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The effect of herbivorous zooplankton on summer phytoplankton standing crops in Placid Lake, British… Krause, Edith 1984

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THE EFFECT OF HERBIVOROUS ZOOPLANKTON ON SUMMER PHYTOPLANKTON STANDING CROPS IN PLACID LAKE, BRITISH COLUMBIA by E d i t h K r a u s e BSc (Hon.) 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 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (D e p a r t m e n t 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 a s 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 6 December 1984 © E d i t h K r a u s e , 1984 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of ix^y'lv ^  The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date 2b DE-6 (3/81) i i ABSTRACT U n d e r s t a n d i n g t h e impact w h i c h g r a z e r s have on t h e i r p r e y i s of v i t a l i m p o r t a n c e i n u n d e r s t a n d i n g how a q u a t i c e c o s y s t e m s f u n c t i o n . In an a t t e m p t t o c o n t r i b u t e t o t h i s u n d e r s t a n d i n g , t h i s s t u d y e x a m i n e d , a t t h r e e l e v e l s , t h e e f f e c t s of z o o p l a n k t o n on p h y t o p l a n k t o n b i o m a s s i n P l a c i d Lake i n summer. E x a m i n a t i o n of s e l e c t i v e f e e d i n g by 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 _in s i t u e n c l o s u r e s r e v e a l e d t h a t s i n g l e p h y t o p l a n k t o n c e l l s i n t h e ran g e of 6-20 Mm l o n g were t h e p r e f e r r e d f o o d of t h e s e o r g a n i s m s . C o l o n i a l a l g a e , when d o m i n a t e d by t h e c y a n o p h y t e Mer i s m o p e d i a , d i d not a p p e a r t o be g r a z e d . The e f f e c t of z o o p l a n k t o n b i o m a s s on p h y t o p l a n k t o n biomass was examined i n ir^ s i t u e n c l o s u r e s . G e n e r a l l y , p h y t o p l a n k t o n b i o m a s s d e c r e a s e d o n l y i n e n c l o s u r e s where i n i t i a l z o o p l a n k t o n b i o m a s s was v e r y low or v e r y h i g h . A s i m p l e model b a s e d on t h e c l a s s i c a l l o g i s t i c model of p r e d a t o r - p r e y i n t e r a c t i o n s was d e v e l o p e d t o e x p l a i n e v e n t s i n t h e e n c l o s u r e s . I c o n c l u d e d t h a t i n summer, P l a c i d Lake p h y t o p l a n k t o n depend on n u t r i e n t s r e m i n e r a l i z e d by z o o p l a n k t o n f o r g r o w t h . G r a z i n g a p p e a r s t o be an i m p o r t a n t r e g u l a t i n g mechanism o f t h 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 t h e s p r i n g but not summer. A t h i r d l e v e l of s t u d y i n v o l v e d e x a m i n a t i o n o f t h e r e s p o n s e s o f p h y t o p l a n k t o n t o l a k e p e r t u r b a t i o n , namely r e m o v a l of z o o p l a n k t o n , compared t o p l a n k t o n p a t t e r n s i n p r e v i o u s and s u b s e q u e n t y e a r s . In y e a r s l a c k i n g z o o p l a n k t o n m a n i p u l a t i o n , major i n c r e a s e s i n z o o p l a n k t o n b i o m a s s i n mid s p r i n g were followed by phytoplankton biomass increases i n l a t e s p r i n g . During the f i r s t h arvesting season, J u l y and August 1979, a 50% reduction i n zooplankton biomass was obtained. An enormous bloom of the i n e d i b l e Merismopedia developed. I hypothesized that removal of zooplankton caused a shortage of a v a i l a b l e b i o l o g i c a l l y r e a c t i v e nitrogen which became l i m i t i n g to eukaryotic phytoplankton, a l l o w i n g Mer i smopedia, a blue-green alga which may be able to f i x n i t r o g e n , to t h r i v e . In summer, the p o s i t i v e e f f e c t of zooplankton on phytoplankton v i a n u t r i e n t r e m i n e r a l i z a t i o n appeared to be more s i g n i f i c a n t than the negative e f f e c t of gr a z i n g . During the second h a r v e s t i n g season, May, June, and J u l y 1980, no decrease in zooplankton biomass was apparent. Instead of the usual p a t t e r n of zooplankton biomass increase preceeding the phytoplankton biomass i n c r e a s e , both increases occurred simultaneously. I concluded that harvesting delayed the r i s e i n zooplankton biomass and decreased the grazing pressure on phytoplankton, a l l o w i n g i t to peak e a r l i e r . Grazing may thus be s i g n i f i c a n t in spring i n slowing phytoplankton growth. Seasonal v a r i a t i o n s were introduced to the model for the enclosure experiments to help understand the normal plankton patterns i n P l a c i d Lake. The time l a g between maximum s o l a r r a d i a t i o n and lake temperature, and the e f f e c t s of these two p h y s i c a l parameters on phytoplankton and zooplankton growth appear to be instrumental i n e s t a b l i s h i n g the pattern of plankton biomass dynamics observed i n P l a c i d Lake. i v TABLE OF CONTENTS ABSTRACT i i L I S T OF TABLES . . . i x L I S T OF FIGURES x i i ACKNOWLEDGMENTS x v i CHAPTER I - INTRODUCTION 1 CHAPTER II - GENERAL METHODS AND MATERIALS 10 I n t r o d u c t i o n 10 P h y s i c a l d a t a 14 S a m p l i n g f o r z o o p l a n k t o n 14 S a m p l i n g f o r p h y t o p l a n k t o n 15 E n c l o s u r e c o n s t r u c t i o n 16 C o u n t i n g z o o p l a n k t o n 18 C o u n t i n g p h y t o p l a n k t o n 19 V D a t a a n a l y s i s . .....20 CHAPTER I I I - SELECTIVE FEEDING 22 I n t r o d u c t i o n 22 Methods and m a t e r i a l s 30 R e s u l t s 36 D i s c u s s i o n 49 CHAPTER IV - ENCLOSURE EXPERIMENTS AND MODEL 62 I n t r o d u c t i o n 62 A l o g i s t i c model f o r p h y t o p l a n k t o n g r o w t h ....65 A model f o r z o o p l a n k t o n d y n a m i c s 78 Methods and m a t e r i a l s 83 R e s u l t s 90 Z o o p l a n k t o n d y n a m i c s 90 P h y t o p l a n k t o n d y n a m i c s 99 v i C o m p a r ing r e a l and model d y n a m i c s 114 Di s c u s s i o n 146 Z o o p l a n k t o n c o m p o s i t i o n 146 P h y t o p l a n k t o n c o m p o s i t i o n 148 Community d y n a m i c s - c h o o s i n g a model 149 P h y t o p l a n k t o n model p a r a m e t e r v a l u e s 153 C o n c l u s i o n s 1 59 CHAPTER V - LAKE ZOOPLANKTON DYNAMICS 162 I n t r o d u c t i o n 162-.Methods and m a t e r i a l s 164 R e s u l t s 170 P h y s i c a l c o n d i t i o n s 170 P l a n k t o n d y n a m i c s 173 Di s c u s s i o n 191 v i i P l a n k t o n d y n a m i c s 191 M o d e l l i n g l a k e p l a n k t o n d y n a m i c s 202 LITERATURE CI TED 211 APPENDIX I . R a i n f a l l t o t h e U.B.C. R e s e a r c h F o r e s t , 1 977-1 982. 225 APPENDIX I I . T e m p e r a t u r e i s o c l i n e s i n P l a c i d L a k e , 1977-1982 228 APPENDIX I I I . C o u n t i n g P h y t o p l a n k t o n 234 I n t r o d u c t i o n 234 Methods and m a t e r i a l s 236 R e s u l t s and d i s c u s s i o n 236 APPENDIX IV. Response of p h y t o p l a n k t o n b i o m a s s and c o m p o s i t i o n t o g r a z i n g by s e l e c t e d z o o p l a n k t o n s p e c i e s 240 APPENDIX V. I n i t i a l r e s p o n s e of p h y t o p l a n k t o n biomass and c o m p o s i t i o n t o v a r i a t i o n i n i n i t i a l z o o p l a n k t o n biomass 255 v i i i APPENDIX V I . L o n g - t e r m r e s p o n s e 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 b i o m a s s and c o m p o s i t i o n t o v a r i a t i o n i n i n i t i a l z o o p l a n k t o n biomass ....276 APPENDIX V I I . Z o o p l a n k t o n c o m p o s i t i o n and b i o m a s s , P l a c i d L a k e , 1977-1982 318 APPENDIX V I I I . 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 b i o m a s s , P l a c i d L a k e , 1977-1982 324 i x L I S T OF TABLES 1-1 The p r o p o r t i o n of a v e r a g e d a i l y p r o d u c t i o n consumed by z o o p l a n k t o n , l i t e r a t u r e r e v i e w 2 1-2 The p r o p o r t i o n of p h y t o p l a n k t o n n u t r i e n t r e q u i r e m e n t s u p p l i e d by z o o p l a n k t o n e x c r e t i o n , l i t e r a t u r e r e v i e w 7 111 — 1 Z o o p l a n k t o n f o o d s i z e r a n g e s , l i t e r a t u r e r e v i e w 29 111-2 D e n s i t i e s of z o o p l a n k t o n s t o c k e d i n e n c l o s u r e s 32 111-3 N a u p l i i c o n t a m i n a t i o n i n e n c l o s u r e s , June 1981 43 111-4 F e e d i n g of z o o p l a n k t o n on v a r i o u s s i z e s of p h y t o p l a n k t o n c e l l s 44 111 — 5 F i l t e r i n g r a t e s on p h y t o p l a n k t o n s i z e c l a s s e s s i g n i f i c a n t l y g r a z e d 47 I I I - 6 Maximum z o o p l a n k t o n f i l t e r i n g r a t e s , l i t e r a t u r e r e v i e w 56 IV- 1 Biomass of z o o p l a n k t o n s t o c k e d i n e n c l o s u r e s 84 IV-2 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 s on 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 b i o m a s s i n e n c l o s u r e s 92 X IV-3 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 s on r e l a t i v e z o o p l a n k t o n c o m p o s i t i o n i n e n c l o s u r e s 95 IV-4 I n i t i a l t r e a t m e n t e f f e c t s on p h y t o p l a n k t o n b i o m a s s , e x p e r i m e n t 3 (May 1981 ) 100 IV-5 L o n g - t e r m (6 weeks) t r e a t m e n t e f f e c t s on p h y t o -p l a n k t o n b i o m a s s , e x p e r i m e n t 3 ( M a y - J u l y 1981) 102 IV-6 L o n g - t e r m (6 weeks) t r e a t m e n t e f f e c t s on p h y t o p l a n k t o n b i o m a s s , e x p e r i m e n t 4 ( A u g u s t - S e p t e m b e r 1981) 103 IV-7 P h y t o p l a n k t o n taxonomy d a t a f o r l o n g - t e r m o b s e r v a t i o n s of e x p e r i m e n t 3 ( M a y - J u l y 1981) 106 IV-8 P h y t o p l a n k t o n taxonomy d a t a f o r l o n g - t e r m o b s e r v a t i o n s of e x p e r i m e n t 4 ( A u g u s t -September 1981) 108 IV-9 C a l c u l a t e d model p a r a m e t e r s 145 IV- 10 T h e o r e t i c a l v a l u e s o f "b" 156 V- 1 S e a s o n a l n u t r i e n t i n p u t t o P l a c i d Lake v i a r a i n f a l l and z o o p l a n k t o n e x c r e t i o n 171 V-2 M a j o r c o l o n y t a x a i n P l a c i d L a k e , 1977-1981 190 S e a s o n a l v a r i a t i o n i n model p a r a m e t e r s x i i L I S T OF FIGURES 11 — 1 P l a c i d Lake, B r i t i s h C o l u m b i a 11 II -2 T o t a l p h o s p h o r u s , t o t a l n i t r o g e n and N:P i n P l a c i d Lake, 1982. Taken from 13 11-3 E n c l o s u r e d e s i g n 17 I I - 4 E n c l o s u r e a n c h o r a g e 17 I I I — 1 - I n i t i a l p h y t o p l a n k t o n p a r t i c l e s i z e d i s t r i b u t i o n I I - 5 i n e n c l o s u r e s i n 1) June 2) J u l y 3) A u g u s t 4) August ( Diaphanosoma ) 5) September. Biomass i n #g 0c-l f r e s h w e i g h t 37 I I I - 6 Lake t e m p e r a t u r e a t s u r f a c e and 1 m, June t o O c t o b e r , 1981 42 IV- 1 P h y t o p l a n k t o n i s o c l i n e , c l a s s i c a l l o g i s t i c model 68 IV-2 P h y t o p l a n k t o n i s o c 1 i n e , " l o g i s t i c - p l u s - e n r i c h m e n t model 68 IV-3 The e f f e c t of n o n - g r a z a b l e , n i t r o g e n - f i x i n g c o l o n i a l b l u e - g r e e n a l g a e on i s o c l i n e s of g r a z a b l e , n o n - n i t r o g e n - f i x i n g p h y t o p l a n k t o n 75 x i i i IV-4 Z o o p l a n k t o n i s o c l i n e , c l a s s i c a l l o g i s t i c model 79 IV-5 Z o o p l a n k t o n i s o c l i n e , p r e d a t o r t h r e s h h o l d model 79 IV-6 Z o o p l a n k t o n i s o c l i n e , p r e d a t o r - p r e y t h r e s h h o l d model .81 IV-7 O s c i l l a t i o n s p r e d i c t e d by L o t k a - V o l t e r r a p r e d a t o r - p r e y model 82 IV-8 O s c i l l a t i o n s p r e d i c t e d by " l o g i s t i c - p l u s - e n r i c h m e n t " p r e y model and p r e d a t o r 0 0 p r e y t h r e s h h o l d p r e d a t o r model 82 IV-9 I n i t i a l - c h a n g e s i n 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 b i o m a s s i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 2 ( A p r i l - M a y 1 981) 115 IV-10 I n i t i a l c h a n g e s i n 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 b i o m a s s i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 3 (May 1981) 116 IV-11 I n i t i a l c h a n g e s i n 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 b i o m a s s i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 4 (August 1981) 117 IV-12 Long t e r m (6 weeks) t r a j e c t o r i e s i n p h y t o p l a n k t o n -z o o p l a n k t o n d y n a m i c s i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 1 ( J u l y - S e p t e m b e r 1980) 120 x i v IV-13 Long t e r m (6 weeks) t r a j e c t o r i e s i n p h y t o p l a n k t o n -z o o p l a n k t o n d y n a m i c s i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 3 ( M a y - J u l y 1981) 126 IV-14 Long t e r m (6 weeks) t r a j e c t o r i e s i n p h y t o p l a n k t o n -z o o p l a n k t o n d y n a m i c s i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 4 ( A u g u s t - S e p t e m b e r 1980) 135 IV-15 E x p e r i m e n t a l e n c l o s u r e r e s u l t s , t o t a l p h y t o p l a n k t o n d y n a m i c s i n e n r i c h e d e n c l o s u r e s , w i t h and w i t h o u t z o o p l a n k t o n 141 IV-16 E x p e r i m e n t a l e n c l o s u r e r e s u l t s , s i n g l e - c e l l e d p h y t o p l a n k t o n d y n a m i c s i n e n r i c h e d e n c l o s u r e s , w i t h and w i t h o u t z o o p l a n k t o n 142 IV-17 E x p e r i m e n t a l e n c l o s u r e r e s u l t s , t o t a l p h y t o p l a n k t o n d y n a m i c s i n u n e n r i c h e d e n c l o s u r e s , w i t h and w i t h o u t z o o p l a n k t o n 143 IV-18 E x p e r i m e n t a l e n c l o s u r e r e s u l t s , s i n g l e - c e l l e d p h y t o p l a n k t o n d y n a m i c s i n u n e n r i c h e d e n c l o s u r e s , w i t h and w i t h o u t z o o p l a n k t o n 144 IV-19 P h y t o p l a n k t o n i s o c l i n e s d e t e r m i n e d from c a l c u l a t e d p a r a m e t e r v a l u e s 158 XV IV- 20 P h y t o p l a n k t o n i s o c l i n e s d e t e r m i n e d from r e v i s e d p a r a m e t e r v a l u e s 160 V- 1 H a r v e s t i n g n e t d e s i g n 165 V-2 H a r v e s t i n g n e t use 167 V-3, S o l a r r a d i a t i o n and l a k e t e m p e r a t u r e , P l a c i d L a k e , a - f 1977-1982 174 V-4, T o t a l p h y t o p l a n k t o n (Mg / 1 f r e s h w e i g h t ) and a - f z o o p l a n k t o n (Ug/ 1 d r y w e i g h t ) b i o m a s s , P l a c i d L a k e , 1977-1982 177 V-5 C l a d o c e r a n f e r t i l i t y , P l a c i d L a k e , 1977-1982 183 V-6 Mean s e a s o n a l c h a n g e s i n p h y t o p l a n k t o n (tig / 1 f r e s h w e i g h t ) and z o o p l a n k t o n (Mg/ 1 d r y w e i g h t ) b i o m a s s i n P l a c i d L a k e , 1977-1982 203 V-7 S e a s o n a l c h a n g e s i n p h y t o p l a n k t o n - z o o p l a n k t o n e q u i l i b r i a , " l o g i s t i c - p l u s - e n r i c h m e n t " p r e y model, t h r e s h - h o l d p r e d a t o r 0 0 p r e y p r e d a t o r model 207 V-8 S e a s o n a l c h a n g e s i n p h y t o p l a n k t o n - z o o p l a n k t o n e q u i l i b r i a , " l o g i s t i c - p l u s - e n r i c h m e n t " p r e y model, t h r e s h - h o l d p r e y ( V o l t e r r a ) p r e d a t o r model 208 x v i ACKNOWLEDGMENTS I would l i k e t o e x p r e s s may g r a t i t u d e t o a l l the p e o p l e who have h e l p e d me w i t h t h i s t h e s i s . T hank-you t o C a r l W a l t e r s and B i l l N e i l l f o r d i r e c t i o n and a d v i c e . Thank-you a l s o t o Don R o b i n s o n and Dave Z i t t i n , who t a u g h t me how t o use t h e computer, and t o t h e r e s t of t h e D a t a C e n t r e S t a f f f o r t h e i r i n v a l u a b l e h e l p . An army of f i e l d and l a b a s s i s t a n t s a l s o d e s e r v e c r e d i t : Sue Beachy, B a r b a r a F r a s e r , M a r l y c e F r i e s e n , A l i c e K l a s s e n , Henry K r a u s e , I r e n e K r a u s e , A l b e r t Leung, L i l y L im, B i l l N e i l l , R o b e r t a O l e n i c k , A d o l f P e n n e r , Shannon R e g i e r , Don R o b i n s o n , M i c h e l l e S c h o f i e l d and C a r l W a l t e r s . T h a n k - y o u t o L i z K r a u s e , Henry K r a u s e and D r . A.G. L e w i s f o r h e l p w i t h m a n u s c r i p t p r e p a r a t i o n . A s p e c i a l t h a n k s t o L y d i a Penner and K a t i e K r a u s e f o r months o f t i r e l e s s (and f r e e ) b a b y s i t t i n g , and t o Don R o b i n s o n and Wendy H i r d , A l f and L i z K r a u s e , R o b e r t a O l e n i c k , J o y Bowerman and many o t h e r f r i e n d s a t I.A.R.E. And L a n g l e y M e n n o n i t e F e l l o w s h i p f o r m o r a l s u p p o r t and en c o u r a g e m e n t . F i n a l l y , and most o f a l l , my t h a n k s t o Henry and R a c h e l , my f a n c l u b , whose b e l i e f i n me kept me g o i n g . T h i s s t u d y was s p o n s o r e d i n p a r t by g r a n t s from t h e N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l . 1 CHAPTER I - INTRODUCTION T h i s s t u d y i s c o n c e r n e d w i t h some l i m i t i n g i n f l u e n c e s of z o o p l a n k t o n on p h y t o p l a n k t o n p o p u l a t i o n g rowth. Our knowledge c o n c e r n i n g t h e i n f l u e n c e of z o o p l a n k t o n on p h y t o p l a n k t o n p o p u l a t i o n s i n f r e s h w a t e r i s s t i l l l i m i t e d ( F r o s t 1980). T h e o r e t i c a l l y , z o o p l a n k t o n c o u l d a f f e c t p h y t o p l a n k t o n p o p u l a t i o n s t h r o u g h g r a z i n g and t h r o u g h n u t r i e n t r e c y c l i n g . T h i s b r i e f i n t r o d u c t i o n w i l l r e v i e w some of t h e l i t e r a t u r e r e l e v a n t t o t h i s s t u d y . T h e r e i s c o n s i d e r a b l e c o n t r o v e r s y i n t h e l i t e r a t u r e a b o u t t h e i m p o r t a n c e of g r a z i n g p r e s s u r e i n r e g u l a t i n g 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 f r e s h w a t e r . A l t h o u g h t h e r e i s s u b s t a n t i a l e v i d e n c e t h a t z o o p l a n k t o n may c o n t r o l 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 t h e ocean ( s e e T a b l e 1-1 f o r e x a m p l e s ) , t h i s i s not t h e c a s e i n f r e s h w a t e r . Lund (1965) c o n c l u d e d from h i s o b s e r v a t i o n s of d i a t o m g r o w th i n l a k e s of t h e E n g l i s h Lake D i s t r i c t t h a t t h e amount of p h y t o p l a n k t o n s u s t a i n e d i n f r e s h w a t e r i s l i m i t e d p r i m a r i l y by p h y s i c a l and c h e m i c a l f a c t o r s , s u c h as l i g h t and n u t r i e n t s , t h u s d e m o n s t r a t i n g a f u n d a m e n t a l d i f f e r e n c e between m a r i n e and f r e s h w a t e r p h y t o p l a n k t o n p o p u l a t i o n d y n a m i c s . F i n d e n e g g ( 1 9 6 9 ) , w o r k i n g on a deep o l i g o t r o p h i c S w i s s l a k e , f o u n d t h a t n a n n o p l a n k t o n ( c e l l s l e s s t h an 50 um i n d i a m e t e r ) s t a n d i n g c r o p s were g e n e r a l l y low, even when t h e y were n p t b e i n g g r a z e d , b e c a u s e of n u t r i e n t l i m i t a t i o n . He f o u n d t h a t n e t p l a n k t o n ( c e l l s g r e a t e r t h a n 50 nm i n d i a m e t e r ) were not g r a z e d . Lehman e t a l . (1975) c o n c l u d e d from t h e p r e d i c t i o n s of 2 TABLE 1-1 LOCATION % AVERAGE DAILY PRODUCTION CONSUMED SOURCE Bras D'Or Lake 100 H a r g r a v e and Geen 1970 M o r r i s o n ' s Pond 66 H a r g r a v e and Geen 1970 He a r t Lake (summer) H e a r t Lake ( w i n t e r ) Drowned Bog Lake 100 + 10 114 Haney 1973 Haney 1973 Haney 1973 H a l l s Lake 6.6 Haney 1973 Lake E r k e n , Sweden 100 + Nauwerck 1963 F l a d e n Ground (May) 100 + W i l l i a m s and L i n d l e y 1980 c o a s t a l C a l i f o r n i a (La J o l l a ) Georges Bank, New Y o r k B i g h t 40 - 50 E n r i g h t 1969 50 Dagg and T u r n e r 1982 3 a computer model o f p h y t o p l a n k t o n p o p u l a t i o n d y n a m i c s t h a t g r a z i n g o n l y becomes a major m o r t a l i t y a gent when p h y t o p l a n k t o n growth has been s l o w e d s u b s t a n t i a l l y t h r o u g h p h y s i c a l l i m i t a t i o n s . U n l i k e Lund, W r i g h t ' s (1965) s t u d y of D a p h n i a i n Canyon F e r r y R e s e r v o i r , Montana, d e m o n s t r a t e d a s i g n i f i c a n t c o r r e l a t i o n between d e c l i n e i n c h l o r o p h y l l c o n c e n t r a t i o n s and i n c r e a s e i n z o o p l a n k t o n a b u n d a n c e . He c o n c l u d e d t h a t g r a z i n g was r e s p o n s i b l e f o r t h e d e c l i n e of t h e s p r i n g bloom of p h y t o p l a n k t o n . Nauwerck (1963) a l s o f o u n d s i g n i f i c a n t r e g r e s s i o n s between a l g a l m o r t a l i t y and z o o p l a n k t o n a b u n d a n c e s i n Lake E r k e n . C u s h i n g (1976) m a i n t a i n e d t h a t , b a s e d on Nauwerck's measured z o o p l a n k t o n f i l t r a t i o n r a t e s , p h y t o p l a n k t o n c o u l d o n l y a c c o u n t f o r 10% of t h e z o o p l a n k t o n d i e t , and t h a t even i f t h e whole p r i m a r y p r o d u c t i o n c o u l d be g r a z e d , z o o p l a n k t o n would s t i l l have t o r e l y on d e t r i t u s f o r p a r t of t h e i r d i e t . ( I t s h o u l d be n o t e d t h a t Nauwerck's measured f i l t e r i n g r a t e s a r e s u b s t a n t i a l l y lower than more r e c e n t measurements by Haney (1973) and Buckingham (1978).) G l i w i c z (1975) a l s o f o u n d t h a t n a n n o p l a n k t o n d e c r e a s e d as z o o p l a n k t o n i n c r e a s e d . H a r g r a v e and Geen (1970) and Haney (1973) examined t h e p e r c a p i t a f i l t e r i n g r a t e s of z o o p l a n k t o n from two s a l t w a t e r l a k e s and t h r e e f r e s h w a t e r l a k e s , r e s p e c t i v e l y , and e x t r a p o l a t e d o v e r a l l g r a z i n g p r e s s u r e from f i l t e r i n g r a t e s , p h y t o p l a n k t o n d e n s i t i e s and z o o p l a n k t o n d e n s i t i e s . The e f f e c t of g r a z i n g i n t h e two s a l t w a t e r l a k e s was s i g n i f i c a n t ( see T a b l e 1-1 ). Haney 4 f o u n d t h a t t h e e x t e n t of g r a z i n g v a r i e d s e a s o n a l l y , and t h a t d u r i n g summer, more t h a n 100% of t h e water i n t h e e u p h o t i c zone c o u l d be f i l t e r e d by t h e z o o p l a n k t o n p r e s e n t i n H e a r t L a k e ; i n w i n t e r , t h e z o o p l a n k t o n p r e s e n t c o u l d f i l t e r l e s s t h a n 10% o f t h e e u p h o t i c zone d a i l y . He a l s o f o u n d t h a t t h e i m p o r t a n c e of g r a z i n g i n c o n t r o l l i n g t h 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 v a r i e d w i t h t h e t r o p h i c s t a t u s of l a k e s , so t h a t g r a z i n g was l e a s t e f f e c t i v e i n l i m i t i n g p h y t o p l a n k t o n i n t h e o l i g o t r o p h i c l a k e s t u d i e d . T h i s was due t o low g r a z i n g r a t e s c o u p l e d w i t h m o d e r a t e l y h i g h p r i m a r y p r o d u c t i v i t y . In a d d i t i o n t o i t s e f f e c t on 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 t has been d e m o n s t r a t e d t h a t g r a z i n g by z o o p l a n k t o n has a s i g n i f i c a n t e f f e c t 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 p h y t o p l a n k t o n community. P o r t e r (1972, 1973), w o r k i n g w i t h 500 L e n c l o s u r e s i n a m e s o t r o p h i c l a k e , and Weers and Z a r e t ( 1 9 7 5 ) , w o r k i n g w i t h j a r s s u s p e n d e d iri s i t u i n a m e s o - o l i g o t r o p h i c l a k e , f o u n d t h a t t h e numbers o f some a l g a l s p e c i e s were enhanced, o t h e r s u n a f f e c t e d , and s t i l l o t h e r s d e c r e a s e d i n t h e p r e s e n c e o f z o o p l a n k t o n . (See C h a p t e r I I I f o r f u r t h e r d i s c u s s i o n of s e l e c t i v e f e e d i n g by z o o p l a n k t o n . ) G l i w i c z ( 1 9 7 5 ) , c o m p a r i n g n i n e P o l i s h l a k e s of v a r y i n g t r o p h y , f o u n d t h a t n e t p l a n k t o n i n c r e a s e d a s z o o p l a n k t o n i n c r e a s e d and he h y p o t h e s i z e d t h a t t h i s was due t o d e c r e a s e d c o m p e t i t i o n f o r t h e n e t p l a n k t o n w i t h n a n n o p l a n k t o n , due t o z o o p l a n k t o n g r a z i n g on nannoplankton.- He c i t e d t h i s as t h e r e a s o n why n e t p l a n k t o n i s f a r more p r e v a l e n t i n summer and i n e u t r o p h i c w a t e r s . M c C a u l e y and B r i a n d ( 1 9 7 9 ) , i n e n c l o s u r e e x p e r i m e n t s i n an o l i g o t r o p h i c l a k e , f o u n d t h a t 5 i n c r e a s e d g r a z i n g p r e s s u r e l e d t o g r e a t e r p h y t o p l a n k t o n s p e c i e s d i v e r s i t y ; t h e y s u g g e s t e d t h a t g r a z i n g r e d u c e d c o m p e t i t i o n f o r n u t r i e n t s and t h u s a l l o w e d l e s s c o m p e t i t i v e s p e c i e s t o t h r i v e . A n o t h e r means by w h i c h z o o p l a n k t o n may i n f l u e n c e p h y t o p l a n k t o n growth i s by r e c y c l i n g n u t r i e n t s . Here a g a i n , t h e r e has been some d i s p a r i t y i n t h e l i t e r a t u r e on t h e i m p o r t a n c e of t h i s p r o c e s s . G a r d i n e r (1937) was one of t h e f i r s t t o p o i n t out t h e s i g n i f i c a n c e of n u t r i e n t r e c y c l i n g by z o o p l a n k t o n . R i g l e r (1961) c l a i m e d t h a t z o o p l a n k t o n c o u l d c o n t r i b u t e o n l y 0.5% of t h e t o t a l p h o s p h o r u s t u r n o v e r , and were t h e r e f o r e t r i v i a l c o n t r i b u t o r s . He l a t e r r e t r a c t e d t h i s view ( 1 9 7 3 ) , c i t i n g i n c o r r e c t a s s u m p t i o n s f o r h i s e a r l i e r v i ew. Ketchum ( 1 9 6 2 ) , i n a r e v i e w p a p e r , c o n c l u d e d t h a t r a p i d n u t r i e n t r e c y c l i n g must be o c c u r r i n g and must be a s i g n i f i c a n t s o u r c e of n u t r i e n t s f o r p r i m a r y p r o d u c t i v i t y , s i n c e t h e c o n c e n t r a t i o n of p h o s p h a t e i n l a k e w a t e r a t any g i v e n t i m e i s t o o low t o a c c o u n t f o r t h e amount o f p r o d u c t i o n . S e a s o n a l v a r i a b i l t y i s an i m p o r t a n t q u a l i f i e r of t h e i m p o r t a n c e of n u t r i e n t r e c y c l i n g by z o o p l a n k t o n . D u r i n g summer months, when r u n o f f from r a i n f a l l , t h e major s o u r c e of a l l o c h t h o n o u s n u t r i e n t s , i s o f t e n low, t h e f r e e n u t r i e n t s p r o v i d e d f o r p h y t o p l a n k t o n g r o w t h by z o o p l a n k t o n e x c r e t i o n may be s i g n i f i c a n t . Lehman (1980) f o u n d t h a t i n Lake W a s h i n g t o n , r u n o f f c o n t r i b u t e d l e s s t h a t 10% o f a l l n u t r i e n t s 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 d u r i n g J une t h r o u g h September. He c a l c u l a t e d t h a t z o o p l a n k t o n c o n t r i b u t e d a p p r o x i m a t e l y 90% o f a l l n u t r i e n t s a v a i l a b l e d u r i n g t h i s t i m e . O t h e r a u t h o r s have made 6 s i m i l a r c l a i m s ( s e e T a b l e 1-2). F e r r a n t e (1976) a l s o n o t e d a s e a s o n a l v a r i a t o n i n t h e i m p o r t a n c e of n u t r i e n t r e c y c l i n g by z o o p l a n k t o n . D u r i n g s p r i n g when p h y t o p l a n k t o n g r o w th r a t e s a r e v e r y h i g h and z o o p l a n k t o n numbers a r e low, e x c r e t i o n of p h o s p h a t e by z o o p l a n k t o n i s not enough t o meet t h e demands of p h y t o p l a n k t o n u p t a k e , as i t i s i n summer. R i c h e y (1979) f o u n d t h a t d u r i n g p r e - and e a r l y bloom p e r i o d s ( e a r l y s p r i n g ) , p h o s p h a t e a c c u m u l a t e d o v e r w i n t e r and, s u p p l i e d t h r o u g h v e r t i c a l m i x i n g , was s u f f i c i e n t t o meet p h y t o p l a n k t o n n e e d s . However, i n l a t e s p r i n g and summer, e x c r e t i o n by z o o p l a n k t o n was t h e major s o u r c e of p h o s p h a t e . ( I t s h o u l d be n o t e d t h a t t h e r e i s s u b s t a n t i a l agreement t h a t p h o s p h o r u s t e n d s t o be t h e l i m i t i n g n u t r i e n t i n most f r e s h w a t e r l a k e s ( S c h i n d l e r 1 9 7 7 ) ) . T u r n o v e r of n u t r i e n t s by z o o p l a n k t o n i s e x t r e m e l y r a p i d and has been e s t i m a t e d t o be as h i g h as 50% ( f o r n i t r o g e n , Ganf and B l a z k a 1974) t o 100% ( f o r p h o s p h o r u s , Pomeroy e t a l . 1963) of t h e i r body n u t r i e n t c o n t e n t d a i l y . P e t e r s and R i g l e r (1973) f o u n d t h a t t h e r e c y c l i n g e f f i c i e n c y o f p h o s p h o r u s , t h a t i s , t h e p r o p o r t i o n of p h o s p h o r u s i n g e s t e d w h i c h i s e x c r e t e d , i s e x t r e m e l y h i g h , between 88 and 93%. A n o t h e r s i g n i f i c a n t f a c t o r i n t h e speed of r e c y c l i n g by z o o p l a n k t o n i s t h e form i n w h i c h n u t r i e n t s a r e r e l e a s e d . N i t r o g e n i s e x c r e t e d p r i m a r i l y as ammonia ( E p p l e y e t a l . 1973, D u g d a l e 1976 ), w h i c h i s r e a d i l y u t i l i z a b l e by p h y t o p l a n k t o n . S i x t y t o n i n e t y p e r c e n t o f t h e p h o s p h o r u s r e l e a s e d by z o o p l a n k t o n i s h i g h l y a c t i v e o r t h o p h o s p h a t e ( P e t e r s and Lean 1973). The u p t a k e of t h e s e n u t r i e n t s by p h y t o p l a n k t o n i s r a p i d . When l a k e c o n c e n t r a t i o n s 7 TABLE 1-2 LOCATION % PHYTOPLANKTON NUTRIENT REQUIREMENT SUPPLIED BY ZOOPLANKTON EXCRETION SOURCE Cayuga Lake, New York Bras D'Or Lake 127 + 200 + Barlow and Bishop 1965 Hargrave and Geen 1968 Morrison's Pond 20 + Hargrave and Geen 1968 Long Island Sound Lake Memphramegog 56 12 + Harris 1958 Nakashima and Legett 1980 Heart Lake 54 + Peters and Rigle r 1973 Lake Washington 68 Devol 1979 Lake Washington 93 Lehman 1980 ++ nitrogen only + phosphorus only 8 of z o o p l a n k t o n a r e h e l d i n f i l t e r e d l a k e w a t e r , t h e r e i s a s i g n i f i c a n t i n c r e a s e i n b o t h ammonia and p h o s p h a t e as compared t o c o n t r o l s w i t h o u t z o o p l a n k t o n . However, when n o r m a l l a k e c o n c e n t r a t i o n s of p h y t o p l a n k t o n a r e p r e s e n t , t h e r e i s no s i g n i f i c a n t i n c r e a s e i n n u t r i e n t l e v e l s compared w i t h c o n t r o l s (Ganf and B l a z k a 1974). Z o o p l a n k t o n may a l s o be a b l e t o r e c l a i m n u t r i e n t s l o s t t o p h y t o p l a n k t o n by r e t u r n i n g n u t r i e n t s t o t h e e p i l i m n o n from the h y p o l i m n i o n t h r o u g h d i e l m i g r a t i o n s ( G a r d i n e r 1937, H a r r i s 1959). In some l a k e s , t h e p r i m a r y p r o d u c t i v i t y i s s u c h t h a t z o o p l a n k t o n must r e l y i n p a r t on d e t r i t u s f o r f o o d (Nauwerck 1963, H i l l b r i c h t - I l k o w s k a e t a l . 1972). In t h i s s i t u a t i o n , n u t r i e n t s w h i c h a r e l o c k e d i n t o d e t r i t i s u n t i l d e c o m p o s i t o n i s c o m p l e t e d , and w h i c h may s i n k t o t h e s e d i m e n t s , a r e r e c l a i m e d and r a p i d l y r e c y c l e d . The r e l a t i v e i m p o r t a n c e of n u t r i e n t r e g e n e r a t i o n by z o o p l a n k t o n and f i s h was d e m o n s t r a t e d by Nakashima and L e g g e t t (1980) who f o u n d t h a t i n t h e l a k e t h e y were s t u d y i n g , z o o p l a n k t o n p r o v i d e d 12% of t h e d a i l y summer o r t h o p h o s p h a t e needed by t h e s e s t o n w h i l e f i s h p r o v i d e d o n l y 0.33%. The p u r p o s e of my s t u d y was t o d e t e r m i n e w hether t h e z o o p l a n k t o n p o p u l a t i o n i n P l a c i d L a k e , B r i t i s h C o l u m b i a , has any a p p r e c i a b l e i n f l u e n c e on t h e s t a n d i n g c r o p of p h y t o p l a n k t o n , e i t h e r by g r a z i n g or by n u t r i e n t r e c y c l i n g . I a t t e m p t e d t o examine t h e e f f e c t s of s e l e c t i v e f e e d i n g by z o o p l a n k t o n by i s o l a t i n g 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 s p e c i e s i n e n c l o s u r e s and o b s e r v i n g t h e r e p o n s e of t h e v a r i o u s 9 s i z e c l a s s e s c o m p osing t h e p h y t o p l a n k t o n community. E n c l o s u r e s were a l s o u s e d t o examine t h e e f f e c t s of v a r y i n g z o o p l a n k t o n d e n s i t i e s , and t h u s g r a z i n g p r e s s u r e and t o t a l , z o o p l a n k t o n e x c r e t i o n , on t h 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 . In o r d e r t o i n t e r p r e t t h e s e d a t a , a s i m p l e m a t h e m a t i c a l model, p r e d i c t i n g t h e combined e f f e c t s of g r a z i n g and n u t r i e n t r e m i n e r a l i z a t i o n , was c o n s t r u c t e d . D a t a from a s h o r t - t e r m , s m a l l e n c l o s u r e e x p e r i m e n t were u s e d t o c a l c u l a t e some t e n t a t i v e c o n s t a n t s f o r t h e model. Normal 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 p o p u l a t i o n p a t t e r n s i n P l a c i d Lake were examined. T h e s e d a t a , from t h e y e a r s 1977, 1978, and 1982 were o b t a i n e d w i t h p e r m i s s i o n from C . J . W a l t e r s . In a d d i t i o n t o t h i s , a p e r t u r b a t i o n s t u d y was done i n w h i c h I h a r v e s t e d t h e z o o p l a n k t o n from t h e t o p two m e t e r s of t h e l a k e f o r two summers, 1979 and 1980. H a r v e s t i n g was f o l l o w e d by one summer (1981) of r e p l i c a t i n g t h e t u r b u l e n c e c a u s e d by t h e m o t i o n of t h e o u t b o a r d motor and n e t . The p u r p o s e of t h e h a r v e s t i n g e x p e r i m e n t s was t o see whether r e m o v a l of z o o p l a n k t o n from t h e l a k e would r e s u l t i n any 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 s t a n d i n g c r o p . The c o n t r o l y e a r (1981) was t o o b s e r v e i r r e g u l a r i t i e s c a u s e d by t h e i n c r e a s e d t u r b u l e n c e i n t h e l a k e , i n o r d e r t o s e p a r a t e t h e s e e f f e c t s f r o m t h o s e o f z o o p l a n k t o n r e m o v a l . 10 CHAPTER II - GENERAL METHODS AND MATERIALS INTRODUCTION The f o l l o w i n g a r e t h e g e n e r a l methods and m a t e r i a l s u s e d f o r most p a r t s of t h i s s t u d y . Methods and m a t e r i a l s p e r t a i n i n g o n l y t o v e r y s p e c i f i c p a r t s of t h i s s t u d y a r e d e s c r i b e d a t t h e b e g i n n i n g of e a c h of t h e c h a p t e r s . P l a c i d Lake ( s e e F i g u r e 11 — 1 ) i s a s m a l l bog l a k e i n s o u t h e r n c o a s t a l B r i t i s h C o l u m b i a , l o c a t e d i n t h e 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 R e s e a r c h F o r e s t . The f o r e s t i s i n the C o a s t a l M o u n t a i n s , and t h e a r e a below.600 m e t e r s a l t i t u d e i s c o n s i d e r e d t o be i n t h e C o a s t a l W e s t e r n Hemlock zone ( E f f o r d 1967). The a v e r a g e a n n u a l r a i n f a l l i n t h e f o r e s t o v e r t h e s i x y e a r s s t u d i e d was 194 cm, w i t h "a low of 151 cm i n 1978 and a h i g h of 235 cm i n 1982. J u l y and A u g u s t t e n d e d t o be t h e d r i e s t months, w i t h a mean r a i n f a l l o v e r 1977 t o 1982 of 57 and 70 mm, r e s p e c t i v e l y . November and December t e n d e d t o be t h e w e t t e s t months, w i t h mean r a i n f a l l s o f 287 and 292 mm, r e s p e c t i v e l y ( A p p e n d i x I ) . P l a c i d Lake has a s u r f a c e a r e a of 1.65 h e c t a r e s , and maximum d e p t h of 7 m e t e r s and a mean d e p t h of 4 m e t e r s . I t i s l o c a t e d a t an a l t i t u d e of 510 m e t e r s , and i s n o r m a l l y c o v e r e d w i t h i c e from mid December u n t i l e a r l y A p r i l . I t i s o n l y l i g h t l y c o l o u r e d , and b e c a u s e of t h i s a s w e l l a s i t s s h a l l o w n e s s , most o f t h e l a k e ' s volume i s e u p h o t i c . However, p r o b a b l y b e c a u s e of i t s p r o t e c t e d l o c a t i o n f r o m m i x i n g by wind , 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 (A p p e n d i x I I ) . I t t e n d s t o be a c i d i c , w i t h a pH of 5.5 t o 6.0 ( S h e p p a r d 1973), 11 F i g u r e 11 — 1 P l a c i d L a k e ; d e p t h i s o c l i n e s i n m e t e r s ; t a k e n from S h e p p a r d 1973. 1 2 as i s c h a r a c t e r i s t i c of bog l a k e s . B e c a u s e of i t s s l i g h t a c i d i t y and r e l a t i v e l y low i n o r g a n i c n i t r o g e n c o n c e n t r a t i o n s ( W e r r i n g , u n p u b l i s h e d d a t a ) , P l a c i d Lake would be c o n s i d e r e d o l i g o t r o p h i c by t h e s t a n d a r d s s e t by W e t z e l ( 1 9 7 5 ) . T o t a l p h o s p h o r u s i s r e l a t i v e l y h i g h , r e s u l t i n g i n an o v e r a l l low n i t r o g e n t o p h o s p h o r u s r a t i o ( F i g u r e 11 -2 ) . I t s p h y t o p l a n k t o n c o m p o s i t i o n , b e i n g m o s t l y n a n n o p l a n k t o n and v e r y l i t t l e n e t p l a n k t o n , would a l s o q u a l i f y i t as o l i g o t r o p h i c ( G l i w i c z 1975). F i v e of t h e g e n e r a of p h y t o p l a n k t o n l i s t e d as i n d i c a t i v e o f o l i g o t r o p h i c l a k e s by J a r n e f e l d t (1952, 1956 i n H u t c h i n s o n 1967) a r e among t h e most common p h y t o p l a n k t o n i n P l a c i d L a k e . The p r i n c i p l e members of t h e summer z o o p l a n k t o n community i n P l a c i d Lake a r e 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 H o l o p e d i u m  gibberurn and t h e co p e p o d D i a p t o m u s o r e g o n e n s i s . T h e s e n o r m a l l y c o m p r i s e a p p r o x i m a t e l y 90% o f t h e l a k e ' s z o o p l a n k t o n b i o m a s s . O t h e r z o o p l a n k t o n s p e c i e s i n c l u d e Bosmina l o n q i r o s t r i s , C e r i o d a p h n i a q u a d r a n g u l a , Diaphanasoma b r a c h y u r u m , Polyphemous  p e d i c u l u s , D i aptomus k e n a i , D i a c y c l o p s t h o m a s i , and T r o p o c y c l o p s  p r a s i n u s . R o t i f e r s a r e e x t r e m e l y r a r e . The l a r v a l s t a g e s of t h e midge C h a o b o r u s f l a v i c a n s a r e a l s o f o u n d , a l t h o u g h t h e y t e n d t o r e m a i n n e a r t h e bottom of t h e l a k e d u r i n g t h e day, a v o i d i n g 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 , Salmo c l a r k i i . The p h y t o p l a n k t o n of P l a c i d Lake i s composed m o s t l y of d e l i c a t e f l a g e l l a t e s , s u c h as c h r y s o p h y t e s and c r y p t o m o n a d s , as w e l l as d i a t o m s , d e s m i d s , d i n o f l a g e l l a 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 . O c c a s i o n a l blooms of d i a t o m s , d i n o f l a g e l l a t e s , or 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 s p . o c c u r i n t h e summer. 1 3 F i g u r e 1 1 - 2 T o t a l n i t r o g e n a n d p h o s p h o r u s c o n c e n t r a t i o n s i n P l a c i d L a k e , 1 9 8 2 ; t a k e n f r o m W e r r i n g , u n p u b l . 1 j a 1 4 PHYSICAL DATA S t r a t i f i e d l a k e t e m p e r a t u r e s o v e r t h e d e e p e s t p a r t of t h e l a k e (1 m i n t e r v a l s , from 6 m t o s u r f a c e ) were measured w i t h a t h e r m i s t o r a t one t o t h r e e week i n t e r v a l s t h r o u g h o u t t h e y e a r , e x c e p t d u r i n g t h e p e r i o d of i c e c o v e r . T h e s e measurements were o b t a i n e d from t h e d a t a c o l l e c t e d by Dr. C . J . W a l t e r s . D a i l y i r r a d i a t i o n ( k i l o w a t t s p e r s q u a r e m e t r e ) and r a i n f a l l (mm) d a t a were o b t a i n e d from t h e o f f i c e of t h e 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 R e s e a r c h F o r e s t , l o c a t e d 145 m above s e a l e v e l and 7.5 km s o u t h of P l a c i d L a k e . T h i s was t h e b e s t r a i n f a l l and i r r a d i a t i o n i n f o r m a t i o n a v a i l a b l e . I t i s u n l i k e l y t h a t c o n d i t i o n s were much d i f f e r e n t a t P l a c i d L a k e ; r e l a t i v e r a i n f a l l and i r r a d i a t i o n , , compared between s e a s o n s and y e a r s , would c e r t a i n l y not have d i f f e r e d s i g n i f i c a n t l y between t h e R e s e a r c h F o r e s t o f f i c e v i c i n i t y and t h e P l a c i d Lake a r e a . SAMPLING FOR ZOOPLANKTON Z o o p l a n k t o n s a m p l e s were t a k e n from P l a c i d Lake w i t h a Clark-Bumpus p l a n k t o n n e t , e q u i p p e d w i t h #10 (104 Mm) s i z e mesh. S t r a t i f i e d one m i n u t e l o n g o b l i q u e h a u l s were done o v e r d e p t h s o f 6 t o 4 m, 4 t o 2 m, and 2 m t o s u r f a c e . E a c h tow sampled a p p r o x i m a t e l y 400 - 800 L of l a k e w a t e r , d e p e n d i n g on t h e p l a n k t o n d e n s i t y and t h e v i s c o s i t y of t h e l a k e w a t e r . A c a l i b r a t e d meter on t h e Clark-Bumpus n e t measured t h e e x a c t v o l u m e . The sam p l e s were p r e s e r v e d i n s u g a r e d f o r m a l i n (800 g s u g a r , 500 ml 37% f o r m a l i n and 500 ml w a t e r ) . 1 5 Z o o p l a n k t o n s a m p l e s were t a k e n from e n c l o s u r e s w i t h t h e use of a b a t t e r y - p o w e r e d b i l g e pump. The f l o w r a t e of t h e pump was c a l i b r a t e d a t t h e b e g i n n i n g of e a c h day of s a m p l i n g by m e a s u r i n g the amount of t i m e r e q u i r e d t o f i l l a 50 L g a r b a g e p a i l . Water was t h e n pumped from an e n c l o s u r e o v e r a c o n t i n u o u s range of d e p t h s and f i l t e r e d t h r o u g h a 54 Mm mesh n e t . 50 L samples were t a k e n from t h e 1000 L e n c l o s u r e s and 15 L from t h e 300 L e n c l o s u r e s , about 5 p e r c e n t of t h e e n c l o s u r e volume. L a r g e r samples p r o b a b l y would have been more a c c u r a t e r e p r e s e n t a t i o n s of t h e c o n t e n t s of t h e e n c l o s u r e s , but a l s o m ight have i n t e r f e r e d w i t h e x p e r i m e n t s . SAMPLING FOR PHYTOPLANKTON Water f o r p h y t o p l a n k t o n a n a l y s i s was t a k e n from a p p r o p r i a t e d e p t h s i n P l a c i d Lake w i t h t h e use of a s m a l l r e c r e a t i o n a l v e h i c l e water pump and a l o n g , t h i n p l a s t i c h o s e , t h e end of w h i c h was a t t a c h e d t o a w i n c h w i r e j u s t above t h e w e i g h t . Water was pumped i n t o a b u c k e t as t h e b o a t t r a v e l l e d a c r o s s t h e l a k e and t h e hose end was s i m u l t a n e o u s l y r a i s e d . T h i s r e s u l t e d i n o b l i q u e l y s t r a t i f i e d s a m p l e s . Samples were t a k e n from d e p t h s of 6 t o 4 m, 4 t o 2 m, and 2 m t o s u r f a c e . Water was sa m p l e d from t h e l a r g e e n c l o s u r e s by pumping w i t h a b a t t e r y o p e r a t e d b i l g e pump t h r o u g h a 2.5 cm d i a m e t e r hose w h i c h was r a i s e d from t h e b o t t o m t o t h e s u r f a c e of t h e e n c l o s u r e a t an even r a t e w h i l e t h e water was b e i n g pumped. To sample water from t h e s m a l l e n c l o s u r e s , one end of an u n a t t a c h e d 2.5 cm d i a m e t e r hose was l o w e r e d t o t h e b o t t o m of t h e e n c l o s u r e . The 1 6 o t h e r end ( o u t of t h e w a t e r ) was s e a l e d . The submerged end of t h e hose was t h e n r a i s e d out of t h e w a t e r , t h e s e a l removed, and t h e water d r a i n e d from t h e hose i n t o a b u c k e t . Water samples f o r p h y t o p l a n k t o n a n a l y s i s were p r e s e r v e d w i t h L u g o l ' s S o l u t i o n . ENCLOSURE CONSTRUCTION E n c l o s u r e s c o n s i s t e d o f l a r g e p l a s t i c bags s u s p e n d e d from wood and s t y r o f o a m f l o a t i n g frames ( F i g u r e 1 1 - 3 ) . L a r g e e n c l o s u r e s were made from c l e a r u l t r a v i o l e t l i g h t r e s i s t a n t 4 ml p l a s t i c s h e e t i n g ( c o m m e r c i a l l y u s e d f o r g r e e n h o u s e s ) w h i c h was c u t i n t o a p p r o p r i a t e s i z e d s h e e t s and made i n t o l o n g t u b e s by i r o n i n g t h e two o p p o s i t e edges of e a c h s h e e t t o g e t h e r . The b o t t o m edge o f e a c h tube was t i e d o f f w i t h p o l y p r o p y l e n e rope.' T h e s e l a r g e e n c l o s u r e s were a p p r o x i m a t e l y 0.6 m i n d i a m e t e r , 4.0 m deep, and h e l d 1.0 t o 1.2 c u b i c m of w a t e r . S m a l l e r 300 and 150 L e n c l o s u r e s were a l s o u s e d . These were made from c l e a r 6 m i l p o l y e t h y l e n e drum l i n e r s . The f u l l volume of e a c h drum l i n e r was 300 L; t h e bottom h a l f of a drum l i n e r was t i e d o f f t o r e d u c e t h e volume t o 150 L f o r t h e s m a l l e s t e n c l o s u r e s . The 300 L e n c l o s u r e s were a p p r o x i m a t e l y 0.5 m i n d i a m e t e r and 2 m deep; t h e 150 L e n c l o s u r e s were t h e same d i a m e t e r , but h a l f t h e d e p t h . To a t t a c h t h e bags t o t h e frame, t h e t o p edge of e a c h bag was r o l l e d a r o u n d c e d a r s l a t s w h i c h were n a i l e d t o t h e f l o a t i n g f r a m e . The e n t i r e s t r u c t u r e was a n c h o r e d t o t h e l a k e b o t t o m by means of p o l y p r o p y l e n e r o p e and cement b l o c k s , w i t h l a r g e s t y r o f o a m b l o c k s a t t a c h e d t o t h e r o p e near t h e frame end of t h e F i g u r e 11 -3 E n c l o s u r e d e s i g n F i g u r e I I - 4 E n c l o s u r e a c h o r a g e 17a 4m 18 r o p e s t o p r o v i d e some f l e x i b i l i t y and s t a b i l i t y t o t h e a n c h o r e d p o s i t i o n ( F i g u r e 11-4) . The l a r g e e n c l o s u r e s were a l s o i n d i v i d u a l l y w e i g h t e d down by b r i c k s . E n c l o s u r e s were f i l l e d w i t h water u s i n g a p o r t a b l e g a s -powered pump. The water was f i l t e r e d t h r o u g h 54 urn mesh i n o r d e r t o remove a l l z o o p l a n k t o n but a l l o w p a s s a g e of p h y t o p l a n k t o n . COUNTING ZOOPLANKTON Lake z o o p l a n k t o n samples r e p r e s e n t e d c o n s i d e r a b l e volumes of water and c o n t a i n e d l a r g e numbers of o r g a n i s m s . In most c a s e s , s a m p l e s were s p l i t i n a p l a n k t o n s p l i t t e r and s u b s a m p l e s were, a n a l y s e d . E a c h subsample c o n t a i n e d a p p r o x i m a t e l y 200 o r g a n i s m s o f a t l e a s t one c l a d o c e r a n c a t e g o r y , and 40 i n d i v i d u a l s of a t l e a s t one c o p e p o d c a t e g o r y . E n c l o s u r e z o o p l a n k t o n samples were not s p l i t b e f o r e a n a l y s i s . C o u n t s and measurements were done a t 25X m a g n i f i c a t i o n u s i n g a W i l d M odel M-5 d i s s e c t i n g m i c r o s c o p e , e q u i p p e d w i t h an o c u l a r m i c r o m e t e r . C l a d o c e r a n s were i d e n t i f i e d and c o u n t e d i n t h e c a t e g o r i e s of a d u l t s , g r a v i d f e m a l e s , and j u v e n i l e s f o r t h e s p e c i e s H o l o p e d i u m g i b b e r u m , D a p h n i a r o s e a , Diaphanosoma b r a c h y u r u m , and Polyphemus p e d i c u l u s . M a t u r e H o l o p e d i u m and D a p h n i a males were a l s o c o u n t e d . A l l o t h e r c l a d o c e r a n s were i d e n t i f i e d and c o u n t e d as g r a v i d f e m a l e s and o t h e r s . Ten i n d i v i d u a l s i n e a c h c a t o g o r y of D a p h n i a and H o l o p e d i urn were measured f o r body l e n g t h ( e x c l u d i n g s p i n e s ) , and c l u t c h s i z e s of g r a v i d f e m a l e s were a l s o r e c o r d e d . 