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UBC Theses and Dissertations

Responses of two coexisting cyclopoid copepods to experimental manipulations of food and predators Peacock, Adrienne 1981

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RESPONSES OF TWO COEXISTING CYCLOPOID COPEPODS TO EXPERIMENTAL MANIPULATIONS OF FOOD AND PREDATORS by ADRIENNE PEACOCK B . S c , York U n i v . , T o r o n t o , 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY 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 January 1981 © A d r i e n n e Peacock, 1981 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t 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 , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e V ancouver, Canada V6T 1W5 Date DE-6 (2/79) i i ABSTRACT F a c t o r s c a u s i n g the p o p u l a t i o n l i m i t a t i o n of two s p e c i e s of c y c l o p o i d copepods, T r o p o c y c l o p s p r a s i n u s and C y c l o p s  b i c u s p i d a t u s thomas i , and the subsequent impact of the s e s p e c i e s on the z o o p l a n k t o n community, were i n v e s t i g a t e d i n two montane l a k e s of the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t . In P l a c i d Lake, C•b.thomasi was abundant and T . p r a s i n u s was r a r e , w h i l e i n Gwendoline Lake, T . p r a s i n u s was the more abundant s p e c i e s . F i e l d and l a b o r a t o r y e x p e r i m e n t s were co n d u c t e d t o a s s e s s the importance of such f a c t o r s as f o o d , p h y s i c a l - c h e m i c a l l a k e c h a r a c t e r i s t i c s , and p r e d a t i o n upon the p o p u l a t i o n dynamics of t h e s e s p e c i e s . The f e e d i n g appendages of a d u l t T . p r a s i n u s and C.b.thomasi were compared and found t o be s t r u c t u r a l l y a l i k e , a l t h o u g h a d u l t d i e t s were q u i t e d i s s i m i l a r . T . p r a s i n u s was omnivorous i n the l a t e r i n s t a r s , e a t i n g a l g a e , p r o t o z o a n s and p r o b a b l y dead m a c r o z o o p l a n k t e r s w h i l e C.b.thomasi was markedly c a r n i v o r o u s i n c o p e p o d i d and a d u l t i n s t a r s . S e a s o n a l abundance peaks r e v e a l e d a t e m p o r a l s e p a r a t i o n of the n a u p l i a r i n s t a r s of the s e s p e c i e s i n both l a k e s s t u d i e d . C o n s e q u e n t l y , c o m p e t i t i o n between T . p r a s i n u s and C.b.thomasi was u n l i k e l y . Large s c a l e e n c l o s u r e e x p e r i m e n t s i n P l a c i d Lake, where C.b.thomasi was abundant, showed t h a t l a k e d e n s i t i e s of C.b.thomasi c o u l d l i m i t the abundance of T . p r a s i n u s . S u r v i v o r s h i p c u r v e s and l a b o r a t o r y f e e d i n g e x p e r i m e n t s i n d i c a t e d t h a t the low abundances of T . p r a s i n u s was caused by two i m p o r t a n t f a c t o r s : h i g h T . p r a s i n u s n a u p l i a r m o r t a l i t y even i n the absence of C.b.thomasi, and C.b.thomasi p r e d a t i o n on these T . p r a s i n u s i n s t a r s . When C.b.thomasi was t r a n s p l a n t e d t o e n c l o s u r e s i n Gwendoline Lake, where T . p r a s i n u s was the more abundant c y c l o p o i d copepod, C.b.thomasi was a b l e t o grow and reproduce w i t h i n the c r u s t a c e a n p l a n k t o n community and became as abundant as i n i t s home l a k e . N a u p l i a r i n s t a r s were the most ^ m o r t a l i t y - p r o n e s t a g e s i n both l a k e s . F e r t i l i z a t i o n of a Gwendoline Lake e n c l o s u r e i n c r e a s e d the s u r v i v o r s h i p of C.b.thomasi about 22%. over t h a t observed i n the u n f e r t i l i z e d t r e a t m e n t . However, a d d i t i o n of the midge f l y l a r v a e common t o Gwendoline Lake ( C . t r i v i t t a t u s and C.americanus) t o the e n c l o s u r e p l a n k t o n community r e s u l t e d i n a d e c r e a s e i n the s t a n d i n g c r o p of C.b.thomasi t o a p o i n t a t which samples became d i f f i c u l t t o o b t a i n . W ith Chaoborus i n the e n c l o s u r e , f e r t i l i z a t i o n d i d not enhance the abundance of C. b. thomas'i but r a t h e r , improved the s u r v i v o r s h i p of Chaoborus f i r s t and second i n s t a r l a r v a e , and r e s u l t e d i n an a c c e l e r a t i o n of the C.b.thomasi n a u p l i i d e c l i n e . The impact of C•b.thomasi on o t h e r z o o p l a n k t o n was a l s o examined. C.b.thomasi appeared t o dampen the f l u c t u a t i o n s i n t o t a l z o o p l a n k t o n biomass i n e n c l o s u r e s w i t h o u t a Chaoborus p o p u l a t i o n . However, i n the f e r t i l i z e d t r e a t m e n t w i t h o u t Chaoborus, C.b.thomasi was unable t o t r a c k prey demographic responses and caused no major c o m p o s i t i o n a l changes i n the c r u s t a c e a n community a t e i t h e r h i g h or low n u t r i e n t l e v e l s . Temporal - s p a t i a l o v e r l a p and p r e d a t o r and prey d e v e l o p m e n t a l responses were c r i t i c a l f a c t o r s d e t e r m i n i n g prey s e n s i t i v i t y t o p r e d a t i o n by C.b.thomasi. i v TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v i i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS x i GENERAL INTRODUCTION 1 I . A MORPHOLOGICAL STUDY OF TROPOCYCLOPS PRASINUS FISCHER AND CYCLOPS BICUSPIDATUS THOMASI FORBES 7 I n t r o d u c t i o n 7 M a t e r i a l s And Methods 13 S t r u c t u r e Of The Mouthparts 14 Mouth Region 17 Labrum •. 17 M a n d i b l e s 22 Paragnathae 26 G r a s p i n g Appendages 27 The F i r s t M a x i l l a e 27 The Second M a x i l l a e 34 The M a x i l l i p e d s 35 F u n c t i o n a l I n t e r r e l a t i o n s h i p Of The Mouthparts 39 D i s c u s s i o n 42 I I . EXPERIMENTAL STUDIES ON THE FACTORS LIMITING TROPOCYCLOPS PRASINUS IN AN OLIGOTROPHIC LAKE 45 I n t r o d u c t i o n 45 Study Area 48 M a t e r i a l s And Methods 51 L a b o r a t o r y E x p e r i m e n t s 51 1. D i e t Of T . p r a s i n u s 51 2. F e e d i n g R a t e s : C.b.thomasi On T . p r a s i n u s 55 F i e l d E x p e r i m e n t s 56 P l a n k t o n Sampling 58 P l a n k t o n C o u n t i n g 64 Temperature, Oxygen And G r a z e a b l e Seston 65 R e s u l t s 66 F e e d i n g E x p e r i m e n t s 66 D i e t Of T . p r a s i n u s 66 E f f e c t s Of C.b.thomasi P r e d a t i o n In L a b o r a t o r y E x p e r i m e n t s 71 F i e l d E x p e r i m e n t s 76 S e a s o n a l Abundance Of C.b.thomasi And T . p r a s i n u s ... 76 C. b. thomasi 7 6 T . p r a s i n u s 81 E f f e c t s Of E n c l o s u r e 86 E f f e c t Of C o m p e t i t i o n 96 E f f e c t s Of C.b.thomasi P r e d a t i o n 104 D i s c u s s i o n 112 Summary 117 I I I . RESPONSES OF CYCLOPS BICUSPIDATUS THOMASI TO ALTERATIONS IN THE FOOD AND PREDATOR ENVIRONMENT 119 I n t r o d u c t i o n 119 M a t e r i a l s And Methods 124 F i e l d E x p e r i m e n t s 124 L a b o r a t o r y P r e d a t i o n T e s t s 127 R e s u l t s 128 v i E f f e c t Of I n t r o d u c t i o n On The C.b.thomasi P o p u l a t i o n In The Absence Of Chaoborus 128 E f f e c t Of F e r t i l i z a t i o n In The Absence Of Chaoborus 139 E f f e c t Of Chaoborus At Low N u t r i e n t C o n c e n t r a t i o n s .151 E f f e c t Of F e r t i l i z a t i o n And Chaoborus 161 D i s c u s s i o n 167 Summary 176 IV. EFFECT OF VARYING INVERTEBRATE PREDATOR ABUNDANCE AND FOOD ON A CRUSTACEAN PLANKTON COMMUNITY 179 I n t r o d u c t i o n 179 M a t e r i a l s And Methods 183 Biomass E s t i m a t e s 183 F i e l d E x periments 184 G e n e r a l L i f e H i s t o r i e s 185 Body S i z e And Weight 187 R e s u l t s 190 C l u s t e r A n a l y s e s Of The Community 190 C l a d o c e r a n N u m e r i c a l Response 194 C a l a n o i d Copepod Response 202 C y c l o p o i d Copepod Response 205 Biomass Response Of P h y t o p l a n k t o n And Prey Zooplankton 208 D i s c u s s i o n 218 Summary 226 GENERAL DISCUSSION 227 LITERATURE CITED 238 APPENDICES 253 LIST OF TABLES Table 1. C h a r a c t e r i s t i c s Of P l a c i d And Gwendoline Lakes 49 Table 2. Comparison Of R e p l i c a t e Samples 52 Tab l e 3. Comparison Of 5 T r a n s e c t s 61 Table 4. Mean Length And Dry Weight Of T . p r a s i n u s And D.rosea 67 Table 5. Index Of A s s i m i l a t i o n 69 Table 6. S u r v i v o r s h i p By I n s t a r Of T . p r a s i n u s 70 Tab l e 7. A n a l y s i s Of V a r i a n c e Of Eggs Per Female 100 Table 8. C l u t c h S i z e Of T . p r a s i n u s In P l a c i d Lake 103 Tab l e 9. Common P h y t o p l a n k t o n Genera 143 Table 10. C l u t c h S i z e Of C.b. thomasi 148 Tab l e 11. P r e d a t i o n Rates Of Chaoborus I n s t a r I 153 Table 12. Mean Length And Dry Weights Of Gwendoline Lake Z o o p l a n k t o n 188 v i i i LIST OF FIGURES F i g u r e 1. A d u l t Male T . p r a s i n u s 10 F i g u r e 2. A d u l t Female T . p r a s i n u s With Egg Sacs 10 F i g u r e 3. O r a l Region Of A d u l t Female T . p r a s i n u s . ... 15 F i g u r e 4. F e e d i n g Appendages Of A d u l t Female T . p r a s i n u s 15 F i g u r e 5. A d u l t Male T . p r a s i n u s I n g e s t i n g Prey 18 F i g u r e 6. O r a l C a v i t y Of A d u l t Female C.b.thomasi And T . p r a s i n u s 18 F i g u r e 7. O r a l Face Of Labrum 20 F i g u r e 8. "Large L a b r a l Tooth 20 F i g u r e 9. M a n d i b l e s Of A d u l t Female C.b.thomasi 24 F i g u r e 10. E l o n g a t e .' 24 F i g u r e 11. ' B u c c a l C a v i t y Of A d u l t Female T . p r a s i n u s .... 24 F i g u r e 12. L a t e r a l View Of The F e e d i n g S t r u c t u r e s 28 F i g u r e 13. F e e d i n g Appendages Of A d u l t Female C.b. thomasi 28 F i g u r e 14. View Of P a i r e d Paragnathae 28 F i g u r e 17. Main Sensory Seta On A d u l t Female T . p r a s i n u s . 28 F i g u r e 15. Mouthparts Of A d u l t Female C.b.thomasi 30 F i g u r e 16. M o u t h p a r t s Of A d u l t Female T . p r a s i n u s 32 F i g u r e 18. M a x i l l i p e d Of A d u l t Male C.b.thomasi 36 F i g u r e 19. The P o s i t i o n Of The F e e d i n g A p p a r a t u s ........ 36 F i g u r e 20. Contour Map Of P l a c i d Lake ...62 F i g u r e 21. P r e d a t i o n R a tes 72 F i g u r e 22. Changes In The S t a n d i n g Crop Of C.b.thomasi . 77 F i g u r e 23. % C o m p o s i t i o n Of C.b.thomasi I n s t a r s 79 F i g u r e 24. Changes In The S t a n d i n g Crop Of T . p r a s i n u s .. 82 F i g u r e 25. D i e l V e r t i c a l M i g r a t i o n 84 F i g u r e 26. V a r i a t i o n In Oxygen And P a r t i c u l a t e O r g a n i c M a t t e r 87 F i g u r e 27. Changes In Temperature 89 F i g u r e 28. Changes In The S t a n d i n g Crop Of P l a c i d Lake Zooplankton 91 F i g u r e 29. % Of The Abundant C r u s t a c e a n P o p u l a t i o n s .... 95 F i g u r e 30. P r o p o r t i o n Of O v i g e r o u s A d u l t Females 97 F i g u r e 31. P r o p o r t i o n Of O v i g e r o u s Females Of T . p r a s i n u s •. 101 F i g u r e 32. E s t i m a t e d D a i l y P r e d a t i o n Of C.b.thomasi On T . p r a s i n u s N a u p l i i •. 105 F i g u r e 33. S t a n d i n g Crop Of T . p r a s i n u s In The E x p e r i m e n t s And Lake 108 F i g u r e 34. T . p r a s i n u s S u r v i v o r s h i p By I n s t a r 110 F i g u r e 35. Changes In S t a n d i n g Crop Of C.b.thomasi 129 F i g u r e 36. Depth D i s t r i b u t i o n Of C.b.thomasi 131 F i g u r e 37. % C o m p o s i t i o n Of C.b.thomasi I n s t a r s 133 F i g u r e 38. C.b.thomasi S u r v i v o r s h i p By I n s t a r 137 F i g u r e 39. Changes In The S t a n d i n g Crop Of The I n t r o d u c e d C.b.thomasi .....140 F i g u r e 40. V a r i a t i o n In P h y t o p l a n k t o n Biomass And % C o m p o s i t i o n 145 F i g u r e 41. P r o p o r t i o n Of O v i g e r o u s C.b.thomasi Females .149 F i g u r e 42. E s t i m a t e d % Of C.b.thomasi N a u p l i i Eaten By Chaoborus 154 X F i g u r e 43. V e r t i c a l D i s t r i b u t i o n Of C.b.thomasi And Chaoborus 156 F i g u r e 44. Summer V a r i a t i o n In P a r t i c u l a t e O r g a n i c M a t t e r (POM) 159 F i g u r e 45. V a r i a t i o n In The S t a n d i n g Crop Of Chaoborus L a r v a e . . . . 162 F i g u r e 46. V a r i a t i o n In The S t a n d i n g Crop Of S o l i t a r y R o t i f e r s 164 F i g u r e 47. C l u s t e r A n a l y s e s Of The Treatment E n c l o s u r e s In Gwendoline Lake. 191 F i g u r e 48. V a r i a t i o n In S t a n d i n g Crop Of C l a d o c e r a n C r u s t a c e a n s 195 F i g u r e 49. V a r i a t i o n In S t a n d i n g Crop Of C a l a n o i d Copepods 203 F i g u r e 50. Changes In The S t a n d i n g Crop Of T . p r a s i n u s ..206 F i g u r e 51. A s h - f r e e Dry Weight 209 F i g u r e 52. Changes In The % C o m p o s i t i o n 212 x i ACKNOWLEDGEMENTS Dr. W.E. N e i l l p r o v i d e d b oth the m a t e r i a l and i n t e l l e c t u a l o p p o r t u n i t y t o do t h i s r e s e a r c h . I am p a r t i c u l a r l y g r a t e f u l f o r h i s p a t i e n t , thorough and c o n s t r u c t i v e c r i t i c i s m of e a r l i e r v e r s i o n s of the m a n u s c r i p t . I would a l s o l i k e t o thank my committee, Dr.P.A. L a r k i n , D r . J . Myers, Dr.T. G. N o r t h c o t e and Dr.T.Parsons f o r h e l p f u l comments on the t e x t . In a d d i t i o n , Dr. N o r t h c o t e p r o v i d e d the o p p o r t u n i t y t o work w i t h both W.J.P. Smyly and Dr.M.A. Chapman, and he was generous i n making v a r i o u s e s s e n t i a l p i e c e s of equipment a v a i l a b l e t o me throughout the c o u r s e of the s t u d y . W.J.P. Smyly p r o v i d e d i n v a l u a b l e h e l p , b oth i n the f i e l d and i n the l a b o r a t o r y . Dr.D.J. McQueen and Dr.M.A. Chapman were a l s o v e r y h e l p f u l a t d i f f e r e n t s t a g e s of the s t u d y . I am e s p e c i a l l y g r a t e f u l t o Deborah Henderson and Bea Schroeder f o r t h e i r generous a s s i s t a n c e w i t h the s c a n n i n g e l e c t r o n m i c r o g r a p h s . N e i l G i l b e r t k i n d l y p r o v i d e d s t a t i s t i c a l a d v i c e . Ken A s h l e y gave p r a c t i c a l a s s i s t a n c e t h a t has w i t h s t o o d the t e s t of t i m e , and was h e l p f u l a t a l l t i m e s . Among the many peopl e who p r o v i d e d m a t e r i a l , moral and i n t e l l e c t u a l s u p p o r t , I would p a r t i c u l a r l y l i k e t o thank: E r i c S h o u b r i d g e , T e r e s a S k i n n a r l a n d , M a r i a Weston, R a c h e l l e B o u f f a r d , P a t r i c k M i c h i e l , P e t e r Delaney, Susan Krepp, Douglas Heard, R i c k S t a n l e y and D a v i d Levy. D u r i n g t h i s s tudy I was s u p p o r t e d by a N a t i o n a l R esearch C o u n c i l S c h o l a r s h i p , a 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 Tea c h i n g A s s i s t a n t s h i p . 1 GENERAL INTRODUCTION E x p l a n a t i o n s f o r the d i s t r i b u t i o n and abundance of s p e c i e s have p l a y e d a c e n t r a l r o l e i n . the development of e c o l o g i c a l thought from the e a r l y f o r m u l a t i o n s of Darwin and W a l l a c e . W a l l a c e (1903) s t a t e d t h a t i t was the resemblances r a t h e r than the d i v e r s i t i e s t h a t he observed i n the d i s t a n t c o n t i n e n t s and i s l a n d s t h a t were the most d i f f i c u l t t o e x p l a i n . Freshwater systems were p a r t i c u l a r l y p u z z l i n g t o the e a r l y e c o l o g i s t s because l a k e s , or a t l e a s t l a k e s and t h e i r d r a i n a g e b a s i n s , seemed l i k e i s l a n d s , s e p a r a t e d from each o t h e r by b a r r i e r s of l a n d . In The Or i g i n of Spec i e s , Darwin r e c a l l e d h i s s u r p r i s e i n f i n d i n g t h a t "not o n l y have many f r e s h w a t e r s p e c i e s , b e l o n g i n g t o d i f f e r e n t c l a s s e s , an enormous ( g e o g r a p h i c ) • r a n g e , but a l l i e d s p e c i e s p r e v a i l i n a remarkable manner throughout the w o r l d " . Yet i n s p i t e of the e v i d e n c e t h a t many f r e s h w a t e r organisms were remar k a b l y c o s m o p o l i t a n , l a k e t o l a k e d i f f e r e n c e s were o f t e n g r e a t . In 1887 S.A. Forbes d e s c r i b e d the f r e s h w a t e r l a k e as a microcosm wherein each s p e c i e s was f u n c t i o n a l l y i n t e r c o n n e c t e d w i t h the whole s p e c i e s assemblage, and a change i n one s p e c i e s ' abundance i n f l u e n c e d a l l o t h e r s i n the community. The microcosm an a l o g y was p r o b a b l y s t i m u l a t e d by o b s e r v a t i o n s on the patchy i n t e r l a k e d i s t r i b u t i o n of spec i e s . A l t h o u g h Darwin c o n f i n e d h i s remarks t o more v i s u a l l y o b v i o u s f r e s h w a t e r o r g a n i s m s , the widesp r e a d o c c u r r e n c e of many 2 genera i n the o r d e r Copepoda l e d S e w e l l (1956) t o use t h i s group as e v i d e n c e f o r the c o n t i n e n t a l d r i f t t h e o r y . F r e s h w a t e r Copepoda a r e both g e o g r a p h i c a l l y w i d e s p r e a d and l o c a l l y d i s c o n t i n u o u s (Gurney, 1931; R y l o v , 1963; Pennak, 1957). Yet p r o d i g i o u s d i s p e r s a l c a p a b i l i t i e s a r e on r e c o r d . B e a l (1881) d e s c r i b e d a "shower of C y c l o p s q u a d r i c o r n i s " i n Iowa. A f t e r a r a i n he noted t h a t the ground had a b l o o d - l i k e appearance and, on e x a m i n a t i o n , h a l f a teaspoon c o n t a i n e d about 500 C y c l o p s . B e a l ' s r e p o r t was undoubtedly an u n u s u a l mode of d i s p e r s a l . N e v e r t h e l e s s , copepod s p e c i e s are r e m a r k a b l y u b i q u i t o u s i n f r e s h w a t e r systems even though i n t e r l a k e d i f f e r e n c e s are o f t e n g r e a t , p a r t i c u l a r l y i n the abundances of s p e c i e s . Pennak (1957), i n a study of 57 l a k e s i n C o l o r a d o , o b s e r v e d t h a t the most abundant copepod s p e c i e s a t any one time a c c o u n t e d f o r about 80% of a l l copepod i n d i v i d u a l s p r e s e n t . He c o n c l u d e d t h a t t h i s r e s u l t was ' t y p i c a l ' of l i m n e t i c z o o p l a n k t o n communities. I f copepods p o s s e s s such extreme powers of d i s p e r s a l , why are t h e r e so few abundant s p e c i e s w i t h i n a g i v e n community? The r e c o g n i t i o n t h a t s p e c i e s i n t e r a c t i o n s may be of major importance i n s t r u c t u r i n g f r e s h w a t e r z o o p l a n k t o n communities i s a r e l a t i v e l y r e c e n t development (Hrbacek, 1962; Brooks and Dodson, 1965). The r e t u r n t o Forbe's p e r s p e c t i v e has encouraged i n v e s t i g a t i o n by e x p e r i m e n t a t i o n p r o b a b l y because a n i m a l s a r e e a s i e r t o m a n i p u l a t e than t h e i r n a t u r a l e n v i r o n m e n t s . With the use of ' c o n t r o l l e d d i s t u r b a n c e s ' , some i n v e s t i g a t o r s have been a b l e t o p r o v i d e m e c h a n i s t i c i n t e r p r e t a t i o n s of the d i s t r i b u t i o n and abundance p a t t e r n s of 3 some z o o p l a n k t o n s p e c i e s (e.g. H u r l b u r t et a l . , 1972; N o r t h c o t e e t a l . , 1978; N e i l l , 1978; Lynch, 1979). However, e x p l a n a t i o n s f o r the d i s t r i b u t i o n and abundance of c y c l o p o i d copepods are s t i l l vague, p r i m a r i l y because the r o l e of these m i c r o c r u s t a c e a n s w i t h i n the p l a n k t o n community has remained somewhat o b s c u r e . N e v e r t h e l e s s , the p o t e n t i a l importance of c y c l o p o i d copepods i n s t r u c t u r i n g l a c u s t r i n e communities has been g e n e r a l l y r e c o g n i z e d (Gurney, 1931; R y l o v , 1963). Many d i f f e r e n t r o l e s w i t h i n the p l a n k t o n community have been a s c r i b e d t o t h e s e t i n y c r u s t a c e a n s . R y l o v (1963) p o i n t s out t h e i r importance as i n t e r m e d i a t e h o s t s f o r many p a r a s i t i c Cestodes and Nematodes which p a r a s i t i z e v e r t e b r a t e s , i n c l u d i n g humans. V a r i o u s p l a n k t i v o r o u s f i s h a re r e p o r t e d t o f e e d on c y c l o p o i d s a l t h o u g h F r y e r (1957b) s u g g e s t s t h a t an abundant c y c l o p o i d p o p u l a t i o n may a c t u a l l y l i m i t f i s h p r o d u c t i o n because the a d u l t c y c l o p o i d s a t t a c k newly hat c h e d f r y . I t i s known t h a t many s p e c i e s of c y c l o p o i d copepods are d e m o n s t r a t a b l y c a r n i v o r o u s i n the l a t e c o p e p o d i d i n s t a r s ( F r y e r , 1957a,b; McQueen, 1969; Anderson, 1970a; K e r f o o t , 1978; B r a n d l and Fernando, 1974,75,78). However, i t i s s t i l l a m atter of debate whether c y c l o p o i d s have any s i g n i f i c a n t e f f e c t even on o t h e r z o o p l a n k t o n ( H a l l et a l . , 1976; Lane, 1979; Lynch, 1979). The i n f l u e n c e of c y c l o p o i d copepods on the d i s t r i b u t i o n and abundance of o t h e r c y c l o p o i d s p e c i e s i s a l s o u n c l e a r . H u t c h i n s o n (1957) d e s c r i b e s s e v e r a l s t u d i e s which suggest t h a t t e m p o r a l s e g r e g a t i o n of „planktonic c y c l o p o i d s p e c i e s by t h e i n t e r c a l a t i o n of a d i a p a u s e i n t h e i r l i f e h i s t o r y i s c r i t i c a l i n p e r m i t t i n g two or more c y c l o p o i d s p e c i e s t o c o - o c c u r . 4 Smyly (1978) s u g g e s t s t h a t C y c l o p s abyssorum S a r s and M e s o c y c l o p s l e u c k a r t i ( C l a u s ) , c o - o c c u r r a r e l y i n the E n g l i s h Lake D i s t r i c t ( a l t h o u g h b oth s p e c i e s are w i d e l y d i s t r i b u t e d ) because d i f f e r e n t temperature optima r e s u l t i n the h e r b i v o r o u s i n s t a r s of the s l o w e r growing s p e c i e s b e i n g e a t e n by the c o p e p o d i d i n s t a r s of the f a s t e r growing s p e c i e s . However, u n r a v e l i n g the v a r i o u s f a c t o r s which produce obs e r v e d abundance p a t t e r n s i s o f t e n d i f f i c u l t and the a b i l i t y of c y c l o p o i d s p e c i e s t o c o - e x i s t may be m e c h a n i s t i c a l l y complex. Pennak (1957) p o i n t s out t h a t a t any one time o n l y one s p e c i e s i n each " f u n c t i o n a l or 'food n i c h e ' " i s the r u l e - t h a t i s , one c y c l o p o i d copepod s p e c i e s per community. Pennak adds t h a t i f two s p e c i e s of l i m n e t i c c y c l o p o i d s are found t o g e t h e r they a r e almost i n v a r i a b l y of d i f f e r e n t genera and he assumes t h a t t h i s p a t t e r n r e f l e c t s d i f f e r e n t ' f o o d - n i c h e s ' . Recent e x p e r i m e n t a l s t u d i e s ( B r a n d l and Fernando, 1978; Lane, 1979; Lynch, 1979), p o t e n t i a l l y p r o v i d i n g more d e f i n i t i v e e v i d e n c e , have l e f t the r o l e of c y c l o p o i d copepods s t i l l somewhat ambiguous because they e i t h e r use u n r e a l i s t i c d e n s i t i e s of p r e d a t o r and p r e y , i n a p p r o p r i a t e t e m p o r a l or s p a t i a l s c a l e s , p o t e n t i a l l y s t r e s s e d a n i m a l s or e x t r a p o l a t e the e f f e c t s of the c y c l o p o i d copepods from o t h e r s t u d i e s s u f f e r i n g from t h e s e e x p e r i m e n t a l problems. In t h i s s t u d y I combined s m a l l s c a l e l a b o r a t o r y e x p e r i m e n t s and m o r p h o l o g i c a l s t u d i e s w i t h l a r g e s c a l e i_n s i t u p e r t u r b a t i o n s and demographic o b s e r v a t i o n s of l a k e p o p u l a t i o n s t o m e c h a n i s t i c a l l y e x p l a i n the r e l a t i v e abundances of two' c o - e x i s t i n g c y c l o p o i d copepods, T r o p o c y c l o p s p r a s i n u s ( F i s c h e r ) 1860 and C y c l o p s b i c u s p i d a t u s 5 thomasi ( F o r b e s ) 1882 (= D i a c y c l o p s thomasi ( K i e f e r , 1 9 7 8 ) . I n t e n s i v e s a m p l i n g r e v e a l e d that,- where the one s p e c i e s was abundant, the o t h e r was s c a r c e . To e x p l a i n the mechanisms u n d e r l y i n g t h i s p a t t e r n I f o c u s e d my study on two nearby o l i g o t r o p h i c l a k e s where the r e l a t i v e abundances of the two s p e c i e s appeared t o be r e v e r s e d . I then examined 1) the m o r p h o l o g i c a l f e a t u r e s of the f e e d i n g a p p a r a t u s i n C.b.thomasi and T . p r a s i n u s 2) the r e s p e c t i v e f e e d i n g h a b i t s of the two s p e c i e s 3) the p o p u l a t i o n dynamics of both s p e c i e s i n the two l a k e s and i n ' d i s t u r b e d ' e n c l o s u r e s and 4) the way i n which c o m p e t i t i o n , p r e d a t i o n and l a k e n u t r i e n t l e v e l s i n t e r a c t i n the f i e l d t o a f f e c t abundances of C.b.thomasi , T . p r a s i n u s and the z o o p l a n k t o n community. The work i s p r e s e n t e d i n f o u r s e c t i o n s . S e c t i o n one examines the f e e d i n g s t r u c t u r e s of T . p r a s i n u s and"C.b.thomasi . S e c t i o n two d e s c r i b e s e x p e r i m e n t a l m a n i p u l a t i o n s of one z o o p l a n k t o n community, undertaken t o i n v e s t i g a t e the C.b.thomasi - T . p r a s i n u s i n t e r a c t i o n . S e c t i o n t h r e e d i s c u s s e s the C.b.thomasi t r a n s p l a n t e x p e r i m e n t s . I t r a n s f e r r e d C.b.thomasi from P l a c i d Lake t o nearby Gwendoline Lake and a l t e r e d both the d e n s i t y of Chaoborus (a predaceous midge l a r v a ) and the n u t r i e n t environment. S e c t i o n f o u r d e s c r i b e s the e f f e c t on the Gwendoline Lake z o o p l a n k t o n community of v a r y i n g b oth i n v e r t e b r a t e p r e d a t o r p o p u l a t i o n s and n u t r i e n t l e v e l s . 6 I . A MORPHOLOGICAL STUDY OF TROPOCYCLOPS PRASINUS FISCHER AND CYCLOPS BICUSPIDATUS THOMASI FORBES 7 INTRODUCTION In most a n i m a l s t h e r e i s a c l o s e r e l a t i o n s h i p between the s t r u c t u r e of the mouthparts and the d i e t ( H a s s a l l , 1 9 7 7 ) . W i t h i n the spectrum of a p p a r e n t l y a v a i l a b l e f o o d , i t i s o f t e n p o s s i b l e t o i d e n t i f y the p r e c i s e segment t h a t the a n i m a l can use by s t u d y i n g the t o o l s w i t h which the a n i m a l e a t s . Gwyne and B e l l (1968) were a b l e t o e x p l a i n the r e l a t i o n s h i p s between l a r g e v e r t e b r a t e h e r b i v o r e s i n a g r a z i n g s u c c e s s i o n by examining s i m p l e d i f f e r e n c e s i n d e n t i t i o n . U n l i k e many v e r t e b r a t e s , i n v e r t e b r a t e gut c o n t e n t s a re o f t e n d i f f i c u l t t o stud y and mouthpart morphology may p r o v i d e the f i r s t i n s i g h t i n t o the n a t u r e of the d i e t (Room, 1975). I s l e y (1944) showed t h a t a v a r i e t y of grasshopper s p e c i e s c o u l d be i d e n t i f i e d as g r a n i v o r e , h e r b i v o r e or c a r n i v o r e by examining t h e i r m a n d i b l e s . Anraku a n d Omori (1963) s u r v e y e d ' s e v e r a l - s p e c i e s of marine copepods and showed t h a t t h e r e i s a c l o s e r e l a t i o n s h i p between the s t r u c t u r e of the mouthparts and the type of food t a k e n . I t o h (1970) a l s o examined marine copepods and he was a b l e t o d i f f e r e n t i a t e h e r b i v o r e s , omnivores and c a r n i v o r e s on the b a s i s of an "Edge i n d e x " d e r i v e d from measurements of the c u t t i n g edges of the mandible. Z a r e t (1978) suggests t h a t the f e e d i n g morphology of l a c u s t r i n e a n i m a l s i s d e t e r m i n e d p r i m a r i l y by c o m p e t i t i o n . However, i n f r e s h w a t e r c y c l o p o i d copepods the c o r r e l a t i o n between mouthpart morphology and food s p e c i f i c i t y doesn't seem o b v i o u s ( F r y e r , 1 9 5 7 a ) . I l l u s t r a t i o n s of i n d i v i d u a l appendages of v a r i o u s c y c l o p o i d copepod s p e c i e s ( S ars,1918; Gurney,1931) a r e remarkable f o r t h e i r s i m i l a r i t y . In s p i t e of t h i s , Fryer. (1957a,b) was a b l e t o i d e n t i f y 8 d i s t i n c t l y d i f f e r e n t d i e t s ( h e r b i v o r o u s v e r s u s c a r n i v o r o u s as a d u l t s ) even w i t h i n a c l o s e l y r e l a t e d group of c y c l o p o i d copepods. Lewis (1979) s t a t e s t h a t a d u l t Thermocycylops  h y a l i n u s a r e u n e q u i v o c a l l y h e r b i v o r o u s and t h a t "mouthpart m o r p h o l o g i e s c o n f i r m t h i s c o n c l u s i o n " - , a l t h o u g h he does not p r o v i d e i l l u s t r a t i o n s . F r y e r (1957a) assembled i n d i v i d u a l appendages t o i l l u s t r a t e how they f u n c t i o n in v i v o and suggested t h a t the f e e d i n g mechanism i s s i m i l a r a c r o s s d i e t s . A l t h o u g h t h e r e has been some i n t e r e s t i n the type of food taken (Smyly, 1970; B r a n d l and Fernando 1974, 75, 78) and f o o d -s e e k i n g b e h a v i o u r (e.g. S t r i c k l e r and B a l , 1973; K e r f o o t , 1978) t h e r e has been l i t t l e advance i n our knowledge of s p e c i e s - s p e c i f i c d i f f e r e n c e s i n the f e e d i n g t o o l s s i n c e F r y e r ' s c l a s s i c paper, and many common s p e c i e s have never been d e s c r i b e d i n ' d e t a i l . C y c l o p o i d copepods a r e among the most c o s m o p o l i t a n of a n i m a l s . They are found i n a wide v a r i e t y of f r e s h w a t e r h a b i t a t s , from c a v e s , w e l l s and s m a l l p u d d l e s t o the open waters of l a r g e l a k e s . Only a few s p e c i e s a r e t y p i c a l l y l i m n e t i c , most b e i n g b e n t h i c - l i t t o r a l c r e a t u r e s . L i k e o t h e r copepods, the c y c l o p o i d copepod b e g i n s l i f e as a n a u p l i u s ; subsequent l a r v a l s t a g e s a r e c l a s s i f i e d as c o p e p o d i d i n s t a r s . There a r e n o r m a l l y s i x n a u p l i a r and s i x c o p e p o d i d i n s t a r s i n f r e e - l i v i n g copepods (Elgmork and Langeland,1970). The molt from n a u p l i u s VI t o co p e p o d i d I r e s u l t s i n a d i s t i n c t change of form, from a t y p i c a l s o f t - b o d i e d n a u p l i u s t o an u n m i s t a k a b l e 'copepodan' form ( w i t h an e l o n g a t e , c h i t i n o u s segmented body). R e p r o d u c t i o n i n a l l the Copepoda i s always b i s e x u a l . In male 9 c y c l o p o i d copepods, u n l i k e c a l a n o i d copepods, both antennae are g e n i c u l a t e ( f i g . l ) . These antennae a r e used f o r g r a s p i n g the female d u r i n g c o p u l a t i o n , when the sperm i s t r a n s f e r r e d t o the female i n a c h i t i n o u s package c a l l e d the spermatophore. The females s t o r e the sperm i n the spermatheca, i n t o which the o v i d u c t s open on the g e n i t a l segment, and the eggs a r e f e r t i l i z e d as they a re l a i d . One mating p r o v i d e s enough sperm t o l a s t the e n t i r e l i f e of the female (Hutchinson,1957) . Lewis e t a l . (1971) observe t h a t as many as 9 p a i r s of egg sacs can be l a i d per a d u l t female. The eggs a r e c a r r i e d l a t e r a l l y i n two egg sacs ( F i g . 2 ) . A l l the f r e e - l i v i n g a d u l t C y c l o p o i d a are r a p t o r i a l , whether they a re h e r b i v o r e s or c a r n i v o r e s ( H u t c h i n s o n , 1 9 6 7 ) . The f e e d i n g appendages appear s t o u t and s p i n y , a p p r o p r i a t e f o r a g r a s p i n g , but not a f i l t e r i n g , mode of food a c q u i s i t i o n . F r y e r (1957b) s u g g e s t s t h a t , i n g e n e r a l , l a r g e s p e c i e s t e n d t o be c a r n i v o r o u s as a d u l t s w h i l e the s m a l l e r s p e c i e s remain h e r b i v o r e s t hroughout t h e i r l i f e h i s t o r y . As the f e e d i n g a p p a r a t u s i s g r o s s l y s i m i l a r among many s p e c i e s examined ( S a r s , 1918; Gurney, 1931; F r y e r , 1957a; R y l o v , 1963) s m a l l d i f f e r e n c e s i n s t r u c t u r e and/or s i z e must be c r u c i a l i n p e r m i t t i n g the h a n d l i n g of the d i f f e r e n t t y p e s of food i t e m s , p l a n t or a n i m a l . T h i s c h a p t e r p r e s e n t s d e t a i l s of mouthpart s t r u c t u r e i n two u n r e l a t e d but e x c e p t i o n a l l y common l i m n e t i c c y c l o p o i d copepods, T r o p o c y c l o p s p r a s i n u s ( F i s c h e r ) 1860 and C y c l o p s  b i c u s p i d a t u s thomasi (Forbes) 1882. A l t h o u g h C.b.thomasi and T . p r a s i n u s c o e x i s t i n the o l i g o t r o p h i c montane l a k e s of the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t , where one 10 F i g u r e 1. A d u l t male T . p r a s i n u s w i t h g e n i c u l a t e antennae. S c a l e - 50 urn F i g u r e 2. A d u l t female T . p r a s i n u s w i t h egg s a c s . S c a l e -50 um 12 s p e c i e s i s abundant, the o t h e r i s s c a r c e , and the f e e d i n g a p p a r a t u s may be i m p o r t a n t i n e x p l a i n i n g the o b s e r v e d d i s t r i b u t i o n and abundance p a t t e r n i n t h e s e l a k e s . As s m a l l d i f f e r e n c e s i n o r a l appendages can be t r a n s l a t e d i n t o l a r g e d i f f e r e n c e s i n f u n c t i o n a l morphology, the mouthparts of both s p e c i e s a r e compared. Under a l i g h t m i c r o s c o p e (LM) the g e n e r a l mouthpart morphology appears v e r y s i m i l a r between the two s p e c i e s . Both s p e c i e s a r e q u i t e s m a l l and d e t a i l , which may be c r i t i c a l t o the a n i m a l , i s d i f f i c u l t t o see under a LM . T h e r e f o r e a d e t a i l e d study was undertaken u s i n g a s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM). T h i s study r e p r e s e n t e d a f i r s t s t e p i n u n d e r s t a n d i n g the r e l a t i o n s h i p s of these a n i m a l s w i t h each o t h e r and w i t h t h e i r e nvironment. 13 MATERIALS AND METHODS The mouthparts of T . p r a s i n u s and C.b.thomasi were p r e p a r e d f o r s t r u c t u r a l s t u d i e s under.the LM by mounting the a n i m a l s on s e p a r a t e s l i d e s i n r e d p o l y v i n y l l a c t o p h e n o l and removing the c e p h a l o t h o r a x i n t a c t . I n d i v i d u a l mouthparts were then removed u s i n g e l e c t r o l y t i c a l l y sharpened t u n g s t e n n e e d l e s and were drawn w i t h the a i d of a camera l u c i d a . In a d d i t i o n , c o pepodids IV t o a d u l t of T . p r a s i n u s and C.b.thomasi were p r e p a r e d f o r s c a n n i n g e l e c t r o n m i c r o s c o p y . Over 50 d i f f e r e n t a d u l t a n i m a l s of both s p e c i e s were examined and the a n i m a l s showed l i t t l e v a r i a t i o n , a t l e a s t i n the mouthpart s t r u c t u r e . Best r e s u l t s were o b t a i n e d w i t h a n i m a l s taken d i r e c t l y from the l a k e , a n a e s t h e t i z e d w i t h c a r b o n a t e d water, and then d e h y d r a t e d w i t h a s e r i e s of e t h a n o l s o l u t i o n s of i n c r e a s i n g c o n c e n t r a t i o n , from 10% t o 100%. The a n i m a l s were c r i t i c a l - p o i n t d r i e d , c o a t e d w i t h g o l d / p a l l a d i u m and mounted f o r e x a m i n a t i o n . 14 STRUCTURE OF THE MOUTHPARTS In both T . p r a s i n u s and C.b.thomasi the o r a l c a v i t y i s b o r d e r e d a n t e r i o r l y by the labrum, l a t e r a l l y by the m a n d i b l e s and p o s t e r i o r l y by paragnathae ( F i g . 3 ; p ) . From b e h i n d and l a t e r a l t o the p a i r e d paragnathae a r i s e t h r e e p a i r e d f e e d i n g appendages: the f i r s t m a x i l l a e or m a x i l l u l e s , the second m a x i l l a e and the m a x i l l i p e d s . The second m a x i l l a e o v e r l a p the f i r s t w h i l e the m a x i l l i p e d s a r e s l i g h t l y below and i n s i d e the second m a x i l l a e (Fig.4;m2,ml,mx). 15 F i g u r e 3. O r a l r e g i o n of a d u l t female T . p r a s i n u s . L e t t e r s r e p r e s e n t : l a - labrum, m - 'mustache' or l a b r a l s e t a e , ma - m a n d i b l e s , p - paragnathae, ml - f i r s t m a x i l l a e . S c a l e - 50 urn F i g u r e 4. F e e d i n g appendages of a d u l t female T . p r a s i n u s . L e t t e r s a re d e f i n e d as i n d i c a t e d above and i n a d d i t i o n : m2 - second m a x i l l a , mx - m a x i l l i p e d . S c a l e ^ - 50 urn 17 MOUTH REGION Labrum The a n t e r i o r of the labrum has a prominent b e a k l i k e s t r u c t u r e . The 'beak' becomes a m e d i a l groove p o s t e r i o r l y which s e p a r a t e s the two s i d e s of a t r i a n g u l a r f l a t t e n e d p l a t e . Each s i d e bears a row of r e g u l a r l y spaced s t o u t h a i r s , b i l a t e r a l l y s p l i t t o l o o k l i k e a s p a r s e b r u s h mustache (Fig.3;m). When the prey item i s l a r g e enough, th e s e h a i r s make c o n t a c t w i t h the food b e i n g i n g e s t e d ( F i g . 5 ; p r ) . The h a i r s a r i s e from the lower edge of the f l a t t e n e d , m e d i a l l y c l e f t p l a t e which almost f o l d s under t o form a r i d g e of s m a l l t e e t h on the f r e e m a rgin, c u l m i n a t i n g i n a. s i n g l e l a r g e c o a r s e * ' t o o t h ' a t e i t h e r edge ( F i g . 3 ) . The s m a l l t e e t h are s i t u a t e d m e d i a l l y , d i r e c t l y over the o r a l c a v i t y , and they appear t o be s m a l l e r and f i n e r i n the c e n t r e , becoming l a r g e r and somewhat s p l a y e d out toward the l a t e r a l edge ( F i g . 6 ; l a ) . A l l the t e e t h p o i n t toward the mouth c a v i t y . S e v e r a l d u c t s open on the i n t e r n a l ( o r a l ) f a c e of the labrum. A l t h o u g h t h e i r f u n c t i o n i s unknown, F r y e r (1957a), s u g g e s t s t h a t these are the openings of the s a l i v a r y d u c t s l e a d i n g from l a b r a l g l a n d s . Near these d u c t s a r e b i l a t e r a l l y s y m m e t r i c a l patches of s h o r t s t o u t s p i n y s e t a e , a l l of which p o i n t toward the o r a l c a v i t y . T h i s a r e a i s remarkably s i m i l a r between the two . s p e c i e s , T . p r a s i n u s and C.b.thomasi ( F i g . 