change s i g n i f i c a n t l y o v e r t h e s t u d y p e r i o d . Too few g r a v i d i n d i v i d u a l s of t h e t h e s e two s p e c i e s were c o l l e c t e d t o a d e q u a t e l y d e t e r m i n e c h a n g e s i n t h e mean b r o o d s i z e s o r body s i z e s of g r a v i d f e m a l e s . (2) E f f e c t s of E n c l o s u r e The s e q u e n c e o f n u m e r i c a l dominance of t h e major h e r b i v o r o u s z o o p l a n k t o n s p e c i e s i n t h e c o n t r o l e n c l o s u r e s was e s s e n t i a l l y t h e same as i n t h e l a k e ( F i g s . 3 0 - 3 6 ) , but t h e r e were some d i f f e r e n c e s i n t h e m a g n i t u d e and t i m i n g of t h e s e c h a n g e s . D e n s i t i e s of H o l o p e d i u m i n t h e c o n t r o l s i n June and J u l y were c o n s i d e r a b l y l o w e r t h a n i n t h e l a k e ( F i g . 3 0 ) . H o l o p e d i u m v i r t u a l l y d i s a p p e a r e d f r o m a l l of t h e c o n t r o l e n c l o s u r e s by e a r l y A u g u s t . S t a t i s t i c a l a n a l y s i s showed t h a t t h e s e a s o n a l mean d e n s i t i e s i n t h e c o n t r o l s were s i g n i f i c a n t l y l o w e r t h a n i n t h e l a k e ( T a b l e s 10 and 11). However, mean b r o o d s i z e s were not s i g n i f i c a n t l y d i f f e r e n t ( T a b l e 1 2 ) . T h e r e was no s i g n i f i c a n t d i f f e r e n c e i n mean d e n s i t i e s o r mean b r o o d s i z e s between l i t t o r a l and l i m n e t i c c o n t r o l s . 1 53 F i g u r e 37. Mean body s i z e s o f a d u l t , g r a v i d f e m a l e and j u v e n i l e D a p h n i a r o s e a 154 JUN Daphnia rosea Mean Body Sizes (mm.) adults faaalaa/agga Juvanllaa Enclosure 1 ( l i t t o r a l control) saaaonal Maan - 1.18 Baaaonal Haan • 0.88 JLY A I M S E P OCT NOV Enclosure 2 ( l imnetic control) Baaaonal Mean - 1.14 Saaaonal Maan - 0.82 JLY SEP OCT NOV Enclosure 5 ( l i t t o r a l control) Saaaonal Moan » 1.22 JUN JLY OCT NOV Enclosure 6 ( l imnetic control) Baaaonal Haan - 1.22 ->o. _ ^ -» _ Saaaonal Maan - 0.83 JLY SEP OCT NOV LAKE Baaaonal Ma an - 1.18 r Saaaonal Ma an - 0.83. JUN JLY AUB SEP OCT NOV Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) Saaaonal Maan - 1.27 Baaaonal Haan - 0.88 JUN JLY SEP OCT NOV Enclosure 4 (l imnetic f e r t i l i z e d ) Saaaonal Ma an - l.SO Baaaonal Haan - 0.88 JUN JLY AUB SEP OCT NOV Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) Saaaonal Maan - 1.20 Saaaonal Haan - 0.84 JUN JLY AUB SEP OCT NOV Enclosure 8 (l imnetic f e r t i l i z e d ) aj Haan - 1.37 Baaaonal Haan • O.BS JLY AUB OCT NOV 1 5 5 F i g u r e 38. Mean b r o o d s i z e s of D a p h n i a r o s e a LAKE M e a n b r o o d s i z e s o f D a p h n i a R o s e a CO D Ol o J L Y AUS SEP OCT NOV Enclosure 1 ( l i t t o r a l control) J L Y AUB S E P OCT NOV Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) n oi J L Y AUB S E P OCT NOV Enclosure 2 (limnetic control) Enclosure 4 (limnetic f e r t i l i z e d ) CO CO tn M J L Y AUS S E P OCT NOV J L Y AUB SEP OCT NOV Enclosure 5 Enclosure 7 ( l i t t o r a l control) ( l i t t o r a l f e r t i l i z e d ) J L Y AUS S E P OCT NOV J L Y AUB SEP OCT NOV Enclosure 6 Enclosure B (limnetic control) (limnetic f e r t i l i z e d ) J L Y AUB S E P OCT NOV J L Y AUB S E P OCT NOV 1 57 F i g u r e 39. Mean body s i z e s of a d u l t Diaphanosoma brachyurum LAKE 158 Diaphanosoma Adults Mean Body Sizes (mm.) 2 * 8 a a a o n a l Mann 0 . 7 0 JUN J L Y AUB SEP OCT NOV a J U N Enclosure 1 ( l i t t o r a l control) S a a a o n a l Maan » 0 . 7 0 J L Y AUS SEP OCT NOV Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) a d S a a a o n a l Naan - 0 . 7 3 JUN J L Y AUB SEP OCT NOV o J U N Enclosure 2 (limnetic control) •n - o . B S J L Y AUB 8EP OCT NOV Enclosure A (limnetic f e r t i l i z e d ) a o o S a a a o n a l Naan - 0 . 7 B J L Y AUB SEP OCT NOV oi o O J U N Enclosure 5 ( l i t t o r a l control) S a a a o n a l Maan - 0 . 7 0 J L Y AUB S E P OCT NOV Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) at o a 0 . 7 4 J U N J L Y AUB SEP OCT NOV o Enclosure 6 (limnetic control) S a a a o n a l Maan - 0 . 7 1 J U N J L Y AUB BEP OCT NOV Enclosure B (limnetic f e r t i l i z e d ) o o S a a a a n a l Maan • 0 . 7 4 JUN J L Y AUS SEP OCT NOV 1 5 9 F i g u r e 40. Mean body s i z e s of a d u l t , g r a v i d f e m a l e and j u v e n i l e D i aptomus o r e g o n e n s i s 160 Diaptomus oregonensis Mean Body Sizes (mm.) Enclosure 1 ( l i t t o r a l control) B a a a o n a l Haan - 0 . 9 4 \ _ ,_. B a a a o n a l Haan - 0 . 8 4 JUN J L Y AUB S E P OCT NOV Enclosure 2 (l imnetic control) B a a a o n a l Haan - 0 . 8 4 JUN J L Y AUB S E P OCT NOV Enclosure 5 ( l i t t o r a l control) B a a a o n a l Haan - 0 . 8 4 / N x S a a a o n a l Haan - 0 . 8 7 JUN J L Y AUB S E P OCT NOV Enclosure 6 ( l imnetic control) B a a a o n a l Haan - 0 . 8 4 B a a a o g a l H a a q - 0 . 8 7 JUN J L Y AUB S E P OCT NOV CO o c : n L A K E S a a a o n a l Haan V S a a a o n a l Haan - 0 . 6 6 JUN J L Y AUB SEP OCT NOV o n Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) B a a a o n a l Haan « 1 . 0 3 \ 7 B a a a o n a l Haan - 0 , 6 6 JUN J L Y AUB SEP OCT NOV Enclosure 4 (l imnetic f e r t i l i z e d ) B a a a o n a l Haan - 0 . 8 S B a a a o n a l Haan - 0 . 8 7 JUN J L Y AUB SEP OCT NOV Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) B a a a o n a l Haan - 1 . 0 0 B a a a o n a l Haan 0 . 8 8 -JUN J L Y AUB 8EP OCT NOV o to Enclosure B (l imnetic f e r t i l i z e d ) B a a a o n a l Haan - 1 . 0 0 _- - / A / * ^-xtr " • M ^ l t " f a n , - , 0,7,0 JUN J L Y AUB SEP OCT NOV T a b l e 1 0 . M e a n d e n s i t i e s ( t h o u s a n d s / c u b i c m e t e r ) o f t h e d o m i n a n t z o o p l a n k t o n s p e c i e s 1 n t h e 0 - 2 m d e p t h s t r a t u m . S p e c 1 e s T r e a t m e n t m e a n s ( t h o u s a n d s / c u b i c m e t e r ) L a k e L i t t o r a l C o n t r o l s L I m n e t 1 c C o n t r o l s L 1 t t o r a l F e r t i l i z e d L i m n e t 1 c F e r t 1 1 1 z e d D a p h n i a r o s e a 4 8 0 0 4 7 0 0 2 0 0 0 3 5 0 0 0 3 4 0 0 0 D i a p h a n o s o m a b r a c h y u r u m 7 4 0 0 1 0 0 0 0 2 5 0 0 2 6 3 0 0 3 4 5 0 0 D i a p t o m u s o r e g o n e n s i s 8 7 0 0 5 3 0 0 0 5 0 0 1 0 5 0 0 1 6 0 0 0 B o s m l n a l o n g l r o s t r l s 2 0 0 0 9 0 0 0 3 0 0 0 1 2 0 0 0 3 8 0 0 0 H o l o p e d i u m c M b b e r u m 5 0 0 0 1 0 0 0 2 0 0 0 5 0 0 1 0 0 0 ON H 1 T a b l e 1 1 . R e s u l t s o f 3 w a y a n a l y s i s o f v a r i a n c e o n l o g 1 0 o f t h e d e n s i t i e s o f t h e d o m i n a n t z o o p l a n k t o n s p e c i e s I n t h e 0 - 2 m d e p t h s t r a t u m . N u m b e r s 1 n b r a c k e t s a r e p r o b a b i l i t i e s o f n o s i g n i f i c a n t t r e a t m e n t e f f e c t ( F i , n f o r l a k e v s c o n t r o l c o m p a r i s o n s , F i , i i » f o r c o n t r o l v s f e r t i l i z e d c o m p a M s o s , a n d F i , n f o r a l l o t h e r c o m p a r i s o n s . T h e l e t t e r s T a n d L I n d i c a t e s i g n i f i c a n t t i m e o r l o c a t i o n w i t h i n l a k e e f f e c t s , r e s p e c t i v e l y . T h e l i s t e d t r e a t m e n t 1 s t h e o n e t h a t I s s i g n i f i c a n t l y h i g h e r ( a t a = 0 . 0 5 ) . S p e c i e s T r e a t m e n t C o m p a r i s o n L a k e v s C o n t r o l v s L i t t o r a l C o n t r o l L i t t o r a l F e r t 1 1 i z e d L i t t o r a l C o n t r o l L 1 m n e t 1 c C o n t r o l C o n t r o l F e r t i 1 i z e d L i m n e t i c C o n t r o l L i m n e t i c F e r t 1 1 I z e d L i t t o r a l F e r t i 1 I z e d L 1 m n e t 1 c F e r t 1 1 i z e d D . r o s e a L ( O . O O O ) T F ( O . O O O ) T L 1 t t ( 0 . 0 0 3 ) T L * n s ( 0 . 3 0 6 ) T L F ( O . O O O ) T F ( 0 . 0 0 0 ) D . b r a c h y u r u m n s ( 0 . 8 8 8 ) T * F ( O . O O O ) T L 1 t t ( 0 . . 0 1 4 ) T n s ( 1 . 0 0 0 ) T F ( 0 . 0 2 1 ) T L F ( O . O O O ) T D . o r e g o n e n s i s n s ( 0 . 4 5 7 ) T * n s ( 0 . 0 8 1 ) T * n s ( 0 . 8 8 8 ) T * n s ( 0 . 0 7 7 ) T * F ( 0 . 0 4 7 ) T n s ( 0 . 7 1 3 ) T L ' B . l o n g i r o s t r I s C ( 0 . 0 2 4 ) L F ( 0 . 0 0 5 ) T L L 1 t t ( 0 . 0 0 0 ) L i m ( O . O O O ) n s ( 0 . 8 6 3 ) F ( O . O O O ) T L H . q i b b e r u m L ( O . O O O ) T C ( O . O O O ) T n s ( 0 . 3 1 3 ) T L 1 m ( 0 . 0 0 3 ) T L C ( O . O O O ) T n s ( 0 . 0 7 6 ) T L n s = n o t s i g n i f i c a n t a t a = 0 . 0 5 F , L i t t , L i m , o r C - i n t e r a c t i o n s i g n i f i c a n t a t a = 0 . 0 5 * = v a r i a n c e h e t e r o g e n e o u s a t a = 0 . 0 5 T a b l e 1 2 . R e s u l t s o f n o n - p a r a m e t r i c o n e w a y a n a l y s t s o f v a r i a n c e ( K r u s k a l -W a l l l s t e s t ) o n t h e m e a n b r o o d s i z e s o f t h e d o m i n a n t z o o p l a n k t o n s p e c i e s i n t h e 0 - 2 m d e p t h s t r a t u m . N u m b e r s I n b r a c k e t s a r e t h e v a l u e s o f H a d j u s t e d f o r t i e s . I f H > x ' ( d f = 2 , a = 0 . 0 5 ) = 3 . 8 4 1 , t h e l i s t e d t r e a t m e n t 1 s t h e o n e t h a t 1 s s i g n i f i c a n t l y h i g h e r . T h e " * * " i n d i c a t e s t h e s a m p l e s i z e w a s s m a l l , b u t f o r a l l c a s e s a t t h e s a m p l e s i z e t e s t e d , t h e r e s u l t s w e r e n o t s i g n i f i c a n t l y d i f f e r e n t . T r e a t m e n t C o m p a r i s o n S p e c i e s L a k e C o n t r o l L i t t o r a l L i t t o r a l L i t t o r a l L i m n e t i c C o n t r o l F e r t i l i z e d C o n t r o l C o n t r o l v s v s C o n t r o l F e r t 1 1 1 z e d v s L 1 m n e t 1 c C o n t r o l v s L i m n e t i c F e r t i 1 i z e d v s L i t t o r a l F e r t i 1 i z e d v s L i m n e t i c F e r t 1 1 i z e d D . r o s e a n s ( 0 4 9 0 ) F ( 1 1 4 8 0 0 ) n s ( 0 6 4 5 ) L i m ( 0 3 0 6 ) F ( 2 3 . 0 6 0 ) F ( 2 8 . 3 6 0 ) D . b r a c h y u r u m n s ( 0 3 7 8 ) F ( 9 3 6 0 ) n s ( 0 5 0 2 ) n s ( 1 5 5 7 ) n s ( 3 . 4 1 9 ) * * D . o r e q o n e n s 1 s n s ( 0 0 0 2 ) F ( 17 6 9 0 ) n s ( 0 3 3 3 ) L 1 t t ( 4 5 2 2 ) F ( 9 . 4 9 1 ) n s ( 1 . 4 9 0 ) § • 1 o n q 1 r o s t r 1 s C ( 4 6 2 9 ) F ( 9 1 1 3 0 ) n s ( 2 8 3 6 ) L i m ( 9 1 1 5 ) F ( 1 0 . 8 9 0 ) F ( 2 1 . 9 0 0 ) H . g i b b e r u m n s ( 1 4 3 6 ) F ( 7 3 9 3 ) n s ( 0 2 3 9 ) n s ( 0 0 0 4 ) *.* * * C . q u a d r a n q u l a n s ( 0 0 8 8 ) F ( 2 7 9 8 0 ) n s ( 0 4 0 6 ) L 1 m ( 4 0 2 5 ) n s ( 1 . 4 3 5 ) * * n s = n o t s i g n i f i c a n t a t a = 0 . 0 5 F . L l t t , L i m , o r C = I n t e r a c t i o n s i g n i f i c a n t a t a = 0 . 0 5 .ON 1 64 D e n s i t i e s of D a p h n i a i n t h e c o n t r o l e n c l o s u r e s r e a c h e d maximum l e v e l s a bout two weeks e a r l i e r t h a n i n t h e l a k e ( F i g u r e 3 1 ) . W i t h t h e e x c e p t i o n of l i t t o r a l c o n t r o l e n c l o s u r e 1, maximum d e n s i t i e s were s l i g h t l y l o w e r t h a n i n t h e l a k e a t t h e ti m e o f peak abundance. The more r a p i d i n c r e a s e t o maximum d e n s i t i e s t h a t o c c u r r e d i n t h e c o n t r o l s may have been f a c i l i t a t e d by t h e e x c l u s i o n of r e s i d e n t c u t t h r o a t t r o u t from the e n c l o s u r e s . As i n t h e l a k e , D a p h n i a p o p u l a t i o n s i n t h e c o n t r o l e n c l o s u r e s began t o d e c l i n e s h a r p l y i n l a t e J u l y ; however, t h e s e d e c l i n e s r e s u l t e d i n lo w e r d a p h n i d d e n s i t i e s t h a n i n t h e l a k e , so t h a t mean l a k e d e n s i t i e s were s i g n i f i c a n t l y h i g h e r t h a n i n t h e c o n t r o l s ( T a b l e s 10 and 1 1 ) . Mean b r o o d s i z e s d i d n o t d i f f e r ( T a b l e 12) nor d i d mean body s i z e s ( T a b l e 1 3 ) . A l s o , t h e r e were no s i g n i f i c a n t d i f f e r e n c e s i n e i t h e r p a r a m e t e r between l i t t o r a l and l i m n e t i c c o n t r o l s . Diaphanosoma p o p u l a t i o n s i n t h e l i t t o r a l c o n t r o l e n c l o s u r e s r e s p o n d e d i n a s i m i l a r f a s h i o n t o l a k e p o p u l a t i o n s e x c e p t t h a t t h e l i t t o r a l c o n t r o l p o p u l a t i o n l e v e l s were h i g h e r t h a n i n t h e l a k e ( F i g u r e 3 2 ) . The a v e r a g e d e n s i t y i n t h e l i m n e t i c c o n t r o l s was much l o w e r t h a n i n t h e l i t t o r a l e n c l o s u r e s and t h e l a k e ( T a b l e s 10 and 11). S t a t i s t i c a l a n a l y s i s showed t h a t t h e p o o l e d means of t h e d e n s i t i e s of t h e l i t t o r a l and l i m n e t i c p o p u l a t i o n s were n o t s i g n i f i c a n t l y d i f f e r e n t f r o m l a k e mean d e n s i t i e s . However, mean l i t t o r a l d e n s i t i e s were o n l y r o u g h l y s i m i l a r ( b e i n g s l i g h t l y h i g h e r ) t o mean l a k e d e n s i t i e s (ANOVA, F ( 1 ) 3 0 ) = 3.60, p = 0.067, a = 0.05), b e i n g s l i g h t l y h i g h e r , and mean l i m n e t i c d e n s i t i e s were s i g n i f i c a n t l y l o w e r t h a n mean l a k e T a b l e 1 3 . R e s u l t s o f n o n - p a r a m e t r i c o n e w a y a n a l y s i s o f v a r i a n c e ( K r u s k a l - W a l l i s t e s t ) o n t h e m e a n b o d y s i z e s o f 3 o f t h e d o m i n a n t z o o p l a n k t o n s p e c i e s i n t h e 0 - 2 m d e p t h s t r a t u m . N u m b e r s I n b r a c k e t s a r e t h e v a l u e s o f H a d j u s t e d f o r t i e s . I f H > x ' ( d f = 2 , a - 0 . 0 5 ) = 3 . 8 4 1 , t h e l i s t e d t r e a t m e n t 1 s t h e o n e t h a t 1 s s i g n i f i c a n t l y h i g h e r . S p e c i e s L a k e v s T r e a t m e n t C o m p a r i s o n C o n t r o l v s L 1 t t o r a l C o n t r o l L i t t o r a l F e r t i 1 i z e d L i t t o r a l C o n t r o l L i m n e t 1 c C o n t r o l C o n t r o l F e r t i 1 i z e d L I m n e t i c C o n t r o l L i m n e t i c F e r t i 1 i z e d L i t t o r a l F e r t i 1 i z e d L i m n e t i c F e r t i 1 i z e d D . r o s e a A d u l t O u v e n i I e G r a v i d f e m a l e s n s ( 0 . 7 6 4 ) F ( 2 0 . 4 8 0 ) n s ( 0 3 3 2 ) n s ( 3 . 2 4 1 ) F ( 6 . 3 1 9 ) F ( 1 4 . 3 2 0 ) n s ( O . O O O ) n s ( 0 . 9 3 7 ) n s ( 0 . 5 3 8 ) n s ( 0 . 0 6 0 ) n s ( 0 . 1 2 4 ) n s ( 1 . 4 0 1 ) n s ( 0 . 1 3 5 ) F ( 2 5 . 5 8 0 ) n s ( 0 . 1 9 8 ) n s ( 2 . 0 4 5 ) F ( 9 . 6 3 0 ) F ( 1 4 . 3 2 0 ) D . o r e q o n e n s l s M a l e F e m a l e C o p e p o d l t e n s ( 0 . 0 0 2 ) n s ( 2 . 8 9 0 ) n s ( 0 . 0 0 3 ) F ( 4 9 . 2 9 0 ) F ( 4 5 . 8 5 0 ) n s ( 0 . 3 2 3 ) n s ( 0 . 5 0 1 ) n s ( 1 . 7 0 4 ) n s ( 0 . 8 2 4 ) n s ( 3 . 3 9 9 ) n s ( 2 . 3 9 1 ) n s ( 0 . 4 8 9 ) F ( 3 8 . 4 4 0 ) F ( 2 2 . 7 4 0 ) n s ( 0 . 2 9 1 ) F ( 1 9 . 4 4 0 ) F ( 2 3 . 8 3 0 ) n s ( 0 . 1 4 0 ) D . b r a c h y u r u m A d u l t s n s ( 0 . 0 0 0 ) F ( 2 4 . 5 1 0 ) n s ( 0 . 0 0 6 ) L i m ( 6 . 5 9 1 ) F ( 1 5 . 7 6 0 ) F ( 3 5 . 5 9 0 ) n s = n o t s i g n i f i c a n t a t a - 0 . 0 5 F , L i t t , L i m , o r C = i n t e r a c t i o n s i g n i f i c a n t a t a = 0 . 0 5 166 d e n s i t i e s (ANOVA, F ( 1 j 3 2 ) = 10.14, p < 0.05). Mean b r o o d s i z e s were s i m i l a r i n a l l t h r e e h a b i t a t s ( T a b l e 12). The e n c l o s e d p o p u l a t i o n s o f D. o r e g o n e n s i s e x h i b i t e d s e a s o n a l t r e n d s t h a t were s i m i l a r t o t h o s e o c c u r r i n g i n t h e l a k e ( F i g u r e 3 3 ) . The mean d e n s i t i e s o f Diaptomus i n t h e 0-2 m d e p t h s t r a t u m ( F i g u r e 29) o f t h e e n c l o s u r e s were n o t s i g n i f i c a n t l y d i f f e r e n t f r o m t h o s e i n t h e l a k e o r between l i t t o r a l and l i m n e t i c e n c l o s u r e s ( T a b l e s 10 and 1 1 ) . However, when t h e z o o p l a n k t o n d a t a were examined o v e r a l l d e p t h s , i t became c l e a r t h a t D. o r e g o n e n s i s were more a b u n d a n t a t d e p t h s below 2m. When a l l c o u n t s o f D. o r e g o n e n s i s o v e r a l l d e p t h s were a n a l y z e d t o g e t h e r , t h e r e s u l t s showed t h a t t h e l i m n e t i c mean d e n s i t i e s were not s i g n i f i c a n t l y d i f f e r e n t f r o m t h e mean l a k e d e n s i t i e s (ANOVA, F ( 1 i 5 0 ) = 0.00, P = 1.0) but t h e l i t t o r a l mean d e n s i t i e s were s i g n i f i c a n t l y l o w e r (ANOVA, F ( 1 ) 5 0 ) = 8.96, p = 0.004, a = 0.05). Mean d e n s i t i e s of D i a p t o m u s i n t h e l i t t o r a l e n c l o s u r e s were a l s o s i g n i f i c a n t l y l o w e r t h a n i n t h e l i m n e t i c e n c l o s u r e s . F i n a l l y , w i t h r e s p e c t t o D. o r e g o n e n s i s p o p u l a t i o n d y n a m i c s , t h e mean b r o o d s i z e s of a n i m a l s i n t h e c o n t r o l e n c l o s u r e s were not s i g n i f i c a n t l y d i f f e r e n t f r o m l a k e a n i m a l s o r between e n c l o s u r e s ( T a b l e 1 2 ) . The l i f e c y c l e o f P l a c i d L a k e ' s D. o r e g o n e n s i s has n e v e r been f u l l y documented, however, i t a p p e a r s t o be s i m i l a r t o t h e l i f e c y c l e o f t h i s s p e c i e s d e s c r i b e d by L a i and C a r t e r (1970) and t o t h e l i f e c y c l e of Diaptomus t y r e l l i , i t s c o u n t e r p a r t i n o t h e r l a k e s o f t h e U.B.C. R e s e a r c h f o r e s t ( O l e n e c k , 1983; Marmorek, 1983). Samples were f i r s t t a k e n i n 1982 i n May, 167 s h o r t l y a f t e r i c e - o u t . At t h a t t i m e , t h e p o p u l a t i o n of D. o r e g o n e n s i s a p p e a r e d t o c o n s i s t m o s t l y of a d u l t c o p e p o d s , and a few n a u p l i i (D. R o b i n s o n , p e r s o n a l c o m m u n i c a t i o n ) . T h i s i s s i m i l a r t o t h e p a t t e r n o b s e r v e d i n p r e v i o u s y e a r s ( W a l t e r s , u n p u b l . d a t a ) . Thus, i t a p p e a r s t h a t t h i s s p e c i e s o v e r w i n t e r s as a d u l t s i n t h i s l a k e . L e s s t h a n 1 % of t h e f e m a l e s a t t h i s t i m e were g r a v i d . The f e m a l e s became i n c r e a s i n g l y g r a v i d (10-60 % g r a v i d ) i n J u n e . T h i s was f o l l o w e d by a p u l s e of n a u p l i i (peak abundance a p p r o x . 1 0 / l i t e r ) w h i c h p e a k e d i n mid J u n e , t h e t a i l end of wh i c h c a n be seen i n ( F i g u r e 4 1 ) . T h i s p u l s e of n a u p l i i p r o b a b l y r e p r e s e n t e d t h e f i r s t a n n u a l g e n e r a t i o n and p r o b a b l y a r o s e from r e s t i n g eggs as w e l l as from a d u l t s . The s t r u c t u r e of t h e p o p u l a t i o n ( r a t i o of c o p e p o d i t e s : a d u l t s ) r e m a i n e d r e l a t i v e l y c o n s t a n t o v e r t h e summer w i t h m i n o r f l u c t u a t i o n s i n d e n s i t y s u g g e s t i n g t h a t t h e r e was l i t t l e d e v e l o p m e n t beyond t h e c o p e p o d i t e s t a g e . V e r y few f e m a l e s were g r a v i d d u r i n g t h e summer. Peaks of c o p e p o d i t e abundance a p p e a r e d t o f o l l o w s m a l l p e a k s i n n a u p l i a r d e n s i t y w h i c h o c c u r r e d t h r o u g h o u t t h e summer. T h e s e s m a l l p u l s e s of n a u p l i i may have been t h e p r o g e n y of t h e a d u l t c o p e p o d s t h a t r e m a i n e d f r o m t h e May a d u l t c l a s s , o r t h e y may have been h a t c h e d from r e s t i n g e g g s . F i n a l l y , by e a r l y September, t h e m a j o r i t y o f c o p e p o d i t e s m a t u r e d i n t o a d u l t s . However, t h i s was n o t f o l l o w e d by a c o r r e s p o n d i n g i n c r e a s e i n n a u p l i i p r o d u c t i o n , d e s p i t e t h e f a c t t h a t many o f t h e f e m a l e s were c a r r y i n g eggs a t t h i s t i m e . C o o l e y (1971) d e s c r i b e d s u b i t a n e o u s eggs of D. o r e g o n e n s i s a s b e i n g g r e e n i n c o l o u r , w h i l e D a u n e r i e r , o r r e s t i n g e ggs, a r e 1 68 F i g u r e 41. D e n s i t i e s of c a l a n o i d n a u p l i i . Note t h e change i n s c a l e of t h e Y - a x i s on t h e p l o t s f o r t h e f e r t i l i z e d e n c l o s u r e s . L A K E 8 D e n s i t i e s o f C a l a n o i d N a u p l i i N, O fl » * O J U N J L Y AUG SEP OCT Enclosure 1 ( U t t o r a l control) Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) 3 8 JUN J L Y AUB 8BP OCT J U N J L Y AUS SEP OCT J U N Enclosure 2 (l imnetic control) 3 8 Enclosure 4 (l imnetic f e r t i l i z e d ) J L Y AUB SEP OCT JUN J L Y AUB SEP OCT Enclosure 5 ( l i t t o r a l control) Enclosure 7 ( l i t t o r a l f e r t i l i z e d ) o J U N J L Y AUB 8EP OCT JUN J L Y AUS SEP OCT Enclosure 6 (l imnetic control) Enclosure B (l imnetic f e r t i l i z e d ) 8 J U N J L Y AUB S E P OCT JUN J L Y AUB SEP OCT 170 r e d d i s h - b r o w n . A l l of t h e eggs t h a t I o b s e r v e d b e i n g c a r r i e d by a d u l t D. O r e g o n e n s i s were of t h e l a t t e r c o l o u r . Thus, u n l e s s t h e egg t y p e of t h e s e c o p e p o d s , i n t h i s l a k e , i s not d i s t i n g u i s h a b l e by c o l o u r , t h e l a t e summer egg p r o d u c t i o n by a d u l t D. o r e g o n e n s i s a p p e a r s t o have c o n s i s t e d l a r g e l y of r e s t i n g e g g s . I f so, t h e s e eggs p r o b a b l y spawn much of t h e n e x t y e a r ' s s p r i n g h a t c h . I t a p p e a r s , t h e n , t h a t i n 1982, D. o r e g o n e n s i s i n t h i s l a k e were b i v o l t i n e w i t h an e a r l y summer ( J u n e - J u l y ) g e n e r a t i o n of young and a f a l l p r o d u c t i o n of o v e r w i n t e r i n g e g g s . C e r i o d a p h n i a were r a r e i n t h e l a k e b u t i n c r e a s e d c o n s p i c u o u s l y i n some o f t h e c o n t r o l e n c l o s u r e s ( F i g u r e 3 4 ) . S t a t i s t i c a l a n a l y s i s was not p e r f o r m e d t o d e t e r m i n e whether c o n t r o l d e n s i t i e s were s i g n i f i c a n t l y d i f f e r e n t f r o m l a k e d e n s i t i e s , however, i t seems t h a t t h i s may have been t h e c a s e i n a t l e a s t t h r e e of t h e f o u r c o n t r o l e n c l o s u r e s . The mean b r o o d s i z e s o f C e r i o d a p h n i a i n t h e c o n t r o l s were not s i g n i f i c a n t l y d i f f e r e n t f r o m t h o s e i n t h e l a k e ( T a b l e 1 2 ) . Bosmina l o n g i r o s t r i s , w h i c h l i k e C e r i o d a p h n i a i s r a r e i n t h e l a k e ( N e i l l , 1978), became q u i t e numerous i n t h e 0-2 m d e p t h s t r a t u m i n a l l of t h e c o n t r o l s ( F i g u r e 3 5 ) . D e n s i t i e s ( T a b l e s 10 and 11) and mean b r o o d s i z e s ( T a b l e 12) o f Bosmina i n t h e c o n t r o l s were s i g n i f i c a n t l y h i g h e r i n t h e l a k e . D e n s i t i e s o f Bosmina i n l i t t o r a l e n c l o s u r e s were s i g n i f i c a n t l y h i g h e r t h a n i n l i m n e t i c e n c l o s u r e s ( T a b l e 11) but mean b r o o d s i z e s were n o t ( T a b l e 1 2 ) . D i a c y c l o p s t h o m a s i was much more a b u n d a n t t h r o u g h o u t t h e 171 summer i n t h e c o n t r o l e n c l o s u r e s t h a n i n t h e l a k e ( F i g u r e 3 6 ) . I t s i n c r e a s e must have r e s u l t e d from an e n h a n c e d f o o d s u p p l y . Two p o s s i b l e f o o d i t e m s (Bosmina and C e r i o d a p h n i a ) were more abundant i n t h e c o n t r o l e n c l o s u r e s t h a n i n t h e l a k e . McQueen (1969) has shown D i a c y c l o p s t h o m a s i t o p r e y upon C e r i o d a p h n i a and L y n c h (1979) r e p o r t s p r e d a t i o n by C y c l o p s v e r n a l i s , a v e r y s i m i l a r s p e c i e s , on b o t h C e r i o d a p h n i a and Bosmina. (3) E f f e c t s o f F e r t i l i z a t i o n E f f e c t s o f f e r t i l i z a t i o n on z o o p l a n k t o n abundance ( F i g u r e 30-36) were p a r t i c u l a r l y e v i d e n t w i t h r e s p e c t t o t h e C l a d o c e r a . The c o p e p o d D. o r e g o n e n s i s a l s o i n c r e a s e d s i g n i f i c a n t l y i n d e n s i t y o v e r t h e c o n t r o l p o p u l a t i o n s , b u t not t o t h e same e x t e n t as t h e c l a d o c e r a n s . The c o p e p o d D i a c y c l o p s t h o m a s i d e c r e a s e d i n r e s p o n s e t o n u t r i e n t a d d i t i o n . The d e n s i t i e s of D. r o s e a , D. b r a c h y u r u m , C. q u a d r a n g u l a and B. l o n g i r o s t r i s i n c r e a s e d s i g n i f i c a n t l y o v e r t h o s e i n t h e c o n t r o l s ( T a b l e s 10 and 11). Mean d e n s i t i e s of D. r o s e a and D. b r a c h y u r u m were not s i g n i f i c a n t l y d i f f e r e n t between l i t t o r a l and l i m n e t i c e n c l o s u r e s . Mean d e n s i t i e s o f Bosmina were s i g n i f i c a n t l y h i g h e r i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s t h a n i n t h e l i t t o r a l e n c l o s u r e s , a r e s u l t of an e x t r e m e l y l a r g e bloom of t h e s e a n i m a l s i n September - O c t o b e r i n e n c l o s u r e 4 ( F i g u r e 3 5 ) . D e n s i t i e s o f H o l o p e d i u m were l a r g e l y u n a f f e c t e d by n u t r i e n t a d d i t i o n , e x c e p t t h a t t h i s s p e c i e s showed a b r i e f r e s u r g e n c e f o l l o w i n g a major d e c l i n e i n D a p h n i a abundance i n e n c l o s u r e 4 i n e a r l y J u l y ( F i g u r e 3 1 ) . H o l o p e d i u m d i s a p p e a r e d 1 72 r a p i d l y f r o m a l l o f t h e o t h e r f e r t i l i z e d e n c l o s u r e s ( w i t h i n one week) a f t e r t h e i n i t i a t i o n of f e r t i l i z a t i o n ( F i g u r e 3 0 ) . The median b r o o d s i z e s of a l l C l a d o c e r a were a l s o s i g n i f i c a n t l y h i g h e r under f e r t i l i z a t i o n t h a n i n c o n t r o l s ( T a b l e 1 2 ) . L i m n e t i c D a p h n i a , Bosmina and C e r i o d a p h n i a a l l had l a r g e r b r o o d s i z e s t h a n t h e i r l i t t o r a l c o u n t e r p a r t s , s u g g e s t i n g t h a t l i m n e t i c a n i m a l s had a b e t t e r q u a l i t y f o o d b a s e . . F u r t h e r , t h e mean body s i z e s of g r a v i d D a p h n i a and n o n - g r a v i d a d u l t D a p h n i a and Diaphanosoma i n t h e f e r t i l i z e d e n c l o s u r e s were l a r g e r t h a n i n c o n t r o l s ( T a b l e 13, F i g u r e s 37 and 39, r e s p e c t i v e l y ) . T h e r e was no body s i z e d i s t i n c t i o n between l i t t o r a l and l i m n e t i c f e r t i l i z e d e n c l o s u r e s . A n a l y s i s of v a r i a n c e on t h e mean d e n s i t i e s of D i a p t o m u s  o r e g o n e n s i s i n t h e 0-2 m d e p t h s t r a t u m of t h e e n c l o s u r e s showed t h a t t h e mean d e n s i t i e s of t h e s e c o p e p o d s i n t h e f e r t i l i z e d e n c l o s u r e s (13.2 ± S.E.=1.85 i n d i v i d u a l s / l i t e r ) were n o t s i g n i f i c a n t l y d i f f e r e n t from t h o s e i n t h e c o n t r o l s (7.0 ± S.E.=0.83 i n d i v i d u a l s / l i t e r ) , n or were t h e d e n s i t i e s i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s (16.7 ± S.E.=3.3 i n d i v i d u a l s / l i t e r ) s i g n i f i c a n t l y d i f f e r e n t from t h e l i m n e t i c c o n t r o l s (8.7 ± S.E. = 1.1 i n d i v i d u a l s / 1 i t e r ) ( T a b l e s 10 and 1 1 ) . T h e s e same r e s u l t s were o b t a i n e d w i t h r e s p e c t a n a l y s i s o f t h e d e n s i t i e s o f D i a p t o m u s i n t e g r a t e d o v e r a l l d e p t h s b ut t h e p l o t s of t h e d e n s i t y of t h e o r g a n i s m s ( F i g u r e s 29 and 33) s u g g e s t t h a t t h e above s i g n i f i c a n c e t e s t s may be m i s l e a d i n g . W h i l e t h e mean s e a s o n a l d e n s i t y i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s was o b v i o u s l y h i g h e r t h a n i n t h e l i m n e t i c c o n t r o l s , t h e sample 1 73 v a r i a n c e s were s t a t i s t i c a l l y v e r y h e t e r o g e n e o u s ( F s = 4.37 > F < 3 o _ 3 o > ( a = 0.05) = 2 . 0 7 ) . A t - t e s t of e q u a l i t y of t h e means of two samples whose v a r i a n c e s a r e f o u n d t o be u n e q u a l ( S o k a l and R o h l f , 1981) d e t e r m i n e d t h a t t h e two means were c l e a r l y s i g n i f i c a n t l y d i f f e r e n t ( t s = 3.81 > t ( 3 0 ) (a = 0.001) = 3.65). S i m i l a r l y , t h e o v e r a l l mean d e n s i t i e s of D i a p t o m u s i n t h e p o o l e d f e r t i l i z e d e n c l o s u r e s were s i g n i f i c a n t l y h i g h e r t h a n i n t h e c o n t r o l s ( t s = 24.26 » t ( 6 0 ) ( a = 0.001) = 3 . 