1 9 Copepod n a u p l i i were not i d e n t i f i e d t o s p e c i e s , but a l l were c o u n t e d i n one c a t e g o r y . Immature c a l a n o i d c o p e p o d s were a l s o n ot i d e n t i f i e d t o s p e c i e s but were c o u n t e d as s m a l l (<1.2 mm) and l a r g e (>1.2 mm) c o p e p o d i t e s . M a t u r e c a l a n o i d c o p e p o d s were i d e n t i f i e d t o s p e c i e s and were c a t e g o r i z e d as m a l e s , non-g r a v i d f e m a l e s , and g r a v i d f e m a l e s . F r e e eggs of e a c h s p e c i e s were a l s o c o u n t e d . F i v e i n d i v i d u a l s of t h e s p e c i e s D i aptomus  o r e g o n e n s i s were measured f o r t o t a l body l e n g t h as were f i v e i n d i v i d u a l s i n e a c h c a t e g o r y o f Diaptomus k e n a i . C l u t c h s i z e s of g r a v i d D i aptomus k e n a i f e m a l e s were a l s o r e c o r d e d . C y c l o p o i d c o p e p o d s , o t h e r than T r o p o c y c l o p s p r a s i n u s , were n o t i d e n t i f i e d by s p e c i e s , sex, or d e v e l o p m e n t a l s t a g e s o t h e r t h a n g r a v i d f e m a l e s and o t h e r s . I n d i v i d u a l s . o f T r o p o c y c l o p s p r a s i n u s were c a t e g o r i z e d a s g r a v i d f e m a l e s and o t h e r s . F r e e c y c l o p o i d eggs were not i d e n t i f i e d t o s p e c i e s . C h i r o n o m i d l a r v a e were a l l c o u n t e d as one c a t e g o r y . C h a o b o r u s f l a v i c a n s l a r v a e were c o u n t e d a c c o r d i n g t o d e v e l o p m e n t a l s t a g e . COUNTING PHYTOPLANKTON S i x t y ml of e a c h sample were p o u r e d i n t o a 100 ml g r a d u a t e d c y l i n d e r and a l l o w e d t o s t a n d f o r 72 h o u r s t o a l l o w s u s p e n d e d m a t e r i a l s t o s e t t l e . The t o p 40 ml were t h e n s i p h o n e d o f f and t h e b o t t o m 20 ml, w h i c h c o n t a i n e d t h e s e t t l e d c e l l s , were t r a n s f e r r e d t o a s e t t l i n g chamber (a p l e x i g l a s s c y l i n d e r 2cm i n d i a m e t e r , 25 ml i n volume, w i t h a m i c r o s c o p e c o v e r s l i p bottom) 20 and a l l o w e d t o s e t t l e 24 h o u r s . These t i m e s were worked out by t e c h n i c i a n T h e r e s a S k i n n a r l a n d . An i n v e r t e d W i l d Model #M-40 m i c r o s c o p e (400X m a g n i f i c a t i o n ) was us e d t o do v i s u a l c o u n t s of t h e p l a n k t o n c e l l s . C e l l s were c o u n t e d i n e i g h t c a t e g o r i e s : u n i c e l l u l a r c e l l s i n t h e s i z e c a t e g o r i e s <2 urn, 2 -6 M m , 6 - 1 0 M m , 10 - 14 M m , 14 -20 M m , and > 20 ixm i n t h e i r g r e a t e s t d i m e n s i o n , as w e l l as c o l o n i e s and f i l a m e n t s . The two most abundant t a x o n o m i c a l g r o u p s i n e a c h c a t e g o r y were a l s o r e c o r d e d . The number of m i c r o s c o p e f i e l d s c o u n t e d v a r i e d f o r e a c h s i z e c a t e g o r y and e a c h sample. F o r t h e s m a l l e s t s i z e c a t e g o r y , <2 M m d i a m e t e r , number of c e l l s p e r f i e l d were v i s u a l l y e s t i m a t e d f o r 2-4 f i e l d s . F o r t h e 2 - 6 M m c a t e g o r y , s u f f i c i e n t f i e l d s were c o u n t e d , c h o s e n a t random, t o p r o d u c e a minimum c o u n t of a p p r o x i m a t e l y 200. F o r a l l o t h e r c a t e g o r i e s , t h e number of f i e l d s c o u n t e d i n c l u d e d 100 c e l l s i n e a c h c a t e g o r y , or a maximum of 30 - 50 f i e l d s . To o b t a i n a r e a s o n a b l e d e g r e e of q u a n t i t a t i v e a c c u r a c y , 30 f i e l d s or 100 c e l l s was t h e minimum sample s i z e f o r m i c r o s c o p i c c o u n t s ( A p p e n d i x I I I ) . The upper l i m i t was a l o g i s t i c a l c o n s i d e r a t i o n . Biomass v a l u e s f o r a l l c a t e g o r i e s were o b t a i n e d from Marmorek ( 1 9 8 3 ) . DATA ANALYSIS Most of t h e s t a t i s t i c a l d a t a a n a l y s i s i n v o l v e d a n a l y s i s of v a r i a n c e and Duncan's M u l t i p l e Range t e s t s , u s i n g t h e computer p a c k a g e U B C - G e n l i n . P l a n k t o n d a t a were t r a n s f o r m e d u s i n g n a t u r a l l o g a r i t h m s . P r o b a b i l i t y o f e r r o r l e s s t h a n or e q u a l t o 5% was t h e a c c e p t e d l e v e l of s i g n i f i c a n c e . P r o b a b i l i t y of 21 h o m o g e n e i t y of v a r i a n c e g r e a t e r t h a n 1% was c o n s i d e r e d a c c e p t a b l e . R e g r e s s i o n a n a l y s i s and l e a s t s q u a r e s f i t l i n e c a l c u l a t i o n s were done u s i n g p r o g r a m s I d e v e l o p e d from L a r k i n (1976) and S o k a l and R o h l f (1981) . S e v e r a l n o n - p a r a m e t r i c t e s t s were a l s o u s e d , i n c l u d i n g t h e K r u s k a l - W a l l i s T e s t , Spearman's Rank C o r r e l a t i o n T e s t and t h e W i l c o x o n - M a t c h e d P a i r s - S i g n e d Rank T e s t ( L a r k i n 1976). 22 CHAPTER I I I - SELECTIVE FEEDING INTRODUCTION Some models of a q u a t i c g r a z i n g have assumed t h a t a l l p h y t o p l a n k t o n p r e s e n t a r e g r a z e d , i n e q u a l p r e f e r e n c e , by 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 community; a p a r a l l e l a s s u m p t i o n r e g a r d i n g t h e t e r r e s t r i a l w o r l d would not be c o n s i d e r e d . In t h e l a k e e x p e r i m e n t s ( C h a p t e r V ) , I o b s e r v e d t h a t d i f f e r e n t g r o u p s w i t h i n t h e p h y t o p l a n k t o n community r e s p o n d e d d i f f e r e n t l y t o c h a n g e s i n z o o p l a n k t o n d e n s i t y . One p o s s i b l e r e a s o n f o r t h i s p a t t e r n was t h a t not a l l p h y t o p l a n k t o n a r e g r a z e d by z o o p l a n k t o n . In o r d e r t o d e a l more a d e q u a t e l y w i t h i n t e r p r e t a t i o n of r e s u l t s f r o m t h e community e x p e r i m e n t s , I d e c i d e d t o d e t e r m i n e r o u g h l y which p h y t o p l a n k t o n g r o u p s were g r a z e d by t h e z o o p l a n k t o n . T h i s was done b o t h t h r o u g h r e v i e w i n g t h e l i t e r a t u r e on s e l e c t i v e f e e d i n g by t h e z o o p l a n k t o n s p e c i e s of P l a c i d L a k e , and t h r o u g h a s e r i e s of f i e l d e x p e r i m e n t s . Many s t u d i e s done d u r i n g t h e p a s t 20 y e a r s have been d e v o t e d t o t h e p r o b l e m o f whether c r u s t a c e a n z o o p l a n k t o n f e e d s e l e c t i v e l y on d i f f e r e n t p h y t o p l a n k t o n c e l l t y p e s and s i z e s . Most f r e s h w a t e r h e r b i v o r o u s z o o p l a n k t o n a r e p r i m a r i l y f i l t e r -f e e d e r s , and a c e r t a i n amount of s e l e c t i o n o c c u r s p a s s i v e l y t h r o u g h t h e f i l t e r i n g p r o c e s s . C o n t r o v e r s y e x i s t s whether a c t i v e s e l e c t i o n o c c u r s t h r o u g h f i l t e r f e e d i n g . The p r o c e s s o f f i l t e r - f e e d i n g d i f f e r s s i g n i f i c a n t l y between t h e C l a d o c e r a and t h e Copepoda, as w e l l a s between d i f f e r e n t s p e c i e s w i t h i n e a c h g r o u p . In g e n e r a l among c l a d o c e r a n s , t h e 23 t h o r a c i c l e g s c r e a t e p o s t e r i o r l y - m o v i n g water c u r r e n t s w h i c h c a r r y f o o d p a r t i c l e s t h r o u g h t h e v a l v e s . The e n d i t e s of t h e s e l e g s have s p e c i a l s e t u l e - b e a r i n g s e t a e w h i c h form a s i e v e t h r o u g h w h i c h t h e water i s f i l t e r e d . Food p a r t i c l e s c a u g h t i n t h i s f i l t e r a r e t h e n c o l l e c t e d i n t h e m i d - v e n t r a l f o o d g r o o v e , l o c a t e d a l o n g t h e b a s e s of t h e t h o r a c i c l e g s , and a r e p a s s e d f o r w a r d t o t h e m a n d i b l e s and t h e mouth (Edmondson 1959, J o r g e n s e n 1966). T h e o r e t i c a l l y , p a r t i c l e s s m a l l e r t h a n t h e s p a c e s between t h e s e t u l e s w i l l p a s s t h r o u g h t h e s i e v e . However, many p a r t i c l e s v e r y much s m a l l e r t h a n t h e p o r e s i z e of f i l t e r s a r e c a u g h t due t o c l o g g e d f i l t e r s , d i r e c t i n t e r c e p t i o n o f p a r t i c l e s by t h e s e t u l e s , e l e c t r o s t a t i c a t t r a c t i o n s , or a number of o t h e r p h y s i c a l mechanisms ( S h e l d o n and S u t c l i f f e 1969, R u b e n s t e i n and K o h l 1977, Malone e t a l . 1979). P a r t i c l e s t o o l a r g e t o f i t between t h e v a l v e s can a l s o not be i n g e s t e d , c r e a t i n g a p a s s i v e " f o o d s i z e r a n g e " f o r e a c h o r g a n i s m . The r e s u l t s of s e v e r a l s e l e c t i v e f e e d i n g s t u d i e s have s u g g e s t e d t h a t t h e maximum s i z e of p a r t i c l e s i n g e s t e d by c l a d o c e r a n s i s r e l a t e d t o body s i z e (maximum c a r a p a c e gape) of t h e g r a z e r , w h i l e t h e minimum s i z e i s more or l e s s i n d e p e n d e n t of body s i z e ( B r o o k s and Dodson 1965, B u r n s 1968a, Lampert 1974, N a d i n - H u r l e y and Duncan 1976, L y n c h 1978). B e c a u s e of t h i s , l a r g e r f i l t e r f e e d e r s t e n d t o have l a r g e r f o o d s i z e r a n g e s ( G l i w i c z 1980). A l t h o u g h t h e r e i s l i t t l e e v i d e n c e of d e l i b e r a t e s e l e c t i v e f e e d i n g by most c l a d o c e r a n s , t h e upper s i z e l i m i t o f p a r t i c l e s i n g e s t e d may be d e p e n d e n t on 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 and 24 c o n c e n t r a t i o n . At h i g h c o n c e n t r a t i o n s of l a r g e c e l l s , f i l a m e n t s and c o l o n i e s , some c l a d o c e r a n s may a d j u s t t h e i r c a r a p a c e gape t o d e c r e a s e t h e maximum s i z e of p a r t i c l e s i n g e s t e d and a v o i d i n t e r f e r e n c e of f i l t e r i n g by t h e l a r g e r c e l l s and c o l o n i e s . F i l t e r i n g of t h e s e l a r g e r p a r t i c l e s may make t i g h t c l o s u r e of the f i l t e r i n g a p p a r a t u s d i f f i c u l t , r e s u l t i n g i n t h e l o s s of s m a l l p a r t i c l e s when t h e water i s f o r c e d t h r o u g h t h e f i l t e r s ( G l i w i c z 1977, G l i w i c z and S i e d l a r 1980). At h i g h c o n c e n t r a t i o n s of f i l a m e n t o u s b l u e - g r e e n a l g a e , l a r g e c l a d o c e r a n s i n c r e a s e t h e i r r e j e c t i o n r a t e s and t h i s i n t e r u p t s t h e r h y t h m i c pumping of t h e t h o r a c i c l i m b s , r e s u l t i n g i n d e c r e a s e d f i l t e r i n g r a t e s ( Burns 1968b, Webster and P e t e r s 1978, P o r t e r and McDonough 1984). The minimum s i z e of p a r t i c l e s may a l s o be a d j u s t e d . Some c h y d o r i d s a r e a b l e t o a d j u s t t h e gap between t h e l a s t p a i r of t h o r a c i c l i m b s w hich s e a l t h e f i l t e r chamber p o s t e r i o r l y . By i n c r e a s i n g t h e gap t h e y a l l o w , t h e e s c a p e o f v e r y s m a l l p a r t i c l e s but a l s o i n c r e a s e t h e i r l i k e l i h o o d of c a p t u r i n g l a r g e r p a r t i c l e s by t h e r e s u l t i n g i n c r e a s e d w ater f l o w t h r o u g h t h e f i l t e r i n g a p p a r a t u s ( F r y e r 1968, DeMott 1980). In a d d i t i o n t o t h i s , t h e f i l t e r i n g a p pendages of c h y d o r i d s a r e m o d i f i e d f o r g r a s p i n g l a r g e p a r t i c l e s a s w e l l as f i l t e r i n g s m a l l p a r t i c l e s ( F r y e r 1968). Demott and K e r f o o t (1982) have o b s e r v e d s i m i l a r m o d i f i c a t i o n s i n Bosmina. T h i s g r a s p i n g f u n c t i o n makes d e l i b e r a t e s e l e c t i o n p o s s i b l e . The c l a s s i c a l c o n c e p t i s t h a t t h e r e a r e two methods of f i l t e r f e e d i n g by c o p e p o d s . Among some c a l a n o i d c o p e p o d s , 25 i n c l u d i n g t h e m a r i n e g e n e r a C a l a n u s and P s e u d o c a l a n u s and t h e f r e s h w a t e r genus D i a p t o m u s , water c u r r e n t s moving p o s t e r i o r l y a l o n g t h e v e n t r a l m i d l i n e , p a s t t h e m a x i l l a e and t h e n o ut l a t e r a l l y a r e c r e a t e d by v i b r a t i o n s of t h e m a x i l l a e and t h e m a n d i b u l a r p a l p s . On t h e s e c o n d m a x i l l a e a r e l o n g s e t u l e -b e a r i n g s e t a e , w h i c h form t h e s i e v e t h r o u g h w h i c h t h e water i s f i l t e r e d . P a r t i c l e s c a u g h t by t h e s i e v e a r e s c r a p e d o f f by t h e e n d i t e s of t h e f i r s t m a x i l l a e and s p e c i a l i z e d s e t a e on t h e m a x i l l i p e d e s , and a r e p a s s e d on t o t h e m a n d i b l e s and mouth. In o t h e r c a l a n o i d s , s u c h as t h e m a r i n e genus A c a r t i a , and t h e c y c l o p o i d c o p e p o d s , t h e m a x i l l a e c a n be e x t e n d e d t o be used as sw e e p i n g n e t s . R e c e n t r e s e a r c h ( A l c a r e z e t a l . 1980) has c a s t some doubt on whether, t h e c l a s s i c a l c o n c e p t of f i l t e r - f e e d i n g i n c o p e p o d s i s c o r r e c t . In a d d i t i o n t o f i l t e r f e e d i n g , c a l a n o i d c o p e p o d s may a l s o f e e d r a p t o r i a l l y . P a r t i c l e s t o o l a r g e t o be h a n d l e d by t h e f i l t e r i n g mechanism may s t i l l be c a u g h t f o r f o o d by d i r e c t s e i z i n g . The uses of r a p t o r i a l and f i l t e r f e e d i n g modes do not o c c u r s i m u l t a n e o u s l y , but t h e o r g a n i s m s a r e a b l e t o s w i t c h back and f o r t h e a s i l y ( P o u l e t 1974). The c h o i c e of mode depends on t h e n a t u r e , c o n c e n t r a t i o n , and s i z e d i s t r i b u t i o n of f o o d p a r t i c l e s ( J o r g e n s e n 1975). S i n c e most c o p e p o d s t e n d t o combine b o t h r a p t o r i a l and f i l t e r f e e d i n g , t h e s i e v e s u t i l i z e d by t h e f i l t e r i n g p r o c e s s t e n d t o be c o a r s e r and l e s s e f f i c i e n t a t f i l t e r i n g o u t s m a l l p a r t i c l e s t h a n t h o s e of t h e c l a d o c e r a n s ( J o r g e n s e n 1966, 1975). D i s t a n c e s between s e t u l e s m o s t l y v a r y between 5 t o 10 nm ( G a u l d 1966, J o r g e n s e n 1975). Thus w h i l e 26 D a p h n i a and s m a l l e r c l a d o c e r a n s a r e a b l e t o f e e d e q u a l l y on b a c t e r i a and s m a l l a l g a l c e l l s (Bogdan and McNaught 1975, DeMott 1982) c a l a n o i d c o p e p o d s a p p e a r t o have poor f i l t e r i n g e f f i c i e n c i e s f o r c e l l s s m a l l e r t h a n 5 jum i n d i a m e t e r ( H a r g a v e and Geen 1970, McQueen 1970). T h e r e i s c o n s i d e r a b l e c o n t r o v e r s y whether f e e d i n g p r e f e r e n c e i n c o p e p o d s w i t h r e s p e c t t o p a r t i c l e s i z e i s b e h a v i o u r a l l y d e t e r m i n e d o r a f u n c t i o n of s i e v i n g e f f i c i e n c i e s . P a r t o f t h e p r o b l e m i s t h e d i f f i c u l t y i n d i s t i n g u i s h i n g between r a p t o r i a l and f i l t e r f e e d i n g modes, g i v e n t h a t r a p i d s w i t c h i n g of modes may o c c u r , i n s t u d i e s where d a t a a r e c o l l e c t e d by a l l o w i n g c o p e p o d s t o f e e d on an a s s e m b l a g e of m i x e d s i z e p a r t i c l e s and m o n i t o r i n g c h a n g e s i n t h e s i z e d i s t r i b u t i o n as a r e s u l t of f e e d i n g . Many a u t h o r s seem t o have assumed t h a t r a p t o r i a l f e e d i n g i s n e g l i g i b l e and have t r i e d t o e x p l a i n f o o d s i z e s e l e c t i o n a l m o s t s o l e l y on t h e b a s i s o f f i l t e r f e e d i n g . W i l s o n ( 1 9 7 3 ) , w o r k i n g w i t h A c a r t i a t o n s a , f o u n d t h a t t h e c o p e p o d s seemed t o a s s e s s 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 and a d j u s t t h e i r f i l t e r i n g or r a p t o r i a l p o s t u r e s t o c a t c h most e f f i c i e n t l y c e l l s s l i g h t l y l a r g e r t h a n t h e b i o m a s s peak. S i m i l a r " b e h a v i o u r s " have been o b s e r v e d i n P s e u d o c a l a n u s m i n u t u s ( P o u l e t 1973,1974) and t h r e e D i aptomus s p e c i e s , i n c l u d i n g D. o r e g o n e n s i s (Richman e t a l . 1980). However, t h e l a c k of m e c h a n o r e c e p t o r s o r m u s c l e s i n t h e s e t a e or s e t u l e s makes a d j u s t m e n t of t h e p o r e s i z e o f t h e f i l t e r s i m p o s s i b l e ( F r i e d m a n and S t r i c k l e r 1975). Many a u t h o r s ( P a r s o n e t a l . 1967, Boyd 1976, Lehman 1976, N i v a l and N i v a l 1976, F r o s t 1977, B a r l o w and 27 M o n t e i r o 1979) have a r g u e d t h a t , r e g a r d l e s s of any r a p t o r i a l f e e d i n g w h i c h may o c c u r , t h i s o b s e r v e d s e l e c t i v i t y can be a c c o u n t e d f o r by 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 , f i l t e r i n g r a t e s , and, most i m p o r t a n t l y t h e p r o b a b i l i t y o r e f f i c i e n c y of c a t c h i n g a p a r t i c u l a r s i z e d c e l l . Boyd (1976) has r e f e r r e d t o the f i l t e r i n g a p p a r a t u s of c o p e p o d s as " l e a k y s i e v e s " . L eakage from t h e s i e v e , p a r t i c u l a r l y of s m a l l p a r t i c l e s , o c c u r s as a r e s u l t of d i s t e n s i o n of t h e s e t u l e mesh as p a r t i c l e s i m p i n g e on i t . The l a r g e r a p a r t i c l e i s , p r o v i d e d i t i s s m a l l enough t o be h a n d l e d by t h e o r g a n i s m , t h e l e s s l i k e l y i t w i l l be l o s t t h r o u g h " l e a k a g e " , hence t h e g r e a t e r t h e e f f i c e n c y of c a p t u r e . In g e n e r a l , s m a l l e r o r g a n i s m s , e s p e c i a l l y w i t h i n a g i v e n s p e c i e s , have s m a l l mesh s i z e s i n t h e i r f i l t e r i n g a p p a r a t u s and a r e t h e r e f o r e a b l e t o f i l t e r s m a l l p a r t i c l e s a t g r e a t e r e f f i c i e n c i e s t h a n l a r g e r o r g a n i s m s ( N i v a l and N i v a l 1976). A c t i v e s e l e c t i v e f i l t e r f e e d i n g by c o p e p o d s has not been r u l e d o ut e n t i r e l y , but may o c c u r on t h e b a s i s of c h e m i c a l c o m p o s i t o n or " t a s t e " r a t h e r t h a n s i z e . T h i s i s s u g g e s t e d by th e p r e s e n c e of c h e m o r e c e p t o r s on t h e m o u t h p a r t s of some c o p e p o d s ( F r i e d m a n and S t r i c k l e r 1975, F r i e d m a n 1980) and t h e o b s e r v a t i o n s of some c o p e p o d s a c t i v e l y r e j e c t i n g c e r t a i n p a r t i c l e s o n l y a f t e r b r i n g i n g them t o t h e mouth (Co n o v e r 1966). Z o o p l a n k t o n may a v o i d f e e d i n g on some c e l l t y p e s b e c a u s e t h e c e l l s r e l e a s e t o x i n s or f e e d i n g i n h i b i t i n g s u b s t a n c e s ( R y t h e r 1954, A r n o l d 1971, P o r t e r and O r c u t t 1980, Lampert 1981). O t h e r p h y t o p l a n k t o n c e l l s may be i n g e s t e d but not d i g e s t e d ( P o r t e r 1973,1975,1976,1977, D e l n f a n t e 1978); some 28 t y p e s may a c t u a l l y b e n e f i t from gut p a s s a g e by a b s o r b i n g d i g e s t e d n u t r i e n t s from t h e a n i m a l s ' g u t s ( P o r t e r 1976). The l i t e r a t u r e i s f u l l of r e p o r t s of s t u d i e s of s i z e s e l e c t i v e f e e d i n g 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 . I r e s t r i c t e d my r e v i e w t o t h o s e s p e c i e s and c l o s e l y r e l a t e d ones w h i c h a r 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 . D u r i n g t h e summer months t h e s e a r e 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 , Diaptomus  o r e g o n e n s i s , Diaphanosoma b r a c h y u r u m , 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 g i r o s t r i s , C h y d o r i s s p h a e r i c u s , and T r o p o c y c l o p s  p r a s i n u s . Diaptomus k e n a i a r e p r e s e n t i n t h e s p r i n g and f a l l b ut go i n t o d i a p a u s e d u r i n g t h e summer months. Diaphanosoma i s s c a r c e t h r o u g h o u t most of t h e y e a r , e x c e p t d u r i n g l a t e summer, when i t i s one of t h e major components of t h e z o o p l a n k t o n community. T a b l e 111 — 1 summarizes my f i n d i n g s . I was u n a b l e t o f i n d any i n f o r m a t i o n on s e l e c t i v e f e e d i n g by H o l o p e d i u m g i b b e r u m or D i a ptomus k e n a i . The d a t a a v a i l a b l e from C e r i o d a p h n i a , Diaphanasoma, C h y d o r i s , and T r o p o c y c l o p s a r e not v e r y s p e c i f i c . C o m p a r i s o n between my s t u d y r e s u l t s and t h o s e r e p o r t e d i n t h e l i t e r a t u r e i s sometimes d i f f i c u l t , s i n c e I a n a l y z e d my p h y t o p l a n k t o n samples m a n u a l l y w h i l e most s t u d i e s i n t h e l i t e r a t u r e u s e d e l e c t r o n i c p a r t i c l e c o u n t e r s . H a r b i s o n and M c A l i s t e r (1980) have d e m o n s t r a t e d t h a t s i e v e s , whether i n a n i m a t e o r f i I t e r - f e e d i n g a p p e n d a g e s , s o r t p a r t i c l e s on t h e b a s i s of l i n e a r d i m e n s i o n s ( n o t volumes) so t h a t t h e maximum l i n e a r d i m e n s i o n i s t h e l i m i t i n g f a c t o r . T h i s p r o p e r t y l i m i t s t h e u s e f u l n e s s of e l e c t r o n i c c o u n t i n g i n s t r u m e n t s i n s e l e c t i v e f e e d i n g s t u d i e s , s i n c e t h e s e i n s t r u m e n t s measure c e l l s on t h e TABLE I I I - l Zooplankton food s i z e ranges zooplankton species Daphnia rosea ingested preferred D. rosea D. pulex D. pulex D. galeata Daphnia sp.' Bosmina  long i s r o s t r i s B_. l o n g i r o s t r i s B_. long i r o s t r i s Ceriodaphnia  r e t i c u l a t a Diaphanosoma sp. Chydoris sp. Diaptomus  minutus D. oregonensis D. oregonensis lum) 0.2 6.6 < 31 1.5 28, 2-31 < 22 > 22 > 50 2.5-45 < 14 0.2 6.6 2-5 5-12 < 50 < 50 > 22 < 22 2.5 6 -(urn) rejected source (um) 12-31 30 12 > 50 > 50 < 5 DeMott 198 2 Burns 1968 Laropert 1974 Lynch 1978 Bogdan and McNaught 1975 Gliwicz 1980 Bogdan and G i l b e r t 1982 Burns 1968 DeMott 1982 Lynch 1978 Gliwicz 1980 Gliwicz 1980 Bogdan and McNaught 1975 Richman et a l . 1980 McQueen 1970 30 b a s i s of volume r a t h e r t h a n l i n e a r d i m e n s i o n . Some a u t h o r s have a t t e m p t e d t o improve on volume measurements by c o n v e r t i n g them i n t o " s p h e r i c a l d i a m e t e r e q u i v a l e n t s " , but s i n c e few l i v i n g c e l l s a r e p e r f e c t or even a p p r o x i m a t e s p h e r e s , t h i s c o n v e r s i o n i s m e r e l y c o s m e t i c . In T a b l e 111 — 1 I have c o n v e r t e d some of t h e volume s i z e s t o s p h e r i c a l d i a m e t e r e q u i v a l e n t s i n o r d e r t o s t a n d a r d i z e t h e s i z e u n i t s u s e d and t o f a c i l i t a t e c o m p a r i s o n between s t u d i e s . In g e n e r a l , i t a p p e a r s t h a t a l l but t h e v e r y l a r g e s t s i z e s o f u n i c e l l u l a r a l g a e and b a c t e r i a a r e a v a i l a b l e a s f o o d f o r some members of 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 community. G e l a t i n o u s c o l o n i a l g r e e n and b l u e - g r e e n and f i l a m e n t o u s b l u e - g r e e n a l g a e a p p e a r g e n e r a l l y t o be a v o i d e d a s f o o d . METHODS AND MATERIALS S e l e c t i v e f e e d i n g e x p e r i m e n t s were d e s i g n e d s i m i l a r l y t o t h o s e of G l i w i c z ( 1 9 7 5 ) , i n w h i c h v a r i o u s d e n s i t i e s of D a p h n i a  p u l e x were h e l d f o r f i v e d a y s i n 100 L e n c l o s u r e s c o n t a i n i n g pond w a t e r . He sampled f o r p h y t o p l a n k t o n a t t h e o n s e t of t h e e x p o s u r e and a g a i n a t t h e end, and compared c e l l s i z e c o m p o s i t i o n of t h e p h y t o p l a n k t o n b i o m a s s between s a m p l e s . I n s t e a d of v a r y i n g t h e numbers of z o o p l a n t o n , I v a r i e d t h e z o o p l a n k t o n s p e c i e s and compared t h e c h a n g e s i n p h y t o p l a n k t o n c o m p o s i t i o n t o a c o n t r o l c o n t a i n i n g no z o o p l a n k t o n . Z o o p l a n k t o n g u t a n a l y s i s was not done b e c a u s e o n l y t h o s e p h y t o p l a n k t o n g r o u p s t h a t a r e not q u i c k l y d i g e s t e d can be i d e n t i f i e d f r o m gut s a m p l e s . S i n c e I was p a r t i c u l a r l y i n t e r e s t e d t o see w h i c h of 31 t h e n a t u r a l l y o c c u r r i n g p h y t o p l a n k t o n s i z e g r o u p s were i n g e s t e d , methods i n v o l v i n g a r t i f i c i a l p a r t i c l e s s u c h as p l a s t i c beads or r a d i o a c t i v e l y l a b e l l e d c u l t u r e d p h y t o p l a n k t o n were not u s e d . The major d i s a d v a n t a g e of t h e method us e d i s t h a t z o o p l a n k t o n f e e d i n g a c t i v i t i e s c an c a u s e c o n s i d e r a b l e f r a g m e n t a t i o n of c o l o n i e s , and o b s c u r e t h e e f f e c t s of f o o d c o n s u m p t i o n a l o n e . T h i s has been o b s e r v e d t o be t r u e of D iaptomus o r e g o n e n s i s (Richman e t a l . 1980) and t h e m a r i n e c o p e p o d A c a r t i a c l a u s i ( B a r l o w and M o n t e i r o 1979). An a r r a y of 10 e n c l o s u r e s , each h o l d i n g 150 L, was u s e d f o r t h e s e e x p e r i m e n t s . The e n c l o s u r e s were f i l l e d w i t h l a k e water f i l t e r e d t h r o u g h 54 Mm Nytex mesh t o remove a l l m a c r o z o o p l a n k t o n . Water samples f o r p h y t o p l a n k t o n were t a k e n and t h e n 1000 t o 2000 i n d i v i d u a l s of s e l e c t e d z o o p l a n k t o n s p e c i e s were added t o e a c h e n c l o s u r e . Maximum l a k e d e n s i t i e s i n 1981 f o r H o l o p e d i u m , D a p h n i a and D i a p t o m u s o r e g o n e n s i s were a p p r o x i m a t e l y 10, 5, and 2 a n i m a l s p e r l i t r e , r e s p e c t i v e l y , so t h e s e d e n s i t i e s ( T a b l e 111-2) were between .5 and 8.5 t i m e s maximum l a k e d e n s i t i e s . P h y t o p l a n k t o n s a m p l e s were t h e n t a k e n a t 4 t o 6 h o u r i n t e r v a l s f o r 48 t o 72 h o u r s . S h o r t e x p e r i m e n t d u r a t i o n t i m e s and h i g h d e n s i t i e s of z o o p l a n k t o n were us e d t o r e d u c e t h e c o n f o u n d i n g f a c t o r of a l g a l g r o w t h . P r e s u m a b l y , a l g a l g r o w t h o v e r 72 h o u r s o r l e s s s h o u l d be o n l y moderate and t h e g r a z i n g p r e s s u r e from t h e h i g h d e n s i t y of g r a z e r s u s e d s h o u l d be s u f f i c i e n t t o d e m o n s t r a t e s i g n i f i c a n t t r e n d s i n g r a z e r p r e f e r e n c e . The e x p e r i m e n t s were done a t f o u r d i f f e r e n t t i m e s , J u n e , J u l y , A u g u s t , and September, 1981. TABLE III-2 Zooplankton Biomass in Enclosures zooplankton species animal density animal density at s t a r t at end (#/l) (ug/1) (#/l) (ug/1) ( r e p l i c a t e enclosures) June 1981: 1 2 1 2 Holopedium gibberum 5.0 18. .1 3. 6 1. ,7 13.0 6.2 Daphnia rosea 5.0 26, .5 15. 6 11. ,7 82.7 62.0 Diaptomus kenai 5.0 131, .2 3. 8 3. .9 99.7 102.3 Diaptomus oreqonensis 5.0 12, .7 8. 2 7. .2 20.8 18.3 J u l y 1981: Holopedium gibberum 13.5 48. ,9 23. 1 17. 7 83.6 64.1 Daphnia rosea 13.5 90, .5 30. 0 13. 0 201.1 87.1 Diaptomus kenai 10.1 265. .0 4. 7 17. 0* 123.3 446.1 Diaptomus oregonensis 13.5 42, .3 15. 6 13. 3 48.9 41.7 August 1081: Holopedium gibberum 11.3 40. .9 0. 1 0. 8 . 0.4 2.9 Daphnia rosea 11.3 80. .4 3. 1 3. 0 22.1 21.3 Diaptomus oreqonensis 11.3 35. ,4 3. 9 4. 3 12.2 13.5 Diaphanosoma brachyurum 9.0 15. .3 11. 1 8. 3 18.9 14.1 September 1981: Holopedium gibberum 1.7.0 61. 5 11. 3 8. 2 40.9 29.7 Daphnia rosea 17.0 121. 0 27. 6 4. 4 196.4 31.3 Diaptomus oregonensis 17.0 53. 3 11. 2 32. 3 35.1 101.2 * Most of these were dead at the time of sampling, but after preservation i n formalin, i t was d i f f i c u l t to d i s t i n g u i s h between those that had been dead and those that had been a l i v e at the time of sampling. 3 3 The z o o p l a n k t o n u s e d i n t h e s e e x p e r i m e n t s were c o l l e c t e d by v e r t i c a l h a u l s from P l a c i d L a k e , or (when a p a r t i c u l a r s p e c i e s was t e m p o r a r i l y s c a r c e i n P l a c i d L a k e ) from G w e n d o l i n e Lake, u s i n g a 0.5 m d i a m e t e r , 220 jum mesh s i z e p l a n k t o n n e t . Z o o p l a n k t o n s p e c i e s were f i r s t c r u d e l y s o r t e d u s i n g v a r i o u s m e s h - s i z e d s i e v e s t o c o n c e n t r a t e o r g a n i s m s of p a r t i c u l a r s i z e r a n g e s . From t h e c o n c e n t r a t e s , i n d i v i d u a l s of t h e t a r g e t s p e c i e s were s o r t e d m a n u a l l y w i t h p i p e t t e s . Most of t h e abun d a n t g r a z e r s p e c i e s were t e s t e d e x c e p t f o r C e r i o d a p h n i a and Bosmina, w h i c h were t o o d i f f i c u l t t o i d e n t i f y and s e p a r a t e i n l a r g e numbers. I n i t i a l p h y t o p l a n k t o n samples were t a k e n from t h e e n c l o s u r e s b e f o r e a n i m a l s were a d d e d . Z o o p l a n k t o n s p e c i e s were a s s i g n e d t o e n c l o s u r e by l o t t e r y . T h e r e were two r e p l i c a t e s f o r e a c h t r e a t m e n t . At t h e end of t h e e x p e r i m e n t s , t h e e n c l o s u r e s were s a m p l e d f o r z o o p l a n k t o n t o d e t e r m i n e t h e z o o p l a n k t o n s u r v i v a l r a t e and t h e e n c l o s u r e s were e m p t i e d u s i n g a c a l i b r a t e d pump t o d e t e r m i n e t h e i r a c t u a l volume. Sample a n a l y s i s . was done a c c o r d i n g t o t h e p r o c e d u r e s d e s c r i b e d i n C h a p t e r I I . Two methods of s t a t i s i t i c a l a n a l y s i s were u s e d . L i n e a r r e g r e s s i o n s of p h y t o p l a n k t o n v e r s u s t i m e were c a l c u l a t e d f o r e a c h t r e a t m e n t f o r o b s e r v a t i o n s from t h e f i r s t 24 h o u r s o f t h e e x p e r i m e n t s , s i n c e c h a n g e s i n p h y t o p l a n k t o n d e n s i t y a p p e a r e d l i n e a r d u r i n g t h e f i r s t 24 h o u r s . R e g r e s s i o n f o r e x p e r i m e n t a l t r e a t m e n t s were compared t o t h o s e f o r t h e c o n t r o l s t o t e s t f o r s i g n i f i c a n t d i f f e r e n c e s . Two-way a n a l y s i s of v a r i a n c e , w i t h t i m e and t r e a t m e n t a s v a r i a b l e s , was us e d t o 34 analyze the data for the e n t i r e observation period. For each s i z e category of phytoplankton the r e s u l t s of a given species were compared to those of the c o n t r o l (no zooplankton) over time and tested for s i g n i f i c a n c e at a p r o b a b i l i t y of error of 0.05. (Time was d i v i d e d i n t o four i n t e r v a l s , and the r e s u l t s from each of these i n t e r v a l s were lumped together for the a n a l y s i s . ) Slopes and y - i n t e r c e p t s obtained from the regression a n a l y s i s of observations from the f i r s t 24 hours were used to c a l c u l a t e grazing rates for those treatments where s i g n i f i c a n t grazing e f f e c t s were detected e i t h e r by regression a n a l y s i s or by a n a l y s i s of variance. To c a l c u l a t e a phytoplankton growth r a t e , r, i n the absence of grazers, I used the r e l a t i o n s h i p of dx/dt = rx where x i s the phytoplankton population density and t time. The s o l u t i o n of t h i s equation i s : X(t) = X(0)e(exp r t ) and r = l n ( X ( t ) / X ( 0 ) ) / t Growth rates were c a l c u l a t e d for relevant phytoplankton s i z e c l a s s e s from enclosures without zooplankton. F i l t e r i n g r a t e s for each of the s i g n i f i c a n t combinations of zooplankton species 35 and phytoplankton size classes were then calculated using the relationship dx/dt = rx - cxy where y i s the grazer density and c is the f i l t e r i n g or clearance rate. The solution to this function i s : X(t) = X(0)e(exp rt-cy) and c = (r - In (X( t ) / X ( 0 ) ) / ( y t ) (This model i s discussed further in Chapter IV.) since grazing pressure changed as zooplankton populations grew or declined during the experiment, "minimum" and "maximum" f i l t e r i n g rates for each species were calculated, based on stocked and end densities of grazers. I was not s a t i s f i e d with using a mean zooplankton density to calculate one f i l t e r i n g rate since I had no information regarding the time frame of mortalities or recruitments of the o r i g i n a l l y stocked zooplankton populations. For example, i t i s possible that many of those animals which died died as the result of handling, soon after being stocked, and may not have grazed in the enclosures at a l l . A f i l t e r i n g rate calculated on the basis of a mean zooplankton density would in that case be an overestimation. For those zooplankton species and experiments where s i g n i f i c a n t grazing effects were 36 o b s e r v e d , Spearmans's rank c o r r e l a t i o n c o e f f i c i e n t s were c a l c u l a t e d t o d e t e r m i n e whether c o r r e l a t i o n s e x i s t e d between t h e r e l a t i v e b i o m a s s e s o f the c e l l s i z e c l a s s e s and r e l a t i v e f i l t e r i n g r a t e s . Taxonomic d a t a f o r t h e s e t r e a t m e n t s were a l s o c h e c k e d f o r c h a n g e s i n dominant p h y t o p l a n k t o n s p e c i e s o v e r t i m e . RESULTS I n i t i a l p h y t o p l a n k t o n b i o m a s s and p a r t i c l e s i z e d i s t r i b u t i o n s f o r e a c h of t h e e x p e r i m e n t s a r e shown i n F i g u r e s 111 — 1 t o 111- 5. V a r i a t i o n between i n i t i a l c o n d i t i o n s f o r e a c h e x p e r i m e n t may be due t o p a t c h y p h y t o p l a n k t o n d i s t r i b u t i o n i n t h e l a k e , where water f o r t h e e n c l o s u r e s was o b t a i n e d . V a r i a t i o n i n t h e d a t a may a l s o be due t o s a m p l i n g and c o u n t i n g e r r o r s . S t o c k e d z o o p l a n k t o n f o r e a c h of t h e e x p e r i m e n t s a r e shown i n T a b l e I I I - 2 . S u b s t a n t i a l i n c r e a s e s (>40%) i n d e n s i t i e s of o r g a n i s m s o v e r s t o c k e d d e n s i t i e s d u r i n g t h e c o u r s e of t h e e x p e r i m e n t s were o b s e r v e d f o r b o t h June D a p h n i a e n c l o s u r e s , one J u l y and one September D a p h n i a e n c l o s u r e , b o t h June D i a p t o m u s  o r e g o n e n s i s e n c l o s u r e s , one September D. o r e g o n e n s i s e n c l o s u r e and b o t h J u l y H o l o p e d i u m e n c l o s u r e s . Poor s u r v i v a l r a t e s of t h e e x p e r i m e n t a l a n i m a l s were o b s e r v e d f o r H o l o p e d i u m , D a p h n i a , and D i a p t o m u s o r e g o n e n s i s d u r i n g A u g u s t when t e m p e r a t u r e s e x c e e d e d 20 C, and f o r Diaptomus k e n a i i n J u l y . Diaphanosoma, w h i c h was a l s o t e s t e d i n A u g u s t , showed good s u r v i v a l r a t e s . (See T a b l e 111-2.) The s u r f a c e and 1 m t e m p e r a t u r e s f o r P l a c i d L a k e f o r June 1 g u r e s 111 — 1 t o 111-5 Mean i n i t i a l p h y t o p l a n k t o n b i o m a s s (jug/1 f r e s h w e i g h t ) and c o m p o s i t i o n i n e n c l o s u r e s f o r e x p e r i m e n t s done i n June, J u l y , August and September, 1 981 37a % 80-1 60" 40" 20" c o l > 2 0 1 4 - 2 0 1 0 - 1 4 6 - 1 0 2 - 6 < 2 200-O) 13 100-wmm IIIII i l l ss 1 1 1 i l l miim ti m II J U N E A , ro 0) —& IV V o i I o 1 ro o o 1 ro o o s i z e c l a s s 38 80-6041 401 2 OH c o l > 20 14-20 10-14 6-1 0 2-6 <2 400i 300 200] 100 0 ro ro i JULY ? S * o - 1 W A O s ize c l a s s v ro o o o 39 % 80" 401 20 col > 20 14-20 10-14 6-10 2-6 I J<2 % 80 601 40 20 col > 20 14-20 10-14 6-10 2-6 <2 200 100^ 0 200-U) 100 0 A ro A ro AUGUST co - L I o _L I o - 1 s ize c l a s s ro i CD I ro o v ro o o o AUGUST g f f (D i aphanosoma) ro CO I CO ->• a ^ i i - 1 ro s ize c l a s s v ro o o o 40 40 20-^ c o l >20 1 4 - 2 0 1 0 - 1 4 6 - 1 0 2 - 6 < 2 2 0 0 100 S E P T E M B E R / s ro ro i OJ 1 o 1 •r> >2 CO 0) o 1 i ro o o s ize c l a s s 41 t o O c t o b e r 9, 1981 a r e g i v e n i n F i g u r e I I I - 6 . ( T e m p e r a t u r e s i n e n c l o s u r e s s h o u l d be s i m i l a r . ) At the t i m e of t h e June e x p e r i m e n t s , n e a r - s u r f a c e t e m p e r a t u r e s were a r o u n d 13 C, i n J u l y t h e y were a r o u n d 16 C, i n August a p p r o x i m a t e l y 21 C f o r b o t h s e t s of e x p e r i m e n t s , and i n September, a r o u n d 18 C. A c o m p l i c a t i n g f a c t o r f o r t h e June e x p e r i m e n t s was t h e p r e s e n c e of l a r g e numbers of s m a l l c opepod n a u p l i i ( T a b l e 111-3) i n a l l of t h e June e n c l o s u r e s e x c e p t f o r t h o s e w h i c h a l s o c o n t a i n e d D. k e n a i . A p p a r e n t l y , t h e wrong m e s h - s i z e d Nytex n e t t i n g (104 Mm i n s t e a d of 54 nm) was used i n f i l t e r i n g t h e l a k e w a ter as t h e e n c l o s u r e s were f i l l e d , a l l o w i n g n a u p l i i t o p a s s i n t o t h e e n c l o s u r e s . t h i s p r o b l e m was not d e t e c t e d u n t i l l a t e r e x a m i n a t i o n of t h e z o o p l a n k t o n s a m p l e s . However, s i n c e t h e n a u p l i i were a l s o p r e s e n t i n t h e c o n t r o l s , t h e r e s u l t s f r o m t h e o t h e r s p e c i e s a r e s t i l l c o n s i d e r e d t o be v a l i d . N a u p l i i were v e r y s c a r c e i n t h e e n c l o s u r e s w i t h D. k e n a i , and e n c l o s u r e s w i t h D. o r e g o n e n s i s c o n t a i n e d s i g n i f i c a n t l y h i g h e r numbers of n a u p l i i t h a n a l l t h e o t h e r e n c l o s u r e s . The r e s u l t s of t h e f e e d i n g e x p e r i m e n t s a r e g i v e n i n T a b l e 111-4 and A p p e n d i x IV. In g e n e r a l , t h e s i g n i f i c a n t r e s u l t s s u p p o r t l i t e r a t u r e r e p o r t s w i t h r e s p e c t t o t h e f e e d i n g s i z e r a n g e s o f t h e v a r i o u s z o o p l a n k t o n s p e c i e s t e s t e d . R e g r e s s i o n a n a l y s i s of d a t a f r o m t h e f i r s t 24 h o u r s y i e l d e d s i g n i f i c a n t g r a z i n g e f f e c t s on p h y t o p l a n k t o n of t h e 6-10 Mm s i z e c a t e g o r y by D a p h n i a i n t h e June and August e x p e r i m e n t s and o f t h e 10-14 and 14-20 Mm s i z e c a t e g o r i e s by D a p h n i a i n t h e A u g u s t g u r e 111-6 Lake t e m p e r a t u r e ( d e g r e e s C e l s i u s ) a t s u r f a c e and 1 m, June - September, 1981 TABLE III-3 N a u p l i i Contamination In Enclosures, June 1981 enclosure zooplankton species tested n a u p l i i I c o n t r o l 5.19 2 D. kenai 0.11 3 D. oreqonensis 24.64 4 D. kenai 0.06 5 Daphnia 6.42 6 D. oregonensis 44.28 7 ' Daphnia 9.60 8 Holopedium 10.08 9 c o n t r o l 11.70 10 Holopedium 11.22 TABLE III-4 Feeding of Zooplankton on various sizes of phytoplankton a) June 1981 zooplankton species <2 phytoplankton s i z e 2-6 6-10 10-14 (um) 14-20 >20 colonies Daphnia rosea NS NS ++ NS + + NS Holopedium gibberum NS NS + + + + NS Diaptomus oregonensis NS NS NS + + NS NS Diaptomus kenai + NS + + + NS b) J u l y 1981 zooplankton species <2 phytoplankton s i z e 2-6 6-10 10-14 (um) 14-20 >20 colonies Daphnia rosea NS NS NS NS NS NS + Holopedium gibberum NS NS NS NS NS NS NS Diaptomus oregonensis NS NS ++ NS NS NS NS Diaptomus kenai NS NS NS C) NS August : NS L981 NS NS zooplankton species <2 phytoplankton s i z e 2-6 6-10 10-14 (um) 14-20 >20 colonies Daphnia rosea NS NS - ++ ++ - -Holoped ium g ibberum NS NS NS . NS NS NS NS Diaptomus oreqonensis NS NS NS NS + NS NS Diaphanosoma brachyurum NS NS NS NS NS .• NS NS d) September 1981 , zooplankton species <2 phytoplankton s i z e 2-6 6-10 10-14 (um) 14-20 >20 colonies Daphnia rosea NS NS NS NS NS NS -Holoped ium gibberum NS NS " NS NS NS NS -Diaptomus NS NS NS NS NS NS NS oregonensis NS not s i g n i f i c a n t ++ s i g n i f i c a n t grazing (regression analysis) + s i g n i f i c a n t grazing (analysis of variance) s i g n i f i c a n t phytoplankton growth (regression analysis) s i g n i f i c a n t phytoplankton growth (analysis of variance) 45 e x p e r i m e n t . A s i g n i f i c a n t g r a z i n g e f f e c t by D i aptomus  o r e g o n e n s i s on c e l l s o f t h e 6-10 urn c l a s s was d e t e c t e d i n t h e J u l y e x p e r i m e n t . Two-way a n a l y s i s of v a r i a n c e on d a t a from t h e e n t i r e o b s e r v a t i o n p e r i o d y i e l d e d more r e s u l t s . W i t h t h e e x c e p t i o n of t h e June e x p e r i m e n t s w i t h D i a p t o m u s k e n a i , t h e r e s u l t s of w h i c h s u g g e s t t h a t D. k e n a i g r a z e on p h y t o p l a n k t o n c e l l s s m a l l e r than 2 / i m , none of t h e e x p e r i m e n t s showed s i g n i f i c a n t e f f e c t s of g r a z e r s on t h e two s m a l l e s t p h y t o p l a n k t o n s i z e c a t e g o r i e s . The June e x p e r i m e n t r e s u l t s s u g g e s t t h a t t h o s e s i z e g r o u p s s i g n i f i c a n t l y g r a z e d a r e t h e 6-10 nm (by Daphn 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 , D i a p t o m u s k e n a i ), 10-14 M m (by H o l o p e d i u m , D. k e n a i , D i a p t o m u s o r e g o n e n s i s ) , 14-20 M m (by a l l f o u r s p e c i e s t e s t e d ) and t h e >20 M m (by a l l g r o u p s but D. o r e g o n e n s i s ' ) . T h e r e was no e v i d e n c e t h a t any of t h e f o u r s p e c i e s g r a z e d on a l g a l c o l o n i e s i n June.' The J u l y , A u g u s t , and September e x p e r i m e n t r e s u l t s s u g g e s t no s i g n i f i c a n t g r a z i n g of t h e 6-10 M m c e l l s ; t h i s g r o u p d i d s i g n i f i c a n t l y b e t t e r i n J u l y i n t h e p r e s e n c e of D. o r e g o n e n s i s and i n August i n t h e p r e s e n c e of D a p h n i a t h a n t h e c o n t r o l g r o u p . The August r e s u l t s s u g g e s t D a p h n i a f e d on t h e 10-14 M m g r o u p ; t h e r e a r e no o t h e r s i g n f i c a n t r e s u l t s f o r t h e l a s t t h r e e months w i t h r e s p e c t t o g r a z i n g on t h i s s i z e c a t e g o r y . The A u g u s t r e s u l t s a l s o s u g g e s t D a p h n i a and D. o r e g o n e n s i s f e d on t h e 14-20 M m g r o u p . The >20 M m g r o u p d i d s i g n i f i c a n t l y b e t t e r i n A u g u s t i n t h e p r e s e n c e of Daphn i a than t h e c o n t r o l ; a l l o t h e r r e s u l t s f o r t h i s g r o u p d u r i n g J u l y , A u g u s t , and September were i n s i g n i f i c a n t . O n l y i n t h e J u l y 46 e x p e r i m e n t s i s t h e r e any s u g g e s t i o n t h a t a l g a l c o l o n i e s were g r a z e d ; t h e s e r e s u l t s s u g g e s t Daphn i a g r a z e on t h e c o l o n i e s . The A u g u s t and September r e s u l t s s u g g e s t e x a c t l y t h e o p p o s i t e -t h a t c o l o n i e s do b e t t e r i n t h e p r e s e n c e of D a p h n i a t h a t i n t h e i r a b s e n c e . T h i s was a l s o s u g g e s t e d by t h e r e s u l t s f o r H o l o p e d i u m f o r September. D u r i n g J u l y , t h e d ominant t a x o n i n t h e c o l o n y c a t e g o r y was G l a e o c y s t i s s p . ; i n A u g u s t and September i t was M e r i s m o p e d i a sp. No c h a n g e s i n d o m i n a n t s p e c i e s of any of t h e s i z e c l a s s e s of p h y t o p l a n k t o n were o b s e r v e d f o r t h o s e t r e a t m e n t s e x h i b i t i n g s i g n i f i c a n t g r a z i n g e f f e c t s . B e c a u s e of c h a n g e s i n h e r b i v o r e d e n s i t i e s e i t h e r due t o r e c r u i t m e n t or m o r t a l i t y d u r i n g t h e e x p e r i m e n t s , i t i s d i f f i c u l t t o c a l c u l a t e a c c u r a t e g r a z i n g r a t e s o f i n d i v i d u a l s . T h e r e f o r e minimum and maximum g r a z i n g r a t e s , b a s e d on s t o c k e d ( i n i t i a l ) and r e t r i e v e d ( f i n a l ) z o o p l a n k t o n d e n s i t i e s , were c a l c u l a t e d ( T a b l e I I I - 5 ) . W i t h i n t h e range of c e l l s i z e s i n g e s t e d by D a p h n i a , t h e s m a l l e r c e l l s (6-10 um) were f i l t e r e d f r o m t h e w a ter a t t h e f a s t e s t r a t e and f i l t e r i n g r a t e s d e c r e a s e d as c e l l s i z e i n c r e a s e d i n t h e J une e x p e r i m e n t . In A u g u s t the o p p o s i t e was o b s e r v e d a l t h o u g h t h e l a r g e s t s i z e c l a s s (>20 urn) e x h i b i t e d no s i g n i f i c a n t g r a z i n g e f f e c t . Dominant s p e c i e s i n e a c h s i z e g r o u p were s i m i l a r d u r i n g t h e two months. U n l i k e June and A u g u s t , i n J u l y t h e r e was a s i g n i f i c a n t g r a z i n g e f f e c t of Daphn i a on c o l o n i e s . In June and A u g u s t , M e r i s m o p e d i a was d o minant c o l o n i a l a l g a w h i l e G l a e o c y s t i s was dominant i n J u l y . T h e r e was no r e c o g n i z a b l e r e l a t i o n s h i p between f i l t e r i n g r a t e s TABLE III -5 F i l t e r i n g rates for phytoplankton s i z e c l asses s i g n i f i c a n t l y grazed. zooplankton phytoplankton minimum maximum s e l e c t i v i t y species size c l a s s f i l t e r i n g f i l t e r i n g c o e f f i c i e n t (um) (1/d/ug 2p) (1/d/ug zp) Daphnia rosea (June 1981) 6-10 * * 10 -14 0. 