7 ; l a o ) , as i s the e n t i r e l a b r a l r e g i o n . The o n l y 18 F i g u r e 5. A d u l t male T . p r a s i n u s i n g e s t i n g p r e y . L e t t e r s r e p r e s e n t : l a - labrum, pr - p r e y . S c a l e - 50 urn F i g u r e 6. O r a l c a v i t y of a d u l t female C.b.thomasi and T . p r a s i n u s . a. O r a l c a v i t y of an a d u l t female C.b.thomasi. b. The same a r e a i n an a d u l t female T . p r a s i n u s . L e t t e r s r e p r e s e n t : p - p aragnathae, ma -m a n d i b l e s , ml - f i r s t m a x i l l a e , m2 - second m a x i l l a e , mx - m a x i l l i p e d s . S c a l e - 50 urn 20 F i g u r e 7. O r a l f a c e of labrum a. O r a l f a c e of the labrum of an a d u l t C.b.thomasi female. b. The same are a i n an a d u l t female T . p r a s i n u s showing s i m i l a r f e a t u r e s . L e t t e r s r e p r e s e n t : l a o - o r a l f a c e of labrum, s - s e t a e on o r a l f a c e of labrum, t - t o o t h - l i k e s t r u c t u r e s , s t - s m a l l ' t o o t h ' observed between the l a r g e and s m a l l l a b r a l t e e t h on T . p r a s i n u s . F i g u r e 8. Large l a b r a l t o o t h of a d u l t female C.b.thoma'si. S c a l e - 50 urn 22 d i f f e r e n c e s a r e i n the l o c a t i o n and number of the s t o u t s p i n i f o r m s e t a e on the i n n e r f a c e of the labrum ( T . p r a s i n u s has more) and the e x i s t e n c e of a s m a l l e x t r a t o o t h between the l a r g e c o a r s e t o o t h and the s m a l l e r m e d i a l t e e t h i n T . p r a s i n u s . T h i s s m a l l t o o t h i s not p r e s e n t i n C.b.thomasi ( f i g . 7 ; t , s t and 8; t ) . M a n d i b l e s The m a n d i b l e s a r e s i t u a t e d l a t e r a l l y r e l a t i v e t o the o r a l c a v i t y , below the second antennae and above the f i r s t m a x i l l a e (Fig.6;ma). Each mandible c o n s i s t s of a s t o u t s h a f t or gnathobase whose p r o x i m a l end a r t i c u l a t e s w i t h the c e p h a l o t h o r a x s t e r n i t e and whose d i s t a l e x t r e m i t y i s drawn out i n t o a h o r i z o n t a l l y f l a t t e n e d arm t e r m i n a t i n g i n a ' t o o t h e d ' m a s t i c a t o r y edge. F r y e r (1957a) a c c u r a t e l y d e s c r i b e d the t w i s t i n g of the m a n d ibular gnathobase a l o n g t h i s d i s t a l b l a d e such t h a t the t o o t h e d edge l i e s a l o n g the l i n e of the oesophagus and not a c r o s s i t ( F i g . 6,9;d). The c u s p i d edge of t h i s b l a d e appear b i f i d and, a t p o i n t s , t r i f i d i n the l a t e r a l view ( F i g . 6 , 7 ; t ) . In C.b.thomasi the ' t o o t h ' p r o x i m a l t o the o r a l c a v i t y i s e l o n g a t e , almost t w i c e the l e n g t h of the o t h e r p o i n t s . T h i s ' t o o t h ' i s s i m i l a r i n g i r t h t o the o t h e r s but d i f f e r s i n s t r u c t u r e i n t h a t the edges are c u t i n t o s h o r t s p u r s or s e r r a t i o n s which p o i n t toward the d i s t a l end ( F i g . 6 , 1 0 ; j u s t above and t o the l e f t of d ) . U n l i k e M a c r o c y c l o p s a l b i d u s ( J u r i n e ) ( F r y e r , 1 9 5 7 a ) , t h i s p r o j e c t i o n i s not more s l e n d e r or d e l i c a t e i n s t r u c t u r e than the o t h e r t e e t h on the d i s t a l edge of the mandible a l t h o u g h , as suggested by 23 F r y e r (1957a), i t p r o b a b l y does s e r v e a s e n s o r y f u n c t i o n . I t p r o j e c t s i n t o the o r a l c a v i t y and i s not apparent i n T . p r a s i n u s , a l t h o u g h i n t h i s s p e c i e s the t o o t h c l o s e s t t o the o r a l c a v i t y appears s l i g h t l y l o n g e r than the r e s t of the ' t e e t h ' and may a l s o have a s e n s o r y f u n c t i o n . There i s a l s o a s m a l l , f i n g e r -l i k e p r o j e c t i o n , which seems p e c u l i a r t o T . p r a s i n u s , a r i s i n g from the b u c c a l c h i t i n j u s t a n t e r i o r t o the paragnathae ( F i g . l l j f ) . T h i s p r o j e c t i o n has s e r r a t e edges r a t h e r l i k e t h o s e on the 'sensory' cusp of C.b.thomasi but i s a t t a c h e d t o the s t e r n i t e r a t h e r than a r i s i n g from the m a n d i b u l a r b l a d e . I t s f u n c t i o n i s p r o b a b l y s e n s o r y because i t doesn't seem s t o u t enough f o r g r a s p i n g or c r u s h i n g . On the v e n t r a l s u r f a c e of the m a n d i b u l a r gnathobase, toward the l a t e r a l edge, i s a much-reduced p a l p c o n n e c t e d t o the b a s a l p l a t e by a moveable j o i n t ( F i g . 6,12 ;"to the l e f t of 'ma' i n b o t h ) . T h i s p a l p b e a r s t h r e e p o s t e r i o r l y d i r e c t e d s e t a e . Two of t h e s e s e t a e are e x t r e m e l y l o n g . In T . p r a s i n u s they r e a c h back beyond the p o s t e r i o r edge of the c e p h a l o t h o r a x ( F i g . 4 ) , w h i l e i n C.b.thomasi they appear t o be somewhat s h o r t e r ( F i g . 1 3 ) . The t h i r d s e t a i s q u i t e s h o r t i n both s p e c i e s . In T . p r a s i n u s the s e t a l y i n g v e n t r a l l y appears plumose, b e a r i n g two rows of r e g u l a r l y spaced f i n e h a i r s or s e t u l e s , from the t i p t o almost the p a l p a l o r i g i n . The d o r s a l l y s i t u a t e d s e t a appears h a i r l e s s from the p a l p t o mid-l e n g t h where i t shows the same plumose p a t t e r n of f i n e h a i r s t o the d i s t a l e x t r e m i t y . F r y e r (1957a) r e p o r t s u s i n g the movement of t h e s e s e t a e , r e f l e c t i n g as they do the movements of the arm of the m a n d i b l e s , t o i n t e r p r e t the mode of a c t i o n of 24 F i g u r e 9. M a n d i b l e s of a d u l t female C.b.thomasi showing ' t e e t h ' on the d i s t a l end, paragnathae and f i r s t m a x i l l a . L e t t e r s r e p r e s e n t : ma - m a n d i b l e , d - d i s t a l end of m a n d i b l e , ml - f i r s t m a x i l l a , ms - main s e n s o r y s e t a of f i r s t m a x i l l a , p - p a r a g n a t h a . S c a l e - 5 um F i g u r e 10. E l o n g a t e d i r e c t l y above and t o the l e f t of 'd', j u s t below the labrum, t h i s ' t o o t h ' resembles a s t o u t s e t a e more than i t does the r e s t of the ' t e e t h ' on the d i s t a l end of the mandible. T h i s s t r u c t u r e was o b v i o u s on C.b.thomasi o n l y . S c a l e - 5 um F i g u r e 11. B u c c a l c a v i t y of a d u l t female T . p r a s i n u s showing the f i n g e r - l i k e p r o j e c t i o n from the b u c c a l c a v i t y . L e t t e r s r e p r e s e n t : f - f i n g e r - l i k e p r o j e c t i o n a r i s i n g from the b u c c a l c h i t i n . S c a l e - 5 um 26 these m i n i s c u l e appendages i n l i v e M . a l b i d u s . T h e i r l o c a t i o n and f i n e s t r u c t u r e s u g gests a s e n s o r y f u n c t i o n . Paragnathae The p a i r e d paragnathae l i e b e h i n d the d i s t a l arms of the mandi b l e s ( F i g . 1 4 ) . They form a p a i r of s t o u t c h i t i n o u s p r o j e c t i o n s t h a t a r i s e from the c e p h a l o t h o r a x s t e r n i t e and a r e d i s t i n c t from the appendages. J o i n e d m e d i a l l y , each arm i s s l i g h t l y s u b d i v i d e d i n t o two l o b e s a t the p o i n t most d i s t a l from the body. The m e d i a l l o b e on each s i d e i s c o v e r e d w i t h a s t r i p of s h o r t , c o a r s e s e t a e both a n t e r i o r l y and p o s t e r i o r l y . I f one p e e r s toward the o r a l c a v i t y from the p o s t e r i o r s i d e of the p a r a g n a t h a e , the d o r s a l l y l o c a t e d setae p r o j e c t t h e i r s t i f f t i p s , a l m o s t l i k e s e r r a t i o n s , i n t o the m e d i a l c l e f t t h r o u g h which f o o d must pass t o re a c h the o r a l c a v i t y ( F i g . 1 4 ) . The c l e f t i t s e l f i s smooth and h a i r l e s s . In M . a l b i d u s F r y e r (1957a) has d e s c r i b e d these paragnathae as " s m a l l s e r r a t e d c h i t i n o u s knobs, a t t a c h e d t o the l a t e r a l e x t r e m i t i e s of a c h i t i n o u s b a r " . A c c o r d i n g t o H a r t o g (1888), t h i s p o s t - o r a l -bar can c o n t r a c t f o r m i n g a V. T h i s causes the edges of the paragnathae t o come t o g e t h e r " l i k e minute p i n c e r s " ( F r y e r , 1 9 5 7 a ) . There a r e no s e r r a t e d edges i n e i t h e r T . p r a s i n u s or C.b.thomasi a l t h o u g h the s p i n i f o r m s e t a e t h a t p r o t r u d e m e d i a l l y may have the same e f f e c t , h o l d i n g the prey when the paragnathae c o n t r a c t . A n i m a l s o b s e r v e d w i t h food i n t h e i r g r a s p c o n f i r m the p i n c e r - l i k e f u n c t i o n of the paragnathae ( F i g . 5 ) . 27 GRASPING APPENDAGES The F i r s t M a x i l l a e The f i r s t m a x i l l a e are v e r y s t o u t g r a s p i n g appendages which a r t i c u l a t e w i t h the s t e r n i t e j u s t below the gnathobase of the mandibles ( F i g . 4 , 6 , 1 3 ) . Each c o n s i s t s of a broad f l a t t e n e d base which c u r v e s over m e d i a l l y and narrows a t i t s d i s t a l e x t r e m i t y t o form t h r e e s t i f f h e a v i l y c h i t i n i z e d c l a w s . On the v e n t r a l s i d e of t h e s e a p p a r e n t l y f i x e d c l a w s l i e s a s t o u t s e t a which appears t o have a moveable base (Fig.9,16,17;ms). On the i n n e r , m e d i a l f a c e of the gnathobase are s e v e r a l s h o r t , s p i n i f o r m s e t a e . A l l these s e t a e p o i n t toward the mouth. One of the most i n t e r e s t i n g ' o f "these s e t a e i s what F r y e r ( l 9 5 7 a ) c a l l s the "main sensory s e t a " of the i n n e r f a c e of the f i r s t m a x i l l a . U n l i k e the o t h e r s , t h i s s e t a appears t o have a f l e s h y s t a l k and i s on a moveable base. I t resembles a ' b o t t l e - b r u s h ' i n appearance as i t has s e v e r a l w h o r l s of s e t u l e s , one s e t o r i g i n a t i n g near the base and the o t h e r around the t i p (Fig.9,15,16 ,17 ) . The b a s a l s e t u l e s a r e l o n g e r than those a t the d i s t a l e x t r e m i t y and a l l the s e t u l e s appear q u i t e s t i f f and s p i n y . On the o u t e r a n t e r i o r s i d e of the f i r s t m a x i l l a l i e s a p a l p w i t h two l o b u l e s , each of which bear m o d e r a t e l y l o n g s e t a e - the p o s t e r i o r b e a r i n g f o u r and the a n t e r i o r one t h r e e . These a r c h over the d i s t a l e x t r e m i t y of the f i r s t m a x i l l a , the a n t e r i o r s p l a y i n g over the mouth r e g i o n and the p o s t e r i o r 28 F i g u r e 12. L a t e r a l view of the f e e d i n g s t r u c t u r e s of a d u l t female C.b.thomasi showing how the m a n d i b u l a r p a l p i s c o n n e c t e d t o the b a s a l p l a t e by a moveable j o i n t . The p a l p bears 3 s e t a e , 2 l o n g and one s h o r t . L e t t e r s r e p r e s e n t : ma - mandible ( p a l p j u s t t o l e f t of 'ma'), ml - f i r s t m a x i l l a , m2 - second m a x i l l a , mx - m a x i l l i p e d S c a l e - 50 urn F i g u r e 13. F e e d i n g appendages of a d u l t female C.b.thomasi . S c a l e - 50 um F i g u r e 14. View of p a i r e d paragnathae i n a d u l t female T . p r a s i n u s viewed from the p o s i t i o n of the f i r s t m a x i l l a e . S c a l e - 5 um F i g u r e 17. Main sensory s e t a on a d u l t female T . p r a s i n u s showing the l o n g s t u r d y s e t a e and w i d e l y spaced s e t u e s on the m a x i l l i p e d . S c a l e - 50 um 30 F i g u r e 15. Mouthparts of a d u l t female C.b.thomasi as drawn under a l i g h t m i c r o s c o p e u s i n g a camera l u c i d a a t t a c h m e n t . A r a b i c numbers r e p r e s e n t : 1 - mandible - f i r s t m a x i l l a 3 - second m a x i l l a 4 - m a x i l l i p e d . 31 32 F i g u r e 16. Mouthparts of a d u l t female T . p r a s i n u s drawn as d e s c r i b e d above. Mouthpart numbers are the same as F i g . 15. 33 34 l o b u l e s e t a e r e a c h i n g over the v e n t r a l s i d e of the p o s t e r i o r l o b u l e . T h e i r p o s i t i o n was v a r i a b l e i n some p i c t u r e s s u g g e s t i n g c o n s i d e r a b l e independent movement. R y l o v (1963) s u g g e s t s t h a t t h e s e l o b u l a t e outgrowths of t h e p a l p are r u d i m e n t s of the exo-and e n d o p o d i t e o b s e r v e d i n c a l a n o i d copepods. There are d i f f e r e n c e s between the p a l p a l s e t a e i n C.b.thomasi and T . p r a s i n u s . A l l the s e t a e on the former are l o n g and h a i r l e s s except f o r the most d i s t a l s e t a on the a n t e r i o r l o b u l e , which has s h o r t s e r r a t e s e t a e . By c o n t r a s t , T . p r a s i n u s has f i n e s e t u l e s on a l l the p a l p a l s e t a e . The Second M a x i l l a e The second m a x i l l a e , are s t o u t j o i n t e d appendages which a r t i c u l a t e b a s a l l y w i t h the s t e r n i t e of the c e p h a l o t h o r a x j u s t below and s l i g h t l y , l a t e r a l t o the f i r s t m a x i l l a e . U n l i k e the l a t t e r , t h e r e are s e v e r a l w e l l - d e f i n e d segments of which the f i r s t two ( c a l l e d the p r o t o p o d i t e ) are s i g n i f i c a n t l y l a r g e r than the o t h e r s ( F i g . 1 6 , 1 7 ) . The segments d i s t a l t o the p r o t o p o d i t e form the e n dopodite ( R y l o v , 1 9 6 3 ) . The a n t e r i o r j o i n t of the e n dopodite t e r m i n a t e s d i s t a l l y on the i n n e r margin i n one l a r g e c l a w - l i k e p r o j e c t i o n w i t h s e r r a t e edges a l o n g i t s d i s t a l e x t r e m i t y . On the a n t e r i o r j o i n t t h e r e i s a l s o a s t u r d y s e t a w i t h s p i n i f o r m s e t u l e s and a s l e n d e r s i m p l e s e t a a t i t s base. The p o s t e r i o r j o i n t i s l e s s r o b u s t and has two plumed s e t a e . The most d i s t a l j o i n t of the e n d o p o d i t e bears f i v e s e t a e , two of which a r e l a r g e w i t h s e r r a t e t i p s ( F i g . 6 , 1 2 ) . A c c o r d i n g t o F r y e r (1957a)' the most imp o r t a n t 35 sensory s e t a e are the two pappose s e t a e s i t u a t e d on the i n n e r f a c e of the b a s a l segment. Some photographs (not shown here) show t h a t these s e t a e are c l e a r l y c a p a b l e of movement independent of the e n t i r e appendage. The second m a x i l l a e move l a t e r a l l y , l i k e the f i r s t m a x i l l a e , but the former l i m b s appear to have more a n t e r o -p o s t e r o movement than the f i r s t m a x i l l a e . The second m a x i l l a e hangs below the c e p h a l o t h o r a x i n some SEM photographs w h i l e i n o t h e r s they r e a c h a n t e r i o r l y as f a r as the mouth, w i t h the t i p s of the d i s t a l e x t r e m i t y t w i s t e d up and c o n v e r g i n g . The o n l y d i f f e r e n c e between the second m a x i l l a e i n T . p r a s i n u s and C.b.thomasi• i s the presence of some s h o r t s p i n i f o r m s e t a e on the p o s t e r i o r b a s a l segment of T . p r a s i n u s . T h i s a r e a i s q u i t e smooth, w i t h o u t s e t a e , i n C.b.thomasi. The M a x i l l i p e d s These appendages a r t i c u l a t e w i t h t h e ' c h i t i n o u s s t e r n i t e j u s t below and m e d i a l t o the second m a x i l l a e . A l t h o u g h somewhat s i m i l a r i n s t r u c t u r e t o the l a t t e r , the m a x i l l i p e d s are more s l i g h t l y b u i l t and more a t t e n u a t e d than the second m a x i l l a e ( F i g . 1 5 , 1 6 ) . The s e t a e a r e l o n g and s t u r d y w i t h w i d e l y spaced (about 4 um), s p i n y s e t u l e s (Fig.4,13,18;mx). The b a s a l segment has t h r e e s e t a e , a l l of which a r e c a p a b l e of independent movement. The s e t a e on the d i s t a l segments a r e s i m i l a r -in s t r u c t u r e t o those on the f i r s t e xcept t h a t they t e r m i n a t e i n s e r r a t e s e t a e . The most d i s t a l s e t a i s d i f f e r e n t , more a t t e n u a t e d - almost d e l i c a t e - and smooth u n t i l the t e r m i n a l s e r r a t i o n s ( F i g . 4 , 1 8 ) . 36 F i g u r e 18. M a x i l l i p e d of a d u l t male C.b.thomasi showing the l o n g s t u r d y s e t a e and w i d e l y spaced s e t u l e s on the m a x i l l i p e d . S c a l e - 50 um F i g u r e 19. The p o s i t i o n of the f e e d i n g a p p a r a t u s of an a d u l t female T . p r a s i n u s showing the sweeping a c t i o n of t h i s appendages when h a n d l i n g clumps of f o o d . S c a l e - 50 um 31 38 Movement of these appendages i s p r o b a b l y q u i t e s i m i l a r t o the second m a x i l l a e . L i k e the l a t t e r , they have a complex system of i n t e r n a l m u s c l e s , o b v i o u s under the LM, which p e r m i t s i n d i v i d u a l segments some independent movement. The m a x i l l i p e d s a r e i d e n t i c a l i n s t r u c t u r e between the two s p e c i e s except t h a t T . p r a s i n u s has many more c l u s t e r s of s h o r t s p i n i f o r m s e t a e , - on the v e n t r a l p o s t e r i o r f a c e of the f i r s t two b a s a l segments and on the d o r s a l f a c e of the b a s a l segment ( F i g . 4 ) . On C.b.thomasi t h e r e a r e o n l y two sp a r s e p a t c h e s of thes e s e t a e - on the v e n t r a l f a c e of the second segment ( F i g . 1 8 ) . 39 FUNCTIONAL INTERRELATIONSHIP OF THE MOUTHPARTS As d e s c r i b e d by F r y e r (1957a), the f i r s t m a x i l l a e a re the p r i m a r y g r a s p i n g appendages. The. second m a x i l l a e and m a x i l l i p e d s p l a y t h e i r p a r t i n h e l p i n g t o st e a d y the p r e y , h o l d i n g or c r a d l i n g i t p r i o r t o p u s h i n g i t f o r w a r d . Some of the SEM p i c t u r e s caught T . p r a s i n u s i n the a c t of e a t i n g a food i t e m and they i l l u s t r a t e the f u n c t i o n a l f l e x i b i l i t y of the f e e d i n g appendages. As the a n i m a l s a re momen t a r i l y r e l a x e d p r i o r t o f i x a t i o n w i t h e t h a n o l , some of the p r e s s u r e a p p l i e d i n the l i v i n g s i t u a t i o n i s r e l a x e d but the p o s i t i o n s a r e g e n e r a l l y m a i n t a i n e d . F i g . 5 shows a s m a l l food item b e i n g eaten by a male T . p r a s i n u s . The f i r s t m a x i l l a e c l a s p the prey on each s i d e w h i l e the paragnathae p r o v i d e a ' v i s e ' v e n t r a l l y ( F i g . 5 ) . T h i s i m m o b i l i z a t i o n a l l o w s the mandibular cusps t o t e a r away a t a f i x e d t a r g e t from each s i d e . Both the muscle a c t i o n d e s c r i b e d by Hartog (1888) and the s h o r t s p i n y s e t u l e s appear t o be used i n p r o v i d i n g t h i s ' v i s e ' e f f e c t . The l a b r a l 'mustache' c o n t a c t s the prey as the prey i s f o r c e d down a g a i n s t the l a b r a l t e e t h and presumably p r o v i d e s s e n s o r y i n f o r m a t i o n about the p o s i t i o n of the prey p r i o r t o c o n t a c t w i t h the s h o r t s e t a e on the i n n e r f a c e of the labrum. These l a t t e r s e t a e a r e s t i f f and s p i n y i n appearance ( a l l p o i n t toward the o r a l c a v i t y ) and may f u n c t i o n as ' t e e t h ' or h o l d i n g d e v i c e s t o p r e v e n t the prey from s l i p p i n g back away from the o r a l c a v i t y . I t i s a l s o p o s s i b l e t h a t they f u n c t i o n as s e n s o r y i n s t r u m e n t s . The two l a r g e o u t e r l a b r a l t e e t h appear ( F i g . 5 ) t o a c t as s t a b i l i z e r s . The prey i s pushed between t h e s e two c o a r s e c h i t i n o u s p r o j e c t i o n s . The paragnathae may p e r m i t more 40 c e n t e r i n g of the prey w h i l e the m a n d i b l e s 'punch' the food i n t o the d e s i r a b l e s i z e f o r the l a b r a l t e e t h and o r a l c a v i t y . The cusps on the mandibles are c l e a r l y c a p a b l e of t e a r i n g p i e c e s from l a r g e food masses a l t h o u g h F r y e r (1975a) s u g g e s t s t h a t they are not used t o any marked e x t e n t f o r m a s t i c a t i n g f o o d . E v i d e n c e t o support t h i s view i s p r o v i d e d by the f a c t t h a t the i n g e s t i o n of the c o r a c i d i a of c e s t o d e s of the genus T r i a e n o p h o r u s by C y c l o p s b i c u s p i d a t u s does not damage the p a r a s i t e l a r v a e ( M i l l e r , 1 9 5 2 ) . I f the m a n d i b l e s s i m p l y 'punch' the food i n t o the a p p r o p r i a t e shape f o r i n g e s t i o n , s m a l l food items or the food mass i n the c e n t r e of a l a r g e food i t e m c o u l d be i n g e s t e d i n t a c t . I t i s i m p o s s i b l e t o observe whether the l a b r a l t e e t h are s i m p l y h o l d i n g t o o l s or a c t u a l l y p l a y some r o l e i n t e a r i n g l a r g e r prey i t e m s . S t r u c t u r a l l y they appear c a p a b l e of the l a t t e r f u n c t i o n as they seem t o be h i n g e d t o the r e s t of the labrum ( F i g l 6 ) . As a r e s u l t of n a r c o t i z a t i o n the second m a x i l l a e and m a x i l l i p e d s have become d i s p l a c e d , a l t h o u g h t h e i r c r a d l i n g a c t i o n can s t i l l be seen ( F i g . 5 ) . The p a l p a l s e t a e of the f i r s t m a x i l l a e a r e a l s o d i s p l a c e d from t h a t i n l i f e where they u s u a l l y s p l a y out over the p r e y . A l t h o u g h the second m a x i l l a e and m a x i l l i p e d s a r e c l e a r l y u n s u i t a b l e f i l t e r i n g a p p a r a t u s , they may be u s e f u l i n sweeping l a r g e clumps of food m a t e r i a l toward the mouth r e g i o n , where the f i r s t m a x i l l a e can g r a s p i t . T h i s type of a c t i o n i s s uggested by F i g . 1 9 . The s e r r a t e t i p s o bserved on the s e t a e of the second m a x i l l a e and m a x i l l i p e d s a r e p r o b a b l y i m p o r t a n t i n s e n s i n g or ' t a s t i n g ' the q u a l i t y of the food b e f o r e i t i s pushed f o r w a r d t o the m a n d i b l e s . Sheldon and L a v e r a c k (1970) 41 showed t h a t the s e r r a t e s e t a e i n the mouthparts of the European l o b s t e r Homarus gammorus (L.) are s e n s i t i v e t o c h e m i c a l s t i m u l i . T h i s f u n c t i o n might e x p l a i n why o n l y the t i p s a r e s e r r a t e . I t would seem u n l i k e l y t h a t c h e m o s e n s i t i v e s e t a e would a l s o be used f o r h o l d i n g prey because the p o s s i b l i t y of damage t o t h e i r s e n s o r y s u r f a c e s would be i n c r e a s e d . As a g e n e r a l i z a t i o n , T . p r a s i n u s has more s e t a e and s e t u l e s than C.b.thomasi. The plumose s e t a e on the mandibular p a l p of T . p r a s i n u s may p r o v i d e a f i n e r - g r a i n e d s e n s o r y i n p u t than the s i m p l e s e t a e of C.b.thomasi. The p a t c h e s of s h o r t s p i n i f o r m s e t a e on the two p a i r s of p o s t e r i o r f e e d i n g appendages of T . p r a s i n u s a re p r o b a b l y a l s o s ensory i n s t r u m e n t s . I f T . p r a s i n u s i s a f u n c t i o n a l omnivore p r e f e r r i n g s m a l l e r food i t e m s , t h i s s p e c i e s may need t o be more d i s c r i m i n a t i n g than C.b.thomasi .to s e l e c t between food and non-food p a r t i c l e s i n the environment. Gross movement of the food i t e m w i l l , not p r o v i d e the sensory cue f o r T . p r a s i n u s , t h a t i t a p p a r e n t l y does f o r C.b.thomasi ( K e r f o o t , 1 9 7 8 ) , i f c o p e p o d i d and a d u l t i n s t a r s feed on n o n - m o t i l e a l g a e . 42 DISCUSSION The s t r u c t u r e of T . p r a s i n u s and C.b.thomasi mouthparts a r e remark a b l y s i m i l a r a t the l e v e l of d e t a i l p o s s i b l e w i t h the LM, a l t h o u g h a d u l t female T . p r a s i n u s appendages a r e a p p r o x i m a t e l y 15% s m a l l e r than C.b.thomasi. In the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t Lakes, T . p r a s i n u s a d u l t s appear i n the p l a n k t o n i n e a r l y August, a t a time when the major p r o p o r t i o n of the C.b.thomasi p o p u l a t i o n i s i n copepodid i n s t a r s IV and V. Mouthpart s t r u c t u r e between the two s p e c i e s i s so s i m i l a r (even i n s i z e i n the s e s t a g e s ) t h a t one might p r e d i c t , from a study of the f e e d i n g appendages a l o n e , t h a t i n t e n s e c o m p e t i t i o n i s the reason f o r t h e i r i n a b i l i t y t o t h r i v e t o g e t h e r . However, under SEM some d i f f e r e n c e s i n mouthpart s t r u c t u r e a r e a p p a r e n t . In g e n e r a l , T . p r a s i n u s has more sensory equipment (more s e t a e and s e t u l e s ) than C.b. thomasi.- The most s t r i k i n g d i f f e r e n c e s a re the l o n g e r , more plumose s e t a e of the man d i b u l a r p a l p and the plumose s e t a e ( r a t h e r than s i m p l e s e t a e as i n C.b.thomasi) of the p a l p of the f i r s t m a x i l l a . The more f i l a m e n t o u s setae may r e f l e c t a g r e a t e r s e n s i t i v i t y t o the food environment than i s p o s s i b l e f o r C.b.thomasi , p e r m i t t i n g f i n e r d i s c r i m i n a t i o n between e d i b l e and i n e d i b l e p a r t i c l e s . A l t e r n a t i v e l y , the f i n e r c o n s t r u c t i o n of the a d u l t T . p r a s i n u s f e e d i n g a p p a r a t u s may have no f u n c t i o n a l i m p o r t a n c e . Thus the p r a c t i c a l s t r u c t u r e may be i d e n t i c a l , a l t h o u g h s l i g h t l y s m a l l e r i n s i z e . As C.b.thomasi i s a v i g o r o u s c a r n i v o r e (McQueen, 1969; Anderson,1970a; B r a n d l and Fernando, 1975, 1978), the remarkable s i m i l a r i t y between C.b.thomasi and T . p r a s i n u s i n a d u l t mouthpart s t r u c t u r e may i n d i c a t e t h a t T . p r a s i n u s i s a l s o 43 an a c t i v e c a r n i v o r e i n the l a t e r i n s t a r s . I f t h i s i s t r u e , T . p r a s i n u s may e x p e r i e n c e i n t e n s e c o m p e t i t i o n f o r s c a r c e food r e s o u r c e s ' such t h a t a d u l t females may have d i f f i c u l t y s e q u e s t e r i n g s u f f i c i e n t food energy f o r egg p r o d u c t i o n . The r e l a t i v e r a r i t y of T . p r a s i n u s i n P l a c i d Lake may t h e r e f o r e be the r e s u l t of a c o m p e t i t i v e i n t e r a c t i o n w i t h C.b.thomasi. The l a t t e r s p e c i e s may have an advantage i n t h a t i t appears i n the s p r i n g and t h e r e f o r e r e p roduces b e f o r e t h e r e i s c o m p e t i t i o n from T . p r a s i n u s f o r f o o d . In any c a s e , i t i s o b v i o u s from t h i s study t h a t a d u l t mouthpart s t r u c t u r e s a r e too s i m i l a r between C.b.thomasi and T . p r a s i n u s t o p r o v i d e c l e a r e v i d e n c e of d i e t a r y d i f f e r e n c e s . EXPERIMENTAL STUDIES ON THE FACTORS LIMITING TROPOCYCLOPS PRASINUS IN AN OLIGOTROPHIC LAKE 45 INTRODUCTION The r e s o l u t i o n of the i n t e r a c t i o n s t h a t d e t e r m i n e an organism's d i s t r i b u t i o n and abundance i s a b a s i c problem i n e c o l o g y . D i s s e c t i n g the c a u s a t i v e c o n n e c t i o n s w h i l e m a i n t a i n i n g some semblance of r e a l i t y can be v e r y d i f f i c u l t . The l i m n e t i c c r u s t a c e a n community i n temperate a q u a t i c ecosystems o f f e r s s e v e r a l advantages i n examining d i s t r i b u t i o n / a b u n d a n c e q u e s t i o n s because many s p e c i e s a re u b i q u i t o u s and yet each community i s r e l a t i v e l y s i m p l e i n s t r u c t u r e . P a t a l a s (1971) noted t h a t i t i s c h a r a c t e r i s t i c of thes e communities " t h a t o n l y a few s p e c i e s a r e g e n e r a l l y abundant, the r e s t r e m a i n i n g s c a r c e " . E x p l a n a t i o n s f o r t h i s phenomenon a r e numerous, but can be summarized by two b a s i c a l l y d i f f e r e n t c o n c e p t u a l models which are e x e m p l i f i e d by the f o l l o w i n g r e s e a r c h : those s t u d i e s t h a t emphasize the r o l e of ' h a b i t a t ' i n l i m i t i n g an organism's success and those t h a t f o c u s on the importance of p r e d a t i o n . The importance of some ' h a b i t a t ' l i m i t a t i o n i n s t r u c t u r i n g the o b s e r v e d s p e c i e s a s s o c i a t i o n i s emphasized by many i n v e s t i g a t o r s , a l t h o u g h the r e l e v a n t f e a t u r e may be p h y s i c a l ( P a t a l a s , 1971), c h e m i c a l ( S p r u l e s , 1975), s p a t i a l (Sandercock, 1967; Watson and Smallman, 1971), or some l i m i t a t i o n i n the q u a n i t i t y or q u a l i t y of a v a i l a b l e food ( P a t a l a s , 1972; S p r u l e s , 1972). However, o t h e r s argue t h a t p r e d a t o r - p r e y i n t e r a c t i o n s a r e of major -importance i n s t r u c t u r i n g p l a n k t o n communities (Brooks and Dodson, 1965; H a l l et a l . , 1970; Dodson, 1974; H a l l et a l . , 1976; Lane, 1978; Z a r e t , 1978). S e v e r a l s t u d i e s have suggested t h a t even i n v e r t e b r a t e p r e d a t o r s have the p o t e n t i a l 46 t o a c t as s i g n i f i c a n t m o r t a l i t y agents l i m i t i n g the s u c c e s s of o t h e r l i m n e t i c m a c r o c r u s t a c e a n s (McQueen, 1969; Dodson, 1972; Fedorenko, 1973; Confer and C o o l e y , 1977; Boers and C a r t e r , 1978; Lane, 1979). Among the i n v e r t e b r a t e p r e d a t o r s , c y c l o p o i d copepods appear t o have the a b i l i t y t o i n f l u e n c e the abundance of o t h e r z o o p l a n k t e r s and they may even dampen t h e i r own p o p u l a t i o n numbers through i n t r a s p e c i f i c p r e d a t i o n (McQueen, 1969; Lane, 1979). However, w i t h i n any one l a k e t h e r e a re r a r e l y more than one or two n u m e r i c a l l y dominant c y c l o p o i d copepod s p e c i e s a l t h o u g h t h e s e s p e c i e s may be wide s p r e a d a c r o s s many d i f f e r e n t communities (Pennak, 1957; P a t a l a s , 1971). U s i n g c l a s s i c a l d e s c r i p t i v e t e c h n i q u e s , i t has proven q u i t e d i f f i c u l t t o c o n c l u s i v e l y demonstrate whether some ' h a b i t a t ' l i m i t a t i o n i s a l l - i m p o r t a n t t o thes e copepods or whether p r e d a t o r s p l a y the key r o l e . T r o p o c y c l o p s p r a s i n u s ( F i s c h e r ) 1 8 6 0 and C y c l o p s  b i c u s p i d a t u s thomasi (Forbes)1882 (Yeatman, 1959) a r e two v e r y common s p e c i e s of c y c l o p o i d copepods. T . p r a s i n u s i s c o n s i d e r e d a warm water form (Guerney, 1931; R y l o v , 1963; Yeatman, 1959), a l t h o u g h T.p.mexicanus i s common i n deep water b o d i e s ( P a t a l a s , 1971). C.b.thomasi i s a s p e c i e s d e s c r i b e d by P a t a l a s (1971) as common i n l a r g e , deep l a k e s i n the E x p e r i m e n t a l Lakes Area i n n o r t h w e s t e r n O n t a r i o and r a r e l y found i n s m a l l , s h a l l o w l a k e s . However, i t i s common i n s h a l l o w l a k e s elsewhere ( P a t a l a s , 1964). C a r l (1940) d e s c r i b e s C.b.thomasi as o c c u r r i n g i n ponds or l a k e s r e g a r d l e s s of s i z e or a l k a l i n i t y and T . p r a s i n u s as " w i d e l y d i s t r i b u t e d " . Anderson (1974) d e s c r i b e s C.b.thomasi as common i n c o l d low 47 s a l i n i t y l a k e s a l t h o u g h W h i t t a k e r and F a i r b a n k s (1958) r e p o r t C.b.thomasi i n l a k e s of h i g h s a l i n i t i e s and W i l l i a m s (1977) c o l l e c t e d C.b.thomasi from Chesapeake Bay, a marine environment. There does not seem t o be any easy i d e n t i f i c a t i o n of c y c l o p o i d s p e c i e s w i t h l a k e ' t y p e ' , and i t seems p r o b a b l e t h a t s p e c i e s i n t e r a c t i o n s a r e i m p o r t a n t i n c r e a t i n g some of the obser v e d d i s t r i b u t i o n and abundance p a t t e r n s . T h i s study was c a r r i e d out t o e x p l o r e two q u e s t i o n s r e l a t i n g t o the d i s t r i b u t i o n and abundance of c y c l o p o i d copepods: are c y c l o p o i d copepod i n t e r a c t i o n s i m p o r t a n t i n d e t e r m i n i n g the dominant c y c l o p o i d copepod s p e c i e s w i t h i n a g i v e n community? and what p a r t does ' h a b i t a t ' l i m i t a t i o n v e r s u s p r e d a t o r - p r e y i n t e r a c t i o n s p l a y i n r e s t r i c t i n g the number of c o e x i s t i n g c y c l o p o i d copepod s p e c i e s ? T h i s study e x p e r i m e n t a l l y examines the f a c t o r s which determine the r e l a t i v e abundance of T . p r a s i n u s and C.b.thomasi i n a s m a l l o l i g o t r o p h i c l a k e i n c o a s t a l B r i t i s h Columbia. 48 Study Area The montane l a k e s of the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t a r e p a r t i c u l a r l y s u i t a b l e systems i n which t o study c y c l o p o i d copepod i n t e r a c t i o n s because year t o year d i f f e r e n c e s a r e remarkably s m a l l i n s p i t e of a dynamic s e a s o n a l p a t t e r n ( W a l t e r s , u n p u b . d a t a ) . In a d d i t i o n , f o u r of these l a k e s have been sampled r e g u l a r l y over the l a s t s i x y e a r s so the s e a s o n a l p a t t e r n s a r e known ( N o r t h c o t e and C l a r o t t o , 1975; N e i l l , 1978; N o r t h c o t e e t a l . , 1978). A l t h o u g h c y c l o p o i d copepods a r e p r e s e n t i n a l l f o u r l a k e s , o n l y two s p e c i e s a r e abundant, T . p r a s i n u s and C.b.thomasi. Table 1 summarizes some of the 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 c h a r a c t e r i s t i c s of two of t hese l a k e s , one where T . p r a s i n u s i s r e l a t i v e l y abundant (Gwendoline Lake) and the o t h e r ( P l a c i d Lake) where T . p r a s i n u s i s q u i t e r a r e . D e s p i t e some p h y s i c a l - c h e m i c a l d i f f e r e n c e s , b o th l a k e s c o n t a i n the same assemblage of c r u s t a c e a n z o o p l a n k t o n ( w i t h the s i n g l e e x c e p t i o n of a c a l a n o i d copepod). The main d i f f e r e n c e s i n the c r u s t a c e a n communities a r e i n the r e l a t i v e abundances of s p e c i e s . There are a l s o d i f f e r e n t s p e c i e s of midge f l y l a r v a e , Chaoborus, between the two l a k e s and P l a c i d Lake c o n t a i n s t r o u t (Salmo c l a r k i c l a r k i ) whereas Gwendoline Lake does n o t . T . p r a s i n u s reaches h i g h e r d e n s i t i e s i n the deeper and c o o l e r Gwendoline Lake ( t o a summer peak of 0.5 a n i m a l s per l i t r e compared t o 0.1 per l i t r e i n P l a c i d Lake) a l t h o u g h the food base i n P l a c i d does not seem t o d i f f e r s i g n i f i c a n t l y from t h a t i n Gwendoline Lake. A l g a l biomass ( v a r y i n g between 0.04 and 0.08 mg per l i t r e ' a s h - f r e e d r y weight) and p a r t i c u l a t e T a b l e 1 Some p h y s i c a l , c h e m i c a l a n d b i o l o g i c a l c h a La ke. * r a c t e r i s t i c s o f P l a c i d a n d G w e n d o l i n e CH AF.ACTERISTICS P l a c i d ,-e.ndoline e l e v a t i o n , n. D r a i n a g a e a r e a , h a . S n r f a c e a r e a , h a . Ma xiffiu.T d e p t h , n. C o l o u r , P t u n i t s T r a n s p a r e n c y ( S e c c h i d e p t h , in.) pH T o t a l C a r b o n (mg 1-1) T o t a l P h o s p h o r u s ,fr>g 1 - 1 ) T o t a l O r q a n i c N i t r o g e n (Eg 1"1) T o t a l D i s s o l v e d S o l i d s (mg 1-1) 510 UU 1.6 7 2 0 - 2 5 ';-& 6 . 6 5.0 0. 006 0.17 17-23 522 81 13. C 27 15 5-7 6. 6 3.C 0.003 0.05 18 C R U S T A C E A 200PLANKTON S P S C I 2 S + O i a r i o m v i s i s e j ^ a i nia^toraus oregonensis D i a c ^ o ^ u s l e n t o n u s H i i l f n o s o m a h n a c h v t i r n m SISIODS SiciLSpi da t us t h o m a s i lEP£2CJ£&l2£5 p r . i s i n us A A A A A A A A A A P A B R A A A R P A * a d a p t e d f r o n \ e i l l ( 1 9 7 8 ) t Key t o s y n b c l s : A = a b u n d a n t R = r a r e a b s e n t 50 o r g a n i c m a t t e r c o n c e n t r a t i o n s ( p a r t i c l e s p a s s i n g t h r o u g h a 73 um mesh net) a r e low i n both l a k e s (from 0.3 t o 1.4 mg per a s h -f r e e d r y w e i g h t ) . B a c t e r i a l p o p u l a t i o n s a l s o appear t o be low (<10 5 c e l l s per m l ) . P l a c i d Lake seems more s u i t a b l e h a b i t a t than Gwendoline Lake f o r T . p r a s i n u s , b e i n g s h a l l o w and g e n e r a l l y the warmer of the two l a k e s - t h a t i s , more p o n d l i k e . I chose P l a c i d Lake f o r study because C.b.thomasi i s much more abundant than T . p r a s i n u s i n t h i s l a k e i n s p i t e of the f a c t t h a t P l a c i d Lake seems p h y i c a l l y more s u i t e d t o T . p r a s i n u s . 