4 6 ) . S i m i l a r r e s u l t s were a l s o o b t a i n e d f o r t h e d e n s i t i e s o f D i aptomus i n t e g r a t e d o v e r a l l d e p t h s . The median b r o o d s i z e s o f D i a p t o m u s i n t h e f e r t i l i z e d e n c l o s u r e s were a l s o s i g n i f i c a n t l y h i g h e r t h a n i n c o n t r o l s ( T a b l e 1 2 ) . L i t t o r a l c o p e p o d s had l a r g e r b r o o d s i z e s t h a n d i d l i m n e t i c c o p e p o d s . F i n a l l y , t h e mean body s i z e s of male and f e m a l e a d u l t D. o r e g o n e n s i s were s i g n i f i c a n t l y l a r g e r under f e r t i l i z a t i o n ( T a b l e 1 3 ) . F e r t i l i z a t i o n r e s u l t e d i n a s l i g h t l y g r e a t e r p r o p o r t i o n of D. o r e g o n e n s i s p o p u l a t i o n r e a c h i n g a d u l t h o o d ( F i g u r e 3 3 ) . T h i s may have c o n t r i b u t e d t o t h e d r a m a t i c i n c r e a s e i n t h e d e n s i t i e s o f n a u p l i i t h a t o c c u r r e d i n t h e f e r t i l i z e d e n c l o s u r e s ( F i g u r e 4 1 ) . However, n a u p l i i numbers i n no way c o r r e s p o n d e d t o t h e numbers o f eggs b e i n g c a r r i e d by g r a v i d D. o r e g o n e n s i s f e m a l e s . T h e r e were so few g r a v i d f e m a l e s i n s a m p l e s i t p r e c l u d e d s t a t i s t i c a l a n a l y s i s , y e t numbers o f n a u p l i i i n c r e a s e d e n o r m o u s l y . I s u s p e c t t h a t f e r t i l i z a t i o n g r e a t l y e n h a n c e d t h e s u r v i v a l o f n a u p l i i h a t c h i n g f r o m r e s t i n g e g g s . The i n c r e a s e i n n a u p l i a r d e n s i t y r e s u l t e d i n s i g n i f i c a n t l y g r e a t e r numbers o f c o p e p o d i t e s , but o n l y i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s 174 ( F i g u r e 3 3 ) . The g r e a t e r d e n s i t i e s o f c o p e p o d i t e s i n t h e two l i m n e t i c f e r t i l i z e d e n c l o s u r e s , i n A u g u s t , d i d n o t r e s u l t i n s i g n i f i c a n t l y h i g h e r d e n s i t i e s o f a d u l t D. o r e g o n e n s i s i n September and O c t o b e r t h a n o c c u r r e d i n t h e c o n t r o l s o r t h e l a k e . F i n a l l y , a s i n t h e c o n t r o l s , t h e l a t e summer a p p e a r a n c e of a d u l t s o f t h i s s p e c i e s , d i d not r e s u l t i n a f u r t h e r i n c r e a s e i n n a u p l i a r d e n s i t i e s . The p r e d a c e o u s D i a c y c l o p s t h o m a s i e x h i b i t e d a n e g a t i v e r e s p o n s e t o n u t r i e n t a d d i t i o n compared t o t h e c o n t r o l p o p u l a t i o n s ( F i g u r e 3 6 ) . T h i s d e c r e a s e may have been due t o t h e more p r o n o u n c e d a n a e r o b i c c o n d i t i o n s t h a t o c c u r r e d i n t h e h y p o l i m n i a of t h e f e r t i l i z e d e n c l o s u r e s i n t h e l a t t e r p a r t of t h e summer ( F i g u r e 6 ) . T h i s p o s s i b i l i t y w i l l be d i s c u s s e d l a t e r . F e r t i l i z a t i o n d i d not a f f e c t t h e s e q u e n c e o f s h i f t s i n n u m e r i c a l dominance o f t h e major h e r b i v o r o u s z o o p l a n k t o n , but as i n t h e c o n t r o l s , t h e r e were some e n c l o s u r e - s p e c i f i c d i f f e r e n c e s i n t h e m a g n i t u d e and t i m i n g of t h e s e s h i f t s . D e n s i t i e s of H o l o p e d i u m i n t h e f e r t i l i z e d e n c l o s u r e s d i d n o t i n c r e a s e i n abundance s h o r t l y a f t e r e n c l o s u r e as t h e y d i d i n t h e c o n t r o l s . I n s t e a d , t h e few i n d i v i d u a l s t h a t were p r e s e n t r a p i d l y d e c l i n e d i n numbers ( F i g u r e 3 0 ) . T h i s r e s p o n s e may have been due t o some f o r m of c o m p e t i t i v e i n t e r a c t i o n w i t h t h e D a p h n i a , whose p o p u l a t i o n s were r a p i d l y i n c r e a s i n g ( F i g u r e 3 1 ) . D e n s i t i e s o f D a p h n i a began i n c r e a s i n g v e r y s h o r t l y a f t e r f e r t i l i z a t i o n began, a t a r a t e w h i c h p e r s i s t e d ( i n two e n c l o s u r e s - 3 and 8 ) , w i t h m i n o r f l u c t u a t i o n s u n t i l l a t e J u l y . 175 D e n s i t i e s i n t h e f e r t i l i z e d e n c l o s u r e s were a p p r o x i m a t e l y 5X h i g h e r t h a n i n t h e i r r e s p e c t i v e c o n t r o l s a t t h i s t i m e . Then t h e p o p u l a t i o n s began t o d e c l i n e , a s t h e y d i d i n t h e c o n t r o l s , but i n a much l e s s a b r u p t f a s h i o n and t o minimum mid summer d e n s i t i e s 10-20 t i m e s h i g h e r t h a n i n t h e c o n t r o l s . P o p u l a t i o n s r e m a i n e d "low" u n t i l e a r l y September, t h e n began i n c r e a s i n g a g a i n a t a r a t e s i m i l a r t o t h a t i n J u n e - J u l y . C l e a r l y , D a p h n i a r o s e a d o m i n a t e d t h e z o o p l a n k t o n community i n t h e f e r t i l i z e d e n c l o s u r e s i n t e r m s o f abundance and b i o m a s s f o r most of t h e s e a s o n . Maximum d e n s i t i e s o f D a p h n i a a t t h e t i m e s o f peak abundance i n a l l f e r t i l i z e d e n c l o s u r e s , i n t h e 0-2 m d e p t h s t r a t u m where d e n s i t i e s were g r e a t e s t ( F i g u r e 2 9 ) , e x c e e d e d 100 i n d i v i d u a l s / l i t e r compared t o maximum d e n s i t i e s o f abo u t 10 i n d i v i d u a l s / l i t e r i n t h e c o n t r o l s . A l s o , d e n s i t i e s p e a k e d i n a b i m o d a l f a s h i o n r e a c h i n g maximum l e v e l s i n J u l y and a g a i n i n September ( F i g u r e 3 1 ) . D e n s i t i - e s of D a p h n i a i n t h e c o n t r o l s and t h e l a k e p e a k e d o n l y once i n J u l y and were a t t h e i r l o w e s t i n t h e l a t t e r p a r t of t h e summer. The a u t u m n a l peaks i n abundance i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s were much r e d u c e d compared t o t h o s e i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . S i g n i f i c a n t l y h i g h e r d e n s i t i e s of Bosmina were a s s o c i a t e d w i t h t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s ( T a b l e s 10 and 1 1 ) . T h i s was due t o an enormous "bloom" o f t h i s s p e c i e s i n e n c l o s u r e 4 i n l a t e summer ( F i g u r e 3 5 ) . D e n s i t i e s o f Bosmina of t h i s m a g n i t u d e were not a p p a r e n t i n any o t h e r e n c l o s u r e . W i t h one e x c e p t i o n ( e n c l o s u r e 7 i n e a r l y J u l y ) t h e p o p u l a t i o n s o f Diaphanosoma i n t h e f e r t i l i z e d e n c l o s u r e s 176 a c h i e v e d n u m e r i c a l dominance a t t h e same t i m e as t h e c o n t r o l and l a k e p o p u l a t i o n s , d e c l i n i n g a b r u p t l y i n l a t e A u g u s t - e a r l y September ( F i g u r e 3 2 ) . D e n s i t i e s i n e n c l o s u r e 4 i n l a t e J u l y e a r l y A u g u s t r e a c h e d a maximum of 127 i n d i v i d u a l s / l i t e r , h i g h e r t h a n f o r any o t h e r s p e c i e s e x a m i n e d . D e n s i t i e s of C e r i o d a p h n i a i n t h e f e r t i l i z e d e n c l o s u r e s were on a v e r a g e 3-5 t i m e s h i g h e r t h a n i n t h e c o n t r o l s d u r i n g peak abundance p e r i o d s i n September ( F i g u r e 3 4 ) . D e n s i t i e s were h i g h e s t i n e n c l o s u r e 3. The C e r i o d a p h n i a p o p u l a t i o n i n t h i s e n c l o s u r e began t o i n c r e a s e more r a p i d l y t h a n t h e p o p u l a t i o n s i n t h e o t h e r f e r t i l i z e d e n c l o s u r e s s t a r t i n g i n m i d - A u g u s t . C o i n c i d e n t w i t h t h i s r a p i d r i s e i n C e r i o d a p h n i a abundance was a d r a m a t i c d e c r e a s e i n t h e abundance of C y c l o p s i t s c h i e f p r e d a t o r ( F i g u r e 3 6 ) . One f i n a l n o t e o f i n t e r e s t i s t h a t t h e p r o p o r t i o n of d a p h n i d s i n t h e t o t a l D a p h n i a p o p u l a t i o n t h a t were a d u l t s , b a s e d on t h e body s i z e o f t h e s m a l l e s t g r a v i d i n d i v i d u a l s ( c a . 1.1 mm), was g r e a t e r i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s t h a n i n t h e i r r e s p e c t i v e c o n t r o l s ( F i g u r e 4 2 ) . T h i s i s i m p o r t a n t i n t h a t i f t h e o b j e c t i v e o f an e x e r c i s e i s t o i n c r e a s e t h e b i o m a s s of f o o d f o r f i s h , i t would be a l s o b e n e f i c i a l t o i n c r e a s e t h e s i z e of t h e f i s h f o o d t o t h e e x t e n t t h a t t h e f i s h w i l l u t i l i z e i t . S i n c e i t has been shown above t h a t t h e a p p a r e n t minimum body s i z e o f z o o p l a n k t o n e a t e n by f i s h i n P l a c i d Lake i s 1.0 mm ( N o r t h c o t e and C l a r o t t o , 1975), i t i s c l e a r t h a t t h i s o b j e c t i v e has been met, a t l e a s t i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . The p r o p o r t i o n of a d u l t d a p h n i d s i n t h e l i t t o r a l f e r t i l i z e d 177 F i g u r e 42. P r o p o r t i o n of a d u l t D a p h n i a r o s e a i n t h e l a k e , and c o n t r o l and f e r t i l i z e d e n c l o s u r e s 1 7 8 Proportion of Daphnia That Are Adults LAKE O ' o JUN J L Y AUB S E P OCT NOV Enclosure 1 ( l i t t o r a l control) J U N J L Y AUB SEP OCT NOV Enclosure 3 ( l i t t o r a l f e r t i l i z e d ) o o JUN J L Y AUB 8EP OCT NOV Enclosure 2 ( l imnetic control) J U N J L Y AUB S E P OCT NOV Enclosure 4 (l imnetic f e r t i l i z e d ) J U N J L Y AUB S E P OCT NOV Enclosure 5 ( l i t t o r a l control) JUN J L Y AUB S E P OCT NOV Fnclosure 7 ( l i t t o r a l f e r t i l i z e d ) JUN J L Y AUB 8EP OCT NOV Enclosure 6 ( l imnetic control) Enclosure 8 (l imnetic f e r t i l i z e d ) o J U N J L Y AUB S E P OCT NOV JUN J L Y AUB SEP OCT NOV 1 79 e n c l o s u r e s r e m a i n e d t h e same as i n t h e c o n t r o l s and t h e l a k e . 3.A.3 Summary of Z o o p l a n k t o n R e s u l t s T o t a l z o o p l a n k t o n b i o m a s s a f t e r t h e J u l y maximum was s i g n i f i c a n t l y l o w e r i n t h e c o n t r o l e n c l o s u r e s t h a n i n t h e l a k e . Z o o p l a n k t o n b i o m a s s d i d n o t d i f f e r s i g n i f i c a n t l y between l i t t o r a l and l i m n e t i c c o n t r o l s . Z o o p l a n k t o n b i o m a s s i n t h e f e r t i l i z e d e n c l o s u r e s i n c r e a s e d 4 - f o l d and 7 - f o l d o v e r c o n t r o l s i n t h e l i t t o r a l and l i m n e t i c f e r t i l i z e d e n c l o s u r e s r e s p e c t i v e l y . Z o o p l a n k t o n b i o m a s s i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s was s i g n i f i c a n t l y h i g h e r t h a n i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s . Q u a l i t a t i v e c h a n g e s i n z o o p l a n k t o n community s t r u c t u r e i n t h e c o n t r o l e n c l o s u r e s , as compared t o t h e l a k e i n c l u d e d , a r e d u c t i o n i n numbers of l a r g e c l a d o c e r a n s , an i n c r e a s e i n numbers o f s m a l l c l a d o c e r a n s and an i n c r e a s e i n numbers o f t h e major p r e d a t o r on t h e s m a l l c l a d o c e r a n s , D i a c y c l o p s t h o m a s i . L i t t o r a l p o p u l a t i o n s of t h e c o p e p o d D. o r e g o n e n s i s were s i g n i f i c a n t l y s m a l l e r t h a n l i m n e t i c p o p u l a t i o n s . L i m n e t i c p o p u l a t i o n s of t h e l i t t o r a l c l a d o c e r a n D. b r a c h y u r u m were s i g n i f i c a n t l y s m a l l e r t h a n l i t t o r a l p o p u l a t i o n s . Mean body s i z e s and mean b r o o d o f a l l z o o p l a n k t e r s e x c e p t Bosmina, whose mean b r o o d s i z e i n c r e a s e d , were t h e same as f o r l a k e a n i m a l s . Q u a l i t a t i v e c h a n g e s i n t h e z o o p l a n k t o n community s t r u c t u r e i n t h e f e r t i l i z e d e n c l o s u r e s compared t o c o n t r o l s i n c l u d e d a m a s s i v e i n c r e a s e i n t h e numbers o f t h e c l a d o c e r a n s D a p h n i a r o s e a and D. b r a c h y u r u m and m o derate i n c r e a s e s i n a l l o t h e r 180 z o o p l a n k t e r s , e x c e p t H o l o p e d i u m . D a p h n i a r o s e a , w h i c h was o n l y n u m e r i c a l l y dominant i n t h e c o n t r o l s and t h e l a k e i n J u l y , a l s o became n u m e r i c a l l y d ominant i n t h e l a t t e r p a r t of summer i n t h e f e r t i l i z e d e n c l o s u r e s . A l l of t h e o t h e r s p e c i e s became n u m e r i c a l l y d ominant i n t h e same c h r o n o l o g i c a l s e q u e n c e t h a t o c c u r r e d i n t h e c o n t r o l s and l a k e , a l t h o u g h t h e i r peak d e n s i t i e s were about 5X h i g h e r t h a n i n c o n t r o l s . The body s i z e s and mean b r o o d s i z e s o f a l l of t h e z o o p l a n k t e r s were s i g n i f i c a n t l y h i g h e r u n der f e r t i l i z a t i o n . The mean p e r c e n t a g e of a d u l t , h ence l a r g e r , D a p h n i a i n c r e a s e d f r o m 33.75 % i n t h e l i m n e t i c c o n t r o l s t o 51.30 % i n th e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . P e r c e n t a g e s of a d u l t D a p h n i a i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s r e m a i n e d unchanged from c o n t r o l s . 3.5 EFFECTS OF FERTILIZATION ON CUTTHROAT TROUT DIETS T h i s s e c t i o n has two p u r p o s e s . F i r s t , I w i s h t o i l l u s t r a t e t h a t i t may be i m p r a c t i c a l t o s t o c k h a t c h e r y r e a r e d c u t t h r o a t t r o u t f r y (0.2 - 0.3 gm) i n t o B.C.'s c o a s t a l m o u n t a i n l a k e s i n d e n s i t i e s s u g g e s t e d by t h e B r i t i s h C o l u m b i a F i s h and W i l d l i f e ' s f i s h s t o c k i n g f o r m u l a . S econd, I w i s h t o d e m o n s t r a t e t h a t t h e z o o p l a n k t o n b i o m a s s c u l t i v a t e d by n u t r i e n t a d d i t i o n s i n my e x p e r i m e n t a l e n c l o s u r e s was e x t e n s i v e l y u t i l i z e d by t h e t r o u t i n d i g e n o u s t o P l a c i d L a k e . On June 14, 1982, 50 - 55 c u t t h r o a t t r o u t f r y ( 3 6 0 0 / h e c t a r e ) were ad d e d t o e a c h e n c l o s u r e ( T a b l e 14). The p u r p o s e o f s t o c k i n g t h e f i s h i n t h e e n c l o s u r e s was t o measure 181 Table 14. The average fork l e n g t h (mm) amd weight (grams) and numbers of c u t t h r o a t t r o u t f r y p l a n t e d i n each of the c o n t r o l and f e r t i l i z e d e n c l o s u r e s i n June, 1982. En c l o s u r e Mean S i z e Mean Weight Number P l a n t e d no. (mm) (grams) 1 24.6 0.24 58 2 25.0 0.29 53 3 24.2 0.26 53 4 26.2 0.29 53 5 23.9 0.27 53 6 25.4 0.28 53 7 25.4 0.30 53 8 24.9 0.28 53 1 8 2 and compare t h e r e l a t i v e s u r v i v a l and growth o f t h e f r y under low ( c o n t r o l ) , h i g h ( f e r t i l i z e d ) , l i t t o r a l and l i m n e t i c f o o d l e v e l s . S t o c k i n g d e n s i t i e s were d e t e r m i n e d u s i n g a l a k e s t o c k i n g f o r m u l a d e v e l o p e d and employed by t h e B r i t i s h C o l u m b i a F i s h and W i l d l i f e b r a n c h ( S t r i n g e r e t a l . 1980) f o r t h e s t o c k i n g o f r a i n b o w t r o u t i n t h e Kamloops-Okanagan r e g i o n of B r i t i s h C o l u m b i a . U n f o r t u n a t e l y , a l l o f t h e s t o c k e d f i s h d i s a p p e a r e d f r o m a l l of t h e e n c l o s u r e s w i t h i n two weeks. I t i s d o u b t f u l t h a t t h e y e s c a p e d s i n c e d a i l y i n s p e c t i o n s o f t h e e n c l o s u r e s were made t o e n s u r e t h a t t h e r e were no h o l e s o r r e n t s t h r o u g h w h i c h t h e y c o u l d e s c a p e . However, I c a n n o t be c e r t a i n t h a t t h e y d i d n o t f i n d some way o u t . I c o u l d f i n d no s i g n of them i n s i d e t h e e n c l o s u r e o r e l s e w h e r e i n t h e l a k e ; t h e y were e i t h e r e a t e n by p r e d a t o r s , o r moved i n t o p r o t e c t i v e r e f u g e s ( u n d e r t h e Sphagnum mat o r , i f t h e y e s c a p e d t o t h e l a k e , i n t h e i n l e t or o u t l e t s t r e a m s , e t c . ) . The o n l y c o n c l u s i o n I c o u l d draw from t h e d i s a p p e a r a n c e of t h e p l a n t e d t r o u t f r y , was t h a t f r y s u r v i v a l was n o t e n h a n c e d d e s p i t e p r o t e c t i n g them from p r e d a t i o n by i n d i g e n o u s a d u l t f i s h t h r o u g h e n c l o s u r e , and d e s p i t e i n c r e a s e d f o o d l e v e l s i n t h e f e r t i l i z e d e n c l o s u r e s . B e g i n n i n g i n mid J u l y , however, s e v e r a l g r o u p s o f a d u l t f i s h were o b s e r v e d c r u i s i n g and f e e d i n g w i t h i n t h e e n c l o s u r e s , and, a t t i m e s , f i s h were seen jumping o v e r t h e e n c l o s u r e p e r i m e t e r s , i n t o and o u t o f t h e e n c l o s u r e s . A l l of t h e e n c l o s u r e s were b e i n g i n v a d e d by e x t r a n e o u s f i s h from t h e l a k e . S i n c e I had l o s t t h e o p p o r t u n i t y t o measure c h a n g e s i n g r o w t h 183 and s u r v i v a l o f j u v e n i l e c u t t h r o a t t r o u t , I f e l t t h a t any c o n c e r t e d e f f o r t t o remove and e x c l u d e e x t r a n e o u s t r o u t f r o m t h e e n c l o s u r e s would have been t o o d i f f i c u l t , t o o c o s t l y and t o o t i m e c o n s u m i n g . B e s i d e s , t h e i n v a d e r f i s h p r o v i d e d an e x c e l l e n t o p p o r t u n i t y t o see w hether t h e s e p a r t i c u l a r f i s h would e x p l o i t an e n h a n c e d f o o d base i n t h e form of c r u s t a c e a n z o o p l a n k t o n , s i n c e t h e s e f i s h have been shown t o be c o n s i s t e n t p l a n k t i v o r e s i n o t h e r s t u d i e s ( A n d r u s a k and N o r t h c o t e , 1971; S h e p h e r d , 1973; N o r t h c o t e and C l a r o t t o , 1975). The r e s u l t s of f i s h stomach a n a l y s i s on f i s h n e t t e d ( w i t h g i l l n e t s ) i n t h e e n c l o s u r e s show t h a t t h e t r o u t u t i l i z e d t h e e n h a n c e d z o o p l a n k t o n s t a n d i n g c r o p , p a r t i c u l a r l y D a p h n i a ( T a b l e 1 5 ) . Mean numbers of D a p h n i a i n t h e stomachs of f i s h n e t t e d i n t h e f e r t i l i z e d e n c l o s u r e s were, on a v e r a g e , 27 t i m e s h i g h e r t h a n i n t h e c o n t r o l s o r t h e l a k e . D. r o s e a was an i m p o r t a n t p r e y i t e m o v e r t h e whole summer i n t h e f e r t i l i z e d e n c l o s u r e s . F i s h i n t h e c o n t r o l e n c l o s u r e s and t h e l a k e a p p e a r e d t o f e e d e x t e n s i v e l y on t h e D a p h n i a and H o l o p e d i u m i n t h e e a r l y summer but began t o r e l y more h e a v i l y on t e r r e s t r i a l f l y i n g i n s e c t s , e p h e m e r o p t e r a n and z y g o p t e r a n l a r v a e , c h i r o n o m i d l a r v a e and C h a o b o r u s as c r u s t a c e a n z o o p l a n k t o n became l e s s a b u n d a n t , i n A u g u s t . F i s h i n t h e f e r t i l i z e d e n c l o s u r e s r a r e l y t o o k t e r r e s t r i a l f l y i n g i n s e c t s o r i n s e c t l a r v a e . Two o u t o f t h e t h r e e f i s h c a u g h t i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s i n A u g u s t were f o u n d t o have e a t e n many t e r r e s t r i a l i n s e c t s . V e r y few z o o p l a n k t o n were i n t h e i r g u t s w h i c h l e a d s me t o b e l i e v e t h a t t h e s e two f i s h were o n l y r e c e n t T a b l e 1 5 . M e a n n u m b e r s o f p r e y I t e m s 1 n t h e s t o m a c h s o f c u t t h r o a t t r o u t c a p t u r e d 1 n P l a c i d L a k e , a n d i n t h e c o n t r o l a n d e x p e r i m e n t a l l y f e r t i l i z e d e n c l o s u r e s I n 1 9 8 2 . A d a s h i n d i c a t e s t h a t t h o s e s p e c i e s w e r e n o t p r e s e n t i n t h e g u t . P r e y L A K E L i t t o r a l C o n t r o l s L i m n e t i c C o n t r o l s L i t t o r a l F e r t i l i z e d L i m n e t i c F e r t i l i z e d S p e c i e s J u l y A u g S e p J u l y A u g S e p d u l y A u g S e p d u l y A u g S e p d u l y A u g S e p H o i o p e d l u m 9 8 8 3 6 7 4 3 - 1 5 1 - 0 . 5 - 0 . 7 3 9 2 - -D a p h n i a 3 1 2 0 8 4 4 3 9 0 1 4 4 0 6 1 N 2 4 2 5 2 6 0 1 1 1 2 0 6 3 1 4 2 8 4 5 2 1 2 D . k e n a i - - - - - 4 7 - 0 - - - 2 2 5 4 7 -' D . o r e q o n e n s i s - - - - - 0 . 3 - - - - 2 - 0 . 5 T e r r e s t r 1 a l 1 n s e c t s - 9 4 8 - 8 2 0 - 2 D 0 . 3 3 2 1 - - -C h i r o n o m i d s - 3 3 - - 1 5 - - 4 A - 3 2 2 0 . 1 6 1 3 C h a o b o r o u s f l a v l c a n s - 2 2 - • - 2 0 . 7 0 . 5 T A 0 . 8 - 0 . 3 1 5 0 . 3 0 . 3 P o l y p h e m u s - - - - - - -A - - - - 1 5 -E p h e m e r o p t e r a n / Z y g o p t e r a n 1 a r v a e - 4 6 - - 14 - 8 - - - - - -O t h e r s 1 0 0 . 3 0 . 4 2 0 . 5 1 2 0 . 3 0 . 3 - - - -N o . o f 6 4 7 3 4 1 3 2 7 3 3 5 4 6 S t o m a c h s 185 i m m i g r a n t s t o t h e e n c l o s u r e s . H o l o p e d i u m and D i a p t o m u s k e n a i were f o u n d i n a few c u t t h r o a t t r o u t stomachs a t t i m e s when t h o s e s p e c i e s were not r e c o r d e d or a p p e a r e d i n v e r y low d e n s i t i e s i n p l a n k t o n samples t a k e n a t t h e same t i m e ( c f . D. K e n a i i n A u g u s t i n t h e f e r t i l i z e d e n c l o s u r e s , T a b l e 1 5 ) . P e r h a p s t h e s e s p e c i e s were c o n c e n t r a t e d n e a r t h e b o t t o m o f t h e l a k e where t h e s a m p l i n g g e a r c o u l d n o t be u s e d e f f e c t i v e l y . I f s o , t r o u t f e e d i n g on t h e s e z o o p l a n k t e r s would have o c c u r r e d below 5m, a f u l l 2m below t h e maximum d e p t h sampled. • T h e s e t r o u t t e n d t o move i n t o d e e p e r w a t e r s (4-6 m) as t h e summer p r o g r e s s e s ( A n d r u s a k and N o r t h c o t e , 1971). T h i s would e x p l a i n t h e low numbers of f i s h c a p t u r e d i n 3m n e t s i n A u g u s t and September ( T a b l e 1 5 ) . 1 86 4.0 DISCUSSION 4.j_ E f f e c t s of E n c l o s u r e I s o l a t i o n of t h e w a t e r column w i t h i n p l a s t i c o r wooden e n c l o s u r e s ( 1 i m n o c o r r a l s ) f o r t h e p u r p o s e of s i m u l a t i n g n a t u r a l l a k e c o n d i t i o n s i s a method t h a t has been employed by l i m n o l o g i s t s f o r v a r i o u s i n v e s t i g a t i o n s s i n c e t h e e a r l y 1960's (Goldman, 1962; F r e s h w a t e r B i o l o g i c a l A s s o c i a t i o n , 1963). L i m n o c o r r a l s a l l o w t h e i n v e s t i g a t o r t o p e r f o r m s e v e r a l s h o r t -t e r m e x p e r i m e n t s i n a l a k e s i m u l t a n e o u s l y . F u r t h e r , c o n t r o l s can e a s i l y be e s t a b l i s h e d t o j u d g e t h e e f f e c t s o f b i o t i c or a b i o t i c m a n i p u l a t i o n s on t h e c h e m i c a l e n v i r o n m e n t o r on p h y t o p l a n k t o n o r z o o p l a n k t o n community e c o l o g y . W i t h s m a l l (1-2 m d i a m e t e r ) e n c l o s u r e s i t i s d i f f i c u l t t o m a i n t a i n n a t u r a l p h y t o p l a n k t o n o r z o o p l a n k t o n p o p u l a t i o n s f o r l o n g enough p e r i o d s t o e v a l u a t e s e a s o n a l c h a n g es i n b i o c e n o s e s . Over l o n g p e r i o d s , p l a n k t o n c o m m u n i t i e s i n s m a l l e n c l o s u r e s come t o r e s e m b l e pond o r l a k e l i t t o r a l c o m m u n i t i e s . In r e s p o n s e t o t h i s p r o b l e m , l i m n o l o g i s t s (Lund, 1972; R e y n o l d s , 1979) began t o i n v e s t i g a t e t h e use o f l a r g e ( a p p r o x . 100 m 2) e n c l o s u r e s w i t h t h e hope t h a t t h e y w o u l d b e t t e r r e p r e s e n t t h e s t u d y l a k e ' s p h y s i c a l , c h e m i c a l and b i o l o g i c a l e n v i r o n s . To d a t e , few s t u d i e s have documented whether l o n g - t e r m impoundment of n a t u r a l l a k e w a t e r s i n s i d e l a r g e l i m n o c o r r a l s i n any way a l t e r s t h e p h y s i c a l , c h e m i c a l o r b i o l o g i c a l e n v i r o n m e n t . In many l i m n o c o r r a l i n v e s t i g a t i o n s , t h e a b i o t i c o r b i o t i c p a r a m e t e r s measured i n t h e c o n t r o l e n c l o s u r e s a r e s e l d o m 187 compared t o t h o s e i n t h e p a r e n t l a k e . S i n c e t h e v a l u e of an e x p e r i m e n t , where t h e r e s u l t s a r e t o be a p p l i e d t o t h e l a k e o r l a k e s a s a whole, depends on t h e b e h a v i o u r of t h e d y n a m i c s of t h e c o n t r o l e n c l o s u r e s r e s e m b l i n g t h o s e i n t h e l a k e , t h i s i s a r e l e v a n t c o n c e r n . 4.J_.J_ On P h y s i c a l C o n d i t i o n s and Water C h e m i s t r y My r e s u l t s show t h a t t h e p h y s i c a l and c h e m i c a l e n v i r o n m e n t i n s i d e l a r g e e n c l o s u r e s g e n e r a l l y r e m a i n e d u n a l t e r e d f r o m a m bient c o n d i t i o n s i n t h i s s m a l l , s h a l l o w p o l y m i c t i c l a k e f o r 5 months i n t h e summer o f 1982. M o r e o v e r , t h e c h e m i c a l e n v i r o n m e n t s i n e n c l o s e d l i t t o r a l r e g i o n s were q u a l i t a t i v e l y s i m i l a r t o t h o s e i n e n c l o s e d l i m n e t i c r e g i o n s . T h e r e was o n l y one anomalous c h e m i c a l r e s u l t . T h a t o c c u r r e d i n l i t t o r a l c o n t r o l e n c l o s u r e 1 and i t i s d o u b t f u l t h a t t h i s r e s u l t was due t o e n c l o s u r e . M i d t o l a t e summer e p i l i m n e t i c pH v a l u e s i n t h i s e n c l o s u r e r o s e t o much h i g h e r l e v e l s (pH = 8.0) t h a n i n a l l o t h e r t r e a t m e n t e n c l o s u r e s , i n c l u d i n g t h e f e r t i l i z e d ones ( F i g u r e 15). The pH e x c e e d e d 7.0 f o r most o f t h e l a t t e r p a r t of t h e summer, d e c l i n i n g t o n o r m a l l e v e l s a t f a l l o v e r t u r n . One mechanism w h i c h may e x p l a i n s u c h an i n c r e a s e i n pH i s an o c c u r r e n c e o f a p h y t o p l a n k t o n or e p i b e n t h i c a l g a l bloom t a k i n g up s i g n i f i c a n t q u a n t i t i e s o f C 0 2 ( c a u s i n g a s h i f t i n t h e c a r b o n a t e - b i c a r b o n a t e e q u i l i b r i u m t o w a r d s c a r b o n a t e ) , r e s u l t i n g i n an o v e r a l l d e c r e a s e i n h y d r o g e n i o n c o n t e n t o f t h e w a t e r . The d a t a g i v e no i n d i c a t i o n t h a t a p h y t o p l a n k t o n bloom o c c u r r e d . 