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The effects of lake acidification on zooplankton community structure and phytoplankton-zooplankton interactions… Marmorek, David R. 1983

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EFFECTS OF LAKE ACIDIFICATION ON ZOOPLANKTON' COMMUNITY STRUCTURE AND PHYTOPLANKTON-ZOOPLANKTON INTERACTIONS: AN EXPERIMENTAL APPROACH by . DAVID MARMOREK B.E.S., U n i v . of W a t e r l o o , 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA June 1983 © D a v i d Marmorek, 1983 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 his 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 ^oolo^y  The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date a* ABSTRACT The e f f e c t s o f 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 c o m m u n i t i e s an d 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 were i n v e s t i g a t e d by means o f two f i e l d e x p e r i m e n t s i n E u n i c e L a k e , an o l i g o t r o p h i c , low a l k a l i n i t y l a k e i n t h e C o a s t a l Range M o u n t a i n s o f B r i t i s h C o l u m b i a . B o t h e x p e r i m e n t s were c a r r i e d o u t in s i t u u s i n g e i g h t p o l y o l e f i n e n c l o s u r e s , e a c h h o l d i n g 28,000 l i t e r s o f l a k e w a t e r a n d p l a n k t o n . From J u l y t o O c t o b e r 1979, a c i d i f i c a t i o n w i t h H 2 S O a and e n r i c h m e n t w i t h NH aN0 3 and H 3POi, were a p p l i e d t o e n c l o s u r e s b o t h a s s e p a r a t e t r e a t m e n t s and i n c o m b i n a t i o n . A c i d i f i c a t i o n a l o n e l o w e r e d t h e e p i l i m n e t i c pH t o 5.6, but d i d n o t a f f e c t z o o p l a n k t o n , p h y t o p l a n k t o n , o r t r a n s p a r e n c y . E n r i c h m e n t a l o n e i n c r e a s e d c h l o r o p h y l l a c o n c e n t r a t i o n s , t h e b i o m a s s o f e d i b l e a l g a l c e l l s a nd z o o p l a n k t o n b i o m a s s . When a c i d i f i c a t i o n and e n r i c h m e n t were combined, b i o t i c p r o c e s s s i n g o f NH J |N0 3 l o w e r e d t h e pH t o 5.4, c a u s i n g h i g h m o r t a l i t y t o t h e z o o p l a n k t o n community d o m i n a n t , D a p h n i a r o s e a . The d e c l i n e o f D a p h n i a a l l o w e d c h l o r o p h y l l a c o n c e n t r a t i o n s t o i n c r e a s e 6-9 f o l d . I t a l s o l e d t o major c h a n g e s i n t h e s p e c i e s c o m p o s i t i o n , s i z e s t r u c t u r e and a m p l i t u d e o f b i o m a s s f l u c t u a t i o n s o f t h e z o o p l a n k t o n community. P h y t o p l a n k t o n a p p e a r e d much more a f f e c t e d by a c i d - i n d u c e d c h a n g e s t o z o o p l a n k t o n g r a z i n g t h a n by d i r e c t a b i o t i c e f f e c t s o f a c i d i f i c a t i o n . i i i The 1979 e x p e r i m e n t s u g g e s t e d t h a t i f l a k e s o f pH 5.0 t o 5.5 a r e e n r i c h e d , t h e p r o b a b i l i t y o f n u i s a n c e a l g a l blooms may be i n c r e a s e d , p a r t i c u l a r l y i f t h e h e r b i v o r o u s community d o m i n a n t s a r e b o t h l a r g e a n d a c i d - s e n s i t i v e . In May 1980, u n e n r i c h e d e n c l o s u r e s were a c i d i f i e d o v e r a t e n day p e r i o d t o pH 5.5, 5.0 o r 4.5, and t h e n m a i n t a i n e d a t c o n s t a n t pH f o r s e v e n weeks. Though t h e a c i d t o l e r a n c e o f D. r o s e a was v e r y s i m i l a r i n b o t h y e a r s ' e x p e r i m e n t s , t h e c o p e p o d D i a p t o m u s t y r r e l l i was more s e n s i t i v e t o a c i d i f i c a t i o n i n 1980 t h a n i n 1979, due t o d i f f e r e n c e s i n e i t h e r l i f e h i s t o r y s t a g e , t e m p e r a t u r e o r f o o d . H i g h r a t e s o f a c i d i f i c a t i o n i n c r e a s e d t o x i c i t y n e a r i n c i p i e n t l e t h a l l e v e l s i n b o t h D. r o s e a and Bosmina l o n q i r o s t r i s . A c i d i f i c a t i o n t o pH 5.5, 5.0 and 4.5 d e c r e a s e d mean z o o p l a n k t o n b i o m a s s by 20%, 63% and 74%. However, b o t h c h l o r o p h y l l a c o n c e n t r a t i o n s a n d r o t i f e r b i o m a s s i n c r e a s e d w i t h t h e l e v e l o f a c i d i f i c a t i o n , due t o a p p a r e n t r e l e a s e s f r o m g r a z i n g and c o m p e t i t i o n ( r e s p e c t i v e l y ) . B o t h my 1979 e n c l o s u r e e x p e r i m e n t and w h o l e - l a k e m a n i p u l a t i o n s p e r f o r m e d e l s e w h e r e s u g g e s t e d t h a t a c i d i c l a k e s m i g h t show i n c r e a s e d f l u c t u a t i o n i n z o o p l a n k t o n b i o m a s s o v e r c i r c u m n e u t r a l l a k e s . A n a l y s e s o f u n p u b l i s h e d z o o p l a n k t o n d a t a from O n t a r i o a c i d i c l a k e s s u p p o r t t h i s s u g g e s t i o n . In g e n e r a l , t h e d i r e c t i o n o f z o o p l a n k t o n community change i s d e t e r m i n e d by t h e i n t e r s e c t i o n o f a c i d i f i c a t i o n e p i s o d e s w i t h t h e s p a t i a l and t e m p o r a l d i s t r i b u t i o n s o f a c i d - s e n s i t i v e s p e c i e s , and t h e c o m p e t i t i v e r e l a t i o n s h i p s w i t h i n t h e community a t t h e t i m e o f a c i d i f i c a t i o n . B o t h s p e c i e s d i s t r i b u t i o n s and i v c o m p e t i t i v e r e l a t i o n s h i p s a r e s e n s i t i v e t o s e a s o n a l c h a n g e s i n t e m p e r a t u r e and n u t r i e n t s , t o w h i c h z o o p l a n k t o n l i f e h i s t o r i e s have been f i n e l y t u n e d . L o n g - t e r m a c i d i f i c a t i o n may c r e a t e " h o l e s " i n t h e t e m p o r a l o r g a n i z a t i o n o f z o o p l a n k t o n c o m m u n i t i e s , w i t h no a c i d - t o l e r a n t s p e c i e s a v a i l a b l e w i t h t h e a p p r o p r i a t e l i f e h i s t o r y a n d t e m p e r a t u r e r e s p o n s e p h y s i o l o g y t o f i l l them. V TABLE OF CONTENTS ABSTRACT i i L I S T OF TABLES x L I S T OF FIGURES x i i i ACKNOWLEDGEMENTS .'. x v i i I . EXPERIMENTAL ACIDIFICATION AND FERTILIZATION OF ENCLOSED PLANKTON SYSTEMS (Summer 1979) 2 1. INTRODUCTION 3 1.1 H i s t o r i c a l and T h e o r e t i c a l B a c k g r o u n d 3 1.2 R e s e a r c h O b j e c t i v e s 13 2. MATERIALS AND METHODS 15 2.1 S t u d y S i t e 15 2.2 E x p e r i m e n t a l Methods 20 2.2.1 P h y s i c a l and C h e m i c a l measurements 27 2.2.2 P h y t o p l a n k t o n 28 ( i ) Methods U s e d 28 ( i i ) P r e c i s i o n o f A l g a l C e l l Volume a n d C h l o r o p h y l l D e t e r m i n a t i o n s 31 2.2.3 Z o o p l a n k t o n 34 2.2.4 S t a t i s t i c a l Methods 37 2.3 E x p e r i m e n t a l D e s i g n 40 3. RESULTS 46 3.1 P h y s i c a l and C h e m i c a l C o n d i t i o n s 46 3.1.1 P r e c i p i t a t i o n and T e m p e r a t u r e 46 3.1.2 Hydrogen Ion C o n c e n t r a t i o n and A l k a l i n i t y .... 51 v i ( i ) E n c l o s u r e E f f e c t s 52 ( i i ) E f f e c t o f H + i o n L o a d i n g i n P r e c i p i t a t i o n .. 59 ( i i i ) E f f e c t s o f E x p e r i m e n t a l A c i d i f i c a t i o n 59 ( i v ) E f f e c t s o f E x p e r i m e n t a l F e r t i l i z a t i o n 62 (v) E f f e c t s o f E x p e r i m e n t a l A c i d i f i c a t i o n and F e r t i l i z a t i o n 63 3.1.3. T r a n s p a r e n c y 64 3.1.4 Summary o f P h y s i c a l and C h e m i c a l R e s u l t s 68 3.2 Z o o p l a n k t o n 70 3.2.1 E f f e c t s o f E n c l o s u r e 72 3.2.2 E f f e c t s o f A c i d i f i c a t i o n 95 3.2.3 E f f e c t s o f F e r t i l i z a t i o n 98 3.2.4 E f f e c t s o f Combined A c i d i f i c a t i o n a n d F e r t i l i z a t i o n 100 ( i ) T o t a l Z o o p l a n k t o n B iomass and Response o f C l a d o c e r a 101 ( i i ) R e sponse o f D i a p t o m i d Copepods 107 3.2.5 V e r t i c a l D i s t r i b u t i o n 109 3.3 P h y t o p l a n k t o n 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 118 3.3.1 E f f e c t s o f E n c l o s u r e 118 3.3.2 E f f e c t s o f A c i d i f i c a t i o n — 138 3.3.3 E f f e c t s o f F e r t i l i z a t i o n 139 3.3.4 E f f e c t s o f A c i d i f i c a t i o n and F e r t i l i z a t i o n ...141 ( i ) J o i n t F l u c t u a t i o n s i n C h l o r o p h y l l , P h a e o p h y t i n and T o t a l Z o o p l a n k t o n Biomass 141 ( i i ) E s t i m a t e s o f A l g a l B i o mass by C e l l C o u n t s ..144 v i i ( i i i ) E f f e c t s o f L o s s o f D a p h n i a on H e r b i v o r e C o n t r o l o f P h y t o p l a n k t o n and P h y t o p l a n k t o n Community S i z e S t r u c t u r e 147 4. DISCUSSION 158 4.1 E n c l o s u r e E f f e c t s 159 4.2 E v a l u a t i o n o f t h e P h y s i c a l and C h e m i c a l E f f e c t s o f t h e E x p e r i m e n t a l A c i d i f i c a t i o n 165 4.3 B i o t i c P r o c e s s i n g o f Ammonium N i t r a t e A d d i t i o n s ...172 4.4 Z o o p l a n k t o n Community S t r u c t u r e 177 4.4.1 C l a d o c e r a n s 181 ( i ) D a p h n i a r o s e a 181 M u l t i - L a k e S u r v e y s 183 L a b o r a t o r y E x p e r i m e n t s 184 Whole Lak e M a n i p u l a t i o n s 189 ( i i ) Diaphanosoma b r a c h y u r u m 191 ( i i i ) Bosmina l o n q i r o s t r i s 194 ( i v ) C h y d o r u s s p h a e r i c u s 198 (v) C e r i o d a p h n i a p u l c h e l l a 201 4.4.2 Copepods 202 4.5 P h y t o p l a n k t o n 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 203 I I . EXPERIMENTAL ACIDIFICATION OF ENCLOSED ZOOPLANKTON COMMUNITIES ( S p r i n g 1980) 21 1 1. INTRODUCTION 212 2. MATERIALS AND METHODS 215 2.1 E x p e r i m e n t a l D e s i g n and E x e c u t i o n 215 2.2 S t a t i s t i c a l and S i m u l a t i o n A n a l y s e s 220 v i i i 3. RESULTS 223 3.1 T e m p e r a t u r e and pH Measurements 223 3.2 O v e r a l l E f f e c t s on t h e P l a n k t o n Community 229 3.2.1 E f f e c t s o f E n c l o s u r e 229 3.2.2 Z o o p l a n k t o n Biomass and Community C o m p o s i t i o n .236 3.2.3 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 239 3.3 P e r f o r m a n c e o f P a r t i c u l a r Taxa under A c i d S t r e s s ...242 3.3.1 D a p h n i a r o s e a 242 3.3.2 H o l o p e d i u m q i b b e r u m 258 3.3.3 Bosmina l o n q i r o s t r i s 261 3.3.4 Di a p t o m u s k e n a i 264 3.3.5 Diaptomus t y r r e l l i 268 3.3.6 D i a c y c l o p s t h o m a s i and c o p e p o d n a u p l i i 275 3.3.7 R o t i f e r s 280 4. DISCUSSION 287 4.1 Changes i n Z o o p l a n k t o n Biomass and Community S t r u c t u r e 287 4.1.1 C o m p a r i s o n s w i t h O t h e r S t u d i e s 287 4.1.2 C o m p a r i s o n s w i t h t h e 1979 e x p e r i m e n t 295 4.2 P e r f o r m a n c e o f P a r t i c u l a r Taxa Under A c i d S t r e s s ...297 I I I SUMMARY AND CONCLUDING DISCUSSION 305 1. SUMMARY 306 2. CONCLUDING DISCUSSION 313 2.1 T e m p o r a l F l u c t u a t i o n s o f P l a n k t o n Systems i n A c i d i c L a k e s 313 2.2 R e l a t i v e I m p o r t a n c e o f T o x i c i t y , C o m p e t i t i o n and P r e d a t i o n i n A c i d i f y i n g L a k e s 319 2.3 I m p l i c a t i o n s f o r H i g h e r T r o p h i c L e v e l s and .Lake Management 325 REFERENCES CITED 329 APPENDIX A: COMPUTER PROGRAM USED TO CALCULATE ALGAL CELL VOLUMES 354 APPENDIX B: LENGTH-WEIGHT REGRESSIONS USED FOR CALCULATION OF ZOOPLANKTON BIOMASS (1979 and 1980) 361 APPENDIX C: THREE-WAY ANALYSES OF VARIANCE OF ALKALINITY AND H + CONCENTRATIONS ( 1979) 363 APPENDIX D: REGRESSION ANALYSES OF SECCHI DEPTH-CHLOROPHYLL a RELATIONSHIPS ( 1979) 370 APPENDIX E : ZOOPLANKTON LENGTHS (1979) 379 APPENDIX F: PHYTOPLANKTON VOLUMES BY TAXA FOR EACH CYLINDER AND SAMPLING DATE ( 1979) 382 APPENDIX G: METHODS OF ESTIMATING DAPHNIA MORTALITY RATES (1980) 392 APPENDIX H: SENSITIVITY ANALYSIS OF THREE STAGE MODEL FOR ESTIMATING DAPHNIA MORTALITY RATES (1980) 395 X L I S T OF TABLES T a b l e 1. P h y s i c a l and c h e m i c a l p r o p e r t i e s , and c r u s t a c e a n z o o p l a n k t o n community, o f E u n i c e L a k e 18 T a b l e 2. E x p e r i m e n t a l t r e a t m e n t s 26 T a b l e 3. V a r i a b i l i t y i n p h y t o p l a n k t o n b i o m a s s e s t i m a t e s due t o low numbers o f c o u n t e d c e l l s 32 T a b l e 4. T e s t o f p r e c i s i o n o f z o o p l a n k t o n s a m p l i n g method . 35 T a b l e 5. H y p o t h e s e s t e s t e d by 1979 e x p e r i m e n t 41 T a b l e 6. I n d i c a t o r v a r i a b l e s m o n i t o r e d i n 1979, and t h e i r j u s t i f i c a t i o n 43 T a b l e 7. Mean pH and a l k a l i n i t y by t r e a t m e n t and d e p t h s t r a t u m 57 T a b l e 8. D i f f e r e n c e s between t r e a t m e n t s i n mean water column a l k a l i n i t y on e a c h s a m p l i n g d a t e 58 T a b l e 9. Hydrogen i o n l o a d i n g f r o m p r e c i p i t a t i o n (1979) ... 60 T a b l e 10. Mean z o o p l a n k t o n community b i o m a s s by t r e a t m e n t and d a t e , and r e s u l t s o f a n a l y s e s of v a r i a n c e on i n t e r - t r e a t m e n t d i f f e r e n c e s 77 T a b l e 11. Two-way a n a l y s e s o f v a r i a n c e on z o o p l a n k t o n a b undance, egg d e n s i t i e s , and mean c l u t c h s i z e p e r o v i g e r o u s f e m a l e 78 T a b l e 12. L e n g t h s o f D a p h n i a r o s e a i n E u n i c e Lake and t h e c o n t r o l s 94 T a b l e 13. R e s u l t s o f a n a l y s e s o f v a r i a n c e on c h l o r o p h y l l measurements i i Q x i T a b l e 14. R e s u l t s of a n a l y s e s of v a r i a n c e on a l g a l volumes i n p a r t i c u l a r s i z e c l a s s e s 122 T a b l e 15. Mean p e r c e n t a g e o f a l g a l volume i n p a r t i c u l a r s i z e c l a s s e s 123 T a b l e 16. D i f f e r e n c e s between c y l i n d e r s 3AF and 7AF i n t o t a l a l g a l b i o m a s s a n d bi o m a s s o f C h r y s o c a p s a c o l o n i e s 145 T a b l e 17. Assumed f i l t e r i n g r a t e s f o r s p e c i e s i n F i g u r e 27 150 T a b l e 18. Mean a l g a l b i o m a s s by m a j o r s i z e c l a s s e s i n t r e a t m e n t s F and AF, b e f o r e and a f t e r t h e c o l l a p s e o f D a p h n i a i n AF 154 T a b l e 19. Summary o f 1979 r e s u l t s r e l e v a n t t o h y p o t h e s e s on e n c l o s u r e e f f e c t s 160 T a b l e 20. T o t a l aluminum c o n c e n t r a t i o n s i n a c i d i f i e d and n o n - a c i d i f i e d l a k e s i n Norway and O n t a r i o 168 T a b l e 21. Summary of 1979 r e s u l t s r e l e v a n t t o h y p o t h e s e s on z o o p l a n k t o n c o m m u n i t i e s 178 T a b l e 22. Summary of l i t e r a t u r e s h o w i n g c hanges i n numbers of c r u s t a c e a n z o o p l a n k t o n s p e c i e s a c r o s s l a k e s o f d i f f e r e n t pH 180 T a b l e 23. Summary o f e x p e r i m e n t a l s t u d i e s o f t h e pH t o l e r a n c e o f d a p h n i d s 185 T a b l e 24. Summary of 1979 r e s u l t s r e l e v a n t t o h y p o t h e s e s on p h y t o p l a n k t o n 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 204 T a b l e 25. C o r r e s p o n d e n c e between 1979 and 1980 e x p e r i m e n t a l t r e a t m e n t s 218 T a b l e 26. E x p e c t e d a n d o b s e r v e d amounts o f s u l f u r i c a c i d added t o c y l i n d e r s 228 T a b l e 27. Mean pH l e v e l s June 5 t o J u l y 16 230 T a b l e 28. Mean z o o p l a n k t o n community b i o m a s s by t r e a t m e n t and d a t e , and r e s u l t s o f a n a l y s e s o f v a r i a n c e on i n t e r - t r e a t m e n t d i f f e r e n c e s 237 T a b l e 29. Mean b r o o d s i z e s o f D a p h n i a r o s e a 246 T a b l e 30. Mean p e r c e n t a g e s o f o v i g e r o u s f e m a l e D a p h n i a r o s e a .246 T a b l e 31. R e s u l t s o f a n a l y s e s of v a r i a n c e on d e n s i t i e s of c l a d o c e r a n s and c l a d o c e r a h eggs 248 T a b l e 32. R e s u l t s o f a n a l y s e s o f v a r i a n c e on d e n s i t i e s of d i a p t o m i d c o p e p o d s and l o o s e c a l a n o i d eggs 267 T a b l e 33. M a j o r c h a n g e s i n D i a p t o m u s t y r r e l l i c o p e p o d i t e s i n t r e a t m e n t s M a n d H 274 T a b l e 34. R e s u l t s of a n a l y s e s o f v a r i a n c e on d e n s i t i e s o f c y c l o p o i d c o p e p o d s , c o p e p o d n a u p l i i and r o t i f e r s 281 T a b l e 35. C o m p a r i s o n o f t o t a l z o o p l a n k t o n b i o m a s s i n 1980 l i m n o c o r r a l s and O n t a r i o l a k e s 288 T a b l e B1. L e n g t h - w e i g h t r e g r e s s i o n s and mean z o o p l a n k t e r w e i g h t s 362 T a b l e D1. L i n e a r r e g r e s s i o n s o f i n v e r s e S e c c h i d e p t h and t o t a l c h l o r o p h y l l a, by t r e a t m e n t 372 T a b l e H1. S e n s i t i v i t y a n a l y s i s o f e s t i m a t e d m o r t a l i t y r a t e s o f j u v e n i l e D a p h n i a r o s e a 396 x i i i L I S T OF FIGURES F i g u r e 1. L o c a t i o n o f s t u d y s i t e 16 F i g u r e 2. P l a n v i e w o f l i m n o c o r r a l d e s i g n 21 F i g u r e 3. L o c a t i o n o f l i m n o c o r r a l s i n E u n i c e L a k e 23 F i g u r e 4. D a i l y p r e c i p i t a t i o n (1979) 47 F i g u r e 5. D a i l y v a r i a t i o n i n a i r t e m p e r a t u r e (1979) 47 F i g u r e 6. E u n i c e Lake water t e m p e r a t u r e s d u r i n g 1979 49 F i g u r e 7. 1979 pH l e v e l s by d e p t h and t i m e , and t i m i n g o f c h e m i c a l a d d i t i o n s 53 F i g u r e 8. D e p t h - t i m e d i a g r a m s o f a l k a l i n i t y i s o p l e t h s d u r i n g 1979 55 F i g u r e 9. Mean S e c c h i d i s c d e p t h s by t r e a t m e n t and t i m e ... 65 F i g u r e 10. C u m u l a t i v e b i o m a s s of major z o o p l a n k t o n s p e c i e s i n 1979 e x p e r i m e n t 73 F i g u r e 11. P e r c e n t c o m p o s i t i o n o f t o t a l z o o p l a n k t o n b i o m a s s i n 1979 e x p e r i m e n t ... 75 F i g u r e 12. D e n s i t i e s o f D a p h n i a r o s e a by age c l a s s 79 F i g u r e 13. D a p h n i a r o s e a r e p r o d u c t i o n 81 F i g u r e 14. D e n s i t i e s o f D i a p t o m u s t y r r e l l i 83 F i g u r e 15. D e n s i t i e s o f D i a p t o m u s k e n a i 85 F i g u r e 16. D e n s i t i e s o f l o o s e c a l a n o i d c o p e p o d eggs and n a u p l i i 87 F i g u r e 17. D e n s i t i e s o f Bosmina l o n g i r o s t r i s 89 F i g u r e 18. V e r t i c a l d i s t r i b u t i o n s o f a d u l t forms o f z o o p l a n k t o n 110 x i v F i g u r e 19. V e r t i c a l d i s t r i b u t i o n s o f immature forms o f z o o p l a n k t o n , and Diaphanosoma b r a c h y u r u m 112 F i g u r e 20. L i v e c h l o r o p h y l l a and p h a e o p h y t i n a 120 F i g u r e 21. A l g a l b i o m a s s by m a j o r s i z e c l a s s e s 124 F i g u r e 22. P e r c e n t c o m p o s i t i o n o f a l g a l b i o m a s s by d e t a i l e d s i z e c l a s s e s 126 F i g u r e 23. P h y l e t i c c o m p o s i t i o n o f a l g a l volume 128 F i g u r e 24. P r o p o r t i o n o f l i v e c h l o r o p h y l l a 131 F i g u r e 25. C h l o r o p h y l l a p e r u n i t a l g a l b i o m a s s 134 F i g u r e 26. Phase p l a n e t r a j e c t o r i e s t h r o u g h t i m e o f t o t a l z o o p l a n k t o n b i o m a s s and l i v e c h l o r o p h y l l a c o n c e n t r a t i o n s 136 F i g u r e 27. E s t i m a t e d f i l t e r i n g r a t e s o f z o o p l a n k t o n s p e c i e s 149 F i g u r e 28. D e s i g n o f 1980 e x p e r i m e n t 216 F i g u r e 29. E u n i c e Lake water t e m p e r a t u r e s d u r i n g 1980 224 F i g u r e 30. 1980 pH l e v e l s by d e p t h and t i m e 226 F i g u r e 31 . 1980 t o t a l z o o p l a n k t o n b i o m a s s 231 F i g u r e 32. P e r c e n t c o m p o s i t i o n o f t o t a l z o o p l a n k t o n b i o m a s s i n 1980 233 F i g u r e 33. Mean c o n c e n t r a t i o n s o f l i v e c h l o r o p h y l l a on J u l y 19, 1980 240 F i g u r e 34. D e n s i t i e s o f D a p h n i a r o s e a by age c l a s s (May 13-29) 243 F i g u r e 35. D e n s i t i e s o f D a p h n i a r o s e a by age c l a s s (May 29-J u l y 16) 243 F i g u r e 36. I n s t a n t a n e o u s m o r t a l i t y r a t e s o f j u v e n i l e D a p h n i a r o s e a , as e s t i m a t e d f r o m 3 - s t a g e model 249 F i g u r e 37. I n s t a n t a n e o u s m o r t a l i t y r a t e s o f a d u l t D a p h n i a r o s e a , a s e s t i m a t e d f r o m 3 - s t a g e model 251 F i g u r e 38. Mean m o r t a l i t y r a t e s o f a d u l t D a p h n i a r o s e a v e r s u s pH e x p o s u r e ( u s i n g 3 - s t a g e model) 254 F i g u r e 39. Mean m o r t a l i t y r a t e s o f D a p h n i a r o s e a v e r s u s pH e x p o s u r e ( u s i n g 2 - s t a g e model) 256 F i g u r e 40. D e n s i t i e s o f H o l o p e d i u m g i b b e r u m 259 F i g u r e 41. D e n s i t i e s o f Bosmina l o n q i r o s t r i s 262 F i g u r e 42. D e n s i t i e s o f D i a p t o m u s k e n a i 265 F i g u r e 43. D e n s i t i e s o f l o o s e c a l a n o i d eggs 269 F i g u r e 44. D e n s i t i e s o f D i a p t o m u s t y r r e l l i 271 F i g u r e 45. D e n s i t i e s o f D i a c y c l o p s t h o m a s i 276 F i g u r e 46. D e n s i t i e s o f c o p e p o d n a u p l i i 278 F i g u r e 47. D e n s i t i e s o f K e r a t e l l a and K e l l i c o t t i a 282 F i g u r e 48. D e n s i t i e s o f P o l y a r t h r a and C o n o c h i l u s 285 F i g u r e 49. P r i m a r y e c o l o g i c a l i n t e r a c t i o n s i n t h e 1979 and 1980 f i e l d e x p e r i m e n t s ...307 F i g u r e 50. C o e f f i c i e n t s of v a r i a t i o n of May t o November t o t a l z o o p l a n k t o n b i o m a s s 316 F i g u r e C1. R e s u l t s o f 3-way a n a l y s e s of v a r i a n c e c o m p a r i n g [ H + ] and a l k a l i n i t y i n t h e c o n t r o l s w i t h E u n i c e Lake ...364 F i g u r e C2. R e s u l t s o f 3-way a n a l y s e s o f v a r i a n c e c o m p a r i n g [H*] and a l k a l i n i t y i n t h e c o n t r o l s w i t h t r e a t m e n t F ...366 F i g u r e C3. R e s u l t s o f 3-way a n a l y s e s o f v a r i a n c e c o m p a r i n g [ H + ] and a l k a l i n i t y i n t r e a t m e n t s A and AF 368 F i g u r e D1. N o n - l i n e a r r e g r e s s i o n s o f S e c c h i d i s c d e p t h x v i v s . t o t a l c h l o r o p h y l l a 374 F i g u r e D2. D i s t r i b u t i o n o f r e s i d u a l S e c c h i d e p t h s from S e c c h i d e p t h - c h l o r o p h y l l r e g r e s s i o n on p o o l e d d a t a ...377 F i g u r e E 1 . Mean l e n g t h s o f z o o p l a n k t o n by t r e a t m e n t and t i m e (1979) 380 xvi i ACKNOWLEDGEMENTS I wish to express my grat i tude to the many people who have provided st imulat ion and support during my graduate studies at the Univers i ty of B r i t i s h Columbia. My thesis advisors , Ken H a l l and B i l l N e i l l , e n t h u s i a s t i c a l l y encouraged my research ideas, gave guidance to mould these ideas into testable hypotheses, and, by frequently sharing the ir extensive knowledge, helped me to place my resu l t s in a wider context. I thank Dr. J . Walters and the s taf f of the U . B . C . Research Forest , both for permission to conduct my research and for f r i end ly help when I required spec ia l ass i s tance . The successful setup of my f i r s t f i e l d experiment i s due large ly to the t i r e l e s s , creat ive and unpaid assistance of Peter G i l l i e s , to whom I am p a r t i c u l a r l y g r a t e f u l . Betty Bronson, Alan Carruthers , K a r l E n g l i s h , Ken H a l l , J u l i e Hunter, Granger Jung, Ken Lertzman, Peter Morrison, B i l l N e i l l , Roberta Olenick , Robert Saint P i e r r e , Mark Wel ls , John Werring, Suzy Woolf and Itsuo Yesaki helped with fieldwork at various times, most of them ra iny . Robert Saint P i e r r e , Itsuo Yesaki , Granger Jung and John Werring helped in the enumeration of plankton samples, saving me many months and e x i s t e n t i a l c r i s e s . Thanks also to Itsuo for draf t ing such elegant f igures , to Cathi Lowe for typing my f i r s t d r a f t , to Edgard Guindon for his computer program to estimate zooplankton morta l i ty rates , and to Roberta Olenick for measuring zooplankton lengths in exchange for programming (such x v i i i a d e a l ! ) . ESSA L t d . n o t o n l y p a i d f o r t y p i n g s e r v i c e s , but p a t i e n t l y a c c e p t e d my s c h i z o i d s t u d e n t / c o n s u l t a n t e x i s t e n c e . I n t e r a c t i o n s w i t h t h e s t u d e n t s and f a c u l t y o f t h e I n s t i t u t e o f A n i m a l R e s o u r c e E c o l o g y have g r e a t l y e n r i c h e d my u n d e r s t a n d i n g o f t h e s c i e n c e o f e c o l o g y , and t h e r e b y i n d i r e c t l y i m p r o v e d t h e q u a l i t y o f t h i s t h e s i s . D i r e c t improvements were due t o t h e c o n s t r u c t i v e a n d s t i m u l a t i n g comments o f Ken H a l l , P e t e r M o r r i s o n , B i l l N e i l l and C a r l W a l t e r s ( a l l o f U.B.C.) and Norm Yan ( o f t h e O n t a r i o M i n i s t r y o f E n v i r o n m e n t ) . I thank Norm a l s o f o r o u r open and e x c i t i n g e x c h a n g e s o f i d e a s , m a n u s c r i p t s , u n p u b l i s h e d d a t a and f r i s b e e s . To t h e o v e r one m i l l i o n z o o p l a n k t e r s I k i l l e d i n t h e s e e x p e r i m e n t s , I c a n o f f e r o n l y my hope t h a t t h r o u g h t h i s t h e s i s t h e i r d e a t h s may save many t r i l l i o n s o f t h e i r b r e t h r e n f r o m l a k e a c i d i f i c a t i o n . A t t i m e s , a t h e s i s i s a s much a p s y c h o l o g i c a l c h a l l e n g e as a s c i e n t i f i c one. T h e s e t i m e s were made much e a s i e r by t h e p a t i e n t u n d e r s t a n d i n g o f my f a m i l y (who l e a r n e d n e v e r t o a s k "When?"), t h e w o n d e r f u l f r i e n d s h i p o f Ken L e r t z m a n ( f r o m T i n b e r g e n t o Telemann t o t e l e m a r k i n g ) , and t h e s t r o n g , l o v i n g s u p p o r t o f B e t t y B r o n s o n (my s h i n i n g l i g h t when I c a n b a r e l y s e e ) . R e s e a r c h f u n d s f o r t h i s work were p r o v i d e d by o p e r a t i n g g r a n t s f r o m 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 C o u n c i l o f Canada (NSERC) t o D r s . K . J . H a l l and W.E. N e i l l , and a N a t u r a l , A p p l i e d and H e a l t h S c i e n c e s g r a n t f r o m t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a . P e r s o n a l s u p p o r t came f r o m an NSERC s c h o l a r s h i p , a x i x U n i v e r s i t y of B r i t i s h Columbia Graduate F e l l o w s h i p , and a Teaching A s s i s t a n t s h i p . "Sometimes t h e l i g h t ' s a l l s h i n i n g on me, O t h e r t i m e s I c a n b a r e l y s e e . L a t e l y i t o c c u r s t o me, What a l o n g , s t r a n g e t r i p i t ' s b e e n . . . " G a r c i a e t a l . 1970. EXPERIMENTAL ACIDIFICATION AND FERTILIZATION OF ENCLOSED PLANKTON SYSTEMS (SUMMER 1979) 3 1. INTRODUCTION J_.J_ H i s t o r i c a l and T h e o r e t i c a l B a c k g r o u n d S i n c e t h e m i d d l e o f t h e 19th C e n t u r y , man has known t h a t a c i d p r e c i p i t a t i o n d e r i v e d f r o m a t m o s p h e r i c e m i s s i o n s c a n damage b o t h n a t u r a l s y s t e m s and man-made s t r u c t u r e s ( S m i t h 1852, c i t e d i n C o w l i n g 1980). However, t h e s p a t i a l e x t e n t and s e v e r i t y o f p o t e n t i a l e c o l o g i c a l i m p a c t s o f a c i d p r e c i p i t a t i o n were f i r s t r e c o g n i z e d o n l y i n 1 9 6 8 . In t h a t y e a r , t h e S w e d i s h s c i e n t i s t S v a n t e Oden p r e s e n t e d r e s u l t s c l e a r l y p o i n t i n g t o t h e l o n g r a n g e t r a n s p o r t o f e m i s s i o n s i n n o r t h e r n E u r o p e and t h e i n c r e a s i n g a c i d i t y o f b o t h p r e c i p i t a t i o n and a q u a t i c systems i n p a r t s o f S c a n d i n a v i a (Oden 1968, c i t e d i n C o w l i n g 1980). Oden h y p o t h e s i z e d t h a t s u c h c h e m i c a l c h a n g e s c o u l d l e a d t o w i d e s p r e a d l o s s e s o f f i s h e r i e s . S i n c e 1968, a l a r g e number o f m e t e o r o l o g i s t s , s o i l s c i e n t i s t s , c h e m i s t s , l i m n o l o g i s t s , b i o l o g i s t s , and f o r e s t e r s , p r i m a r i l y i n S c a n d i n a v i a a n d N o r t h A m e r i c a , have c o n d u c t e d r e s e a r c h t o i d e n t i f y and u n d e r s t a n d a c i d p r e c i p i t a t i o n phenomena. T h e i r i n t e n s i v e and e x t e n s i v e s c i e n t i f i c i n v e s t i g a t i o n s have been p a r a l l e l e d by w i d e s p r e a d and v o c a l p u b l i c c o n c e r n a b o u t t h e e f f e c t s o f a c i d p r e c i p i t a t i o n . My r e s e a r c h c o n c e r n s t h e i m p a c t s o f 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 . W i t h t h e p l e t h o r a o f c o m p r e h e n s i v e r e v i e w a r t i c l e s and books a v a i l a b l e on a q u a t i c e f f e c t s o f a c i d p r e c i p i t a t i o n , a d e t a i l e d l i t e r a t u r e r e v i e w i s n o t n e c e s s a r y h e r e ( s e e B r a e k k e 1976, Gorham 1976, Yan 1978, Aimer e t 4 a l . 1978, O v e r r e i n e t a l . 1980, NRCC 1981, H a i n e s 1981). However, i n o r d e r t o s e t a c l e a r c o n t e x t f o r my r e s e a r c h , I s h a l l p r e s e n t a b r i e f o v e r v i e w o f l a k e a c i d i f i c a t i o n . L ake a c i d i f i c a t i o n c a n be t r a c e d back t o s h o r t o r l o n g r a n g e t r a n s p o r t ( i . e . 0-2000 km) o f g a s e o u s e m i s s i o n s o f s u l f u r and n i t r o g e n o x i d e s ( L i k e n s e t a l . 1979). These e m i s s i o n s may be d e p o s i t e d t h r o u g h d i r e c t d i f f u s i o n o f g a s e s t o s o i l s a nd v e g e t a t i o n , o r may u n d e r g o c h e m i c a l t r a n s f o r m a t i o n s t o s u l f a t e and n i t r a t e compounds t h a t r e a c h t h e g r o u n d e i t h e r i n p a r t i c u l a t e o r l i q u i d p h a s e (Yan 1978). O x i d a t i o n o f s u l f u r and n i t r o g e n o x i d e s g e n e r a t e s f r e e h y d r o g e n i o n s w h i c h c a n p o t e n t i a l l y a c i d i f y " s e n s i t i v e " s o i l s , s t r e a m s o r l a k e s (NRCC 1981). The p r i m e i n d i c a t o r o f a l a k e ' s s e n s i t i v i t y t o a c i d i f i c a t i o n i s i t s a l k a l i n i t y . A l k a l i n i t y i s d e f i n e d as t h e t o t a l q u a n t i t y o f base ( u s u a l l y i n e q u i l i b r i u m w i t h b i c a r b o n a t e o r c a r b o n a t e ) t h a t c an be t i t r a t e d w i t h a s t r o n g a c i d ( H u t c h i n s o n 1957; pg. 6 6 7 ) . The t i m e s c a l e o f a c i d i f i c a t i o n o f l a k e s i s g e n e r a l l y n o t known p r e c i s e l y due t o t h e a b s e n c e o r u n r e l i a b i l i t y of h i s t o r i c a l d a t a . A l t h o u g h some p u b l i c a t i o n s document t e m p o r a l c h a n g e s t o t h e c h e m i s t r y and b i o l o g y ( m o s t l y f i s h e r i e s ) o f a c i d i f y i n g l a k e s and r i v e r s ( B e a m i s h and H a r v e y 1972, D i c k s o n 1975, Watt 1979, Farmer e t a l . 1980, O v e r r e i n e t a l . 1980), t h e v a s t m a j o r i t y o f r e p o r t s r e l y on s p a t i a l c o m p a r i s o n s o f w a t e r s o f d i f f e r e n t a c i d i t y . A w e a l t h o f s u c h s p a t i a l e v i d e n c e has now a c c u m u l a t e d i n N o r t h A m e r i c a a n d S c a n d i n a v i a . Of s p e c i a l 5 i n t e r e s t t o p l a n k t o n s y s t e m s i s t h e e v i d e n c e showing t h a t r e l a t i v e t o o l i g o t r o p h i c l a k e s o f pH g r e a t e r t h a n 6.0, l a k e s of pH l e s s t h a n 5.0 c o n s i s t e n t l y c o n t a i n : • fewer s p e c i e s and much l o w e r a b u n d a n c e s o f f i s h ( r e v i e w e d i n O v e r r e i n e t a l . 1980, NRCC 1981, H a i n e s 1981); • fewer numbers o f z o o p l a n k t o n s p e c i e s and d i f f e r e n t community d o m i n a n t s (Aimer e t a l . 1974, S p r u l e s 1975a and 1975b, H e n d r e y and W r i g h t 1976, R o f f and K w i a t k o w s k i 1977, Hobaek and Raddum 1980, N i l s s e n 1980a, K e l l e r 1981); • r e d u c e d t o t a l z o o p l a n k t o n b i o m a s s (Yan and S t r u s 1980); • d i f f e r e n t z o o b e n t h i c community s t r u c t u r e ( i n t erms o f b o t h t a x o n o m i c and f u n c t i o n a l g r o u p s ) and r e d u c e d s p e c i e s d i v e r s i t y ( R o f f and K w i a t k o w s k i 1977, M o s s b e r g and N y b e r g 1979, O k l a n d and O k l a n d 1980); • d i f f e r e n t t a x o n o m i c c o m p o s i t i o n and r e d u c e d s p e c i e s d i v e r s i t y o f t h e p h y t o p l a n k t o n community ( A i m e r e t a l . 1974, Hendrey and W r i g h t 1976, K w i a t k o w s k i a n d R o f f 1976, Yan and S t o k e s 1978, Yan 1979), but s i m i l a r a l g a l b i o m a s s (NRCC 1981, A i m e r e t a l . 1978); • i n c r e a s e d S e c c h i d e p t h t r a n s p a r e n c i e s (Aimer e t a l . 1978, NRCC 1981); and • i n c r e a s e d c o n c e n t r a t i o n s o f aluminum, manganese an d o t h e r m e t a l s (Aimer e t a l . 1978, D i c k s o n 1980). 6 C o m p a r i s o n s o f key v a r i a b l e s a c r o s s a g r a d i e n t of l a k e a c i d i t i e s a r e n o t a new i d e a ( s e e Skadowski 1926, M a n u j l o v a 1949). W h i l e s u c h s t u d i e s do p r o v i d e an u n d e r s t a n d i n g of t h e o v e r a l l p a t t e r n s a s s o c i a t e d w i t h l a k e a c i d i f i c a t i o n , t h e y do n o t r e v e a l t h e c a u s a l pathways by w h i c h a c i d i f i c a t i o n a l t e r s t h e c h e m i s t r y and b i o l o g y o f l a k e s . N i l s s e n (1980b) h a s p o i n t e d o u t t h a t many s u r v e y s o f a q u a t i c o r g a n i s m s i n a c i d l a k e s n e g l e c t t o c o n s i d e r t h e r o l e s o f c o m p e t i t i o n , p r e d a t i o n and o t h e r p r o c e s s e s i n s h a p i n g t h e d i r e c t i o n o f e c o s y s t e m change under a c i d i f i c a t i o n . Too o f t e n , i t i s i m p l i c i t l y assumed t h a t pH a n d o t h e r a b i o t i c p a r a m e t e r s a f f e c t e c o s y s t e m components i n d e p e n d e n t l y o f b i o t i c i n t e r a c t i o n s . R o f f and K w i a t k o w s k i (1977) s t r e s s e d t h e l i m i t a t i o n s o f t h e i r s y n o p t i c s u r v e y s f o r o b s e r v i n g b i o t i c i n t e r a c t i o n s . I n r e v i e w i n g t h e s i m i l a r i t i e s between s p e c i e s d i v e r s i t y v s . pH c u r v e s f o r p h y t o p l a n k t o n and z o o p l a n k t o n f r o m s i x O n t a r i o l a k e s , t h e a u t h o r s c o n c l u d e d t h a t i t was n o t p o s s i b l e t o c h o o s e between r e d u c e d f o o d a v a i l a b i l i t y and t h e d i r e c t a c t i o n o f pH a s p o s s i b l e c a u s e s f o r t h e r e d u c t i o n i n z o 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 . F o r t u n a t e l y , t h e r e i s some e x p e r i m e n t a l work a v a i l a b l e t o s u p p l e m e n t s y n o p t i c s u r v e y s and m o n i t o r i n g s t u d i e s . D e t a i l e d s t u d i e s o f t h e e f f e c t s o f n e u t r a l i z a t i o n a n d / o r f e r t i l i z a t i o n o f s e v e r a l O n t a r i o a c i d l a k e s ( D i l l o n e t a l . 1979, Yan and L a f r a n c e 1981, Yan and D i l l o n 1981) have r e v e a l e d t h e r e l a t i v e r a t e s o f r e c o v e r y o f d i f f e r e n t e c o s y s t e m components, and i n p a r t i c u l a r t h e r e s p o n s e s o f p l a n k t o n s y s t e m s t o l a r g e i n c r e a s e s i n b o t h pH and n u t r i e n t c o n t e n t . Whole l a k e and e n c l o s u r e e x p e r i m e n t s a t 7 t h e E x p e r i m e n t a l L a k e s A r e a ( S c h i n d l e r 1980, S c h i n d l e r e t a l . 1980a and 1980b, M u l l e r 1980, M a l l e y e t a l . 1982) c o n t i n u e t o y i e l d v a l u a b l e i n f o r m a t i o n c o n c e r n i n g t h e p h y s i c a l , c h e m i c a l and b i o l o g i c a l p r o c e s s e s a f f e c t e d by l a k e a c i d i f i c a t i o n . F i n a l l y , l a b o r a t o r y and f i e l d t o x i c i t y s t u d i e s ( Skadowski 1926, J a c e n k o 1928, I v a n o v a 1969, K r i n g and O ' B r i e n 1976, Havas 1980, M a l l e y 1980, W a l t o n e t a l . 1982) a r e e s s e n t i a l t o s e p a r a t e d i r e c t t o x i c i t y f r o m i n d i r e c t b i o t i c e f f e c t s . I n s p i t e o f t h e above r e s e a r c h , many c r i t i c a l t h e o r e t i c a l and a p p l i e d q u e s t i o n s r e m a i n u n r e s o l v e d . From a t h e o r e t i c a l p e r s p e c t i v e , we c a n c o n s i d e r a c i d i f i c a t i o n as a r e l a t i v e l y r a p i d s e l e c t i o n o f a new and s m a l l e r s e t o f s p e c i e s , and t h e n a s k : Does t h e s i m p l i f i e d , a c i d i f i e d s y s t e m s t i l l r e t a i n v a r i o u s o r g a n i z a t i o n a l p r o p e r t i e s o f more d i v e r s e , c i r c u m n e u t r a l s y s t e m s ? Which p r o p e r t i e s a r e d ominant i n d e t e r m i n i n g t h e d i r e c t i o n o f c h ange 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 c o m m u n i t i e s d u r i n g l a k e a c i d i f i c a t i o n ? In t h e f o l l o w i n g p a r a g r a p h s , I l i s t s e v e r a l key p r o p e r t i e s o f p l a n k t o n s y s t e m s and d i s c u s s t h e i r r e l e v a n c e t o l a k e a c i d i f i c a t i o n . N e i t h e r t h e p r o p e r t i e s l i s t e d , n o r t h e l i t e r a t u r e c i t e d a r e i n t e n d e d as c o m p r e h e n s i v e summaries o f c u r r e n t knowledge o f p l a n k t o n s y s t e m s . 