51 MATERIALS AND METHODS L a b o r a t o r y E x p e r i m e n t s 1. D i e t of T . p r a s i n u s Food l i m i t a t i o n i s o f t e n c i t e d as one of the f a c t o r s l i m i t i n g the s u c c e s s of z o o p l a n k t o n p o p u l a t i o n s ( G l i w i c z , 1969; H a l l et a l . , 1970; L e B r a s s e u r and Kennedy, 1972). S e v e r a l s t u d i e s have shown t h a t C.b.thomasi i s predaceous i n the l a t e r i n s t a r s (McQueen, 1969; Anderson, 1970). However, the t r o p h i c s t a t u s of T . p r a s i n u s i s unknown. F r y e r (1957b) su g g e s t s t h a t s m a l l e r c y c l o p o i d copepod s p e c i e s tend t o be h e r b i v o r o u s , even as a d u l t s . I f T . p r a s i n u s i s h e r b i v o r o u s i n the c o pepodid and a d u l t i n s t a r s , t h i s s p e c i e s may be competing w i t h c a l a n o i d copepod and c l a d o c e r a n g r a z e r s f o r s c a r c e f o o d r e s o u r c e s . A l t e r n a t i v e l y , i f the l a t e r i n s t a r s are predaceous on o t h e r m a c r o z o o p l a n k t e r s , T . p r a s i n u s may s u f f e r from r e s o u r c e and/or i n t e r f e r e n c e c o m p e t i t i o n w i t h l a t e i n s t a r C.b.thomasi. In o r d e r t o u n d e r s t a n d the r o l e of T . p r a s i n u s i n the P l a c i d Lake food environment, the a b i l i t y of t h i s s p e c i e s t o i n g e s t and a s s i m i l a t e a l g a e was f i r s t t e s t e d . To a c c o m p l i s h t h i s , f o u r s p e c i e s of a l g a e commonly found i n the R esearch F o r e s t l a k e s (Dickman, 1968; S t e i n , 1975) were f e d t o n a u p l i a r , c o pepodid and a d u l t T . p r a s i n u s . Pure c u l t u r e s were o b t a i n e d from the l a b o r a t o r y of D r . J . S t e i n , 52 Dept. of Botany, U.B.C. and the I n d i a n a U n i v e r s i t y C u l t u r e C o l l e c t i o n of A l g a e . I c u l t u r e d the a l g a e on m o d i f i e d Chu's medium (Fogg,1966) and i l l u m i n a t e d the c u l t u r e s by f l u o r e s c e n t l i g h t a t 20±2°C. Algae f o r use as r a d i o a c t i v e food were s a t u r a t i o n l a b e l e d by ad d i n g 20 u Ci of NaH 1 4C0 i n 1 ml. of s t e r i l e d i s t i l l e d water t o 50 ml. of l o g growth phase c u l t u r e 2 days b e f o r e use. A c t i v e l y growing a l g a l c e l l s at a known c o n c e n t r a t i o n (determined by haemocytometer c o u n t s ) were suspended i n l a k e water which had been f i l t e r e d t h r o u g h a g l a s s f i b r e f i l t e r . E i g h t 300 ml b o t t l e s l i n e d w i t h a 73 um mesh bag f i t t e d w i t h a d r a w - s t r i n g were f i l l e d w i t h the a l g a l - w a t e r m i x t u r e and 30-70 a n i m a l s were p l a c e d i n each b o t t l e . The b o t t l e s were mounted on a s l o w l y r o t a t i n g wheel a t 0.25 rpm. F r e s h l y c o l l e c t e d z o o p l a n k t o n were p r e c o n d i t i o n e d f o r 4 hours p r i o r t o the t e s t by i n c u b a t i n g the a n i m a l s w i t h the food type and a t the temperature t o be used i n the e x p e r i m e n t . A f t e r the a c c l i m a t i o n p e r i o d the n o n - r a d i o a c t i v e a l g a l s u s p e n s i o n was si p h o n e d away and the b o t t l e s were r e f i l l e d w i t h a r a d i o a c t i v e d i e t of the same c o n c e n t r a t i o n and t e m p e r a t u r e . Loss of a n i m a l s , p a r t i c u l a r l y when n a u p l i i were used, was p r e v e n t e d by s i p h o n i n g o u t s i d e the mesh bag. The a n i m a l s were i n c u b a t e d f o r 8-10 h o u r s , and then washed f r e e of the l a b e l l e d food w i t h f i l t e r e d l a k e water and exposed f o r 1 hour t o n o n - l a b e l l e d food i n the same manner. A f t e r t h i s p e r i o d the a n i m a l s were removed from the b o t t l e s i n the mesh bag, s o r t e d by i n s t a r group and s p e c i e s , a n a e s t h e t i z e d i n c a r b o n a t e d w a t e r , k i l l e d i n hot water and t r a n s f e r r e d t o g l a s s s c i n t i l l a t i o n v i a l s , each c o n t a i n i n g 0.2 ml of P r o t o s o l t i s s u e s o l u b i l i z e r . Each v i a l 53 was t i g h t l y capped and p l a c e d i n an oven a t 40°C o v e r n i g h t . Upon c o o l i n g , 10 ml of t o l u e n e s c i n t i l l a t i o n s o l u t i o n (4 g PPO 1, 0.2 g POPOP 2 i n 1 l i t r e t o l u e n e ) were added. I checked p o s s i b l e background c o u n t s caused by inadequate washing, e t c . by c a r r y i n g a group of f r e s h l y k i l l e d a n i m a l s t h r o u g h the e x p e r i m e n t a l s t e p s . T h i s background was always n e g l i g i b l e . A l l e x p e r i m e n t s were conducted a t an a l g a l c e l l c o n c e n t r a t i o n of about 20 x 10 3 c e l l s per ml. Whenever d i f f e r e n t T . p r a s i n u s i n s t a r s had been f e d t o g e t h e r , e q u a l numbers of each stage were i n c l u d e d . Food r a d i o a c t i v i t y was det e r m i n e d by c o u n t i n g s e v e r a l 2 ml a l i q u o t s of the food s u s p e n s i o n f i l t e r e d on t o 0.45 HA M i l l i p o r e membranes and d i s s o l v i n g t h e s e i n 10 ml Bray's l i q u i d s c i n t i l l a t o r ( B r a y , 1960). Samples of i d e n t i c a l n o n - r a d i o a c t i v e d i e t were c o n c e n t r a t e d f o r w e i g h i n g by f i l t e r i n g 100 ml on t o a preashed t a r e d g l a s s f i b r e f i l t e r . I d r i e d t h e s e samples o v e r n i g h t a t 60±2°C, weighed them on a Cahn e l e c t r o b a l a n c e , ashed the samples a t 600°C f o r 3 hours and reweighed t o determine a s h -f r e e d r y w e i g h t . D e t e c t i o n and measurement of r a d i o a c t i v i t y was done by l i q u i d s c i n t i l l a t i o n s p e c t r o m e t r y u s i n g . a S e a r l e I s o c a p 300 l i q u i d s c i n t i l l a t i o n c o u n t e r . Counts were c o n v e r t e d t o d i s i n t e g r a t i o n s per minute (DPM) u s i n g an e x t e r n a l s t a n d a r d t o determine the e f f i c i e n c y of c o u n t i n g ( S o r o k i n , ^PO = 2 , 5 - d i p h e n y l o x a z o l e 2P0P0P = 1 , 4 - b i s - [ 2 - ( 5 - p h e y l o x a z o l y l ) ] - b e n z e n e 54 1966; Wolfe and S c h e l s k e , 1967). I d e t e r m i n e d d r y w e i g h t s f o r a l l groups of a n i m a l s used i n the e x p e r i m e n t s by s o r t i n g f r e s h l y c o l l e c t e d z o o p l a n k t o n by i n s t a r and s p e c i e s and t r a n s f e r r i n g them t o preashed and t a r e d g l a s s f i b r e f i l t e r s . A f t e r d r y i n g a t 60°C f o r 2 days, a t l e a s t t h r e e r e p l i c a t e s of 40 or more a n i m a l s were weighed on a Cahn e l e c t r o b a l a n c e . I o b s e r v e d T . p r a s i n u s e a t i n g p r o t o z o a n s i n c u l t u r e v e s s e l s , s u g g e s t i n g t h a t T . p r a s i n u s i s an omnivore. To t e s t the a c c e p t a b i l i t y of a p l a n t v e r s u s an a n i m a l d i e t I f e d c o p e p o d i d I on a u n i - a l g a l (not b a c t e r i a - f r e e ) and a u n i -p r o t o z o a n d i e t . Copepodids were r e a r e d i n d i v i d u a l l y at 20±1°C i n 20 ml of l a k e water c l e a r e d t h r o u g h a 0.45 um HA M i l l i p o r e f i l t e r . I added C h l o r e l l a v u l g a r i s a t 20 X 10' c e l l s / m l t o 40 v i a l s and Paramecium sp. a t approx. 5 X 1 0 2 a n i m a l s / m l t o an a d d i t i o n a l 40 v i a l s . A c c o r d i n g t o B r a n d l (1973), both food d e n s i t i e s were i n e x c e s s of d a i l y needs ( v i s u a l checks ensured t h a t abundant food was always p r e s e n t b e f o r e the next food a d d i t i o n ) . The food d e n s i t i e s t h e r e f o r e r e p r e s e n t ' o p t i m a l ' c o n d i t i o n s on these d i e t s . The water was changed, food added and i n s t a r a n a l y z e d under a d i s s e c t i n g scope a t 25X m a g n i f i c a t i o n , every o t h e r day. Male and female a d u l t s from t h i s experiment were put t o g e t h e r i n p a i r s , the number of females d e v e l o p i n g eggs noted and the egg-sacs d i s s e c t e d and eggs c o u n t e d . 55 2. F e e d i n g r a t e s : C.b.thomasi on T . p r a s i n u s • As C.b.thomasi ( I V , V and a d u l t ) a r e the most common st a g e s i n the l a k e when T . p r a s i n u s appears i n the p l a n k t o n , these i n s t a r s were used as p r e d a t o r s i n e x p e r i m e n t s t o dete r m i n e p r e d a t i o n r a t e s of C.b.thomasi on T . p r a s i n u s . The d e s i g n i n c l u d e d s i n g l e and m u l t i s p e c i e s prey e x p e r i m e n t s t o a s s e s s the importance of a l t e r n a t i v e prey i n a l t e r i n g p r e d a t o r impact. To check the e f f e c t of c o n t a i n e r s i z e on p r e d a t i o n r a t e s , s i x copepodid V C.b.thomasi were p l a c e d i n 1, 1.5 and 2 l i t r e b e a k e r s w i t h T . p r a s i n u s n a u p l i i a t a d e n s i t y of 50 a n i m a l s per l i t r e and p l a c e d i n an e n v i r o n m e n t a l chamber f o r 24 hours a t 20±1°C. There was no d i f f e r e n c e i n p r e d a t i o n r a t e between the 3 beaker s i z e s , and 1 l i t r e v e s s e l s were used f o r a l l subsequent p r e d a t i o n t r i a l s . S i x p r e d a t o r s and a known c o n c e n t r a t i o n of prey were added t o each beaker. I o b t a i n e d T . p r a s i n u s n a u p l i i by h a t c h i n g the eggs from w i l d o v i g e r o u s females i n the l a b o r a t o r y . A l l o t h e r prey were o b t a i n e d d i r e c t l y from the l a k e . The e x p e r i m e n t s were c a r r i e d out i n semidarkness (beakers c o v e r e d w i t h f o i l ) , and p l a c e d i n a c o n s t a n t temperature chamber a t 20±1°C f o r 24 h o u r s . At the end of t h i s p e r i o d a l l a n i m a l s were s i e v e d t h r o u g h a 54 um p l a n k t o n mesh and r i n s e d i n t o a p e t r i d i s h f o r c o u n t i n g a t 25X m a g n i f i c a t i o n under a d i s s e c t i n g m i c r o s c o p e . F i v e r e p l i c a t e c o n t r o l s c o n t a i n i n g 20 T . p r a s i n u s but no C.b.thomasi were used t o a s s e s s n a t u r a l m o r t a l i t y ( t h e r e was none) and c o u n t i n g v a r i a t i o n ( l e s s than- 2 % ) . A l t e r n a t i v e prey s p e c i e s were p r o v i d e d t o e v a l u a t e C.b.thomasi impact on T . p r a s i n u s n a u p l i i when p r e s e n t e d w i t h a c h o i c e . These s p e c i e s i n c l u d e d ( i n 56 d e s c e n d i n g o r d e r of abundance) immature Diaphanasoma brachyurum (<1.0mm), immature Daphnia r o s e a (<1.0mm), c a l a n o i d n a u p l i i , and C.b.thomasi copepodids I-111. F i f t e e n a l t e r n a t i v e prey per l i t r e were p r o v i d e d a t a T . p r a s i n u s p r e y d e n s i t y of 1 n a u p l i u s / l i t r e and 5 n a u p l i i / l i t r e , 30 a l t e r n a t i v e s per l i t r e a t 10 T . p r a s i n u s n a u p l i i / l i t r e and 45 a l t e r n a t i v e s a t 30 T . p r a s i n u s n a u p l i i / l i t r e . D e n s i t i e s of a l t e r n a t i v e prey' i n the 5 T . p r a s i n u s n a u p l i i / l i t r e e x p e r i m e n t a l beakers were the mean d e n s i t i e s e n c ountered a t 1.5 m depth i n P l a c i d Lake i n August, 1976. Prey eaten were not r e p l a c e d d u r i n g the 24 hour p e r i o d . F i e l d E x p e r i m e n t s The importance of p r e d a t i o n v e r s u s food l i m i t a t i o n on the su c c e s s of T . p r a s i n u s was e x p e r i m e n t l y examined under f i e l d c o n d i t i o n s . The e f f e c t of v e r t e b r a t e p r e d a t o r s was not s t u d i e d because Shepherd (1970) found t h a t c y c l o p o i d s were r a r e l y p r e s e n t i n the gut c o n t e n t s of the o n l y f i s h i n the l a k e , c u t t h r o a t t r o u t . I t a l s o seemed improbable t h a t f i s h would s e l e c t i v e l y e l i m i n a t e the s m a l l e r of the two c y c l o p o i d s p e c i e s . Salamander p r e d a t i o n ( T a r i c h a g r a n u l o s a , Ambystoma  g r a c i l e ) i s c o n s i d e r e d u n l i k e l y as such a n i m a l s a r e o n l y p l a n k t i v o r o u s f o r a s h o r t p e r i o d i n the midsummer ( G i g u e r e , 1973) and do not c o i n c i d e w i t h the c y c l o p o i d c y c l e s of s e a s o n a l abundance. Among i n v e r t e b r a t e p r e d a t o r s , o n l y Chaoborus l a r v a e and C.b.thomasi were s u f f i c i e n t l y abundant i n the p l a n k t o n t o have an i m p o r t a n t impact. Fedorenko (1973), e x t r a p o l a t i n g from s i n g l e s p e c i e s l a b o r a t o r y e x p e r i m e n t s , 57 argued t h a t Chaoborus t r i v i t t a t u s l a r v a e c o u l d account f o r as much as 30% of the c a l a n o i d copepod m o r t a l i t y i n a nearby l a k e i n the Research F o r e s t . However, N e i l l (1978) found no d e t e c t a b l e net impact on any members of the z o o p l a n k t o n community when Chaoborus f l a v i c a n s d e n s i t i e s were i n c r e a s e d by 3X t h a t of the l a k e i n l a r g e i_n s i t u e n c l o s u r e s sampled from J u l y t h r o u g h September, 1975. However, a d u l t numbers of C.b.thomasi d e c l i n e d f a s t e r i n the Chaoborus bags than i n those t r e a t m e n t s w i t h o u t Chaoborus ( N e i l l , pers.comm.). Experiments r e p e a t e d i n the s p r i n g of 1976 a g a i n showed no net impact on the prey z o o p l a n k t o n abundance a t P l a c i d Lake d e n s i t i e s of the p r e d a t o r ( N e i l l , pers.comm.). McQueen (1969), i n a study on nearby M a r i o n Lake, c o n c l u d e d t h a t 25-35% of the C.b.thomasi p o p u l a t i o n p e r i s h e d through c a n n i b a l i s m . As C.b.thomasi i s abundant i n P l a c i d Lake, p o s s i b l e i n t e r a c t i o n s between the two c y c l o p o i d copepod s p e c i e s were e v a l u a t e d t h r o u g h m a n i p u l a t i o n s of the z o o p l a n k t o n community. The m a n i p u l a t i o n s were c a r r i e d out i n l a r g e ir\ s i t u e x p e r i m e n t a l e n c l o s u r e s from l a t e June u n t i l November, 1976. The e n c l o s u r e s were s i m i l a r i n d e s i g n t o tho s e used by N e i l l (1978). They were made of c l e a r 4 m i l p o l y e t h y l e n e p l a s t i c bags, 1.5 m wide and as deep as the deepest p a r t of the l a k e (6 m) but not i n c o n t a c t w i t h the sediments because they were s e a l e d a t the bottom. These tubes were suspended from a wooden and p o l y s t y r e n e frame f l o a t i n g on the s u r f a c e . The e n c l o s u r e s h e l d a p p r o x i m a t e l y 10* l i t r e s of water and were f i r s t f i l l e d by pumping l a k e water through a 54 jum mesh p l a n k t o n n e t . T h i s removed a l l the c r u s t a c e a n s y e t p e r m i t t e d 58 passage of g r a z a b l e s e s t o n . N a t u r a l l a k e d e n s i t i e s of P l a c i d Lake c r u s t a c e a n s were then added t o the e n c l o s u r e s from p o o l e d z o o p l a n k t o n samples o b t a i n e d from d e p t h - s t r a t i f i e d h o r i z o n t a l tows w i t h a Clark-Bumpus sampler. There were no f i s h or Chaoborus l a r v a e i n any e n c l o s u r e s . Three e x p e r i m e n t a l c o n d i t i o n s were produced: (1) T . p r a s i n u s p l u s a l l P l a c i d Lake z o o p l a n k t o n , i n c l u d i n g C.b.thomasi (2) T . p r a s i n u s p l u s P l a c i d Lake z o o p l a n k t o n , e x c l u d i n g C.b.thomasi (3) T . p r a s i n u s a l o n e . A l l z o o p l a n k t o n were c o l l e c t e d i n P l a c i d Lake and added a t P l a c i d Lake d e n s i t i e s w i t h the e x c e p t i o n of T . p r a s i n u s . T h i s s p e c i e s was added a t 1.5X the low d e n s i t y found i n P l a c i d Lake t o p e r m i t adequate s a m p l i n g . P l a n k t o n s a m p l i n g Z o o p l a n k t o n s a m p l i n g was done ev e r y 5-7 days i n these e n c l o s u r e s and i n the l a k e w i t h an e l e c t r i c pump, c a p a b l e of pumping 25 l i t r e s per minute. Each sample was c o l l e c t e d by f i l t e r i n g t h r o u g h a 54 um p l a n k t o n n e t . P o s s i b l e s e l e c t i v i t y by t h i s s a m p l i n g gear was checked by add i n g n o n o v i g e r o u s r e p r e s e n t a t i v e s of the community t o be sampled, Diaptomus sp., C.b.thomasi, and Daphnia r o s e a , t o f o u r 180 l i t r e a q u a r i a i n known c o n c e n t r a t i o n s and r e s a m p l i n g a f t e r 5 h o u r s . A C h i -square a n a l y s i s t o t e s t f o r r e p l i c a b i l i t y of the pump samples showed t h a t a l l t h r e e s p e c i e s were o b t a i n e d randomly (Table 2 ) . T h i s s a m p l i n g method was thus c o n s i d e r e d s u i t a b l e f o r Cn T a b l e Comparison of 4 r e p l i c a t e samples of r e p r e s e n t a t i v e z o o p l a n k t o n i n 130 1, a q u a r i a u s i n g t h e e l e c t r i c pump (mean w i t h S.E»). mean mean added (S,E. ) r e c o v e r e d (S.E.) X2" p Dia^tomos so. 1. 3 (0.0 1) 1, 2 (0. 02) 5. 62 >0.05 D. ro s e a 1. 2 (0.005) 1. 1 (0.02) 4.0 >0.05 C.b.thomasi 0.6 (0.002) 0.5 (0.01) 3. 3 >0,05 60 e s t i m a t i n g the abundance of the P l a c i d Lake c r u s t a c e a n community. Copepods a r e noted f o r an e f f e c t i v e escape response (Drenner et a l . , 1978). T h e r e f o r e I f i e l d - t e s t e d the r e p l i c a b i l i t y of the pump samples f o r the copepods i n P l a c i d Lake by t a k i n g f o u r tows i n c l o s e s u c c e s s i o n a t the same depth on May 7, 1976. I cou n t e d a l l D.oreqonensi s, D.kenai and C.b.thomasi c o l l e c t e d . A C h i - s q u a r e a n a l y s i s showed t h a t t h e s e copepods were o b t a i n e d w i t h e q u a l e f f i c i e n c y (Table 3 ) . T h i s r e s u l t c o n f i r m e d the a q u a r i a t e s t s w i t h r e p r e s e n t a t i v e c r u s t a c e a n p o p u l a t i o n s and showed t h a t the copepod s p e c i e s found i n P l a c i d Lake c o u l d be sampled w i t h r e p l i c a b l e r e s u l t s w i t h the e l e c t r i c pump. In the e n c l o s u r e s , 3 samples of 50 l i t r e s each were taken r e p r e s e n t i n g 3 de p t h s : s u r f a c e , mid-depth and bottom. In the l a k e , 100 l i t r e samples were taken a t one metre i n t e r v a l s from s u r f a c e t o bottom (5.5m), a l o n g a t r a n s e c t a t the deepest p a r t of the l a k e ( F i g . 2 0 , T r a n s e c t A ) . Three a d d i t i o n a l t r a n s e c t s ( F i g . 2 0 , B, C, D) were sampled, a l l of which were i n d i f f e r e n t p a r t s of the l i t t o r a l zone. The same s a m p l i n g gear used i n the e n c l o s u r e s was used i n the l a k e . T a b l e 3 Comparison of 4 t r a n s e c t s taken a t 2.5 m. a l o n g t r a n s e c t A i n P l a c i d Lake., v a l u e s a re g i v e n i n numbers per 100 l i t r e s . Tow No. D.oreqonensis O.kenai C.b.thomasi 1 273 73 238 2 276 90 242 3 2 67 70 246 4 248 58 240 •g 2. 45 7. 16 0.35 p >0.05 0.05 • >0.05 62 F i g u r e 20. Contour map of P l a c i d Lake i n the U.B.C. Research F o r e s t . Sampling t r a n s e c t s are i n d i c a t e d by A, B, C and D. B, C and D r e p r e s e n t d i f f e r e n t p a r t s of the l i t t o r a l zone. Squares i n d i c a t e the l o c a t i o n of the e x p e r i m e n t a l e n c l o s u r e s . 64 P l a n k t o n c o u n t i n g Samples from the e n c l o s u r e s were examined i_n t o t o under 25X m a g n i f i c a t i o n f o r s p e c i e s ' abundance, r e p r o d u c t i v e c o n d i t i o n and i n s t a r d i s t r i b u t i o n (Yeatman, 1959; Torke, 1974). Lake samples were always co u n t e d i n t h e i r e n t i r e t y f o r T . p r a s i n u s , or whenever numbers were below 100 a n i m a l s , f o r a l l o t h e r s p e c i e s ; o t h e r w i s e , samples were d i l u t e d t o 900 ml and two or t h r e e r e p l i c a t e s of a p p r o x i m a t e l y 150 ml were taken f o r e x a m i n a t i o n , u s i n g the subsampler 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 (1975). Counts from t h e s e subsamples were averaged and a d j u s t e d t o 100 1 of l a k e volume sampled. The t o t a l numbers of r o t i f e r s , c l a d o c e r a n s by s p e c i e s , d i a p t o m i d n a u p l i i , and d i a p t o m i d copepodids by s p e c i e s were r e c o r d e d , as w e l l as any m i t e s , Chaoborus l a r v a e . and c h i r o n o m i d l a r v a e . C.b.thomasi and T . p r a s i n u s p o p u l a t i o n s enumerated as n a u p l i i I , I I , I I I , IV, V and c o p e p o d i d I , I I , I I I , IV, V, a d u l t s , a d u l t f e m a l e s , a d u l t females w i t h eggs and eggs. The number of eggs per c l u t c h was a l s o r e c o r d e d f o r the c l a d o c e r a n and copepod s p e c i e s . The same methods were used t o c o l l e c t and a n a l y s e samples d u r i n g a 24 hour s a m p l i n g regime t o p r o v i d e d a t a on the v e r t i c a l m i g r a t i o n p a t t e r n s of the z o o p l a n k t o n i n the e n c l o s u r e s and i n the l a k e . 65 Temperature, Oxygen and G r a z e a b l e Seston Temperature and oxygen p r o f i l e s i n P l a c i d Lake were mo n i t o r e d from s u r f a c e t o the 5.5 depth s t r a t u m a t 1 m i n t e r v a l s u s i n g a Y e l l o w S p r i n g s I n s t r u m e n t s temperature/oxygen meter from May t o November. B i w e e k l y water samples were c o l l e c t e d from . a l l d e p t h s , p o o l e d and ' f i x e d ' w i t h L u g o l ' s S o l u t i o n ( V o l l e n w e i d e r , 1969) f o r l a b o r a t o r y e x a m i n a t i o n of p a r t i c u l a t e o r g a n i c matter c o n c e n t r a t i o n s . In the l a b o r a t o r y , a l l p a r t i c l e s l e s s than 73 um were c o l l e c t e d on a g l a s s f i b r e f i l t e r , d r i e d o v e r n i g h t a t 60±2°C, and weighed on a Cahn e l e c t r o b a l a n c e , ashed a t 600°C f o r 3 hours and reweighed t o p e r m i t c a l c u l a t i o n of the a s h - f r e e d ry w e i g h t . S i m i l a r l y , water samples were c o l l e c t e d b i w e e k l y f o r e x a m i n a t i o n of a l g a l s i z e and t y p e . 100 ml of l a k e water was f i x e d w i t h L u g o l ' s S o l u t i o n a n d . f i l t e r e d onto a 0.45 um M i l l i p o r e f i l t e r . A l l the f i l t e r s were c l e a r e d i n Cedar Wood o i l f o r 10 days, mounted on g l a s s s l i d e s and counted w i t h a phase c o n t r a s t microscope a t 500X m a g n i f i c a t i o n . C e l l s were counted by s i z e c l a s s and type t o a minimum of 200 c e l l s per f i e l d or 10 o c u l a r f i e l d s . S i z e c l a s s e s were enumerated as <2, 3-6, 7-10, 11-14, 14-20, >20 micrometres and c e l l type was i d e n t i f i e d as di a t o m , s o l i t a r y , c o l o n i a l or f i l a m e n t o u s . S p e c i e s were i d e n t i f i e d where p o s s i b l e a c c o r d i n g t o P r e s c o t t (1970). The f i e l d s t o be counted were randomly s e l e c t e d b e f o r e c o u n t i n g began. I n i t i a l c o u n t i n g a c r o s s the di a m e t e r of s e v e r a l c l e a r e d f i l t e r s showed t h a t the c e l l s were randomly d i s t r i b u t e d a c r o s s the s u r f a c e ( d e t e r m i n e d by a P o i s s o n d i s t r i b u t i o n and X 2 ) and t h e r e f o r e t h i s s a m p l i n g method was a p p r o p r i a t e . 66 RESULTS FEEDING EXPERIMENTS D i e t of T . p r a s i n u s To e v a l u a t e the s t a t u s of T . p r a s i n u s i n the t r o p h i c s t r u c t u r e of P l a c i d Lake, f e e d i n g e x p e r i m e n t s were conducted i n the l a b o r a t o r y . I f T . p r a s i n u s i s h e r b i v o r o u s through a l l i n s t a r s , the abundance and t e m p o r a l s p a c i n g of the o t h e r g r a z e r s i n the system c o u l d be c r i t i c a l i n d e t e r m i n i n g the s u c c e s s of T . p r a s i n u s . A l t e r n a t i v e l y , i f a n i m a l food s o u r c e s are r e q u i r e d , T . p r a s i n u s might l i m i t i t s own p o p u l a t i o n t h r o u g h c a n n i b a l i s m , or compete w i t h C.b.thomasi f o r f o o d . A measure of t h i s s p e c i e s ' a b i l i t y t o a s s i m i l a t e a l g a e was o b t a i n e d by f e e d i n g the most p a l a t a b l e ( B r a n d l , 1973) of the a l g a e commonly found i n the Research F o r e s t (Dickman, 1968; S t e i n , 1975). S o r o k i n ' s (1968) "index of a s s i m i l a t i o n " , the a s s i m i l a t e d f r a c t i o n of the carbon consumed (Appendix A ) , was c a l c u l a t e d t o compare the r e l a t i v e e f f e c t i v e n e s s of f e e d i n g among d i f f e r e n t i n s t a r s and t o p e r m i t comparison w i t h known h e r b i v o r e s . Carbon c o n t e n t was assumed t o be 50% of the a s h - f r e e d ry weight of the a n i m a l ( V o l l e n w e i d e r , 1969). Mean w e i g h t s used i n c a l c u l a t i n g a s s i m i l a t i o n a r e shown i n Table 4. T h i s method of e s t i m a t i n g a s s i m i l a t i o n i s a r e l a t i v e measure, undoubtedly a ^ T a b l e 4 Mean l e n g t h and dry weight of D.rosea and T . p r a s i n u s •\a frcm P l a c i d Lake, 1976. Length (mm) Dry Weight {ag) D.rosea 1.32 14.20 T . p r a s i n u s a d u l t c o p e p o d i d n a u p l i i 0.70 0.45 0. 10 2.57 1.14 0. 17 68 low e s t i m a t e of a s s i m i l a t i o n as i t must assume t h a t t h e r e i s no l o s s of absorbed carbon t h r o u g h r e s p i r a t i o n or e x c r e t i o n . However, i t i s adequate f o r the purpose of comp a r i s o n . The r e s u l t s of th e s e e x p e r i m e n t s a r e shown i n T a b l e 5. T . p r a s i n u s seemed t o u t i l i z e a l l a l g a l t y p e s l e s s e f f i c i e n t l y than the c l a d o c e r a n , Daphnia r o s e a . However, v a l u e s f o r the a s s i m i l a t i o n of Ochromonas and C h l o r e l l a a re comparable t o those of a predaceous c y c l o p o i d on a c l a d o c e r a n prey ( S o r o k i n , 1968). T h i s r e s u l t s u g g e s t s t h a t a t l e a s t some a l g a l t y p e s are e f f i c i e n t l y a s s i m i l a t e d by T . p r a s i n u s n a u p l i i . A s s i m i l a t i o n e f f i c i e n c y d e c r e a s e s by the a d u l t i n s t a r . T . p r a s i n u s a d u l t s i n c u l t u r e v e s s e l s were obser v e d f e e d i n g on p r o t o z o a n s on the s u r f a c e and c a n n i b a l i s m was not o b s e r v e d . When T . p r a s i n u s appeared i n the p l a n k t o n i n l a t e J u l y p a r t i c u l a t e o r g a n i c m a t t e r i n the l a k e was v e r y low, as w'as the s t a n d i n g c r o p of p h y t o p l a n k t o n . C o n s e q u e n t l y , food l i m i t a t i o n seemed a l i k e l y h a z a r d f o r a p o p u l a t i o n t h a t f i r s t appeared i n the p l a n k t o n d u r i n g t h i s p e r i o d . However, i f a p l a n t or a n i m a l d i e t was e q u a l l y a c c e p t a b l e an o p p o r t u n i s t i c s p e c i e s might be a b l e t o c i r c u m v e n t t h i s d i f f i c u l t y . I t i s a l s o p o s s i b l e t h a t T . p r a s i n u s might p r e f e r m i c r o z o o p l a n k t e r s and o n l y supplement i t s d i e t w i t h a l g a e . To a s s e s s the a c c e p t a b i l i t y of an a l g a l d i e t v e r s u s an a n i m a l food s o u r c e , copepodids were r e a r e d on a u n i - a l g a l (but not b a c t e r i a - f r e e ) and a u n i - p r o t o z o a n d i e t from copepodid I t o a d u l t . S u r v i v o r s h i p , p r o p o r t i o n of o v i g e r o u s f e m a l e s , and number of eggs per female were u n a f f e c t e d by the type of d i e t p r o v i d e d ( T a b l e 6 ) . T h i s r e s u l t s u g g e s t s t h a t T . p r a s i n u s i s p r o b a b l y T a b l e 5 Index of a s s i m i l a t i o n e s t i m a t e d f o r T.prasinns and ^•I2s_ea or. d i f f e r e n t f o o d s p e c i e s a t 15 + 2 C. n=8 FOOD CONSUMERS Ca/C (SE) cfo ^ o r g 2 I a !• P£i*sinus a d u l t 19 .2 (2.3) c a - 5 32 , 7 (1.4) 0 1-3 2 1.8 (1 -2) N3-6 35 . 2 (0,8) D. r o s e a a d u l t 39 .6 (3 .2) Ochromonas T. p r a s i n u s a d u l t ' 2 0 . 5 ( 2 . 1 ) C4-5 25. 3 ( 1.9) C i - 3 26. 7 ( 1 .4) N3-6 29.6 (1.1) D.rosea a d u l t 35.1 (2.0) Chlamgggmgnas T. p r a s i n u s a d u l t 13.1 (1.2) C4-5 14.6 (0.9) C1-3 18.3 (1.2) N3-6 20. 4 (0.7) Scenedesrous JB£§.siiius a d u l t 2.1 (0.2) C4-5 1.0 (0.1) C1-3 0 .2 (0.1) D. r o s e a a d u l t 19.6 (1.2) T a b l e 6 S u r v i v o r s h i p front c o pepodid I t o a d u l t T . p r a s i n u s , subsequent egg development and number o f eggs per f e m a l e (SE) on a p l a n t v e r s u s a n i m a l d i e t . n=5 % S u r v i v a l % Egg Development No.Eggs/Female P r o t o z o a 92 81 16 (0. 9) Algae 83 79 15 (1.1) 71 o p p o r t u n i s t i c i n the w i l d , t a k i n g p l a n t or a n i m a l food as a v a i l a b l e . However, as a l g a e do not appear t o be a s s i m i l a t e d as e f f i c i e n t l y by T . p r a s i n u s as by the h e r b i v o r o u s c l a d o c e r a n t e s t e d , T . p r a s i n u s may be a t a d i s a d v a n t a g e when both m i c r o z o o p l a n k t e r s and a l g a l food r e s o u r c e s ; a r e low. T . p r a s i n u s was a l s o observed a p p a r e n t l y e a t i n g dead z o o p l a n k t e r s i n c u l t u r e v e s s e l s . E f f e c t s Of C.b.thomasi P r e d a t i o n i n L a b o r a t o r y E x p e r i m e n t s T . p r a s i n u s n a u p l i i and copepodids were f e d a l o n e and w i t h a l t e r n a t i v e prey t o C.b.thomasi copepodids IV, V and a d u l t s . In e x p e r i m e n t s w i t h T . p r a s i n u s n a u p l i i I-VI as prey ( F i g . 2 1 a ) , p r e d a t i o n r a t e reached a maximum of 1.9 prey per p r e d a t o r per l i t r e per day a t the h i g h e s t prey d e n s i t y of 30 prey per l i t r e . T h i s p r e d a t i o n r a t e was • h i g h e r than the r a t e o b t a i n e d by McQueen (1969) a t about 190 C.b.thomasi n a u p l i i per l i t r e , the lo w e s t C.b.thomasi n a u p l i a r prey c o n c e n t r a t i o n he t e s t e d . Anderson, (1970a) found a mean prey consumption per p r e d a t o r per day of 6.3 f o r C y c l o p s v e r n a l i s but o n l y 0.19 f o r C.b.thomasi . The mean p r e d a t o r - t o - p r e y s i z e r a t i o i n An d e r s o n s ' s • s t u d y was 0.85/1 whereas the r a t i o of C.b.thomasi p r e d a t o r t o T . p r a s i n u s prey i n t h i s s t u d y was about 2.6/1. T h i s s i z e r a t i o was comparable t o t h a t of C . v e r n a l i s i n Anderson's s t u d y . However, both Anderson and McQueen used u n n a t u r a l l y h i g h prey d e n s i t i e s . Jamieson (1977) used more r e a l i s t i c p rey d e n s i t i e s and found t h a t c o p e p o d i d IV i n s t a r s of Meso c y c l o p s l e u k a r t i k i l l e d about 1.5 c a l a n o i d n a u p l i i per l i t r e .per p r e d a t o r per day a t a prey c o n c e n t r a t i o n of 30 pr e y 72 c F i g u r e 21. P r e d a t i o n r a t e s f o r C.b.thomasi copepodids IV, V and a d u l t s on T . p r a s i n u s n a u p l i i and copepodids w i t h and w i t h o u t a l t e r n a t i v e prey as a f u n c t i o n of d e n s i t y . S i x r e p l i c a t e s a t each prey d e n s i t y a r e shown. E q u a t i o n s f o r the f i t t e d r e g r e s s s i o n s l i n e s a r e g i v e n , PR r e p r e s e n t s the p r e d a t i o n r a t e and x r e p r e s e n t s the prey d e n s i t y . A. PR = 0.07x°• 8 7 r 2=0.85 B. PR = 0.05x° • 5 3 r 2=0.88 C. PR = O.Olx + 0.08 r 2=0.66 D. PR = O.Olx + 0.02 r 2=0.50 13 Ac p/ JOicpdJ 74 per l i t r e . When a l t e r n a t i v e prey were added ( F i g . 2 1 b ) , the p r e d a t i o n r a t e was e s s e n t i a l l y unchanged a t the lowe s t prey c o n c e n t r a t i o n (mean of 0.05 prey per p r e d a t o r per d a y ) . Even a t the h i g h e s t prey c o n c e n t r a t i o n of 30 n a u p l i i per l i t r e w i t h a l t e r n a t i v e prey the p r e d a t i o n r a t e s between those e x p e r i m e n t s w i t h a l t e r n a t i v e prey and those w i t h o u t a l t e r n a t i v e s were not s i g n i f i c a n t l y d i f f e r e n t (X 2=0.64, df=5). McQueen (1969) a l s o found t h a t C.b.thomasi f e d a t s i m i l a r r a t e s on s e l e c t e d prey s p e c i e s when t e s t e d i n mixed and s i n g l e prey assemblages. However 1 T . p r a s i n u s n a u p l i i / l i t r e was s l i g h t l y h i g h e r than d e n s i t y e s t i m a t e s f o r T . p r a s i n u s n a u p l i i i n the l a k e . The e q u a t i o n y = 0.05x°-53 gave the be s t f i t t o the d a t a ( r 2 = 0.88), where x=prey d e n s i t y . . Thus the e s t i m a t e d l a k e prey d e n s i t y was used t o e x t r a p o l a t e the p r e d a t i o n r a t e f o r each date on which p r e d a t i o n c o u l d occur (August 11 t o October 1 8 ) . The p r e d a t i o n r a t e on T . p r a s i n u s copepodids I -IV w i t h a s m a l l e r and p r o b a b l y more a t t r a c t i v e a l t e r n a t i v e p r e y , Diaptomus n a u p l i i , (Fig.21d) was 0.04 a t 5 prey per l i t r e , lower than the p r e d a t i o n r a t e on T . p r a s i n u s copepodids I -IV a l o n e (Fig.'21c).- When a l t e r n a t i v e prey were p r e s e n t , assuming a s t r a i g h t l i n e r e l a t i o n s h i p ( r 2 = 0 . 4 3 , y=-0.02+0.Olx), the p r e d a t i o n r a t e of the C.b.thomasi p r e d a t o r s on T . p r a s i n u s copepodids dropped t o z e r o by 2 prey per l i t r e . As t h i s c o n c e n t r a t i o n of T . p r a s i n u s prey was h i g h e r than t h a t observed i n the l a k e (maximum T . p r a s i n u s copepodid d e n s i t i e s were .0.4 per l i t r e ) , p r e d a t i o n on t h e s e s t a g e s was p r o b a b l y not i m p o r t a n t . The T . p r a s i n u s p o p u l a t i o n was s u f f i c i e n t l y reduced 75 by the c o p e p o d i d i n s t a r s t h a t few p r e d a t o r s were e n c o u n t e r e d . Jamieson (1977) showed t h a t s w i t c h i n g d i d not occur when a d u l t female M e s o c y c l o p s were f e d l a r g e and s m a l l C e r i o d a p h n i a . M e s o c y c l o p s k i l l e d i t s prey as a s i m p l e f u n c t i o n of the r e l a t i v e abundance of the prey i n the environment. I t seemed p r o b a b l e t h a t the v e r y low p r e d a t i o n r a t e s of C.b.thomasi on T . p r a s i n u s c o p e p o d i d s , even a t 5 prey per l i t r e , was a r e f l e c t i o n of apparent s c a r c i t y . The tendency f o r T . p r a s i n u s copepodids t o c o n c e n t r a t e near the s u r f a c e , o b s e r v e d i n l a b o r a t o r y a q u a r i a , might have reduced encounter r a t e s . The e x t e n t t o which t h i s b e h a v i o u r o c c u r s i n n a t u r e i s unknown but i t appeared t h a t the brunt of C.b.thomasi p r e d a t i o n p r o b a b l y f e l l on the n a u p l i a r s t a g e s of T . p r a s i n u s . 76 FIELD EXPERIMENTS S e a s o n a l Abundance of C.b.thomasi and T . p r a s i n u s C.b.thomasi C.b.thomasi was the dominant c y c l o p o i d copepod i n P l a c i d Lake. A f t e r i c e broke up, the f i r s t i n d i v i d u a l s caught were st a g e IV and V ( F i g . 2 2 ) . By mid-May these copepodids had moulted t o the a d u l t stage and produced eggs by the end of May. Eggs ha t c h e d i n e a r l y June. By the end of June n a u p l i i had moulted t o copepodids and by the m i d d l e of J u l y more than h a l f the p o p u l a t i o n had reached c o p e p o d i d IV s t a g e . The m a j o r i t y of the p o p u l a t i o n was u s u a l l y r e p r e s e n t e d by o n l y two or t h r e e i n s t a r s ( F i g . 2 3 ) . T h i s u n i f o r m i t y of i n s t a r on any g i v e n s a m p l i n g day has been noted i n many n a t u r a l c y c l o p o i d copepod p o p u l a t i o n s , and has been a t t r i b u t e d t o c a n n i b a l i s m (Smyly, 1961; McQueen, 1969; Anderson, 1970b). A s m a l l p u l s e of eggs was produced i n e a r l y September but most of the p o p u l a t i o n remained i n the c o p e p o d i d IV and V s t a g e s u n t i l they d i s a p p e a r e d from the p l a n k t o n i n e a r l y November. 77 F i g u r e 22. Changes i n the s t a n d i n g c r o p of C.b.thomasi i n P l a c i d Lake i n 1976. 79 F i g u r e 23. % C o m p o s i t i o n of C•b.thomasi i n s t a r s i n P l a c i d Lake i n 1976. L e t t e r s i n d i c a t e : N l - naupius 1, CI - c o p e p o d i d I , AD - a d u l t . May 31 50 25A N1 CI i r AD June 20 50-, O O O N1 CI AD July 22 50-, 25H N1 l r ci AD 81 T . p r a s i n u s T h i s s p e c i e s o c c u r r e d i n low numbers i n P l a c i d Lake. The main p u l s e began i n the p l a n k t o n i n m i d - J u l y , a p p a r e n t l y emerging as copepodids I I - a d u l t ( F i g . 2 4 ) . Because numbers were always low, the n a u p l i i and copepodids of T . p r a s i n u s were c o l l e c t i v e l y d e s c r i b e d . By e a r l y August egg sacs were ob s e r v e d . N a u p l i i were b l u e - g r e e n i n c o l o u r and when f i r s t c a u g ht, c o u l d be d i s t i n g u i s h e d from those of C.b.thomasi. The number of T . p r a s i n u s i n the p l a n k t o n i n c r e a s e d t h r o u g h August-September, but by the b e g i n n i n g of November a r a p i d d e c l i n e i n the numbers c o l l e c t e d was e v i d e n t . By l a t e O c t o b e r - e a r l y November s e v e r a l i n s t a r s were p r e s e n t , s u g g e s t i n g t h a t the p o p u l a t i o n o v e r w i n t e r s i n a v a r i e t y of s t a g e s , a l t h o u g h p r i n c i p a l l y i n copepod IV, V and* a d u l t . In Gwendoline Lake, where T . p r a s i n u s was more numerous, a s i m i l a r p a t t e r n was o b s e r v e d . The u n i f o r m i t y of i n s t a r on each s a m p l i n g date noted i n C.b.thomasi was not o b s e r v e d i n t h i s p o p u l a t i o n . T . p r a s i n u s does not appear t o undergo any marked d i e l m i g r a t i o n , u n l i k e C.b.thomasi ( F i g . 2 5 ) . T h i s s a m p l i n g a l s o shows t h a t d i f f e r e n c e s between the l a k e and e n c l o s u r e s were m i n i m a l . The two c y c l o p o i d copepod p o p u l a t i o n s appear t o m a i n t a i n b a s i c a l l y the same p a t t e r n s of movement i n the e n c l o s u r e s as i n the l a k e . 82 F i g u r e 24. Changes i n the s t a n d i n g c r o p of T . p r a s i n u s i n P l a c i d Lake i n 1976. 84 F i g u r e 25. D i e l v e r t i c a l m i g r a t i o n of C.b.thomasi (on the l e f t i n d i c a t e d as Cbt) and T . p r a s i n u s (on the r i g h t i n d i c a t e d as Tp) as r e p r e s e n t e d by abundance on Aug. 25, 1976. S o l i d c i r c l e s r e p r e s e n t P l a c i d Lake and open c i r c l e s r e p r e s e n t the mean of two r e p l i c a t e e n c l o s u r e s . D e n s i t y per l i t r e as shown. Time — hours 0900 1500 2100 0300 Number per Litre 86 E f f e c t s of E n c l o s u r e Many complex i n t e r a c t i o n s a r e p o s s i b l e i n the v a r y i n g environment of the l a k e which a r e d i f f i c u l t t o mimic i n s h o r t -term l a b o r a t o r y e x p e r i m e n t s . T h e r e f o r e i t was i m p o r t a n t t o examine the f a c t o r s l i m i t i n g the s u c c e s s of T . p r a s i n u s under f i e l d c o n d i t i o n s . T e s t s were conducted by i n t r o d u c i n g T . p r a s i n u s a t P l a c i d Lake d e n s i t i e s i n t o l a r g e in s i t u e x p e r i m e n t a l e n c l o s u r e s i n P l a c i d Lake. The community was m a n i p u l a t e d i n these e n c l o s u r e s t o produce d i f f e r e n t c o n d i t i o n s of p r e d a t i o n and c o m p e t i t i o n f o r the s c a r c e food r e s o u r c e s . F i g u r e 26 shows time s e r i e s data f o r oxygen and p a r t i c u l a t e o r g a n i c m a t t e r . Oxygen v a l u e s were i d e n t i c a l a t the s u r f a c e . S m a l l d i f f e r e n c e s i n oxygen l e v e l s between the l a k e and e n c l o s u r e s o c c u r r e d a t the deepest depth measured. How-ever, t h i s s h o u l d not have a f f e c t e d T . p r a s i n u s , a s p e c i e s which was never c o l l e c t e d deeper than 3.5 m. There were no d i f f e r e n c e s i n temperature p r o f i l e s between l a k e and e n c l o s u r e s ( F i g . 2 7 ) . P a r t i c u l a t e o r g a n i c m a t t e r i n c l u d e d a l l d e t r i t u s and p h y t o p l a n k t o n l e s s than 73 um i n d i a m e t e r . In an o l i g o t r o p h i c system such as P l a c i d Lake, s m a l l changes i n the p a r t i c u l a t e o r g a n i c m a t t e r c o u l d r e p r e s e n t s i g n i f i c a n t changes i n the food base. However, d i f f e r e n c e s between the l a k e and e n c l o s u r e s were g e n e r a l l y s m a l l , as were d i f f e r e n c e s between e n c l o s u r e s . The n u m e r i c a l l y i m p o r t a n t z o o p l a n k t o n s p e c i e s a r e shown i n F i g . 