1 8 8 Though I have no d i r e c t e v i d e n c e t o r u l e o u t a bloom of e p i b e n t h i c a l g a e , t h e r e were no v i s i b l e s i g n s t h a t s u c h a bloom m i g h t have o c c u r r e d ( p e r s o n a l o b s e r v a t i o n ) . W e l c h (1952, page 122) r e p o r t e d t h a t e p i l i m n e t i c w a t e r s i n l a k e s h a v i n g a sphagnum d o m i n a t e d l i t t o r a l r e g i o n can e x h i b i t s t r i k i n g l y d i f f e r e n t pH l e v e l s i n a r e a s i n c l o s e p r o x i m i t y t o e a c h o t h e r , however, no e x a m p les o r c a u s a l mechanisms were g i v e n . In t h e n e x t few p a r a g r a p h s , I w i l l a t t e m p t t o r e s o l v e why t h e n u t r i e n t c o n c e n t r a t i o n s i n t h e l i t t o r a l c o n t r o l e n c l o s u r e s were t h e same as i n t h e l i m n e t i c c o n t r o l e n c l o s u r e s . L i t t o r a l m a c r o p h y t e s a r e i m p o r t a n t i n t h e c y c l i n g of n u t r i e n t s , f u n c t i o n i n g as b o t h a s o u r c e ( R i g l e r , 1964; McRoy, 1972; W a l l s t e n , 1979) and a s i n k ( C o f f i n e t a l . , 1949; Hayes and C o f f i n , 1951; McRoy, 1972) f o r i n o r g a n i c r e a c t i v e p h o s p h o r o u s . However, R i g l e r (1964) showed t h a t - r e t u r n o f r e a c t i v e p h o s p h o r o u s from t h e l i t t o r a l t o t h e e p i l i m n i o n d u r i n g t h e summer i n s m a l l l a k e s was 20 % g r e a t e r t h a n l o s s . The l i t t o r a l i s a l s o a m a j o r s i t e o f ' n i t r o g e n f l u x . I n t e r n a l s o u r c e s o f n i t r o g e n t o t h e w ater column a r e l a r g e l y f r o m t h e s e d i m e n t s (Keeney, 1973). M i x i n g o f s u r f i c i a l s e d i m e n t s by water movements, a c t i v i t i e s of b e n t h i c o r g a n i s m s and gas b u b b l e s , and s i m p l e d i f f u s i o n p r o c e s s e s w i l l a l l c o n t r i b u t e t o t h e N s u p p l y o f o v e r l y i n g w a t e r s . T h e s e p r o c e s s e s would have a more p r o f o u n d i m p a c t on l i t t o r a l e p i l i m n e t i c n i t r o g e n c o n c e n t r a t i o n s t h a n t h e same p r o c e s s e s o c c u r r i n g i n d e e p e r h y p o l i m n e t i c w a t e r s would have on l i m n e t i c e p i l i m n i o n c o n c e n t r a t i o n s . T r a n s l o c a t i o n o f n u t r i e n t s f r o m t h e l i t t o r a l zone t o t h e e p i l i m n i o n o f l a k e s has 189 been w e l l documented ( S c h r o d e r , 1975; L i e , 1977). T h i s , one c o u l d a r g u e , c o u l d l e a d t o h i g h e r n u t r i e n t c o n c e n t r a t i o n s i n e p i l i m n e t i c w a t e r s i n l i t t o r a l e n c l o s u r e s t h a n i n l i m n e t i c e n c l o s u r e s , s i n c e n u t r i e n t t r a n s p o r t t o t h e l i m n e t i c zone was o b s t r u c t e d by an impermeable b a r r i e r and t h e volume of t h e e p i l i m n i o n was s m a l l e r i n t h e l i t t o r a l t h a n i n t h e l i m n e t i c e n c l o s u r e s . I f o u n d , i n f a c t , t h a t i n t h e c o n t r o l e n c l o s u r e s , t h e l i t t o r a l e p i l i m n i o n had r o u g h l y t h e same n u t r i e n t c o n c e n t r a t i o n s as i n t h e l i m n e t i c z o n e . The d o m i n a n t l i t t o r a l m a c r o p h y t e i n a l l o f t h e l i t t o r a l e n c l o s u r e s was Sphagnum sp p . We know from work done i n m i r e s and p e a t l a n d s (Moore and B e l l a m y , 1974) and i n bog l a k e s ( C o f f i n e t a l . f 1949; Hayes and C o f f i n , 195.1; R i g l e r , 1956) t h a t Sphagnum c a n f u n c t i o n as a n u t r i e n t t r a p . Thus, i t may be t h a t b o t h N and P r e l e a s e d f r o m l i t t o r a l m a c r o p h y t e s and s e d i m e n t s i n P l a c i d L a k e were bound up i n t h e p e a t r a t h e r t h a n c y c l e d i n t o t h e e p i l i m n i o n . P a b s o r p t i o n by Sphagnum has been documented i n s t u d i e s done i n bog l a k e s ( C o f f i n e t a l . , 1949; Hayes and C o f f i n , 1951; R i g l e r , 1956). A l s o , Sphagnum mats a r e m a j o r s i t e s o f c a t i o n f l u x i n l a k e s (Gorham, 1956; Clymo, 1963; C r a i g i e and Mass, 1966). S i n c e t h e main i n o r g a n i c f o r m o f n i t r o g e n r e l e a s e d by s e d i m e n t s i s t h e c a t i o n ammonium (Keeney, 1973), i t t o o was p r o b a b l y t a k e n up by Sphagnum. Thus, i t i s n o t s u r p r i s i n g t h a t t h e l i t t o r a l c o n t r o l e n c l o s u r e s d i d not d e v e l o p h i g h e r c o n c e n t r a t i o n s o f i n o r g a n i c n u t r i e n t s i n t h e i r e p i l i m n e t i c w a t e r s . I t a p p e a r s t h a t u n d e r n a t u r a l c o n d i t i o n s t h e c o n c e n t r a t i o n s o f i n o r g a n i c n u t r i e n t s i n 1 90 P l a c i d L a k e , may be m a i n t a i n e d a t an e q u i l i b r i u m l e v e l where t h e r a t e of s u p p l y i s b a l a n c e d by t h e r a t e o f l o s s . I f so, one s h o u l d n o t e x p e c t a n e t i n c r e a s e i n i n o r g a n i c n u t r i e n t c o n c e n t r a t i o n s i n t h e w a t e r column u n l e s s t h e water r e c e i v e s a s i g n i f i c a n t amount o f n u t r i e n t . In l a k e s n ot a f f e c t e d by d i r e c t human a c t i v i t i e s , n u t r i e n t i n c r e m e n t s o c c u r i n t h e form o f p r e c i p i t a t i o n , r u n o f f and s t r e a m d i s c h a r g e . T h e s e p r o c e s s e s a r e a l s o t h e major a l l o c h t h o n o u s s o u r c e s o f f r e e n u t r i e n t s t o P l a c i d L a k e . T h a t p r e c i p i t a t i o n i s a m a j o r n u t r i e n t s o u r c e i s c l e a r l y i l l u s t r a t e d by t h e c o n s p i c u o u s i n c r e a s e s i n n u t r i e n t l e v e l s i n t h e w a t e r column d u r i n g t h e o n l y two major p l u v i a l e v e n t s t h a t o c c u r r e d i n t h e summer of 1982, i n A u g u s t and September ( F i g u r e s 8 - 1 5 ) . The n u t r i e n t p u l s e s a r e a l s o e v i d e n t even i n t h e l i m n e t i c e n c l o s u r e s w h i c h would r e c e i v e no s u r f a c e r u n o f f o r s t r e a m c a r r i e d n u t r i e n t s , o n l y t h o s e t h a t a r e r a i n b o r n e . However, t h i s i n p u t r a r e l y o c c u r r e d o v e r t h e summer and i t was r a p i d l y d e p l e t e d by a l g a l a s s i m i l a t i o n a n d / o r a p r o b a b l e n e t f l u x i n t o t h e s e d i m e n t s . S i n c e t h e r e were no o b v i o u s d i f f e r e n c e s i n t h e n u t r i e n t r e g i m e s o f t h e . l i t t o r a l and l i m n e t i c e n c l o s u r e s and a l l o c h t h o n o u s n u t r i e n t s o u r c e s were m i n i m a l , i t i s l i k e l y t h a t a u t o c h t h o n o u s n u t r i e n t r e c y c l i n g mechanisms l i k e z o o p l a n k t o n a s s i m i l a t i o n and e x c r e t i o n ( H a r g r a v e and Geen, 1968; R i c h e y , 1979; T a y l o r , 1984) and s e d i m e n t d e s o r p t i o n and d i f f u s i o n o r r e s u s p e n s i o n and d i f f u s i o n p r o c e s s e s ( S y e r s e t a_l. , 1973; Keeney, 1973; L i j k l e m a e t a l . , 1983; T w i n c h and P e t e r s , 1984) 191 s u p p l i e d a l m o s t a l l of t h e n u t r i e n t s a v a i l a b l e f o r b i o l o g i c a l p r o d u c t i o n d u r i n g t h e summer months. N u t r i e n t r e m i n e r a l i z a t i o n by z o o p l a n k t o n does not p r o v i d e i n o r g a n i c n u t r i e n t s a t r a t e s c o m p a r a b l e t o r a t e s of s u p p l y of n u t r i e n t s f r o m a l l o c h t h o n o u s s o u r c e s d u r i n g w i n t e r months (Lehman, 1980). However, t h e r a t e o f s u p p l y of n u t r i e n t s f r o m t h e f o r m e r p r o c e s s i s s u f f i c i e n t t o s u p p l y a s i z a b l e f r a c t i o n o f t h e d a i l y N and P r e q u i r e m e n t s of t h e p h y t o p l a n k t o n d u r i n g summer months when e x t e r n a l s o u r c e s a r e a t a minimum (Lehman, 1980). K r a u s e (1984) has shown t h a t t h e z o o p l a n k t o n i n P l a c i d L a k e a r e i m p o r t a n t i n t h i s r e g a r d i n y e a r s when a l l o c h t h o n o u s n u t r i e n t s o u r c e s a r e m i n i m a l . The w ater t e m p e r a t u r e p r o f i l e f o r P l a c i d Lake ( F i g u r e 5) shows t h a t t h e r m a l s t r a t i f i c a t i o n i n 1982 was weak and no d e f i n i t e s t a b l e e p i l i m n i o n e x i s t e d . A l s o , o n l y 17 % of t h e t o t a l l a k e volume l i e s below 4 m ( T a b l e 1 ) , t h e d e p t h a t w h i c h a weak d e n s i t y g r a d i e n t a p p e a r e d t o e x i s t . T h i s l e a d s me t o b e l i e v e t h a t t h e e p i l i m n i o n d e p t h f o r 1982 was e s s e n t i a l l y t h e e n t i r e d e p t h of t h e l a k e . Thus, v e r t i c a l f l u x o f r e l e a s e d i n o r g a n i c n u t r i e n t s f r o m s e d i m e n t s may a l s o have been an i m p o r t a n t i n t e r n a l n u t r i e n t s o u r c e . B e c a use t h e l a k e i s s h e l t e r e d f r o m s t r o n g w i n d s , i t i s d o u b t f u l t h a t enough wind e n e r g y c a n be g e n e r a t e d t o r e s u l t i n s i g n i f i c a n t n u t r i e n t f l u x e s by d i r e c t e n t r a i n m e n t of t h e mixed w a t e r s o v e r t h e s e d i m e n t s ( S t a u f f e r and A r m s t r o n g , 1984). T h e r e f o r e , I s u r m i s e t h a t d i f f u s i o n of n u t r i e n t s i n t o t h e e p i l i m n i o n f r o m s e d i m e n t p o r e w a t e r s was a more l i k e l y s o u r c e of f r e e i n o r g a n i c n u t r i e n t s . 1 9 2 4 2 On P h y t o p l a n k t o n S i n c e t h e p h y s i c a l and c h e m i c a l r e g i m e s i n t h e e n c l o s u r e s were c l o s e r e p l i c a t e s of e a c h o t h e r and t h e l a k e , i t i s n o t s u r p r i s i n g t h a t t h e v e r t i c a l and s e a s o n a l d i s t r i b u t i o n s of c h l o r o p h y l l a i n a l l c o n t r o l e n c l o s u r e s were s i m i l a r t o t h o s e i n t h e l a k e . However, s i g n i f i c a n t l y h i g h e r c o n c e n t r a t i o n s of p h a e o p h y t i n o c c u r r e d i n a l l c o n t r o l e n c l o s u r e s a t 2 - 4 m. I s u g g e s t t h a t t h i s may have been due t o s e d i m e n t a t i o n o f s e s t o n on t h e weak d e n s i t y g r a d i e n t t h a t e x i s t e d a t 4 m. Thus, one c o n s e q u e n c e o f e n c l o s u r e may be t h a t m e t a l i m n e t i c e n t r a i n m e n t of t h e water column may have been impeded, p r e v e n t i n g t h e r e s u s p e n s i o n of t h e a c c u m u l a t e d dead and d y i n g p l a n k t o n . T h i s i s p o s s i b l e s i n c e t h e c i r c u l a t i o n i n d u c i n g a c t i o n of t h e wind w o u l d have been impeded by t h e f l o t a t i o n g e a r a t t h e s i d e s of t h e e n c l o s u r e s . The d i f f e r e n c e i n t h e mid summer p h y l e t i c c o m p o s i t i o n of t h e a l g a e between l i m n e t i c e n c l o s u r e 2 , w h i c h was d o m i n a t e d by t h e b l u e - g r e e n a l g a M e r i s m o p e d i a , and a l l o f t h e o t h e r c o n t r o l s and t h e l a k e i s p u z z l i n g s i n c e g r o s s p h y s i c a l and c h e m i c a l c h a r a c t e r i s t i c s were t h e same f o r a l l c a s e s . One r e a s o n f o r t h i s d i f f e r e n c e may be t h a t c a l a n o i d c o p e p o d s r a t h e r t h a n c l a d o c e r a n s d o m i n a t e d t h e mid summer z o o p l a n k t o n a s s e m b l a g e s o f t h e l i m n e t i c c o n t r o l s . They a l s o o c c u r r e d i n much h i g h e r d e n s i t i e s t h a n i n t h e o t h e r c o n t r o l s or t h e l a k e . C a l a n o i d c o p e p o d s have a much g r e a t e r c a p a b i l i t y t h a n do c l a d o c e r a n s f o r s e l e c t i n g f o o d b a s e d on s i z e (Richman e t a_l. 1 9 8 0 ; A l c a r a z e t a l . 1 9 8 0 ) , t a s t e ( P o u l e t and M a r s o t , 1 9 7 8 , 1 9 8 0 ) and shape 1 93 (McNaught e t a l . 1980). G i v e n t h i s , i t i s p o s s i b l e t h a t D. o r e g o n e n s i s can a v o i d even s m a l l amounts of t o x i c or unmanageable b l u e - g r e e n a l g a e i n t h e h e t e r o g e n e o u s m i x t u r e of f o o d p a r t i c l e s t h a t makes up t h e i r e n v i r o n m e n t . S e l e c t i v e f e e d i n g by D. o r e g o n e n s i s may have r e s u l t e d i n a l a r g e r p o o l of f r e e n u t r i e n t s b e i n g made a v a i l a b l e t o t h e u n g r a z e d b l u e - g r e e n s and s u b s e q u e n t l y r e s u l t e d i n t h e i r dominance o f t h e a l g a l community. T h i s argument assumes t h a t t h e more p a s s i v e l y f e e d i n g C l a d o c e r a c a n n o t o r do n o t a v o i d i n g e s t i n g t o x i c a l g a e t h a t a p p e a r i n low abundance r e l a t i v e t o n o n - t o x i c f o r m s ; t h e r e i s e v i d e n c e t o s u p p o r t t h i s a s s u m p t i o n ( P o r t e r , 1977). A s i m i l a r , b u t l e s s p r o n o u n c e d dominance o f b l u e - g r e e n a l g a e o c c u r r e d i n l i m n e t i c e n c l o s u r e 6 ( F i g u r e 2 2 ) . E n c l o s u r e 6 had s i g n i f i c a n t l y h i g h e r numbers of Diaphanosoma ( 5 - 6 / l i t e r ) t h a n e n c l o s u r e 2 ( 2 - 3 / l i t e r ) d u r i n g mid summer, so t h e r e may have been a s i g n i f i c a n t g r a z i n g p r e s s u r e e x e r t e d on t h e b l u e - g r e e n s i n t h i s e n c l o s u r e n e g a t i n g t h e e f f e c t of s e l e c t i v e f e e d i n g by t h e c o p e p o d s . i . * l ' 3 _ On Z o o p l a n k t o n T h e r e was a r a p i d e x t i n c t i o n o f H o l o p e d i u m from a l l of t h e l i m n o c o r r a l s w i t h i n two weeks a f t e r e n c l o s u r e . H o l o p e d i u m has a l w a y s d i s a p p e a r e d f r o m s m a l l e r e n c l o s u r e s i n o t h e r e x p e r i m e n t s e v e n where f i s h were e x c l u d e d ( O l e n e c k , 1983; Marmorek, 1983; N e i l l , p e r s . comm.). I do n o t b e l i e v e t h a t t h e d i s a p p e a r a n c e of H o l o p e d i u m from my l a r g e bags was due t o e n c l o s u r e s i n c e t h e e n c l o s u r e s r e s e m b l e d t h e l a k e i n a l l m e a s u r e d a s p e c t s of 1 94 n u t r i e n t s and f o o d ; r a t h e r , i t may have been due p r e d a t i o n on z o o p l a n k t o n by c u t t h r o a t t r o u t , w h i c h i n v a d e d a l l t r e a t m e n t e n c l o s u r e s and were p r o b a b l y more dense i n t h e e n c l o s u r e s t h a n i n t h e l a k e ( d i s c u s s e d b e l o w ) . T h i s may a l s o e x p l a i n why d e n s i t i e s o f D a p h n i a d u r i n g l a t e summer were s i g n i f i c a n t l y l o w e r i n a l l c o n t r o l e n c l o s u r e s compared t o t h e l a k e . D e n s i t i e s of Diaphanosoma were s i g n i f i c a n t l y l o w e r i n t h e l i m n e t i c c o n t r o l s t h a n i n t h e l i t t o r a l c o n t r o l s or t h e l a k e ( T a b l e 1 1 ) . T h i s i s n o t s u r p r i s i n g s i n c e D. b r a c h y u r u m i s most commonly f o u n d i n t h e l i t t o r a l m a r g i n s of l a k e s and i s l i m n e t i c o n l y i n s m a l l l a k e s ( B r o o k s , 1959). D. b r a c h y u r u m i n P l a c i d Lake had u n f e t t e r e d a c c e s s t o t h e l i m n e t i c r e g i o n of t h e l a k e and may have had c o n s t a n t r e c r u i t m e n t from l a r g e r l i t t o r a l p o p u l a t i o n s . Thus, t h e y were r e l a t i v e l y a b undant a t t h e l i m n e t i c l a k e s a m p l i n g s i t e s . However, t h e p o p u l a t i o n s o f Diaphanosoma i n t h e l i m n e t i c c o n t r o l s a r o s e from o r g a n i s m s t h a t were " e n c l o s e d " when t h e s p e c i e s ' d e n s i t i e s were low, i n e a r l y J u n e , and t h e r e was no r e c r u i t m e n t from th e l i t t o r a l zone, so t h e p o p u l a t i o n r e m a i n e d s m a l l . L e s s t h a n 10 % o f t h e p o p u l a t i o n c o n s i s t e d o f g r a v i d f e m a l e s w h i c h s u g g e s t s t h a t t h e i r l e v e l of r e p r o d u c t i o n may have been t o o low t o a l l o w t h e p o p u l a t i o n t o do more t h a n m a i n t a i n i t ' s numbers. In c o n t r a s t , d e n s i t i e s o f Diaphanosoma were s i g n i f i c a n t l y h i g h e r i n t h e l i t t o r a l e n c l o s u r e s t h a n i n b o t h t h e l i m n e t i c e n c l o s u r e s and t h e l a k e . T h e r e a r e 2 p o s s i b l e e x p l a n a t i o n s f o r t h i s a s i d e f r o m t h e f a c t t h a t Diaphanosoma i s , g e n e r a l l y , a l i t t o r a l c l a d o c e r a n ( B r o o k s , 1959) and would be e x p e c t e d t o 1 9 5 o c c u r i n g r e a t e r d e n s i t i e s t h e r e . F i r s t , t h e l i t t o r a l p o p u l a t i o n was " e n c l o s e d " and c o u l d n ot d i s p e r s e , so t h e r e was no d i l u t i o n of p o p u l a t i o n numbers. Second, r e d u c e d o v e r a l l a b u n d a n c e s of H o l o p e d i u m and D a p h n i a i n t h e e n c l o s u r e s , compared t o l a k e numbers, may have a l l e v i a t e d some form of c o m p e t i t i v e i n t e r a c t i o n a c t i n g t o h o l d down Diaphanosoma numbers i n t h e l a k e . W h i l e o b s e r v e d d e n s i t i e s of D. o r e g o n e n s i s were s i g n i f i c a n t l y l o w e r i n t h e l i t t o r a l c o n t r o l s t h a n i n l i m n e t i c c o n t r o l s and t h e l a k e ( F i g u r e 3 3 ) , i t i s p o s s i b l e t h a t t h e a c t u a l d e n s i t i e s o f D. o r e g o n e n s i s i n t h e l i t t o r a l e n c l o s u r e s were as h i g h a s i n t h e l a k e and t h e l i m n e t i c e n c l o s u r e s . P e l a g i c c r u s t a c e a n s a r e known t o e x h i b i t a b e h a v i o u r p a t t e r n known a s " U f e r f l u c h t " or " a v o i d a n c e of s h o r e " i n r e s p o n s e t o * c h a n g i n g l i g h t r e g i m e s ( S i e b e r t , 1980). T h i s p a t t e r n o f movement o c c u r s as shadows c a s t by s h o r e l i n e f e a t u r e s l e n g t h e n d u r i n g t h e day. The c o p e p o d s m i g r a t e o u t o f t h e shaded a r e a s . I f D. o r e g o n e n s i s e x h i b i t s s u c h a b e h a v i o u r , most of t h e c o p e p o d s i n t h e l i t t o r a l e n c l o s u r e s would m i g r a t e t o w a r d s t h e e n c l o s u r e w a l l s e p a r a t i n g t h e l i t t o r a l f r o m t h e l i m n e t i c zone ( t h a t would o c c u r i n t h e e n c l o s u r e s on t h e s o u t h s i d e o f t h e l a k e ) , o r t o w a r d s t h e s i d e o f t h e e n c l o s u r e l e a s t l i k e l y t o c a u s e s h a d i n g ( t h e n o r t h s i d e o f t h e e n c l o s u r e s on t h e n o r t h w e s t s i d e o f t h e l a k e ) . T h i s m i g r a t i o n m i g h t have o c c u r r e d , s i n c e t h e a f t e r n o o n sun was a l m o s t a l w a y s p a r t i a l l y b l o c k e d by m o u n t a i n s and t r e e s l i n i n g t h e l a k e ' s s o u t h e r n and s o u t h w e s t e r n s h o r e . T h i s b l o c k a g e o f t h e sun r e s u l t e d i n shadows f a l l i n g on 1 96 l i t t o r a l e n c l o s u r e s 1 and 3 o v e r most of t h e summer. Such " a v o i d a n c e " m i g r a t i o n s , i f t h e y o c c u r r e d , would l e a d t o b i a s e d e s t i m a t e s of p o p u l a t i o n s i z e b e c a u s e t h e a n i m a l s would be removed from t h e "open w a t e r " o f t h e s e e n c l o s u r e s ( t h e a r e a t h a t was s a m p l e d ) . The . f a c t t h a t d e n s i t i e s o f D. o r e g o n e n s i s i n l i t t o r a l e n c l o s u r e 5 were s i g n i f i c a n t l y h i g h e r t h a n i n l i t t o r a l e n c l o s u r e 1 (ANOVA, F , 1 j 3 3 , = 7.95, p = 0.008) makes t h i s s u r m i s e seem p r o b a b l e s i n c e a n i m a l s i n e n c l o s u r e 5, b e c a u s e o f i t s l o c a t i o n , c o u l d n o t be a f f e c t e d by shadows. OR I n t e n s i t y of T r o u t P r e d a t i o n on Z o o p l a n k t o n I a r g u e d above t h a t f i s h p r e d a t i o n was l i k e l y t o be much more i n t e n s e i n t h e e n c l o s u r e s t h a n i n t h e l a k e , r e s u l t i n g i n some p r o f o u n d c h a n g e s i n t h e d e n s i t i e s o f H o l o p e d i u m and D a p h n i a t h a t c o u l d not be a t t r i b u t e d t o o t h e r f e a t u r e s of t h e e n c l o s u r e s . T h i s s e c t i o n p r e s e n t s t h e b a s i s f o r t h a t argument. As n o t e d e a r l i e r , i n mid t o l a t e J u l y s e v e r a l a d u l t c u t t h r o a t t r o u t were o b s e r v e d s e v e r a l t i m e s o v e r t h e summer ( o b s e r v a t i o n s made from s u r f a c e ) i n s i d e a l l of t h e e n c l o s u r e s . As many a s 5-10 f i s h were o b s e r v e d a t any g i v e n t i m e . B a s e d on a s u r f a c e a r e a o f a p p r o x i m a t e l y 0.015 h e c t a r e , I c o n s e r v a t i v e l y e s t i m a t e f i s h abundance was on t h e o r d e r of 300-600 f i s h / h e c t a r e i n s i d e t h e e n c l o s u r e s . The o n l y a v a i l a b l e p o p u l a t i o n e s t i m a t e f o r f i s h i n t h e l a k e was made by S h e p h e r d ( 1 9 7 3 ) . He e s t i m a t e d t h e p o p u l a t i o n t o be a r o u n d 300 f i s h o r 160 f i s h / h e c t a r e ( b a s e d on a s u r f a c e a r e a of 1.87 h e c t a r e s ) . One c o u l d a r g u e t h a t t h e f i s h p o p u l a t i o n i n P l a c i d Lake i n 1982 was n o t t h e same a s i n 1 97 1973, o r t h a t t h e o r i g i n a l e s t i m a t e may have been t o o low, but t h e r e a r e no d a t a t o s u p p o r t t h e s e o b j e c t i o n s . T h u s , b a s e d on a v a i l a b l e e s t i m a t e s , i t a p p e a r s t h a t f i s h abundance i n s i d e t h e e n c l o s u r e s may have been from 2-6 t i m e s h i g h e r t h a n i n t h e l a k e . The d a t a a r e c o n s i s t e n t w i t h t h e h y p o t h e s i s t h a t f i s h p r e d a t i o n was more i n t e n s e i n t h e e n c l o s u r e s . D e n s i t i e s of H o l o p e d i u m and D a p h n i a i n a l l of t h e c o n t r o l e n c l o s u r e s d e c l i n e d t o much l o w e r l e v e l s t h a n i n t h e l a k e , t o t h e e x t e n t t h a t a d u l t s o f b o t h s p e c i e s were u n d e t e c t a b l e ( o r n e a r l y so) i n t h e e n c l o s u r e s by l a t e A u g u s t ( F i g u r e s 30 and 3 1 ) . A l s o , t h e body s i z e s o f a l l a d u l t (and e g g - b e a r i n g ) d a p h n i d s i n t h e c o n t r o l e n c l o s u r e s , w h i c h r a n g e d between 1.3 and 1.6 mm i n l e n g t h b e f o r e f i s h i n v a s i o n , d e c r e a s e d s h a r p l y t o a r o u n d 1.0-1.1 mm w i t h i n 1-2 weeks a f t e r t h e f i s h were f i r s t d e t e c t e d ( F i g u r e 3 7 ) . S i n c e t h e m a j o r i t y o f t h e t r o u t i n P l a c i d L a k e a r e u s u a l l y f o u n d n e a r t h e l a k e ' s s o u t h s h o r e ( S h e p h e r d , 1973), t h e p r o b a b i l i t y of more f i s h e n c o u n t e r s o c c u r r i n g w i t h i n t h e s o u t h s i d e e n c l o s u r e s (1 and 2, F i g u r e 3) may e x p l a i n why D a p h n i a body s i z e s d e c l i n e d more r a p i d l y i n t h o s e e n c l o s u r e s t h a n i n t h e c o n t r o l e n c l o s u r e s s i t u a t e d on t h e n o r t h w e s t s i d e o f t h e l a k e ( e n c l o s u r e s 5 and 6, F i g u r e 3 ) . D a p h n i a j u v e n i l e s a l s o e x h i b i t e d a s l i g h t d e c r e a s e i n body s i z e as t h e summer p r o g r e s s e d ( F i g u r e 3 7 ) . S i m i l a r r e d u c t i o n s i n t h e s i z e s o f D a p h n i a a l s o o c c u r r e d i n t h e l a k e , however, a mean body s i z e o f between 1.0-1.1 mm was n o t r e a l i z e d u n t i l m i d-September, more t h a n one month l a t e r t h a n i n t h e e n c l o s u r e s ( F i g u r e 3 7 ) . O t h e r w o r k e r s ( G r e e n , 1967; W e l l s , 1970; Warshaw, 1972; N o r t h c o t e and C l a r o t t o , 1975) have o b s e r v e d 198 s i m i l a r r e d u c t i o n s i n o f f s p r i n g and a d u l t s i z e i n z o o p l a n k t o n s p e c i e s e x p o s e d t o f i s h p r e d a t i o n . O v e r s h a d o w i n g t h e a s s u m p t i o n t h a t f i s h p r e d a t i o n was r e s p o n s i b l e f o r t h e r e d u c e d body s i z e s of d a p h n i d s i n t h e c o n t r o l s and t h e l a k e , a r e r e p o r t s t h a t f o o d q u a n t i t y ( H a l l , 1964; B r o m b i l l a , 1980), f o o d q u a l i t y (Taub and D o l l a r , 1969) and wa t e r t e m p e r a t u r e ( C u l v e r , 1980) have a i l been i m p l i c a t e d i n t h e c o n t r o l o f body s i z e i n D a p h n i a . However, w i t h r e s p e c t t o c h a n g e s i n t h e body s i z e s o f d a p h n i d s i n t h i s s t u d y , t h e s e 3 f a c t o r s c an be r u l e d o u t . T h e r e i s no e v i d e n c e t h a t t h e f o o d q u a n t i t y ( A p p e n d i x 3) o r f o o d q u a l i t y ( F i g u r e s 22 and 24) c h a n g e d i n t h e c o n t r o l s compared t o t h e l a k e , p a r t i c u l a r l y o v e r t h e t i m e t h a t t h e s i z e s of d a p h n i d s i n t h e c o n t r o l e n c l o s u r e s d r o p p e d most r a p i d l y ( l a t e J u l y - e a r l y A u g u s t ) . D u r i n g t h i s t i m e t h e d e n s i t y o f g r a z a b l e a l g a e i n a l l c o n t r o l s and t h e l a k e were s i m i l a r . A l s o , t h e r e were no major d i f f e r e n c e s i n t h e s p e c i e s c o m p o s i t i o n o f t h e a l g a e i n t h e c o n t r o l s and t h e l a k e . I f t e m p e r a t u r e was a f a c t o r c o n t r o l l i n g d a p h n i d body s i z e i n t h i s s y s t e m , t h e n d a p h n i d s i z e s i n t h e f a l l s h o u l d have r e t u r n e d t o e a r l y summer l e v e l s i n r e s p o n s e t o d e c r e a s i n g w a t e r t e m p e r a t u r e s ( C u l v e r , 1980). T h i s d i d not o c c u r . T h u s , f i s h p r e d a t i o n i s t h e most l i k e l y mechanism c o n t r o l l i n g t h e abundance and body s i z e s o f t h e two m a j o r p r e y s p e c i e s i n P l a c i d L a k e . T h i s i s r e i n f o r c e d by t h e o b s e r v a t i o n t h a t d u r i n g l a t e summer, when p r e d a t i o n on z o o p l a n k t o n by t h e t r o u t i s most i n t e n s e ( N o r t h c o t e and C l a r o t t o , 1975; N e i l l , 1978), t h e body s i z e s o f t h e d a p h n i d s d e c l i n e d t o t h e i r l o w e s t 1 99 l e v e l s (1.0-1.2 mm). T h i s s u g g e s t s t h a t 1.0 mm may be t h e minimum body s i z e of z o o p l a n k t e r s upon w h i c h t h e c u t t h r o a t t r o u t f e e d . I n d e e d , t h e body s i z e s of Diaphanosoma ( F i g u r e 39) and D. o r e g o n e n s i s ( F i g u r e 4 0 ) , two s p e c i e s w h i c h a r e o n l y r a r e l y e a t e n by t h e t r o u t , s e l d o m e x c e e d t h i s " t h r e s h o l d " s i z e l i m i t . As n o t e d e a r l i e r , s m a l l e r c l a d o c e r a n s were much more abundant i n t h e c o n t r o l e n c l o s u r e s t h a n i n t h e l a k e . T h i s o b s e r v e d s h i f t from a p o p u l a t i o n d o m i n a t e d n u m e r i c a l l y by l a r g e r c l a d o c e r a n s ( i n t h i s c a s e H o l o p e d i u m , D a p h n i a and Diaphanosoma) t o one d o m i n a t e d by s m a l l e r c l a d o c e r a n s i s c h a r a c t e r i s t i c of z o o p l a n k t o n c o m m u n i t i e s s u b j e c t e d t o i n t e n s e p r e d a t i o n by v i s u a l l y f e e d i n g p r e d a t o r s ( L y n c h , 1978, 1980; Z a r e t , l 9 8 0 ) . S i n c e t h e a l g a l f o o d r e s o u r c e s i n t h e c o n t r o l e n c l o s u r e s were not s i g n i f i c a n t l y d i f f e r e n t i n t e r m s of q u a l i t y and q u a n t i t y f r o m t h o s e i n t h e l a k e , d i f f e r e n c e s i n f o o d were not l i k e l y a c a u s a l f-actor i n t h e o b s e r v e d s p e c i e s s h i f t . The a l m o s t c o m p l e t e e r a d i c a t i o n of H o l o p e d i u m from a l l of t h e c o n t r o l e n c l o s u r e s i s a l s o s u g g e s t i v e of more i n t e n s e f i s h p r e d a t i o n o c c u r r i n g w i t h i n t h e e n c l o s u r e s . The s e a s o n a l d e c l i n e i n t h i s s p e c i e s t h a t o c c u r r e d i n b o t h t h e l a k e and t h e c o n t r o l s i s s i m i l a r t o d e c l i n e s n o t e d by S t e n s o n (1973) f o r l a k e s i n Sweden. He f o u n d v e r y s i g n i f i c a n t n e g a t i v e c o r r e l a t i o n s between i n c r e a s e d f i s h a c t i v i t y , a bundance and numbers o f f i s h f r y and H o l o p e d i u m d e n s i t y . Not one o f s e v e r a l p h y s i c a l o r c h e m i c a l v a r i a b l e s t e s t e d were c o r r e l a t e d w i t h d e c l i n e s i n H o l o p e d i u m . 200 4.2 E f f e c t s o f F e r t i l i z a t i o n The p r i n c i p a l p u r p o s e of t h i s s e c t i o n i s t o e s t a b l i s h t h e e f f e c t s o f f r e q u e n t a d d i t i o n s o f i n o r g a n i c n u t r i e n t s c o n t a i n i n g N and P i n an a t o m i c r a t i o o f 40:1 (21:1 by w e i g h t ) on t h e a b i o t i c and b i o t i c c h a r a c t e r i s t i c s o f P l a c i d L a k e . 4.2.j_ On Water C h e m i s t r y From t h e d a t a i n S e c t i o n 3.2.2 i t i s q u i t e a p p a r e n t t h a t t h e c o n c e n t r a t i o n s of a l l of t h e n u t r i e n t s p e c i e s examined i n c r e a s e d i n t h e f e r t i l i z e d e n c l o s u r e s compared t o t h e c o n t r o l s . Some of t h e o b s e r v e d c h a n g e s were v e r y i n t e r e s t i n g , p a r t i c u l a r l y w i t h r e s p e c t t o i n o r g a n i c N and P. (1) P h o s p h o r o u s ( N H „ ) 2 H P O a was a d d e d i n t h r e e e q u a l d o s e s (3.3 u g P / l i t e r of e p i l i m n i o n ) e a c h week f o r 16 weeks t o t h e f e r t i l i z e d e n c l o s u r e s so as t o r a i s e t h e a l l o c h t h o n o u s p h o s p h a t e i n p u t by 10 u g P / l i t e r o f e p i l i m n i o n p e r week. A d d i t i o n s o f t h i s m a g n i t u d e and f r e q u e n c y r e s u l t e d i n d e t e c t a b l e l e v e l s o f o r t h o - P o c c u r r i n g i n a l l o f t h e f e r t i l i z e d e n c l o s u r e s , a t a l l d e p t h s , u n t i l e a r l y A u g u s t ( F i g u r e 8 ) . E a r l y summer c o n c e n t r a t i o n s o f o r t h o - P i n t h e s e e n c l o s u r e s r e a c h e d as h i g h a s 10 u g / l i t e r , t h e amount o f P b e i n g added on a w e e k l y b a s i s . T h i s i s i n d i r e c t c o n t r a s t t o o b s e r v a t i o n s made i n o t h e r l a k e f e r t i l i z a t i o n s t u d i e s ( C o f f i n e t a l . , 1949; R i g l e r , 1956; S c h i n d l e r e t a l . , 1971,1973; S t o c k n e r e t a l . , 1980), where s i m i l a r o r h i g h e r p h o s p h a t e l o a d i n g r a t e s 201 were a p p l i e d t o whole l a k e s but e p i l i m n e t i c c o n c e n t r a t i o n s o f o r t h o - P were f o u n d t o be u n d e t e c t a b l e w i t h i n 1-2 h o u r s a f t e r f e r t i l i z a t i o n . Why d i d s u c h r a p i d d e p l e t i o n of t h e added p h o s p h a t e not o c c u r i n my e n c l o s u r e s ? I p r o p o s e 4 p o s s i b l e mechanisms w h i c h may have i n t e r a c t e d t o p r o d u c e t h i s a p p a r e n t l a c k of P u t i l i z a t i o n . T h e r e may be o t h e r s , but I f e e l t h e s e a r e t h e most i m p o r t a n t . 1) The f i r s t few f e r t i l i z a t i o n e v e n t s may have s a t u r a t e d t h e c e l l p h o s p h a t e q u o t a r e q u i r e m e n t o f t h e e x i s t i n g p h y t o p l a n k t o n s u c h t h a t f r e q u e n t s u b s e q u e n t a d d i t i o n s may n o t have been t a k e n up so r a p i d l y as i f t h e p h y t o p l a n k t o n had been i n a P - s t a r v e d s t a t e . 