0. 025 0033 0. 0 . 068 0090 .60 .08 1 4 - 2 0 0 . 0082 0 . 022 .20 >20 0 . 0052 0 . 014 .12 t o t a l 0. 0047 0 . 013 Daphnia rosea (July 1981) colonies 0. 0018 0 . 0022 Daphnia rosea (August 1981) 6-10 10-14 0 . 0. 018 030 0 . 0 . 061 100 . 2 1 .34 14 -20 0. 039 0 . 034 .45 t o t a l 0. 014 0. 048 Holopedium qibberum (June 1981) 6-10 10-14 0. 0. ,0031 ,0074 0 . 0. 0059 014 . 1 1 .27 14 -20 0. .012 0. ,023 .44 >20 0. .0052 0. ,0098 .19 t o t a l 0. ,016 0. 030 Diaptomus oreqonensis (June 1981) 10-14 14-20 0. 0. .0040 .059 0. o. ,0062 ,091 .06 .94 t o t a l 0. .010 o. .016 Diaptomus oregonensis (July 1981) 14 -20 0. ,0153 0. 0164 Diaptomus kenai (June 1981) < 2 * * 2 -6 0. 0. ,0047 0011 0 . 0 . 0061 0014 .26 .06 6 -10 0 . 8.052 0 . 0068 .29 10 -14 0 . 0012 0 . 0015 .07 14 -20 0 . 0032 0 . 0042 .18 >20 0 . 0025 0 . 0032 .14 t o t a l 0. 0035 0 . 0045 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 but I f e l t b i o l o g i c a l l y important 48 and c e l l s i z e f o r D i a p t o m u s k e n a i i n J u n e . H i g h e s t c l e a r a n c e r a t e s were f o r c e l l s of t h e 6-10 Mm s i z e g r o u p ; t h e s e were a l s o t h e most abundant c e l l s i n terms o f b i o m a s s . Low f i l t e r i n g r a t e s f o r t h e 10-14 Mm s i z e c l a s s a r e p e c u l i a r s i n c e dominant s p e c i e s i n t h i s c a t e g o r y were t h e same as some of t h o s e of t h e 6-10 Mm c a t e g o r y and some of t h e 14-20 Mm c a t e g o r y ; no b i o l o g i c a l r e a s o n f o r t h i s low f i l t e r i n g r a t e i s a p p a r e n t . H o l o p e d i u m g i b b e r u m e x h i b i t e d f a s t e s t f i l t e r i n g r a t e s f o r t h o s e c e l l s i n t h e 14-20 Mm s i z e c l a s s , w i t h a l o w e r r a t e f o r c e l l s >20 Mm and d e c r e a s i n g r a t e s w i t h d e c r e a s i n g c e l l s i z e f o r c e l l s s m a l l e r t h a n 14 Mm. Diaptomus o r e g o n e n s i s e x h i b i t e d a s i g n i f i c a n t g r a z i n g e f f e c t on a v e r y narrow r a n g e of c e l l s i z e s . In t h e A u g u s t e x p e r i m e n t , f i l t e r i n g r a t e s were h i g h e s t f o r t h e c e l l s i n t h e 14-20 Mm c a t e g o r y , t h e l a r g e r of t h e two c a t e o g r i e s w i t h i n i t s f e e d i n g r a n g e . F o r i n t e r e s t ' s s a k e , s e l e c t i v i t y c o e f f i c i e n t s ( C h e s s o n 1978) were c a l c u l a t e d and a r e shown i n T a b l e 111- 5 . Because of t h e way i n w h i c h t h e y a r e c a l c u l a t e d , t h e y do not p r o v i d e a d d i t i o n a l i n s i g h t on s e l e c t i v e f e e d i n g above t h e i n f o r m a t i o n g a i n e d f r o m f i l t e r i n g r a t e s . No c o r r e l a t i o n s were d e t e c t e d between t h e r e l a t i v e b i o m a s s e s of c e l l s i z e c l a s s e s and r e l a t i v e c l e a r a n c e r a t e s f o r d i f f e r e n t c e l l s i z e c l a s s e s . 49 DISCUSSION H i g h v a r i a n c e s i n t h e d a t a may have o b s c u r e d some o t h e r w i s e s i g n i f i c a n t r e s u l t s , p a r t i c u l a r l y i n t h e l a r g e c e l l c a t e g o r i e s (>20 Mm, 14-20 Mm) where s t a n d a r d d e v i a t i o n s were g e n e r a l l y a r o u n d 20% of t h e mean, a few g r e a t e r t h a n 30%. Data f o r t h e s m a l l e s t s i z e c a t e g o r y (<2 Mm) a l s o e x h i b i t e d s t a n d a r d d e v i a t i o n s a r o u n d 20%. S t a n d a r d d e v i a t i o n s f o r t h e 2-6, 6-10 and 10-14 Mm c e l l s g e n e r a l l y were a r o u n d 10%. A d e c r e a s e of r o u g h l y t w i c e t h e s t a n d a r d d e v i a t i o n would be needed t o p r o d u c e c l e a r l y s i g n i f i c a n t g r a z i n g e f f e c t s . T h i s means t h a t z o o p l a n k t o n would need t o be a b l e t o f i l t e r a t t h e v e r y l e a s t 20-40% of t h e water volume i n o r d e r t o p r o d u c e t h i s k i n d of d e c r e a s e . C o n s i d e r i n g t h e low z o o p l a n k t o n b i o m a s s e s i n some of t h e e n c l o s u r e s and t h e g e n e r a l range of f i l t e r i n g r a t e s , i t i s not s u r p r i s i n g t h a t some of t h e e x p e r i m e n t s p r o d u c e d no s i g n i f i c a n t r e s u l t s . At a f i l t e r i n g r a t e of 0.0096 1/Mg z o o p l a n k t o n / d a y (mean c a l c u l a t e d from my d a t a ; a c t u a l f i l t e r i n g r a t e s a r e l i k e l y l o w e r ) , o v e r a p e r i o d of 72 h o u r s , a minimum z o o p l a n k t o n biomass of 14 M g / l would be needed t o f i l t e r 40% of t h e water volume. The t r i a l on Diaphanosoma was run f o r o n l y 48 h o u r s , w h i c h i s p r o b a b l y why no s i g n i f i c a n t r e s u l t s were o b t a i n e d f o r t h i s s p e c i e s . I n s p i t e of t h e s e p r o b l e m s , t h e r e s u l t s on f e e d i n g r a n g e s of 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 were f a i r l y c o n s i s t e n t w i t h d a t a i n t h e l i t e r a t u r e . The f e e d i n g range f o r D a p h n i a r o s e a was s i m i l a r t o t h a t f o u n d f o r D a p h n i a p u l e x ( L y n c h 1978) wh i c h i s s i m i l a r i n s i z e t o D. r o s e a , e x c e p t t h a t t h e lo w e r s i z e l i m i t i n 50 my r e s u l t s was h i g h e r (6 um as compared t o 2 Mm) . DeMott (1 9 8 2 ) , i n l a b o r a t o r y e x p e r m e n t s , f o u n d t h a t D. r o s e a would e a t A e r o b a c t e r c e l l s (0.2 Mm d i a m e t e r ) w i t h e q u a l p r e f e r e n c e t o Chlamydomonas (6.6 Mm d i a m e t e r ) when f e d a m i x t u r e of t h e two. Because f i l t e r i n g e f f i c i e n c y t e n d s t o i n c r e a s e w i t h i n c r e a s i n g c e l l s i z e ( F r o s t 1977), i t i s q u e s t i o n a b l e whether h i g h e f f i c i e n c i e s a t f i l t e r i n g b a c t e r i a w o u l d be o b s e r v e d f o r D a p h n i a f e e d i n g on 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 , where most of t h e p h y t o p l a n k t o n biomass i s i n c e l l s g r e a t e r t h a n 6.6 Mm i n d i a m e t e r . On t h e o t h e r hand, p e r h a p s g r a z i n g o f z o o p l a n k t o n on v e r y s m a l l p h y t o p l a n k t o n c e l l s i s n o t o b s e r v e d b e c a u s e g r o w t h r a t e s of t h e s m a l l c e l l s a r e s u f f i c i e n t t o compensate f o r g r a z i n g l o s s e s (Banse 1976, S c h l e s i n g e r e t a l . 1981). C o n f l i c t i n g r e s u l t s (June compared t o J u l y ) c o n c e r n i n g D a p h n i a f e e d i n g on c o l o n i e s may be due t o 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 of t h e c o l o n y c a t e g o r y . D u r i n g J u l y , t h e d o m i n a n t t a x o n i n t h e c o l o n y c a t e g o r y was G l a e o c y s t i s s p . ; i n A u g u s t and September i t was M e r i s m o p e d i a s p . W h i l e M e r i s m o p e d i a i s a g e l a t i n o u s b l u e - g r e e n , G l a e o c y s t i s i s a g r e e n a l g a . They a l s o d i f f e r i n m o r p h o l o g y . A l t h o u g h i n d i v i d u a l G l a e o c y t i s c e l l s a r e l a r g e r t h a n t h o s e of M e r i s m o p e d i a , c o l o n i e s n o r m a l l y c o n s i s t of f o u r c e l l s a r r a n g e d i n a t e t r a h e d r o n and may measure l e s s t h a n 20 Mm i n d i a m e t e r . M e r i s m o p e d i a c o l o n i e s v a r y i n number o f c e l l s . The c e l l s a r e a r r a n g e d i n l a r g e p l a t e s , one c e l l t h i c k . The maximum l i n e a r d i m e n s i o n of a M e r i s m o p e d i a c o l o n y may be i n e x c e s s of 50 Mm. C h e m i c a l d i f f e r e n c e s between t h e two s p e c i e s may a l s o e x i s t . 51 The c e l l s i z e f e e d i n g r a n g e of H o l o p e d i u m was s i m i l a r t o t h a t of D a p h n i a . I was u n a b l e t o f i n d any i n f o r m a t i o n i n t h e l i t e r a t u r e on t h e f e e d i n g range of H o l o p e d i u m . My r e s u l t s on t h e c e l l s i z e f e e d i n g range of Diaptomus  o r e g o n e n s i s were v e r y s i m i l a r t o t h o s e of McQueen ( 1 9 7 0 ) . McQueen's c e l l s i z e s were measured i n terms of volume, w h i l e mine u s e d maximum l i n e a r d i m e n s i o n . C o n v e r t i n g h i s volume range t o s p h e r i c a l e q u i v a l e n t d i a m e t e r s y i e l d e d a f e e d i n g range between 6 and 12 urn d i a m e t e r . The f e e d i n g r a n g e I o b s e r v e d was between 6 and 20 Mm maximum l i n e a r d i m e n s i o n . S i n c e s m a l l c e l l s t e n d t o be a p p r o x i m a t e l y s p h e r i c a l , t h e low e r l i m i t s we o b s e r v e d were more or l e s s i d e n t i c a l . The volume of a 12 Mm d i a m e t e r s p h e r e i s a p p r o x i m a t e l y e q u a l t o t h a t of a c y l i n d e r 9 Mm i n d i a m e t e r and 20 Mm l o n g w h i c h i s v e r y c l o s e t o t h e d i m e n s i o n s of th e l a r g e s t c e l l s w i t h i n t h e f e e d i n g range I o b s e r v e d . The s i z e r a n g e f o r D. o r e q o n e n s i s r e p o r t e d by Richman e t a l . (1980) was c o n s i d e r a b l y b r o a d e r (2.5 - 30 Mm s p h e r i c a l e q u i v a l e n t d i a m e t e r ) . The c e l l s i z e f e e d i n g range o f Diaptomus k e n a i was q u i t e b r o a d . The a p p a r e n t h i g h e f f i c i e n c y a t f i l t e r i n g v e r y s m a l l c e l l s was u n e x p e c t e d . B a r l o w and M o n t e i r o (1979) f o u n d t h a t c o p e p o d s i n c r e a s e d t h e i r f e e d i n g r a n g e s a s c o m p e t i t i o n f o r f o o d i n c r e a s e d . T h e r e f o r e , t h e b r o a d f e e d i n g range o b s e r v e d may have been due t o a b n o r m a l l y h i g h d e n s i t i e s of D. k e n a i i n t h e e n c l o s u r e s . F i l t e r i n g r a t e s c an e a s i l y be u s e d t o c a l c u l a t e p r e f e r e n c e c o e f f i c i e n t s ( C h e s s o n 1978). The C h e s s o n p r e f e r e n c e 52 c o e f f i c i e n t , A, f o r a g i v e n p r e y t y p e , i , i s c a l c u l a t e d a s : A ( i ) = ( r ( i ) / n ( i ) ) / I ( r ( j ) / n ( j ) ) where r ( i ) i s t h e p r o p o r t i o n of p r e y i i n t h e d i e t , n ( i ) i s t h e p r o p o r t i o n of p r e y i n t h e e n v i r o n m e n t , and j = 1 t o m. S i n c e i t i s not p o s s i b l e t o d i r e c t l y measure from my d a t a t h e p r o p o r t i o n of any c e l l s i z e c l a s s i n t h e d i e t , we can assume t h a t r ( i ) = ( p ( i ) g ( i ) ) / I ( p ( j ) g ( j ) ) where p ( i ) i s t h e d e n s i t y of p h y t o p l a n k t o n s i z e i and g ( i ) t h e o b s e r v e d f i l t e r i n g r a t e on p h y t o p l a n k t o n i . The p r o p o r t i o n of p h y t o p l a n k t o n I i n t h e e n v i r o n m e n t can be c a l c u l a t e d a s n ( i ) = p ( i ) / I ( p ( j ) ) . The p r e f e r e n c e c o e f f i c i e n t c a n t h e n be c a l c u l a t e d a s A ( i ) = ( p ( i ) g ( i ) ) / Z ( p ( j ) g ( j ) ) X Z ( p ( i ) ) / p ( i ) = g ( i ) / Z ( g ( j ) ) T h e r e a r e two s c h o o l s o f t h o u g h t on t h e c a u s e of f o o d s i z e p r e f e r e n c e by z o o p l a n k t o n . Lehman (1976) f e e l s t h a t z o o p l a n k t o n 53 o p t i m i z e t h e i r r e t u r n s e n e r g e t i c a l l y ; t h e r e f o r e t h e y s h o u l d p r e f e r c e l l s j u s t s l i g h t l y l a r g e r t han t h o s e most numerous. F r o s t (1977) c l a i m s t h a t f o o d p r e f e r e n c e i s r e l a t e d t o mesh s i z e and t h a t t h e l a r g e r t h e c e l l , w i t h i n t h e bounds of what t h e o r g a n i s m s a r e a b l e t o i n g e s t , t h e g r e a t e r t h e e f f i c i e n c y o f c a p t u r e . Most r e s e a r c h e r s f e e l t h a t f o o d p r e f e r e n c e i s a f u n c t i o n of b o t h t h e s e f a c t o r s (Boyd 1976, N i v a l and N i v a l 1976, B a r l o w and M o n t e i r o 1979), t h a t t h e p r o b a b i l i t y of c a p t u r e i s t h e p r o d u c t of t h e p r o b a b i l i t y of e n c o u n t e r and t h e e f f i c i e n c y of c a p t u r e of a p a r t i c u l a r s i z e c l a s s of p h y t o p l a n k t o n . My p r e f e r e n c e r e s u l t s s u p p o r t t h i s l a t t e r t h e o r y . In J u n e , H o l o p e d i u m and D i aptomus o r e g o n e n s i s f i l t e r e d l a r g e c e l l s (14-20 Mm) a t t h e h i g h e s t r a t e . B o t h o r g a n i s m s were i n r e l a t i v e l y low d e n s i t i e s i n t e r m s of b i o m a s s ( T a b l e I I I - 2 ) i n t h e e n c l o s u r e s , s i n c e b i o m a s s p e r i n d i v i d u a l i s low f o r b o t h s p e c i e s . T h e r e f o r e c o m p e t i t i o n f o r f o o d was p r o b a b l y a l s o m i n i m a l and t h e p r o b a b l i l i t y of e n c o u n t e r o f f o o d c e l l s was l e s s i m p o r t a n t t h a n e f f i c i e n c y of c a p t u r e , so l a r g e c e l l s were f i l t e r e d a t t h e h i g h e s t r a t e . Daphn i a and Diaptomus kena i i n June f i l t e r e d t h e most abundant c e l l s , i n t h e 6-10 Mm c a t e g o r y , a t t h e h i g h e s t r a t e . T h e s e o r g a n i s m s were i n r e l a t i v e l y h i g h d e n s i t i e s i n t e r m s of b i o m a s s i n t h e i r e n c l o s u r e s and c o m p e t i t i o n f o r f o o d was t h e r e f o r e a l s o h i g h . P r o b a b i l i t y of e n c o u n t e r was i m p o r t a n t i n t h e s e c a s e s s i n c e o r g a n i s m s c a n n o t a f f o r d t o p a s s o v e r f o o d when c o m p e t i t i o n f o r f o o d i s i n t e n s e . By d e c r e a s i n g t h e c a r a p a c e gape, c l a d o c e r a n s a r e a b l e t o a v o i d i n t e r f e r e n c e by t h e r a r e l y e n c o u n t e r e d l a r g e c e l l s and t h e r e f o r e f i l t e r s m a l l c e l l s 54 more e f f i c i e n t l y ( G l i w i c z and S i e d l a r 1980). Copepods a r e a l s o a b l e t o i n c r e a s e t h e i r f e e d i n g s i z e range under t h e s t r e s s of c o m p e t i t i o n (Barlow and M o n t e i r o 1979), a l t h o u g h t h e y do n o t p o s s e s s t h e mechanism f o r i n c r e a s i n g e f f i c i e n c y of c a p t u r e o f s m a l l c e l l s t h a t c l a d o c e r a n s do. H a r g r a v e and Geen (1970) and McQueen (1970) have s u g g e s t e d t h a t c o p e p o d s t e n d t o have p o o r f i l t e r i n g e f f i c i e n c i e s f o r c e l l s l e s s t h a n 5 Mm i n d i a m e t e r . B e c a u s e of t h i s and b e c a u s e t h e c e l l s s m a l l e r t h a n 2 jum make up o n l y a b o u t 7% of t h e t o t a l p h y t o p l a n k t o n , I am u n a b l e t o e x p l a i n t h e h i g h f i l t e r i n g r a t e s of Diaptomus k e n a i f o r t h o s e c e l l s s m a l l e r t h a t 2 Mm i n d i a m e t e r . S e l e c t i v i t y c o e f f i c i e n t s and f i l t e r i n g r a t e s d i f f e r s u b s t a n t i a l l y l e s s between s i z e c l a s s e s f o r D. k e n a i than f o r D a p h n i a and may s i m p l y be a r e f l e c t i o n of c e l l a bundance of e a c h s i z e c l a s s . A l t h o u g h c o m p e t i t i o n f o r f o o d was p r o b a b l y a s h i g h i n t h e A u g u s t Daphn i a e n c l o s u r e s as i n t h e June e n c l o s u r e s , t h e maximum f i l t e r i n g r a t e was o b s e r v e d f o r t h e l a r g e s t s i z e c a t e g o r y w i t h i n t h e f e e d i n g r a n g e , t h e 14-20 Mm g r o u p . The 6-10 Mm c a t e g o r y , w h i l e c o m p r i s i n g 47% 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 i n Ju n e , o n l y c o m p r i s e d 21% 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 A u g u s t . T h e r e f o r e t h e r e was l e s s b e n e f i t f o r Daphn i a t o i n c r e a s e i t s e f f i c i e n c y a t c a p t u r i n g t h e s e s m a l l c e l l s a t t h e expe n s e of b e i n g u n a b l e t o c a p t u r e t h e v e r y l a r g e c e l l s . U n f o r t u n a t e l y , t h i s does n o t e x p l a i n why t h e a p p a r e n t upper l i m i t i n f e e d i n g r a n g e i s l o w e r i n A u g u s t than i n J u n e . I t may j u s t be due t o e r r o r . H i g h v a r i a n c e s i n t h e c o n t r o l s a s w e l l a s t h e e x p e r i m e n t a l d a t a may have o b s c u r e d t h e s e r e s u l t s s i n c e t h e f i l t e r i n g r a t e s c a l c u l a t e d 55 from r e g r e s s i o n s f o r D a p h n i a f e e d i n g on c e l l s >20 urn (0.009 0.034 1/d/jug z o o p l a n k t o n ) a r e r e a s o n a b l e . B e c a u se of c h a n g e s i n g r a z e r p o p u l a t i o n s , "minimum" and "maximum" f i l t e r i n g r a t e s were c a l c u l a t e d . The "maximum" f i l t e r i n g r a t e s f o r t o t a l p h y t o p l a n k t o n c a l c u l a t e d a r e h i g h compared t o t h o s e i n t h e l i t e r a t u r e ( T a b l e 111-6) . In some c a s e s , s u c h as D a p h n i a i n A u g u s t , my c a l c u l a t e d f i l t e r i n g r a t e s a r e more t h a n two o r d e r s of m a g n i t u d e g r e a t e r t h a n v a l u e s f o u n d i n t h e l i t e r a t u r e . "Minimum" r a t e s a r e a l s o h i g h (mean t o t a l f i l t e r i n g r a t e s f o r a l l s p e c i e s i n June of t h i s s t u d y was 0.0086 1/d/iug z o o p l a n k t o n as compared t o t h e mean of maximum r a t e s f o r a l l s p e c i e s i n T a b l e 111-6, 0.0049 1/d / V g z o o p l a n k t o n ). Buckingham (1978) f o u n d t h a t a t t h e low s e s t o n c o n c e n t r a t i o n s common t o P l a c i d Lake (<1000 u g / 1 ) , f i l t e r i n g r a t e s v a r i e d d r a m a t i c a l l y w i t h c h a n g e s i n f o o d p a r t i c l e c o n c e n t r a t i o n s , and t h i s was t h e r e a s o n f o r v a r i a t i o n i n h e r f i l t e r i n g r a t e s ( T a b l e 111 — 6 ) . I t may a l s o be a major r e a s o n f o r h i g h e r r a t e s measured i n t h i s s t u d y compared t o t h o s e i n t h e l i t e r a t u r e as w e l l as v a r i a t i o n between months. F i l t e r i n g r a t e s f o r Daphn i a were h i g h e r i n A u g u s t when p h y t o p l a n k t o n b i o m a s s was l o w e r t h a n i n J u n e . T h e r e f o r e , w h i l e t h e c o m p a r i s o n of c l e a r a n c e r a t e s of d i f f e r e n t c e l l s i z e c l a s s e s 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 i c e s i s u s e f u l i n d e t e r m i n i n g f o o d p r e f e r e n c e s , t h e r a t e s measured i n t h i s s t u d y a r e not a d e q u a t e f o r c a l c u l a t i n g o v e r a l l f e e d i n g r a t e s I t i s d i f f i c u l t t o a s s e s s t h e e f f e c t o f c h a n g i n g z o 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 o v e r a l l g r a z i n g r a t e . In 56 TABLE III-6 Maximum Zooplankton Filtering Rates zooplankton filt e r i n g rate source species (ug/d/ug zooplankton) Bosmina coregoni 0.00125 Nauwerck 1963 B. lonqirostris 0.0011 Haney 1973 Ceriodaphnia 0.0004 Nauwerck 1963 quadrangula 0.0029 Haney 1973 0.0029 Buckingham 1978 C. reticulata 0.003 O'Brien 1974 Chydoris sp. 0.0105 Haney 1973 Daphnia rosea 0.0019 Burns and Rigler 1967 0.0017 Haney 1973 0.0019 Buckingham 1978 Diaphahosoma 0.0007 Nauwerck 1963 brachyurum 0.0009 Haney 1973 0.0005 Gliwicz 1977 Diaptomus 0.00125 Richman 1966 oregonensis 0.005 McQueen 1970 0.0003 Haney 1973 0.0072 Buckingham 1978 0.0132 Buckingham 1978 Holopedium gibberum 0.0096 Buckingham 1978 0.0336 Buckingham 1978 57 s p r i n g and f a l l , t h e h e r b i v o r e p o p u l a t i o n i s d o m i n a t e d by D a p h n i a , H o l o p e d i u m , Diaptomus k e n a i and D. o r e g o n e n s i s . D a p h n i a has been c l a s s e d as a "warm" s p e c i e s by Buckingham (1978) s i n c e i t e x h i b i t s maximum f e e d i n g r a t e s a t 20 C. Burns and R i g l e r (1967) a l s o o b s e r v e d maximum f i l t e r i n g r a t e s f o r D. r o s e a a t 20 C. H o l o p e d i u m , Diaptomus kena i and D. o r e g o n e n s i s have been c l a s s e d as " c o o l " s p e c i e s s i n c e t h e y f e e d a t maximum r a t e s a t c o o l e r t e m p e r a t u r e s (8 C f o r H o l o p e d i u m and D. k e n a i and 12 C f o r D. o r e g o n e n s i s . In summer D. k e n a i i s r e p l a c e d by Diaphanosoma, a n o t h e r "warm" s p e c i e s . The o v e r a l l g r a z i n g r a t e t h u s i s a f a i r l y complex f u n c t i o n of s p e c i e s c o m p o s i t i o n , t e m p e r a t u r e , and z o o p l a n k t o n d e n s i t y . 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 may a l s o be a f a c t o r d e t e r m i n i n g g r a z i n g r a t e s ( Burns 1968b, Webster and P e t e r s 1978, G l i w i c z and S i e d l a r 1980, P o r t e r and McDonough 1984). I t a p p e a r s from s e l e c t i v i t y r e s u l t s t h a t t h e c e l l s of t h e 14-20 /am c a t e g o r y p r o b a b l y u n d e r g o h i g h e s t l o s s due t o g r a z i n g . O t h e r g r o u p s (6-14 um, >20 Mm) e x p e r i e n c e i n c r e a s i n g l o s s e s t o g r a z i n g as c o m p e t i t i o n f o r f o o d between z o o p l a n k t o n i n c r e a s e s . The o n l y g r o u p which seems q u i t e c l e a r l y not t o be u t i l i z e d as f o o d i s t h e c o l o n y g r o u p when 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 s p . T h i s i n f o r m a t i o n i s of p a r t i c u l a r v a l u e s i n c e most of t h e major p h y t o p l a n k t o n blooms i n b o t h t h e e n c l o s u r e s and t h e l a k e were composed p r i m a r i l y o f M e r i s m o p e d i a s p . C o l o n y m o r p h o l o g y may be p a r t l y r e s p o n s i b l e f o r t h e z o o p l a n k t o n a p p a r e n t a v o i d a n c e of t h i s s p e c i e s . C h e m i c a l p r o p e r t i e s of M e r i s m o p e d i a may a l s o be i m p o r t a n t . When A r n o l d (1971) t r i e d 58 c u l t u r i n g Daphn i a on a d i e t of Mer i smopedia, he kept t h e water i n t h e c u l t u r e s t i r r i n g so t h a t most M e r i s m o p e d i a c e l l s were e i t h e r s i n g l e o r i n p a i r s , not i n l a r g e p l a t e s . N e v e r t h e l e s s , f e e d i n g r a t e s of D a p h n i a on M e r i s m o p e d i a were low compared t o f e e d i n g r a t e s on C h l o r e l l a , and s u r v i v a l and r e p r o d u c t i o n of Daphn i a f e d Mer i s m o p e d i a was p o o r e r t h a n of Daphn i a i n d i s t i l l e d w a t e r . The s i g n i f i c a n t a b s e n c e of n a u p l i i i n t h e June e n c l o s u r e s w i t h D i aptomus k e n a i compared t o o t h e r e n c l o s u r e s i s i n t e r e s t i n g . The z o o p l a n k t o n samples were t a k e n a t t h e end of t h e run of t h e e x p e r i m e n t s and t h e r e i s no r e a s o n t o suppose t h a t t h e s e e n c l o s u r e s were not a l s o c o n t a m i n a t e d w i t h n a u p l i i as were t h e o t h e r s a t t h e o n s e t of t h e e x p e r i m e n t s . T h i s s u g g e s t s t h a t D. k e n a i were f e e d i n g on n a u p l i i , an h y p o t h e s i s s u p p o r t e d by s u b s e q u e n t l a b o r a t o r y f e e d i n g s t u d i e s and gut a n a l y s e s done by Bowerman ( 1 9 8 3 ) . S i g n i f i c a n t l y h i g h e r numbers of n a u p l i i i n t h e e n c l o s u r e s w i t h Diaptomus o r e g o n e n s i s may have been t h e o f f s p r i n g of t h e D. o r e g o n e n s i s s t o c k e d i n t h e e n c l o s u r e s . D u r i n g t h e manual s o r t i n g p r o c e d u r e , t h e r e t e n d e d t o be a b i a s t o w a r d s l a r g e i n d i v i d u a l s w i t h eggs s i n c e t h e s e were most r e a d i l y i d e n t i f i e d . T h i s p r o c e d u r a l phenomenon may a l s o be 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 i n c r e a s e d numbers of o r g a n i s m s i n some e n c l o s u r e s a t t h e end of e a c h e x p e r i m e n t compared t o what had been s t o c k e d ( e g . H o l o p e d i u m i n J u l y ) . S o r t i n g i r r e g u l a r i t i e s may a l s o have c a u s e d i n e q u i t i e s between e n c l o s u r e s ( e g . D a p h n i a i n J u l y ) , s i n c e t h e f i r s t b a t c h of o r g a n i s m s s o r t e d would have c o n t a i n e d more of t h e l a r g e r , e g g -59 b e a r i n g i n d i v i d u a l s t h an t h e s e c o n d b a t c h . Of t h e f i v e s e t s o f e x p e r i m e n t s done, o n l y t h e f i r s t one (June 1981) was v e r y s u c c e s s f u l . The o t h e r s f a i l e d on the whole t o show c o n s i s t e n t l y s i g n f i c a n t (p=0.05) d i f f e r e n c e s between t h e t e s t e n c l o s u r e s and t h e c o n t r o l s . T h e r e a r e s e v e r a l p o s s i b l e r e a s o n s f o r t h i s f a i l u r e . In o r d e r t o make t h e e x p e r i m e n t s l o g i s t i c a l l y f e a s a b l e , t h e e n c l o s u r e s had t o be s m a l l (100-200 L) and s h a l l o w (<1 m). C o n s e q u e n t l y t h e water t e m p e r a t u r e s i n t h e e n c l o s u r e s were h i g h . B e c a u s e of t h i s , some of t h e o r g a n i s m s may have been f i l t e r -f e e d i n g a t q u i t e r e d u c e d r a t e s , p a r t i c u l a r l y i n A u g u s t . Buckingham (1978) f o u n d t h a t i n P l a c i d L a k e , H o l o p e d i u m g i b b e r u m and D i a p t o m u s k e n a i e x h i b i t e d maximum f e e d i n g r a t e s a t 8 C. Dia p t o m u s t y r r e l l i , a s p e c i e s w h i c h i s s i m i l a r i n s i z e and d i s t r i b u t i o n t o D. o r e g o n e n s i s d e m o n s t r a t e d maximum f e e d i n g r a t e s a t 12 C, a l t h o u g h measurements a t l o w e r t e m p e r a t u r e s were not made. At 15 C, f e e d i n g r a t e s of H o l o p e d i u m had d r o p p e d t o s l i g h t l y more t h a n one q u a r t e r t h e maximum r a t e , D. k e n a i t o o n e - s i x t h , and D. t y r r e l l i t o a p p r o x i m a t e l y 5% t h e maximum f e e d i n g r a t e s . At 19 C, D. k e n a i and D. t y r r e l l i had v i t u a l l y s t o p p e d f e e d i n g . D a p h n i a r o s e a , on t h e o t h e r hand, e x h i b i t e d maximum f e e d i n g r a t e s a t 20 C. T h e r e f o r e h i g h t e m p e r a t u r e s c a n n o t be t h e r e a s o n f o r i t s p o o r p e r f o r m a n c e i n my s t u d y . Diaphanosoma, b e c a u s e i t i s s c a r c e t h r o u g h o u t t h e y e a r e x c e p t i n summer, a l s o was e x p e c t e d t o p e r f o r m b e t t e r . The p r o b l e m of c h a n g e s i n t e m p e r a t u r e m i g h t have been r e d u c e d by u s i n g P o r t e r ' s (1973) method of t y i n g o f f t h e t o p s of t h e e n c l o s u r e s and 60 s u s p e n d i n g them u n d e r w a t e r . However, c o n s i d e r i n g t h e f r e q u e n c y and t i m e s o f s a m p l i n g i n t h e p r e s e n t s t u d y , t h i s would have been l o g i s t i c a l l y d i f f i c u l t and would have c a u s e d c o n s i d e r a b l e d i s t u r b a n c e t o the e n c l o s u r e s . A s o l u t i o n might have been t o sample l e s s o f t e n (once e v e r y 24 h o u r s ) , t a k i n g s e v e r a l samples a t a t i m e , and u s i n g P o r t e r ' s method. I c h o s e t o sample more f r e q u e n t l y b e c a u s e I hoped t o o b s e r v e c h a n g e s i n f o o d p r e f e r e n c e as w e l l as e d i b i l i t y , t h a t i s " s w i t c h i n g " from more t o l e s s p r e f e r r e d f o o d i t e m s a s t h e more p r e f e r r e d ones were g r a z e d . A n o t h e r c o m p l i c a t i n g f a c t o r i s n u t r i e n t e n r i c h m e n t o f t h e e n c l o s u r e s due t o e x c r e t i o n by t h e z o o p l a n k t o n . S i n c e t h e d e n s i t i e s of a n i m a l s u s e d i n J u l y , A u g u s t , and September were c o n s i d e r a b l y h i g h e r t h a n t h o s e u s e d i n June, the e n r i c h m e n t e f f e c t w o u l d be g r e a t e r . In some of t h e l a t e r e n c l o s u r e s , i n some s i z e c a t e g o r i e s , g r o w t h of t h 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 was o b s e r v e d . I do not know how t h i s c o u l d have been e n t i r e l y a v o i d e d . The a n i m a l s had been h e l d a t h i g h d e n s i t i e s f o r 10-12 h o u r s d u r i n g t h e s o r t i n g p r o c e s s b e f o r e b e i n g i n t r o d u c e d i n t o t h e e n c l o s u r e s . B e c a u se of t h i s t h e i r g u t s were p r o b a b l y f a i r l y empty of f a e c a l m a t e r i a l . C h a n g i n g t h e water i n w h i c h t h e y had been h e l d s h o r t l y b e f o r e i n t r o d u c i n g them t o t h e e n c l o s u r e s would have h e l p e d t o a v o i d i n t r o d u c i n g l a r g e amounts of f a e c a l m a t e r i a l i n t o t h e e n c l o s u r e s . U n f o r t u n a t e l y , t h i s was not done. I know of no way t o a v o i d t h e e x c r e t i o n by z o o p l a n t k o n . One way t o mask i t would be t o e n r i c h t h e e n c l o s u r e s a r t i f i c i a l l y t o n o n - l i m i t i n g n u t r i e n t l e v e l s . P r o b a b l y t h e most s i g n i f i c a n t p r o b l e m w i t h t h e f e e d i n g 61 e x p e r i m e n t r e s u l t s was c a u s e d by c o u n t i n g e r r o r s . Samples f o r t h e J u l y , A u g u s t , and September e x p e r i m e n t s were c o u n t e d by two p e o p l e , m y s e l f and a t e c h n i c i a n . T h i s r e s u l t e d i n h i g h l y v a r i a b l e c o u n t s . In many c a s e s , r e s u l t s were n o t s t a t i s t i c a l l y s i g n i f i c a n t b e c a u s e of v e r y l a r g e v a r i a n c e s . R e c o u n t s on p a r t i c u l a r l y bad samples were done, but t h e r e a r e p r o b l e m s w i t h d o i n g r e c o u n t s ; c o l o n i e s t e n d t o b r e a k up w i t h e x c e s s i v e h a n d l i n g , and c e l l s a r e l o s t i n t h e p r o c e s s of t r a n s f e r r i n g samples from one c o n t a i n e r t o a n o t h e r . In r e t r o s p e c t , i t would have been w i s e r t o have one t e c h n i c i a n do a l l t h e c o u n t i n g , a t l e a s t a l l t h e samples from a g i v e n e x p e r i m e n t . 62 CHAPTER IV - ENCLOSURE EXPERIMENTS AND MODEL INTRODUCTION Z o o p l a n k t o n d e n s i t y has been i m p l i c a t e d as an i m p o r t a n t d e t e r m i n a n t o f p h y t o p l a n k t o n a b u n d a n c e . One o f t h e c o n c l u s i o n s of Haney's work (1973) was t h a t t h e i m p o r t a n c e of g r a z i n g as a p h y t o p l a n k t o n l i m i t i n g f a c t o r was r e l a t e d t o l a k e t r o p h y ; more p r e c i s e l y , he h y p o t h e s i z e d t h a t p h y t o p l a n k t o n l o s s e s due t o g r a z i n g a r e p r o p o r t i o n a l t o z o o p l a n k t o n d e n s i t y . He f o u n d t h a t i n o l i g o t r o p h i c H a l l ' s L a k e , z o o p l a n k t o n were t o o s p a r s e t o be a b l e t o l i m i t p h y t o p l a n k t o n t h r o u g h g r a z i n g . S i m i l a r c o n c l u s i o n s were drawn by F i n d e n e g g (1969) w o r k i n g i n o l i g o t r o p h i c S w i s s l a k e s . O ne.might e x p e c t t h e same t o be t r u e of t h e e f f e c t o f n u t r i e n t r e g e n e r a t i o n by z o o p l a n k t o n : t h a t t h i s t o o might be p r o p o r t i o n a l t o t h e d e n s i t y o f z o o p l a n k t o n i n a s y s t e m . W h i l e t h e a c t u a l amount o f n u t r i e n t r e g e n e r a t e d i n a e u t r o p h i c l a k e m ight be c o n s i d e r a b l y h i g h e r t h a n i n an o l i g o t r o p h i c l a k e , n u t r i e n t r e g e n e r a t i o n i n a l a k e a l r e a d y r i c h i n n u t r i e n t s m ight be q u i t e s u p e r f l u o u s , w h i l e i n a n u t r i e n t p oor l a k e even a s m a l l c o n t r i b u t i o n t o t h e f r e e n u t r i e n t p o o l m i g h t have a s u b s t a n t i a l i m p a c t on p h y t o p l a n k t o n g r o w t h . W a l t e r s e t a l . (1980) a r g u e d t h e o p p o s i t e , b a s e d on a model of s i x l a k e s s t u d i e d i n t h e IBP symposium. They c o n c l u d e d t h a t g r a z i n g e f f e c t s a r e l a r g e s t i n o l i g o t r o p h i c l a k e s , and a r e i n e f f e c t i v e a s p h y t o p l a n k t o n r e g u l a t o r s i n e u t r o p h i c l a k e s . W o r k i n g i n a m e s o t r o p h i c l a k e , G l i w i c z ( 1 9 7 5 ) , i n e n c l o s u r e e x p e r i m e n t s , f o u n d t h a t v a r y i n g t h e d e n s i t y o f D a p h n i a p u l e x 63 a f f e c t e d b o t h p h o t o s y n t h e t i c r a t e s and p h y t o p l a n k t o n c o m p o s i t i o n . P h o t o s y n t h e t i c r a t e s i n c r e a s e d a t . h i g h e r h e r b i v o r e d e n s i t i e s and n o n - g r a z a b l e forms of p l a n k t o n , l a r g e " n e t p l a n k t o n " , a l s o i n c r e a s e d d i s p r o p o r t i o n a t e l y i n t h e p h y t o p l a n k t o n community. G l i w i c z h y p o t h e s i z e d t h a t g r a z i n g c a u s e s h i g h e r n u t r i e n t t u r n o v e r and t h u s h i g h e r n u t r i e n t a v a i l a b i l i t y f o r p h y t o p l a n k t o n g r o w t h . He a l s o s u g g e s t e d t h a t n a n n o p l a n k t o n t e n d t o d o m i n a t e p h y t o p l a n k t o n i n t h e a b s e n c e of g r a z e r s b e c a u s e t h e y a r e s u p e r i o r c o m p e t i t o r s f o r n u t r i e n t s due t o t h e i r h i g h e r s u r f a c e a r e a : volume r a t i o s and t h e r e f o r e have h i g h e r g r o w t h r a t e s . N e t p l a n k t o n a r e o n l y a b l e t o compete when n a n n o p l a n k t o n d e n s i t i e s a r e c o n t r o l l e d by g r a z e r s . M c C a u l e y and B r i a n d ( 1 9 7 9 ) , w o r k i n g w i t h e n c l o s u r e s i n an o l i g o t r o p h i c l a k e , f o u n d t h a t s p e c i e s d i v . e r s i t y o f t h e n o n - g r a z a b l e forms d e c r e a s e d w i t h d e c r e a s i n g g r a z i n g p r e s s u r e , p r e s u m a b l y due t o g r e a t e r c o m p e t i t i o n f o r n u t r i e n t s w i t h t h e g r a z a b l e f o r m s . P o r t e r (1972, 1973) examined t h e e f f e c t of g r a z i n g p r e s s u r e on t h e s t a n d i n g c r o p of v a r i o u s p h y t o p l a n k t o n g r o u p s i n e n c l o s u r e s i n . a m e s o t r o p h i c l a k e . She f o u n d t h a t i n t h e a b s e n c e of g r a z e r s , t h e s m a l l (2-30 jum) e d i b l e f orms s u c h as s m a l l f l a g e l l a t e s , d i a t o m s , and l a r g e c h r y s o p h y t e s i n c r e a s e d , w h i l e t h e 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 e d e c r e a s e d i n a b u n d a n c e . No c h a n g e s were o b s e r v e d i n t h e l a r g e (>30 M m ) forms s u c h as l a r g e d e s m i d s or d i n o f l a g e l l a t e s or i n t h e v e r y s m a l l c h r y s o p h y t e s and b l u e - g r e e n a l g a e . In t h e p r e s e n c e o f g r a z e r s , t h e o p p o s i t e was o b s e r v e d t h o s e g r o u p s w h i c h had i n c r e a s e d i n t h e a b s e n c e of g r a z e r s were s u p p r e s s e d and t h e g e l a t i n o u s g r e e n c o l o n i e s w h i c h had d e c r e a s e d 64 i n t h e a b s e n c e of g r a z e r s , i n c r e a s e d as g r a z e r s i n c r e a s e d . The o t h e r s were u n a f f e c t e d . Weers and Z a r e t (1975) d i d s i m i l a r e x p e r i m e n t s u s i n g j a r s s u s p e n d e d i n a m e s o - o l i g o t r o p h i c l a k e i n Panama, and a l s o f o u n d t h a t v a r i o u s s p e c i e s of p h y t o p l a n k t o n r e s p o n d e d d i f f e r e n t l y t o t h e p r e s e n c e of z o o p l a n k t o n . R e d f i e l d (1980) s t u d i e d t h e e f f e c t s of normal d i u r n a l and n o c t u r n a l z o o p l a n k t o n d e n s i t i e s on p h y t o p l a n k t o n p r o d u c t i v i t y i n e n c l o s u r e e x p e r i m e n t s i n s u b a l p i n e C a s t l e L a k e . He f o u n d t h a t i n t h e summer e x p e r i m e n t s a f t e r 72 h o u r s , t h e p r e s e n c e of z o o p l a n k t o n e nhanced p r i m a r y p r o d u c t i v i t y , and t h a t i n c r e a s e s i n a l g a l p r o d u c t i v i t y were r e l a t e d s i g n i f i c a n t l y t o z o o p l a n k t o n b i o m a s s . In t h e f a l l , z o o p l a n k t o n had a n e g a t i v e e f f e c t on p h y t o p l a n k t o n p r o d u c t i v i t y . My p a r t i c u l a r i n t e r e s t was i n e x a m i n i n g t h e e f f e c t v a r y i n g l e v e l s of z o o p l a n k t o n would have on t h 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 , p a r t i c u l a r l y a t z o o p l a n k t o n d e n s i t i e s below t h o s e t h a t w ould p r o d u c e a s u b s t a n t i a l g r a z i n g e f f e c t . I f n u t r i e n t r e g e n e r a t i o n by z o o p l a n k t o n i s an i m p o r t a n t s o u r c e o f f r e e n u t r i e n t s f o r p h y t o p l a n k t o n g r o w t h and i f t h e m a i n t a i n a n c e of t h e n u t r i e n t p o o l i n a s y s t e m i s de p e n d e n t t o some e x t e n t on t h e t u r n o v e r of n u t r i e n t s between t h e s e two t r o p h i c l e v e l s , t h e n t h e a b s e n c e or n e a r - a b s e n c e of g r a z e r s m i g h t have a d e l e t e r i o u s e f f e c t on t h 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 . Most e n c l o s u r e e x p e r i m e n t s d e s c r i b e d i n t h e l i t e r a t u r e d i d not d e a l w i t h t h i s i s s u e , but were p r i m a r i l y c o n c e r n e d w i t h t h e e f f e c t of g r a z i n g on p h y t o p l a n k t o n community c o m p o s i t i o n . R e d f i e l d d e a l t w i t h t h e q u e s t i o n o f e n r i c h m e n t by z o o p l a n k t o n , b u t i n terms of 65 p h y t o p l a n k t o n p r o d u c t i v i t y r a t h e r t h a n community biomass or p o p u l a t i o n s i z e . Most r e p o r t e d e x p e r i m e n t s were of s h o r t d u r a t i o n ( P o r t e r , 4 d a y s ; Weers and Z a r e t , 24 h o u r s ; G l i w i c z , 5 d a y s ) , t o o s h o r t p e r h a p s t o o b s e r v e any e n r i c h m e n t e f f e c t s of z o o p l a n k t o n on p h y t o p l a n k t o n d e n s i t y . E n c l o s u r e e x p e r i m e n t s were done t o o b s e r v e t h e e f f e c t s of v a r y i n g z o o p l a n k t o n d e n s i t i e s on 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 . To a i d i n i n t e r p r e t a i o n of t h e e x p e r i m e n t a l r e s u l t s , a s i m p l e m a t h e m a t i c a l model, b u i l t on c l a s s i c a l m o d e l s , was d e v i s e d . T h a t model i s d e s c r i b e d n e x t , as p r e a m b l e and m o t i v a t i o n t o t h e e x p e r i m e n t a l methods and d e s i g n . A Log i s t i c M o d e l F o r P h y t o p l a n k t o n Growth The c l a s s i c a l l o g i s t i c model ( L o t k a 1925, V o l t e r r a 1926; r e v i e w e d by S m i t h , 1974) i s : d x / d t = rx (1 - x/k) (1) H e r e , x i s t h e p o p u l a t i o n d e n s i t y , r i s t h e i n t r i n s i c r a t e of i n c r e a s e , t i s t i m e , and k i s 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 f o r t h a t p o p u l a t i o n . The i n t r i n s i c r a t e of i n c r e a s e , by d e f i n i t i o n ( A n d r e w a r t h a and B i r c h 1954 r e v i e w e d i n K r e b s 1972) i s t h e r a t e of i n c r e a s e i n t h e a b s e n c e of n u t r i e n t l i m i t a t i o n . The term (1 - x/k) a c t s as a m o d i f i e r t o " r " so t h a t t h e r a t e o f i n c r e a s e i s r e l a t e d t o t h e c a p a c i t y o f t h e e n v i r o n m e n t f o r growth of t h e p o p u l a t i o n . I t assumes t h a t : 1. One v a r i a b l e , r , i s a d e q u a t e t o d e s c r i b e t h e c h a n g e s i n 66 p o p u l a t i o n , x, d e n s i t y . I t i s assumed t h a t c h a n g e s i n t h e o v e r a l l g r o w t h r a t e due t o c h a n g e s i n age and ( i n a c a s e s u c h as 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 o f t h e p o p u l a t i o n , o c c u r r a p i d l y and c o n s i s t e n t l y enough so t h a t a r e p e a t a b l e r e l a t i o n s h i p i s m a i n t a i n e d between s t a t i s t i c a l a v e r a g e r a t e s (measured by r ) and a v e r a g e p o p u l a t i o n s i z e . 2. Per c a p i t a g r o w t h , m o r t a l i t y and m e t a b o l i c r a t e s v a r y l i n e a r l y w i t h t o t a l d e n s i t y , x. 3. Any t i m e l a g s i n g r o w t h o r m o r t a l i t y can be i g n o r e d . 4. r and k a r e c o n s t a n t , or a t l e a s t v a r y s l o w l y compared t o t h e p o p u l a t i o n d e n s i t y , x. V o l t e r r a (1926, i n S m i t h , 1974) e xpanded t h i s t o i n c l u d e p r e d a t i o n as an i m p o r t a n t component i n p o p u l a t i o n g r o w t h : d x / d t = rx (1 - x/k) - c x y (2) H e r e , y i s t h e p r e d a t o r p o p u l a t i o n s i z e and c i s t h e p e r p r e d a t o r p r e d a t i o n r a t e . When z o o p l a n k t o n a r e t h e p r e d a t o r s , c i s t h e f i l t e r i n g r a t e . T h i s expanded v e r s i o n a l s o assumes t h a t : 5. c, t h e p r e d a t i o n r a t e , i s c o n s t a n t or c h a n g i n g s l o w l y r e l a t i v e t o x and y so t h a t t h e r a t e a t w h i c h the p r e y p o p u l a t i o n i s e a t e n i s p r o p o r t i o n a l t o t h e p r o d u c t of t h e p r e y and p r e d a t o r d e n s i t i e s . Any " b e h a v i o u r a l " o r p h y s i o l o g i c a l r e s p o n s e s ( i e . , n o n l i n e a r f u n c t i o n a l r e s p o n s e s ) on t h e p a r t o f p r e d a t o r s t o c h a n g e s i n p r e y d e n s i t i e s a r e i g n o r e d . Richman (1966) w o r k i n g w i t h D i aptomus o r e q o n e n s i s , K e r s t i n g (1978) 67 w o r k i n g w i t h D a p h n i a magna, and Buckingham ( 1 9 7 8 ) , who s t u d i e d 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 , f o u n d t h a t 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 do d i s p l a y a f u n c t i o n a l r e s p o n s e , s u c h t h a t f i l t e r i n g r a t e s d e c r e a s e as s e s t o n c o n c e n t r a t i o n s (x) i n c r e a s e . However, t h i s d e c r e a s e m ight be s m a l l o v e r t h e ra n g e of s e s t o n c o n c e n t r a t i o n s f o u n d i n P l a c i d L a k e (Buckingham 1978). S i m i l a r o b s e r v a t i o n s have been made by many o t h e r r e s e a r c h e r s (Richman 1966, B u r n s and R i g l e r 1967, P a r s o n s e t a l . 