2 8 . V a r i a t i o n between r e p l i c a t e e n c l o s u r e s was s m a l l , p a r t i c u l a r l y w i t h i n the C y c l o p s p r e s e n t and C y c l o p s absent t r e a t m e n t s . Other organisms I o c c a s i o n a l l y sampled i n low 87 F i g u r e 26. V a r i a t i o n i n oxygen and p a r t i c u l a t e o r g a n i c m atter (POM) shown f o r P l a c i d Lake and the e x p e r i m e n t a l e n c l o s u r e s . C i r c l e s on the POM d a t a r e p r e s e n t the mean of 2 r e p l i c a t e e n c l o s u r e s and v e r t i c a l bars r e p r e s e n t the 95% c o n f i d e n c e i n t e r v a l . Squares r e p r e s e n t oxygen at 4.5 m.; s o l i d symbols r e p r e s e n t the e n c l o s u r e s and open symbols r e p r e s e n t the l a k e . CO d-l mGfQM Ajp 6ui) -uouoo lAJOd ueBffl 89 F i g u r e 27. Changes i n temperature a t 0.5 m. and 4.5 m. f o r P l a c i d Lake i n 1976. C i r c l e s r e p r e s e n t 0.5 m. and t r i a n g l e s r e p r e s e n t 4.5 m. Open symbols r e p r e s e n t the l a k e and s o l i d symbols r e p r e s e n t the e n c l o s u r e d a t a . Do a j n j . D J 8 d UU 9 J _ 91 F i g u r e 28. Changes i n the s t a n d i n g c r o p of P l a c i d Lake z o o p l a n k t o n showing s e a s o n a l v a r i a t i o n of the n u m e r i c a l l y i m p o r t a n t z o o p l a n k t o n i n r e p l i c a t e e n c l o s u r e s i n 1976. C i r c l e s r e p r e s e n t the t r e a t m e n t w i t h C.b.thomasi, squares r e p r e s e n t the t r e a t m e n t w i t h o u t C.b.thomasi, and t r i a n g l e s i n d i c a t e the T . p r a s i n u s a l o n e t r e a t m e n t . Open and s o l i d symbols are r e p l i c a t e s of the same treatment,. z o o o o 9 o o o ° o O n i«i - ° V ri N - O 93 numbers were Polyphemus, Bosmina, Chydorus and s o l i t a r y r o t i f e r s . A l t h o u g h Polyphemus i s d e s c r i b e d as c a r n i v o r o u s , e a t i n g p r o t o z o a n s , r o t i f e r s and "minute c r u s t a c e a n s " ( B r o o k s , 1959), Anderson (1970a) found no e v i d e n c e of p r e d a t i o n on d i a p t o m i d or c y c l o p i d n a u p l i i by t h i s s p e c i e s and the numbers i n my e n c l o s u r e s were always too low t o have a s i g n i f i c a n t impact on c r u s t a c e a n prey (maximum of 0.13 per l i t r e ) . M i t e s and c h i r o n o m i d s were a l s o o c c a s i o n a l l y sampled. By l a t e September-October, chiromomids were r e g u l a r l y sampled i n a l l e n c l o s u r e s a t about 0.7 t o 1 a n i m a l per l i t r e . I c o l l e c t e d C e r i o d a p h n i a i n a l l r e p l i c a t e s of a l l t r e a t m e n t s but always i n low numbers w i t h the e x c e p t i o n of the T . p r a s i n u s a l o n e t r e a t m e n t . In s p i t e of the f a c t t h a t I i n d i v i d u a l l y p i p e t t e d the T . p r a s i n u s f o r i n t r o d u c t i o n t o t h i s t r e a t m e n t , c o n t a m i n a t i n g c l a d o c e r a n s Cer iodaphn i a and D.rosea became abundant ( F i g . 2 8 ) . V a r i a t i o n between r e p l i c a t e s was much g r e a t e r i n the T . p r a s i n u s a l o n e t r e a t m e n t than i n e i t h e r of the o t h e r two t r e a t m e n t s and the v a r i a t i o n was caused by appearance of t h e s e two c l a d o c e r a n s . T h i s v a r i a t i o n was p r o b a b l y due t o the a c c i d e n t a l n a t u r e of the i n t r o d u c t i o n - r a t h e r l i k e the appearance of a garden 'weed'. D.rosea was a n u m e r i c a l l y dominant z o o p l a n k t e r i n a l l the t r e a t m e n t s but C e r i o d a p h n i a was n o t . T h i s l a t t e r s p e c i e s became abundant, r e l a t i v e t o i t s d e n s i t y i n the o t h e r f o u r e n c l o s u r e s , o n l y i n the t r e a t m e n t where T . p r a s i n u s and D.rosea were the s o l e r e p r e s e n t a t i v e s of the c r u s t a c e a n p l a n k t o n . There was a 5 t o 10 f o l d i n c r e a s e i n Cer i o d a p h n i a abundance i n t h i s t r e a t m e n t and t h i s enhancement o c c u r r e d i n both r e p l i c a t e s , s u g g e s t i n g t h a t c o m p e t i t i v e 94 i n t e r a c t i o n s might n o r m a l l y p r e v e n t C e r i o d a p h n i a from i n c r e a s i n g i n number. E n c l o s u r e does not appear t o a f f e c t v e r t i c a l d i s t r i b u t i o n ( F i g . 2 9 ) , s u g g e s t i n g t h a t the e n c l o s e d c r u s t a c e a n p o p u l a t i o n s p r o b a b l y i n t e r a c t i n the same way as they do i n the l a k e . There i s one n o t a b l e d i f f e r e n c e , however. T . p r a s i n u s seems t o be over r e p r e s e n t e d i n the C y c l o p s absent t r e a t m e n t a t 2.5 m. r e l a t i v e t o the same depth i n the C y c l o p s p r e s e n t t r e a t m e n t and i n the l a k e . T h i s r e s u l t i s t y p i c a l of the August samples s u g g e s t i n g t h a t T . p r a s i n u s i s more v e r t i c a l l y homogenous i n the absence of C.b.thomasi , u n l i k e e i t h e r D.rosea or D.oregonensis I t i s not c l e a r , however, whether t h i s 'expansion of t e r r i t o r y ' i s due t o C.b.thomasi / T . p r a s i n u s i n t e r a c t i o n or whether t h i s r e s u l t s i m p l y r e f l e c t s the i n c r e a s e d T . p r a s i n u s numbers i n the C y c l o p s - f r e e t r e a t m e n t . E f f e c t of C o m p e t i t i o n N e i l l (1978) showed t h a t Daphnia r o s e a , one of the dominant g r a z e r s i n the system, f i n d s P l a c i d Lake a poor food environment d u r i n g the summer months. B i o a s s a y s of r e p r o d u c t i v e i n d i c a t o r s a r e p r o b a b l y the b e s t measures of changes i n n u t r i t i o n a l q u a n t i t y and/or q u a l i t y i n the absence of any d a t a on s e l e c t i v i t y i n f e e d i n g . C o n s e q u e n t l y , changes i n the r e p r o d u c t i v e output of the two most i m p o r t a n t g r a z e r s i n the l a k e , D.rosea and Diaptomus o r e g o n e n s i s , a r e p r o b a b l y the best measure of any d i f f e r e n c e s between e n c l o s u r e s w i t h r e s p e c t t o the food base. The p r o p o r t i o n of o v i g e r o u s a d u l t females of D.rosea and Diaptomus o r e g o n e n s i s ( F i g . 3 0 ) shows t h a t the 95 F i g u r e 29. % of the abundant c r u s t a c e a n p o p u l a t i o n s - the % of C.b.thomasi , D.orgonensis and D.rosea a t 0.5, 2.5, and 4.5 m. i n the C.b.thomasi e n c l o s u r e , the e n c l o s u r e w i t h o u t C.b.thomasi and P l a c i d Lake on J u l y 13th and August 1 7 t h , 1976. T . p r a s i n u s i s shown o n l y f o r August 17th as t h i s s p e c i e s was not p r e s e n t i n the p l a n k t o n on J u l y 1 3 t h , 1976. Aug 17 T. prasinu s o.sm o lake with Cyclops without Cyclops % of population July 13 o.5m C.b. thomasi D. oregonensts m 233 D. rosea mi 4 .5 m 4,5m gure 30. P r o p o r t i o n of o v i g e r o u s a d u l t females of a) D.oregonensis and b) D.rosea d u r i n g the summer of 1976. Symbols r e p r e s e n t : c i r c l e s - mean of the r e p l i c a t e e n c l o s u r e s w i t h C y c l o p s , squares - mean of r e p l i c a t e e n c l o s u r e s w i t h o u t C y c l o p s , t r i a n g l e s - mean of r e p l i c a t e e n c l o s u r e s of the T . p r a s i n u s 'alone' t r e a t m e n t . 99 t r e n d s a r e the same between e n c l o s u r e s ( a n a l y s i s d e s c r i b e d i n Appendix B ) . An a n a l y s i s of v a r i a n c e on the eggs per female d a t a f o r t h e s e two s p e c i e s show t h a t t h e r e were no d i f f e r e n c e s between e n c l o s u r e s or t r e a t m e n t s (Table 7 ) . Data f o r D.oregonensis i n c l u d e the e n c l o s u r e s w i t h and w i t h o u t C.b.thomasi. The mean c l u t c h s i z e was 4.4+0.2 (±1SE), n=122. Data f o r D.rosea i n c l u d e a l l the e n c l o s u r e s as t h i s s p e c i e s invaded the T . p r a s i n u s a l o n e t r e a t m e n t and became abundant by the m i d d l e of August. There was no d i f f e r e n c e i n c l u t c h s i z e between any of the e n c l o s u r e s (Mean=l.8±0.1, n=174) As T . p r a s i n u s can f u n c t i o n h e r b i v o r o u s l y and P l a c i d Lake i s a low food environment, the e f f e c t of c o m p e t i t i o n between t h i s s p e c i e s and o t h e r h e r b i v o r o u s z o o p l a n k t o n was examined by comparing T . p r a s i n u s r e p r o d u c t i o n i n the. e n c l o s u r e s w i t h o u t predaceous z o o p l a n k t o n w i t h the e n c l o s u r e s where o n l y T . p r a s i n u s was added. There appeared t o be no d i f f e r e n c e between these two t r e a t m e n t s . The i n d i c a t o r s of r e p r o d u c t i v e o u t p u t , p r o p o r t i o n of o v i g e r o u s females ( F i g . 3 1 ) and number of eggs per female ( T a b l e 8 ) , show t h a t T . p r a s i n u s e x p e r i e n c e d e s s e n t i a l l y the same food environment i n the T . p r a s i n u s a l o n e t r e a t m e n t as i n the o t h e r t r e a t m e n t s . T h i s might have been an i n d i c a t i o n t h a t food r e s o u r c e s were not l i m i t i n g T . p r a s i n u s . However, the e v e r - p r e s e n t 'weed' i n the system, D.rosea, appeared i n the T . p r a s i n u s a l o n e e n c l o s u r e s and became numerous. N e i l l (1978) e s t i m a t e s t h a t D.rosea c o n t r i b u t e s more than 75% of the summertime g r a z i n g p r e s s u r e on l a k e s e s t o n . The appearance of D.rosea i n the T . p r a s i n u s a l o n e e n c l o s u r e s a p p a r e n t l y e l i m i n a t e d any b e n e f i t T . p r a s i n u s g a i n e d TABLE 7 One-way a n a l y s i s of v a r i a n c e of the eggs per female of D.orgonensis i n the e n c l o s u r e s i n P l a c i d Lake d u r i n g J u l y and August, 1976. Source o f v a r i a t i o n df SS MS F Between 5 5.6 1,1 0.7 ns W i t h i n 116 197.7 1.7 One-way a n a l y s i s of v a r i a n c e of the eggs per female o f D.rosea i n the P l a c i d Lake e n c l o s u r e s d u r i n g J u l y and August, 1976. Source of v a r i a t i o n df SS MS F Between W i t h i n 3. 1 223.3 0.6 1.3 0.8 ns 101 F i g u r e 31. P r o p o r t i o n of o v i g e r o u s females of T . p r a s i n u s f o r the same time p e r i o d and t r e a t m e n t s as i n d i c a t e d i n F i g . 30. MONTHS TABLE 8 Mean C l u t c h s i z e (+ 1 SE) of T . p r a s i n u s i n P l a c i d Lake and i n the e x p e r i m e n t a l e n c l o s u r e s d u r i n g August-September, 1976. Cy c l o p s Without C y c l o p s T r o p o c y c l o p s a l o n e Lake 17.6 + 0.99 17.3 + 0.95 15.9 + L 15 17. 5 * 0. 81 17.5 + 0.84 16.9 + 0,88 16.9 «• 0.53 Source o f v a r i a t i o n Between E r r o r a n a l y s i s of v a r i a n c e df SS MS 6 133 38. 5 2125. 4 6.4 15.9 F 0. 4 as T o t a l 139 2163,9 ns,not s i g n i f i c a n 104 from the removal of the o t h e r g r a z e r s i n the system, or food was never l i m i t i n g i n the e n c l o s u r e s . E f f e c t s Of C.b.thomasi P r e d a t i o n The p r e d a t i o n m o r t a l i t y which c o u l d have been caused by C.b.thomasi i n the l a k e was e s t i m a t e d by combining a l a b o r a t o r y e s t i m a t e d r a t e of p r e d a t i o n w i t h the f i e l d e s t i m a t e s of p r e d a t o r and prey s t a n d i n g s t o c k s . W ith prey d e n s i t i e s of 1 T . p r a s i n u s prey per l i t r e , l a b o r a t o r y e x p e r i m e n t s showed t h a t 0.05 T . p r a s i n u s n a u p l i i c o u l d be eaten per C.b.thomasi p r e d a t o r per day. To e x t r a p o l a t e t o l a k e d e n s i t i e s , I used the e q u a t i o n y=0.05X° ' 3 . The c o n c e n t r a t i o n of C.b.thomasi p r e d a t o r s was e s t i m a t e d by t a k i n g the s t a n d i n g s t o c k of C.b.thomasi found each s a m p l i n g day above 3.5 m, as t h i s was the deepest p o i n t a t which T . p r a s i n u s was c o l l e c t e d . Combining these e s t i m a t e s , I c a l c u l a t e d the % of T . p r a s i n u s n a u p l i i t h a t c o u l d be removed d a i l y by C.b.thomasi p r e d a t o r s (Appendix C ) . From August 4 t h t o O c t . 1 8 t h , an average of about 2.6% per day c o u l d be l o s t t o C.b.thomasi p r e d a t i o n ( F i g . 3 2 ) . The l a b o r a t o r y e s t i m a t e d r e s i d e n c e time f o r T . p r a s i n u s n a u p l i i was about 9.8 days a t 1976 P l a c i d Lake t e m p e r a t u r e s . I f 2.6% per day c o u l d be l o s t t o C.b.thomasi p r e d a t i o n , newly hatched n a u p l i i had about a 23-25% chance of b e i n g removed b e f o r e r e a c h i n g c o p e p o d i d i n s t a r I . C l e a r l y the predaceous i n s t a r s of C.b.thomasi had the p o t e n t i a l t o i n f l i c t c o n s i d e r a b l e m o r t a l i t y on T . p r a s i n u s n a u p l i i , even a t low P l a c i d Lake d e n s i t i e s . McQueen (1969) e s t i m a t e d t h a t 31% of the C.b.thomasi 105 F i g u r e 32. E s t i m a t e d d a i l y p r e d a t i o n of C.b.thomasi on T . p r a s i n u s n a u p l i i f o r a l l C.b.thomasi copepodids IV, and a d u l t s above 3.5 m. on T . p r a s i n u s n a u p l i i from Aug. 4 t h t o Oct. 18th i n P l a c i d Lake i n 1976. % Eaten O cn 10/ 107 n a u p l i i s t a n d i n g s t o c k and 30% of the d i a p t o m i d n a u p l i i were eaten by the c a r n i v o r o u s i n s t a r s of C.b.thomasi i n M a r i o n Lake. As C.b.thomasi i n P l a c i d Lake reaches d e n s i t i e s comparable t o t h a t i n McQueen's study a r e a , the importance of t h i s p r e d a t o r t o the s u c c e s s of T . p r a s i n u s was t e s t e d i n the f i e l d . F i e l d t e s t i n g i s c r i t i c a l because s m a l l s p a t i a l / t e m p o r a l d i s c o n t i n u i t i e s can s e v e r e l y b i a s r e s u l t s and p r e d i c t i o n s based s o l e l y on l a b o r a t o r y e x p e r i m e n t s . T . p r a s i n u s i n c r e a s e d d r a m a t i c a l l y i n both t r e a t m e n t s where C.b.thomasi was e x c l u d e d ( F i g . 3 3 ) , c o n f i r m i n g the i n d i r e c t p r e d i c t i o n t h a t the presence of t h i s c y c l o p o i d p r e d a t o r was an i m p o r t a n t c o n s t r a i n t on the s u c c e s s of T . p r a s i n u s i n P l a c i d Lake. The T . p r a s i n u s p o p u l a t i o n expanded i n a s i m i l a r manner i n both t r e a t m e n t s where C.b.thomasi was e x c l u d e d , s u g g e s t i n g t h a t D.oregonensis had no s i g n i f i c a n t impact on T . p r a s i n u s , because t h i s c a l a n o i d copepod was' absent from the T . p r a s i n u s a l o n e t r e a t m e n t . L a b o r a t o r y f e e d i n g e x p e r i m e n t s i n d i c a t e d t h a t the p r i m a r y impact of C.b.thomasi p r e d a t i o n was e x p e r i e n c e d by T . p r a s i n u s n a u p l i i r a t h e r than the c o p e p o d i d i n s t a r s . I a s s e s s e d the v a l i d i t y of t h i s p r e d i c t i o n by c a l c u l a t i n g s u r v i v o r s h i p c u r v e s ( F i g . 3 4 ) from f i e l d p o p u l a t i o n d a t a (Gehrs and R o b e r t s o n , 1975) and l a b o r a t o r y e s t i m a t e d development t i m e s (Appendix D) u s i n g the mean abundance of the T . p r a s i n u s p o p u l a t i o n i n the C y c l o p s p r e s e n t and C y c l o p s absent t r e a t m e n t s . The p o s i t i v e skew on the s u r v i v o r s h i p c u r v e s showed t h a t the g r e a t e s t m o r t a l i t y o c c u r r e d i n the e a r l y l i f e s t a g e s . By the N3 i n s t a r about 70% of the n a u p l i i had p e r i s h e d i n both t r e a t m e n t s , s u g g e s t i n g t h a t t h e s e s t a g e s were not s i g n i f i c a n t l y a f f e c t e d by C.b.thomasi 108 F i g u r e 33. S t a n d i n g c r o p of T . p r a s i n u s i n the e x p e r i m e n t s and l a k e f o r the summer of 1976. Symbols r e p r e s e n t the t r e a t m e n t s as e x p l a i n e d f o r F i g u r e 30 except t h a t the open c i r c l e s i n d i c a t e the l a k e v a l u e s . 110 F i g u r e 34. T . p r a s i n u s s u r v i v o r s h i p by i n s t a r i n P l a c i d Lake from August t o October, 1977. I n t e r v a l on the x - a x i s i s s c a l e d t o r e p r e s e n t the r e l a t i v e d u r a t i o n of each i n s t a r i n t e r v a l shown. /// 'I i r 1 1 1 r 1 Egg 3 6 I II III IV V N C I 112 p r e d a t i o n . - T . p r a s i n u s s u r v i v o r s h i p was l o w e r , however, by the N6 stage i n the e n c l o s u r e s w i t h C y c l o p s and t h i s t r e n d c o n t i n u e d u n t i l i n s t a r C I I . C o n t r a s t i n g t h i s d e c l i n e w i t h the p a t t e r n i n the t r e a t m e n t w i t h o u t C y c l o p s showed t h a t the N3 t o CI i n t e r v a l was the most s e n s i t i v e t o C.b.thomasi p r e d a t i o n . DISCUSSION The g r e a t e s 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 between o t h e r w i s e s i m i l a r l a k e s i s o f t e n i n the r e l a t i v e abundance of s p e c i e s , e s p e c i a l l y the r a r e s p e c i e s . Many a t t e m p t s have been made t o c o r r e l a t e t h e s e p a t t e r n s w i t h b i o t i c (Anderson, 1970a,b; Anderson, 1974; Anderson and R a a s v e l d t , 1974; Pope and C a r t e r , 1975; N o r t h c o t e and C l a r o t t o , 1975; Boers and C a r t e r , 1978) or a b i o t i c ( S p r u l e s , - 1975; P a t a l a s , 1971) f e a t u r e s of the l a k e s under s t u d y , or t o e x t r a p o l a t e from n e c e s s a r i l y s m a l l s c a l e l a b o r a t o r y e x p e r i m e n t s t o s p a t i a l l y and t e m p o r a l l y complex p l a n k t o n communities (McQueen, 1969; Fedorenko, 1973; C o n f e r and C o o l e y , 1977). However, the c o m p l e x i t y of dynamic i n t e r a c t i o n s o f t e n confounds both approaches i n p r o v i d i n g e x p l a n a t i o n s f o r the causes of the o b s e r v e d p a t t e r n s . T h i s study used l a r g e s c a l e i_n s i t u e x p e r i m e n t a l e n c l o s u r e s t o examine the f a c t o r s l i m i t i n g the p o p u l a t i o n of T . p r a s i n u s i n the c o n t e x t of the s p a t i a l l y , t e m p o r a l l y complex community found i n P l a c i d Lake. T . p r a s i n u s i s r a r e i n P l a c i d Lake, i n c o n t r a s t t o i t s r e l a t i v e s u c c e s s i n nearby Gwendoline, a l a k e w i t h s i m i l a r c h e m i c a l c h a r a c t e r i s t i c s and z o o p l a n k t o n community c o m p o s i t i o n . 113 L a b o r a t o r y f e e d i n g e x p e r i m e n t s showed t h a t T . p r a s i n u s was a b l e t o i n g e s t and a s s i m i l a t e p h y t o p l a n k t o n e f f i c i e n t l y even as a d u l t s . There was no e v i d e n c e t h a t T . p r a s i n u s c a n n i b a l i z e d i t s own young or preyed on o t h e r l i v i n g c r u s t a c e a n z o o p l a n k t e r s . C o n s e q u e n t l y the predaceous i n s t a r s of C.b.thomasi c o u l d not compete w i t h T . p r a s i n u s . C o m p e t i t i o n between the two s p e c i e s c o u l d o n l y occur d u r i n g the e a r l y l i f e s t a g e s , n a u p l i a r or e a r l y c o pepodid i n s t a r . However, the main egg p u l s e and n a u p l i a r development o c c u r i n August-September i n T . p r a s i n u s , w h i l e i n C.b.thomasi t h e s e events o c c u r r e d i n May-June. I t h e r e f o r e c o n c l u d e t h a t C.b.thomasi i s not competing w i t h T . p r a s i n u s f o r s c a r c e food i t e m s . P r e d a t i o n e x p e r i m e n t s show t h a t , a t low prey d e n s i t i e s (1 per l i t r e ) and i n the presence of a l t e r n a t i v e pre.y, p r e d a t i o n r a t e s are low. These e x p e r i m e n t s i n d i c a t e d t h a t even though T . p r a s i n u s numbers were low i n P l a c i d Lake, the n a u p l i a r i n s t a r s were not immune t o p r e d a t i o n . C a l c u l a t i o n s u s i n g the l a b o r a t o r y e s t i m a t e d p r e d a t i o n r a t e and P l a c i d Lake p o p u l a t i o n d a t a f o r T . p r a s i n u s and C.b.thomasi i n d i c a t e d t h a t C.b.thomasi c o u l d i n f l i c t s u b s t a n t i a l m o r t a l i t y on T . p r a s i n u s , removing about 20-26% of the n a u p l i a r r e c r u i t s . L a b o r a t o r y e x p e r i m e n t s showed t h a t , f o r T . p r a s i n u s copepodids i n the presence of a l t e r n a t i v e p r e y , p r e d a t i o n became n e g l i g i b l e a t c o n c e n t r a t i o n s below 5 prey per l i t r e . T h i s prey d e n s i t y was never reached i n the l a k e and t h e r e f o r e p r e d a t i o n m o r t a l i t y was p r o b a b l y not i m p o r t a n t t o c o p e p o d i d i n s t a r s of T . p r a s i n u s . T . p r a s i n u s n a u p l i i p r o b a b l y r e p r e s e n t e d the ' b o t t l e n e c k ' most v u l n e r a b l e t o p r e d a t i o n by C.b.thomasi . 114 E x p e r i m e n t a l e n c l o s u r e s p l a c e d i n P l a c i d Lake examined the causes f o r the p o p u l a t i o n l i m i t a t i o n of T . p r a s i n u s and showed t h a t removal of T . p r a s i n u s by C.b.thomasi g r e a t l y enhanced r e c r u i t m e n t i n the T . p r a s i n u s p o p u l a t i o n . T h i s o c c u r r e d even i n the presence of non-predaceous z o o p l a n k t e r s - p o s s i b l e c o m p e t i t o r s f o r s m a l l food p a r t i c l e s . As the c l u t c h - s i z e and the number of o v i g e r o u s females was not 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 , the l a r g e r p o p u l a t i o n s of T . p r a s i n u s i n the e n c l o s u r e s w i t h o u t C.b.thomasi c o u l d not be due t o g r e a t e r f e c u n d i t y . S u r v i v o r s h i p c u r v e s were c o n s t r u c t e d t o e v a l u a t e the l o c a t i o n s of major m o r t a l i t i e s i n the l i f e h i s t o r y of T . p r a s i n u s . These c u r v e s showed t h a t the NIV t o CI i n t e r v a l was the most s e n s i t i v e t o m o r t a l i t y i n the presence of C.b.thomasi . Predaceous forms of C.b.thomasi' eat the n a u p l i i of c a l a n o i d copepods and c y c l o p o i d copepods w i t h no e v i d e n c e of s e l e c t i o n (McQueen, 1969; Smyly, pers.comm.; p e r s . o b s . ) . C o n s e q u e n t l y T . p r a s i n u s emerges from di a p a u s e t o f a c e a non-d i s c r i m i n a t i n g p r e d a t o r w i t h a d e v e l o p m e n t a l advantage due t o an e a r l i e r p o s i t i v e n u m e r i c a l response t o a l t e r n a t i v e food s o u r c e s . H a b i t a t h e t e r o g e n e i t y i n the l i m n e t i c r e g i o n of an o l i g o t r o p h i c l a k e i s p r o b a b l y m i n i m a l compared t o l i t t o r a l , b e n t h i c or t e r r e s t r i a l h a b i t a t . That which e x i s t s i s p r i n c i p a l l y the r e s u l t of c h e m i c a l g r a d i e n t s , temperature s t r a t i f i c a t i o n and l i g h t a t t e n u a t i o n ( f o r v i s u a l p r e d a t o r s ) . As p r e d a t o r y copepods hunt v i a mechanoreceptors ( K e r f o o t , 1978), the prey of such a p r e d a t o r cannot f i n d a r e f u g e by d i e l m i g r a t i o n i n t o .the dark b e n t h i c a r e a . By August the main body 115 of the C.b.thomasi p o p u l a t i o n i s found i n the c o o l e r deeper waters of the l a k e and the best s t r a t e g y f o r T . p r a s i n u s i s t o remain near the s u r f a c e . By r e m a i n i n g near the s u r f a c e throughout the e n t i r e day, some s e g r e g a t i o n from the main body of the C.b.thomasi p r e d a t o r p o p u l a t i o n i s p o s s i b l e . Without t h i s p a r t i a l s p a t i a l r e f u g e , a l l of the h i g h l y v u l n e r a b l e T . p r a s i n u s n a u p l i i r e c r u i t i n g d u r i n g August can t h e o r e t i c a l l y be eaten by C.b.thomasi . I t i s i n t e r e s t i n g t h a t the depth d i s t r i b u t i o n of T . p r a s i n u s changes i n the absence of C. b.thomasi such t h a t a g r e a t e r % of the p o p u l a t i o n i s found a t a deeper de p t h , a l t h o u g h t h i s i s c i r c u m s t a n t i a l , i t i s e v i d e n c e of an i n t e r a c t i o n . Thus the tendency t o remain near the s u r f a c e may be the e x p l a n a t i o n f o r the c o n t i n u e d p e r s i s t e n c e of T . p r a s i n u s i n P l a c i d Lake. The n u m e r i c a l response of the T . p r a s i n u s p o p u l a t i o n i s the same i n both t r e a t m e n t s w i t h o u t C.b.thomasi i n s p i t e of the f a c t t h a t o n l y the f i l t e r - f e e d i n g c l a d o c e r a n s C e r i o d a p h n i a and D. r o s e a a re p r e s e n t i n the T . p r a s i n u s 'alone' t r e a t m e n t . C o n s e q u e n t l y , D . o r e g o n e n s i s , p r e s e n t and abundant i n the o t h e r t r e a t m e n t w i t h o u t C.b.thomasi , cannot be a s i g n i f i c a n t m o r t a l i t y agent f o r T . p r a s i n u s a l t h o u g h Lane (1978) r e p o r t s t h a t t h i s s p e c i e s i s predaceous. As t h e r e a r e no o t h e r p o t e n t i a l l y i m p o r t a n t p r e d a t o r s , and no i n d i c a t i o n t h a t food l e v e l s a r e d i f f e r e n t among t r e a t m e n t s , food l i m i t a t i o n may be the cause of the h i g h NI t o N I I I m o r t a l i t y a c r o s s a l l t r e a t m e n t s . A l t h o u g h the stage a t which n a u p l i a r f e e d i n g b e g i n s i s not known, R i g l e r and Co o l e y (1974) r e p o r t t h a t food must be s u p p l i e d t o second i n s t a r c a l a n o i d n a u p l i i i f 116 s i g n i f i c a n t m o r t a l i t y i s t o be a v e r t e d . These a u t h o r s c o n c l u d e t h a t o n l y a few eggs c o n t a i n s u f f i c i e n t s t o r e d energy t o p e r m i t c o n t i n u e d growth t o N3 w i t h o u t e x t e r n a l l y p r o v i d e d f o o d . C o n s e q u e n t l y , i t appears t h a t T . p r a s i n u s e x p e r i e n c e s a combined b o t t l e n e c k of p r e d a t o r - i n d u c e d m o r t a l i t y and food l i m i t a t i o n d u r i n g i t s e a r l y l i f e s t a g e s . About 80% of the p o p u l a t i o n p e r i s h e s between the egg and c o p e p o d i d I s t a g e s w i t h o u t any p r e d a t o r i n the system. The i n v e r t e b r a t e p r e d a t o r C.b.thomasi f u r t h e r reduces the a l r e a d y low s u r v i v o r s h i p t h r o u g h p r e d a t i o n on the e a r l y n a u p l i a r i n s t a r s . C o n s e q u e n t l y , the t e m p o r a l s t r a t e g y of T . p r a s i n u s l e a v e s t h i s warm water form v u l n e r a b l e t o the predaceous s t a g e s of C.b.thomasi w h i l e the s p a t i a l s t r a t e g y s e p a r a t e s T . p r a s i n u s from the g r e a t e s t p r o p o r t i o n of the p r e d a t o r p o p u l a t i o n and p e r m i t s the p e r s i s t e n c e of T . p r a s i n u s , a t low d e n s i t i e s , i n t h i s o l i g o t r o p h i c l a k e . SUMMARY T . p r a s i n u s i s an omnivore but i s not c a n n i b a l i s t i c and does not prey on o t h e r m i c r o c r u s t a c e a n s . C.b.thomasi can remove about 3% of the T . p r a s i n u s n a u p l i i d a i l y d u r i n g the peak p e r i o d of n a u p l i a r p r o d u c t i o n or about 2.2% d a i l y a veraged over the e n t i r e i n t e r v a l when T . p r a s i n u s n a u p l i i a r e p r e s e n t i n the p l a n k t o n . P r e d a t i o n by C.b.thomasi on the T . p r a s i n u s p o p u l a t i o n i n P l a c i d Lake i s reduced by v e r t i c a l s e p a r a t i o n and low prey d e n s i t y . Removing C.b.thomasi from the e n c l o s u r e d r a m a t i c a l l y i n c r e a s e s the s t a n d i n g s t o c k of T . p r a s i n u s . S u r v i v o r s h i p c u r v e s suggest t h i s i n c r e a s e i s due t o improved s u r v i v o r s h i p d u r i n g the NIV t o CI . i n t e r v a l when C.b.thomasi i s removed. 118 I I I . RESPONSES OF CYCLOPS BICUSPIDATUS THOMASI TO ALTERATIONS IN THE FOOD AND PREDATOR ENVIRONMENT 119 INTRODUCTION C.b.thomasi i s a. l i m n e t i c c y c l o p o i d copepod, common throughout temperate N o r t h America ( C a r l , 1940; Rawson and Moore, 1944; Reed, 1964; P a t a l a s , 1971; P a t a l a s , 1972; Anderson, 1974). A l t h o u g h g e o g r a p h i c a l l y w i d e s p r e a d , C.b.thomasi g e n e r a l l y e x h i b i t s a n o n - c o n t a g i o u s d i s t r i b u t i o n . I t s p r e s e nce i n one l a k e does not p r e d i c t i t s d i s t r i b u t i o n or abundance i n nearby l a k e s , or even i n l a k e s w i t h i n the same d r a i n a g e system. T h i s p a t c h y d i s t r i b u t i o n on a r e g i o n a l b a s i s i s c h a r a c t e r i s t i c of most f r e s h w a t e r l i m n e t i c c r u s t a c e a n s p e c i e s and P a t a l a s (1971) c o n c l u d e s t h a t the c o m p o s i t i o n of the f r e s h w a t e r c r u s t a c e a n p l a n k t o n community w i t h i n a r e g i o n i s d e t e r m i n e d by l a k e morphology. However, Anderson (1974) r e p o r t s t h a t A c a n t h o c y c l o p s v e r n a l i s and C.b.thomasi are too w i d e s p r e a d t>o be c o r r e l a t e d w i t h p h y s i c a l / c h e m i c a l f a c t o r s and s u g g e s t s t h a t c o m p e t i t i o n and/or p r e d a t i o n may be more i m p o r t a n t i n e x p l a i n i n g the d i s t r i b u t i o n and abundance of t h e s e o r g a n i s m s . Zaret (1978) argues t h a t p r e d a t i o n i s the p r i n c i p a l o r g a n i z i n g f a c t o r of f r e s h w a t e r z o o p l a n k t o n communities and t h a t a b i o t i c f a c t o r s and food r e s o u r c e s a r e c o m p a r a t i v e l y i n s i g n i f i c a n t . A p u z z l i n g a s p e c t of c y c l o p o i d copepod d i s t r i b u t i o n i s the f a c t t h a t a s p e c i e s which i s abundant i n one l a k e i s o f t e n r a r e , a l t h o u g h p r e s e n t , i n a nearby a p p a r e n t l y s i m i l a r l a k e . In a s t u d y of 340 l a k e s and ponds, Anderson (1974) shows t h a t when C.b.thomasi and C y c l o p s v e r n a l i s occur t o g e t h e r , the l a t t e r i s abundant o n l y when the former i s r e l a t i v e l y low i n number. The dynamics of the i n t e r a c t i o n may be complex 120 because Anderson (1972) s u g g e s t s t h a t the presence of f i s h may favour C.b.thomasi over C y c l o p s v e r n a l i s i n t h e s e l a k e s . However, Smyly (1976a) r e p o r t s t h a t a r e d u c t i o n i n Chaoborus  f l a v i c a n s p r e d a t i o n i n h i s e x p e r i m e n t a l e n c l o s u r e s r e s u l t s i n g r e a t e r abundances of both C . b i c u s p i d a t u s and C . v e r n a l i s . T h i s paper examines the r e l a t i v e importance of these . v a r i o u s e x p l a n a t i o n s i n m a i n t a i n i n g a low abundance of C.b.thomasi i n an o l i g o t r o p h i c montane l a k e i n c o a s t a l B r i t i s h C olumbia. The 1976 e x p e r i m e n t s i n P l a c i d Lake i n d i c a t e d t h a t the presence of a l a r g e C.b.thomasi p o p u l a t i o n i n a f i s h l a k e ( P l a c i d Lake) was an i m p o r t a n t c a u s a l f a c t o r r e s t r i c t i n g the success of the c y c l o p o i d copepod T . p r a s i n u s . The r e l a t i v e l y g r e a t e r abundance of T . p r a s i n u s i n nearby Gwendoline Lake (a l a k e w i t h o u t f i s h a t the time of the s t u d y ) seemed an improbable f a c t o r t o be i n v o l v e d i n ' l i m i t i n g C.b.thomasi because the' l i f e h i s t o r i e s of the two c y c l o p o i d s p e c i e s were s i m i l a r between P l a c i d Lake and Gwendoline Lake. In the former l a k e , C.b.thomasi c l e a r l y had the advantage i n the C.b.thomasi- T . p r a s i n u s i n t e r a c t i o n and t h e r e was no e v i d e n c e t h a t f i s h or Chaoborus f l a v i c a n s were i n v o l v e d i n m a i n t a i n i n g the g r e a t e r abundance of C.b.thomasi. C.b.thomasi was found i n Gwendoline Lake a t c o u n t a b l e d e n s i t i e s , even though the numbers were low, thus s u g g e s t i n g t h a t d i s p e r s a l was not a problem. Gwendoline Lake i s deeper than P l a c i d (mean of 13.4 m. compared t o mean of 4.3 m. i n P l a c i d Lake) but s e v e r a l a u t h o r s ( P a t a l a s , 1971; S p r u l e s , 1977) a s s o c i a t e C.b.thomasi w i t h deeper c o l d e r l a k e s . Other a u t h o r s , however, have found C.b.thomasi dominant under a wide range of c o n d i t i o n s ( C a r l , 121 1940; W h i t t a k e r and F a i r b a n k s , 1958; Anderson, 1974). In the G reat L a k e s , P a t a l a s (1972) found an i n c r e a s i n g abundance of C.b.thomasi a s s o c i a t e d w i t h i n c r e a s i n g n u t r i e n t s . P a t a l a s showed t h a t C . v e r n a l i s i s abundant o n l y where C.b.thomasi i s not but he a t t r i b u t e s t h i s r e s u l t t o depth and temperature p r e f e r e n c e s ( C.b.thomasi p r e f e r r i n g g r e a t e r depth and c o l d e r t e m p e r a t u r e ) . In P l a c i d Lake the C.b.thomasi p o p u l a t i o n e x p e r i e n c e s g r e a t e s t m o r t a l i t y i n the n a u p l i a r s t a g e s . McQueen (1969) e s t i m a t e d t h a t 25-30% of the j u v e n i l e s c o u l d p e r i s h t h r o u g h c a n n i b a l i s m i n nearby M a r i o n Lake. Other s t u d i e s (Smyly, 1961; Anderson, 1970b) have suggested t h a t c a n n i b a l i s m e l i m i n a t e s the n a u p l i a r and e a r l y c o p e p o d i d i n s t a r s which d e v e l o p a f t e r the main p u l s e of r e c r u i t s , r e s u l t i n g i n a narrow d i s t r i b u t i o n of i n s t a r s t a g e s and body s i z e s . w i t h i n a g i v e n lake-. T h i s c a n n i b a l i s t i c b e h a v i o u r c o u l d have a s e l f -dampening e f f e c t on p o p u l a t i o n s i z e . Lane (1979) s u g g e s t s t h a t t h i s s p e c i e s can e f f e c t i v e l y r e g u l a t e i t s own p o p u l a t i o n t h r o u g h i n t r a s p e c i f i c p r e d a t i o n which becomes p a r t i c u l a r l y i n t e n s e a t h i g h d e n s i t i e s . However, i t seems p o s s i b l e t h a t a t low d e n s i t i e s the c a n n i b a l i s t i c h a b i t of C.b.thomasi may e x a c e r b a t e i n t r a s p e c i f i c m o r t a l i t y , a l r e a d y h i g h i n the e a r l y d e v e l o p m e n t a l s t a g e s , such t h a t t h i s s p e c i e s r e q u i r e s a g r e a t e r food r e s o u r c e base t o p r o s p e r n u m e r i c a l l y . Gwendoline Lake i s , on a v e r a g e , s l i g h t l y lower i n o r g a n i c carbon than P l a c i d Lake ( W a l t e r s , unpub.data). Even s m a l l d i f f e r e n c e s i n the a v a i l a b l e food may be c r i t i c a l l y i m p o r t a n t t o the e a r l y l i f e ' s t a g e s i n such d i l u t e environments and t h e r e i s e v i d e n c e t h a t 122 some copepods r e q u i r e food e a r l y i n t h e i r development. For example, i n l a b o r a t o r y e x p e r i m e n t s w i t h a c a l a n o i d copepod, R i g l e r and Cooley (1974) found t h a t the d u r a t i o n of the second n a u p l i a r i n s t a r was v a r i a b l e and m o r t a l i t y was h i g h u n l e s s some a l g a l food was p r o v i d e d . These a u t h o r s c o n c l u d e d t h a t o n l y a few eggs have enough s t o r e d food t o c a r r y them t h r o u g h t o the t h i r d n a u p l i a r i n s t a r . I f n u t r i t i o n a l d i f f i c u l t i e s a r e en c o u n t e r e d by C.b.thomasi n a u p l i i i n a d d i t i o n t o i n t r a s p e c i f i c p r e d a t i o n p r e s s u r e , j u v e n i l e m o r t a l i t y may be so h i g h t h a t the p o p u l a t i o n cannot i n c r e a s e i n number even though the few s u r v i v i n g i n d i v i d u a l s may do v e r y w e l l . One major d i f f e r e n c e between the two study l a k e s i s t h a t the Chaoborus s p e c i e s found i n Gwendoline Lake a r e p l a n k t o n i c a l l day and some i n d i v i d u a l s a re p r e s e n t throughout the y e a r . In P l a c i d Lake the Chaoborus s p e c i e s i s C . f l a v i c a n s , a s p e c i e s which i s b e n t h i c by day and p r e s e n t i n the p l a n k t o n o n l y at n i g h t . T h i s d i f f e r e n c e may s u b s t a n t i a l l y i n c r e a s e the p r e d a t i o n p r e s s u r e on the C.b.thomasi p o p u l a t i o n i n Gwendoline Lake and r e s u l t i n a low abundance of C.b.thomasi. However, Anderson and R a a s v e l d t (1974), i n a survey of 50 l a k e s and ponds i n A l b e r t a and B r i t i s h Columbia, found t h a t mean c y c l o p o i d copepod d e n s i t i e s were h i g h e r i n communities which i n c l u d e d a Chaoborus p o p u l a t i o n , a l t h o u g h they d i d not examine the mechanisms i n v o l v e d i n p r o d u c i n g the c o r r e l a t i o n . D i f f e r e n t s t a g e s i n the l i f e h i s t o r y of C.b.thomasi may be d i f f e r e n t i a l l y s u s c e p t a b l e t o v a r i a t i o n s i n c o m p e t i t i o n , p r e d a t i o n or changes i n the p h y s i c a l / c h e m i c a l environment. A l l t h e s e f a c t o r s can p o t e n t i a l l y p l a y a p a r t i n d e t e r m i n i n g 123 the u l t i m a t e s u c c e s s of C.b.thomasi i n Gwendoline Lake. S p e c i f i c hypotheses which emerge from the s e v e r a l a l t e r n a t i v e e x p l a n a t i o n s a r e the f o l l o w i n g : 1) the p h y s i c a l or c h e m i c a l environment i n Gwendoline Lake i s u n f a v o u r a b l e t o C.b.thomasi 2) food r e s o u r c e s i n Gwendoline Lake are t o o low t o support C.b.thomasi whose c a n n i b a l i s t i c b e h a v i o u r i s u n s u i t e d t o an e x t r e m e l y o l i g o t r o p h i c environment 3) i n v e r t e b r a t e p r e d a t i o n by Chaoborus l i m i t s the C.b.thomasi p o p u l a t i o n i n Gwendoline Lake 4) p r e d a t i o n by Chaoborus p l u s the low food l e v e l s l i m i t the C.b.thomasi p o p u l a t i o n but the e f f e c t of p r e d a t i o n can be overcome by i n c r e a s i n g food r e s o u r c e s , p a r t i c u l a r l y t o the n a u p l i a r s t a g e s . These h y p o t h e s i s were t e s t e d e x p e r i m e n t a l l y t o e x p l a i n the way i n which p r e d a t i o n , c o m p e t i t i o n and p h y s i c a l / c h e m i c a l f a c t o r s i n t e r a c t t o determine the d i s t r i b u t i o n and abundance of the c y c l o p o i d copepod, C.b.thomasi. 