2 ) The g r o w t h o f t h e p h y t o p l a n k t o n i n P l a c i d Ltake may n o t have been l i m i t e d by p h o s p h o r o u s t h e r e f o r e , p h o s p h o r o u s demand was low. 3) U n t i l l a t e summer, b i o m a s s g r a z e r l i m i t e d ) a l g a l f orms s m a l l e r forms m i g h t have been t h e added n u t r i e n t . o f l a r g e r (and p e r h a p s n o t was low and g r a z e r l i m i t e d u n a b l e t o t a k e up a l l o f 4) The a d d e d p h o s p h o r o u s may have been r a p i d l y a s s i m i l a t e d by t h e p h y t o p l a n k t o n but z o o p l a n k t o n g r a z i n g p r e s s u r e c o u l d have been so i n t e n s e t h a t a l l o f t h e e n s u i n g p h y t o p l a n k t o n p r o d u c t i o n c o u l d be c r o p p e d and t h e 202 n u t r i e n t s r e c y c l e d by t h e z o o p l a n k t o n a t a r a t e e q u a l t o or g r e a t e r t h a n p h y t o p l a n k t o n u p t a k e r a t e s . I w i l l d i s c u s s e a c h p o s s i b i l i t y i n t h e o r d e r p r e s e n t e d a b o v e . The n u t r i e n t a d d i t i o n e x p e r i m e n t s c i t e d above i n v o l v e d e i t h e r o n e - t i m e a d d i t i o n s of p h o s p h a t e ( C o f f i n e t a l . 1949; R i g l e r , 1956) or once w e e k l y a d d i t i o n s of N and P o v e r 15-20 weeks d u r i n g t h e summer ( S c h i n d l e r e t a_l. , 1971,1973; S t o c k n e r e t a l . , 1980) t o p h o s p h o r o u s - l i m i t e d p h y t o p l a n k t o n c o m m u n i t i e s . P - u p t a k e by p h o s p h o r o u s s t a r v e d a l g a e i s r a p i d and i s a f u n c t i o n of b o t h i n t e r n a l and e x t e r n a l s u b s t r a t e c o n c e n t r a t i o n s (Rhee, 1980). When t h e i n t e r n a l c o n c e n t r a t i o n of P i s low, u p t a k e r a t e i s h i g h and v i c e v e r s a . T h e r e f o r e , p h o s p h o r o u s s t a r v e d a l g a e w i l l have e x t r e m e l y r a p i d P - u p t a k e r a t e s . J e a n j e a n e t a l . (1970, c i t e d i n Halman and S t i l l e r , 1974) r e p o r t e d maximum P-uptake r a t e s , f o r v e r y d e n s e p o p u l a t i o n s of P - s t a r v e d a l g a e , of 2.17 X 10 6 u g / l i t e r / h o u r . More c o n s e r v a t i v e e s t i m a t e s o f P-uptake by p h o s p h o r o u s s t a r v e d l a k e p h y t o p l a n k t o n p o p u l a t i o n s r a n g e from 0-30 u g / l i t e r / h o u r (Halman and S t i l l e r , 1974; Lean and P i c k , 1981) a t e x t e r n a l P c o n c e n t r a t i o n s r a n g i n g f r o m 0-50 u g / l i t e r P ( f o r a l g a l b i o m a s s e s o f 5000 u g / l i t e r i n t h e f i r s t s t u d y and 300 u g / l i t e r i n t h e l a t t e r ) . V i r t u a l l y a l l measurements of P-uptake by p h y t o p l a n k t o n a r i s e f r o m s t u d i e s of t h e r e s p o n s e of P - s t a r v e d a l g a e t o a s i n g l e d ose of n u t r i e n t . I have not f o u n d one r e p o r t t h a t a d d r e s s e s t h e q u e s t i o n of how p h y t o p l a n k t o n would r e s p o n d t o s u b s e q u e n t , r e p e a t e d a d d i t i o n s o f P. However, i t i s p o s s i b l e t o 203 v i s u a l i z e a s c e n a r i o i n w h i c h one dose o f p h o s p h a t e may be enough t o s a t u r a t e t h e P-debt ( n u t r i e n t r e q u i r e m e n t ) o f a p h o s p h o r o u s d e f i c i e n t a l g a l community, t h e r e b y r e d u c i n g t h e demand f o r , and t h e r a t e o f u p t a k e o f , t h a t n u t r i e n t , t o t h e d e g r e e t h a t s u b s e q u e n t a d d i t i o n s o f t h a t n u t r i e n t w o u l d be t a k e n up v e r y s l o w l y o r n o t a t a l l ( P a u l J . H a r r i s o n , p e r s o n a l c o m m u n i c a t i o n ) . B e c a u s e p h o t o s y n t h e s i s and c e l l g r o w t h a r e s u p p r e s s e d when a l g a e a r e r e p l e n i s h i n g n u t r i e n t s t o r e s (Lean and P i c k , 1981), c e l l d i v i s i o n may n o t o c c u r f o r s e v e r a l d a y s ( G e r h a r t and L i k e n s , 1975). S i n c e I f e r t i l i z e d e v e r y 48 h o u r s , i n t h e t i m e f o l l o w i n g a n u t r i e n t a d d i t i o n , a l g a l P - d e b t s may have been s a t u r a t e d ( a s s u m i n g t h e y were d e f i c i e n t i n p h o s p h a t e t o b e g i n w i t h ) and a l g a l b i o m a s s may n o t have y e t begun t o i n c r e a s e . T h e r e f o r e , a d d i t i o n a l p h o s p h a t e would have been more or l e s s s u r p l u s t o a l g a l r e q u i r e m e n t s . Weekly f e r t i l i z a t i o n s g i v e a l g a e more t i m e t o a s s i m i l a t e t h e added n u t r i e n t and r e t u r n a P - s t a r v e d s t a t e , so t h a t t h e n e x t n u t r i e n t p u l s e w o u l d be more l i k e l y t o be a s s i m i l a t e d i m m e d i a t e l y a f t e r i t was made a v a i l a b l e . I t c o u l d be a r g u e d t h a t i n t h e s t u d i e s o f S c h i n d l e r e_t a l . (1971, 1973) and S t o c k n e r e t . a l . (1980) p h y t o p l a n k t o n b i o m a s s l e v e l s were h i g h e r so t h e r e were more a l g a e a v a i l a b l e t o t a k e up t h e a d d e d n u t r i e n t s , but t h i s was n o t t h e c a s e . T o t a l c h l o r o p h y l l v a l u e s ( u n c o r r e c t e d f o r p h a e o p h y t i n ) i n t h e SEP l a k e s p r i o r t o f e r t i l i z a t i o n , a v e r a g e d 0.88 u g / l i t e r ( S t e p h e n s and S t o c k n e r , 1983). L e v e l s i n L a k e 227 were s l i g h t l y h i g h e r , 1-5 u g / l i t e r ( S c h i n d l e r , 1973). T h e s e v a l u e s a r e v e r y c l o s e t o 204 t h e v a l u e s o b s e r v e d i n P l a c i d Lake i n 1982 (0.96 u g / l i t e r , t h i s s t u d y , T a b l e 6 ) . I t s h o u l d be p o i n t e d o u t t h a t i n b o t h of t h e s e s t u d i e s , t h e n u t r i e n t s , p e r a d d i t i o n , were added i n amounts 3 t i m e s (10 u g / l i t e r P) g r e a t e r t h a n I added t o my e n c l o s u r e s a t any one t i m e (3.3 u g / l i t e r P ) , y e t t h e p h o s p h a t e was s t i l l d e p l e t e d w i t h i n one h o u r . Above, I a l l u d e d t o t h e p o s s i b l i t y t h a t t h e g r o w t h r a t e of a l g a e i n P l a c i d L a k e may not have been l i m i t e d a t a l l by p h o s p h o r u s , hence t h e demand f o r t h i s n u t r i e n t was low and t h e added n u t r i e n t was n o t b e i n g r a p i d l y a s s i m i l a t e d . I w i l l now a d d r e s s t h i s f u r t h e r . N u t r i e n t l i m i t a t i o n can be i n f e r r e d from n i t r o g e n : p h o s p h o r o u s r a t i o s i n p h y t o p l a n k t o n . H e a l e y (1975) and H e a l e y and H e n d z e l (1980) s u g g e s t e d t h a t N:P r a t i o s > 10 i m p l y p h o s p h o r o u s l i m i t a t i o n . The a v e r a g e summer N:P r a t i o of t h e a l g a e i n P l a c i d L a k e , i n 1982, was 9, w h i c h i s i n d i c a t i v e of n i t r o g e n r a t h e r t h a n p h o s p h o r o u s l i m i t a t i o n ( H e a l e y and H e n d z e l , 1980). S i n c e t h e r a t e of P -uptake i s a s a t u r a t i n g f u n c t i o n of b o t h i n t e r n a l and e x t e r n a l s u b s t r a t e c o n c e n t r a t i o n s ( H e a l e y , 1973; Rhee, 1980), t h i s means t h a t P - u p t a k e r a t e s by a l g a e w i t h low N:P r a t i o s w i l l be s l o w e r t h a n i f t h e a l g a e were p h o s p h o r o u s l i m i t e d (N:P > 10), and g r o w t h r a t e w i l l be u n a f f e c t e d by e x t e r n a l P c o n c e n t r a t i o n . B i o m a s s e s o f a l g a e s i m i l a r t o t h o s e f o u n d i n t h e f e r t i l i z e d e n c l o s u r e s i n t h e e a r l y summer (200-300 u g / l i t e r d r y w e i g h t ) , i n a s y s t e m where P was known t o be t h e l i m i t i n g n u t r i e n t ( K o o t e n e y L a k e , B . C ) , were f o u n d t o t a k e up p h o s p h a t e a t a r a t e of 0.20 u g / l i t e r / h r a t an e x t e r n a l P c o n c e n t r a t i o n o f a r o u n d 3 u g / l i t e r 205 (Lean and P i c k , 1981), t h e c o n c e n t r a t i o n I was a d d i n g a t e a c h a d d i t i o n . I f t h e s e c o n d i t i o n s p r e v a i l e d i n my e n c l o s u r e s , t h i s l e v e l of a l g a l b i o m a s s , i f i t was P - l i m i t e d , s h o u l d have d e p l e t e d a t l e a s t 1.0 ug o f t h e added P w i t h i n 5 h r s . In f a c t , t h e c o n c e n t r a t i o n s of i n o r g a n i c p h o s p h o r o u s i n my f e r t i l i z e d e n c l o s u r e s showed no a p p a r e n t d e c l i n e up t o 6 h o u r s a f t e r t h e n u t r i e n t was adde d . P h o s p h a t e c o n c e n t r a t i o n s were f o u n d t o be much h i g h e r (6-10 u g / l i t e r ) a f t e r a d d i t i o n t h a n what was b e i n g added (3.3 u g / l i t e r ) , s u g g e s t i n g t h a t even v e r y l i t t l e P from t h e p r e v i o u s n u t r i e n t a d d i t i o n had been t a k e n up. T h i s i s c o n s i s t e n t w i t h t h e above s u g g e s t i o n t h a t p h o s p h a t e was not i n g r e a t demand i n t h i s s y s t e m . I t a p p e a r s t h e n , t h a t t h e low s t a n d i n g s t o c k of a l g a e i n P l a c i d L a k e may not be a r e s u l t o f a l a c k of p h o s p h a t e a s we have b e l i e v e d . R a t h e r , t h e r e may be enough p h o s p h a t e a v a i l a b l e t o s u p p o r t maximum g r o w t h r a t e s o f t h e a l g a e a t e x i s t i n g , o r even s l i g h t l y h i g h e r b i o m a s s l e v e l s . T h e r e i s e v i d e n c e t o s u p p o r t t h i s h y p o t h e s i s . Z o o p l a n k t o n r e m o v a l e x p e r i m e n t s , p e r f o r m e d i n e n c l o s u r e s i n P l a c i d L a k e , r e v e a l e d t h a t a l g a l b i o m a s s i n c r e a s e d by as much as 50 % i n e n c l o s u r e s w i t h o u t z o o p l a n k t o n o v e r t h o s e w i t h z o o p l a n k t o n ( K r a u s e , 1984). ( I t s h o u l d be m e n t i o n e d t h a t t h e s e e n c l o s u r e s were d o s e d w i t h s m a l l q u a n t i t i e s o f f e r t i l i z e r but t h e y were o n l y s u f f i c i e n t t o r e p l a c e t h e p h o s p h a t e t h a t would n o r m a l l y be r e c y c l e d by t h e z o o p l a n k t o n ) . The l a t t e r p o i n t , t h a t z o o p l a n k t o n r e m o v a l e x p e r i m e n t s r e s u l t e d i n i n c r e a s e d a l g a l b i o m a s s , s u g g e s t s a t h i r d mechanism, 206 z o o p l a n k t o n g r a z i n g , w h i c h may p a r t i a l l y e x p l a i n t h e a p p a r e n t l a c k of P- u p t a k e by t h e a l g a e . The p h y t o p l a n k t o n t h a t d o m i n a t e d t h e a l g a l b i o m a s s a t t h e o n s e t o f f e r t i l i z a t i o n were s m a l l (<20 um), e d i b l e a l g a l f o r m s , making up as much as 70-90 % of t h e b i o m a s s , d e p e n d i n g on t h e e n c l o s u r e ( F i g u r e 2 6 ) . L a r g e , u n g r a z a b l e a l g a e were r e l a t i v e l y s c a r c e . T h e s e s m a l l a l g a e must have been g r o w i n g and d i v i d i n g r a p i d l y f o l l o w i n g f e r t i l i z a t i o n a s e v i d e n c e d by t h e e x p o n e n t i a l i n c r e a s e i n t h e numbers of D a p h n i a r o s e a , t h e j u v e n i l e s and a d u l t s o f w h i c h f e e d on a l g a e <20 um i n s i z e ( B u r n s , 1968a; N e i l l , 1975; K r a u s e , 1984). The e n s u i n g g r a z i n g r a t e must have been s u c h t h a t 100 % o f t h e new a l g a e p r o d u c e d , was b e i n g c r o p p e d , s i n c e n e i t h e r a l g a l b i o m a s s , nor t h e p e r c e n t c o m p o s i t i o n of t h e s e s m a l l a l g a l f orms i n t h e t o t a l b i o m a s s , i n c r e a s e d . Thus, z o o p l a n k t o n a p p e a r e d t o be r e m o v i n g any new c e l l s w h i c h would o t h e r w i s e be t a k i n g up t h e added P. I t may be t h e n , t h a t t h e s m a l l b i o m a s s of a l g a e r e m a i n i n g i n t h e s y s t e m was u n a b l e t o t a k e up a l l of t h e add e d P. T h e r e i s a weakness t o t h i s argument however. I f t h e a l g a e were n o t P - l i m i t e d , a s s u g g e s t e d by N:P r a t i o s , t h e n a d d i n g P s h o u l d have had no e f f e c t on a l g a l g r o w t h r a t e s . T h e r e f o r e , i n o r d e r t o i n c r e a s e c e l l d i v i s i o n r a t e s under t h e s e c o n d i t i o n s , some o t h e r n u t r i e n t i n t h e f e r t i l i z e r must have been l i m i t i n g c e l l d i v i s i o n r a t e s . The N:P r a t i o s s u g g e s t t h a t n i t r o g e n was t h e l i m i t i n g n u t r i e n t . T h i s w i l l be d i s c u s s e d l a t e r . P e r h a p s z o o p l a n k t o n g r a z i n g and t h e i r s u b s e q u e n t r e c y c l i n g o f n u t r i e n t i n tandem were r e s p o n s i b l e f o r t h e o b s e r v e d p h o s p h a t e p o o l . T h e r e i s no doubt t h a t z o o p l a n k t o n 207 r e m i n e r a l i z a t i o n of P would c o n t r i b u t e some of t h e P t h a t was m e a s u r e d . However, i t does not seem l i k e l y , g i v e n p u b l i s h e d z o o p l a n k t o n r e m i n e r a l i z a t i o n r a t e s , t h a t t h e amount of P m easured i n t h e f e r t i l i z e d e n c l o s u r e s c o u l d be w h o l l y a t t r i b u t e d t o z o o p l a n k t o n e x c r e t i o n . P e t e r s and R i g l e r (1973) d e t e r m i n e d t h a t o n l y an e s t i m a t e d 14.8 % o f t h e t o t a l P of t h e t r o p h o g e n i c zone o f e u t r o p h i c H e a r t L a k e , O n t a r i o , was r e g e n e r a t e d d a i l y , d i r e c t l y o r i n d i r e c t l y , by z o o p l a n k t o n g r a z e r s . The g r e a t e s t p o r t i o n o f t h e i n g e s t e d P was c o n v e r t e d i n t o z o o p l a n k t o n b i o m a s s . I c a l c u l a t e d z o o p l a n k t o n e x c r e t i o n r a t e s f o r t h e a n i m a l s i n t h e f e r t i l i z e d e n c l o s u r e s u s i n g t h e e q u a t i o n f o r e x c r e t i o n r a t e "E" ( u g / l i t e r / h o u r ) d e r i v e d by P e t e r s and R i g l e r ( 1 9 7 3 ) . The e q u a t i o n i s E = 0.0286 e ( 0 - 0 3 7 8 T + ° - ° 0 0 0 1 C " 3 ' 3 4 P ) KW" 0- 3 8 3 ( 1 ) where T i s t e m p e r a t u r e i n d e g r e e s C e l s i u s , C i s f o o d c o n c e n t r a t i o n i n c e l l s / m l , P i s p h o s p h o r o u s c o n c e n t r a t i o n i n t h e f o o d i n ug/ml and W i s d r y w e i g h t o f z o o p l a n k t o n i n mg. T h i s e q u a t i o n was d e r i v e d f o r e x c r e t i o n by D a p h n i a and i s v a l i d i n t h i s c a s e b e c a u s e D a p h n i a made up t h e g r e a t e s t p r o p o r t i o n of t h e b i o m a s s i n t h e f e r t i l i z e d e n c l o s u r e s a t t h i s t i m e . The mean s u r f a c e t e m p e r a t u r e f o r J u l y , 1982 i n P l a c i d Lake was 18.5 °C. The a v e r a g e c o n c e n t r a t i o n o f f o o d i n t h e e p i l i m n i o n o f t h e f e r t i l i z e d e n c l o s u r e s was n o t e a s y t o d e t e r m i n e b e c a u s e z o o p l a n k t o n g r a z i n g w a s'very i n t e n s e , but a 208 c o n s e r v a t i v e measure would be 1 X 10" c e l l s / m l . P a r t i c u l a t e P measurements i n t h e e p i l i m n i o n o f t h e e n c l o s u r e s a v e r a g e d 9-15 u g / l i t e r . Not a l l o f t h i s was p h y t o p l a n k t o n , but t o be c o n s e r v a t i v e , I c h o s e a v a l u e o f 10 u g / l i t e r P (0.01 ug/ml P ) . The a v e r a g e d r y w e i g h t of z o o p l a n k t o n i n t h e e n c l o s u r e s was a r o u n d 0.250 m g / l i t e r . U s i n g t h e s e v a l u e s , E = 0.105 u g / l i t e r / h o u r , w h i c h was r e m a r k a b l y c l o s e t o t h e r a t e of p h o s p h a t e r e g e n e r a t i o n by D a p h n i a r o s e a (0.08 u g / l i t e r / h o u r ) , m e a sured by P e t e r s and Lean ( 1 9 7 3 ) , and by a n a t u r a l l a k e z o o p l a n k t o n community (0.076 - 0.49 u g / l i t e r / h o u r ) measured by Lehman ( 1 9 8 0 ) . My v a l u e may have been s l i g h t l y o v e r e s t i m a t e d b e c a u s e I was l i b e r a l i n my d e t e r m i n a t i o n s o f f o o d a v a i l a b i l i t y and i t s P c o n t e n t . The P - u p t a k e r a t e o f 0.20 u g / l i t e r / h o u r f o r t h e P - s t a r v e d K o o t e n e y L a k e p h y t o p l a n k t o n b i o m a s s l e v e l s ( m e n t i o n e d a b o v e ) , s u g g e s t s t h a t , a t t h a t l e v e l o f b i o m a s s , t h e a l g a e i n my e n c l o s u r e s c o u l d be t a k i n g up P a t more t h a n t w i c e t h e r a t e t h a n i t c o u l d be s u p p l i e d by t h e z o o p l a n k t o n . Thus, w h i l e t h e z o o p l a n k t o n i n my f e r t i l i z e d e n c l o s u r e s may have c o n t r i b u t e d a s i g n i f i c a n t p r o p o r t i o n o f t h e P made a v a i l a b l e f o r p h y t o p l a n k t o n g r o w t h , i t may have been t a k e n up i m m e d i a t e l y t h e r e b y making i t u n d e t e c t a b l e i n t h e water c o l u m n . T h e r e f o r e , i t i s u n l i k e l y t h a t z o o p l a n k t o n were r e s p o n s i b l e f o r t h e d e t e c t a b l e p h o s p h a t e p o o l i n t h e f e r t i l i z e d e n c l o s u r e s e x c e p t i n so f a r as t h e y l i m i t e d p h y t o p l a n k t o n b i o m a s s i n c r e a s e s and hence t h e a l g a l b i o m a s s a v a i l a b l e t o t a k e up and h o l d P i n p a r t i c u l a t e form. The e v i d e n c e p r o v i d e d t h u s f a r s u g g e s t s t h a t , a t t h e 209 b i o m a s s l e v e l s of a l g a e p r e s e n t i n P l a c i d Lake i n e a r l y summer, p h o s p h o r o u s was n o t i m p o s i n g m a j o r l i m i t a t i o n s on a l g a l g r o w t h r a t e s or abundance. Added p h o s p h a t e was not u s e d up as q u i c k l y as w o u l d be e x p e c t e d f o r a P - s t a r v e d a l g a l community. C o u l d i t be t h a t t h e s p r i n g - e a r l y summer g r o w t h o f p h y t o p l a n k t o n i n t h i s l a k e i s l i m i t e d more by t h e a v a i l a b i l i t y of n i t r o g e n r a t h e r t h a n p h o s p h o r o u s ? The low r a t i o s o f TN:TP s u g g e s t t h a t t h i s may be s o . (2) N i t r o g e n and P o s s i b l e N i t r o g e n L i m i t a t i o n s on P h y t o p l a n k t o n Growth T h e r e i s c i r c u m s t a n t i a l e v i d e n c e t h a t i n o r g a n i c n i t r o g e n may be i n s h o r t s u p p l y d u r i n g t h e s p r i n g and e a r l y summer i n one of t h e o t h e r U.B.C. r e s e a r c h f o r e s t l a k e s . Dickman (1968) f o u n d t h a t t h e a d d i t i o n o f n i t r a t e n i t r o g e n t o e n c l o s u r e s i n M a r i o n Lake i n d u c e d s i g n i f i c a n t l y h i g h e r s p r i n g and e a r l y summer p r i m a r y p r o d u c t i o n v a l u e s t h a n d i d p h o s p h a t e . F u r t h e r , added n i t r o g e n d i s a p p e a r e d 4 t i m e s f a s t e r (20 u g / l i t e r / d a y ) t h a n p h o s p h a t e (5 u g / l i t e r / d a y ) (Dickman, 1968 - F i g u r e 4 ) . T h i s s u g g e s t s t h a t i n o r g a n i c n i t r o g e n may have been i n g r e a t e r demand. I n o r g a n i c n i t r o g e n i n t h e f o r m o f N H „ + and N0 3~ d i d not d i s a p p e a r r a p i d l y from my e n c l o s u r e s f o l l o w i n g a d d i t i o n ( F i g u r e s 11 and 12), e x c e p t i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s . C o n c e n t r a t i o n s of b o t h N H „ + and N0 3~ were as h i g h ( o r h i g h e r , i n t h e c a s e o f N0 3~) 4-6 h r s a f t e r f e r t i l i z a t i o n a s t h e c o n c e n t r a t i o n of n u t r i e n t a d d e d . T h i s does n o t mean t h a t 210 n i t r o g e n was not i n demand. Maximum N H „ + and N0 3~ u p t a k e r a t e s f o r . p h y t o p l a n k t o n ( i n b o t h f r e s h w a t e r o l i g o t r o p h i c - N - 1 i m i t e d and e u t r o p h i c - N - s u f f i c i e n t s y s t e m s , and i n t h e s e a ) , a r e commonly below 0.2 u g / l i t e r / h o u r ( D u g d a l e and G o e r i n g , 1967; T o e t z e_t a l . , 1973; Murphy, 1980; Berman e t a l . , 1984). At t h e s e r a t e s , i t w ould be i m p o s s i b l e t o measure any g r e a t d e c l i n e s i n n u t r i e n t a d d i t i o n s t h a t measured 75 and 60 u g / l i t e r ( N H „ + and N0 3~, r e s p e c t i v e l y ) i n t h e 4-6 h o u r s a f t e r f e r t i l i z a t i o n when n u t r i e n t s a m p l e s were t a k e n . I t would have t a k e n 5 h o u r s f o r t h e p h y t o p l a n k t o n t o d e p l e t e o n l y 1 ug of t h e added n u t r i e n t , a s s u m i n g maximum u p t a k e was o c c u r r i n g . T h i s slow r a t e o f N-u p t a k e , w h i c h o c c u r s a s a s a t u r a t i n g f u n c t i o n of N c o n c e n t r a t i o n , may e x p l a i n t h e a p p a r e n t l a g p e r i o d s i n a l g a l b i o m a s s i n c r e a s e s f o l l o w i n g combined N and P a d d i t i o n s i n t h e w a t e r s of U.B.C. R e s e a r c h f o r e s t l a k e s (Dickman, 1968; W a l t e r s e t a_l. , i n p r e s s ; t h i s s t u d y ) and i n t h e a p p a r e n t l a c k o f r e s p o n s e o f a l g a e t o a d d i t i o n s of o n l y N i n s h o r t t e r m e n c l o s u r e s t u d i e s i n t h e same w a t e r s (W.E. N e i l l , p e r s o n a l c o m m u n i c a t i o n ) . I f t h e g r o w t h o f an a l g a l p o p u l a t i o n was l i m i t e d by t h e a v a i l a b i l i t y of i n o r g a n i c n i t r o g e n and t h e u p t a k e of added n i t r o g e n was v e r y slow d e s p i t e i t s i n c r e a s e d a v a i l a b i l i t y , t h e n one w o u l d e x p e c t a l g a l b i o m a s s t o i n c r e a s e o n l y a f t e r s u f f i c i e n t N had been t a k e n up so as t o a l l o w f o r g r o w t h and d i v i s i o n . M o r e o v e r , t h e N:P r a t i o a t w h i c h o p t i m a l c e l l u l a r g r o w t h o c c u r s a p p e a r s t o be s p e c i e s s p e c i f i c (Rhee and Gotham, 1980). Thus, i n t h e same n u t r i e n t . e n v i r o n m e n t , t h e g r o w t h o f some s p e c i e s may 21 1 be more r a p i d t han f o r o t h e r s . A s s u m i n g t h a t t h e e x t r a c e l l u l a r N:P r a t i o s f a v o u r e d maximum g r o w t h of a s p e c i e s t h a t was i n i t i a l l y f o u n d i n v e r y low numbers i n t h e p h y t o p l a n k t o n soup, t h e n r a p i d a l g a l b i o m a s s i n c r e a s e s m i g h t not o c c u r u n t i l t h i s ( o r t h e s e ) s p e c i e s r e a c h e d numbers s u f f i c i e n t t o g e n e r a t e a "bloom". Thus, w h i l e I do n o t have measurements of t h e k i n e t i c s of n i t r o g e n u p t a k e i n my s y s t e m , t h e low c o n c e n t r a t i o n s of i n o r g a n i c N, t h e a p p a r e n t slow r a t e o f P - u p t a k e , e v i d e n c e t h a t a d d i t i o n o f n i t r a t e s t i m u l a t e d g r e a t e r p r i m a r y p r o d u c t i v i t y t h a n t h e a d d i t i o n of o n l y p h o s p h a t e i n a n e a r b y l a k e (Dickman, 1968) and t h e low r a t i o s o f c e l l u l a r N:P (PN:PP), a l l s u g g e s t t h a t i n t h e e a r l y summer, t h e p r i m a r y p r o d u c t i v i t y of P l a c i d Lake may be r e s t r i c t e d by n i t r o g e n d e f i c i e n c y . How w o u l d N r a t h e r t h a n P l i m i t a t i o n a r i s e i n a l a k e i n a l i m n o l o g i c a l r e g i o n i n w h i c h t h e a v e r a g e TN:TP r a t i o o f l a k e s i s 89:1 ( S t o c k n e r and S h o r t r e e d , 1985), a c l e a r i n d i c a t i o n of P l i m i t a t i o n ? The TN:TP r a t i o of n e a r b y G w e n d o l i n e L a k e , o n l y 500 m t o t h e n o r t h w e s t a v e r a g e s 30:1 (W.E. N e i l l , p e r s o n a l c o m m u n i c a t i o n ) . I b e l i e v e t h a t t h e answer l i e s i n t h e f a c t t h a t Sphagnum p e a t mats d o m i n a t e t h e l i t t o r a l v e g e t a t i o n of P l a c i d L a k e . In l i g h t of t h e h i g h c a t i o n e x c h a n g e c a p a c i t y of Sphagnum (Clymo, 1963), t h e ammonium c a t i o n ( N H „ + ) s h o u l d have a p r o p e n s i t y t o w a r d s b e i n g i n c o r p o r a t e d i n t o t h e p e a t . The i o n i c c o m p o s i t i o n o f t h e w a t e r s i n t h e v i c i n i t y of t h e Sphagnum mats t h e n , s h o u l d be one t h a t i s low i n N H a + . T h i s l o s s f a c t o r was 212 e v i d e n t i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s , where NH„ + c o n c e n t r a t i o n s were 3-5 t i m e s l o w e r t h a n i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s ( F i g u r e 12), w h i c h d i d n o t have a water Sphagnum i n t e r f a c e . T h e r e may a l s o have been g r e a t e r n e t l o s s p r o c e s s e s of N H „ + f l u x t o t h e e p i l i m n e t i c s e d i m e n t - w a t e r i n t e r f a c e i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s s i n c e ammonia i s known t o be s t r o n g l y s o r b e d t o p a r t i c u l a t e and c o l l o i d a l p a r t i c l e s ( W e t z e l , 1975) and t o c l a y s and o r g a n i c c o l l o i d s i n s e d i m e n t s ( T o e t z , 1970; B r e z o n i k , 1972; Keeney e t a l . , 1973; R o s e n f e l d , 1979) and t h i s s o r p t i o n can be r a p i d ( B r o o k s , 1969). B a c t e r i a i n v o l v e d i n t h e n i t r o g e n c y c l e , b o t h a e r o b i c n i t r o g e n f i x i n g and n i t r i f y i n g b a c t e r i a a r e v i r t u a l l y a b s e n t f r o m p e a t b e c a u s e of e x t r e m e p h y s i c a l c o n d i t i o n s o f h i g h pH and low 0 2 (Moore and B e l l a m y , 1974). T h i s i s of c o n s i d e r a b l e i m p o r t a n c e b e c a u s e any N H „ + p r e s e n t i n o r a r o u n d t h e p e a t i s u n l i k e l y t o b e - r e c y c l e d as N0 3~ i n t h e a b s e n c e of n i t r i f y i n g b a c t e r i a . T h u s , s u c h n u t r i e n t s a r e e f f e c t i v e l y removed from c i r c u l a t i o n . Any n i t r a t e p r o d u c e d under t h e s e c o n d i t i o n s i s p r o b a b l y u t i l i z e d as r a p i d l y as i t i s p r o d u c e d . T h i s would e x p l a i n why [ N 0 3 ~ ] was below t h e l i m i t o f d e t e c t i o n (<20 u g / l i t e r ) a t a l l t i m e s i n t h e l a k e ( F i g u r e 1 1 ) . T h i s m ight a l s o e x p l a i n t h e l o w e r e p i l i m n e t i c N0 3" c o n c e n t r a t i o n s i n t h e l i t t o r a l compared t o t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . An a b s e n c e of n i t r i f y i n g b a c t e r i a due t o t h e p r e s e n c e o f t h e Sphagnum would mean t h a t much of t h e added ammonium would not be o x i d i z e d t o n i t r a t e . C o n v e r s e l y , t h e l a c k o f a water - Sphagnum i n t e r f a c e i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s means t h a t 213 n i t r i f y i n g b a c t e r i a may have been more abundant i n t h e open w a t e r , c o n v e r t i n g much o f t h e added N H „ + t o N0 3~, hence , t h e a p p a r e n t a c c u m u l a t i o n of N0 3~ ( F i g u r e 11). (3) pH change and Ammonium Nitrate-Ammonium P h o s p h a t e A d d i t i o n s The l a c k of a s i g n i f i c a n t change i n t h e h y d r o g e n i o n c o n t e n t o f t h e e p i l i m n e t i c w a t e r s i n t h e f e r t i l i z e d e n c l o s u r e s compared t o t h e c o n t r o l s ( F i g u r e 15) i s i n t e r e s t i n g and u n e x p e c t e d . N e l s o n and Edmondson ( 1 9 5 5 ) , S m i t h (1967) and S c h i n d l e r e t a l . (1971, 1973) a l l n o t e d s i g n i f i c a n t i n c r e a s e s i n e p i l i m n e t i c pH v a l u e s f o l l o w i n g f e r t i l i z a t i o n . In a l l of t h e s e s t u d i e s , b i o l o g i c a l l y m e d i a t e d p h o t o s y n t h e t i c u t i l i z a t i o n o f C 0 2 was i m p l i c a t e d i n t h e o b s e r v e d pK c h a n g e s . P r i m a r y p r o d u c t i o n l e v e l s (10 - 50 u g / l i t e r C h i a) a t w h i c h t h e h i g h e s t pH l e v e l s ( 9.0, up from 7.0) o c c u r r e d f o l l o w i n g t h e f e r t i l i z a t i o n of Lake 227 ( S c h i n d l e r e t a l . , 1971, 1973), were s i m i l a r t o t h o s e o b t a i n e d i n my l i m n e t i c f e r t i l i z e d e n c l o s u r e s (10 - 40 u g / l i t e r C h i a, F i g u r e 16) w i t h o u t n o t i c e a b l e i n c r e a s e s i n pH. H i g h l e v e l s of ammonium i o n added t o my f e r t i l i z e d e n c l o s u r e s may be, i n p a r t , r e s p o n s i b l e f o r t h e o b s e r v e d l a c k of change i n pH. Ammonium i n d u c e d s u p p r e s s i o n o f a l g a l a s s i m i l a t i o n o f n i t r a t e and may have c o n t r i b u t e d s u f f i c i e n t H + i o n s t h r o u g h i t s own s u b s e q u e n t a s s i m i l a t i o n so as t o b u f f e r i n c r e a s e s i n pH c a u s e d by C 0 2 d e p l e t i o n . Ammonium was not added i n t h e s t u d i e s c i t e