1967, Burns 1968b, F r o s t 1972, O ' B r i e n 1974, Lam and F r o s t 1976, G l i w i c z 1977). ( H u n t l e y ( 1 9 8 1 ) , however, f o u n d no s a t u r a t i o n f e e d i n g i n t h r e e c o p e p o d s i n t h e L a b r a d o r Sea.) F i g u r e IV-1 shows t h e shape of th e p h y t o p l a n k t o n i s o c l i n e ( d x / d t = 0) as d e s c r i b e d by t h e c l a s s i c a l l o g i s t i c m o d e l . W i t h i n p h y t o p l a n k t o n d y n a m i c s , s i n k i n g may a l s o be a major c a u s e o f p o p u l a t i o n d e c l i n e ( H u t c h i n s o n 1967). I n c l u d i n g a s i n k i n g r a t e , s, i n e q u a t i o n 2 y i e l d s d x / d t = r x ( l - x/k) - c x y - sx (3) W i t h i n t h e p l a n k t o n s y s t e m , c a r r y i n g c a p a c i t y d u r i n g summer i s d e t e r m i n e d p r i m a r i l y by t h e n u t r i e n t budget of t h e s y s t e m ( W e t z e l 1975). Lehman (1980) h a s s u g g e s t e d t h a t d u r i n g summer months, o u t s i d e n u t r i e n t i n p u t t o L a k e W a s h i n g t o n i s n e g l i g i b l e . I f t h i s i s t r u e of P l a c i d L a k e , we c a n d e s c r i b e t o t a l n u t r i e n t s , N ( t ) i n t h e e p i l i m n i o n a t any d i s c r e t e t i m e a s : N ( t ) = N ( f ) + Ax + By (4) 68 F i g u r e IV-1 P r e y i s o c l i n e p r e d i c t e d by t h e c l a s s i c a l l o g i s t i c ( w i t h p r e d a t i o n ) model F i g u r e IV-2 P h y t o p l a n k t o n i s o c l i n e p l u s - e n r i c h m e n t " model p r e d i c t e d by t h e " l o g i s t i c -OO 0) 69 where N ( f ) i s f r e e n u t r i e n t s , Ax i s n u t r i e n t s c o n t a i n e d i n p h y t o p l a n k t o n , and By i s n u t r i e n t s c o n t a i n e d i n z o o p l a n k t o n (A and B a r e n u t r i e n t c o n t e n t s per u n i t x and y, r e s p e c t i v e l y ) . T h i s assumes t h a t n u t r i e n t s c o n t a i n e d i n o t h e r forms, s u c h as f i s h , b a c t e r i a , d e t r i t i s or n e u s t o n a r e e i t h e r n e g l i g i b l e or u n a v a i l a b l e t o t h e p l a n k t o n s y s t e m and t h a t z o o p l a n k t o n , by way of e x c r e t i o n , a r e t h e major c o n t r i b u t o r s t o t h e f r e e n u t r i e n t p o o l . Ganf and B l a z k a (1974) f o u n d t h a t when t h e two t r o p h i c l e v e l s a r e t i g h t l y c o u p l e d , as t h e y p r e s u m a b l y would be i n summer, r a p i d t u r n o v e r of n u t r i e n t s o c c u r s and t h e amount of f r e e n u t r i e n t s ( N ( f ) ) p r e s e n t a t any d i s c r e t e t i m e i s n e g l i g i b l e . I f we assume t h a t n u t r i e n t i n p u t t o and l o s s from t h e e p i l i m n i o n a r e s m a l l and a p p r o x i m a t e l y e q u a l , as i s t h e c a s e i n summer when r a i n f a l l i s low, t h e n t h e t o t a l n u t r i e n t c o n t e n t of t h e e p i l i m n i o n c a n be a p p r o x i m a t e d by t h e sum of n u t r i e n t s t i e d up i n p h y t o p l a n k t o n and i n z o o p l a n k t o n . C o n v e r t i n g n u t r i e n t s t o p h y t o p l a n k t o n biomass e q u i v a l e n t s y i e l d s a c a r r y i n g c a p a c i t y , k, e x p r e s s e d as k = ax + by (5) where b = B/A. R e w r i t i n g e q u a t i o n 3 y i e l d s d x / d t = r x ( l - x / ( x + b y ) ) - cxy - sx (6) The model assumes t h a t t h e v a l u e s of " s " and "b" a r e c o n s t a n t b u t o f c o u r s e , i n a r e a l l a k e s i t u a t i o n , t h i s i s not 70 t r u e . S i n k i n g r a t e s v a r y w i t h 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 , w i t h s i z e and m o r p h o l o g y of c e l l s ( r e v i e w e d i n H u t c h i n s o n 1967), as w e l l as n u t r i t i o n a l s t a t e and g r o w t h p h a s e of c e l l s ( T i t m a n and K i l h a m 1976). The v a l u e of b (= B/A) may a l s o v a r y s i n c e t h e n i t r o g e n c o n t e n t of p h y t o p l a n k t o n c e l l s may v a r y c o n s i d e r a b l y w i t h n u t r i e n t a v a i l a b i l i t y ( S y r e t t 1962), r a n g i n g f r om 8-10 % when n u t r i e n t s a r e abundant t o 1-2 % when n u t r i e n t s a r e s c a r c e , as t h e y a r e i n summer. When z o o p l a n k t o n m e t a b o l i c demands a r e h i g h , t h a t r a t i o of n u t r i e n t s i n g e s t e d p e r u n i t z o o p l a n k t o n b i o m a s s , and hence t h e t o t a l n u t r i e n t c o n t e n t o f z o o p l a n k t o n b i o m a s s i s h i g h e r t h a n when m e t a b o l i c demands a r e low. M e t a b o l i c demands a r e r e f l e c t e d i n e x c r e t i o n r a t e s . T h e s e v a r y w i t h s p e c i e s and s i z e of o r g a n i s m s . S m a l l e r o r g a n i s m s , w h i c h r e s p i r e more q u i c k l y t h a n l a r g e r o n es, a l s o r e c y c l e n u t r i e n t s more q u i c k l y ( J o h a n n e s 1964, Ba r l o w and B i s h o p 1965, Ganf and B l a z k a , 1974, F e r r a n t e 1976). ( I n c i d e n t l y , T a y l o r and Le a n ( 1981 ) d i s a g r e e w i t h t h i s . They found' t h a t e x c r e t i o n r a t e s o f t h e c i l i a t e a n d ' r o t i f e r t h e y s t u d i e d were slow and c o m p a r a b l e t o t h o s e of c r u s t a c e a n z o o p l a n k t o n . They r e a s o n e d t h a t a l t h o u g h m i c r o z o o p l a n k t o n r e s p i r e a t h i g h e r r a t e s , t h e y a l s o have h i g h e r a s s i m i l a t i o n and growth e f f i c i e n c i e s . ) S c a v i a and M c F a r l a n d (1982) f o u n d t h a t t h e s t a g e o f l i f e h i s t o r y a l s o makes a d i f f e r e n c e i n t h e r a t e of n u t r i e n t r e l e a s e . Daphn i a magna p h o s p h o r u s r e l e a s e r a t e a v e r a g e d 6.7 t i m e s g r e a t e r d u r i n g and j u s t a f t e r e c d y s i s t h a n a t o t h e r t i m e s . R a t e s f o r f e m a l e s c a r r y i n g mature embryos were lo w e r t h a n f o r f e m a l e s c a r r y i n g y o u n g e r embryos. The n u t r i t i o n a l s t a t e of t h e a n i m a l s and v a l u e 71 of t h e i r f o o d a l s o a f f e c t n u t r i e n t e x c r e t i o n r a t e s . R e l e a s e of p h o s p h o r u s from s t a r v i n g a n i m a l s i s l e s s t h a n t h a t from w e l l - f e d ones ( R i g l e r 1964). S c a v i a and G a r d n e r (1982) f o u n d t h a t e x c r e t i o n r a t e s f o r z o o p l a n k t o n f e d d e t r i t u s and b a c t e r i a were h i g h e r than f o r z o o p l a n k t o n f e d a l g a e . Ganf and B l a z k a (1974) f o u n d t h a t e x c r e t i o n and r e s p i r a t i o n r a t e s a r e t e m p e r a t u r e d e p e n d e n t and v a r y d i u r n a l l y , p o s s i b l y due t o d i u r n a l f e e d i n g p a t t e r n s . D e s p i t e a l l t h e s e r e a s o n s why s and b s h o u l d not be c o n s t a n t o v e r l o n g t i m e s c a l e s , I f e e l t h a t i t i s v a l i d t o use them as c o n s t a n t s i n c o m p a r i n g t h e r e s u l t s between e x p e r i m e n t a l e n c l o s u r e s . S i n c e t h e z o o p l a n k t o n and p h y t o p l a n k t o n f o r t h e e n c l o s u r e s i n e a c h of t h e e x p e r i m e n t s were c o l l e c t e d a t t h e same ti m e from t h e same s o u r c e , t h e c o m p o s i t i o n o f t h e i n i t i a l z o o p l a n k t o n and p h y t o p l a n k t o n a s s e m b l a g e s s h o u l d have d i f f e r e d l i t t l e between e n c l o s u r e s . P h y s i c a l c o n d i t i o n s s u c h as r a d i a t i o n and t e m p e r a t u r e a l s o s h o u l d not have d i f f e r e d s i g n i f i c a n t l y between e n c l o s u r e s w i t h i n a g i v e n e x p e r i m e n t . T h e r e f o r e , t h e v a l u e s of a and b s h o u l d be a p p r o x i m a t e l y e q u a l f o r a l l t h e e n c l o s u r e s . The o n l y p o s s i b l e s o u r c e of v a r i a n c e was t h e n u t r i t i o n a l s t a t e of t h e z o o p l a n k t o n , s i n c e c o m p e t i t i o n f o r f o o d may have v a r i e d between e n c l o s u r e s and m i g h t a c t u a l l y v a r y c o n s i d e r a b l y d u r i n g t h e c o u r s e of t h e e x p e r i m e n t . The g e n e r a l shape o f t h e p h y t o p l a n k t o n i s o c l i n e ( d x / d t = 0) i s shown i n F i g u r e IV-2. The c u r v a t u r e i n t h e i s o c l i n e i s a r e s u l t of t h e b a l a n c e between t h e n e g a t i v e e f f e c t o f z o o p l a n k t o n on p h y t o p l a n k t o n b i o m a s s by g r a z i n g and t h e p o s i t i v e e f f e c t by 72 n u t r i e n t r e m i n e r a l i z a t i o n . At h i g h z o o p l a n k t o n d e n s i t i e s , t h e l o s s of p h y t o p l a n k t o n due t o g r a z i n g i s g r e a t e r t h a n p h y t o p l a n k t o n p r o d u c t i v i t y . At low z o o p l a n k t o n d e n s i t i e s , when p h y t o p l a n k t o n a r e l a r g e l y d e pendent on n u t r i e n t s r e m i n e r a l i z e d by z o o p l a n k t o n , g r o w t h r a t e s of p h y t o p l a n k t o n a r e low due t o low a v a i l a b i l i t y o f n u t r i e n t s , and t h e l o s s of p h y t o p l a n k t o n due t o s i n k i n g i s g r e a t e r t h a n p h y t o p l a n k t o n p r o d u c t i v i t y . An a s p e c t o f p h y t o p l a n k t o n d y n a m i c s w h i c h must be c o n s i d e r e d i s t h e f u n d a m e n t a l d i f f e r e n c e i n d y n a m i c s between c o l o n i a l b l u e - g r e e n a l g a e , w h i c h make up an o v e r w h e l m i n g m a j o r i t y of t h e c o l o n y c a t e g o r y i n P l a c i d L a k e , and s i n g l e -c e l l e d e u k a r y o t i c p h y t o p l a n k t o n . The a b i l i t y of many b l u e -g r e e n s t o f i x a t m o s p h e r i c n i t r o g e n p r o v i d e s them w i t h a s o u r c e o f n i t r o g e n when b i o l o g i c a l l y r e a c t i v e forms of n i t r o g e n s u c h as ammonia and n i t r a t e a r e s c a r c e . A l s o , many s m a l l b l u e - g r e e n s have been f o u n d t o p o s s e s s s u p e r i o r a b i l i t y t o t a k e up p h o s p h a t e s a t e x t r e m e l y low c o n c e n t r a t i o n s o f t h a t n u t r i e n t (Lehman 1982). T h e s e two f a c t o r s p r e v e n t b l u e - g r e e n s from b e i n g n u t r i e n t l i m i t e d under low n u t r i e n t c o n d i t i o n s t o t h e same e x t e n t t h a t e u k a r y o t i c a l g a e a r e . A n o t h e r d i f f e r e n c e i n t h e i r d y n a m i c s was d i s c u s s e d i n C h a p t e r I I I . I t i s p r o b a b l e t h a t many s p e c i e s of c o l o n i a l b l u e - g r e e n a l g a e , i n c l u d i n g M e r i s m o p e d i a , t h e most common i n P l a c i d L a k e , do not e x p e r i e n c e g r a z i n g p r e s s u r e t o any a p p r e c i a b l e e x t e n t and c e r t a i n l y l e s s t h a n most s i n g l e - c e l l e d e u k a r y o t i c p h y t o p l a n k t o n s p e c i e s . F o r t h e s e r e a s o n s , i t i s d o u b t f u l t h a t a c l o s e c o u p l i n g between c o l o n i a l b l u e - g r e e n a l g a e and h e r b i v o r o u s z o o p l a n k t o n e x i s t s , and t h e 73 model p r e s e n t e d i s u n l i k e l y t o be a r e a s o n a b l e a p p r o x i m a t i o n of t h e p o p u l a t i o n d y n a m i c s o f some c o l o n i a l b l u e - g r e e n a l g a e . However, many of t h e c o l o n i a l a l g a e a r e not b l u e - g r e e n s , and s i n c e d i s t i n c t i o n s were not made between s p e c i e s i n p h y t o p l a n k t o n c o u n t s , i t i s i m p o s s i b l e t o e s t i m a t e what p r o p o r t i o n of t h e c o l o n i a l p h y t o p l a n k t o n a t any p a r t i c u l a r t i m e were u n g r a z a b l e , n i t r o g e n - f i x i n g f o r m s . Even i f a l l c o l o n i a l b i o m a s s c o m p r i s e d u n g r a z a b l e , n i t r o g e n - f i x i n g forms, we would s t i l l be l e f t w i t h t h e p r o b l e m of how t o s e p a r a t e t h e d y n a m i c s of t h e s e two d i s t i n c t i v e g r o u p s of p h y t o p l a n k t o n . A s i m p l e s o l u t i o n m i g h t be t o i g n o r e t h e c o l o n i a l p h y t o p l a n k t o n a l t o g e t h e r and d e a l o n l y w i t h t h e s i n g l e -c e l l e d p h y t o p l a n k t o n . T h i s might be an a d e q u a t e a p p r o a c h i f i n t e r a c t i o n s between t h e two g r o u p s were m i n i m a l . G l i w i c z and S i e d l a r (1980) have f o u n d t h a t t h e p r e s e n c e of h i g h d e n s i t i e s of g e l a t i n o u s c o l o n i a l a l g a e may i n t e r f e r e w i t h n o r m a l f i l t e r i n g by z o o p l a n k t o n and t h e r e f o r e might r e d u c e t h e g r a z i n g p r e s s u r e on some c o n s t i t u e n t s of t h e s i n g l e - c e l l e d p h y t o p l a n k t o n . A l t h o u g h many c o l o n i a l b l u e - g r e e n a l g a e may be s e l f - s u f f i c i e n t i n n i t r o g e n , c o m p e t i t i o n w i t h s i n g l e - c e l l e d p h y t o p l a n k t o n f o r o t h e r n u t r i e n t s , p a r t i c u l a r l y p h o s p h a t e , must o c c u r . A more complex s o l u t i o n would be t o l o o k a t t h e s y s t e m i n t h r e e d i m e n s i o n s , s e p a r a t i n g t h e d y n a m i c s of s i n g l e - c e l l e d and c o l o n i a l p h y t o p l a n k t o n : d x / d t = rx (1 - x / ( x + by - e z ) ) - cxy - sx (7) 74 where x i s t h e s i n g l e - c e l l e d p h y t o p l a n k t o n p o p u l a t i o n , r t h e i n t r i n s i c g r o w t h r a t e of t h e s i n g l e - c e l l e d p h y t o p l a n k t o n , and z i s t h e c o l o n i 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 . B e c a u s e of n u t r i e n t l e a k a g e from b l u e - g r e e n c e l l s , a u t o l y s i s and 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 b a c t e r i a , some o f t h e n u t r i e n t s from b l u e -g r e e n a l g a e w i l l become a v a i l a b l e f o r o t h e r p h y t o p l a n k t o n g r o w t h . G i v e n t h a t c o l o n i a l b l u e - g r e e n a l g a e a r e not g r a z a b l e , a s i g n i f i c a n t p o r t i o n o f t h e n u t r i e n t s t a k e n up by t h e s e c e l l s w i l l be l o s t from t h e p h y t o p l a n k t o n - z o o p l a n k t o n community and t h e r e f o r e must be s u b t r a c t e d from t h e c a r r y i n g c a p a c i t y f o r e d i b l e p h y t o p l a n k t o n . "e" i s t h e p r o p o r t i o n of n u t r i e n t s bound by b l u e - g r e e n a l g a e w h i c h w i l l be l o s t from t h e p h y t o p l a n k t o n -z o o p l a n k t o n community. The e f f e c t s t h i s a d d i t i o n have on t h e d y n a m i c s o f t h e s i n g l e - c e l l e d p h y t o p l a n k t o n a r e shown i n F i g u r e I V - 3 . The maximum v a l u e of x, t h e v a l u e of y a t which t h i s o c c u r s , and t h e l o w e r y - i n t e r c e p t , a r e a f f e c t e d , but t h e g e n e r a l shape of t h e f u n c t i o n i s not c h a n g e d , nor i s t h e upper y-i n t e r c e p t . The e f f e c t s o f t h e p r e s e n c e of g e l a t i n o u s c o l o n i a l a l g a e on t h e f i l t e r i n g r a t e s of z o o p l a n k t o n a r e s t i l l not d e a l t w i t h ; t h a t would add g r e a t e r c o m p l e x i t y , as would t h e a d d i t i o n of t h e e f f e c t s of v a r y i n g d e n s i t i e s o f c a r n i v o r o u s z o o p l a n k t o n . The f a c t o r s not d e a l t w i t h i n t h i s s i m p l e model a r e l e g i o n . However, b e c a u s e o f t h e g e n e r a l c r u d e n e s s o f t h e e x p e r i m e n t a l d a t a , i t i s u n l i k e l y t h a t even th e r e f i n e m e n t o f e q u a t i o n (7) o v e r e q u a t i o n (6) i s n e c e s s a r y , o r t h a t t h e added c o m p l e x i t y w i l l be s u b s t a n t i a t e d by t h e d a t a . A s i m p l e a l t e r n a t i v e , and t h e one u s e d , i s t o q u a l i t a t i v e l y a s s e s s whether c o l o n i a l a l g a e 75 F i g u r e I V - 3 T h e e f f e c t o f c o l o n i a l b l u e - g r e e n a l g a e o n p h y t o p l a n k t o n i s o c l i n e s a s p r e d i c t e d b y t h e " l o g i s t i c -p l u s - e n r i c h m e n t " m o d e l p r e y ( x ) 76 appear to f o l l o w the dynamics of grazable, non-nitrogen f i x i n g phytoplankton, as p r e d i c t e d by the model, and i f they do not, whether they have any appreciable e f f e c t on the growth of the s i n g l e - c e l l e d phytoplankton biomass. The f o l l o w i n g steps were used to c a l c u l a t e values of the model constants. Under c o n d i t i o n s of u n l i m i t e d , l o g i s t i c growth, i n the absence of grazers: dx/dt = rx. (8) a We must assume that since the c e l l s are g e n e r a l l y q u i t e small and they are growing at a r a p i d r a t e , s i n k i n g i s n e g l i g i b l e (Titman a X ( t ) = X(0) e x p ( r t ) . (9) If values f o r X(t) and X(0) can be obtained, we can solve f o r r: r = l n ( X ( t ) / X ( 0 ) ) / t . (10) Under c o n d i t i o n s without n u t r i e n t l i m i t a t i o n i n the presence of grazers: dx/dt = rx - cxy. (11) The s o l u t i o n of t h i s equation 77 X ( t ) = X ( 0 ) x e ( e x p ( r t - c y ) ) . (12) G i v e n v a l u e s f o r y, X ( 0 ) , and X ( t ) , and u s i n g t h e c a l c u l a t e d v a l u e f o r r : c = ( r - l n ( X ( t ) / X ( 0 ) ) ) / y . (13) Under c o n d i t i o n s of extreme n u t r i e n t l i m i t a t i o n , w i t h o u t g r a z e r s , we assume t h a t c e l l g r o w t h i s n e g l i g i b l e , and: d x / d t = - s x , (14) where s i s a s i n k i n g r a t e . F i n a l l y , under c o n d i t i o n s of n u t r i e n t l i m i t a t i o n , w i t h g r a z e r s : d x / d t = rx - r x 2 / ( x + by) - c x y - sx (15) B e c a u s e t h i s f u n c t i o n i s u n s o l v a b l e by n o r m a l i n t e g r a t i o n , t h e R u n g e - K u t t a method of n u m e r i c a l a n a l y s i s ( R o s s e r and de Boor 1979) i s u s e d t o s o l v e f o r X ( t ) . T h i s i s done f o r a v a r i e t y o f v a l u e s of b and t h e v a l u e of b w h i c h r e s u l t s i n t h e l e a s t sum o f s q u a r e s of t h e d i f f e r e n c e between c a l c u l a t e d and o b s e r v e d v a l u e s f o r X i s c h o s e n as t h e b e s t e s t i m a t e of t h e c o n s t a n t v a l u e . 78 A Model F o r Z o o p l a n k t o n Dynamics L i t t l e has been s a i d u n t i l now about z o o p l a n k t o n d y n a m i c s . Two t r a d i t i o n a l s i m p l e m o d e l s e x i s t w h i c h may e x p l a i n z o o p l a n k t o n d y n a m i c s ( S m i t h 1974). The c l a s s i c a l " L o t k a V o l t e r r a " model i s a r e a s o n a b l e s t a r t i n g p o i n t : d y / d t = gxy - my (16) where y i s t h e p r e d a t o r p o p u l a t i o n , x t h e p r e y p o p u l a t i o n , g t h e f i l t e r i n g r a t e t i m e s t h e f o o d c o n v e r s i o n e f f i c i e n c y , and m t h e m o r t a l i t y r a t e . T h i s e q u a t i o n p r o d u c e s a t h r e s h h o l d p r e y d e n s i t y a t w h i c h no g r o w th i n t h e p r e d a t o r p o p u l a t i o n o c c u r s ( F i g u r e I V - 4 ) . . I f t h i s model i s v a l i d f o r p l a n k t o n d y n a m i c s , t h e n i n e n c l o s u r e e x p e r i m e n t s , i n i t i a l i n c r e a s e i n z o o p l a n k t o n b i o m a s s s h o u l d o c c u r whenever p h y t o p l a n k t o n d e n s i t y e x c e e d s a c e r t a i n t h r e s h h o l d (x = m/g), i n d e p e n d e n t o f t h e z o o p l a n k t o n b i o m a s s . A s e c o n d model ( F i g u r e IV-5) r e p r e s e n t s a s i t u a t i o n where t h e p r e d a t o r has a c o n s t a n t f o o d i n t a k e , r e g a r d l e s s of t h e p r e y d e n s i t y . R a t h e r t h a n b e i n g l i m i t e d by f o o d a v a i l a b i l i t y , t h e p r e d a t o r i s s e l f - l i m i t e d , p r o b a b l y by some k i n d of b e h a v i o u r a l c o n s t r a i n t s u c h as t e r r i t o r i a l i t y . T h i s i s e x p r e s s e d a s : d y / d t = gy - ey - d y 2 (17) 7 9 F i g u r e IV-4 P r e d a t o r i s o c l i n e as p r e d i c t e d by the V o l t e r r a p r e d a t o r (Prey t h r e s h h o l d ) model F i g u r e IV-5 P r e d a t o r i s o c l i n e as p r e d i c t e d by the p r e d a t o r t h r e s h h o l d model p r e d a t o r ( y ) CD GL < -—, OL II o 0 p r e d a t o r ( y ) dy/dt=0 80 The p r e y p o p u l a t i o n i s not c o n t r o l l e d by t h e p r e d a t o r ; i t t o o must be s e l f - l i m i t i n g . A l t h o u g h f e e d i n g r a t e s a r e r e l a t i v e l y c o n s t a n t , 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 a r e s e l f - r e g u l a t i n g . T h e i r d i s t r i b u t i o n , p a r t i c u l a r l y i n an o l i g o t r o p h i c l a k e t e n d s t o be s p a r s e , so s p a c e i s h a r d l y a c o n s t r a i n t t o t h e i r g r o w t h . 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 t h i s model p r o v i d e s an a d e q u a t e d e s c r i p t i o n of z o o p l a n k t o n d y n a m i c s . An a l t e r n a t i v e model ( F i g u r e IV-6) r e p r e s e n t s a s i t u a t i o n where . p r e d a t o r s r e q u i r e a minimum r a t i o n f o r g r o w t h . I t i s e x p r e s s e d a s : d y / d t = gxy(1 - ( y / x ) / k ) - my) (18) where k i s a maximum s u s t a i n a b l e p r e d a t o r : p r e y r a t i o ( i f m = 0) and i s f u n c t i o n a l l y t h e c o n v e r s i o n e f f i c i e n c y o f p h y t o p l a n k t o n b i o m a s s t o z o o p l a n k t o n b i o m a s s . T h i s p r o d u c e s a t h r e s h h o l d r a t i o above w h i c h t h e r e i s n e t d e c r e a s e i n p r e d a t o r p o p u l a t i o n s i z e . I f t h i s model i s v a l i d f o r p l a n k t o n d y n a m i c s , u n l i k e t h e V o l t e r r a model, i n i t i a l c h a n g e s i n z o o p l a n k t o n biomass w i l l be d e p e n d e n t on b o t h i n i t i a l 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 b i o m a s s . B o t h t h e c l a s s i c a l L o t k a - V o l t e r r a p r e d a t o r - p r e y m o d els and t h e a l t e r n a t i v e s w h i c h I have p r e s e n t e d p r e d i c t p r e d a t o r - p r e y o s c i l l a t i o n s w h i c h , p l o t t e d on a p r e d a t o r - p r e y phase d i a g r a m , t r a v e l i n a c o u n t e r - c l o c k w i s e d i r e c t i o n a r o u n d a s t a b l e e q u i l i b r i u m p o i n t ( F i g u r e s I V - 7 , 8 ) . g u r e I V - 6 P r e d a t o r i s o c l i n e p r e d i c t e d by t h e p r e d a t o r / p t h r e s h h o l d model p r e d a t o r ( y ) 82 F i g u r e IV-7 P r e d a t o r - p r e y d y n a m i c s p r e d i c t e d by t h e L o t k a -V o l t e r r a model F i g u r e IV-8 P r e d a t o r - p r e y d y n a m i c s p r e d i c t e d by t h e " l o g i s t i c -p l u s - e n r i c h m e n t " p r e y model and t h e p r e d a t o r / p r e y t h r e s h h o l d p r e d a t o r model p r e d a t o r ( y ) 83 METHODS AND MATERIALS C o n s t r u c t i o n of e n c l o s u r e s was d e s c r i b e d i n C h a p t e r I I . E x p e r i m e n t s u s i n g 1200 L e n c l o s u r e s were run a t f o u r d i f f e r e n t t i m e s . The g e n e r a l s e t up o f t h e e x p e r i m e n t s was s i m i l a r each t i m e . Lake water was pumped i n t o t h e e n c l o s u r e s , f i l t e r e d f i r s t t h r o u g h a 54 Mm mesh t o remove z o o p l a n k t o n . Then v a r y i n g numbers of c o l l e c t e d z o o p l a n k t o n were added t o e a c h e x p e r i m e n t (see T a b l e s I V - 1 , a - d ) . F o r t h r e e of t h e e x p e r i m e n t s , s i x d i f f e r e n t z o o p l a n k t o n d e n s i t i e s i n c l u d i n g a "no z o o p l a n k t o n " t r e a t m e n t were s e t up, w i t h two e n c l o s u r e s f o r e a c h d e n s i t y . F o r one o f t h e e x p e r i m e n t s I had hoped t o s e t up c o m b i n a t i o n s of t h r e e p h y t o p l a n k t o n d e n s i t i e s and t h r e e z o o p l a n k t o n d e n s i t i e s . My method f o r a l t e r i n g p h y t o p l a n k t o n d e n s i t y was u n s a t i s f a c t o r y so I was l e f t w i t h s i x r e p l i c a t e s o f t h r e e d i f f e r e n t z o o p l a n k t o n d e n s i t i e s . E a c h e n c l o s u r e was sampled f o r p h y t o p l a n k t o n p r i o r t o t h e a d d i t i o n of a n i m a l s . I n t h e f i r s t e x p e r i m e n t (14 J u l y 25 A u g u s t , 1980), 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 were sampled a t sev e n day i n t e r v a l s . A f t e r e x a m i n i n g t h e r e s u l t s I d e c i d e d t o sample more o f t e n i n s u b s e q u e n t e n c l o s u r e e x p e r i m e n t s , p a r t i c u l a r l y i n t h e i n i t i a l s t a g e s . In o r d e r t o o b s e r v e t h e i n i t i a l e f f e c t s of v a r y i n g z o o p l a n t k o n d e n s i t i e s on t h e p h y t o p l a n k t o n p o p u l a t i o n , t h e s e c o n d e x p e r i m e n t (30 A p r i l - 5 May, 1981) was r u n o n l y f i v e d a y s and 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 a t t w e l v e hour i n t e r v a l s . Z o o p l a n k t o n was sampl e d o n l y on t h e s e c o n d and l a s t d a y s . In t h e t h i r d e x p e r i m e n t (20 May - 2 J u l y , 1981), e n c l o s u r e s were sampled f o r p h y t o p l a n k t o n e v e r y t w e l v e h o u r s f o r t h e f i r s t 96 h o u r s ; f o r t h e f o u r t h e x p e r i m e n t T A B L E I V - l a - E x p e r i m e n t 1 E n c l o s u r e s t o c k e d i n i t i a l z o o p l a n k t o n b i o m a s s ( u g / 1 ) 1 0 2 40 3 10 5 80 7 20 8 0 10 3 13 3 14 10 15 40 1 6 80 18 20 a c t u a l i n i t i a l z o o p l a n k t o n b i o m a s s ( u g / 1 ) 0.3 4 9 . 8 9.0 113.2 11.4 2.0 2 . 4 4 . 9 13.2 37 . 3 75.5 13.2 85 TABLE I V - l b - E x p e r i m e n t 2 E n c l o s u r e s t o c k e d i n i t i a l a c t u a l i n i t i a l z o o p l a n k t o n biomass z o o p l a n k t o n b i o m a s s (ug/1) (ug/1) 1 50 36.2 2 5.0 7.1 3 2.5 3.0 4 0.0 0.0 5 25 27.1 6 2.5 1.7 7 5.0 4.5 8 0.5 1.2 9 50 66.9 10 0.0 0.0 11 0.5 0.0 12 25 32.7 TABLE I V - l c - E x p e r i m e n t 3 E n c l o s u r e s t o c k e d i n i t i a l a c t u a l i n i t i a l z o o p l a n k t o n b i o m a s s z o o p l a n k t o n biomass (ug/1) (ug/1) 1 0.5 0.7 2 5 7 3 0.5 0.6 4 0.5 0.2 5 50 49.6 6 0.5 0.5 7 50 45.7 8 0.5 0.5 9 50 24.9 10 5 5.5 11 5 4.4 12 50 33.4 13 5 7.6 14 0.5 1.1 15 50 37.4 16 5 7.4 17 50 27.4 18 5 2.1 87 TABLE I V - l d - E x p e r i m e n t 4 E n c l o s u r e s s t o c k e d i n i t i a l a c t u a l i n i t i a l z o o p l a n k t o n b i o m a s s z o o p l a n k t o n b i o m a s s ug/1 (ug/1) 1 7.5 10.5 2 1.5 1.6 3 3.5 3.1 4 3.5 4.4 5 1.5 1.2 6 0 0.4 7 15 14.9 8 7.5 5.1 9 35 33.6 10 0 1.0 11 15 16.5 12 35 31.0 88 (13 A u g u s t - 26 September, 1981), I was a b l e t o sample more o f t e n , e v e r y f o u r t o s i x h o u r s f o r t h e f i r s t 72 h o u r s . A f t e r t h i s i n i t i a l p e r i o d , - p h y t o p l a n k t o n samples were t a k e n e v e r y t h r e e days and z o o p l a n k t o n s a m p l e s e v e r y 6 d a ys f o r b o t h t h e t h i r d and f o u r t h e x p e r i m e n t s . The K r u s k a l - W a l l i s t e s t was u s e d t o d e t e r m i n e whether i n i t i a l z o o p l a n k t o n b i o m a s s f o r t h e v a r i o u s t r e a t m e n t s were s i g n i f i c a n t l y d i f f e r e n t from e a c h o t h e r . S t a t i s t i c a l a n a l y s i s i n v o l v e d two-way a n a l y s i s of v a r i a n c e e x a m i n i n g t h e e f f e c t s of t r e a t m e n t ( z o o p l a n k t o n b i o m a s s ) o v e r t i m e on t h e b i o m a s s of p h y t o p l a n k t o n i n e a c h p h y t o p l a n k t o n s i z e c a t e g o r y f o r e a c h of t h e e x p e r i m e n t s . Log t r a n s f o r m a t i o n s were done on a l l p h y t o p l a n k t o n b i o m a s s d a t a t o s a t i s f y t h e a n a l y s i s of v a r i a n c e r e q u i r e m e n t f o r n o r m a l d i s t r i b u t i o n o f d a t a . Two-way a n a l y s i s of v a r i a n c e was u s e d t o d e t e r m i n e s i g n i f i c a n t d i f f e r e n c e s between t r e a t m e n t s i n c h a n g es 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 , as w e l l as b i o m a s s of D a p h n i a , H o l o p e d i u m , s m a l l c l a d o c e r a n s , c a l a n o i d c o p e p o d s , t o t a l h e r b i v o r e s , and c y c l o p o i d c o p e p o d s f o r t h e l o n g - t e r m e n c l o s u r e r e s u l t s . I n i t i a l r e s p o n s e 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 b i o m a s s i n e a c h e n c l o s u r e were t e s t e d f o r s i g n i f i c a n c e u s i n g Spearman's rank c o r r e l a t i o n c o e f f i c i e n t s . B ecause t h i s i s a s i m p l e non-p a r a m e t r i c t e s t b a s e d on r a n k s , no t r a n s f o r m a t i o n s o f d a t a were n e c e s s a r y . F o r e a c h e n c l o s u r e , a s e r i e s of t e s t s , b e g i n n i n g w i t h t h e f i r s t t h r e e p h y t o p l a n k t o n o b s e r v a t i o n s , were done f o r i n c r e a s i n g numbers o f c o n s e c u t i v e o b s e r v a t i o n s , u n t i l a s i g n i f i c a n t r e s p o n s e was d e t e c t e d . T e s t s on z o o p l a n k t o n d a t a were done a t 5 d a y s , 21 d a y s , and 14 d a y s r e s p e c t i v e l y f o r 89 e x p e r i m e n t s 2, 3, and 4. One-way a n a l y s i s of v a r i a n c e , t o d e t e r m i n e s i g n i f i c a n t change o v e r t i m e f o r t o t a l , s i n g l e - c e l l e d and c o l o n i a l p h y t o p l a n k t o n , was a l s o done on l o n g - t e r m o b s e r v a t i o n s f o r e a c h e n c l o s u r e . These d a t a were t r a n s f o r m e d u s i n g n a t u r a l l o g a r i t h m s . I n i t i a l z o o p l a n k t o n and p h y t o p l a n k t o n b i o m a s s p o i n t s f o r e a c h e n c l o s u r e and g e n e r a l d i r e c t i o n s of c h a n g e o f 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 c r e a s e o r d e c r e a s e ) were p l o t t e d on z o o p l a n k t o n - p h y t o p l a n k t o n a x e s f o r t h e s h o r t t e r m o b s e r v a t i o n s of e x p e r i m e n t t h r e e (May - J u l y 1981) a n d t h e l o n g term o b s e r v a t i o n s of e x p e r i m e n t s one ( J u l y - A u g u s t 1980), t h r e e (May - J u l y 1981), and f o u r ( August - September 1 9 8 1 ) . In l a t e August 1981, e i g h t 300 L e n c l o s u r e s were s e t up. F o u r were " e n r i c h e d " by i n c u b a t i n g them w i t h h i g h d e n s i t i e s (500/1) o f z o o p l a n k t o n f o r 48 h o u r s ; a f t e r t h i s t i m e , z o o p l a n k t o n were f i l t e r e d o ut of t h e w a t e r . P h y t o p l a n k t o n d e n s i t i e s were t h u s r e d u c e d by g r a z i n g and f r e e n u t r i e n t l e v e l s were p r e s u m a b l y e l e v a t e d t h r o u g h t h e e x c r e t i o n by t h e z o o p l a n k t o n . Of t h e f o u r e n r i c h e d and f o u r " n o r m a l " e n c l o s u r e s , two o f e a c h c o n t a i n e d no z o o p l a n k t o n and two c o n t a i n e d n o r m a l l a k e d e n s i t i e s of z o o p l a n k t o n . Once z o o p l a n k t o n had been added t o e n c l o s u r e s , 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 e v e r y 4 t o 6 h o u r s f o r 46 h o u r s . I n i t i a l p h y t o p l a n k t o n g r o w t h r e s p o n s e s i n t h e e n r i c h e d e n c l o s u r e s were u s e d t o c a l c u l a t e t h e i n t r i n s i c g r o w t h r a t e , r . The d i f f e r e n c e i n p h y t o p l a n k t o n growth i n e n c l o s u r e s w i t h and w i t h o u t z o o p l a n k t o n was u s e d t o c a l c u l a t e c, t h e f i l t e r i n g r a t e . (An " r " v a l u e was a l s o c a l c u l a t e d from t h e 90 i n i t i a l g r o w t h i n n o r m a l 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 , i n o r d e r t o be a b l e t o c a l c u l a t e a f i l t e r i n g r a t e f o r t h e s e e n c l o s u r e s t o s u p p l e m e n t th e one c a l c u l a t e d from t h e e n r i c h e d e n c l o s u r e s . ) R a t e s of d e c l i n e t h a t f o l l o w e d i n i t i a l g rowth i n b o t h e n r i c h e d and u n e n r i c h e d e n c l o s u r e s were u s e d t o c a l c u l a t e s i n k i n g r a t e s , " s " . P h y t o p l a n k t o n p o p u l a t i o n c h a n ges i n u n e n r i c h e d e n c l o s u r e s w i t h z o o p l a n k t o n were u s e d t o c a l c u l a t e two s e t s o f v a l u e s f o r "b", u s i n g b o t h f i l t e r i n g r a t e s . Model p a r a m e t e r v a l u e s were c a l c u l a t e d f o r b o t h t o t a l p h y t o p l a n k t o n and s i n g l e - c e l l e d p h y t o p l a n k t o n o n l y . RESULTS S h o r t - t e r m c h a n g 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 e n c l o s u r e s f o r e x p e r i m e n t s 2 ( A p r i l - May 1981), 3 (May 1981) and 4 ( A u g u s t 1981) a r e shown i n A p p e n d i x V. L o n g - t e r m c h a n g e s i n 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 f o r e x p e r i m e n t s 1 ( J u l y -September 1980), 3 (May - J u l y 1981) and 4 ( A u g u s t - September 1981) a r e shown i n A p p e n d i x V I . Z o o p l a n k t o n Dynamics K r u s k a l - W a l l i s t e s t s c o n f i r m e d t h a t t h e d i f f e r e n c e s i n z o o p l a n k t o n c o n c e n t r a t i o n s between t h e t r e a t m e n t s f o r e a c h of t h e e x p e r i m e n t s were s i g n i f i c a n t . K r u s k a l - W a l l i s t e s t s were a l s o done on s u b s e q u e n t d a t e s f o r e a c h o f e x p e r i m e n t s 1, 3, and 4. F o r e x p e r i m e n t 1, z o o p l a n k t o n 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 between t r e a t m e n t s f o r t h e f i r s t t h r e e weeks; a f t e r 91 t h i s t i m e , r e c r u i t m e n t i n low i n i t i a l z o o p l a n k t o n e n c l o s u r e s and m o r t a l i t y i n h i g h i n i t i a l z o o p l a n k t o n e n c l o s u r e s o b l i t e r a t e d d i f f e r e n c e s between t r e a t m e n t s . In s p i t e of c h a n g e s i n t h e z o o p l a n k t o n c o m m u n i t i e s , z o o p l a n k t o n d e n s i t i e s r e m a i n e d s i g n i f i c a n t l y d i f f e r e n t between t r e a t m e n t s i n e x p e r i m e n t s 3 and 4 f o r t h e f u l l 6 weeks of t h e e x p e r i m e n t s . S i g n i f i c a n t c h a n g e s o c c u r r e d i n t h e t o t a l h e r b i v o r o u s z o o p l a n k t o n b i o m a s s of a l m o s t a l l t h e e n c l o s u r e s i n a l l t h e e x p e r i m e n t s . C o n s i s t e n t l y , t h e t r e n d was f o r i n c r e a s i n g 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 w i t h low t o medium i n i t i a l z o o p l a n k t o n d e n s i t i e s , and d e c r e a s i n g i n t h o s e w i t h h i g h i n i t i a l d e n s i t i e s . Changes i n d e n s i t i e s of t h e v a r i o u s z o o p l a n k t o n g r o u p s f o r e a c h t r e a t m e n t of e a c h e x p e r i m e n t a r e shown i n T a b l e I V - 2 , a - c . Changes i n . c l a d o c e r a n : c o pepod, h e r b i v o r e : e a r n i voire and "warm":"cool" c l a d o c e r a n r a t i o s a r e shown i n T a b l e I V - 3 , a - c . In e x p e r i m e n t 1 ( J u l y - A u g u s t 1980) a l l t h e z o o p l a n k t o n g r o u p s t e s t e d e x h i b i t e d 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 s , and a l l b e h a v e d s i m i l a r l y . At low i n i t i a l z o o p l a n k t o n d e n s i t i e s , t h e y e i t h e r i n c r e a s e d or d i d not change s i g n i f i c a n t l y ; a t h i g h i n i t i a l z o o p l a n k t o n d e n s i t i e s , a l l e x c e p t t h e s m a l l c l a d o c e r a n s d e c r e a s e d s i g n i f i c a n t l y . No r e g u l a r p a t t e r n s of z o 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 change were o b s e r v e d . The r a t i o of c l a d o c e r a n s t o c o p e p o d s i n c r e a s e d s i g n i f i c a n t l y o n l y i n t h e z o o p l a n k t o n e x c l u s i o n t r e a t m e n t . The r a t i o of "warm" c l a d o c e r a n s p e c i e s ( D a p h n i a , Diaphanosoma, C e r i o d a p h n i a , Bosmina, and C h y d o r i s ) t o t h e " c o o l " s p e c i e s ( H o l o p e d i u m ) i n c r e a s e d i n t h e z o o p l a n k t o n TABLE IV-2a:.. E x p e r i m e n t 1 ( J u l y - A u g u s t 1980) Changes i n z o 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 t r e a t m e n t ( i n i t i a l 0 3 10 s p e c i e s D a p h n i a r o s e a 0 + 0 H o l o p e d i u m gibberum 0 0 0 s m a l l c l a d o c a r n s + + 0 Diaptomus o r e g o n e n s i s 0 0 0 c y c l o p o i d copepods 0 0 0 t o t a l h e r b i v o r s + + 0 + - i n c r e a s e - - d e c r e a s e 0 - no change S - s i g n i f i c a n t at p=.05 NS - not s i g n i f i c a n t z o o p l a n k t o n b i o m a s s , ug/1) 20 40 80 t r e a t m e n t e f f e c t + - - S 0 S 0 0 0 S 0 S - - - S 0 - - S TABLE IV-2b E x p e r i m e n t 3 ( M a y - J u l y 1981) "Changes i n zooplankton species composition t r e a t m e n t s p e c i e s 0.5 D a p h n i a r o s e a + Ho l o p e d ium gibberum 0 s m a l l c l a d o c e r a n s + Diaptomus o r e g o n e n s i s 0 c y c l o p o i d copepods + t o t a l h e r b i v o r e s ++ + - i n c r e a s e - - d e c r e a s e 0 - no change S - s i g n i f i c a n t at p=.05 NS - not s i g n i f i c a n t i n i t i a l z o o p l a n k t o n b i o m a s s , ug/1) 5 50 t r e a t m e n t e f f e c t + + NS - - S + + NS + + S + 0 S + 0 s TABLE I V - 2c Experiment 4 (August - September 1981) ,Changes i n zooplankton species composition treatment ( i n i t i a l zooplankton biomass, ug/1) species 0 1.5 3.5 7.5 15 35 treatment e f f e c t s Daphnia rosea 0 + + 0 0 - S Holopedium gibberum 0 0 - - - - NS small cladocerans + + 0 0 0 0 s Diaptomus oregonensis ++ -H- + + + + s cyclopoid copepods + + 0 4- NS t o t a l herbivores + + + 0 0 0 S + - increase 0 - no change - - decrease S - s i g n i f i c a n t at p=.05 NS - not s i g n i f i c a n t -p-TABLE IV -3a Experiment 1 (July-August 1981) Changes i n zooplankton r a t i o s i n enclosure experiments treatment ( i n i t i a l zooplankton biomass ug/1) r a t i o s of : 0 3 10 20 40 cladocerans : copepods + warm :cool cladocerans + 0 + herbivores .carnivores + 0 0 + + - increase - - decrease 0 - no change TABLE IV -3b Experiment 3 (May-July 1981) Changes i n zooplankton r a t i o s i n enclosure experiments treatment ( i n i t i a l zooplankton biomass ug/1) r a t i o s of: .5 5 50 cladocerans : copepods warm : cool cladocerans 0 + + herbivores : carnivores 0 + - increase - - decrease 0 - no change TABLE I V - 3c Experiment 4 (August-September 1981) Changes i n zooplankton r a t i o s i n enclosure experiments treatment ( i n i t i a l zooplankton biomass ug/1) r a t i o s of: 0 1.5 3.5 7.5 cladocerans : copepods 0 0 0 -warm : cool cladocerans 0 0 0 + herbivores : carnivores + 0 0 0 + - increase - - decrease 0 - no change 98 e x c l u s i o n , 10 M g / l , and 80 ug/1 z o o p l a n k t o n t r e a t m e n t s . The r a t i o of h e r b i v o r e s t o c a r n i v o r e s i n c r e a s e d i n t h e z o o p l a n k t o n e x c l u s i o n and 20 M g / l z o o p l a n k t o n t r e a t m e n t s . In e x p e r i m e n t 3 (May - J u l y 1981) Daphn i a and s m a l l c l a d o c e r a n s e x h i b i t e d no 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 s . I n c r e a s e s i n t o t a l h e r b i v o r e and c y c l o p o i d c o p e p o d d e n s i t i e s o c c u r r e d a t low and i n t e r m e d i a t e i n i t i a l z o o p l a n k t o n d e n s i t i e s ; no s i g n i f i c a n t c h a n g e s were o b s e r v e d a t t h e h i g h i n i t i a l z o o p l a n k t o n d e n s i t y . H o l o p e d i u m e x h i b i t e d no s i g n i f i c a n t change a t t h e low i n i t i a l z o o p l a n k t o n d e n s i t y but s i g n i f i c a n t d e c r e a s e s a t i n t e r m e d i a t e and h i g h z o o p l a n k t o n d e n s i t i e s . D. o r e g o n e n s i s a l s o e x h i b i t e d no s i g n i f i c a n t change a t low i n i t i a l z o o p l a n k t o n b i o m a s s ; a t i n t e r m e d i a t e and h i g h z o o p l a n k t o n d e n s i t i e s , s i g n i f i c a n t i n c r e a s e s were o b s e r v e d . The r a t i o of c l a d o c e r a n s t o c o p e p o d s d e 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 h i g h z o o p l a n k t o n d e n s i t y t r e a t m e n t . No s i g n i f i c a n t c h a n g e s were o b s e r v e d i n o t h e r t r e a t m e n t s . The r a t i o of "warm" t o " c o o l " c l a d o c e r a n s i n c r e a s e d i n t r e a t m e n t s i n v o l v i n g i n t e r m e d i a t e and h i g h i n i t i a l z o o p l a n k t o n d e n s i t i e s . No c h a n g e s i n t h e r e l a t i v e a b u ndances o f h e r b i v o r e s and c a r n i v o r e s were o b s e r v e d . In e x p e r i m e n t 4 (August - September 1981) c h a n g e s i n 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 between t r e a t m e n t s f o r a l l g r o u p s e x c e p t H o l o p e d i u m and c y c l o p o i d c o p e p o d s . S i g n i f i c a n t i n c r e a s e s i n t o t a l h e r b i v o r e b i o m a s s were o b s e r v e d a t t h e t h r e e l o w e s t z o o p l a n k t o n d e n s i t y t r e a t m e n t s ; no s i g n i f i c a n t c h a n g e s were o b s e r v e d a t h i g h e r z o o p l a n k t o n 99 d e n s i t i e s . "Blooms" of Chydor i s i n two e n c l o s u r e s (one z o o p l a n k t o n e x c l u s i o n e n c l o s u r e s and one 1.5 uq/l z o o p l a n k t o n d e n s i t y e n c l o s u r e s ) c a u s e d s i g n i f i c a n t i n c r e a s e s i n s m a l l c l a d o c e r a n s a t t h o s e t r e a t m e n t s . The b i o m a s s of D. o r e g o n e n s i s i n c r e a s e d d r a m a t i c a l l y a t t h e two l o w e s t z o o p l a n k t o n d e n s i t i e s ; l e s s s u b s t a n t i a l i n c r e a s e s were o b s e r v e d a t 3.5 M g/1 and 7.5 M g/1 z o o p l a n k t o n d e n s i t i e s . At t h e two h i g h e s t z o o p l a n k t o n d e n s i t i e s , D. o r e g o n e n s i s b i o m a s s i n c r e a s e d f o r t h e f i r s t f o u r weeks, t h e n d e c r e a s e d back t o i n i t i a l l e v e l s . The r a t i o of c l a d o c e r a n s t o c o p e p o d s d e c r e a s e d s i g n i f i c a n t l y a t the two h i g h i n i t i a l z o o p l a n k t o n d e n s i t y t r e a t m e n t s . The r a t i o of "warm" t o " c o o l " c l a d o c e r a n s p e c i e s i n c r e a s e d s i g n i f i c a n t l y o n l y a t 7.5 M g/1 i n i t i a l z o o p l a n k t o n d e n s i t y . The r a t i o of h e r b i v o r e s t o c a r n i v o r e s i n c r e a s e d a t t h e z o o p l a n k t o n e x c l u s i o n t r e a t m e n t and d e c r e a s e d a t t h e two h i g h e s t i n i t i a l z o o p l a n k t o n d e n s i t i e s . P h y t o p l a n k t o n Dynamics Two-way a n a l y s i s of v a r i a n c e f o r t r e a t m e n t v e r s u s t i m e on t h e i n i t i a l o b s e r v a t i o n s f o r e x p e r i m e n t s 2 (May 1981) and 4 (Au g u s t 1981) showed no s i g n i f i c a n t d i f f e r e n c e s between t r e a t m e n t s f o r any of t h e p h y t o p l a n k t o n s i z e c a t e g o r i e s and no s i g n i f i c a n t c h a n g e s i n t h e bio m a s s of any of t h e s i z e c a t e g o r i e s . A n a l y s i s f o r t h e i n i t i a l o b s e r v a t i o n s of e x p e r i m e n t 3 (May 1981) showed s i g n i f i c a n t l y d i f f e r e n t r e s p o n s e s i n p h y t o p l a n k t o n biomass t o t r e a t m e n t s , w h i c h a r e t a b u l a t e d i n T a b l e I V - 4 . G e n e r a l l y , 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 s t a n d i n g TABLE IV-4 SHORT TERM (5 DAY'S) OBSERVATIONS OF ENCLOSURES IN EXPERIMENT 3 (MAY - JULY, 19 81) t r e a t m e n t c e l l s i z e c l a s s 2 2-6 6-10 1C-14 14-20 20 c o l o n i e s t o t a l low z o o p l a n k t o n (0.5 ug/1) + medium z o o p l a n k t o n (5 ug/1) + 0 0 + h i b h z o o p l a n k t o n (50 ug/1) a n a l y s i s of v a r i a n c e NS.: NS S NS + - i n c r e a s e - - d e c r e a s e 0 - no s i g n i f i c a n t change S - s i g n i f i c a n t at p=.05 NS - not s i g n i f i c a n t 101 c r o p t o low z o o p l a n k t o n d e n s i t i e s was t o d e c r e a s e o v e r t i m e . O n l y c o l o n i e s i n c r e a s e d when g r a z e r s were s c a r c e . At i n t e r m e d i a t e z o o p l a n k t o n d e n s i t i e s , t h e r e were i n c r e a s e s i n t h e d e n s i t i e s of s m a l l p h y t o p l a n k t o n (< 10 um). At h i g h z o o p l a n k t o n d e n s i t i e s , o n l y t h e s m a l l e s t c e l l s (< 2 um) and c o l o n i e s d i d n o t d e c r e a s e i n b i o m a s s . In t h e a n a l y s i s of l o n g e r term e f f e c t s , o v e r t h e e n t i r e d u r a t i o n of t h e e x p e r i m e n t , s i g n i f i c a n t i n c r e a s e s i n b i o m a s s were o b s e r v e d i n most of t h e e n c l o s u r e s i n e x p e r i m e n t 1 ( J u l y -A u g u s t 1980), but 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 p h y t o p l a n k t o n growth t r e n d s between t r e a t m e n t s . T h i s may have been due i n p a r t t o l a c k of s i g n i f i c a n t d i f f e r e n c e s between t r e a t m e n t s a f t e r t h e f i r s t 3 weeks. No s i g n i f i c a n t d e c r e a s e s were o b s e r v e d i n any o f t h e p h y t o p l a n k t o n s i z e c a t e g o r i e s i n any o f t h e e n c l o s u r e s . 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 s were f o u n d f o r e x p e r i m e n t s 3 (May - J u l y 1981) ( T a b l e IV-5) and 4 ( A u g u s t September 1981) ( T a b l e I V - 6 ) . In e x p e r i m e n t 3, s i g n i f i c a n t d i f f e r e n c e s between r e s p o n s e s t o t r e a t m e n t s were f o u n d f o r a l l s i z e c l a s s e s e x c e p t t h e two s m a l l e s t (< 6 um). At low z o o p l a n k t o n d e n s i t i e s , s i g n i f i c a n t i n c r e a s e s i n p h y t o p l a n k t o n biomass were o b s e r v e d i n t h e 10-14 Mm s i z e c l a s s and c o l o n i e s . A d e c r e a s e was o b s e r v e d i n t h e 6-10 Mm c e l l s and t h e l a r g e s t s i n g l e c e l l s , > 20 Mm, as w e l l as t o t a l b i o m a s s . At i n t e r m e d i a t e z o o p l a n k t o n d e n s i t i e s , t h e r e was a s i g n i f i c a n t i n c r e a s e i n c o l o n y b i o m a s s , and s i g n i f i c a n t d e c r e a s e s i n b i o m a s s e s of 6-10 Mm and > 20 Mm c e l l c l a s s e s , as w e l l as a s i g n i f i c a n t d e c r e a s e i n t o t a l b i o m a s s . At h i g h TABLE IV-5 LONG TERM (6 WEEKS) OBSERVATIONS OF ENCLOSURES IN EXFERIMENT 3(MAY-JULY,198 1) c e l l s i z e c l a s s treatment 2 2-6 6-10) 10-14 14-20 20 c o l o n i e s t o t a l low z o o p l a n k t o n 0 0 - + 0 - + (.5 ug/1) medium z o o p l a n k t o n 0 0 - 0 0 - + (5 ug/1) high z o o p l a n k t o n 0 0 + + + + + + ( 5 0 U R / 1 ) a n a l y s i s of NS NS v a r i a n c e r e s u l t s + - i n c r e a s e - - decrease 0 - no s i g n i f i c a n t change S - s i g n i f i c a n t at p=.05 NS - not s i g n i f i c a n t o T A B L E I V-6 L O N G T E R M(6 W E E K S ) O B S E R V A T I O N S OF E N C L O S U R E S I N E X P E R I M E N T 4 ( A U G U S T -S E P T E M B E R 1981) c e l l s i z e c l a s s t r e a t m e n t 2 2-6 6-10 10-14 14-20 20 c o l o n i e s t o t ; 0 z o o p l a n k t o n 0 0 0 + 0 0 + + 1.5 z o o p l a n k t o n 0 0 - - - 0 + 0 3.5 z o o p l a n k t o n 0 0 0 0 - 0 + 0 7.5 z o o p l a n k t o n 0 0 - + 0 0 + + 15 z o o p l a n k t o n 0 0 0 + 0 0 + + 35 z o o p l a n k t o n 0 0 - + 0 0 + + a n a l y s i s of NS NS s s s NS c s v a r i a n c e r e s u l t s + - i n c r e a s e - - d e c r e a s e 0 - no change S - s i g n i f i c a n t at p=.05 NS - not s i g n i f i c a n t 1 04 z o o p l a n k t o n d e n s i t i e s , t h e r e w e r e s i g n i f i c a n t f l u c t u a t i o n s i n b i o m a s s f o r t h e c e l l s i n t h e 6 - 2 0 Mm r a n g e - a n i n i t i a l d e c r e a s e , f o l l o w e d b y a s u b s t a n t i a l i n c r e a s e , t h e n a d e c r e a s e a g a i n . T h e o v e r a l l e f f e c t w as a s i g n i f i c a n t i n c r e a s e i n b i o m a s s . F o r t h e > 20 Mm c e l l s , c o l o n i e s , a n d t o t a l b i o m a s s , t h e r e w e r e s i g n i f i c a n t i n c r e a s e s . I n e x p e r i m e n t 4 , ' no 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 s w e r e o b s e r v e d f o r t h e s m a l l e s t c e l l s (< 6 Mm) o r f o r t h e l a r g e s t s i n g l e c e l l s (> 20 Mm) ( T a b l e I V - 6 ) . T h e r e w e r e s i g n i f i c a n t i n c r e a s e s i n t o t a l b i o m a s s i n t h e 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 a n d t h e e n c l o s u r e s w i t h i n t e r m e d i a t e a n d h i g h c o n c e n t r a t i o n o f z o o p l a n k t o n , b u t no s i g n i f i c a n t c h a n g e s i n t h o s e e n c l o s u r e s w i t h l o w z o o p l a n k t o n . I t i s d i f f i c u l t t o p i c k o u t a n y p a r t i c u l a r t r e n d i n t r e a t m e n t e f f e c t s o n p h y t o p l a n k t o n b i o m a s s . I n a l l e n c l o s u r e s , c o l o n y b i o m a s s i n c r e a s e d b u t t h e r e w e r e s i g n i f i c a n t d i f f e r e n c e s i n p a t t e r n s t h a t o c c u r r e d . T h e m o s t m o d e s t i n c r e a s e s o c c u r r e d i n t h e 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 a n d t h o s e w i t h v e r y l o w z o o p l a n k t o n b i o m a s s . T h e b i o m a s s a f t e r 6 w e e k s w a s a p p r o x i m a t e l y d o u b l e t h a t a t t h e b e g i n n i n g . T h e g r e a t e s t i n c r e a s e o c c u r r e d i n t h e e n c l o s u r e s 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 b u t a f t e r a maximum was r e a c h e d , c o l o n y b i o m a s s d r o p p e d b a c k down t o i n i t i a l l e v e l s . T h e e n c l o s u r e s w i t h i n t e r m e d i a t e c o n c e n t r a t i o n s o f z o o p l a n k t o n ( w i t h t h e e x c e p t i o n o f some w i l d f l u c t u a t i o n s i n t h e 7 . 