124 MATERIALS AND METHODS F i e l d E x p e r i m e n t s To e v a l u a t e how c o m p e t i t i o n and p r e d a t i o n i n t e r f a c e w i t h n u t r i e n t l e v e l s i n Gwendoline Lake, l a r g e _in s i t u e x p e r i m e n t s were conducted from mid-May u n t i l l a t e September. These e n c l o s u r e s were i d e n t i c a l t o those used i n P l a c i d Lake the year b e f o r e . T h e r e f o r e o n l y the water and the z o o p l a n k t o n community d i f f e r e d from the p r e v i o u s e x p e r i m e n t s i n P l a c i d Lake where the C.b.thomasi p o p u l a t i o n was abundant i n s i d e the e n c l o s u r e s . The bags were f i l l e d i n the same way by pumping l a k e water through a 54 pm mesh p l a n k t o n net t h a t removed a l l m a c r o z o o p l a n k t e r s but p e r m i t t e d g r a z a b l e s e s t o n t o pass t h r o u g h . C.b.thomasi, c o l l e c t e d i n P l a c i d Lake and s e p a r a t e d from the o t h e r c r u s t a c e a n s by a s e r i e s of s i e v e s , was added t o the C y c l o p s - s u p p l e m e n t e d e n c l o s u r e s i n Gwendoline Lake a t P l a c i d Lake d e n s i t i e s . N a t u r a l l a k e d e n s i t i e s of Gwendoline Lake c r u s t a c e a n macrozooplankton were added t o a l l e n c l o s u r e s from p o o l e d z o o p l a n k t o n samples S i x e x p e r i m e n t a l d i s t u r b a n c e s were p r o d u c e d : t h r e e d i f f e r e n t communities and two n u t r i e n t l e v e l s . The t h r e e d i f f e r e n t communities were the f o l l o w i n g : one i n c l u d e d Chaoborus, Gwendoline Lake c r u s t a c e a n s and P l a c i d Lake d e n s i t i e s of C.b.thomasi; one c o n t a i n e d o n l y Gwendoline Lake c r u s t a c e a n s ; and one c o n t a i n e d Gwendoline Lake c r u s t a c e a n s p l u s P l a c i d Lake d e n s i t i e s of C.b.thomasi but e x c l u d e d 125 Chaoborus. P l a s t i c s c r e e n i n g was used t o p r e v e n t e g g - l a y i n g by a d u l t Chaoborus i n the f o u r C h a o b o r u s - f r e e e n c l o s u r e s . Chaoborus a d u l t s were a l l o w e d t o l a y eggs i n the r e m a i n i n g two e n c l o s u r e s . One e n c l o s u r e of each community 'type' r e c e i v e d a 'high ' f e r t i l i z e r t r e a t m e n t w h i l e one r e c e i v e d no f e r t i l i z e r . To produce t h e 'h i g h ' f e r t i l i z e r c o n d i t i o n , a p h o s p h a t e - n i t r a t e f e r t i l i z e r (atomic r a t i o 1:10) was used (NaH^PO^and KN0 3) a t a phosphate c o n c e n t r a t i o n of 500 ,ug 1" 1 4PO^ . N u t r i e n t s were added o n l y once i n mid-May, a l t h o u g h p a r t i a l m i x i n g of the water w i t h i n the e n c l o s u r e s was c a r r i e d out v i a o c c a s i o n a l b u b b l i n g d u r i n g June, J u l y and August. As h i g h m o r t a l i t y o c c u r r e d d u r i n g the n a u p l i a r i n s t a r s , the f e r t i l i z e r a d d i t i o n was i n t e n d e d t o enhance a l g a l d e n s i t i e s d u r i n g n a u p l i a r p r o d u c t i o n i n May and June. The h i g h n u t r i e n t c o n c e n t r a t i o n was not i n t e n d e d t h e r e f o r e t o mimic any n a t u r a l l y o c c u r r i n g c o n d i t i o n but r a t h e r t o p r o v i d e s u b s t a n t i a l l y i n c r e a s e d food abundance t o the h e r b i v o r o u s s t a g e s of C.b.thomasi. Zooplankton sam p l i n g was done every 4-7 days i n thes e e n c l o s u r e s and i n the two l a k e s ( P l a c i d and Gwendoline) w i t h an e l e c t r i c b i l g e pump. Each 100 l i t r e sample was f i l t e r e d t h r o u g h a 54 um p l a n k t o n net and p r e s e r v e d i n s u g a r - f o r m a l i n For most s p e c i e s , samples were examined _in t o t o under 25X m a g n i f i c a t i o n f o r s p e c i e s ' abundances and r e p r o d u c t i v e c o n d i t i o n . I f v i s u a l e x a m i n a t i o n suggested t h a t a s p e c i e s ' abundance exceeded 500 i n d i v i d u a l s , then the number of a n i m a l s i n the sample was e s t i m a t e d by c o u n t i n g a l l the a n i m a l s i n 1/10 to 1/5 of the sample. Temperature and oxygen p r o f i l e s were o c c a s i o n a l l y m o n i t o r e d w i t h a Y e l l o w S p r i n g s I n s t r u m e n t s 126 temperature/oxygen metre. B i w e e k l y water samples were c o l l e c t e d f o r d e t e r m i n a t i o n of the a s h - f r e e d r y we i g h t s of p a r t i c l e s p a s s i n g t h r o u g h a 30 um mesh as p r e v i o u s l y d e s c r i b e d . T h i s assessment of ' g r a z a b l e s e s t o n ' was combined w i t h m i c r o s c o p i c e x a m i n a t i o n of a l g a l s i z e c o m p o s i t i o n and c a t e g o r y ( c o l o n i a l , f i l a m e n t o u s , b l u e - g r e e n , e t c . ) . Water samples f i x e d i n ' L u g o l ' s ' s o l u t i o n f o r p h y t o p l a n k t o n enumeration, had t o be c o n c e n t r a t e d f o r c o u n t i n g . A subsample (100 ml of about 300 ml) was taken and a l l o w e d t o s e t t l e f o r 24 hou r s . S e v e n t y - f i v e ml of the s u p e r n a t a n t was then s i p h o n e d o f f and the r e m a i n i n g 25 ml a l l o w e d t o s e t t l e f o r a f u r t h e r 20 hours p r i o r t o c o u n t i n g . U s i n g an Utermohl's ' i n v e r t e d ' m i c r o s c o p e , a minimum of 200 i n d i v i d u a l s ( at l e a s t 100 of n u m e r i c a l l y i m p o r t a n t c e l l s ) was counted under 400X m a g n i f i c a t i o n . Measurements r e q u i r e d f o r c o n v e r s i o n - t o c e l l volume were taken f o r the most common t y p e s i n each s i z e and c a t e g o r y . Where p o s s i b l e , p h y t o p l a n k t o n was i d e n t i f i e d t o s p e c i e s . Length measurements of female c y c l o p o i d copepods were made u s i n g an o c u l a r micrometer i n a d i s s e c t i n g scope, a t 50X m a g n i f i c a t i o n . Metasomal l e n g t h was measured t o a v o i d i n a c c u r a c i e s caused by f o r m a l i n induced c o n t r a c t i o n or ex p a n s i o n of the body segments (Smyly, 1976b). 127 L a b o r a t o r y P r e d a t i o n T e s t s Crop c o n t e n t s suggest Chaoborus t r i v i t t a t u s i n s t a r I feeds p r i m a r i l y on n a u p l i i and r o t i f e r s ( N e i l l and Peacock, 1980). As C.b.thomasi n a u p l i i a re abundant i n P l a c i d Lake d u r i n g peak abundances of Chaoborus i n s t a r I i n Gwendoline Lake, these l a r v a e were o f f e r e d C.b.thomasi n a u p l i i from P l a c i d Lake as prey i n the presence of 4 a l t e r n a t i v e p o s s i b l e prey from Gwendoline Lake. A l l a n i m a l s used were* f r e s h l y c o l l e c t e d . The e x p e r i m e n t s were conducted at Gwendoline Lake mid-June temperature a t about 3 m. (15±2°C) i n seven 2 l i t r e b eakers and l a s t e d 24 hours under a l i g h t r d a r k regime of 16 hours l i g h t and 8 hours dark. P r e d a t o r and prey were added t o s i e v e d Gwendoline Lake water a t normal l a k e d e n s i t i e s w i t h the e x c e p t i o n of C.b.thomasi n a u p l i i , which were added a t P l a c i d Lake d e n s i t i e s . The number of prey k i l l e d - per ' p r e d a t o r per day was deduced by comparing the number of l i v i n g and dead prey r e t r i e v e d i n the Chaoborus beakers w i t h those o b s e r v e d i n 2 c o n t r o l beakers a t the end of the 24 hour p e r i o d . As t h e r e was no m o r t a l i t y i n the c o n t r o l b e a k e r s , the dead prey were i n c l u d e d i n the number of prey k i l l e d per p r e d a t o r per day. 128 RESULTS E f f e c t of I n t r o d u c t i o n on the C.b.thomasi P o p u l a t i o n i n the Absence of Chaoborus As C.b.thomasi i s r a r e i n Gwendoline Lake, a d u l t s were i n t r o d u c e d from P l a c i d Lake t o f o u r e x p e r i m e n t a l e n c l o s u r e s , a t P l a c i d Lake d e n s i t i e s , t o a s s e s s t h i s s p e c i e s ' a b i l i t y t o c o e x i s t w i t h Gwendoline Lake z o o p l a n k t e r s i n Gwendoline Lake water. I e l i m i n a t e d the o n l y p r e d a t o r s s u f f i c i e n t l y abundant t o have an impact on C.b.thomasi, the phantom midge f l y l a r v a e Chaoborus, from two of t h e s e e n c l o s u r e s . In the absence of Chaoborus, C.b.thomasi had no d i f f i c u l t y s u r v i v i n g and r e c r u i t i n g i n both ' the u n f e r t i l i z e d and the f e r t i l i z e d 'Gwendoline Lake' t r e a t m e n t ( F i g . 3 5 a - c ) . Not o n l y does the C.b.thomasi p o p u l a t i o n s u r v i v e and r e c r u i t but v e r t i c a l d i s t r i b u t i o n a l s o f o l l o w s a s i m i l a r p a t t e r n i n the Gwendoline Lake e n c l o s u r e t o t h a t o b s e r v e d i n P l a c i d Lake ( F i g . 3 6 ) . As i n P l a c i d Lake, a d u l t s a r e found throughout the water column, c o p e p o d i d i n s t a r s peak a t a p p r o x i m a t e l y 2.5 m. and the m a j o r i t y of the n a u p l i i a re found below 2.5 m. T h e r e f o r e i n t r o d u c t i o n t o the a l i e n Gwendoline Lake environment does not a f f e c t the g e n e r a l p a t t e r n of development or s p a c i n g b e h a v i o u r of the i n t r o d u c e d C.b.thomasi p o p u l a t i o n . The c o n t r i b u t i o n of each i n s t a r t o the t o t a l p o p u l a t i o n w i t h i n the Gwendoline Lake e n c l o s u r e s was s i m i l a r t o the per ce n t c o m p o s i t i o n o b s e r v e d i n P l a c i d Lake i n June ( F i g . 3 7 ) . 129 F i g u r e 35. Changes i n s t a n d i n g c r o p of C.b.thomasi i n P l a c Lake and i n the f e r t i l i z e d and u n f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e s i n Gwendoline Lake from May t o September, 1977. /do 131 F i g u r e 36. Depth d i s t r i b u t i o n of C.b.thomasi f o r C.b.thomasi n a u p l i i , copepodids and a d u l t s i n P l a c i d Lake and i n the u n f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t i n Gwendoline Lake on June 7, 1977. 133 F i g u r e 37. % c o m p o s i t i o n of C.b.thomasi i n s t a r s i n the f e r t i l i z e d and u n f e r t i l i z e d 'Chaoborus-Cyclops' t r e a t m e n t , the f e r t i l i z e d and u n f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t (no Chaoborus i n t h i s t r e a t m e n t ) , and i n P l a c i d Lake i n 1977. /5V-% C o m p o s i t i o n o f C . b . t h o m a s i i n s t a r s f e r t i l i z e d Chaoborus -C y c l o p s J u n e 6 N 1-3 N 4 - 6 CI e n P cm civ c v adult J u l y 2 7 NI -3 N 4 - 6 c, • C l l cm civ cv adult u n f e r t i l i z e d C h a o b o r u s -C y c l o p s D i i • u f e r t i l i z e d u n f e r t i l i z e d P l a c i d C y c l o p s C y c l o p s L a k e • • Z] Zl Zl z z I 1 20% 135 T h i s s i m i l a r i t y was m a i n t a i n e d o n l y i n the u n f e r t i l i z e d e n c l o s u r e w i t h o u t Chaoborus by the end of J u l y . As the Gwendoline Lake e n c l o s u r e s were i d e n t i c a l i n s i z e and d e p t h , and t emperature d i d not d i f f e r between the e n c l o s u r e s , the d i f f e r e n t age s t r u c t u r e o b s e r v e d i n the f e r t i l i z e d Chaoborus-f r e e e n c l o s u r e was p r o b a b l y not r e l a t e d t o a b i o t i c f a c t o r s but t o the absence of Chaoborus. S u r v i v o r s h i p c u r v e s were c a l c u l a t e d u s i n g the method of Gehrs and Robertson (1975) t o p e r m i t a c o m p a r a t i v e assessment of m o r t a l i t y p a t t e r n s between the C h a o b o r u s- f r e e e n c l o s u r e p o p u l a t i o n s and t h a t i n P l a c i d Lake. I n s t a r d u r a t i o n s were d e r i v e d from the spring/summer g e n e r a t i o n by u s i n g the i n t e r v a l between the f i r s t appearance of the f i r s t i n d i v i d u a l of i n s t a r n+1 and i n s t a r n (Comita, 1972). The d i f f e r e n c e a p p r o x i m a t e s the d u r a t i o n of i n s t a r n i n . a p o p u l a t i o n - where o v e r l a p of s u c c e s s i v e g e n e r a t i o n s i s m i n i m a l . I n s t a r d u r a t i o n s ( a p p l i c a b l e t o the spring/summer g e n e r a t i o n ) used i n d e v e l o p i n g t h e s e l i f e t a b l e s are g i v e n i n Appendix C. The main body of the C.b.thomasi p o p u l a t i o n o v e r w i n t e r s i n c o pepodid IV and V s t a g e s . There are few a d u l t s i n the f a l l p l a n k t o n when compared w i t h the s p r i n g d e n s i t i e s , s u g g e s t i n g t h a t o n l y a s m a l l p r o p o r t i o n of the 5 t h c o p e p o d i d i n s t a r s moult t o the a d u l t stage b e f o r e the s p r i n g . C o n s e q u e n t l y , a p parent m o r t a l i t y shown by the s u r v i v o r s h i p c u r v e s i n the c o p e p o d i d i n s t a r s IV and V may be u n r e a l i s t i c a l l y h i g h based on f a l l p o p u l a t i o n s i z e s . However, s u r v i v o r s h i p c u r v e s are p r o b a b l y r e p r e s e n t a t i v e of r e a l m o r t a l i t y p a t t e r n s u n t i l the 4 t h c o p epodid i n t e r v a l . The r e s u l t i n g s u r v i v o r s h i p c u r v e s a r e 136 both p o s i t i v e l y skewed, s i g n i f y i n g h i g h e a r l y m o r t a l i t y f o r immature i n s t a r s ( F i g . 3 8 ) . G r e a t e s t m o r t a l i t y o c c u r s i n the egg t o copepodid I i n t e r v a l i n both p o p u l a t i o n s . N a u p l i a r s t a g e s a r e c l e a r l y the most v u l n e r a b l e p e r i o d ; s u r v i v o r s h i p i n c r e a s e s i n the c o p e p o d i d s t a g e s . B u r g i s (1971) a l s o r e p o r t s h i g h m o r t a l i t y i n the n a u p l i a r i n s t a r s of a t r o p i c a l c y c l o p o i d copepod s p e c i e s . S u b s t i t u t i n g the Gwendoline Lake community f o r the P l a c i d Lake community does not a l t e r the p a t t e r n of n a u p l i a r m o r t a l i t y . T h i s r e s u l t s u g g e s t s t h a t the heavy n a u p l i a r m o r t a l i t y o b s e r v e d i n P l a c i d Lake i s the r e s u l t of d i f f u s e or i n t r a s p e c i f i c c o m p e t i t i o n f o r s c a r c e food r e s o u r c e s and/or i n t r a s p e c i f i c p r e d a t i o n r a t h e r than an i n t e r a c t i o n w i t h an abundant p o p u l a t i o n of Diaptomus o r e g o n e n s i s , the o n l y member of the P l a c i d Lake c r u s t a c e a n p l a n k t o n community which i s not found i n Gwendoline Lake. In any c a s e , d i f f e r e n t i a l m o r t a l i t y i n C h a o b o r u s - f r e e Gwendoline Lake environment i s not the cause of the low abundance of C.b.thomasi i n t h i s l a k e . 137 F i g u r e 38. C.b.thomasi s u r v i v o r s h i p by i n s t a r i n the e x p e r i m e n t a l e n c l o s u r e s w i t h o u t Chaoborus l a r v a e i n Gwendoline Lake and i n P l a c i d Lake f o r the s p r i n g g e n e r a t i o n of C.b.thomasi i n 1977. I n t e r v a l on the x-a x i s i s s c a l e d to r e p r e s e n t the r e l a t i v e d u r a t o n of each i n s t a r i n t e r v a l shown. /3f E g g 3 6 I II III IV V N C 139 E f f e c t of F e r t i l i z a t i o n i n the Absence of Chaoborus In the absence of Chaoborus, adding f e r t i l i z e r t o the Gwendoline Lake e n c l o s u r e d r a m a t i c a l l y i n c r e a s e d s u r v i v o r s h i p of C.b.thomasi ( F i g . 3 8 , F i g . 3 9 a , b ) . As o b s e r v e d i n P l a c i d Lake and the u n f e r t i l i z e d t r e a t m e n t , m o r t a l i t y r a t e s were h i g h e s t i n the egg t o N3 i n t e r v a l a l t h o u g h t h e s e r a t e s were lower than i n P l a c i d Lake or i n the u n f e r t i l i z e d t r e a t m e n t . However, the f e r t i l i z e d e n c l o s u r e e x p e r i e n c e d r e l a t i v e l y l i t t l e m o r t a l i t y i n the N4-6 t o CI i n t e r v a l compared t o t h a t o b s e r v e d i n the u n f e r t i l i z e d bag. In the l a t t e r environment, a p p r o x i m a t e l y 76% of the eggs produced f a i l e d t o d e v e l o p t o the CI stage w h i l e i n the f e r t i l i z e d environment o n l y 32% p e r i s h e d by i n s t a r C I . C.b.thomasi, known t o be c a r n i v o r o u s and c a n n i b a l i s t i c i n the l a t e r c o p epodid and a d u l t i n s t a r s (McQueen, 1969; Anderson, 1970; Lane, 1976, 78, 7 9 ) , was the o n l y p r e d a t o r i n t h e s e e n c l o s u r e s . However, l o s s e s t o i n t r a s p e c i f i c p r e d a t i o n s h o u l d have been lower i n the u n f e r t i l i z e d e n c l o s u r e s as a d u l t s d e c l i n e d f a s t e r and d i s a p p e a r e d by mid-June (as i n P l a c i d Lake) whereas a d u l t s were c o n t i n o u s l y p r e s e n t i n c o u n t a b l e numbers throughout the season i n the f e r t i l i z e d t r e a t m e n t ( F i g . 3 9 a , b ) . The h i g h m o r t a l i t y a s s o c i a t e d w i t h the n a u p l i a r i n s t a r s might be the r e s u l t of inadequate n u t r i t i o n . As n a u p l i i a r e h e r b i v o r o u s ( F r y e r , 1957b, Smyly, 1970), I compared p a r t i c u l a t e o r g a n i c m a t t e r , p h y t o p l a n k t o n volume biomass and. type c o m p o s i t i o n between the f e r t i l i z e d and u n f e r t i l i z e d e n c l o s u r e s t o a s s e s s a v a i l a b l e food r e s o u r c e s . P h y t o p l a n k t o n c e l l c o u n t s were c o n v e r t e d t o c e l l volume by assuming the c e l l form t o 140 F i g u r e 39. Changes i n the s t a n d i n g c r o p of the i n t r o d u c e d C.b.thomasi ( n a u p l i i , c o p e p o d i d s , and a d u l t s ) f o r the f o u r Gwendoline Lake e n c l o s u r e s i n 1977 from May t o September. C i r c l e s r e p r e s e n t n a u p l i i , squares r e p r e s e n t copepodids and t r i a n g l e s i n d i c a t e a d u l t s . N U M B E R PER L I T R E 142 c o r r e s p o n d t o s i m p l e , g e o m e t r i c a l s o l i d s ( s p h e r e , cone or c y l i n d e r ) (Findenegg, 1969). P h y t o p l a n k t o n were a s s i g n e d t o the f o l l o w i n g groups based on e x t e n s i v e e v i d e n c e of r e l a t i v e e d i b i l i t y (e.g. Burns, 1968; A r n o l d , 1971; S c h i n d l e r , 1971; P o r t e r , 1973; G l i w i c z , 1975; N a d i n - H u r l e y and Duncan, 1976; G l i w i c z , 1977; P o r t e r , 1977) : <2um ( s m a l l , c o c c o i d c e l l s , m a i n l y b a c t e r i a ) , 2-20 um (nannoplankton as d e f i n e d by G e l i n and R i p l ( 1 9 7 8 ) ) and >20 um ( l a r g e , s o l i t a r y c e l l s , m a i n l y d i a t o m s ) , c o l o n i a l and f i l a m e n t o u s . McCauley and B r i a n d (1979) t r e a t e d a l l s p e c i e s l a r g e r than 50 um and a l l b l u e - g r e e n a l g a e , i r r e s p e c t i v e of s i z e , as i n e d i b l e . A l t h o u g h t h i s d i s t i n c t i o n might be v a l i d f o r f i I t e r - f e e d e r s , such a d e f i n i t i o n seemed unwarranted f o r r a p t o r i a l f e e d e r s . My c r i t e r i a i n c l u d e d s i z e , shape and ' q u a l i t y ' based on the above l i t e r a t u r e . . . C o n s e q u e n t l y , I d i s t i n q u i s h f i l a m e n t o u s a l g a e as a group because most s p e c i e s o b s e r v e d were b l u e - g r e e n , c o n s i d e r e d a poor n u t r i t i v e source by some a u t h o r s ( e . g . A r n o l d , 1971), but not c o l o n i e s , most of which were green a l g a e . C o l o n i e s were r e l a t i v e l y uncommon and t h e r e f o r e I combined t h i s group w i t h s o l i t a r y >20 um c e l l s ( p r i m a r i l y ' e d i b l e ' diatoms) because a l l of thes e s p e c i e s were p r o b a b l y a v a i l a b l e t o r a p t o r i a l f e e d e r s and many a v a i l a b l e t o f i l t e r -f e e d e r s ( P o r t e r , 1 9 7 7 ) . B a c t e r i a formed a group because, b e i n g <2 um, i n d i v i d u a l s c e l l s p r o b a b l y p r e s e n t e d h a n d l i n g problems; nannoplankton were grouped t o g e t h e r because a l l thes e s m a l l green a l g a l c e l l s were p r o b a b l y d e s i r a b l e food i t e m s . T a b l e 9 l i s t s t he most common groups e n c o u n t e r e d and t h e i r e s t i m a t e d c e l l volume. T a b l e 1 sorce p h y s i c a l , c h e m i c a l a n d b i o l o g i c a l c h a r a c t e r i s t i c s o f P l a c i d a n d G w e n d o l i n e L a k e . * C H A R A C T E R I S T I C S P l a c i d G w e n d o l i n e 510 522 an 81 1.6 13. 7 27 F l e v a t i o n , m. D r a i n a g a e a r e a , h a . S u r f a c e a r e a , h a . K a x i i u i d e p t h , a. i t. < C o l o u r , P+ u n i t s 2 0 - 2 5 15 T r a n s p a r e n c y ( S e c c h i d e p t h , m.) 'i-6 5-7 PH . 6.6 6.6 T o t a l C a r b o n (mg 1-1) 5.0 3.0 T o t a l P h o s p h o r u s f n g 1-1) 0.006 0.003 T o t a l O r g a n i c N i t r o g e n (eg 1-1) 0.17 0.05 T o t a l D i s s o l v e d S o l i d s {.tig 1-1) 17-23 18 CRUSTACEAN ZOOPLANKTON S P E C I E S + r>i a r j^onius *e_naJL Dia£tomus o r ^ g o n e n s i s fi3£]}Qi3 rosea Hibherun Hl2Ell3.Ii osoffia brachvurum J3i22irilli I c ^ n i r o s t r i s Xiod a nhji i a S i j a d r a n o u l a vcj.ons p r a s i n us * a d a p t e d f r o n N e i l l ( 1 9 7 8 ) «• Key t o s y m b o l s : A •= a b u n d a n t R = r a r e a b s e n t <J0 144 P h y t o p l a n k t o n s t a n d i n g c r o p , as measured by c e l l volume d i d not d i f f e r between the h i g h and low n u t r i e n t e n c l o s u r e s u n t i l the second week i n June ( F i g . 4 0 a - g ) . At t h i s p o i n t the s t a n d i n g c r o p i n the f e r t i l i z e d e n c l o s u r e s i n c r e a s e d r e l a t i v e t o the low n u t r i e n t e n c l o s u r e s and remained h i g h e r throughout the summer months. The i n c r e a s e i n volume biomass was accompanied by a s u b s t a n t i a l s h i f t i n a l g a l c o m p o s i t i o n from a predominance of nannoplankton t o net p l a n k t o n (here i n c l u d i n g >20 um s o l i t a r y c e l l s , c o l o n i a l and/or f i l a m e n t o u s t y p e s ) ( F i g . 4 0 a - g ) . By June 2 4 t h , the b l u e - g r e e n a l g a e O s c i l l a t o r i a and Lyngbya were the most common forms. T h i s change i n p h y t o p l a n k t o n c o m p o s i t i o n was not accompanied by i n c r e a s e d m o r t a l i t y r a t e s i n the C.b.thomasi p o p u l a t i o n . When the a l g a l s h i f t o c c u r r e d , the m a j o r i t y of the C.b.thomasi p o p u l a t i o n was i n the N4-6 t o CI s t a g e s . S u r v i v o r s h i p i n ' the f e r t i l i z e d e n c l o s u r e d u r i n g t h i s i n t e r v a l was h i g h ( F i g . 3 8 ) . Many a u t h o r s ( A r n o l d , 1971; P o r t e r , 1973; N a d i n - H u r l e y and Duncan, 1976) c o n s i d e r b l u e -green a l g a e a poor q u a l i t y food f o r f i l t e r - f e e d i n g c l a d o c e r a n s . The n a u p l i a r i n s t a r s of copepods have g e n e r a l l y been i g n o r e d i n z o o p l a n k t o n f e e d i n g s t u d i e s and i t i s p o s s i b l e t h a t the r a p t o r i a l mode of f e e d i n g common t o c y c l o p o i d copepods o b v i a t e s the h a n d l i n g problems e x p e r i e n c e d by f i l t e r - f e e d e r s w i t h l a r g e f i l a m e n t s ( G l i w i c z , 1975). A l t h o u g h b l u e - g r e e n a l g a e a r e g e n e r a l l y c o n s i d e r e d l e s s n u t r i t i v e food s o u r c e s than o t h e r t y p e s of a l g a e and some s p e c i e s (eg. M i c r o c y s t i s a e r u g i n o s a , Anabaena f l o s - a q u a e ) may even be t o x i c ( G e n t i l e and Maloney, 1969; G e n t i l e , 1971; C a r m i c h a e l e t a l , 1975; P o r t e r , 1977), 145 F i g u r e 40. V a r i a t i o n i n p h y t o p l a n k t o n biomass and % c o m p o s i t i o n . The t o t a l p h y t o p l a n k t o n volume biomass i s based on e s t i m a t e d c e l l volume and the % c o m p o s i t i o n i s composed of f o u r groups as e x p l a i n e d i n the t e x t . The groups a r e i n d i c a t e d by: <2 - l e s s than 2 um, nann -nannoplankton (2 - 20 um), >20-col - g r e a t e r than 20 um c e l l s and p h y t o p l a n k t o n c o l o n i e s , f i l a m e n t s -f i l a m e n t o u s a l g a e . 147 t h e r e appears t o be c o n s i d e r a b l e v a r i a t i o n i n the a b i l i t y of z o o p l a n k t o n t o a s s i m i l a t e d i f f e r e n t s p e c i e s of b l u e - g r e e n a l g a e ( S c h i n d l e r , 1971). Lewis (1979) found t h a t the p r o d u c t i o n of an h e r b i v o r o u s c y c l o p o i d copepod, Thermocyclops h y a l i n u s , was p o s i t i v e l y c o r r e l a t e d w i t h the abundance of diatoms and of b l u e - g r e e n a l g a e . I n f a n t e (1978) has a l s o shown t h a t c y c l o p o i d s i n Lake V a l e n c i a , V e n e z u e l a , i n g e s t and d i g e s t l a r g e amounts of Lyngbya 1 i m n e t i c a . T h i s i s c l e a r l y an ar e a which r e q u i r e s f u r t h e r i n v e s t i g a t i o n b e f o r e g e n e r a l i z a t i o n s can c o n f i d e n t l y be made. I a n a l y z e d d i f f e r e n c e s i n f e c u n d i t y by c o u n t i n g the eggs produced per female and the p r o p o r t i o n of o v i g e r o u s females i n the p o p u l a t i o n s of both the h i g h and low n u t r i e n t t r e a t m e n t s over the e g g - b e a r i n g p e r i o d i n May and June. A two-way a n a l y s i s of v a r i a n c e was used t o d i f f e r e n t i a t e between the e f f e c t on c l u t c h s i z e due t o f e r t i l i z a t i o n and t h a t due t o p r e d a t i o n (Table 1 0 ) . F e r t i l i z a t i o n was the o n l y f a c t o r t h a t s i g n i f i c a n t l y a f f e c t e d c l u t c h s i z e (p<0.01). Smyly s i m i l a r l y found t h a t i n c r e a s i n g a v a i l a b l e food i n c r e a s e d the mean c l u t c h s i z e . Smyly was a b l e t o almost double the mean c l u t c h s i z e of female a d u l t C y c l o p s abyssorum when he a r t i f i c i a l l y f e d t h e s e a n i m a l s A r t e m i a n a u p l i i i n excess of d a i l y need. In the absence of Chaoborus, the p r o p o r t i o n of o v i g e r o u s female C.b.thomasi i s a l s o d i f f e r e n t between the f e r t i l i z e d and u n f e r t i l i z e d e n c l o s u r e s ( F i g . 4 1 , a r c s i n t r a n s f o r m e d d a t a a n a l y z e d as d e s c r i b e d i n Appendix B, t=3.05, df=6, p<.01). However, i t i s c l e a r from F i g . 4 1 t h a t t h i s d i f f e r e n c e i s due t o an extended p e r i o d of egg p r o d u c t i o n i n the h i g h n u t r i e n t T a b l e 10 C l u t c h s i z e (+1 SE) o f C.b.thomasi i n e x p e r i m e n t a l e n c l o s u r e s i n May-June 1977. U n f e r t i l i z e d p r e d a t o r F e r t i l i z e d p r e d a t o r U n f e r t i l i z e d F e r t i l i z e d 21.0 • 1.34 25.7 + 1.30 21.0 + 0.94 26.0 * 1.88 a n a l y s i s of v a r i a n c e Source o f v a r i a t i o n df SS MS F P e r t i l i z a t i o n 1 140. 16 140. 16 14. 06** Preda t o r 1 0. 16 0. 16 0. 0 2 ns F e r t i l i z a t i o n x P r e d a t o r 1 0, 18 0. 18 0. 02 ns E r r o r 20 199. 33 9. 97 T o t a l 2 3 3 3 9. 83 14. 78 Note: P r e d a t o r i n d i c a t e s p r e s ence o f Chaoborus, f e r t i l i z e d / u n f e r t i l i z e d as d e s c r i b e d i n t h e t e x t . **P<0.01; n s , not s i g n i f i c a n t 149 F i g u r e 41. P r o p o r t i o n of o v i g e r o u s C.b.thomasi females f o r 3 t r e a t m e n t e n c l o s u r e s d u r i n g May-June i n Gwendoline Lake i n 1977. 151 e n c l o s u r e r a t h e r than a g r e a t e r p r o p o r t i o n of o v i g e r o u s females throughout the e g g - b e a r i n g p e r i o d . T h i s i s c o n f i r m e d by a t -t e s t on the p r o p o r t i o n of o v i g e r o u s females d u r i n g the f i r s t f o u r weeks (p=0.2). F e r t i l i z a t i o n t h e r e f o r e enhances egg p r o d u c t i o n i n t h i s p o p u l a t i o n by i n c r e a s i n g the number of eggs per c l u t c h and p o s s i b l y by e x t e n d i n g the e g g - p r o d u c i n g p e r i o d . Smyly (1973) has a l s o suggested t h a t i n c r e a s e d food r e s o u r c e s r e s u l t i n l o n g e r b r e e d i n g p e r i o d s . E f f e c t of Chaoborus at Low N u t r i e n t C o n c e n t r a t i o n s The presence of Chaoborus i n the u n f e r t i l i z e d e n c l o s u r e d r a s t i c a l l y reduced C.b.thomasi s t a n d i n g s t o c k t o a l e v e l where sam p l i n g became d i f f i c u l t ( F i g . 3 9 d ) . The f i r s t sample of the e n c l o s u r e s showed .the i n i t i a l d e n s i t i e s of a d u l t female C.b. thomasi t o be s i m i l a r - 2.8 x 10 2/m 3 i n the Chaoborus t r e a t m e n t compared t o 2.4 x 10 2/m 3 i n the C h a o b o r u s - f r e e e n c l o s u r e . There might have been some p r e d a t i o n by two year o l d 4 t h i n s t a r Chaoborus t r i v i t t a t u s on the a d u l t female C.b.thomasi . However, I used a Mann-Whitney U t e s t t o a n a l y z e d i f f e r e n c e s i n the s t a n d i n g s t o c k of a d u l t C.b.thomasi females between the Chaoborus p r e s e n t and the Chaoborus absent u n f e r t i l i z e d e n c l o s u r e s and t h e r e was no s t a t i s t i c a l d i f f e r e n c e (p=.84). In s p i t e of t h i s r e s u l t and the p r e v i o u s f i n d i n g t h a t t h e r e was no d i f f e r e n c e i n the number of eggs produced per female between Chaoborus and Chaoborus - f r e e e n c l o s u r e s , t h e r e were fewer n a u p l i i sampled i n the p r e s ence of Chaoborus ( F i g . 3 9 c ) . T h i s r e s u l t was not due t o a lower p r o p o r t i o n of o v i g e r o u s females ( F i g . 4 1 ) . Copepodid i n s t a r s were always 152 s c a r c e i n the Chaoborus e n c l o s u r e s when compared t o the e n c l o s u r e w i t h o u t Chaoborus . The c r i t i c a l p e r i o d f o r the C.b.thomasi p o p u l a t i o n appeared t o be d u r i n g the n a u p l i a r i n s t a r s . To e s t i m a t e the impact of Chaoborus on C.b.thomasi n a u p l i i , f i r s t i n s t a r Chaoborus were exposed t o C.b.thomasi n a u p l i i i n the presence of o t h e r prey a t d e n s i t i e s commonly found i n Gwendoline Lake. Chaoborus i n s t a r I was chosen because n a u p l i i and r o t i f e r s a r e common i n c r o p c o n t e n t s ( N e i l l and Peacock,1980), Fedorenko (1973) observed t h a t C.americanus i n s t a r I I f e d l e a s t e f f i c i e n t l y on c a l a n o i d n a u p l i i and C.b.thomasi n a u p l i a r d e c l i n e s c o i n c i d e d w i t h the appearance of f i r s t i n s t a r Chaoborus i n the p l a n k t o n . F e e d i n g r a t e s of Chaoborus i n s t a r I are shown i n T a b l e 11. The f r a c t i o n of the s t a n d i n g c r o p of C.b.thomasi n a u p l i i t h a t c o u l d be removed by i n s t a r I Chaoborus p r e d a t i o n was c a l c u l a t e d u s i n g p r e d a t o r s t a n d i n g s t o c k v a l u e s found i n the Chaoborus u n f e r t i l i z e d e n c l o s u r e and the C.b.thomasi n a u p l i a r d e n s i t i e s observed i n the u n f e r t i l i z e d Chaoborus - f r e e e n c l o s u r e ( F i g . 4 2 ) . C a l c u l a t i o n s are d e s c r i b e d i n Appendix C. The e s t i m a t e d mean pe r c e n t a g e of C.b.thomasi n a u p l i i t h a t c o u l d be removed d a i l y by Chaoborus i n s t a r I was 6.9%. As the e s t i m a t e d r e s i d e n c e time f o r C.b.thomasi n a u p l i i was 17.5 days, i t was c o n c e i v a b l e t h a t a l l the C.b.thomasi n a u p l i i c o u l d be e a t e n . A l t h o u g h a s p a t i a l r e f u g e d i d not seem l i k e l y as 1 s t i n s t a r l a r v a e were found i n the 1-5 m. l a y e r s throughout the day ( F i g . 4 3 ) , C.b.thomasi was not d r i v e n t o e x t i n c t i o n i n the u n f e r t i l i z e d e n c l o s u r e , nor i n the l a k e . However, i t was c l e a r t h a t 1 s t i n s t a r Chaoborus had the p o t e n t i a l t o i n f l i c t d r a s t i c l o s s e s on T a b l e 11 P r e d a t i o n r a t e s o f Chaoborus l a r v a l i n s t a r I on C.b.thomasi n a u p l i i i n the presence of o t h e r prey (Mean number eaten per l i t r e per 24 h o u r s ) . n=5 I n i t i a l prey Prey k i l l e d / l i t r e Prey k i l l e d / l i t r e / p r e d a t o r / d a y C. b. th omasi 10 Diapatomas n a u p l i i 3 P i a ptomus c o p e p o d i d s 2 C I - C I I I D. rosea <1.0 mm 2 Diaphanosoma 1 <1.0 mm 4. 4 1 . 9 0.0 0.2 0.2 2. 2 1.0 0.0 0. 1 0. 1 154 F i g u r e 42. E s t i m a t e d % of C.b.thomasi n a u p l i i e a t e n by Chaoborus - the C.b.thomasi n a u p l i i i n the u n f e r t i l i z e d • ' C y c l o p s ' e n c l o s u r e which c o u l d be eaten by Chaoborus i n s t a r I l a r v a e as found i n the u n f e r t i l i z e d 'Chaoborus-C y c l o p s ' t r e a t m e n t . 55/ 156 F i g u r e 43. V e r t i c a l d i s t r i b u t i o n of C.b.thomasi and Chaoborus - C.b.thomasi i n s t a r s i n P l a c i d Lake and Gwendoline Lake (on l e f t ) and Chaoborus i n s t a r s i n Gwendoline Lake (on r i g h t ) over a 24 hour p e r i o d i n June, 1978. L e t t e r s i n d i c a t e the i n s t a r of the p l a n k t o n shown: N - C.b.thomasi n a u p l i i , C - C.b.thomasi c o p e p o d i d s , AD - C.b.thomasi a d u l t s . Roman numerals i n d i c a t e the i n s t a r s of Chaoborus . 158 the C.b.thomasi p o p u l a t i o n , a l t h o u g h when numbers became low, encounter r a t e s might have become so r a r e t h a t p r e d a t i o n m o r t a l i t y might have been s u f f i c i e n t l y reduced t o a l l o w some i n d i v i d u a l s t o r e a c h m a t u r i t y . Fedorenko (1973) a l s o observed t h a t a l l 1 s t , 2nd and young 3rd i n s t a r s of C . t r i v i t t a t u s l a r v a e were found above the t h e r m o c l i n e a t a l l times of the day. As C.b.thomasi n a u p l i i remained a t about 2-5 m. d u r i n g the e n t i r e day, t h e r e was c l e a r l y c o n s i d e r a b l e s p a t i a l o v e r l a p between p r e d a t o r and p r e y . P a r t i c u l a t e o r g a n i c matter and p h y t o p l a n k t o n c e l l volume were s i m i l a r between Chaoborus and Ch a o b o r u s - f r e e e n c l o s u r e s ( F i g . 4 4 ) . The p e r c e n t c o m p o s i t i o n of a l g a l t y p e s was remarkably a l i k e i n p a t t e r n between the two t r e a t m e n t s , a l t h o u g h the c h r o n o l o g y was s l i g h t l y d i f f e r e n t ( F i g . 4 0 b , d ) . Net p l a n k t o n became the dominant type i n August, about a week e a r l i e r i n the C h a o b o r u s - f r e e e n c l o s u r e w h i l e the Chaoborus t r e a t m e n t d e v e l o p e d l a r g e green c o l o n i e s i n s t e a d of the b l u e -green f i l a m e n t s which were dominant i n the former e n c l o s u r e . T h i s development p r o b a b l y had l i t t l e e f f e c t on the C.b.thomasi p o p u l a t i o n as the b u l k of the p o p u l a t i o n had reached the c a r n i v o r o u s i n s t a r s i n both e n c l o s u r e s by August ( F i g . 3 9 d ) . 159 F i g u r e 44. Summer v a r i a t i o n i n p a r t i c u l a t e o r g a n i c matter (POM) f o r Gwendoline Lake and the e x p e r i m e n t a l e n c l o s u r e s . POM v a l u e s a r e d e r i v e d from p o o l e d samples from the s u r f a c e , 2.5 m., and 4.5 m. d e p t h s . C i r c l e s r e p r e s e n t the 'Chaoborus-Cyclops' t r e a t m e n t , square r e p r e s e n t the ' C y c l o p s ' t r e a t m e n t , t r i a n g l e s r e p r e s e n t the ' p r e d a t o r - f r e e ' e n c l o s u r e s and x i n d i c a t e s the l a k e v a l u e s . S o l i d symbols r e p r e s e n t f e r t i l i z e d t r e a t m e n t s and open symbols r e p r e s e n t u n f e r t i l i z e d e n c l o s u r e s . ( , .1 ' iM A j p -Suu ) - o u o o lAJOd 161 E f f e c t of F e r t i l i z a t i o n and Chaoborus F i r s t i n s t a r Chaoborus l a r v a e were found i n g r e a t numbers i n b o th the f e r t i l i z e d and u n f e r t i l i z e d e n c l o s u r e s ( F i g . 4 5 ) . In the u n f e r t i l i z e d c o n d i t i o n s , however, r e l a t i v e l y few de v e l o p e d i n t o second i n s t a r l a r v a e . T h i s r e s u l t was c o n s i s t e n t w i t h the o b s e r v a t i o n s of Fedorenko (1973) and N e i l l and Peacock (1980) t h a t r e l a t i v e l y few Chaoborus s u r v i v e d t o the o l d e r i n s t a r s i n the s e o l i g o t r o p h i c l a k e s . The massive e a r l y j u v e n i l e m o r t a l i t y d e c r e a s e d s i g n i f i c a n t l y i n the h i g h n u t r i e n t e n c l o s u r e as many more i n s t a r I d e v e l o p e d i n t o i n s t a r I I and I I I . N e i l l and Peacock (1980) showed t h a t t h i s improved s u r v i v o r s h i p was due t o s o l i t a r y r o t i f e r blooms a t the h i g h n u t r i e n t l e v e l s . S o l i t a r y r o t i f e r numbers ( K e l l i c o t t i a , K e r a t e l l a and Lecane spp.) i n c r e a s e d d r a m a t i c a l l y i n the f e r t i l i z e d e n c l o s u r e ( F i g . 4 6 ) and t h i s r o t i f e r e x p l o s i o n was c o r r e l a t e d w i t h i n c r e a s i n g Chaoborus i n s t a r I s u r v i v o r s h i p . The e f f e c t of t h i s improved Chaoborus s u r v i v o r s h i p was t o i n c r e a s e the r a t e of C.b.thomasi d e c l i n e . In c o n t r a s t t o the Ch a o b o r u s - f r e e h i g h n u t r i e n t e n c l o s u r e , few C.b.thomasi co p e p o d i d i n s t a r s were r e c r u i t e d from the l a r g e n a u p l i a r s t a n d i n g s t o c k and, u n l i k e the u n f e r t i l i z e d Chaoborus e n c l o s u r e , no a d u l t s were sampled i n August or September ( F i g . 3 9 d ) . T h i s summer d e c l i n e d i d not appear t o be r e l a t e d t o food r e s o u r c e s . Adding n u t r i e n t s t o a Chaoborus e n c l o s u r e r e s u l t e d i n changes p a r a l l e l i n g t h a t observed i n the e n r i c h e d Chaoborus-f r e e e n c l o s u r e i n both p a r t i c u l a t e o r g a n i c m a tter and p h y t o p l a n k t o n s t a n d i n g c r o p . As i n the u n f e r t i l i z e d 162 F i g u r e 45. V a r i a t i o n i n the s t a n d i n g c r o p of Chaoborus l a r v a e . Roman numerals i n d i c a t e the l a r v a l i n s t a r ( I , I I and I I I a r e shown) of Chaoborus i n f e r t i l i z e d and u n f e r t i l i z e d 'Chaoborus-Cyclops' e n c l o s u r e s and i n Gwendoline Lake d u r i n g the summer of 1977. /C3 N U M B E R P E R L I T R E 164 F i g u r e 46. V a r i a t i o n i n the s t a n d i n g c r o p of s o l i t a r y r o t i f e r s o bserved i n the 'Chaoborus-Cyclops' t r e a t m e n t s and Gwendoline Lake d u r i n g the summer of 1977. /£5 166 e n c l o s u r e s , the p a t t e r n of change i n p e r c e n t c o m p o s i t i o n of a l g a l t y p e s was s i m i l a r a l t h o u g h the t i m i n g of ev e n t s was s l i g h t l y d i f f e r e n t . F i l a m e n t s , m a i n l y b l u e - g r e e n s , O s c i l l a t o r i a and Lyngbya, became the overwhelming dominants i n June, about a week e a r l i e r i n the Chaoborus p r e s e n t e n c l o s u r e and t h i s dominance was m a i n t a i n e d u n t i l l a t e August ( F i g . 4 0 a ,c ) . 167 DISCUSSION C.b.thomasi i s abundant i n P l a c i d Lake, a s m a l l montane l a k e w i t h a r e s i d e n t p o p u l a t i o n of c u t t h r o a t t r o u t , and s c a r c e i n Gwendoline Lake, a nearby l a r g e r , deeper l a k e w i t h o u t a f i s h p o p u l a t i o n . The e x p e r i m e n t a l t r a n s p l a n t a t i o n of C.b.thomasi from P l a c i d Lake t o an e n c l o s u r e i n Gwendoline Lake r e v e a l e d t h a t C.b.thomasi c o u l d s u r v i v e and r e c r u i t i n the f i s h l e s s l a k e environment when the predaceous Chaoborus s p e c i e s p r e s e n t i n t h i s l a k e was removed. C o n s e q u e n t l y the low abundance of C.b.thomasi i n Gwendoline Lake c o u l d not be e x p l a i n e d by the h y p o t h e s i s t h a t p h y s i c a l / c h e m i c a l d i f f e r e n c e s between t h i s l a k e and P l a c i d Lake were u n f a v o u r a b l e t o C.b".thomasi. There was no i n d i c a t i o n t h a t food r e s o u r c e s were too low t o s u pport an abundant p o p u l a t i o n of C.b.thomasi, comparable t o t h a t found i n P l a c i d Lake. A measure of the 'grazable- s e s t o n ' p a r t i c u l a t e o r g a n i c m atter - showed t h a t the u n f e r t i l i z e d e n c l o s u r e s were i n d i s t i n q u i s h a b l e from Gwendoline Lake. Food r e s o u r c e s were t h e r e f o r e p r o b a b l y s i m i l a r t o those e x p e r i e n c e d i n the l a k e y e t C.b.thomasi was always s c a r c e i n Gwendoline Lake and abundant i n the e n c l o s u r e s w i t h o u t Chaoborus. C.b.thomasi was a b l e t o i n c r e a s e i n the C h a o b o r u s - f r e e e n c l o s u r e s even though t h e r e was e v i d e n c e t h a t food was l i m i t i n g i n Gwendoline Lake. ' Adding n u t r i e n t s t o Gwendoline Lake water i n the e n c l o s u r e s i n c r e a s e d the number of eggs per c l u t c h (mean of 25.7 compared t o a mean of 21.0 per c l u t c h , p<.01.) and extended the p e r i o d d u r i n g which o v i g e r o u s females were p r e s e n t i n the p o p u l a t i o n . The l a t t e r e f f e c t s u g g e sted t h a t the i n c r e a s e i n the t o t a l .number of eggs produced i n the 168 f e r t i l i z e d e n c l o s u r e might be due t o a l o n g e r female a d u l t l i f e t h us p o s s i b l y , p e r m i t t i n g an i n c r e a s e i n the number of broods per female. In A c a n t h o c y c l o p s v i r i d i s , Smyly (1970) found t h a t the mean number of broods v a r i e d w i t h the d i e t , from 1.5 broods on a l g a e t o 8.6 broods on A r t e m i a n a u p l i i . T h i s a u t h o r a l s o found l o n g e v i t y v a r i e d w i t h d i e t , from 67.9 days on A r t e m i a t o 106.4 days on Chydorus s p h a e r i c u s . In Gwendoline Lake, Chydorus spp. i n c r e a s e d i n a l l the f e r t i l i z e d e n c l o s u r e s and may have s e r v e d as an i m p o r t a n t food source f o r the l a t e r i n s t a r s of C.b.thomasi. C.b.thomasi s u r v i v o r s h i p i n the u n f e r t i l i z e d e n c l o s u r e showed a m o r t a l i t y p a t t e r n s i m i l a r t o t h a t o b s e r v e d i n P l a c i d Lake. The g r e a t e s t m o r t a l i t y o c c u r r e d i n the egg t o CII i n t e r v a l i n both e n v i r o n m e n t s . S e v e r a l a u t h o r s (Gehrs and Robertson,. 