1) I n t e r a c t i o n s between p h y s i c a l / c h e m i c a l c o n d i t i o n s and  p l a n k t o n i c o r g a n i s m s . S e a s o n a l c h a n g e s i n s o l a r r a d i a t i o n , l a k e t e m p e r a t u r e s a n d p r e c i p i t a t i o n a l t e r b o t h t h e r a t e o f g r o w t h 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 , and t h e s p a t i a l d i s t r i b u t i o n o f o r g a n i s m s and compounds w i t h i n t h e l a k e . In a c i d l a k e s , t h e 8 d i s t r i b u t i o n o f h y d r o g e n i o n s may be s t r o n g l y a f f e c t e d by t h e t h e r m a l r e g i m e . L a t e w i n t e r i n v e r s e t e m p e r a t u r e s t r a t i f i c a t i o n h as been o b s e r v e d t o p r e v e n t m i x i n g o f a c i d m e l t w a t e r s , c r e a t i n g a c i d i c s u f a c e l a y e r s i n a c i d l a k e s ( H e n r i k s e n a n d W r i g h t 1977; H u l t b e r g 1977; J e f f r i e s e t a l . 1979). Summer s t r a t i f i c a t i o n may r e s t r i c t a c i d l o a d i n g s t o t h e e p i l i m n i o n ( S c h i n d l e r e t a l . 1980a). D u r i n g p e r i o d s w i t h s t r o n g pH g r a d i e n t s , do s u r f a c e - o r i e n t e d z o o p l a n k t o n s p e c i e s s u f f e r s i g n i f i c a n t l y h i g h e r m o r t a l i t y t h a n d e e p - d w e l l i n g s p e c i e s ? Do v e r t i c a l d i s t r i b u t i o n s s h i f t d u r i n g s u c h p e r i o d s ? The a c t i v i t i e s o f p h y t o p l a n k t o n and b a c t e r i a c a n i n t h e m s e l v e s a f f e c t l a k e a l k a l i n i t y a nd pH (NRCC 1981). Can t h e s e a c t i v i t i e s change t h e pH s u f f i c i e n t l y t o a l t e r t h e t o x i c i t y o f l a k e w a t e r t o z o o p l a n k t o n ? A c i d i f i c a t i o n may a f f e c t w a t e r t r a n s p a r e n c y by c h e m i c a l means, s u c h a s c o p r e c i p i t a t i o n o f aluminum a n d humic m a t e r i a l s (Aimer e t a l . 1978), o r c o l o u r c h a n g e s i n d i s s o l v e d m a t e r i a l ( S c h i n d l e r 1980). However, i t i s w e l l known t h a t t h e t r a n s p a r e n c y o f o l i g o t r o p h i c l a k e s i s a l s o s t r o n g l y a f f e c t e d by c h a n g e s i n a l g a l b i o m a s s o r c h l o r o p h y l l a c o n c e n t r a t i o n s ( C a r l s o n 1977, D i l l o n a nd R i g l e r 1975). As l a k e s a c i d i f y , what a r e t h e r e l a t i v e i m p o r t a n c e s o f t h e s e a b i o t i c and b i o t i c f a c t o r s ? 2) The a b i l i t y o f t h e h e r b i v o r o u s z o o p l a n k t o n community t o  e f f i c i e n t l y u t i l i z e t h e a v a i l a b l e p a r t i c l e s p e c t r u m ( s e e G l i w i c z and H i l b r i c h t - I l k o w s k a 1972, L y n c h 1977, G l i w i c z 1977, N i l s s e n 1 978). C u r r e n t s t u d i e s i n d i c a t e t h a t z o o p l a n k t o n c o m m u n i t i e s i n 9 O n t a r i o a c i d l a k e s c o n t a i n s m a l l e r - s i z e d community d o m i n a n t s and l o w e r t o t a l z o o p l a n k t o n b i o m a s s t h a n c i r c u m n e u t r a l l a k e s ( S p r u l e s 1975a, Yan and S t r u s 1980, S p r u l e s 1981). Do t h e s e c h a n g e s r e d u c e t h e a b i l i t y o f z o o p l a n k t o n t o e f f i c i e n t l y g r a z e t h e e n t i r e s i z e s p e c t r u m o f a v a i l a b l e a l g a e , a s s u g g e s t e d by Yan and S t r u s ( 1 980)? 3) The a b i l i t y o f p h y t o p l a n k t o n a s s e m b l a g e s t o r e s i s t  g r a z i n g by h e r b i v o r o u s z o o p l a n k t o n ( s e e P o r t e r 1977, M c C a u l e y and B r i a n d 1977). P h y t o p l a n k t o n c a n e s c a p e h e r b i v o r e g r a z i n g by b l o o m i n g p r i o r t o t h e emergence o f z o o p l a n k t o n f r o m o v e r w i n t e r i n g s t a g e s , o r by m o r p h o l o g i c a l a d a p t a t i o n s s u c h a s l a r g e c e l l s , l o n g f i l a m e n t s , d u r a b l e c e l l w a l l s o r g e l a t i n o u s s h e a t h s w h i c h i n h i b i t i n g e s t i o n or d i g e s t i o n by z o o p l a n k t o n ( P o r t e r 1977). Do t h e p h y t o p l a n k t o n a s s e m b l a g e s o f a c i d l a k e s s t i l l m a i n t a i n t h e t e m p o r a l , m o r p h o l o g i c a l , and c h e m i c a l f e a t u r e s t h a t p r e v e n t o v e r g r a z i n g by z o o p l a n k t o n ? 4) The l e v e l o f n u t r i e n t s and p r i m a r y p r o d u c t i o n ( s e e G l i w i c z 1969a and 1969b, N i l s s e n 1978). Grahn (1974) s u g g e s t e d t h a t a c i d i f i c a t i o n may p r o d u c e a s e l f - a c c e l e r a t i n g " o l i g o t r o p h i c a t i o n " , whereby i n c r e a s i n g l a k e a c i d i t y s h i f t e d t h e decomposer community from b a c t e r i a t o f u n g i , c r e a t i n g n u t r i e n t s h o r t a g e s t h a t c o n s t r a i n e d p r o d u c t i o n a t a l l h i g h e r t r o p h i c l e v e l s . C o n s i s t e n t w i t h t h e o l i g o t r o p h i c a t i o n h y p o t h e s i s , K w i a t k o w s k i and R o f f (1976) f o u n d t h a t a c i d l a k e s c o n t a i n e d l o w e r r a t e s o f t o t a l w a t e r column p r i m a r y p r o d u c t i o n t h a n c i r c u m n e u t r a l l a k e s . However, more r e c e n t s t u d i e s have f o u n d no e v i d e n c e o f r e d u c e d p r i m a r y p r o d u c t i o n i n a c i d l a k e s (Aimer e t 10 a l . 1978, D i l l o n e t a l . 1979, S c h i n d l e r 1980). Does a c i d i f i c a t i o n c a u s e r e d u c t i o n s i n z o o p l a n k t o n p r o d u c t i o n t h r o u g h l o w e r p r i m a r y p r o d u c t i o n ? 5) The r e s p o n s e o f p l a n k t o n s y s t e m s t o e x p e r i m e n t a l  f e r t i l i z a t i o n . I n c r e a s e s i n z o o p l a n k t o n p r o d u c t i o n f o l l o w n u t r i e n t e n r i c h m e n t o f o l i g o t r o p h i c l a k e s , a t b o t h c i r c u m n e u t r a l ( S m i t h 1969, L e b r a s s e u r e t a l . 1972, N e i l l and P e a c o c k 1980) and a c i d i c pH l e v e l s (Yan a n d L a f r a n c e 1981). However, p l a n k t o n s y s t e m s i n l a k e s o f pH l e s s t h a n 4.5 may be l e s s a b l e t o a b s o r b c o n t i n u e d n u t r i e n t e n r i c h m e n t t h a n a r e c i r c u m n e u t r a l s y s t e m s , due t o i m b a l a n c e s i n p l a n t - h e r b i v o r e o r p r e d a t o r - p r e y i n t e r a c t i o n s (Yan and L a f r a n c e 1 9 81). Do t h e p l a n k t o n s y s t e m s i n l a k e s o f i n t e r m e d i a t e pH (5.0 t o 6.0) r e s p o n d t o i n c r e a s e d n u t r i e n t s i n t h e same manner as t h o s e o f c i r c u m n e u t r a l l a k e s , o r a r e symptoms o f a l t e r e d e c o s y s t e m f u n c t i o n i n g a l r e a d y a p p a r e n t ? 6) The r o l e o f 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 d e t e r m i n i n g  z o o p l a n k t o n community s t r u c t u r e ( s e e B r o o k s and Dodson 1965, N e i l l 1974 and 1975a, H a l l e t a l . 1976, H rbacek 1978, L y n c h 1978, K e r f o o t and Demott 1980, O l e n i c k 1982). Recent s t u d i e s have r e v e a l e d many c o m p l e x i t i e s i n c o m p e t i t i o n between h e r b i v o r o u s z o o p l a n k t o n , i n c l u d i n g d i f f e r e n t s e n s i t i v i t i e s o f age c l a s s e s t o r e s o u r c e c h a n g e s ( N e i l l 1975a) and t e m p o r a l f l u c t u a t i o n s i n b o t h r e s o u r c e s and c o m p e t i t i v e a b i l i t i e s ( L y n c h 1978). S i n c e a c i d s t r e s s c a n remove h e r b i v o r o u s community d o m i n a n t s , c o m p e t i t i v e r e l a t i o n s h i p s may be of g r e a t i m p o r t a n c e i n d e t e r m i n i n g s p e c i e s r e p l a c e m e n t s . (See N i l s s e n (1980a) r e g a r d i n g c o m p e t i t i o n i n N o r w e g i a n a c i d l a k e s . ) Does 11 a c i d i f i c a t i o n a f f e c t t h e outcome o f c o m p e t i t i o n between h e r b i v o r e s ? 7) The e f f e c t s o f v e r t e b r a t e and i n v e r t e b r a t e p r e d a t o r s on  z o o p l a n k t o n community s t r u c t u r e ( s e e H r b a c e k 1962, B r o o k s and Dodson 1965, Dodson 1970, L y n c h 1977, N o r t h c o t e e t a l . 1978, L y n c h 1979, N e i l l and P e a c o c k 1980, Z a r e t 1980, N e i l l 1 9 81a). Over t h e l a s t 20 y e a r s many s t u d i e s have d e m o n s t r a t e d t h a t p r e d a t o r s c a n e f f e c t t h e s t r u c t u r e o f z o o p l a n k t o n c o m m u n i t i e s i n c i r c u m n e u t r a l l a k e s . Does t h e i m p o r t a n c e and f o r m o f p r e d a t i o n change w i t h a c i d i f i c a t i o n ? E r i c k s s o n e t a l . (1980) an d H e n r i k s o n e t a l . (1980) u s e d r e s u l t s f r o m c i r c u m n e u t r a l l a k e s t o s p e c u l a t e t h a t l o s s o f f i s h p o p u l a t i o n s f r o m S w e d i s h a c i d l a k e s may i n c r e a s e t h e abundance o f i n v e r t e b r a t e p r e d a t o r s ( p a r t i c u l a r l y C h a o b o r u s l a r v a e ) , c a u s i n g a s h i f t t o l a r g e r s i z e d h e r b i v o r e s . E r i c k s o n and c o - w o r k e r s f u r t h e r s u g g e s t t h a t t h e p r e d a t o r - i n d u c e d s i z e s h i f t c o u l d d e c r e a s e n u t r i e n t c y c l i n g , c o n t r i b u t i n g t o t h e o l i g o t r o p h i c a t i o n e f f e c t s h y p o t h e s i z e d by G r a h n ( 1 9 7 4 ) . N i l s s e n (1980a) p r o p o s e d t h a t t h e i n v e r t e b r a t e p r e d a t o r H e t e r o c o p e s a l i e n s p l a y e d a s i z e - s t r u c t u r i n g r o l e 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 o f N o r w e g i a n a c i d l a k e s . To d a t e , o n l y Yan and L a f r a n e e (1981) have 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 t h a t i n v e r t e b r a t e p r e d a t i o n c a n have an i m p o r t a n t i m p a c t on c r u s t a c e a n z o o p l a n k t o n i n a c i d l a k e s . However, t h e i r t h r e e y e a r s t u d y o c c u r r e d i n an e x p e r i m e n t a l l y f e r t i l i z e d a c i d l a k e . A r e i n v e r t e b r a t e p r e d a t o r s a s i m p o r t a n t i n u n e n r i c h e d a c i d l a k e s ? 12 In a d d i t i o n t o a d d r e s s i n g t h e t h e o r e t i c a l q u e s t i o n s p o s e d above, r e s e a r c h on a c i d - s t r e s s e d p l a n k t o n s y s t e m s has p r a c t i c a l b e n e f i t s f o r managers r e s p o n s i b l e f o r m o n i t o r i n g , p r o t e c t i n g or r e s t o r i n g l a k e s t h a t a r e s e n s i t i v e t o a c i d p r e c i p i t a t i o n , z o o p l a n k t o n community s t r u c t u r e i n t e g r a t e s " unseen" e p i s o d i c e v e n t s , p r o v i d i n g a v a l u a b l e i n d i c a t o r o f b i o t i c change i n l a k e s w i t h i n t h e pH r a n g e 4.7-5.3. Such " t r a n s i t i o n l a k e s " ( H e n r i k s e n 1980) a r e c h a r a c t e r i z e d by p e r i o d s o f s e v e r e pH f l u c t u a t i o n s w i t h d e l e t e r i o u s b i o t i c e f f e c t s . In t r a n s i t i o n l a k e s , t h e s p r i n g pH commonly d e c r e a s e s 1 t o 1.5 pH u n i t s w i t h i n a one t o two week p e r i o d and t h e n r e t u r n s t o above pH 6.0 ( J e f f r i e s e t a l . 1979). C h e m i c a l m o n i t o r i n g p r o g r a m s c a n e a s i l y m i s s s u c h c r i t i c a l , e p i s o d i c , a b i o t i c e v e n t s . A knowledge o f p l a n k t o n community c h a n g e s i n d u c e d by a c i d i f i c a t i o n c a n u s e f u l l y s u p p l e m e n t c h e m i c a l i n d i c a t o r s o f l a k e s t a t u s . In many S c a n d i n a v i a n a n d a few O n t a r i o l a k e s o f pH > 5.0, n e u t r a l i z i n g a g e n t s s u c h a s c a l c i u m c a r b o n a t e and c a l c i u m h y d r o x i d e have been u s e d t o c o u n t e r a c t a l k a l i n i t y l o s s e s . S u c c e s s f u l i m p l e m e n t a t i o n of s u c h a c t i o n s n o t o n l y r e q u i r e s knowledge o f t h e c a p a c i t y o f w a t e r and s e d i m e n t s t o c h e m i c a l l y n e u t r a l i z e added b a s e , but a l s o demands an u n d e r s t a n d i n g o f t h e s e n s i t i v i t y o f p l a n k t o n c o m m u n i t i e s t o sudden pH f l u c t u a t i o n s , and t h e amount of b i o t i c a l k a l i n i t y g e n e r a t i o n by a l g a e and b a c t e r i a . I f a c i d i f i c a t i o n i n d u c e s f o o d s h o r t a g e s f o r z o o p l a n k t o n i n l a k e s of pH 5.0-6.0, t h e n m o d e r a t e n u t r i e n t e n r i c h m e n t m i g h t a l l e v i a t e s u c h c o n d i t i o n s . V e r y l i t t l e i s known a b o u t e i t h e r 13 t h e a v a i l a b i l i t y o f f o o d f o r z o o p l a n k t o n o r t h e c h e m i c a l and b i o l o g i c a l e f f e c t s o f f e r t i l i z a t i o n , i n t h i s i n t e r m e d i a t e pH r a n g e . In some a c i d l a k e s of pH < 4.5, s c i e n t i s t s have combined l a k e n e u t r a l i z a t i o n and f e r t i l i z a t i o n t o r e s t o r e n o n t o x i c pH l e v e l s a n d i n c r e a s e z o o b e n t h i c and z o o p l a n k t o n p r o d u c t i o n p r i o r t o t h e r e i n t r o d u c t i o n o f f i s h ( D i l l o n e t a l . 1979). O c c a s i o n a l l y , t h e s e m a n i p u l a t i o n s have u n i n t e n t i o n a l l y c a u s e d n e a r t o t a l e l i m i n a t i o n of t h e z o o p l a n k t o n community ( D i l l o n e t a l . 1979, Yan and L a f r a n c e 1981). What f a c t o r s i n f l u e n c e t h e a b i l i t y o f p l a n k t o n s y s t e m s t o a b s o r b n e u t r a l i z a t i o n a n d e n r i c h m e n t p e r t u r b a t i o n s ? j_.2 R e s e a r c h O b j e c t i v e s My o v e r a l l g o a l s i n t h i s r e s e a r c h were t o improve upon e x i s t i n g methods f o r s t u d y i n g t h e e f f e c t s o f l a k e a c i d i f i c a t i o n on p l a n k t o n s y s t e m s , t o a d d r e s s some o f t h e f o r e g o i n g t h e o r e t i c a l q u e s t i o n s , and t o p r o v i d e u s e f u l i n f o r m a t i o n f o r l a k e management. To a c h i e v e t h e s e g o a l s , I f o r m u l a t e d the f o l l o w i n g s e v e n r e s e a r c h o b j e c t i v e s : 1. t o e v a l u a t e t h e use o f l i m n o c o r r a l s ( l a r g e p l a s t i c e n c l o s u r e s f l o a t i n g i n a l a k e ) i n s i m u l a t i n g t h e e f f e c t s o f l a k e a c i d i f i c a t i o n on p l a n k t o n s y s t e m s ; 2. t o examine t h e a b i o t i c and b i o t i c c o n t r o l s on a l k a l i n i t y , pH and t r a n s p a r e n c y under e x p e r i m e n t a l a c i d i f i c a t i o n and f e r t i l i z a t i o n . 3. t o d e t e r m i n e t h e e f f e c t s o f e x p e r i m e n t a l 14 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 p o p u l a t i o n s , community s t r u c t u r e a n d b i o m a s s , i n t h e a b s e n c e o f p r e d a t i o n ; 4. t o d e t e r m i n e whether e x p e r i m e n t a l a c i d i f i c a t i o n c a n c a u s e s t a r v a t i o n i n c r u s t a c e a n z o o p l a n k t o n by a l t e r i n g t h e a v a i l a b i l i t y o f e d i b l e p h y t o p l a n k t o n ; 5. i f (4) i s t r u e , t o a s s e s s whether n u t r i e n t e n r i c h m e n t d u r i n g e x p e r i m e n t a l a c i d i f i c a t i o n c a n a l l e v i a t e s u c h s t a r v a t i o n e f f e c t s ; 6. t o compare t h e r e s p o n s e s of a c i d - s t r e s s e d and c i r c u m n e u t r a l p l a n k t o n s y s t e m s t o e x p e r i m e n t a l e n r i c h m e n t ; and 7. t o a s s e s s t h e r e l a t i v e i m p o r t a n c e s o f a c i d t o x i c i t y , c o m p e t i t i o n and f o o d s u p p l y i n c h a n g 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 e x p e r i m e n t a l l y a c i d i f i e d z o o p l a n k t o n c o m m u n i t i e s . S e c t i o n I o f t h i s t h e s i s d e s c r i b e s a f i e l d e x p e r i m e n t I c o n d u c t e d f r o m m i d - J u l y t o m i d - O c t o b e r i n 1979, t a r g e t e d t o t h e above r e s e a r c h o b j e c t i v e s . Q u e s t i o n s s t i m u l a t e d by t h e r e s u l t s o f t h e 1979 e x p e r i m e n t were a d d r e s s e d i n a s e c o n d f i e l d e x p e r i m e n t t h a t r a n from mid-May t o m i d - J u l y , 1980 ( S e c t i o n I I ) . 15 2. MATERIALS AND METHODS 2.J_ S t u d y S i t e E n c l o s u r e e x p e r i m e n t s were p e r f o r m e d i n 1979 i n E u n i c e L a k e , a montane l a k e i n t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a (U.B.C.) R e s e a r c h F o r e s t , l o c a t e d e a s t o f V a n c o u v e r n e a r Haney, B r i t i s h C o l u m b i a ( F i g u r e 1 ) . T a b l e 1 summarizes some o f E u n i c e L a k e ' s p h y s i c a l and c h e m i c a l p r o p e r t i e s , and i t s z o o p l a n k t o n community. The g e o l o g y and s o i l s o f t h e s u r r o u n d i n g a r e a a r e d e s c r i b e d i n E f f o r d (1967) and F e l l e r (1975 and 1977).. B r i e f l y , t h e a r e a i s u n d e r l a i n by m a s s i v e , i g n e o u s b e d r o c k o f h o r n e b l e n d e d i o r i t e , w i t h g e n e r a l l y deep, p e r m e a b l e p o d z o l i c t i l l s . E u n i c e L a k e i s an a p p r o p r i a t e l o c a t i o n f o r a c i d i f i c a t i o n e x p e r i m e n t s . I t s a l k a l i n i t y , t o t a l d i s s o l v e d s o l i d s and t o t a l p h o s p h o r u s l e v e l s a r e a l l low ( T a b l e 1 ) , and t y p i c a l o f l a k e s i n b o t h t h e C o a s t a l Range o f B r i t i s h C o l u m b i a and a r e a s o f E a s t e r n Canada t h a t a r e c o n s i d e r e d s e n s i t i v e t o a c i d p r e c i p i t a t i o n (NRCC 1981, D i l l o n e t a l . 1978). E x p o r t s o f a n i o n s and c a t i o n s i n a c a l i b r a t e d w a t e r s h e d w i t h i n t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a R e s e a r c h F o r e s t ( F e l l e r 1975) a r e s t r i k i n g l y s i m i l a r t o c a l i b r a t e d w a t e r s h e d s i n t h e Muskoka H a l i b u r t o n R e g i o n o f O n t a r i o (NRCC 1981). F e l l e r (1975, and u n p u b l i s h e d d a t a ) h a s m o n i t o r e d t h e c h e m i s t r y and volume o f p r e c i p i t a t i o n f a l l i n g i n t h e U.B.C. R e s e a r c h F o r e s t on a w e e k l y b a s i s d u r i n g t h e l a s t d e c a d e . The a n n u a l a v e r a g e a t m o s p h e r i c d e p o s i t i o n o f H + i s 45 ± 6.6 m e q « m ~ 2 « y r ~ 1 (SE; n = 8 ) , and t h e mean pH o f p r e c i p i t a t i o n e v e n t s i s 4.7, compared t o 67 meq»irr 2 » y r ~ 1 and pH 4.2 i n t h e 16 F i g u r e 1. L o c a t i o n of study s i t e . From N e i l l (1978) 17 18 Table 1. Physical, chemical, and biological characteristics of Eunice Lake. Crustacean densities are maximum summer densities in limnetic zone, averaged over the entire water column. Adapted from Northcote and Clarotto (1975), Olenick (1982), and Neill (unpublished data). Elevation (m) 480 Drainage area (ha) 191 Surface area (ha) 18.2 Maximum depth (m) 42 Mean depth (m) _x 15.8 Total dissolved solids (mg*l~ ) 16 Mean pH x 6.4 Mean alkalinity (ueq*l~ )* 90 Total phosphorus (yg«l~ ) 5 Colour (Pt units) 10 Transparency (secchi disc; m) 6-10 Epilimnion depth, late summer (m) 4-6 Hypolimnion O 2 minimum (mg/1) 0.6 Crustacean density (#/100 1) Daphnia rosea 233 Holopedium gibberun 124 Diaphanosoma brachyurum 253 Polyphemus pediculus 13 Ceriodaphm'a pulchella rare Bosmina 1ongi rostris 230 Diaptomus kenai 70 Diaptomus leptopus 0.5 Diaptomus-tyrrelli 800 Cyclopoid copepods 27 Years zooplankton sampled 1967, 1969, 1971, 1972. * Total fixed end point (pH 4.5) alkalinity. Inflection point alkalinity is 70 yeq-1"1. 19 M u s k o k a - H a l i b u r t o n R e g i o n o f O n t a r i o (NRCC 1981). S p r i n g pH d e p r e s s i o n s a r e commonly o b s e r v e d i n t h e M u s k o k a - H a l i b u r t o n A r e a o f O n t a r i o ( J e f f r i e s e t a l . 1979) i n l a k e s t h a t a r e c h e m i c a l l y q u i t e s i m i l a r t o E u n i c e L a k e . Measurements i n E u n i c e L a k e a r e i n s u f f i c i e n t t o a s c e r t a i n whether u n u s u a l a l k a l i n i t y d e p r e s s i o n s o c c u r s e a s o n a l l y , o r whether a c i d d e p o s i t i o n i n t h e l a s t d ecade h a s been s u f f i c i e n t t o g r a d u a l l y e r o d e l a k e a l k a l i n i t i e s . The f u n c t i o n a l r e l a t i o n s h i p between a l k a l i n i t y and pH i s c a l l e d a t i t r a t i o n c u r v e . T h i s n o n l i n e a r c u r v e s t e e p e n s c o n s i d e r a b l y i n t h e pH r a n g e o f 5.0 t o 6.0. E u n i c e L a k e ' s mean pH has n o t c h a n g e d s i n c e 1970 ( N o r t h c o t e and C l a r o t t o 1975), w h i c h i s e x p e c t e d s i n c e c u r r e n t pH and a l k a l i n i t y l e v e l s a r e above t h e s t e e p p a r t o f t h e l a k e w a t e r ' s t i t r a t i o n c u r v e . The E u n i c e L a k e z o o p l a n k t o n community c o n t a i n s s e v e r a l s p e c i e s commonly f o u n d i n W e s t e r n N o r t h A m e r i c a ( C a r l 1940; Edmundson 1959; A n d e r s o n 1974), n o t a b l y w i t h i n h i g h e l e v a t i o n , low a l k a l i n i t y w a t e r b o d i e s t h a t a r e r e l a t i v e l y s e n s i t i v e t o l a k e a c i d i f i c a t i o n . S i n c e some o f t h e s e l a k e s a r e e x p e r i e n c i n g a l k a l i n i t y d e c l i n e s a t t r i b u t a b l e t o a c i d p r e c i p i t a t i o n ( L e w i s 1982), an u n d e r s t a n d i n g o f a c i d i f i c a t i o n e f f e c t s on E u n i c e L a k e z o o p l a n k t o n i s o f immediate and w i d e s p r e a d r e l e v a n c e . 20 2.2 E x p e r i m e n t a l Methods In l a t e J u n e 1979, e i g h t l i m n o c o r r a l s were p l a c e d i n E u n i c e L a k e . T h e s e e n c l o s u r e s were u s e d f o r e x p e r i m e n t a l a c i d i f i c a t i o n a nd f e r t i l i z a t i o n . T h i s s e c t i o n d e s c r i b e s a l l e x p e r i m e n t a l and a n a l y t i c a l methods u s e d ; t h e s p e c i f i c h y p o t h e s e s t e s t e d a r e d e s c r i b e d i n S e c t i o n 2.3. L i m n o c o r r a l d e s i g n a n d s e t u p was v e r y s i m i l a r t o t h a t d e s c r i b e d by N e i l l ( 1 9 7 8 ) . The e n c l o s u r e s were c o n s t r u c t e d o f t r a n s l u c e n t p o l y o l e f i n f a b r i c s u s p e n d e d f r o m wood a n d p o l y s t y r e n e f l o a t s ( F i g u r e 2 ) , and m e a s u r e d 2 m i n d i a m e t e r by 9 m deep. The t o p 66 cm o f t h e c y l i n d e r s were c o a t e d w i t h a s e c o n d l a y e r o f p o l y o l e f i n , c o l o u r e d o r a n g e t o p r e v e n t u l t r a v i o l e t d e g r a d a t i o n o f t h e m a t e r i a l ; no d e g r a d a t i o n was a p p a r e n t two y e a r s l a t e r . The b o t t o m o f t h e e n c l o s u r e s were s e a l e d and n o t i n c o n t a c t w i t h l a k e s e d i m e n t s , t h e r e b y e l i m i n a t i n g a p o t e n t i a l s o u r c e o f a c i d n e u t r a l i z i n g b a s e s , p o t e n t i a l l y t o x i c m e t a l s , z o o p l a n k t o n e m e r g i n g f r o m o v e r w i n t e r i n g e ggs, and p h y t o p l a n k t o n d e v e l o p i n g from r e s t i n g s t a g e s . T h i s a s p e c t o f t h e d e s i g n i n c r e a s e d e x p e r i m e n t a l c o n t r o l and p h y s i c a l s t a b i l i t y of t h e e n c l o s u r e s , a t t h e e x p e n s e o f some l o s s o f c h e m i c a l a n d b i o l o g i c a l r e a l i s m . A f t e r b e i n g a n c h o r e d n e a r t h e w e s t e r n s h o r e o f E u n i c e L a k e ( F i g u r e 3 ) , t h e 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 w a t e r f r o m s e v e r a l d e p t h s , f i l t e r e d t h r o u g h a 54 nm n e t t o remove c r u s t a c e a n z o o p l a n k t o n , n a u p l i i and most r o t i f e r s b u t p e r m i t g r a z a b l e s e s t o n t o p a s s . Z o o p l a n k t o n were c o l l e c t e d w i t h t o w n e t s from s e v e r a l l a k e d e p t h s and p l a c e d i n c o o l s t o r a g e 21 F i g u r e 2. P l a n view of l i m n o c o r r a l d e s i g n 22 50x100 mm Fir <d 1.57 m Exterior of cylinder at 0.5 m below surface. (o.38mm woven polyolefin) 1.32 m Interior of cylinder nailed to float at surface. 10 cm Styrofoam 3 mm steel wire capped on outside with tygon tubing (looped through middle of foam) wm ^2L 12 mm plywood 23 F i g u r e 3. L o c a t i o n of 1 i m n o c o r r a l s i n E u n i c e Lake. Depth c o n t o u r s from Hume (1978). 25 c o n t a i n e r s a l o n g t h e s h o r e . L a k e d e n s i t i e s and 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 e s t i m a t e d from a p o o l e d sample o f 10 r e p l i c a t e h a u l s of a 54 Mm W i s c o n s i n n e t t h r o u g h t h e upper 10 m o f w a t e r n e a r t h e l a k e ' s d e e p e s t p o i n t . A f t e r making a d d i t i o n s t o t h e s t o r a g e c o n t a i n e r s t o remove b i a s e s i n s p e c i e s c o m p o s i t i o n , s u f f i c i e n t v o l u m e s o f " p l a n k t o n s o u p " were added f r o m t h e s t o r a g e c o n t a i n e r s t o y i e l d l a k e d e n s i t i e s o f 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 (21 a n i m a l s » L ~ 1 ) . I n e s t i m a t i n g l a k e d e n s i t i e s , I assumed t h e W i s c o n s i n n e t t o be 35% e f f i c i e n t (W.E. N e i l l p e r s . comm. ) . E u n i c e L a k e c u t t h r o a t t r o u t were e x c l u d e d f r o m t h e c y l i n d e r s . Two r e p l i c a t e s o f t h r e e e x p e r i m e n t a l t r e a t m e n t s were ran d o m l y a s s i g n e d t o s i x o f t h e e n c l o s u r e s ; t h e o t h e r 2 s e r v e d a s c o n t r o l s ( T a b l e 2 ) . E x p e r i m e n t a l a c i d i f i c a t i o n s w i t h r e a g e n t - g r a d e H 2 S O 0 were p e r f o r m e d on s i x o c c a s i o n s i n t r e a t m e n t s A ( a c i d i f i c a t i o n ) a n d AF ( a c i d i f i c a t i o n p l u s f e r t i l i z a t i o n ) , by a d d i n g a c i d t o t h e s u r f a c e and m i x i n g w i t h an o a r . In t o t a l , a p p r o x i m a t e l y 62 neq»L~ 1 o f H* were ad d e d t o t h e e p i l i m n i o n o f e a c h a c i d i f i e d c y l i n d e r . T r e a t m e n t s F and AF r e c e i v e d 10 /ug»L~ 1 o f P ( a s d i s s o l v e d r e a g e n t g r a d e H 3 P 0 4 ) and 150 Mg*L" 1 o f N (as d i s s o l v e d r e a g e n t g r a d e NH f lN0 3) on f o u r o c c a s i o n s , p r o v i d i n g an N:P m o l e c u l a r r a t i o o f a p p r o x i m a t e l y 33:1. ( T h e s e c o n c e n t r a t i o n s a r e b a s e d on a 4 m deep e p i l i m n i o n . ) I s a m p l e d w a t e r , p h y t o p l a n k t o n , c h l o r o p h y l l a nd z o o p l a n k t o n by means o f a 9 m l o n g , 2.5 cm d i a m e t e r h o s e , c o n n e c t e d t o a b a t t e r y o p e r a t e d b i l g e pump t h a t d e l i v e r e d 34 L « m i n _ 1 . B o t h Table 2. Experimental Treatments. Treatment Cylinder Numbers Total Quantities of Chemicals Added to Epilimnion (top 4 m) C: control 2C 6C no chemical additions F: fertilization IF 8F 40 ug/L P and 600 ug/L N* A: acidification 4A 5A 62 ueq/L of H+** AF: acidification and fertilization 3AF 7AF treatments A and F combined ** divided equally over four additions. divided unequally over six additions of sulfuric acid. 27 E u n i c e Lake and t h e e n c l o s u r e s were s a m p l e d w e e k l y , and a l l s a m p l e s were t a k e n p r i o r t o any s c h e d u l e d c h e m i c a l a d d i t i o n s . 2.2.J_ P h y s i c a l and C h e m i c a l measurements C h e m i c a l s a m p l e s (300 ml) were t a k e n w e e k l y , from d e p t h s o f 1, 3, 5, and 7 m . I p e r f o r m e d pH measurements and f i x e d end p o i n t a l k a l i n i t y t i t r a t i o n s ( t o pH 4.5 w i t h 0.01 N s u l f u r i c a c i d ) w i t h i n 4 t o 24 h o u r s o f s a m p l i n g , u s i n g a b u f f e r -c a l i b r a t e d F i s h e r Accumet pH meter o r F i s h e r A u t o m a t i c T i t r a t o r . The two i n s t r u m e n t s d i d n o t d i f f e r i n t h e i r measured a l k a l i n i t y , a nd y i e l d e d s t a n d a r d e r r o r s w i t h i n 1.5% o f t h e mean. A l l measurements were p e r f o r m e d on s t i r r e d 100 ml s u b s a m p l e s e q u i l i b r a t e d t o room t e m p e r a t u r e . In May 1980, Gran t i t r a t i o n s (Stumm and Morgan 1970) were p e r f o r m e d on 3 l a k e w a t e r s a m p l e s t o d e t e r m i n e t h e i n f l e c t i o n p o i n t o r b i c a r b o n a t e a l k a l i n i t y ( T a b l e 1 ) . To examine t r e a t m e n t x t i m e x d e p t h v a r i a t i o n s i n b o t h h y d r o g e n i o n c o n c e n t r a t i o n s and a l k a l i n i t i e s , t h r e e - w a y a n a l y s e s o f v a r i a n c e (ANOVA) were p e r f o r m e d on u n t r a n s f o r m e d d a t a u s i n g t h e p r o g r a m UBC GENLIN ( G r e i g a n d B e r r i n g 1977). Mean pH was d e t e r m i n e d f r o m mean [H+], S i n c e t h e s t a n d a r d e r r o r s o f pH means a r e n o t s y m m e t r i c , I computed a "mean s t a n d a r d e r r o r " t o s i m p l i f y p r e s e n t a t i o n o f r e s u l t s : [ p H ( h i g h ) - p H ( l o w ) ] pSE = (1) 2 where pSE = mean s t a n d a r d e r r o r o f pH; 28 p H ( h i g h ) = - l o g ([H+] - S E ) ; p H(low) = - l o g ([H+] + S E ) ; [H+] = mean h y d r o g e n i o n c o n c e n t r a t i o n ; and SE = s t a n d a r d e r r o r o f [H+], S e c c h i d i s c d e p t h s were measured w e e k l y , t o t h e n e a r e s t 0.25 m, and compared t o 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 u s i n g l i n e a r r e g r e s s i o n a n a l y s i s . Water t e m p e r a t u r e s were r e c o r d e d on e a c h s a m p l i n g d a t e by pumping water from 1 m d e p t h i n t e r v a l s o v e r a thermometer a c c u r a t e t o ± 0.5° C. D e p t h - t i m e i s o c l i n e s o f t e m p e r a t u r e , pH and a l k a l i n i t y were p r o d u c e d u s i n g t h e s u b r o u t i n e CNTOUR, d e v e l o p e d a t t h e U n i v e r s i t y o f A l b e r t a C omputing C e n t r e and a d a p t e d by Susan M a i r o f t h e UBC Computing C e n t r e and m y s e l f t o r u n a t UBC. 2.2.2 P h y t o p l a n k t o n (i) Methods U s e d F o r p h y t o p l a n k t o n a n a l y s i s , a 300 ml w a t e r sample was o b t a i n e d f r o m a m i x t u r e c o n t a i n i n g 1000 ml o f water pumped from e a c h o f f o u r d e p t h s ( 1 , 3, 5, and 7 m), and p r e s e r v e d i n L u g o l ' s s o l u t i o n . A 100 ml s u bsample was a l l o w e d t o s e t t l e i n a g r a d u a t e d c y l i n d e r f o r 24-36 h. The t o p 75 ml o f t h e s u p e r n a t a n t was t h e n d e c a n t e d and t h e r e m a i n i n g 25 ml l e f t t o s e t t l e i n a c o u n t i n g chamber f o r a f u r t h e r 24 h. U s i n g an i n v e r t e d , p h a s e - c o n t r a s t m i c r o s c o p e a t 400X m a g n i f i c a t i o n , two o r more m i c r o s c o p e f i e l d s were examined u n t i l a t l e a s t 200 c e l l s g r e a t e r t h a n 2 um had been c o u n t e d . A l l c e l l s were c l a s s i f i e d a c c o r d i n g t o t h e i r shape ( s e e A p p e n d i x A f o r c a t e g o r i e s ) , s i z e 29 (two measurements f o r e a c h shape) and t y p e ( s i n g l e , c o l o n y o r f i l a m e n t o u s ) . As w e l l a s b e i n g measured, c e l l s and c o l o n i e s were c l a s s i f i e d a c c o r d i n g t o t h e i r maximum d i m e n s i o n i n t o one o f se v e n s i z e c l a s s e s (< 2, 2-5, 5-9, 9-13, 13-18, 18-30 and > 30 um) w h i l e f i l a m e n t s were a s s i g n e d t o a s e p a r a t e c l a s s . Where p o s s i b l e , t a x o n o m i c i d e n t i f i c a t i o n t o genus was p e r f o r m e d f o r th e most common f o r m s , u s i n g P r e s c o t t ( 1 9 7 8 ) , S m i t h (1951) and S t e i n ( 1 9 7 5 ) . A FORTRAN computer p r o g r a m ( A p p e n d i x A) was u s e d t o compute t o t a l c e l l , c o l o n y , and f i l a m e n t v o l u m e s (and bi o m a s s ) by s i z e c l a s s , t a x o n and phylum. I n c o n v e r t i n g a l g a l volume t o b i o m a s s , I assumed c e l l s h ad a s p e c i f i c g r a v i t y o f 1.0. The volume o f D i n o b r y o n c o l o n i e s was computed a s t h e sum of s e p a r a t e l o r i c a v o l u m e s . The p r o p o r t i o n o f t o t a l a l g a l volume i d e n t i f i e d t a x o n o m i c a l l y v a r i e d w i t h t h e t r e a t m e n t . I n t h e l a k e , c o n t r o l and A c y l i n d e r s a m p l e s , t h e p e r c e n t o f p h y t o p l a n k t o n volume i d e n t i f i e d t a x o n o m i c a l l y a v e r a g e d 87% and was n e v e r l e s s t h a n 70%. However, i n e n c l o s u r e s 1F, 8F, and 3AF, an a v e r a g e o f o n l y 59.0% o f t h e p h y t o p l a n k t o n volume was i d e n t i f i e d . The o b s e r v e d p r o p o r t i o n o f p h y t o p l a n k t o n volume i n any phylum s h o u l d t h e r e f o r e be r e g a r d e d o n l y a s a r o u g h minimum e s t i m a t e o f t h e t r u e f r a c t i o n . F o r c h l o r o p h y l l a n a l y s e s , one l i t e r w a t e r samples ( c o n t a i n i n g 250 ml from e a c h o f 1, 3, 5 and 7 m d e p t h s ) were g e n t l y f i l t e r e d t h r o u g h g l a s s f i b e r f i l t e r s , u s i n g a f i e l d hand pump. The f i l t e r s were i m m e d i a t e l y f r o z e n and s t o r e d i n t h e d a r k t o be l a t e r a n a l y z e d on a T u r n e r M o d e l I I I F l u o r o m e t e r f o r 30 c h l o r o p h y l l a and p h a e o p h y t i n a a c c o r d i n g t o S t r i c k l a n d and P a r s o n s ( 1 9 6 8 ) . The f l u o r o m e t e r was c a l i b r a t e d u s i n g a s p e c t r o p h o t o m e t e r and a 12-day o l d ( e a r l y s t a t i o n a r y p h a s e ) c u l t u r e o f t h e d i n o f l a g e l l a t e P r o r o c e n t r u m minimum S c h i l l e r , o b t a i n e d f r o m N o r t h e a s t P a c i f i c C u l t u r e C o l l e c t i o n (NEPCC) and grown i n a r t i f i c i a l s e a w a t e r medium. S i n c e p h a e o p h y t i n a r e p r e s e n t s d e g r a d e d o r d i g e s t e d c h l o r o p h y l l a, I u s e d l i v e c h l o r o p h y l l a a s an i n d e x o f a v a i l a b l e f o o d f o r z o o p l a n k t o n . To d e t e c t g r o s s c h a n g e s i n t h e f o o d q u a l i t y and p h y s i o l o g i c a l s t a t e o f t h e p h y t o p l a n k t o n , I computed t h e r a t i o s o f l i v e c h l o r o p h y l l a c o n c e n t r a t i o n s t o t o t a l a l g a l b i o m a s s , and t h e r a t i o s o f l i v e c h l o r o p h y l l a t o p h a e o p h y t i n a . P e r i p h y t o n (on t h e s i d e s and b o t t o m o f t h e e n c l o s u r e s ) were n o t q u a n t i t a t i v e l y s a m p l e d . However, v i s u a l o b s e r v a t i o n c o n f i r m e d t h a t s u c h a l g a e were s p a r s e i n u n f e r t i l i z e d c y l i n d e r s , b ut s i g n i f i c a n t i n t h e f e r t i l i z e d o n e s . A l g a l s i z e c l a s s v o l u m e s , c h l o r o p h y l l a c o n c e n t r a t i o n s and p h a e o p h y t i n a c o n c e n t r a t i o n s were l o g - t r a n s f o r m e d and s u b j e c t e d t o two way a n a l y s e s o f v a r i a n c e ( t r e a t m e n t x t i m e ) . S i g n i f i c a n c e l e v e l s t h e r e f o r e r e f e r t o d i f f e r e n c e s i n g e o m e t r i c means. Mean v a l u e s q u o t e d a r e a r i t h m e t r i c means u n l e s s o t h e r w i s e s t a t e d . 31 ( i i ) P r e c i s i o n o f A l g a l C e l l Volume and C h l o r o p h y l l  D e t e r m i n a t i o n s I u s e d b o t h c h l o r o p h y l l a c o n c e n t r a t i o n s and t o t a l c e l l volume a s e s t i m a t o r s o f a l g a l b i o m a s s . C h l o r o p h y l l a c o n c e n t r a t i o n s r e f l e c t n o t o n l y t h e b i o m a s s o f a l g a e i n t h e w a t e r , b u t a l s o t h e c e l l c h l o r o p h y l l c o n t e n t , w h i c h i s known t o v a r y w i t h l i g h t l e v e l s , n u t r i e n t s u p p l y , a l g a l g r o w t h p h a s e , s p e c i e s c o m p o s i t i o n a n d pH ( D a l e y a n d Brown 1973, N i c h o l l s a n d D i l l o n 1978). A l t h o u g h t o t a l a l g a l c e l l v o l u m e s a r e t h e r e f o r e g e n e r a l l y c o n s i d e r e d more r e l i a b l e m e a sures o f a l g a l b i o m a s s t h a n c h l o r o p h y l l a c o n c e n t r a t i o n s , t h i s i s o n l y a s s u r e d i f a d e q u a t e numbers o f c e l l s a r e c o u n t e d under t h e m i c r o s c o p e t o r e d u c e s u b s a m p l i n g e r r o r t o a r e a s o n a b l e l e v e l . U n f o r t u n a t e l y , t h e c r i t e r i o n I u s e d t o d e c i d e on t h e number o f m i c r o s c o p i c f i e l d s t o be e x amined ( s u f f i c i e n t t o y i e l d more t h a n 200 c e l l s i n a l l s i z e c a t e g o r i e s ) r e s u l t e d i n v e r y low numbers o f c e l l s a nd low p r e c i s i o n i n most s i z e c a t e g o r i e s , as shown i n columns 3 and 4 o f T a b l e 3. C o m p a r i n g c o l u m n s 5 w i t h 6, and columns 7 and 8 w i t h 9 a n d 10, i l l u s t r a t e s t h a t t h e o b s e r v e d mean 95% c o n f i d e n c e l i m i t s on a l g a l v o l u m e s i n e a c h s i z e c l a s s a r e w e l l p r e d i c t e d ( f o r s i x o f e i g h t s i z e c l a s s e s ) s o l e l y by P o i s s o n f r e q u e n c y d i s t r i b u t i o n c o n f i d e n c e l i m i t s on t h e mean numbers o f c o u n t e d c e l l s . T h a t i s , t h e o b s e r v e d w i t h i n t r e a t m e n t v a r i a t i o n i n a l g a l v o l u m e s a p p e a r e d l a r g e l y due t o c o u n t i n g ( s u b s a m p l i n g ) e r r o r , r a t h e r t h a n r e a l d i f f e r e n c e s between r e p l i c a t e s . The o n l y e x c e p t i o n s t o t h i s p a t t e r n were t h e 2-5 urn and f i l a m e n t s i z e c l a s s e s , w h i c h showed g r e a t e r t h a n p r e d i c t e d w i t h i n Table 3. Variability in phytoplankton volume estimates due to low numbers of counted cells. Size Class of Algae (ym) (1) Mean Cell Volume ± 2 (SE) (ym3) (2) Total # Cells Measured in All Samples (3) Mean # "Cells" Counted/ Sample2 (4) (5) Predicted 95% C.I.