5 ug/1 z o o p l a n k t o n d e n s i t y e n c l o s u r e s ) , e x h i b i t e d g r o w t h o f a " l o g i s t i c " p a t t e r n a n i n i t i a l r a p i d i n c r e a s e w h i c h e v e n t u a l l y l e v e l l e d o f f t o r e a s o n a b l y s t a b l e q u a n t i t i e s , a t a b o u t 3 t i m e s t h e i n i t i a l 1 05 c o n c e n t r a t i o n . In t h e z o o p l a n k t o n e x c l u s i o n e n c l o s u r e s , t h e 10-14 jum c e l l s i n c r e a s e d s i g n i f i c a n t l y . At t h e l o w e s t c o n c e n t r a t i o n of z o o p l a n k t o n (1.5 uq/1), t h e r e were s i g n i f i c a n t d e c r e a s e s i n c e l l s i n t h e range of 6-20 Mm. A s i g n i f i c a n t d e c r e a s e a l s o o c c u r r e d i n t h e b i o m a s s of 14-20 um.cells a t t h e se c o n d l o w e s t z o o p l a n k t o n d e n s i t y (3.5 ug/1)'. S i g n i f i c a n t d e c r e a s e s o c c u r r e d i n - t h e 10-14 j i m g r o u p a t t h e t h r e e h i g h e s t z o o p l a n k t o n d e n s i t i e s (7.5 Mg/1, 15 Mg/1, 35 uq/1). P h y t o p l a n k t o n t a x o n o m i c d a t a f o r t h e 1980 e n c l o s u r e s was not d e t a i l e d enough t o d e t e c t any changes o v e r t i m e , or d i f f e r e n c e s between t r e a t m e n t s . No n o t i c e a b l e 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 were o b s e r v e d d u r i n g t h e i n i t i a l s t a g e s of t h e e n c l o s u r e e x p e r i m e n t s done i n 1981. P h y t o p l a n k t o n t a x o n o m i c . d a t a f o r t h e l o n g - t e r m o b s e r v a t i o n s of e x p e r i m e n t s 3 (May - J u l y 1981) and 4 (August - September 1981) a r e shown i n T a b l e s IV-7 and IV-8. In e x p e r i m e n t 3, a few n o t a b l e d i f f e r e n c e s between t r e a t m e n t s e x i s t e d i n 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 o v e r t h e f u l l d u r a t i o n of t h e e x p e r i m e n t . The s m a l l d e s m i d , C o s m o c l a d i u m d r o p p e d o ut of t h e e n c l o s u r e s 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 a f t e r two d a y s , t h e n r e o c c u r r e d i n t h e f o u r t h week. At low and medium z o o p l a n k t o n d e n s i t i e s , C o s m o c l a d i u m p e r s i s t e d f o r a p p r o x i m a t e l y two weeks. M a l l o m o n a s , a l a r g e c h r y s o p h y t e , became l e s s common i n e n c l o s u r e s w i t h low z o o p l a n k t o n t h a n t h e o t h e r e n c l o s u r e s a f t e r a b o u t two and a h a l f weeks. O p h i o c y t i u m , a l a r g e x a n t h o p h y t e , was a l s o most common i n e n c l o s u r e s w i t h h i g h z o o p l a n k t o n TABLE IV-7 - Taxonomic Data, Experiment 3 No. of samples MAY JUNE JULY where group SPECIES TREATMENT 20 22 25 28 1 4 7 10 13 16 19 22 25 29 2 common Ochvomonas High X X X X X X X X X X X X X X X 15 Medium X X X X X X X X X X X X X X X 15 Low X X X X X X X X X X X X X X X 15 Thlamydomonas High X X X X X X X X. . X X X X X X 14 Medium X X X X X X X X X X X X X 13 Low X X X X X X X X X X X X X X 14 Oo cyst-is High X X X X X X X X X X X 11 Medium X X X X X X X X X .X X X 12 Low X X X X X X X X X X X X 12 coccoid green High X X X X X X X X 8 Medium X X X X X X X 7 Low X X X X X X X X 8 small desmid High X X X X X 5 Cosmocladium Medium X X X X X X X 7 Low X X X X X X 6 medium desmid High X X X X X X X X X X X X 12 Cosmariim Medium X X X X X X X X X X X X X 13 Low X X X X X X X X X X X X X X X 15 Mallomonas High X X X X X X X X X X X X X X 14 Medium X X X X X X X X X X X X 12 Low X X X X X X X X X X X 11 Cryptomonas High X X X X X X X X X X X X X X 14 Medium X X X X X X X X X X X X X X X 15 Low X X X X X X X X X X X X X X 14 o MAY TABLE IV-7 Cont'd JUNE No. of samples JULY where group SPECIES TREATMENT 20 22 25 28 1 4 7 10 13 16 19 22 25 29 2 common pennate diatoms High X X X X X X X X X X X X X X 14 Medium X X X X X X X X X X X X X X 14 Low X X X X X X X X X X X X X 13 Mouestia High . X X X X X X X X X X 10 Medium X X X X X X 6 Low X X X X X X 6 large desmids High X 1 Medium X X X X 4 Low X X X X X X X X 8 Dinobryon High X - X X X X X X X X X 10 Medium X X X X X X X X 8 Low X X X X 4 Merismopedia High X X X X X X X X X X X X 12 Medium X X X X X X X X X X 10 Low X X X X X X X X X X X X X 13 Glaeocystis High X X X X X X X X X X X X X X X 15 Medium X X X X X X X X X X X X X X X 15 Low X X X X X X X X X X X X X X 14 Saenedesmus High Medium Low Chroococcus High Medium Low X common taxa for that date SPECIES TABLE IV-8 - Taxonomic Data, Experiment 4 No. of samples AUGUST SEPTEMBER where group TREATMENT 14 16 18 21 24 27 30 2 5 8 11 14 17 20 23 26 common ug/1 Odhromonas Ch lamydomonas 0 X X X X X X X X X X X X X X X X 16 1.5 X X X X X X X X X X X X X X X X 16 3.5 X X X X X X X X X X X X X X X 15 7.5 X X X X X X X X X X X X X X X X 16 15 X X X X X X X X X X X X X X X X 16 35 X X X X X X X X X . X X X X X X X 16 0 X X X X X X X X X X 10 1.5 X X X X X X X X X X X 11 3.5 X X X X X X X X X X 10 7.5 X X X X X X X X X X X X X X 14 15 X X X X X X X X X X X X X 13 35 X X X X X X X X X X X X 12 small cryptophyte 0 X X X X X X X X X 9 1.5 X X X X X X X X X X 10 3.5 X X X X X X X X X 9 7.5 X X X X X X X 7 15 X X X X X X X X X 9 35 X X X X X X X X X X X 11 Oocystis 0 X X X X X X X X X 9 1.5 X X X X X 5 3.5 X X X X X X X 7 7.5 X X X X X X X X 8 15 X X X X X 5 35 X X X 3 TABLE IV-8 - Taxonomic D a t a , E x p e r i m e n t 4 C o n t ' d No. o f samples AUGUST SEPTEMBER where group SPECIES TREATMENT 14 16 18 21 24 27 30 2 5 8 11 14 17 20 23 26 common ug/1 Mouestia 0 X X X X X X X 7 1.5 X X X X X X X X X X 10 3.5 X X X X X 5 7.5 X X X X X X X X 8 15 X X X X X X X X X 9 35 X X X X X X X X X 9 Merismopedia 0 X X X X X X X X X X X X X X X X 16 1.5 X X X X X X X X X X X X X X X X 16 3.5 X X X X X X X X X X X X X X X X 16 7.5 X X X X X X X X X X X X X X X X 16 15 X X X X X X X X \r X X X X X X X 16 35 X X X X X X X" X X X X X X X X X 16 Chroocoocus 0 X X X X X X X X X X X 11 1.5 X X X X X X X X X X X 11 3.5 X X X X X X X X X X X 11 7.5 X X X X X X X X X X X X 12 15 X X X X X X X X X X X X 12 35 X X X X X X X X X X X X 12 Glaeooystis 0 X X X X X •:-5 1 . 5 X X X X X 5 3.5 X X 2 7.5 X X X 3 15 X X 2 35 X X X 3 SPECIES Matlomonas Cryptomonas d i n o f l a g e l l a t e s pennate diatoms TABLE IV-8 - Taxonomic D a t a , E x p e r i m e n t 4 C o n t ' d No. o f samples where group AUGUST SEPTEMBER TREATMENT 14 16 18 21 24 27 30 2 5 8 11 14 17 20 23 26 commi ug/1 0 X X X X X X X X X 9 1.5 X X X X X X X X X X X X X X 14 3.5 X X X X X X X X X X X X 12 7.5 X X X X X X X X X X X X X X X X 16c 15 X X X X X X X X X X X X X X X X 15 35 X X X X X X X X X X X X X X X 15 0 X X X X X X X X X X V i t X X . X X X 16 1.5 X X X X X X X X X X X X X 13 3.5 X X X X X X X X X X X X 132 7.5 X X X X X X X X X X X X X X 14 15 X X X X X X X X X X X X X X X 15 35 X X X X X X X X X X X X X X X X 16 0 X X X X X X X X X 9 1 . 5 X X X X X 5 3.5 X X X 3 7.5 X X X X X X 6 15 X X X X X 5 35 X X X 3 • 0 X X X X X X X X X X X X X X X X 16 1 . 5 X V X X X X X X X X X X X X 14 3.5, X X X X X X X X X X X X X X X 15 7 . 5 X X X X X X X X X X X X X X X 15 . 15 X X X X X X X X X X X X X 13 35 X X X X X X X X X X X X X X ' x X 16 TABLE IV-8 - Taxonomic Data, Experiment 4 Cont'd AUGUST SEPTEMBER SPECIES TREATMENT 14 16 18 21 24 27 30 2 5 8 11 14 17 20 23 26 c o m m i ug/1 coccoid green 0 X X X X X X 6 1.5 X X X 3 3.5 X X X X X X 6 7.5 X X X 3 15 X X X X X X X 7 35 X X X X X X 6 Chroomonas 0 X X X X X 5 1.5 X X X ;3 3.5 X X X X 4 7.5 X X X X X X X 7 15 X X X X X 5 35 X X X 3 Cosmocladium 0 X X X X X X X X 8 1.5 X X X X X X X 7 3.5 X X X X X 5 7.5 X X X X X X X X 8 15 X X X X 4 35 X X X X X X 6 No. of samples where group Cosmarivm 0 X X X X X X 6 1.5 X X X X X X 6 3.5 X X X X X X X 7 7.5 X X X X 4 15 X X X X 4 35 TABLE IV-8 - Taxonomic Data , E x p e r i m e n t 4 C o n t ' d No. o f sampl AUGUST SEPTEMBER where g r o u SPECIES TREATMENT 14 16 18 21 24 27 30 2 5 8 11 14 17 20 23 26 common ug/1 Scenedesmus large desmids 0 1 . 3 , 7 15 35 0 1 3. 7 15 35 X X X X X X X X X X X X X X X X X X X 6 0 0 0 0 0 1 3 6 4 2 4 1 1 3 d e n s i t i e s . T h i s was a l s o o b s e r v e d o f D i n o b r y o n . L a r g e d e s m i d s (>20 /nm), i n c l u d i n g A n k i s t r o d e s m u s , were common on o n l y one s a m p l i n g d a t e i n h i g h z o o p l a n k t o n e n c l o s u r e s ; t h e y were most common i n low z o o p l a n k t o n e n c l o s u r e s . G l a o e c y s t i s was t h e dominant c o l o n i a l a l g a i n ' a l l e n c l o s u r e s ; M e r i s m o p e d i a was a l s o common. In e x p e r i m e n t 4 (August - September 1981 ) t h e r e were a l s o o n l y a few d i f f e r e n c e s between t r e a t m e n t s i n p h y t o p l a n k t o n c o m p o s i t i o n c h a n g e s . The g r e e n c o l o n y O o c y s t i s d i s a p p e a r e d from t h e h i g h e s t z o o p l a n k t o n d e n s i t y e n c l o s u r e s a f t e r t h e f i r s t f o u r d a y s of t h e e x p e r i m e n t . I t p e r s i s t e d f o r t h r e e weeks i n t h e z o o p l a n k t o n e x c l u s i o n e n c l o s u r e s . O t h e r u n i c e l l u l a r c o c c o i d g r e e n a l g a e a p p e a r e d f i r s t i n t h e e n c l o s u r e s w i t h t h e h i g h e s t z o o p l a n k t o n d e n s i t y , a f t e r a p p r o x i m a t e l y one week, but became common i n a l l t h e e n c l o s u r e s d u r i n g t h e l a s t t h r e e weeks. The p y r r o p h y t e Chroomonas was most common a t 7.5 M g/1 z o o p l a n k t o n d e n s i t i e s , l e a s t common a t 1.5 uq/1 and 35 M g/1 z o o p l a n k t o n d e n s i t i e s . As i n e x p e r i m e n t 3, M a l l o m o n a s was l e a s t common a t l o w e s t z o o p l a n k t o n d e n s i t y , i n t h e z o o p l a n k t o n e x c l u s i o n e n c l o s u r e s . D i n o f l a g e l l a t e s were most common i n t h e z o o p l a n k t o n e x c l u s i o n e n c l o s u r e s and l e a s t common a t z o o p l a n k t o n d e n s i t i e s of 3.5 M g/1 and 35 uq/1. U n l i k e e x p e r i m e n t 3, l a r g e d e s m i d s (>20 Mm) were l e a s t common i n t h e z o o p l a n k t o n e x c l u s i o n e n c l o s u r e s , b e i n g common on o n l y one s a m p l i n g d a t e . The c o l o n i a l g r e e n G l a e o c y s t i s was most common i n z o o p l a n k t o n e x c l u s i o n and low (1.5 M g/1 ) z o o p l a n k t o n d e n s i t i e s . The c o l o n i a l d e s m i d Scenedesmus o c c u r r e d o n l y i n t h e z o o p l a n k t o n 1 1 4 e x c l u s i o n e n c l o s u r e s . M e r i s m o p e d i a was t h e dominant c o l o n i a l form i n a l l e n c l o s u r e s . C o m p a r i n q R e a l and Model Dynamics The p h y t o p l a n k t o n i s o c l i n e s p r o d u c e d by e q u a t i o n 7 p r e d i c t t h e immediate s h o r t - t e r m c h a n g e s i n p h y t o p l a n k t o n biomass from a g i v e n s t a r t i n g p o i n t . Because of i n t e r a c t i o n s w i t h z o o p l a n k t o n , o s c i l l a t i o n s i n p h y t o p l a n k t o n b i o m a s s s h o u l d o c c u r o v e r l o n g e r t i m e d u r a t i o n s . T h i s i s a l s o t r u e of z o o p l a n k t o n b i o m a s s : t h e models o n l y p r e d i c t immediate c h a n g e s from a g i v e n s t a r t i n g p o i n t . P h y t o p l a n k t o n - z o o p l a n k t o n phase d i a g r a m s f o r e x p e r i m e n t s 2 ( A p r i l - M a y 1981), 3 (May 1981) and 4 (August 1981) ( F i g u r e s IV-9,10,11) were u s e d t o compare' s h o r t - t e r m c h a n g es i 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 p o p u l a t i o n s i n t h e e n c l o s u r e s w i t h p r e d i c t i o n s made by t h e mo d e l . A r r o w s from e a c h s t a r t i n g p o i n t show t h e g e n e r a l d i r e c t i o n o f change ( d e c r e a s e o r i n c r e a s e ) o f e a c h g r o u p ( 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 ) d u r i n g t h e i n i t i a l s t a g e s of t h e e x p e r i m e n t . I n i t i a l c h a n g e s i n p h y t o p l a n k t o n b i o m a s s were d e t e c t a b l e a t v a r i o u s t i m e d u r a t i o n s f o r t h e v a r i o u s e n c l o s u r e s . In most e n c l o s u r e s , s t a t i s t i c a l l y s i g n i f i c a n t i n i t i a l r e s p o n s e s were d e t e c t e d w i t h i n t h e f i r s t 120 h o u r s ; i n two e n c l o s u r e s , s i g n i f i c a n t i n i t i a l r e s p o n s e s were not d e t e c t e d u n t i l 12 d a y s . B e c a u s e of o s c i l l a t i o n s of v a r y i n g f r e q u e n c y i n p h y t o p l a n k t o n b i o m a s s growth t r e n d s , i t was n o t p o s s i b l e t o use a s t a n d a r d t i m e d u r a t i o n f o r t e s t i n g f o r s i g n i f i c a n t i n i t i a l c h a n g e . 1 1 5 F i g u r e IV-9 I n i t i a l c h a n g e s i n p h y t o p l a n k t o n b i o m a s s (ug / 1 f r e s h w e i g h t ) and z o o p l a n k t o n b i o m a s s (nq/1 d r y w e i g h t ) i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 2 ( A p r i l - M a y 1 981 ) i 11 EXP.2 n 100 200 300 400 phytoplankton ug/l 1 1 6 F i g u r e IV-10 I n i t i a l c h a n g e s i n p h y t o p l a n k t o n b i o m a s s (Mg/1 f r e s h w e i g h t ) and z o o p l a n k t o n b i o m a s s (Mg/1 d r y w e i g h t ) i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 3 ( M a y - J u l y 1981) 116a EXP.3 50 40 o>30 3 C O ^ 2 0 i5 a o o N 10 0 4* 200 400 600 800 phytoplankton ug/l 1 1 7 F i g u r e IV-11 I n i t i a l c h a n g e s i n p h y t o p l a n k t o n b i o m a s s (uq/l f r e s h w e i g h t ) and z o o p l a n k t o n b i o m a s s (uq/l d r y w e i g h t ) i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 4 ( A u g u s t -September 1981) EXP.4 4 200 400 600 800 phytoplankton ug/1 1 18 O n l y f o r low i n i t i a l p h y t o p l a n k t o n c o n c e n t r a t i o n s d i d low i n i t i a l z o o p l a n k t o n b i o m a s s e s r e s u l t i n an i n c r e a s e i n p h y t o p l a n k t o n b i o m a s s . At h i g h i n i t i a l p h y t o p l a n k t o n c o n c e n t r a t i o n s and low i n i t i a l z o o p l a n k t o n b i o m a s s e s , d e 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 were g e n e r a l l y o b s e r v e d . At h i g h i n i t i a l p h y t o p l a n k t o n c o n c e n t r a t i o n s and moderate i n i t i a l z o o p l a n k t o n b i o m a s s e s , i n c r e a s e s or no s i g n i f i c a n t c h a n g e s i n p h y t o p l a n k t o n biomass were g e n e r a l l y o b s e r v e d , w i t h a few e x c e p t i o n s o c c u r r i n g a t v e r y h i g h i n i t i a l p h y t o p l a n k t o n c o n c e n t r a t i o n s . The e f f e c t of h i g h i n i t i a l z o o p l a n k t o n b i o m a s s e s v a r i e d . In e x p e r i m e n t 3, i n i t i a l z o o p l a n k t o n b i o m a s s e s g r e a t e r t h a n 10 ug/1 p r o d u c e d i n i t i a l d e 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 . In e x p e r i m e n t s 2 and 4, i n i t i a l z o o p l a n k t o n b i o m a s s e s g r e a t e r t h a n 20 /ug/1 r e s u l t e d i n i n i t i a l i n c r e a s e s o r no change i n p h y t o p l a n k t o n b i o m a s s , w i t h t h e e x c e p t i o n of t h e e n c l o s u r e i n e x p e r i m e n t 2 w i t h t h e h i g h e s t i n i t i a l z o o p l a n k t o n b i o m a s s , 67 uq/l, where a d e c r e a s e i n p h y t o p l a n k t o n biomass was o b s e r v e d . S i n c e c o l o n y b i o m a s s c h a n g e d v e r y l i t t l e i n t h e i n i t i a l s t a g e s of e x p e r i m e n t s 2, 3 and 4, i t i s u n l i k e l y t h a t t h e b i o m a s s o f c o l o n i e s had a s i g n i f i c a n t e f f e c t on p h y t o p l a n k t o n d y n a m i c s . S i g n i f i c a n t i n i t i a l c h a n g e s i n z o o p l a n k t o n b i o m a s s i n most i n d i v i d u a l e n c l o s u r e s were not s t a t i s t i c a l l y d e t e c t a b l e b e c a u s e of t h e l o n g t i m e i n t e r v a l s between s a m p l e s . A minimum of 3 o b s e r v a t i o n s w i t h c o n s e c u t i v e i n c r e a s e s o r d e c r e a s e s was n e c e s s a r y t o show s i g n i f i c a n t change a c c o r d i n g t o t h e Spearman's 1 1 9 Rank C o r r e l a t i o n T e s t . In the c a s e of e x p e r i m e n t 2, 3 s a m p l e s were t a k e n w i t h i n 5 d a y s . In e x p e r i m e n t 3, b e c a u s e o f s a m p l i n g d i f f i c u l t i e s a t day 14, t h e f i r s t 3 o b s e r v a t i o n s i n c l u d e d day 21. Changes i n z o o p l a n k t o n b i o m a s s from s t a r t i n g c o n d i t i o n s a r e shown i n F i g u r e s IV-9,10,11; o n l y t h o s e c h a n g e s shown i n F i g u r e IV-9 can be t r u l y c o n s i d e r e d t o be i n i t i a l c h a n g e s . No p a r t i c u l a r t r e n d s i n c h a n g e s i n z o o p l a n k t o n b iomass w i t h r e s p e c t t o s t a r t i n g c o n d i t i o n s were d e t e c t e d . Long terms o b s e r v a t i o n s (6 weeks) of e x p e r i m e n t s 1 ( J u l y -September 1980), 3 (May - J u l y 1981) and 4 ( A u g u s t - September 1981) a r e p l o t t e d as t r a j e c t o r i e s i n p r e d a t o r - p r e y phase p l o t s ( F i g u r e s I V - 1 2 , 1 3 , 1 4 ) . A l t h o u g h i t a p p e a r s t h a t i n some e n c l o s u r e s ( e g . exp. 1 e n c l . 1) t h e ' s y s t e m c h a n g e d i n a c o u n t e r - c l o c k w i s e d i r e c t i o n , i n o t h e r s ( e g . exp. 1 e n c l . 18), t h e change was i n a c l o c k w i s e d i r e c t i o n ; i n o t h e r s ( e g . exp. 3 e n c l . 2) t h e d i r e c t i o n of change s w i t c h e d midway. C o n s i s t e n t l y o c c u r r i n g c o u n t e r - c l o c k w i s e o s c i l l a t i o n s a r o u n d a s t a b l e e q u i l i b r i u m p o i n t , as p r e d i c t e d by t h e m o d e l s , were not o b s e r v e d . Changes i n p h y t o p l a n k t o n i n e n r i c h e d and u n e n r i c h e d 300 L e n c l o s u r e s were u s e d t o c a l c u l a t e v a l u e s f o r t h e model p a r a m e t e r s . The mean v a l u e s of t h e g r o w t h t r e n d s i n r e p l i c a t e t r e a t m e n t s were s u b j e c t e d t o a l e a s t s q u a r e s f i t t o a s t r a i g h t l i n e ; t h e y - i n t e r c e p t was u s e d as t h e v a l u e f o r Xo, Xt was t h e v a l u e a l o n g t h e l i n e a t 24 h o u r s , a n d t h e v a l u e s of t h e v a r i o u s c o n s t a n t s were c a l c u l a t e d f o r t o t a l , s i n g l e - c e l l e d , and c o l o n i a l 120 F i g u r e I V - 1 2 , a - l L o n g - t e r m (6 weeks) p h y t o p l a n k t o n - z o o p l a n k t o n d y n a m i c s i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 1 ( J u l y -September 1980), 8 day i n t e r v a l s 0 1000 2000 3000 4000 PHYTOPLANKTON (UG/L) 121 122 PHYTOPLANKTON (UG/L) 0 1000 1 2000 T 1 1 1 1 r 0 1000 2000 PHYTOPLANKTON (UG/L) 124 401 20-ENCL. 13 0 1000 2000 40-1 CD ZD O 20" ENCL. 14 < 0 O 40" O N 20H 000 2000 ENCL. 15 0' 500 1500 PHYTOPLANKTON (UG/L) 3000 1 2 5 1 26 F i g u r e I V - 1 3 , a - r L o n g - t e r m ' ( 6 weeks) p h y t o p l a n k t o n - z o o p l a n k t o n d y n a m i c s i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 3 (May-J u l y 1981), 7 day i n t e r v a l s 126a 127 129 0 200 400 600 PHYTOPLANKTON (UG/L) 130 6Ch ENCL. 15 0 2 0 0 4 0 0 6 0 0 8 0 0 PHYTOPLANKTON (UG/L) 133 134 ENCL. 17 0 2 0 0 4 0 0 6 0 0 PHYTOPLANKTON (UG/L) 135 F i g u r e I V - 1 4 , a - l L o n g - t e r m (6 weeks) p h y t o p l a n k t o n - z o o p l a n k t o n d y n a m i c s i n e x p e r i m e n t a l e n c l o s u r e s , e x p e r i m e n t 4 ( A u g u s t - S e p t e m b e r 1981), 7 day i n t e r v a l s 135a E N C L . I PHYTOPLANKTON (UG/L) 136 10 0 ^ 400 r 855 1 [200 PHY TO PLANK TQN (UG/L) 137 E N C L . 6 0 2 0 0 4 0 0 6 0 0 8 0 0 1000 P H Y T O P L A N K T O N (UG/L) 138 0 200 400 . 600 P H Y T O P L A N K T O N (UG/L) 139 1 40 p h y t o p l a n k t o n . The r e s u l t s of i n i t i a l ( " u n l i m i t e d " ) growth of p h y t o p l a n k t o n i n t h e e n r i c h e d 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 , w h i c h o c c u r r e d f o r t h e f i r s t 28 h o u r s , were u s e d t o c a l c u l a t e v a l u e s of r . The r e s u l t s o f g r o w t h d u r i n g t h e f i r s t 28 h o u r s i n e n r i c h e d e n c l o s u r e s w i t h z o o p l a n k t o n were u s e d t o c a l c u l a t e v a l u e s f o r c. D a t a from t h e e n r i c h e d and u n e n r i c h e d 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 a f t e r g r o w t h had l e v e l l e d o f f and n u t r i e n t s e x c r e t e d by z o o p l a n k t o n had r u n o u t , a f t e r 28 h o u r s , were u s e d t o c a l c u l a t e s, t h e s i n k i n g r a t e . The v a l u e f o r b, t h e p h y t o p l a n k t o n b i o m a s s e q u i v a l e n t of n u t r i e n t s c o n t a i n e d i n z o o p l a n k t o n , was c a l c u l a t e d f r o m d a t a f r o m e n c l o s u r e s w i t h z o o p l a n k t o n a f t e r 28 h o u r s . (See F i g u r e s IV-15 t o IV-18.) V a l u e s f o r t h e model c o n s t a n t s a r e shown i n T a b l e I V - 9 . B e c a use i n i t i a l p h y t o p l a n k t o n d e n s i t i e s i n e n r i c h e d e n c l o s u r e s w i t h z o o p l a n k t o n were c o n s i d e r a b l y h i g h e r t h a n e n r i c h e d 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 , t h e r e i s some u n c e r t a i n t y whether g r o w t h r a t e s i n t h e e n c l o s u r e s w i t h z o o p l a n k t o n were as h i g h as t h o s e 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 . T h i s c o u l d i n t r o d u c e a c o n s i d e r a b l e o v e r e s t i m a t i o n o f t h e c a l c u l a t e d f i l t e r i n g r a t e . B e c a u s e of t h i s , a f i l t e r i n g r a t e was a l s o c a l c u l a t e d f o r u n e n r i c h e d e n c l o s u r e s . V a l u e s f o r b were c a l c u l a t e d f o r b o t h v a l u e s o f c . 141 F i g u r e IV-15 Changes i n 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 (uq/l f r e s h w e i g h t ) i n e n r i c h e d e n c l o s u r e s , w i t h and w i t h o u t z o o p l a n k t o n 1 42 Figure IV-16 Changes i n s i n g l e - c e l l e d phytoplankton biomass ( u g / 1 fre s h weight) i n enriched enclosures, with and without zooplankton 143 F i g u r e IV-17 Changes i n 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 (ug/1 f r e s h w e i g h t ) i n u n e n r i c h e d e n c l o s u r e s , w i t h and w i t h o u t z o o p l a n k t o n T O T A L P H Y T O P L AN K T O N - U N ENR ICHED 6004 400 200H 0 600H 4001 20CH 0 NO Z O O P L A N K T O N Z O O P L A N K T O N 10 20 30 40 1 44 F i g u r e IV-18 Changes i n s i n g l e - c e l l e d p h y t o p l a n k t o n b i o m a s s (ug/1 f r e s h w e i g h t ) i n u n e n r i c h e d e n c l o s u r e s , w i t h and w i t h o u t z o o p l a n k t o n SINGLE-CELLED PHYTOPLANKTON-UNENRICHED 500' 400-D) ZD 300 200 5001 400 m ZD 3001 2001 100 NO ZOOPLANKTON ZOOPLANKTON 0 10 20 30 40 hours 145 TABLE IV-9 CALCULATED MODEL CONSTANTS TOTAL PP SINGLE-CELLED PP ONLY .90 .71 .0133* .014* .0021** .003** .36 * .24 ** 2 .08 2. 98 * c a l c u l a t e d from e n r i c h e d e n c l o s u r e s ** c a l c u l a t e d from u n e n r i c h e d e n c l o s u r e s 1 46 DISCUSSION Z o o p l a n k t o n C o m p o s i t i o n In s p i t e of c h a n g e s i n d e n s i t i e s 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 o v e r t h e d u r a t i o n of t h e e x p e r i m e n t s , 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 s g e n e r a l l y c o n s i s t e n t w i t h e x p e c t a t i o n s were o b s e r v e d . T h i s s u g g e s t s t h a t 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 i n i t i a l s t a g e s of t h e e x p e r i m e n t s were s u f f i c i e n t t o i n f l u e n c e t h e d y n a m i c s o f t h e p h y t o p l a n k t o n . R e l a t i v e changes i n s p e c i e s c o m p o s i t i o n o f z o o p l a n k t o n were g e n e r a l l y c o n s i s t e n t w i t h e x p e c t a t i o n s . The g e n e r a l i n c r e a s e o v e r t i m e i n t h e r a t i o of "warm" t o " c o o l " c l a d o c e r a n s i n a l l t h e e n c l o s u r e s was e x p e c t e d s i n c e t e m p e r a t u r e s were n e a r e r maximum f e e d i n g t e m p e r a t u r e s f o r "warm" s p e c i e s t h a n t h e maximum f e e d i n g t e m p e r a t u r e s f o r t h e " c o o l " s p e c i e s (Buckingham 1978). G r e a t e r r e l a t i v e i n c r e a s e s o f t h e b i o m a s s of w a r m r c o o l s p e c i e s a t h i g h e r z o o p l a n k t o n d e n s i t i e s (as d e m o n s t r a t e d b e s t by e x p e r i m e n t 3) were e x p e c t e d s i n c e c o m p e t i t i o n f o r f o o d was g r e a t e r . S u p e r i o r c o m p e t i t o r s , t h e "warm" s p e c i e s i n warm wa t e r , s h o u l d d o m i n a t e . T h i s r e l a t i v e i n c r e a s e o f "warm" s p e c i e s t o " c o o l " s p e c i e s s u g g e s t s an i n c r e a s e i n p e r c a p i t a f e e d i n g r a t e s ; a l t h o u g h t o t a l d e n s i t i e s of h e r b i v o r e s d e c r e a s e d i n e n c l o s u r e s w i t h h i g h i n i t i a l z o o p l a n k t o n d e n s i t i e s , t h e o v e r a l l g r a z i n g p r e s s u r e may not have d e c r e a s e d t o t h e same e x t e n t . I n e n c l o s u r e s w i t h low i n i t i a l z o o p l a n k t o n d e n s i t i e s , t h e i n c r e a s e i n g r a z i n g p r e s s u r e may have been g r e a t e r t h a n t h e i n c r e a s e i n z o o p l a n k t o n d e n s i t y . 1 47 Richman and Dodson (1983) fo u n d t h a t D i a p t o m u s t e n d e d t o do b e t t e r t h a n D a p h n i a under c o n d i t i o n s of s t r e s s , where f o o d q u a l i t y i s low, and f o o d q u a n t i t y i s e i t h e r v e r y h i g h o r v e r y low. D a p h n i a t e n d e d t o do b e t t e r under c o n d i t i o n s of h i g h f o o d q u a l i t y and i n t e r m e d i a t e f o o d q u a n t i t y . T h i s may e x p l a i n why c l a d o c e r a n : c o p e p o d r a t i o s t e n d e d t o d e c r e a s e a t h i g h z o o p l a n k t o n d e n s i t i e s , s i n c e t h e s e c o n d i t i o n s a r e more s t r e s s f u l ( c o m p e t i t i o n f o r f o o d i s h i g h , t h e r e i s a g r e a t e r l i k e l i h o o d t h a t f o o d q u a l i t y w i l l d e c r e a s e as f a v o u r e d f o o d i s g r a z e d away) and i n c r e a s e a t low z o o p l a n k t o n d e n s i t i e s ( c o m p e t i t o n f o r f o o d i s low, h i g h q u a l i t y f o o d can p e r s i s t ) . F i l t e r i n g r a t e s i n t h e l i t e r a t u r e (Haney 1973, Buckingham 1978) as w e l l as f i l t e r i n g r a t e s measured i n t h i s s t u d y ( C h a p t e r I I I ) g e n e r a l l y i n d i c a t e t h a t c l a d o c e r a n s a r e f a s t e r f i l t e r f e e d e r s t h a n c o p e p o d s . I f c l a d o c e r a n : c o p e p o d r a t i o s d e c r e a s e as z o o p l a n k t o n d e n s i t y i n c r e a s e s , t h i s may r e d u c e th e d i f f e r e n c e s i n g r a z i n g p r e s s u r e w i t h d i f f e r e n c e s i n z o o p l a n k t o n d e n s i t y . Changes i n h e r b i v o r e : e a r n i v o r e r a t i o s g e n e r a l l y were i n c r e a s e s a t low i n i t i a l z o o p l a n k t o n d e n s i t i e s and d e c r e a s e s a t h i g h i n i t i a l z o o p l a n k t o n d e n s i t i e s . At low z o o p l a n k t o n d e n s i t i e s , f o o d f o r t h e h e r b i v o r e s was r e l a t i v e l y a b undant w h i l e f o o d f o r t h e c a r n i v o r e s was n o t . At h i g h i n i t i a l z o o p l a n k t o n d e n s i t i e s , h e r b i v o r e p o p u l a t i o n s d e c r e a s e d p r e s u m a b l y due t o h i g h c o m p e t i t i o n f o r f o o d . A l a r g e f o o d base f o r t h e c a r n i v o r e s a l l o w e d f o r i n i t i a l g r o w t h o r a t l e a s t l e s s d e c l i n e . 1 48 P h y t o p l a n k t o n Compos i t i o n P o r t e r (1972, 1973), Weers and Z a r e t (1975) and M c C a u l e y and B r i a n d (1979) have d e m o n s t r a t e d t h a t w e a k l y g r a z e d c e l l s i z e s have d e m o g r a p h i c a d v a n t a g e s o v e r p r e f e r r e d s i z e s i n t h e p r e s e n c e of 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 ; t h e s e a d v a n t a g e s a r e l o s t i n t h e a b s e n c e of g r a z e r s . T h i s d i f f e r e n c e c a u s e s q u a l i t a t i v e s h i f t s i n dominance of c e l l t y p e s w i t h c h a n g e s i n g r a z i n g p r e s s u r e . The s e l e c t i v e f e e d i n g s t u d y d e s c r i b e d i n C h a p t e r I I I showed t h a t s i n g l e - c e l l e d p h y t o p l a n k t o n w i t h i n t h e r a n g e o f 6 - 20 Mm l o n g e s t l i n e a r d i m e n s i o n , p a r t i c u l a r l y t h e l a r g e r o n es, s h o u l d e x p e r i e n c e t h e g r e a t e s t g r a z i n g p r e s s u r e i n P l a c i d L a k e . I n c r e a s e s i n t h e numbers o f 10 - 14 Mm l o n g c e l l s a t l.ow z o o p l a n k t o n d e n s i t i e s i n e x p e r i m e n t 3 may t h e r e f o r e be due t o r e d u c e d g r a z i n g p r e s s u r e on t h e s e c e l l s . Reduced g r a z i n g p r e s s u r e may a l s o e x p l a i n t h e i n c r e a s e o f 10 - 14 Mm l o n g c e l l s i n t h e z o o p l a n k t o n e x c l u s i o n e n c l o s u r e s i n e x p e r i m e n t 4. S i g n i f i c a n t d e c r e a s e s i n numbers of 10 - 14 Mm l o n g c e l l s a t t h e t h r e e h i g h e s t z o o p l a n k t o n d e n s i t i e s i n e x p e r i m e n t 4 may be due t o i n c r e a s e d g r a z i n g p r e s s u r e . D e c r e a s e s i n numbers of 6-10 Mm and >20 Mm l o n g c e l l s a t low and i n t e r m e d i a t e z o o p l a n k t o n d e n s i t i e s i n e x p e r i m e n t 3 and of 14-20 Mm l o n g c e l l s a t low z o o p l a n k t o n d e n s i t i e s i n e x p e r i m e n t 4 may be due t o i n c r e a s e d c o m p e t i t i o n f o r n u t r i e n t s and r e d u c e d g r o w th r a t e s due t o n u t r i e n t s h o r t a g e s . I n c r e a s e d n u t r i e n t s u p p l y a t h i g h z o o p l a n k t o n d e n s i t i e s , i n e x p e r i m e n t 3, may be why l a r g e s i n g l e c e l l s , >20 Mm l o n g , i n c r e a s e d i n numbers. B e c a u s e s e l e c t i v e f e e d i n g o c c u r s , c h a n g e s i n p h y t o p l a n k t o n 1 49 s p e c i e s c o m p o s i t i o n can a l s o be a t t r i b u t e d t o c h a n g e s i n g r a z i n g p r e s s u r e . In e x p e r i m e n t 3, t h r e e l a r g e s p e c i e s , O p h i o c y t i u m , D i n o b r y o n , and M a l l o m o n a s , were more common i n e n c l o s u r e s 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 . T h i s r e s u l t i s i n agreement w i t h M c C a u l e y and B r i a n d (1979) who f o u n d t h a t s p e c i e s d i v e r s i t y i n c r e a s e d w i t h g r a z i n g p r e s s u r e b e c a u s e c o m p e t i t i o n f o r n u t r i e n t s d e c r e a s e d , a l l o w i n g s p e c i e s o t h e r w i s e r a r e t o i n c r e a s e . O t h e r s p e c i e s , s u c h as C o s m o c l a d i u m , O o c y s t i s , and Scenedesmus d i d b e t t e r a t low z o o p l a n k t o n d e n s i t i e s , s u g g e s t i n g t h a t t h e s e s p e c i e s may be g r a z e d and u n a b l e t o compete w i t h o t h e r forms under heavy g r a z i n g p r e s s u r e . Weers and Z a r e t (1975) f o u n d t h a t O o c y s t i s l a c u s t r i s and Scenedesmus b i j u g a were among t h e n i n e s p e c i e s w h i c h d e c r e a s e d i n t h e p r e s e n c e of z o o p l a n k t o n . C a u s e s of c o n f l i c t i n g r e s u l t s f o r o t h e r g r o u p s , s u c h as t h e l a r g e d e s m i d s and d i n o f l a g e l l a t e s a r e u n r e s o l v e d . Community Dynamics ^ C h o o s i n g a Model The c l a s s i c a l l o g i s t i c m odel, w i t h p r e d a t i o n e f f e c t s added, p r e d i c t s t h a t t h e r a t e a t w h i c h a p r e y p o p u l a t i o n grows d e c r e a s e s as p r e d a t o r d e n s i t y i n c r e a s e s . In a l o g i s t i c model t h a t i n c l u d e s e n r i c h m e n t by p r e d a t o r s , low p r e d a t o r d e n s i t i e s may r e s u l t i n d e c r e a s e s i n t h e p r e y p o p u l a t i o n due t o low a v a i l a b i l i t y o f n u t r i e n t s f o r g r o w t h . I n c r e a s e s i n t h e p r e y p o p u l a t i o n o c c u r a t medium p r e d a t o r d e n s i t i e s . A t much h i g h e r p r e d a t o r d e n s i t i e s , p r e d a t i o n p r e s s u r e c a u s e s d e c r e a s e s i n t h e p r e y p o p u l a t i o n . The i n i t i a l r e s p o n s e s of t h e p h y t o p l a n k t o n b i o m a s s e s i n t h e 1 50 3 e x p e r i m e n t s where i n i t i a l r e s p o n s e s were m o n i t o r e d ( F i g u r e IV-9,10,11) do not p r o v i d e good s u p p o r t f o r t h e c l a s s i c a l l o g i s t i c ( w i t h p r e d a t i o n ) model. The i n i t i a l c h a n g e s were s u c h t h a t I was u n a b l e t o draw a l i n e a r b o undary between t h o s e c o n d i t i o n s w h i c h p r o d u c e d i n i t i a l i n c r e a s e s and t h o s e w h i c h p r o d u c e d i n i t i a l d e c r e a s e s . I n s t e a d , t h e r e s u l t s p r o v i d e b e t t e r s u p p o r t f o r my " l o g i s t i c - p l u s - e n r i c h m e n t " model. E x c e p t a t v e r y low i n i t i a l p h y t o p l a n k t o n b i o m a s s e s , low z o o p l a n k t o n b i o m a s s e s r e s u l t e d i n a d e c r e a s e i n p h y t o p l a n k t o n b i o m a s s , w h i l e h i g h e r i n i t i a l z o o p l a n k t o n b i o m a s s e s a t c o m p a r a b l e i n i t i a l p h y t o p l a n k t o n b i o m a s s e s p r o d u c e d 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 b i o m a s s . ( I t a p p e a r s t h a t f r e e n u t r i e n t s were h i g h e r i n e n c l o s u r e s w i t h v e r y low i n i t i a l p h y t o p l a n k t o n b i o m a s s t h a n i n t h o s e w i t h h i g h e r i n i t i a l p h y t o p l a n k t o n b i o m a s s . C e l l l y s i s , c a u s e d by t h e water pump d u r i n g s e t - u p p r o c e d u r e s , may have o c c u r r e d i n t h e e n c l o s u r e s w i t h low i n i t i a l p h y t o p l a n k t o n b i o m a s s , and c o n t r i b u t e d t o t h e f r e e n u t r i e n t p o o l . How t h i s c o u l d o c c u r i n o n l y some e n c l o s u r e s i s u n c l e a r . ) T h i s s u g g e s t s t h a t z o o p l a n k t o n do c o n t r i b u t e s i g n i f i c a n t l y t o p h y t o p l a n k t o n p r o d u c t i v i t y . At h i g h i n i t i a l z o o p l a n k t o n b i o m a s s e s , d e 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 , as were p r e d i c t e d by b o t h m o d e l s , were o b s e r v e d , w i t h t h e e x c e p t i o n of e x p e r i m e n t 4 ( A u g u s t 1981). In e x p e r i m e n t 4, a t h i g h i n i t i a l z o o p l a n k t o n b i o m a s s e s ( a r o u n d 50 Mg/1), no s i g n i f i c a n t c h a n g e s i n p h y t o p l a n k t o n biomass were o b s e r v e d w i t h i n t h e f i r s t 12 days of t h e e x p e r i m e n t . T h e r e a r e two 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 e d i f f e r e n t r e s p o n s e s i n May and A u g u s t . F i r s t , f i l t e r i n g r a t e s d e c r e a s e as s e s t o n 151 c o n c e n t r a t i o n s i n c r e a s e (Richman 1966, Buckingham 1978). The d i f f e r e n c e i n i n i t i a l p h y t o p l a n k t o n c o n e n t r a t i o n s between e x p e r i m e n t s done a t d i f f e r e n t t i m e s of t h e y e a r i s seen most c l e a r l y when c o m p a r i n g e x p e r i m e n t 2 ( A p r i l - May 1981), when i n i t i a l p h y t o p l a n k t o n c o n c e n t r a t i o n s were a r o u n d 100-300 uq/l, w i t h e x p e r i m e n t 4 (A u g u s t 1981) when i n i t i a l p h y t o p l a n k t o n c o n c e n t r a t i o n s were a r o u n d 400-800 ug/1. Th e s e d i f f e r e n c e s r e f l e c t s e a s o n a l d i f f e r e n c e s i n p h y t o p l a n k t o n c o n c e n t r a t i o n s i n t h e l a k e . P h y t o p l a n k t o n c o m p o s i t i o n may a l s o be an i m p o r t a n t f a c t o r ( A p p e n d i x V ) . In A p r i l and May, c o l o n i a l p h y t o p l a n k t o n made up o n l y a v e r y s m a l l p r o p o r t i o n of 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 , g e n e r a l l y l e s s t h a n 10%. (Some e r r o r i n c o l o n y b i o m a s s was i n t r o d u c e d when r e c o u n t s were done; b r e a k a g e of c o l o n i e s l e d t o i n c r e a s e d c o l o n y c o u n t s and h i g h e r b i o m a s s e s t i m a t e s . ) In a d d i t i o n , t h e dominant c o l o n y t a x o n i n s p r i n g was G l a e o c y s t i s, w h i c h a p p e a r s t o be g r a z a b l e ( C h a p t e r I I I ) . In A u g u s t , c o l o n y b i o m a s s g e n e r a l l y made up 20-30% of 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 , and t h e dominant t a x o n was t h e i n e d i b l e M e r i s m o p e d i a ( C h a p t e r I I I ) . The i n e d i b i l i t y of t h e s e c o l o n i e s as w e l l as t h e p o s s i b i l i t y t h a t a t h i g h c o n c e n t r a t i o n s t h e y m i g h t have i n t e r f e r e d w i t h f i l t e r i n g by z o o p l a n k t o n ( G l i w i c z and S i e d l a r 1980), may have l e d t o f u r t h e r r e d u c t i o n s i n f i l t e r i n g r a t e s i n A u g u s t as compared t o A p r i l and May. I t a p p e a r s t h a t g r a z i n g i m p a c t p e r u n i t z o o p l a n k t o n biomass d e c r e a s e s from s p r i n g t o summer. The h i g h e s t d e n s i t i e s of z o o p l a n k t o n u s e d i n e x p e r i m e n t 4 were s l i g h t l y h i g h e r t h a n z o o p l a n k t o n d e n s i t i e s n o r m a l l y f o u n d i n P l a c i d Lake i n Au g u s t ( s e e C h a p t e r V, F i g u r e 6 ) , s u g g e s t i n g 1 52 t h a t t h e g r a z i n g e f f e c t i n l i m i t i n g p h y t o p l a n k t o n b i o m a s s i n l a t e summer may be m i n i m a l a t b e s t . I n i t i a l c h a n g e s from s t a r t i n g p o i n t s i n z o o p l a n k t o n do not s u p p o r t e i t h e r t h e c l a s s i c a l p r e y t h r e s h h o l d or t h e p r e d a t o r -p r e y t h r e s h h o l d m o d e l s . They may be due i n p a r t t o p o o r d a t a , s i n c e few o b s e r v a t i o n s of z o o p l a n k t o n b i o m a s s were made i n t h e i n i t i a l s t a g e s of e x p e r i m e n t s 3 and 4. S i g n i f i c a n t c h a n g e s i n z o o p l a n k t o n b i o m a s s i n e x p e r i m e n t 2 were o b s e r v e d i n o n l y 2 e n c l o s u r e s ; t h e s e a r e i n s u f f i c i e n t d a t a from w h i c h t o draw c o n c l u s i o n s . The t r a j e c t o r i e s o f l o n g - t e r m (6 week) z o o p l a n k t o n -p h y t o p l a n k t o n d y n a m i c s do not f o l l o w t h e g e n e r a l p a t t e r n p r e d i c t e d by a l l of t h e p r e d a t o r - p r e y model c o m b i n a t i o n s , t h a t i s , c o u n t e r - c l o c k w i s e o s c i l l a t i o n s a r o u n d a s t a b l e e q u i l i b r i u m p o i n t . T h i s s u g g e s t s t h a t t h e models may be i n a d e q u a t e t o d e s c r i b e p l a n k t o n d y n a m i c s . However, t h e t r a j e c t o r i e s may a l s o be t h e p r o d u c t of i n a d e q u a t e s a m p l i n g r e g i m e s ; s i g n i f i c a n t c h a n g e s i n 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 b i o m a s s e s may have been m i s s e d between s a m p l i n g t i m e s . H a n d l i n g of z o o p l a n k t o n i n s e t t i n g up t h e e n c l o s u r e s may have c o n t r i b u t e d t o a b n o r m a l l y h i g h m o r t a l i t y r a t e s i n t h e i n i t i a l s t a g e s of t h e e x p e r i m e n t s . T h i s a p p e a r s t o have o c c u r r e d i n e x p e r i m e n t 1 ( J u l y - S e p t e m b e r 1980), where d i f f e r e n c e s i n t r e a t m e n t s ( z o o p l a n k t o n b i o m a s s ) were o b s c u r e d a f t e r 3 weeks. I r r e g u l a r m o r t a l i t y r a t e s would a l t e r t h 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 d y n a m i c s p a t t e r n s . A n o t h e r f a c t o r w h i c h may have c o n t r i b u t e d t o t h e i r r e g u l a r i t i e s o f t h e t r a j e c t o r i e s i s t h e p o s s i b i l i t y t h a t t h e p h y t o p l a n k t o n -1 53 z o o p l a n k t o n e q u i l i b r i u m p o i n t i s not s t a b l e . T h i s w i l l be d i s c u s s e d i n some d e t a i l i n C h a p t e r V. In s h o r t , t h e p r e d a t o r -p r e y e q u i l i b r i u m p o i n t f o r any s y s t e m i s s t a b l e o n l y i f t h e p a r a m e t e r s w h i c h d e f i n e i t , s u c h as growth r a t e s , p r e d a t i o n r a t e s , e t c . , do not cha n g e . Over a 6 week p e r i o d , p h y s i c a l c o n d i t i o n s and z o o p l a n k t o n and p h y t o p l a n k t o n d e n s i t i e s , w h i c h a f f e c t t h e s e p a r a m e t e r s , may change s i g n i f i c a n t l y , p r o d u c i n g a s h i f t i n t h e e q u i l i b r i u m p o i n t . P h y t o p l a n k t o n Model P a r a m e t e r V a l u e s A l t h o u g h t h e v a l u e s o f t h e model c o n s t a n t s c a l c u l a t e d from one e x p e r i m e n t i n l a t e summer may not a p p l y t o a l l s e a s o n s , t h e g e n e r a l p a t t e r n s h o u l d h o l d t r u e . The v a l u e s c a l c u l a t e d p r o v i d e an a p p r o x i m a t e i d e a of t h e m a g n i t u d e of t h e c o n s t a n t s . S e a s o n a l v a r i a t i o n of t h e model c o n s t a n t s and t h e i r i m p l i c a t i o n s w i l l be d i s c u s s e d i n C h a p t e r V. The v a l u e s c a l c u l a t e d f o r d a i l y i n t r i n s i c g r o w t h r a t e , 0.71 and 0.90, seem w e l l w i t h i n t h e t h e o r e t i c a l r a n g e . D o u b l i n g t i m e s i n n o n - l i m i t e d c u l t u r e s v a r y from 2 h o u r s f o r t h e b l u e -g r e e n A n a c y s t i s n i d u l a n s t o n e a r l y 100 h o u r s f o r t h e x a n t h o p h y t e Monodus s u b t e r r a n s , ( d a i l y r v a l u e s o f 8.32 and 0.17 r e s p e c t i v e l y ) , w i t h most f a l l i n g between 7 and 30 h o u r s ( d a i l y r v a l u e s o f 2.38 and 0 . 