1975; Boers and C a r t e r , 1978) have observed a * s i m i l a r p a t t e r n i n c a l a n o i d p o p u l a t i o n s , a l t h o u g h R i g l e r and Cooley (1974) found t h a t a l l f i v e c o h o r t s of S k i s t o d i a p t o m u s  o r e q o n e n s i s shared a h i g h s u r v i v a l t o N3. Gehrs and Robertson (1975) suggest t h a t the h i g h m o r t a l i t y o c c u r r i n g i n the N6 sta g e may be a s s o c i a t e d w i t h m o r p h o l o g i c a l changes o c c u r r i n g as the organism moults from l a r v a l t o c o p e p o d i t e form. There i s e v i d e n c e t h a t the N5-N6 s t a g e s a r e more s e n s i t i v e t o e n v i r o n m e n t a l s t r e s s . Karanas e t a l . (1979) showed t h a t the N5-N6 s t a g e s of A c a r t i a c l a u s i i , a marine c a l a n o i d copepod, were u n u s u a l l y s e n s i t i v e t o m i d u l t r a v i o l e t r a d i a t i o n . T o l e r a n c e i n c r e a s e d w i t h age w i t h the e x c e p t i o n of s t a g e s N5-N6 which were even more, s e n s i t i v e than the N3-N4 i n s t a r s . I grouped n a u p l i i by e a r l y (N1-N3) and l a t e (N4-N6) 169 n a u p l i a r i n s t a r s i n t h i s s tudy and I found s i g n i f i c a n t m o r t a l i t y o c c u r r e d d u r i n g both i n t e r v a l s . B u r g i s (1971) a l s o found h i g h n a u p l i a r m o r t a l i t y i n the e a r l y s t a g e s of, Thermocyclops h y a l i n u s a l t h o u g h Lewis (1979) c o n c l u d e d t h e r e was r e l a t i v e l y l i t t l e n a u p l i a r m o r t a l i t y i n the same s p e c i e s . Gehrs and Robertson (1975) found t h a t a l a b o r a t o r y p o p u l a t i o n of Diaptomus c l a v i p e s had l i t t l e m o r t a l i t y i n the egg t o N4 i n t e r v a l but i n the f i e l d p o p u l a t i o n 84% f a i l e d t o d e v e l o p t o the N4 s t a g e . In s p i t e of h i g h n a u p l i a r m o r t a l i t y , the s u r v i v o r s h i p c u r v e s from the C h a o b o r u s - f r e e e n c l o s u r e s suggest t h a t C.b.thomasi s h o u l d not e x p e r i e n c e any more d i f f i c u l t y s u r v i v i n g and r e c r u i t i n g i n the Gwendoline Lake environment than i t does i n P l a c i d Lake, where t h i s s p e c i e s i s abundant. However, the predaceous midge f l y l a r v a e , Chaoborus d i f f e r s i n s p e c i e s and be h a v i o u r between the two l a k e s . C . f l a v i c a n s , a s m a l l s p e c i e s which i s b e n t h i c d u r i n g the d a y l i g h t hours, i s found i n P l a c i d Lake. In Gwendoline Lake, C . t r i v i t t a t u s and C.americanus a r e the i n d i g e n o u s s p e c i e s , w i t h the former the more abundant s p e c i e s . Both s p e c i e s a r e e n t i r e l y p l a n k t o n i c and both a r e l a r g e r i n s i z e than C . f l a v i c a n s , C . t r i v i t t a t u s b e i n g the l a r g e s t . Fedorenko (1973) c o n c l u d e s t h a t t h i s p r e d a t o r can have a s u b s t a n t i a l impact on z o o p l a n k t o n communities, a l t h o u g h N e i l l and Peacock (1980) found t h a t Chaoborus does not have a major demographic e f f e c t on i n d i g e n o u s s p e c i e s i n low n u t r i e n t e n v i r o n m e n t s . S i m i l a r l y , N o r t h c o t e et a l . (1978) were unable t o f i n d any s i g n i f i c a n t e f f e c t on s p e c i e s c o m p o s i t i o n or on s e a s o n a l abundances of c r u s t a c e a n p l a n k t e r s when Chaoborus 170 was e f f e c t i v e l y e l i m i n a t e d from E u n i c e Lake by s a l m o n i d p r e d a t i o n . C.b.thomasi was i n t r o d u c e d i n t o e n c l o s u r e s i n Gwendoline Lake w i t h o u t Chaoborus, i d e n t i c a l t o those d e s c r i b e d above, t o t e s t the h y p o t h e s i s t h a t Chaoborus p r e v e n t e d the e x p a n s i o n of the C.b.thomasi p o p u l a t i o n i n t h i s l a k e . A l t e r n a t i v e l y , p r e d a t i o n by Chaoborus might be an i m p o r t a n t f a c t o r l i m i t i n g the s u c c e s s of C.b.thomasi o n l y when food r e s o u r c e s were low and C.b.thomasi j u v e n i l e m o r t a l i t y was a l r e a d y h i g h . A l t h o u g h C.b.thomasi reproduced i n the e n c l o s u r e s w i t h Chaoborus, n a u p l i a r r e c r u i t m e n t d i d not r e s u l t i n the c o p e p o d i d r e c r u i t m e n t o b s e r v e d i n the e n c l o s u r e s w i t h o u t Chaoborus. Smyly (1976b) found t h a t i n s t a r IV l a r v a e of C . t r i v i t t a t u s a t e more C.b.thomasi a d u l t s than e i t h e r the l a r g e r a d u l t Diaptomus ke n a i or the s m a l l e r a d u l t T . p r a s i n u s when o f f e r e d a c h o i c e , a l t h o u g h d e n s i t i e s used i n the e x p e r i m e n t s were h i g h e r than those found i n Gwendoline Lake. A d u l t numbers of C.b.thomasi d e c r e a s e d more r a p i d l y i n the Chaoborus e n c l o s u r e s u g g e s t i n g t h a t t h e r e might have been some l o s s of a d u l t females t o the o v e r w i n t e r i n g f o u r t h i n s t a r C . t r i v i t t a t u s l a r v a e . However, the p r o p o r t i o n of a d u l t female C.b.thomasi p r o d u c i n g eggs and the number of eggs per c l u t c h were not s i g n i f i c a n t l y d i f f e r e n t between the two t r e a t m e n t s and the number of females p r e s e n t d u r i n g the r e p r o d u c t i v e p e r i o d i n May-June were not s t a t i s t i c a l l y d i f f e r e n t . T h i s r e s u l t showed t h a t t h e r e was a s i m i l a r p o t e n t i a l f o r i n c r e a s e between the e n c l o s u r e s both w i t h and w i t h o u t Chaoborus . A l g a l s t a n d i n g c r o p s i n the u n f e r t i l i z e d e n c l o s u r e s w i t h 171 and w i t h o u t Chaoborus were remarkably s i m i l a r , b o th i n volume biomass and i n the c o m p o s i t i o n of a l g a l t y p e s . T h e r e f o r e , i n c r e a s e d m o r t a l i t y i n the Chaoborus t r e a t m e n t due t o n a u p l i a r s t a r v a t i o n was u n l i k e l y , and t h e r e was no reason t o expect i n t r a s p e c i f i c p r e d a t i o n t o be g r e a t e r i n t h i s s i t u a t i o n . Gwendoline Lake i s a low food environment f o r Chaoborus. G r e a t e s t m o r t a l i t y o c c u r s i n the f i r s t and second i n s t a r s ( N e i l l and Peacock, 1980). R o t i f e r p o p u l a t i o n s a re low i n the u n f e r t i l i z e d e n c l o s u r e s , as they a r e i n the l a k e . A l a b o r a t o r y experiment u s i n g a l t e r n a t i v e c r u s t a c e a n prey a t r e a l i s t i c l a k e d e n s i t i e s showed t h a t f i r s t i n s t a r Chaoborus a t e more C.b.thomasi n a u p l i i than the a l t e r n a t i v e s - c a l a n o i d n a u p l i i and immature c l a d o c e r a n s . T h i s r e s u l t may be due t o s i z e and/or b e h a v i o u r . C.b.thomasi n a u p l i i a r e s m a l l e r than the c a l a n o i d n a u p l i i ( D . l e p t o p u s and D.kenai) found i n Gwendoline Lake and appear t o swim i n a q u i c k j e r k y f a s h i o n r a t h e r than the smooth g l i d i n g motion of the c a l a n o i d n a u p l i i . As Chaoborus l o c a t e prey v i a mechanoreceptors ( G i g u e r e and D i l l , 1979), t h i s b e h a v i o u r may make them more v u l n e r a b l e . In a d d i t i o n , D.leptopus may have a te m p o r a l r e f u g e as t h i s s p e c i e s produces eggs i n March ( i n some y e a r s a t l e a s t ) , r e a c h i n g c o p e p o d i t e s t a g e s by June when 1 s t i n s t a r Chaoborus appear (M.A.Chapman,pers.comm.). G e r r i t s e n (1978) i n v e s t i g a t e d the p o t e n t i a l p r e d a t o r y impact of a Chaoborus sp. (he d i d not i d e n t i f y the s p e c i e s ) on C y c l o p s s c u t i f e r n a u p l i i . He argues t h a t n a u p l i i a r e c r y p t i c t o the v i b r a t i o n - s e n s i t i v e Chaoborus p r e d a t o r s because n a u p l i i a r e l e s s a c t i v e than copepodids or a d u l t s . Fedorenko 172 (1973) a l s o found t h a t Chaoborus l a r v a e i n s t a r I I d i d not feed e f f e c t i v e l y on c a l a n o i d n a u p l i i . However, i t i s c l e a r t h a t Chaoborus i n s t a r I can s e l e c t i v e l y remove r o t i f e r s ( N e i l l and Peacock,1980) and t h a t t h i s s u s c e p t i b i l i t y cannot be p r e d i c t e d by e x amining swimming speeds a l o n e . C l e a r l y the d e v e l o p m e n t a l response and s p a t i a l / t e m p o r a l i n t e r a c t i o n of p r e d a t o r and prey must be i m p o r t a n t f a c t o r s i n a s s e s s i n g prey v u l n e r a b i l i t y . F u r t h e r m o r e , the s p e c t r a of prey items a v a i l a b l e t o a s m a l l Chaoborus l a r v a are e x t r e m e l y l i m i t e d , p a r t i c u l a r l y i n o l i g o t r o p h i c l a k e s where r o t i f e r s a r e g e n e r a l l y not abundant. Lewis (1979) notes the d i s c r e p a n c i e s between growth r a t e s based on c o h o r t development i n the f i e l d and t h a t p o s s i b l e based on p u b l i s h e d f e e d i n g r a t e d a t a . He f i n d s t h a t even w i t h h i g h a s s i m i l a t i o n and growth e f f i c i e n c i e s , l a b o r a t o r y e s t i m a t e s of p rey i n t a k e are much too low. Lewis sug-gests t h a t p a r t of the problem may be e x p l a i n e d by the tendency of most i n v e s t i g a t o r s t o f o c u s on the l a s t i n s t a r , the i n s t a r he e s t i m a t e s t o have the l o w e s t growth r a t e . U s i n g C.b.thomasi p o p u l a t i o n d a t a from the C h a o b o r u s - f r e e e n c l o s u r e and 1 s t i n s t a r Chaoborus from the Chaoborus e n c l o s u r e , I e s t i m a t e d the C.b.thomasi n a u p l i a r p o p u l a t i o n which c o u l d be removed by 1 s t i n s t a r Chaoborus l a r v a e ( F i g . 4 6 ) . I t was c l e a r t h a t the p r o b a b i l t y of b e i n g eaten was e x t r e m e l y h i g h over the e n t i r e 17.5 day r e s i d e n c e time of an average n a u p l i a r C.b.thomasi copepod. . As a p p r o x i m a t e l y 76% of the n a u p l i a r p o p u l a t i o n p e r i s h e d b e f o r e r e a c h i n g the c o pepodid i n s t a r s even i n the absence of Chaoborus, the a d d i t i o n of t h i s p r e d a t o r had c a t a s t r o p h i c e f f e c t s on C.b.thomasi copepodid 173 r e c r u i t m e n t . Lewis (1977) examined gut c o n t e n t s of a l l 4 i n s t a r s of Chaoborus i n t r o p i c a l Lake Lanao and found c y c l o p o i d n a u p l i i seldom r e p r e s e n t e d . T h i s a u t h o r c o n c l u d e d t h a t some p r e d a t o r a v o i d a n c e mechanism must be o p e r a t i n g and s uggested i t may be b e h a v i o u r a l - l a c k of motion c a u s i n g the n a u p l i i t o be c r y p t i c t o Chaoborus l a r v a e as s u ggested by G e r r i t s e n (1978). However, the o n l y r o t i f e r commonly e n c o u n t e r e d i n the gut c o n t e n t s was K e r a t e l l a - a l o r i c a t e r o t i f e r . Fedorenko (1975) found t h a t s m a l l , s o f t - b o d i e d prey such as copepod n a u p l i i and n o n l o r i c a t e r o t i f e r s were seldom d e t e c t e d i n c r o p a n a l y s i s and she a t t r i b u t e d t h i s r e s u l t t o r a p i d d i g e s t i o n of t h e s e food t y p e s i n the l a r v a l c r o p . Lewis (1979) conceded t h a t r a p i d p r o c e s s i n g i n the c r o p might have c o n t r i b u t e d t o h i s r e s u l t . F e r t i l i z a t i o n appeared t o a c c e l e r a t e the e l i m i n a t i o n of C.b.thomasi n a u p l i i , even though s u r v i v o r s h i p was g r e a t l y enhanced by the a d d i t i o n of f e r t i l i z e r t o Chaoborus - f r e e e n c l o s u r e s . Adding n u t r i e n t s caused a r o t i f e r bloom and g r e a t l y improved j u v e n i l e Chaoborus s u r v i v o r s h i p . A s i m i l a r response was observed by N e i l l (pers.comm.), such t h a t even the r - s t r a t e g i s t c l a d o c e r a n s were unable t o s u r v i v e the impact of Chaoborus p r e d a t i o n . Chaoborus s u r v i v o r s h i p improved about 2 0 - f o l d over low n u t r i e n t t r e a t m e n t s i n the ' i n t e r m e d i a t e ' f e r t i l i z e r a d d i t i o n s ( N e i l l and Peacock, 1978). As C.b.thomasi improved i t s s u r v i v o r s h i p o n l y about 4 - f o l d a t the 'h i g h ' f e r t i l i z e r l e v e l , i t seemed u n l i k e l y t h a t C.b.thomasi c o u l d n u m e r i c a l l y escape t h i s p r e d a t o r even a t l e s s extreme n u t r i e n t l e v e l s . 174 These r e s u l t s i n d i c a t e t h a t t h e p r e d a t o r component of the b i o t i c environment has more e f f e c t on the abundance of C.b.thomasi than l a k e d e p t h , temperature or even n u t r i e n t l e v e l . The g e n e r a l i t y of t h i s r e s u l t i s d i f f i c u l t t o e v a l u a t e . P a t a l a s (1971) examines o n l y the c r u s t a c e a n p l a n k t o n community i n h i s study of 45 l a k e s i n the E x p e r i m e n t a l Lakes A r e a . Anderson and R a a s v e l d t (1974) found a weak p o s i t i v e c o r r e l a t i o n (p<0.30) between mean c y c l o p o i d d e n s i t i e s and the presence of Chaoborus spp. but he does not d i f f e r e n t i a t e between c h a o b o r i d s p e c i e s i n g r o u p i n g h i s c ommunities. Both C.americanus and C . f l a v i c a n s a r e found i n h i s study a r e a . Pope and C a r t e r (1975) found t h a t a l l the l a k e s i n the Matamek R i v e r System were dominated by c y c l o p o i d copepods, p r i m a r i l y C y c l o p s s c u t i f e r , d u r i n g J u l y and l a t e August-September and they r e p o r t e d no d i f f e r e n c e between those l a k e s w i t h h i g h d e n s i t i e s of Chaoborus spp. ( m a i n l y C.americanus i n the f i s h l e s s l a k e s ) and no f i s h and those l a k e s w i t h f i s h . However, Von Ende (1979), i n a s t u d y of 4 bog l a k e s i n the Upper P e n i n s u l a of M i c h i g a n , r e p o r t s t h a t C.b.thomasi i s abundant o n l y i n a f i s h l a k e w i t h a s m a l l , b e n t h i c - b y - d a y s p e c i e s of Chaoborus , C . p u n c t i p e n n i s . I f the p r i n c i p a l impact of p r e d a t i o n i s on the n a u p l i a r i n s t a r s , r a t h e r than the c o p e p o d i d or a d u l t , the r e s u l t s of Lynch (1979) may i n d i c a t e an i n t e r a c t i o n between C . v e r n a l i s and C.americanus. Lynch c o n c l u d e s t h a t C . v e r n a l i s i s not i n f l u e n c e d by Chaoborus p r e d a t i o n but he does not p r o v i d e any e x p l a n t i o n f o r the l a r g e n a u p l i a r m o r t a l i t y o b s e r v e d i n the p r e s e nce of abundant Chaoborus. N a u p l i a r abundance d a t a a l s o 175 suggest t h a t the June d e c l i n e of C . v e r n a l i s , i d e n t i c a l i n b oth h i g h and low Chaoborus c o n d i t i o n s , r e p r e s e n t s the end of a g e n e r a t i o n r a t h e r than e v i d e n c e of immunity t o Chaoborus p r e d a t i o n . However, as c y c l o p o i d and c a l a n o i d n a u p l i i a r e not d i f f e r e n t i a t e d and no i n s t a r or r e p r o d u c t i v e d a t a a r e p r e s e n t e d , t h e r e a r e no d a t a w i t h which t o d i s c r i m i n a t e between the two h y p o t h e s e s . However, f u t u r e s t u d i e s s h o u l d c o n s i d e r t h i s p o s s i b i l i t y . Most p l a n k t o n i c i n v e r t e b r a t e p r e d a t o r s a r e organisms w i t h l i m i t e d f u n c t i o n a l and n u m e r i c a l responses ( H a l l et a l , 1976) and t h i s g e n e r a l i z a t i o n i s p a r t i c u l a r l y t r u e i n o l i g o t r o p h i a systems where the p r e d a t o r e x i s t s i n a v e r y d i l u t e food environment. T h i s study shows t h a t s p a t i a l / t e m p o r a l i n t e r a c t i o n and the abundance of a l t e r n a t i v e prey must be e v a l u a t e d b e f o r e the e f f i c a c y of an i n v e r t e b r a t e p r e d a t o r can be a c c u r a t e l y a s s e s s e d . SUMMARY C.b.thomasi can s u r v i v e and reproduce i n e n c l o s u r e s f i l l e d w i t h Gwendoline Lake water and and Gwendoline Lake c r u s t a c e a n p l a n k t o n . The n a u p l i a r s t a g e s r e p r e s e n t the d e v e l o p m e n t a l b o t t l e n e c k f o r C.b.thomasi p o p u l a t i o n s i n Gwendoline Lake and t h i s m o r t a l i t y p a t t e r n i s s i m i l a r t o t h a t o b s e r v e d i n P l a c i d Lake. F e r t i l i z a t i o n of the Gwendoline Lake e n c l o s u r e s i n c r e a s e s s u r v i v o r s h i p of C.b.thomasi about 22% over t h a t o b s e r v e d i n the u n f e r t i l i z e d t r e a t m e n t . Adding the midge f l y l a r v a e C . t r i v i t t a t u s and C.americanus t o the Gwendoline Lake e n c l o s u r e s d r a s t i c a l l y reduces the abundance of C.b.thomasi. In the p r e s e n c e of Chaoborus , f e r t i l i z a t i o n 177 does not i n c r e a s e the abundance of C.b.thomasi but r a t h e r , improves the s u r v i v o r s h i p of Chaoborus and t h i s r e s u l t s i n the r a p i d d e c l i n e of C.b.thomasi n a u p l i i . 178 IV. EFFECT OF VARYING INVERTEBRATE PREDATOR ABUNDANCE AND FOOD ON A CRUSTACEAN PLANKTON COMMUNITY 179 INTRODUCTION L i m n e t i c c r u s t a c e a n z o o p l a n k t o n l i v e i n a r e l a t i v e l y u n s t r u c t u r e d h a b i t a t w i t h l i t t l e a pparent p r o t e c t i o n from t h e i r p r e d a t o r s . Yet the p e r s i s t e n c e and o f t e n h i g h abundance of l a c u s t r i n e prey s p e c i e s , even i n o l i g o t r o p h i c systems, suggest an i n s e n s i t i v i t y t o p r e d a t o r i n f l u e n c e . T h i s appearance of immunity might e x p l a i n why t h e r e was l i t t l e i n t e r e s t i n the r o l e of p r e d a t o r s i n d e t e r m i n i n g the c o m p o s i t i o n of f r e s h w a t e r prey communities u n t i l the c l a s s i c papers of Hrbacek (1962) and Brooks and Dodson (1965). S i n c e t h a t time c o n s i d e r a b l e e v i d e n c e has been amassed t o show t h a t f i s h can a l t e r the c o m p o s i t i o n of c r u s t a c e a n prey ( H u r l b e r t , Z e d l e r and F a i r b a n k s , 1972; Z a r e t , 1978; O ' B r i e n , 1979). F i s h p l a n k i v o r e s s e l e c t i v e l y remove prey which a r e l a r g e i n s i z e or v i s i b l e t h r o u g h p i g m e n t a t i o n and/or c o n s p i c u o u s l o c o m o t i o n ( Z a r e t , 1978). Thus the presence of f i s h p r e d a t o r s f a v o u r s prey s p e c i e s which have a s m a l l b o d y - s i z e , reduced p i g m e n t a t i o n or reduced motion ( N o r t h c o t e and C l a r o t t o , 1975; Z a r e t , 1975; Z a r e t and K e r f o o t , 1975; N o r t h c o t e e t a l . , 1978; Lynch, 1979). The r o l e of i n v e r t e b r a t e p r e d a t o r s i n c a u s i n g changes i n the c o m p o s i t i o n and abundance of prey p o p u l a t i o n s i s more c o n t r o v e r s i a l but some r e s e a r c h e r s ( e . g . Lane, 1979) suggest t h e s e p r e d a t o r s a r e a t l e a s t as i m p o r t a n t as f i s h i n s h a p i n g prey dynamics. There are s e v e r a l reasons why the impact of i n v e r t e b r a t e p r e d a t o r s i s d i f f i c u l t t o d e f i n e . I n v e r t e b r a t e p r e d a t o r s , u n l i k e f i s h p r e d a t o r s , u s u a l l y have a l i f e e x p e c t a n c y o n l y s l i g h t l y l o n g e r than t h e i r p r e y . Thus t h e r e i s l i t t l e t i m e , r e l a t i v e t o f i s h , f o r i n v e r t e b r a t e p r e d a t o r s t o 180 have an impact. There i s a l s o l i t t l e f l e x i b i l i t y i n the prey type t h a t they can l o c a t e and handle ( H a l l e t a l . , 1976; Z a r e t , 1978). The prey c h a r a c t e r i s t i c s which a f f e c t the s u c c e s s of an i n v e r t e b r a t e p r e d a t o r a r e o f t e n d i f f i c u l t t o d e f i n e because they r e p r e s e n t a complex f u n c t i o n of s i z e , shape, b e h a v i o u r , t a c t i l e or c h e m i c a l d e t e c t a b i l i t y ( L i and L i , 1979), and s p a t i a l and t e m p o r a l i n t e r a c t i o n s ( L e w i s , 1977). Attempts t o d e s c r i b e the p r e d a t o r - p r e y r e l a t i o n s e x p e r i m e n t l y have been confounded by these problems (McQueen, 1969; Dodson, 1974; Fedorenko, 1975; K e r f o o t , 1979; Lane, 1979; Lynch, 1979). P r e d a t o r y impact and the t e m p o r a l a v a i l a b i l i t y of food f o r both p r e d a t o r and prey are i n t e r r e l a t e d and d i f f i c u l t t o uncouple i n examining communities composed of s m a l l , s h o r t -l i v e d a q u a t i c a n i m a l s . However Lane (1979), u s i n g f l u o r e s c e n t l a b e l l i n g t e c h n i q u e s , e s t i m a t e s t h a t C y c l o p s b i c u s p i d a t u s . can c r o p 33% of the Daphnia p o p u l a t i o n a t 5 m d u r i n g June and 69% of t h i s p o p u l a t i o n a t 20 m. A l t h o u g h Lane e s t i m a t e s lower r a t e s of prey consumption by t h i s p r e d a t o r d u r i n g J u l y and August, i t i s c l e a r t h a t C . b i c u s p i d a t u s has a major impact on Daphnia biomass i n G u l l Lake. However, G u l l Lake i s a r e l a t i v e l y n u t r i e n t r i c h system and t h e r e f o r e may r e p r e s e n t an environment where C . b i c u s p i d a t u s can e x e r c i s e h i g h p r e d a t i o n r a t e s per p r e d a t o r w i t h r e l a t i v e l y l i t t l e o v e r a l l i n f l u e n c e on p r e y abundance. Z a r e t (1978) s u g g e s t s t h a t i f p r e d a t i o n and c o m p e t i t i o n a r e the main d e t e r m i n a n t s of community s p e c i e s c o m p o s i t i o n , n u t r i e n t s w i l l have t h e i r g r e a t e s t e f f e c t on the n u m e r i c a l l e v e l s of s p e c i e s r a t h e r than on c o m p o s i t i o n of s p e c i e s . H a l l e t a l . (1976) c o n c l u d e t h a t i n v e r t e b r a t e 181 p r e d a t o r s r a r e l y cause prey e x t i n c t i o n s , u n l i k e v e r t e b r a t e p r e d a t o r s , because they have poor f u n c t i o n a l and n u m e r i c a l responses r e l a t i v e t o t h e i r p r e y . Lynch (1979) r e p o r t s t h a t the i n v e r t e b r a t e p r e d a t o r Chaoborus reaches s u f f i c i e n t l y h i g h d e n s i t i e s i n P l e a s a n t Pond t o cause prey e x t i n c t i o n s but t h a t a s m a l l e r p r e d a t o r , the c y c l o p o i d copepod C y c l o p s v e r n a l i s , does not. T h e r e f o r e i n a n u t r i e n t r i c h environment prey w i t h h i g h r e p r o d u c t i v e r a t e s may n u m e r i c a l l y swamp a s m a l l p r e d a t o r w i t h a poor f u n c t i o n a l response and low r a t e s of r e c r u i t m e n t such t h a t no net impact on prey p o p u l a t i o n numbers i s d e t e c t e d . However, i n an o l i g o t r o p h i c system these m i c r o c r u s t a c e a n p r e d a t o r s may have a s i g n i f i c a n t n u m e r i c a l and/or c o m p o s i t i o n a l e f f e c t on prey p o p u l a t i o n s because prey are a l r e a d y f o o d -l i m i t e d and abundance i s - low. F u r t h e r m o r e , i t i s a l s o p o s s i b l e t h a t the impact of predaceous c y c l o p o i d copepods i s a l t e r e d by the presence of o t h e r i n v e r t e b r a t e p r e d a t o r s such t h a t the combined e f f e c t i s g r e a t e r than t h a t p r e d i c t e d from examining e i t h e r p o p u l a t i o n a l o n e . S m a l l i n v e r t e b r a t e p r e d a t o r s may i n f l u e n c e r e l a t i o n s between h e r b i v o r o u s c r u s t a c e a n s p e c i e s by r e d u c i n g the number of c o m p e t i t o r s and t h e r e b y i n c r e a s i n g s c a r c e food r e s o u r c e s . A l t e r n a t i v e l y , i n s u f f i c i e n t energy may be a v a i l a b l e t o s u p p o r t a s u b s t a n t i a l p r e d a t o r y t r o p h i c l e v e l because prey d e n s i t y and p r o d u c t i v i t y a re low. Hence p r e d a t i o n may be unimportant under o l i g o t r o p h y and o n l y become of consequence under more p r o d u c t i v e c o n d i t i o n s . In t h i s study I examined, the impact of C.b.thomasi on the r e l a t i v e and a b s o l u t e abundances of h e r b i v o r o u s p r e y i n l a r g e 182 i n s i t u p e r t u r b a t i o n s of n u t r i e n t s , C y c l o p s and Chaoborus d e n s i t i e s . By u s i n g f i e l d e n c l o s u r e s I was a b l e t o e x p e r i m e n t a l l y m a n i p u l a t e the i n i t i a l c o m p o s i t i o n of the z o o p l a n k t o n communities and t r a c k n u m e r i c a l responses through time w h i l e e n s u r i n g t h a t e n v i r o n m e n t a l v a r i a b l e s i n f l u e n c e d a l l t r e a t m e n t s i n a s i m i l a r manner. By f e r t i l i z i n g water b o d i e s of i d e n t i c a l s i z e and shape, the manner i n which c o m p e t i t i o n and p r e d a t i o n i n t e r f a c e w i t h n u t r i e n t l e v e l s c o u l d be m o n i t o r e d w i t h o u t the c o n f o u n d i n g e f f e c t s of d i f f e r e n t l a k e m o r p h o l o g i e s . T h i s approach a l s o a l l o w s p r e d a t o r impact t o be a s s e s s e d w i t h o u t the problem of e s t i m a t i n g ' p r e d a t o r e f f e c t w i t h a t e c h n i q u e which does not d i s c r i m i n a t e between pr e y k i l l e d by p r e d a t i o n and those d y i n g from o t h e r causes. The e x p e r i m e n t s were t h e r e f o r e d e s i g n e d t o 1) compare the impact of C.b. thomasi p r e d a t i o n on the z o o p l a n k t o n communi-ty when C y c l o p s i s the o n l y p r e d a t o r w i t h t h a t when C y c l o p s i s combined w i t h another i n v e r t e b r a t e p r e d a t o r which has a more i m p r e s s i v e f u n c t i o n a l response 2) t o compare these e f f e c t s w i t h those produced i n the same communities by a l t e r i n g the n u t r i e n t environment i n s i d e - b y - s i d e e n c l o s u r e s 3) t o compare c r u s t a c e a n community dynamics under the above c o n d i t i o n s w i t h t h e responses of the same g r a z e r community when i n v e r t e b r a t e p r e d a t o r s a r e a b s e n t . 183 MATERIALS AND METHODS Biomass e s t i m a t e s Dry w e i g h t s were d e t e r m i n e d f o r a l l c y c l o p o i d copepods as d e s c r i b e d i n s e c t i o n t h r e e . I e s t i m a t e d the biomass of a l l c l a d o c e r a n s and c a l a n o i d copepods by c o n v e r t i n g l e n g t h measurements t o dry we i g h t . A l l measurements were.made u s i n g a c a l i b r a t e d o c u l a r micrometer i n a d i s s e c t i n g m i c r o s c o p e a t 50X m a g n i f i c a t i o n . For C l a d o c e r a the d i s t a n c e between the a n t e r i o r end of the head ( e x c l u d i n g helmet p r o j e c t i o n s i f p r e s e n t ) and the p o s t e r i o r margin of the v a l v e s ( e x c l u d i n g s p i n e ) was measured. For c a l a n o i d copepods, the metasome p l u s urosome ( e x c l u d i n g c a u d a l rami) was c o n s i d e r e d body l e n g t h . Means of a t l e a s t 40 a n i m a l s were c o n v e r t e d t o d r y weight u s i n g e q u a t i o n s d e r i v e d from Research F o r e s t Lake a n i m a l s ( N e i l l , unpub.data) or from Dumont e t a l . ( 1 9 7 5 ) . P h y t o p l a n k t o n c e l l volumes, e s t i m a t e d as d e s c r i b e d i n S e c t i o n Three, were c o n v e r t e d t o a s h - f r e e d r y weight u s i n g the c r i t e r i a of V o l l e n w e i d e r (1969). 184 F i e l d e x p e r i m e n t s The e x p e r i m e n t a l d e s i g n and s a m p l i n g methods were a l s o d e s c r i b e d i n s e c t i o n t h r e e . A l l e x p e r i m e n t s were c a r r i e d out i n Gwendoline Lake. .There were 6 t r e a t m e n t s : 3 d i f f e r e n t communities and 2 l e v e l s of n u t r i e n t s . I s h a l l r e f e r t o the community w i t h l a k e d e n s i t i e s of the Gwendoline Lake c r u s t a c e a n s but no Chaoborus as ' p r e d a t o r - f r e e ' ; the community w i t h the above p l u s added P l a c i d Lake d e n s i t i e s of C.b.thomasi as the ' C y c l o p s ' t r e a t m e n t ; and the l a t t e r community, i n c l u d i n g a l l the above p l u s Gwendoline Lake d e n s i t i e s of Chaoborus as the 'Chaoborus-Cyclops' community. The 2 n u t r i e n t l e v e l s were c r e a t e d by f e r t i l i z i n g one h a l f of the community t r e a t m e n t e n c l o s u r e s and l e a v i n g the o t h e r h a l f w i t h u n a l t e r e d Gwendoline Lake water. A s i n g l e l a r g e dose of a p h o s p h a t e - n i t r a t e f e r t i l i z e r (NaH^ PO^and KNO^, a t a r a t i o of 1:10) was added i n mid-May at a phosphate c o n c e n t r a t i o n of 500 ug PCy per l i t r e . P a r t i a l m i x i n g was c a r r i e d out by o c c a s i o n a l b u b b l i n g d u r i n g June, J u l y and August. The ' h i g h ' n u t r i e n t t r e a t m e n t was not d e s i g n e d t o s i m u l a t e any ' n a t u r a l ' c o n d i t i o n but r a t h e r t o s u b s t a n t i a l l y i n c r e a s e food r e s o u r c e s d u r i n g e a r l y June and t h e r e b y enhance s p r i n g c r u s t a c e a n r e c r u i t m e n t . C l u s t e r a n a l y s e s were performed on the d a t a t o d e s c r i b e the r e l a t i v e s p e c i e s abundances between t r e a t m e n t s d u r i n g two time i n t e r v a l s , May-June and August. The time p e r i o d s were chosen t o r e f l e c t i n i t i a l c o n d i t i o n s / reponses and the p e r i o d of g r e a t e s t d i v e r g e n c e , r e s p e c t i v e l y , thus f a c i l i t a t i n g the i d e n t i f i c a t i o n of s p e c i e s g r o u p i n g s t h r o u g h time and between t r e a t m e n t s . The raw d a t a m a t r i x c o m p r i s i n g the n u m e r i c a l 185 abundance of 21 s p e c i e s / s t a g e s was m o d i f i e d t o make the t r e a t m e n t e f f e c t s a d d i t i v e by a p p l i c a t i o n of the l o g ( x + l ) t r a n s f o r m a t i o n ( S o k a l and R o l f , 1969). Pearson product-moment c o r r e l a t i o n c o e f f i c i e n t s were used as the b a s i s f o r UPGMA c l u s t e r i n g (unweighted p a i r - g r o u p method u s i n g the a r i t h m e t i c average of the c o e f f i c i e n t s between a sample c a n d i d a t e f o r a d m i s s i o n and t h a t of members of an e x t a n t c l u s t e r (Sneath and S o k a l , 1 9 7 3 ) ) . T h i s c l u s t e r i n g t e c h n i q u e y i e l d e d the be s t r e p r e s e n t a t i o n of r e l a t i o n s h i p s among m a t r i c e s as measured by c o p h e n e t i c c o r r e l a t i o n (Sneath and S o k a l , 1973). The a n a l y s e s were performed u s i n g the NT-SYs•package of programs d e v e l o p e d by F . J . R o h l f , J . Rispaugh and D. K i r k a t the S t a t e U n i v e r s i t y of New York a t Stony Brook. G e n e r a l L i f e H i s t o r i e s The two major p l a n k t o n i n v e r t e b r a t e p r e d a t o r s i n c l u d e d i n the e x p e r i m e n t a l d e s i g n are Chaoborus l a r v a e and the c y c l o p o i d copepod C.b.thomasi. Both p r e d a t o r s a re found i n Gwendoline Lake a l t h o u g h C.b.thomasi i s r a r e . Of the 2 Chaoborus s p e c i e s found i n Gwendoline Lake, C.americanus i s u n i v o l t i n e w h i l e C . t r i v i t t a t u s may ta k e 2 y e a r s t o mature. The l a t t e r c o m p r i s e s 90-95% of the Chaoborus l a r v a e ( N e i l l , 1980). A l t h o u g h p u p a t i o n , emergence and r e p r o d u c t i o n o c c u r s i n l a t e s p r i n g t o e a r l y summer, the 2 year c y c l e of the most abundant s p e c i e s , C . t r i v i t t a t u s , r e s u l t s i n a s u s t a i n e d abundance t h r o u g h o u t the year of l a r g e r l a r v a e t h a t prey upon c r u s t a c e a n s . F i r s t and second i n s t a r l a r v a e a r e p r e s e n t f o r a r e l a t i v e l y s h o r t p e r i o d from June through mid-186 J u l y and th e s e i n s t a r s prey upon s m a l l a n i m a l s , r o t i f e r s and n a u p l i i ( N e i l l and Peacock, 1980). Because C.b.thomasi was p r e s e n t but r a r e i n Gwendoline Lake, I i n t r o d u c e d C.b.thomasi t o 4 e n c l o s u r e s a t d e n s i t i e s r e a l i s t i c f o r nearby P l a c i d Lake. As d e s c r i b e d i n s e c t o n t h r e e , i n t r o d u c t i o n t o the Gwendoline Lake e n c l o s u r e s i n the absence of Chaoborus d i d not appear t o d i s r u p t the s e a s o n a l dynamics of C.b.thomasi as observed i n P l a c i d Lake. C a r n i v o r o u s i n s t a r s of C.b.thomasi were p r e s e n t d u r i n g May and June, when r e p r o d u c t i o n o c c u r r e d . A d u l t m o r t a l i t y e s c a l a t e d by l a t e June, l e a v i n g s m a l l p a r t i c l e - f e e d i n g n a u p l i i t o r e p r e s e n t the s p e c i e s . The n a u p l i i d e v e l o p e d t o c o p e p o d i d i n s t a r s by m i d - J u l y , becoming i n c r e a s i n g c a r n i v o r o u s w i t h each m o l t . By August the C.b.thomasi p o p u l a t i o n was a g a i n dominated by c a r n i v o r o u s i n s t a r s . The h e r b i v o r o u s prey s p e c i e s i n Gwendoline Lake a l s o r e p r o d u c e d i n the s p r i n g - D.leptopus r e p r o d u c i n g as e a r l y as March i n 1977 (M.A. Chapman, unpub.data). By l a t e May D.kenai was r e p r o d u c i n g and the c l a d o c e r a n s , D.rosea and Holopedium qibberum, were common. Bosmina l o n q i r o s t r i s was p r e s e n t i n low numbers. The o n l y o t h e r c l a d o c e r a n commonly en c o u n t e r e d i n Gwendoline Lake, Diaphanosoma brachyurum, was a summer s p e c i e s and appeared i n the p l a n k t o n i n J u l y . T . p r a s i n u s was the o n l y r e l a t i v e l y abundant c y c l o p o i d copepod and d i d not appear i n the p l a n k t o n i n c o u n t a b l e numbers u n t i l l a t e J u l y . T . p r a s i n u s began r e p r o d u c t i o n i n August and d i s a p p e a r e d from the p l a n k t o n a g a i n by e a r l y November. The c a l a n o i d copepods u s u a l l y produce no more than two 187 g e n e r a t i o n s per y e a r ; T . p r a s i n u s produces o n l y one. In c o n t r a s t t o slow copepod r e c r u i t e r s , the c l a d o c e r a n s a r e a l l m u l t i v o l t i n e , p r o d u c i n g p a r t h e n o g e n i c young t h r o u g h the s p r i n g -summer p e r i o d and r e s t i n g eggs i n l a t e f a l l . T y p i c a l l y , c l a d o c e r a n p o p u l a t i o n s d e c l i n e d u r i n g l a t e J u l y and August and i n c r e a s e a g a i n i n e a r l y September. Body S i z e and Weight The spectrum of z o o p l a n k t o n s i z e s observed w i t h i n the e n c l o s u r e s i n Gwendoline Lake are shown i n T a b l e 12. The a d u l t mean l e n g t h of both c y c l o p o i d s p e c i e s was <1.0 mm. C.b.thomasi a d u l t s were s l i g h t l y l a r g e r (max. of 1.1 mm) than those of T . p r a s i n u s (max. of 0.9 mm). T . p r a s i n u s n a u p l i i were d i s t i n g u i s h e d from C.b.thomasi n a u p l i i by t h e i r green c o l o r a t i o n . A huge c y c l o p o i d copepod-, M a c r o c y c l o p s a l b i d u s , (max.length 2.6 mm) was p r e s e n t i n low numbers throughout the season i n both n u t r i e n t t r e a t m e n t s , a l t h o u g h somewhat more abundant (but always <0.2 i n d i v i d u a l s / l i t r e ) i n the f e r t i l i z e d e n c l o s u r e s . M e s o c y c l o p s edax was i r r e g u l a r l y sampled, as were the c l a d o c e r a n s , L a t o n a sp., C e r i o d a p h n i a sp., and S c h a p h o l e b e r i s sp. A l l t h e s e r a r e s p e c i e s were more common i n the f e r t i l i z e d e n c l o s u r e s than the u n f e r t i l i z e d t r e a t m e n t s . C y c l o p o i d n a u p l i i and r o t i f e r s p r o b a b l y r e p r e s e n t e d the s m a l l e s t p r e y i n the community,along w i t h neonates of Chydorus and Bosmina. -Although d i a p t o m i d n a u p l i i and copepodids were not d i f f e r e n t i a t e d t o s p e c i e s , the mean s i z e s of the copopodids c o r r e s p o n d e d t o Fedorenko's (1973) ' s m a l l ' group, perhaps s u g g e s t i n g t h a t t h e s e i n s t a r s were D.leptopus c o p e p o d i d s . TABLE 12. Mean l e n g t h and dry w e i g h t s of Gwendoline Lake z o o p l a n k t o n used i n c o n v e r s i o n s t o biomass. S p e c i e s Length (mm) Dry Weight (ug/animal) C.b.thomasi a d u l t c o p e p o d i d n a u p l i us T . p r a s i n u s a d u l t cope podi d n a u p l i u s p . k e n a i D. l e p t o r u i s a d u l t a d u l t P i a Ftomus c o p e p o d i d P i a pto mas n a u p l i u s D. r o s e a H o l o p e d i um  Bosmina  Ch ydorns R o t i f e r s Other c y c l o p o i d s Other c l a d o c e r a n s 0.91 0.55 0. 13 0.70 0.45 0.10 2.41 1.60 1.33 0.36 1.32 1.23 0.40 0.35 1.73 0,98 5. 07 1. 66 0. 37 2. 57 1. 14 0. 24 49. 73* 23. 02* 14.96* 0. 71* 7. 60** 12.62** 1.80* • 1. 48* 0. 28** 26.79* 2.95* * from Dumont ( 1975) ** from l e n g t h / w e i g h t r e g r e s s i o n s on Research F o r e s t Lake a n i m a l s ( N e i l l , unpub.data) 189 Mean s i z e of the c a l a n o i d n a u p l i i i n t h i s s tudy (0.36 mm) was s i m i l a r t o t h a t of Fedorenko's (0.35 mm) i n which both s p e c i e s of d i a p t o m i d s were a l s o grouped as c a l a n o i d n a u p l i i . C o n s e q u e n t l y i t was d i f f i c u l t t o s p e c u l a t e on the r e l a t i v e r e p r e s e n t a t i o n of the two c a l a n o i d copepod s p e c i e s based on s i z e , a l t h o u g h poor r e p r e s e n t a t i o n of D.kenai a d u l t s i n September i n a l l t r e a t m e n t s suggested t h a t most s u r v i v i n g n a u p l i i were D . l e p t o p u s . 