d a b o v e , o n l y n i t r a t e . The a s s i m i l a t i o n of t h e a d d e d n i t r a t e w o u l d have compounded i n c r e a s e s i n pH due t o 214 C 0 2 u t i l i z a t i o n , by i n c r e a s i n g t h e h y d r o x y l i o n c o n c e n t r a t i o n as shown by t h e f o l l o w i n g e q u a t i o n f o r t h e a s s i m i l a t i o n of n i t r a t e by a l g a e (Brewer and Goldman, 1976); 1 0 6 ( C 0 2 ) + 1 3 0 ( H 2 0 ) + I6NO3- --> ( C H 2 0 ) 1 0 6 ( N H 3 ) 1 6 + 160H + + 1060 2 (3) Most s p e c i e s of a l g a e show a p r e f e r e n c e f o r u t i l i z i n g NH„ + o v e r N0 3" when b o t h a r e p r e s e n t ( M a c l s a a c and D u g d a l e , 1968,1972; B r e z o n i k , 1972; E p p l e y e t a l . , 1979; S y r e t t , 1981). In f a c t , t h e a d d i t i o n o f N H „ + c a n l e a d t o a r a p i d c e s s a t i o n of n i t r a t e u t i l i z a t i o n i n b a t c h c u l t u r e ( S y r e t t , 1981). M c C a r t h y e t a l . (1977) showed t h a t t h i s i n h i b i t i o n of n i t r a t e a s s i m i l a t i o n by ammonium can a l s o o c c u r i n n a t u r a l p h y t o p l a n k t o n p o p u l a t i o n s and t h a t t h e e x t e n t o f t h e i n h i b i t i o n was d e p e n d e n t on t h e NH„ + c o n c e n t r a t i o n . C o n c e n t r a t i o n s o f N H « + i n e x c e s s -"of 15-30 u g / l i t e r a l m o s t t o t a l l y s u p p r e s s e d N0 3~ u t i l i z a t i o n by C h e s a p e a k e Bay p h y t o p l a n k t o n . S i n c e I was a d d i n g a p p r o x i m a t e l y 70 ug N H „ - N / l i t e r t h r e e t i m e s w e e k l y f o r 16 weeks, i t seems r e a s o n a b l e t o assume t h a t N0 3~ a s s i m i l a t i o n i n t h e f e r t i l i z e d e n c l o s u r e s w o u l d be s e v e r e l y i n h i b i t e d and t h e a l g a e would be p r e f e r e n t i a l l y a s s i m i l a t i n g NH„ + . A s s i m i l a t i o n of N H „ + by a l g a e c o u l d s u p p l y h y d r o g e n i o n s t o c ompensate f o r l o s s e s r e s u l t i n g f r o m C 0 2 d e p l e t i o n , as s u g g e s t e d a b o v e . The e q u a t i o n d e s c r i b i n g t h i s p r o c e s s i s as f o l l o w s (Brewer and Goldman, 1976); 1 0 6 ( C 0 2 ) + 1 0 6 ( H 2 0 ) + 1 6 N H „ + --> ( C H 2 0 ) 1 0 6 ( N H 3 ) 1 6 + 16H + + 1060 2 215 (4) T h i s shows t h a t e a c h m o l e c u l e of NH„ + a s s i m i l a t e d by a l g a e g e n e r a t e s one e q u i v a l e n t of s t r o n g a c i d . However, f o r e a c h m o l e c u l e of C 0 2 consumed, one h y d r o g e n i o n i s removed from t h e s y s t e m due t o s h i f t s i n t h e c a r b o n a t e - b i c a r b o n a t e e q u i l i b r i u m ( S c h i n d l e r , 1973). T h e r e f o r e , 6 t i m e s as many h y d r o g e n i o n s a r e s t i l l b e i n g l o s t f o r e a c h one p r o d u c e d by t h e a s s i m i l a t i o n o f NH a + . T h i s i m b a l a n c e may be p a r t i a l l y o f f s e t by t h e s y n c h r o n o u s l y o c c u r r i n g p r o c e s s of NH„ + o x i d a t i o n by n i t r i f y i n g b a c t e r i a . The o v e r a l l n i t r i f i c a t i o n r e a c t i o n ( W e t z e l , 1975) p r o d u c e s 2 m o l e c u l e s o f H + i o n f o r e v e r y m o l e c u l e of N H « + consumed: NH(,+ + 2 0 2 — > N0 3- + H 20 + 2H + (5) U n f o r t u n a t e l y , I d i d not q u a n t i f y t h e e x t e n t t o w h i c h n i t r i f i c a t i o n o c c u r r e d i n t h e f e r t i l i z e d e n c l o s u r e s . F u r t h e r m o r e , few d a t a a r e a v a i l a b l e i n t h e l i t e r a t u r e t o e n a b l e one t o make r e a s o n a b l e a s s u m p t i o n s a b o u t p r o b a b l e r a t e s o f n i t r i f i c a t i o n b a s e d on t r o p h i c l e v e l s o r p h y s i c a l - c h e m i c a l c o n d i t i o n s o f t h e w a t e r s i n q u e s t i o n . D e s p i t e t h e p a u c i t y of i n f o r m a t i o n r e g a r d i n g n i t r i f i c a t i o n p r o c e s s e s i n t h e water column of l a k e s , many b e l i e v e t h a t n i t r i f i c a t i o n c a n s i g n i f i c a n t l y r e d u c e t h e q u a n t i t y o f ammonium ( B r e z o n i k , 1972; H a l l e t a l . f 1978; C h r i s t o f i , 1981; S t e w a r t e t a l . , 1982) and t h e pH ( G e r l e t t i and P r o v i n i , 1977) i n t h e w a t e r column d u r i n g 216 summer s t r a t i f i c a t i o n . In t h e l a t t e r s t u d y , i t was f o u n d t h a t t h e n i t r i f i c a t i o n o f ammonium l e d t o t h e a c i d i f i c a t i o n of Lake O r t a . T h i s i s e v i d e n c e t h a t n i t r i f i c a t i o n c an c o n t r i b u t e s u b s t a n t i a l amounts of h y d r o g e n i o n t o t h e water c o l u m n . F u r t h e r , a c t i v e c a t i o n e x c h a n g e p r o c e s s e s o c c u r r i n g i n t h e w a l l s o f Sphagnum moss and f r e e o r g a n i c a c i d s s e c r e t e d by Sphagnum may be r e s p o n s i b l e f o r t h e a c i d i t y i n many bog s y s t e m s (Clymo, 1964, 1966), but Clymo (1966) has shown t h a t t h e c o n t r i b u t i o n o f t h e l a t t e r t o t h e o v e r a l l a c i d i t y of bog w a t e r s i s r e l a t i v e l y low (< 30 % o f H +) However, due t o t h e n a t u r e o f t h e c o n s t r u c t i o n o f t h e e n c l o s u r e s , b o t h o f t h e s e p r o c e s s e s w ould have o n l y been i m p o r t a n t i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s . F i n a l l y , t h e d i f f u s i o n o f C 0 2 i n t o l a k e w ater from t h e a t m o s p h e r e may compensate f o r l o s s e s due t o a l g a l a s s i m i l a t i o n , but t h i s i s d e p e n d e n t on many a b i o t i c f a c t o r s (Emerson ejt a l . , 1973). 4.2.2 On P h y t o p l a n k t o n From t h e d a t a i n S e c t i o n 3.3.1, i t i s q u i t e a p p a r e n t t h a t p h y t o p l a n k t o n p r o d u c t i o n i n c r e a s e d i n t h e f e r t i l i z e d e n c l o s u r e s a t a l l d e p t h s compared t o t h e c o n t r o l s . N u t r i e n t e n r i c h m e n t r e s u l t e d i n a 7 - f o l d i n c r e a s e i n t o t a l p h y t o p l a n k t o n biomass i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s compared t o c o n t r o l s . The l i t t o r a l f e r t i l i z e d e n c l o s u r e s r e s p o n d e d w i t h o n l y a 3 - f o l d i n c r e a s e i n t o t a l b i o m a s s compared t o t h e i r c o n t r o l s . T h i s r e s u l t a r o s e d e s p i t e t h e same c o n c e n t r a t i o n s o f n u t r i e n t added t o b o t h l i t t o r a l and l i m n e t i c h a b i t a t s . The d i f f e r e n c e s i n t h e 217 m a g n i t u d e s o f t h e r e s p o n s e s of t h e e n h a n c e d p h y t o p l a n k t o n c o m m u n i t i e s r e f l e c t t h e d i f f e r e n c e s , d i s c u s s e d a b o v e , i n t h e a v a i l a b i l i t y of f r e e i n o r g a n i c n u t r i e n t s (1) S h i f t s i n S p e c i e s C o m p o s i t i o n An i n t e r e s t i n g change i n t h e p r i m a r y p r o d u c e r s r e s u l t i n g from f e r t i l i z a t i o n i s t h e s h i f t from an a l g a l a s s e m b l a g e d o m i n a t e d by i n e d i b l e b l u e - g r e e n a l g a e and l a r g e h a r d - t o - h a n d l e ( f r o m t h e p e r s p e c t i v e of t h e z o o p l a n k t o n ) c h r y s o p h y t e s t o w a r d s one d o m i n a t e d by s m a l l , h i g h l y n u t r i t i o u s , e d i b l e g r e e n a l g a e . T h i s change was p r e d i c t e d t o o c c u r under t h e f e r t i l i z a t i o n r e g i m e employed and was e x p e c t e d t o g r e a t l y b e n e f i t t h e l a r g e r c l a d o c e r a n z o o p l a n k t o n . The s p e c i e s c o m p o s i t i o n o f t h e a l g a l a s s e m b l a g e s i n t h e f e r t i l i z e d e n c l o s u r e s d i f f e r e d between t h e l i t t o r a l and l i m n e t i c z o n e s ( F i g u r e 2 2 ) . C h r y s o p h y t e s d o m i n a t e d t h e a l g a l a s s e m b l a g e i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s from l a t e J u l y t o l a t e A u g u s t whereas c h r y s o p h y t e s were l a r g e l y a b s e n t from a l l l i m n e t i c e n c l o s u r e s , i n c l u d i n g t h e u n f e r t i l i z e d o n e s . The mechanism r e s p o n s i b l e f o r t h i s d i f f e r e n c e was p r o b a b l y r e l a t e d t o t h e h y d r o m e c h a n i c a l p r o p e r t i e s of t h e l i t t o r a l zone r a t h e r t h a n t h e d i f f e r e n c e s i n n u t r i e n t a v a i l a b i l i t y ( d i s c u s s e d above) s i n c e f r e e i n o r g a n i c n u t r i e n t s were a v a i l a b l e i n t h e same r e l a t i v e p r o p o r t i o n s i n b o t h f e r t i l i z e d h a b i t a t s . They d i f f e r e d o n l y i n t h e o v e r a l l c o n c e n t r a t i o n . T u r b u l e n t m i x i n g o f t h e e p i l i m n e t i c w a t e r s i n t h e l i t t o r a l zone would r e s u l t i n a g i t a t i o n and r e s u s p e n s i o n o f l i t t o r a l s e d i m e n t s , a c o n d i t i o n 218 w h i c h w o u l d not o c c u r i n t h e l i m n e t i c z o n e . T h i s might r e s u l t i n t h e r e s u s p e n s i o n of v e g e t a t i v e s p o r e s o r p r o p a g u l e s o f a l g a l s p e c i e s w h i c h would o t h e r w i s e be c o n f i n e d t o t h e s e d i m e n t s . Many c h r y s o p h y t e s i n c l u d i n g , D i n o b r y o n , S y n u r a , M a l l o m o n a s , and U r o g l e n a ( a l l e x c e p t t h e l a t t e r a r e p r e s e n t i n P l a c i d L a k e ) form p e r s i s t e n t , s i l i c e o u s c y s t s ( R e y n o l d s , 1982). C y s t s a r e p a s s i v e p a r t i c l e s t h a t c a n s e t t l e t o s e d i m e n t s t o s e e d r e c u r r e n t blooms and can be r e s u s p e n d e d from b o t t o m s e d i m e n t s i n t o e n v i r o n m e n t s f a v o u r a b l e f o r g e r m i n a t i o n . R e y n o l d s (1982) r e l a t e d t h e poor d e v e l o p m e n t of C e r a t i u m (a d i n o f l a g g e l a t e ) p o p u l a t i o n s i n Lund t u b e s compared t o t h e s u r r o u n d i n g w a t e r s , t o a v e r y low i n o c c u l a t i o n of c y s t s b e c a u s e t h e t u b e s were h y d r o l o g i c a l l y i s o l a t e d a t t h e t i m e t h e c y s t s were r e s u s p e n d e d f r o m s e d i m e n t s . An a n a l o g o u s s i t u a t i o n may have e x i s t e d i n my l i m n e t i c e n c l o s u r e s i n t h a t once c h r y s o p h y t e s were l o s t t o t h e h y p o l i m n i o n t h r o u g h s i n k i n g , t h e y would n o t be r e p l a c e d by r e s u s p e n d e d e n c y s t e d f o r m s . The r e s u s p e n s i o n o f c y s t s c o u l d c o n c e i v a b l y have o c c u r r e d t h e whole summer l o n g i n t h e l i t t o r a l zone of t h e l a k e a l l o w i n g t h e l i t t o r a l e n c l o s u r e s t o m a i n t a i n l a r g e c h r y s o p h y t e p o p u l a t i o n s . I s o l a t i o n o f t h e l i m n e t i c w a t e r s by e n c l o s u r e w o u l d have e l i m i n a t e d t h e " s e e d i n g " phenomenon t h e r e b y c h a n g i n g t h e s u c c e s s i o n a l p r o g r e s s i o n of t h e a l g a e . In t h e c a s e of t h e l i m n e t i c c o n t r o l s , b l u e - g r e e n a l g a e became d o m i n a n t s . In t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s , g r e e n a l g a l s p e c i e s w h i c h were n o r m a l l y r a r e i n t h e s y s t e m were f a v o u r e d . 219 (2) D e l a y e d Biomass I n c r e a s e A n o t h e r p a r t i c u l a r l y s t r i k i n g f e a t u r e of t h e r e s p o n s e of t h e a l g a l p o p u l a t i o n t o n u t r i e n t a d d i t i o n i s t h a t t h e t o t a l p h y t o p l a n k t o n b i o m a s s d i d n o t i n c r e a s e s i g n i f i c a n t l y i n any of t h e f e r t i l i z e d e n c l o s u r e s , o v e r c o n t r o l l e v e l s , u n t i l 6-8 weeks a f t e r f e r t i l i z a t i o n commenced. T h i s i s shown by c h a n g e s i n S e c c h i d e p t h ( F i g u r e 7 ) , p h y t o p l a n k t o n c e l l v o l u m e s ( F i g u r e 16) and c h l o r o p h y l l a ( F i g u r e 1 7 ) . S i m i l a r d e l a y s i n d e t e c t a b l e i n c r e a s e s i n p h y t o p l a n k t o n b i o m a s s have been r e p o r t e d f o l l o w i n g n u t r i e n t a d d i t i o n s t u d i e s i n two o t h e r U.B.C. r e s e a r c h f o r e s t l a k e s (Dickman, 1968; W a l t e r s e t a l . , i n p r e p ) . I h y p o t h e s i z e t h a t t h i s r e s u l t was due t o t h e m a s s i v e i n c r e a s e i n t h e a b u n d a n c e s of c l a d o c e r a n z o o p l a n k t o n , i n p a r t i c u l a r D a p h n i a  r o s e a and Diaphanosoma b r a c h y u r u m , t h a t o c c u r r e d i m m e d i a t e l y a f t e r f e r t i l i z a t i o n began ( F i g u r e s 2 9 ) . The s u p e r v e n i n g g r a z i n g p r e s s u r e s a p p e a r t o have been i n t e n s e enough t o r e s u l t i n t h e c r o p p i n g o f a l m o s t 100 % o f t h e e n h a n c e d a l g a l p r o d u c t i o n , s i n c e a l g a l b i o m a s s i n c r e a s e d v e r y s l o w l y or not a t a l l ( F i g u r e 16) u n t i l t h i s g r a z i n g p r e s s u r e was a l l e v i a t e d . T h i s d i d n o t o c c u r u n t i l t h e p o p u l a t i o n s o f D a p h n i a d e c l i n e d s i g n i f i c a n t l y i n l a t e J u n e o r e a r l y A u g u s t , d e p e n d i n g on t h e e n c l o s u r e examined ( F i g u r e s 2 9 ) . D a p h n i a and o t h e r c l a d o c e r a n s u s u a l l y a c c o u n t f o r 80 % of t h e t o t a l z o o p l a n k t o n community g r a z i n g r a t e i n m e s o t r o p h i c and e u t r o p h i c n a t u r a l l a k e s ( P o r t e r , 1977). Maximum community g r a z i n g r a t e s as h i g h as 114 % ( t h e e n t i r e volume o f w a t e r ) d u r i n g one day have been r e p o r t e d f o r o l i g o t r o p h i c and bog l a k e s 220 whose dominant z o o p l a n k t o n were D a p h n i a (Haney, 1973). M a j o r p e a k s i n g r a z i n g a c t i v i t y were c o r r e l a t e d w i t h i n c r e a s e s i n t h e p o p u l a t i o n d e n s i t y o f t h e z o o p l a n k t o n . I e s t i m a t e d z o o p l a n k t o n community f i l t e r i n g r a t e s f o r my e n c l o s u r e s by a p p l y i n g t h e maximum z o o p l a n k t o n community f i l t e r i n g r a t e (3 m l / ug z o o p l a n k t o n / day) e s t i m a t e d f o r P l a c i d L a k e z o o p l a n k t o n by Buckingham (1968) t o t h e mean ( T a b l e 9) and maximum ( F i g u r e 25) z o o p l a n k t o n b i o m a s s l e v e l s i n my e n c l o s u r e s . In my c o n t r o l e n c l o s u r e s and t h e l a k e , c l a d o c e r a n s d o m i n a t e d t h e z o o p l a n k t o n community f o r much of t h e summer, however, t h e mean z o o p l a n k t o n community g r a z i n g r a t e (% of t o t a l w a ter column p e r day) a v e r a g e d o n l y 15 % o v e r t h e summer w i t h maxima r e a c h i n g a b o u t 30 % a t peak b i o m a s s p e r i o d s . In c o n t r a s t , D a p h n i a numbers e x p l o d e d i n t h e f e r t i l i z e d e n c l o s u r e s r e s u l t i n g i n h i g h e r o v e r a l l b i o m a s s e s . T o t a l z o o p l a n k t o n community g r a z i n g r a t e s i n c r e a s e d a c c o r d i n g l y t o an a v e r a g e f o r t h e summer o f 48 % i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s and 80 % i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s . Maximum r a t e s r e a c h e d as h i g h as 140 % a t peak b i o m a s s p e r i o d s . The f i l t e r i n g r a t e s e s t i m a t e d by Buckingham (1968) a r e h i g h e r t h a n t h e e q u i v a l e n t mean f i l t e r i n g r a t e (0.5 m l / ug z o o p l a n k t o n / d a y , a s s u m i n g a mean z o o p l a n k t e r w e i g h t of 5 ug) t h a t I e s t i m a t e d f o r t h e z o o p l a n k t o n s p e c i e s i n Drowned Bog Lake (Haney, 1973, T a b l e 4 ) , upon w h i c h t h i s c o m p a r i s o n i s b a s e d . R e g a r d l e s s , t h e f i g u r e s show t h a t g r a z e r s c a n e x e r t s i g n i f i c a n t e f f e c t s on p h y t o p l a n k t o n a b u n d a n c e . Thus, i n c r e a s i n g t h e abundance o f c l a d o c e r a n s t h r o u g h f e r t i l i z a t i o n i n c r e a s e s t h e o v e r a l l z o o p l a n k t o n g r a z i n g 221 p r e s s u r e . T h i s may e x p l a i n why p h y t o p l a n k t o n b i o m a s s d i d n o t i n c r e a s e d u r i n g t h e f i r s t 6-8 weeks f o l l o w i n g f e r t i l i z a t i o n ( F i g u r e 16) as one m i g h t e x p e c t . Z o o p l a n k t o n were a p p a r e n t l y a b l e t o s u p p r e s s a l g a l b i o m a s s i n c r e a s e s . T h i s s u p p r e s s i o n of a l g a l b i o m a s s a p p e a r s t o have been a l l e v i a t e d by d e c l i n e s i n t h e numbers of z o o p l a n k t o n ( F i g u r e 25) w h i c h may have r e s u l t e d from s u c h f a c t o r s as s p e c i e s s u c c e s s i o n of t h e p h y t o p l a n k t o n ( P o r t e r , 1977; G l i w i c z , 1980), o v e r g r a z i n g o f t h e e d i b l e a l g a e ( d i s c u s s e d l a t e r ) , f i s h p r e d a t i o n ( a l s o d i s c u s s e d l a t e r ) o r some o t h e r e n v i r o n m e n t a l p e r t u r b a t i o n . The r a t e o f p h y t o p l a n k t o n r e n e w a l i n t h e f e r t i l i z e d e n c l o s u r e s d u r i n g t h e e a r l y summer z o o p l a n k t o n bloom a p p e a r e d t o match l o s s e s due t o g r a z i n g . The f a c t t h a t p h y t o p l a n k t o n b i o m a s s i n t h e f e r t i l i z e d e n c l o s u r e s i n c r e a s e d s l i g h t l y r a t h e r t h a n d e c r e a s e d f i n r e s p o n s e t o i n c r e a s e d g r a z i n g a c t i v i t y s t r o n g l y s u g g e s t s t h a t f e r t i l i z a t i o n e n a b l e d t h e p h y t o p l a n k t o n t o renew 100% o r more o f t h e a l g a l s t a n d i n g c r o p p e r day. T h i s i s i n a c c o r d a n c e w i t h t h e o b s e r v a t i o n t h a t p h y t o p l a n k t o n r e n e w a l r a t e s i n most n a t u r a l l a k e s of i n t e r m e d i a t e t o e x t r e m e e u t r o p h y a r e commonly a r o u n d 100 % (Haney, 1973). The g r a z i n g r a t e of t h e z o o p l a n k t o n r e a c h e d maxima a p p r o a c h i n g 140 % o f t h e water column p e r day on s e v e r a l o c c a s i o n s , y e t no o b s e r v a b l e d e p r e s s i o n i n t h e b i o m a s s ( F i g u r e 24) o r d e n s i t y ( A p p e n d i x 3) of g r a z a b l e a l g a e o c c u r r e d . 222 (3) S i z e F r a c t i o n of E n h a n c e d A l g a e I t i s c l e a r from F i g u r e 23 t h a t f e r t i l i z a t i o n f a v o u r e d t h e p r o d u c t i o n o f a l g a l s p e c i e s <20 um n t h e i r maximum l i n e a r d i m e n s i o n (MLD) s t a r t i n g i n l a t e A u g u s t i n a l l of t h e f e r t i l i z e d e n c l o s u r e s , but i t i s not c l e a r what s i z e range o f p a r t i c l e s was e n h a n c e d i n e a r l y summer. T h i s i s b e c a u s e v i r t u a l l y a l l of t h e e n h a n c e d p h y t o p l a n k t o n p r o d u c t i o n was g r a z e d by z o o p l a n k t o n . F u r t h e r , t h e p a r t i c l e s i z e d i s t r i b u t i o n of t h e a l g a l b i o m a s s was no d i f f e r e n t from t h a t i n t h e c o n t r o l s or t h e l a k e d u r i n g t h i s p e r i o d ( F i g u r e 2 4 ) . However, s i n c e i t a p p e a r s t h a t v i r t u a l l y a l l o f t h e e a r l y summer a l g a l b i o m a s s e n h a n c e d by f e r t i l i z a t i o n was g r a z e d away by t h e z o o p l a n k t o n , i t i s p o s s i b l e , knowing t h e s i z e r a n g e of a l g a e t h a t w o u l d be g r a z e d by t h e z o o p l a n k t o n a s s e m b l a g e , t o d e t e r m i n e t h e s i z e r a n g e s of t h e a l g a l p a r t i c l e s t h a t were e n h a n c e d by f e r t i l i z a t i o n a t t h i s t i m e . T h e r e a r e s e v e r a l r e p o r t s i n t h e l i t e r a t u r e t h a t document t h e s i z e s p e c t r u m of f o o d p a r t i c l e s s e l e c t i v e l y f e d upon by p a r t i c u l a r z o o p l a n k t o n s p e c i e s . K r a u s e (1984) p r e s e n t s a r e v i e w of t h e a v a i l a b l e d a t a r e l a t i n g t o t h e h e r b i v o r o u s z o o p l a n k t o n of P l a c i d L a k e . T o g e t h e r w i t h t h i s and w i t h t h e r e s u l t s of h e r s p e c i e s s p e c i f i c g r a z i n g e x p e r i m e n t s , she c o n c l u d e d t h a t a l l o f t h e i m p o r t a n t g r a z e r s i n P l a c i d l a k e f e e d s e l e c t i v e l y on a l g a l c e l l s ( o f P l a c i d Lake spp.) w i t h a maximum l i n e a r d i m e n s i o n r a n g i n g f r o m 2-20 um ( K r a u s e , 1984). T h e s e d a t a s u g g e s t t h a t t h e huge i n c r e a s e s i n t h e D. r o s e a and D. b r a c h y u r u m p o p u l a t i o n s t h a t o c c u r r e d i n my f e r t i l i z e d e n c l o s u r e s s h o r t l y a f t e r f e r t i l i z a t i o n commenced, i n J u n e , must 223 have been f u e l e d by i n c r e a s e s i n t h e p h y t o p l a n k t o n w i t h i n t h e p r e f e r r e d s i z e r a n g e s (6-20 um MLD) f o r t h e s e s p e c i e s . A l s o , t h e s e l a r g e c l a d o c e r a n p o p u l a t i o n i n c r e a s e s i m p l y h i g h j u v e n i l e s u r v i v a l . N e i l l (1975) has shown t h a t t h e j u v e n i l e s u r v i v a l of t h e m i c r o c r u s t a c e a n s i s e s p e c i a l l y s e n s i t i v e t o f o o d c o n c e n t r a t i o n . N e i l l (1981) c o n f i r m s t h i s f o r D a p h n i a r o s e a i n P l a c i d Lake w a t e r s . D a p h n i a j u v e n i l e s were f o u n d t o e x p l o i t f o o d p a r t i c l e s i n t h e 2-10 um s i z e r a n g e . T h e r e f o r e , i n o r d e r f o r t h e j u v e n i l e c l a d o c e r a n s i n my s y s t e m t o a t t a i n b e t t e r s u r v i v a l , t h e r e must have been an enhancement of a l g a l forms i n t h e 2-10 um s i z e r a n g e as w e l l . W i t h t h e e x c e p t i o n of D. o r e g o n e n s i s (Richman e t a l . , 1980), none of t h e i m p o r t a n t g r a z e r s f o u n d i n P l a c i d Lake have been r e p o r t e d t o f e e d upon f o o d p a r t i c l e s > 20 um MLD w i t h any d e g r e e of s e l e c t i v i t y o r e f f i c i e n c y . W h i l e Richman e t a l . (1980) f o u n d t h a t D. o r e g o n e n s i s f r o m Lake Winnebago, W i s c o n s i n , f e d s e l e c t i v e l y on f o o d p a r t i c l e s w i t h i n a s i z e r a n g e of 25-30 um, K r a u s e (1984) f o u n d t h a t D. o r e g o n e n s i s i n P l a c i d l a k e r e s t r i c t e d t h e i r f o o d i n t a k e t o p a r t i c l e s i n t h e 14-20 um s i z e r a n g e . T h i s i s i m p o r t a n t b e c a u s e i f none of t h e g r a z e r s i n t h e f e r t i l i z e d e n c l o s u r e s f e d on p a r t i c l e s >20 um MLD, and i f t h e s e p a r t i c l e s were b e i n g e n h a n c e d by f e r t i l i z a t i o n , one would e x p e c t t o see a s i g n i f i c a n t i n c r e a s e i n t h e i r d e n s i t y . T h i s d i d n o t o c c u r . T h e r e were s l i g h t i n c r e a s e s i n t h e d e n s i t i e s of a l g a l p a r t i c l e s i n t h e 20-30 um s i z e r a n g e d u r i n g J u l y and A u g u s t i n l i t t o r a l f e r t i l i z e d e n c l o s u r e s 3 and 7 and l i m n e t i c f e r t i l i z e d 224 e n c l o s u r e 4 ( A p p e n d i x 3 ) , but t h e i n c r e a s e s were not s t a t i s t i c a l l y s i g n i f i c a n t . F u r t h e r m o r e , where s m a l l i n c r e a s e s i n t h i s s i z e f r a c t i o n d i d o c c u r , t h e y were composed a l m o s t e x c l u s i v e l y o f t h e c r y p t o p h y t e s Cryptomonas o v a t a and Cry p t o m o n a s e r o s a , b o t h of w h i c h , t h o u g h r e l a t i v e l y l a r g e , a r e r e a d i l y consumed by D a p h n i a ( I n f a n t e and A b e l l a , 1985) and D. o r e g o n e n s i s (McQueen, 1967). In c o n t r a s t t h e 20-30 um s i z e f r a c t i o n i n t h e c o n t r o l s was d o m i n a t e d by t h e i n e d i b l e b l u e -g r e e n a l g a e M e r i s m o p e d i a and C h r o o c o c c u s o v e r t h e e n t i r e summer ( F i g u r e s 22 and 2 4 ) . The o b s e r v e d i n c r e a s e s i n t h e d e n s i t i e s o f p a r t i c l e s >20 um t h a t o c c u r r e d i n l i t t o r a l f e r t i l i z e d e n c l o s u r e 7 and l i m n e t i c f e r t i l i z e d e n c l o s u r e 8 i n O c t o b e r ( A p p e n d i x 3) were p r i m a r i l y due t o a bloom o f D i n o b r y o n . My r e p o r t e d d e n s i t i e s a r e b i a s e d upward i n t h a t t h e s e numbers r e p r e s e n t c o u n t s of s i n g l e l o r i c a s r a t h e r t h a n c o m p l e t e c o l o n i e s w h i c h were l a r g e r and l e s s a b u n d a n t . A major i n c r e a s e i n t h e d e n s i t y o f a l g a e >30 um was a l s o r e c o r d e d f o r a s i n g l e d a t e , i n J u n e , i n l i t t o r a l f e r t i l i z e d e n c l o s u r e 3 ( A p p e n d i x 3 ) . T h i s t o o i s a r t i f a c t . The sample was c o n t a m i n a t e d by l a r g e d i a t o m s w h i c h had p r o b a b l y been r e s u s p e n d e d from l i t t o r a l s e d i m e n t s by SCUBA d i v e r s who had been w o r k i n g i n s i d e t h a t e n c l o s u r e e a r l i e r i n t h e day. Thus, i t a p p e a r s t h a t o n l y t h o s e a l g a l f o o d p a r t i c l e s i n th e o v e r a l l s i z e r a n g e o f 2-20 um were t h e most l i k e l y b e n e f i c i a r i e s o f t h e a d o p t e d f e r t i l i z a t i o n r e g i m e ; s m a l l , f r e q u e n t d o s e s o f i n o r g a n i c n i t r o g e n and p h o s p h o r o u s a t a h i g h N:P r a t i o . 225 (4) C o n c l u s i o n In c o n c l u s i o n , f e r t i l i z e r was added a t a h i g h N:P r a t i o (40:1 a t o m i c , 21:1 by w e i g h t ) t o i s o l a t e d s e c t i o n s of t h e l i t t o r a l and l i m n e t i c z o n e s o f t h e l a k e . The o b j e c t i v e was t o promote i n c r e a s e s i n a l g a l b i o m a s s , w h i l e p r o m o t i n g a s h i f t i n a p h y t o p l a n k t o n a s s e m b l a g e from one w h i c h has been d o m i n a t e d i n p a s t y e a r s ( K r a u s e , 1984) by l a r g e (> 20 um), i n e d i b l e b l u e -g r e e n a l g a e and l a r g e , u n g r a z e d c h r y s o p h y t e s , t o w a r d s one t h a t f a v o u r e d dominance by s m a l l , e d i b l e a l g a l f o r m s i n t h e s i z e r a n g e s s e l e c t i v e l y f e d upon by t h e z o o p l a n k t o n i n t h e l a k e . The r e s u l t s show c l e a r l y t h a t t h e r e was a d e f i n i t e s h i f t i n t h e s p e c i e s c o m p o s i t i o n of t h e a l g a l a s s e m b l a g e . B l u e - g r e e n a l g a e c o m p r i s e d l e s s t h a n 5 % of t h e t o t a l a l g a l b i o m a s s i n t h e f e r t i l i z e d e n c l o s u r e s . They were r e p l a c e d by c h l o r o p h y t e s and c r y p t o p h y t e s . L a r g e c h r y s o p h y t e s were v i r t u a l l y e l i m i n a t e d from t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s but m a i n t a i n e d a p r e s e n c e i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s . The d i s a p p e a r a n c e o f t h e c h r y s o p h y t e s from a l l l i m n e t i c e n c l o s u r e s and t h e i r p e r s i s t e n c e i n t h e l i t t o r a l e n c l o s u r e s was p r o b a b l y m e d i a t e d by d i f f e r e n c e s i n l i t t o r a l - l i m n e t i c h y d r o d y n a m i c p r o c e s s e s . F e r t i l i z a t i o n d i d n o t enhance t h e i r abundance where t h e y d i d o c c u r . A d r a m a t i c i n c r e a s e i n t h e abundance and b i o m a s s o f c l a d o c e r a n z o o p l a n k t o n w h i c h f e e d s e l e c t i v e l y on a l g a e i n t h e 2-20 um s i z e r a n g e s t r o n g l y s u g g e s t s t h a t t h i s was t h e p a r t i c l e s i z e r a n g e e n h a n c e d by f e r t i l i z a t i o n i n t h e e a r l y p a r t o f t h e summer. L e s s t h a n 20 % and more t h a n 80 % o f t h e l a t e summer a l g a l b i o m a s s c o n s i s t e d 226 of p a r t i c l e s < 20 um i n s i z e i n t h e c o n t r o l and t h e f e r t i l i z e d e n c l o s u r e s , r e s p e c t i v e l y . 