5 5 ) ( r e v i e w e d i n Fogg 1966). The f i l t e r i n g r a t e s c a l c u l a t e d f r o m t h e e n r i c h e d e n c l o s u r e s , a t 0.013 and 0.014 L/day / u g z o o p l a n k t o n a r e q u i t e h i g h when compared t o f i l t e r i n g r a t e s r e c o r d e d i n t h e l i t e r a t u r e , w h i c h a r e g e n e r a l l y a b o u t o n e - f i f t h t h i s r a t e ( s e e 1 54 T a b l e 1 1 1 - 6 ) . The most p r o b a b l e s o u r c e of e r r o r , as was a l r e a d y d i s c u s s e d , i s t h e q u e s t i o n a b l e a s s u m p t i o n t h a t growth of p h y t o p l a n k t o n was o c c u r r i n g a t an i n t r i n s i c r a t e i n t h e e n c l o s u r e s w i t h z o o p l a n k t o n . R e d o i n g t h e c a l c u l a t i o n s w h i l e a s s u m i n g no net growth of p h y t o p l a n k t o n r e s u l t s i n a f i l t e r i n g r a t e of 0.0024, which i s i n good agreement w i t h r a t e s r e c o r d e d i n t h e l i t e r a t u r e . C a l c u l a t i n g a f i l t e r i n g r a t e from t h e e n c l o s u r e s w i t h normal l a k e w a t e r , a s s u m i n g t h a t growth o c c u r r e d i n t h e e n c l o s u r e s w i t h z o o p l a n k t o n a t t h e same r a t e as t h e 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 , r e s u l t s i n an e s t i m a t e of 0.0030 L/ug z o o p l a n k t o n / d a y . T h i s does not a c c o u n t f o r an e n r i c h m e n t e f f e c t by z o o p l a n k t o n , b u t , as was d i s c u s s e d e a r l i e r , s i n c e t h e r e i s a t i m e l a g i n t h e e n r i c h m e n t e f f e c t , t h i s a s s u m p t i o n may be v a l i d . B ecause my c a l c u l a t i o n s were b a s e d on one o b s e r v a t i o n w h i c h was s u b j e c t t o c r i t i c i s m , w h i l e t h o s e of Haney and Buckingham were measured d i r e c t l y u s i n g t r a c e r t e c h n i q u e s , I f e e l t h a t t h e i r measured f i l t e r i n g r a t e s a r e p r o b a b l y c l o s e r t o r e a l i t y t h a n mine. S i n k i n g r a t e s i n t h e l i t e r a t u r e a r e g i v e n i n terms of s p e e d s ( m e t e r s p e r day) a t w h i c h c e l l s s i n k , r a t h e r t h a n as p o p u l a t i o n l o s s r a t e s . The r e a s o n f o r t h i s i s b e c a u s e t h e d e p t h s o f e u p h o t i c z o n e s v a r y s u b s t a n t i a l l y between l a k e s . J u d g i n g from l i g h t p e n e t r a t i o n and t e m p e r a t u r e i s o c l i n e d a t a from S h e p p a r d (1 9 7 3 ) , t h e e u p h o t i c zone i n P l a c i d Lake i n summer i s a p p r o x i m a t e l y f o u r m e t e r s deep. Assuming no c e l l growth and homogenous d i s t r i b u t i o n o f c e l l s , a s i n k i n g r a t e of 1 meter p e r day w o u l d r e s u l t i n a l o s s o f one q u a r t e r of t h e l a k e 1 55 p h y t o p l a n k t o n p o p u l a t i o n from t h e e p i l i m n i o n , e q u i v a l e n t t o a d a i l y s i n k i n g r a t e o f -0.29. Smayda and B o l y n (1965, 1966 r e v i e w e d i n W e t z e l 1975) r e c o r d e d s i n k i n g s p e e d s f o r d i a t o m s of t h e s i z e r a n g e o f t h o s e f o u n d i n P l a c i d L a k e r a n g i n g from 0.1 t o 1.35 m e t e r s p e r day. ( B e c a u s e of t h e i r s i l i c a f r u s t u l e s , d i a t o m s t e n d t o have r a t h e r h i g h s i n k i n g r a t e s ( H u t c h i n s o n 1967). T h e r e f o r e , s i n k i n g r a t e s f o r mixed p h y t o p l a n k t o n s h o u l d p r o b a b l y be lower t h a n t h e s e . ) The s i n k i n g s p e e d s r e c o r d e d by Smayda and B o l y n a r e e q u i v a l e n t t o P l a c i d L a k e - s p e c i f i c s i n k i n g r a t e s of -0.025 t o -0.412 p e r day. When s i n k i n g r a t e s as e s t i m a t e d f r om 2 meter deep e n c l o s u r e s , of -0.24 and -0.36, a r e c o n v e r t e d t o s i n k i n g r a t e s o v e r a 4 meter d e p t h , t h e r a t e s a r e -0.11 and -0.16, r e s p e c t i v e l y , which a r e w e l l w i t h i n t h e t h e o r e t i c a l r a n g e . T i t m a n and K i l h a m (1976) have f o u n d t h a t s t a t i o n a r y phase p o p u l a t i o n s ( p o p u l a t i o n s w h i c h a r e not g r o w i n g ) s i n k , on a v e r a g e , f o u r t i m e s f a s t e r t h a n e x p o n e n t i a l l y g r o w i n g p o p u l a t i o n s . T h i s c r e a t e s a s i g n i f i c a n t p r o b l e m w i t h t h e model, s i n c e " s " may v a r y s u b s t a n t i a l l y w i t h i n a s h o r t t i m e f r a m e . I t a l s o s u g g e s t s t h a t a v a l u e of " s " c a l c u l a t e d f r o m a n o n - g r o w i n g p o p u l a t i o n , a s i t was, w i l l p r o b a b l y be o v e r e s t i m a t e d . The c a l c u l a t e d v a l u e f o r b, the p h y t o p l a n k t o n b i o m a s s e q u i v a l e n t o f n u t r i e n t s i n z o o p l a n k t o n , 2.98, i s b a s e d on a f i l t e r i n g r a t e o f 0.003 L/d/uq z o o p l a n k t o n . T h i s v a l u e i s w i t h i n t h e t h e o r e t i c a l r a n g e ( T a b l e I V - 1 0 ) . The v a l u e b a s e d on a f i l t e r i n g r a t e of 0.014 L/d/jug z o o p l a n k t o n i s g r e a t e r t h a n 1000, f a r beyond t h e t h e o r e t i c a l r a n g e . ( T h i s i s f u r t h e r r e a s o n t o b e l i e v e t h a t a f i l t e r i n g r a t e of 0.014 L/d/ug z o o p l a n k t o n i s T A B L E IV-10 T H E O R E T I C A L V A L U E S FOR " b " P L A N K T O N GROUP DRY WEIGHT A S % W E T WEIGHT NITROGEN C O N T E N T % DRY WEIGHT T H E O R E T I C A L V A L U E S FOR **b" T O T A L P H Y T O -P L A N K T O N 17.5 - 21.3 (1,2) 22 - 27 (3) 2 . 7 9 - 9 . 1 2 (4) B L U E -G R E E N A L G A E 15.7 (1,4) 5.76 (4) Z O O -P L A N K T O N 11 .6 - 16.3 (6) 8.2 - 11 .2 (6) 4.71 - 33.38 0.90 - 11 .43 ** (1) Strathman 1967 (2) Westlake 1969 (3) Myers 1962 (4) Parsons et a l . 1961 (5) Ricketts 1966 (6) Beers 1966 * calculated for nitrogen * * calculated for phosphorus b = nutrient content of zooplankton / nutrient content of phytoplankton / 100 / dry weight as % wet weight of phytoplankton 1 57 e r r o n e o u s l y h i g h . ) The p h y t o p l a n k t o n i s o c l i n e o b t a i n e d from t h e s e c o n s t a n t s i s shown i n F i g u r e IV-19. G e n e r a l l y , t h e e q u i l i b r i u m p h y t o p l a n k t o n p o p u l a t i o n s s u g g e s t e d by t h i s i s o c l i n e a r e low when compared t o a c t u 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 . The p h y t o p l a n k t o n i s o c l i n e i s d e s c r i b e d by x = b y ( 1 - c y / r - s / r ) . (20) c y / r + s / r In t h i s r e l a t i o n s h i p , b a c t s as a s c a l i n g f a c t o r and t h e e n t i r e i s o c l i n e c o u l d be i n c r e a s e d t o h i g h e r p h y t o p l a n k t o n d e n s i t i e s by i n c r e a s i n g b. The p h y t o p l a n k t o n i s o c l i n e i n F i g u r e IV-19 o c c u r s a t r e a l i s t i c z o o p l a n k t o n b i o m a s s e s but a t u n r e a l i s t i c a l l y low p h y t o p l a n k t o n b i o m a s s e s . The v e r t i c a l p o s i t i o n i n g of t h e p h y t o p l a n k t o n i s o c l i n e i n t h 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 phase d i a g r a m i s l a r g e l y d e p e n d e n t on t h e r a t i o of r : c . I am s a t i s f i e d t h a t my c a l c u l a t e d v a l u e s f o r t h e s e two p a r a m e t r e s a r e i n r e a s o n a b l y good agreement w i t h l i t e r a t u r e v a l u e s and a r e r e a l i s t i c . However, my v a l u e s f o r " s " and "b" a r e p r o b a b l y q u i t e low. The r e a s o n f o r b e l i e v i n g " s " i s low has a l r e a d y been d i s c u s s e d . N u t r i e n t r e g e n e r a t i o n r a t e s r e p o r t e d by Pomeroy e t a l . ( l 9 6 3 ) and Lehman (1980) ( T a b l e 1-2) s u g g e s t s t h a t t h e r e may be s i g n i f i c a n t v a r i a t i o n i n e x c r e t i o n and n u t r i e n t r e g e n e r a t i o n r a t e s , w h i c h a f f e c t n u t r i e n t c o n t e n t of z o o p l a n k t o n b i o m a s s . Some of t h e s e v a r i a t i o n s , p a r t i c u l a r l y i n r e s p o n s e t o t e m p e r a t u r e c h a n g e s , a r e d i s c u s s e d i n C h a p t e r V. I have c h o s e n 1 58 F i g u r e IV-19 P h y t o p l a n k t o n i s o c l i n e s as p r e d i c t e d by c a l c u l a t e d p a r a m e t e r v a l u e s phytoplankton ug/1 1 59 v a l u e s of 0.1. and 8.0 f o r " s " and "b" r e s p e c t i v e l y , and t h e p h y t o p l a n k t o n i s o c l i n e p r o d u c e d by t h e s e v a l u e s i s shown i n F i g u r e IV-20. C o n c l u s i o n s The i n i t i a l r e s p o n s e s of p h y t o p l a n k t o n i n e x p e r i m e n t s 2, 3 and 4 s u p p o r t t h e i d e a t h a t e n r i c h m e n t by z o o p l a n k t o n i s an i m p o r t a n t a s p e c t of p h y t o p l a n k t o n - z o o p l a n k t o n d y n a m i c s and t h a t t h e y a r e b e s t d e s c r i b e d by t h e l o g i s t i c p l u s e n r i c h m e n t model. D e 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 under g r a z i n g p r e s s u r e a t 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 o b s e r v e d i n s p r i n g but not summer, s u g g e s t i n g t h a t a t z o o p l a n k t o n d e n s i t i e s common t o P l a c i d Lake., g r a z i n g may be l e s s i m p o r t a n t i n c o n t r o l l i n g t h e s i z e of t h 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 summer t h a n i n s p r i n g . To some e x t e n t , t h i s s u p p o r t s the f i n d i n g s of F i n d e n e g g (1969) and Haney (1973) t h a t g r a z i n g i s not an i m p o r t a n t f a c t o r i n l i m i t 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 i n o l i g o t r o p h i c l a k e s ; t h i s may be t r u e i n summer but not i n s p r i n g . G r a z i n g may a l s o be an i m p o r t a n t f a c t o r i n d e t e r m i n i n g 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 p h y t o p l a n k t o n , i n summer as w e l l a s s p r i n g , s i n c e even w i t h t h e c r u d e o b s e r v a t i o n s ( w i t h r e s p e c t t o taxonomy) of t h i s s t u d y , e f f e c t s on c o m p o s i t i o n were e v i d e n t . T h i s i s i n agreement w i t h t h e f i n d i n g s of M c C a u l e y and B r i a n d ( 1 9 7 9 ) . B e c a u s e growth i 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 i n t h e a b s e n c e of z o o p l a n k t o n was o n l y o b s e r v e d i n t h e i n i t i a l s t a g e s of e x p e r i m e n t s 2, 3, and 4 when i n i t i a l p h y t o p l a n k t o n c o n c e n t r a t i o n s were v e r y low, i t a p p e a r s t h a t f r e e n u t r i e n t 1 60 F i g u r e IV-20 P h y t o p l a n k t o n i s o c l i n e as p r e d i c t e d by r e v i s e d p a r a m e t e r v a l u e s . 160a 250i 161 c o n c e n t r a t i o n s i n the water must have been q u i t e low. However, even a t r e l a t i v e l y low z o o p l a n k t o n d e n s i t i e s (eg. 10 ug/1 ) z o o p l a n k t o n were a b l e t o e x c r e t e enough n u t r i e n t s t o s t i m u l a t e p h y t o p l a n k t o n growth. T h i s s u g g e s t s t h a t 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 dynamics are c l o s e l y l i n k e d and t h a t z o o p l a n k t o n a re a b l e t o i n f l u e n c e p h y t o p l a n k t o n t h rough n u t r i e n t r e g e n e r a t i o n , p a r t i c u l a r l y i n summer when i n p u t s of n u t r i e n t s from o u t s i d e the l a k e a re s m a l l . 162 CHAPTER V - LAKE ZOOPLANKTON DYNAMICS INTRODUCTION E a r l y t h e o r e t i c a l work on p r e d a t o r - p r e y d y n a m i c s ( L o t k a 1925, V o l t e r r a 1926, r e v i e w e d i n K r e b s 1972) s u g g e s t e d t h a t p r e d a t i o n s h o u l d p r o d u c e c y c l i c p a t t e r n s i n p r e y and p r e d a t o r p o p u l a t i o n s . P r e y p o p u l a t i o n s s h o u l d d e c l i n e as p r e d a t o r p o p u l a t i o n s i n c r e a s e , and r e c o v e r a f t e r p r e d a t o r p o p u l a t i o n s c o l l a p s e due t o s t a r v a t i o n . The g e n e r a l p a t t e r n of a p r e y peak f o l l o w e d by a p r e d a t o r peak was d e m o n s t r a t e d e x p e r i m e n t a l l y by Gause (1934 ), and H u f f a k e r ( 1 9 5 8 ) , a l t h o u g h b o t h f o u n d t h a t c o n s i d e r a b l e m a n i p u l a t i o n was r e q u i r e d t o p r o d u c e c y c l i n g . R i l e y (1963) o b s e r v e d t h i s p a t t e r n of p r e y and p r e d a t o r peaks o c c u r r i n g y e a r l y i n t h e m a r i n e p l a n k t o n community a t G e o r g e s Bank. S i m i l a r p a t t e r n s have been o b s e r v e d i n f r e s h w a t e r . W r i g h t (1965) a r g u e d t h a t t h e p a t t e r n ( p r e y peak t h e n p r e d a t o r peak) i n F e r r y Canyon R e s e r v o i r i s e v i d e n c e t h a t z o o p l a n k t o n can c o n t r o 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 i n f r e s h w a t e r t h r o u g h g r a z i n g . S i m i l a r p h y t o p l a n k t o n - z o o p l a n k t o n p a t t e r n s have been r e c o r d e d i n c a s e s t u d i e s of s e v e r a l o t h e r t e m p e r a t e l a k e s (Nauwerck 1963, G r a n b e r g 1970, Coveney e t a l . 1977, Skogheim and Rognerud 1978). W r i g h t ' s i n t e r p r e t a t i o n of t h e p l a n k t o n d y n a m i c s i n F e r r y Canyon R e s e r v o i r s u g g e s t s t h a t t h e p a t t e r n of p h y t o p l a n k t o n - z o o p l a n k t o n s u c c e s s i o n i n a l a k e may p r o v i d e some i n s i g h t t o t h e i n t e r a c t i o n s between t h e s e two t r o p h i c l e v e l s . F i n d e n e g g (1969) and Haney (1973) b o t h p r e s e n t e d e v i d e n c e t h a t f i l t e r i n g r a t e s were t o o low t o make g r a z i n g an i m p o r t a n t 163 p h y t o p l a n k t o n r e g u l a t i n g f a c t o r i n t h e o l i g o t r o p h i c l a k e s t h e y s t u d i e d . The r e s u l t s of my e n c l o s u r e e x p e r i m e n t s s u g g e s t t h a t t h i s may be t r u e of P l a c i d Lake as w e l l , a t l e a s t i n t h e summertime. C o n t r o l l e d p e r t u r b a t i o n of a community c a n be a u s e f u l t o o l i n r e s e a r c h i n g t h e d y n a m i c s of t h e community. Because t h e c a u s e of t h e p e r t u r b a t i o n i s known, i n t e r p r e t a t i o n of community r e s p o n s e s do not have t o d e a l w i t h p o s s i b l e i n d i r e c t c a u s e s of the p e r t u r b a t i o n and t h e i r e f f e c t s , as i s t h e c a s e w i t h n a t u r a l p e r t u r b a t i o n s . As a major c o n t r o l l e d d i s t u r b a n c e , I c h o s e t o h a r v e s t P l a c i d Lake z o o p l a n k t o n w i t h a v e r y l a r g e n e t . The r a t i o n a l e b e h i n d t h i s h a r v e s t i n g was: i f g r a z i n g p r e s s u r e l i m i t s t h 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 summer i n P l a c i d L a k e , t h e n t h e r e m o v a l of g r a z e r s s h o u l d r e s u l t i n a l a r g e r s t a n d i n g c r o p . I f n u t r i e n t r e g e n e r a t i o n by z o o p l a n k t o n i s c r u c i a l f o r p h y t o p l a n k t o n growth, t h e n t h e r e m o v a l of z o o p l a n k t o n may r e s u l t i n a d e c l i n e i n t h 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 . To o b s e r v e t h e no r m a l p a t t e r n 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 s u c c e s s i o n i n P l a c i d L a k e , a d d i t i o n a l 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 e n s u s d a t a f o r 1977, 1978, and 1982 were o b t a i n e d from C. J . W a l t e r s . To a s s e s s t h e impact o f p h y s i c a l p a r a m e t e r s on t h i s s u c c e s s i o n , l a k e t e m p e r a t u r e d a t a were a l s o o b t a i n e d from D r . W a l t e r s , and s o l a r r a d i a t o n and r a i n f a l l d a t a were o b t a i n e d from t h e U.B.C. R e s e a r c h F o r e s t o f f i c e . In t h e summers of 1979 and 1980, e f f o r t s were made t o remove a s u b s t a n t i a l p o r t i o n of t h e z o o p l a n k t o n b i o m a s s . 1981 was us e d as a c o n t r o l y e a r ; p h y s i c a l p e r t u r b a t i o n s ( s t i r r i n g by b o a t 1 64 motor, e t c . ) were p r o d u c e d i n a manner s i m i l a r t o t h e h a r v e s t i n g p r o c e d u r e , w i t h o u t a c t u a l l y r e m o v i n g any z o o p l a n k t o n . D u r i n g a l l t h r e e y e a r s , w e e k l y o r b i w e e k l y 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 samples were c o l l e c t e d d u r i n g t h e g r o w i n g s e a s o n t o m o n i t o r t h e r e s p o n s e of t h e p l a n k t o n community. METHODS AND MATERIALS Z o o p l a n k t o n were removed from t h e t o p two m e t e r s of t h e P l a c i d Lake w i t h t h e use of a v e r y l a r g e p l a n k t o n n e t . The mouth of t h e net was r e c t a n g u l a r , 2 x 5 m, c o n s t r u c t e d from one i n c h g a l v a n i z e d s t e e l p i p e . The net i t s e l f was made of ( a p p r o x i m a t e l y ) 300 um p o r e s i z e p o l y e s t e r f a b r i c , a t t a c h e d t o th e mouth frame w i t h h e a v i e r m a t e r i a l . The n e t d e s i g n e v o l v e d from t h e f i r s t s e a s o n (1979) o f use t o t h e s e c o n d ( 1 9 8 0 ) . The f i r s t y e a r , t h e n e t f u n n e l l e d s h a r p l y from t h e mouth, i n l e s s t h a n 3 m , t o a 3 m l o n g , 0.5 m d i a m e t e r s l e e v e . An 80 L g a r b a g e b u c k e t , w h i c h had l a r g e h o l e s c u t i n t o t h e s i d e s and was l i n e d w i t h n e t t i n g m a t e r i a l , was a t t a c h e d t o t h e end of t h e s l e e v e w i t h a r u b b e r s t r a p . The b u c k e t c o u l d be removed and e m p t i e d . T h i s n e t e x p e r i e n c e d c o n s i d e r a b l e d r a g c a u s e d by i t s s h a r p f u n n e l shape, and t e n d e d t o c l o g q u i c k l y . D u r i n g t h e s e c o n d s e a s o n , a 3 m h o r i z o n t a l l e n g t h was added t o t h e mouth o f t h e net so t h a t i t f u n n e l l e d l e s s a b r u p t l y and had a l a r g e r f i l t e r i n g a r e a t o slow c l o g g i n g ( F i g u r e V - 1 ) . T h r e e l a r g e (0.03 m 3) s t y r o f o a m b l o c k s were a t t a c h e d t o t h e t o p of t h e net mouth frame, and s i x p o l y p r o p y l e n e r o p e s were a t t a c h e d t o t h e frame, one a t e a c h c o r n e r and one a t t h e c e n t r e of e a c h of t h e 5 m 1 65 F i g u r e V-1 H a r v e s t i n g net d e s i g n 165a 1 66 s i d e s . The o t h e r ends of t h e s e r o p e s were a t t a c h e d t o a m e t a l r i n g , w h i c h was a t t a c h e d to. a 15 m l o n g , 2.5 cm t h i c k p o l y p r o p y l e n e r o p e , w h i c h was a t t a c h e d t o t h e b o a t . A 15 ho r s e p o w e r o u t b o a r d motor was u s e d t o d r i v e t h e b o a t and p u l l t h e n e t ( F i g u r e V - 2 ) . N o r m a l l y , 5 h of t o w i n g were done on each o c c a s i o n . At a speed of 1 km p e r h, i t took a p p r o x i m a t e l y 3.5 h t o tow t h e n e t o v e r t h e e q u i v a l e n t of t h e l a k e ' s a r e a . Because of t h e r e d i s t r i b u t i o n z o o p l a n k t o n by s t i r r i n g f r o m t h e boat motor, and i n e f f i c i e n c i e s of t h e n e t c a u s e d by t u r b u l e n c e a t the mouth of t h e n e t and c l o g g i n g of t h e mesh, f i l t e r i n g e f f i c i e n c y was f a r l e s s t h a n 100%. However, r e d i s t r i b u t i o n of z o o p l a n k t o n from d e e p e r w a t e r , where z o o p l a n k t o n d e n s i t i e s were o f t e n g r e a t e r t h a n n e a r e r t h e s u r f a c e , t o t h e t o p 2 m d u r i n g the h a r v e s t p r o c e s s p r o b a b l y o c c u r r e d , i n c r e a s i n g t h e removal e f f i c i e n c y by r e d u c i n g t h e d i l u t i o n of z o o p l a n k t o n i n t h e t o p 2 m. F o r l o g i s t i c a l r e a s o n s , i t was i m p o s s i b l e t o p r e c i s e l y measure t h e a c t u a l amount of z o o p l a n k t o n removed f r o m t h e l a k e w i t h e a c h h a r v e s t i n g e f f o r t . My rough e s t i m a t e w o u l d be t h a t 25-50% o f t h e z o o p l a n k t o n i n t h e t o p 2 m o f t h e l a k e were removed on e a c h h a r v e s t d a t e . In 1979, z o o p l a n k t o n r e m o v a l was s t a r t e d t h e f i r s t week of J u l y and c o n t i n u e d once p e r week u n t i l t h e f i r s t week of September. In 1980, z o o p l a n k t o n r e m o v a l was s t a r t e d t h e l a s t week of May and c o n t i n u e d once p e r week u n t i l t h e end o f J u l y . R e g u l a r s a m p l i n g of l a k e z o o p l a n k t o n and p h y t o p l a n k t o n was done, u s u a l l y one or two d a y s p r i o r t o h a r v e s t i n g , t o m o n i t o r t h e i r c h a n g e s . On one o c c a s i o n , (12 A u g u s t 1979) a sample o f t h e 1 67 F i g u r e V - 2 H a r v e s t i n g n e t u s a g e 1 1 68 h a r v e s t e d z o o p l a n k t o n was kept and compared t o t h e l a k e z o o p l a n k t o n f o r z o o p l a n k t o n c o m p o s i t i o n and s i z e . D u r i n g 1981, t h e net frame, w i t h o n l y some n e t t i n g m a t e r i a l a t t a c h e d t o c r e a t e d r a g , was towed a r o u n d t h e l a k e once a week f o r 4 h from t h e end of May u n t i l t h e b e g i n n i n g of September. The p u r p o s e o f t h i s p r o c e d u r e was t o r e p r o d u c e t h e t u r b u l e n c e c r e a t e d by t h e boat motor and n e t , i n o r d e r t o s e p a r a t e e f f e c t s of t u r b u l e n c e from t h e e f f e c t s of z o o p l a n k t o n r e m o v a l . A n a l y s i s of d a t a f o r t h e e f f e c t s o f h a r v e s t i n g on 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 u s e d two-way a n a l y s i s of v a r i a n c e and Duncan's M u l t i p l e Range t e s t s on t h e l o g -t r a n s f o r m e d mean d e n s i t y of p l a n k t o n o v e r a l l 3 d e p t h s f o r ea c h d a t e . D a t a were g r o u p e d t o g e t h e r on a m o n t h l y b a s i s . C o m p a r i s o n s o f d e n s i t i e s between t h e 5 months of May t h r o u g h September, between t h e 6 y e a r s 1977 t h r o u g h 1982 were done f o r eac h of t h e p h y t o p l a n k t o n s i z e c a t e g o r i e s p l u s t o t a l p h y t o p l a n k t o n . S i m i l a r c o m p a r i s o n s were done f o r t h e z o o p l a n k t o n c a t e g o r i e s of t o t a l z o o p l a n k t o n , t o t a l h e r b i v o r e s , p r e d a t o r y c y c l o p o i d c o p e p o d s , 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 , s m a l l c l a d o c e r a n s ( i n c l u d i n g a l l c l a d o c e r a n s e x c e p t 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 ) and c a l a n o i d c o p e p o d s . U s i n g t h i s method I was a b l e t o d e t e c t s i g n i f i c a n t s e a s o n a l t r e n d s d u r i n g t h e c o n t r o l y e a r s as w e l l as t e s t f o r s i g n i f i c a n t a b e r a t i o n s i n 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 d e n s i t y and c o m p o s i t i o n d u r i n g t h e y e a r s o f l a k e p e r t u r b a t i o n . To a c c o u n t f o r any p o s s i b l e i r r e g u l a r i t i e s c a u s e d by u n u s u a l weather p a t t e r n s d u r i n g t h e e x p e r i m e n t a l s e a s o n s , I a l s o compared 169 s e a s o n a l t r e n d s i n P l a c i d Lake t o t h o s e i n n e a r b y K a t h e r i n e Lake f o r 1977 t h r o u g h 1982. A l t h o u g h K a t h e r i n e Lake i s not a bog l a k e , i t p o s s e s s e s s i m i l a 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 s p e c i e s i n s i m i l a r d e n s i t i e s t o P l a c i d L a k e . K a t h e r i n e Lake a l s o has a p o p u l a t i o n of D o l l y V a r d e n c h a r , S a l v e l i n u s malma. Data f o r K a t h e r i n e L a k e were o b t a i n e d from C . J . W a l t e r s . C o m p a r i s o n s between l a k e s were done f o r each of t h e p h y t o p l a n k t o n c a t e g o r i e s as w e l l as t o t a l z o o p l a n k t o n , p r e d a t o r y c y c l o p o i d c o p e p o d s , c a l a n o i d c o p e p o d s , 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 , s m a l l c l a d o c e r a n s ( a l l t h e r e s t ) , and c o p e p o d and c l a d o c e r a n f e r t i l i t y ( measured i n terms of p e r c e n t a d u l t s w h i c h were g r a v i d f e m a l e s ) . W i l c o x o n - m a t c h e d p a i r s - s i g n e d rank t e s t s ( L a r k i n 1976) were use d t o compare s p e c i e s c o m p o s i t i o n between h a r v e s t e d z o o p l a n k t o n and l a k e z o o p l a n k t o n from s i m i l a r d a t e s . The t e s t s compared t h e p e r c e n t a g e of t o t a l z o o p l a n k t o n numbers t h a t " l a r g e " and " s m a l l " z o o p l a n k t o n r e p r e s e n t e d i n h a r v e s t e d and l a k e s a m p l e s . " s m a l l " z o o p l a n k t o n i n c l u d e d n a u p l i i , c y c l o p o i d c o p e p o d s , Diaphanosoma, Polyphemous, Bosmina, and C e r i o d a p h n i a ; " l a r g e " z o o p l a n k t o n i n c l u d e d D i a p t o m u s o r e g o n e n s i s , H o l o p e d i u m , Daphn i a and D i aptomus kena i . P h y s i c a l d a t a f o r P l a c i d Lake and t h e U.B.C. R e s e a r c h F o r e s t were o b t a i n e d and two-way a n a l y s i s of v a r i a n c e was used t o compare a n n u a l t r e n d s i n r a i n f a l l , s o l a r r a d i a t i o n and l a k e t e m p e r a t u r e measured a t a d e p t h o f 2 m between t h e 6 y e a r s s t u d i e d . Mean d a i l y r a i n f a l l and s o l a r r a d i a t i o n f o r e a c h month and mean t e m p e r a t u r e f o r e a c h month were compared between months and y e a r s . 1 70 RESULTS P h y s i c a l C o n d i t i o n s M o n t h l y r a i n f a l l f o r y e a r s 1977 t h r o u g h 1982 and mean m o n t h l y r a i n f a l l f o r t h e s i x y e a r s a r e shown i n A p p e n d i x I . U s i n g a n n u a l r a i n and s u r f a c e r u n - o f f n u t r i e n t c o n c e n t r a t i o n measurements from F e l l e r ( 1 9 7 7 ) , c o n v e r t e d t o ug-AT/L, and l a k e and d r a i n a g e a r e a s from S h e p p a r d ( 1 9 7 3 ) , I e s t i m a t e d t h e mean m o n t h l y n u t r i e n t i n p u t due t o r a i n f a l l t o P l a c i d L a k e . I a l s o e s t i m a t e d , b a s e d on m o n t h l y mean z o o p l a n k t o n d e n s i t i e s f r o m t h i s s t u d y and l a k e volume e s t i m a t e s f r o m S h e p p a r d ( 1 9 7 3 ) , t h e t o t a l l a k e z o o p l a n k t o n b i o m a s s and i t s c o n t r i b u t i o n v i a e x c r e t i o n t o t h e f r e e n u t r i e n t p o o l ( T a b l e V - 1 ) . E x c r e t i o n r a t e s were o b t a i n e d f r o m Lehman ( 1 9 8 0 ) . (See T a b l e 111 - 10.) In g e n e r a l , November and December a r e t h e w e t t e s t months of t h e y e a r w h i l e J u l y and A u g u s t a r e t h e d r i e s t . N u t r i e n t c o n c e n t r a t i o n s i n r a i n and s u r f a c e r u n - o f f t e n d t o be h i g h e r when i t r a i n s l e s s ( F e l l e r 1977). However, the e s t i m a t e s of n u t r i e n t i n p u t t o P l a c i d Lake v i a r a i n and s u r f a c e r u n - o f f a r e b a s e d on t h e a s s u m p t i o n t h a t e v e r y d r o p of r a i n t h a t f a l l s i n t h e d r a i n a g e a r e a w i l l end up a s s u r f a c e r u n o f f i n t h e l a k e . D u r i n g th e d r y summers, i t i s p r o b a b l e t h a t o n l y r a i n t h a t f a l l s d i r e c t l y on t h e l a k e w i l l end up i n i t . T h e r e f o r e i n most c a s e s , t h e n u t r i e n t i n p u t s e s t i m a t e d a r e o v e r - e s t i m a t e d and s h o u l d be c o n s i d e r e d o n l y as maximum p o s s i b l e i n p u t s . Lehman's (1980) e x c r e t i o n r a t e s were measured i n l a t e September. However i t i s l i k e l y t h a t e x c r e t i o n r a t e s would be h i g h e r i n summer. S i n c e e x c r e t i o n i s t h e r e l e a s e TABLE V - l Nutrient Input to Plac i d Lake Via R a i n f a l l and Zooplankton Excretion Month Nitrogen Nitrogen Nitrogen Phosphorus Phosphorus Phosphorus Total input v i a input v i a zooplankton/rain input v i a input v i a zooplankton/rain N/P r a i n zooplankton input r a i n zooplankton input (g) (g) (g) (g) JANUARY 84.8 250.3 3.0 28.1 28.9 1.0 11.6 FEBRUARY 141.9 252.4 1.8 47.0 29.1 0.6 5.2 MARCH 87.1 367.2 4.2 29.1 42.4 1.5 6.4 APRIL 90.0 517.5 5.8 29.8 59.7 2.0 6.7 MAY 58.8 815.9 13.9 19.5 94.1 4.8 7.7 JUNE 57.7 1253.8 21.7 19.1 144.7 7.6 8.0 JULY 32.9 1652.4 50.2 10.7 190.7 17.8 8.4 AUGUST 40.4 1298.8 32.1 13.4 149.9 11.2 8.2 SEPTEMBER 80.8 840.2 10.4 26.8 96.9 3.6 7.4 OCTOBER 103.8 912.6 8.8 34.4 105.3 3.1 7.3 NOVEMBER 166.1 766.7 4.6 55.0 88.5 1.6 6.5 DECEMBER 168.4 193.2 1.1 55.8 22.3 0.4 4.6 1 72 of m e t a b o l i c w a s t e s r e s u l t i n g f r o m r e s p i r a t i o n , e x c r e t i o n r a t e s s h o u l d v a r y w i t h r e s p i r a t i o n r a t e s . S i n c e r e s p i r a t i o n r a t e s i n c r e a s e c u r v i l i n e a r l y w i t h t e m p e r a t u r e ( P r o s s e r and Brown 1961), i t i s r e a s o n a b l e t o assume t h a t e x c r e t i o n r a t e s s h o u l d a l s o be t e m p e r a t u r e d e p e n d e n t . T h e r e f o r e , i t i s p r o b a b l e t h a t n u t r i e n t c o n t r i b u t i o n s t o the l a k e by z o o p l a n k t o n i n t h e summer a r e u n d e r e s t i m a t e d i n T a b l e V-1. Because of t h e s e i n h e r e n t e r r o r s i n t h e e s t i m a t i o n of n u t r i e n t i n p u t by r a i n and by z o o p l a n k t o n , t h e r a t i o s of i n p u t by z o o p l a n k t o n v e r s u s i n p u t by r a i n s h o u l d be r e a d as minimum v a l u e s . The i n p u t o f n i t r o g e n and p h o s p h o r u s by z o o p l a n k t o n r e l a t i v e t o t h e i n p u t by r a i n i s h i g h e s t d u r i n g t h e • summer months, June t h r o u g h A u g u s t , and l o w e s t d u r i n g t h e w i n t e r months, December t h r o u g h F e b r u a r y . I n p u t of n u t r i e n t by 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 i s t h e o r e t i c a l l y g r e a t e r t h a n by r a i n f a l l and r u n - o f f f o r b o t h n i t r o g e n and p h o s p h o r u s e v e r y month e x c e p t t h e w i n t e r months when r a i n f a l l may c o n t r i b u t e more p h o s p h o r u s . A n o t h e r p o i n t of i n t e r e s t i s t h a t t h e N:P r a t i o s o f n u t r i e n t s added t o t h e l a k e by r a i n f a l l and e x c r e t e d by z o o p l a n k t o n i s low compared t o t h e N:P c o n t e n t o f p h y t o p l a n k t o n , 12:1 - 18:1 ( r e v i e w e d i n P a r s o n s and T a k a h a s h i 1973 ) . A n a l y s i s o f v a r i a n c e was u s e d t o compare s e a s o n a l t r e n d s i n r a i n f a l l , s o l a r r a d i a t i o n , and l a k e t e m p e r a t u r e between t h e 6 y e a r s of t h e s t u d y . No s t a t i s t i c a l l y 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 r e n d s i n l a k e t e m p e r a t u r e s were o b s e r v e d . S o l a r r a d i a t i o n was s i g n i f i c a n t l y l o w e r i n 1981 t h a n a l l o t h e r y e a r s e x c e p t 1980, p a r t i c u l a r l y i n May, June, and J u l y . S o l a r r a d i a t i o n i n May and 173 June 1980 and J u l y 1982 was a l s o low compared t o most o t h e r y e a r s . S o l a r r a d i a t i o n p eaked i n J u l y i n 1978, 1979, and 1980; i n 1981 i t p e a k e d i n A u g u s t w h i l e i n 1977 and 1982 i t p e a k e d i n J u n e . (See F i g u r e s V-3 a - f . ) The y e a r s 1980 t h r o u g h 1982 were s i g n i f i c a n t l y r a i n i e r t h a n t h e p r e v i o u s t h r e e y e a r s . No p a r t i c u l a r c h a n g e s i n r a i n f a l l p a t t e r n s were o b s e r v e d d u r i n g t h e s e t h r e e y e a r s compared t o 1977 t h r o u g h 1979. November and December 1980 were r e l a t i v e l y wet compared t o o t h e r y e a r s , as were J u l y 1981 and J a n u a r y and F e b r u a r y 1982. P l a n k t o n Dynamics U n l i k e t h e p a t t e r n s of p l a n k t o n s u c c e s s i o n d e s c r i b e d by W r i g h t (1965) and o t h e r s , i n P l a c i d Lake between t h e y e a r s 1977 ( d e g r e e s C e l s i u s ) f o r t h e U.B.C. R e s e a r c h F o r e s t and P l a c i d L a k e , r e s p e c t i v e l y , 1977-1982 t o 1982 ( w i t h t h e e x c e p t i o n s of 1980 and 1982), z o o p l a n k t o n biomass i n c r e a s e s p r e c e e d 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 by two weeks t o two months ( F i g u r e s V-4 a - f ) . In 1980, i n c r e a s e s i n 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 b i o m a s s o c c u r r e d s i m u l t a n e o u s l y , i n e a r l y J u n e . P h y t o p l a n k t o n d a t a f o r 1982 were u n u s u a l - b i o m a s s n e v e r e x c e e d e d 300 ug/L f r e s h w e i g h t , and p e a k s i n p h y t o p l a n k t o n b i o m a s s c o m p a r a b l e t o o t h e r y e a r s d i d not o c c u r . D e c l i n e s i n 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 biomass d i d not f o l l o w a r e g u l a r p a t t e r n . In 1977, 1978, and 1980, major d e c r e a s e s i n z o o p l a n k t o n b i o m a s s o c c u r r e d a p p r o x i m a t e l y one month p r i o r t o d e 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 1979 and 1981, p h y t o p l a n k t o n d e c l i n e s p r e c e e d e d z o o p l a n k t o n d e c l i n e s , by two 1 74 F i g u r e V - 3 , a - f T o t a l m o n t h l y i r r a d i a n c e (kw/m 2) and mean l a k e t e m p e r a t u r e 176 40 30 E ^ 2 0 LU O < DC or 1981 30 t-25 H m 20 £ m 15 > —I c io S m o O —I 1 i J F M A M J J A S O N D month 0 60 50 WE 4 0 o I 20 < DC 1982 10 0 30 25 H rn 20 ^ m 15 > 10 I J F M A M J J A S O N D month 1 77 F i g u r e V-4, a - f 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 (uq/1 f r e s h w e i g h t ) and z o o p l a n k t o n b i o m a s s (ug/1 d r y w e i g h t ) i n P l a c i d L a k e , 1977-1982 1977 r2000 -o J F M A M J J A S O N D m o n t h 1979 F3000 J F M A M J J A ' S O N D m o n t h J F M A M J J A S O NTD m o n t h 1981 1982 J F M A M J J A S O N D month 180 months i n 1979 and one month i n 1981. S t a t i s t i c a l c o m p a r i s o n s of h a r v e s t e d and l a k e z o o p l a n k t o n s amples showed t h a t z o o p l a n k t o n i n t h e " l a r g e " c a t e g o r y c o m p r i s e d a s t a t i s t i c a l l y g r e a t e r p e r c e n t a g e of t h e z o o p l a n k t o n i n t h e h a r v e s t e d sample t h a n i n t h e l a k e samples ( p = . 0 l ) . S m a l l z o o p l a n k t o n were s i g n i f i c a n t l y more abundant i n t h e l a k e samples than t h e h a r v e s t e d sample (p=.05). In g e n e r a l z o o p l a n k t o n b i o m a s s i n P l a c i d Lake i n c r e a s e d i n s p r i n g , r e m a i n e d h i g h i n summer and t h e n d e c r e a s e d i n f a l l . B r i e f but s u b s t a n t i a l f l u c t u a t i o n s o c c u r r e d o c c a s i o n a l l y d u r i n g t h e summer months. C o m p a r i s o n s i n z o o p l a n k t o n t r e n d s were made between P l a c i d Lake and K a t h e r i n e L a k e, t o c o n t r o l f o r e n v i r o n m e n t a l c a u s e s of p l a n k t o n b i o m a s s c h a n g e s . G e n e r a l l y c h a n g e s i n z o o p l a n k t o n b i o m a s s d u r i n g May .through September.were q u i t e s i m i l a r i n t h e two l a k e s . No s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e s were d e t e c t e d i n t h e y e a r s 1980, 1981, or 1982. In 1978, m o n t h l y a v e r a g e z o o p l a n k t o n b i o m a s s peaked i n June i n b o t h l a k e s ; however t h e m a g n i t u d e of change was s i g n i f i c a n t l y g r e a t e r i n P l a c i d L a k e . In 1977 z o o p l a n k t o n b i o m a s s t r e n d s i n K a t h e r i n e Lake were u n u s u a l . I n s t e a d of t h e u s u a l i n c r e a s e s i n z o o p l a n k t o n b i o m a s s i n t h e summer, z o o p l a n k t o n b i o m a s s d e c r e a s e d s t e a d i l y f r o m May t h r o u g h A u g u s t , t h e n r e c o v e r e d s l i g h t l y i n September. T r e n d s i n P l a c i d Lake i n 1977 were s i m i l a r t o most o t h e r y e a r s . D u r i n g t h e summer o f 1979, z o o p l a n k t o n biomass i n P l a c i d L a k e d e c r e a s e d s i g n i f i c a n t l y , by more t h a n 50%, i n e a r l y A u g u s t and s t a y e d r e l a t i v e l y low f o r s i x weeks. T h i s p a t t e r n was not o b s e r v e d any o t h e r y e a r i n P l a c i d L a k e , nor d i d i t o c c u r 181 i n K a t h e r i n e Lake i n 1979. R e g u l a r p a t t e r n s i n c o p e p o d and c l a d o c e r a n biomass were o b s e r v e d . N o r m a l l y c l a d o c e r a n b i o m a s s i n c r e a s e d i n s p r i n g and summer, p e a k i n g i n J u l y or A u g u s t , t h e n d e c r e a s e d i n f a l l . Copepod b i o m a s s , s p e c i f i c a l l y D. o r e g o n e n s i s , g e n e r a l l y d e c r e a s e d i n t h e summer and r e c o v e r e d s l i g h t l y by September. No r e g u l a r p a t t e r n was d e t e c t e d f o r c y c l o p o i d c o p e p o d s . As e x p e c t e d from c o m p a r i s o n s of h a r v e s t e d and l a k e z o o p l a n k t o n s a m p l e s , h a r v e s t i n g i n 1979 p r o d u c e d a s i g n i f i c a n t d i f f e r e n c e i n c h a n g e s i n t h e b i o m a s s of c l a d o c e r a n s . C l a d o c e r a n b i o m a s s d e c r e a s e d by more t h a n 50% from J u l y t o A u g u s t ; no s i m i l a r c h a n g e s i n c l a d o c e r a n biomass d u r i n g t h i s t i m e p e r i o d were o b s e r v e d i n K a t h e r i n e L a k e . C a l a n o i d c o p e p o d biomass d i d n o t change s i g n i f i c a n t l y d u r i n g t h e h a r v e s t p e r i o d i n 1979, nor d i d t h e biomass of c y c l o p o d c o p e p o d s o r s m a l l c l a d o c e r a n s . T r e n d s of t h e s e o r g a n i s m s were s i m i l a r i n b o t h l a k e s f o r t h i s t i m e p e r i o d . In 1980, no s i g n i f i c a n t c h a n g e s i n t h e biomass of any o f t h e z o o p l a n k t o n g r o u p s were o b s e r v e d d u r i n g t h e h a r v e s t p e r i o d of May t h r o u g h J u l y . T h i s s u g g e s t s t h a t h a r v e s t i n g d i d not have much impact on t h e z o o p l a n k t o n community i n 1980. However, s m a l l c l a d o c e r a n s i n c r e a s e d i n b i o m a s s s i g n i f i c a n t l y from May t o June e v e r y y e a r e x c e p t 1980. T o t a l c l a d o c e r a n 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 from May t o June i n 1978, 1979, and 1982. S i m i l a r o b s e r v a t i o n s were made i n K a t h e r i n e L a k e . However, u n l i k e o t h e r y e a r s and u n l i k e K a t h e r i n e L a k e , c y c l o p o i d c opepod b i o m a s s d e c r e a s e d s i g n i f i c a n t l y from J u l y t o A u g u s t and a g a i n i n 182 September 1980. In 1981 t h e r e were no s i g n i f i c a n t c h a n g e s d u r i n g t h e months of May t o September i n t h e biomass o f c a l a n o i d c o p e p o d s o r s m a l l c l a d o c e r a n s . As i n a l l o t h e r y e a r s e x c e p t 1980, i n c r e a s e s i n s m a l l c l a d o c e r a n s o c c u r r e d from May t o J u n e . As i n 1980, a s i g n i f i c a n t d e c r e a s e i n c y c l o p o i d c o p e p o d s o c c u r r e d from June t o A u g u s t . Copepod and c l a d o c e r a n f e r t i l i t y was a l s o examined. Copepod f e r t i l i t y i n K a t h e r i n e Lake i n t h e summer months was g e n e r a l l y e x t r e m e l y low and b e c a u s e of t h i s , s t a t i s t i c a l a n a l y s i s on K a t h e r i n e Lake c o p e p o d f e r t i l i t y was not p o s s i b l e . In P l a c i d L a k e , 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 t r e n d s of c o p e p o d f e r t i l i t y between t h e s i x y e a r s s t u d i e d . Copepod f e r t i l i t y t e n d e d t o i n c r e a s e s l i g h t l y i n t h e summer and t h e n d e c r e a s e i n September. Lowest f e r t i l i t y was o b s e r v e d i n September i n e a c h of t h e s i x y e a r s . S i g n i f i c a n t t r e n d d i f f e r e n c e s i n c l a d o c e r a n f e r t i l i t y o v e r t h e s i x y e a r s s t u d i e d were o b s e r v e d i n b o t h P l a c i d and K a t h e r i n e L a k e s ( F i g u r e V - 5 ) . G e n e r a l l y t h e t r e n d i n c l a d o c e r a n f e d c u n d i t y was one o f s t e a d i l y d e c r e a s i n g f e r t i l i t y d u r i n g t h e summer months, w i t h some r e c o v e r y i n September. T h i s t r e n d was s t a t i s t i c a l l y s i g n i f i c a n t i n P l a c i d Lake i n t h e y e a r s 1977, 1978, 1981 and 1982 and i n K a t h e r i n e L a k e i n t h e y e a r s 1977, 1978 and 1981. In P l a c i d Lake i n 1979, no r e c o v e r y i n September was o b s e r v e d . In P l a c i d Lake i n 1980 and K a t h e r i n e Lake i n y e a r s 1979, 1980 and 1982, no s t a t i s t i c a l l y s i g n i f i c a n t c h a n g e s i n c l a d o c e r a n f e r t i l i t y were o b s e r v e d . However, c l a d o c e r a n f e r t i l i t y i n K a t h e r i n e i n 1979 and 1982 d i d f o l l o w t h e d e c r e a s i n g t r e n d d u r i n g summer; r e c o v e r y 1 83 F i g u r e V-5 C l a d o c e r a n f e r t i l i t y i n P l a c i d L a k e , 1977-1982 183a 0 ivi—' J A ' s month 1 84 was o b s e r v e d i n 1979 but not 1982. The most s i g n i f i c a n t e x c e p t i o n t o t h e " n o r m a l " p a t t e r n was o b s e r v e d i n b o t h l a k e s i n 1980. C l a d o c e r a n f e r t i l i t y i n c r e a s e d i n t h e e a r l y p a r t of t h e summer (May, J u n e , J u l y ) . In P l a c i d , i t t h e n d r o p p e d s u d d e n l y i n A ugust and r e c o v e r e d i n September; i n K a t h e r i n e t h e d e c r e a s e was more g r a d u a l , s t i l l o c c u r r i n g i n September. In b o t h l a k e s , f e r t i l i t y was h i g h e r i n 1980 d u r i n g t h e months of June t h r o u g h September t h a n any o t h e r y e a r . S p e c i e s c o m p o s i t i o n o f t h e y e a r l y z o o p l a n k t o n b i o m a s s e s a r e shown i n A p p e n d i x V I I . The s e a s o n a l d i s t r i b u t i o n of t h e common s p e c i e s f o l l o w e d f a i r l y r e g u l a r p a t t e r n s . H o l o p e d i u m g i b b e r u m i s t h e d o m i n a n t c l a d o c e r a n i n e a r l y s p r i n g and u s u a l l y a l m o s t d i s a p p e a r s by t h e end of A u g u s t . D a p h n i a r o s e a biomass i n c r e a s e s as H o l o p e d i u m b i o m a s s d e c r e a s e s , u s u a l l y mid t o l a t e s p r i n g . D a p h n i a i s by f a r t h e dominant s p e c i e s i n terms of b i o m a s s i n t h e l a k e . Diaphanosoma b r a c h y u r u m o n l y o c c u r s i n t h e summer. Bosmi na l o n g i r o s t r i s and Cer i odaphn i a q u a d r a n g u l a g e n e r a l l y o c c u r i n low numbers t h r o u g h o u t t h e y e a r . Diaptomus  k e n a i o c c u r s i n t h e e a r l y s p r i n g , t h e n goes i n t o d i a p a u s e f o r t h e summer, and o f t e n has n o t r e a p p e a r e d by t h e t i m e s a m p l i n g i s h a l t e d f o r t h e w i n t e r . D i aptomus o r e q o n e n s i s i s t h e most common c o p e p o d i n P l a c i d L a k e . I t o c c u r s t h r o u g h o u t t h e y e a r but t e n d s t o be more abundant i n s p r i n g and f a l l . The c a r n i v o r o u s c y c l o p o i d c o p e p o d s , t h e dominant one b e i n g D i a c y c l o p s t h o m a s i , a l s o o c c u r t h r o u g h o u t t h e y e a r ; t h e i r b i o m a s s d o e s n o t f l u c t u a t e g r e a t l y , but t h e y do t e n d t o be more abundant i n s p r i n g and summer. 185 Some a n o m a l i e s i n z o 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 o c c u r r e d d u r i n g t h e t h r e e y e a r s of l a k e p e r t u r b a t i o n . H a r v e s t i n g z o o p l a n k t o n i n 1979 too k i t s t o l l a l m o s t e x c l u s i v e l y on t h e two l a r g e s t c l a d o c e r a n s , H o l o p e d i um and Daphn i a . D u r i n g t h e t i m e t h a t Daphn i a p o p u l a t i o n s were d e p r e s s e d , a l a r g e Cer i o d a p h n i a p o p u l a t i o n was e s t a b l i s h e d . At i t s peak, C e r i o d a p h n i a made up more t h a n h a l f of the t o t a l z o o p l a n k t o n b i o m a s s . As Daphn i a r e c o v e r e d and as w i n t e r s e t i n , C e r i o d a p h n i a numbers d r o p p e d . In 1980, C e r i o d a p h n i a were a g a i n abundant r e l a t i v e t o o t h e r y e a r s , e s p e c i a l l y d u r i n g J u l y , and September t h r o u g h e a r l y O c t o b e r . However, i n t h i s y e a r t h e i n c r e a s e was not c o i n c i d e n t a l w i t h d e c r e a s e s i n o t h e r c l a d o c e r a n s , nor was i t as l a r g e as i n 1979. In 1981, when no z o o p l a n k t o n were removed from t h e l a k e but t u r b u l e n c e was r e p l i c a t e d , t h e H o l o p e d i u m p o p u l a t i o n d e c r e a s e d d r a m a t i c a l l y a t t h e end of Ju n e , b e f o r e t h e D a p h n i a p o p u l a t i o n was l a r g e . I t r e c o v e r e d s l i g h t l y by e a r l y A u g u s t , t h e n d e c l i n e d as i t n o r m a l l y d o e s . An u n u s u a l o c c u r r e n c e i n 1981 was t h e e s t a b l i s h m e n t of a l a r g e Bosmina p o p u l a t i o n i n l a t e September t o mid November. At t h i s t i m e , Bosmina made up more t h a n 30% of t h e l a k e ' s z o o p l a n k t o n b i o m a s s . P a t t e r n s of 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 i n P l a c i d Lake v a r i e d from y e a r t o y e a r , but i n g e n e r a l , 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 i n c r e a s e d i n s p r i n g , s t a y e d h i g h d u r i n g summer, and d e c r e a s e d a g a i n i n f a l l . S i m i l a r t r e n d s were o b s e r v e d i n K a t h e r i n e L a k e . O c c a s s i o n a l blooms o f d i a t o m s o r c o l o n i a l b l u e -g r e e n a l g a e c a u s e d s i g n i f i c a n t d i f f e r e n c e s i n p h y t o p l a n k t o n p o p u l a t i o n t r e n d s between t h e s i x y e a r s s t u d i e d . In 1977, a 186 bloom of 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 d e v e l o p e d i n l a t e J u l y and p e r s i s t e d t h r o u g h t h e end of September. In 1978, a bloom of l a r g e c e l l s ( d i a t o m s , D i n o b r y o n and O p h i o c y t ium ) i n mid J u l y was f o l l o w e d by a M e r i s m o p e d i a bloom i n e a r l y A u g u s t w h i c h l a s t e d u n t i l mid September. In 1979, c o i n c i d e n t a l w i t h t h e d r a m a t i c d e c r e a s e i n z o o p l a n k t o n b i o m a s s t h e r e was a huge M e r i s m o p e d i a bloom, t w i c e t h e m a g n i t u d e o f t h e one i n 1977. The bloom l a s t e d u n t i l mid September, when z o o p l a n k t o n b i o m a s s was r e c o v e r i n g . In 1980 a mixed s p e c i e s bloom o f d i a t o m s , D i n o b r y o n and c o l o n i a l g r e e n and 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 a t e J u n e . The d i a t o m bloom was s h o r t - l i v e d but t h e c o l o n y bloom, a l b e i t a r e l a t i v e l y s m a l l one, l a s t e d u n t i l l a t e September t h e c o m p o s i t i o n of t h e bloom i n 1980 ch a n g e d s e a s o n a l l y . The g r e e n G l a o e c y s t i s was t h e dominant form i n A p r i l , May, and J u n e ; i t was s t i l l i m p o r t a n t i n J u l y but by A u g u s t , t h e b l u e - g r e e n M e r i s m o p e d i a d o m i n a t e d s i g n i f i c a n t l y , and c o n t i n u e d t o d o m i n a t e u n t i l O c t o b e r , when t h e bloom d e c l i n e d s u b s t a n t i a l l y . A n o t h e r bloom of d i a t o m s o c c u r r e d b r i e f l y i n l a t e J u l y . In 1981, d i a t o m blooms were e x t r e m e l y common. The f i r s t bloom began i n e a r l y May and l a s t e d u n t i l e a r l y J u n e . Two v e r y l a r g e blooms o c c u r r e d i n l a t e June and l a t e A u g u s t . A f o u r t h d i a t o m bloom o c c u r r e d i n O c t o b e r . D u r i n g t h e b r i e f o b s e r v a t i o n p e r i o d i n 1982, no s i g n i f i c a n t c hanges i n e i t h e r 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 or b i o m a s s o f any c o n s t i t u e n t g r o u p were o b s e r v e d . 