* on # CelIs/Sample3 As # Cells As % of Mean # Cells/S. (6) Observed 95% C.I. on Within Treatment Mean Algal Volume1* (7) (8) Predicted 95% C.I. on Total Counted Vol. In % of (ym)3 5 Total (9) (10) Observed 95% C.I. on Total Counted Vol. In % of (ym)3 6 Total < 2 0.2681 0 15 ± 8.2 2-5 6.4± 1.3 21605 202 ±27.9 5-9 150± 56 776 7.2 ± 5.8 9-13 479± 210 277 2.6 ± 3.8 13-18 532± 192 2211 20.7 ± 9.4 18-30 2979±1226 234 2.2 ± 3.6 > 30 1315± 315 504 4.7 ± 5.0 Fil. 10± 6 3656 34.2 ±12 Total 29263 288.6 ± 55% + 57% ± 220 0.9% ± 263 ± 1.0% ± 14% + 53% ± 179 0.7% ± 942 ± 3.7% ± 80% + 94% ± 870 3.4% ± 892 ± 3.5% ±146% + 128% ± 1820 7.1% ± 1293 ± 5.0% ± 45% + 67% ± 5001 19.6% ± 5162 ±20.0% ±163% + 146% ±10724 42.0% ±12707 ±49.3% ±106% + 101% ± 6575 25.8% ± 3936 ±15.3% ± 35% + 124% ± 118 0.4% ± 565 ± 2.2% + 55% (25507) 100% (25760)7 100% 1 Volume assumed constant at 26.8 ym3/100 cells in < 2 ym category. Cells in < 2 ym category counted in batches of 100. Mean # cells/sample is therefore approximately 1500, but items counted = 15. 2 Based on 107 samples. Colonies are regarded as single "cells", except for Dinobryon sp. (See Methods.) 3 Based on Poisson distribution (Ricker 1937); calculated as 0.5*(upper limit-lower limit). •* Computed as mean of 1.96(SE)/X x 100, calculated for 47 cases (4 treatments x 12 dates - 1), where X = mean algal volume for date and treatment. 5 Computed from (C.I. on # cells/sample) * (mean cell volume) (i.e. ignoring cell size variation). 6 Mean C.I. computed from confidence intervals for each date and treatment's mean volumes. 7 Actual C.I. for total algal volume is less than sum of C.I.'s for each size class, since "errors" cancel. Observed C.I. was ± 17911 ym3or 70% of sum of C.I.'s. * Confidence interval 33 t r e a t m e n t v a r i a t i o n , s u g g e s t i n g r e a l d i f f e r e n c e s between r e p l i c a t e s i n t h e volumes o f t h e s e s i z e c l a s s e s . S u b s a m p l i n g e r r o r s were v o l u m e t r i c a l l y most s i g n i f i c a n t i n t h e 13-18, 18-30, and > 30 /urn s i z e c l a s s e s , s i n c e s m a l l e r r o r s i n e s t i m a t i n g t h e numbers o f l a r g e c e l l s o r c o l o n i e s c a u s e s c o n s i d e r a b l e e r r o r s i n t h e e s t i m a t e d t o t a l a l g a l v olume. A s i n g l e l a r g e c o l o n y o f C h r y s o c a p s a s p . f o r example, made up 50% o f t h e c o u n t e d p h y t o p l a n k t o n volume i n c y l i n d e r 3AF on J u l y 26. The o b s e r v e d mean 95% c o n f i d e n c e l i m i t s on w i t h i n - t r e a t m e n t t o t a l a l g a l volume ( e x p r e s s e d a s a p e r c e n t a g e o f t h e mean) were ± 55%. T h i s i s much l e s s t h a n t h e mean o f t h e c o n f i d e n c e i n t e r v a l s f o r e a c h s i z e c l a s s , p r e s u m a b l y s i n c e u n d e r e s t i m a t e s i n some s i z e c l a s s e s c a n c e l l e d o v e r e s t i m a t e s i n o t h e r s i z e c l a s s e s . The e q u i v a l e n t l y - c a l c u l a t e d w i t h i n - t r e a t m e n t 95% c o n f i d e n c e i n t e r v a l f o r c h l o r o p h y l l a p l u s p h a e o p h y t i n a was ± 23%. In v i e w o f t h e s u b s a m p l i n g e r r o r s on a l g a l volume e s t i m a t e s , a n d t h e h i g h e r p r e c i s i o n o f c h l o r o p h y l l a measurements, I r e l i e d p r i m a r i l y on t h e l a t t e r f o r e x a m i n i n g c h a n g e s o v e r t i m e . However, between t r e a t m e n t c o m p a r i s o n s o f a r i t h m e t i c o r g e o m e t r i c mean a l g a l c e l l v olumes a v e r a g e d o v e r 12 s a m p l i n g d a t e s s t i l l s e r v e d t o i n d i c a t e m a j o r t r e a t m e n t e f f e c t s , s i n c e t h e e s t i m a t e d g r a n d means had c o n f i d e n c e l i m i t s of a p p r o x i m a t e l y ± 16% (55 x . s q r t ( 1 / 1 2 ) ) . S i n c e t h e i m p r e c i s i o n o f my a l g a l b i o m a s s e s t i m a t e s a f f e c t e d t h e r a t i o s o f c h l o r o p h y l l t o a l g a l b i o m a s s , I d i s c u s s p r i m a r i l y t h e o v e r a l l mean r a t i o s , r a t h e r t h a n t h e time s e r i e s . 34 2.2.3 Z o o p l a n k t o n D e p t h - i n t e g r a t e d z o o p l a n k t o n s a m p l e s were t a k e n by pumping f o r 30 sec f r o m e i g h t d e p t h s : 0.5 m; 1.5 m; 2.5 m; 3.5 m; 4.5 m; 5.5 m; 6.5 m; and 7.5 m, w i t h t h e pump o u t f l o w p a s s i n g t h r o u g h a 54 urn mesh n e t . D e p t h - s t r a t i f i e d s a m p l e s ( f o u r 2 m s t r a t a ) were t a k e n i n s t e a d o f i n t e g r a t e d s a m p l e s on A u g u s t 1 and September 6, 1979. The hose and b i l g e pump method o f z o o p l a n k t o n s a m p l i n g has been p r e v i o u s l y shown t o p r o d u c e e x c e l l e n t r e p l i c a b i l i t y w i t h m i n i m a l s a m p l i n g b i a s ( P e a c o c k 1981, N e i l l 1 9 8 1 a ) . Samples were p r e s e r v e d i n a s u c r o s e - f o r m a l d e h y d e m i x t u r e (Haney and H a l l 1975), and en u m e r a t e d by s p e c i e s , s e x , s t a g e o f m a t u r i t y , r e p r o d u c t i v e c o n d i t i o n , and number o f e g g s , u s i n g Edmondson (1959) and Pennak (1978) f o r t a x o n o m i c i d e n t i f i c a t i o n . U s i n g t h e s u b s a m p l e r d e s c r i b e d i n N o r t h c o t e and C l a r o t t o ( 1 9 7 5 ) , s u c c e s s i v e s i x t h s o f t h e samp l e s were e n u m e r a t e d u n t i l 200 i n d i v i d u a l s o f e a c h s p e c i e s were e n c o u n t e r e d . L o o s e c o p e p o d and c l a d o c e r a n e g g s were a s s i g n e d p r o p o r t i o n a t e l y t o s p e c i e s b a s e d on t h e abundance o f a t t a c h e d e g g s . I t e s t e d t h e o v e r a l l p r e c i s i o n o f t h e z o o p l a n k t o n s a m p l i n g and c o u n t i n g method by s a m p l i n g c y l i n d e r 8F t w i c e on O c t o b e r 5, 1979 and p r o c e s s i n g t h e two samp l e s t h r o u g h two d i f f e r e n t f i e l d a s s i s t a n t s ( T a b l e 4 ) . C o e f f i c i e n t s o f v a r i a t i o n i n T a b l e 4 t h e r e f o r e r e f l e c t v a r i a b i l i t y due t o f o u r s o u r c e s : i n c o n s i s t e n c y i n f i e l d s a m p l i n g t e c h n i q u e ; z o o p l a n k t o n p a t c h i n e s s i n s p a c e and t i m e ; e r r o r s i n s u b s a m p l i n g ; and i d e n t i f i e r b i a s and e r r o r . Compared t o s i m i l a r s t u d i e s o f s a m p l i n g e r r o r (Wiebe e t a l . 1968, G i b s o n and G r i c e 1977), t h e c o e f f i c i e n t s o f v a r i a t i o n Table 4. Test of precision of zooplankton sampling method. Cylinder 8F was sampled twice on October 5, 1979 and the two samples counted by different individuals. Species No. Animals Sample A in Sample* Sample B X S i o o - s / ; D. kenai 24 18 21 4.2 20.2 D. tyrrelli 164 146 155 12.7 8.2 Daphnia rosea 213 218 216 3.5 1.6 Bosmina longirostris 3072 2754 2913 224.9 7.7 Diaphanosoma brachyunum 18 37 28 13.4 48.9 Chydorus sphaericus 1470 1221 1346 55.7 13.1 Acroperus harpae 27 25 26 1.1 5.4 Loose Cladoceran eggs 38 16 27 15.6 57.6 Loose Copepod eggs 137 153 145 11.3 7.8 * 134 liters ** Coefficient of variation. 36 i n T a b l e 4 a r e r e l a t i v e l y low. U s i n g 25X (and o c c a s i o n a l l y 50X) m a g n i f i c a t i o n , 20 a d u l t s and 20 j u v e n i l e s o f e a c h s p e c i e s were m e a s u r e d f r o m e a c h t r e a t m e n t on f o u r o c c a s i o n s : J u l y 26, A u g u s t 9, A u g u s t 23, and September 13. C l a d o c e r a n measurements e x t e n d e d f r o m t h e a n t e r i o r t i p o f t h e c a r a p a c e t o i t s p o s t e r i o r , e x c l u d i n g a l l a c c e s s o r y s t r u c t u r e s s u c h a s h e l m e t s o r c a u d a l s p i n e s . Copepods were measured from t h e a n t e r i o r t i p o f t h e c e p h a l o t h o r a x t o t h e p o s t e r i o r e n d o f t h e urosome, n o t i n c l u d i n g s e t a e . T h e s e l e n g t h s were c o n v e r t e d t o j u v e n i l e and a d u l t b i o m a s s e s t i m a t e s u s i n g l e n g t h - w e i g h t r e g r e s s i o n s f r o m P e a c o c k ( u n p u b l i s h e d d a t a ) , Dumont e t a l . 1975 and B o t t r e l l e t a l . 1976; s e e A p p e n d i x B. The o v e r a l l mean a n d v a r i a n c e o f e a c h s p e c i e s ' l e n g t h s were computed f o r s p e c i f i c d a t e s b a s e d on t h e p r o p o r t i o n s o f j u v e n i l e s and a d u l t s p r e s e n t . S e p a r a t e one-way ANOVAs and Tukey m u l t i p l e r a n g e t e s t s were p e r f o r m e d on u n t r a n s f o r m e d z o o p l a n k t o n t o t a l b i o m a s s e s f o r e a c h d a t e . Numbers o f a n i m a l s , t o t a l numbers o f eggs and mean b r o o d s i z e s were l o g - t r a n s f o r m e d and a n a l y z e d w i t h 2-way ANOVA ( t r e a t m e n t X t i m e ) . N ote t h a t l o g ( b r o o d s i z e ) = l o g ( e g g s ) - l o g ( o v i g e r o u s f e m a l e s ) (2) T h e r e f o r e , i f b o t h l o g ( e g g s ) a n d l o g ( o v i g e r o u s f e m a l e s ) a r e n o r m a l l y d i s t r i b u t e d , l o g ( b r o o d s i z e ) w i l l a l s o be n o r m a l l y d i s t r i b u t e d . 37 To compare t h e t r a n s i e n t b e h a v i o u r o f a c i d i f i e d and c i r c u m n e u t r a l s y s t e m s e x p o s e d t o i d e n t i c a l f e r t i l i z e r a d d i t i o n s , I p l o t t e d t h e s i m u l t a n e o u s c h a n g e s i n t o t a l z o o p l a n k t o n b i o m a s s and l i v e c h l o r o p h y l l a s p h a s e p l a n e t r a j e c t o r i e s . 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 were e s t i m a t e d by m u l t i p l y i n g mean s p e c i e s d e n s i t i e s by p u b l i s h e d f i l t e r i n g r a t e s m e asured a t s i m i l a r t e m p e r a t u r e s b u t c i r c u m n e u t r a l pH (Haney 1973, Buckingham 1978). 2!.2.4 S t a t i s t i c a l Methods "As f a r as t h e laws o f m a t h e m a t i c s r e f e r t o r e a l i t y , t h e y a r e n o t c e r t a i n ; and a s f a r a s t h e y a r e c e r t a i n , t h e y do n o t r e f e r t o r e a l i t y . " E i n s t e i n U n i v a r i a t e a n a l y s e s o f v a r i a n c e (ANOVA) were u s e d i n t h r e e d i f f e r e n t ways i n t h i s s t u d y : ( i ) one-way t r e a t m e n t c o m p a r i s o n s o f 5 t r e a t m e n t s on s p e c i f i c d a t e s , w i t h Tukey m u l t i p l e r a n g e t e s t s ; ( i i ) two-way ( t r e a t m e n t x t i m e ) c o m p a r i s o n s o f two t r e a t m e n t s o v e r a l l s a m p l i n g d a t e s ; and ( i i i ) t h r e e - w a y ( t r e a t m e n t x t i m e x d e p t h ) c o m p a r i s o n s o f two t r e a t m e n t s o v e r a l l s a m p l i n g d a t e s . One-way ANOVAs ( a c r o s s a l l t r e a t m e n t s on a s i n g l e s a m p l i n g d a t e ) u s e d t h e Tukey m u l t i p l e r a n g e t e s t t o compare a l l p a i r s o f t r e a t m e n t means. The Tukey t e s t i s a r e l a t i v e l y c o n s e r v a t i v e 38 t e s t w h i c h a c c o u n t s f o r t h e o v e r a l l r a n g e i n t r e a t m e n t means i n a s s e s s i n g t h e s i g n i f i c a n c e o f i n d i v i d u a l p a i r e d c o m p a r i s o n s ( L i 1964). The a d v a n t a g e o f t r e a t m e n t x t i m e c o m p a r i s o n s i s t h a t o v e r a l l t r e a t m e n t e f f e c t s c a n be i s o l a t e d f r o m t e m p o r a l f l u c t u a t i o n s due t o s e a s o n a l f o r c e s . G r e e n and Hobson (1970) u s e d 2-way ANOVAs i n t h i s manner t o a n a l y z e t h e s p a t i a l and t e m p o r a l s t r u c t u r e o f an i n t e r t i d a l community. The most i m p o r t a n t a s s u m p t i o n s o f ANOVA a r e t h a t t h e w i t h i n g r o u p e r r o r s a r e i n d e p e n d e n t ( o f e a c h o t h e r and o f t h e g r o u p mean), n o r m a l l y d i s t r i b u t e d and homogeneous among g r o u p s ( G l a s s e t a l . 1972, G r e e n 1979). I n a l l a p p l i c a t i o n s o f ANOVA t o t h e e n c l o s u r e s , t h e " g r o u p s " c o n s i s t e d o f two l i m n o c o r r a l s e x p o s e d t o t h e same t r e a t m e n t , and measured on a s i n g l e d a y . I n d e p e n d e n c e o f w i t h i n g r o u p e r r o r s f r o m e a c h o t h e r was a s s u r e d s i n c e t r e a t m e n t s were r a n d o m l y a s s i g n e d t o e n c l o s u r e s , and e n c l o s u r e s r e c e i v i n g t h e same t r e a t m e n t c o u l d i n no way p h y s i c a l l y a f f e c t one a n o t h e r . (Note t h a t t h e i n d e p e n d e n c e a s s u m p t i o n i s v i o l a t e d i f one i n c l u d e d . w i t h i n t h e same g r o u p two measurements t a k e n f r o m t h e same e n c l o s u r e on s u c c e s s i v e s a m p l i n g d a t e s ) . I n d e p e n d e n c e o f w i t h i n g r o u p e r r o r s f r o m w i t h i n g r o u p means was t e s t e d p r i o r t o e a c h ANOVA by e x a m i n i n g t h e l i n e a r r e g r e s s i o n ( T a y l o r 1961, G r e e n 1979): l o g S 2 = l o g a + b l o g X (3) where S 2 = w i t h i n g r o u p sample v a r i a n c e ; X = w i t h i n g r o u p sample mean; l o g a = r e g r e s s i o n i n t e r c e p t , and 39 b = r e g r e s s i o n s l o p e . When b was c l o s e t o 0, no t r a n s f o r m a t i o n was p e r f o r m e d . I n most c a s e s , b was w i t h i n t h e r a n g e 1.75 t o 2, and t h e d a t a t h e r e f o r e s u b j e c t e d t o t h e l o g a r i t h m i c t r a n s f o r m a t i o n Z = l o g ( X ) , o r i f t h e d a t a i n c l u d e d z e r o v a l u e s , Z = l o g ( X + 1 ) ( G r e e n 1979). T h i s t r a n s f o r m a t i o n a l s o s e r v e d t o make t h e w i t h i n g r o u p e r r o r s more n o r m a l l y d i s t r i b u t e d , a l t h o u g h I d i d n o t t e s t f o r n o r m a l i t y s t a t i s t i c a l l y . Note t h a t s t a t i s t i c a l t e s t s on l o g - t r a n s f o r m e d d a t a a s s e s s t h e s i g n i f i c a n c e o f d i f f e r e n c e s between g e o m e t r i c , n o t a r i t h m e t i c means. Homog e n e i t y o f r e s i d u a l v a r i a n c e s was t e s t e d w i t h B a r t l e t t ' s t e s t , u s i n g t h e computer p r o g r a m UBC GENLIN ( G r e i g and B j e r r i n g 1977); h e t e r o g e n e o u s v a r i a n c e s a r e n o t e d i n t h e R e s u l t s s e c t i o n . G l a s s e t a l . (1972) showed t h a t n o n n o r m a l i t y and h e t e r o g e n e i t y o f v a r i a n c e s a f f e c t t h e s i g n i f i c a n c e l e v e l o f t w o - t a i l e d s t a t i s t i c a l t e s t s o n l y s l i g h t l y , p r o v i d e d t h a t t h e s t u d y h as a b a l a n c e d d e s i g n . A l l s t a t i s t i c a l t e s t s between e n c l o s u r e s were b a l a n c e d (2 r e p l i c a t e s / g r o u p ) . ANOVAs between t h e c o n t r o l e n c l o s u r e s and t h e l a k e were u n b a l a n c e d , s i n c e o n l y one sample was o b t a i n e d f r o m E u n i c e L a k e . T h e s e t e s t s t h e r e f o r e examined t h e d i f f e r e n c e s between t h e l a k e and t h e c o n t r o l s r e l a t i v e t o t h e v a r i a t i o n w i t h i n t h e two c o n t r o l c y l i n d e r s . 40 2.3 E x p e r i m e n t a l D e s i g n The f o u r t r e a t m e n t s a p p l i e d t o t h e e n c l o s u r e s were i n t e n d e d t o t e s t a s e t of h y p o t h e s e s c o n c e r n i n g t h e e f f e c t s o f a c i d i f i c a t i o n , f e r t i l i z a t i o n , and e n c l o s u r e ( T a b l e 5 ) . A l t h o u g h most o f t h e s e h y p o t h e s e s were p r e s e n t e d i n my r e s e a r c h p r o p o s a l (Marmorek 1979), o t h e r h y p o t h e s e s e v o l v e d d u r i n g t h e c o u r s e o f my f i e l d work and d a t a a n a l y s i s . I a t t e m p t e d t o f a l s i f y e a c h h y p o t h e s i s by c o m p a r i n g key i n d i c a t o r v a r i a b l e s , b o t h between t r e a t m e n t s and o v e r t i m e ( T a b l e 6 ) . The h y p o t h e s e s l i s t e d i n T a b l e 5 r e q u i r e some e l a b o r a t i o n . W i t h r e s p e c t t o h y p o t h e s e s 2 ( i ) and 5 ( i ) , I was p a r t i c u l a r l y i n t e r e s t e d i n whether c h a n g e s i n z o o p l a n k t o n p o p u l a t i o n s were p r i m a r i l y due t o d i r e c t t o x i c i t y , c h a n g e s i n c o m p e t i t i o n , a l t e r e d f o o d s u p p l y o r a c o m b i n a t i o n of t h e s e f a c t o r s . S i m i l a r l y , I was i n t e r e s t e d i n w hether p h y t o p l a n k t o n community s h i f t s were p r i m a r i l y due t o a b i o t i c c o n d i t i o n s , o r t o c h a n g e s i n h e r b i v o r e f i l t e r i n g ( f o l l o w i n g f r o m h y p o t h e s e s 2 ( i i ) and 5 ( i i ) ) . I n t h e a b s e n c e o f p r i m a r y p r o d u c t i v i t y and z o o p l a n k t o n g r a z i n g measurements, and w i t h o u t p e r f o r m i n g l a b o r a t o r y t o x i c i t y t e s t s , i t i s n o t a l w a y s p o s s i b l e t o c h o o s e between t h e s e a l t e r n a t e e x p l a n a t i o n s . However, I u s e d c a r e f u l e x a m i n a t i o n o f t h e v a r i a t i o n i n s e v e r a l i n d i c a t o r s ( b o t h o v e r t i m e and between t r e a t m e n t s ) t o s u g g e s t w h i c h mechanisms were more p r o b a b l e . B e f o r e t e s t i n g whether e n r i c h m e n t a l l e v i a t e s a c i d - i n d u c e d s h o r t a g e s o f p h y t o p l a n k t o n f o o d f o r z o o p l a n k t o n ( h y p o t h e s i s 4 ( i i )) i t i s n e c e s s a r y t o show t h a t : 1) s u c h f o o d s h o r t a g e s do o c c u r w i t h a c i d i f i c a t i o n 41 T a b l e 5. H y p o t h e s e s t e s t e d by t h e 1979 e x p e r i m e n t N u l l H y p o t h e s i s T e s t 1. E n c l o s u r e o f p l a n k t o n s y s t e m s an d r e m o v a l o f c u t t h r o a t t r o u t p r e d a t i o n d o e s n o t a f f e c t l i g h t t r a n s m i s s i o n , pH, a l k a l i n i t y , p h y t o p l a n k t o n , o r z o o p l a n k t o n . 2. G r a d u a l , e x p e r i m e n t a l a c i d i f i -c a t i o n o f o l i g o t r o p h i c w a t e r s t o pH 5.0: ( i ) d o e s n o t a f f e c t z o o p l a n k t o n p o p u l a t i o n s o r community b i o m a s s ; ( i i ) d o e s n o t a f f e c t p h y t o p l a n k -t o n community b i o m a s s o r s i z e c o m p o s i t i o n ; ( i i i ) d oes n o t a f f e c t t h e a b i o t i c component of l i g h t e x t i n c -t i o n ; ( i v ) i s n o t a f f e c t e d by b i o t i c g e n e r a t i o n o r c o n s u m p t i o n o f a l k a l i n i t y ; a n d (v) r e a l i s t i c a l l y s i m u l a t e s l a k e a c i d i f i c a t i o n . Compare a l l i n d i c a t o r s i n c o n t r o l c y l i n d e r s (C) and E u n i c e L a k e ( L K ) . Compare z o o p l a n k t o n p e r f o r -mance i n A and C r e l a t i v e t o b o t h pH d i s t r i b u t i o n o v e r , d e p t h and t i m e , and p h y t o -p l a n k t o n b i o m a s s and s i z e c o m p o s i t i o n . Compare p h y t o p l a n k t o n i n d i c a -t o r s i n A and C r e l a t i v e t o b o t h pH o v e r t i m e , and e s t i m a t e s o f z o o p l a n k t o n g r a z i n g p r e s s u r e . Compare s e c c h i d e p t h -c h l o r o p h y l l a r e g r e s s i o n l i n e s i n A and C. Compare t h e o r e t i c a l and o b s e r v e d a l k a l i n i t y d e c r e a s e s i n A. Compare c h a n g e s i n key i n d i c a t o r s w i t h o b s e r v a t i o n s f r o m m u l t i p l e l a k e s u r v e y s and whole l a k e e x p e r i m e n t s . 42 T a b l e 5 C o n t i n u e d ... N u l l H y p o t h e s i s T e s t 3. M o d e r a t e n u t r i e n t e n r i c h m e n t of c i r c u m n e u t r a l w a t e r s : ( i ) d o e s n o t a f f e c t a l k a l i n i t y , pH o r t r a n s p a r e n c y ; ( i i ) d o e s n o t a f f e c t z o o p l a n k t o n p o p u l a t i o n s o r community b i o m a s s ; and ( i i i ) d o e s n o t change t h e a v a i l -a b i l i t y o f p h y t o p l a n k t o n f o o d f o r z o o p l a n k t o n . 4. M o d e r a t e n u t r i e n t e n r i c h m e n t d u r i n g e x p e r i m e n t a l a c i d i f i c a t i o n t o pH 5.0: ( i ) d o e s n o t a l t e r a l k a l i n i t y , pH o r t r a n s p a r e n c y from t h e l e v e l s o b s e r v e d by a c i d i f i -c a t i o n a l o n e ; and ( i i ) d o e s n o t a l l e v i a t e a c i d -i n d u c e d s h o r t a g e s o f p h y t o p l a n k t o n f o o d f o r z o o p l a n k t o n . 5. G r a d u a l , e x p e r i m e n t a l a c i d i f i c a t i o n o f m o d e r a t e l y e n r i c h e d w a t e r s : ( i ) ( i i ) same as 2 ( i ) , 2 ( i i ) ( i i i ) d o e s n o t a l t e r t h e a b i l i t y o f p l a n k t o n s y s t e m s t o a b s o r b e n r i c h m e n t p e r t u r b a t i o n s ; and ( i v ) d o e s n o t a l t e r t r a n s p a r e n c y f r o m t h e l e v e l s o b s e r v e d i n n o n - a c i d i f i e d , m o d e r a t e l y e n r i c h e d w a t e r s . Compare a l k a l i n i t y , pH a n d t r a n s p a r e n c y i n F and C. Compare i n d i c a t o r s o f z o o p l a n k t o n p e r f o r m a n c e i n F and C. Compare p h y t o p l a n k t o n i n d i c a t o r s i n F and C. Compare a l k a l i n i t y , pH and t r a n s p a r e n c y i n AF and A. Compare i n d i c a t o r s o f z o o p l a n k t o n p e r f o r m a n c e i n AF and A. P e r f o r m c o m p a r i s o n s l i s t e d f o r 2 ( i ) and 2 ( i i ) , b u t f o r t r e a t m e n t s F and AF. Compare j o i n t f l u c t u a t i o n s 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 F and AF. Compare t r a n s p a r e n c y i n AF and F. Table 6. Indicator variables monitored in the 1979 study, and their justification. Variables Monitored to Indicate Physico-Chemical Conditions: • temperature • pH • alkalinity • secchi depth Phytoplankton: • chlorophyll a^  • pheophytin £ • algal cell counts Zooplankton: • species densities by age class • lengths thermocline; metabolic rates abiotic stress degree of acification (acid balance-biotic effects on alkalinity abiotic and biotic effects on transparency algal biomass biotic effects on transparency chlorophyll degradation, including digestion by zooplankton size composition and edibility of phytoplankton taxonomic composition total algal biomass overall effects of life histories and perturbations growth effects (copepods) age structure (cladocerans) biomass 44 Table 6. Continued Variables Monitored to Indicate Zooplankton (continued): • mean brood size, percent of ovigerous females, total egg production • vertical distribution food availability reproductive status of population birth rates actual exposure to abiotic stress overlap in species' spatial distribution alone; zooplankton in the A and AF enclosures are exposed to the same degree of ac id stress (hypothesis 4 ( i ) ) . 46 3. RESULTS I ' l P h y s i c a l and C h e m i c a l C o n d i t i o n s 3_._1_.J_ P r e c i p i t a t i o n and T e m p e r a t u r e D a i l y p r e c i p i t a t i o n d u r i n g t h e e x p e r i m e n t i s shown i n F i g u r e 4. S i n c e t h e e x p e r i m e n t a l e n c l o s u r e s were i s o l a t e d f r o m w a t e r s h e d f l u s h i n g and m a t e r i a l s e x p o r t , p r e c i p i t a t i o n c o u l d have i n c r e a s e d d i f f e r e n c e s between t h e l a k e and t h e e n c l o s u r e s . D u r i n g t h e f i r s t month o f t h e e x p e r i m e n t however, t h e c o m p l e t e a b s e n c e o f p r e c i p i t a t i o n ( F i g u r e 4) r e d u c e d f l u s h i n g and m a t e r i a l s e x p o r t t o a minimum. T h i s d r y p e r i o d , w h i c h r e s u l t e d i n a h i g h f o r e s t f i r e r i s k on l a n d and l i k e l y e x a c e r b a t e d n u t r i e n t s h o r t a g e s i n t h e l a k e , was ended by 24 mm o f r a i n f a l l d u r i n g A u g u s t 14-22, f o l l o w e d by 126.7 mm o f r a i n f a l l d u r i n g September 1 t o 8. The l a s t m a j o r s t o r m e v e n t s o f t h e e x p e r i m e n t o c c u r r e d September 26-29, c o n t r i b u t i n g 32.8 mm o f r a i n . D a i l y a i r t e m p e r a t u r e minima and maxima a r e shown i n F i g u r e 5, a n d l a k e water t e m p e r a t u r e s by d e p t h - t i m e i s o t h e r m s i n F i g u r e 6. The d a s h e d l i n e s on F i g u r e 6 o u t l i n e t h e u p p e r a n d l o w e r b o u n d a r i e s o f t h e t h e r m o c l i n e , and were d e t e r m i n e d by e s t i m a t i n g t h e p l a n e o f maximum d e c r e a s e i n t e m p e r a t u r e ( H u t c h i n s o n 1957; pg. 428) f r o m v e r t i c a l d e p t h p r o f i l e s . G r a p h i c a l d e t e r m i n a t i o n o f m e t a l i m n i o n t h i c k n e s s ( W e t z e l 1975; p g . 70) showed t h e upper m e t a l i m n e t i c b o u n d a r y t o be a l w a y s w i t h i n 0.5 m o f t h e s t a r t o f t h e t h e r m o c l i n e , due t o t h e s h a r p d e c r e a s e i n t e m p e r a t u r e t h e r e . The t e m p e r a t u r e change below t h e 47 F i g u r e 4. D a i l y p r e c i p i t a t i o n (1979) . F i g u r e 5. D a i l y v a r i a t i o n i n a i r t e m p e r a t u r e (1979) . .en T E M P E R A T U R E (°C) 01 o ro o ( j i O ro Ol O Ol ID ro-T OJ ro o oi4 DAILY P R E C I P I T A T I O N (mm) m Q oi o cn i i i 1 1 b Ol o 01 o ro Ol 49 F i g u r e 6. E u n i c e L a k e water t e m p e r a t u r e s d u r i n g 1979 . D ashed l i n e s i n d i c a t e u p p e r and l o w e r l i m i t s o f m e t a l i m n i o n . D E P T H m OS 51 t h e r m o c l i n e was more g r a d u a l . F i g u r e 6 i l l u s t r a t e s t h e g r a d u a l t r a n s f e r o f h e a t f r o m t h e l a k e s u r f a c e t o l o w e r d e p t h s , w i t h t h e i s o t h e r m s between 8°C and 14°C s i n k i n g a t r a t e s o f 0.4-0.7 m/month o v e r t h e c o u r s e o f t h e e x p e r i m e n t . More s i g n i f i c a n t however, a r e t h e c h a n g e s i n t h e s t r e n g t h and p o s i t i o n o f t h e t h e r m o c l i n e . D u r i n g t h e l a s t two weeks of J u l y , warm d a y t i m e t e m p e r a t u r e s h e a t e d t h e upper 2 m t o above 22°C, and t h e t h e r m o c l i n e r e m a i n e d a t t h e 3 t o 5 m d e p t h i n t e r v a l . The s t r a t i f i c a t i o n s h a r p e n e d c o n s i d e r a b l y d u r i n g t h e f i r s t two weeks o f A u g u s t , w i t h l o w e r n i g h t t i m e t e m p e r a t u r e s ( F i g u r e 5) l i k e l y r e s p o n s i b l e f o r c o o l i n g t h e u p p e r 2 m . A v e r y s t r o n g t h e r m o c l i n e between 4 and 5 m had formed by t h e s e c o n d week o f A u g u s t , but t h e n became more g r a d u a l , s t r e t c h i n g between 4 and 6 m by A u g u s t 30. The s e r i e s o f m a j o r s t o r m s i n t h e f i r s t week o f September ( F i g u r e 4) c o o l e d and mixed t h e upper 4 m, s h i f t i n g t h e t h e r m o c l i n e down t o t h e 5-7 m s t r a t u m f o r t h e r e m a i n d e r o f t h e e x p e r i m e n t . F o l l o w i n g t h e s t o r m s i n t h e f i r s t week o f September, t h e t h e r m a l s t r u c t u r e r e m a i n e d e s s e n t i a l l y s t a b l e u n t i l t h e end o f t h e month, when a n o t h e r m a j o r s t o r m c o o l e d and m i x e d t h e t o p 4 m t o a homogeneous t e m p e r a t u r e o f 16°C. 3_.1.2 Hy d r o g e n Ion C o n c e n t r a t i o n and A l k a l i n i t y The pH and a l k a l i n i t y o v e r t i m e and d e p t h a r e i l l u s t r a t e d i n F i g u r e s 7 a n d 8 r e s p e c t i v e l y , w i t h mean v a l u e s f o r e a c h t r e a t m e n t i n T a b l e 7. Mean d i f f e r e n c e s i n c o m p o s i t e a l k a l i n i t y (1-7 m) between t r e a t m e n t s a r e shown f o r e a c h sample d a t e i n 52 T a b l e 8. A p p e n d i x C d e t a i l s t h e r e s u l t s of t h r e e - w a y a n a l y s e s of v a r i a n c e ( t r e a t m e n t , t i m e and d e p t h ) of c y l i n d e r a l k a l i n i t e s and H + c o n c e n t r a t i o n s ; o n l y a summary of t h e s e a n a l y s e s i s p r e s e n t e d h e r e . (i_) E n c l o s u r e E f f e c t s D i d e n c l o s u r e a f f e c t a l k a l i n i t y and pH l e v e l s ? The e n c l o s u r e o f w ater s i g n i f i c a n t l y r e d u c e d mean e p i l i m n e t i c (1-3 m) a l k a l i n i t i e s ( T a b l e 7 a n d A p p e n d i x C ) . E p i l i m n e t i c a l k a l i n i t i e s i n E u n i c e Lake and t h e c o n t r o l s d i f f e r e d by more t h a n 25 ueq»L~ 1 on A u g u s t 1 and A u g u s t 30 ( F i g u r e 8 ) . B o t h t h e l a k e and t h e c o n t r o l c y l i n d e r s showed a n e t d e c l i n e i n e p i l i m n i o n a l k a l i n i t y o v e r A u g u s t , t h o u g h t h e l a k e showed g r e a t e r t e m p o r a l f l u c t u a t i o n s ( F i g u r e 8 a n d A p p e n d i x C ) . E p i l i m n e t i c pH l e v e l s were l o w e r i n t h e c o n t r o l s t h a n i n E u n i c e Lake ( F i g u r e 7 and T a b l e 7) but d i f f e r e n c e s were s m a l l r e l a t i v e t o t h e a l k a l i n i t y gap. T h i s i s b e c a u s e water has a h i g h b u f f e r i n g c a p a c i t y a t pH v a l u e s c l o s e t o 6.4, t h e a p p r o x i m a t e e q u i v a l e n c e p o i n t o f b i c a r b o n a t e a n d c a r b o n i c a c i d (Stumm a n d Morgan 1970). At 5-7 m, c o n t r o l c y l i n d e r mean a l k a l i n i t i e s and pH l e v e l s were s i g n i f i c a n t l y h i g h e r t h a n E u n i c e Lake measurements, and d i f f e r e d i n d e p t h g r a d i e n t s as w e l l ( A p p e n d i x C ) . I n summary, t h e c o n t r o l s showed s m a l l e r v e r t i c a l g r a d i e n t s and t e m p o r a l f l u c t u a t i o n s i n a l k a l i n i t y t h a n d i d E u n i c e L a k e . L i g h t l i m i t a t i o n from l i m n o c o r r a l s h a d i n g l i k e l y c o n t r i b u t e d t o t h e s e e n c l o s u r e e f f e c t s ( d i s c u s s e d i n S e c t i o n 4 . 1 ) . 53 F i g u r e 7. 1979 pH l e v e l s by d e p t h and t i m e , and t i m i n g o f c h e m i c a l a d d i t i o n s . V a l u e s a t 3 m ( n o t shown f o r c l a r i t y ) were a l w a y s w i t h i n 0.05 u n i t s o f pH a t 1 m. B a r s show neq o f H* added t o t o t a l c y l i n d e r volume from s u l f u r i c a c i d ; d o t s i n d i c a t e t i m i n g o f n u t r i e n t a d d i t i o n s . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 54 J 9 7 9 p H by D E P T H L A Y E R + 1- 1 m J ACID ADDITION x * 5 m • NUTRIENT ADDITION o——o 7 m m © 23 1 9 16 23 30 6 13 20 21 3 II J I «JG. | SEPT. | OCI. 23 I 9 16 23 30 6 13 21 J t AUG | SEPT. 27 S I) | OCT. 23 1 J I 16 23 30 6 13 20. 27 S I SEPT. | OCT. HI © 23 1 9 16 23 30 8 13 20 21 5 II J I . WC. I SEPT. | OCt. SAMPLING DATE 23 I 9 16 23 30 6 13 20 27 S II J | BUG. I SEPT. | OCT. ...m©,.. 16 23 30 6 13 20 27 5 11 HJC. | SEPT. J OCI. ••..,.:CF)... 1 9 16 23 30 6 13 20 27 5 I «*. | SEPT. | OCI. ERA 23 I 9 16 23 30 6, 13 20 21 5 11 J I tUd. J SEPT. | OCI. m © 23 1 B ' IB 23 30 6 19 70 37 5 II J I «*• I SEPT. | OCI. SAMPLING OflTE 55 F i g u r e 8. D e p t h - t i m e d i a g r a m s o f a l k a l i n i t y i s o p l e t h s d u r i n g 1979 . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 56 Table 7. Mean pH and alkalinity by treatment and depth stratum (mean ± s.e. (n)). pH Alkalinity (ueq/1) Treatment !  1-7 m 1-3 m 5-7 m 1-7 m 1-3 m 5-7 m Lake 6. ,45 + 0.03 (43) 6. 61 + 0.03 (21) 6. 34 + 0.03 (22) 89 + 2.4 (31) 98 + 2.9 (15) 81 + 2.3 (16) C 6. ,44 + 0.01 (86) 6. ,51 + 0.02 (43) 6. 39 + 0.02 .(43) 84 + 0.9 (61) 82 + 1.1 (31) 87 + 1.3 (30) F 6. ,45 + 0.02 (86) 6. ,52 + 0.03 (43) 6. 38 0.02 (43) 79 + 1.2 (63) 74 + 1.2 (32) 84 + 1.7 (31) A 5. ,85 + 0.04 (96) 5. ,72 + 0.05 (48) 6. 06 + 0.05 (48) 62 + 2.3 (63) 49 + 2.6 (31) 74 + 2.2 (32) AF 5. ,60 + 0.05 (96) 5. ,42 + 0.06 (48) 5. 91 + 0.10 (48) 59 + 2.7 (64) 43 + 2.5 (32) 73 + 2.9 (32) U l 5 8 Table 8. Differences between treatments in mean water column alkalinity (ueq/1) on each sampling date. Column 1: Effect of enclosure. Column 2: Cumulative amount of acid added to A enclosures as H2SG\. Column 3: Observed effect of acidification. Column 4: Amount of acid added to F enclosures as fertilizer. Column 5: Observed effect of fertilization on circumneutral waters. Column 6: Amount of acid added to AF enclosures as fertilizer. Column 7: Observed effect of fertilization on acidifying waters. Change in Composite Alkalinity Due to Treatment (ueq/1) nate Control Acidification in Fertilization in Fertilization in Enclosures A Enclosures F Enclosures AF Enclosures (1) (2) (3) (4) (5) (6) (7) u+ f H + from H + from _ (C-LK) " Q (y o m i s ^  H3P04 and (F-C) H3P04 and (AF-A) HzSO. KR-l) NH,N03* Nh\N03 July 16 -5 0 -4 0 3 0 1 Aug. 1 -14 -5 -7 -0.38 -3 -0.38 -6 9 +3 -8 -11 -0.57 -3 -0.56 -6 16 -5 -18 -22 -0.57 -4 -0.56 -3 23** -6 -18 -33 -0.77 -3 -0.72 2 30 ? -18 -17 -0.77 -7 -0.72 -7 Sept. 6 -4 -18 -20 -0.77 -7 -0.72 -4 .13 -3 -23 -26 -0.77 -8 -0.72 -2 27 -1 -31 -41 -0.77 -10 -0.72 -3 Difference not calculable due to missing data. Calculated from dissociation constants for H3P0i* and NhUN03at 20°C in pure water, and mean epilimnetic pH for each treatment (Kester and Pytokowicz 1967, Thurston et al. 1974). About 98% of the H+ comes from H3P04 at the observed pH levels. Only two samples taken for 0-4 m and 4-8 m, rather than four samples. 59 ( i i ) E f f e c t o f H* i o n L o a d i n g i n P r e c i p i t a t i o n A c a l c u l a t i o n o f t h e t o t a l H + l o a d i n g f r o m p r e c i p i t a t i o n d u r i n g t h e c o u r s e o f t h e e x p e r i m e n t i s shown i n T a b l e 9. Even a l l o w i n g f o r a 20% i n c r e a s e f o r e v a p o r a t i v e c o n c e n t r a t i o n (Yan and L a f r a n c e 1981), t h e p r e c i p i t a t i o n c o u l d o n l y have i n c r e a s e d e p i l i m n e t i c [H+] ( m a x i m a l l y d e c r e a s e d a l k a l i n i t y ) w i t h i n t h e e n c l o s u r e s by 0.247 ,ueq»L" 1. Hence H + l o a d i n g f r o m t h e at m o s p h e r e d u r i n g t h i s e x p e r i m e n t was n e g l i g i b l e r e l a t i v e t o t h e o b s e r v e d r a n g e o f f l u c t u a t i o n i n e p i l i m n i o n a l k a l i n i t y . ( i i i ) E f f e c t s o f E x p e r i m e n t a l A c i d i f i c a t i o n The amount o f s u l f u r i c a c i d added t o t h e A e n c l o s u r e s i s shown by t h e b a r s on F i g u r e 7, e x p r e s s e d a s *ieq«L~ 1 o f H + a v e r a g e d o v e r t h e e n t i r e volume o f t h e c y l i n d e r . Was t h e a c i d i f i c a t i o n of t h e A c y l i n d e r s a f f e c t e d by b i o t i c g e n e r a t i o n or c o n s u m p t i o n o f a l k a l i n i t y ? T h i s q u e s t i o n c a n be a n s w e r e d by c o m p a r i n g t h e e x p e c t e d H + l o a d i n g t o t r e a t m e n t A ( t h e s e c o n d column o f T a b l e 8) w i t h t h e o b s e r v e d d i f f e r e n c e i n mean a l k a l i n i t y between t r e a t m e n t s C and A ( t h e t h i r d c o l u m n ) . The A e n c l o s u r e s showed c o m p o s i t e a l k a l i n i t i e s 4 /ueq»L~ 1 l e s s t h a n t h e c o n t r o l s on J u l y 16 p r i o r t o any a c i d a d d i t i o n s . When t h i s i n i t i a l d i f f e r e n c e i s d e d u c t e d f r o m s u b s e q u e n t (A-C) a l k a l i n i t y d i f f e r e n c e s t h e mean r a t i o o f o b s e r v e d t o e x p e c t e d a l k a l i n i t y d i f f e r e n c e s i s 0.98 ± 0 . 1 1 (SE; n = 8 ) . Hence, i t a p p e a r s t h a t t h e a d d e d s u l f u r i c a c i d was c l o s e t o 100% e f f i c i e n t i n r e m o v i n g a l k a l i n i t y . The e x c e p t i o n a l l y l a r g e a l k a l i n i t y d i f f e r e n c e on A u g u s t 23 was due t o an i n c r e a s e i n t h e c o n t r o l s ' mean Table 9. Hydrogen ion1 loading from precipitation during 1979 experiment. Total M H H+ Loadinq/ Potential Date of Collection Rainfall2 '"z 5 • Enclosures3 Increase in (m) o r K a i n (ueq) Epilimnetic (H+)4 (yeq/1) Aug. 7-27 0.024 6.0 50 0.005 Aug. 27-Sept. 13 0.1267 5.1 2094 0.2 Sept. 18-29 0.0332 5.2 436 0.042 Total 0.1839 2580 0.247 1 Source: Dr. Michael Feller, Department of Forestry, U.B.C. Precipitation Sampling Stations 6 km from Eunice Lake. 2 From Figure 3.1. 3 Calculated assuming surface area of enclosure =• 2.08 m2. " Calculated assuming epilimnetic depth = 4 km and allowing 20% for evaporative concentration (Yan and Lafrance 1981). 61 a l k a l i n i t y r a t h e r t h a n a d e c r e a s e i n t h e A e n c l o s u r e s . I n c o n c l u s i o n , n e t a l k a l i n i t y c h a n g e s due t o b i o t a were n e g l i g i b l e , c o n s i s t e n t w i t h h y p o t h e s i s 2 ( i v ) on page 41. The t h e r m a l r e g i m e , c l e a r l y a f f e c t e d t h e v e r t i c a l d i s t r i b u t i o n o f added H + . When a l k a l i n i t y c o n t o u r s i n t h e A e n c l o s u r e s a r e compared w i t h t h e c o n t r o l s ( F i g u r e 8 ) , i t i s e v i d e n t t h a t a l t h o u g h some a c i d was a b l e t o p e n e t r a t e t h e s h a r p A u g u s t t h e r m o c l i n e a t 5 m, b o t t o m w a t e r a l k a l i n i t i e s l a g g e d b e h i n d s u r f a c e c o n c e n t r a t i o n s by two t o t h r e e weeks. C o i n c i d e n t w i t h w i n d m i x i n g and wea k e n i n g o f t h e t h e r m o c l i n e i n t h e f i r s t h a l f o f September, a l k a l i n i t i e s d r o p p e d i n t h e 4-6 m z o n e . A l k a l i n i t y i n c r e a s e s a t t h e 7 m l e v e l d u r i n g t h i s p e r i o d were p r e s u m a b l y due t o C 0 2 p r o d u c t i o n f r o m m i c r o b i a l m e t a b o l i s m ( t o be d i s c u s s e d i n S e c t i o n 4 . 