4.2._3 On Z o o p l a n k t o n Community S t r u c t u r e The p r i m a r y e f f e c t s of f e r t i l i z a t i o n , as r e f l e c t e d i n t h e z o o p l a n k t o n , i n c l u d e d s i g n i f i c a n t i n c r e a s e s i n t h e t o t a l b i o m a s s of a l l of t h e t r e a t e d e n c l o s u r e s , compared t o c o n t r o l s , and i n c r e a s e s i n t h e d e n s i t y ( e x c e p t f o r H o l o p e d i u m gibberum ) and egg p r o d u c t i o n o f a l l of t h e i m p o r t a n t g r a z e r s . The body s i z e s of a d u l t D a p h n i a r o s e a , D i a p t o m u s o r e g o n e n s i s and Diaphanosoma  b r a c h y u r u m were a l s o i n c r e a s e d under f e r t i l i z a t i o n . J u v e n i l e body s i z e s o f t h e s e s p e c i e s d i d n o t i n c r e a s e . B iomass i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s i n c r e a s e d 7-f o l d o v e r l i m n e t i c c o n t r o l s w h i l e b i o m a s s i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s i n c r e a s e d o n l y 3 - f o l d o v e r l i t t o r a l c o n t r o l s . D a p h n i a r o s e a c o n s i s t e n t l y a c c o u n t e d f o r >50% o f t h e t o t a l b i o m a s s f o r t h e e n t i r e s e a s o n i n a l l o f t h e t r e a t e d e n c l o s u r e s . The d i f f e r e n c e s i n b i o m a s s between l i t t o r a l a nd l i m n e t i c e n c l o s u r e s c an be a t t r i b u t e d e n t i r e l y t o t h e e f f e c t s o f n u t r i e n t a d d i t i o n on t h e p h y t o p l a n k t o n , a s d e m o n s t r a t e d i n t h e p r e v i o u s s e c t i o n . The d i f f e r e n t i a l m a g n i t u d e s o f t h e i n c r e a s e s between l i t t o r a l a nd l i m n e t i c b i o m a s s e s were p r o p o r t i o n a l t o t h e m a g n i t u d e s o f t h e i n c r e a s e s i n p h y t o p l a n k t o n b i o m a s s i n t h e r e s p e c t i v e l a k e z o n e s . The p e r f o r m a n c e o f t h e i n d i v i d u a l z o o p l a n k t o n s p e c i e s i n t h e n u t r i e n t e n r i c h e d e n c l o s u r e s d i f f e r e d i n some r e s p e c t s between l i t t o r a l a nd l i m n e t i c h a b i t a t s . A l s o , t h e r e were some 227 e n c l o s u r e s p e c i f i c r e s p o n s e s p e c u l i a r t o some of t h e s p e c i e s e x a mined w h i c h may have been r e l a t e d t o f a c t o r s o t h e r t h a n f e r t i l i z a t i o n , s u c h as p r e d a t i o n or i n t e r s p e c i f i c c o m p e t i t i o n . In o r d e r t o d e a l w i t h e a c h r e s u l t e f f e c t i v e l y , I w i l l d i s c u s s e a c h s p e c i e s s e p a r a t e l y and a t t e m p t t o i d e n t i f y whether f o o d a v a i l a b i l i t y , c o m p e t i t i o n , p r e d a t i o n , t e m p o r a l o r s p a t i a l i d i o m o r p h i s m s or some c o m b i n a t i o n of t h e s e f a c t o r s were r e s p o n s i b l e f o r i t s o b s e r v e d p a t t e r n o f abundance. (1) H o l o p e d i u m g i b b e r u m I t was s u r p r i s i n g t o f i n d t h a t H o l o p e d i u m f a r e d worse under f e r t i l i z a t i o n t h a n i t d i d i n c o n t r o l e n c l o s u r e s . In t h e c o n t r o l s , H o l o p e d i u m d e n s i t i e s , w h i c h were l o w e r t h a n l a k e v a l u e s a t t h e o n s e t o f e n c l o s u r e , i n c r e a s e d i n abundance ( t o n e a r - l a k e l e v e l s ) s h o r t l y a f t e r t h e e x p e r i m e n t began. In t h e f e r t i l i z e d e n c l o s u r e s , t h e few H o l o p e d i u m t h a t were p r e s e n t a t th e o n s e t o f f e r t i l i z a t i o n , d e c l i n e d r a p i d l y i n numbers d e s p i t e i n c r e a s e d l e v e l s o f f o o d a v a i l a b i l i t y . T h a t f o o d was a v a i l a b l e i n abundance d u r i n g t h i s e a r l y d e c l i n e i s e v i d e n c e d by t h e f a c t t h a t t h e few H o l o p e d i u m p r e s e n t b e f o r e t h e d e c l i n e showed d r a m a t i c a l l y i n c r e a s e d l e v e l s of egg p r o d u c t i o n . Some o f t h e g r a v i d i n d i v i d u a l s t h a t were sampled were f o u n d t o be c a r r y i n g a s many as 30-40 e g g s . The maximum number o f eggs i n any H o l o p e d i u m i n t h e c o n t r o l s was 8, f o u n d i n o n l y one i n d i v i d u a l . T h i s s t r o n g l y s u g g e s t s t h a t H o l o p e d i u m i n t h e f e r t i l i z e d e n c l o s u r e s b e n e f i t t e d f r o m e n h a n c e d l e v e l s o f f o o d p r o d u c t i o n but some o t h e r f a c t o r a c t e d t o p r e v e n t them from 228 c a p i t a l i z i n g on the resource. One such f a c t o r may have been f i s h p r e d a t i o n . However, The e a r l y demise of the Holopedium c o u l d not have been due to f i s h p r e d a t i o n s i n c e f i s h had not yet invaded the e n c l o s u r e s . A l s o , p r e d a t i o n on Holopedium by the j u v e n i l e c u t t h r o a t t r o u t that were stocked in the e n c l o s u r e s can be r u l e d out as a f a c t o r s i n c e the same d e n s i t i e s of f i s h were stocked i n the c o n t r o l e n c l o s u r e s with no l i k e w i s e e f f e c t . T h i s i s not to say that Holopedium were not a f f e c t e d by the f i s h that invaded the e n c l o s u r e s l a t e r , i n J u l y , as was suggested f o r c o n t r o l p o p u l a t i o n s (see above). P r e d a t i o n by the i n v a d i n g a d u l t c u t t h r o a t t r o u t may e x p l a i n why Holopedium were almost completely e r a d i c a t e d from a l l of the e n c l o s u r e s , both c o n t r o l and f e r t i l i z e d . The presence of Holopedium in the guts of f i s h captured i n the e n c l o s u r e s , at a l l times over the summer (Table 15) support t h i s assumption. Another f a c t o r which may have prevented Holopedium from c a p i t a l i z i n g on the enhanced food resource, i s i n t e r s p e c i f i c c ompetition with Daphnia rosea, which i n c r e a s e d d r a m a t i c a l l y i n number, almost i n s t a n t a n e o u s l y , f o l l o w i n g f e r t i l i z a t i o n . In support of t h i s are the o b s e r v a t i o n s that Holopedium decreased in numbers almost as r a p i d l y as the daphnids i n c r e a s e d , and, that the d e n s i t y of Holopedium exploded i n l i m n e t i c f e r t i l i z e d e n c l o s u r e 4 f o l l o w i n g a r a p i d d e c l i n e i n the numbers of Daphnia, which f e l l from over 100 i n d i v i d u a l s / l i t e r to l e s s than 1 0 / l i t e r in l e s s than two weeks. A l s o , the f e e d i n g r a t e s of D. rosea, under the c o n d i t i o n s of food c o n c e n t r a t i o n ( c a . 300-500 229 u g / l i t e r d r y w e i g h t ) and t e m p e r a t u r e (16 °C t o 20 °C) p r e s e n t i n t h e e n c l o s u r e s d u r i n g t h e e a r l y p a r t o f t h e e x p e r i m e n t , may have been 3-4 t i m e s h i g h e r t h a n f o r H o l o p e d i u m (0.1 v s 0.03 ug/ug z o o p l a n k t o n / h r , r e s p e c t i v e l y , Buckingham, 1978), w h i c h m i g h t have g i v e n t h e D a p h n i a a c o m p e t i t i v e a d v a n t a g e . (2) D a p h n i a r o s e a L a r g e c l a d o c e r a n s l i k e D a p h n i a a r e an i m p o r t a n t f o o d f o r f i s h i n l a k e s . S i n c e one of t h e main o b j e c t i v e s of t h i s s t u d y was t o enhance t h e f o o d f o r f i s h i n t r o u t l a k e s by a f e r t i l i z a t i o n scheme f a v o u r i n g t h e growth o f D a p h n i a f o o d , I w i l l d i s c u s s t h e r e s p o n s e s of t h i s s p e c i e s i n some d e t a i l . D e n s i t i e s o f D a p h n i a r o s e a i n c r e a s e d d r a m a t i c a l l y f o l l o w i n g n u t r i e n t a d d i t i o n . They p e r s i s t e d a s t h e b i o m a s s dominant f o r t h e e n t i r e summer w h i l e D a p h n i a i n u n f e r t i l i z e d e n c l o s u r e s and t h e l a k e d e c l i n e d by l a t e J u l y . None of t h e o t h e r z o o p l a n k t o n s p e c i e s e x h i b i t e d s u c h a d r a m a t i c p r o l o n g a t i o n o f t h e i r n o rmal s e a s o n a l c y c l e s . I n 3 e n c l o s u r e s o u t of 4, t h e c u r v e r e p r e s e n t i n g t h e p o p u l a t i o n s i z e of D. r o s e a i n t h e f e r t i l i z e d e n c l o s u r e s was somewhat b i m o d a l , w i t h h i g h d e n s i t i e s i n t h e e a r l y summer and f a l l , a nd l o w e r d e n s i t i e s ( b ut s t i l l h i g h e r t h a n i n t h e c o n t r o l s ) i n t h e mid-summer months. I n t h e f o u r t h c a s e , t h a t of e n c l o s u r e 8, d e n s i t i e s o f D a p h n i a i n c r e a s e d r a p i d l y i n i t i a l l y , r e m a i n e d s t a b l e t h r o u g h mid-summer, t h e n i n c r e a s e d a g a i n i n t h e f a l l . T h e r e was no o b s e r v a b l e d e c l i n e i n e i t h e r d e n s i t i e s o f D a p h n i a ( F i g u r e 31) o r t o t a l z o o p l a n k t o n b i o m a s s ( F i g u r e 2 5 ) . 230 The p a t t e r n of p o p u l a t i o n g r o w t h , d e s c r i b e d f o r t h e o t h e r t h r e e f e r t i l i z e d e n c l o s u r e s , i s o f t e n i n t e r p r e t e d as a r e f l e c t i o n of a f o o d - l i m i t e d s y s t e m where z o o p l a n k t o n i n c r e a s e i n numbers w i t h t h e s p r i n g p h y t o p l a n k t o n bloom u n t i l n u t r i e n t d e p l e t i o n of e p i l i m n e t i c w a t e r s r e s u l t s i n d e c r e a s e d l e v e l s o f p h y t o p l a n k t o n p r o d u c t i o n and t h e f o o d s u p p l y i s o v e r g r a z e d . O v e r g r a z i n g r e s u l t s i n a d e c l i n e i n z o o p l a n k t o n numbers w h i c h i s m a i n t a i n e d by low r a t e s o f a l g a l p r o d u c t i o n and a dominance o f l a r g e n e t p h y t o p l a n k t o n c h a r a c t e r i s t i c o f summer a l g a l a s s e m b l a g e s . N u t r i e n t r e c h a r g e t o t h e e p i l i m n i o n from h y p o l i m n e t i c w a t e r s a t f a l l o v e r t u r n r e s u l t s i n i n c r e a s e d p h y t o p l a n k t o n p r o d u c t i o n and s u b s e q u e n t i n c r e a s e s i n z o o p l a n k t o n numbers. Why d i d t h e numbers of D a p h n i a d e c l i n e i n t h e f e r t i l i z e d e n c l o s u r e s d u r i n g t h e mid-summer d e s p i t e c o n t i n u e d f e r t i l i z a t i o n t o keep f o o d l e v e l s h i g h ? Was i t a r e s u l t o f D a p h n i a d e m o g r a p h i c s o r s i m p l e p r e d a t o r - p r e y o s c i l l a t i o n s (Murdoch and M c C a u l e y , 1985), f i s h p r e d a t i o n , o v e r g r a z i n g o f t h e g r a z e a b l e a l g a e , i n t e r f e r e n c e d u r i n g blooms o f n e t p h y t o p l a n k t o n , o r a c o m b i n a t i o n o f t h e s e f a c t o r s ? I w i l l a d d r e s s e a c h o f t h e s e c o n c e r n s i n t h e o r d e r I have p r e s e n t e d them. Murdoch and M c C a u l e y (1985) i n d i c a t e t h a t D a p h n i a p o p u l a t i o n s r a i s e d i n t h e l a b o r a t o r y under c o n d i t i o n s of c o n s t a n t f o o d and t e m p e r a t u r e , c a n e x h i b i t r e g u l a r c y c l i c f l u c t u a t i o n s i n d e n s i t y w i t h a p e r i o d o f 20-40 d a y s . Such o s c i l l a t i o n s a r e deemed t o be a p r o p e r t y of D a p h n i a demography and have n o t h i n g t o do w i t h p r e d a t o r - p r e y o s c i l l a t i o n s . Demography r e f e r s t o t h e dynamic b a l a n c e of a p o p u l a t i o n w i t h 231 r e g a r d t o d e n s i t y and c a p a c i t y f o r e x p a n s i o n o r d e c l i n e ( i . e . g r o w t h r a t e s , d i s t r i b u t i o n , f e c u n d i t y , m o r t a l i t y ) I f t h e f l u c t u a t i o n s i n d a p h n i d d e n s i t y t h a t o c c u r r e d i n my e n c l o s u r e s were a r e s u l t o f d e m o g r a p h i c a l c h a n g e s i n p o p u l a t i o n s i z e , s h o u l d t h e y not have been more r e g u l a r i n t h e i r p a t t e r n of r i s e and f a l l ? The d a p h n i d d e c l i n e s a l l o c c u r r e d r a p i d l y , o v e r one s a m p l i n g p e r i o d (1 week). R e c o v e r y was g e n e r a l l y v e r y slow and t o o k s e v e r a l weeks. T h e s e r a p i d d e c l i n e s s u g g e s t t h a t some mechanism o t h e r t h a n D a p h n i a d e m o g r a p h i c s was o c c u r r i n g . M o r e o v e r , i n two ( e n c l o s u r e s 4 and 7) o u t o f t h e 3 e n c l o s u r e s w i t h major d e c l i n e s l e a d i n g t o low mid-summer d e n s i t i e s , t h e d r o p o c c u r r e d a t t h e same t i m e as i n t h e l a k e and c o n t r o l p o p u l a t i o n s . In one o f t h e c a s e s ( e n c l o s u r e 4 ) , t h e r e s u l t i n g r e c o v e r y f o l l o w e d t h e c l a s s 1 d y n a m i c s d e s c r i b e d by Murdoch and M c C a u l e y ( 1 9 8 5 ) . In t h e o t h e r ( e n c l o s u r e 7 ) , t h e d a p h n i d p o p u l a t i o n r e m a i n e d low f o r t h e ""duration of t h e e x p e r i m e n t w i t h one b r i e f r e s u r g e n c e i n numbers ( w h i c h c o u l d have been a random s a m p l i n g e r r o r ) o c c u r r i n g i n t h e f a l l . W i t h r e s p e c t t o p r e d a t o r - p r e y o s c i l l a t i o n s , none a r e r e a l l y e v i d e n t . D e n s i t i e s of g r a z a b l e a l g a e do i n c r e a s e f o l l o w i n g d e c l i n e s i n t h e numbers of D a p h n i a ( A p p e n d i x 3 ) , but t h e i n c r e a s e s a r e more s u g g e s t i v e o f an a l g a l s p e c i e s o b t a i n i n g o p t i m a l g r o w t h c o n d i t i o n s and g r o w i n g so f a s t t h a t t h e d a p h n i d g r a z i n g c o u l d not h o l d them i n c h e c k , r e s u l t i n g i n an a l g a l b l o om. Thus, t h e r e s u l t s do n o t c l e a r l y f i t any of t h e dynamic p a t t e r n s o f D a p h n i a p o p u l a t i o n s d e s c r i b e d by Murdoch and 232 M c C a u l e y ( 1 9 8 5 ) . T h e r e must have been some o t h e r , l e s s c l e a r e x p l a n a t i o n s f o r t h e D a p h n i a d y n a m i c s I o b s e r v e d . Were t h e D a p h n i a p o p u l a t i o n s i n t h e f e r t i l i z e d e n c l o s u r e s f o o d - l i m i t e d d u r i n g t h e mid-summer d e s p i t e c o n t i n u o u s n u t r i e n t a d d i t i o n s t o keep f o o d l e v e l s h i g h , o r was some o t h e r f a c t o r r e s p o n s i b l e f o r t h e low mid-summer d a p h n i d d e n s i t i e s ? My r e s u l t s s u g g e s t t h a t D a p h n i a i n f e r t i l i z e d e n c l o s u r e s d u r i n g mid-summer were n o t f o o d - l i m i t e d . I f t h e l o w e r mid-summer d e n s i t i e s were a r e s u l t o f f o o d l i m i t a t i o n , one would e x p e c t t h a t l i t t l e e n e r g y would be a l l o t t e d f o r t h e p r o d u c t i o n of e g g s . H a l l (1964) s u g g e s t e d t h a t t h e a v e r a g e b r o o d s i z e o f an e q u i l i b r i u m p o p u l a t i o n o f D a p h n i a (one t h a t has r e a c h e d t h e c a r r y i n g c a p a c i t y of t h e e n v i r o n m e n t i n t e r m s o f f o o d s u p p l y ) s h o u l d be 1.0 e g g s / a d u l t of l e s s ( H a l l , 1964). In f a c t , d u r i n g t h e months of J u l y , A u g u s t and September, i n my s y s t e m , t h e a v e r a g e b r o o d s i z e s o f t h e D a p h n i a i n t h e l i t t o r a l and l i m n e t i c f e r t i l i z e d e n c l o s u r e s r a n g e d between 1.5 and 6.0 e g g s / a d u l t ( F i g u r e 3 8 ) . M o r e o v e r , t h e p r o p o r t i o n o f g r a v i d a d u l t d a p h n i d s r a n g e d from 50-90 % o f t h e p o p u l a t i o n o v e r t h e same p e r i o d . T h e s e d a t a a r e h a r d l y s u g g e s t i v e o f p o p u l a t i o n s i n e q u i l i b r i u m . I s u g g e s t t h a t t h e mid-summer D a p h n i a p o p u l a t i o n s i z e s i n t h e f e r t i l i z e d e n c l o s u r e s were m a i n t a i n e d by p r e d a t i o n r a t h e r t h a n low f o o d s u p p l y . The p r e d a t o r was p r o b a b l y c u t t h r o a t t r o u t . F i s h p r e d a t i o n on z o o p l a n k t o n i n t h i s s y s t e m was d i s c u s s e d i n some d e t a i l w i t h r e s p e c t t o c h a n g e s i n t h e z o o p l a n k t o n community s t r u c t u r e o f t h e c o n t r o l e n c l o s u r e s compared t o t h a t o f t h e l a k e , t h e r e f o r e , I w i l l d e a l w i t h i t 233 b r i e f l y h e r e . E v i d e n c e f o r f i s h p r e d a t i o n c a u s i n g t h e s e r e d u c e d mid-summer d e n s i t i e s o f d a p h n i d s i n t h e f e r t i l i z e d e n c l o s u r e s i s s p a r s e b u t , I b e l i e v e , s u b s t a n t i v e . R e c a l l t h a t t h e p r i m a r y z o o p l a n k t o n p r e y o f P l a c i d Lake c u t t h r o a t t r o u t a r e H o l o p e d i u m g i b b e r u m and D a p h n i a r o s e a ( N o r t h c o t e and C l a r o t t o , 1975). They a r e a l s o s e l e c t i v e i n t h e i r c h o i c e of f o o d b a s e d on s i z e , w i t h t h e i r p l a n k t i v o r y l a r g e l y r e s t r i c t e d t o H o l o p e d i u m and D a p h n i a o v e r 1.0 mm l o n g ( N o r t h c o t e and C l a r o t t o , 1975). The mean body s i z e s of t h e a d u l t d a p h n i d s d e c l i n e d s h a r p l y t o a maximum o f 1.1 mm from 1.3 t o 1.6 mm i n J u l y when f i s h p r e s e n c e was f i r s t n o t e d . A l s o , t h e d e n s i t i e s o f D a p h n i a f e l l t o l o w e r l e v e l s a f t e r t h e f i s h i n v a s i o n t h a n a t any o t h e r t i m e d u r i n g t h e summer, e x c e p t p e r h a p s , a t t h e b e g i n n i n g o f t h e e x p e r i m e n t . F i n a l l y , D a p h n i a were f o u n d i n abundance i n t h e stomachs o f f i s h c a p t u r e d w i t h i n t h e f e r t i l i z e d e n c l o s u r e s . D a p h n i d s i n t h e g u t s of t h e s e f i s h numbered i n t h e t h o u s a n d s (max = 7194 f o r one age 2+ f i s h ) i n J u l y t h r o u g h September. In c o n t r a s t , d a p h n i d s i n t h e g u t s o f f i s h c a p t u r e d w i t h i n t h e c o n t r o l e n c l o s u r e s numbered i n t h e h u n d r e d s (max = 160 f o r one unmeasured a d u l t ) i n J u l y , f a l l i n g t o o n l y t e n s (max = 88) t h r o u g h A u g u s t and September. T h e s e o b s e r v a t i o n s a r g u e f o r a s i g n i f i c a n t r o l e of f i s h p r e d a t i o n i n s t r u c t u r i n g t h e D a p h n i a p o p u l a t i o n s i n t h e f e r t i l i z e d e n c l o s u r e s . W h i l e t h e numbers o f d a p h n i d s / l i t e r were on a v e r a g e o n l y 5X h i g h e r t h a n c o n t r o l v a l u e s a t any g i v e n t i m e d u r i n g t h e e x p e r i m e n t , t h e numbers o f d a p h n i d s consumed by f i s h i n c r e a s e d by a f a c t o r o f 100 - 1000. 234 M a j o r s h o r t - t e r m (1-2 weeks) d e c l i n e s i n t h e p o p u l a t i o n s i z e s of D a p h n i a , w h i c h were not a p p a r e n t i n any of t h e o t h e r f e r t i l i z e d e n c l o s u r e s o r t h e c o n t r o l s , o c c u r r e d i n e n c l o s u r e s 4 and 7 i n J u l y ( F i g u r e 3 1 ) . (The d e c l i n e s a r e not as e v i d e n t f o r e n c l o s u r e 7 i n F i g u r e 31 b e c a u s e o f t h e l o g - s c a l e a x i s . ) These d e c l i n e s were p a r a l l e l e d by v e r y s h a r p d e c r e a s e s i n t h e a v e r a g e b r o o d s i z e ( F i g u r e 38) and, i n e n c l o s u r e 7 o n l y , by a d e c r e a s e i n t h e p r o p o r t i o n of g r a v i d f e m a l e s . T h i s l e d me t o b e l i e v e t h a t t h e r e may have been e n c l o s u r e s p e c i f i c i n c i d e n c e ' s of o v e r g r a z i n g of t h e f o o d s u p p l y due t o e x t r e m e l y r a p i d i n c r e a s e s i n t h e numbers of D a p h n i a . S i n c e t h e p h y t o p l a n k t o n d a t a gave no c l e a r i n d i c a t i o n t h a t t h e f o o d r e s o u r c e i n t h e s e e n c l o s u r e s had been o v e r g r a z e d , I d e c i d e d t o t e s t whether t h e s t a n d i n g s t o c k of z o o p l a n k t o n , p r e c e d i n g t h e s e d e c l i n e s , would have been c a p a b l e of d e c i m a t i n g t h e a v a i l a b l e f o o d s u p p l y . Peak z o o p l a n k t o n b i o m a s s e s p r e c e e d i n g t h e d e c l i n e s a v e r a g e d 450 u g / l i t e r ( F i g u r e 2 5 ) . Buckingham (1978) e s t i m a t e d maximum f i l t e r i n g r a t e s f o r P l a c i d Lake D. r o s e a t o be a r o u n d 1-1.5 m l / ug z o o p l a n k t o n / day f o r z o o p l a n k t e r s e x p o s e d t o f o o d c o n c e n t r a t i o n s i n t h e ran g e of 100-500 u g / l i t e r d r y w e i g h t . T h e s e f i l t e r i n g r a t e s a r e a p p r o p r i a t e t o use i n e s t i m a t i n g t h e t o t a l community f i l t e r i n g r a t e s i n my e n c l o s u r e s d u r i n g t h e s e b i o m a s s p e a k s s i n c e 70-90 % o f t h e z o o p l a n k t o n b i o m a s s a t t h e s e t i m e s was made up o f D a p h n i a . A l s o , t h i s r a n g e of g r a z i n g r a t e s i s c o m p a r a b l e t o t h e ran g e of f i l t e r i n g r a t e s r e p o r t e d f o r 1.3 -1.6 mm D a p h n i a p u l e x ( r o u g h l y t h e s i z e r a n g e of D. r o s e a i n my e n c l o s u r e s ) by Haney (1985, h i s f i g u r e s 1 and 2 ) . I f one 235 assumes t h a t t h e a v e r a g e w e i g h t of a 1.3-1.6 D. p u l e x t o be a r o u n d 10 ug ( d e r i v e d f r o m my b i o m a s s e s t i m a t e of 450 u g / l i t e r o f a x e n i c m o n o c u l t u r e of D a p h n i a d i v i d e d by t h e maximum number of i n d i v i d u a l s p e r l i t e r , 45, see F i g u r e 31) t h e n Haney's f i l t e r i n g r a t e s a r e 1.0-1.5 m l / ug z o o p l a n k t o n / day. A s s u m i n g t h e s e f i l t e r i n g r a t e s a r e r e a l i s t i c e s t i m a t e s of t h e g r a z i n g p r e s s u r e e x e r t e d i n my e n c l o s u r e s a t peak b i o m a s s t i m e s , t h e n t h e g r a z e r s were o n l y t a k i n g 45-70 % of t h e p h y t o p l a n k t o n b i o m a s s p e r d a y . I have a l r e a d y shown above t h a t p h y t o p l a n k t o n r e n e w a l r a t e s i n t h e f e r t i l i z e d e n c l o s u r e s a p p e a r t o be s u f f i c i e n t t o r e s u l t i n more t h a n 100 % o f t h e s t a n d i n g s t o c k i n one day. In s u p p o r t o f t h i s , e x a m i n a t i o n of a s h o r t b u r s t of a l g a l b i o m a s s o v e r 2 weeks i n e n c l o s u r e 8 i n A u g u s t ( F i g u r e 17) s u g g e s t s t h a t t h e n e t d a i l y g r o w t h r a t e of p h y t o p l a n k t o n b i o m a s s i n t h e f e r t i l i z e d e n c l o s u r e s c o u l d be as h i g h as 0.6 p e r day. T h i s was h i g h enough t o p e r m i t a bloom t o o c c u r , s i n c e z o o p l a n k t o n were r e m o v i n g o n l y a b o u t 45-70 % o f t h e a l g a l b i o m a s s d a i l y . T h u s , i t a p p e a r s t h a t t h e s h a r p d e c l i n e s i n d a p h n i d d e n s i t i e s i n e n c l o s u r e s 4 and 7, i n J u l y , may n o t have been due t o o v e r g r a z i n g o f t h e f o o d s u p p l y . In a c c o r d a n c e w i t h t h i s , l a r g e p o p u l a t i o n s o f o t h e r h e r b i v o r o u s z o o p l a n k t o n l i k e H o l o p e d i u m ( i n e n c l o s u r e 4 ) , Diaphanosoma and D. o r e g o n e n s i s , a l l s p e c i e s w h i c h a p p e a r t o f e e d on t h e same s i z e s p e c t r u m of f o o d a s D a p h n i a , a l m o s t i n s t a n t a n e o u s l y i n c r e a s e d i n number t o r e p l a c e D a p h n i a as t h e community d o m i n a n t s . T h i s s u g g e s t s t h a t t h e a v a i l a b i l i t y o f f o o d was not l i k e l y t o have been a f a c t o r 236 a s s o c i a t e d w i t h t h e s e o b s e r v e d D a p h n i a d e c l i n e s . I s t h e r e t h e n some o t h e r f a c t o r r e l a t e d t o f o o d s u p p l y o t h e r t h a n f o o d q u a n t i t y t h a t may have r e d u c e d t h e c o m p e t i t i v e a d v a n t a g e o f D. r o s e a t h e r e b y a l l o w i n g Diaphanosoma o r D i a p t o m u s  o r e g o n e n s i s t o o u t c o m p e t e t h e d a p h n i d s ? T h e r e i s e v i d e n c e i n t h e l i t e r a t u r e t h a t t h e q u a l i t y of f o o d may be an i m p o r t a n t f a c t o r i n t h e t e m p o r a l d y n a m i c s of l a r g e c l a d o c e r a n s s u c h as D a p h n i a . G l i w i c z (1980) f o u n d t h a t summer blooms of n e t p h y t o p l a n k t o n ( > 50 um) were r e s p o n s i b l e f o r d e c r e a s e s i n t h e e f f e c t i v e f i l t e r i n g r a t e , f e c u n d i t y and d e n s i t i e s o f t h e " s p r i n g " c l a d o c e r a n D a p h n i a c u c u l l a t a . The d e c l i n e i n d a p h n i d s was a c c o m p a n i e d by i n c r e a s e s i n Diaphanosoma b r a c h y u r u m and C h y d o r i s s p h a e r i c u s w h i c h were p o s t u l a t e d by G l i w i c z (1980) t o be l e s s a f f e c t e d by t h e p r e s e n c e of l a r g e a l g a e t h a t m i g h t i n t e r f e r e w i t h f i l t r a t i o n . T h i s i m p l i e s t h a t w h i l e t h e q u a n t i t y of g r a z a b l e a l g a e may n o t be l i m i t i n g , t h e D a p h n i a were n o t a b l e t o h a r v e s t i t e f f e c t i v e l y and were e x c l u d e d by s p e c i e s w h i c h were more e f f i c i e n t f i l t e r f e e d e r s i n a low q u a l i t y f o o d e n v i ronment. S i m i l a r f i n d i n g s were r e p o r t e d by Richman and Dodson (1983) f o r c l a d o c e r a n - c o p e p o d i n t e r a c t i o n s . T h e s e w o r k e r s f o u n d t h a t even a t a h i g h f o o d a b u n d a n c e , D a p h n i a p u l e x was e x c l u d e d by D i a p t o m u s s i c i l o i d e s i n a c o m p e t i t i o n e x p e r i m e n t when t h e p r o p o r t i o n of n e t p l a n k t o n (as number of c e l l s ) i n t h e t e s t medium e x c e e d e d 40 %. The r e a s o n g i v e n f o r t h e c o m p e t i t i v e a d v a n t a g e o f t h e c o p e p o d o v e r t h e c l a d o c e r a n was t h a t t h e 237 s e l e c t i v e f e e d i n g a b i l i t y of c o p e p o d s a l l o w s them t o f e e d more e f f i c i e n t l y i n a l o w - q u a l i t y f o o d e n v i r o n m e n t by s e l e c t i n g e d i b l e forms and a v o i d i n g i n e d i b l e s p e c i e s . The c l a d o c e r a n s on t h e o t h e r hand, expend much of t h e i r e n e r g y r e j e c t i n g c o l l e c t e d f o o d when n e t p h y t o p l a n k t o n a r e f o u n d i n a b u ndance. Thus t h e c o p e p o d has a g r e a t e r e n e r g e t i c e f f i c i e n c y t h a n t h e c l a d o c e r a n w h i c h a l l o w s i t t o o utcompete t h e c l a d o c e r a n when f o o d q u a l i t y i s p o o r . Net p h y t o p l a n k t o n were abundant i n e n c l o s u r e s 4 and 7 i n J u l y when t h e a f o r e - m e n t i o n e d p e r i o d s of D a p h n i a d e c l i n e o c c u r r e d . In e n c l o s u r e 4 and 7, n e t p h y t o p l a n k t o n c o n s t i t u t e d 65 and 50-70 % o f t h e p h y t o p l a n k t o n b i o m a s s , r e s p e c t i v e l y a t t h e s e t i m e s ( F i g u r e 2 4 ) . B o t h d a p h n i d d e n s i t y and b r o o d s i z e d e c r e a s e d i n a c c o r d a n c e w i t h t h e o b s e r v a t i o n s o f G l i w i c z ( 1 9 8 0 ) . T h e r e f o r e , n e t p h y t o p l a n k t o n i n t e r f e r e n c e c a n e x p l a i n t h e s e e n c l o s u r e - s p e c i f i c s h a r p d e c l i n e s i n D. r o s e a a b u n d a n c e . In c o n c l u s i o n , D a p h n i a r o s e a g r e a t l y b e n e f i t e d from t h e a l g a l enhancement b r o u g h t on by n u t r i e n t a d d i t i o n . P e r i o d i c i n c r e a s e s i n t h e n e t p h y t o p l a n k t o n i n some o f t h e e n c l o s u r e s a p p e a r t o have had d e v a s t a t i n g e f f e c t s on t h e p e r f o r m a n c e of t h e d a p h n i d s . T h i s i s i m p o r t a n t i n t h a t i t s u g g e s t s t h a t t o a c h i e v e maximum p r o d u c t i o n o f D a p h n i a a s z o o p l a n k t o n i c f i s h f o o d , a t t e m p t s must be made t o keep n e t p h y t o p l a n k t o n t o a minimum. F e r t i l i z a t i o n w i t h h i g h N:P r a t i o f e r t i l i z e r s may be one way t o c o n t r o l n e t p h y t o p l a n k t o n by e n h a n c i n g s m a l l c h l o r o c o c c o i d s and f l a g e l l a t e s t h a t a r e more c h a r a c t e r i s t i c o f p h o s p h o r o u s l i m i t e d , g e n e r a l l y o l i g o t r o p h i c l a k e s a s i s shown i n t h i s s t u d y . 