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 p a t t e r n s were s t a t i s t i c a l l y s i m i l a r i n P l a c i d and K a t h e r i n e L a k e s i n 1977, when l a r g e b l u e -g r e e n blooms o c c u r r e d i n b o t h l a k e s f r o m l a t e J u l y t h r o u g h 187 September, i n 1982, when l i t t l e o f i n t e r e s t o c c u r r e d i n e i t h e r l a k e , and i n 1981. The l a c k o f s i g n i f i c a n t d i f f e r e n c e s between l a k e s i n 1981 i s an a r t i f a c t of e x t r e m e l y h i g h v a r i a n c e s i n t h e m o n t h l y means of p h y t o p l a n k t o n b i o m a s s i n P l a c i d Lake due t o t h e d r a m a t i c c h a n g e s i n biomass c a u s e d by blooms. T h e r e was l i t t l e c hange i n p h y t o p l a n k t o n community s t r u c t u r e i n K a t h e r i n e Lake i n 1981, w h i l e f o u r c o n c u r r e n t d i a t o m blooms o c c u r r e d i n P l a c i d L a k e . T r e n d s were s t a t i s t i c a l l y d i f f e r e n t i n 1978, a y e a r when l i t t l e s e a s o n a l v a r i a t i o n i n t h e p h y t o p l a n k t o n p o p u l a t i o n o c c u r r e d i n K a t h e r i n e w h i l e blooms o f d i a t o m s and b l u e - g r e e n s o c c u r r e d i n P l a c i d , and i n 1979 and 1980, t h e two y e a r s of h a r v e s t i n g i n P l a c i d L a k e . B l u e - g r e e n blooms o c c u r r e d i n b o t h l a k e s i n 1979 a l t h o u g h t h e one i n K a t h e r i n e o c c u r r e d a b o u t a month l a t e r and was o n l y o n e - t h i r d t h e m a g n i t u d e of t h e one i n P l a c i d L a k e . In 1980, d i a t o m blooms o c c u r r e d i n June i n b o t h l a k e s ; however t h e d i a t o m bloom i n P l a c i d i n A u g u s t was not r e p e a t e d i n K a t h e r i n e . A b l u e - g r e e n bloom s i m i l a r t o t h e one w h i c h o c c u r r e d i n P l a c i d i n A u g u s t o c c u r r e d i n K a t h e r i n e i n September. In a d d i t i o n t o c h a n g e s i n b i o m a s s by c o l o n i e s and l a r g e s i n g l e c e l l s d i s c u s s e d above, s e a s o n a l c h a n g e s i n t h e s e v e r a l s i z e c a t e g o r i e s of s m a l l e r s i n g l e c e l l e d p h y t o p l a n k t o n c a n be see n i n A p p e n d i x V I I I . No r e c o g n i z a b l e p a t t e r n s and 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 t r e n d s between y e a r s o r between l a k e s were o b s e r v e d i n t h e b i o m a s s o f c e l l s s m a l l e r t h a n 2 um d i a m e t e r . F o r t h e c e l l s i z e g r o u p of 2-6 Mm d i a m e t e r , no r e c o g n i z a b l e p a t t e r n of s e a s o n a l change was o b s e r v e d i n P l a c i d 188 L a k e , but d i f f e r e n c e s between y e a r s were s i g n i f i c a n t . No s i g n i f i c a n t c h a n g e s o c c u r r e d from May t o September i n 1977, 1978, 1979, or 1981. In 1980, a s i g n i f i c a n t d e c r e a s e o c c u r r e d from June t o J u l y . In 1982, a s i g n i f i c a n t i n c r e a s e o c c u r r e d from J u l y t o A u g u s t . No r e c o g n i z a b l e p a t t e r n was o b s e r v e d i n K a t h e r i n e Lake e i t h e r . No c h a n g e s s i m i l a r t o t h o s e i n w h i c h o c c u r r e d i n P l a c i d Lake i n 1980 o r 1982 o c c u r r e d i n K a t h e r i n e L a k e ; s i g n i f i c a n t c h a n g e s were a l s o not d e t e c t e d i n 1977, 1979 or 1981. In 1978 a s i g n i f i c a n t d e c r e a s e was o b s e r v e d from May t o September. No r e c o g n i z a b l e p a t t e r n or s i g n i f i c a n t d i f f e r e n c e s between y e a r s or l a k e s were o b s e r v e d f o r t h e c e l l s i z e c l a s s of 6-10 Mm maximum l i n e a r d i m e n s i o n . Biomass of t h e g r o u p of 10-14 Mm c e l l s seemed t o d e c r e a s e from s p r i n g t o summer, t h e n . r e c o v e r a g a i n i n f a l l , however t h e s e c h a n g e s were no t s i g n i f i c a n t most y e a r s . 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 t r e n d s between y e a r s or l a k e s . In P l a c i d L a k e , t h e r e was no r e c o g n i z a b l e p a t t e r n i n bi o m a s s c h a n g e s of c e l l s of t h e 14-20 Mm g r o u p and no s i g n i f i c a n t t r e n d d i f f e r e n c e s between y e a r s were d e t e c t e d . No s i g n i f i c a n t c h a n g e s i n t h e b i o m a s s of t h e s e c e l l s o c c u r r e d i n K a t h e r i n e Lake i n any y e a r s e x c e p t 1980 when a s i g n i f i c a n t d e c r e a s e o c c u r r e d from J u l y t o A u g u s t , f o l l o w e d by a s i g n i f i c a n t i n c r e a s e from A u g u s t t o September. In e n c l o s u r e e x p e r i m e n t s ( C h a p t e r IV) I f o u n d t h a t , g i v e n t h e d e g r e e of d e t a i l of t h e p h y t o p l a n k t o n t a x o n o m i c d a t a r e c o r d e d , c h a n g e s i n s p e c i e s c o m p o s i t i o n of t h e s i n g l e - c e l l e d p h y t o p l a n k t o n w i t h g r a z i n g p r e s s u r e were o n l y v e r y m i n o r . In s e l e c t i v e f e e d i n g e x p e r i m e n t s ( C h a p t e r I I I ) , p h y t o p l a n k t o n 189 t a x o n o m i c d a t a were n o t d e t a i l e d enough f o r me t o d e t e r m i n e whether any of t h e s i n g l e - c e l l e d p h y t o p l a n k t o n s p e c i e s were d e f i n i t e l y e d i b l e or n o t . T h e r e was e v i d e n c e t h a t some c o l o n i a l s p e c i e s ( e g . G l a e o c y s t i s ) may be g r a z e d w h i l e o t h e r s ( e g . Mer i smopedia ) a r e n o t . P r e l i m i n a r y e x a m i n a t i o n of l a k e p h y t o p l a n k t o n t a x o n o m i c d a t a a l s o d i d not r e v e a l any n o t e w o r t h y c h a n g e s i n t h e c o m p o s i t i o n o f t h e s i n g l e - c e l l e d p h y t o p l a n k t o n . No s e l e c t i v e g r a z i n g of s i n g l e c e l l e d p h y t o p l a n k t o n was a p p a r e n t . T h e r e f o r e , I r e s t r i c t e d my o b s e r v a t i o n s o f l a k e p h y t o p l a n k t o n t a x o n o m i c d a t a t o t h e c o m p o s i t i o n of blooms and t h e s e a s o n a l c h a n g e s i n c o l o n y t a x a ( T a b l e V - 2 ) . S e a s o n a l s u c c e s s i o n of c o l o n i e s f o l l o w e d q u i t e a r e g u l a r p a t t e r n . The g r e e n a l g a e G l a e o c y s t i s, O o c y s t i s and Scenedesmus and t h e c h r y s o p h y t e D i n o b r y o n were common i n s p r i n g . The b l u e -g r e e n Mer i smopedia was a l s o common and by June and J u l y was g e n e r a l l y t h e dominant f o r m . In A u g u s t , Mer i smopedia c o m p r i s e d t h e b u l k of t h e c o l o n y b i o m a s s . The g e l a t i n o u s b l u e - g r e e n C h r o o c o c c u s was a l s o common i n t h e summer. In l a t e September and O c t o b e r , O o c y s t i s, Scenedesmus and D i n o b r y o n were r e e s t a b l i s h e d a s s i g n i f i c a n t components of t h e c o l o n y f r a c t i o n 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 . C o l o n y s u c c e s s i o n i n 1981 was c o n s i d e r a b l y d i f f e r e n t from t h e p r e v i o u s f o u r y e a r s . L i t t l e d a t a were a v a i l a b l e f o r A p r i l and May; c o l o n y c o m p o s i t i o n d u r i n g t h e s e months a p p e a r e d t o be s i m i l a r t o p r e v i o u s y e a r s . However, i n June M e r i s m o p e d i a , t h e do m i n a n t c o l o n y by t h i s t i m e i n o t h e r y e a r s , was n o t even common. The June 1981 c o l o n y b i o m a s s was d o m i n a t e d by t h e g r e e n TABLE V-2 - MAJOR COLONY TAXA MONTH APRIL MAY JUNE 1977 Merismopedia(Z) Glaeocystis (1) 1978 Glaeocystis(S) Merismopedia(1) Glaeocystis (15) Oocystis (6) Merismopedia(l) Merismopediai'6) Glaeocystis (Q ) Dinobryon(4) YEAR 1979 Glaeocystis(Z) Scenedesmus(2) Merismopedia(2) Merismopedia (15) Glaeocystis (?) Oocystis (7) Merismopedia (17) Glaeocystis (10) Scenedesmus (.4) JULY Merismopedia(9) AUGUST Merismopedia(4) Merismopedia912) Merismopedia(24) Chroococcus (4) Glaeocystis (6) Dinobryon(4) Chroococcus (5) Merismopedia(ZO) Chroococcus (10) Glaeocystis (2) 1980 Glaeocystis (Z) Scenedesmus(2) Merismopedia(1) Glaeocystis (10) Merismopedia (6) Glaeocystis (7) Merismopedia(6) 1981 Merismopedia(l) Glaeocystis (1) Oocystis (1) Oocystis (5) Dinobryon(4) Glaeocystis (2) Merismopedid(l2) Chroococcus(10) Glaeocystis (10) Merismopediai'5) Chroococcus (4) Oocystis(l) Merismopedia(lS) Merismopedia(10) Chroococcus,(6) Glaeocystis (4) Glaeocystis (1) Chroococcus (1) SEPTEMBER Merismopediai'8) Merismopedia(9) Merismopedia(lZ) Merismopediai'7') Chroococcus(1) Glaeocystis(6) Glaeocystis (6) Chroococcus (1) Scenedesmus(1) Scenedesmus(1) Merismopedia (17) Chroococcus(9) OCTOBER Oocystis (2) Merismopedia(9) Merismopedia(l) Glaeocystis (Z) Chroococcus(Z) Merismopedia(4) Merismopedia(16) Chroococcus (Z) Chroococcus (9) Dinobryon(l) Scenedesmus (1) *numbers i n brackets - number of sample counts where species was common no colony taxonomy data available f or 1982 191 a l g a e O o c y s t i s and t h e c h r y s o p h y t e D i n o b r y o n , n e i t h e r of w h i c h were o b s e r v e d as t h e dominant f o r m a t any o t h e r t i m e . O o c y s t i s r e m a i n e d common i n J u l y and Mer i s mopedia d i d not become t h e d o m i n a n t form u n t i l A u g u s t . F a l l p a t t e r n s were s i m i l a r t o p r e v i o u s y e a r s . DISCUSSION P l a n k t o n Dynamics The e f f e c t s of t h e h a r v e s t i n g e f f o r t i n 1979 on t h e z o o p l a n k t o n biomass i n P l a c i d L a k e a r e q u i t e e v i d e n t . Z o o p l a n k t o n biomass d e c r e a s e d by more t h a n 60%, a d e c r e a s e u n p a r a l l e l e d i n K a t h e r i n e L a k e . In s p i t e of t h i s m a s s i v e d e c r e a s e 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 , t h e biomass o f c o p e p o d s , p a r t i c u l a r l y c y c l o p o i d c o p e p o d s , r e m a i n e d c o n s t a n t d u r i n g h a r v e s t i n g p e r i o d . T h i s may have been due t o i n c r e a s e d n a u p l i a r s u r v i v a l i n t h e e v e n t of d e c r e a s e d c o m p e t i t o n f o r f o o d w i t h c l a d o c e r a n s (Bowerman 1983) as w e l l a s a l o w e r s u s c e p t i b i l i t y t o c a p t u r e by t h e h a r v e s t i n g t e c h n i q u e . The d e c r e a s e i n g r a z i n g p r e s s u r e was t h e r e f o r e even g r e a t e r t h a n t h e d e c r e a s e i n z o o p l a n k t o n b i o m a s s . In a d d i t i o n t o t h e e f f e c t on g r a z i n g p r e s s u r e , t h e r e m o v a l of l a r g e q u a n t i t i e s o f z o o p l a n k t o n from t h e s y s t e m amounted t o r e m o v a l of t h e n u t r i e n t s w h i c h t h e y c o n t a i n . T h i s r e d u c t i o n i n t o t a l n u t r i e n t s c o u p l e d w i t h t h e s u b s e q u e n t r e d u c t i o n i n n u t r i e n t r e m i n e r a l i z a t i o n r a t e s p r o b a b l y r e s u l t e d i n r e d u c e d n u t r i e n t a v a i l a b i l t y t o p h y t o p l a n k t o n . At t h e same t i m e , t h e o c c u r r e n c e s of s e v e r a l l a r g e d i a t o m blooms 192 d u r i n g t h e c o n t r o l y e a r , 1981, s u g g e s t t h a t t h e h a r v e s t i n g p r o c e s s may have c r e a t e d enough v e r t i c a l m i x i n g t o r e t u r n n u t r i e n t s t o t h e e p i l i m n i o n . C o i n c i d e n t w i t h t h e d e c l i n e i n z o o p l a n k t o n b i o m a s s i n 1979 was a t w o - f o l d i n c r e a s e i n p h y t o p l a n k t o n b i o m a s s . However, u n l i k e 1981, v i r t u a l l y a l l of t h i s i n c r e a s e was due t o a t remendous bloom of t h e i n e d i b l e c o l o n i a l 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 sp. F e r r a n t e (1976) f o u n d i n h i s s t u d y l a k e t h a t a l t h o u g h ' M e r i s m o p e d i a was d o m i n a n t i n most p h y t o p l a n k t o n s a m p l e s , i t was not f o u n d i n any z o o p l a n k t o n gut a n a l y s e s . A r n o l d (1971) f o u n d t h a t M e r i s m o p e d i a was d e t r i m e n t a l t o Daphn i a p u l e x s u r v i v a l . In s e l e c t i v e f e e d i n g s t u d i e s ( C h a p t e r I I I ) , I c o n c l u d e d t h a t M e r i s m o p e d i a was p r o b a b l y not g r a z e d s i n c e t h e p r e s e n c e o f g r a z e r s i n most c a s e s t e n d e d t o e n h a n c e M e r i s m o p e d i a s t a n d i n g c r o p s . Why s h o u l d g r a z e r r e m o v a l f a v o u r growth of a s p e c i e s t h a t t h e o r e t i c a l l y s h o u l d not be l i m i t e d by g r a z e r s ? A s i m i l a r phenomenon was o b s e r v e d by L y n c h and S h a p i r o (1981) who removed g r a z e r s from one h a l f o f P l e a s a n t Pond by i n t r o d u c i n g p l a n k t i v o r o u s f i s h . They f o u n d t h a t a s f i s h removed l a r g e z o o p l a n k t o n g r a z e r s from P l e a s a n t Pond S o u t h , i n i t i a l l y a d e n s e growth of t h e e d i b l e d e s m i d A n k i s t r o d e s m u s s p . d e v e l o p e d , s u c c e e d e d by s e v e r a l s p e c i e s of s m a l l e r p h y t o p l a n k t o n , i n c l u d i n g t h e b l u e - g r e e n C h r o o c o c c u s d i s p e r s u s . E v e n t u a l l y , as f i s h removed v i r t u a l l y a l l t h e c l a d o c e r a n s and c o p e p o d s , t h i s growth was r e p l a c e d by s e v e r a l o t h e r s p e c i e s o f b l u e - g r e e n a l g a e . L y n c h and S h a p i r o a l s o f o u n d t h a t p h y t o p l a n k t o n b i o m a s s t o t o t a l p h o s p h o r u s r a t i o s d e c r e a s e d as g r a z e r b i o m a s s d e c r e a s e d , 1 93 s u g g e s t i n g t h a t p h o s p h o r u s became l e s s l i m i t i n g t o p h y t o p l a n k t o n g r o w t h as g r a z e r b i o m a s s d e c r e a s e d . Reduced n u t r i e n t r e m i n e r a l i z a t i o n of b i o l o g i c a l l y r e a c t i v e n i t r o g e n may have l e d t o n i t r o g e n l i m i t a t i o n of e u k a r y o t i c p h y t o p l a n k t o n and g i v e n a c o m p e t i t i v e a d v a n t a g e t o 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 e . As was m e n t i o n e d i n C h a p t e r I , b e c a u s e P l a c i d Lake i s a bog l a k e , t h e r a t i o of n i t r o g e n t o p h o s p h o r u s t e n d s t o be r e l a t i v e l y low. N u t r i e n t i n p u t and r e c y c l i n g r a t e s ( T a b l e V-1) a l s o s u g g e s t n u t r i e n t l o a d i n g w i t h low N:P r a t i o s . A l t h o u g h t o t a l p h y t o p l a n k t o n growth i n f r e s h w a t e r i s n o r m a l l y l i m i t e d by p h o s p h o r u s ( S c h i n d l e r 1977), l i m i t e d a v a i l a b i l i t y of n i t r o g e n (and o t h e r n u t r i e n t s ) may be an i m p o r t a n t f a c t o r d e t e r m i n i n g t h e c o m p o s i t i o n of t h e p h y t o p l a n k t o n community. R e s u l t s of e n r i c h m e n t s t u d i e s ( F l e t t e t a l . 1980) have d e m o n s t r a t e d t h a t t h e a d d i t i o n of n u t r i e n t s w i t h low N:P r a t i o s d o e s t e n d t o r e s u l t i n b l u e - g r e e n blooms s i n c e o t h e r forms soon become n i t r o g e n l i m i t e d . The r e m o v a l of z o o p l a n k t o n i n 1979 may have d e c r e a s e d t h e a v a i l a b i l i t y of u t i l i z a b l e forms of n i t r o g e n r e q u i r e d by n o n - n i t r o g e n f i x i n g t a x a t o t h e e x t e n t t h a t t h e y became n i t r o g e n l i m i t e d . T h e i r c o m p e t i t i o n w i t h n i t r o g e n - f i x i n g t a x a f o r p h o s p h a t e would d e c l i n e , a l l o w i n g t h e b l u e - g r e e n s t o t h r i v e . A l t h o u g h I was u n a b l e t o f i n d from t h e l i t e r a t u r e w hether M e r i s m o p e d i a a r e n i t r o g e n - f i x i n g , B othe (1982) f o u n d t h a t i n g e n e r a l , t h o s e b a c t e r i a f o r m i n g g e l a t i n o u s s h e a t h s , as M e r i s m o p e d i a d o e s , a r e a b l e t o f i x n i t r o g e n . A c y a n o p h y t e c l o s e l y r e l a t e d t o M e r i s m o p e d i a , C h r o o c o c c u s , i s a b l e t o f i x n i t r o g e n ( S t e w a r t 1973). N i t r o g e n - f i x i n g b a c t e r i a and b l u e -193a Page 194 missed i n numbering. 1 95 g r e e n a l g a e have been c i t e d a s i m p o r t a n t s o u r c e s of b i o l o g i c a l l y r e a c t i v e n i t r o g e n f o r l a k e s ( H o m e and D i l l a r d 1972, H o m e and Goldman 1972, S c h i n d l e r 1977). N i t r o g e n f i x e d by t h e s e o r g a n s i m s may become a v a i l a b l e t o o t h e r p h y t o p l a n k t o n v i a d i r e c t l e a k a g e from c e l l s , p r e d a t i o n and e x c r e t i o n by g r a z e r s , or by a u t o l y s i s . However, t h e p r e s e n c e of a m u c i l a g e s h e a t h a r o u n d t h e b l u e - g r e e n a l g a l c e l l s may slow t h e l e a k a g e p r o c e s s , and the p r e s e n c e of ammonium or n i t r a t e s i n t h e m u c i l a g e s h e a t h may a c t u a l l y i n h i b i t n i t r o g e n - f i x i n g ( r e v i e w e d i n W e t z e l 1975). S i n c e M e r i s m o p e d i a i s not v e r y e d i b l e and may c a u s e m o r t a l i t y of t h o s e g r a z e r s w h i c h f e e d on i t ( A r n o l d 1971), r e c y c l i n g r a t e s of n i t r o g e n v i a g r a z e r s may a l s o be low. A l t h o u g h n i t r o g e n - f i x i n g may p r o v i d e t h e n i t r o g e n - f i x i n g o r g a n i s m s w i t h t h e n i t r o g e n t h e y need and may c o n t r i b u t e t o t h e t o t a l n i t r o g e n c o n t e n t o f a l a k e , i t may not b e n e f i t o t h e r p h y t o p l a n k t o n i n t h e s h o r t t e r m . The s u c c e s s of c o l o n i a l b l u e - g r e e n a l g a e has a l s o been a t t r i b u t e d t o t h e i r m i n i m a l l o s s e s t o s i n k i n g , s i n c e many s p e c i e s , p a r t i c u l a r l y t h e s c u m - f o r m i n g ones, p o s s e s s gas v a c u o l e s w h i c h make them e x t r e m e l y b u o y a n t ( R e y n o l d s and Walsby 1975). A l t h o u g h M e r i s m o p e d i a does not p o s s e s s gas v a c u o l e s ( W h i t t o n 1973), i t may d e r i v e c o n s i d e r a b l e buoyancy from i t s c o n f i g u r a t i o n ( l a r g e f l a t s h e e t s ) . Murphy e t a l . (1976) and K e a t i n g (1977,1978) have d e m o n s t r a t e d t h a t some b l u e - g r e e n a l g a e have a l l e l o p a t h i c e f f e c t s on o t h e r a l g a e and a r e t h u s a b l e t o d o m i n a t e . A l t h o u g h M e r i s m o p e d i a was not m e n t i o n e d i n any of t h e s e p a p e r s , A r n o l d ' s (1971) o b s e r v a t i o n t h a t M e r i s m o p e d i a i s d e t r i m e n t a l t o D a p h n i a 196 s u r v i v a l and g r o w t h s u g g e s t s t h a t i t may p r o d u c e t o x i c m e t a b o l i t e s w h i c h have an a n t i b i o t i c e f f e c t . ( I n c i d e n t l y , t h e a d v e r s e e f f e c t of M e r i s m o p e d i a on z o o p l a n k t o n may have c o n t r i b u t e d t o low f e c u n d i t y i n A u g u s t and September of 1979 and h e l p e d t o d e p r e s s t h e z o o p l a n k t o n p o p u l a t i o n i n A u g u s t . ) The a d v a n t a g e t o b l u e - g r e e n s of t h i s a l l e l o p a t h i c e f f e c t i s o n l y i n c u r r e d when b l u e - g r e e n a l g a e a r e a l r e a d y a t r e l a t i v e l y h i g h d e n s i t i e s , s i n c e t h e a l l e l o p a t h i c s u b s t a n c e s a r e not g r o w t h -i n h i b i t i n g t o o t h e r a l g a e a t h i g h d i l u t i o n s ( K e a t i n g 1978). A l l e l o p a t h y may e x p l a i n why b l u e - g r e e n blooms l a s t so l o n g , r e l a t i v e t o , f o r example, d i a t o m blooms, but i t does not e x p l a i n t h e e s t a b l i s h m e n t of t h e blooms. S m i t h and K a l f f (1983) have f o u n d t h a t p h o s p h a t e u p t a k e r a t e s . and g r o w t h r a t e s of c o l o n i a l b l u e - g r e e n a l g a e a r e r e l a t i v e l y slow and t h a t b l u e - g r e e n s a r e not p a r t i c u l a r l y good c o m p e t i t o r s f o r p h o s p h a t e , e s p e c i a l l y a t low p h o s p h a t e c o n c e n t r a t i o n s . E x t e n s i v e r e s e a r c h of n u t r i e n t c o m p e t i t i o n between p h y t o p l a n k t o n s p e c i e s ( T i l m a n 1977, T i l m a n e t a l . 1982, Sommer 1983,1984) has shown t h a t i n c o n t i n u o u s c u l t u r e a t s t e a d y s t a t e c o n d i t i o n s , d i f f e r e n t p h y t o p l a n k t o n s p e c i e s c a n c o e x i s t o n l y when a l l a r e l i m i t e d by d i f f e r e n t r e s o u r c e s . When t h e s p e c i e s a r e l i m i t e d by t h e same r e s o u r c e , t h e one w i t h t h e g r e a t e s t a b i l i t y t o p r o c u r e and u t i l i z e t h a t r e s o u r c e w i l l e v e n t u a l l y e x c l u d e t h e o t h e r s . The s u c c e s s of c o l o n i a l b l u e -g r e e n a l g a e , w h i c h a r e p r i m a r i l y l i m i t e d by p h o s p h a t e a v a i l a b i l i t y , i n t h e p r e s e n c e o f many p h y t o p l a n k t o n s p e c i e s w h i c h a r e s u p e r i o r c o m p e t i t o r s f o r p h o s p h a t e s u g g e s t s t h a t 1 97 p h o s p h a t e c a n n o t be t h e l i m i t i n g r e s o u r c e f o r t h e s e o t h e r s p e c i e s . The c o i n c i d e n c e of t h i s tremendous b l u e - g r e e n s u c c e s s w i t h r e d u c t i o n i n n u t r i e n t r e g e n e r a t i o n s u g g e s t s t h a t n i t r o g e n i s a major l i m i t i n g f a c t o r f o r t h e n o n - n i t r o g e n f i x i n g p h y t o p l a n k t o n . I t i s n o t e w o r t h y t h a t d u r i n g 1981, when no z o o p l a n k t o n r e m o v a l o c c u r r e d and n u t r i e n t s may a c t u a l l y have been r e t u r n e d t o t h e e p i l i m n i o n v i a a r t i f i c i a l m i x i n g , no s i g n i f i c a n t bloom of b l u e - g r e e n a l g a e o c c u r r e d . The a p p a r e n t d e f i c i e n c y of n i t r o g e n i n 1979 s u g g e s t s t h a t t h e i n p u t of n u t r i e n t s v i a m i x i n g was i n s i g n i f i c a n t r e l a t i v e t o n u t r i e n t r e m o v a l v i a h a r v e s t i n g . A x l e r e t a l . (1980) f o u n d t h a t summer p h y t o p l a n k t o n p o p u l a t i o n s i n o l i g o t r o p h i c C a s t l e Lake were n i t r o g e n d e f i c i e n t and t h a t 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 sometimes l i m i t e d p r i m a r y p r o d u c t i v i t y . In a s u b s e q u e n t s t u d y , A x l e r e t a l . (1981) f o u n d t h a t ammonium a s s i m i l a t i o n r a t e s of p h y t o p l a n k t o n were s t r o n g l y c o r r e l a t e d w i t h 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 ; t h i s c o r r e l a t i o n e x t e n d e d t o s i g n i f i c a n t d i e l v a r i a t i o n i n ammonium u p t a k e and e x c r e t i o n . They c o n c l u d e d t h a t p h y t o p l a n k t o n were s u b s t a n t i a l l y d e p e n d e n t on n i t r o g e n r e m i n e r a l i z e d by z o o p l a n k t o n f o r g r o w t h . A n o t h e r f a c t o r w h i c h p o i n t s t o n i t r o g e n l i m i t a t i o n of e u k a r y o t i c p h y t o p l a n k t o n g i v i n g c o m p e t i t i v e a d v a n t a g e t o b l u e -g r e e n a l g a e i s t h a t once e s t a b l i s h e d , b l u e - g r e e n s p e r s i s t i n t h e l a k e u n t i l f a l l when t h e r m a l s t r a t i f i c a t i o n b r e a k s down ( A p p e n d i x I I ) and t h e r e t u r n o f n u t r i e n t s from t h e s e d i m e n t s t o t h e e p i l m n i o n by v e r t i c a l m i x i n g may o c c u r . At t h i s t i m e , b i o l o g i c a l l y a c t i v e n i t r o g e n s h o u l d be a v a i l a b l e f o r e u k a r y o t i c 1 98 p h y t o p l a n k t o n g r o w t h . The p e r s i s t e n c e o f b l u e - g r e e n blooms u n t i l f a l l o c c u r r e d not o n l y i n 1979, but e v e r y o t h e r y e a r when b l u e - g r e e n blooms o c c u r r e d . The s u c c e s s o f C e r i o d a p h n i a as a r e s u l t of h a r v e s t i n g may be due t o i t s d e m o g r a p h i c a d v a n t a g e o v e r Daphn i a , one of i t s main c o m p e t i t o r s ( L y n c h 1978). C o m p a r i s o n s of l a k e and h a r v e s t e d z o o p l a n k t o n showed t h a t l a r g e c l a d o c e r a n s were more s u s c e p t i b l e t o h a r v e s t i n g and t h a t s m a l l c l a d o c e r a n s were r e l a t i v e l y l e s s a f f e c t e d . The h i g h d e n s i t y of g e l a t i n o u s b l u e -g r e e n c o l o n i e s p r e s e n t may a l s o have f a v o u r e d t h e s m a l l e r c l a d o c e r a n s . Webster and P e t e r s (1978) and P o r t e r and McDonough (1984) f o u n d t h a t i n t h e p r e s e n c e of f i l a m e n t o u s b l u e - g r e e n a l g a e , 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 e x p e n d e d c o n s i d e r a b l e t i m e and e n e r g y c l e a r i n g t h e i r f i l t e r i n g a p p a r t u s of unwanted b l u e - g r e e n a l g a e . S m a l l c l a d o c e r a n s d i d n o t have t o r e j e c t t h e s e a l g a e b e c a u s e t h e f i l a m e n t s were t o o l a r g e t o g e t t h r o u g h t h e c a r a p a c e gape i n t h e f i r s t p l a c e . The d r a m a t i c d e c r e a s e i n z o o p l a n k t o n b i o m a s s t h a t o c c u r r e d i n 1979 as a r e s u l t of h a r v e s t i n g d i d not o c c u r i n 1980. A n a l y s i s of v a r i a n c e done on m o n t h l y means of z o o p l a n k t o n o b s e r v a t i o n s d i d not d e t e c t any s i g n i f i c a n t d i f f e r e n c e s between months i n 1980. However t h i s d o e s not mean t h a t h a r v e s t i n g had no e f f e c t . 1980 was t h e o n l y y e a r of t h e s i x s t u d i e d when s p r i n g z o o p l a n k t o n and p h y t o p l a n k t o n p o p u l a t i o n s i n c r e a s e d s i m u l t a n e o u s l y i n s t e a d of t h e u s u a l p a t t e r n o f p h y t o p l a n k t o n i n c r e a s e s l a g g i n g b e h i n d z o o p l a n k t o n i n c r e a s e s . (The u s u a l p a t t e r n of z o o p l a n k t o n i n c r e a s e s p r e c e e d i n g p h y t o p l a n k t o n 199 i n c r e a s e s d i d o c c u r i n K a t h e r i n e Lake i n 1980.) Removal o f z o o p l a n k t o n may have d e l a y e d t h e z o o p l a n k t o n i n c r e a s e and d e c r e a s e d g r a z i n g p r e s s u r e enough t o a l l o w an i n c r e a s e i n p r i m a r y p r o d u c t i o n . T h e o r e t i c a l l y , w h i l e t h e e f f e c t s o f s t i m u l a t e d p h y t o p l a n k t o n g r o w t h due t o 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 may o v e r c o m p e n s a t e f o r g r a z i n g l o s s e s i n summer, i n s p r i n g when t e m p e r a t u r e s a r e s t i l l low and z o o p l a n k t o n a s s i m i l a t i o n e f f i c i e n c i e s a r e h i g h , t h i s c o m p e n s a t o r y e f f e c t may not o c c u r . The t i m i n g of t h e h a r v e s t i n g may be a major r e a s o n f o r t h e l a c k of a s i g n i f i c a n t d e c r e a s e i n t h e g r a z e r p o p u l a t i o n . Even i n 1979, w h i l e h a r v e s t i n g was begun a t t h e b e g i n n i n g o f J u l y , i t was not u n t i l A u g u s t , when f e r t i l i t y was l o w e r , t h a t h a r v e s t i n g had an o b s e r v a b l e impact on t h e g r a z e r b i o m a s s . In June, when f e r t i l i t y was c o n s i d e r a b l y h i g h e r , p a r t i c u l a r l y i n 1980, z o o p l a n k t o n p r o d u c t i o n must have compensated f o r l o s s e s due t o h a r v e s t , s i n c e t h e h a r v e s t i n g e f f o r t , and t h u s t h e a c t u a l amount of z o o p l a n k t o n removed from t h e l a k e , was g r e a t e r i n 1980 t h a n 1979. The e f f e c t of z o o p l a n k t o n r e m o v a l and r a p i d r e c o v e r y o f t h e z o o p l a n k t o n p o p u l a t i o n , t h a t i s , h i g h z o o p l a n k t o n t u r n o v e r r a t e , i s u n c l e a r . I t may have meant an i n c r e a s e i n g r a z i n g p r e s s u r e r a t h e r t h a n t h e i n t e n d e d d e c r e a s e . Removal o f z o o p l a n k t o n a l s o i m p l i e s t h e r e m o v a l o f n u t r i e n t s from t h e s y s t e m . However, b e c a u s e z o o p l a n k t o n biomass was s t i l l h i g h , even t h o u g h t h e t o t a l n u t r i e n t b a s e o f t h e s y s t e m was r e d u c e d , r a p i d r e m i n e r a l i z a t i o n r a t e s s h o u l d have e n s u r e d n u t r i e n t a v a i l i b i l i t y f o r p h y t o p l a n k t o n g r o w t h . However, t h e t h e o r y of 200 n u t r i e n t c o n s e r v a t i o n s u g g e s t s t h a t t o t a l b i o m a s s s h o u l d have been l o w e r i n 1980, but a c t u a l t o t a l b i o m a s s was s i m i l a r t o o t h e r y e a r s . S i n c e t h e e f f e c t s of h a r v e s t i n g i n 1980 on t h e z o o p l a n k t o n p o p u l a t i o n i s u n c l e a r , t h e e f f e c t s on p h y t o p l a n k t o n a r e even l e s s d i s c e r n a b l e . D i a t o m blooms d i d o c c u r , as i n 1981, but t o a much l e s s e r e x t e n t . The o c c u r r e n c e of a d i a t o m bloom i n 1978 s u g g e s t s t h a t d i a t o m blooms may be normal e p i s o d i c o c c u r r e n c e s and t h e r e f o r e i t i s d i f f i c u l t t o a t t r i b u t e them, p a r t i c u l a r l y when t h e y a r e s m a l l as i n 1980, t o l a k e p e r t u r b a t i o n . The o c c u r r e n c e o f a l o n g - l i v e d bloom of b l u e - g r e e n a l g a e , from June t o t h e end o f September s u g g e s t s t h a t t h e r e m o v a l of z o o p l a n k t o n may have had an impact on t h e n u t r i e n t a v a i l a b i l i t y . However, as w i t h d i a t o m blooms, b l u e - g r e e n blooms a r e a l s o a n o r m a l l a k e o c c u r r e n c e and u n l e s s t h e y a r e l a r g e and c l e a r l y c o i n c i d e n t w i t h l a k e p e r t u r b a t i o n i t i s d i f f i c u l t t o a t t r i b u t e them t o l a k e p e r t u r b a t i o n . Lake p e r t u r b a t i o n has p r o v i d e d some i n s i g h t on t h e r e l a t i v e i m p o r t a n c e of t h e e f f e c t s of g r a z i n g and 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 on p h y t o p l a n k t o n p o p u l a t i o n s . The f a c t t h a t z o o p l a n k t o n r e m o v a l i n summer of 1979 c o n f e r r e d a c o m p e t i t i v e a d v a n t a g e t o a n o n - g r a z a b l e b l u e - g r e e n a l g a but had no n e t e f f e c t on t h e p o p u l a t i o n of e u k a r y o t i c p h y t p l a n k t o n s u g g e s t s t h a t n o n - n i t r o g e n f i x i n g p h y t o p l a n k t o n a r e de p e n d e n t on z o o p l a n k t o n f o r ammonia. F u r t h e r m o r e , t h i s p o s i t i v e e f f e c t of z o o p l a n k t o n on p h y t o p l a n k t o n g r o w t h a t l e a s t c o m p e n s a t e s f o r p h y t o p l a n k t o n l o s s e s t o g r a z i n g . Removal o f z o o p l a n k t o n i n 201 s p r i n g of 1981 a l t e r e d t h e e a r l y d y n a m i c s of the p l a n k t o n community and t h i s may have been due t o a d e c r e a s e i n g r a z i n g p r e s s u r e on p h y t o p l a n k t o n , s u g g e s t i n g t h a t g r a z i n g may have more of an impact on t h 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 i n s p r i n g t h a n i n summer. W h i l e S c h i n d l e r (1977) has p r o v i d e d c o n v i n c i n g arguments t h a t p h o s p h o r u s i s t h e n u t r i e n t w h i c h l i m i t s p r i m a r y p r o d u c t i o n i n l a k e s , n i t r o g e n may be a t l e a s t as i m p o r t a n t i n l i m i t i n g t h e o v e r a l l p r o d u c t i o n , s i n c e t h e a v a i l a b i l i t y of b i o l o g i c a l l y a c t i v e n i t r o g e n , ammonia and n i t r a t e , may have p r o f o u n d e f f e c t s on t h e c o m p o s i t i o n o f t h e p r i m a r y p r o d u c t i o n ( F l e t t e t a l . 1980). We have o b s e r v e d t h a t t h e f e r t i l i t y of z o o p l a n k t o n i n P l a c i d Lake as w e l l a s t h r e e o t h e r 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 d r o p s c o n s i d e r a b l y e v e r y summer. The commonly a c c e p t e d r e a s o n i s t h a t i n c r e a s e d t e m p e r a t u r e s i n summer r e s u l t i n i n c r e a s e d m e t a b o l i c demands and t h e r e f o r e lower c o n v e r s i o n e f f i c i e n c y of f o o d i n t o o f f s p r i n g . However, H a l l (1964) f o u n d no s i g n i f i c a n t e f f e c t of t e m p e r a t u r e on D a p h n i a r e p r o d u c t i v e r a t e s . Food a v a i l a b i l i t y or q u a l i t y may a l s o have an impact on f e r t i l i t y . N e i l l (1981) f o u n d t h a t h i g h t e m p e r a t u r e s had a d e l e t e r i o u s e f f e c t on j u v e n i l e D a p h n i a r o s e a s u r v i v a l and growth a t low s e s t o n c o n c e n t r a t i o n s , but not a t h i g h s e s t o n c o n c e n t r a t i o n s . S y r e t t (1980) f o u n d t h a t t h e n i t r o g e n c o n t e n t o f n i t r o g e n d e f i c i e n t p h y t o p l a n k t o n c e l l s d r o p p e d from 8-10% t o l e s s t h a n 2%. S i n c e n i t r o g e n i s e s s e n t i a l t o p r o t e i n s y n t h e s i s and growth, i t f o l l o w s t h a t c o n v e r s i o n e f f i c i e n c y of low n i t r o g e n - c o n t e n t f o o d w i l l be low. In t h i s way, n i t r o g e n 202 d e f i c i e n c y may n o t o n l y l i m i t some p h y t o p l a n k t o n g r o w t h , b u t z o o p l a n k t o n g r o w t h a s w e l l . M o d e l l i n g L a k e P l a n k t o n D y n a m i c s T h 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 s u c c e s s i o n p a t t e r n o f z o o p l a n k t o n p e a k s f o l l o w e d b y p h y t o p l a n k t o n p e a k s w e r e n o t f o u n d i n a n y a c c o u n t s i n t h e l i t e r a t u r e , b u t h a v e b e e n o b s e r v e d i n t h r e e o t h e r 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 ( W a l t e r s , p e r s . comm..). T h i s p a t t e r n i s c o u n t e r - i n t u i t i v e s i n c e a l l t h e c l a s s i c a l t h e o r y a n d r e s e a r c h o n p r e d a t o r - p r e y d y n a m i c s s u g g e s t s t h a t c h a n g e s i n p r e d a t o r b i o m a s s m u s t f o l l o w c h a n g e s i n p r e y b i o m a s s . I n t h e L o t k a - V o l t e r r a m o d e l : d x / d t = r x ( l - x / k ) - c x y d y / d t = g x y - my x a n d y a r e p r e y a n d p r e d a t o r b i o m a s s , r e s p e c t i v e l y , r t h e i n t r i n s i c g r o w t h r a t e o f p r e y , k t h e c a r r y i n g c a p a c i t y f o r p r e y , c t h e p e r p r e d a t o r p r e d a t i o n r a t e , g t h e p r o d u c t o f t h e p e r p r e d a t o r p r e d a t i o n r a t e a n d t h e f o o d c o n v e r s i o n e f f i c i e n c y , a n d m t h e p r e d a t o r m o r t a l i t y r a t e . T h i s m o d e l p r e d i c t s t h a t p r e d a t o r - p r e y d y n a m i c s , when p l o t t e d o n a p r e d a t o r v e r s u s p r e y p l a n e , o s c i l l a t e i n a c o u n t e r - c l o c k w i s e d i r e c t i o n a r o u n d a s t a b l e e q u i l i b r i u m p o i n t . A c t u a l c h a n g e s i n P l a c i d L a k e p l a n k t o n o c c u r i n a c l o c k w i s e d i r e c t i o n ( F i g u r e V - 6 ) . One o f t h e m a j o r a s s u m p t i o n s o f t h e L o t k a - V o l t e r r a m o d e l i s t h a t t h e 203 F i g u r e V-6 S e a s o n a l c h a n g e s i n p h y t o p l a n k t o n b i o m a s s (uq/l f r e s h w e i g h t ) and z o o p l a n k t o n b i o m a s s (uq/l d r y w e i g h t ) f o r P l a c i d Lake, mean f o r 1977-1982 A P R I L 0 20*0 4d0 600 8d0 10b0 1200 14'00 16'00 phytoplankton ug/l 204 p a r a m e t e r s w h i c h d e f i n e t h e s t a b l e e q u i l i b r i u m p o i n t a r e c o n s t a n t s . F o r b i o l o g i c a l r e a s o n s , t h i s a s s u m p t i o n i s not v a l i d f o r most of t h e p a r a m e t e r s i n v o l v e d , w h i c h may change c o n s i d e r a b l y s e a s o n a l l y . P h y t o p l a n k t o n g r o w t h r a t e s ("r") a r e l i g h t d e p e n d e n t ( r e v i e w e d i n P a r s o n s and T a k a h a s h i 1973). E x c r e t i o n and c o n v e r s i o n e f f i c i e n c i e s a r e i m p o r t a n t f a c t o r s i n t h e model p r e s e n t e d i n C h a p t e r IV: d x / d t = r x ( l - x / ( x + b y ) ) - cxy - sx d y / d t = g x y ( l - y/xk) - my (where a i s t h e s i n k i n g r a t e f o r p h y t o p l a n k t o n ; b, a measure of p h y t o p l a n k t o n g r o w t h . a l l o w e d by n u t r i e n t s c o n t a i n e d i n z o o p l a n k t o n , and k t h e c o n v e r s i o n e f f i c i e n c y o f p h y t o p l a n k t o n t o z o o p l a n k t o n b i o m a s s ) . E x c r e t i o n r a t e s ( r e l a t e d t o "b") a r e p r o p o r t i o n a l t o l a k e t e m p e r a t u r e w h i l e c o n v e r s i o n e f f i c i e n c y ("k") d e c r e a s e s as t e m p e r a t u r e s i n c r e a s e . I t c a n be assumed, as Buckingham (1978) has o b s e r v e d , t h a t f i l t e r i n g r a t e s a r e t e m p e r a t u r e i n d e p e n d e n t , and t h e r e f o r e I w i l l assume t h a t " c " d o e s not c hange s i g n i f i c a n t l y s e a s o n a l l y . ( T h i s a s s u m p t i o n i s n o t e n t i r e l y c o r r e c t s i n c e f o o d p a r t i c l e d e n s i t y , w h i c h may change s e a s o n a l l y , does have a s i g n i f i c a n t e f f e c t on f i l t e r i n g r a t e s . See C h a p t e r I I I f o r r e f e r e n c e s . ) I w i l l a l s o assume t h a t p r e d a t i o n on h e r b i v o r e s i s u n r e l a t e d t o l a k e t e m p e r a t u r e o r s o l a r r a d i a t i o n , a l t h o u g h t h i s a s s u m p t i o n may a l s o not be e n t i r e l y c o r r e c t . 205 S e a s o n a l v a r i a t i o n s were i n c o r p o r a t e d i n t o t h e model p a r a m e t e r s by a s s u m i n g p o s i t i v e l i n e a r r e l a t i o n s h i p s between r and s o l a r r a d i a t i o n and b and l a k e t e m p e r a t u r e , and a n e g a t i v e l i n e a r r e l a t i o n s h i p between k and l a k e t e m p e r a t u r e . By d e f i n i t i o n , g = ck; g i s t h e r e f o r e a l s o t e m p e r a t u r e - d e p e n d e n t . Maximum v a l u e s f o r r , b and k were c h o s e n from o b s e r v a t i o n s of e n c l o s u r e e x p e r i m e n t s ( C h a p t e r IV) and t h e l i t e r a t u r e ( P a r s o n s and T a k a h a s h i 1973). Then, u s i n g t h e mean s o l a r r a d i a t i o n and l a k e t e m p e r a t u r e v a l u e s f o r P l a c i d Lake d a t a 1977-1982, r e l a t i o n a l v a l u e s of r w i t h s o l a r r a d i a t i o n , and b and k w i t h l a k e t e m p e r a t u r e were c a l c u l a t e d . T h e s e v a l u e s were u s e d t o c a l c u l a t e p a r a m e t e r v a l u e s f o r e a c h month o f May t h r o u g h September ( T a b l e V - 3 ) . V a l u e s f o r c, s and m were assumed t o be c o n s t a n t ; v a l u e s f o r c and s a t 0.003 ug/ug z o o p l a n k t o n / d a y and 0.1/day r e s p e c t i v e l y , as d e t e r m i n e d i n C h a p t e r IV, and m a t 0.1/day, a f t e r H e b e r t 1978. I s o c l i n e s f o r z o o p l a n k t o n and p h y t o p l a n k t o n were c a l c u l a t e d and a r e shown i n F i g u r e V-7. S i n c e I was u n a b l e t o d e m o n s t r a t e t h a t t h e p r e d a t o r model w h i c h I i n t r o d u c e d i n C h a p t e r IV b e t t e r d e s c r i b e d p l a n k t o n d y n a m i c s t h a n t h e V o l t e r r a p r e d a t o r m o del, I a l s o p l o t t e d t h e V o l t e r r a p r e d a t o r i s o c l i n e s w i t h p r e y i s o c l i n e s o b t a i n e d from my " l o g i s t i c - p l u s - e n r i c h m e n t " model ( F i g u r e V - 8 ) . B o t h s e t s of i s o c l i n e s y i e l d a c l o c k w i s e d i r e c t i o n i n change o f t h e e q u i l i b r i u m p o i n t i n t h e p r e d a t o r - p r e y p l a n e . T h i s dynamic i s t h e r e s u l t of t h e l a g t i m e between c h a n g e s i n s o l a r r a d i a t i o n and l a k e t e m p e r a t u r e ( F i g u r e s V-3 a - f ) . In May and J u n e , l a k e t e m p e r a t u r e s a r e s t i l l r e a s o n a b l y low, r e s u l t i n g i n h i g h TABLE V-3 SEASONAL VARIATION IN MODEL PARAMETERS MONTH SOLAR LAKE r b k g RADIATION TEMPERATURE May 2 8 . 3 9 . 4 .73 5 . 8 3 .21 .00063 June 3 3 . 6 13.1 . 8 6 8 . 1 2 .17 .00051 J u l y 3 5 . 0 . 15.9 . 9 9 . 8 6 .14 .00042 August 2 7 . 5 19.5 .71 12.09 .11 .00033 September 2 0 . 4 14.6 . 5 2 9 . 0 5 .15 .00045 207 F i g u r e V - 7 P r e d i c t e d s e a s o n a l c h a n g e s i n p h y t o p l a n k t o n -z o o p l a n k t o n e q u i l i b r i a b a s e d on " l o g i s t i c - p l u s -e n r i c h m e n t " p r e y model and p r e d a t o r / p r e y t h r e s h h o l d 208 Figure V-8 Predicted seasonal changes in phytoplankton-zooplankton equilibria based on "logistic-plus-enrichment" prey model and Volterra (prey threshhold) p h y t o p l a n k t o n ug/1 209 c o n v e r s i o n e f f i c i e n c i e s of p h y t o p l a n k t o n t o z o o p l a n k t o n . B e c a u s e s o l a r r a d i a t i o n i s a l r e a d y h i g h a t t h i s t i m e of y e a r , p h y t o p l a n k t o n p r o d u c t i v i t y i s h i g h and much of i t i s q u i c k l y c o n v e r t e d t o z o o p l a n k t o n b i o m a s s . T h i s dynamic has been o b s e r v e d i n t h e c e n t r a l N o r t h P a c i f i c where no s p r i n g blooms of p h y t o p l a n k t o n a r e o b s e r v e d even t h o u g h p h y t o p l a n k t o n p r o d u c t i v i t y i s h i g h . L a t e s p r i n g z o o p l a n k t o n maxima s u g g e s 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 v i t y i s n e v e r t r a n s l a t e d i n t o p r o d u c t i o n b e c a u s e of e x t r e m e l y r a p i d r e s p o n s e of g r a z e r s t o i n c r e a s e s i n f o o d a v a i l a b i l i t y ( H e i n r i c h 1962). A l t h o u g h I am r e a s o n a b l y c o n v i n c e d t h a t g r a z i n g i s not an i m p o r t a n t p h y t o p l a n k t o n p o p u l a t i o n r e g u l a t i n g mechanism i n summer, i t may be i m p o r t a n t i n s p r i n g and f a l l . R e d f i e l d (1980) who a l s o worked a t an o l i g o t r o p h i c l a k e , o b s e r v e d t h a t t h e n e g a t i v e g r a z i n g e f f e c t of z o o p l a n k t o n on p h y t o p l a n k t o n p r o d u c t i v i t y was much more p r o n o u n c e d i n September t h a n i n J u l y o r A u g u s t , even t h o u g h z o o p l a n k t o n p o p u l a t i o n s were h i g h e r i n J u l y and A u g u s t . He s u g g e s t e d t h a t l o w e r t e m p e r a t u r e s r e s u l t e d i n lo w e r e x c r e t i o n r a t e s so t h a t t h e p o s i t i v e e f f e c t of s t i m u l a t e d p h y t o p l a n k t o n g r o w t h by 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 was not g r e a t enough t o compensate f o r t h e d e l e t e r i o u s e f f e c t of g r a z i n g . B e c a u s e f i l t e r i n g r a t e s i n c l a d o c e r a n s i n c r e a s e as f o o d p a r t i c l e c o n c e n t r a t i o n s d e c r e a s e t o some lower l i m i t (Richman 1966), i n f a l l , a s p h y t o p l a n k t o n p o p u l a t i o n s d r o p , f i l t e r i n g r a t e s may a c t u a l l y i n c r e a s e , c o m p e n s a t i n g f o r t h e l o w e r z o o p l a n k t o n p o p u l a t i o n . The same p r i n c i p l e s s h o u l d h o l d t r u e f o r s p r i n g , when t e m p e r a t u r e s and p l a n k t o n p o p u l a t i o n s a r e low. In a d d i t i o n 210 t o t h i s , f r e e n u t r i e n t c o n c e n t r a t i o n s from v e r t i c a l m i x i n g and r a i n f a l l a r e h i g h e r t h a n i n summer, so 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 i s not as c r u c i a l t o p h y t o p l a n k t o n g r o w t h . In J u l y , t e m p e r a t u r e i n c r e a s e s p r o d u c e h i g h m e t a b o l i c demands and low c o n v e r s i o n e f f i c i e n c i e s , s l o w i n g t h e growth of z o o p l a n k t o n p o p u l a t i o n s . At t h e same t i m e , i n c r e a s i n g s o l a r r a d i a t i o n r e s u l t s i n even h i g h e r p h y t o p l a n k t o n g r o w th r a t e s . In A u g u s t a d e c r e a s e i n s o l a r r a d i a t i o n s l o w s t h e growth of p h y t o p l a n k t o n ; i n c r e a s e s i n t e m p e r a t u r e r e s u l t i n a s u b s t a n t i a l d e c r e a s e of z o o p l a n k t o n b i o m a s s as growth and r e p r o d u c t i o n r a t e s a r e t o o low t o compensate f o r m o r t a l i t y . In September t e m p e r a t u r e s d r o p and z o o p l a n k t o n g r o w th r a t e s i n c r e a s e , r e d u c i n g t h e net l o s s of z o o p l a n k t o n b i o m a s s . P h y t o p l a n k t o n g r o w th r a t e s and hence b i o m a s s c o n t i n u e t o d e c l i n e w i t h r e d u c e d s o l a r r a d i a t i o n and 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 . S h o r t - t e r m d y n a m i c s a r o u n d any g i v e n e q u i l i b r i u m p o i n t may s t i l l be i n a c o u n t e r -c l o c k w i s e d i r e c t i o n but i t may not be p o s s i b l e t o o b s e r v e t h i s w i t h t h e k i n d of s a m p l i n g s c h e d u l e employed i n t h i s s t u d y . However, t h e e q u i l i b r i u m p o i n t s and t h e r e f o r e t h e e n t i r e s y s t e m , i s moving i n a c l o c k - w i s e d i r e c t i o n . 21 1 LITERATURE CITED A l c a r e z , M., G.A. P a f f e n h o e 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 c o p e p o 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 P r e s s o f New E n g l a n d , H a n o v e r , New H a m p s h i r e . 793 pp. A n d r e w a r t h a , H.G. And L.C. B i r c h . 1954. The D i s t r i b u t i o n and  Abundance o f A n i m a l s . U n i v e r s i t y of C h i c a g o P r e s s , C h i c a g o . 