1 ) . R e d u c t i o n s i n pH i n t h e A t r e a t m e n t s were l a r g e l y r e s t r i c t e d t o t h e upper 4 m d u r i n g A u g u s t ( F i g u r e 7 ) . F o l l o w i n g a major a c i d a d d i t i o n on A u g u s t 9 (10 ueq»L~ 1 ) t h e mean pH i n t h e 1-3 m l a y e r o f t h e A e n c l o s u r e s d r o p p e d f r o m 6.3 ± 0.00 t o 5.72 ± 0.06. I n t h e a b s e n c e o f a c i d a d d i t i o n s between A u g u s t 9 and September 6, t h e e p i l i m n e t i c pH r e c o v e r e d , p r o b a b l y due t o g e n e r a t i o n o f a l k a l i n i t y by p h o t o s y n t h e t i c N0 3~ u p t a k e and m i x i n g w i t h l e s s a c i d w a ter a t 5 m; by September 6, t h e v e r t i c a l pH g r a d i e n t between 1 and 5 m had d i s a p p e a r e d . I t was n o t u n t i l l a t e September t h a t t h e pH a t 5 m d r o p p e d below 6.0. 6 2 ( i v ) E f f e c t s o f E x p e r i m e n t a l F e r t i l i z a t i o n E p i l i m n e t i c a l k a l i n i t i e s were on a v e r a g e 8 y e q « L " 1 l o w e r i n t h e F c y l i n d e r s t h a n i n t h e c o n t r o l s ( T a b l e 7 ) , a d i f f e r e n c e w h i c h was h i g h l y s i g n i f i c a n t s t a t i s t i c a l l y ( A p p e n d i x C ) . A t 5-7 m, t h e mean d i f f e r e n c e i n a l k a l i n i t i e s was o n l y 3 veq*L~1, but s t i l l s i g n i f i c a n t ( T a b l e 7; A p p e n d i x C ) . U n l i k e t h e a c i d i f e d c y l i n d e r s ( t r e a t m e n t A ) , t h e f e r t i l i z e d c y l i n d e r s were p r o b a b l y a f f e c t e d by b i o t i c c o n t r o l s on a l k a l i n i t y . Column 5 of T a b l e 8 c o n t a i n s t h e mean o b s e r v e d a l k a l i n i t y d i f f e r e n c e between t h e C and F t r e a t m e n t s ( F - C ) , c a l c u l a t e d f r o m 1-7 m c o m p o s i t e a l k a l i n i t i e s . C o m p a r i n g column 5 t o column 4 ( t h e t h e o r e t i c a l c h e m i c a l c o n t r i b u t i o n s o f H* from d i s s o c i a t i o n o f H 3PO 0 and N H „ N 0 3 ) i t seems t h a t b i o t i c r a t h e r t h a n a b i o t i c p r o c e s s i n g o f t h e added n u t r i e n t s must have been r e s p o n s i b l e f o r t h e o b s e r v e d l o s s o f a l k a l i n i t y i n t h e F c y l i n d e r s . P r e f e r e n t i a l a l g a l a s s i m i l a t i o n o f N H „ * o v e r N0 3~ and b a c t e r i a l n i t r i f i c a t i o n a p p e a r t o be t h e two most l i k e l y c a u s e s o f t h e a l k a l i n i t y d e c r e a s e ( t o be d i s c u s s e d i n S e c t i o n 4 . 1 ) . Note t h a t t h e l o s s o f a l k a l i n i t y i n t h e F c y l i n d e r s was i n s u f f i c i e n t t o d e c r e a s e t h e pH ( F i g u r e 7; T a b l e 7 ) . O b s e r v e d [H*] d i f f e r e n c e s between t r e a t m e n t F and t h e c o n t r o l 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 ( A p p e n d i x C ) . T h e r e f o r e , p a r t o f n u l l h y p o t h e s i s 3 ( i ) (page 41) was c o n t r a d i c t e d ( t h a t m o derate n u t r i e n t e n r i c h m e n t o f c i r c u m n e u t r a l w a t e r s d o e s n o t a f f e c t a l k a l i n i t y ) , a nd p a r t was n o t c o n t r a d i c t e d ( t h a t pH i s n o t a f f e c t e d ) . 63 (v) E f f e c t s of E x p e r i m e n t a l A c i d i f i c a t i o n and F e r t i l i z a t i o n D u r i n g t h e week a f t e r t h e major a c i d a d d i t i o n on A u g u s t 9, when t h e mean e p i l i m n e t i c pH i n t h e A e n c l o s u r e s d r o p p e d f r o m 6.3 t o 5.72, t h e AF e n c l o s u r e s showed a much l a r g e r d e c r e a s e , f r o m 6.24 ± 0.06 t o 5.30 ± 0.10 ( F i g u r e 7 ) . T h i s h a d tr e m e n d o u s b i o l o g i c a l r e p e r c u s s i o n s i n AF ( t o be d i s c u s s e d ) . S i n c e t h e same amount o f H2SO., was a d d e d t o t h e A and AF e n c l o s u r e s p r i o r t o A u g u s t 16, and d i s s o c i a t i v e c o n t r i b u t i o n s o f H* from NH aNO 3 and H3PO., were n e g l i g i b l e ( T a b l e 8 ) , t h e g r e a t e r a c i d i f i c a t i o n o f AF must have been due t o b i o t i c p r o c e s s i n g o f t h e added n u t r i e n t s . S u ch n u t r i e n t p r o c e s s i n g r e s u l t e d i n a h i g h l y s i g n i f i c a n t d i f f e r e n c e i n o v e r a l l mean e p i l i m n e t i c a l k a l i n i t i e s between t r e a t m e n t s A and AF ( T a b l e 7; A p p e n d i x C ) . Though t h e a l k a l i n i t y d i f f e r e n c e between t r e a t m e n t s A a n d AF was on a v e r a g e 2 ueq»L~1 l e s s t h a n t h e d i f f e r e n c e between C and F, n u t r i e n t - i n d u c e d a l k a l i n i t y l o s s e s r e s u l t e d i n a much g r e a t e r s u p p l e m e n t a r y pH d e c l i n e i n AF t h a n F. T h i s i s b e c a u s e t h e c o n c u r r e n t H 2 S O « a d d i t i o n s t o t h e AF t r e a t m e n t s d e c r e a s e d t h e pH t o a l e v e l (5.7) where t h e b u f f e r i n g c a p a c i t y was t h e o r e t i c a l l y a b o u t 53% o f t h a t a v a i l a b l e i n t h e F t r e a t m e n t a t pH 6.5 (Stumm and Morgan 1970). ( B u f f e r i n g c a p a c i t y i s t h e s l o p e o f t h e t i t r a t i o n c u r v e , o r d ( p H ) / d ( a l k a l i n i t y ) (Stumm and Morgan 1970).) D i f f e r e n c e s i n pH between t r e a t m e n t s A and AF c l o s e l y r e f l e c t e d t h e a l k a l i n i t y d i f f e r e n c e s . Over t h e c o u r s e o f t h e e x p e r i m e n t , t h e mean e p i l i m n e t i c pH was s i g n i f i c a n t l y l o w e r i n t r e a t m e n t AF (5.42 ± 0.06) t h a n i n t r e a t m e n t A (5.72 ± 0.05) 64 ( A p p e n d i x C ) . T h e r e f o r e , i n c o n t r a s t t o n u l l h y p o t h e s i s 4 ( i ) (page 4 1 ) , moderate n u t r i e n t e n r i c h m e n t d u r i n g e x p e r i m e n t a l a c i d i f i c a t i o n t o pH 5.0 d i d a l t e r a l k a l i n i t y a nd pH f r o m t h e l e v e l s o b s e r v e d by a c i d i f i c a t i o n a l o n e . 3._1_.3. T r a n s p a r e n c y The n u l l h y p o t h e s e s c o n c e r n i n g w ater t r a n s p a r e n c y (page 41) f o c u s s e d on t h e e f f e c t s o f e n c l o s u r e , t h e e f f e c t s o f e x p e r i m e n t a l a c i d i f i c a t i o n ( p a r t i c u l a r l y on t h e a b i o t i c component o f w a t e r t r a n s p a r e n c y ) , and t h e e f f e c t s o f e x p e r i m e n t a l f e r t i l i z a t i o n ( i n b o t h c i r c u m n e u t r a l and a c i d i c w a t e r s ) . T h e s e q u e s t i o n s were a p p r o a c h e d by b o t h g r a p h i c a l c o m p a r i s o n o f S e c c h i d e p t h c h a n g e s o v e r t i m e ( t h i s s e c t i o n ) , and s t a t i s t i c a l c o m p a r i s o n s o f between t r e a t m e n t d i f f e r e n c e s i n S e c c h i d e p t h v s . c h l o r o p h y l l a n o n l i n e a r r e g r e s s i o n s (summarized i n t h i s s e c t i o n and d e t a i l e d i n A p p e n d i x D ) . R e l a t i v e t o t h e l a k e , t r a n s p a r e n c y was r e d u c e d i n t h e c o n t r o l s d u r i n g A u g u s t and O c t o b e r , b u t s l i g h t l y g r e a t e r i n September ( F i g u r e 9 ) . E n c l o s u r e i n c r e a s e d l i g h t e x t i n c t i o n due t o a b i o t i c f a c t o r s , most l i k e l y s h a d i n g f r o m t h e f l o a t s o f t h e l i m n o c o r r a l s ( A p p e n d i x D ) . D i d e x p e r i m e n t a l a c i d i f i c a t i o n a f f e c t t r a n s p a r e n c y ? On September 13 and 20, S e c c h i d e p t h i n t r e a t m e n t A a v e r a g e d 6.9 ± 0.13 m, compared t o o n l y 6.1 ± 0.07 m i n t h e c o n t r o l s ( F i g u r e 9 ) . T h i s t r a n s p a r e n c y d i f f e r e n c e o c c u r r e d when t h e mean e p i l i m n e t i c pH i n t h e A c y l i n d e r s was 5.92 ± 0.05 ( v e r s u s 6.39 ± 0.03 i n t h e c o n t r o l s ) , and t h e t o t a l c h l o r o p h y l l a 65 F i g u r e 9 . Mean S e c c h i d i s c d e p t h s by t r e a t m e n t and t i m e . E r r o r b a r s show ± SE. A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 66 AUGUST SEPTEMBER OCT. 67 c o n c e n t r a t i o n s ( c h l o r o p h y l l p l u s p h a e o p h y t i n ) i n A and C were s i m i l a r (0.93 ± 0.14 and 0.89 ± 0.07 M 9 # L " 1 r e s p e c t i v e l y ) . O v e r a l l mean S e c c h i d e p t h s i n t r e a t m e n t A (7.14 ± 0.23 m) were n o t s i g n i f i c a n t l y d i f f e r e n t f r o m t h e c o n t r o l s (6.72 ± 0.35 m). F i n a l l y , r e g r e s s i o n a n a l y s e s o f S e c c h i d i s c d e p t h s v e r s u s t o t a l c h l o r o p h y l l ( A p p e n d i x D) d i d n o t show s i g n i f i c a n t c h a n g e s i n l i g h t e x t i n c t i o n w i t h e x p e r i m e n t a l a c i d i f i c a t i o n ( t r e a t m e n t A ) . In c o n c l u s i o n , t h e r e i s i n s u f f i c i e n t e v i d e n c e t o c o n t r a d i c t h y p o t h e s i s 2 ( i i i ) (page 4 1 ) , t h a t e x p e r i m e n t a l a c i d i f i c a t i o n ( a t l e a s t t o pH 5.5) does n o t a f f e c t t h e a b i o t i c component o f l i g h t e x t i n c t i o n . S e c c h i d e p t h s i n t r e a t m e n t F were v e r y s i m i l a r t o t h e c o n t r o l s f o r most o f t h e e x p e r i m e n t , but were s i g n i f i c a n t l y l o w e r ( p a r t i c u l a r l y i n 8F) d u r i n g c h l o r o p h y l l p e a k s on A u g u s t 1 a n d September 2 7 - O c t o b e r 5 ( F i g u r e 9; F i g u r e 20, page 1 2 0 ) . Between t r e a t m e n t d i f f e r e n c e s on t h e s e d a t e s were s u f f i c i e n t t o r e j e c t p a r t o f n u l l h y p o t h e s e s 3 ( i ) : t h a t n u t r i e n t e n r i c h m e n t o f c i r c u m n e u t r a l w a t e r s d o e s n o t a f f e c t t r a n s p a r e n c y . In t h e AF e n c l o s u r e s , S e c c h i d e p t h s t r a c k e d t h e l a r g e f l u c t u a t i o n s i n c h l o r o p h y l l ( F i g u r e 20, page 1 2 0 ) . D u r i n g t h e p e r i o d f r o m A u g u s t 9 t o 23, when t h e s e c y l i n d e r s showed s i x t o s e v e n f o l d i n c r e a s e s i n t o t a l c h l o r o p h y l l , mean S e c c h i d e p t h d e c r e a s e d f r o m 5.8 ± 0.2 m t o 3.8 ± 0.8 m. Th e s e d r a m a t i c c h a n g e s i n c h l o r o p h y l l and t r a n s p a r e n c y were a s s o c i a t e d w i t h m a j o r a c i d - i n d u c e d d e c r e a s e s i n t o t a l z o o p l a n k t o n b i o m a s s ( S e c t i o n 3 . 3 . 5 ) . Due t o t h e h i g h c h l o r o p h y l l l e v e l s , t h e f r a c t i o n o f s u b s u r f a c e l i g h t a t t e n u a t e d by c h l o r o p h y l l was 68 s i g n i f i c a n t l y h i g h e r i n t r e a t m e n t AF t h a n i n t h e o t h e r t r e a t m e n t s ( A p p e n d i x D ) . In c o n c l u s i o n , l o w e r S e c c h i d e p t h s i n t r e a t m e n t AF t h a n t r e a t m e n t A c o n t r a d i c t p a r t of h y p o t h e s i s 4 ( i ) (page 4 1 ) : t h a t n u t r i e n t e n r i c h m e n t d u r i n g a c i d i f i c a t i o n t o pH 5.0 d o e s n o t a l t e r t r a n s p a r e n c y f r o m t h e l e v e l s o b s e r v e d by a c i d i f i c a t i o n a l o n e . N u l l h y p o t h e s i s 5 ( i v ) ( t h a t a c i d i f i c a t i o n o f m o d e r a t e l y e n r i c h e d w a t e r s d o e s n o t a l t e r i t s t r a n s p a r e n c y ) was a l s o c o n t r a d i c t e d , s i n c e S e c c h i d e p t h s were much l o w e r i n AF t h a n F ( F i g u r e 9 ) . _3.J..4. Summary o f P h y s i c a l and C h e m i c a l R e s u l t s 1. A s t r o n g t h e r m o c l i n e a t 4-6 m p e r s i s t e d t h r o u g h o u t t h e d r y , warm months o f J u l y and A u g u s t , b e f o r e w e a k e n i n g w i t h t h e s t o r m s a n d c o o l w e a t h e r i n Septe m b e r . 2. E n c l o s u r e o f water r e d u c e d mean e p i l i m n e t i c a l k a l i n i t y by 16 neq»L~1 and r a i s e d mean m e t a l i m n e t i c a l k a l i n i t y by 6 ,ueq»L~L. However, mean pH l e v e l s c h a n g e d by l e s s t h a n 0.1 u n i t s . 3. B i o t i c p r o c e s s e s d i d n o t c a u s e a n e t change i n a l k a l i n i t y i n t r e a t m e n t A, s i n c e e x p e r i m e n t a l a d d i t i o n s o f s u l f u r i c a c i d t o t h e A c y l i n d e r s removed a s much a l k a l i n i t y a s was e x p e c t e d f r o m l a b o r a t o r y t i t r a t i o n o f l a k e w a t e r . 4. Due t o summer t h e r m a l s t r a t i f i c a t i o n , pH l e v e l s a t 5 m i n A and AF r e m a i n e d above 6.0 u n t i l l a t e .September. I n b o t h t r e a t m e n t s F and AF, f e r t i l i z a t i o n s i g n i f i c a n t l y r e d u c e d e p i l i m n e t i c a l k a l i n i t i e s , t h r o u g h b i o t i c p r o c e s s i n g o f n i t r o g e n a d d i t i o n s . The a l k a l i n i t y l o s s d i d n o t a f f e c t pH l e v e l s i n F, but r e d u c e d t h e e p i l i m n e t i c pH i n AF by 0.4 u n i t s r e l a t i v e t o A. F e r t i l i z a t i o n - i n d u c e d a l k a l i n i t y l o s s e s r e d u c e d t h e b u f f e r i n g c a p a c i t y o f AF c y l i n d e r w a t e r s , i n c r e a s i n g t h e s e n s i t i v i t y o f pH l e v e l s t o s u b s e q u e n t a c i d a d d i t i o n s . E n c l o s u r e o f w a t e r i n c r e a s e d l i g h t e x t i n c t i o n due t o f a c t o r s o t h e r t h a n c h l o r o p h y l l , p r o b a b l y l i m n o c o r r a l s h a d i n g . E x p e r i m e n t a l a c i d i f i c a t i o n d i d n o t a f f e c t t h e a b i o t i c component of l i g h t e x t i n c t i o n , but d i d i n c r e a s e t r a n s p a r e n c i e s s l i g h t l y r e l a t i v e t o t h e c o n t r o l s . F e r t i l i z a t i o n d e c r e a s e d t r a n s p a r e n c y i n b o t h t r e a t m e n t s F and AF, but t o a much g r e a t e r e x t e n t i n AF, where S e c c h i d e p t h s d e c r e a s e d 2 m between A u g u s t 9 and 23. 70 3.2 Z o o p l a n k t o n In t h i s s e c t i o n , I d e s c r i b e t h e b i o m a s s f l u c t u a t i o n , s p e c i e s s u c c e s s i o n , and v e r t i c a l d i s t r i b u t i o n o f 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 w i t h i n t h e e x p e r i m e n t a l e n c l o s u r e s . The c o m m u n i t i e s a r e compared b o t h t o one a n o t h e r and t o E u n i c e L a k e . The z o o p l a n k t o n c o m m u n i t i e s c o n s i s t e d a l m o s t e x c l u s i v e l y o f c r u s t a c e a n s w i t h i n t h e s u b c l a s s e s C l a d o c e r a and Copepoda. The most i m p o r t a n t c l a d o c e r a n s p e c i e s ( i n o r d e r o f mean b i o m a s s a c r o s s a l l t r e a t m e n t s ) were D a p h n i a r o s e a , C h y d o r u s s p h a e r i c u s , H o l o p e d i u m gibberurn, Bosmina l o n g i r o s t r i s , Diaphanosoma  b r a c h y u r u m , and C e r i o d a p h n i a p u l c h e l l a . The two o t h e r i m p o r t a n t s p e c i e s were c a l a n o i d c o p e p o d s , D i a p t o m u s t y r r e l l i and D i a p t o m u s  k e n a i , w h i c h t o g e t h e r a v e r a g e d 20% o f t h e t o t a l z o o p l a n k t o n b i o m a s s a c r o s s a l l s a m p l e s . O t h e r c r u s t a c e a n z o o p l a n k t o n s p e c i e s commonly i d e n t i f i e d i n t h e samples were t h e c l a d o c e r a n s Polyphemus p e d i c u l u s and S c a p h o l e b e r i s k i n q i , t h e c a l a n o i d c o p e p o d D i a p t o m u s l e p t o p u s and t h e c y c l o p o i d c o p e p o d D i a c y c l o p s  t h o m a s i , b u t t h e s e made n e g l i g i b l e c o n t r i b u t i o n s t o t o t a l z o o p l a n k t o n b i o m a s s . R o t i f e r s were n e v e r a b u n d a n t , w i t h o n l y K e r a t e l l a s p . r e a c h i n g d e n s i t i e s a s h i g h a s 2 p e r l i t e r , and t h a t on o n l y one o c c a s i o n . L a r v a e o f t h e genus C h i r o n o m u s were o c c a s i o n a l l y f o u n d i n s a m p l e s from c y l i n d e r s r e c e i v i n g f e r t i l i z e r a d d i t i o n s , e s p e c i a l l y i f t h e s a m p l i n g hose a c c i d e n t a l l y b r u s h e d t h e s i d e o r b o t t o m o f t h e c y l i n d e r . S i n c e t h e r e s u l t s i n t h i s s e c t i o n a r e p r e s e n t e d by t r e a t m e n t , r a t h e r t h a n by s p e c i e s , a few comments a r e n e c e s s a r y t o f a m i l i a r i z e t h e r e a d e r w i t h b o t h t h e l o c a t i o n o f t a b l e s and 71 f i g u r e s , and t h e i r i n t e r p r e t a t i o n . The c a l c u l a t e d b i o m a s s o f t h e e i g h t m a j o r s p e c i e s l i s t e d above a r e shown f o r a l l c y l i n d e r s i n F i g u r e 10, and expanded i n t o a g r a p h o f p e r c e n t a g e c o m p o s i t i o n i n F i g u r e 11. S t a t i s t i c a l a n a l y s e s o f t o t a l z o o p l a n k t o n b i o m a s s a r e summarized i n T a b l e 10, f o r e a c h d a t e a c r o s s a l l t r e a t m e n t s . A n a l y s e s o f v a r i a n c e o f t h e abu n d a n c e , egg p r o d u c t i o n and c l u t c h s i z e s o f t h e m a j o r s p e c i e s a r e shown f o r p a r t i c u l a r t r e a t m e n t c o m p a r i s o n s ( a c r o s s a l l d a t e s ) i n T a b l e 11. Some o f t h e numbers on w h i c h t h e s e a n a l y s e s a r e b a s e d a r e shown g r a p h i c a l l y i n F i g u r e s 12 t o 17. When e x a m i n i n g t h e g r a p h s of D. t y r r e l l i and D. k e n a i abundance and egg p r o d u c t i o n ( F i g u r e s 14 t o 16), i t i s i m p o r t a n t t o c o n s i d e r t h e l i f e h i s t o r i e s o f t h e s e d i a p t o m i d c o p e p o d s . Though not documented p r e c i s e l y , t h e l i f e h i s t o r i e s g e n e r a l l y c o n f o r m t o t h e p a t t e r n s d e s c r i b e d by Chapman a n d G r e e n ( u n p u b l i s h e d d a t a ) and summarized by O l e n i c k ( 1 9 8 2 ) . I n E u n i c e L a k e , D. t y r r e l l i n a u p l i i emerge f r o m r e s t i n g eggs i n A p r i l o r May and r e a c h a d u l t h o o d by J u l y o r A u g u s t . T h e s e a d u l t s p r o d u c e s u b i t a n e o u s e g g s , y i e l d i n g a s e c o n d g e n e r a t i o n w i t h i t s peak o f a d u l t s i n September o r O c t o b e r . The l a r g e r D i aptomus k e n a i h a t c h from e g g s ( l i k e l y b o t h s u b i t a n e o u s and r e s t i n g e g g s ) i n December, r e a c h i n g r e p r o d u c t i v e m a t u r i t y by A p r i l . In May and J u n e , t h e s e a d u l t s p r o d u c e e i t h e r s u b i t a n e o u s eggs ( w h i c h m a t u r e i n t o a d u l t s by A u g u s t and September) o r r e s t i n g eggs ( w h i c h o n l y h a t c h i n l a t e A u g u s t o r S e p t e m b e r ) . The d i a p t o m i d c o p e p o d p o p u l a t i o n s i n a l l o f t h e e x p e r i m e n t a l c y l i n d e r s a p p e a r e d t o c o n f o r m t o t h e s e g e n e r a l i z e d l i f e h i s t o r y p a t t e r n s . Copepod 72 n a u p l i i d e n s i t i e s ( n o t i d e n t i f i e d t o s p e c i e s b u t p r o b a b l y D. t y r r e l l i ) d e c l i n e d t o l e s s t h a n 10 p e r 100 L by m i d - A u g u s t ( F i g u r e 1 6 ) . D. t y r r e l l i d e n s i t i e s were h i g h l y v a r i a b l e , b ut showed a c o n s i s t e n t t r a n s i t i o n f r o m c o p e p o d i t e s t o a d u l t s o v e r A u g u s t , w i t h v e r y few c o p e p o d i t e s p r e s e n t a f t e r September 13 ( F i g u r e 1 4 ) . D. k e n a i a l s o moved from t h e c o p e p o d i t e t o a d u l t s t a g e d u r i n g J u l y and A u g u s t ; I f o u n d v i r t u a l l y no D. k e n a i c o p e p o d i t e s a f t e r A u g u s t 30. L o o s e c a l a n o i d eggs ( n o t i d e n t i f i e d t o s p e c i e s ) showed h i g h e s t abundance i n l a t e A u g u s t and September, c o i n c i d e n t w i t h t h e s e x u a l m a t u r a t i o n o f b o t h d i a p t o m i d s ( F i g u r e 1 6 ) . C l a d o c e r a n s , u n l i k e c o p e p o d s , do n o t show d i s t i n c t l y s e p a r a b l e c o h o r t s , and r e p r o d u c e a s e x u a l l y t h r o u g h o u t s p r i n g and summer. M a l e s and s e x u a l e p h i p p i a l eggs a p p e a r i n t h e f a l l , u s u a l l y c o i n c i d i n g w i t h a d e c l i n e i n f o o d c o n d i t i o n s o r t e m p e r a t u r e . A l t h o u g h l e n g t h s o f t h e a n i m a l s d i d v a r y b o t h between t r e a t m e n t s and s e a s o n a l l y ( s e e A p p e n d i x E) t h e e f f e c t s o f t h e s e v a r i a t i o n s on b i o m a s s were n e g l i g i b l e r e l a t i v e t o f l u c t u a t i o n s i n d e n s i t i e s . I t h e r e f o r e make r e f e r e n c e t o s t a t i s t i c a l a n a l y s e s o f s p e c i e s abundance t o e x p l a i n v a r i a t i o n s i n b i o m a s s . 3.2._1_ E f f e c t s o f E n c l o s u r e R e p l i c a t i o n w i t h i n t h e c o n t r o l c y l i n d e r s was e x c e l l e n t ( F i g u r e 1 0 ) . The m a j o r e f f e c t s o f e n c l o s u r e ( a s d e t e r m i n e d by c o m p a r i n g t h e c o n t r o l s t o t h e l a k e ) were a r a p i d d e c l i n e o f H o i o p e d i u r n g i b b e r u m , c h a n g e s i n t h e s e a s o n a l d y n a m i c s o f D a p h n i a 73 F i g u r e 10. C u m u l a t i v e b i o m a s s o f major z o o p l a n k t o n s p e c i e s i n 1979 e x p e r i m e n t . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 75 F i g u r e 11. P e r c e n t c o m p o s i t i o n o f t o t a l z o o p l a n k t o n b i o m a s s i n 1979 e x p e r i m e n t . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 76 Table 10. Mean zooplankton community biomass by treatment and date. Probability of no treatment effect in column labelled "P" obtained from analyses of variance on total biomass values (untransformed unless stated). Results of Tukey multiple range tests show treatments listed in ascending order of biomass from left to right. Underlining indicates homogeneous subsets i.e. treatments are not significantly different from one another at stated probability level. Zooplankton Community Biomass by Tukey Significant Differences Treatment (X±SE)in yg/L n = 2 replicates/treatment Date P C F A AF LK a = 0.10 a = 0.05 July 16 96±19 49± 6 54± 3 76± 7 26 56± 9 76±20 61 ±10 59±0.3 Aug. 1 76± 9 117+17 109±22 120± 4 9 65±18 89±29 100±16 85±16 16 65± 5 73± 8 51± 2 24±12 23 34± 9 99±11 38± 8 41 ±19 30 20± 3 98±28 22± 3 43±14 Sept. 6 33±21 66±21 15± 1.4 225±36 13 18± 8 76±11 32±15 101±27 27 19± 3 76± 5 15± 1.2 132±33 Oct. 11 9±0.3 48±25 11± 2** 58±30 All Dates (2-way 45± 6 79± 6 46+ 7 87±13 ANOVA) * 0.10 ns ns 47 0.67 ns ns 77 0.29 ns ns 31 0.44 ns ns 61 0.06 AF-A-LK-C-F ns 61 0.09 C-A-AF-LK-F ns 24 0.10 C-A-LK-AF-F ns 48 0.01 A-C-LK-F-AF A-C •LK-F-AF 30 0.09 C-LK-A-F-AF ns 30 0.03 same as a=0.05 A-C •LK-F-AF 83 0.25h ns ns \9±6 (10"5)H C-A-LK-AF-F C-A •LK-AF-F * Sample destroyed. ** Sample from October 5 used for 6C since Oct. 11 sample lost. h Variances heterogeneous at p < 0.05. H Variances heterogenous at p< 0.01. ns Differences not significant at stated a level. Table 11. Two-way analyses of variance on lop-transformed zooplankton species abundance, egg production, and brood size per ovigerous female. If p < 0.05, the treatment listed is the one that was sicmificantly hiaher. See Materials and Methods for explanation of log transform applied to brood sizes,, Number in brackets is probability of significant treatment effect (Fi 5n for C vs. LK; F i j 2 2 for all other comparisons). Most time effects (Fi . 0 , 2 2 ) were significant at p < 0.01. Species and Variable C vs. LK Treatment Comparison C vs. A C vs. F F vs. AF A vs. AF D. kenai: log (#) log(# eggs) log (brood size) ns(0.11)h ns(O.ll) ns(0.36) ns ns ns (0.54)H (0.7) (0.94) ns(0.86)h ns(0.96) ns(0.79) ns ns ns (0.058) (0.38) (0.46) AF(0.0002) ns(0.2) ns(0.2) D. tyrrelli: log (#) log(# eggs) log(brood size) ns(0.2)h ns(0.09) ns(0.25) ns ns ns (0.06) (0.36) (0.94) F(0.05) F(0.002)i ns(0.08) F ns ns (0.006)IH [0.64)i r0.24)I ns(0.9) AF(10"s)h AF(0.01) D. rosea: log(#) log(# eggs) log(brood size) LK(0.001)IH LK(0.006) ns(0.46) ns ns ns (0.68) [0.76) (0.32) F(10"5)IH F(10"5) F(0.014) F( F ns 10"5)IH 0.0009)H 0.19) A(10"5)i AF(0.03) ns(0.07)h B. longirostris: log(#) log(# eggs) log(brood size) C(0.08) C(0.0004) ns(0.10) CI C ns '0.05) (0.02) 0.20) F(10"5)Ih F(10"5) ns(0.07) ns AF ns 0.84) 0.03) 0.08) AF(10'5)IH AF(10"5)H ns(0.14) C. sphaericus: log(#) C(10"5)IH ns 0.28) F(0.02) ns 0.83) ns(0.06) D. brachyurum: log(#) C(0.01) ns (0.88) F(0.0005) AF (0.004)H AF(10_5)iH C. pulchella: log(#) C(10_5)IH ns (0.10)H F(0.02)H AF (0.002)H AF(10"5)IH Calanoid eggs: log(#) C(0.01) nsi 0.57)H ns(0.12) AF( 0.004) AF(10"5) Copepod nauplii: log(#) ns(0.13) C( 0.04) ns(0.10) nsl 0.46) AF(0.0036) ns Not significant at a = 0.05. h Variances heterogeneous at a = 0.05. H Variances heterogeneous at a = 0.01. i Treatment * time interaction significant at a = 0.05. I Interaction significant at a = 0.01. - 4 0 0 79 F i g u r e 12. D e n s i t i e s o f D a p h n i a r o s e a by age c l a s s . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 80 1979 DAPHNIA AGE C L A S S E S PUUUBUUUUU1 FoWoVAwa DAPHNIA ADULTS DAPHNIA JUVENILES DAPHNIA EGGS o o U J > LAKE | OCT. O O J 0 0 0 H ] © ±rrrfft - ooooooooooo . O O O O O O O O O O O O l p o o o o o o o.o oooooooooooooooo nnnnnnnnr... „ ~ OO go O O O BfT. .Ji 26 JULT 16 2 3 RUG. 13 SEPT. | OCT. O O > "obit*— e"o°o0°oooV I n n n n r n n n n r n n n n r n r n n n n n f i n * h — -^j tl •oooooo ooeoooQO oo on oo ooooo&s, _ _*a n — « r ~. n r> rff TVfi fflifn /•, IV 0 ( u u i M 4 m a ( j i a n a j 3 ( w u f i j u u u u i 16 2 6 JULT 1 S 16 RUG. 23 30 13 SEPT. 5 11 OCT. 16 2 3 2 6 1 9 16 23 30 6 13 27 11 16 26 1 9 16 23 30 6 13 27 11 JULT | RUG. | SEPT. | OCT. JULT | RUG. | SEPT. | OCT. A > - SSSSSSSSSSS oc ocfcei [o oo oo o o o o oo oo oo o o o o °_o.o_°-0_0°_c,. \ ^o^ass--q o o o oT^  v.'v.'v^v 16 26 1 JULT | 16 23 RUG. 13 SEPT. 27 11 | OCT. 21 ZD (_) oooooo. I .——uwuuuuO OOO 0 OG • Xjowwooooooooooooooooal*. - V. loooooooooooooooooooo ooocrtrv* IbOOOOOaoODOOOCiODOODOOOC 7 E S -I) ooo ooo oooooooooooooooo ip ooo oo o o o o o oooooooooooooo o oo oo fOOOIlOncnyiinnijYnnnfifftnnynn^nnnr,' 2 6 JULY 16 23 RUG. 13 SEPT. 27 1! | OCT. ) 6 26 1 9 16 23 30 6 13 27 U 16 26 ] 9 I E 23 30 6 13 27 11 JULT | AUG. | SEPT. | OCT. JULT | AUG. | 5EPT. | OCT. SAMPLING DATE SAMPLING DATE 81 F i g u r e 13. Daphnia rosea r e p r o d u c t i o n : % o v i g e r o u s females and mean brood s i z e s . Number of o v i g e r o u s females i n d i c a t e d i f l e s s than t e n . A b b r e v i a t i o n s used f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . BROOD SIZE I ; OVIG. BROOD SIZE & ; OVIG. BROOD SIZE i. X OVIG. BROOD SIZE I X OVIG. 0 - at t 85 (0 TJ z o 8* 3) O O C CI H O z BROOD SIZE t, % OVIG. BROOD SIZE I % OVIG. BROOD SIZE I % OVIG. BROOD SIZE i. J OVIG. BROOD SIZE I % OVIG. 83 i Figure 14. Densi t ies of Diaptomus t y r r e l l i . Graphs show attached eggs only; loose eggs in Figure 16. Abbreviations used for treatments: C=controls; F = f e r t i l i z a t i o n only; A=ac id i f i ca t ion only; AF=ac id i f i ca t ion and f e r t i l i z a t i o n . 84 ISO 1979 DIAPTOMUS TYRRELLI _) too o o 490 + + ADULTS X- -X COPEPODITES PER 300 EGGS * ISO 7S0 O O Qi UJ Q_ n i i d © k * - " X v " . ""v. ••»--—•».—i i , »- *r 750 600 —I o 450 o — 300 UJ # ISO 0 ISO 600 _J O 450 o 300 UJ a. » ISO 0 TS0 600 o « 0 o Di 11 i 300 D_ 150 2326 1 JULT | E © 16 23 DUG. 30 6 13 SEPT. 21 -1J I OCT. v.';-ii-23 26 ] JULT | 9 16 23 30 6 13 RUG. | SEPT. 27 1) I OCT. 26 1 9 16 23 30 6 13 J A T | BUG. | SEPT. 27 II I OCT. I D © 19*1-— - ' J " - J ! : - - v-r--: 26 I 9 16 23 30 6 ]3 27 II JULT | DUG. | SEPT. | OCT. SRMPLING DATE LAKE A . 16 26 I 9 16 23 30 6 13 20 JULT | DUG. | SEPT. n n I OCT. TS0 COO — I o 450 o Qi 300 UJ tt >so I ] © \ V • K - - » - . o o Qi UJ 16 26 I 9 16 23 30 6 13 JULT | RUG. | SEPT. i 5 II OCT. -T — 16 26 1 JULT | M A . 16 23 30 6 13 (UG. | SEPT. 27 II I OCT. O o UJ 26 I 9 16 23 30 6 13 JULT | 27 II 1 OCT. 776 o o Qi UJ Q. * 300 i AFV '• 26 I 9 16 23 30 6 13 27 1] JLT | RUG. | SEPT. | OCT. SAMPLING DATE 85 Figure 15. Densi t ies of Diaptomus kenai . Graphs show attached eggs only; loose eggs in Figure 16. Abbreviations used for treatments: C=controls; F = f e r t i l i z a t i o n only; A=ac id i f i ca t ion only; AF=ac id i f i ca t ion and f e r t i l i z a t i o n . 86 BO 1979 DIAPTOMUS KENAI _ J 64 o o 48 + -+ ADULTS •"^ cm 32 X X COPEPODITES U J * 16 o • EGGS LAKE —=«---r G ~Jt 26 I 9 16 23 30 6 13 20 21 11 JULT | BUG. | SEPT. | OCT. m© K >' v •& ^ Q 16 23 26 1 JULT | 16 23 30 6 13 | OCT. CD© / \ 16 23 26 1 JULT | 16 23 30 6 13 21 11 BUG. I SEPT. | OCT. 26 1 9 16 23 30 6 13 JULT | BUG. | SEPT. 21 11 | OCT. 13J (AF 26 1 9 16 23 30 6 13 21 II JULT | RUG. | SCPI. I OCT. SAMPLING DATE _ ) 64 o o 46 en | , | 32 Q _ » 16 0 SO 64 _ l O O 48 | ! | 32 Q _ =» 16 0 B0 _ J 64 O O 48 32 U J 16 0 80 64 _ i o o 48 1 OL 11 | 32 # 16 m© V - ^ . ^ T Y 16 26 1 jur I a © 16 23 30 6 13 RUG. I SEPT. 21 5 11 | OCT. 16 26 1 JULT | 16 23 30 6 13 21 11 RUG. I SEPT. I OCT. 16 26 1 JULT | s © /\ 1 i \ : i \: i \* i < ' .' i 16 23 30 6 13 RUG. I SEPT. 21 11 | OCT. > i W ! \ / I W ' -16 26 1 9 16 23 30 6 13 21 11 JULT | RUG. | SEPT. | OCT. SAMPLING DATE 87 F i g u r e 16. D e n s i t i e s o f l o o s e c a l a n o i d c o p e p o d eggs and n a u p l i i . N o te t h a t l o g a r i t h m i c s c a l e d i f f e r s s l i g h t l y between g r a p h s . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 88 1979 COPEPOD EGGS • NAUPLII + -+ LOOSE CALANOID EGGS x- x COPEPOD NAUPLII o o UJ 0_ LAKE 7< — _ x 26 1 J I 16 23 30 6 13 20 27 11 BUG. | SEPT. | OCT. O O U J Q_ # m © 16 26 1 JULT | 16 23 30 6 13 BUG. | SEPT. 27 11 I OCT. O o 100 i -U J Q_ # 10 in© — * 16 26 1 9 16 23 30 6 13 JULT | BUG. | SEPT. 27 5 | 0 O o UJ B © 2326 I 9 16 23 30 6 13 27 11 JULT | BUG. | SEPT. | OCT. O O — ' 100 C C U J C L # s © X--K' V,' 16 26 1 JULT | 16 23 30 6 13 27 11 BUG. | SEPT. | OCT. O O CC U J O' o 1 0 « 0 0 1000 100 a ® * -X x- \ X \ / \ / U J Q_ 26 1 . 9 16 23 30 6 13 JULT | BUG. | SEPT. 27 11 | OCT. • -X =>x x^ 26 1 9 16 23 30 6 13 27 11 JULT | BUG. | SEPT. | OCT. SAMPLING DATE o o en U J C L > h X,' 16 26 1 JULT | 16 23 30 6 13 AUG. | SEPT. 27 11 | OCT. 4000 1000 O O en U J Q . 0 too f - - _x-' - x \ / 16 26 1 9 16 23 30 6 13 27 II JULT | BUG. | SEPT. | OCT. SAMPLING DATE 89 Figure 17. Densi t ies of Bosmina l o n g i r o s t r i s . Note that logarithmic scale d i f f e r s s l i g h t l y between graphs. Abbreviations used for treatments: C=controls; F = f e r t i l i z a t i o n only; A=ac id i f i ca t ion only; AF=ac id i f i ca t ion and f e r t i l i z a t i o n . 90 1979 BOSMINA LONGIROSTRIS + + FEMALES x -x JUVENILES EGGS 16 26 1 JULT | 16 23 30 6 13 RUG. | SEPT. SAMPLING DATE 21 II | OCT. 16 26 1 JULT | 16 23 30 6 13 27 I I RUG. | SEPT. | OCT. SAMPLING DATE 91 r o s e a , r e p l a c e m e n t o f D a p h n i a r o s e a by C h y d o r u s s p h a e r i c u s a s community d o m i n a n t , and an e a r l i e r "shutdown" o f t h e z o o p l a n k t o n community. E n c l o s u r e a l s o a f f e c t e d t h e l e n g t h s o f some z o o p l a n k t o n s p e c i e s . H. g i b b e r u m o f t e n h as d i f f i c u l t i e s s u r v i v i n g i n l i m n o c o r r a l s d u r i n g warm-water c o n d i t i o n s (W.E. N e i l l , p e r s . comm.; Buckingham 1978). The l o s s o f H. g i b b e r u m ( i n a l l t r e a t m e n t s ) was not a t t r i b u t a b l e t o my 1 0 m e n c l o s u r e s b e i n g t o o s h a l l o w f o r t h i s s p e c i e s t o r e a c h optimum d e p t h , s i n c e s t r a t i f i e d samples on A u g u s t 1 showed H. g i b b e r u m c o n c e n t r a t e d i n t h e u p p e r 2 m o f c y l i n d e r s 1F, 8F, 4A, and 7AF ( t h e o n l y e n c l o s u r e s w i t h s u f f i c i e n t numbers o f a n i m a l s t o c a l c u l a t e a v e r t i c a l d i s t r i b u t i o n ) . A l s o , H. g i b b e r u m s u f f e r e d a s i m i l a r d e c l i n e i n a n o t h e r l i m n o c o r r a l e x p e r i m e n t c o n d u c t e d i n E u n i c e L a k e i n 1979, b u t w i t h 15 m e n c l o s u r e s ( R . J . O l e n i c k , p e r s . comm.). S i n c e t h e l a k e was n o t sample d on J u l y 16, I c a n n o t be c e r t a i n w h ether a l l o f t h e s p e c i e s were p r o p e r l y s t o c k e d i n t h e e n c l o s u r e s . However, t h e J u l y 26 samples s u g g e s t t h a t t h e r e p l a c e m e n t o f H. g i b b e r u m by D a p h n i a r o s e a may have been p a r t i a l l y due t o i n i t i a l u n d e r r e p r e s e n t a t i o n o f H. g i b b e r u m a n d o v e r r e p r e s e n t a t i o n o f D. r o s e a ( F i g u r e s 10, 11 a n d 1 2 ) . (Note t h a t t h e mean w e i g h t o f a d u l t H. g i b b e r u m u s e d i n bi o m a s s c a l c u l a t i o n s was t w i c e t h a t o f D. r o s e a ) . Peak b i o m a s s l e v e l s o f D. r o s e a i n t h e c o n t r o l c y l i n d e r s ( A u g u s t 1 i n 6C, A u g u s t 9 i n 2C) were about d o u b l e t h e b i o m a s s o f l a k e D. r o s e a on A u g u s t 1. T h i s was due e i t h e r t o i n i t i a l o v e r s t o c k i n g o f D. r o s e a , o r an i n c r e a s e i n r e s o u r c e s c a u s e d by t h e c o l l a p s e o f H. g i b b e r u m , 92 o r b o t h f a c t o r s . However, D. r o s e a c o u l d n o t m a i n t a i n t h i s b i o m a s s i n t h e c o n t r o l c y l i n d e r s , a s e v i d e n c e d by low egg d e n s i t i e s on t h e peak d a t e s and s u b s e q u e n t s h a r p d e c r e a s e s i n abundance ( F i g u r e 1 2 ) . In t h e l a k e , D. r o s e a d e n s i t i e s o s c i l l a t e d w i t h a p e r i o d o f a b o u t t h r e e weeks, t h e t r o u g h s o f t h e o s c i l l a t i o n s o c c u r r i n g A u g u s t 9, A u g u s t 30, and September 20, w i t h maximum d e n s i t i e s on O c t o b e r 11 ( F i g u r e 1 2 ) . T h e r e i s a s t r i k i n g d i f f e r e n c e between t h e c o l l a p s e o f t h e c o n t r o l c y l i n d e r p o p u l a t i o n s i n September and t h e s h a r p r e c o v e r y of t h e l a k e ' s p o p u l a t i o n . S t a t i s t i c a l a n a l y s i s c o n f i r m e d t h a t D. r o s e a p o p u l a t i o n s i n t h e c o n t r o l s b e h a v e d d i f f e r e n t l y f r o m t h e E u n i c e L a k e p o p u l a t i o n . When a l l c o u n t s o f D. r o s e a made o v e r t h e t h r e e month e x p e r i m e n t a r e a n a l y z e d t o g e t h e r , l a k e p o p u l a t i o n s showed s i g n i f i c a n t l y h i g h e r g e o m e t r i c mean d e n s i t y and egg d e n s i t i e s t h a n t h e c o n t r o l s ( T a b l e 1 1 ) . Mean b r o o d s i z e s were n o t s i g n i f i c a n t l y d i f f e r e n t however ( T a b l e 1 1 ) . As w e l l a 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 d i f f e r e n c e s i n abundance ( i . e . a t r e a t m e n t e f f e c t ) , t h e c o n t r a s t i n D. r o s e a t e m p o r a l p a t t e r n s between t h e l a k e and t h e c o n t r o l s p r o d u c e d a s i g n i f i c a n t (p < 0.01) t r e a t m e n t x t i m e i n t e r a c t i o n . T h i s d i f f e r e n c e i n t e m p o r a l p a t t e r n s may have been p a r t i a l l y due t o f i s h p r e d a t i o n c o n t r o l l i n g t h e s i z e o f t h e l a k e p o p u l a t i o n . W i t h t h e d e c l i n e o f D. r o s e a i n t h e c o n t r o l c y l i n d e r s a t t h e end o f A u g u s t , t h e g e n e r a l l y l i t t o r a l c l a d o c e r a n , C h y d o r u s  s p h a e r i c u s , became d o m i n a n t i n t e r m s o f b i o m a s s ( F i g u r e 1 1 ) . C. s p h a e r i c u s d e n s i t i e s were s i g n i f i c a n t l y h i g h e r i n t h e c o n t r o l 93 c y l i n d e r s t h a n i n t h e l a k e ( T a b l e 1 1 ) . When t e s t e d t o g e t h e r w i t h o t h e r t r e a t m e n t s u s i n g a Tukey m u l t i p l e r a n g e t e s t , t h e a r i t h m e t i c mean z o o p l a n k t o n b i o m a s s i n t h e c o n t r o l s (45 ± 2 1 /zg»L" 1) was n o t s i g n i f i c a n t l y d i f f e r e n t f r o m t h e l a k e (49 ± 15 uq»L~ 1) ( T a b l e 1 0 ) . The most o b v i o u s d i f f e r e n c e between t h e l a k e a n d t h e c o n t r o l s was t h e l o n g e r p e r s i s t e n c e o f t h e l a k e z o o p l a n k t o n community t h r o u g h t h e autumn. Between September 27 and O c t o b e r 11, t h e mean z o o p l a n k t o n b i o m a s s o f t h e c o n t r o l s f e l l f r o m 19 ± 3 t o 9 ± 0.3 M g « L " 1 , w h i l e t h e l a k e b i o m a s s grew f r o m 30 t o 83 u g » L " 1 ( F i g u r e 1 0 ) . E n c l o s u r e a f f e c t e d t h e l e n g t h s o f t h e l a r g e r c r u s t a c e a n s , p r o b a b l y due t o t h e r e m o v a l o f c u t t h r o a t t r o u t s i z e - s e l e c t i v e p r e d a t i o n . The mean l e n g t h s o f a d u l t and j u v e n i l e D. r o s e a were c o n s i s t e n t l y l o w e r i n E u n i c e Lake t h a n i n t h e c o n t r o l s ( T a b l e 12 and A p p e n d i x E ) , w h i c h s u p p o r t s t h e view t h a t s i z e - s e l e c t i v e f i s h p r e d a t i o n a f f e c t e d D a p h n i a . On September 13, t h e mean l e n g t h o f D. k e n a i a d u l t s was c o n s i d e r a b l y l e s s i n E u n i c e L a k e (1.82 ± 0.070 ( 1 0 ) ) t h a n i n t h e c o n t r o l c y l i n d e r s (1.94 ± 0.024 ( 1 4 ) ) , t h o u g h n o t s i g n i f i c a n t l y d i f f e r e n t (p=0.08). By c o n t r a s t , t h e mean l e n g t h s o f t h e s m a l l e r s p e c i e s D i aptomus  t y r r e l l i and Bosmina l o n g i r o s t r i s d i d n o t d i f f e r s i g n i f i c a n t l y between E u n i c e Lake and t h e c o n t r o l s on any o f t h e f o u r d a t e s e x a m i n e d . T h e s e o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e r e s u l t s o f N o r t h c o t e e t a l . ( 1 9 7 8 ) , who f o u n d t h a t c u t t h r o a t t r o u t (added t o E u n i c e L a k e i n 1974 and 1975) g r e a t l y p r e f e r r e d D. k e n a i and D. r o s e a t o D. t y r r e l l i and B. l o n g i r o s t r i s . I n t e r e s t i n g l y , 94 T a b l e 12. L e n g t h s o f D a p h n i a r o s e a i n E u n i c e L a k e and t h e c o n t r o l s , compared u s i n g S t u d e n t t - t e s t s on e a c h d a t e . A b b r e v i a t i o n s : C = c o n t r o l s ; L K = E u n i c e L a k e ; p = p r o b a b i l i -t y o f s i g n i f i c a n t i n t e r - t r e a t m e n t d i f f e r e n c e ( i f l e s s t h a n 0 . 