238 O c c u r r e n c e s o f n e t p h y t o p l a n k t o n were g r e a t l y m i n i m i z e d o v e r t h o s e t h a t a p p e a r e d i n c o n t r o l s , however, i t a p p e a r s t h a t t h e i r p r e s e n c e i s not t o t a l l y s u p p r e s s e d by t h i s method o f f e r t i 1 i z a t i o n . (3) Diaphanosoma b r a c h y u r u m Diaphanosoma a r e a major component o f t h e summer z o o p l a n k t o n a s s e m b l a g e i n t h e U.B.C. R e s e a r c h F o r e s t l a k e s . D e n s i t i e s o f t h i s s p e c i e s a r e h i g h e r i n P l a c i d Lake t h a n i n most of t h e o t h e r l a k e s i n t h e r e s e a r c h f o r e s t ( N o r t h c o t e and C l a r o t t o , 1975). Maximum d e n s i t i e s i n P l a c i d L a k e s e l d o m e x c e e d 10 i n d i v i d u a l s / l i t e r ( N o r t h c o t e and C l a r o t t o , 1975; K r a u s e , 1984; t h i s s t u d y ) . T y p i c a l l y , a b u n d a n c e s o f Diaphanosoma a r e low d u r i n g t h e s p r i n g , b e g i n t o i n c r e a s e i n e a r l y J u l y r e a c h i n g maximum d e n s i t i e s by e a r l y A u g u s t , and d e c l i n e s h a r p l y i n September ( K r a u s e , 1984; t h i s s t u d y ) . T h i s s e a s o n a l c y c l e of r i s e and f a l l was n o t d i s r u p t e d f o l l o w i n g f e r t i l i z a t i o n . However, a d r a m a t i c n u m e r i c a l r e s p o n s e , t h a t was h i g h l y v a r i a b l e from one e n c l o s u r e t o a n o t h e r , o c c u r r e d a s a r e s u l t of f e r t i l i z a t i o n . Maximum d e n s i t i e s i n two o f t h e f o u r f e r t i l i z e d e n c l o s u r e s e x c e e d e d 100 i n d i v i d u a l s / l i t e r . Maximum d e n s i t i e s i n t h e o t h e r two f e r t i l i z e d e n c l o s u r e s r a n g e d between 40 and 60 i n d i v i d u a l s / l i t e r . T h e s e maxima were a t t a i n e d a t d i f f e r e n t t i m e s d u r i n g t h e summer. The v a r i a b i l i t y i n t h e n u m e r i c a l r e s p o n s e o f t h e Diaphanosoma between t h e f e r t i l i z e d e n c l o s u r e s a p p e a r s t o have 239 been s t r o n g l y i n f l u e n c e d by t h e p r e s e n c e of D a p h n i a r o s e a . The g r e a t e s t i n c r e a s e s i n a b u n d a n c e s of Diaphanosoma o c c u r r e d i n c o n j u n c t i o n w i t h major d e c l i n e s i n t h e d a p h n i d s . T h i s was a l s o t r u e f o r Diaphanosoma p o p u l a t i o n s i n t h e l a k e and c o n t r o l s . Diaphanosoma a p p e a r e d t o p e r f o r m much b e t t e r i n t h e f e r t i l i z e d e n c l o s u r e s where d a p h n i d a b u n d a n c e s d r o p p e d below 20 i n d i v i d u a l s / l i t e r . T h i s was p a r t i c u l a r l y n o t i c e a b l e d u r i n g t h e e a r l y summer c r a s h e s of t h e d a p h n i d p o p u l a t i o n s i n e n c l o s u r e s 4 and 7. In e n c l o s u r e s 3 and 8, where e a r l y and mid-summer d a p h n i d d e n s i t i e s a l w a y s e x c e e d e d 20 i n d i v i d u a l s / l i t e r , Diaphanosoma numbers r e m a i n e d below a b o u t 30-40 i n d i v i d u a l s / l i t e r . O n l y when d a p h n i d d e n s i t i e s i n e n c l o s u r e 3 d e c l i n e d p r e c i p i t o u s l y i n l a t e A u g u s t , p r o b a b l y a s a r e s u l t o f i n t e n s i v e p r e d a t i o n p r e s s u r e by f i s h e s , d i d Diaphanosoma i n c r e a s e t o maximum d e n s i t i e s f o r t h a t e n c l o s u r e . The i n t e r a c t i o n between t h e s e two c l a d o c e r a n s s u g g e s t s t h e e x i s t e n c e of some f o r m o f e x p l o i t a t i o n c o m p e t i t i o n . The s u g g e s t i o n t h a t D a p h n i a r o s e a and Diaphanosoma  b r a c h y u r u m compete i s n o t a new one. Marmorek (1983) c o n c l u d e d t h a t i n c r e a s e s i n d e n s i t i e s o f Diaphanosoma i n E u n i c e L a k e e n c l o s u r e s t h a t were s u b j e c t e d t o combined t r e a t m e n t s o f a c i d i f i c a t i o n and n u t r i e n t e n r i c h m e n t , were l a r g e l y due t o a c i d i f i c a t i o n i n d u c e d d e c l i n e s o f D. r o s e a . F u r t h e r , N e i l l (1984) f o u n d t h a t D. r o s e a r e m o v a l from f e r t i l i z e d e n c l o s u r e s r e s u l t e d i n s u b s t a n t i a l i n c r e a s e s i n t h e d e n s i t i e s o f D. b r a c h y u r u m . C o m p e t i t i o n between t h e s e two s p e c i e s was n o t i n f e r r e d by N e i l l ( 1 9 8 4 ) , t h o u g h h i s d a t a s u p p o r t t h e 240 h y p o t h e s i s . D e s p i t e i n c r e a s e d d e n s i t i e s o f Diaphanosoma i n t h e t h e f e r t i l i z e d e n c l o s u r e s (> 1 0 0 / l i t e r i n e n c l o s u r e s 4 and 7 ) , Diaphanosoma were n e v e r f o u n d i n t h e stomachs o f any of 28 f i s h were c a p t u r e d i n t h o s e e n c l o s u r e s , nor i n any o f t h e stomachs of 30 f i s h n e t t e d i n t h e c o n t r o l e n c l o s u r e s and t h e l a k e o v e r t h e summer. N o r t h c o t e and C l a r o t t o (1975) f o u n d t h a t Diaphanosoma was v e r y r a r e l y e a t e n by P l a c i d Lake c u t t h r o a t . P o s s i b l y t h e i r s m a l l s i z e (<0.8 mm) and t r a n s p a r e n t b o d i e s o r t h e i r c r y p t i c swimming b e h a v i o u r ( B r o o k s , 1968) e x p l a i n t h e i r a b s e n c e from f i s h s t o m a c h s . (4) Bosmina l o n g i r o s t r i s Bosmina l o n g i r o s t r i s i s a p a n m i c t i c c l a d o c e r a n w h i c h c a n t o l e r a t e a l l e x t r e m e s r e l a t i n g t o l a k e morphometry, t e m p e r a t u r e and w a t e r c h e m i s t r y ( P a t a l a s , 1971; C a r t e r e t a_l. , 1980). I t i s o f t e n a dominant member of t h e z o o p l a n k t o n community i n bog l a k e s (Haney, 1973), however, i t a p p e a r s t h a t i t s dominance i s r e s t r i c t e d t o l a k e s where a l l o t h e r c l a d o c e r a n s a r e p o o r l y r e p r e s e n t e d ( P a t a l a s , 1971, t a b l e 1; P a t a l a s and S a l k i , 1973, t a b l e 2 ) , o r , i f t h e y a r e p r e s e n t a t a l l , t h e y a r e s m a l l i n s i z e ( i . e . D a p h n i a c a t a w b a , D a p h n i a r e t r o c u r v a , and Diaphanosoma  b r a c h y u r u m ) . S i n c e t h e z o o p l a n k t o n a s s e m b l a g e i n P l a c i d Lake i s n u m e r i c a l l y d o m i n a t e d by l a r g e r c l a d o c e r a n s l i k e D a p h n i a r o s e a and H o l o p e d i u m g i b b e r u m f o r most o f t h e summer, i t i s n o t s u r p r i s i n g t h a t B. l o n g i r o s t r i s i s f o u n d o n l y i n v e r y low 241 numbers ( < 1 / I i t e r ) . B. l o n g i r o s t r i s d i d a p p e a r i n s i g n i f i c a n t l y g r e a t e r d e n s i t i e s i n t h e f a l l i n t h e l a k e when t h e d e n s i t i e s of a l l of t h e o t h e r c l a d o c e r a n s were a t t h e i r l o w e s t l e v e l s of t h e summer and i n e a r l y A u g u s t i n t h e c o n t r o l e n c l o s u r e s when d e n s i t i e s of D. r o s e a and H o l o p e d i u m were s h a r p l y r e d u c e d . D e s p i t e i t s o c c u r r e n c e i n l a k e s of a l l t y p e s , Bosmina  l o n g i r o s t r i s has been t a k e n t o be an a n i m a l i n d i c a t o r of e u t r o p h i c a t i o n ( B r o o k s , 1969). T h i s a c c o r d i s b a s e d s o l e l y on t h e f a c t t h a t t h i s b o s m i n i d i s o f t e n t h e o n l y one f o u n d i n e u t r o p h i c l a k e s , h a v i n g r e p l a c e d a l l o t h e r s p e c i e s o f Bosmina t h a t p r e v i o u s l y o c c u r r e d i n t h e l a k e s p r i o r t o e u t r o p h i c a t i o n . However, i t i s n o t c l e a r why Bosmina l o n g i r o s t r i s a c t u a l l y becomes a community do m i n a n t i n l a k e s f o l l o w i n g e u t r o p h i c a t i o n . I t s p e r f o r m a n c e may be d i r e c t l y i n f l u e n c e d by c h e m i c a l e n r i c h m e n t p r o c e s s e s o r a l t e r a t i o n s i n t h e p o p u l a t i o n s of p l a n k t i v o r o u s f i s h f o l l o w i n g e u t r o p h i c a t i o n ( B r o o k s , 1969). Some r e p o r t s i n t h e l i t e r a t u r e s u g g e s t t h a t Bosmina  l o n g i r o s t r i s may not be e n h a n c e d as a d i r e c t r e s u l t o f n u t r i e n t e n r i c h m e n t . Edmondson and L i t t (1982) f o u n d t h a t Bosmina  l o n g i r o s t r i s a c t u a l l y became more ab u n d a n t i n L a k e W a s h i n g t o n a f t e r sewage d i v e r s i o n r e s u l t e d i n a r e v e r s a l o f t h e e u t r o p h i c a t i o n p r o c e s s . A l s o , McNaught e_t al. (1983) f o u n d t h a t t h e p r o p o r t i o n o f B. l o n g i r o s t r i s i n t h e z o o p l a n k t o n community of Saginaw Bay, Lake Huron, r e m a i n e d c o n s t a n t f o l l o w i n g n u t r i e n t ' d i v e r s i o n and an improvement i n w ater q u a l i t y . My s t u d y shows t h a t Bosmina l o n g i r o s t r i s may n o t be an 242 i n d i c a t o r of e u t r o p h i c a t i o n i f i t has h i s t o r i c a l l y been t h e o n l y b o s m i n i d i n t h e l a k e b e i n g i n v e s t i g a t e d . My r e s u l t s show t h a t i n c r e a s e s i n t h e abundance of B. l o n g i r o s t r i s f o l l o w i n g n u t r i e n t e n r i c h m e n t were c o n f i n e d t o o n l y one e n c l o s u r e , l i m n e t i c f e r t i l i z e d e n c l o s u r e 4, even t h o u g h a l l o f t h e f e r t i l i z e d e n c l o s u r e s were s u b j e c t e d t o t h e same f e r t i l i z a t i o n r e g i m e . The d e n s i t y o f b o s m i n i d s i n e n c l o s u r e 4 e x c e e d e d 1 0 0 0 / l i t e r i n O c t o b e r . D e n s i t i e s i n a l l of t h e o t h e r f e r t i l i z e d e n c l o s u r e s were below 1 0 / l i t e r . The u n u s u a l l y h i g h d e n s i t y r e p o r t e d f o r o n l y one s a m p l i n g d a t e i n O c t o b e r may have been a s a m p l i n g a r t i f a c t . I may have e n c o u n t e r e d a Bosmina "swarm" d u r i n g s a m p l i n g . Bosmina swarming has been r e p o r t e d t o o c c u r i n o t h e r bog l a k e s . Haney (1973) r e p o r t s t h a t one sample from Drowned Bog L a k e n e a r D o r s e t , O n t a r i o c o n t a i n e d i n e x c e s s o f 20,000 b o s m i n i d s / 1 i t e r . Normal l a k e d e n s i t i e s r a n g e d between 1000-3000 b o s m i n i d s / l i t e r . A l s o , J a r n e f e l t (1956) r e p o r t e d o c c u r r e n c e s of d e n s e c l o u d s of b o s m i n i d s i n s e v e r a l bog l a k e s i n F i n l a n d . R e g a r d l e s s o f t h e p o s s i b i l i t y of a swarm c o n t r i b u t i n g t o t h e u n u s u a l l y h i g h d e n s i t y r e c o r d e d i n O c t o b e r , i t i s c l e a r t h a t B. l o n g i r o s t r i s i n e n c l o s u r e 4 d e r i v e d some b e n e f i t from f e r t i l i z a t i o n . T h i s i s s u p p o r t e d by t h e f a c t t h a t t h e b o s m i n i d s i n t h i s e n c l o s u r e p r o g r e s s i v e l y i n c r e a s e d i n d e n s i t y s t a r t i n g i n l a t e A u g u s t . The q u e s t i o n i s , why d i d f e r t i l i z a t i o n o n l y b e n e f i t B. l o n g i r o s t r i s i n o n l y one o f t h e f e r t i l i z e d e n c l o s u r e s ? I b e l i e v e t h a t t h e i m p r o v e d p e r f o r m a n c e o f t h e b o s m i n i d s i n e n c l o s u r e 4 was a r e s u l t o f two f a c t o r s : a l a r g e r 243 i n c r e a s e i n t h e q u a n t i t y of s m a l l a l g a l c e l l s i n d u c e d by f e r t i l i z a t i o n t h a n o c c u r r e d i n a l l of t h e o t h e r f e r t i l i z e d e n c l o s u r e s , and a r e d u c t i o n i n c o m p e t i t i v e l i m i t a t i o n of B. l o n g i r o s t r i s by D. r o s e a i n d u c e d by i n t e n s i v e f i s h p r e d a t i o n e x e r t e d upon t h e l a t t e r . W i t h r e g a r d t o t h e f i r s t h y p o t h e s i s , t h e p h y t o p l a n k t o n t h a t bloomed (80-90 % o f t h e t o t a l b i o m a s s ) i n e n c l o s u r e 4 i n l a t e A u g u s t were v e r y s m a l l forms of A n k i s t r o d e s m u s . The a v e r a g e maximum l i n e a r d i m e n s i o n f o r t h e s e c e l l s (14.3 um) was much s m a l l e r t h a n t h e a v e r a g e f o r t h e same s p e c i e s i n a l l of t h e o t h e r f e r t i l i z e d e n c l o s u r e s (19.8 um) w h i c h d e v e l o p e d s i g n i f i c a n t c o n c e n t r a t i o n s of A n k i s t r o d e s m u s but not Bosmina. A l s o , t h e maximum d e n s i t y of t h e s e a l g a l c e l l s i n e n c l o s u r e 4 (1.1 X 10 B c e l l s / l i t e r ) was s i g n i f i c a n t l y h i g h e r t h a n i n i t s r e p l i c a t e , e n c l o s u r e 8 (7.5 X 10 7 c e l l s / l i t e r ) o r i n t h e l i t t o r a l e n c l o s u r e s (1-2 X 10 7 c e l l s / l i t e r ) . B u r n s (1968a) f o u n d t h a t t h e maximum s i z e of p a r t i c l e s t h a t c a n be i n g e s t e d by B. l o n g i r o s t r i s was v e r y c l o s e t o 20 um. The a v e r a g e s i z e o f t h e most abundant a l g a l p a r t i c l e s i n e n c l o s u r e 4 a t t h e t i m e of t h e Bosmina o u t b r e a k f e l l w e l l s h o r t of t h i s maximum, w h i l e i n a l l o f t h e o t h e r f e r t i l i z e d e n c l o s u r e s , t h e a v e r a g e p a r t i c l e s i z e was c l o s e r t o t h e maximum s i z e i n g e s t i b l e . T h e r e f o r e , much more f o o d of manageable s i z e was a v a i l a b l e t o t h e b o s m i n i d s i n e n c l o s u r e 4 t h a n t o t h o s e i n t h e o t h e r f e r t i l i z e d e n c l o s u r e s . W i t h r e s p e c t t o c o m p e t i t i v e i n t e r a c t i o n s w i t h D. r o s e a , t h e r e i s e v i d e n c e i n t h e l i t e r a t u r e t h a t t h e s e two c l a d o c e r a n s 244 do compete. N e i l l (1983) f o u n d t h a t r e m o v a l o f D. r o s e a f r o m n u t r i e n t e n r i c h e d e n c l o s u r e s r e s u l t e d i n d r a m a t i c i n c r e a s e s i n B. l o n g i r o s t r i s . Marmorek (1983) f o u n d t h a t when D. r o s e a d i e d out d u r i n g combined a c i d i f i c a t i o n and n u t r i e n t e n r i c h m e n t e x p e r i m e n t s , B. l o n g i r o s t r i s d e v e l o p e d a more d r a m a t i c and s u s t a i n e d n u m e r i c a l r e s p o n s e t h a n i n e n c l o s u r e t h a t r e c e i v e d o n l y n u t r i e n t a d d i t i o n s but r e t a i n e d t h e i r D a p h n i a p o p u l a t i o n s . F i n a l l y , L y n c h (1978) f o u n d t h a t i n a s e r i e s of c o m p e t i t i o n e x p e r i m e n t s i n e n c l o s u r e s i n n u t r i e n t - r i c h P l e a s a n t Pond, Bosmina was c o n s i s t e n t l y o u t - c o m p e t e d by D a p h n i a . The e x t e n t of t h e c o m p e t i t i v e i n t e r a c t i o n s between t h e s e two s p e c i e s i s g r e a t l y r e g u l a t e d by r e s o u r c e a v a i l a b i l i t y ( K e r f o o t and DeMott, 1980) and s i z e - s p e c i f i c p r e d a t i o n ( B r o o k s , 1969). The f o r m e r w o r k e r s showed t h a t t h e l e v e l of c o m p e t i t i v e i n t e r a c t i o n between D a p h n i a and Bosmina was p a r t i c u l a r l y i n t e n s e under l a b o r a t o r y c o n d i t i o n s where b o t h s p e c i e s were f o r c e d t o s h a r e a s i n g l e l i m i t i n g r e s o u r c e , s m a l l a l g a l p a r t i c l e s . Under n a t u r a l l a k e c o n d i t i o n s w h i c h s u p p l i e d a more d i v e r s e s p e c t r u m of f o o d p a r t i c l e s , D a p h n i a and Bosmina c o u l d c o e x i s t . The f o o d r e s o u r c e i n e n c l o s u r e 4 i n l a t e A u g u s t c o n s i s t e d a l m o s t e x c l u s i v e l y of a s i n g l e r e s o u r c e , s m a l l A n k i s t r o d e s m u s , but t h e amount o f f o o d a v a i l a b l e s u g g e s t s t h a t i t may have been s u f f i c i e n t t o m a i n t a i n a c o e x i s t i n g p o p u l a t i o n o f D a p h n i a and Bosmina p r o v i d e d t h a t t h e D a p h n i a p o p u l a t i o n was p r e v e n t e d i n some way from i n c r e a s i n g i n a b u n d a n c e . I n t e n s e s i z e - s p e c i f i c p r e d a t i o n e x e r t e d on t h e D a p h n i a c o u l d have s u p p r e s s e d D a p h n i a p r o d u c t i o n t o a l e v e l where t h e s m a l l e r Bosmina c o u l d have 245 e s c a p e d from p r o b a b l e c o n d i t i o n s o f f o o d l i m i t a t i o n . I n d e e d , a b u n d a n c e s o f D a p h n i a i n e n c l o s u r e 4, b e f o r e t h e Bosmina i n c r e a s e , were much low e r t h a n c o u l d be a c c o u n t e d f o r by t h e q u a l i t y and q u a n t i t y o f t h e a v a i l a b l e f o o d s u p p l y ; a f o o d s u p p l y t h a t c o u l d be e f f i c i e n t l y h a r v e s t e d by t h e b o s m i n i d s . Had t h e r e n o t been f i s h p r e d a t i o n e x e r t e d i n e n c l o s u r e 4, i t i s q u i t e l i k e l y t h a t t h e Bosmina would n o t have r e a c h e d s u c h h i g h d e n s i t i e s b e c a u s e t h e y would have been o u t - c o m p e t e d by t h e D a p h n i a . In c o n c l u s i o n , n u t r i e n t e n r i c h m e n t a l o n e d i d n o t r e s u l t i n an i n c r e a s e i n t h e d e n s i t i e s o f Bosmina i n t h e f e r t i l i z e d e n c l o s u r e s . Where t h e b o s m i n i d s d i d a p p e a r t o b e n e f i t f r o m f e r t i l i z a t i o n , a l a r g e r e d u c t i o n i n t h e numbers of one o f i t s c h i e f c o m p e t i t o r s ( t h e d a p h n i d s ) , t h r o u g h s i z e - s e l e c t i v e p r e d a t i o n , seems t o have been n e c e s s a r y . (5) C e r i o d a p h n i a q u a d r a n g u l a As w i t h t h e u n i q u e b o s m i n i d , C. q u a d r a n g u l a i s a h i s t o r i c a l l y r a r e component o f t h e P l a c i d Lake z o o p l a n k t o n community w h i c h sometimes a p p e a r s i n m o d e r a t e numbers i n t h e h y p o l i m n i o n o f t h e l a k e (W.E. N e i l l , p e r s o n a l c o m m u n i c a t i o n ) . D e n s i t i e s o f t h i s a n i m a l n e v e r e x c e e d e d 1 p e r l i t e r ( a v e r a g e d o v e r t h e e n t i r e w a ter column) i n 1982. The s p e c i e s ' s e a s o n a l abundance p r o f i l e was m o n o c y c l i c , w i t h maximum d e n s i t i e s o c c u r r i n g i n t h e f a l l . B o t h e n c l o s u r e and f e r t i l i z a t i o n r e s u l t e d i n e n h a n c e d l e v e l s of C e r i o d a p h n i a p r o d u c t i o n , however, e x c e p t f o r i t s 246 p e r f o r m a n c e i n l i t t o r a l f e r t i l i z e d e n c l o s u r e 3, i n c r e a s e s i n t h e a b u n d a n c e s of t h i s s p e c i e s were i n s i g n i f i c a n t compared t o t h o s e of t h e o t h e r h e r b i v o r e s i n t h e s y s t e m . B a s e l i n e i n c r e a s e s of C e r i o d a p h n i a i n b o t h t h e c o n t r o l and f e r t i l i z e d t r e a t m e n t s , i . e . t h e e n c l o s u r e e f f e c t , were p r o b a b l y f o o d r e l a t e d . The p r e f e r r e d f o o d o f C e r i o d a p h n i a q u a d r a n g u l a i s b a c t e r i a (Gophen e t a l . , 1974; Smyly, 1975; DeMott, 1985). D e n s i t i e s o f C. q u a d r a n g u l a were a l w a y s h i g h e s t i n a l l t r e a t m e n t e n c l o s u r e s i n t h e 4-6 m d e p t h s t r a t u m . G i v e n t h e i r p r e f e r e n c e f o r m i c r o b i a l m a t t e r , t h e s e a n i m a l s were p r o b a b l y g r a z i n g on d e c o m p o s i n g d e t r i t u s t h a t had s e t t l e d o u t f rom t h e e p i l i m n e t i c w a t e r s . D e c r e a s e d e n t r a i n m e n t of t h e m e t a l i m n e t i c w a t e r s i n t h e c o n t r o l e n c l o s u r e s (a p r e v i o u s l y d i s c u s s e d e f f e c t of e n c l o s u r e ) may have r e s u l t e d i n a h i g h e r t h a n u s u a l a c c u m u l a t i o n o f m i c r o b i a l m a t t e r i n t h e t h e t h e r m a l l y i n d u c e d d e n s i t y g r a d i e n t a t a r o u n d 4m. I f t h i s i s t r u e , t h e n t h e i n c r e a s e d d e n s i t i e s o f C e r i o d a p h n i a i n t h e c o n t r o l s compared t o t h e l a k e may have been due t o t h i s u n e x p e c t e d , n o n - n u t r i e n t i n d u c e d , f o o d e n r i c h m e n t . H i g h e r d e n s i t i e s of d e t r i t a l m a t t e r ( l e a d i n g t o i n c r e a s e d m i c r o b i a l p r o d u c t i o n ) i n t h e f e r t i l i z e d e n c l o s u r e s , r e s u l t i n g d i r e c t l y f r o m i n c r e a s e d a l g a l p r o d u c t i o n i n d u c e d by n u t r i e n t e n r i c h m e n t , would a c c o u n t f o r t h e h i g h e r d e n s i t i e s o f C e r i o d a p h n i a i n t h e f e r t i l i z e d t r e a t m e n t s . The p e r f o r m a n c e o f C e r i o d a p h n i a i n a l l o f t h e e n c l o s u r e s may have been f u r t h e r e n h a n c e d by p r e d a t i o n - i n d u c e d o r s e a s o n a l l y i n d u c e d d e p r e s s i o n s i n t h e numbers o f a l l o f t h e 247 o t h e r f i l t e r - f e e d i n g c l a d o c e r a n s t h a t o c c u r r e d d u r i n g t h e l a t t e r p a r t o f t h e summer. T h i s may have a l l e v i a t e d some form of c o m p e t i t i v e i n t e r a c t i o n t h a t may have r e s t r a i n e d C e r i o d a p h n i a p o p u l a t i o n g r o w t h . As t h e d e n s i t i e s of t h e o t h e r major c l a d o c e r a n s , t h e D a p h n i a and t h e Diaphanosoma, d e c l i n e d i n t h e l a t t e r p a r t of t h e summer, t h e C e r i o d a p h n i a a p p e a r e d i n g r e a t e r numbers i n t h e e p i l i m n e t i c w a t e r s . T h i s s u g g e s t s t h a t t h e C e r i o d a p h n i a may have been c o m p e t i t i v e l y e x c l u d e d f r o m t h e e p i l i m n i o n by t h e l a r g e r , more e f f i c i e n t f i l t e r - f e e d e r s . T h e r e i s a l s o some c i r c u m s t a n t i a l e v i d e n c e t h a t s u g g e s t s t h a t t h e C e r i o d a p h n i a s e a s o n a l abundance c y c l e may be c o n t r o l l e d by c o p e p o d p r e d a t i o n . The c y c l o p o i d c o p e p o d D i a c y c l o p s t h o m a s i i s t h e n u m e r i c a l l y d ominant c o p e p o d t h r o u g h o u t t h e e a r l y and mid- summer months i n P l a c i d L a k e . T h i s c o p e p o d i s known t o be an o b l i g a t e c a r n i v o r e and i t i s a l s o known t o a c t i v e l y f e e d on C e r i o d a p h n i a (McQueen, 1969). D e n s i t i e s of C e r i o d a p h n i a i n a l l o f t h e e n c l o s u r e s a t t a i n e d s e a s o n a l maxima o n l y a f t e r a l a t e summer d e c l i n e i n t h e d e n s i t i e s of D i a c y c l o p s . T h i s was p a r t i c u l a r l y e v i d e n t i n l i t t o r a l f e r t i l i z e d e n c l o s u r e 3. D e n s i t i e s of D i a c y c l o p s d e c l i n e d much e a r l i e r a nd much more s h a r p l y i n t h i s e n c l o s u r e t h a n i n a l l o f t h e o t h e r e n c l o s u r e s . T h i s d e c l i n e was p a r a l l e l e d by a d r a m a t i c i n c r e a s e i n t h e numbers o f C e r i o d a p h n i a . 248 (6) D i aptomus o r e g o n e n s i s A n u m e r i c a l r e s p o n s e by D. o r e g o n e n s i s t o n u t r i e n t a d d i t i o n was o n l y r e a l l y e v i d e n t i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s , a l t h o u g h f e r t i l i z a t i o n s i g n i f i c a n t l y i n c r e a s e d D. o r e g o n e n s i s egg p r o d u c t i o n and mean body s i z e i n b o t h t h e l i t t o r a l and l i m n e t i c f e r t i l i z e d e n c l o s u r e s . T h i s d i f f e r e n t i a l r e s p o n s e may be a s a m p l i n g a r t i f a c t . I f t h e phenomenon of " a v o i d a n c e of s h o r e " t h a t I d i s c u s s e d p r e v i o u s l y , i s e x h i b i t e d by t h e s e c o p e p o d s , t h e n i t may have been t h a t a major p o r t i o n of t h e c o p e p o d p r o d u c t i o n i n t h e l i t t o r a l f e r t i l i z e d e n c l o s u r e s was not b e i n g s a m p l e d . D e s p i t e t h i s p o s s i b l e b i a s , i t i s c l e a r t h a t t h e c o p e p o d s r e s p o n d e d l e s s d r a m a t i c a l l y t o f e r t i l i z a t i o n t h a t t h e c l a d o c e r a n s . The o n s e t of f e r t i l i z a t i o n , i n J u n e , c o i n c i d e d w i t h what a p p e a r e d t o be a p r o d u c t i o n o f a new g e n e r a t i o n of D. o r e g o n e n s i s n a u p l i i . T hus, one wo u l d have e x p e c t e d t h a t i f f e r t i l i z a t i o n b e n e f i t t e d t h i s s p e c i e s , t h e r e would have been a l a r g e i n c r e a s e i n t h e numbers o f d i a p t o m i d s l a t e r i n t h e y e a r . In f a c t , numbers o f t h e c o p e p o d s i n c r e a s e d o n l y m o d e s t l y , compared t o t h e c l a d o c e r a n s , and compared t o t h e numbers of n a u p l i i p r e s e n t i n t h e w a t e r column i n J u l y . F u r t h e r , o n l y a s m a l l p r o p o r t i o n o f t h e c o p e p o d i t e s i n t h e s e e n c l o s u r e s m a t u r e d i n t o a d u l t s . The l a c k o f an o v e r a l l n u m e r i c a l r e s p o n s e i n t h e s e c o p e p o d s c a n p r o b a b l y be a t t r i b u t e d t o poor j u v e n i l e ( n a u p l i i t o e a r l y c o p e p o d i t e ) s u r v i v a l w h i l e t h e l a c k of s u r v i v a l and d e v e l o p m e n t o f t h e l a t e summer c o p e p o d i t e s may have been due t o c o m p e t i t i v e i n t e r a c t i o n s w i t h t h e abu n d a n t c l a d o c e r a n s . 249 M o r t a l i t y o f D. o r e g o n e n s i s n a u p l i i must have been h i g h i n my f e r t i l i z e d e n c l o s u r e s . N a u p l i a r d e n s i t i e s r e a c h e d as h i g h as 8 0 / l i t e r i n e n c l o s u r e 4 i n l a t e J u l y but t h i s t r a n s l a t e d i n t o a maximum of o n l y 50 c o p e p o d i t e ' s / l i t e r by m i d - A u g u s t . Whether t h i s h i g h m o r t a l i t y was a r e s u l t o f c o m p e t i t i o n between j u v e n i l e c l a d o c e r a n s and t h e n a u p l i i or some o t h e r f a c t o r i s a m a t t e r of s p e c u l a t i o n s i n c e I have no d a t a t o s u p p o r t e i t h e r c l a i m . C o m p e t i t i v e l i m i t a t i o n of t h e c o p e p o d s by t h e c l a d o c e r a n s , i f i t d i d e x i s t , was p r o b a b l y most s t r o n g l y e x p r e s s e d i n t h e e a r l y summer when t h e n a u p l i i were most a b u n d a n t . The r a p i d i n c r e a s e i n t h e numbers o f j u v e n i l e c l a d o c e r a n s t h a t o c c u r r e d a t t h i s t i m e may have r e s u l t e d i n g r e a t l y r e d u c e d s u r v i v a l r a t e s f o r t h e c o p e p o d n a u p l i i . T h i s i s a s s u m i n g t h a t b o t h a n i m a l s e x p l o i t t h e same f o o d r e s o u r c e . A l t h o u g h t h e s t a g e a t w h i c h n a u p l i a r f e e d i n g on a l g a e commences i s unknown, R i g l e r and C o o l e y (1974) r e p o r t t h a t f o o d must be s u p p l i e d t o s e c o n d i n s t a r D. o r e g o n e n s i s i f s i g n i f i c a n t m o r t a l i t y i s t o be a v e r t e d . The f o o d added t o t h e i r z o o p l a n k t o n c u l t u r e s c o n s i s t e d of s m a l l f orms o f g r e e n a l g a e s u c h as C h l o r e l l a s p . , S e l e n a s t r u m s p . and Chlamydomonas r e i n h a r d t i i , w h i c h range i n s i z e f r o m 3-10 um ( P r e s c o t t , 1962), t h e same s i z e s p e c t r u m e x p l o i t e d by s m a l l d a p h n i d s ( N e i l l , 1975). S i n c e j u v e n i l e d a p h n i d s were p r e s e n t i n g r e a t numbers and were i n c r e a s i n g i n numbers a l m o s t e x p o n e n t i a l l y , i t seems r e a s o n a b l e t o assume some form of c o m p e t i t i v e i n t e r a c t i o n was o c c u r r i n g . A n o t h e r f a c t o r w h i c h may have l e d t o r e d u c e d s u r v i v a l , was t h a t d u r i n g t h e e a r l y p a r t of summer t h e c y c l o p o i d c o p e p o d D i a c y c l o p s t h o m a s i were p r e s e n t i n 250 l a r g e numbers. T h e s e c o p e p o d s a r e known t o be p r e d a t o r y on t h e n a u p l i i o f c a l a n o i d c o p e p o d s and c a n s i g n i f i c a n t l y r e d u c e t h e p o p u l a t i o n of d i a p t o m i d n a u p l i i i n a m a t t e r o f d a y s (McQueen, 1967, 1 9 6 8 ) . The a p p a r e n t l y poor s u r v i v a l and d e v e l o p m e n t of D. o r e g o n e n s i s c o p e p o d i t e s i n A u g u s t i n t h e l i m n e t i c f e r t i l i z e d e n c l o s u r e s may be r e l a t e d t o t h e f a c t t h a t t h i s s p e c i e s , i n t h i s l a k e , i s t h o u g h t t o f e e d on t h e same s i z e r a n g e of f o o d p a r t i c l e s t h a t a r e s e l e c t i v e l y browsed by a l l of t h e more e f f i c i e n t c l a d o c e r a n s p e c i e s ( K r a u s e , 1984). B e c a u s e c l a d o c e r a n s have h i g h e r f i l t e r i n g r a t e s ( W e t z e l , 1975) and c a n f i l t e r a w i d e r range of p a r t i c l e s i z e s t h a n c o p e p o d s (Bogdan and McNaught, 1975), t h e y were p r o b a b l y a b l e t o s u p p r e s s c o p e p o d numbers, p o s s i b l y t h r o u g h f o o d l i m i t a t i o n . C o n v e r s e l y , i f t h i s d i a p t o m i d i n t h i s l a k e e x p l o i t s l a r g e r f o o d p a r t i c l e s t h a n t h e c l a d o c e r a n s ( c a . 