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 of 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 . j_6 :906-920. A x l e r , R.P. 1980. S t i m u l a t i o n of n i t r a t e u p t a k e and p h o t o s y n t h e s i s by molybdenum i n C a s t l e L a k e , C a l i f o r n i a . Can. J . F i s h . A q u a t . S c i . 37 :707~712. A x l e r , R.P., G.W. R e d f i e l d and C R . Goldman. 1981. The i m p o r t a n c e of r e g e n e r a t e d n i t r o g e n t o p h y t o p l a n k t o n p r o d u c t i v i t y i n a s u b a l p i n e l a k e . E c o l o g y 62 :345-354. Banse, K a r l . 1976. R a t e s of g r o w t h , r e s p i r a t i o n and p h o t o s y n t h e s i s o f u n i c e l l u l a r a l g a e as r e l a t e d t o c e l l s i z e - a r e v i e w . J . P h y c o l . J_2 : 1 3 5— 1 4 0 . B a r l o w , J . P . And J.W. B i s h o p . 1965. P h o s p h a t e r e g e n e r a t i o n by z o o p l a n k t o n i n Cayuga L a k e . L i m n o l . O c e a n o g r . 1 P ( s u p p l . ) :R15-R24. B a r l o w , J . P . And J . D . C M o n t e i r o . 1979. S e l e c t i v e g r a z i n g by z o o p l a n k t o n p o p u l a t i o n s i n Southhampton w a t e r . M a r i n e B i o l o g y 53 :335-344. B e e r s , J.R. 1966. S t u d i e s on t h e c h e m i c a l c o m p o s i t i o n of t h e major z o o p l a n k t o n g r o u p s i n t h e S a r g a s s o Sea o f f Bermuda. L i m n o l . O c e a n o g r . :520-528. Bogdan, K e n n e t h G. And J o hn J . G i l b e r t . 1982. S e a s o n a l p a t t e r n s o f f e e d i n g by n a t u r a l p o p u l a t i o n s o f K e r a t e l l a , P o l y a r t h r a , and Bosmina : c l e a r a n c e r a t e s , s e l e c t i v i t e s , and c o n t r i b u t i o n s t o community g r a z i n g . L i m n o l . O c e a n o g r . 27 :918-934. Bogdan, K.G. And D.C. McNaught. 1975. S e l e c t i v e f e e d i n g by D i a ptomus and D a p h n i a . V e r h . I n t e r n a t . V e r e i n . L i m n o l . 19 :2935-2942. 2 1 2 Bothe, H. 1982. Nitrogen f i x a t i o n . I_n N.G. Carr and B.A. Whitton The Biology of Cyanobacteria, B l a c k w e l l , 688 pp. Bowerman, J.E. 1983. The r e l a t i v e importance of food a v a i l a b i l t y and predation to the j u v e n i l e s u r v i v a l of Diacyclops thomasi . MSc Thesis, U n i v e r s i t y of B r i t i s h Columbia Boyd, CM. 1976. S e l e c t i o n of p a r t i c l e s i z e s by f i l t e r -feeding copepods: a plea for reason. Limnol. Oceanogr. 2j_ : 1 75. Brooks, J.L. And S.I. Dodson. 1965. Predation, body s i z e , and composition of plankton. Science 150 :28~35. Buckingham, Sandra L. 1978. Fun c t i o n a l responses and feeding s t r a t e g i e s of freshwater f i I t e r - f e e d i n g zooplankton. PhD Thesis, U n i v e r s i t y of B r i t i s h Columbia Burns, C.W. 1968a. The r e l a t i o n s h i p between body s i z e of f i l t e r - f e e d i n g cladocera and the maximum s i z e of p a r t i c l e ingested. Limnol. Oceanogr. j_3 :675-678. . 1968b. D i r e c t observations of mechanisms r e g u l a t i n g feeding behaviour, of Daphn i a , i n lakewater. I n t . Rev. Gesamten Hyd r o b i o l . 53 :83- 100. Burns, C.W. And F.H. R i g l e r . 1967. Comparison of f i l t e r i n g r a t es of Daphn i a i n lake water and i n suspension of yeast. Limnol. Oceanogr. J_2 :492-502. Chesson, J . 1978. Measuring preference i n s e l c e t i v e p r e d a t i o n . Ecology 59 :211-215. Conover, Robert J . 1966. Feeding on large p a r t i c l e s by Calanus hyperboreus (Kroyer), pp 187-194. I_n H. Barnes [ed.] , Some contemporary stud i e s in mar ine sc ience, A l l e n and Unwin. Coveney, M.F., G. Cronberg, M. E n e l l , K. Larsson and L. Olafsson. 1977. Phytoplankton, zooplankton and b a c t e r i a - standing crop and production r e l a t i o n s h i p s in a eutrophic lake. Oikos 2j5 :5 - 2 1 . Cushing, D.H. 1976. Grazing i n Lake Erken. Limnol. Oceanogr. 2J_ :349-356. Dagg, Michael J . And J e f f e r s o n T. Turner. 1982. The impact of copepod grazing on the phytoplankton of Georges Bank and the New York Bight . Can. J . F i s h . Aquat. S c i . 39 :979-990. 213 De I n f a n t e , A. 1 9 7 8 . N a t u r a l f o o d of h e r b i v o r o u s z o o p l a n k t o n of Lake V a l e n c i a V e n e z u e l a . A r c h . H y d r o b i o l . 8_2 : 3 4 7 -3 5 8 . De M o t t , W i l l i a m R. 1 9 8 2 . F e e d i n g s e l e c t i v i t i e s and r e l a t i v e i n g e s t i o n r a t e s of D a p h n i a and Bosmina . L i m n o l . O c e a n o g r . 27 : 518-527. De M o t t , W.R. And W.C. K e r f o o t . 1 9 8 2 . C o m p e t i t i o n among c l a d o c e r a n s : N a t u r e of t h e i n t e r a c t i o n between Bosmina and D a p h n i a . E c o l o g y 63 : 1 9 4 9 - 1 9 6 6 . D e v o l , A l l a n H. 1 9 7 9 . Z o o p l a n k t o n r e s p i r a t i o n and i t s r e l a t i o n t o p l a n k t o n d y n a m i c s i n two l a k e s of c o n t r a s t i n g t r o p h i c s t a t e . L i m n o l . O c e a n o g r . 24_ : 8 9 3 -9 0 5 . D u g d a l e , R.C. 1 9 7 6 . N u t r i e n t c y c l e s . I_n C u s h i n g , D.H. And J . J . Walsh [ e d s . ] , The E c o l o g y of t h e S e a s , B l a c k w e l l . E f f o r d , I . E . 1 9 6 7 . T e m p o r a l and s p a t i a l d i f f e r e n c e s i n p h y t o p l a n k t o n p r o d u c t i v i t y i n M a r i o n L a k e , B r i t i s h C o l u m b i a . J . F i s h . Res. B o a r d Can. 24^ : 2 2 8 3 - 2 3 0 7 . E n r i g h t , J . T . 1 9 6 9 . Z o o p l a n k t o n g r a z i n g r a t e s e s t i m a t e d u nder f i e l d c o n d i t i o n s . E c o l o g y 5_0 : 1 0 7 0 - 1 0 7 5 . E p p l e y , R i c h a r d W., Edward H. R e n g e r , E l i z a b e t h L. V e n r i c k , and M i c h a e l M. M u l l i n . 1 9 7 3 . A s t u d y of p l a n k t o n d y n a m i c s and n u t r i e n t c y c l i n g i n t h e c e n t r a l g y r e of t h e N o r t h P a c i f i c Ocean. L i m n o l . O c e a n o g r . J_8 : 5 3 4 - 5 5 1 . F e l l e r , M.C. 1 9 7 7 . N u t r i e n t movement t h r o u g h w e s t e r n h e m l o c k - w e s t e r n r e d c e d a r e c o s y s t e m s 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 . E c o l o g y 58 : 1 2 6 9 - 1 2 8 3 . F e r r a n t e , John G. 1 9 7 6 . The r o l e of z o o p l a n k t o n i n t h e i n t r a b i o c o e n i t i c p h o p h o r u s c y c l e and f a c t o r s a f f e c t i n g p h o p h o r u s e x c r e t i o n i n a l a k e . H y d r o b i o l o g i a 4_9 : 2 0 3 -2 1 4 . F i n d e n e g g , I . 1 9 6 9 . R e l a t i o n s h i p between s t a n d i n g c r o p and p r i m a r y p r o d u c t i v i t y , pp 2 7 1 - 2 8 9 . J_n Goldman, C R . [ e d . ] , P r i m a r y P r o d u c t i v i t y i n A q u a t i c E n v i r o n m e n t s , U n i v e r s i t y of C a l i f o r n i a P r e s s , 464 pp. F l e t t , R . J . , D.W. S c h i n d l e r , R.D. H a m i l t o n and N.E.R. C a m p b e l l . 1 9 8 0 . N i t r o g e n f i x a t i o n i n C a n a d i a n P r e c a m b r i a n S h i e l d L a k e s . Can. J . F i s h . A q u a t . S c i . 37 : 4 9 4 - 5 0 5 . 214 F r i e d m a n , Marc M. 1980. C o m p a r a t i v e m o r p h o l o g y and f u n c t i o n a l s i g n i f i c a n c e of c o p e p o d r e c e p t o r s and o r a l . . s t r u c t u r e s , pp 185-197 I_n K e r f o o t , W.C, t 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 P r e s s o f New E n g l a n d , London, 793 pp. F r i e d m a n , M.M. And J.R. S t r i c k l e r . 1975. C h e m o r e c e p t o r s and f e e d i n g i n c a l a n o i d c o p e p o d s ( A r t h r o p o d a : C r u s t a c e a ) . P.N.A.S. 72 :4185-4188. F r o s t , B.W. 1972. E f f e c t s of s z i e and c o n c e n t r a t i o n of f o o d p a r t i c l e s on t h e f e e d i n g b e h a v i o u r o f t h e m a r i n e p l a n k t o n i c c o p e p o d C a l a n u s p a c i f i c u s . )u L i m n o l . O c e a n o g r . 17 :805-815. . 1977. F e e d i n g b e h a v i o u r of C a l a n u s pac i f i c u s i n m i x t u r e s of f o o d p a r t i c l e s . L i m n o l . O c e a n o g r . 2/2 :472-491 . . 1980. G r a z i n g , pp 465-491. In M o r r i s , I [ e d . ] , The P h y s i o l o g i c a l E c o l o g y of P h y t o p l a n k t o n , B l a c k w e l l . 62 5 pp. F r y e r , G. 1968. E v o l u t i o n and a d a p t i v e r a d i a t i o n i n t h e C h y d o r i d a e ( C r u s t a c e a : C l a d o c e r a ) : A s t u d y i n c o m p a r a t i v e f u n c t i o n a l m o r p h o l o g y and e c o l o g y . P h i l . T r a n s . R. Soc. L o n d . S e r . B 254 : 2 2 l - 3 8 5 . G a n f , G.G. And P. B l a z k a . 1974. Oxygen u p t a k e , ammonia and p h o s p h a t e e x c r e t i o n by z o o p l a n k t o n o f a s h a l l o w e q u a t o r i a l l a k e ( L a k e G e orge, U g a n d a ) . L i m n o l . O c e a n o g r . j_9 :313~325. G a r d i n e r , A . C 1937. P h o s p h a t e p r o d u c t i o n by p l a n k t o n i c a n i m a l s . J . C o n s e i l . Perm. I n t e r n . E x p l o r a t i o n Mer J_2 :144-146. G a u l d , D.T. 1966. The swimming and f e e d i n g of p l a n k t o n i c c o p e p o d s , pp313-334. I_n B a r n e s , H. [ e d . ] , Some  C o n t e m p o r a r y S t u d i e s i n Mar i n e Sc i e n c e , A l l e n and Unwin. Gause, G.F. 1934. The S t r u g g l e f o r E x i s t e n c e . W i l l i a m s and W i l k i n s , B a l t i m o r e , 163 pp. G l i w i c z , Z. M a c i e j . 1975. E f f e c t of z o o p l a n k t o n g r a z i n g on p h o t o s y n t h e t i c a c t i v i t y and c o m p o s i t i o n o f p h y t o p l a n t k o n . V e r h . I n t e r n a t . V e r e i n . L i m n o l . J_9 :1490-1497. 215 . 1977a. F o o d s i z e s e l e c t i o n and s e a s o n a l s u c c e s s i o n of f i l t e r f e e d i n g z o o p l a n k t o n i n an e u t r o p h i c l a k e . E k o l . P o l . 25 :179-225. . 1980. F i l t e r i n g r a t e s , f o o d s i z e s e l e c t i o n , and f e e d i n g r a t e s i n c l a d o c e r a n s - a n o t h e r a s p e c t of i n t e r s p e c i f i c c o m p e t i t i o n i n f i l t e r - f e e d i n g z o o p l a n k t o n . 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 P r e s s of New E n g l a n d , London, 793 pp. G l i w i c z , Z. M a c i e j and Ewa S i e d l a r . 1980. Food s i z e l i m i t a t i o n and a l g a e i n t e r f e r i n g w i t h f o o d c o l l e c t i o n i n D a p h n i a . A r c h . H y d r o b i o l . 8_8 : 1 55—1 77. G r a n b e r g , K. 1970. S e a s o n a l f l u c t u a t i o n s i n numbers and b i o m a s s o f t h e p l a n k t o n of Lake P a a r j a r v i , s o u t h e r n F i n l a n d . Ann. Z o o l . F e n n i c i 1_ : 1—24-H a l l , D.J. 1964. An e x p e r i m e n t a l a p p r o a c h t o t h e d y n a m i c s o f a n a t u r a l p o p u l a t i o n of Daphn i a g a l e a t a mendotae . E c o l o g y 45 :94-112. Haney, J . F . 1973. An i n s i t u e x a m i n a t i o n of t h e g r a z i n g a c t i v i t i e s o f n a t u r a l z o o p l a n k t o n c o m m u n i t i e s . A r c h . H y d r o b i o l . 72 :87-132. H a r b i s o n , G.R. And V . L . M c A l i s t e r . 1980. F a c t and a r t i f a c t i n c o p e p o d f e e d i n g e x p e r i m e n t s . L i m n o l . O c e a n o g r . 2_5 :971-981 . H a r g r a v e , B.T. And G.H. Geen. 1968. P h o s p h o r u s e x c r e t i o n by z o o p l a n k t o n . L i m n o l . O c e a n o g r . J_3 : 332-342 . . 1970. E f f e c t s of c o p e p o d g r a z i n g on two 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 . J . F i s h . R e s . B o a r d Can. 27 :1395-. H a r r i s , Eugene. 1959. The n i t r o g e n c y c l e i n Long I s l a n d Sound. B u l l . Bingham O c e a n o g r a p h i c C o l l e c t i o n _1_7 :31-64. H e b e r t , P a u l D.N. 1978. The p o p u l a t i o n b i o l o g y o f D a p h n i a ( C r u s t a c e a : D a p h n i d a e ) . B i o . Rev. 5_3 :387-426. H e i n r i c h , A.K. 1962. The l i f e h i s t o r i e s of p l a n k t o n a n i m a l s and s e a s o n a l c y c l e s o f p l a n k t o n c o m m u n i t i e s i n t h e o c e a n s . J . Du C o n s e i l I n t e r n . Pour 1 7 e x p l o r . De l a Mer 27 :15-24. 216 H i l l b r i c h t - I l k o w s k a , A., I . S p o d n i e w s k a , T. Weglenska and A. K a r a b i n . 1972. The s e a s o n a l v a r i a t i o n of some e c o l o g i c a l e f f i c i e n c i e s and p r o d u c t i o n r a t e s i n t h e p l a n k t o n community of s e v e r a l P o l i s h l a k e s of d i f f e r e n t t r o p h y , pp 111-127. I_n Z. K a j a k and A. H i l l b r i c h t -I l k o w s k a [ e d . ] , P r o d u c t i v t y p r o b l e m s of f r e s h w a t e r s , IBP-UNESCO Symposium, K a z i m i e r z D o l n y , P o l a n d , 1970, p r o c e e d i n g s , P o l i s h S c i e n t i f i c P u l i s h e r s , Warsawa. Home, A . J . And C R . Goldman. 1972. N i t r o g e n f i x a t i o n i n C l e a r L a k e , C a l i f o r n i a . I . S e a s o n a l v a r i a t i o n and t h e r o l e of h e t e r o c y s t s . L i m n o l . O c e a n o g r . _T7 :678-692. Home, A . J . And J . E . D i l l a r d . 1972. N i t r o g e n f i x a t i o n i n C l e a r L a k e , C a l i f o r n i a . I I . S y n o p t i c s t u d i e s on t h e autumn Anabaena bloom. L i m n o l . O c e a n o g r . j_7 : 693-703 . H u f f a k e r , C B . 1958. E x p e r i m e n t a l s t k u d i e s on p r e d a t i o n : d i s p e r s i o n f a c t o r s and p r e d a t o r - p r e y o s c i l l a t i o n s . H i l g a r d i a 27 :343-383. H u n t l e y , Mark. 1981. N o n s e l e c t i v e , n o n s a t u r a t e d f e e d i n g by t h r e e c a l a n o i d c o p e p o d s p e c i e s i n t h e L a b r a d o r Sea. L i m n o l . O c e a n o g r . 26 :831-842. H u t c h i n s o n , G.E. 1967. A T r e a t i se on L i m n o l o g y , 2. I n t r o d u c t i o n t o l a k e B i o l o g y and t h e L i m n o p l a n k t o n . John W i l e y and Sons, I n c . , New Yo r k , N.Y., 1115 pp. J a r n e f e l d t , H. 1952. P l a n k t o n a l s I n d i k a t o r d e r T o p h i e g r u p p e n d e r Seen. Suomal. T i e d e a k t . Toim. ( A n n l s A c a d . S c i . F e n n . ) , s e r A.IV B i o l . j_8 :29 pp.. 1956. Zur L i m n o l o g i e s e i n i g e r Gewaessen F i n n l a n d s . XVI. M i t b e s o n d e r e r B e r u e c k s i c h t i g u n g des P l a n k t o n s . Suomal. E l a i n - j a k a s i r t . S e r r . Van. E l a i n J u l k . ( A n n l s Z o o l . Soc. Vanamo) j_7 :2 01 pp. J o h a n n e s , R.E. 1964. P h o s p h o r u s e x c r e t i o n and body s i z e i n m a r i n e a n i m a l s : m i c r o z o o p l a n k t o n and n u t r i e n t r e g e n e r a t i o n . S c i e n c e 146 :923-924. J o r g e n s e n , C B . 1966. B i o l o g y of S u s p e n s i o n F e e d i n g . Pergammon P r e s s L t d . , O x f o r d , 375 pp. . 1975. C o m p a r a t i v e p h y s i o l o g y of s u s p e n s i o n f e e d i n g . Ann. Rev. P h y s i o l . 3_7 :57-79. K e a t i n g , K . I . 1977. A l l e l o p a t h i c i n f l u e n c e on b l u e - g r e e n bloom s e q u e n c e i n a e u t r o p h i c l a k e . S c i e n c e 196 :885-887. 217 K e a t i n g , K . I . 1978 . B l u e - g r e e n a l g a l i n h i b i t i o n of d i a t o m g r o w t h : t r a n s i t i o n from m e s o t r o p h i c t o e u t r o h p i c community s t r u c t u r e . S c i e n c e 199 : 9 7 1 - 9 7 3 . Ketchum, B.H. 1962. R e g e n e r a t i o n o f n u t r i e n t s by z o o p l a n k t o n . R a p p t . P r o c e s - V e r b a u x R e u n i o n s C o n s e i l Perm. I n t e r n . E x p l o r a t i o n Mer 153 : 1 4 2 - 1 4 7 . K e r s t i n g , K. 1978. Some f e a t u r e s of f e e d i n g , r e s p i r a t i o n and en e r g y c o n v e r s i o n of Daphn i a magna . H y d r o b i o l o g i a 5_9 : 1 1 3 - 1 2 0 . K i b b y , H.V. 1971 . E f f e c t of t e m p e r a t u r e on t h e f e e d i n g b e h a v i o u r of Daphn i a r o s e a . L i m n o l . O c e a n o g r . j_6 : 5 8 0 -581 . K r e b s , C . J . 1972 . E c o l o g y - The E x p e r i m e n t a l A n a l y s i s of D i s t r i b u t i o n and Abundance. H a r p e r and Row, P u b l i s h e r s , New Yo r k , 694 pp. Lam, R.K. And B. F r o s t . 1976 . Model of c o p e p o d f i l t e r i n g r e s p o n s e t o c h a n g e s i n s i z e and c o n c e n t r a t i o n of f o o d . L i m n o l . O c e a n o g r . 2J_ : 4 9 0 - 5 0 0 . L a m p e r t , W. 1974 . A method f o r d e t e r m i n i n g f o o d s e l e c t i o n by z o o p l a n k t o n . L i m n o l . O c e a n o g r . J_9 : 9 9 5 - 9 9 8 . . 1981 . T o x i c i t y o f t h e b l u e - g r e e n M i c r o c y s t i s - a e r o q i n o s a : E f f e c t i v e d e f e n s e mechanisms a g a i n s t g r a z i n g p r e s s u r e by D a p h n i a . I n t . V e r . T h e o r . Angew. L i m n o l . V e r h . 2J_ : 1 4 3 6 - 1 4 4 0 . Lehman, J . T . 1976. The f i l t e r f e e d e r as an o p t i m a l f o r a g e r , and t h e p r e d i c t e d s h a p e s of f e e d i n g c u r v e s . L i m n o l . O c e a n o g r . 2J_ : 50 1 - 5 1 6 . . 1980 . N u t r i e n t r e c y c l i n g a s an i n t e r f a c e between a l g a e and g r a z e r s i n f r e s h w a t e r c o m m u n i t i e s , p p 2 5 l - 2 6 3 . 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 P r e s s of New E n g l a n d , H a n o v e r , New Ham p s h i r e , 793 pp. . 1982 . P h o s p h o r u s d y n a m i c s of t h e p r o c a r y o t i c n a n n o p l a n k t o n i n a M i c h i g a n l a k e . L i m n o l . O c e a n o g r . 27 : 8 2 8 - 8 3 8 . Lehman, J.T., D.B. B o t k i n , and G.E. L i k e n s . 1975 . The a s s u m p t i o n s and r a t i o n a l e s of a computer model of p h y t o p l a n t k t o n p o p u l a t i o n d y n a m i c s . L i m n o l . O c e a n o g r . 20 : 3 4 3 - 3 6 4 . 218 Lehman, J . T . And D. S c a v i a . 1982. M i c r o - s c a l e p a t c h i n e s s of n u t r i e n t s i n p l a n k t o n c o m m u n i t i e s . S c i e n c e 216 :729-730. L o t k a , A . J . 1925. E l e m e n t s of P h y s i c a l B i o l o g y . R e p r i n t e d by Dover P u b l i c a t i o n s , New Y o r k , 1956. Lund, J.W.G. 1965. The e c o l o g y of t h e f r e s h w a t e r p h y t o p l a n k t o n . B i o l . Rev. 4_0 :231-293. Lund, J.W.G, C. K i p l i n g , and E. L e C r e n . 1958. The i n v e r t e d m i c r o s c o p e method of e s t i m a t i n g a l g a l numbers and the s t a t i s t i c a l b a s i s of e s t i m a t i o n s by c o u n t i n g . H y d r o b i o l o g i a _M_ :143-170. L y n c h , M i c h a e l . 1978. Complex i n t e r a t i o n s between n a t u r a l c o e x p l o i t e r s - D a p h n i a and C e r i o d a p h n i a . E c o l o g y 59 :552-564. L y n c h , M. And J . S h a p i r o . 1981. P r e d a t i o n , e n r i c h m e n t , and p h y t o p l a n k t o n community s t r u c t u r e . L i m n o l . O c e a n o g r . 26 :86-102. Malo n e , T.C., M.B. C h e r v i n , and D.C. Boardman. 1979. E f f e c t s of 22-ym s c r e e n s on s i z e - f r e q u e n c y d i s t r i b u t i o n s of s u s p e n d e d p a r t i c l e s and b i o m a s s e s t i m a t e s o f p h y t o p l a n k t o n s i z e f r a c t i o n s . L i m n o l . O c e a n o g r . 24 :956-960. Marmorek, D. 1983. The e f f e c t of l a k e a c i d i f i c a t i o n on z o o p l a n k t o n community s t r u c t u r e and p h y t o p l a n k t o n -z o o p l a n k t o n i n t e r a c t i o n s : an e x p e r i m e n t a l a p p r o a c h . MSc T h e s i s , 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 . M c C a u l e y , Edward and F r e d e r i c B r i a n d . 1979. Z o o p l a n k t o n g r a z i n g and p h y t o p l a n k t o n s p e c i e s r i c h n e s s : F i e l d t e s t s of t h e p r e d a t i o n h y p o t h e s i s . L i m n o l . O c e a n o g r . 2_4 :243-252. McQueen, D.J. 1970. G r a z i n g r a t e s and f o o d s e l e c t i o n i n Diap t o m u s o r e g o n e n s i s ( C o p e p o d a ) , M a r i o n L a k e , B r i t i s h C o l u m b i a . J . F i s h . Res. Bd. Can. 2T7 : 13-20. Murphy, T.P., D.R.S. Lean and C. N a l e w a j k o . 1976. B l u e - g r e e n a l g a e : T h e i r e x c r e t i o n of i r o n - s e l e c t i v e c h e l a t o r s e n a b l e s them t o d o m i n a t e o t h e r a l g a e . S c i e n c e 192 :900-902. M y e r s , J . 1962. L a b o r a t o r y c u l t u r e s . I_n L e w i n , R.A. [ e d . ] , P h y s i o l o g y and b i o c h e m i s t r y o f a l g a e , A c a d e m i c P r e s s , New Y o r k , NY. 219 N a d i n - H u r l e y , C M . And A. Duncan. 1976. A c o m p a r i s o n of d a p h n i d gut p a r t i c l e s w i t h t h e s e s t o n i c p a r t i c l e s p r e s e n t i n two Thames V a l l e y r e s e r v o i r s t h r o u g h o u t 1970 and 1971. F r e s h w a t e r B i o l . 6 : 109-124. Nakashima, B r i a n S. And W i l l i a m C. L e g g e t t . 1980. The r o l e of f i s h e s i n t h e r e g u l a t i o n of p h o s p h o r u s a v a i l a b i l i t y i n l a k e s . Can. J . F i s h . A q u a t . S c i . 37 :1540-1549. Nauwerck, A. 1963. D i e B e z i e h u n g e n z w i s c h e n Z o o p l a n k t o n und P h y t o p l a n k t o n im See E r k e n . Symb. Bo t . U p s a l . XVII :5-1 63 . N e i l l , W.E. 1981. D e v e l o p m e n t a l r e s p o n s e of j u v e n i l e D a p h n i a  r o s e a t o e x e p r i m e n t l a a l t e r a t i o n of t e m p e r a t u r e and n a t u r a l s e s t o n c o n c e n t r a t i o n . Can. J . Z o o l . 3_8 : 1357-1 362. N i v a l , P. And S. N i v a l . 1976. P a r t i c l e r e t e n t i o n e f f i c i e n c i e s of an h e r b i v o r o u s copepod, A c a r t i a c l a u s i ( a d u l t and c o p e p o d i d s t a g e s ) . E f f e c t s on g r a z i n g . L i m n o l . O c e a n o g r . 2J_ :24-38. O ' B r i e n , W. J o h n . 1974. F i l t e r i n g r a t e of C e r i o d a p h n i a  r e t i c u l a t a i n pond w a t e r s of v a r y i n g p h y t o p l a n t k o n c o n c e n t r a t i o n s . A m e r i c a n M i d l a n d N a t u r a l i s t 9_1_ :509-512. P a r s o n s , T.R., R . J . L e B r a s s e r , and J.D. F u l t o n . 1967. Some o b s e r v a t i o n s on t h e d e pendence of z o o p l a n k t o n g r a z i n g on t h e c e l l s i z e and c o n c e n t r a t i o n of p h y t o p l a n k t o n b l ooms. J o u r n a l o f t h e O c e a n o g r a p h i c a l S o c i e t y of J a p a n 23 :10-17. P a r s o n s , T.R., K. S t e p h e n s and J.D.H. S t r i c k l a n d . 1961. On t h e c h e m i c a l c o m p o s i t i o n of e l e v e n s p e c i e s o f m a r i n e p h y t o p l a n k t e r s . J . F i s h . Res. Bd. Canada ]_8 : 1001-1016. P a r s o n s , T.R. And M. T a k a h a s h i . 1973. B i o l o g i c a l O c e a n o g r a p h i c P r o c e s s e s . Pergamon P r e s s , Don M i l l s , O n t a r i o , 186 pp. P e t e r s , R o b e r t and D a v i d L e a n . 1973. The c h a r a c t e r i z a t i o n o f s o l u b l e p h o p h o r u s r e l e a s e d by l i m n e t i c z o o p l a n k t o n . L i m n o l . O c e a n o g r . J_8 :270-279. P e t e r s , R.H. And F.H. R i g l e r . 1973. P h o s p h o r u s r e l e a s e by D a p h n i a . L i m n o l . O c e a n o g r . J_8 :821 -839. Pomeroy, L.R., H.R. Mathews amd H.S. M i n . 1963. E x c r e t i o n o f p h o s p h a t e and s o l u b l e o r g a n i c p h o s p h o r u s compounds by z o o p l a n k t o n . L i m n o l . O c e a n o g r . 8 :50-55. 220 P o r t e r , K a r e n G l a u s . 1972. Amethod f o r t h e i n s i t u s t u d y of z o o p l a n k t o n g r a z i n g e f f e c t s on a l g a l s p e c i e s c o m p o s i t i o n and s t a n d i n g c r o p . L i m n o l . O c e a n o g r . 17 : 9 1 3 - 9 1 7 . . 1973. S e l e c t i v e g r a z i n g and d i f f e r e n t i a l d i g e s t i o n o f a l g a e by z o o p l a n k t o n . N a t u r e 244 :179-180. . 1975. V i a b l e gut p a s s a g e o f g e l a t i n o u s g r e e n a l g a e i n g e s t e d by Daphn i a . V e r h . I n t . V e r e i n . L i m n o l . J_9 : 2 8 4 0 - 2 8 5 0 . . 1976. Enhancement of a l g a l growth and p r o d u c t i v i t y by g r a z i n g z o o p l a n k t o n . S c i e n c e . 192 : 1332-1 334 . . 1977. The p l a n t - a n i m a l i n t e r f a c e i n f r e s h w a t e r e c o s y s t e m s . A m e r i c a n S c i e n t i s t 6_5 : 159-170 . P o r t e r , K.G. And R. McDonough. 1984 . The e n e r g e t i c c o s t of r e s p o n s e t o b l u e - g r e e n a l g a l f i l a m e n t s by c l a d o c e r a n s . L i m n o l . O c e a n o g r . 2_9 : 3 6 5 - 3 6 9 . P o r t e r , K.G. And J.D. O r c u t t j r . 1980. N u t r i t i o n a l a d e q u a c y , m a n a g e a b i l i t y , and t o x i c i t y as f a c t o r s t h a t d e t e r m i n e t h e f o o d q u a l i t y of g r e e n and b l u e - g r e e n a l g a e f o r D a p h n i a . 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 P r e s s of New E n g l a n d , Hanover, New H a m p s h i r e , 793 pp. P o u l e t , S.A. 1973 . G r a z i n g of P s e u d o c a l a n u s m i n u t u s on n a t u r a l l y o c c u r r i n g p a r t i c u l a t e m a t t e r . L i m n o l . O c e a n o g r . JJ3 : 564-573 . . 1974. S e a s o n a l g r a z i n g of P s e u d o c a l a n u s m i n u t u s on p a r t i c l e s . Mar. B i o l . 2_5 : 1 0 9 - 1 2 3 . P r o s s e r , C.L. And F.A. Brown, j r . 1961 . Comparat i v e A n i m a l  P h y s i o l o g y , s e c o n d e d i t i o n . W.B. S a u n d e r s Company, P h i l a d e l p h i a , 688 pp. R e d f i e l d , G.W. 1980. The e f f e c t of z o o p l a n k t o n on p h y t o p l a n k t o n p r o d u c t i v i t y i n t h e e p i l i m n i o n of a s u b a l p i n e l a k e . H y d r o b i o l o g i a 7_0 : 2 l 7 - 2 2 4 . R e y n o l d s , C.S. And A.E. Wa l s b y . 1975 . W a t e r - b l o o m s . B i o l . Rev. 50 : 4 3 7 - 4 8 0 . R i c h e y , J e f f r e y E. 1979 . P a t t e r n s o f p h o s p h o r u s s u p p l y and u t i l i z a t i o n i n Lake W a s h i n g t o n and F i n d l e y L a k e . L i m n o l . O c e a n o g r . 24 : 9 0 6 ~ 9 1 6 . 221 Richman, S. 1966. The e f f e c t of p h y t o p l a n k t o n c o n c e n t r a t i o n on t h e f e e d i n g r a t e of Diaptomus o r e g o n e n s i s . V e r h . I n t . V e r . L i m n o l . J_6 :.392-398. Richman, S.E., S.A. Bohon and S.E R o b b i n s . 1980. G r a z i n g i n t e r a c t i o n s among f r e s h w a t e r c a l a n o i d c o p e p o d s , pp 219-233 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 P r e s s o f New E n g l a n d , London, 793 pp. R i c k e t t s , T.R. 1966. On t h e c h e m i c a l c o m p o s i t i o n of some u n i c e l l u l a r a l g a e . Phytochem. 5_ :67-76. R i g l e r , F.H. 1961. The u p t a k e and r e l e a s e of i n o r g a n i c p h o s p h o r u s by Daphn i a magna S t r a u s . L i m n o l . O c e a n o g r . 6 : 165-174. 1964. The c o n t i b u t i o n of z o o p l a n k t o n t o t h e t u r n o v e r of p h o s p h o r u s i n t h e e p i l i m n i o n of l a k e s . Can. F i s h . C u l t u r i s t 32 :3-9. . 1973. A dynamic view of t h e p h o s p h o r u s c y c l e i n l a k e s , pp 539-572. In G r i f f i t h , E . J . , A. B e e t o n , J.M. S p e n c e r and D.T. M i t c h e l l [ e d s . ] , E n v i r o n m e n t a l  P h o s p h o r u s Handbook, John W i l e y & Sons. 718 pp.. R u b e n s t e i n , D.I. And M.A.R. K o e h l . 1977. The mechanism of ' f i l t e r f e e d i n g : some t h e o r e t i c a l c o n s i d e r a t i o n s . Am. Nat . J_M : 98 1 -994 . R y t h e r , J.H. 1954. I n h i b i t o r y e f f e c t s of p h y t o p l a n k t o n upon t h e f e e d i n g of D a p h n i a magna w i t h r e f e r n c e t o gro w t h , r e p r o d u c t i o n , and s u r v i v a l . E c o l o g y 3_5 :522-533. S c a v i a , D. And W.S. G a r d n e r . 1982. K i n e t i c s of n i t r o g e n and p h o s p h o r u s r e l e a s e i n v a r y i n g f o o d s u p p l i e s by Daphn i a  magna . H y d r o b i o l o g i a 9_6 : 105-1 11. S c a v i a , D. And M.J. M c F a r l a n d . 1982. P h o s p h o r u s r e l e a s e p a t t e r n s and t h e e f f e c t s o f r e p r o d u c t i v e s t a g e and e c d y s i s i n D a p h n i a magna . Can. J . F i s h . A q u a t . S c i . 39 :1310-1314. S c h i n d l e r , D.W. 1977. E v o l u t i o n of p h o s p h o r u s l i m i t a t i o n i n l a k e s . S c i e n c e . 195 :260-262. S c h l e s i n g e r , D.A., L.A. M o l o t and B . J . S h u t e r . 1981. S p e c i f i c g r o w t h r a t e s of f r e s h w a t e r a l g a e i n r e l a t i o n t o c e l l s i z e and l i g h t i n t e n s i t y . Can. J . F i s h . A q u a t . S c i . 38 :1052-1058 . 222 S h e l d o n , R.W. And W. H. S u t c l i f f e . 1969. R e t e n t i o n of m a r i n e p a r t i c l e s by s c r e e n s and f i l t e r s . L i m n o l . O c e a n o g r . 14 :441-444. S h e p a r d , B.G. 1973. T e m p o r a l and s p a t i a l d i f f e r e n c e s i n d e v e l o p m e n t of 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 , B r i t i s h C o l u m b i a . MSc T h e s i s , 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 Skogheim, O.K. And S. Rognerud. 1978. R e c e n t c h a n g e s i n p l a n k t o n c o m m u n i t i e s and p r e s e n t t r o p h i c s t a t e of Lake S t e i n s f j o r d . A r c h . H y d r o b i o l . 83 :179-199. S m i t h , J . Maynard. 1974. M o d e l s i n E c o l o g y . Cambridge U n i v e r s i t y P r e s s , G r e a t B r i t a i n , 146 pp. S m i t h , R.E. And J . K a l f f . 1982. The e f f e c t of p h o s p h o r u s l i m i t a t i o n on a l g a l growth r a t e s : E v i d e n c e from a l k a l i n e p h o s p h a t a s e . Can. J . F i s h . A q u a t . S c i . 3_8 :1421-1427. Sommer, U l r i c h . 1983. N u t r i e n t c o m p e t i t i o n between p h y t o p l a n k t o n s p e c i e s i n m u l t i s p e c i e s c h e m o s t a t e x p e r i m e n t s . A r c h . H y d r o b i o l . i :399-416. 1984. The par a d o x of t h e p l a n k t o n : F l u c t u a t i o n s o f p h o s p h o r u s a v a i l a b i l i t y m a i n t a i n d i v e r s i t y of p h y t o p l a n k t o n i n f l o w - t h r o u g h c u l t u r e s . L i m n o l . O c e a n o g r . 2_9 :633-635. S t e w a r t , W.D.P. 1973. N i t r o g e n f i x a t i o n , pp 260-278. I_n C a r r , N G. And B.A W h i t t o n [ e d s . ] , The B i o l o g y of B l u e - g r e e n A l g a e , B l a c k w e l l S c i e n t i f i c P u b l i c a t i o n s , 676 pp. S t r a t h m a n n , R.R. 1967. E s t i m a t i n g t h e o r g a n i c c a r b o n c o n t e n t of p h y t o p l a n k t o n from c e l l volume or pla s m a volume. L i m n o l . O c e a n o g r . J_2 :41 1—418. S y r e t t , P . J . 1962. N i t r o g e n a s s i m i l a t i o n , p.171-188. In L e w i n , R.A. [ e d . ] , P h y s i o l o g y and b i o c h e m i s t r y of  a l g a e , Academic P r e s s , New Y o r k , NY. T a y l o r , W.D. And D.R.S. L e a n . 1981. R a d i o t r a c e r e x p e r i m e n t s on p h o s p h o r u s u p t a k e and r e l e a s e by l i m n e t i c m i c r o z o o p l a n k t o n . Can. J . F i s h . A q u a t . S c i . 3_8 : 1316 — 1 321 . T i l m a n , D. 1977. R e s o u r c e c o m p e t i t i o n between p l a n k t o n i c a l g a e : an e x p e r i m e n t a l and t h e o r e t i c a l a p p r o a c h . E c o l o g y 58 :338-348. 223 T i l m a n , D., S.S. K i l h a m and P. R i l h a m . 1982. P h y t o p l a n k t o n community e c o l o g y : The r o l e of l i m i t i n g n u t r i e n t s . Ann. Rev. E c o l . S y s t . 13 :349-372. T i t m a n , D. 1976. S i n k i n g i n f r e s h w a t e r p h y t o p l a n k t o n : some e c o l o g i c a l i m p l i c a t i o n s o f c e l l n u t r i e n t s t a t u s and p h y s i c a l m i x i n g p r o c e s s e s . L i m n o l . O c e a n o g r . 2J[ :409-417. V e n r i c k , E l i z a b e t h L. 1978. How many c e l l s t o c o u n t ? , pp 167-180. I_n S o u r n i a , A. [ e d . ] , P h y t o p l a n k t o n M a n u a l , UNESCO, 377 pp. V o l l e n w e i d e r , R.A. 1969. A manual on methods f o r m e a s u r i n g  pr imary p r o d u c t i o n i n a q u a t i c e n v i r o n m e n t s . IBP Handbook No. 12, 213 pp. V o l t e r r a , V. 1926a. V a r i a z i o n e e f l u t t u a z i n i d e l numero d ' i n d i v i d u i i n s p e c i e a n i m a l i c o n v i v e n t i . A c c a d . N a z i o n a l e L i n c e i ( s e r . 6) 2 : 31 — 113. 1926b. F l u c t u a t i o n s i n t h e abundance o f a s p e c i e s c o n s i d e r e d m a t h e m a t i c a l l y . N a t u r e 118 :558~560. W a l t e r s , C . J . , R.A. Park and J . F . Koonce. 1980. Dynamic models o f l a k e e c o s y s t e m s . I_n Le C r e n , E.D. And R.H. M c C o n n e l l [ e d s . ] , The F u n c t i o n i n g of F r e s h w a t e r  E c o s y s t e m s , IBP 22, Cambridge U n i v e r s i t y P r e s s , G r e a t B r i t a i n . W e b s t e r , K.E. And R.H. P e t e r s . 1978. Some s i z e - d e p e n d e n t i n h i b i t i o n s of l a r g e r c l a d o c e r a n f i l t e r e r s i n f i l a m e n t o u s s u s p e n s i o n s . L i m n o l . O c e a n o g r . 2_3 : 12 38-1245. Weers, E.T. And T . Z a r e t . 1975. G r a z i n g e f f e c t s on n a n n o p l a n k t o n i n Gatun L a k e , Panama. V e r h . I n t e r n a t . V e r e i n . L i m n o l . J_9 : 1480-1483. W e t z e l . 1975. L i m n o l o g y . W.B. S a u n d e r s Company, P h i l a d e l p h i a , 743 pp. W h i t t o n , B.A. 1973. F r e s h w a t e r p l a n k t o n . I_n N.G. C a r r and B.A. W h i t t o n [ e d s . ] , The B i o l o g y of B l u e - g r e e n A l g a e , B l a c k w e l l , 676 pp. W i l l i a m s , R. And J.A. L i n d l e y . 1980. P l a n k t o n of t h e F l a d e n G r ound d u r i n g FLEX76 I I I . V e r t i c a l d i s t r i b u t i o n , p o p u l a t i o n d y n a m i c s and p r o d u c t i o n of C a l a n u s  f i n m a r c h i c u s ( C r u s t a c e a : C o p e p o d a ) . Mar. B i o l . 60 :47-56. 2 2 4 W i l s o n , D.S. 1 9 7 3 . Food s i z e s e l e c t i o n among c o p e p o d s . E c o l o g y 5 4 . . - 9 0 9 - 9 1 4 . W r i g h t , John C. 1 9 6 5 . The p o p u l a t i o n d y n a m i c s and p r o d u c t i o n of Daphn i a i n Canyon F e r r y R e s e r v o i r , Montana. L i m n o l . O c e a n o g r . _ K ) : 5 8 3 - 5 9 0 . 225 A P P E N D I X I Monthly r a i n f a l l (mm) to the U.B.C. Research Forest, 1977-1982 400 0 J F M A M J J A S O N ' D month rainfall mm rainfall mm ro ro 400 M A M J J A S month 228 APPENDIX II T e m p e r a t u r e i s o c l i n e s ( d e g r e e s C e l s i u s , d e p t h i n m) i n P l a c i d L a k e , 1977-1982 1978 1980 1981 month 234 APPENDIX I I I - COUNTING PHYTOPLANKTON I n t r o d u c t i o n The g e n e r a l method o f t r e a t i n g and c o u n t i n g p h y t o p l a n k t o n has been d e s c r i b e d i n C h a p t e r I I . In t h e o r i g i n a l a n a l y s i s of s amples c o l l e c t e d by C . J . W a l t e r s , o n l y enough f i e l d s t o c o n t a i n a t o t a l o f 200 c e l l s of s i z e s g r e a t e r t h a n 2 Mm were c o u n t e d . T h i s may have been i n f l u e n c e d by Lund e t a l . ( 1 9 5 8 ) , who s a i d t h a t a c o u n t i n g a c c u r a c y of 50% was q u i t e a d e q u a t e , and t h a t one was s a f e i n c o u n t i n g no more t h a n 100 a l g a e t o o b t a i n t h i s l e v e l of a c c u r a c y . They f e l t t h a t t h e c o u n t i n g e r r o r , even a t 50%, was s m a l l r e l a t i v e t o t h e s a m p l i n g e r r o r . However, u n l i k e our o b l i q u e s t r a t i f i e d s a m p l i n g , t h e i r samples were t a k e n from d i s c r e t e s t a t i o n s , and were much more s u b j e c t t o s a m p l i n g e r r o r due t o p a t c h y p l a n k t o n d i s t r i b u t i o n . M a r g a l e f ( V o l l e n w e i d e r , 1974) s u g g e s t e d t h a t c e l l e n u m e r a t i o n s h o u l d be done i n two s t a g e s : f i r s t , t h e e n t i r e b o t t o m a r e a of t h e chamber s h o u l d be s c a n n e d under low m a g n i f i c a t i o n t o c o u n t a l l t h e l a r g e f o r m s . S e c o n d l y , t h e n a n n o p l a n k t o n o f two c r o s s e d d i a m e t e r t r a n s e c t s o f t h e b o t t o m of t h e chamber s h o u l d be c o u n t e d , a t h i g h m a g n i f i c a t i o n . He a l s o s u g g e s t e d t h a t i f e x t r e m e a c c u r a c y i s r e q u i r e d , r e p l i c a t e s a m p l e s i n s e v e r a l d i f f e r e n t s i z e d chambers s h o u l d be e n u m e r a t e d . F o r t a x o n o m i c a c c u r a c y , a t l e a s t 100 i n d i v i d u a l s o f e v e r y i m p o r t a n t s p e c i e s s h o u l d be c o u n t e d . T h e r e i s a v a s t d i f f e r e n c e between t h e methods p r e s c r i b e d by Lund and by M a r g a l e f . Our o r i g i n a l method was more s t r i n g e n t 2 3 5 t h a n t h a t s u g g e s t e d b y L u n d . H o w e v e r , t h e r e w e r e some s e r i o u s q u e s t i o n s r e g a r d i n g t h e a c c u r a c y o f o u r c o u n t s , p a r t i c u l a r l y s i n c e t h e y m i g h t i n c l u d e o n l y v e r y f e w o f t h e l a r g e c e l l s . A c o u n t i n g e r r o r o f o n l y a f e w l a r g e c e l l s c o u l d i n t r o d u c e s u b s t a n t i a l e r r o r i n t o t h e o v e r a l l b i o m a s s e s t i m a t e . On t h e o t h e r h a n d , M a r g a l e f ' s p r e s c r i p t i o n s e e m e d e x t r e m e . O u r o r i g i n a l m e t h o d n o r m a l l y i n v o l v e d c o u n t i n g 2 t o 6 f i e l d s a t h i g h p o w e r ; M a r g a l e f ' s w o u l d i n v o l v e c o u n t i n g a p p r o x i m a t e l y 150 f i e l d s . S i n c e I was i n t e r e s t e d i n q u a n t i t a t i v e a c c u r a c y w i t h r e s p e c t t o c e l l s i z e , n o t t a x o n o m y , a n d c o n s i d e r i n g t h e n u m b e r o f s a m p l e s t o be a n a l y z e d , a m o r e p r a c t i c a l a p p r o a c h was n e e d e d . V e n r i c k ( 1 9 7 8 ) r e v e i w e d t h e r e c o m m e n d a t i o n s o f s e v e r a l r e s e a r c h e r s ; t h e s e f e l l g e n e r a l l y b e t w e e n t h e t w o e x t r e m e s o f L u n d a n d M a r g a l e f . S h e c o n c l u d e d t h a t t h e d i s p a r i t y i n r e c o m m e n d a t i o n s r e f l e c t e d t h e d i f f e r e n c e s i n l a b o r a t o r y m e t h o d s a n d t h e t h e o r e t i c a l c o n c e p t s a n d a s s u m p t i o n s on w h i c h t h e m e t h o d s w e r e b a s e d . F u n d a m e n t a l d i f f e r e n c e s i n t h e t y p e s a n d d e n s i t i e s o f c e l l s i n t h e s a m p l e s a l s o w e r e m a j o r s o u r c e s o f d i s a g r e e m e n t . My c o n c l u s i o n , a f t e r e x a m i n i n g t h e l i t e r a t u r e a n d c o n s i d e r i n g my own r e q u i r e m e n t s f o r a c c u r a c y a n d p r e c i s i o n , a s w e l l a s t i m e c o n s t r a i n t s , w a s t o d o a f e w e x t e n s i v e c o u n t s t o f i n d t h e m i n i m u m n u m b e r o f f i e l d s a n d c e l l s I n e e d e d t o c o u n t f o r m o r e r e a s o n a b l e q u a n t i t a t i v e a c c u r a c y t h a n we h a d p r e v i o u s l y h a d . 236 Methods and M a t e r i a l s U s i n g two n o r m a l l y s e t - u p samples f o r t h e i n v e r t e d m i c r o s c o p e , up t o 50 f i e l d s were c o u n t e d f o r t h e l a r g e c e l l c a t e g o r i e s (>10 Mm), 20 f i e l d s f o r t h e 6-10 Mm c a t e g o r y , and 5 t o 10 f o r t h e <6 Mm c a t e g o r y . C e l l c o u n t s f o r e a c h f i e l d were r e c o r d e d . Then, t o t a l c e l l c o u n t s p e r f i e l d , up t o t h e l a t e s t f i e l d c o u n t e d , were c a l c u l a t e d and t h e s e were p l o t t e d a g a i n s t number of f i e l d s c o u n t e d . R e s u l t s and D i s c u s s i o n At t h e b e g i n n i n g o f t h e c o u n t i n g , t h e mean number of c e l l s p e r f i e l d f l u c t u a t e d c o n s i d e r a b l y , , but as t h e number of f i e l d s c o u n t e d i n c r e a s e d , t h e v a r i a b i l i t y d e c r e a s e d . (See F i g u r e s A I I I - 1 t o A I I I - 6 . ) I a r b i t r a r i l y c h o s e an e r r o r l e v e l of 5% of t h e o v e r a l l mean as a c u t - o f f - t h e p o i n t a t w h i c h 95% a c c u r a c y was a c h i e v e d i n terms o f e i t h e r f i e l d s c o u n t e d where c e l l c o u n t s were low o r number of c e l l s c o u n t e d where c e l l c o u n t s were h i g h . G e n e r a l l y , f o r t h o s e c a t e g o r i e s l a r g e r t h a n 10 Mm l o n g e s t d i m e n s i o n , f i e l d s c o u n t e d was t h e l i m i t i n g c r i t e r i o n : o v e r a l l i t was n e c e s s a r y t o c o u n t a t l e a s t 20-40 f i e l d s t o a c h i e v e 95% a c c u r a c y as compared t o t h e mean a t 50 f i e l d s . A s t a n d a r d of 30 f i e l d s was c h o s e n . F o r t h e 6-10 Mm s i z e c a t e g o r y , a minimum of 100 c e l l s were c o u n t e d . O c c a s i o n a l l y , t h i s 100 c e l l l i m i t was a l s o u s e d f o r t h e l a r g e r c e l l s i n s a m p l e s where t h e c o u n t s f o r t h e l a r g e r c e l l c a t e g o r i e s were q u i t e h i g h . F o r t h e 2-6 Mm s i z e 237 F i g u r e s A I I I - 1 t o A I I I - 6 Changes i n mean c e l l c o u n t s w i t h i n c r e a s i n g f i e l d s c o u n t e d cells/field 8£z 2 3 9 c a t e g o r y , a c u t - o f f of 2 0 0 c e l l s was c h o s e n , and t h e e s t i m a t e s f o r t h e <2 um c e l l s were made from t h e number of f i e l d s r e q u i r e d t o c o u n t 2 0 0 of t h e 2 - 6 Mm c e l l s . APPENDIX I V Response of phytoplankton biomass (ug/1 fresh weight) composition to grazing by selected zooplankton species KEY TO FIGURES JUNE control JUNE Daphnia rosea BXK 241a JUNE Diaptomus oregonensis 242a JUNE Holopedium gibberum 3 C X K O . 6 - L c f . ia- S 4 . 30> 3 B - « • « • 3 4 . b O -TIKE 4 0 O - . JULY control TIM-: TIMZ JULY Daphnia rosea axi: TIME JULY Diaptomus kenai 1 E T O - . TIKE JULY Diaptomus oregonensis 1,1 m: TlfcE JULY Holopedium gibberum ere. AUGUST control ET.O-. TlfcE 246 AUGUST Daphnia rosea Til£ AUGUST Diaptomus oregonensis TlfcE 248 AUGUST Holopedium gibberum J U J K NXU' AUGUST control (Diaphanosoma) 251 SEPTEMBER control DXD' T 11XX3: SEPTEMBER Daphnia rosea VT.O: TIKE SEPTEMBER Diaptomus oregonensis lpro-. THE 254 SEPTEMBER Holopedium gibberum jxro-iCO-TIME i t x » x Tii£ 255 APPENDIX V I n i t i a l r e s p o n s e of p h y t o p l a n k t o n b i o m a s s (uq/1 f r e s h w e i g h t ) and c o m p o s i t i o n t o v a r i a t i o n i n i n i t i a l z o o p l a n k t o n biomass (ug/1 d r y w e i g h t ) K E Y TO F I G U R E S A-C E L L SIZE (um) E X P . 2 0.5 UG/L "TENCL. 4 TIKE 257 EXP. 2 2.5 U G / L "TENCL. 6 "T ENCL. 3 1 3 3 TINE 258 259 EXP. 2 25. UG/L E N C L . 2 f v . S 1/ , H V/ \ \ /' \ ts Y \ / \ in w ••••\\\\ 1 1 i£. B - i . S B . « • En- T E . tn- 9> ire> iff> Tilvll 260 EXP. 2 50 UG/L X ENCL. 9 0' L O . a > 30* 4 0 - 5U» 6 0 . >*«• 0 0 - 3 > ICG* 1 1 0 - 1 3 J TitvE 261 EXP. 3 LOW INITIAL ZOOPLANKTON ENCL. I 7tx>-.a HOURS 262 0. 12- 24 - 36 ' 48- ED- 75> B4> H O U R S 263 265 266 267 268 HOURS 269 EXP 4 0 UG/L "7 ENCL. 6 E X P 4 1.5 UG/L 272 273 E X P 4 7 . 5 U G /L O b ' 1 5 - I B ' ttA- 3 0 - 3 E . 3 4 * b 0 « 5 G - 7 £ « TH,;:: EXP. 4 15 UG/L ^"'T ENCL.7 1GC0-. THE "TENCL. II lHiO-. TIME: 275 E X P . 4 35 U G / L "TENCL. 9 TiLE 2 7 6 APPENDIX VI L o n g - t e r m r e s p o n s e of p h y t o p l a n k t o n (ug / 1 f r e s h w e i g h t ) and z o o p l a n k t o n b i o m a s s ( u g / 1 d r y w e i g h t ) and c o m p o s i t i o n t o v a r i a t i o n i n i n i t i a l z o o p l a n k t o n b i o m a s s (ug / 1 d r y w e i g h t ) K E Y TO F IGURES ZOOPLANKTON--cladocerans calanoid copepods carnivores P H Y T O P L A N K T O N colonies >20 um cells 6 - 2 0 um cells < 6 um cells 276a 277 EXP. I ENCL. 2 4_ , , , r 1 1 " 1 1 1 0 8 16 24 32 40 DAYS 278 6 0 1 EXPI ENCL. 3 D A Y S 279 280 EXP. I ENCL. 7 - i 1 1 r 16 2 4 3 2 D A Y S 80 EXP. I ENCL. 8 60^ 40-^  4_ , , , . r 1 1 1 • r ~ 0 8 16 24 32 40 DAYS 282 80-1 EXP. I E N C L . 10 8 0 E X P I ENCL. 13 601 D A Y S 284 285 801 EXR I ENCL. 15 60-4 0 o 2 0 ZD CO 00 < o £D 0 -\ 1 1 1 1 1 1 1 r 2500-2000 -15001 10001 5001 Q _ | , , 1 r— 1 1 1 1 r 0 8 16 24 32 4 0 D A Y S 286 E X P I E N C L . 16 8 0 E X P . I E N C L . 18 601 4 0 H DAYS 288 E X P 3 E N C L . I 2 0 10 0 Z> $ 800-1 o m 6 0 0 400 2 0 0 1 |CLAD0CERANS r~"-]CALAN0l D l„„ J COPE PODS • CARNIVORES 16 24 DAYS EXP 3 ENCL.2 290 E X P 3 E N C L . 3 4 0 " 301 0 T 1 1 1 1 1 \ 1 1 1 1 0 8 16 2 4 32 4 0 D A Y S 291 - \ 1 1 1 1 1 [ ( I 0 8 16 24 32 DAYS 293 E X P 3 E N C L . 6 30-1 201 101 O-l 1 1 1 1 1 1 1 1 1 1— 0 8 16 2 4 3 2 4 0 DAYS 295 296 E X P 3 ENCL. 10 30H 0^ 1 1 1 1 1 1 1 1 1 1 — 0 8 16 2 4 32 4 0 DAYS 298 EXP. 3 ENCL. II 30-20-101 H 1 1 1 1 1 1 1 1 i 1 0 8 16 24 32 40 DAYS 299 300 EXP 3 ENCL. 13 30-1 20H 3800-I co CO o OD 600-400 200 O H 1 1 1 1 1 1 ' 1 ' 0 8 16 24 32 40 DAYS 301 302 0 8 16 24 32 40 DAYS EXP. 3 ENCL. 16 H 1 1 1—; 1 1 1 1 ' 0 8 16 24 32 DAYS 305 E X P 3 E N C L . 18 201 I0H _j 0 - i i ' \ 1 1 1 1 1 i i I H 1 1 1 1 1 1 1 1 i i 0 8 16 24 32 4 0 DAYS 306 EXR 4 ENCL. I 20-C H 1 1 1 1 1 1 1 1 1 1 0 8 16 24 32 40 DAYS EXP. 4 ENCL. 2 20\ 0-J 1 1 1 1 1 1 1 1 1 1 0 8 16 24 32 40 DAYS 308 EXP. 4 ENCL. 3 EXP. 4 ENCL. 4 201 101 0-c/)IOOO < o co 8 0 0 600 4 0 0 2 0 0 0-j 1 1 1 1 1 1 1 1 1 1—! 1 0 8 16 24 32 40 DAYS 310 0 1 1 1 1 " 1 1 1 ' 1 1 — 0 8 16 24 32 40 DAYS 311 401 EXR 4 ENCL. 6 \ CO ZD CO to < o CQ I500H 313 314 0 8 16 24 32 40 DAYS 315 316 EXP. 4 ENCL. 11 3 0 1 0 8 16 24 32 40 DAYS 317 EXP. 4 ENCL. 12 O ' l i • 1 1 1 1 1 1 ' ' 0 8 16 24 32 40 DAYS 318 APPENDIX V I I Z o o p l a n k t o n c o m p o s i t i o n and bi o m a s s ( Mg/l d r y w e i g h t ) , P l a c i d L a k e , 1977-1982 K E Y T O F I G U R E S Hi Holopedium gibberum IS Daphnia rosea H other cladocerans 111 Diaptomus kenai tZ] Diaptomus oreqonensis Hi cyclopoid copepods HH miscellaneous 319 320 180-160-140-120-- 100-1980 322 801 323 324 APPENDIX V I I I 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 b i o m a s s (ug/1 f r e s h w e i g h t ) , P l a c i d L a k e , 1977-1982 V K E Y T O F I G U R E S colonies • cells >20 um • cells 6 - 2 0 u m i 1 cells < 6 um 240CH 2000 1600^  c 1200 o c cd Q. O Q. 800H 400 325 140 On 1978 1200^  SMOOO^  c o c 8001 §• 600 J A S m o n t h 1979 3200H 2800H 327 1600 1400 1200-1000^  O) o 800 a 600 •5 4001 200 0 M J J A S O month 328 6610 3200i 2800 2400-- 20001 =J § 1600 •*-> c 5 1200 a o -»—> £ 800 a 400-M J J A S m o n t h o 1400i 1200H oi 1000 c £ 800 c 03 a o a 6 0 0 400H 1982 200 

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