1 ) ; ns=no s i g n i f i c a n t d i f f e r e n c e . D a t e T r e a t - Mean L e n g t h o f D a p h n i a r o s e a (mm)±SE (n) ment J u v e n i l e s A d u l t s J u l y 26 C 0 .80 + 0.027 (20) 1 .32 ± 0.034 (20) LK 0 .77 + 0.057 (10) 1 .24 ± 0.027 (10) ns ns Aug. 5 C 0 .83 + 0.024 (20) 1 .31 ± 0.033 (20) LK 0 .78 + 0.053 (10) 1 .21 ± 0.035 (10) ns p=0.07 Aug. 23 C 0 .95 + 0.017 (20) 1 .42 ± 0.038 (20) LK 0 .82 + 0.037 (10) 1 .25 ± 0.035 (10) P= 0.002 P=0.01 S e p t . 13 C 0 .97 + 0.030 (13) 1 .37 ± 0.038 (17) LK 0 .81 + 0.029 (10) 1 .27 ± 0.040 (10) P= 0.002 p=0.09 95 N o r t h c o t e e t a l . a t t r i b u t e d c h a n g e s i n t h e l e n g t h s o f c r u s t a c e a n s p r i m a r i l y t o i n c r e a s e d c o m p e t i t i o n between h e r b i v o r e s ( f o l l o w i n g t h e e l i m i n a t i o n o f C h a o b o r u s p r e d a t i o n ) r a t h e r t h a n t o i n c r e a s e d f i s h p r e d a t i o n . By c o n t r a s t , t h e l e n g t h c h a n g e s o b s e r v e d i n my e x p e r i m e n t s u g g e s t t h a t f i s h p r e d a t i o n d i d have a s i g n i f i c a n t e f f e c t on E u n i c e L a k e z o o p l a n k t o n . The d i f f e r e n c e between my r e s u l t s a n d t h o s e o f N o r t h c o t e e t a l . may be due t o c h a n g e s i n t h e d e n s i t y o f f i s h i n E u n i c e Lake ( h i g h e r i n 1979 t h a n 1976 (W.E. N e i l l , p e r s . comm.)). F i s h p r e d a t i o n may have a l s o a f f e c t e d t h e d e n s i t i e s o f D. r o s e a and D. k e n a i . As r e p o r t e d a b ove, D. r o s e a d e n s i t i e s were s i g n i f i c a n t l y h i g h e r i n E u n i c e Lake t h a n i n t h e c o n t r o l s . Mean d e n s i t i e s o f D. k e n a i a d u l t s and j u v e n i l e s were c o n s i d e r a b l y h i g h e r i n t h e c o n t r o l c y l i n d e r s (0.22 ± 0.028 and 0.10 ± 0.03 L - 1 r e s p e c t i v e l y ) t h a n i n E u n i c e L a k e (0.08 ± 0.011 and 0.041 ± 0.016 L " 1 ) but s t a t i s t i c a l l y n o t s i g n i f i c a n t l y d i f f e r e n t (p = 0.11; T a b l e 1 1 ) . 3.2.2 E f f e c t s o f A c i d i f i c a t i o n When c o m p a r i s o n s were made between t r e a t m e n t s A a n d C o v e r t h e e n t i r e l e n g t h o f t h e e x p e r i m e n t , t h e r e were no h i g h l y s i g n i f i c a n t (p < 0.01) s t a t i s t i c a l d i f f e r e n c e s i n a b u n d a n c e s , egg p r o d u c t i o n o r b r o o d s i z e s o f any o f t h e m a j o r s p e c i e s ( T a b l e 1 1 ) . O n l y Bosmina l o n g i r o s t r i s and c o p e p o d n a u p l i i showed d i f f e r e n c e s s i g n i f i c a n t a t p < 0.05. Though t h e c o n t r o l s showed s i g n i f i c a n t l y h i g h e r B. l o n g i r o s t r i s d e n s i t i e s and egg 96 p r o d u c t i o n (p = 0.05 and 0.02 r e s p e c t i v e l y ) , t h e v e r y low b i o m a s s o f B. l o n q i r o s t r i s i n b o t h t h e c o n t r o l s and t r e a t m e n t A ( F i g u r e 11) makes t h i s r e s u l t o f n e g l i g i b l e i m p o r t a n c e a t t h e community l e v e l . S i m i l a r l y , t h e h i g h v a r i a b i l i t y a n d low d e n s i t i e s o f n a u p l i i i n b o t h t r e a t m e n t s C and A ( F i g u r e 16) d i m i n i s h e s t h e i m p o r t a n c e o f t h e s t a t i s t i c a l l y s i g n i f i c a n t (p = 0.04) d i f f e r e n c e i n n a u p l i i between t h e s e two t r e a t m e n t s . I f a c i d i f i c a t i o n were t o c a u s e a c u t e t o x i c i t y , one would e x p e c t t o see m o r t a l i t y f o l l o w i n g t h e A u g u s t 9 and September 20 a c i d a d d i t i o n s , w h i c h g e n e r a t e d t h e s h a r p e s t pH d e c l i n e s ( F i g u r e 1 2 ) . The d e g r e e o f e x p o s u r e t o s u c h pH d r o p s v a r i e d w i t h t h e v e r t i c a l d i s t r i b u t i o n o f t h e s p e c i e s , s i n c e o n l y t h e u p p e r 4 m were s t r o n g l y a c i d i f i e d (See S e c t i o n 3.2.5). What happened a f t e r t h e A u g u s t 9 a c i d a d d i t i o n ? The m a j o r d i f f e r e n c e between t r e a t m e n t s A and C d u r i n g A u g u s t 9 t o 16 was t h e s h a r p e r d r o p 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 and D. r o s e a d e n s i t i e s i n A ( F i g u r e s 10 and 1 2 ) . S i n c e D. r o s e a a b u n d a n c e s i n A s u b s e q u e n t l y s t a b i l i z e d and were v e r y s i m i l a r t o C ( F i g u r e 12), i t seems u n l i k e l y t h a t D. r o s e a m o r t a l i t y i n A d u r i n g A u g u s t 9-16 was i n d u c e d by t h e c o n c u r r e n t d r o p i n mean pH from 6.3 t o 5.7. The D. r o s e a p o p u l a t i o n s i n A and C were n o t e x a c t l y i n p h a s e . T h i s l e a d s me t o b e l i e v e t h a t t h e o b s e r v e d d i f f e r e n c e s between A and C on A u g u s t 9 were due t o my s a m p l i n g s c h e d u l e b r a c k e t i n g t h e peak a b u n d a n c e s o f D. r o s e a i n t h e c o n t r o l s , b ut t r a c k i n g t h e p o p u l a t i o n i n A. 97 A f t e r t h e September 20 a c i d d o s e , t h e a b u n d a n c e s o f D. r o s e a ( F i g u r e 12) were t o o low t o c l e a r l y d e t e c t any i n c r e a s e d m o r t a l i t y . A v e r a g i n g t o g e t h e r f o u r a c i d i f i c a t i o n a n d f o u r c o n t r o l c y l i n d e r s a m p l e s t a k e n a f t e r September 20 r e v e a l e d l o w e r d e n s i t i e s o f D. t y r r e l l i i n A (0.19 ± 0.06 L " 1 ) t h a n C (1.00 ± 0 . 5 0 ) . The pH i n t h e u p p e r 4 m o f t h e A c y l i n d e r s a v e r a g e d 5.36 d u r i n g September 20 t o O c t o b e r 11, t h e most a c i d c o n d i t i o n s o b s e r v e d i n t h i s t r e a t m e n t . A l t h o u g h a c i d - i n d u c e d m o r t a l i t y o f D. t y r r e l l i i s a p o s s i b l e e x p l a n a t i o n o f t h e o b s e r v e d d e c l i n e s i n A c y l i n d e r s , t h e e v i d e n c e i s n o t c o m p e l l i n g due t o t h e r e l a t i v e l y low d e n s i t i e s o f a n i m a l s p r i o r t o t h e a c i d i f i c a t i o n . N u l l h y p o t h e s i s 2 ( i ) (page 41) s t a t e d t h a t g r a d u a l , e x p e r i m e n t a l a c i d i f i c a t i o n o f o l i g o t r o p h i c w a t e r s t o pH 5.0 d o e s n o t a f f e c t any z o o p l a n k t o n p o p u l a t i o n s o r community b i o m a s s . The t r e a t m e n t A c y l i n d e r s d i d n o t r e a c h pH 5.0 u n t i l t h e v e r y end o f t h e e x p e r i m e n t , so t h a t t h e n u l l h y p o t h e s i s was o n l y t e s t e d d e f i n i t i v e l y down t o pH 5.6. I t i s i m p r e s s i v e t h a t a t pH 5.6 ( a l m o s t a t e n f o l d i n c r e a s e i n [ H + ] ) t h e z o o p l a n k t o n i n t r e a t m e n t A c y l i n d e r s a p p e a r e d t o be r e p l i c a t e s o f t h e c o n t r o l s ; t h e r e were no d i f f e r e n c e s i n z o o p l a n k t o n p o p u l a t i o n s o r b i o m a s s . H y p o t h e s i s 4 ( i i ) (page 41) s t a t e d t h a t n u t r i e n t e n r i c h m e n t d u r i n g a c i d i f i c a t i o n d o e s n o t a l l e v i a t e a c i d - i n d u c e d s h o r t a g e s of p h y t o p l a n k t o n f o o d . S i n c e z o o p l a n k t o n i n t r e a t m e n t A showed no e v i d e n c e o f i n c r e a s e d f o o d l i m i t a t i o n ( r e l a t i v e t o t h e c o n t r o l s ) , t h i s h y p o t h e s i s became i r r e l e v a n t t o pH l e v e l s o f 5.6 o r h i g h e r . 98 3.2.3 E f f e c t s o f F e r t i l i z a t i o n The f u n c t i o n o f t r e a t m e n t F was f i r s t t o t e s t whether n u t r i e n t a d d i t i o n s removed f o o d s h o r t a g e s ( p a r t i c u l a r l y f o r t h e most a c i d - s e n s i t i v e z o o p l a n k t o n s p e c i e s ) and s e c o n d , t o s e r v e as a c o n t r o l f o r t h e e f f e c t s o f a c i d i f i c a t i o n i n t h e AF c y l i n d e r s . Though f e r t i l i z a t i o n had no a p p a r e n t e f f e c t on H. q i b b e r u m o r D. k e n a i , i t s i g n i f i c a n t l y b e n e f i t e d D. r o s e a , B. l o n q i r o s t r i s , D. t y r r e l l i , C. s p h a e r i c u s , and D. b r a c h y u r u m . The two f e r t i l i z a t i o n r e p l i c a t e s were v i r t u a l l y i d e n t i c a l , t h o u g h B. l o n q i r o s t r i s was c o n s i d e r a b l y more ab u n d a n t i n 1F t h a n 8F. E f f e c t s o f f e r t i l i z a t i o n on z o o p l a n k t o n b i o m a s s were most e v i d e n t i n t h e C l a d o c e r a . T h i s was e x p e c t e d g i v e n t h e i r c o n t i n u o u s r e c r u i t m e n t and r a p i d r a t e s o f d e v e l o p m e n t . D. r o s e a and B. l o n g i r o s t r i s i n c r e a s e d s i g n i f i c a n t l y o v e r t h e c o n t r o l s i n d e n s i t i e s o f b o t h a n i m a l s and eggs ( T a b l e 1 1 ) . Mean b r o o d s i z e s of D. r o s e a ( F i g u r e 13) and B. l o n g i r o s t r i s were a l s o h i g h e r under f e r t i l i z a t i o n (1.37 ± 0.07 and 1.12 ± 0.03 r e s p e c t i v e l y ) t h a n i n t h e c o n t r o l s (1.05 ± 0.13 and 0.90 ± 0.09) b u t t h e s e d i f f e r e n c e s were s t a t i s t i c a l l y s i g n i f i c a n t o n l y f o r D. r o s e a ( T a b l e 1 1 ) . D. r o s e a r e m a i n e d a b u n d a n t f o r much l o n g e r i n t r e a t m e n t F t h a n i n t h e c o n t r o l s , c a u s i n g a h i g h l y s i g n i f i c a n t t r e a t m e n t x t i m e i n t e r a c t i o n t e r m ( T a b l e 1 1 ) . C. s p h a e r i c u s a p p e a r e d i n t h e community one week e a r l i e r i n t r e a t m e n t F t h a n i n C o r A. I t s b i o m a s s and d e n s i t i e s were s i g n i f i c a n t l y g r e a t e r i n F t h a n i n t r e a t m e n t A o r t h e c o n t r o l s ( F i g u r e 10 and T a b l e 1 1 ) . 99 Copepods r e s p o n d 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 t h r o u g h more r a p i d g r o w t h and h i g h e r egg p r o d u c t i o n , w h i c h a f t e r a t i m e l a g t h a t i s d e p e n d e n t on g e n e r a t i o n t i m e and l i f e h i s t o r y , shows up i n i n c r e a s e d abundance ( s e e S m i t h 1969). F e r t i l i z a t i o n h ad s e v e r a l p o s i t i v e e f f e c t s on D. t y r r e l l i . R e l a t i v e t o t h e c o n t r o l s , t r e a t m e n t F a d u l t s showed s i g n i f i c a n t l y h i g h e r a t t a c h e d egg p r o d u c t i o n (p = 0.002; T a b l e 11) and t h e g e o m e t r i c mean d e n s i t y o f a d u l t s p l u s c o p e p o d i t e s was a l s o s i g n i f i c a n t l y h i g h e r (p = 0.05; T a b l e 1 1 ) . Over A u g u s t 23 and 30 ( i . e j u s t a f t e r t h e n u t r i e n t a d d i t i o n s ) t h e r e were much h i g h e r mean d e n s i t i e s o f l o o s e c a l a n o i d eggs i n t h e F t u b e s (8.5±2.9 e g g s » L " 1 (SE;n=4)) t h a n i n t h e c o n t r o l s (1.5±0.6 e g g s » L " 1 ) . When a n a l y z e d o v e r t h e e n t i r e e x p e r i m e n t however, t h e g e o m e t r i c mean d e n s i t i e s o f l o o s e c a l a n o i d eggs i n F were n o t s i g n i f i c a n t l y h i g h e r (p = 0.09; T a b l e 1 1 ) . S i m i l a r l y , D. t y r r e l l i mean c l u t c h s i z e s and p e r c e n t o v i g e r o u s f e m a l e s were h i g h e r on A u g u s t 23 and 30 i n F ( 2.18 ± 0.12 e g g s / f e m a l e and 5.56 ± 1.58% o v i g e r o u s f e m a l e s ) t h a n i n C (1.00 ± 0.35 e g g s / f e m a l e and 1.23 ± 0.44% r e s p e c t i v e l y ) , b u t were n o t s i g n i f i c a n t l y d i f f e r e n t when compared o v e r t h e whole s e a s o n . S i n c e l o o s e c a l a n o i d eggs were n o t i d e n t i f i e d t o s p e c i e s , I c a n n o t s a y d e f i n i t i v e l y whether D. t y r r e l l i o r D. k e n a i was r e s p o n s i b l e f o r l a t e A u g u s t i n c r e a s e s i n l o o s e e g g s . However, 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 t h e t o t a l number o f eggs c a r r i e d by D. t y r r e l l i i n F t u b e s ( r e l a t i v e t o t h e c o n t r o l s ) and no s u c h e f f e c t i n D. k e n a i (C v s . F; T a b l e 11). As t h e s e two d i a p t o m i d c o p e p o d s d i d n o t a p p e a r t o be d i f f e r e n t i a l l y p r o n e t o 100 egg l o s s i n s a m p l i n g o r l a b o r a t o r y h a n d l i n g , I s u s p e c t t h a t D. t y r r e l l i c o n t r i b u t e d more t o t h e l a r g e p o o l o f l o o s e c a l a n o i d e g g s . In summary, D. t y r r e l l i a p p e a r e d t o b e n e f i t more from f e r t i l i z a t i o n t h a n d i d D. k e n a i . Not s u r p r i s i n g l y , t h e mean z o o p l a n k t o n b i o m a s s o v e r a l l s a m p l e s was s i g n i f i c a n t l y h i g h e r i n t h e f e r t i l i z e d c y l i n d e r s t h a n i n t h e c o n t r o l s ( T a b l e 1 0 ) . I n c o n c l u s i o n , t r e a t m e n t F f a l s i f i e d n u l l h y p o t h e s i s 3 ( i i ) (page 4 1 ) , t h a t m o d e r a t e n u t r i e n t e n r i c h m e n t does n o t a f f e c t z o o p l a n k t o n p o p u l a t i o n s o r community b i o m a s s . However, e n r i c h m e n t d i d n o t s h i f t t h e o v e r a l l z o o p l a n k t o n community s t r u c t u r e . 3_.2.4 E f f e c t s o f Combined A c i d i f i c a t i o n and F e r t i l i z a t i o n The AF t r e a t m e n t had i n e s s e n c e two " c o n t r o l s " . R e l a t i v e t o t r e a t m e n t F, t h e AF c y l i n d e r s showed t h e e f f e c t s o f a c i d i f i c a t i o n upon a n u t r i e n t e n r i c h e d s y s t e m . When compared t o t r e a t m e n t A, AF i l l u s t r a t e s t h e e f f e c t s o f e n r i c h m e n t o f a c i d i f y i n g w a t e r s . I i n t e n d e d t h i s l a t t e r c o m p a r i s o n t o r e v e a l whether i n c r e a s e d f o o d r e s o u r c e s b e n e f i t t e d a c i d - s t r e s s e d z o o p l a n k t o n (page 13). However, t h e s i g n i f i c a n t l y l o w e r pH l e v e l s i n AF d e s t r o y e d one o f t h e n e c e s s a r y c o n d i t i o n s f o r e x a m i n i n g t h i s q u e s t i o n , namely, t h a t t h e a c i d s t r e s s w o u l d be e q u i v a l e n t i n b o t h f o o d l i m i t e d (A) and f o o d r i c h (AF) e n v i r o n m e n t s . Z o o p l a n k t o n would have more f o o d i n AF t h a n i n A, but would c o n c u r r e n t l y have t o d e a l w i t h g r e a t e r a c i d s t r e s s . T h i s r e p r e s e n t s aJi o v e r l y s t r o n g t e s t o f f o o d l i m i t a t i o n . A f t e r 101 p e r f o r m i n g t h e pH measurements on A u g u s t 16, I c o n s i d e r e d i n c r e a s i n g t h e a c i d a d d i t i o n s t o t h e A c y l i n d e r s t o p r o d u c e s i m i l a r pH l e v e l s t o AF. However, due to. t h e d a n g e r o f o v e r a c i d i f y i n g t h e A c y l i n d e r s ( w h i c h would weaken t h e t e s t o f f o o d l i m i t a t i o n ) , and my u n c e r t a i n t y a t t h e t i m e a s t o why pH d i f f e r e d i n A and AF, I d e c i d e d t o c o n t i n u e t o g i v e t h e same amount o f a c i d t o t h e two t r e a t m e n t s . I n e v e r t h e l e s s was a b l e t o t e s t t h e h y p o t h e s i s o f f o o d l i m i t a t i o n , by e x a m i n i n g a c i d -s t r e s s e d z o o p l a n k t o n p o p u l a t i o n s i n AF. In t h e f o l l o w i n g p a g e s , p a t t e r n s o f change 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 and c l a d o c e r a n c o m p o s i t i o n a r e d e s c r i b e d f i r s t , f o l l o w e d by a n a l y s e s o f t h e r e s p o n s e s o f t h e two d i a p t o m i d c o p e p o d s . The AF t r e a t m e n t p r o d u c e d t h e most i n t e r e s t i n g r e s u l t s o f my e x p e r i m e n t , and i s t h e r e f o r e r e p o r t e d i n c o n s i d e r a b l y g r e a t e r d e t a i l t h a n t h e o t h e r t r e a t m e n t s . (_i) T o t a l Z o o p l a n k t o n Biomass and Respo n s e o f C l a d o c e r a Changes i n community c o m p o s i t i o n i n AF c e n t r e d on c l a d o c e r a n s p e c i e s r e p l a c e m e n t s , t r i g g e r e d l a r g e l y by t h e l o s s o f D a p h n i a  r o s e a . From J u l y 16 t o A u g u s t 9 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 and b i o m a s s were e s s e n t i a l l y i d e n t i c a l i n AF and F ( F i g u r e 10; T a b l e 1 0 ) . D u r i n g t h i s p e r i o d , D. r o s e a p o p u l a t i o n s i n t h e AF and F c y l i n d e r s had shown v e r y s i m i l a r b e h a v i o u r , i n terms o f b o t h d e n s i t i e s o f a n i m a l s ( F i g u r e 12) and r e p r o d u c t i v e i n d i c a t o r s ( F i g u r e 1 3 ) . In t r e a t m e n t F, o v e r t h e f o u r s a m p l e s f r o m J u l y 16 t o A u g u s t 9, t h e mean p e r c e n t a g e o f o v i g e r o u s f e m a l e s , mean b r o o d s i z e , and mean p e r c e n t a g e o f j u v e n i l e s i n 102 t h e p o p u l a t i o n were ( r e s p e c t i v e l y ) 25 ± 4.7% o v i g e r o u s , 1.41 ± 0.10 eggs p e r f e m a l e and 52.5 ± 3.9% j u v e n i l e s . In AF c y l i n d e r s t h e s e s t a t i s t i c s were v e r y s i m i l a r : 27 ± 4.9% o v i g e r o u s , 1.54 ± 0.16 eggs p e r f e m a l e and 50.4 ± 4.1% j u v e n i l e s r e s p e c t i v e l y . However, major pH d r o p s between A u g u s t 9 and 16 ( F i g u r e 7) were a s s o c i a t e d w i t h m a s s i v e m o r t a l i t y o f D. r o s e a j u v e n i l e s and a d u l t s i n b o t h AF t u b e s ( F i g u r e 1 2 ) . I t i s i n t e r e s t i n g t h a t D. r o s e a r e d u c t i o n s were l e s s s e v e r e i n 7AF (81.7 % between A u g u s t 9 and 16) t h a n i n 3AF ( 9 8 . 4 % ) . T h i s v a r i a t i o n i n r e s p o n s e o f t h e s e two r e p l i c a t e s was n o t l i k e l y due t o d i f f e r e n c e s i n e p i l i m n e t i c pH; t h e A u g u s t 9 t o 16 pH d e c l i n e i n 7AF (6.24 ± 0.05 t o 5.34 ± 0.10) was v e r y s i m i l a r t o t h a t o f 3AF (6.24 ± 0.05 t o 5.29 ± 0.10). I t i s p o s s i b l e t h a t D. r o s e a was d i s t r i b u t e d d e e p e r i n 7AF t h a n 3AF on A u g u s t 9 a n d t h e r e f o r e l o s t fewer i n d i v i d u a l s i n t h e a c i d e p i l i m n i o n . T h i s h y p o t h e s i s was s u g g e s t e d by my s t r a t i f i e d s a m p l e s f r o m A u g u s t 1, w h i c h showed 97% of t h e D. r o s e a j u v e n i l e s and 93% o f t h e a d u l t s above 4 m i n 3AF, whereas i n 7AF t h i s s t r a t u m c o n t a i n e d o n l y 41% o f t h e j u v e n i l e s and 58% o f t h e a d u l t s . I t i s n o t c l e a r why t h e v e r t i c a l d i s t r i b u t i o n s were d i f f e r e n t on A u g u s t 1, nor wh e t h e r t h e c o n t r a s t i n d i s t r i b u t i o n s o f D. r o s e a was m a i n t a i n e d u n t i l t h e a c i d a d d i t i o n s o f A u g u s t 9. Due t o t h e c o l l a p s e o f D. r o s e a , t h e A u g u s t 16 mean t o t a l z o o p l a n k t o n b i o m a s s i n AF c y l i n d e r s (24 ± 12 M g # L " 1 ) was a b o u t one t h i r d t h a t o f t h e F c y l i n d e r s (73 ± 8 j u g ^ L " 1 ) , ( F i g u r e 10 and T a b l e 1 0 ) . Mean t o t a l z o o p l a n k t o n b i o m a s s i n AF r e m a i n e d a t 41-43% o f t h a t i n F f o r A u g u s t 23 and 30. D. r o s e a n e v e r 103 r e c o v e r e d i n 3AF, but i n 7AF a d u l t s p e r s i s t e d a t low d e n s i t i e s (0.50 ± 0.12 L _ 1 ) from A u g u s t 23 u n t i l September 13, d u r i n g w h i c h t i m e mean e p i l i m n e t i c pH v a l u e s v a r i e d f r o m 5.5 ± 0.1 t o 5.74 ± 0.01). D. r o s e a was a b s e n t however f r o m s a m p l e s t a k e n i n 7AF f o l l o w i n g t h e September 20 a c i d i f i c a t i o n , w h i c h r e d u c e d t h e mean e p i l i m n e t i c pH from 5.77 ± 0.025 t o 5.15 ± 0.05 ( s e e F i g u r e s 7 and 1 2 ) . These r e s u l t s s u g g e s t t h a t a pH o f 5.3 t o 5.4 c a u s e s a c u t e t o x i c i t y i n D. r o s e a j u v e n i l e s and a d u l t s . How d i d t h e z o o p l a n k t o n community r e s p o n d t o t h e l o s s o f D. r o s e a ? F i g u r e 14 shows an enormous i n c r e a s e i n D. t y r r e l l i a d u l t s i n c y l i n d e r 3AF between A u g u s t 16 and 23 ( f r o m 2.8 L " 1 t o 23.4 L " 1 ) and a s m a l l e r but s t i l l s u b s t a n t i a l i n c r e a s e i n 7AF on A u g u s t 30. S i n c e t h e r e were f a r t o o few c o p e p o d i t e s t o have g e n e r a t e d s u c h h i g h numbers o f a d u l t s , I b e l i e v e t h e s e r e s u l t s a r e a n o m a l o u s . The number o f D. t y r r e l l i a d u l t s i n t r e a t m e n t AF q u i c k l y d r o p p e d back down t o " r e a s o n a b l e " l e v e l s , s u g g e s t i n g t h a t t h e p e a k s on A u g ust 23 i n 3AF and A u g u s t 30 i n 7AF were l i k e l y due t o s a m p l i n g e r r o r s . The s a m p l i n g hose may have c o n t a c t e d d ense " p a t c h e s " o f D. t y r r e l l i , o r t h e a n i m a l s may have s u f f e r e d s u b l e t h a l a c i d s t r e s s and been l e s s a b l e t o e s c a p e c a p t u r e . W i t h t h e e x c e p t i o n of D. t y r r e l l i i n t h e s e two s a m p l e s , t h e m a j o r s h o r t t e r m r e s p o n s e t o t h e l o s s o f D. r o s e a was i n t h e C l a d o c e r a , not t h e c o p e p o d s . A f t e r a t i m e l a g o f a b o u t t h r e e weeks, s m a l l , o p p o r t u n i s t i c c l a d o c e r a n s showed d r a m a t i c i n c r e a s e s i n d e n s i t i e s , p a r t i c u l a r l y i n 3AF ( F i g u r e 1 0 ) . The r a p i d g r o w t h o f C h y d o r u s s p h a e r i c u s , and t o a l e s s e r e x t e n t 104 Bosmina l o n g i r o s t r i s and Diaphanosoma b r a c h y u r u m , c a u s e d a 523% i n c r e a s e i n t r e a t m e n t AF mean t o t a l z o o p l a n k t o n b i o m a s s i n t h e week between A u g u s t 30 and September 6 ( T a b l e 10); t h i s i s e q u i v a l e n t t o 26.7% d a y " 1 a s s u m i n g e x p o n e n t i a l g r o w t h . Keen(1967) f o u n d t h a t l a b o r a t o r y p o p u l a t i o n s o f C. s p h a e r i c u s i n c r e a s e d a t 23.7% d a y " 1 u n d e r h i g h f o o d c o n d i t i o n s and t e m p e r a t u r e s o f 25°C, and 15.7% d a y " 1 a t 15°C. S i n c e t h e AF p o p u l a t i o n s o f C. s p h a e r i c u s were m o s t l y f o u n d a t 5-7 m and t e m p e r a t u r e s o f 8-14°C ( S e c t i o n 3.2.5), t h e o b s e r v e d r a t e o f i n c r e a s e was e x c e p t i o n a l l y h i g h . A s h i f t i n t h e d i s t r i b u t i o n o f C. s p h a e r i c u s f r o m t h e s i d e s o f t h e e n c l o s u r e s t o t h e c e n t r e may have been p a r t i a l l y r e s p o n s i b l e f o r t h e v e r y l a r g e i n c r e a s e (See d i s c u s s i o n i n S e c t i o n 4.4.1 ( i v ) ) . In s p i t e o f t h e s e s h a r p b i o m a s s i n c r e a s e s i n AF t u b e s , o v e r a l l g e o m e t r i c mean d e n s i t i e s o f B. l o n g i r o s t r i s were n o t s i g n i f i c a n t l y d i f f e r e n t between t r e a t m e n t s AF and F ( F i g u r e 17 and T a b l e 1 1 ) . The same was t r u e f o r C. s p h a e r i c u s a l t h o u g h mean d e n s i t i e s d u r i n g A u g u s t 30 t o O c t o b e r 11 were s e v e n t i m e s h i g h e r i n AF t h a n F (75 ± 32 L " 1 and 10.3 ± 2.5 L " 1 r e s p e c t i v e l y ) . N ote t h a t C. s p h a e r i c u s f l u c t u a t i o n s i n b o t h AF c y l i n d e r s were o u t o f phase w i t h o t h e r s m a l l c l a d o c e r a n s d u r i n g most o f September ( F i g u r e 1 1 ) . I d i d f i n d 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 AF and F i n g e o m e t r i c mean d e n s i t i e s o f D. b r a c h y u r u m and C e r i o d a p h n i a p u l c h e l l a . T h e s e two c l a d o c e r a n s were r a r e i n b o t h F c y l i n d e r s , common i n 7AF d u r i n g September a n d abu n d a n t i n 3AF on s e v e r a l s a m p l i n g d a t e s . (D. b r a c h y u r u m r e a c h e d peak d e n s i t i e s o f 21.3 L " 1 on September 105 13 i n 3AF.) C y l i n d e r 3AF showed h i g h e r t o t a l b i o m a s s t h a n 7AF between September 6 and 27 ( F i g u r e 1 0 ) . T h i s d i f f e r e n c e was p r i m a r i l y due t o h i g h e r p o p u l a t i o n s o f B. l o n g i r o s t r i s , D. b r a c h y u r u m and C. p u l c h e l l a i n 3AF, t h o u g h mean d e n s i t i e s o f C h y d o r u s  s p h a e r i c u s d u r i n g September t e n d e d t o be h i g h e r i n 3AF (91 ± 37 a n i m a l s « L - 1 ) t h a n i n 7AF (47 ± 1 9 L " 1 ) a s w e l l . The v a r i e d r e s p o n s e i n t h e AF r e p l i c a t e s may be i n d i r e c t l y due t o d i f f e r e n c e s i n t h e A u g u s t 9-16 D. r o s e a d e c l i n e s ( t o be d i s c u s s e d i n S e c t i o n 4 . 4 ) . In c o n t r a s t t o t h e r e l a t i v e l y c o n s t a n t t o t a l z o o p l a n k t o n b i o m a s s o f t h e F t u b e s , AF t o t a l z o o p l a n k t o n b i o m a s s showed a l a r g e a m p l i t u d e 3 week f l u c t u a t i o n d u r i n g September, w i t h p e a k s on September 6 and 27 ( F i g u r e 1 0 ) . By O c t o b e r 11, t h e t o t a l b i o m a s s i n AF was a g a i n s i m i l a r t o F (58 ± 30 and 48 ± 25 jxg»L~ 1 r e s p e c t i v e l y ) . The a r i t h m e t i c mean t o t a l b i o m a s s i n AF o v e r t h e whole e x p e r i m e n t was n o t s i g n i f i c a n t l y d i f f e r e n t f r o m F ( T a b l e 10), t h o u g h t h e v a r i a b i l i t y i n AF ( a s measured by o v e r a l l s t a n d a r d e r r o r ) was t w i c e t h a t o f t h e F c y l i n d e r s . S t a t i s t i c a l t e s t s on d e n s i t i e s , egg p r o d u c t i o n , and b r o o d s i z e s g e n e r a l l y c o n f i r m e d t h e above p a t t e r n s i n c l a d o c e r a n a bundance and s u c c e s s i o n , b u t f o r D. r o s e a y i e l d e d some s u r p r i s i n g r e s u l t s ( T a b l e 1 1 ) . The d e n s i t y o f a d u l t p l u s j u v e n i l e D. r o s e a , and t h e t o t a l number o f d a p h n i d e g g s , were s i g n i f i c a n t l y l o w e r i n AF t h a n F, as w o u l d be e x p e c t e d . However, t h e r e was no s i g n i f i c a n t d i f f e r e n c e i n o v e r a l l g e o m e t r i c mean b r o o d s i z e s p e r o v i g e r o u s f e m a l e between 106 t r e a t m e n t s F and AF (1.37 ± 0.07 and 1.53 ± 0.24 e g g s / f e m a l e r e s p e c t i v e l y ) . Numbers o f o v i g e r o u s D. r o s e a f e m a l e s c o u n t e d i n 3AF were t o o low t o g i v e r e l i a b l e mean b r o o d s i z e s a f t e r A u g u s t 9 ( s e e F i g u r e 13). However, c y l i n d e r 7AF mean b r o o d s i z e s between A u g u s t 23 and September 13 were a l l b a s e d on more t h a n 15 o v i g e r o u s f e m a l e s p e r sample (mean o f 31 ± 9.3) and hence t h e sample s i z e s a r e s u f f i c i e n t l y l a r g e t o y i e l d m e a n i n g f u l s t a t i s t i c a l c o m p a r i s o n s . C o m p a r i n g c y l i n d e r 7AF w i t h t r e a t m e n t F c y l i n d e r s between A u g u s t 23 and September 13, I f o u n d t h e mean p e r c e n t a g e o f o v i g e r o u s f e m a l e s and mean b r o o d s i z e s ( u n c o r r e c t e d f o r body s i z e ) t o be much h i g h e r i n t h e a c i d -s t r e s s e d p o p u l a t i o n o f c y l i n d e r 7AF (38.3 ± 4.8% a n d 3.28 ± 0.23 r e s p e c t i v e l y ) t h a n i n t h e two F c y l i n d e r s (20.0 ± 3.7% and 1.16 ± 0.04). D i f f e r e n c e s i n b r o o d s i z e s and p e r c e n t a g e s o f o v i g e r o u s f e m a l e s between c y l i n d e r 7AF and t r e a t m e n t F were p a r t i a l l y due t o d i f f e r e n c e s i n t h e age s t r u c t u r e o f t h e p o p u l a t i o n s . The mean p e r c e n t a g e o f j u v e n i l e D. r o s e a was much l o w e r i n 7AF (4.4 ± 1.6% j u v e n i l e s ) t h a n i n t h e F c y l i n d e r s (14.6 ± 4 . 9 % ) . S i n c e t h e p o s t n a t a l d e v e l o p m e n t t i m e o f D. r o s e a j u v e n i l e s a t 18-20°C, n e u t r a l pH and " h i g h " s e s t o n c o n c e n t r a t i o n (1.0 m g » L " 1 ) i s 6 t o 7 d a y s ( N e i l l 1981b), more j u v e n i l e s " s h o u l d " have been seen i n 7AF i n l a t e A u g u s t and e a r l y S eptember. T h e i r a b s e n c e s u g g e s t s t h a t f o l l o w i n g A u g u s t 9, t h e " b o t t l e n e c k " f o r D. r o s e a was s u r v i v a l t h r o u g h t h e embryo a n d / o r n e o n a t e s t a g e s . The a d u l t D. r o s e a i n 7AF were a b l e t o p r o d u c e eggs a t a r e l a t i v e l y h i g h 107 r a t e ( p o s s i b l y a s s i s t e d by a r e d u c t i o n i n i n t r a s p e c i f i c c o m p e t i t i o n ) b u t a t pH l e v e l s of 5.4 t o 5.6 r e l a t i v e l y few of t h e s e eggs d e v e l o p e d i n t o f i r s t i n s t a r j u v e n i l e s . (The p o s s i b l e r e a s o n s f o r t h i s a r e d i s c u s s e d i n S e c t i o n 4 . 4 . 1 ( i ) . ) T h r o u g h l a t e A u g u s t and September, t h e p o p u l a t i o n age s t r u c t u r e i n 7AF became s t r o n g l y w e i g h t e d t o w a r d s l a r g e r , o l d e r a d u l t s w i t h ample b r o o d p o u c h e s . T h i s i s e v i d e n t f r o m t h e s i g n i f i c a n t l y h i g h e r mean l e n g t h s o f D. r o s e a a d u l t s on September 13 i n t r e a t m e n t AF (1.79 ± 0.051 (n=17)) t h a n i n t r e a t m e n t F (1.46 ± 0.046 ,n=20); (p<0.000l ( t - t e s t ) , see A p p e n d i x E ) . S c a l i n g b r o o d s i z e s a c c o r d i n g t o body l e n g t h s ( H e b e r t 1978) would l i k e l y y i e l d s i m i l a r b r o o d s i z e s f o r t r e a t m e n t s F and AF between A u g u s t 23 and September 13. D. r o s e a d e n s i t i e s were s i g n i f i c a n t l y l o w e r i n AF t h a n A, i n s p i t e o f s i g n i f i c a n t l y g r e a t e r mean egg d e n s i t i e s i n AF ( T a b l e 1 1 ) . The t o x i c i t y e f f e c t s c a u s e d by t h e l o w e r pH i n t r e a t m e n t AF a p p a r e n t l y overwhelmed t h e b e n e f i t s o f i n c r e a s e d f o o d . ( i i ) R esponse o f D i a p t o m i d Copepods The c o p e p o d r e s p o n s e t o a c i d i f i c a t i o n and f e r t i l i z a t i o n was d i v e r g e n t : D i a p t o m u s t y r r e l l i a p p e a r e d a c i d s e n s i t i v e b u t Diaptomus k e n a i was a c i d t o l e r a n t . D. t y r r e l l i a d u l t s d i d n o t - d e c l i n e i n AF a f t e r t h e A u g u s t 9 a c i d s o l u t i o n , b u t d e c l i n e d r a p i d l y t o z e r o a f t e r t h e September 20 a c i d i f i c a t i o n ( F i g u r e 1 4 ) ) . By c o n t r a s t , D. t y r r e l l i m a i n t a i n e d t h e i r d e n s i t i e s d u r i n g September 13-27 i n t h e F c y l i n d e r s . 108 D. t y r r e l l i a d u l t s were t h e r e f o r e s l i g h t l y l e s s a c i d s e n s i t i v e t h a n D. r o s e a a d u l t s and j u v e n i l e s . Even i n c l u d i n g t h e h i g h peak ( s u s p e c t e d s a m p l i n g e r r o r ) on A u g u s t 23 i n Bag 3AF, t h e g e o m e t r i c mean d e n s i t i e s o f D. t y r r e l l i were s i g n i f i c a n t l y h i g h e r i n F t h a n AF ( T a b l e 1 1 ) . T h i s r e s u l t i s due t o t h e c o n s i s t e n t l y low d e n s i t i e s o f D. t y r r e l l i i n AF c y l i n d e r s a f t e r September 13. A t t a c h e d egg p r o d u c t i o n by D. t y r r e l l i d i d n o t d i f f e r s i g n i f i c a n t l y between AF and F ( T a b l e 1 1 ) . U n l i k e D. t y r r e l l i , D i a p t o m u s k e n a i showed no n e g a t i v e i m p a c t s f r o m a c i d i f i c a t i o n i n AF, and i n some r e s p e c t s p e r f o r m e d b e t t e r t h e r e t h a n i n t r e a t m e n t F. D. k e n a i g e o m e t r i c mean d e n s i t i e s were h i g h e r i n AF t h a n F, (0.30 ± 0.018 and 0.20 ± 0.019 L " 1 r e s p e c t i v e l y ) , a l t h o u g h t h i s d i f f e r e n c e was n o t s i g n i f i c a n t (p = 0.058, T a b l e 1 1 ) . When a n a l y z e d o v e r t h e whole e x p e r i m e n t , g e o m e t r i c mean d e n s i t i e s o f a t t a c h e d D. k e n a i eggs a l s o d i d n o t d i f f e r s i g n i f i c a n t l y between F a n d AF ( T a b l e 1 1 ) . However, mean d e n s i t i e s o f a t t a c h e d D. k e n a i eggs were h i g h e r i n AF t h a n F a f t e r t h e c o l l a p s e o f D a p h n i a ( i . e . f r o m A u g u s t 23 t o O c t o b e r 1 1 ) : 0.47 ± 0.14 e g g s » L " 1 i n AF; a n d 0.19 ± 0.055 e g g s » L " 1 i n F. A c l e a r e r d i f f e r e n c e between AF and F i s i n t h e g e o m e t r i c mean number o f l o o s e c o p e p o d e g g s , w h i c h were s i g n i f i c a n t l y h i g h e r i n AF (p = 0.004; T a b l e 11 and F i g u r e 1 6 ) . A l t h o u g h t h e s e l o o s e eggs were n o t i d e n t i f i e d t o s p e c i e s , t h e t i m i n g o f c a l a n o i d c o p e p o d a t t a c h e d egg p r o d u c t i o n s u g g e s t s t h a t ( u n l i k e i n t r e a t m e n t F) many o f t h e l o o s e eggs c o u l d be due t o D. k e n a i ( F i g u r e s 14 and 1 5 ) . I h y p o t h e s i z e t h a t t h e a c i d - i n d u c e d 109 e l i m i n a t i o n o f D. r o s e a i n t r e a t m e n t AF i n c r e a s e d t h e a v a i l a b i l i t y o f r e s o u r c e s t o D. k e n a i r e s u l t i n g i n g r e a t e r a t t a c h e d and l o o s e egg p r o d u c t i o n . A l s o , t h e mean l e n g t h o f D. k e n a i a d u l t s was s i g n i f i c a n t l y h i g h e r ( p = 0 . 0 0 l 2 , t - t e s t ) i n t r e a t m e n t AF (2.06 ± 0.025, n=20) t h a n i n t r e a t m e n t F (1.91 ± 0.039, n=17) on September 13, s u g g e s t i n g i m p r o v e d g r o w t h i n AF. 3.2.5 V e r t i c a l D i s t r i b u t i o n B o t h c o p e p o d s and c l a d o c e r a n s commonly show v e r t i c a l m i g r a t i o n s n e a r s u n s e t a n d s u n r i s e ( W e t z e l 1975). My two d a y t i m e s t r a t i f i e d s a m p l i n g s on A u g u s t 1 and September 6) were t a k e n between t h e h o u r s o f 10 a.m. and 5 p.m., and t h e r e f o r e r e p r e s e n t o n l y t h e d a y t i m e d i s t r i b u t i o n . N o n e t h e l e s s , t h e s e d a t a g i v e a p r e l i m i n a r y i n d i c a t i o n o f : 1. t h e mean t e m p e r a t u r e s a t w h i c h eggs and j u v e n i l e s o f e a c h s p e c i e s were d e v e l o p i n g ; 2. t h e d e g r e e t o w h i c h p a r t i c u l a r s p e c i e s were e x p o s e d t o a c i d e p i l i m n e t i c w a t e r s ; 3. t h e s p a t i a l o v e r l a p i n d a y t i m e d i s t r i b u t i o n s o f d i f f e r e n t s p e c i e s , and 4. t r e a t m e n t e f f e c t s on v e r t i c a l d i s t r i b u t i o n s . V e r t i c a l d i s t r i b u t i o n s o f a d u l t f o r m s o f s e l e c t e d s p e c i e s a r e shown i n F i g u r e s 18 a n d 19. N e i t h e r t h e A u g u s t 1 n o r September 6 s a m p l i n g s r e v e a l e d v e r t i c a l m i g r a t i o n s f o r any o f t h e s p e c i e s when t h e i r v e r t i c a l d i s t r i b u t i o n s were examined i n a s e q u e n c e c o r r e s p o n d i n g t o t h e t i m e o f s a m p l i n g . A c c u r a t e 110 F i g u r e 18. V e r t i c a l d i s t r i b u t i o n s o f a d u l t forms o f z o o p l a n k t o n . G r a p h s show % o f p o p u l a t i o n a t e a c h o f f o u r s t r a t a : 0-2 m; 2-4 m; 4-6 m; and 6-8 m.; V e r t i c a l d i s t r i b u t i o n s o m i t t e d i f fewer t h a n 10 a n i m a l s c o u n t e d . ' S p e c i e s a b b r e v i a t i o n s : KA = D. k e n a i a d u l t s ; TA = D. t y r r e l l i a d u l t s ; DA = D. r o s e a a d u l t s ; DOF = D. r o s e a o v i g e r o u s f e m a l e s BA = B. l o n g i r o s t r i s a d u l t s ; CHYA = C. s p h a e r i c u s a d u l t s . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . I l l < < X o J « - i s / n J L J 1 g V < m I 1,1(1 \ (m A pt 6, 1979 < g V to 1 o V to < i-Iwl k < J g J J o V < CD I V, in ilk u _ O Q A t i l l 1 I r f l C D cn ri> < (IT) A. J I toi n to < < 1 -1 , /I mi h to L < 1 H J/ U J O V h i "E CM t o CM CC Cvl I D CVI O ( M CD CM t O C J CD CM ID O C M o to li-0 0 L L < < o % LL. _ l o -o •8-c o-8 o-8 o • O-o 112 F i g u r e 19. V e r t i c a l d i s t r i b u t i o n s o f immature forms o f z o o p l a n k t o n , a n d Diaphanosoma b r a c h y u r u m . G r a p h s show % o f p o p u l a t i o n a t e a c h o f f o u r s t r a t a : 0-2 m; 2-4 m; 4-6 m; and 6-8 m.; V e r t i c a l d i s t r i b u t i o n s o m i t t e d i f fewer t h a n 10 a n i m a l s c o u n t e d . S p e c i e s a b b r e v i a t i o n s : T J = D. t y r r e l l i j u v e n i l e s ; D J = D. r o s e a j u v e n i l e s ; B J = B. l o n q i r o s t r i s j u v e n i l e s ; CHJ=C. s p h a e r i c u s j u v e n i l e s ; N = c o p e p o d n a u p l i i ; DIA = D. b r a c h y u r u m a d u l t s and j u v e n i l e s . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . D A T E B T A H G H ( m ) A u g . 1 , 1 9 7 9 S e p t . 6 , 1 9 7 9 T J DJ B J N T J D J B J C H J N D I A 2 C 2-6 <I0 6 C 2 6; — 1— <I0 8 F 2 6 : 2 <I0 <I0 <I0 <I0 <I0 1 r 4 A 6 2 6 <I0 < I 0 5 A 2 <I0 < I 0 3 A F 2-6 : <I0 <I0 < 7 A F 2 6 z z <I0 <I0 L K 2 6 5^ w <I0 <I0 <I0 <I0 < 10 < I 0 0-H—i—(-100 0-1—I—H00 O-i—I—HOO 0-H—I—HOO 0 I I—HOO % I I I I I I I I I 1 O—l—l—HOO 0-H—I—HOO 0—I—I—HOO 0-H—I—I-100 0-H—I—HOO I—1 1 14 a s s e s s m e n t o f t i m e of day e f f e c t s on v e r t i c a l d i s t r i b u t i o n s w o uld have r e q u i r e d s e v e r a l 24-hour s a m p l i n g s o f t h e e n c l o s u r e s i n random o r d e r . Some c o n s i s t e n t t r e n d s i n s p e c i e s ' v e r t i c a l d i s t r i b u t i o n s a r e s e e n i n F i g u r e s 18 and 19. D i a p t o m u s k e n a i a d u l t s were m o s t l y i n t h e c o o l e r w a t e r s below 4 m and t h e r e f o r e s h i e l d e d f r o m t h e a c i d e p i l i m n i o n . I n s u f f i c i e n t numbers o f D. k e n a i c o p e p o d i t e s were c o u n t e d t o w a r r a n t g r a p h s o f t h e i r v e r t i c a l d i s t r i b u t i o n . Of t h e 15 v e r t i c a l d i s t r i b u t i o n s o f D. k e n a i a d u l t s i n F i g u r e 18, t h e g r e a t e s t p r o p o r t i o n o f a n i m a l s was a t 6-8 m i n t e n c a s e s , and a t 4-6 m i n f o u r . The p a t t e r n o f c o n c e n t r a t i n g a t l o w e r d e p t h s i s s h a r p e n e d by t h e f a c t t h a t i n 12 o f t h e s e 14 "deep peak" c a s e s t h e p r o p o r t i o n s w i t h i n t h e d e n s e s t 2 m s t r a t u m were g r e a t e r t h a n 50%. F e d o r e n k o (1975a, 1975b) f o u n d a d i s t i n c t r e v e r s e v e r t i c a l m i g r a t i o n i n E u n i c e L a k e D. k e n a i ( i . e . d e e p e r a t n i g h t ) , b u t t h i s p a t t e r n may have c h a n g e d w i t h t h e e l i m i n a t i o n o f C h a o b o r u s i n 1976. No c l e a r d i f f e r e n c e s i n D. k e n a i d i s t r i b u t i o n were e v i d e n t between t r e a t m e n t s . Though most commonly f o u n d a t 4-6 m, D i aptomus t y r r e l l i a d u l t s were much more e q u i t a b l y d i s t r i b u t e d t h r o u g h t h e w a t e r column t h a n were D. k e n a i , B. l o n g i r o s t r i s , o r C. s p h a e r i c u s . C o p e p o d i t e s were on a v e r a g e 2 m h i g h e r i n t h e w a t e r c o l u m n , most commonly c o n c e n t r a t e d a t 2-4 m. In g e n e r a l , v a r i a b i l i t y i n D. t y r r e l l i v e r t i c a l d i s t r i b u t i o n s a p p e a r e d a s l a r g e w i t h i n t r e a t m e n t s a s between. S i g n i f i c a n t f r a c t i o n s o f t h e a d u l t and j u v e n i l e p o p u l a t i o n s i n A and AF c y l i n d e r s were f o u n d i n t h e 115 e p i l i m n i o n , and t h e r e f o r e s u s c e p t i b l e t o a c i d s t r e s s . F e d o r e n k o (1975a, 1975b) o b s e r v e d no v e r t i c a l m i g r a t i o n i n Diaptomus  t y r r e l l i . O l e n i c k (1982) d e t e r m i n e d t h e d i u r n a l and n o c t u r n a l v e r t i c a l d i s t r i b u t i o n s o f e n c l o s e d E u n i c e Lake z o o p l a n k t o n on J u l y 5-6 and September 18-19, 1979. On b o t h d a t e s , d i u r n a l s a m p l e s showed 50% o f D. t y r r e l l i a d u l t s and 50% o f t h e c o p e p o d i t e s a t 0-2m ( p e r c e n t a g e s b a s e d on t o t a l numbers o f a n i m a l s i n a l l e n c l o s u r e s ) . N o c t u r n a l d i s t r i b u t i o n s were a b o u t 2m l o w e r . I t i s n o t c l e a r why t h e d i u r n a l d i s t r i b u t i o n o f D. t y r r e l l i was somewhat h i g h e r i n O l e n i c k ' s e n c l o s u r e s t h a n i n mine. D a p h n i a r o s e a f e m a l e s ( b o t h o v i g e r o u s and n o n o v i g e r o u s ) were e v e n l y d i s t r i b u t e d i n a manner s i m i l a r t o D. t y r r e l l i a d u l t s , a n d were t h e r e f o r e a l s o w i t h i n t h e a c i d i f i e d z o n e . J u v e n i l e D. r o s e a showed d i s t r i b u t i o n s s t r i k i n g l y s i m i l a r t o D. t y r r e l l i c o p e p o d i t e s i n 5 o f t h e 12 c a s e s where b o t h s p e c i e s c o n t a i n e d s u f f i c i e n t numbers t o p e r m i t c o m p a r i s o n ; i n o n l y one o f 12 c a s e s ( c y l i n d e r 4A on September 6) were t h e two s p e c i e s ' j u v e n i l e f o rms d i s t r i b u t e d v e r y d i f f e r e n t l y . R e p l i c a t i o n w i t h i n C, A, and F c y l i n d e r s was good enough on A u g u s t 1 t o p e r m i t c a u t i o u s c o m p a r i s o n s between t h e s e t r e a t m e n t s ; a d u l t s a n d j u v e n i l e s were e q u i t a b l y (and s i m i l a r l y ) d i s t r i b u t e d i n t r e a t m e n t A and t h e c o n t r o l s , b u t showed c o n s i s t e n t c l u m p i n g a t 2-4 m i n t h e F c y l i n d e r s . I n S e c t i o n 3 . 2 . 4 ( i ) , I n o t e d t h a t t h e r e l a t i v e l y d e e p d i s t r i b u t i o n o f D. r o s e a a d u l t s and j u v e n i l e s i n c y l i n d e r 7AF on A u g u s t 1 may have a c c o u n t e d f o r t h e b e t t e r s u r v i v a l o f D. r o s e a 116 i n 7AF t h a n 3AF. O l e n i c k ( u n p u b l i s h e d d a t a ) o b s e r v e d day t o n i g h t s h i f t s i n t h e v e r t i c a l d i s t r i b u t i o n of E u n i c e Lake D. r o s e a i n o n l y 8 o f 21 s t r a t i f i e d s a m p l e s , and 6 o f t h e s e 8 were downward s h i f t s . T h e r e f o r e , my d a y t i m e s a m p l e s a r e p r o b a b l y r e a s o n a b l e e s t i m a t o r s o f maximum a c i d e x p o s u r e . Bosmina l o n g i r o s t r i s was t h e most s u r f a c e - o r i e n t e d s p e c i e s o f t h e community a n d t h e r e f o r e e x p e r i e n c e d t h e g r e a t e s t e x p o s u r e t o a c i d c o n d i t i o n s . C l u m p i n g i n t h e 0-2 m s t r a t u m was e v i d e n t f o r a d u l t s i n 11 o f 18 s a m p l e s , and f o r j u v e n i l e s i n 11 o f 16 c a s e s . In t h e s e c a s e s , t h e p r o p o r t i o n s o f a n i m a l s a t 0-2 m were r e l a t i v e l y h i g h , a v e r a g i n g 75 ± 3.1% (n = 2 2 ) . B. l o n g i r o s t r i s was c o n s i s t e n t l y s e g r e g a t e d f r o m C h y d o r u s s p h a e r i c u s , b u t t h e s e two s p e c i e s a r e o f t e n f o u n d t o g e t h e r a t n i g h t n e a r t h e s u r f a c e o f t h e w a t e r (W.E. N e i l l , p e r s . comm. ) . C h y d o r u s s p h a e r i c u s was n o t i n s u f f i c i e n t a b undance on Au g u s t 1 t o examine i t s v e r t i c a l d i s t r i b u t i o n . On September 6, b o t h j u v e n i l e s and a d u l t s were c l u m p e d a t 6-8 m i n a l l e i g h t c y l i n d e r s a m p l e s , w i t h mean p r o p o r t i o n s o f 83 ± 5% and 84 ± 4%. I f t h o s e c h y d o r i d s s a m p l e d a t 6-8 m r e m a i n e d t h e r e c o n s i s t e n t l y , t h e y would n e v e r have been e x p o s e d t o pH l e v e l s below 5.9. However, C. s p h a e r i c u s h as f r e q u e n t l y been r e c o r d e d a s d i s p l a y i n g an upward v e r t i c a l m i g r a t i o n a f t e r s u n s e t (Meyers 1980, W.E. N e i l l p e r s . comm. ), and t h e r e f o r e was l i k e l y e x p o s e d t o t h e more a c i d z o n e . The v e r t i c a l d i s t r i b u t i o n s o f more minor c o n s t i t u e n t s o f t h e p l a n k t o n ( H o l o p e d i u m g i b b e r u m , c o p e p o d n a u p l i i , and Diaphanosoma  brachyurum) were a l s o e x a m i n e d . In t h e f i v e c a s e s where 1 17 s u f f i c i e n t numbers were p r e s e n t t o e s t i m a t e t h e v e r t i c a l d i s t r i b u t i o n , most H. g i b b e r u m were i n t h e 0-2 m s t r a t u m . Most c o p e p o d n a u p l i i were f o u n d n e a r t h e s u r f a c e as w e l l ; 9 o f t h e 11 g r a p h s i n F i g u r e 19 show more t h a n h a l f t h e n a u p l i i i n t h e t o p 2 m. D. b r a c h y u r u m o n l y had s u f f i c i e n t numbers f o r a n a l y s i s i n t r e a t m e n t AF, where i t was c e n t e r e d a b o u t t h e 2-4 m s t r a t u m . 118 3.3 P h y t o p l a n k t o n 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 _3._3.J_ E f f e c t s o f E n c l o s u r e The p h y t o p l a n k t o n o f t h e c o n t r o l e n c l o s u r e s were compared t o E u n i c e L a k e t o see i f d i f f e r e n c e s were a p p a r e n t i n e i t h e r mean c o n d i t i o n s o r t e m p o r a l p a t t e r n s . T a b l e 13 shows t h a t mean c h l o r o p h y l l and p h a e o p h y t i n a l e v e l s i n t h e c o n t r o l s were n o t s i g n i f i c a n t l y d i f f e r e n t f r o m E u n i c e L a k e . However, 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 d i d not peak so s h a r p l y i n t h e c o n t r o l c y l i n d e r s as i n t h e l a k e d u r i n g September ( F i g u r e 2 0 ) , p e r h a p s due t o g r e a t e r n u t r i e n t r e p l e n i s h m e n t i n t h e l a k e f o l l o w i n g t h e s t o r m s of September 1 t o 9 ( F i g u r e 4, page 4 7 ) . T h e s e s t o r m s would have t r a n s p o r t e d l a r g e q u a n t i t i e s o f n u t r i e n t s f r o m b o t h E u n i c e and p a r t i c u l a r l y G w e n d o l i n e L a k e s ' w a t e r s h e d s , s i n c e G w e n d o l i n e was e x p e r i m e n t a l l y f e r t i l i z e d i n t h e summer o f 1979 ( C . J . W a l t e r s , p e r s . comm. ). O t h e r p h y t o p l a n k t o n i n d i c a t o r s d i f f e r e d between t h e c o n t r o l s and E u n i c e L a k e . Mean t o t a l a l g a l b i o m a s s was h i g h e r (p = 0.014) i n t h e l a k e (0.85 ± 0 . 1 1 m g « L " 1 ) t h a n i n t h e c o n t r o l s (0.58 ± 0.08), w i t h t h e b u l k o f t h i s b i o m a s s d i f f e r e n c e c o n t r i b u t e d by t h e c o l o n i a l g r e e n a l g a G l e o c y s t i s s p i n t h e 13-18 Mm s i z e c a t e g o r y ( T a b l e s 14 and 15, F i g u r e s 21, 22 and 2 3 ) . T h e s e c o l o n i e s f o r m e d between 52 a n d 90% o f t h e t o t a l p h y t o p l a n k t o n volume i n t h e A u g u s t and September l a k e samples ( A p p e n d i x F ) . P r i o r t o A u g u s t 1, b o t h E u n i c e L a k e a n d c o n t r o l c y l i n d e r s a m p l e s were d o m i n a t e d ( v o l u m e t r i c a l l y ) by c o l o n i e s o f t h e c h r y s o p h y t e D i n o b r y o n , w h i c h were c l a s s i f i e d i n t h e > 30Mm Table 13. Results of analyses of variance on chlorophyll measurements. (See Table IT for abbreviations and degrees of freedom.) Treatment Geometric Means ± SE (yg/1) Variable — LK C F A AF Chlorophyll a 0.39 ± 0.049 0.32 ± 0.036 0.51 ± 0.044 0.36 ± 0.031 1.39 ± 0.11 Pheophytin a 0.38 ± 0.030 0.41 ± 0.025 0.87 ± 0.056 0.42 ± 0.027 1.82 ±0.10 Treatment Comparison Variable C vs. LK C vs. A C vs. F F vs. AF A vs. AF Log (Chlorophyll a) ns (0.13) ns (0.31) F (0.00002) AF (10"5) IH AF (10"5) IH Log (Pheophytin a) ns (0.23) ns (0.81) F (10"5) IH AF (10"5) IH AF (10"s) IH 120 F i g u r e 20. L i v e c h l o r o p h y l l a and p h a e o p h y t i n a. N ote m i s s i n g sample on A u g u s t 9 i n 7AF; e s t i m a t e d c h l o r o p h y l l c o n c e n t r a t i o n ( b a s e d on S e c c h i d e p t h - c h l o r o p h y l l r e g r e s s i o n e q u a t i o n i n T a b l e D1 ) i s 0.5 jig»L" 1. A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . N o te a l s o no s a m p l e s t a k e n between A u g u s t 30 and September 13. 121 1979 CHLOROPHYLL A LIVE CHLOROPHYLL a PHAEOPHYTIN a CD ZD O 3 CJ UJ , ac » L A K E IS 23 I 9 16 23 30 13 20 21 S JULT | RUG. | SEPT. I 0 C D Z D O 3 U J 2 _ J • Z D 19 23 1 9 16 23 30 13 20 21 S M.r | RUG. | SEPT. | 0 C D O 3 l_> U J 2 ac « t_> m© 19 23 1 9 16 ' 23 30 JULT | RUG. | 13 20 21 5 SEPT. | 0 C D C D 3 C J U J 2 > d ' _ J Z D 2 1 o Z D CJ E © 19 23 1 JULT | 16 23 30 RUG. | 13 20 21 5 SEPT. I 0 C D CJ CD 3 C J I— C C 1 Z D C J 19 23 I 9 16 23 30 13 20 21 5 • U . T | RUG. | SEPT. | 0 C D Z D C J C D 3 C J U J 2 L T 1 _ J Z D Z D ° C J 19 23 1 9 16 23 30 13 20 21 S JULT | RUG. | SEPT. | 0 C J g 3 C J U J 2 CO ' _ J Z D ID© ttQnmDi&iiiiip 19 23 1 9 16 23 30 13 20 27 JILT | RUG. I SEPT. Z D C J 19 23 1 9 16 23 30 13 20 21 5 JULT | RUG. | SEPT. | 0 SAMPLING DATE C D Z D CJ g 3 C J > 2 t x 1 _ J Z D ! i ° C J 19 23 1 JULT | 16 23 30 13 20 21 5 RUG. | SEPT. SAMPLING DATE Table 14. Results of analyses of variance on algal volumes in particular size classes. See Table 11 for abbreviations and degrees of freedom. Cell Size Class Used for Volume Comparisons C vs. LK Treatment Comparison C vs. A C vs. F F vs. AF A vs. AF < 13 ym* < 2 ym 2-5 ym 5-9 ym 9-13 ym LK(0.03)IH ns(0.13) C(0.027) LK(10-5)IH ns(0.19) ns(0.19) ns(0.56) ns(0.54) ns(0.78) ns(0.17) F (0.002)IH C (0.037)H ns(0.67) F (0.0002)IH C (0.0036)IH ns(0.86)IH AF(0.046)h AF(0.013)h ns(0.63)I ns(0.17)IH AF(0.001)h ns(0.49) AF(0.026)h AF(0.004)h AF(0.025)Ih 13-18 ym 18-30 ym > 30 ym Filaments LK(0.0006) ns(0.12)H ns(0.94) ns(0.92) ns(0.57) ns(0.8) ns(0.41) ns(0.52)H C (lO-^IH ns(0.28) C (10~5)IH ns(0.4)H ns(0.85) AF(0.023)H AF(0.028)Ih AF(0.019)h A (10"5)IH AF(0.009H A (0.003)1 AF(0.002)H Total Algal Biomass LK(0.014) ns(0.55) C (0.0005) AF(0.005)ih ns(0.60)h * Accumulated from separate size categories. Table 15. Mean percentage of algal volume in particular size classes, by treatment (± SE). Cell Size Class Used for Proportion Comparisons LK Treatment AF < 5 ym* 5-9 ym 9-13 ym 13-18 ym > 18 ym* Fi1aments 3.4 ± 0.5 12.0 ± 2.4 10.5 ± 2.9 20.3 ± 3.3 15.0 ± 2.7 4.3 ± 0.8 57 ± 9.7 33 ± 9.2 2.8 ± 0.6 2.7 ± 0.7 12.8 ± 2.4 4.9 ± 1.0 2.2 ± 0.7 2.2 ± 0.6 2.7 ± 0.6 10.8 ± 2.4 6.3 ± 2.0 38 ± 6.1 38 ± 6.3 18.2 ± 2.9 11.8 ±2.5 45 ± 6.5 46 ± 6.2 36.2 ± 5.5 57.7 ± 5.2 0.19 ± 0.1 0.4 ± 0.2 0.3 ± 0.2 1.8 ± 1.1 4.2 ± 1.3 * Accumulated from separate size categories, 124 F i g u r e 21. A l g a l b i o m a s s by m a j o r s i z e c l a s s e s . See F i g u r e 22 and T a b l e 15 f o r d e t a i l e d s i z e breakdown. A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 125 1979 ALGAL BIOMASS fes&s&a < I 3 M I C R O N S 13-18 M I C R O N S > I 8 M I C R O N S F I L A M E N T S 5 2 to CD ( X C D LAKE 26 I JULT I 16 23 30 6 AUG. | 13 20 27 SEPT. 5 O C T . 13 20 27 5 SEPT. | OCT. C O s i C D C D C X C D 13 20 27 5 SEPT. | OCT. JULT | BUG. | SEPT. | OCT. JULT | BUG. | SEPT. | OCT. 126 F i g u r e 22. P e r c e n t c o m p o s i t i o n o f a l g a l b i o m a s s by d e t a i l e d s i z e c l a s s e s . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 1 2 7 128 F i g u r e 23. P h y l e t i c c o m p o s i t i o n o f a l g a l volume. A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 129 130 s i z e c l a s s due t o t h e i r 45 Mm l o n g l o r i c a s . The s h i f t i n dominance f r o m D i n o b r y o n s p . t o G l e o c y s t i s t h a t o c c u r r e d i n E u n i c e L a k e a b o u t A u g u s t 1 a l s o o c c u r r e d i n t h e c o n t r o l c y l i n d e r s , b u t n o t u n t i l September 6 t o 13 ( F i g u r e 23 and A p p e n d i x F ) . T h e s e d i f f e r e n c e s i n dominant t a x a e x p l a i n why t h e p r o p o r t i o n o f a l g a l volume i n s i z e c l a s s e s g r e a t e r t h a n 13 Mm d i f f e r e d between t h e l a k e and t h e c o n t r o l s ( F i g u r e 22 and T a b l e 1 5 ) . The mean volume i n a l g a l c e l l s l e s s t h a n 13 Mm i n d i a m e t e r was g r e a t e r i n t h e l a k e (0.076 ± 0.012 Mm3) t h a n i n t h e c o n t r o l s (0.060 ± 0.004), a d i f f e r e n c e s i g n i f i c a n t a t p = 0.03 ( T a b l e 1 4 ) . I s u s p e c t t h a t most c e l l s o f t h i s s i z e were c a p a b l e o f b e i n g i n g e s t e d and d i g e s t e d by z o o p l a n k t o n ( B u r n s 1968; P o r t e r 1973, 1977). I t i s p o s s i b l e t h a t c u t t h r o a t t r o u t p r e d a t i o n i n E u n i c e L a k e l e s s e n e d t h e h e r b i v o r o u s g r a z i n g p r e s s u r e on t h e s e s m a l l a l g a l c e l l s , o r t h a t l o w e r l i g h t l e v e l s i n t h e c o n t r o l s a f f e c t e d p r i m a r y p r o d u c t i o n . I t i s d i f f i c u l t t o a s s e s s i n t e r - t r e a t m e n t d i f f e r e n c e s i n t h e p r o p o r t i o n o f l i v e c h l o r o p h y l l a o r mean a l g a l c h l o r o p h y l l c o n t e n t , s i n c e t h e s e r a t i o s a r e s e n s i t i v e t o v a r i a b i l i t y i n b o t h t h e n u m e r a t o r and d e n o m i n a t o r . The p r o p o r t i o n o f l i v e c h l o r o p h y l l a ( F i g u r e 24) r o s e s h a r p l y i n E u n i c e L a k e between A u g u s t 30 and September 13 ( d u r i n g t h e p e r i o d o f h i g h p r e c i p i t a t i o n ) b u t d i d n o t i n c r e a s e i n t h e c o n t r o l s . E x amined o v e r t h e whole e x p e r i m e n t , t h e mean p r o p o r t i o n o f l i v e c h l o r o p h y l l a was h i g h e r i n E u n i c e Lake (49.8 ± 2.6%) t h a n i n t h e c o n t r o l s (41.8 ± 1.6%). Mean a l g a l c h l o r o p h y l l c o n t e n t 131 F i g u r e 24. P r o p o r t i o n o f l i v e c h l o r o p h y l l a. A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 1979 PROPORTION LIVE CHLOROPHYLL C t .7 CHL.a t/<GHL. a + PHAEO.aX 5 CX LAKE 19 23 I 9 16 23 30 JULT | RUG. I 13 20 27 SEPT. 5 OCT. m© 19 23 1 9 16 23 30 JULT | RUG. | 13 20 27 5 SEPT. | OCT. X .7 X (_) e x cn 19 23 ] 9 16 23 30 JULT | RUG. | 13 20 27 SEPT. 5 OCT. B © 19 23 1 JULT I 16 23 30 RUG. | 13 20 27 5 SEPT. I OCT. X (_) e x en ts© 19 23 1 JULT I 16 23 30 RUG. I 13 20 27 SEPT. 5 OCT. a® 19 23 1 9 16 23 30 JULT | RUG. I 13 20 27 5 SEPT. | OCT. X U J o X cn tn® 19 23 1 9 16 23 30 13 20 27 JULT | RUG. | SEPT. 5 OCT. 19 23 1 9 16 23 30 13 20 27 S JULT | RUG. | SEPT. | OCT. SAMPLING DATE UJ > c x cn m © 19 23 1 JULT | 16 23 30 13 20 27 RUG. | SEPT. SAMPLING DATE 5 OCT. 133 ( F i g u r e 25) d i d n o t d i f f e r s i g n i f i c a n t l y between E u n i c e Lake (0.50 ± 0.062 uq c h l o r o p h y l l • (mg a l g a l b i o m a s s ) " 1 ) and t h e c o n t r o l c y l i n d e r s (0.68 ± 0.086). Phase p l a n e s o f t h e s i m u l t a n e o u s c h a n g e s i n l i v e c h l o r o p h y l l a and z o o p l a n k t o n b i o m a s s ( F i g u r e 26) i l l u s t r a t e i n t e r - t r e a t m e n t d i f f e r e n c e s i n p l a n k t o n s y s t e m b e h a v i o u r . I t s h o u l d be s t r e s s e d t h a t c h l o r o p h y l l a i s an i n a c c u r a t e i n d e x o f e d i b l e a l g a l b i o m a s s , and t o t a l z o o p l a n k t o n b i o m a s s r e p r e s e n t s a g r o u p o f h e r b i v o r e s r a t h e r t h a n a s i n g l e h e r b i v o r e . N e v e r t h e l e s s , t h e ph a s e p l a n e s i n F i g u r e 26 i l l u s t r a t e t h e o v e r a l l e f f e c t o f p l a n t - h e r b i v o r e i n t e r a c t i o n s , and s e a s o n a l c h a n g e s i n p r i m a r y and s e c o n d a r y p r o d u c t i o n . I d i d n o t g r a p h p h a s e p l a n e s o f t o t a l z o o p l a n k t o n b i o m a s s v s t o t a l a l g a l b i o m a s s due t o p o o r e s t i m a t i o n o f a l g a l b i o m a s s i n t h e l a r g e r s i z e c l a s s e s ( S e c t i o n 2.2.2 ( i i ) ) . P hase p l a n e t r a j e c t o r i e s i n t h e c o n t r o l c y l i n d e r s d i v e r g e d from t h e p a t h shown i n E u n i c e L a k e . The c o n t r o l c y l i n d e r s b o t h showed an i n i t i a l c y c l e i n l a t e J u l y and e a r l y A u g u s t , b u t t h e n d w i n d l e d t o low z o o p l a n k t o n d e n s i t i e s by l a t e September, d e s p i t e r e l a t i v e l y h i g h c h l o r o p h y l l a c o n c e n t r a t i o n s . By c o n t r a s t , t h e l a k e managed t o m a i n t a 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 r o u g h o u t September and O c t o b e r . I s u s p e c t t h a t t h e O c t o b e r r e v i v a l o f E u n i c e L a k e z o o p l a n k t o n b i o m a s s may have been c a u s e d by i n c r e a s e s i n p r i m a r y p r o d u c t i o n f o l l o w i n g t h e s t o r m s o f e a r l y September, but do n o t have measurements t o t e s t t h i s p o s s i b i l i t y . 134 F i g u r e 25. C h l o r o p h y l l a p e r u n i t a l g a l b i o m a s s . Computed by d i v i d i n g l i v e c h l o r o p h y l l a by t o t a l a l g a l b i o m a s s . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 1 3 5 1979 A L G A L CHLOROPHYLL CONTENT C H L . A / A L G A L B I O M A S S • ALGAL BIOMASS INTERPOLATED (July 19 i 23) O CHLOROPHYLL A INTERPOLATED (July 26 • Sept. 6) U J 20 cx C D —I 10 cx C D CX X C D o L A K E o-- .o ' 19 2 3 2 6 1 S 16 2 3 30 6 13 20 27 S JULT | RUG. | SEPT. I OCT. m© o -O 19 23 26 1 9 16 23 30 6 13 20 27 5 JULT | RUG. | SEPT. | OCT. n © 19 2 3 2 6 1 9 16 23 30 6 13 20 27 5 JULT | RUG. | SEPT. I OCT. CX LD _ J CX LD cx X C D X I C D o O a® 19 2 3 2 6 1 JULT | 16 23 30 6 13 20 27 5 RUG. | SEP1. I OCT. X C D X L D X C D C D o O H ® ] 9 23 26 1 9 16 23 30 6 13 20 27 S JULT | RUG. | SEPT. I OCT. X C D X C D cx 5 . C D ZD C D 0 O 19 2 3 2 6 1 9 16 23 30 6 13 20 27 5 JULT | PUG. | SEPT. I OCT. X C D X C D C D C D C D 0 o s © 19 23 26 1 JULT | 16 23 30 6 13 20 27 S RUG. | SEPT. I OCT. SAMPLING DATE SAMPLING DATE 136 F i g u r e 26. Phase p l a n e t r a j e c t o r i e s t h r o u g h t i m e o f t o t a l z o o p l a n k t o n b i o m a s s and l i v e c h l o r o p h y l l a c o n c e n t r a t i o n s . P o i n t s d e s c r i b e v a l u e s o f t h e s e two v a r i a b l e s on J u l y 16 and 26; A u g u s t 1, 9, 16, 23 and 30; September 6, 13, and 27; and O c t o b e r 11. The t r a j e c t o r y o f e n c l o s u r e 1F p a s s e s t h r o u g h t h e same p o i n t t w i c e . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : C = c o n t r o l s ; F = f e r t i l i z a t i o n o n l y ; A = a c i d i f i c a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . 1 3 7 138 3.3.2 E f f e c t s o f A c i d i f i c a t i o n The b e h a v i o u r o f p h y t o p l a n k t o n i n d i c a t o r s i n t r e a t m e n t A was r e m a r k a b l y s i m i l a r t o t h e c o n t r o l s . None o f t h e p h y t o p l a n k t o n c h a r a c t e r i s t i c s shown i n F i g u r e s 20 t o 25 (and compared i n T a b l e s 13 t o 15) 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 a c i d i f i c a t i o n c y l i n d e r s and t h e c o n t r o l s . O t h e r t h a n d e m o n s t r a t i n g h i g h e r z o o p l a n k t o n b i o m a s s i n e a r l y A u g u s t , t h e A c y l i n d e r p h a s e p l a n e s o f t o t a l z o o p l a n k t o n b i o m a s s and c h l o r o p h y l l ( F i g u r e 26) were q u a l i t a t i v e l y s i m i l a r t o t h e c o n t r o l s . I n t e r m s o f a v a i l a b l e f o o d f o r z o 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 t h e volume o f " s m a l l , m o s t l y e d i b l e " a l g a l c e l l s ( l e s s t h a n 13 ym i n d i a m e t e r ) d i d n o t d e c r e a s e i n t h e A c y l i n d e r s d u r i n g t h e most a c i d p e r i o d ( A u g u s t 9 t o 30) when t h e mean e p i l i m n e t i c pH was 5.72 ± 0.06. T h e r e were a l s o no m a j o r s h i f t s i n o t h e r s i z e c l a s s e s o f a l g a e r e l a t i v e t o t h e c o n t r o l s d u r i n g A u g u s t ( F i g u r e 2 2 ) . Though i t i s p o s s i b l e t h a t t h e t a x o n o m i c c o m p o s i t i o n w i t h i n p a r t i c u l a r s i z e c a t e g o r i e s c o u l d have s h i f t e d t o l e s s p a l a t a b l e s p e c i e s , p a r t i a l t a x o n o m i c i d e n t i f i c a t i o n s d i d n o t r e v e a l any c h a n g e s i n m a j o r t a x a ( A p p e n d i x F ) . N u l l h y p o t h e s i s 2 ( i i ) (page 41) s t a t e d t h a t a c i d i f i c a t i o n t o pH 5.0 d o e s n o t a f f e c t p h y t o p l a n k t o n community b i o m a s s and s i z e c o m p o s i t i o n . The r e s u l t s p r o v i d e no e v i d e n c e a g a i n s t t h i s h y p o t h e s i s , a t l e a s t f o r a c i d i f i c a t i o n t o pH 5.6. Most i m p o r t a n t l y , t h e abundance o f s m a l l , m o s t l y e d i b l e f orms o f a l g a e d i d n o t c h a n g e . T h i s i s c o n s i s t e n t w i t h t h e a b s e n c e o f any e v i d e n c e o f i n c r e a s e d f o o d l i m i t a t i o n o f z o o p l a n k t o n i n 139 t r e a t m e n t A ( S e c t i o n 3 . 2 . 2 ) . 3.3.3 E f f e c t s o f F e r t i l i z a t i o n T r e a t m e n t F 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 c h l o r o p h y l l a and p h a e o p h y t i n a c o n c e n t r a t i o n s t h a n t h e c o n t r o l s ( T a b l e 1 3 ) . W e t z e l (1975, page 353) l i s t 0.3-3 ug»L'1 a s a t y p i c a l r a n g e o f c h l o r o p h y l l a c o n c e n t r a t i o n s ( u n c o r r e c t e d f o r p h a e o p h y t i n a) i n o l i g o t r o p h i c l a k e s , and 2-15 u g # L ~ 1 a s t y p i c a l f o r m e s o t r o p h i c l a k e s . The o b s e r v e d r a n g e o f p i g m e n t c o n c e n t r a t i o n s ( F i g u r e 20) s u g g e s t s t h a t f e r t i l i z a t i o n s h i f t e d t h e c y l i n d e r s t r o p h i c s t a t u s f r o m o l i g o t r o p h i c t o o l i g o - m e s o t r o p h i c . N u t r i e n t e n r i c h m e n t a l s o r e s u l t e d i n l a r g e r a m p l i t u d e p h a s e p l a n e t r a j e c t o r i e s ( F i g u r e 2 6 ) . R a t h e r t h a n d e c l i n i n g 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 l i k e t h e c o n t r o l s , t h e two F c y l i n d e r t r a j e c t o r i e s c o n t i n u e d t o c i r c u l a t e a r o u n d c e n t r o i d s a t a p p r o x i m a t e l y 80 ug»L~1 z o o p l a n k t o n and 0.4 Atg»L~ 1 c h l o r o p h y l l , u n t i l l a t e September. In l a t e September, t h e p h a s e p l a n e t r a j e c t o r i e s i n t r e a t m e n t s C, A and F a l l showed i n c r e a s e d c h l o r o p h y l l a c o n c e n t r a t i o n s ( F i g u r e 2 6 ) . T h e s e i n c r e a s e s were a p p a r e n t l y n o t t h e r e s u l t o f c h a n g e s i n a l g a l c h l o r o p h y l l c o n t e n t ( F i g u r e 2 5 ) , but may have been due t o c l a d o c e r a n s s w i t c h i n g t o i n v e s t i n g e n e r g y i n r e s t i n g eggs r a t h e r t h a n a s e x u a l eggs and new ( a l g a e -c o nsuming) i n d i v i d u a l s . The mean p e r c e n t a g e o f l i v e c h l o r o p h y l l ( F i g u r e 24) was l o w e r i n t r e a t m e n t F (36.6 ± 1.4%) t h a n i n t r e a t m e n t C (41.8 ± 1.6%), t h o u g h c y l i n d e r 1F showed an anomalous peak o f 60.9% on 140 September 13. P h a e o p h y t i n a may have g e n e r a l l y been a l a r g e r f r a c t i o n i n t h e F c y l i n d e r s b e c a u s e of g r e a t e r z o o p l a n k t o n g r a z i n g , w h i c h l e a d s t o d i g e s t i o n o f c h l o r o p h y l l a and i t s c o n v e r s i o n t o p h a e o p h y t i n a ( D a l e y and Brown 1973). The mean t o t a l a l g a l b i o m a s s was s i g n i f i c a n t l y l o w e r (p = 0.0005) i n t h e F c y l i n d e r s (0.33 ± 0.07 m g » L " 1 ) t h a n i n t h e c o n t r o l s , due t o f a r l e s s c e l l volume i n t h e 13-18 um and >30 um s i z e c a t e g o r i e s ( T a b l e 14 shows d i f f e r e n c e s s i g n i f i c a n t a t p < 0.0001). However, t h e mean bi o m a s s o f a l g a l c e l l s l e s s t h a n 13 um ( w h i c h I assume t o be m o s t l y e d i b l e ) was s i g n i f i c a n t l y g r e a t e r i n t h e F c y l i n d e r s (0.12 ± 0.035 m g « L " 1 ) t h a n i n t h e c o n t r o l s (0.060 ± 0.004). The p e r c e n t of t o t a l a l g a l b i o m a s s l e s s t h a n 13 um a v e r a g e d 43.9% i n t r e a t m e n t F, b u t o n l y 17% i n t h e c o n t r o l s ( F i g u r e 22 and T a b l e 1 5 ) . N u t r i e n t e n r i c h m e n t may h a v e p r e f e r e n t i a l l y b e n e f i t t e d s m a l l e r c e l l s , o r may have s i m p l y a l l o w e d p r i m a r y p r o d u c t i o n t o s t a y a h e a d o f z o o p l a n k t o n h e r b i v o r y . G i v e n t h e h i g h e r c h l o r o p h y l l and l o w e r t o t a l a l g a l b i o m a s s i n F t h a n C, i t i s no s u r p r i s e t h a t t h e r a t i o o f t h e s e v a r i a b l e s , an e s t i m a t e o f a l g a l c h l o r o p h y l l c o n t e n t ( F i g u r e 2 2 ) , was h i g h e r i n F (3.7 ± 1.0 jug c h l o r o p h y l l a • mg a l g a e " 1 ) t h a n i n C (0.67 ± 0 . 0 9 ) . E s t i m a t e d a l g a l c h l o r o p h y l l c o n t e n t s ( w h i c h a r e o n l y a p p r o x i m a t e due t o i m p r e c i s e e s t i m a t e s o f a l g a l b i o m a s s ) r e a c h e d maxima o f 17.5 jig c h l o r o p h y l l a • mg a l g a e ' L on A u g u s t 1 i n c y l i n d e r IF and 10.3 on A u g u s t 16 i n 8F. By c o m p a r i s o n , t h e mean i c e f r e e s e a s o n a l g a l c h l o r o p h y l l c o n t e n t i n 31 l a k e s i n t h e Kawartha L a k e s - Bay o f Q u i n t e r e g i o n o f O n t a r i o 141 was about 3.07±0.67 nq c h l o r o p h y l l a • mg a l g a e " 1 (computed from 28 p o i n t s i n F i g u r e 4 of N i c h o l l s and D i l l o n ( 1 9 7 8 ) ) . S i n c e i n t h e f e r t i l i z e d c y l i n d e r s an a v e r a g e o f o n l y 56 ± 5% o f t h e c e l l volume was i d e n t i f i e d t a x o n o m i c a l l y , c o m p a r i s o n s o f p h y l e t i c c o m p o s i t i o n o f t h e p h y t o p l a n k t o n community i n t h e C and F t r e a t m e n t s a r e n o t w o r t h w h i l e . In summary, f e r t i l i z a t i o n i n c r e a s e d t h e a l g a l c h l o r o p h y l l c o n t e n t and abundance o f s m a l l , m o s t l y e d i b l e c e l l s . Though I do n o t have d a t a on p r i m a r y p r o d u c t i o n , t h e r e i s good e v i d e n c e ( f r o m p h y t o p l a n k t o n s i z e - d i s t r i b u t i o n s , p h a e o p h y t i n c o n c e n t r a t i o n s and z o o p l a n k t o n r e p r o d u c t i o n ) o f i n c r e a s e d f o o d a v a i l a b i l i t y t o c r u s t a c e a n z o o p l a n k t o n . T h i s r e s u l t c o n t r a d i c t s n u l l h y p o t h e s i s 3 ( i i i ) (page 4 1 ) , w h i c h s t a t e d t h a t f e r t i l i z a t i o n a l o n e d o e s n o t a f f e c t t h e a v a i l a b i l i t y o f p h y t o p l a n k t o n f o o d f o r z o o p l a n k t o n . 3^.3^'i. E f f e c t s o f A c i d i f i c a t i o n a nd F e r t i l i z a t i o n (i_) J o i n t F l u c t u a t i o n s i n C h l o r o p h y l l , P h a e o p h y t i n and T o t a l  Z o o p l a n k t o n B i o m a s s When a v e r a g e d o v e r t h e whole e x p e r i m e n t , b o t h t h e mean and v a r i a n c e o f a l g a l p igment c o n c e n t r a t i o n s were much h i g h e r i n AF c y l i n d e r s (1.39 ± 0.11 uq*h~1 c h l o r o p h y l l a a n d 1.82 ± 0.10 nq»L~ 1 p h a e o p h y t i n a) t h a n i n F (0.51 ± 0.044 and 0.87 ± 0.056). U n t i l A u g u s t 9, c h l o r o p h y l l and p h a e o p h y t i n a c o n c e n t r a t i o n s were s i m i l a r u n d e r t h e F and AF t r e a t m e n t s ( F i g u r e 2 0 ) . D u r i n g A u g u s t 9 t o 23 (when D a p h n i a r o s e a d e n s i t i e s r a p i d l y d e c l i n e d ) , 142 p h a e o p h y t i n - c o r r e c t e d 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 i n c r e a s e d 910% i n c y l i n d e r 3AF, and a p p r o x i m a t e l y 580% i n c y l i n d e r 7AF. (The A u g u s t 9 c h l o r o p h y l l c o n c e n t r a t i o n i n 7AF i s m i s s i n g f r o m F i g u r e 20, b u t was l i k e l y c l o s e t o 0.5 /ug*L" 1 b a s e d on t h e S e c c h i d e p t h a t t h a t t i m e (5.5 m) and t h e c h l o r o p h y l l - S e c c h i d e p t h r e g r e s s i o n e q u a t i o n i n A p p e n d i x D ) . Were t h e l a r g e i n c r e a s e s i n t r e a t m e n t AF 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 t h e r e s u l t o f r e d u c e d z o o p l a n k t o n g r a z i n g , o r d i d p r i m a r y p r o d u c t i v i t y i n AF c y l i n d e r s i n c r e a s e r e l a t i v e t o t h e F c y l i n d e r s ? W i t h o u t p r i m a r y p r o d u c t i v i t y measurements I c a n n o t d i s c r i m i n a t e between t h e s e two p o s s i b i l i t i e s . However, t h e f a c t t h a t 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 on A u g u s t 23 were 2 ag*L~1 h i g h e r i n 3AF t h a n 7AF i s c o n s i s t e n t w i t h t h e v i e w t h a t t h e c h l o r o p h y l l i n c r e a s e s were due p r i m a r i l y t o t h e l o s s o f D. r o s e a , s i n c e s i g n i f i c a n t l y l o w e r d e n s i t i e s o f d a p h n i d s were p r e s e n t i n 3AF t h a n i n 7AF d u r i n g t h i s p e r i o d ( F i g u r e 12 and S e c t i o n 3 . 2 . 4 ( i ) ) . The p e r c e n t of l i v e c h l o r o p h y l l ( F i g u r e 24) i n c r e a s e d s h a r p l y between A u g u s t 16 and 23 i n b o t h AF c y l i n d e r s , b u t n o t i n t h e F e n c l o s u r e s . T h i s d i f f e r e n c e may have been due t o l o w e r h e r b i v o r e g r a z i n g i n t r e a t m e n t AF, c a u s i n g r e d u c e d c o n v e r s i o n o f c h l o r o p h y l l t o p h a e o p h y t i n ( D a l e y and Brown 1973). The d e c r e a s e i n p e r c e n t p h a e o p h y t i n a i n t r e a t m e n t AF ( i n c r e a s e i n p e r c e n t l i v e c h l o r o p h y l l ) was p r o b a b l y n o t r e l a t e d t o d i r e c t pH e f f e c t s on t h e a l g a e , s i n c e D a l e y (1973) f o u n d t h e r e v e r s e : d e c r e a s e d pH l e d t o an i n c r e a s e d r a t e o f p h a e o p h y t i n a f o r m a t i o n f r o m c h l o r o p h y l l . O v e r a l l , t h e mean % l i v e c h l o r o p h y l l was l o w e r i n 143 F (36.6 ± 1.4%) than i n AF (42.0 ± 1.7%). 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 d e c l i n e d 2 M g # L " 1 i n both AF c y l i n d e r s between August 23 and 30, w i t h very l i t t l e i n c r e a s e i n t o t a l zoop lankton biomass u n t i l the f o l l o w i n g week ( F i g u r e 2 6 ) . These events c r e a t e d "L-shaped" phase p lane t r a j e c t o r i e s i n the AF c y l i n d e r s that were analogous to the path seen i n E u n i c e Lake i n l a t e September, but of much l a r g e r a m p l i t u d e . Changes i n both h e r b i v o r e g r a z i n g and p r i m a r y p r o d u c t i o n were l i k e l y r e s p o n s i b l e f o r the "L-shape" t r a j e c t o r i e s i n A F . I n c r e a s e s i n the food a v a i l a b l e to c l a d o c e r a n zoop lankton l e a d to g r e a t e r f e e d i n g r a t e s (Burns and R i g l e r 1967, Buckingham 1978), deve lopmenta l r a t e s (Weglenska 1971, N e i l l 1981b) and r e p r o d u c t i o n (Weglenska 1971). In treatment A F , h i g h c h l o r o p h y l l a c o n c e n t r a t i o n s on August 23 were f o l l o w e d by h i g h d e n s i t i e s of c l a d o c e r a n eggs on August 30, w i t h development of those eggs i n t o a d u l t s i n the f o l l o w i n g week (See F i g u r e 17 (page 89) f o r Bosmina l o n q i r o s t r i s ) . (With abundant food and temperatures of 1 6 - 1 8 ° C B . l o n q i r o s t r i s and Chydorus s p h a e r i c u s r e q u i r e o n l y 5 to 6 days to r e a c h r e p r o d u c t i v e m a t u r i t y (Goulden et a l . 1978, Weglenska 1971).) The g r a z i n g r a t e s of Bosmina and Chydorus must have been h i g h to have produced such dramat i c i n c r e a s e s i n egg and an imal d e n s i t i e s between August 23 and September 6, and may have been e n t i r e l y r e s p o n s i b l e for the August 23 to 30 d e c l i n e s i n c h l o r o p h y l l a c o n c e n t r a t i o n s . T h i s s u g g e s t i o n i s suppor ted by the sharp decrease i n Chydorus d e n s i t i e s a f t e r September 6, which was very l i k e l y due to the p o p u l a t i o n o v e r s h o o t i n g the a v a i l a b l e food s u p p l y . However, 144 n u t r i e n t l i m i t a t i o n a n d / o r s e l f - s h a d i n g ( n o t e t h e t r a n s p a r e n c y d e c r e a s e s d u r i n g A u g u s t 9 t o 23 i n F i g u r e 9) p e r h a p s a l s o c o n t r i b u t e d t o t h e sudden d e c l i n e i n c h l o r o p h y l l a. The September 6 t o 13 r e d u c t i o n s i n t o t a l z o o p l a n k t o n b i o m a s s ( 6 7 % i n 3AF and 66% i n 7AF) may have a l l o w e d c h l o r o p h y l l t o i n c r e a s e s i g n i f i c a n t l y i n t h e f o l l o w i n g week due t o r e d u c e d h e r b i v o r e g r a z i n g . Two u n c e r t a i n t i e s w h i c h weaken t h i s e x p l a n a t i o n a r e t h e d i e t o f C h y d o r u s s p h a e r i c u s ( d i d i t f e e d o f f b o t h p e r i p h y t i c a l g a e and p h y t o p l a n k t o n ? ) a n d t h e a b s e n c e o f c h l o r o p h y l l measurements on September 6. ( i i ) E s t i m a t e s o f A l g a l Biomass by C e l l C o u n t s R e p l i c a t i o n o f a l g a l b i o m a s s ( e s t i m a t e d by c e l l v o l u m e s ) a p p e a r e d a d e q u a t e i n t h e AF c y l i n d e r s i n t h e < 13 um a n d 13-18 um s i z e c a t e g o r i e s , b u t was v e r y p o o r i n t h e 18-30 um s i z e c a t e g o r y ( F i g u r e 2 1 ) . T a b l e 16 shows t h a t d u r i n g t h e p e r i o d f r o m A u g u s t 1 t o September 13, t h e marked d i f f e r e n c e s i n t o t a l a l g a l b i o m a s s between AF r e p l i c a t e s were due a l m o s t e n t i r e l y t o d i f f e r e n c e s i n t h e abundance of one t a x o n o n l y : l a r g e , s p h e r i c a l c o l o n i e s o f C h r y s o c a p s a s p . N o t e t h a t t h e sum o f d i f f e r e n c e s i n t o t a l a l g a l b i o m a s s between t h e two r e p l i c a t e s o v e r t h i s s i x week p e r i o d was o n l y 0.1% a p a r t f r o m t h e sum o f d i f f e r e n c e s i n C h r y s o c a p s a b i o m a s s ( T a b l e 1 6 ) . The low numbers o f c o l o n i e s c o u n t e d ( T a b l e 16) s u g g e s t t h a t t h e l a r g e w i t h i n - t r e a t m e n t v a r i a t i o n may have been due p r i m a r i l y t o i n a d e q u a t e s u b - s a m p l i n g p r o c e d u r e a s d i s c u s s e d i n S e c t i o n 2.2.2 ( i i ) . T h e s e g l o b u l a r c o l o n i e s ( w h i c h m easured on a v e r a g e 22 jim i n d i a m e t e r and 5600 Table 16. Differences in total algal biomass and biomass of Chrysocapsa colonies between cylinders 7AF and 3AF during the period August 1 to September 13, 1979. Cylinder 3AF 7AF (7AF-3AF) Date Total Algal Biomass (mg-L"1) Chrysocapsa Colonies Total Algal Biomass (mg-L"1) Chrysocapsa colonies Total Algal Biomass (mg-L"1) Chrysocapsa colonies Bi omass (mg-L"1) No. Colonies Biomass (mg-L"1) No. Colonies Biomass (mg-L"1) No. Colonies August 1 0.75 0.18 1 2.28 1.78 10 1.52 1.60 9 9 0.13 0. 