190 RESULTS C l u s t e r A n a l y s e s of the Community I o b s e r v e d c r u s t a c e a n z o o p l a n k t o n community reponses t o f e r t i l i z a t i o n under d i f f e r e n t c o n d i t i o n s of i n v e r t e b r a t e p r e d a t i o n d u r i n g the p e r i o d from mid-May t o l a t e September, 1977. I n i t i a l d e n s i t i e s of a l l z o o p l a n k t o n but C.b.thomasi mimicked Gwendoline Lake d e n s i t i e s . Because the e n c l o s u r e s were f i l l e d w i t h the May assemblage of a n i m a l s and T . p r a s i n u s d i d not appear i n the p l a n k t o n u n t i l m i d - J u l y , T . p r a s i n u s was added t o a l l e n c l o s u r e s t o e q u a l summertime d e n s i t i e s . The n u m e r i c a l abundance of 21 s p e c i e s / s t a g e s ( l o g ( x + l ) t r a n s f o r m e d ) was used t o c a l c u l a t e a c o r r e l a t i o n m a t r i x f o r 7 c o l l e c t i o n d a t e s r e p r e s e n t i n g 49 samples d u r i n g May-June t o i l l u s t r a t e p a t t e r n s of s i m i l a r s p e c i e s abundances between t r e a t m e n t s and the l a k e . The m a t r i x of species'/stages ( r e p r e s e n t i n g s a m p l i n g d a t e s and t r e a t m e n t s ) was then s u b j e c t e d t o c l u s t e r a n a l y s i s t o f a c i l i t a t e d e s c r i p t i o n of s i m i l a r i t i e s between t r e a t m e n t s and Gwendoline Lake. As C.b.thomasi and Chaoborus were e x p e r i m e n t a l v a r i a b l e s , t h e s e s p e c i e s were not i n c l u d e d i n the a n a l y s i s which t h e r e f o r e r e p r e s e n t e d o n l y Gwendoline Lake g r a z e r s . A c l u s t e r of 10 t r e a t m e n t s i s e v i d e n t a t the t o p of the dendrogram ( F i g . 4 7 a ) . The f i r s t 7 of t h e s e r e p r e s e n t the i n i t i a l samples taken on May 17 from a l l 6 t r e a t m e n t s and the l a k e (sampled t o the same depth as the e n c l o s u r e s ) . I n i t i a l 191 F i g u r e 47. C l u s t e r a n a l y s e s of the t r e a t m e n t e n c l o s u r e s i n Gwendoline Lake. A. T h i s f i g u r e shows the c l u s t e r a n a l y s i s of the s p e c i e s abundance d a t a from the t r e a t m e n t e n c l o s u r e s and the l a k e on 7 sampling d a t e s from May 17 (M17) t o June 28 ( J 2 8 ) , 1977. B. T h i s f i g u r e shows the c l u s t e r a n a l y s i s of the s p e c i e s abundance d a t a f o r the t r e a t m e n t e n c l o s u r e on 5 s a m p l i n g d a t e s i n August. Treatment numbers r e p r e s e n t : 1 - f e r t i l i z e d 'Chaoborus-Cyclops' , 2 - f e r t i l i z e d ' p r e d a t o r - f r e e ' , 3 - ^ f e r t i l i z e d ' C y c l o p s ' , 4 -u n f e r t i l i z e d •' C y c l o p s ' *, 5 - u n f e r t i l i z e d ' p r e d a t o r -f r e e ' , 6 - u n f e r t i l i z e d 'Chaoborus-Cyclops' . S a m p l e D o t e T r e a t m e n t M a y 1 7 - J u n e 2 8 -d •a M 17 1 M 17 . 4 M 17 3 M 17. 2 M 17 5 M 17 6 M. 17 7 M 24 4 M 24 2 M 24 3 M 24 1 M 30 3 M 30 1 J 5 3 M 24 7 J 5 2 J 5 e M 24 6 H 30 6 J 13 4 M 24 5 M 30 5 J 5 1 J 13 1 M 30 7 J 5 7 J 13 7 M 30 2 J 5 5 J 13 5 J 13 2 J 13 3 J 13 6 M 30 4 J 5 4 J 21 5 J 28 5 J 21 6 J 26 6 J 21 7 J 28 7 J 21 2 J 28 2 J 2 1 3 J 26 3 J 2 1 4 J 28 4 J 21 1 J 28 1 193 d e n s i t i e s a r e c l e a r l y v e r y s i m i l a r between t r e a t m e n t s and the l a k e . S i m i l a r i t y between e n c l o s u r e s d e c r e a s e s w i t h time and the dendrogram i n d i c a t e s t h i s c h r o n o l o g i c a l o r d e r from t o p t o bottom. E a r l y samples from a l l t r e a t m e n t s a r e c l o s e l y r e l a t e d but t r e a t m e n t s i n c r e a s i n g l y d i f f e r e n t i a t e i n t o more d i s t a n t l y r e l a t e d groups as time p r o g r e s s e s . By the l a t t e r two weeks i n June, r e p r e s e n t e d by J21 and J28, each t r e a t m e n t e s s e n t i a l l y forms a s m a l l c l u s t e r . However, the f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t i s most s i m i l a r t o the f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e , both of which a r e q u i t e d i s t i n c t from the 'Chaoborus-Cyclops' t r e a t m e n t . I n c l u d e d i n the former g r o u p i n g i s the u n f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t , perhaps s u g g e s t i n g t h a t the i n f l u e n c e of C.b.thomasi i n the u n f e r t i l i z e d environment i s t o r e l e a s e more foo d r e s o u r c e s f o r h e r b i v o r e s t h u s c r e a t i n g c o n d i t i o n s more, l i k e the f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e w i t h o u t Chaoborus. The same a n a l y s i s performed on 5 s a m p l i n g d a t e s and the 6 t r e a t m e n t s i n August i n d i c a t e s c o n s i d e r a b l e d i v e r g e n c e i n prey z o o p l a n k t o n among t r e a t m e n t s has o c c u r r e d ( F i g . 4 7 b ) . There appears t o be two l a r g e c l u s t e r s A and B w i t h each group f u r t h e r s p l i t i n t o 4 and 3 groups r e s p e c t i v e l y . The August 31st sample from the f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t a t the bottom of the dendrogram appears t o be q u i t e d i s t i n c t . W i t h i n t e r e s t i n g e x c e p t i o n s , group A and B c o r r e s p o n d t o f e r t i l i z e d v e r s u s u n f e r t i l i z e d e n c l o s u r e s . W i t h i n A, t h e r e i s e v i d e n c e of a c l u s t e r i n g of a l l the August f e r t i l i z e d ' Chaoborus-Cyclops' samples. T h i s i s the o n l y t r e a t m e n t wherein each sample remains most s i m i l a r t o i t s next 194 c h r o n o l o g i c a l sample throughout August, s u g g e s t i n g t h a t community s t r u c t u r e i s most unique w i t h i n t h i s t r e a t m e n t . As i n the l a t e June samples, the samples from the f e r t i l i z e d ' C y c l o p s ' and ' p r e d a t o r - f r e e ' t r e a t m e n t s appear t o c l u s t e r t o g e t h e r and the o n l y samples from an u n f e r t i l i z e d e n c l o s u r e i n c l u d e d i n c l u s t e r A a r e those from the ' C y c l o p s ' e n c l o s u r e . However, l a t e August samples from t h i s t r e a t m e n t are found i n the B c l u s t e r w i t h samples from a l l the r e m a i n i n g u n f e r t i l i z e d t r e a t m e n t s . In terms of the g e n e r a l p a t t e r n , the most a b e r r a n t samples are the A22 and A31 samples from the f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t . The former c l u s t e r s w i t h the u n f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e w h i l e the l a t t e r forms a d i s t i n c t group. C l a d o c e r a n n u m e r i c a l response To u n d e r s t a n d the p o p u l a t i o n abundances which g i v e r i s e t o the c l u s t e r i n g p a t t e r n s , a d e t a i l e d d e s c r i p t i o n of prey dynamics i n response t o n u t r i e n t and p r e d a t o r m a n i p u l a t i o n s i s p r e s e n t e d . F i g . 4 8 (a-h) i l l u s t r a t e s the t r e n d s i n p o p u l a t i o n d e n s i t i e s of major c l a d o c e r a n s p e c i e s o b s e r v e d i n the e x p e r i m e n t a l communities. With the n o t a b l e e x c e p t i o n of H o l o p e d i um, these fecund z o o p l a n k t e r s responded t o f e r t i l i z a t i o n by i n i t i a l l y i n c r e a s i n g i n number a c r o s s a l l t r e a t m e n t s . Holopedium i n c r e a s e d i n the f e r t i l i z e d e n c l o s u r e where n e i t h e r Chaoborus nor C.b.thomasi were p r e s e n t but r a p i d l y d i s a p p e a r e d from a l l the f e r t i l i z e d e n c l o s u r e s by mid-June - e a r l y J u l y . I o b s e r v e d a s i m i l a r p a t t e r n , a l t h o u g h not so r a p i d a d e c l i n e , i n a l l t r e a t m e n t s except the u n f e r t i l i z e d 195 F i g u r e 48. V a r i a t i o n i n s t a n d i n g c r o p of c l a d o c e r a n c r u s t a c e a n s i n Gwendoline Lake,1977. C i r c l e s r e p r e s e n t the 'Chaoborus-Cyclops' e n c l o s u r e s , squares r e p r e s e n t the ' C y c l o p s ' t r e a t m e n t s and t r i a n g l e s r e p r e s e n t the ' p r e d a t o r - f r e e ' t r e a t m e n t s . Open symbols r e p r e s e n t the f e r t i l i z e d t r e a t m e n t s and s o l i d r e p r e s e n t the u n f e r t i l i z e d t r e a t m e n t s . NUMBER P E R LITRE \ 197 'Chaoborus-Cyclops' e n c l o s u r e . As Holopedium was u s u a l l y found below the t h e r m o c l i n e i n Gwendoline Lake by J u l y , and the e n c l o s u r e s mimic P l a c i d Lake where the maximum depth i s 7 m., t h i s d i s a p p e a r a n c e might r e f l e c t temperature s t r e s s . N e i l l (1980) found t h a t d u r i n g the c o l d s p r i n g of 1976, f e r t i l i t y was h i g h e r i n t h i s s p e c i e s (31% o v i g e r o u s w i t h 2.61±0.21 eggs/female) than i n 1977. In my s h a l l o w e r e n c l o s u r e s , f e r t i l i t y was low i n May (maximum of 13% w i t h 2.1±0.17 eggs/female) and r e p r o d u c t i o n e f f e c t i v e l y ceased i n a l l t r e a t m e n t s by mid-June, a l t h o u g h o v i g e r o u s females were d e t e c t e d s p o r a d i c a l l y i n the u n f e r t i l i z e d 'Chaoborus-Cyclops' e n c l o s u r e . The p e r s i s t e n c e of Holopedium i n the l a t t e r t r e a t m e n t suggested an immunity t o Chaoborus p r e d a t i o n and p o s s i b l y , the importance of t h i s p r e d a t o r i n r e l i e v i n g g r a z i n g p r e s s u r e by c r o p p i n g c o m p e t i t o r s . U n l i k e Holopedium, D.rosea r a p i d l y i n c r e a s e d i n abundance i n a l l the h i g h n u t r i e n t e n c l o s u r e s s u r p a s s i n g p o p u l a t i o n d e n s i t i e s observed i n the u n f e r t i l i z e d e n c l o s u r e s i n June. Rates of f e r t i l i t y were s i m i l a r i n a l l t h e f e r t i l i z e d e n c l o s u r e s : 'Chaoborus-Cyclops' - 29% o v i g e r o u s females w i t h 11.23±0.06 (s.e.) eggs/female; ' p r e d a t o r - f r e e ' - 24% o v i g e r o u s females w i t h 10.85±0.80 eggs/female; ' C y c l o p s ' -26% o v i g e r o u s females w i t h 11.20±0.49 eggs/female. A l l the u n f e r t i l i z e d e n c l o s u r e s were l o w e r : 'Chaoborus-Cyclops' - 15% o v i g e r o u s females w i t h 4.43±0.14; ' p r e d a t o r - f r e e ' - 15% o v i g e r o u s w i t h 3.12±o.22 and ' C y c l o p s ' - 14% o v i g e r o u s w i t h 4.12±0.17 eggs/female. The pr e s e n c e of Chaoborus appeared t o moderate D.rosea p o p u l a t i o n i n c r e a s e s and a s i m i l a r , a l t h o u g h 198 l e s s marked, e f f e c t was obser v e d i n the u n f e r t i l i z e d 'Chaoborus-Cyclops' e n c l o s u r e . W h i l e the presence of Chaoborus dampened n u m e r i c a l i n c r e a s e s i n June, the i n c l u s i o n of t h i s p r e d a t o r a l s o appeared t o moderate the August d e c l i n e . In c o n t r a s t t o Chaoborus, the presence of an abundant C.b.thomasi p o p u l a t i o n n e i t h e r b l u n t e d the June i n c r e a s e nor r e s t r a i n e d the August p o p u l a t i o n c r a s h i n the f e r t i l i z e d e n c l o s u r e . However, i n the u n f e r t i l i z e d e n c l o s u r e , D.rosea does not d e c l i n e as d r a s t i c a l l y i n the C.b.thomasi e n c l o s u r e as t h i s p o p u l a t i o n does i n the u n f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e . F e r t i l i t y d e c l i n e d i n a l l e n c l o s u r e s i n August but o n l y i n the ' p r e d a t o r - f r e e ' e n c l o s u r e (both n u t r i e n t enhanced and u n e n r i c h e d ) d i d f e r t i l i t y r a t e s drop t o z e r o . Thus food l i m i t a t i o n was p r o b a b l y the cause of the p o p u l a t i o n c r a s h e s i n the s e t r e a t m e n t s . In' the 'C y c l o p s ' u n f e r t i l i z e d t r e a t m e n t t h e r e averaged about 15% o v i g e r o u s D.rosea females w i t h 1.2±0.23 eggs/female i n August. In the s i m i l a r community but f e r t i l i z e d t r e a t m e n t o n l y 7% c a r r i e d eggs w i t h 1.0±0.16 eggs/female. C o n s e q u e n t l y the presence of 'C y c l o p s ' appeared t o f r e e up more r e s o u r c e s f o r the D.rosea p o p u l a t i o n i n the u n f e r t i l i z e d e n c l o s u r e than i n the h i g h n u t r i e n t t r e a t m e n t . There were r e l a t i v e l y more o v i g e r o u s females i n both the 'Chaoborus-Cyclops' t r e a t m e n t s and i n the f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e i n August - 11% w i t h 1.4±0.79 and 13% w i t h 1.2±0.11 eggs/female i n the f e r t i l i z e d and u n f e r t i l i z e d t r e a t m e n t s r e s p e c t i v e l y . In sum, D.rosea was unable t o i n c r e a s e n u m e r i c a l l y i n the f e r t i l i z e d 'Chaoborus-Cyclops' e n c l o s u r e t o the e x t e n t p o s s i b l e 199 i n the o t h e r two t r e a t m e n t s and t h i s r e s u l t appeared t o be due t o Chaoborus p r e d a t i o n . In c o n t r a s t C.b.thomasi had • r e l a t i v e l y l i t t l e e f f e c t on D. r o s e a d e n s i t i e s . D. r o s e a e x h i b i t e d dynamics i n the f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e s i m i l a r t o t h a t observed i n the f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t . However, i n the u n f e r t i l i z e d e n c l o s u r e C.b.thomasi appeared t o break the August d e c l i n e i n a manner e q u i v a l e n t t o t h a t o b s e r v e d i n the u n f e r t i l i z e d 'Chaoborus-Cyclops' t r e a t m e n t . Thus C.b.thomasi might be i m p o r t a n t t o D.rosea i n low food environments by r e l i e v i n g g r a z i n g p r e s s u r e through p r e d a t i o n on s m a l l h e r b i v o r e s . In h i g h n u t r i e n t s i t u a t i o n s t h i s e f f e c t seemed t o be swamped by c l a d o c e r a n r e p r o d u c t i o n . Bosmina e x h i b i t e d more o s c i l l a t o r y b e h a v i o u r . T h i s s p e c i e s d i d not i n c r e a s e markedly..- i n the h i g h n u t r i e n t e n c l o s u r e over the' u n f e r t i l i z e d t r e a t m e n t s i n both e n c l o s u r e s w i t h o u t Chaoborus. In the f e r t i l i z e d Chaoborus e n c l o s u r e Bosmina remained a t much lower d e n s i t i e s than i n any of the u n f e r t i l i z e d e n c l o s u r e s ( F i g . 4 8 e , f ) . D u r i n g June, over 25% of the p o p u l a t i o n were o v i g e r o u s i n the f e r t i l i z e d 'Chaoborus-C y c l o p s ' e n c l o s u r e compared t o 20% i n the ' p r e d a t o r - f r e e ' and 17% i n the ' C y c l o p s ' f e r t i l i z e d t r e a t m e n t . T h i s r e l a t i v e l y h i g h f e r t i l i t y y e t low p o p u l a t i o n number of Bosmina i n the f e r t i l i z e d 'Chaoborus-Cyclops' e n c l o s u r e was p r o b a b l y due t o s u p p r e s s i o n by Chaoborus p r e d a t i o n . O v i g e r o u s females c o n s t i t u t e d about 12-15% of the p o p u l a t i o n i n a l l the u n f e r t i l i z e d e n c l o s u r e s d u r i n g t h i s p e r i o d and d e n s i t i e s were o n l y s l i g h t l y lower than those i n the. f e r t i l i z e d t r e a t m e n t s . The i n a b i l i t y of Bosmina t o s i g n i f i c a n t l y i n c r e a s e i n number i n 200 the f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e might be caused by c o m p e t i t i o n from the o t h e r s m a l l c l a d o c e r a n , Chydorus ( F i g . 4 8 g,h). By the end of J u l y , 24% of the Bosmina p o p u l a t i o n were o v i g e r o u s but over 31% of the Chydorus p o p u l a t i o n c a r r i e d eggs. Bosmina d e c l i n e d i n e a r l y August w h i l e Chydorus numbers underwent a minor e x p l o s i o n , i n c r e a s i n g almost 5 - f o l d . The Chydorus bloom was s h o r t - l i v e d , however, and o v i g e r o u s females dropped t o 9%. As t h i s d e c l i n e o c c u r r e d , Bosmina i n c r e a s e d . F e r t i l i t y rose u n t i l 23% of the p o p u l a t i o n c a r r i e d eggs. Bosmina abundance d e c l i n e d a g a i n a t the end of August. T h i s d e c r e a s e was not as sharp and was u n r e l a t e d t o Chydorus abundance because the l a t t e r p o p u l a t i o n never r e c o v e r e d from i t s e a r l y August d e c l i n e i n t h i s e n c l o s u r e . Chydorus ( F i g . 4 8 g,h) d e n s i t i e s s t r u c k a c o n t r a s t t o the Bosmina t r e n d s i n a l l the f e r t i l i z e d e n c l o s u r e s . T h i s . s p e c i e s i n c r e a s e d d r a m a t i c a l l y i n a l l t h e s e e n c l o s u r e s when compared t o p o p u l a t i o n numbers i n the u n f e r t i l i z e d e n c l o s u r e s . Chydorus appeared i n the f e r t i l i z e d 'Chaoborus-Cyclops' t r e a t m e n t i n mid-June, about two weeks ahead of i t s appearance i n the o t h e r f e r t i l i z e d t r e a t m e n t s . Over 40% of the p o p u l a t i o n was o v i g e r o u s d u r i n g June a l t h o u g h t h i s h i g h f e r t i l i t y d e c r e a s e d t o about 15% i n J u l y . F e r t i l i t y was lower i n both ' p r e d a t o r -f r e e ' and ' C y c l o p s ' f e r t i l i z e d e n c l o s u r e s , a t 5% and 15% r e s p e c t i v e l y i n l a t e June. D u r i n g August the p r o p o r t i o n of o v i g e r o u s females i n the f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e rose t o over 40%. T h i s h i g h p r o d u c t i o n of young was not r e f l e c t e d i n p o p u l a t i o n i n c r e a s e s , i n s p i t e of the f a c t t h a t the number of eggs each female 201 c a r r i e d was r e l a t i v e l y i n v a r i a n t (2.0±0.01) a c r o s s a l l t r e a t m e n t s . As p r e v i o u s l y d e s c r i b e d , Chydorus e x p e r i e n c e d a b r i e f bloom i n the f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e and p r o b a b l y e n c o u n t e r e d severe food s h o r t a g e s by mid-August because the f e r t i l i t y r a t e s dropped t o about 9%. In the 'Chaoborus-Cyclops' f e r t i l i z e d t r e a t m e n t o n l y 20-25% of the p o p u l a t i o n was o v i g e r o u s d u r i n g August. C o n s e q u e n t l y , the lower Chydorus p o p u l a t i o n numbers i n the f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e was p r o b a b l y caused by C.b.thomasi p r e d a t i o n . In sum, Bosmina was a p p a r e n t l y not an e f f e c t i v e c o m p e t i t o r at e i t h e r n u t r i e n t l e v e l and was v e r y s e n s i t i v e t o Chaoborus p r e d a t i o n i n the f e r t i l i z e d t r e a t m e n t . In c o n t r a s t Chydorus was a b l e t o i n c r e a s e d r a m a t i c a l l y i n a l l f e r t i l i z e d e n c l o s u r e s . Chydorus seemed r e l a t i v e l y immune t o Chaoborus p r e d a t i o n , p o s s i b l y because Chydorus tended t o c o n c e n t r a t e near the water* s u r f a c e . The most abundant p o p u l a t i o n of Chydorus was found i n the f e r t i l i z e d 'Chaoborus-Cyclops' e n c l o s u r e . Lowest Chydorus d e n s i t i e s , among the f e r t i l i z e d t r e a t m e n t s , were found i n the ' C y c l o p s ' e n c l o s u r e , t h u s C.b.thomasi, u n l i k e Chaoborus, a p p a r e n t l y had a s i g n i f i c a n t impact on Chydorus. At low n u t r i e n t l e v e l s Chydorus was l e a s t abundant i n the ' p r e d a t o r -f r e e ' e n c l o s u r e s u g g e s t i n g t h a t Chydorus was a weak c o m p e t i t o r and unable t o s e q u e s t e r food r e s o u r c e s i n the absence of an i n v e r t e b r a t e p r e d a t o r t o r e l e a s e some g r a z i n g p r e s s u r e . 202 C a l a n o i d Copepod Response The Gwendoline Lake c a l a n o i d copepods d i d not i n c r e a s e n u m e r i c a l l y i n response t o f e r t i l i z a t i o n of the e n c l o s u r e s ( f i g . 4 9 a - h ) . D.kenai a d u l t s v a n i s h e d from a l l t r e a t m e n t s by l a t e J u l y and I d i d not sample a d u l t s a g a i n . T h i s d i s a p p e a r a n c e p r o b a b l y r e f l e c t e d temperature s t r e s s as D.kenai a d u l t s were sampled o n l y below the t h e r m o c l i n e i n Gwendoline Lake d u r i n g the summer months. In P l a c i d Lake where maximun depth i s the same as my e n c l o s u r e s , D.kenai a d u l t s a l s o d i s a p p e a r e d from the p l a n k t o n d u r i n g J u l y and August. T h i s phenomenon has been obse r v e d over many y e a r s ( W a l t e r s , unpub.data). C a l a n o i d copepod n a u p l i i were c o n t i n u o u s l y p r e s e n t i n a l l u n f e r t i l i z e d e n c l o s u r e s but were not sampled d u r i n g the mid-August p e r i o d i n a l l the h i g h n u t r i e n t e n c l o s u r e s . C a l a n o i d c opepodids r e c r u i t e d from n a u p l i a r i n s t a r s most s u c c e s s f u l l y i n the u n f e r t i l i z e d C.b.thomasi e n c l o s u r e ( F i g . 4 9 e , f ) . Copepodids d e c r e a s e d t o low l e v e l s i n a l l the f e r t i l i z e d e n c l o s u r e s by e a r l y August and d i s p p e a r e d from the samples i n the u n f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e s by the same t i m e . A g a i n , c a l a n o i d copepodids seemed t o r e c r u i t most e f f e c t i v e l y (of the f e r t i l i z e d t r e a t m e n t s ) i n the ' C y c l o p s ' f e r t i l i z e d e n c l o s u r e . Only i n t h i s t r e atment were copepodids c o n t i n u o u s l y p r e s e n t even though numbers were low. The weakest r e c r u i t m e n t of copepodids was i n the ' p r e d a t o r - f r e e ' e n c l o s u r e s i n both n u t r i e n t t r e a t m e n t s . D .leptopus i s commonly found i n the e p i l i m n i o n i n summer ( W a l t e r s , unpub.data). T h i s t o l e r a n c e f o r h i g h e r temperature 203 F i g u r e 49. V a r i a t i o n i n s t a n d i n g c r o p of c a l a n o i d copepod i n Gwendoline Lake, 1977. The t r e a t m e n t e n c l o s u r e s •Gwendoline Lake are i n d i c a t e d as i n F i g . 48. N U M B E R P E R L I T R E -to? 205 may have c o n t r i b u t e d t o the more p o s i t i v e ' n u m e r i c a l response of D .leptopus a d u l t s i n the e n c l o s u r e s compared t h a t of D.kenai. September v a l u e s were h i g h e r i n the u n f e r t i l i z e d e n c l o s u r e s and t h e r e was l i t t l e community tr e a t m e n t e f f e c t . C y c l o p o i d Copepod Response C.b.thomasi, d e s c r i b e d i n s e c t i o n 3 ( F i g . 3 9 ) , responded p o s i t i v e l y t o f e r t i l i z a t i o n i n the absence of Chaoborus l a r v a e . The i n t e r a c t i o n of both Chaoborus and C.b.thomasi c l e a r l y had a n e g a t i v e impact on the C.b.thomasi p o p u l a t i o n . Thus the 'Chaoborus-Cyclops' e n c l o s u r e s became e s s e n t i a l l y Chaoborus e n c l o s u r e s . T . p r a s i n u s , the o n l y o t h e r n u m e r i c a l l y i m p o r t a n t c y c l o p o i d copepod i n the system, i n c r e a s e d d r a m a t i c a l l y i n the f e r t i l i z e d t r e a t m e n t w i t h o u t e i t h e r Chaoborus or C.b.thomasi ( f i g . 5 0 a , b ) . However, i n the p r e s e n c e of C^b.thomasi, the T . p r a s i n u s p o p u l a t i o n was more abundant i n the u n f e r t i l i z e d e n c l o s u r e . In the u n f e r t i l i z e d 'Chaoborus-Cyclops' t r e a t m e n t T . p r a s i n u s d e n s i t i e s were as h i g h as i n the u n f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e . Thus, Chaoborus l a r v a e were p r o b a b l y not i m p o r t a n t i n r e s t r i c t i n g T . p r a s i n u s numbers i n the u n f e r t i l i z e d environment. There was a s l i g h t dampening e f f e c t i n the f e r t i l i z e d 'Chaoborus-Cyclops' e n c l o s u r e , however ( F i g . 5 0 a , b ) . 206 F i g u r e 50. Changes i n the s t a n d i n g c r o p of T . p r a s i n u s i n Gwendoline Lake e n c l o s u r e s , 1977. Symbols a r e d e s c r i b e d i n the F i g . 48 l e g e n d . N U M B E R P E R L I T R E 208 Biomass Response of P h y t o p l a n k t o n and Prey Zooplankton The e f f e c t of a z o o p l a n k t o n p o p u l a t i o n on community food r e s o u r c e s i s c l o s e l y c o r r e l a t e d w i t h biomass, which v a r i e s g r e a t l y between s p e c i e s and i n s t a r . C o n s e q u e n t l y , I examined the r e l a t i v e e f f e c t of the p r e d a t o r and n u t r i e n t t r e a t m e n t s on p h y t o p l a n k t o n and z o o p l a n k t o n biomass ( f i g . 5 1 a , b ) . Chaoborus was not i n c l u d e d i n the t o t a l z o o p l a n k t o n biomass shown i n F i g . 5 1 b because t h i s z o o p l a n k t e r f u n c t i o n e d e x c l u s i v e l y , i n terms of i t s i n t e r a c t i o n w i t h o t h e r z o o p l a n k t o n s p e c i e s , as p r e d a t o r . However, because c y c l o p o i d copepods were both p r e y and p r e d a t o r the biomass of these s p e c i e s was i n c l u d e d i n the a n a l y s i s of t o t a l p rey z o o p l a n k t o n biomass. Adding n u t r i e n t s t o the Gwendoline Lake e n c l o s u r e s caused a s i g n i f i c a n t i n c r e a s e i n a l g a l biomass ( F i g . 5 1 a ) . In May and e a r l y June, a l l the f e r t i l i z e d e n c l o s u r e s had a h i g h e r biomass of nannoplankton (2-20 um) and t h i s r e s u l t e d i n h i g h e r z o o p l a n k t o n biomass i n each f e r t i l i z e d t r e a t m e n t r e l a t i v e t o i t s u n f e r t i l i z e d e q u i v a l e n t . However, by J u l y and August, b l u e - g r e e n f i l a m e n t o u s a l g a e were the p h y t o p l a n k t o n group which e x p e r i e n c e d the most enhancement i n the f e r t i l i z e d e n c l o s u r e s . S e v e r a l a u t h o r s (eg. A r n o l d , 1971; G l i w i c z , 1975; P o r t e r , 1977) argue t h a t b l u e - green a l g a e can not be u t i l i z e d v e r y e f f e c t i v e l y by z o o p l a n k t o n g r a z e r s , p a r t i c u l a r l y the c l a d o c e r a n s . F i g 51b seems t o a t l e a s t p a r t i a l l y s u p port t h i s view because the l a r g e J u l y - A u g u s t p h y t o p l a n k t o n biomass does n o t , i n g e n e r a l , t r a n s l a t e i n t o h e r b i v o r e biomass. For most of t h i s p e r i o d i n a l l f e r t i l i z e d e n c l o s u r e s the r a t i o of p h y t o p l a n k t o n t o z o o p l a n k t o n biomass was g r e a t e r than 1.0. 209 F i g u r e 51. A s h - f r e e dry weight of t o t a l p h y t o p l a n k t o n and z o o p l a n k t o n biomass. A. A s h - f r e e d ry weight (AFDW) of t o t a l p h y t o p l a n k t o n biomass i n each treatment from May t o September, 1977. Nannoplankton, >20 um c e l l s and c o l o n i e s , and f i l a m e n t s i n d i c a t e d by l i g h t l i n e , w h i t e and dark s t r i p e s r e s p e c t i v e l y . Treatment l e t t e r s r e p r e s e n t : Cy -u n f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t , pf - u n f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t , Ch-Cy - u n f e r t i l i z e d 'Chaoborus-Cyclops' t r e a t m e n t , CyF - f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t , p f F - f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t , Cy-ChF - f e r t i l i z e d 'Chaoborus-Cyclops' t r e a t m e n t . B. Changes i n the AFDW of the t o t a l c r u s t a c e a n - r o t i f e r biomass i n each Gwendoline Lake t r e a t m e n t from May t o September, 1977. L e t t e r s f o r each treatment a r e the same as i n a. a. Phytoplankton b. Zooplankton 211 N e v e r t h e l e s s , t h e r e were d i s t i n c t d i f f e r e n c e s between t r e a t m e n t s . The z o o p l a n k t o n biomass i n the f e r t i l i z e d ' C y c l o p s ' treatment was most s i m i l a r t o i t s u n f e r t i l i z e d c o u n t e r p a r t throughout the summer season. There were d i f f e r e n c e s i n e a r l y June, when the z o o p l a n k t o n biomass i n the u n f e r t i l i z e d e n c l o s u r e was s l i g h t l y h i g h e r . T h i s r e s u l t was p r o b a b l y caused by the f a c t t h a t a d u l t D.kenai were r e l a t i v e l y more numerous ( F i g . 4 9 a , b ) . A s m a l l n u m e r i c a l change i n these l a r g e a n i m a l s t r a n s l a t e d i n t o s i g n i f i c a n t biomass d i f f e r e n c e s , a l t h o u g h the n u m e r i c a l change was w i t h i n the range of s a m p l i n g e r r o r . D u r i n g J u l y the f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e m a i n t a i n e d about 2.5X the z o o p l a n k t o n biomass of the u n f e r t i l i z e d e n c l o s u r e f o r s e v e r a l weeks a f t e r the l a t t e r e x p e r i e n c e d a sharp m i d - J u l y d e c l i n e . - There were two i m p o r t a n t f a c t o r s which c o n t r i b u t e d t o t h i s d i f f e r e n c e : much g r e a t e r C. b.thomasi copepodid r e c r u i t m e n t i n the f e r t i l i z e d t r e a t m e n t and a d e l a y i n the D.rosea d e c l i n e u n t i l e a r l y August ( F i g . 5 2 a,b) c o u p l e d w i t h a h i g h D.rosea p o p u l a t i o n ( F i g . 4 8 b , c ) . The p r o p o r t i o n of the D.rosea biomass i n r e p r o d u c t i v e c o n d i t i o n was h i g h e r d u r i n g J u l y i n the f e r t i l i z e d t r e a t m e n t ( F i g . 5 2 a,b) but d e c l i n e d i n August. A p p a r e n t l y f e r t i l i z a t i o n i n the presence of ' C y c l o p s ' p r o l o n g e d the f a v o u r a b l e food environment f o r the D. r o s e a p o p u l a t i o n a t l e a s t . Both the ' p r e d a t o r - f r e e ' and 'Chaoborus-Cyclops' u n f e r t i l i z e d e n c l o s u r e s e x h i b i t e d lower t o t a l biomass thro u g h o u t and a lower mid-summer d e c l i n e than the u n f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t . The ' p r e d a t o r - f r e e ' reached p a r t i c u l a r l y 212 F i g u r e 52. Changes i n the % c o m p o s i t i o n of c r u s t a c e a n -r o t i f e r biomass i n each t r e a t m e n t e n c l o s u r e i n Gwendoline Lake i n 1977. L e t t e r i n g i n d i c a t e s the z o o p l a n k t o n type and sh a d i n g r e p r e s e n t s the f o l l o w i n g f o u r groups: c a l a n o i d copepods, c y c l o p o i d copepods, c l a d o c e r a n s and r o t i f e r s ( l a t t e r shown as w h i t e ) . L e t t e r i n g r e p r e s e n t s : DK - D.kenai a d u l t s , DL -D.leptopus a d u l t s , immature - immature c a l a n o i d copepods, Tp - T . p r a s i n u s p o p u l a t i o n , c b t -C.b.thomasi p o p u l a t i o n , eye - c y c l o p o i d copepods o t h e r than T . p r a s i n u s or C.b.thomasi , H o i - Holopedium , Dros - D.rosea , Chy - Chydorus and Bos - Bosmina . C r o s s - h a t c h i n g on the c l a d o c e r a n p l a n k t o n i n d i c a t e s the % of the t o t a l i n r e p r o d u c t i v e c o n d i t i o n . 214 low l e v e l s by e a r l y August. However, the f e r t i l i z e d l a t t e r community a c h i e v e d the h i g h e s t t o t a l biomass of any t r e a t m e n t , r e a c h i n g 4.3 m g / l i t r e i n J u l y . The mean r a t i o of p h y t o p l a n k t o n t o z o o p l a n k t o n biomass (P/Z r a t i o ) was 0.74 d u r i n g J u l y but r e v e r s e d i n August t o 4.13. C l e a r l y a h i g h p h y t o p l a n k t o n biomass was a v a i l a b l e i n August but not u t i l i z e d . D.rosea and Chydorus were the most abundant s p e c i e s c a u s i n g the z o o p l a n k t o n peak a t the end of J u l y ( F i g . 4 8 c , g ) . A l t h o u g h Chydorus were e x t r e m e l y numerous, t h i s s p e c i e s ' r e l a t i v e c o n t r i b u t i o n t o t o t a l biomass was minor compared t o D.rosea, r e f l e c t i n g i t s s m a l l s i z e ( F i g . 52d). C o n s e q u e n t l y the August z o o p l a n k t o n biomass d e c l i n e ( f i g . 5 1 b ) was almost e x c l u s i v e l y a d e c l i n e i n the D.rosea p o p u l a t i o n . T h i s d e c r ease was p r o b a b l y caused by s t a r v a t i o n as the p e r c e n t of the D.rosea biomass i n r e p r o d u c t i v e c o n d i t i o n was g r e a t l y reduced by August. A l t h o u g h T . p r a s i n u s i n c r e a s e d i n August and 2 l a r g e c y c l o p o i d copepod, M a c r o c y c l o p s a l b i d u s and Mesocyclops edax became r e l a t i v e l y more numerous, t h e i r combined biomass was low. R e l a t i v e t o the o t h e r two f e r t i l i z e d t r e a t m e n t s , the t o t a l biomass i n the f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t remained low t h r o u g h September. The p h y t o p l a n k t o n c o n t i n u e d t o be dominated by b l u e - g r e e n f i l a m e n t s and l a r g e green c o l o n i e s ; nannoplankton was slow t o r e c o v e r . C o n s e q u e n t l y by the end of September p h y t o p l a n k t o n biomass was h i g h e r i n the u n f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e atment but the D.rosea p o p u l a t i o n ( F i g . 5 2 d ) remained low i n number. In the u n f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t Chydorus was never a b l e t o become abundant ( F i g . 4 8 g,h) and Bosmina r e p l a c e d D.rosea i n 215 r e l a t i v e importance i n the t o t a l biomass ( F i g . 5 2 c ) . In c o n t r a s t t o the f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t , the h i g h n u t r i e n t 'Chaoborus-Cyclops' e n c l o s u r e m a i n t a i n e d a lower t o t a l z o o p l a n k t o n biomass d u r i n g J u l y (from .66 t o 1.23 m g / l i t r e ) , e x p e r i e n c e d a l a t e J u l y - e a r l y August d e p r e s s i o n and i n c r e a s e d most s h a r p l y i n l a t e August ( F i g . 5 1 b ) . The P/Z r a t i o was h i g h e r d u r i n g J u l y and August than t h a t i n the f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e . Howver, b l u e - g r e e n f i l a m e n t s were dominant. The l a t e J u l y / e a r l y August z o o p l a n k t o n d e c l i n e was c o r r e l a t e d w i t h the d e c l i n e of D.rosea arid the r e l a t i v e i n c r e a s e of Chydorus and s o l i t a r y r o t i f e r s ( F i g . 5 2 f ) . A l t h o u g h Chydorus remained e x t r e m e l y abundant, the D.rosea p o p u l a t i o n i n c r e a s e d i n mid-August, c o i n c i d e n t w i t h a p h y t o p l a n k t o n s h i f t t o nannoplankton. T h i s r e t u r n of nannoplankton o c c u r r e d j u s t when l a r g e M a c r o c y c l o p s a l b i d u s became r e l a t i v e l y more common and Chydorus d e c r e a s e d s h a r p l y . The d e c r e a s e i n Chydorus p r o b a b l y r e f l e c t e d food l i m i t a t i o n because r e p r o d u c t i o n a b r u p t l y d e c l i n e d about the end of J u l y ( F i g . 5 2 f ) . A l t h o u g h t h e r e might have been some p r e d a t i o n l o s s t o M a c r o c y c l o p s , t h e s e a n i m a l s were never v e r y numerous. In g e n e r a l , f e r t i l i z a t i o n enhanced p h y t o p l a n k t o n biomass and t h i s i n c r e a s e r e s u l t e d i n c r u s t a c e a n z o o p l a n k t o n biomass i n c r e a s e s a c r o s s a l l communities. However, by l a t e summer t h i s d i f f e r e n c e between f e r t i l i z e d and u n f e r t i l i z e d e n c l o s u r e s was not so d i s t i n c t . W h i l e mean t o t a l c r u s t a c e a n biomass v a l u e s i n September were h i g h e r i n both f e r t i l i z e d t r e a t m e n t s w i t h i n v e r t e b r a t e p r e d a t o r s ( 'Chaoborus-Cyclops' - 1.21 m g / l i t r e , ' C y c l o p s ' - 0.94 m g / l i t r e ) than i n the f e r t i l i z e d 216 ' p r e d a t o r - f r e e ' t r e a t m e n t (0.49 m g / l i t r e ) , z o o p l a n k t o n biomass i n the l a t t e r t r e atment was lower than t h a t i n the u n f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e (0.88 m g / l i t r e ) . T h i s r e s u l t was due t o an i n c r e a s e i n D.rosea, the presence of a c o n s i d e r a b l e C.b.thomasi p o p u l a t i o n i n the ' C y c l o p s ' t r e a t m e n t as w e l l as the reappearance of a d u l t D .leptopus and immature c a l a n o i d copepods ( F i g . 5 2 a ) . None of th e s e s p e c i e s was i m p o r t a n t i n the f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e by September. The mean September c r u s t a c e a n biomass i n the u n f e r t i l i z e d 'Chaoborus-Cyclops' e n c l o s u r e was a l s o s l i g h t l y h i g h e r (0.54 m g / l i t r e ) than i n the f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t and 2X t h a t of the u n f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t (0.25 m g / l i t r e ) . The h i g h e r September biomass i n the community w i t h h i g h e r l e v e l s of i n v e r t e b r a t e p r e d a t i o n was m a i n l y due t o the reappearance of D. l e p t o p u s . These c a l a n o i d s . were most s u c c e s s f u l i n the u n f e r t i l i z e d e n c l o s u r e s w i t h i n v e r t e b r a t e p r e d a t o r s . The p e r c e n t c o m p o s i t i o n of biomass i n May i n a l l t r e a t m e n t s was dominated by the c a l a n o i d copepods. T h i s biomass predominance was l o s t by the end of J u l y i n a l l t r e a t m e n t s because the a d u l t s d i e d and were r e p l a c e d by n a u p l i i . However, i n a l l f e r t i l i z e d e n c l o s u r e s the c a l a n o i d s c o n t i n u e d t o d i m i n i s h i n r e l a t i v e importance throughout the summer and were unable t o make a September ' r e c o v e r y ' , even i n those e n c l o s u r e s w i t h i n v e r t e b r a t e p r e d a t o r s ( F i g . 5 2 b , d , f ) . T h i s r e s u l t was i n c o n t r a s t t o the r e t u r n ( m a i n l y of D.leptopus ) i n both the u n f e r t i l i z e d e n c l o s u r e s w i t h C.b.thomasi and Chaoborus. A l t h o u g h the c a l a n o i d copepods d i d not d i p t o such 217 low l e v e l s of r e l a t i v e importance i n the u n f e r t i l i z e d e n c l o s u r e as t h e s e a n i m a l s d i d i n a l l the f e r t i l i z e d e n c l o s u r e s , the r e c o v e r y was weak r e l a t i v e t o t h a t o b s e r v e d i n the ' p r e d a t o r ' e n c l o s u r e s w i t h Chaoborus and C.b.thomasi. The s i m i l a r i t y of the p e r c e n t c o m p o s i t i o n c o n f i g u r a t i o n s of the c a l a n o i d copepods i n the u n f e r t i l i z e d 'Chaoborus-Cyclops' and ' C y c l o p s ' t r e a t m e n t s s t r o n g l y suggested t h a t the presence of i n v e r t e b r a t e p r e d a t o r s was i m p o r t a n t f o r c a l a n o i d s u r v i v a l , perhaps by c r o p p i n g competing m i c r o c r u s t a c e a h s . In view of s i m i l a r c a l a n o i d copepod r e c o v e r i e s i n both these t r e a t m e n t s , and the d i s s i m i l a r p a t t e r n i n the u n f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e s , a p h y s i c a l e x p l a n a t i o n , such as temperature s t r e s s , seemed t o be too s i m p l e an i n t e r p r e t a t i o n , a t l e a s t f o r D.leptopus d e n s i t i e s . Another g e n e r a l d i f f e r e n c e between f e r t i l i z e d and u n f e r t i l i z e d e n c l o s u r e s was the r o t i f e r abundance. In u n f e r t i l i z e d e n c l o s u r e s , t h e i r c o n t r i b u t i o n t o t o t a l biomass was too low t o d e p i c t (0.002 - 0.01 m g / l i t r e ) except f o r a b r i e f bloom (0.10 m g / l i t r e ) i n the 'Chaoborus-Cyclops' t r e a t m e n t ( F i g . 