25-35 um), as s u g g e s t e d f o r t h i s s p e c i e s by McQueen (1969) and Richman e t a l . ( 1 9 8 0 ) , t h e n t h e s e c o p e p o d s may have been e x p l o i t i n g a p a r t i c l e s i z e s p e c t r u m t h a t was p r e v i o u s l y shown t o have been o n l y m a r g i n a l l y e n h a n c e d by t h e c h o s e n f e r t i l i z a t i o n r e g i m e . (7) D i a c y c l o p s t h o m a s i D i a c y c l o p s t h o m a s i d i d n o t i n c r e a s e i n r e s p o n s e t o f e r t i l i z a t i o n ; on t h e c o n t r a r y , t h e y a p p e a r e d t o s u f f e r a d e c r e a s e i n numbers. A l s o , t h e r e was an e a r l i e r p o p u l a t i o n d e c l i n e t h a n o c c u r r e d i n t h e c o n t r o l e n c l o s u r e s . T h i s i s s u r p r i s i n g s i n c e one of D i a c y c l o p s ' p r e f e r r e d f o o d i t e m s , 251 C e r i o d a p h n i a (McQueen, 1969), i n c r e a s e d i n abundance i n t h e f e r t i l i z e d e n c l o s u r e s . I b e l i e v e t h i s e a r l y d e c l i n e of D. t h o m a s i d e n s i t i e s was a r e s u l t of an i n c r e a s e i n t h e t h i c k n e s s of t h e a n o x i c h y p o l i m n e t i c w ater mass i n t h e f e r t i l i z e d e n c l o s u r e s . The t h i c k n e s s o f t h e a n o x i c zone (<1 m g / l i t e r 0 2 ) i n t h e c o n t r o l s was m i n i s c u l e and o n l y e v i d e n t i n l a t e September i n t h e d e e p e s t w a t e r s (below 6m), whereas t h e a n o x i c zone i n t h e f e r t i l i z e d e n c l o s u r e s e x t e n d e d t o as much as 0.5 m above t h e s e d i m e n t s and, i n some of t h e e n c l o s u r e s , was e v i d e n t a s e a r l y i n t h e s e a s o n as th e b e g i n n i n g of A u g u s t . T h i s i n c r e a s e i n t h e d e p t h of t h e a n o x i c zone must s u r e l y have r e s t r i c t e d h y p o l i m n e t i c d w e l l i n g a n i m a l s t o i n a s m a l l e r volume o f h a b i t a b l e w a t e r . T h i s h y p o t h e s i s i s s u p p o r t e d by t h e o b s e r v a t i o n t h a t d e n s i t i e s o f D. t h o m a s i i n l i m n e t i c f e r t i l i z e d e n c l o s u r e 4, d u r i n g t h e p e r i o d o f most s e v e r e a n o x i a ( l a t e summer), were 2-3 t i m e s h i g h e r t h a n i n any o f t h e o t h e r f e r t i l i z e d e n c l o s u r e s . E n c l o s u r e 4 had a maximum d e p t h of 6m. A l l of t h e o t h e r f e r t i l i z e d e n c l o s u r e s had a maximum d e p t h of o n l y 5m. Th u s , v i r t u a l l y t h e e n t i r e h y p o l i m n i o n i n a l l o f t h e s e e n c l o s u r e s would have been a l m o s t t o t a l l y a n o x i c . E n c l o s u r e 4 c o u l d have c o n t a i n e d a s much as 1.0-1.5 m o f O K - s u f f i c i e n t l i v i n g s p a c e , even t h o u g h t h e oxygen c o n t e n t o f t h i s w a ter was r e l a t i v e l y low. D i a c y c l o p s t h o m a s i c a n s u r v i v e i n w a t e r s c o n t a i n i n g r e l a t i v e l y low c o n c e n t r a t i o n s o f d i s s o l v e d oxygen ( S t r o s s e t a l . , 1961). T h e s e w o r k e r s f o u n d t h a t p r a c t i c a l l y t h e e n t i r e p o p u l a t i o n o f C y c l o p s i n P e t e r - P a u l L a k e , a s m a l l bog l a k e i n G o g e b i c C o u n t y , 252 Michigan, was l o c a t e d i n a two meter zone near the thermocline, which c o n t a i n e d a d i s s o l v e d oxygen c o n c e n t r a t i o n as low as 1.0 m g / l i t e r . 253 5.0 PLACID LAKE CUTTHROAT TROUT 5.1 L i f e H i s t o r y T r a i t s The l i f e h i s t o r y o f t h e c o a s t a l c u t t h r o a t t r o u t (Salmo  c l a r k i R i c h a r d s o n - S c o t t and Crossman, 1973) i s e x t r e m e l y v a r i e d f r o m s t o c k t o s t o c k . T h e r e a r e two forms of t h i s s p e c i e s i n B r i t i s h C o l u m b i a , t h e i n t e r i o r f o r m ( Salmo c l a r k i l e w i s i ) and t h e c o a s t a l f o r m (Salmo c l a r k i c l a r k i ) . The c o a s t a l form has b o t h anadromous and non-anadromous s t o c k s . P l a c i d Lake c u t t h r o a t a r e of t h e l a t t e r t y p e . V e r y l i t t l e i s known ab o u t t h e l i f e h i s t o r y of t h e U.B.C. R e s e a r c h f o r e s t c u t t h r o a t s t o c k s so I w i l l o u t l i n e some of t h e t h i n g s I have l e a r n e d a b o u t t h e P l a c i d L a k e s t o c k . P l a c i d Lake c u t t h r o a t a r e spawned i n g r a v e l l y p o r t i o n s o f t h e downstream r e a c h e s of t h e l a k e ' s o u t f l o w s t r e a m . A few s m a l l r e d d s , a b o u t 10-15 cm i n d i a m e t e r and 5-7 cm deep were f o u n d i n l o o s e g r a v e l , r i f f l e d p o r t i o n s i n a r e a s o f r e l a t i v e l y f l a t r e l i e f , about 500 - 800 m downstream from t h e l a k e . T h e r e may have been some more f a r t h e r down s t r e a m but I d i d n o t i n v e s t i g a t e t h i s . T h i r t y f e m a l e c u t t h r o a t t r o u t were t a k e n f r o m t h e l a k e i n e a r l y O c t o b e r of 1981 t o s e r v e as b r o o d s t o c k f o r t h e f o l l o w i n g y e a r ' s e x p e r i m e n t s . Of t h e s e , a p p r o x i m a t e l y o n e - t h i r d had a l r e a d y spawned, o n e - t h i r d c o n t a i n e d r i p e o r n e a r l y r i p e eggs and o n e - t h i r d were not r e a d y t o spawn but were spawned i n a h a t c h e r y s e t t i n g t h e f o l l o w i n g J a n u a r y . Spawning t h u s e v i d e n t l y o c c u r s o v e r a l o n g p e r i o d of t i m e , a t l e a s t f r o m September t o 254 J a n u a r y . T h i s i s i n a c c o r d a n c e w i t h t h e o b s e r v a t i o n s of D e W i t t (1955) who r e p o r t e d t h a t f e m a l e c u t t h r o a t c o n t a i n i n g r i p e o r n e a r l y r i p e eggs had been t a k e n f r o m s e v e r a l c o a s t a l C a l i f o r n i a s t r e a m s from September t o A p r i l . On t h e o t h e r hand, Dymond (1928) and S c o t t and Crossman (1973) r e p o r t t h a t spawning of c u t t h r o a t i n B r i t i s h C o l u m b i a o n l y o c c u r s f r o m F e b r u a r y t o May. T h i s e m p h a s i z e s t h e v a r i a b i l i t y of t h e l i f e h i s t o r y c h a r a c t e r i s t i c s among t h e v a r i o u s s t o c k s o f t h i s s p e c i e s . F e c u n d i t y o f m ature f e m a l e s i n P l a c i d L a k e i s v e r y low, a v e r a g i n g 120 e g g s / f e m a l e ( a v e r a g e f o r 6 f i s h r a n g i n g i n s i z e f r o m 17-21 cm f o r k l e n g t h ) . S c o t t and Crossman (1973) r e p o r t t h a t a v e r a g e egg number p e r f e m a l e f o r t h i s s p e c i e s , d i s r e g a r d i n g s i z e and l o c a t i o n , s h o u l d be 1100-1700. T h i s s u g g e s t s t h a t P l a c i d Lake c u t t h r o a t a r e p r o b a b l y s e v e r e l y a f f e c t e d by low f o o d a v a i l a b i l i t y s i n c e v e r y l i t t l e e n e r g y i s b e i n g c o n v e r t e d i n t o e g g s . Young o f t h e y e a r p r o b a b l y spend much of t h e i r e a r l y l i f e i n t h e s t r e a m e n v i r o n m e n t . I have no e v i d e n c e t o s u p p o r t t h i s b ut D e W i t t (1955) and S c o t t and Crossman (1973) b o t h o b s e r v e d t h a t young o f t h e y e a r o f t h i s s p e c i e s commonly s p e n t a b o u t 1 y e a r i n t h e s t r e a m e n v i r o n m e n t b e f o r e moving i n t o t h e i r d e s t i n a t i o n h a b i t a t s a s f i n g e r l i n g t r o u t . I e l e c t r o - f i s h e d t h e o u t l e t s t r e a m t o o b t a i n young f i s h t o r e p l a c e t h e ones t h a t had d i s a p p e a r e d f r o m my e n c l o s u r e s , and c a p t u r e d f i f t y - f o u r s m a l l t r o u t r a n g i n g i n s i z e f r o m 2.5-14 cm f o r k l e n g t h . Most o f t h e s e f i s h were l e s s t h a n 8 cm l o n g . O n l y one was 14 cm l o n g . I a l s o s u r v e y e d t h e l a k e u s i n g SCUBA and s n o r k e l i n g g e a r t o seek o u t 255 young f i s h . The s m a l l e s t c u t t h r o a t I o b s e r v e d i n t h e l a k e was a r o u n d 5 cm i n l e n g t h . A l l s i g h t i n g s of j u v e n i l e f i s h o c c u r r e d i n t h e v i c i n i t y of t h e o u t l e t s t r e a m . P e r h a p s t h e s e j u v e n i l e f i s h were j u s t moving i n t o t h e l a k e e n v i r o n m e n t from t h e downstream r e a r i n g a r e a s . I t i s c l e a r from my r e s u l t s and from t h o s e of S h e p h e r d ( 1 9 7 3 ) , A n d r u s a k and N o r t h c o t e (1973) and N o r t h c o t e and C l a r o t t o (1975) t h a t z o o p l a n k t o n , a l m o s t e x c l u s i v e l y l a r g e c l a d o c e r a n s l i k e D a p h n i a and H o l o p e d i u m , i n s e c t l a r v a e ( p r e d o m i n a n t l y d i p t e r a n pupae and t r i c h o p t e r a n l a r v a e ) and f l y i n g a q u a t i c d i p t e r a n s a r e t h e main d i e t a r y components of P l a c i d Lake c u t t h r o a t . Some of t h e l a r g e r i n d i v i d u a l s (> 200 mm) have been known t o e a t s m a l l f i s h ( A n d r u s a k and N o r t h c o t e , 1973; Hume, 1978) w h i c h may p a r t i a l l y e x p l a i n t h e d i s a p p e a r a n c e o f t h e c u t t h r o a t t r o u t f r y I s t o c k e d i n t h e e n c l o s u r e s . T h e r e was a marked s e a s o n a l i t y i n t h e t r o u t d i e t s . L a r g e c l a d o c e r a n z o o p l a n k t o n were t a k e n a l m o s t e x c l u s i v e l y i n t h e l a t e s p r i n g and e a r l y summer when t h e y were a t t h e i r maximum d e n s i t i e s i n t h e water c o l u m n . As t h e summer p r o g r e s s e d , i n s e c t l a r v a e and f l y i n g i n s e c t s became t h e p r e d o m i n a n t p r e y t y p e s . T h e r e was a l s o e v i d e n c e t h a t i n d i v i d u a l f i s h p r e f e r r e d c e r t a i n p r e y t y p e s ( p e r s o n a l o b s e r v a t i o n ) . The g u t s of some o f t h e f i s h I a n a l y z e d c o n t a i n e d D a p h n i a a l m o s t e x c l u s i v e l y . O t h e r s c o n t a i n e d o n l y H o l o p e d i u m and some C h a o b o r u s l a r v a e and s t i l l o t h e r s c o n t a i n e d a m i x t u r e of a l l p r e y t y p e s . One f i s h was f o u n d t o have o n l y D i a p t o m u s k e n a i and s e v e r a l c h i r o n o m i d l a r v a e i n i t s g u t . T h i s was u n u s u a l i n t h a t D. k e n a i were n o t 256 p r e s e n t i n t h e water column a t t h e t i m e ( b a s e d on sample e n u m e r a t i o n s ) . However, t h e p r e s e n c e of c h i r o n o m i d l a r v a e i n t h e gut s u g g e s t s t h a t t h i s f i s h was f e e d i n g n e a r t h e b o t t o m . T h i s s u g g e s t s t h a t D. k e n a i , d u r i n g t h e mid t o l a t e summer months, may be c o n c e n t r a t e d n e a r t h e b o t t o m i n t h i s l a k e , p l a c i n g i t o u t of r e a c h of our t r a d i t i o n a l s a m p l i n g g e a r , t h e b i l g e pump. We t e n d t o a v o i d g e t t i n g t o o c l o s e t o t h e s e d i m e n t s b e c a u s e t h e y a r e e a s i l y d i s t u r b e d and w i l l be s u c k e d up by t h e pump, t h e r e b y c o n t a m i n a t i n g t h e s a m p l e s and making them d i f f i c u l t t o e n u m e r a t e . I t w o u l d be i n t e r e s t i n g t o d e t e r m i n e whether D. k e n a i i s f e e d i n g on e p i s a m m i c p h y t o p l a n k t e r s , s u c h as d i a t o m s a t t h i s t i m e . The v a r i e d d i e t s of i n d i v i d u a l f i s h , i . e . some c o n t a i n i n g b e n t h i c p r e y t y p e s l i k e C h a o b o r u s and c h i r o n o m i d l a r v a e , and d i f f e r e n c e s i n t h e c o l o u r a t i o n o f f i s h e s i n t h e l a k e l e a d s me t o b e l i e v e t h a t t h e r e a r e two s e p a r a t e v a r i e t i e s o f c u t t h r o a t i n t h i s l a k e . One t y p e b e i n g p r e d o m i n a n t l y l i t t o r a l and b e n t h i c o r i e n t e d and t h e o t h e r b e i n g a more l i m n e t i c v a r i e t y . L i t t o r a l o r b e n t h i c morphs of t h e f i s h e x h i b i t a d a r k g o l d e n - g r e e n c o l o u r w i t h v e r y d i s t i n c t p a r r marks and h e a v y s p o t t i n g . P a r r marks were e v i d e n t even on some l a r g e r , o b v i o u s l y a d u l t f i s h . L i m n e t i c t y p e s a r e d a r k o n l y on t h e d o r s a l s i d e , d i s t i n c t l y l i g h t e r on t h e v e n t r a l s i d e , have no p a r r marks and a r e l e s s h e a v i l y s p o t t e d . T h i s i s . a phenomenon w o r t h f u t u r e i n v e s t i g a t i o n . I t may be t h a t t h e d a r k e r c o l o u r e d f i s h a r e s i m p l y i n spawning c o l o u r a t i o n , but i f t h i s i s so t h e n a l m o s t o n e - h a l f o f t h e p o p u l a t i o n i s a l w a y s i n spawning c o n d i t i o n . 257 5.2 F e r t i l i z a t i o n and The D i e t of t h e T r o u t I t i s c l e a r from my r e s u l t s t h a t t h e c u t t h r o a t t r o u t i n t h e f e r t i l i z e d e n c l o s u r e s b e n e f i t e d immensely from t h e enhancement of D a p h n i a r o s e a . D a p h n i d s o c c u r r e d i n e x t r e m e l y l a r g e numbers i n t h e stomachs of f i s h c a p t u r e d i n t h e s e e n c l o s u r e s . In f a c t , t h e abundance and a v a i l a b i l i t y of t h e D a p h n i a a p p e a r t o have e l i c i t e d a s h i f t i n t h e d i e t of t h e s e f i s h . D u r i n g t h e l a t e summer when f l y i n g a q u a t i c i n s e c t s and t h e i r a q u a t i c l a r v a e f o r m e d t h e dom i n a n t component of t h e d i e t of f i s h i n t h e c o n t r o l s and t h e l a k e , t h e f i s h i n t h e f e r t i l i z e d e n c l o s u r e s g o r g e d t h e m s e l v e s on D a p h n i a and l a r g e l y i g n o r e d i n s e c t s and i n s e c t l a r v a e . D e s p i t e t h e f a c t t h a t t h e d e n s i t i e s of a l l o f t h e major h e r b i v o r o u s z o o p l a n k t o n ( e x c e p t Bosmina) i n c r e a s e d a p p r e c i a b l y f o l l o w i n g f e r t i l i z a t i o n , D a p h n i a were t h e o n l y z o o p l a n k t o n t a k e n i n s i g n i f i c a n t numbers by t h e f i s h . Does t h i s mean t h a t enhancement of t h e s m a l l e r z o o p l a n k t o n s p e c i e s was "wasted" i n t h e s e n s e t h a t t h e s e s m a l l e r z o o p l a n k t o n w i l l d e r i v e b e n e f i t s from enhancement b ut n o t c o n t r i b u t e t o t h e g r o w t h of t h e t a r g e t s p e c i e s , t h e c u t t h r o a t t r o u t ? I f t h e answer t o t h e above q u e s t i o n i s " y e s " , t h e n one c o u l d p r o c e e d w i t h a p l a n f o r n u t r i e n t e n r i c h m e n t t h a t does not have t h e m a i n t e n a n c e o f t h e e x i s t i n g s p e c i e s c o m p o s i t i o n of t h e z o o p l a n k t o n as one o f i t s g o a l s , a s i s so o f t e n t h e c a s e ( P a r s o n s e t a l . , 1972; S t o c k n e r e_t a l . , 1980). I n s t e a d , one c o u l d g i v e f u l l a t t e n t i o n t o t h e p r o b l e m of m a x i m i z i n g t h e p r o d u c t i o n o f o n l y t h e l a r g e s p e c i e s o f z o o p l a n k t o n on w h i c h t h e 258 f i s h w i l l f e e d . The answer would be " y e s " i f i t i s o n l y i m p o r t a n t t o c o n s i d e r t h e a d u l t t r o u t i n t h e l a k e But what of th e j u v e n i l e s of t h i s t r o u t s p e c i e s ? I t may be t h a t t h e j u v e n i l e t r o u t would consume s m a l l z o o p l a n k t o n once t h e y e n t e r t h e l a k e u n t i l t h e y r e a c h a s i z e t h a t e n a b l e s them t o h a n d l e t h e l a r g e r c l a d o c e r a n s . The enhancement of s m a l l z o o p l a n k t o n would no t be "empty" i n t h i s r e g a r d . I c o u l d f i n d o n l y two p u b l i s h e d r e p o r t s w h i c h d e t a i l t h e f o o d o f j u v e n i l e c u t t h r o a t t r o u t . H a z z a r d and Madsen (1933) r e p o r t e d t h a t f i n g e r l i n g (2-4 cm) i n t e r i o r c u t t h r o a t , Salmo  c l a r k i l e w i s i ( G i r a r d ) , t a k e n f r o m a t r i b u t a r y of J a c k s o n L a k e , Wyoming, f e d a l m o s t e x c l u s i v e l y on s m a l l immature forms of d i p t e r a n s , m a y f l i e s , c a d d i s f l i e s and s t o n e f l i e s . The two-winged f l i e s , p a r t i c u l a r l y t h e m i d g e s , were t a k e n most f r e q u e n t l y and i n t h e l a r g e s t numbers. No c r u s t a c e a n z o o p l a n k t o n were f o u n d , - p r o b a b l y b e c a u s e z o o p l a n k t o n do not i n h a b i t l o t i c w a t e r s . The q u e s t i o n t h e n a r i s e s , do f i n g e r l i n g c u t t h r o a t t r o u t , once t h e y move i n t o t h e l a k e , f e e d on z o o p l a n k t o n , a f o o d i t e m w i t h w h i c h t h e y a r e c o m p l e t e l y u n f a m i l i a r ? C a l h o u n (1944) f o u n d t h a t t h e f o o d of f i n g e r l i n g b l a c k - s p o t t e d t r o u t , Salmo c l a r k i i h e n s h a w i , i n B l u e L a k e , A l p i n e V a l l e y , C a l i f o r n i a , s t i l l c o n s i s t e d l a r g e l y of c h i r o n o m i d pupae and l a r v a e , and l a n d i n s e c t s . M i c r o c r u s t a c e a n s made up o n l y 5.8 % o f t h e volume o f t h e f o o d i n t h e i r g u t s . C o n v e r s e l y , a s much a s 70 % (by volume) o f f o o d i n t h e g u t s o f a d u l t b l a c k - s p o t t e d t r o u t c o n s i s t e d o f m i c r o c r u s t a c e a n s ( D a p h n i a and c o p e p o d s ) . The a b i l i t y o f f i n g e r l i n g c u t t h r o a t t o consume s m a l l 259 d i p t e r a n f l i e s , w h i c h r a n g e i n s i z e f r o m 2-40 mm ( J o h a n n s e n , 1934; U n s i n g e r , 1963), s u g g e s t s t h a t t h e y s h o u l d be c a p a b l e of c o n s u m i n g t h e l a r g e z o o p l a n k t e r s t h e y w i l l e n c o u n t e r i n t h e l a k e . I f s o , t h e y w i l l p r o b a b l y a v o i d t h e s m a l l e r z o o p l a n k t e r s i f g i v e n t h e c h o i c e . D o b l e and E g g e r s (1978) f o u n d t h a t t h e j u v e n i l e s o c k e y e salmon i n L a k e W a s h i n g t o n were e x t r e m e l y s e l e c t i v e i n t h e i r f o o d c h o i c e , t a k i n g o n l y t h e l a r g e s t z o o p l a n k t o n a v a i l a b l e t o them. N a r v e r (1970) f o u n d t h a t u n d e r y e a r l i n g s o c k e y e i n B a b i n e L a k e , B r i t i s h C o l u m b i a p r e f e r e n t i a l l y consumed t h e l a r g e r z o o p l a n k t e r s i n t h e l a k e , namely D a p h n i a l o n g i s p i n a ( l a r g e r t h a n our D. r o s e a ) and H e t e r o c o p e s e p t e n t r i o n a l i s , a v e r y l a r g e (3-4 mm) d i a p t o m i d c o p e p o d . S m a l l z o o p l a n k t o n i n b o t h l a k e s were consumed o n l y a f t e r t h e l a r g e r s p e c i e s had d e c r e a s e s i n numbers. T h e s e d a t a , a l t h o u g h n o t s p e c i f i c t o c u t t h r o a t t r o u t , s u g g e s t t h a t t h e f i n g e r l i n g t r o u t would p r o b a b l y s e l e c t f o r t h e l a r g e s t z o o p l a n k t o n t h e y c o u l d f i n d , i f t h e y f e e d on them a t a l l . I n t e r e s t i n g l y , Hume (1978) f o u n d t h a t t h e s m a l l e s t c u t t h r o a t t r o u t he c a p t u r e d (70 mm) i n E u n i c e and Loon L a k e s ( i n t h e U.B.C r e s e a r c h f o r e s t ) consumed f a r fewer z o o p l a n k t o n and f a r more s u r f a c e p r e y ( i n s e c t s ) t h a n t h e medium (140-279 mm) and l a r g e (> 280 mm) s i z e d f i s h . P e r h a p s f e e d i n g on z o o p l a n k t o n i s a s k i l l t h a t must be l e a r n e d by n a i v e f i s h . F r e e swimming p r e y c a n a v o i d an e n c o u n t e r w i t h a p r e d a t o r much e a s i e r t h a n a p r e y i t e m t h a t i s t r a p p e d i n t h e s u r f a c e f i l m of t h e l a k e ' s s u r f a c e . The j u v e n i l e t r o u t w o u l d have t o f i r s t l e a r n t h e f r e e swimmer's p r e d a t o r a v o i d a n c e b e h a v i o u r b e f o r e i t c o u l d become e f f i c i e n t a t 260 f e e d i n g on them. Thus, i t seems t h a t t h e a u g m e n t a t i o n of s m a l l z o o p l a n k t e r s t h r o u g h f e r t i l i z a t i o n may v e r y w e l l be "wasted" enhancement i n t e r m s o f f i s h p r o d u c t i o n . U n l e s s one t a k e s i n t o a c c o u n t t h e p o s s i b i l i t y t h a t t h e s e s m a l l a n i m a l s may s e r v e t o i n c r e a s e t h e s u r v i v a l of l a r g e r p r e d a t o r y a q u a t i c i n s e c t l a r v a e w h i c h p r e y on s m a l l c r u s t a c e a n s and w h i c h , i n t u r n , a r e p r e y e d upon by t h e f i s h . But as a d i r e c t , i m m ediate, r a p i d l y r e p r o d u c i n g f o o d s u p p l y , t h e y do not a p p e a r t o be i m p o r t a n t . D e t a i l e d work on t h e f e e d i n g h a b i t s and d i e t a r y r e q u i r e m e n t s o f f i n g e r l i n g t r o u t w o u l d be r e q u i r e d b e f o r e t h i s i d e a can be c o n f i r m e d . 261 6.0 CONCLUSIONS The use o f l a r g e e n c l o s u r e s f o r impounding t h e water column i n s h a l l o w l a k e s i n o r d e r t o s t u d y t h e e f f e c t s of e x p e r i m e n t a l l y i n d u c e d p e r t u r b a t i o n s , on t h e p h y s i c a l , c h e m i c a l or b i o l o g i c a l components of t h e s y s t e m , a p p e a r s t o be a v i a b l e e x p e r i m e n t a l t o o l f o r l i m n o l o g i s t s . S i n c e t h e use of l a r g e l i m n o c o r r a l s d o e s n o t a p p e a r t o a l t e r t h e above p a r a m e t e r s i n r e l a t i o n t o t h e s t u d y l a k e , r e s u l t s f r o m s u c h e x p e r i m e n t s may be u s e d t o p r e d i c t t h e e f f e c t s of s i m i l a r p e r t u r b a t i o n s i f a p p l i e d t o t h e s t u d y l a k e as a w h o l e . The e f f e c t s of f e r t i l i z a t i o n on t h e p h y s i c a l , c h e m i c a l and b i o l o g i c a l d y n a m i c s o f t h e l a k e w a t e r s were more e v i d e n t i n t h e l i m n e t i c zone of t h e l a k e t h a n i n t h e l i t t o r a l z o n e . S m a l l f r e q u e n t d o s e s of f e r t i l i z e r a t h i g h N:P r a t i o s (40:1) r e s u l t e d i n c h a n g e s i n t h e l a k e ' s p h y t o p l a n k t o n a s s e m b l a g e t o t h e e x t e n t t h a t d e l e t e r i o u s b l u e - g r e e n a l g a e were v i r t u a l l y e l i m i n a t e d from t h e a l g a l community. T h i s p r o v i d e s some s u p p o r t f o r t h e h y p o t h e s i s t h a t dominance of p h y t o p l a n k t o n a s s e m b l a g e s by b l u e - g r e e n a l g a e i s f a v o u r e d by a d e f i c i e n c y i n t h e a v a i l a b i l i t y o f i n o r g a n i c n i t r o g e n i n t h e water c o l u m n . The p r e d o m i n a n t s p e c i e s of a l g a e i n t h e f e r t i l i z e d e n c l o s u r e o v e r most of t h e summer c o n s i s t e d o f s m a l l g r e e n a l g a l 262 f o r m s , w h i c h were i n t h e s i z e r a n g e t h a t was g r a z a b l e by t h e z o o p l a n k t o n i n t h e l a k e . T h i s was p a r t i c u l a r l y t r u e f o r t h e l i m n e t i c z o ne. Net p h y t o p l a n k t o n showed no s i g n i f i c a n t i n c r e a s e i n d e n s i t y as a r e s u l t o f f e r t i l i z a t i o n . T h i s r e s u l t p r o v i d e s some s u p p o r t f o r t h e h y p o t h e s i s t h a t s m a l l a l g a l f orms grow f a s t e r t h a n l a r g e a l g a e when c o n d i t i o n s f o r gr o w t h a r e i d e n t i c a l (Banse, 1976) and t h a t n a t u r a l p h y t o p l a n k t o n a s s e m b l a g e s c a n be m a n i p u l a t e d t o p r o d u c e a l g a l c o m m u n i t i e s d o m i n a t e d by s m a l l c e l l s ( T u r p i n and H a r r i s o n . 1980). F e r t i l i z e r i n d u c e d c h a n g e s a t t h e p r i m a r y t r o p h i c l e v e l d i d n o t r e s u l t i n s i g n i f i c a n t c h a n g e s i n t h e s e a s o n a l s e q u e n c e o f s h i f t s i n n u m e r i c a l dominance o f t h e major h e r b i v o r o u s z o o p l a n k t o n s p e c i e s , e x c e p t f o r D a p h n i a . T h i s i s i m p o r t a n t i n t h a t i t s u g g e s t s t h a t some mechanism o t h e r t h a n f o o d abundance o r q u a l i t y , or c o m p e t i t i o n between c o - e x i s t i n g s p e c i e s may d r i v e t h e s e a s o n a l c y c l e of some z o o p l a n k t o n s p e c i e s i n l a k e s . The f o o d r e g i m e t h a t i s n o r m a l l y a v a i l a b l e t o t h e s p e c i e s o f h e r b i v o r o u s z o o p l a n k t o n i n P l a c i d Lake was d r a s t i c a l l y a l t e r e d a s a r e s u l t of f e r t i l i z a t i o n , y e t t h e n o r m a l s e a s o n a l c y c l e s o f abundance i n t h e z o o p l a n k t o n o c c u r r e d . The D a p h n i a were t h e z o o p l a n k t o n w h i c h d e r i v e d t h e g r e a t e s t b e n e f i t f r o m t h i s e n h a n c e d a l g a l b i o m a s s . I t a p p e a r s t h a t t h e c h o s e n f e r t i l i z a t i o n r e g i m e was s u c c e s s f u l 263 i n g e n e r a t i n g t h e s i z e r a n g e and s p e c i e s c o m p o s i t i o n of a l g a e t h a t i n d u c e d good g r o w t h i n t h e D a p h n i a p o p u l a t i o n o v e r most of t h e summer. D a p h n i a d o m i n a t e d t h e z o o p l a n k t o n b i o m a s s i n a l l of t h e f e r t i l i z e d e n c l o s u r e s f o r most of t h e summer i n s t e a d of o n l y d u r i n g e a r l y summer as i n t h e l a k e and c o n t r o l s . 7. The g r o w t h of c u t t h r o a t t r o u t i n t h i s l a k e i s a p p a r e n t l y l i m i t e d by f o o d a v a i l a b i l i t y . However, i t i s q u i t e c l e a r t h a t t h i s f i s h s p e c i e s becomes h i g h l y p l a n k t i v o r o u s g i v e n t h a t t h e r e i s a b u n d a n t z o o p l a n k t o n on w h i c h t o f e e d . The d a t a c l e a r l y show t h a t D a p h n i a i s t h e p r e d o m i n a n t z o o p l a n k t e r i n t h e d i e t s o f t h e s e f i s h d u r i n g t h e mid and l a t e summer months. 8. I t i s l i k e l y t h a t t r o u t of f i n g e r l i n g o r c a t c h a b l e s i z e w o uld o b t a i n g r e a t e r s u r v i v a l r a t e s i f p l a n t e d i n s u c h an a r t i f i c i a l l y e n r i c h e d e n v i r o n m e n t t h a n i f t h e y were p l a n t e d i n t h e t y p i c a l u n p r o d u c t i v e c o a s t a l m o u n t a i n l a k e . However, i t a p p e a r s as t h o u g h f e r t i l i z a t i o n d i d n o t r e s u l t i n i n c r e a s e d g r o w t h and s u r v i v a l o f p l a n t e d , h a t c h e r y r e a r e d , w i l d c u t t h r o a t t r o u t f r y . A v i a n p r e d a t o r s t o o k many of t h e p l a n t e d f i s h , o t h e r s may have been l o s t t o a d u l t f i s h w h i c h i n v a d e d t h e e n c l o s u r e s . T h i s i s i m p o r t a n t b e c a u s e i t s u g g e s t s t h a t p r e d a t i o n i s more i m p o r t a n t t h a n f o o d i n d e t e r m i n i n g t h e s u r v i v a l r a t e o f j u v e n i l e f i s h i n a q u a t i c s y s t e m s . 264 E s t i m a t e s of f i s h y i e l d b a s e d on t h e summer c h l o r o p h y l l a r e l a t i o n s h i p d e r i v e d by O g l e s b y ( 1 9 7 7 ) , s u g g e s t t h a t t h e u n f e r t i l i z e d l a k e w a t e r s would y i e l d o n l y 0.02 g d r y w e i g h t of f i s h / m 2 / y e a r . I f t h e l a k e was t o be f e r t i l i z e d t o t h e l e v e l o b t a i n e d i n t h i s e x p e r i m e n t , f i s h y i e l d s h o u l d r i s e t o a r e s p e c t a b l e l e v e l of 0.5 g d r y w e i g h t of f i s h / m 2 / y e a r , a 25 f o l d i n c r e a s e i n p r o d u c t i o n . The r e s u l t s of t h i s r e s e a r c h have i m p o r t a n t i m p l i c a t i o n s f o r t h e o v e r a l l management o f l a k e s w h i c h a r e s u b j e c t t o c u l t u r a l e u t r o p h i c a t i o n t h r o u g h t h e d i s c h a r g e of w a s t e w a t e r s . Changes i n t h e N:P r a t i o s of l a k e s c an be a c h i e v e d by r e d u c t i o n of P a t t h e p o i n t s o u r c e , o r by a d j u s t i n g t h e N:P r a t i o o f t h e d i s c h a r g e i n some o t h e r way. 265 REFERENCES CITED A l c a r e z , M., G.A. P a f f e n h o f e r and J.R. S t r i c k l e r . 1980. C a t c h i n g t h e a l g a e : a f i r s t a c c o u n t of v i s u a l o b s e r v a t i o n s on f i l t e r - f e e d i n g C a l a n o i d s . I_n K e r f o o t , W.C. [ e d . ] , E v o l u t i o n and e c o l o g y of z o o p l a n k t o n c o m m u n i t i e s . U n i v e r s i t y of N ew E n g l a n d P r e s s , New E n g l a n d , pages 241-248. A m e r i c a n P u b l i c H e a l t h A s s o c i a t i o n . S t a n d a r d Methods. T h i r t e e n t h e d i t i o n , New Y ork ( 1 9 7 1 ) . A n d r u s a k , H., and T.G. N o r t h c o t e . 1971. S e g r e g a t i o n between a d u l t c u t t h r o a t t r o u t (Salmo c l a r k i ) and D o l l y V a r d e n ( S a l v e l i n u s malma) i n s m a l l c o a s t a l B r i t i s h C o l u m b i a l a k e s . J . F i s h . Res. Bd. Canada. 2 8 ( 3 ) : 1259-1268 A r n o l d , D.E. 1971. I n g e s t i o n , a s s i m i l a t i o n , s u r v i v a l and r e p r o d u c t i o n by D a p h n i a p u l e x f e d seven s p e c i e s o f b l u e -g r e e n a l g a e . L i m n o l . O c e a n o g r . 1 6 ( 6 ) : 906-920. B a l l , R.C. 1948. F e r t i l i z a t i o n o f n a t u r a l l a k e s i n M i c h i g a n . T r a n s . Am. F i s h . S o c . 78: 145-155. Banse, K. 1976. 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