0 0.82 0.36 2 0.69 0.36 2 16 0.51 0.04 10* 3.25 2.85 16 2.74 2.80 * 23 0.76 0.36 2 1.16 0.89 10 0.40 0.53 8 30 0.79 0.36 2 1.37 1.07 9 0.58 0.71 7 September 6 0.31 0. 0 1.75 1.42 8 1.44 1.42 8 13 0.14 0. 0 0.35 0.18 1 0.22 0.18 1 Biomass Totals 3.38 0.94 10.97 8.54 7.59 7.60 (Numbers may not sum to totals due to rounding errors.) * Smaller colonies 8umin diameter. (All other colonies were 24umin diameter.) The difference between 7AF and 3AF in No. Colonies is not shown due to this size difference. 146 Mm3 i n volume) are most ly c l e a r muc i lage ( P r e s c o t t 1978), so tha t they would not c o n t r i b u t e much c h l o r o p h y l l . T h i s sugges t ion i s c o n s i s t e n t w i t h observed 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 , which were lower i n 7AF than 3AF i n s p i t e of a much g r e a t e r volume of Chrysocapsa in the former t u b e . Chrysocapsa a l s o caused the r a t i o of c h l o r o p h y l l to a l g a l biomass ( F i g u r e 25) to be much lower in the 7AF (2 .9 ± 0.85 M9 c h l o r o p h y l l mg a l g a l b i om ass" 1 ) than i n 3AF (4 .8 ± 0 . 7 5 ) . Due to C h r y s o c a p s a , the mean biomass i n the 18-30 urn s i z e c l a s s was g r e a t e r i n AF than F . Treatment AF a l s o c o n t a i n e d a g r e a t e r mean biomass of f i l a m e n t s and c e l l s of d iameter above 30 Mm (Table 14 and Appendix F ) . A l t h o u g h Chrysocapsa was more abundant i n AF than F , i t does not f o l l o w tha t a c i d i f i c a t i o n of n u t r i e n t - e n r i c h e d waters f a c i l i t a t e d the growth of C h r y s o c a p s a ; r e l a t i v e l y l a r g e numbers of c o l o n i e s were p r e s e n t i n 7AF on August 1 when the e p i l i m n e t i c pH was s t i l l 6 .15 . The c o n c u r r e n t sharp i n c r e a s e s i n t o t a l z o o p l a n k t o n biomass and Chrysocapsa c o l o n i e s d u r i n g the p e r i o d from J u l y 26 to August 1 i n d i c a t e s t h a t the abundance of these l a r g e c o l o n i e s were not reduced by i n t e n s i v e z o o p l a n k t o n g r a z i n g p r e s s u r e . Consumption of s m a l l e r a l g a l c e l l s by Daphnia may have f a c i l i t a t e d growth of Chrysocapsa through removal of c o m p e t i t o r s , as suggested by Lynch and S h a p i r o (1981) f o r Aphanizomenon. Of p a r t i c u l a r i n t e r e s t was the biomass of s m a l l , mos t ly e d i b l e a l g a l c e l l s < 13 am i n treatment A F , r e l a t i v e t o t rea tments F and A . Over the whole exper iment , there was no s i g n i f i c a n t d i f f e r e n c e between trea tments AF and F i n the mean 147 b i o m a s s l e s s t h a n 13 Mm (0.12 ± 0.14 m g » L ~ 1 i n AF and 0.12 ± 0.035 i n F; p=0.86). However, d i f f e r e n c e s between F and AF i n t h e a l g a l b i o m a s s < 13 Mm were a p p a r e n t i n t h e s e c o n d h a l f o f t h e e x p e r i m e n t , and a r e d i s c u s s e d i n t h e n e x t s e c t i o n i n d e t a i l . L a s t l y , I f o u n d s i g n i f i c a n t l y g r e a t e r mean v o l u m e s o f a l g a e l e s s t h a n 13 Mm d i a m e t e r i n AF t h a n A (p = 0.001; T a b l e 1 4 ) . In a d d i t i o n t o c h l o r o p h y l l measurements and m i c r o s c o p e c e l l c o u n t s , I o b t a i n e d i n f o r m a t i o n on n e t p l a n k t o n c o m p o s i t i o n ( l a r g e r t h a n 54 Mm) f r o m z o o p l a n k t o n s a m p l e s . Net p l a n k t o n became v e r y abundant i n t h e z o o p l a n k t o n s a m p l e s t a k e n i n AF c y l i n d e r s a f t e r A u g u s t 23. Common g e n e r a were T a b e l l a r i a s p p . , Oedogonium spp. and M o u g e o t i a s p p . M o u g e o t i a was m o s t l y d i s t r i b u t e d a t 5-7 m, b a s e d on v e r t i c a l l y s t r a t i f i e d s a m p l e s t a k e n f r o m t h e AF c y l i n d e r s on September 6 a n d O c t o b e r 11. A l t h o u g h t h e above a l g a l g e n e r a were a l s o f o u n d i n z o o p l a n k t o n s a m p l e s from t h e F c y l i n d e r s , t h e i r d e n s i t i e s were c o n s i d e r a b l y l o w e r . ( i i i ) E f f e c t s o f L o s s o f D a p h n i a on H e r b i v o r e C o n t r o l o f  P h y t o p l a n k t o n and P h y t o p l a n k t o n Community S i z e S t r u c t u r e In t h i s s e c t i o n , I compare 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 c o m m u n i t i e s i n t r e a t m e n t s F a n d AF i n t e r m s of t h e i r s i z e s t r u c t u r e and p a r t i c l e r e m o v a l c a p a b i l i t i e s . The o b j e c t i v e o f t h i s c o m p a r i s o n i s t o d e t e r m i n e whether a c i d - i n d u c e d m o r t a l i t y o f D a p h n i a s u b s t a n t i v e l y a l t e r e d t h e a p p a r e n t i m p a c t o f h e r b i v o r e s on t h e p h y t o p l a n k t o n community. T h e r e f o r e , t h e a n a l y s i s examines 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 a t a b e f o r e and 148 a f t e r t h e l o s s o f D. r o s e a i n AF. F i g u r e 27 shows t h e e s t i m a t e d f i l t e r i n g r a t e s f o r e a c h s p e c i e s i n t h e F and AF t r e a t m e n t s , b o t h p r i o r t o D a p h n i a ' s d e c l i n e ( J u l y 16 t o Au g u s t 9 t h ) and f o l l o w i n g i t ( A u g u s t 30 t o O c t o b e r 1 1 ) . The e i g h t s p e c i e s have been g r a p h e d a c c o r d i n g t o t h e i r mean s i z e s t o r e f l e c t z o o p l a n k t o n community s i z e s t r u c t u r e . E s t i m a t e d f i l t e r i n g r a t e s were computed by m u l t i p l y i n g mean s p e c i e s d e n s i t i e s by p u b l i s h e d f i l t e r i n g r a t e s m e a sured a t s i m i l a r t e m p e r a t u r e s b u t c i r c u m n e u t r a l pH ( T a b l e 1 7 ) . T h e s e f i l t e r i n g e s t i m a t e s a r e o n l y a p p r o x i m a t e , s i n c e t h e y assume t h a t 1) t h e s i z e s o f a n i m a l s , s e s t o n 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 , and f o o d q u a l i t y i n H e a r t L a k e (Haney 1973) were s i m i l a r t o t h o s e f o u n d i n t h e F and AF c y l i n d e r s ; and 2) f i l t e r i n g r a t e change a r e n e g l i g i b l e . I w i l l comment on t h e s e two a s s u m p t i o n s b e f o r e d i s c u s s i n g t h e e s t i m a t e d f i l t e r i n g r a t e s . 149 F i g u r e 27. E s t i m a t e d f i l t e r i n g r a t e s o f z o o p l a n k t o n s p e c i e s p r e s e n t i n t r e a t m e n t s F and AF o v e r two t i m e p e r i o d s ; J u l y 16 - A u g u s t 9 ( p r i o r t o c o l l a p s e o f D. r o s e a i n A F ) ; and A u g u s t 30 - O c t o b e r 11 (when o t h e r c l a d o c e r a n s had r e p l a c e d D a p h n i a r o s e a ) . A b b r e v i a t i o n s u s e d f o r t r e a t m e n t s : F = f e r t i l i z a t i o n o n l y ; A F = a c i d i f i c a t i o n and f e r t i l i z a t i o n . S t a n d a r d d e v i a t i o n s a p p l i e d t o mean l e n g t h s t o i l l u s t r a t e t h e r a n g e o f a n i m a l s i z e s and i m p l i e d p a r t i c l e h a n d l i n g c a p a b i l i t y . A v e r a g e o f 68 ± 20.0 (SD) a n i m a l s p e r s p e c i e s m e asured i n f i r s t t i m e p e r i o d and 32 ± 8.5 i n s e c o n d . F i l t e r i n g r a t e s t a n d a r d e r r o r s b a s e d on n = 8 and n = 10 f o r f i r s t and s e c o n d t i m e p e r i o d s r e s p e c t i v e l y . T o t a l f i l t e r i n g r a t e s ( " T o t a l = ") a r e t h e sums o f i n d i v i d u a l s p e c i e s ' mean r a t e s . L a r g e s i z e o f D. r o s e a i n AF d u r i n g A u g u s t 30 -O c t o b e r 11 p e r i o d e x p l a i n e d i n S e c t i o n 3 . 2 . 4 ( i ) . Mean f i l t e r i n g r a t e s p e r i n d i v i d u a l a n i m a l drawn from s o u r c e s l i s t e d i n T a b l e 17. 150 . Total = 6.5 % © r July 16 to August 9 Dr 0 . 4 0 .8 1.2 1.6 Total = 6.8 % July 16 to August 9 Bl Dt Dr Hg 2.0 rh Dk Total = 3.0% © August 30 to October II BI rH -Cs Db Dt J B L_ J , D , Dk 0.4 0.8 Total = 4.0 % © August 30 to October II 1.2 1.6 2.0 B l Bosmina longirostris Cs Chydorus sphaericus Db Diaphanosoma brachyurum Cp Ceriodaphnia pulchella Dt Diaptomus tyre l l i Dr Daphnia rosea Hg Holopedium gibberum Dk Diaptomus kenai B l Cs Db Cp Dt Dr fir 0 .4 O B 1.2 1.6 2.0 " 0.4 0.8 M E A N L E N G T H OF Z O O P L A N K T O N (mm)±SD 2.0 151 T a b l e 17. Assumed f i l t e r i n g r a t e s f o r s p e c i e s i n F i g u r e 27. S p e c i e s F i l t e r i n g R a t e R e f e r e n c e m l » i n d " 1 » d a y " 1 ( 1 ) D a p h n i a r o s e a  D i a ptomus k e n a i  D i a ptomus t y r r e l l i 5.5 16.6 0.48 H o l o p e d i u m g i b b e r u m 9.4 Bosmina l o n g i r o s t r i s 0.44 Diaphanbsoma b r a c h y u r u m 1.6 C e r i o d a p h n i a p u l c h e l l a 4.6 C h y d o r u s s p h a e r i c u s 0.18 Haney (1973) Buckingham ( 1978) ( 2 > Haney (1973) ( 3 ) Haney (1973) Haney (1973) Haney (1973) Haney (1973) Haney (1973) % f i l t e r e d » d a y ~ 1 = ( i n d « m l " 1 ) x ( m l » i n d " ' " d a y " 1 ) X 1 0 0 a s s u m i n g a d r y w e i g h t s e s t o n c o n c e n t r a t i o n o f 5 m g » l i t e r _ 1 , t e m p e r a t u r e s o f 8-12°C and mean D i a p t o m u s k e n a i b i o m a s s of 21.7 /ig. measured f o r D. o r e q o n e n s i s W i t h r e s p e c t t o t h e f i r s t a s s u m p t i o n , t h e l e n g t h s l i s t e d by Haney (1973; page 110) were g e n e r a l l y g r e a t e r t h a n t h o s e shown i n F i g u r e 27, a n d mean s e s t o n l e v e l s i n t r e a t m e n t s AF and F were a b o u t 3-6 t i m e s l o w e r t h a n t h e l e v e l s f o u n d i n H e a r t L a k e (15 m g « l i t e r _ 1 ) . (As s e s t o n was not me a s u r e d i n t h e e n c l o s u r e s , t h i s e s t i m a t e i s b a s e d on mean 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 p r e v i o u s measurements o f l a k e s e s t o n ( N e i l l 1978).) S i n c e l o w e r f o o d abundance r e q u i r e s h i g h e r f i l t e r i n g r a t e s t o o b t a i n t h e same e n e r g y ( B u r n s and R i g l e r 1967), F i g u r e 27 l i k e l y u n d e r e s t i m a t e s t h e p e r c e n t o f e n c l o s e d w a t e r t h e a n i m a l s would 152 have f i l t e r e d p e r day. A s s u m p t i o n 2) may be j u s t i f i a b l e f o r a c i d - t o l e r a n t s p e c i e s , but t h i s i s by no means c e r t a i n . Though i n Haney's s t u d y , B. l o n g i r o s t r i s showed no r e d u c t i o n i n f i l t e r i n g r a t e between c i r c u m n e u t r a l H e a r t L a k e and Bog L a k e (pH 4 . 5 ) , sudden c h a n g e s i n pH may c a u s e t e m p o r a r y d e c r e a s e s i n maximal f i l t e r i n g r a t e s ( I v a n o v a 1969, K r i n g a n d O ' B r i e n 1 9 76). T h e s e c o n s i d e r a t i o n s n o t w i t h s t a n d i n g , t h e r e l a t i v e m a g n i t u d e s o f f i l t e r i n g shown i n F i g u r e 27 a r e l i k e l y r e a s o n a b l e . P r i o r t o t h e d e c l i n e o f D. r o s e a A u g u s t 9 t o 16, t r e a t m e n t AF f i l t e r i n g was v e r y s i m i l a r t o t r e a t m e n t F ( F i g u r e 2 7 ) . From J u l y 16 t o A u g u s t 9, D. r o s e a p r o v i d e d on a v e r a g e a b o u t 80% o f t h e t o t a l z o o p l a n k t o n f i l t e r i n g i n t r e a t m e n t s F and AF. I t i s not known what s i z e c l a s s e s o f a l g a e t h e d a p h n i d s a c t u a l l y consumed. However, I c a n e s t i m a t e t h e maximum s i z e p a r t i c l e i n g e s t e d v i a t h e e m p i r i c a l r e l a t i o n s h i p d e v e l o p e d by B u r n s ( 1 9 6 8 ) : Y = 22X + 4.87 (4) where: Y = d i a m e t e r o f l a r g e s t p l a s t i c b e a d i n g e s t e d (nm), w i t h a 95% c o n f i d e n c e i n t e r v a l o f ±9nm; and X = c a r a p a c e l e n g t h o f a f i l t e r - f e e d i n g c l a d o c e r a n (mm) S u b s t i t u t i n g t h e mean l e n g t h o f D. r o s e a (1.1mm) i n t o e q u a t i o n (4) l e a d s t o t h e s u g g e s t i o n t h a t a l g a l c e l l s up t o 29±9 urn i n d i a m e t e r c o u l d t h e o r e t i c a l l y have been i n g e s t e d by D. r o s e a . 153 From A u g u s t 30 t o O c t o b e r 11, t r e a t m e n t AF had a g r e a t e r mean t o t a l f i l t e r i n g r a t e (4.0%) t h a n F ( 3 . 0 % ) ; ( F i g u r e 2 7 ) . However, t h i s f i l t e r i n g was l a r g e l y p r o v i d e d by s m a l l c l a d o c e r a n s , w h i c h were a p p a r e n t l y n o t so e f f e c t i v e a s t h e F c y l i n d e r z o o p l a n k t o n a t g r a z i n g c h l o r o p h y l l down t o low c o n c e n t r a t i o n s ( F i g u r e 2 6 ) . I h y p o t h e s i z e t h a t t h e l o s s o f D. r o s e a i n AF c r e a t e d an u n b a l a n c e d s i z e d i s t r i b u t i o n o f h e r b i v o r e s , g r e a t l y r e d u c i n g t h e f i l t e r i n g o f l a r g e r a l g a e and t h e r e f o r e a l l o w i n g 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 t o i n c r e a s e . A g a i n u s i n g t h e e m p i r i c a l e q u a t i o n o f B u r n s ( 1 9 6 8 ) , I e s t i m a t e t h a t d u r i n g t h e p e r i o d f r o m A u g u s t 30 t o O c t o b e r 5, a b o u t 85% o f t h e h e r b i v o r o u s f i l t e r i n g i n t h e AF c y l i n d e r s d u r i n g t h e A u g u s t 30 - O c t o b e r 5 p e r i o d would have been by s p e c i e s c o n s u m i n g p r i m a r i l y a l g a l c e l l s l e s s t h a n 13Atm i n s i z e . D. k e n a i was t h e o n l y h e r b i v o r e i n AF l a r g e enough t o f i l t e r l a r g e c e l l s , b u t i t s n u m e r i c a l r e s p o n s e was t o o slow t o f i l l t h e gap l e f t by D. r o s e a ( a t l e a s t w i t h i n t h e t i m e h o r i z o n o f t h i s e x p e r i m e n t ) . D. k e n a i e g g s p r o d u c e d i n f a l l do n o t mature i n t o c o p e p o d i t e s u n t i l t h e f o l l o w i n g F e b r u a r y (A. Chapman, u n p u b l i s h e d d a t a ) . I f t h e z o o p l a n k t o n community c h a n g e s i n AF d e c r e a s e d t o t a l h e r b i v o r e f i l t e r i n g o f t h e a l g a e , b u t i n c r e a s e d t h e f i l t e r i n g o f s m a l l e r p a r t i c l e s , one would e x p e c t g r e a t e r t o t a l a l g a l b i o m a s s i n AF t h a n F, but a s m a l l e r p r o p o r t i o n o f t o t a l a l g a l b i o m a s s i n c e l l s £13jim i n d i a m e t e r . P h y t o p l a n k t o n d a t a were g e n e r a l l y c o n s i s t e n t w i t h t h e s e e x p e c t a t i o n s . T a b l e 18 l i s t s t h e b i o m a s s o f p h y t o p l a n k t o n i n forms w i t h maximum d i m e n s i o n £13ym, and bi o m a s s o f l a r g e r f o r m s . C h r y s o c a p s a c o l o n i e s a r e n o t i n c l u d e d 154 i n T a b l e 18, due t o t h e i r i n e d i b l e n a t u r e and t h e i r s t r o n g b i a s on e s t i m a t e d a l g a l b i o m a s s ( S e c t i o n 3.3.4 ( i i ) ) . T a b l e 18. Mean a l g a l b i o m a s s by majo r s i z e c l a s s e s i n t r e a t m e n t s F and AF, b e f o r e and a f t e r t h e c o l l a p s e o f D a p h n i a i n AF. Biomass o f C h r y s o c a p s a c o l o n i e s n o t i n c l u d e d -se e t e x t f o r e x p l a n a t i o n . Time T r e a t - A l g a l S i z e Mean Biomass % o f Mean T o t a l P e r i o d ment C l a s s ' 1 ) ± SE [mg/L] B i o m a s s J u l y t o Aug. 9 (n=8) 16 AF <13nm > 1 3jLini T o t a l <1 3^m > 1 3 am T o t a l 0.05±0.01 0.09±0.04 0.14±0.04 37, 62, 100, 0.10±0.02 0.20±0.07 0.30±0.07 32 67 100-Aug. 30 t o O c t . 5 £1 3/xm > 1 3 Aim T o t a l (n=12) AF <1 3nm > 13nm T o t a l 0.18±0.06 0.12±0.04 0.30±0.08 59.1 40.9 100 0.11±0.02 0.27±0.05 0.38±0.05 28.8 71.2 100 b a s e d on maximum d i m e n s i o n o f c e l l o r c o l o n y . F i l a m e n t s ( i n c l u d e d i n >13/xm s i z e c l a s s ) were <5% o f t h e t o t a l b i o m a s s i n AF, and <2% i n F ( T a b l e 15 and F i g u r e 2 2 ) . P r i o r t o t h e c o l l a p s e o f D. r o s e a i n t h e AF c y l i n d e r s ( J u l y 16 t o A u g u s t 9) t h e p e r c e n t o f t o t a l a l g a l b i o m a s s ^13um was s i m i l a r i n AF and F ( T a b l e 18) w h i c h i s c o n s i s t e n t w i t h t h e s i m i l a r f i l t e r i n g p a t t e r n t h e s e two t r e a t m e n t s showed d u r i n g t h i s p e r i o d ( F i g u r e 2 7 ) . I t i s s u r p r i s i n g t h a t t h e mean t o t a l a l g a l b i o m a s s was n e a r l y t w i c e a s h i g h i n AF a s i n F d u r i n g t h i s 155 p e r i o d , s i n c e mean c h l o r o p h y l l a l e v e l s were s i m i l a r (0.37 ± 0.07 Axg #L" 1 i n F and 0.42 ± 0.08 y g » L - 1 i n A F ) . I t i s p o s s i b l e t h a t s u b l e t h a l pH l e v e l s i n t r e a t m e n t AF l o w e r e d b o t h t h e a c t u a l f i l t e r i n g r a t e s o f h e r b i v o r e s ( a s shown by K r i n g and O ' B r i e n (1976) f o r D a p h n i a p u l e x ) a n d a l s o t h e c h l o r o p h y l l c o n t e n t o f a l g a l c e l l s . However, I have no d a t a on a c t u a l f i l t e r i n g r a t e s . A f t e r t h e c o l l a p s e o f D a p h n i a (August 30 t o O c t o b e r 5 ) , t r e a t m e n t AF c o n t a i n e d l e s s a l g a l b i o m a s s i n s m a l l c e l l s t h a n d i d t r e a t m e n t F, b o t h q u a n t i t a t i v e l y and as a p e r c e n t o f t o t a l a l g a l b i o m a s s ( T a b l e 1 8 ) . T h e s e r e s u l t s , and t h e h i g h c h l o r o p h y l l a c o n c e n t r a t i o n s i n AF a f t e r t h e l o s s o f D. r o s e a , s u g g e s t t h a t a c i d i f i c a t i o n -i n d u c e d m o r t a l i t y o f D a p h n i a : 1) r e d u c e d o v e r a l l h e r b i v o r e c o n s u m p t i o n o f l a r g e p a r t i c l e s ; a n d 2) l e d t o z o o p l a n k t o n community c h a n g e s w h i c h i n c r e a s e d g r a z i n g p r e s s u r e on s m a l l p a r t i c l e s . H i g h e r 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 i n t r e a t m e n t AF t h a n F i n September m i g h t have been p a r t i a l l y due t o g r e a t e r p r i m a r y p r o d u c t i o n . N u t r i e n t t u r n o v e r may have been f a s t e r i n AF d u r i n g t h i s month, s i n c e s m a l l e r h e r b i v o r e s have f a s t e r m e t a b o l i c r a t e s and h i g h e r P r e l e a s e r a t e s p e r u n i t b i o m a s s ( P e t e r s and R i g l e r 1973, Lehman 1980a). T u r n o v e r r a t e s may a f f e c t b o t h t h e r a t e and q u a l i t y o f p h y t o p l a n k t o n p r o d u c t i o n (Lehman 1980b). W i t h o u t measurements o f p a r t i c l e s i z e - s e g r e g a t e d p r i m a r y p r o d u c t i o n and z o o p l a n k t o n g r a z i n g r a t e s , I c a n n o t q u a n t i t a t i v e l y a s s e s s t h e i m p o r t a n c e o f c h a n g e s i n p r i m a r y p r o d u c t i o n r e l a t i v e t o c h a n g e s i n h e r b i v o r e g r a z i n g . 156 The r e s u l t s i n t h i s l o n g s e c t i o n a r e b e s t summarized by r e f e r r i n g back t o t h e o r i g i n a l n u l l h y p o t h e s e s i n T a b l e 5 (page 4 1 ) . N u l l h y p o t h e s i s 4 ( i i ) s t a t e d t h a t n u t r i e n t e n r i c h m e n t d u r i n g e x p e r i m e n t a l a c i d i f i c a t i o n t o pH 5.0 does n o t a l l e v i a t e a c i d -i n d u c e d s h o r t a g e s o f p h y t o p l a n k t o n f o o d f o r z o o p l a n k t o n . S e c t i o n 3.3.2 d e m o n s t r a t e d t h a t a c i d i f i c a t i o n a l o n e ( t o pH 5.6) d i d n o t d e c r e a s e t h e a v a i l a b i l i t y o f p h y t o p l a n k t o n f o o d . S e c t i o n 3.3.4 showed t h a t f e r t i l i z a t i o n d u r i n g a c i d i f i c a t i o n i n c r e a s e d t h e c h l o r o p h y l l a c o n c e n t r a t i o n s and b i o m a s s o f e d i b l e a l g a e (<13Mm) o v e r t h a t i n t r e a t m e n t A. However, t h e b e n e f i t s o f t h i s i n c r e a s e d f o o d t o a c i d - s e n s i t i v e D a p h n i a r o s e a were overwhelmed by t h e t o x i c i t y o f pH 5.4 water ( S e c t i o n 3.2.4 ( i ) ) . N u l l h y p o t h e s i s 5 ( i i ) s t a t e d t h a t g r a d u a l e x p e r i m e n t a l a c i d i f i c a t i o n o f m o d e r a t e l y e n r i c h e d w a t e r s does n o t a f f e c t p h y t o p l a n k t o n b i o m a s s o r s i z e c o m p o s i t i o n . The r e s u l t s f r o m t r e a t m e n t s F and AF c o n t r a d i c t t h i s h y p o t h e s i s . A f t e r t h e c o l l a p s e o f D. r o s e a i n t r e a t m e n t AF, mean ' c h l o r o p h y l l a c o n c e n t r a t i o n s i n c r e a s e d s i x f o l d , w h i l e i n t r e a t m e n t F c o n c e n t r a t i o n s r e m a i n e d c o n s t a n t . The r e p l a c e m e n t o f D. r o s e a by s m a l l e r h e r b i v o r e s i n t r e a t m e n t AF g r e a t l y i n c r e a s e d t h e e s t i m a t e d f i l t e r i n g r a t e s of a l g a l c e l l s s m a l l e r t h a n 13 Mm, and d e c r e a s e d f i l t r a t i o n o f l a r g e r c e l l s , a p p a r e n t l y c a u s i n g AF a l g a l b i o m a s s t o become d o m i n a t e d by l a r g e r c e l l s . In t r e a t m e n t F (where D. r o s e a p e r s i s t e d ) , a l g a l b i o m a s s was d o m i n a t e d by s m a l l e r c e l l s . 157 N u l l h y p o t h e s i s 5 ( i i i ) s t a t e d t h a t g r a d u a l e x p e r i m e n t a l a c i d i f i c a t i o n o f m o d e r a t e l y e n r i c h e d w a t e r s does n o t a f f e c t t h e a b i l i t y of p l a n k t o n s y s t e m s t o a b s o r b e n r i c h m e n t p e r t u r b a t i o n s . The e x p e r i m e n t d i d n o t p r o c e e d l o n g enough t o r i g o r o u s l y t e s t t h i s h y p o t h e s i s . However, t h e r e s u l t s s u g g e s t t h a t a c i d i f i c a t i o n may i n c r e a s e t h e amount o f p l a n k t o n s y s t e m f l u c t u a t i o n f o l l o w i n g e n r i c h m e n t . D u r i n g t h e s i x weeks f o l l o w i n g t h e c o l l a p s e o f D a p h n i a , t r e a t m e n t AF c h l o r o p h y l l a c o n c e n t r a t i o n s i n c r e a s e d s i x f o l d , t h e n d r o p p e d s h a r p l y as C h y d o r u s s p h a e r i c u s , Bosmina l o n q i r o s t r i s , and Diaphanosoma  b r a c h y u r u m i n c r e a s e d , and t h e n a g a i n i n c r e a s e d a b r u p t l y when t h e s e z o o p l a n k t e r s c o l l a p s e d . By c o n t r a s t , t r e a t m e n t F c h l o r o p h y l l a c o n c e n t r a t i o n s f l u c t u a t e d r e l a t i v e l y l i t t l e d u r i n g t h e same p e r i o d . 158 4. DISCUSSION "I n o u r e n d e a v o r t o u n d e r s t a n d r e a l i t y we a r e somewhat l i k e a man t r y i n g t o u n d e r s t a n d t h e mechanism o f a c l o s e d w a t c h . He s e e s t h e f a c e and t h e moving hands, even h e a r s i t s t i c k i n g , b u t has no way o f o p e n i n g t h e c a s e . I f he i s i n g e n i o u s , he may form some p i c t u r e o f a mechanism w h i c h c o u l d be r e s p o n s i b l e f o r a l l t h e t h i n g s he o b s e r v e s , b u t he may n e v e r be q u i t e s u r e h i s p i c t u r e i s t h e o n l y one w h i c h c o u l d e x p l a i n h i s o b s e r v a t i o n s . " E i n s t e i n (1938; page 31) T h i s e x p e r i m e n t a t t e m p t e d t o s i m u l a t e l a k e a c i d i f i c a t i o n w i t h i n p l a s t i c e n c l o s u r e s o v e r a p e r i o d o f t h r e e months, t h o u g h a c i d i f i c a t i o n a c t u a l l y t a k e s p l a c e i n whole l a k e s (and t h e i r w a t e r s h e d s ) o v e r p e r i o d s g e n e r a l l y g r e a t e r t h a n t e n y e a r s . T h e r e a r e t h r e e m a j o r ways i n w h i c h t h e e x p e r i m e n t a l s e t t i n g and t r e a t m e n t s c o u l d m i s r e p r e s e n t l a k e a c i d i f i c a t i o n : 1) E n c l o s u r e e f f e c t s : d i f f e r e n c e s i n b i o p h y s i c a l c o n d i t i o n s between t h e l a k e and t h e e x p e r i m e n t a l e n c l o s u r e s ; 2) N a t u r e o f t h e p e r t u r b a t i o n : d i f f e r e n c e s between t h e p h y s i c a l and c h e m i c a l c o n d i t i o n s o b s e r v e d i n a c i d i f y i n g l a k e s and t h e c o n d i t i o n s p r o d u c e d by my 159 e x p e r i m e n t a l t r e a t m e n t s . 3) R e s p o n s e t o p e r t u r b a t i o n : d i f f e r e n c e s between l a k e s and e n c l o s u r e s i n t h e i r r e s p o n s e s t o i n c r e a s e d a c i d a n d / o r n u t r i e n t l o a d i n g , due t o t h e l i m i t e d s i z e a n d d u r a t i o n o f t h e e n c l o s u r e e x p e r i m e n t . The f i r s t two i s s u e s a r e c o n s i d e r e d i n S e c t i o n s 4.1 and 4.2. I have a d d r e s s e d t h e t h i r d i s s u e t h r o u g h o u t S e c t i o n s 4.3 t o 4.5, by c o m p a r i n g t h e r e s p o n s e s of p a r t i c u l a r e c o s y s t e m components i n t h i s e x p e r i m e n t w i t h t h o s e o b s e r v e d i n a c i d i c l a k e s . 4.j_ E n c l o s u r e E f f e c t s D i f f e r e n c e s between the c o n t r o l c y l i n d e r s and t h e l a k e do n o t a f f e c t c o m p a r i s o n s between c y l i n d e r s . However, t h e v a l u e o f t h i s e x p e r i m e n t a s a s i m u l a t i o n o f l a k e a c i d i f i c a t i o n d e p e n d s upon k e e p i n g t h e b e h a v i o u r o f t h e c o n t r o l c y l i n d e r s f r o m d i v e r g i n g t o o f a r f r o m t h a t of t h e l a k e . The major r e s u l t s r e l e v a n t t o t h e n u l l h y p o t h e s e s c o n c e r n i n g e n c l o s u r e e f f e c t s a r e l i s t e d i n T a b l e 19. C o n t r o l e n c l o s u r e s showed s i g n i f i c a n t l y l o w e r a l k a l i n i t i e s a nd h y d r o g e n i o n c o n c e n t r a t i o n s t h a n t h e l a k e a t 1 and 3 m, w h i c h I h y p o t h e s i z e was p r i m a r i l y t h e r e s u l t o f l i g h t l i m i t a t i o n on p h o t o s y n t h e t i c u p t a k e o f N 0 3 _ , a p r o c e s s t h a t g e n e r a t e s a l k a l i n i t y (Brewer and Goldman 1976). S i n c e mean a l k a l i n i t i e s were 16 jxeq'L" 1 l e s s i n t h e c o n t r o l s t h a n i n E u n i c e L a k e , a l g a e i n t h e c o n t r o l s would had t o have t a k e n up r o u g h l y 16 A i e q « L ~ 1 160 Table 19. Summary of 1979 results relevant to hypotheses on enclosure effects. Null Hypotheses (from pg 41) Major Results Enclosure of plankton systems and removal of cutthroat trout predation does not affect: light transmission, pH, alkalinity, phytoplankton, and zooplankton falsified: increased light extinction by non-chlorophyll factors in controls (vs. lake) no evidence against hypothesis falsified: epilimnetic alkalinity significantly reduced, metalimnetic alkalinity significantly increased some evidence against hypothesis: controls had decreased biomass of algal cells < 13 yg and Gleocystis, and apparent decrease in primary production near end of experiment some evidence against hypothesis: controls showed collapse of Holopedium gibberum populations, altered dynamics of Daphnia  rosea,appearance of Chydorus sphaericus, lncreased lengths of Daphnia rosea and Diaptomus kenai 161 l e s s N0 3" (0.99 mg»Lr 1 ) t h a n d i d E u n i c e Lake a l g a e (See S e c t i o n 4.3 f o r s t o i c h i o m e t r i c e q u a t i o n s ) . U n f o r t u n a t e l y I have no measurements o f N0 3" l e v e l s t o t e s t whether o r n o t t h i s c o n d i t i o n p r e v a i l e d . N e i l l ( u n p u b l i s h e d d a t a ) made p h o t o m e t e r measurements i n l i m n o c o r r a l s a n c h o r e d i n G w e n d o l i n e L a k e , u p s t r e a m o f E u n i c e L a k e . He f o u n d t h a t l i g h t l e v e l s a t 1m d e p t h were 20% l o w e r i n t h e l i m n o c o r r a l s t h a n i n t h e l a k e , b u t t h a t d i f f e r e n c e s i n l i g h t i n t e n s i t y d e c r e a s e d w i t h d e p t h and were n e g l i g i b l e below 5 m ( N e i l l , p e r s . comm.). Though my e n c l o s u r e s were 0.5 m l a r g e r i n d i a m e t e r t h a n N e i l l ' s , t h e y p r o b a b l y s h a d e d t h e upper w a t e r s a t l e a s t a s much due t o t h e U V - p r o t e c t i v e o r a n g e c o a t i n g on t h e c y l i n d e r ' s t o p 0.7 m. C o n s i s t e n t w i t h t h e h y p o t h e s i s o f l i m n o c o r r a l s h a d i n g i s t h e c o n t r o l c y l i n d e r s ' g r e a t e r kw ( e x t i n c t i o n c o e f f i c i e n t f o r f a c t o r s o t h e r t h a n c h l o r o p h y l l ) . L i g h t l i m i t a t i o n i s p r o b a b l y n e g l i g i b l e i n e x p e r i m e n t s w i t h c y l i n d e r s g r e a t e r t h a n 5 m i n d i a m e t e r . M u l l e r (1980) f o u n d no d i f f e r e n c e i n e x t i n c t i o n c o e f f i c i e n t s o f l i g h t i n t e n s i t y between c o n t r o l t u b e s and l a k e 223; h i s e n c l o s u r e s were 10 m i n d i a m e t e r and 2.5 m deep. A t t h e o t h e r s i z e e x t r e m e , M a r s h a l l and M e l l i n g e r (1980) f o u n d t h a t l i g h t i n t e n s i t i e s i n 8 l i t e r t r a n s l u c e n t p o l y e t h y l e n e c a r b o y s were e q u i v a l e n t t o Lak e M i c h i g a n l i g h t i n t e n s i t i e s 1.7 m d e e p e r . Though l i g h t l i m i t a t i o n seems r e s p o n s i b l e f o r r e d u c e d e p i l i m n e t i c a l k a l i n i t y i n t h e c o n t r o l s , two a l t e r n a t i v e h y p o t h e s e s t o e x p l a i n t h i s o b s e r v a t i o n a r e : 1) n u t r i e n t l i m i t a t i o n i n t h e " l i m n e t i c z o n e " o f t h e t u b e s , due t o e i t h e r p e r i p h y t o n g r o w t h on t h e s i d e s 162 and i s o l a t i o n f r o m w a t e r s h e d n u t r i e n t r e p l e n i s h m e n t , and 2) l o s s o f b i c a r b o n a t e c o n t r i b u t i o n s from t h e w a t e r s h e d t o m a i n t a i n a l k a l i n i t y . P e r i p h y t o n g r o w t h was n o t o b s e r v e d i n t h e t o p 2 m o f t h e c o n t r o l c y l i n d e r s , and was p r o b a b l y n o t s u f f i c i e n t a t 3 m t o c a u s e n u t r i e n t l i m i t a t i o n . The e x t r e m e l y d r y w e a t h e r o f J u l y and A u g u s t ( F i g u r e 4) makes i t u n l i k e l y t h a t w a t e r s h e d f l u s h i n g o f e i t h e r n u t r i e n t s o r b u f f e r s were r e s p o n s i b l e f o r l o w e r a l k a l i n i t i e s d u r i n g t h e s e months. T h e s e two a l t e r n a t e h y p o t h e s e s t h e r e f o r e a p p e a r i n a d e q u a t e t o e x p l a i n l o w e r e p i l i m n e t i c a l k a l i n i t i e s i n t h e c o n t r o l s t h a n i n t h e l a k e . One c o n s e q u e n c e of t h e d i f f e r e n c e i n e p i l i m n e t i c a l k a l i n i t y between t h e c o n t r o l s and E u n i c e L a k e i s t h a t t h e c y l i n d e r s c o n t a i n e d l e s s p h o t o s y n t h e t i c b u f f e r i n g a g a i n s t a c i d i f i c a t i o n t h a n i n t h e l a k e . The e p i l i m n e t i c pH r e d u c t i o n s a c h i e v e d by my a c i d a d d i t i o n s t o A c y l i n d e r s were a b o u t 0.5 pH u n i t s more s e v e r e t h a n would have o c c u r r e d u nder e q u i v a l e n t H + l o a d i n g t o t h e l a k e . T h i s i s e s t i m a t e d f r o m t h e pH d r o p c a u s e d by t h e l a s t 16 /neq»L" 1 d e c l i n e i n a l k a l i n i t y i n t r e a t m e n t A ( I6=the mean a l k a l i n i t y d i f f e r e n c e between t h e c o n t r o l s and E u n i c e L a k e ) . The v e r t i c a l d i s t r i b u t i o n o f pH and a l k a l i n i t y i s s t r o n g l y a f f e c t e d by t h e u t i l i z a t i o n o f C 0 2 i n t h e zone o f p r i m a r y p r o d u c t i o n ( t r o p h o g e n i c l a y e r ) and i t s l i b e r a t i o n i n t h e d e c o m p o s i t i o n o r t r o p h o l y t i c zone ( H u t c h i n s o n 1957; p g . 6 8 5 ) . Deep, o l i g o t r o p h i c l a k e s w i t h o r t h o g r a d e oxygen c u r v e s f r e q u e n t l y show t h e g r a d u a l d e c l i n e i n pH and a l k a l i n i t y w i t h 163 d e p t h e x h i b i t e d by E u n i c e Lake ( i b i d , p g . 6 8 5 ) . U n l i k e t h e l a k e , t h e c o n t r o l c y l i n d e r s showed i n c r e a s e s i n a l k a l i n i t y and pH between 5 and 7 m. S i n c e c o n t r o l t u b e oxygen measurements on A u g u s t 1 ( o n l y d a t e t a k e n ) showed c o n c e n t r a t i o n s o f 9.5 m g « L ~ 1 a t 7 m, i t seems u n l i k e l y t h a t a n a e r o b i c c o n d i t i o n s and d e n i t r i f i c a t i o n c o u l d have g e n e r a t e d t h e a l k a l i n i t y . A l k a l i n i t y i s i n c r e a s e d by a e r o b i c d e c o m p o s i t i o n o f o r g a n i c n i t r o g e n t o N H „ + and OH" (Brewer and Goldman 1976). However, i f t h e N H 4 + so g e n e r a t e d were c o n v e r t e d i n t o N 0 3 " by n i t r i f y i n g b a c t e r i a , w h i c h commonly o c c u r s above a e r o b i c s e d i m e n t s (Keeney 1972), I w o u l d have s e e n a n e t d e c r e a s e r a t h e r t h a n an i n c r e a s e i n a l k a l i n i t y . W i t h o u t n i t r a t e and ammonia measurements i n t h e c y l i n d e r s a n d t h e l a k e , i t i s d i f f i c u l t t o d e d u c e t h e p r o c e s s e s b e h i n d t h e o b s e r v e d a l k a l i n i t y d i f f e r e n c e s a t 5-7 m. The i n c r e a s e i n d e n s i t i e s o f C h y d o r u s s p h a e r i c u s i n t h e c o n t r o l s s i x weeks a f t e r t h e s t a r t o f t h e e x p e r i m e n t may have been a s s i s t e d by t h e e x i s t e n c e o f t h e c y l i n d e r w a l l s . C. s p h a e r i c u s i s a s u b s t r a t e - o r i e n t e d , l i t t o r a l c l a d o c e r a n r a r e l y f o u n d i n t h e l i m n e t i c zone o f E u n i c e Lake ( F r y e r 1968, N o r t h c o t e and C l a r o t t o 1975). M a r s h a l l and M e l l i n g e r (1980) n o t e d v e r y l a r g e i n c r e a s e s i n C h y d o r u s s p h a e r i c u s a f t e r t h r e e weeks i n 8 l i t e r e n c l o s u r e s ( r e l a t i v e t o L a k e M i c h i g a n ) , b u t i n e x p e r i m e n t s t h e y p e r f o r m e d i n 150 m 3 e n c l o s u r e s i n Lake 223, C h y d o r u s s p h a e r i c u s d i d not d i f f e r i n abundance between t h e c o n t r o l s and t h e l a k e . 164 The r a p i d e x t i n c t i o n of H o l o p e d i u m g i b b e r u m i n a l l l i m n o c o r r a l s was i m p o r t a n t , s i n c e t h i s c l a d o c e r a n i s r e l a t i v e l y a c i d t o l e r a n t . ( M a l l e y e t a l . (1982) f o u n d t h a t d e n s i t i e s o f H. g i b b e r u m i n c r e a s e d t w e n t y f o l d a s L a k e 223 was a c i d i f i e d from pH 6.8 t o 5.37.) However, s i n c e t h e b i o m a s s o f H. g i b b e r u m i n E u n i c e L a k e d e c l i n e d t o v e r y low l e v e l s by t h e t h e end o f A u g u s t ( F i g u r e 10), e n c l o s u r e e f f e c t s were n o t s o l e l y r e s p o n s i b l e f o r t h e e x t i n c t i o n o f H. g i b b e r u m i n t h e l i m n o c o r r a l s . Mean d e n s i t i e s o f d a p h n i d s and t h e i r e ggs were l o w e r i n t h e c o n t r o l s t h a n i n E u n i c e L a k e , d a p h n i d l e n g t h s were l o n g e r i n t h e c o n t r o l s , and t h e c o n t r o l p o p u l a t i o n s c o l l a p s e d much s o o n e r t h a n t h e l a k e p o p u l a t i o n . T h e s e d i f f e r e n c e s were p r o b a b l y due p r i m a r i l y t o l o w e r p r i m a r y p r o d u c t i o n i n t h e c o n t r o l s , and t h e r e m o v a l o f c u t t h r o a t t r o u t p r e d a t i o n ( w h i c h w o u l d i n c r e a s e d a p h n i d l e n g t h s and p e r h a p s a l s o c a u s e o v e r g r a z i n g ) . D i f f e r e n c e s between t h e c o n t r o l s and E u n i c e Lake i n d e n s i t i e s and l e n g t h s o f D i a p t o m u s k e n a i a l s o s u g g e s t t h a t s i z e -s e l e c t i v e p r e d a t i o n was i m p o r t a n t i n E u n i c e L a k e . I t would be v a l u a b l e t o r e p e a t t h i s e x p e r i m e n t i n l a r g e r l i m n o c o r r a l s w i t h a few a d d e d f i s h . L a s t l y , i t s h o u l d be s t r e s s e d t h a t w i t h i n s e a s o n e x t i n c t i o n s o f z o o p l a n k t o n i n a c i d i f y i n g e n c l o s u r e s do n o t n e c e s s a r i l y i m p l y y e a r - t o - y e a r e x t i n c t i o n s i n a c i d i f y i n g l a k e s . D i f f e r e n t a b i o t i c c o n d i t i o n s i n a c i d i f y i n g l a k e s may a l t e r t h e l e v e l o f p h y s i o l o g i c a l s t r e s s ; t h e s l o w e r r a t e o f change i n a b i o t i c c o n d i t i o n s i n l a k e s may a l l o w some p o p u l a t i o n s t o become more a c i d t o l e r a n t ; and r e s t i n g e ggs i n l e s s a c i d , b o t t o m w a t e r s ( o r 165 p o p u l a t i o n s i n u n a c i d i f i e d u p s t r e a m l a k e s ) may a l l o w a c i d -s e n s i t i v e s p e c i e s t o p e r s i s t . 4.2 E v a l u a t i o n o f t h e P h y s i c a l and C h e m i c a l E f f e c t s o f t h e E x p e r i m e n t a l A c i d i f i c a t i o n D i d t h e e x p e r i m e n t a l a c i d i f i c a t i o n i n t h i s s t u d y p r o p e r l y s i m u l a t e l a k e a c i d i f i c a t i o n ? T h i s s e c t i o n compares a c i d l a k e s t o t h e a c i d i f i e d l i m n o c o r r a l s w i t h r e s p e c t t o f o u r p r o p e r t i e s : i o n i c c o n t e n t , e f f e c t i v e n e s s o f a c i d i f i c a t i o n i n r e m o v i n g a l k a l i n i t y , r a t e s o f pH c h a n g e , and t r a n s p a r e n c y . The a c i d i f i c a t i o n of w a t e r s h e d s and l a k e s i s c o r r e l a t e d w i t h many c h e m i c a l c h a n g e s t h a t were n o t s i m u l a t e d i n t h i s e x p e r i m e n t . The c h e m i s t r y o f l a k e s w i t h pH l e v e l s s i m i l a r t o t h e AF c y l i n d e r s i s n e a r l y a l w a y s c h a r a c t e r i z e d by i n c r e a s e d c o n c e n t r a t i o n s o f s u l f a t e and aluminum and o f t e n shows e l e v a t e d l e v e l s o f c a l c i u m , manganese, magnesium, m e r c u r y , cadmium, z i n c , and l e a d (Aimer e t a l . 1978, G j e s s i n g e t a l . 1978, H e n r i k s e n and W r i g h t 1978, H u t c h i n s o n e t a l . 1978, S c h e i d e r e t a l . 1979, G a l l o w a y and L i k e n s 1979, D r i s c o l l e t a l . 1980, D i c k s o n 1980). M o b i l i z a t i o n o f m e t a l s f r o m w a t e r s h e d s o i l s ( C r o n a n and S c h o f i e l d 1979) and l a k e s e d i m e n t s ( S c h i n d l e r e t a l . 1980b) were a l s o n o t s i m u l a t e d . A l s o , o n l y s u l f u r i c a c i d was ad d e d t o t h e e n c l o s u r e s , b u t n o t n i t r i c a c i d , w h i c h c o n t r i b u t e s a b o u t a t h i r d o f t h e a c i d i t y i n p r e c i p i t a t i o n i n Norway ( B r a e k k e 1976) and t h e n o r t h e a s t e r n U n i t e d S t a t e s ( L i k e n s e t a l . 1979). 166 E x t r a p o l a t i o n s f r o m my e x p e r i m e n t t o a c i d i f y i n g l a k e s r e q u i r e one t o assume t h a t i n c r e a s e d [ H + ] and [ S 0 4 " 2 ] a r e t h e key i o n i c s h i f t s a f f e c t i n g p l a n k t o n community change i n a c i d l a k e s . T h i s a s s u m p t i o n i s n o t v a l i d f o r h i g h l y m e t a l -c o n t a m i n a t e d a c i d l a k e s ( e . g . l e s s t h a n 20 km downwind o f S u d b u ry, O n t a r i o ) where b o t h 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 c o m m u n i t i e s have l i k e l y been a l t e r e d by h i g h c o p p e r and n i c k e l c o n c e n t r a t i o n s ( H u t c h i n s o n and S t o k e s 1975, W h i t b y e t a l . 1978, Yan and M i l l e r 1981). In a c i d l a k e s d i s t a n t f r o m e m i s s i o n s o u r c e s , i n c r e a s e d aluminum c o n c e n t r a t i o n s a r e t h e most f r e q u e n t l y c i t e d m e t a l c o n t a m i n a t i o n ( r e v i e w e d by NRCC 1981). A l t h o u g h I d i d n o t measure t o t a l aluminum c o n c e n t r a t i o n s , t h e y c o u l d n o t have i n c r e a s e d s i g n i f i c a n t l y w i t h a c i d i f i c a t i o n i n t h e a b s e n c e o f e i t h e r w a t e r s h e d o r s e d i m e n t s o u r c e s o f m e t a l . I t i s n o t known whether aluminum c o n c e n t r a t i o n s i n a c i d i f i e d l a k e s i n c r e a s e pH t o x i c i t y t o z o o p l a n k t o n . E x p e r i m e n t a l s t u d i e s have d e m o n s t r a t e d t h a t t h e c o n c e n t r a t i o n and s p e c i e s o f aluminum g r e a t l y i n f l u e n c e s t h e s u r v i v a l of brown t r o u t (Muniz and L e i v e s t a d 1980) and b r o o k t r o u t f r y ( D r i s c o l l e t a l . 1980, B a k e r and S c h o f i e l d 1980) a t low pH. Low pH k i l l s z o o p l a n k t o n p r i m a r i l y by l o s s o f i o n r e g u l a t i o n l e a d i n g t o h i g h r a t e s o f sodium l o s s ( P o t t s and F r y e r 1979, Havas 1980). Havas (1980) f o u n d t h a t a d d i t i o n s o f 1 mg A l » L ~ 1 t o pond w a t e r d i d n o t change t h e m o r t a l i t y o f D. magna a t pH 4.5 (median s u r v i v a l t i m e was 70 h o u r s ) , b u t a d d i n g 5 mg A l / 1 r e d u c e d median s u r v i v a l t o 10 h o u r s . She a l s o f o u n d t h a t a d d i n g 20 mg A l » L ~ 1 t o pond w a t e r a t 167 pH 4.5 r e d u c e d t h e median s u r v i v a l t i m e of D. m i d d e n d o r f f i a n a f r o m 140 t o 10 h o u r s . S u b l e t h a l e f f e c t s may however o c c u r a t much l o w e r c o n c e n t r a t i o n s : B i e s i n g e r and C h r i s t e n s e n (1972) f o u n d 16% i m p a i r m e n t o f r e p r o d u c t i o n i n D a p h n i a magna a t 0.32 mg A l » L ~ 1 . Mean aluminum c o n c e n t r a t i o n s i n O n t a r i o and N o r w e g i a n a c i d l a k e s o f pH 4.5 a r e a b o u t 15 t o 60 t i m e s l o w e r t h a n Havas' l e t h a l c o n c e n t r a t i o n s , b u t a r e c o m p a r a b l e t o l e v e l s c a u s i n g r e p r o d u c t i v e i m p a i r m e n t i n t h e l a b o r a t o r y ( T a b l e 2 0 ) . I n e x a m i n i n g T a b l e 20, i t i s i m p o r t a n t t o r e c o g n i z e t h a t s p r i n g m e l t e p i s o d e s may g e n e r a t e aluminum c o n c e n t r a t i o n s s i g n i f i c a n t l y h i g h e r t h a n t h e mean and i n c r e a s e t o x i c i t y ( D r i s c o l l e t a l . 1 980). M a l l e y e t a l . (1982) c o n c l u d e d t h a t e x p e r i m e n t a l a c i d i f i c a t i o n o f Lake 223 t o pH 5.37 d i d n o t e l e v a t e e i t h e r h e avy m e t a l s o r aluminum t o t o x i c l e v e l s , b u t t h e i r e x p e r i m e n t d i d n o t a c i d i f y t h e w a t e r s h e d . T h e r e f o r e , t h e r e i s n o t y e t s u f f i c i e n t e v i d e n c e t o r e j e c t t h e a s s u m p t i o n t h a t s u l f u r i c a c i d a d d i t i o n s a l o n e ( w i t h o u t aluminum a d d i t i o n s ) a d e q u a t e l y s i m u l a t e t h e t o x i c i t y e f f e c t s o f 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 c o m m u n i t i e s a t pH l e v e l s above 5.0. At pH l e v e l s o f 4.5 o r l o w e r , t h i s a s s u m p t i o n a p p e a r s more t e n u o u s . The a c i d i f i c a t i o n t r e a t m e n t a p p l i e d i n t h i s e x p e r i m e n t was a p p r o x i m a t e l y 98% e f f e c t i v e i n d e p l e t i n g t h e e n c l o s u r e s ' a l k a l i n i t y . T h i s c o n t r a s t s w i t h major s p r i n g m e l t H* l o a d s t o a c i d i f y i n g l a k e s i n Norway ( H e n r i k s e n and W r i g h t 1977), Sweden ( H u l t b e r g 1977) a n d O n t a r i o ( J e f f r i e s e t a l . 1 9 79). M e l t w a t e r , a l t h o u g h c a u s i n g s h a r p pH d e c l i n e s i n t h e n e a r s u r f a c e w a t e r s , Table 20. Mean and standard deviation of aluminum concentrations in acidified and non-acidified lakes in Norway (Gjessing et al. 1976) and Ontario (NRCC 1981). 168 Location No. Lakes PH Al (mg/1) Norway Central Coastal Western South-central and eastern 52 20' 23 26 6.4 ± 0.64 5.4 ± 0.40 5.22 ± 0.17 4.76 ± 0.33 0.028 ± 0.047 0.086 ± 0.066 0.055 ± 0.019 0.208 ± 0.086 Ontario Muskoka-Haliburton Sudbury Sudbury 14 2 5 5.6 - 6.8 5.5 4.0 - 4.6 0.049 0.069 0.280 - 0.380 169 t e n d s t o be b uoyed up by i n v e r s e s t r a t i f i c a t i o n and moves a c r o s s t h e t o p o f t h e l a k e t o t h e o u t l e t o n l y p a r t i a l l y m i x e d . F o r example, i n Norway's Lak e L a n t j e r n , 85-90% o f t h e a c i d m e l t w a t e r l e a v e s t h e l a k e w i t h o u t d e p l e t i n g a l k a l i n i t y ( H e n r i k s e n and W r i g h t 1977). Summer s t r a t i f i c a t i o n c an c r e a t e i m p o r t a n t r e f u g e s f r o m s u r f a c e a c i d a d d i t i o n s . The t h e r m o c l i n e s h i e l d i n g o f d e e p e r w a t e r s e v i d e n t i n t r e a t m e n t s A and AF was a l s o o b s e r v e d by S c h i n d l e r e t a l . (1980a) i n t h e e x p e r i m e n t a l a c i d i f i c a t i o n o f Lake 223. Had I u s e d b o t h n i t r i c and s u l f u r i c a c i d i n t h e a c i d i f i c a t i o n t r e a t m e n t s , I would have needed t o add more H* t o consume t h e same amount o f a l k a l i n i t y , s i n c e N 0 3 " a s s i m i l a t i o n by a l g a e g e n e r a t e s a l k a l i n i t y (See S e c t i o n 4 . 3 ) . A l s o , i n c r e a s e d N 0 3 " c o n c e n t r a t i o n s would p r o b a b l y have a f f e c t e d 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, and p e r h a p s t h e r a t e o f p r i m a r y p r o d u c t i o n . Under low oxygen c o n d i t i o n s , b i c a r b o n a t e g e n e r a t i o n f r o m s u l f a t e - r e d u c i n g b a c t e r i a c a n g r e a t l y r e d u c e t h e a c i d i f y i n g e f f e c t of summer a d d i t i o n s o f H 2 S O 0 ( S c h i n d l e r e t a l . 1 9 8 0 a ) . In Lake 223 i n 1977, s u c h b u f f e r i n g i n c r e a s e d t h e v o l u m e -w e i g h t e d a v e r a g e a l k a l i n i t y 11 y e q ' L " 1 , and was l a r g e l y r e s p o n s i b l e f o r t h e a d d e d H 2 S O i , b e i n g o n l y 31% e f f i c i e n t a t r e m o v i n g a l k a l i n i t y ( S c h i n d l e r e t a l . 1980a). F e r t i l i z a t i o n o f a c i d i c M o u n t a i n t o p Lake i n O n t a r i o (pH 4.62) a l s o g e n e r a t e d l a r g e i n c r e a s e s i n h y p o l i m n e t i c a l k a l i n i t y a t t r i b u t a b l e t o s u l p h a t e r e d u c t i o n (Yan and L a f r a n c e 1981). K e l l y e t a l . (1982) have shown t h a t t h e h y p o l i m n i a o f a r t i f i c i a l l y e u t r o p h i e d l a k e s 170 c o u l d p o t e n t i a l l y p r o d u c e enough a l k a l i n i t y t o n e u t r a l i z e t y p i c a l l e v e l s o f a c i d d e p o s i t i o n , t h o u g h u n e n r i c h e d l a k e s c o u l d n o t . However, as water i n t h e A e n c l o s u r e s was w e l l o x y g e n a t e d , t h e r e was no p o s s i b i l i t y f o r s u c h b u f f e r i n g . P r e s u m a b l y w i t h d e e p e r c y l i n d e r s e x t e n d i n g i n t o E u n i c e L a k e ' s a n o x i c z o n e , I wou l d a l s o have e n c o u n t e r e d i n c r e a s e s i n h y p o l i m n e t i c a l k a l i n i t y g e n e r a t e d by s u l f a t e - r e d u c e r s . The i m p o r t a n t one u n i t pH change o b s e r v e d A u g u s t 9 t o 16 i n t h e e p i l i m n i o n o f t h e AF e n c l o s u r e s , a l t h o u g h coming i n summer r a t h e r t h a n s p r i n g , i s w i t h i n t h e o b s e r v e d r a n g e o f s p r i n g pH d e p r e s s i o n s i n t h e s u r f a c e w a t e r s o f l a k e s i n C e n t r a l O n t a r i o ( J e f f r i e s e t a l . 1979). T h e r e f o r e , t h o u g h t h e o v e r a l l r a t e o f change i n mean pH was much f a s t e r t h a n has been o b s e r v e d i n a c i d i f y i n g l a k e s ( D i c k s o n 1975), t h e s i m u l a t e d pH d e p r e s s i o n s were r e a s o n a b l e . I t i s d i f f i c u l t t o a s s e s s whether t h e e x p e r i m e n t a l a c i d i f i c a t i o n i n t r e a t m e n t A p r o d u c e d " r e a s o n a b l e " c h a n g e s i n t r a n s p a r e n c y . F i r s t , p u b l i s h e d s t u d i e s s h o w i n g c o r r e l a t i o n s between l a k e a c i d i t y and t r a n s p a r e n c y do n o t r e v e a l whether h i g h t r a n s p a r e n c y i s p r e s e n t b e c a u s e o f a c i d i f i c a t i o n , o r e x i s t e d p r e v i o u s l y (NRCC 1981). S e c o n d , t h e most t h o r o u g h l y m o n i t o r e d c a s e s t u d y a t i n t e r m e d i a t e pH l e v e l s was t h e e x p e r i m e n t a c i d i f i c a t i o n o f Lake 223, w h i c h d i d n o t i n c l u d e a c i d i f i c a t i o n o f t h e w a t e r s h e d . E x p e r i m e n t a l a c i d i f i c a t i o n i n t r e a t m e n t A c a u s e d : 1) a t r a n s i e n t 0.8m i n c r e a s e i n t r a n s p a r e n c y ; 2) no s i g n i f i c a n t c hange i n mean n o n c h l o r o p h y l l l i g h t e x t i n c t i o n ; and 3) a s l i g h t 171 i n c r e a s e i n mean S e c c h i d e p t h s (7.14±0.23 m i n t r e a t m e n t A a t mean pH 5.85, v e r s u s 6.72±0.35 m i n t h e c o n t r o l s a t mean pH 6.44) ( S e c t i o n 3 .13). T h e s e r e s u l t s c a n be compared t o b o t h m u l t i - l a k e s u r v e y s and t h e Lake 223 r e s u l t s . I n a s u r v e y o f 209 l a k e s i n t h e Sudbury A r e a , t h e 25 l a k e s w i t h mean pH 5.5-6.0 had a mean S e c c h i d e p t h of 5.80±0.42 m; 68 l a k e s w i t h mean pH 6.5-7.0 had a mean S e c c h i d e p t h o f 4.80±0.22 m (NRCC 1981). E x p e r i m e n t a l a c i d i f i c a t i o n o f Lak e 223 f r o m pH 6.79 t o 6.08 d u r i n g 1976-1977 was a c c o m p a n i e d by a 0.8m i n c r e a s e i n mean S e c c h i d e p t h s , w i t h no c o n c u r r e n t d e c r e a s e i n a l g a l b i o m a s s ( S c h i n d l e r e t a l . 1980a). Aimer e t a l . (1978) have shown t h a t aluminum c a n r a p i d l y p r e c i p i t a t e humic m a t e r i a l s a t pH 5, and s u g g e s t e d t h a t t h i s mechanism may be p a r t i a l l y r e s p o n s i b l e f o r t h e c l a r i t y o f a c i d l a k e s . Lake 223 aluminum c o n c e n t r a t i o n s were f a r below t h e l e v e l s shown by Aimer e t a l . t o a f f e c t t r a n s p a r e n c y ( S c h i n d l e r e t a l . 1980a); t h i s was v e r y l i k e l y a l s o t h e c a s e i n t h e A c y l i n d e r s . S c h i n d l e r (1980) has h y p o t h e s i z e d t h a t i n c r e a s e s i n c l a r i t y a t pH l e v e l s n e a r 6 may be due t o c h a n g e s i n t h e c o l o u r o f d i s s o l v e d m a t e r i a l s . S i n c e L a k e 223 d i d n o t show c h a n g e s i n a b s o r p t i o n o f b l u e (425 nm) l i g h t ( S c h i n d l e r e t a l . 1980a), t h i s h y p o t h e s i s depends upon a c i d w a t e r s showing d e c r e a s e d a b s o r p t i o n a t l o n g e r w a v e l e n g t h s , a p r e d i c t i o n w h i c h s h o u l d be r e l a t i v e l y e a s y t o t e s t . 172 4.3 B i o t i c P r o c e s s i n g o f Ammonium N i t r a t e A d d i t i o n s A d d i t i o n s o f f e r t i l i z e r were r e s p o n s i b l e f o r 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 e c r e a s e s i n e p i l i m n e t i c a l k a l i n i t y i n b o t h t h e F and AF t r e a t m e n t s ; i n t h e l a t t e r t r e a t m e n t , t h e a l k a l i n i t y d e c r e a s e was o f g r e a t b i o l o g i c a l s i g n i f i c a n c e . I s u g g e s t t h a t t h e s e d e c r e a s e s were due p a r t i a l l y t o p r e f e r e n t i a l a l g a l a s s i m i l a t i o n of NH,* o v e r N0 3~, and p a r t i a l l y t o b a c t e r i a l n i t r i f i c a t i o n . T h e s e two p o s s i b i l i t i e s a r e d i s c u s s e d s e p a r a t e l y b elow. The s t o i c h i o m e t r y of a l g a l NH, + a s s i m i l a t i o n f i r s t p r o p o s e d by R e d f i e l d e t a l . (1963) and r e c e n t l y c o n f i r m e d by Brewer and Goldman (1976) a n d Goldman and Brewer ( 1 9 8 0 ) , i s a s f o l l o w s : l 06CO 2+l06H 2O+16NH f t + — > ( C H 2 0 ) . 0 6 + ( N H 3 ) « 6 + 1 6 H + + 1 0 6 ( 0 2 ) (5) W h i l e e a c h m o l e c u l e o f a s s i m i l a t e d N H « + d e p l e t e s an e q u i v a l e n t q u a n t i t y o f a l k a l i n i t y , t h e r e v e r s e o c c u r s under n i t r a t e a s s i m i l a t i o n : 106C0 2+138H 20+16N0 3- ~ > C H 2 0 ) . 0 6 + ( N H 3 ) . 6 +160H" +1 3 8 ( 0 2 ) (6) S i n c e NH(,N03 i s b a l a n c e d i n a l k a l i n i t y - i n c r e a s i n g and d e p l e t i n g f o rms o f n i t r o g e n , t h e a l k a l i n i t y d e c r e a s e s I o b s e r v e d i n f e r t i l i z e r t r e a t m e n t s c o u l d o n l y have been due t o a l g a l a s s i m i l a t i o n i f NH, + was a s s i m i l a t e d p r e f e r e n t i a l l y . Most o f t h e a l g a l s p e c i e s s t u d i e d t o d a t e show p r e f e r e n t i a l a s s i m i l a t i o n o f N H „ + o v e r N 0 3" when b o t h a r e p r e s e n t . ( P r a t t and Fong 1940; 173 Cramer and Myers 1948; L u i and R o e l s 1972; Brewer and Goldman 1976, B a t e s 1976; M c C a r t h y e t a l . 1977; E p p l e y e t a l . 1979; O l s o n 1980). I t i s p o s s i b l e t h a t t h e change 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 i n d u c e d by my e x p e r i m e n t a l f e r t i l i z a t i o n a f f e c t e d o v e r a l l r a t e s o f NH 0 * and N0 3~ a s s i m i l a t i o n . The mechanism r e s p o n s i b l e f o r p r e f e r e n t i a l N H „ * u p t a k e i s s u p p r e s s i o n o f i n t r a c e l l u l a r n i t r a t e r e d u c t i o n ( f a i l u r e o f i n d u c t i o n of n i t r a t e r e d u c t a s e ) when ammonium i s p r e s e n t i n g r o w i n g c e l l s ( Z i n k and V e l i k y 1977). M c C a r t h y e t a l . (1977) showed t h a t NHft* c o n c e n t r a t i o n s g r e a t e r t h a n 0.5 t o 1.0 Aig-atom N » L " 1 a l m o s t t o t a l l y s u p p r e s s e d N0 3~ u t i l i z a t i o n by a wide v a r i e t y o f p h y t o p l a n k t o n s p e c i e s . S i n c e t h e e p i l i m n i o n o f my f e r t i l i z e d e n c l o s u r e s (F and AF) r e c e i v e d a p p r o x i m a t e l y 5.4 jug-atom N « l i t e r " 1 as NHft* d u r i n g e a c h o f 4 n u t r i e n t a d d i t i o n s ( t o t a l o f 2 1 . 6 ) , i t seems r e a s o n a b l e t h a t N 0 3" u p t a k e was t e m p o r a r i l y s u p p r e s s e d . Under t h e s t o i c h i o m e t r y shown i n e q u a t i o n ( 9 ) , e a c h o f t h e f o u r n u t r i e n t a d d i t i o n s c o u l d have c a u s e d an a l k a l i n i t y r e d u c t i o n o f 5.4 M e q « L ~ 1 a s s u m i n g o n l y NH„ + a s s i m i l a t i o n . To y i e l d t h e 10 neq»L~1 d e c r e a s e i n a l k a l i n i t y o b s e r v e d i n F e n t i r e l y by a l g a l n i t r o g e n p r o c e s s i n g , t h e r a t i o o f NH a +:N0 3~ a l g a l a s s i m i l a t i o n would have t o have been a t l e a s t 1.9 ( i . e . 21 .6/[21 .6-1 0 ] ) . S i m i l a r l y , t h e 8 jueq*L" 1 d e c r e a s e i n AF woul d have r e q u i r e d an NH, +:N0 3" a s s i m i l a t i o n r a t i o o f 1.6. P r a t t and Fong (1940) o b s e r v e d a l k a l i n i t y d e c r e a s e s due t o a l g a l N H „ + u p t a k e , b u t f o l l o w i n g t h e d e p l e t i o n o f NH a* i n t h e i r c u l t u r e s t h e pH r o s e r a p i d l y , w h i c h t h e y a t t r i b u t e d t o 174 a b s o r p t i o n o f n i t r a t e i o n s . G i v e n t h e v e r y h i g h N:P a t o m i c r a t i o o f my n u t r i e n t a d d i t i o n s (33.2:1, w i t h h a l f o f t h e n i t r o g e n a s NH„ + ) i t i s p o s s i b l e t h a t NH f l + n e v e r d i d become d e p l e t e d ; w i t h o u t n u t r i e n t measurements I c a n n o t t e l l . B a c t e r i a l n i t r i f i c a t i o n c a n a l s o d e p l e t e a l k a l i n i t y , by o x i d i z i n g ammonium t o n i t r i c a c i d v i a t h e f o l l o w i n g r e a c t i o n s (Weber and Stumm 1963): 2NH, + + 3 0 2 -"> 2N0 2 + 4H + + 2H 20 (7) 2N0 2 + 0 2 --> 2N0 3- (8) The r e a c t i o n s i n e q u a t i o n s (7) and (8) a r e p e r f o r m e d by t h e b a c t e r i a N i t r o s o m o n a s and N i t r o b a c t e r r e s p e c t i v e l y . Note t h a t e q u a t i o n (7) p o t e n t i a l l y p r o d u c e s t w i c e a s much a c i d i t y a s N H « * a s s i m i l a t i o n by a l g a e ( E q u a t i o n 8 ) . Was n i t r i f i c a t i o n a s i m p o r t a n t i n t r e a t m e n t AF a s i n F? B a c t e r i a l n i t r i f i c a t i o n o f i n d u s t r i a l w a t e r s h e d s has been r e s p o n s i b l e f o r t h e a c i d i f i c a t i o n o f Lake O r t a i n N o r t h e r n I t a l y t o pH v a l u e s n e a r 4 ( G e r l e t t i and P r o v i n i 1978). G e r l e t t i and P r o v i n i 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 t h a t n i t r i f i c a t i o n a t 2 5 ° C p r o c e e d e d c o n s i d e r a b l y s l o w e r a t pH 5 t h a n a t pH 7. I n a d d i t i o n , n a t u r a l L a k e O r t a d e n s i t i e s o f n i t r i f i e r s h ad t o be augmented i n o r d e r t o p r o d u c e m e a s u r a b l e n i t r i f i c a t i o n i n l e s s t h a n two t o t h r e e months. T h e i r r e s u l t s s u g g e s t t h a t l e s s n i t r i f i c a t i o n o c c u r r e d i n t r e a t m e n t AF t h a n F. 175 In r e t r o s p e c t , i t would have been p o s s i b l e t o d i s t i n g u i s h between a l g a l a s s i m i l a t i o n and b a c t e r i a l n i t r i f i c a t i o n w i t h a 24 h r s e r i e s o f NH, +, N0 3" and N0 2~ measurements, s i n c e o n l y n i t r i f i c a t i o n w ould a f f e c t n i g h t t i m e N0 3" c o n c e n t r a t i o n s . A l t e r n a t i v e l y , I c o u l d have examined d a y t i m e a l k a l i n i t y a n d n i t r o g e n c h a n g e s f o l l o w i n g u r e a a d d i t i o n s , s i n c e a l g a l a s s i m i l a t i o n o f u r e a ( C O - ( N H 2 ) 2 ) d o e s n o t p r o d u c e a c i d i t y (Brewer and Goldman 1976), but b a c t e r i a l o x i d a t i o n o f u r e a d o e s (Hunt and Boyd 1981). B i o t i c p r o c e s s i n g o f n i t r o g e n a p p e a r s t o have been i m p o r t a n t i n t h i s e x p e r i m e n t - how i m p o r t a n t i s i t i n low a l k a l i n i t y l a k e s g e n e r a l l y ? T h e r e i s good e v i d e n c e t h a t t h e a c i d i t y o f o l i g o t r o p h i c , p o o r l y b u f f e r e d l a k e s i s q u i t e s e n s i t i v e t o t h e f o r m o f n i t r o g e n a v a i l a b l e t o a l g a e . E x p e r i m e n t a l f e r t i l i z a t i o n o f low a l k a l i n i t y Lake 227 w i t h NaN0 3 c a u s e d t h e pH t o i n c r e a s e f r o m 6.5 t o 10.2 ( S c h i n d l e r 1971 a n d 1973). C r o s s o n L a k e , an o l i g o t o p h i c a c i d l a k e i n O n t a r i o , showed a pH i n c r e a s e f r o m 5.1 t o 6.6 between May and A u g u s t o f 1978, w h i c h was a t t r i b u t e d t o a l g a l N0 3" a s s i m i l a t i o n (NRCC 1981). By c o n t r a s t , f e r t i l i z a t i o n of a c i d i c M o u n t a i n t o p Lake w i t h N H „ N 0 3 c a u s e d t h e e p i l i m n e t i c pH t o d r o p f rom 4.5 t o 4.2 i n t h e summer o f 1977, and from 4.8 t o 4.5 i n 1978 (Yan and L a f r a n c e 1981). Yan and L a f r a n c e ' s a n a l y s i s o f N H „ + and N0 3" c o n c e n t r a t i o n s i n M o u n t a i n t o p showed t h a t a l g a l p r e f e r e n t i a l u p t a k e of N H 4 + o v e r N 0 3 " c o u l d e x p l a i n t h e o b s e r v e d a l k a l i n i t y d e c r e a s e s ; n i t r i f i c a t i o n was d i s c o u n t e d due t o t h e low pH. T h e i r d a t a i n d i c a t e r a t i o s o f N H „ + : N 0 3 ~ a s s i m i l a t i o n o f 1.6, 2.3 and 1.4 d u r i n g 1976, 1977, and 1978 i n 176 M o u n t a i n t o p L a k e ( c a l c u l a t e d o v e r 2.5 t o 3.5 month p e r i o d s ) . T h e s e r a t i o s compare r e a s o n a b l y w e l l w i t h t h e minimum t h e o r e t i c a l a s s i m i l a t i o n r a t i o s of 1.6 and 1.9 t h a t I c a l c u l a t e d were n e c e s s a r y f o r t h e a l k a l i n i t y c h a n g e s i n AF and F ( r e s p e c t i v e l y ) t o be due e n t i r e l y t o a l g a e . Some c o a s t a l m o u n t a i n l a k e s i n B r i t i s h C o l u m b i a have been f e r t i l i z e d w i t h NH f tN0 3 and ammonium p h o s p h a t e t o i n c r e a s e s a l m o n i d p r o d u c t i o n ( S h o r t r e e d e t a l . 1981), b u t no e p i l i m n e t i c a l k a l i n i t y d e c r e a s e s have been o b s e r v e d (K.S. S h o r t r e e d , p e r s . comm. ) . The e n r i c h m e n t was l i k e l y t o o s m a l l t o a f f e c t a l k a l i n i t y : n i t r o g e n a d d i t i o n s were o n l y 30 t o 40 mg N » m ~ 2 , as compared t o 600 m g « m ~ 2 i n e a c h o f my n u t r i e n t a d d i t i o n s . Though i n t h e AF c y l i n d e r s , a l g a l a c t i v i t i e s d e c r e a s e d pH bey o n d t h e l e t h a l t h r e s h o l d f o r a dominant z o o p l a n k t o n s p e c i e s , t h e r e v e r s e may a l s o o c c u r . O ' B r i e n and de N o y e l l e s (1972) d e m o n s t r a t e d t h a t n u t r i e n t e n r i c h m e n t o f s m a l l ponds ( w i t h t r i p l e s u p e r p h o s p h a t e , ammonium n i t r a t e and m u r i a t e o f p o t a s h ) l e d t o an i n c r e a s e i n pH f r o m 9.9 t o 11.0, c a u s i n g m a s s i v e m o r t a l i t y o f C e r i o d a p h n i a r e t i c u l a t a . Why d i d a pH i n c r e a s e o c c u r a f t e r a d d i t i o n s o f N H „ N 0 3 ? A t t h e t e m p e r a t u r e s and pH l e v e l s i n O ' B r i e n and de N o y e l l e ' s e x p e r i m e n t ( a b o u t 21°C and pH 10), r o u g h l y 80% o f t h e adde d NH„ + w o u l d have been c o n v e r t e d t o NH 3 ( T h u r s t o n e t a l . 1974). I s u g g e s t t h a t N H „ + l e v e l s may have been t o o low t o i n h i b i t n i t r a t e r e d u c t i o n . S i n c e n i t r i c a c i d c o n t r i b u t e s a b o u t one t h i r d o f t h e H + i n p r e c i p i t a t i o n i n Norway and t h e n o r t h e a s t e r n U.S., g e n e r a t i o n o f a l k a l i n i t y by n i t r a t e a s s i m i l a t i o n c o u l d be an i m p o r t a n t n a t u r a l 177 b u f f e r i n g mechanism f o r a c i d l a k e s . However, most o f t h e n i t r a t e d e p o s i t e d on w a t e r s h e d s i s t a k e n up by p l a n t s a n d s o i l , and n e v e r r e a c h e s t h e l a k e ( H e n r i k s e n and W r i g h t 1977, D i l l o n e t a l . 1980). I t i s p o s s i b l e t h a t i n l a k e s l i m i t e d by p h o s p h o r u s r a t h e r t h a n n i t r o g e n , c o n t i n u a l n i t r a t e l o a d s f r o m p r e c i p i t a t i o n may e v e n t u a l l y end up a c c u m u l a t i n g i n t h e i n c r e a s i n g l y a c i d w a t e r . Though some a c i d l a k e s show h i g h n i t r a t e l e v e l s r e l a t i v e t o c i r c u m n e u t r a l l a k e s ( G j e s s i n g e t a l . 1976), t h e s e a r e p r o b a b l y i n b a s i n s w i t h v e r y low w a t e r s h e d t o l a k e a r e a r a t i o s ( i . e . low n i t r a t e r e t e n t i o n ) . I have not f o u n d e v i d e n c e i n t h e l i t e r a t u r e showing n i t r a t e a c c u m u l a t i o n s o v e r t i m e i n a c i d l a k e s . T h i s a p p e a r s - t o be a t o p i c w o rthy o f f u r t h e r r e s e a r c h , s i n c e L i k e n s e t a l . (1979) have n o t e d a r e c e n t i n c r e a s e i n t h e r a t i o o f n i t r i c : s u l f u r i c a c i d c o n c e n t r a t i o n s i n p r e c i p i t a t i o n . 4.4 Z o o p l a n k t o n Community S t r u c t u r e The e x p e r i m e n t a l t e s t s of t h e o r i g i n a l n u l l h y p o t h e s e s c o n c e r n i n g z o o p l a n k t o n community s t r u c t u r e a r e summarized i n T a b l e 21. The a b s e n c e o f m e a s u r a b l e a c i d i f i c a t i o n e f f e c t s on community s t r u c t u r e i n t r e a t m e n t A c y l i n d e r s d e m o n s t r a t e s t h a t t h e community c a n t o l e r a t e pH 5.6, a t l e a s t d u r i n g t h e p e r i o d f r o m J u l y t o O c t o b e r . However, t h e n u l l e f f e c t o f t r e a t m e n t A means t h a t a d i s c u s s i o n o f t h e mechanisms by w h i c h a c i d i f i c a t i o n a l t e r s z o o p l a n k t o n c o m m u n i t i e s must f o c u s on t r e a t m e n t AF, and use t h e o t h e r t r e a t m e n t s f o r c o m p a r i s o n s . F o u r m a j o r community c h a n g e s o c c u r r e d i n t h e AF c y l i n d e r s : 1) D. r o s e a d e c l i n e d p r e c i p i t o u s l y A u g u s t 9 t o 16; 1 7 8 Table 21. Summary of 1979 results relevant to hypotheses on zooplankton communities. Null Hypotheses (from pg 41) Major Results 2(i) Gradual experimental acidification of oligotrophic waters to pH 5.0 does not affect zooplankton populations or community biomass 2(ii) Moderate nutrient enrichment of circumneutral waters does not affect zooplankton populations or community biomass 5(i) Gradual, experimental acidification of moderately enriched waters does not affect zooplankton populations or community biomass no evidence against hypothesis down to pH 5.6 (A vs. C) too few animals and samples for adequate test at pH < 5.6 hypothesis falsified (F vs. C) significant increases in densities and reproduction of all species except Diaptomus kenai and Holopedium gibberum significant increases in biomass hypothesis falsified (AF vs. F) mortality of Daphnia  rosea at pH 5.4 led to significant changes in species composition and biomass 179 2) D. b r a c h y u r u m , C. s p h a e r i c u s , B. l o n g i r o s t r i s and C. p u l c h e l l a r e p l a c e d D a p h n i a ; 3) D. t y r r e l l i d i s a p p e a r e d a f t e r September 20; and 4) D. k e n a i showed i m p r o v e d r e p r o d u c t i o n ( r e l a t i v e t o F ) . I c o n s i d e r e a c h s p e c i e s s e p a r a t e l y i n t h i s d i s c u s s i o n , a n d a t t e m p t t o i d e n t i f y whether a c i d t o x i c i t y , c o m p e t i t i o n , f o o d s u p p l y , p r e d a t i o n o r some c o m b i n a t i o n of t h e s e f a c t o r s were r e s p o n s i b l e f o r i t s p e r f o r m a n c e i n t r e a t m e n t AF, r e l a t i v e t o b o t h o t h e r t r e a t m e n t s and o t h e r s t u d i e s . Where a c i d t o x i c i t y a p p e a r s i m p o r t a n t , I a t t e m p t t o i d e n t i f y t h e " c r i t i c a l pH" and t h e most a c i d - s e n s i t i v e l i f e h i s t o r y s t a g e s . The e f f e c t s o f a c i d i f i c a t i o n on p h y t o p l a n k t o n - z o o p l a n k t o n r e l a t i o n s h i p s a r e d i s c u s s e d i n S e c t i o n 4.5. C o m p a r i s o n s o f pH t o x i c i t y between s t u d i e s must a c c o u n t f o r d i f f e r e n t m e t h o d o l o g i e s and t e r m i n o l o g i e s . R e s e a r c h e r s s t u d y i n g a c i d i f i c a t i o n e f f e c t s on z o o p l a n k t o n use t h e t e r m " c r i t i c a l pH" d i f f e r e n t l y d e p e n d i n g on t h e s c a l e o f t h e i n v e s t i g a t i o n . L a b o r a t o r y t e s t s o f a c i d s t r e s s h a v e g e n e r a l l y d e f i n e d c r i t i c a l pH a s t h e pH below w h i c h s h a r p d e c r e a s e s a r e s e e n i n r a t e s o f s u r v i v a l (Havas 1980; P a r e n t and Cheetham 1980), r e p r o d u c t i o n ( P a r e n t and Cheetham 1980) o r i n t r i n s i c r a t e o f p o p u l a t i o n i n c r e a s e ( W a l t o n e t a l . 1981). S u r v e y s o f z o o p l a n k t o n c o m m u n i t i e s a c r o s s many l a k e s o f d i f f e r e n t a c i d i t y have e m p h a s i z e d t h e mean l a k e pH a t w h i c h s i g n i f i c a n t c h a n g e s a r e see n i n s p e c i e s c o m p o s i t i o n , p a r t i c u l a r l y numbers o f s p e c i e s , number o f community d o m i n a n t s o r s p e c i e s d i v e r s i t y i n d i c e s ( summarized i n T a b l e 22, w h i c h i s d i s c u s s e d b e l o w ) . Table 22. Summary of literature showing changes In numbers of crustacean zooplankton species across lakes of different pH. Location 1. LaCloche Mts. , Ontario 2. Greater Sudbury Area, Ontario2 3. Sudbury and Muskota-Hal iburton, Ontario 4. S. Norway' 5. S. Norway 6. S. Sweden3'" 7. Island of Rhum and Yorkshire, U.K. I Lakes Method of Sampled D «es/Lake S a n ) p l 1 n g 47 187 57 27 s 84 74 5 ± 0.1 11 26 ± 5 1 2.7 ± 0.2 1 1 76 urn and 80 umnets 34 L trap; 80um net 95 urn net 90 urn net 75 um net I of Crustacean Species in Lakes with Mean pH In Stated Range (X/lake ± SE (I of Lakes)) 4 0 ' 4 4 4 .5 -4 .9 5.0-5.4 5 .5 -5 .9 6.0-6.4 fi.5 - 6.9 75 pmnet 3.9±0.7 (9) 3.0±0.5(13) 7.2*1.1(10) 11.3±2.0(4) 10.3*1.1(8) 11.0±1(2) 5.3 (12) 6.6 (23) 9.3 (19) 10.1 (23) 10.9 (26) 11.7(32) 3.410.1 (4)' 9.5±1.5 (2)' 11.7±1.2 (4)1 11.9 (1)' 2-5 (2) 2.7 (9) 3.3 (14) 3.7 (6) 3.9 (11) 4.2 (1) 3.9±0.5 (7) - 10.0±0.9 (5) 6 ±1.2 (5) 9.0 (1) 4 (14) 5 (17) 5 (8) 7 (13) 7 (12) 6.4±0.9(14) 6 10.0±1.2(11)' 16.3±1.1 (4) s 13.3±2.2 (3)' 14.8±2.0 (9)6 15.6*1.7(9)' Source Sprules (1975) Keller (1981) Yan and Strus (1980) Hendrey and Wright (1976) Hobaek and Raddum (1980) Aimer et al. (1974) Fryer (1980) 1 Mean t species per sample, not per lake. % Some overlap exists between lakes sampled In locations 1 and 2, and between locations 2 and 3. 1 Standard error not computable from published material. * Only 1 significant digit in mean no. of species Interpretable from published material. s Three humlc acid lakes and 6 alpine lakes not Included here. 6 Includes littoral and limnetic habitats In tarns and ponds. CO o 181 E a c h l e v e l o f a p p r o a c h has i t s w e a k n e s s e s . R e g i o n a l s u r v e y s c a n e a s i l y m i s s c r i t i c a l e p i s o d i c pH d e c l i n e s t h a t a f f e c t z o o p l a n k t o n c o m m u n i t i e s , and a t b e s t p r o v i d e o n l y c o r r e l a t i v e e v i d e n c e f o r t h e p r o c e s s e s r e s p o n s i b l e f o r between l a k e d i f f e r e n c e s . However, a c i d i f i c a t i o n e x p e r i m e n t s i n t h e l a b o r a t o r y o r l i m n o c o r r a l s may n o t a d e q u a t e l y s i m u l a t e t h e c o n d i t i o n s ( o r r a t e o f change i n c o n d i t i o n s ) p r e s e n t i n a c i d i f y i n g l a k e s . S i n c e t h e l i f e h i s t o r y t r a i t s o f c l a d o c e r a n s and c o p e p o d s d i f f e r , f u n d a m e n t a l l y ( A l l e n and G o u l d e n 1980), I w i l l d i s c u s s t h e s e two g r o u p s s e p a r a t e l y . C l a d o c e r a n s (i_) D a p h n i a r o s e a The p r i m a r y e f f e c t o f a c i d i f i c a t i o n i n t h e AF c y l i n d e r s was t h e m o r t a l i t y o f D. r o s e a , w h i c h o c c u r r e d a t a pH o f 5.3 t o 5.4. T h i s m o r t a l i t y was u n d o u b t e d l y a s s o c i a t e d w i t h low pH r a t h e r t h a n s t a r v a t i o n o r p r e d a t i o n , s i n c e : 1) d a p h n i d egg p r o d u c t i o n , a s w e l l as p h y t o p l a n k t o n c e l l s i z e s and 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 a l l i n d i c a t e t h a t f o o d was a b u n d a n t ; 2) t h e o n l y p o t e n t i a l p r e d a t o r s i n t h e AF c y l i n d e r s were D i a c y c l o p s t h o m a s i and D i a p t o m u s k e n a i . D. t h o m a s i i s t o o s m a l l t o p r e y on a d u l t D a p h n i a ( P e a c o c k 1981), and i n any c a s e was f a r t o o r a r e (0.01 L " 1 ) t o have any i m p a c t . D. k e n a i , l a b e l l e d by G e r r i t s e n (1980) a s a p r e d a t o r , has 182 been f o u n d i n t h e UBC R e s e a r c h F o r e s t l a k e s t o be p r i m a r i l y h e r b i v o r o u s , t h o u g h i t may consume r o t i f e r s or n a u p l i i (W.E. N e i l l , E . K r a u s e , and J . Bowerman, p e r s . comm. ). 3) S u b s e q u e n t e x p e r i m e n t s i n 1980 w i t h pH k e p t u n i f o r m o v e r a l l d e p t h s ( S e c t i o n I I ) c o n f i r m e d t h a t D a p h n i a r o s e a shows s i m i l a r a c i d s e n s i t i v i t y u n d e r c o n d i t i o n s o f l o w e r n u t r i e n t s and c o l d e r t e m p e r a t u r e s . R e d u c t i o n s i n t h e abundance o f D. r o s e a d u r i n g A u g u s t 9 t o 16 were l e s s s e v e r e i n c y l i n d e r 7AF t h a n 3AF, i n s p i t e o f i d e n t i c a l pH r e g i m e s . In S e c t i o n 3.2.4 ( i ) , I h y p o t h e s i z e d t h a t t h i s d i f f e r e n c e was due t o between r e p l i c a t e d i f f e r e n c e s i n t h e p r o p o r t i o n s o f t h e c y l i n d e r p o p u l a t i o n s e x p o s e d t o t h e most a c i d zone o f t h e w a t e r column (0-4m). Though d e p t h - s t r a t i f i e d s a mples a r e not a v a i l a b l e f r o m A u g u s t 9 t o t e s t t h i s h y p o t h e s i s , i t n e v e r t h e l e s s seems r e a s o n a b l e . W i t h i n - t r e a t m e n t d i f f e r e n c e s i n t h e v e r t i c a l d i s t r i b u t i o n o f D. r o s e a were p r e s e n t i n t r e a t m e n t AF on A u g u s t 1, and were a l s o 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 ( e . g . t r e a t m e n t F on September 6 ) . Such d i f f e r e n c e s may have been due t o c h a n g e s i n s u n l i g h t d u r i n g t h e day, s i n c e t h e e n c l o s u r e s were s a m p l e d (an d a c i d i f i e d ) s e q u e n t i a l l y r a t h e r t h a n s i m u l t a n e o u s l y . Whatever t h e r e a s o n f o r i t s o c c u r r e n c e , t h e between-r e p l i c a t e d i f f e r e n c e i n t r e a t m e n t AF on A u g u s t 9 u l t i m a t e l y r e s u l t e d i n s i g n i f i c a n t d i f f e r e n c e s i n z o o p l a n k t o n community c o m p o s i t i o n one month l a t e r ( t o be d i s c u s s e d ) . The p r i n c i p l e w h i c h emerges from t h e s e r e s u l t s i s t h a t random v a r i a t i o n i n t h e 183 v e r t i c a l d i s t r i b u t i o n o f a c i d - s e n s i t i v e o r g a n i s m s c a n g r e a t l y r e d u c e ( o r i n c r e a s e ) t h e impact o f a c i d p u l s e s t o s u r f a c e w a t e r s . The r e s u l t s d e m o n s t r a t e c l e a r l y t h a t s t a r v a t i o n o f D. r o s e a v i a a c i d i f i c a t i o n - i n d u c e d c h a n g e s i n f o o d s u p p l y i s n o t l i k e l y . As p h y t o p l a n k t o n community s t r u c t u r e and b i o m a s s a r e u n a f f e c t e d by a c i d i f i c a t i o n t o mean pH 5.7 ( t r e a t m e n t A i n t h i s s t u d y , Yan e t a l . 1977), s t a r v a t i o n e f f e c t s w o