5 2 f ) . In a l l f e r t i l i z e d e n c l o s u r e s the r e l a t i v e c o n t r i b u t i o n of r o t i f e r s was h i g h e r (2 - 3%) and i n the f e r t i l i z e d 'Chaoborus-Cyclops' t r e a t m e n t , r o t i f e r biomass i n c r e a s e d d r a m a t i c a l l y i n August t o 50% of the t o t a l z o o p l a n k t o n biomass. T h i s r e s u l t r e f l e c t e d both a g r e a t i n c r e a s e i n s o l i t a r y r o t i f e r s ( K e r a t e l l a , K e l l i c o t t i a , Lecane) and a d e c r e a s e i n c l a d o c e r a n s , p a r t i c u l a r l y D.rosea and Chydorus ( F i g . 5 2 f ) . As a group, c y c l o p o i d copepods p r o s p e r e d i n a l l the 218 f e r t i l i z e d e n c l o s u r e s r e l a t i v e t o the u n f e r t i l i z e d t r e a t m e n t s . T . p r a s i n u s was most i m p o r t a n t , r e l a t i v e t o o t h e r s p e c i e s , i n both f e r t i l i z e d and u n f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e s , a l t h o u g h numbers were about 2X h i g h e r i n the former t r e a t m e n t . DISCUSSION A l t e r i n g the abundance of the predaceous c y c l o p o i d copepod, C.b.thomasi, had l i t t l e c o m p o s i t i o n a l e f f e c t on the prey s p e c i e s a l t h o u g h c e r t a i n prey s p e c i e s abundances were a l t e r e d by the presence of an abundant C.b.thomasi p o p u l a t i o n . C l u s t e r a n a l y s e s showed t h a t , by August, samples from f e r t i l i z e d e n c l o s u r e s were most d i f f e r e n t from u n f e r t i l i z e d samples. The o n l y t r e a t m e n t t h a t c o n s i s t e n t l y c l u s t e r e d w i t h i t s e l f throughout August was the h i g h n u t r i e n t 'Chaoborus-C y c l o p s ' t r e a t m e n t . C.b.thomasi, a l t h o u g h i n i t i a l l y enhanced i n t h i s t r e a t m e n t t o P l a c i d Lake d e n s i t i e s , was so low i n abundance by J u l y t h a t e s t i m a t e s were u n r e l i a b l e . C o n s e q u e n t l y C.b.thomasi was not the p e r p e t r a t o r of t h i s d i f f e r e n c e . R a t h e r , improved s u r v i v a l of 1 s t and 2nd i n s t a r Chaoborus l a r v a e ( N e i l l and Peacock, 1980) appeared t o cause a major change i n p r e d a t o r impact. Under low n u t r i e n t c o n d i t i o n s these d e v e l o p m e n t a l c o n t r a i n t s p r e v e n t e d Chaoborus from r e s p o n d i n g n u m e r i c a l l y t o i n c r e a s i n g p r e y abundances. C o n v e r s e l y , improved s u r v i v o r s h i p of C.b.thomasi under h i g h n u t r i e n t c o n d i t i o n s w i t h o u t Chaoborus l a r v a e had r e l a t i v e l y l i t t l e e f f e c t on the s t r u c t u r e of the community. There was no c l e a r g r o u p i n g of t h i s t r e atment w i t h i n the August 219 samples. W h i l e the community c o m p o s i t i o n remained r e l a t i v e l y i n v a r i a n t under d i f f e r e n t n u t r i e n t l e v e l s and C.b.thomasi abundance, t h e r e was e v i d e n c e t h a t C.b.thomasi had an e f f e c t on some' pr e y s p e c i e s d e n s i t i e s . A n a l y s i s of the t o t a l c r u s t a c e a n / r o t i f e r biomass s u p p o r t e d Lane's (1979) argument t h a t C.b.thomasi had a dampening e f f e c t on z o o p l a n k t o n abundance. Even under the ' h i g h ' f e r t i l i z e r t r e a t m e n t i n the 'C y c l o p s ' e n c l o s u r e , z o o p l a n k t o n biomass was o n l y 1.2X g r e a t e r than t h a t i n the u n f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t . The p e r c e n t c o m p o s i t i o n of a l g a l t y p e s ( F i g . 4 0 ) was f u r t h e r e v i d e n c e t h a t C.b.thomasi dampens c o m p e t i t i o n f o r s c a r c e food t y p e s i n J u n e - J u l y . A l l f e r t i l i z e d e n c l o s u r e s showed a s h i f t from e d i b l e diatoms and greens (shown as 2-20 um, c o l o n i a l and >20 um) t o r e l a t i v e l y u n d e s i r a b l e f i l a m e n t s ( m a i n l y b l u e - g r e e n a l g a e ) i n mid-June. However,' i n the f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e t h e r e was a s h o r t r e c o v e r y p e r i o d i n l a t e June t o a diatom-green dominated a l g a l community. Thus, a l t h o u g h C.b.thomasi was not a 'keystone' s p e c i e s i n P a i n e ' s sense (1969), t h i s s p e c i e s n e v e r t h e l e s s appeared t o reduce g r a z i n g p r e s s u r e upon the more e d i b l e a l g a e by removing some of the competing g r a z e r s . T h i s i n t e r p r e t a t i o n i s s u p p o r t e d by a comparison of the p o p u l a t i o n dynamics of Chydorus i n the ' C y c l o p s ' v e r s u s the 'Chaoborus-Cyclops' t r e a t m e n t s . The s m a l l - s i z e d Chydorus never reached the n u m e r i c a l abundance i n the former t r e a t m e n t t h a t i t d i d i n the l a t t e r and i n the ' p r e d a t o r - f r e e ' t r e a t m e n t . Chydorus became abundant o n l y when the dominant g r a z e r , D.rosea , d e c l i n e d . The i n a b i l i t y of the Chydorus p o p u l a t i o n t o 220 i n c r e a s e i n d e n s i t y i n the f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t might be due t o C.b.thomasi p r e d a t i o n i n a d d i t i o n t o c o m p e t i t i o n from D.rosea. • Lynch (1979) p r o v i d e s i n d i r e c t e v i d e n c e t h a t C y c l o p s  v e r n a l i s i n f l u e n c e s the abundance of the s m a l l c l a d o c e r a n C e r i o d a p h n i a , a l t h o u g h d a t a from my 1976 e x p e r i m e n t s suggest t h a t C.b.thomasi has no s i g n i f i c a n t n u m e r i c a l impact on C e r i o d a p h n i a . At low n u t r i e n t l e v e l s t h e r e i s no i n d i c a t i o n t h a t C.b.thomasi has a n e g a t i v e impact of Chydorus. In f a c t , the p r e s e n c e s of C.b.thomasi appears t o have a b e n e f i c i a l e f f e c t on p o p u l a t i o n s i z e when d e n s i t i e s of Chydorus i n t h i s t r e a t m e n t a r e compared t o the u n f e r t i l i z e d ' p r e d a t o r - f r e e ' •treatment. A l t h o u g h C.b.thomasi p r e d a t i o n i n June appears t o i n c r e a s e e d i b l e food t y p e s by l o w e r i n g g r a z e r d e n s i t i e s , t h e r e i s no e v i d e n c e t h a t T . p r a s i n u s f u n c t i o n s in. t h i s manner. B l u e - g r e e n f i l a m e n t s dominate the a l g a l community l o n g e r i n August i n the f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e than i n e i t h e r of the o t h e r f e r t i l i z e d t r e a t m e n t s . T . p r a s i n u s i n c r e a s e s s i g n i f i c a n t l y i n t h i s e n c l o s u r e but the p r o l o n g e d predominance of b l u e - g r e e n a l g a e and the f a c t t h a t the c a t a s t r o p h i c d e c l i n e i n z o o p l a n k t o n biomass b e g i n s i n J u l y , l o n g b e f o r e T . p r a s i n u s appears i n the p l a n k t o n , suggest t h a t e d i b l e food s h o r t a g e i s the cause of the z o o p l a n k t o n d e c l i n e . There i s no e v i d e n c e t h a t T . p r a s i n u s ever prey on l i v i n g m i c r o c r u s t a c e a n s , i n s p i t e of numerous l a b o r a t o r y o b s e r v a t i o n s , a l t h o u g h T . p r a s i n u s c l e a r l y does consume p r o t o z o a n s . T . p r a s i n u s p o p u l a t i o n s i z e i s lower i n the u n f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e than i n e i t h e r of the o t h e r two 221 u n f e r t i l i z e d t r e a t m e n t s . In s p i t e of the f a c t t h a t T . p r a s i n u s i s a b l e t o s e q u e s t e r s u f f i c i e n t f o o d r e s o u r c e s t o i n c r e a s e i n the f e r t i l i z e d ' p r e d a t o r - f r e e ' e n c l o s u r e , t h i s s p e c i e s i s unable t o s u s t a i n even the d e n s i t y p o s s i b l e i n the low n u t r i e n t ' C y c l o p s ' e n c l o s u r e i n the f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t . These d a t a c o n f i r m the P l a c i d Lake s t u d i e s showing t h a t T . p r a s i n u s r e c r u i t s p o o r l y i n the presence of C.b.thomasi. Thus c y c l o p o i d i n t e r a c t i o n s can be an i m p o r t a n t f a c t o r i n d e t e r m i n i n g the r e l a t i v e abundance of c y c l o p o i d copepod spec i e s . F e r t i l i z a t i o n i n c r e a s e s the abundance of a l l the c l a d o c e r a n s except Holopedium. The i n a b i l i t y of Holopedium t o n u m e r i c a l l y i n c r e a s e ( t h i s s p e c i e s was not o b s e r v e d i n any samples a f t e r J u l y i n a l l f e r t i l i z e d e n c l o s u r e s ) may be due t o poor c o m p e t i t i v e a b i l i t i e s i n the h i g h e r temperature e p i l i m n i o n environment of the e n c l o s u r e s . Only i n the u n f e r t i l i z e d 'Chaoborus-Cyclops' t r e a t m e n t does Holopedium m a i n t a i n a p o p u l a t i o n l a r g e enough t o sample throughout the summer season. The r a p i d i n c r e a s e i n numbers of the dominant c l a d o c e r a n g r a z e r , D.rosea, s u g g e s t s t h a t food a v a i l a b i l i t y i s an i m p o r t a n t f a c t o r l i m i t i n g the p o p u l a t i o n d e n s i t y of t h i s s p e c i e s i n the low n u t r i e n t w a t e r s of Gwendoline Lake. The p r e s e n c e of C.b.thomasi appears t o r e l e a s e more food r e s o u r c e s i n June but the e f f e c t i s weak and t r a n s i t o r y . The number of eggs per female d o u b l e s i n the f e r t i l i z e d t r e a t m e n t s and the p e r c e n t of the p o p u l a t i o n c a r r y i n g eggs i s about 1.5 X h i g h e r . A l t h o u g h r e p r o d u c t i v e output i s g r e a t e s t i n the f e r t i l i z e d 'Chaoborus-Cyclops' t r e a t m e n t , D.rosea numbers peak at o n l y 1.4 222 X h i g h e r than i n the u n f e r t i l i z e d e n c l o s u r e . However, t h e r e i s a 5 - f o l d i n c r e a s e i n abundance i n the f e r t i l i z e d ' C y c l o p s ' t r e a t m e n t and a 3 - f o l d i n c r e a s e i n the h i g h n u t r i e n t ' p r e d a t o r -f r e e ' e n c l o s u r e . The h i g h Chaoborus p o p u l a t i o n , about 40X over the u n f e r t i l i z e d e n c l o s u r e , c l e a r l y has a s i g n i f i c a n t impact on D.rosea abundance. In both n u t r i e n t c o n d i t i o n s , however, Chaoborus has the g e n e r a l e f f e c t of dampening D.rosea i n c r e a s e s but a l s o m o d e r a t i n g the August d e c l i n e . In c o n t r a s t t o Chaoborus, C.b.thomasi appears t o have t h i s e f f e c t o n l y i n the low n u t r i e n t e n c l o s u r e . In the f e r t i l i z e d e n c l o s u r e the p o p u l a t i o n dynamics of D.rosea resemble the f e r t i l i z e d ' p r e d a t o r - f r e e ' t r e a t m e n t i n p a t t e r n - a r a p i d i n c r e a s e f o l l o w e d by a s h a r p d e c l i n e . R e p r o d u c t i o n d r a s t i c a l l y d e c r e a s e s about 2-3 weeks b e f o r e the c r a s h s u g g e s t i n g the d e c l i n e i n both t h e s e e n c l o s u r e s may be a t t r i b u t e d p r i m a r i l y t o r e s o u r c e d e p l e t i o n . There may be a l s o be a p r e d a t o r e f f e c t on young D.rosea neonates i n the f e r t i l i z e d ' C y c l o p s ' e n c l o s u r e i n e a r l y August. The p r o p o r t i o n of the p o p u l a t i o n r e p r o d u c i n g remains a t about 7% i n t h i s t r e a t m e n t but drops t o z e r o i n the f e r t i l i z e d ' p r e d a t o r -f r e e ' e n c l o s u r e , y e t numbers d e c l i n e f a s t e r i n the former. However, the g e n e r a l p a t t e r n of p o p u l a t i o n i n c r e a s e and c r a s h i s s i m i l a r between the two t r e a t m e n t s , s u g g e s t i n g t h a t C.b.thomasi has l i t t l e c a p a b i l i t y t o r e l i e v e g r a z i n g p r e s s u r e i n the h i g h n u t r i e n t s i t u a t i o n . When abundances a r e l o w e r , as i n the low n u t r i e n t t r e a t m e n t , the l o s s e s t o C.b.thomasi have a r e l a t i v e l y g r e a t e r e f f e c t . T h i s c o n c l u s i o n i s s u p p o r t e d by the g r e a t e r D.rosea f e r t i l i t y r a t e s i n the l a t t e r t r e a t m e n t 223 (15% or about 2X the f e r t i l i z e d e n c l o s u r e ) d u r i n g August. Bosmina i s c l e a r l y d e p r e s s e d by the l a r g e Chaoborus p o p u l a t i o n i n the f e r t i l i z e d Chaoborus t r e a t m e n t . N e i l l (1980) r e p o r t s s i g n i f i c a n t s u p p r e s s i o n of Bosmina p o p u l a t i o n s by Chaoborus p r e d a t i o n i n two y e a r s of e x p e r i m e n t s i n u n f e r t i l i z e d e n c l o s u r e s . Bosmina numbers a r e slower t o i n c r e a s e i n the u n f e r t i l i z e d 'Chaoborus-Cyclops' treatment and t h i s r e s u l t may be due t o p r e d a t i o n . There i s a d e f i n i t e s u p p r e s s i o n of C.b.thomasi by Chaoborus and T . p r a s i n u s by C.b.thomasi. However, l o n g term d e n s i t y e f f e c t s on o t h e r s p e c i e s by e i t h e r C.b.thomasi or Chaoborus a r e d i f f i c u l t t o i d e n t i f y . The responses of the c l a d o c e r a n s , c a l a n o i d and c y c l o p o i d copepods t o f e r t i l i z a t i o n a r e n o t a b l y d i f f e r e n t , r e p r e s e n t i n g d i s t i n c t l y d i f f e r e n t s t r a t e g i e s w i t h i n the z o o p l a n k t o n community. In g e n e r a l , the c a l a n o i d copepods ( r e p r e s e n t e d i n Gwendoline Lake by D.kenai and D.leptopus ) respond l i k e K-s t r a t e g i s t s i n the e n c l o s u r e s . They are unable t o t u r n s h o r t b u r s t s of h i g h food l e v e l s i n t o i n c r e a s e d p o p u l a t i o n numbers. In c o n t r a s t , the h i g h r e p r o d u c t i v e p o t e n t i a l of the c l a d o c e r a n s a l l o w s r a p i d e x p a n s i o n of t h i s group u n t i l r e s o u r c e s a r e d r a s t i c a l l y d e p l e t e d , a t which p o i n t the p o p u l a t i o n s c r a s h . T h i s r e s p o n s e , b e s t e x e m p l i f i e d by D.rosea , i s t h a t of the c l a s s i c a l r - s t r a t e g i s t . A l l a n (1976) c o n c l u d e s t h a t the t h r e e major t a x a of f r e s h w a t e r z o o p l a n k t o n can be ranked w i t h r e s p e c t t o o p p o r t u n i s m i n t h i s o r d e r : r o t i f e r s > c l a d o c e r a n s > copepods. However, my r e s u l t s suggest t h a t i n t e r m e d i a t e between the c l a d o c e r a n s t a t e g y and the c a l a n o i d s t r a t e g y i s 224 t h a t o f - the c y c l o p o i d copepods. Both C.b.thomasi. and T . p r a s i n u s a r e c l e a r l y c a p a b l e of i n c r e a s i n g r e p r o d u c t i v e output i n the f e r t i l i z e d t r e a t m e n t s . A l t h o u g h the p o p u l a t i o n i n c r e a s e i s slow r e l a t i v e t o the c l a d o c e r a n s , r e p r o d u c t i v e output i s s i g n i f i c a n t l y h i g h e r than the c a l a n o i d copepods. However, l i k e the l a t t e r , c y c l o p o i d copepods are l e s s s u s c e p t i b l e t o r e s o u r c e d e p l e t i o n than a re the c l a d o c e r a n s . Hence T . p r a s i n u s i n c r e a s e s n u m e r i c a l l y throughout August, when most c l a d o c e r a n p o p u l a t i o n s a r e i n d e c l i n e , and C.b.thomasi e x p e r i e n c e s no decrease i n numbers d u r i n g t h i s p e r i o d . The omnivorous h a b i t , c o u p l e d w i t h the a b i l i t y of both T . p r a s i n u s and C.b.thomasi to feed on dead c r u s t a c e a n z o o p l a n k t e r s , p r o b a b l y a l l o w s t h i s group t o s u s t a i n h i g h p o p u l a t i o n s under a r e l a t i v e l y broad spectrum of c o n d i t i o n s . C y c l o p o i d s may even u t i l i z e the seemingly i n e d i b l e b l u e - g r e e n a l g a e . Lewis (1979) c o n c l u d e s t h a t Thermocyclops h y a l i n u s i s u s i n g b l u e - g r e e n f i l a m e n t o u s a l g a e i n Lake Lanao, and I n f a n t e (1978) s i m i l a r l y r e p o r t s d i g e s t i o n of f i l a m e n t o u s b l u e - g r e e n s by c y c l o p o i d n a u p l i i . Thus c y c l o p o i d copepods may occupy the g e n e r a l i s t r o l e w i t h i n the z o o p l a n k t o n community, s c a v e n g i n g whatever i s a v a i l a b l e . In summary, C.b.thomasi does appear t o dampen the n u m e r i c a l reponse of c e r t a i n prey s p e c i e s t h r o u g h p r e d a t i o n . Under h i g h f e r t i l i z e r c o n d i t i o n s Chaoborus has the c a p a b i l i t y t o a l t e r community c o m p o s i t i o n , as w e l l as abundance. In c o n t r a s t t o Chaoborus, C.b.thomasi has a g r e a t e r net e f f e c t on most prey s p e c i e s abundance when n u t r i e n t s a r e low. At h i g h n u t r i e n t l e v e l s , C.b.thomasi has v i r t u a l l y no impact on the 225 dynamics of the dominant g r a z e r , D.rosea. At low n u t r i e n t l e v e l s b o th C.b.thomasi and Chaoborus appear t o improve c o n d i t i o n s f o r the c a l a n o i d copepod, D . l e p t o p u s . F i n a l l y , a l t h o u g h C.b.thomasi cannot e f f e c t a c o m p o s i t i o n a l change i n the z o o p l a n k t o n - community, even when food i s s c a r c e , t h i s i n v e r t e b r a t e p r e d a t o r does appear t o be i n s t r u m e n t a l i n r e l i e v i n g g r a z i n g p r e s s u r e such t h a t z o o p l a n k t o n p o p u l a t i o n f l u c t u a t i o n s a r e dampened. SUMMARY Even a t h i g h n u t r i e n t l e v e l s , C.b.thomasi does not cause a c o m p o s i t i o n a l change i n the z o o p l a n k t o n community. C.b.thomasi d e c r e a s e s the n u m e r i c a l response of c e r t a i n s p e c i e s t o f e r t i l i z a t i o n , e.g. Chydorus and T . p r a s i n u s . The presence of C.b.thomasi has l i t t l e e f f e c t on the D.rosea p o p u l a t i o n i n the f e r t i l i z e d t r e a t m e n t but under • low n u t r i e n t c o n d i t i o n s , the p r e s ence of C.b.thomasi appears t o r e l e a s e s c a r c e food r esouces - t h i s e f f e c t i s s i m i l a r t o t h a t of Chaoborus a t low n u t r i e n t l e v e l s . At low n u t r i e n t l e v e l s the p r e s e n c e of C.b.thomasi and Chaoborus appears t o be e s s e n t i a l f o r the c o n t i n u e d p r e s ence of c a l a n o i d copepods d u r i n g the summer months. An abundant C.b.thomasi p o p u l a t i o n seems t o reduce f l u c t u a t i o n s i n t o t a l c r u s t a c e a n biomass at b o th h i g h and low n u t r i e n t l e v e l s . 227 GENERAL DISCUSSION The f e e d i n g s t r u c t u r e s of a d u l t T . p r a s i n u s and C.b.thomasi are r e m a r k a b l y a l i k e , a l t h o u g h a d u l t d i e t s a r e q u i t e d i s s i m i l a r . T . p r a s i n u s i s omnivorous i n the l a t e r i n s t a r s , e a t i n g a l g a e , p r o t o z o a n s and p r o b a b l y dead m a c r o z o o p l a n k t e r s , w h i l e C.b.thomasi i s markedly c a r n i v o r o u s i n the c o p e p o d i d and a d u l t i n s t a r s . S e a s o n a l abundance peaks r e v e a l a t e m p o r a l s e p a r a t i o n of the n a u p l i a r i n s t a r s i n both l a k e s s t u d i e d . C o n s e q u e n t l y , T . p r a s i n u s and C.b.thomasi do not compete. Large s c a l e e n c l o s u r e e x p e r i m e n t s i n P l a c i d Lake show t h a t l a k e d e n s i t i e s of C.b.thomasi l i m i t the n u m e r i c a l i n c r e a s e of T . p r a s i n u s . T h i s impact i s caused by two i m p o r t a n t f a c t o r s : enormous n a u p l i a r m o r t a l i t y - i n the T . p r a s i n u s p o p u l a t i o n , p r o b a b l y caused by s t a r v a t i o n , and C.b.thomasi p r e d a t i o n on t h e s e T . p r a s i n u s i n s t a r s . The a b i l i t y of T . p r a s i n u s t o p e r s i s t i n P l a c i d Lake i n s p i t e of t h e s e d i f f i c u l t i e s i s a t t r i b u t e d t o b e h a v i o u r which r e s u l t s i n s p a t i a l s e p a r a t i o n from the p r i n c i p l e p a r t of the c a r n i v o r o u s C.b.thomasi p o p u l a t i o n . C o n s e q u e n t l y , C.b.thomasi i s unable t o cause p o p u l a t i o n e x t i n c t i o n . H a l l e t a l . (1976) suggest t h a t t h i s r e s u l t i s g e n e r a l l y c h a r a c t e r i s t i c of the impact of i n v e r t e b r a t e p r e d a t o r s on p r e y p o p u l a t i o n s . I c a r r i e d out a t r a n s p l a n t experiment t o e x p l a i n why C.b.thomasi, o b v i o u s l y s u c c e s s f u l i n P l a c i d Lake, i s e x t r e m e l y r a r e i n nearby f i s h l e s s Gwendoline Lake. By p e r m i t t i n g t h e Chaoborus s p e c i e s , common t o Gwendoline Lake ( C . t r i v i t t a t u s and 228 C.americanus) t o l a y eggs i n two e n c l o s u r e s , I showed t h a t the presence of Chaoborus was c r i t i c a l i n r e s t r i c t i n g C.b.thomasi p o p u l a t i o n numbers. V u l n e r a b i l i t y t o Chaoborus p r e d a t i o n was caused by l i m i t e d a l t e r n a t i v e prey and s p a t i a l / t e m p o r a l o v e r l a p of C.b.thomasi n a u p l i i w i t h 1 s t i n s t a r Chaoborus. Adding n u t r i e n t s t o the e n c l o s u r e s s i m p l y e x a c e r b a t e d the p r e d a t o r impact by g r e a t l y i m p r o v i n g Chaoborus s u r v i v o r s h i p . A l t h o u g h C.b.thomasi responded t o f e r t i l i z a t i o n by i n c r e a s i n g the number of eggs per c l u t c h and e x t e n d i n g the e g g - b e a r i n g p e r i o d , t h i s response was not s u f f i c i e n t t o p e r m i t C.b.thomasi t o o u t s t r i p the n u m e r i c a l response of Chaoborus. In c o n t r a s t t o C.b.thomasi, r - s e l e c t e d r o t i f e r s a c h i e v e d the h i g h e s t d e n s i t i e s i n the f e r t i l i z e d 'Chaoborus-Cyclops' e n c l o s u r e - i n s p i t e of h i g h p r e d a t o r abundance. The t e m p o r a l o v e r l a p and prey d e v e l o p m e n t a l response a r e e s s e n t i a l f a c t o r s "in d e t e r m i n i n g prey s e n s i t i v i t y . R a p i d development and p a r t h e n o g e n i c r e p r o d u c t i o n a l l o w the r o t i f e r s t o i n c r e a s e n u m e r i c a l l y i n August, when Chaoborus p r e d a t i o n i s reduced. These l a r v a e molt t o a s i z e a t which r o t i f e r s are no l o n g e r an a t t r a c t i v e p rey i t e m . By August, C.b.thomasi numbers a r e low due t o e a r l i e r Chaoborus p r e d a t i o n and t h i s s p e c i e s ' d e v e l o p m e n t a l response i s such t h a t t h e r e a r e no r e p r o d u c t i v e i n d i v i d u a l s a v a i l a b l e t o take advantage of the o p p o r t u n i t y . As a consequence, the C.b.thomasi p o p u l a t i o n cannot r e c o v e r and p o p u l a t i o n numbers remain low. The impact of C.b.thomasi on o t h e r z o o p l a n k t e r s was examined i n l a r g e s i d e - b y - s i d e e n c l o s u r e s i n Gwendoline Lake w i t h d i f f e r e n t l e v e l s of food abundance and i n v e r t e b r a t e 229 p r e d a t i o n . The presence of C.b.thomasi appeared t o have a dampening e f f e c t on t o t a l z o o p l a n k t o n biomass i n t r e a t m e n t s w i t h o u t a Chaoborus p o p u l a t i o n . However, w h i l e Chaoborus reached a p o i n t a t h i g h n u t r i e n t l e v e l s a t which e a r l y i n s t a r s u r v i v a l was h i g h and p r e d a t o r y impact enormous, C.b.thomasi was unable t o t r a c k prey demographic reponses i n the h i g h f e r t i l i z e r e n c l o s u r e . C.b.thomasi caused no major c o m p o s i t i o n a l changes i n the c r u s t a c e a n community a t e i t h e r n u t r i e n t l e v e l . P a r a d o x i c a l l y , D.rosea and D.leptopus were n u m e r i c a l l y more s u c c e s s f u l i n t r e a t m e n t s w i t h C.b.thomasi than i n those e n c l o s u r e s w i t h o u t an i n v e r t e b r a t e p r e d a t o r . Thus by dampening s m a l l g r a z e r numbers, C.b.thomasi might be r e s p o n s i b l e f o r m a i n t a i n i n g a s u f f i c i e n t food s u p p l y f o r D.rosea and D. l e p t o p u s . W h i l e the n u m e r i c a l abundance of any s p e c i e s i s a t l e a s t p a r t i a l l y dependent upon the suc c e s s of i t s most m o r t a l i t y -prone s t a g e , i d e n t i f i c a t i o n of the c r u c i a l p e r i o d r e q u i r e s a n a l y s i s of the s p a t i a l / t e m p o r a l dynamics w i t h i n the c o n t e x t of the community s p e c i e s assemblage. The tendency t o a t t r i b u t e s i m i l a r responses to d i f f e r e n t s p e c i e s of the same genera or even d i f f e r e n t i n s t a r s of the same s p e c i e s , o f t e n a c r o s s d i f f e r e n t communities, has caused c o n s i d e r a b l e c o n f u s i o n about the mechanisms which govern z o o p l a n k t o n s e a s o n a l abundance f l u c t u a t i o n s and b i o g e o g r a p h i c a l p a t t e r n s . I t i s c l e a r from t h i s study t h a t Chaoborus p r e d a t i o n f a r outweighs food a v a i l a b i l i t y as an i m p o r t a n t v a r i a b l e l i m i t i n g the r e c r u i t m e n t of C.b.thomasi i n Gwendoline Lake. T h i s p r e d a t o r ' s impact cannot be p r e d i c t e d by a p p l y i n g o n l y f e e d i n g r a t e s and 230 p r e f e r e n c e d a t a from o b s e r v a t i o n s on 3rd and 4 t h i n s t a r Chaoborus l a r v a e . I t i s e v i d e n t t h a t the p r e d a t o r y impact of Chaoborus upon the C.b.thomasi p o p u l a t i o n i s a l s o dependent upon the p a r t i c u l a r s p a t i a l and t e m p o r a l i n t e r a c t i o n of p r e d a t o r and prey i n s t a r s , as w e l l as the s c a r c i t y of a l t e r n a t i v e p r e y . N e i l l (1980) shows t h a t c a l a n o i d copepods i n Gwendoline Lake are v u l n e r a b l e t o Chaoborus i n the s p r i n g , when c a l a n o i d d e n s i t i e s a re low and a l t e r n a t i v e prey s c a r c e . I f r e p r o d u c t i o n i s d e l a y e d , summertime r e c o v e r y from s p r i n g t i m e p r e d a t i o n m o r t a l i t y i s d i f f i c u l t and l o n g - t e r m demographic e f f e c t s a r e p o s s i b l e . However, d u r i n g most y e a r s p r e d a t i o n impact i s n e g l i g i b l e . The f a c t t h a t C.b.thomasi i s r a r e i n Gwendoline Lake s u g g e s t s t h a t the n e g a t i v e e f f e c t of Chaoborus on the C.b.thomasi p o p u l a t i o n observed i n t h i s s tudy i s a common event i n Gwendoline Lake. While both c y c l o p o i d and c a l a n o i d copepods were unable t o match c l a d o c e r a n n u m e r i c a l reponse t o i n c r e a s e d n u t r i e n t s , t h e r e were i m p o r t a n t d i f f e r e n c e s between the two copepod groups. F e r t i l i z a t i o n of the e n c l o s u r e s i n c r e a s e d the d e n s i t i e s of both C.b.thomasi and T . p r a s i n u s ( i n the absence of p r e d a t o r s ) and d e c r e a s e d the abundance of both D.kenai and D . l e p t o p u s . P a t a l a s (1972) o b s e r v e d t h a t a " d e c l i n i n g p r o p o r t i o n of d i a p t o m i d s and an i n c r e a s i n g abundance of c y c l o p o i d s ( C.b.thomasi and T . p r a s i n u s mexicanus) and c l a d o c e r a n s " were c h a r a c t e r i s t i c t r e n d s r e l a t e d t o i n c r e a s i n g n u t r i e n t s i n the S t . Lawrence Great Lakes. S i m i l a r l y , H i l l b r e c h t - I l k o w s k a and Weglenska (1973) found i n c r e a s e d c l a d o c e r a n and c y c l o p o i d copepod p r o d u c t i o n and a d e c r e a s e i n 231 c a l a n o i d s i n .response t o i n c r e a s e d food r e s o u r c e s . C y c l o p o i d copepods have two o b v i o u s advantages over f i l t e r - f e e d i n g c a l a n o i d s i n r e s p o n d i n g t o i n c r e a s e d n u t r i e n t s : c y c l o p o i d copepods are c a p a b l e of a g r e a t e r n u m e r i c a l response than c a l a n o i d copepods ( e g g s / c l u t c h range over 100 w h i l e e g g s / c l u t c h i n c a l a n o i d s a r e g e n e r a l l y <30 ( H u t c h i n s o n , 1 9 6 7 ) ) ; and the omnivorous h a b i t a l l o w s c y c l o p o i d s t o e x p l o i t i n c r e a s e s i n b o t h p r i m a r y and secondary p r o d u c t i o n . In a d d i t i o n , some c y c l o p o i d copepods are a p p a r e n t l y c a p a b l e of u t i l i z i n g even b l u e - g r e e n a l g a e ( I n f a n t e , 1978; L e w i s , 1979). Thus a g r e a t e r v a r i e t y of a l g a l t y p e s can be u t i l i z e d d i r e c t l y by n a u p l i a r i n s t a r s and i n d i r e c t l y by c o p e p o d i d and a d u l t s t a g e s c r o p p i n g i n c r e a s e d h e r b i v o r e biomass. I n c r e a s i n g n u t r i e n t s c h a r a c t e r i s t i c a l l y d e c r e a s e s c a l a n o i d copepod abundance (McNaught, 1975), a group p r e d o m i n a n t l y adapted t o the e x p o i t a t i o n of low n u t r i e n t , n u t r i t i o n a l l y d i l u t e e nvironments ( A l l a n , 1976). By c o n t r a s t , c y c l o p o i d copepods ar e a b l e t o c a p i t a l i z e on i n c r e a s e d n u t r i e n t s y e t are a l s o a b l e t o endure l o n g p e r i o d s of s t a r v a t i o n (Smyly, per.comm.; p e r s . o b s . ) . C o n s e q u e n t l y c y c l o p o i d s a r e abundant i n h i g h n u t r i e n t environments but a r e a l s o i m p o r t a n t components of o l i g o t r o p h i c l a k e s . These a n i m a l s seem t o be the g e n e r a l i s t s of the l a c u s t r i n e p l a n k t o n community. However, i n s p i t e of the above, c y c l o p o i d copepod s p e c i e s a r e g e n e r a l l y u n d e r r e p r e s e n t e d r e l a t i v e t o c a l a n o i d copepod s p e c i e s i n u l t r a o l i g o t r o p h i c e n v i r o n m e n t s , such as tho s e found i n the a r c t i c ( R y l o v , 1963; Tash, 1971; Moore, 1978). The ' g e n e r a l i s t ' s t r a t e g y seems t o f a i l under these extreme 232 c o n d i t i o n s . The f e e d i n g s t r u c t u r e s of C.b.thomasi and T . p r a s i n u s and t h e i r r e l a t i v e d e n s i t i e s i n o l i g o t r o p h i c waters may p r o v i d e some i n s i g h t i n t o the g e n e r a l problem c y c l o p o i d copepods have i n l i v i n g i n an e x t r e m e l y d i l u t e environment. F r y e r (1957a) argues c o n v i n c i n g l y t h a t the h e r b i v o r o u s h a b i t i n C y c l o p o i d a r e p r e s e n t s a more r e c e n t l y e v o l v e d t a s t e than the c a r n i v o r o u s d i e t . L a t e copepodid and a d u l t C.b.thomasi a r e p r o b a b l y o b l i g a t e c a r n i v o r e s on m i c r o c r u s t a c e a n s (McQueen, 1969), w h i l e the same i n s t a r s i n T . p r a s i n u s can u t i l i z e a l g a e or p r o t o z o a n s but p r o b a b l y no l i v i n g a n i m a l l a r g e r than a s m a l l r o t i f e r . A c c o r d i n g t o F r y e r ' s r e a s o n i n g , C.b.thomasi b e l o n g s t o a more p r i m i t i v e group, w h i l e the genus T r o p o c y c l o p s r e p r e s e n t s a more r e c e n t l y e v o l v e d c o n d i t i o n . The s m a l l s t r u c t u r a l changes o b s e r v e d i n a d u l t ' mouthparts can then be s a i d t o approximate the s m a l l changes obse r v e d i n o t h e r a n i m a l groups l i k e , f o r example, f i n c h e s , g r a s s h o p p e r s , bees, e t c . (Lack, 1947; Isley . , 1944; B r i a n , 1957), i n which minor changes i n the mouthpart s t r u c t u r e l e a d t o v e r y l a r g e d i f f e r e n c e s i n the type of f o o d t a k e n . F u r t h e r , the o b s e r v a t i o n t h a t the gut s of a l l c y c l o p o i d copepods a r e s h o r t l i k e t hose of c a r n i v o r o u s a n i m a l s ( F r y e r , 1957a), s u g g e s t s t h a t h e r b i v o r o u s c y c l o p o i d copepods a r e not v e r y e f f i c i e n t a t d i g e s t i n g p l a n t m a t t e r . C o n s e q u e n t l y , s p e c i e s which e x p l o i t p l a n t f o o d s o u r c e s t h r o u g h o u t t h e i r l i f e h i s t o r y r e q u i r e a superabundance of food t o be n u m e r i c a l l y s u c c e s s f u l because they a r e not v e r y e f f i c i e n t food p r o c e s s o r s . The omnivorous h a b i t , l i k e t h a t of T . p r a s i n u s (which can fee d on m i c r o z o o p l a n k t e r s as w e l l as 233 a l g a e ) may r e p r e s e n t a t r a n s i t i o n a l s tage from c a r n i v o r y t o h e r b i v o r y . However, t h i s s p e c i e s p o s s e s s e s f e e d i n g a p p a r a t u s remarkably s i m i l a r - t o t h a t of C.b.thomasi, a s p e c i e s which g a t h e r s much l a r g e r a n i m a l food i t e m s . I t seems u n l i k e l y t h a t T . p r a s i n u s , w i t h such r e l a t i v e l y c o a r s e f e e d i n g equipment, can be v e r y e f f i c i e n t a t c o l l e c t i n g f i n e food p a r t i c l e s . Thus T . p r a s i n u s p r o b a b l y f u n c t i o n s best on p r o t o z o a n s , r o t i f e r s and l a r g e a l g a e . In o l i g o t r o p h i c systems, p r o t o z o a n p o p u l a t i o n s a r e o f t e n low, r o t i f e r s and l a r g e a l g a e a r e s c a r c e much of the year and the food environment i s v e r y d i l u t e . C o n s e q u e n t l y , u n p r o d u c t i v e l a k e s may r e p r e s e n t m a r g i n a l h a b i t a t f o r T . p r a s i n u s , a s p e c i e s whose f e e d i n g h a b i t s e v o l v e d i n more p r o d u c t i v e systems. The f a c t t h a t T . p r a s i n u s i s never v e r y abundant i n o l i g o t r o p h i c Gwendoline Lake (an e s s e n t i a l l y p r e d a t o r - f r e e system f o r T . p r a s i n u s ) r e l a t i v e t o C.b.thomasi d e n s i t i e s i n P l a c i d Lake, y e t i n c r e a s e s s i g n i f i c a n t l y when Gwendoline Lake water i s f e r t i l i z e d , s u g g e s t s t h a t the food environment i s not i d e a l f o r T . p r a s i n u s . Assuming t h a t copepodid and a d u l t f e e d i n g s t r u c t u r e s a r e r e l e v a n t i n d i c a t o r s of 'food n i c h e ' and t h a t the food environment i s i m p o r t a n t t o the ' s u c c e s s ' of c y c l o p o i d copepods , some b i o g e o g r a p h i c a l p a t t e r n s emerge. In g e n e r a l , s p e c i e s which a r e h e r b i v o r o u s / o m n i v o r o u s as a d u l t s s h o u l d be most abundant i n p r o d u c t i v e l a k e s and r a r e or absent i n o l i g o t r o p h i c systems. C a r n i v o r e s , on the o t h e r hand, s h o u l d be the o n l y abundant c y c l o p o i d copepods t o t h r i v e i n e x t r e m e l y low n u t r i e n t l a k e s . I f the h e r b i v o r o u s h a b i t e v o l v e d i n systems w i t h h i g h food abundance, warm t o moderate t e m p e r a t u r e s a r e p r o b a b l y a 234 c o n c o m i t a n t r e q u i r e m e n t . P e j l e r (1964) s t a t e s t h a t a r c t i c and s u b a r c t i c l a k e s a re almost always o l i g o t r o p h i c w h i l e t r o p i c a l l a k e s u s u a l l y have a good d e a l i n common w i t h the e u t r o p h i c l a k e s of the temperate zone. On a broad g e o g r a p h i c s c a l e , c a r n i v o r o u s s p e c i e s , but not a d u l t h e r b i v o r e s , s h o u l d be common i n a r c t i c or s u b a r c t i c l a k e s where p r o d u c t i v i t y and temperature i s low (Moore, 1978; P a t a l a s , 1975). H e r b i v o r o u s s p e c i e s s h o u l d be most common i n warmer c l i m a t e s . There i s some e v i d e n c e i n the l i t e r a t u r e t o support t h i s q u a l i t a t i v e g e n e r a l i z a t i o n , a l t h o u g h the d i s t r i b u t i o n and abundance of m i c r o c r u s t a c e a n s r e l a t i v e t o l a k e t r o p h i c s t a t u s i s s t i l l i n c o m p l e t e l y known. E u c y c l o p s and T r o p o c y c l o p s a re not found i n t u n d r a and t a i g a r e g i o n s , a l t h o u g h - b oth genera have c o s m o p o l i t a n s o u t h e r n d i s t r i b u t i o n s ( R y l o v , 1963). S e v e r a l a u t h o r s ( B u r g i s , 1971; I n f a n t e , 1978; L e w i s , 1979) have found t h a t h e r b i v o r o u s s p e c i e s a r e the o n l y abundant c y c l o p o i d s i n t r o p i c a l l a k e s . R y l o v (1963) c o n s i d e r s the A r c t i c s p e c i e s complex t o c o n s i s t p r i m a r i l y of the genus C y c l o p s - a l l a d u l t c a r n i v o r e s . More r e c e n t s u r v e y s have not c o n t r a d i c t e d t h i s g e n e r a l i z a t o n (e.g. Tash, 1971; Moore, 1978). In N o r t h A m e r i c a , P a t a l a s (1975) found T r o p o c y c l o p s  p r a s i n u s mexicanus o n l y i n the S t . Laurence Great Lakes communities w i t h r e l a t i v e l y h i g h t o t a l z o o p l a n k t o n abundance (48-184 i n d i v i d u a l s / 1 and 0.68-3.81 mg/1 wet weight b i o m a s s ) . C.b.thomasi, however, was found i n deep s u b a r c t i c l a k e communities (2.7 i n d i y i d u a l s / 1 and 0.07 mg/1 wet weight biomass) and i n the S t . L a u r e n c e Great L a k e s , - o f t e n a c h i e v i n g n u m e r i c a l dominance. Moore (1978) found o n l y 2 c y c l o p o i d 235 copepod s p e c i e s i n h i s study of 18 l a k e s i n the Canadian a r c t i c and s u b a r c t i c - C.b.thomasi and C y c l o p s s c u t i f e r . Both s p e c i e s a r e c a r n i v o r e s i n the l a t e r i n s t a r s . L i k e C.b.thomasi , C . s c u t i f e r i s a l s o common i n some temperate l a k e s (Elgmork, 1967). S p r u l e s (1975) s u g g e s t s t h a t pH has a major e f f e c t i n the i n d u s t r i a l l y a c i d i f i e d l a k e s of La C l o c h e M o u n t a i n s , O n t a r i o . However, the r e l a t i o n s h i p between food r e s o u r c e s and pH i s complex and many c y c l o p o i d s p e c i e s , i n p a r t i c u l a r T . p r a s i n u s and C.b.thomasi, a r e e u r y i o n i c ( R y l o v , 1963; F r y e r , 1980). Thus the p r i m a r y e f f e c t w i t h r e p e c t t o the c y c l o p o i d s may be f o o d l i m i t a t i o n . C a r t e r e t a l . (1980) c o n c l u d e t h a t p o s t g l a c i a l d i s p e r s a l i s i m p o r t a n t i n e x p l a i n i n g c r u s t a c e a n p l a n k t o n d i s t r i b u t i o n s i n e a s t e r n N o r t h A m e r i c a . However, t h e i r s p e c u l a t i o n s a r e based on one sample from each l a k e c o l l e c t e d over a v a r i e t y of time i n t e r v a l s . In the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t Lakes, June samples may not d e t e c t T . p r a s i n u s and August samples i n Gwendoline Lake a r e u n l i k e l y t o c o l l e c t C.b.thomasi. Rare s p e c i e s , e s s e n t i a l f o r any assessment of s p e c i e s d i s p e r s a l , are a l s o l e s s l i k e l y t o be c o l l e c t e d by 'once-only' s a m p l i n g when a l a r g e mesh net i s used as i n the C a r t e r et a l . ( 1 9 8 0 ) s t u d y (158 um - 239 um) because n a u p l i i can pass through the mesh. Gurney (1931) c a u t i o n s t h a t " a l t h o u g h i n some c a s e s p r e s e n t d i s t r i b u t i o n i s p r o b a b l y due t o the i n f l u e n c e of the g l a c i a l p e r i o d , the g r e a t p o s s i b i l t i e s of p a s s i v e t r a n s p o r t must always be borne i n mind, and s p e c u l a t i o n founded oh g e o g r a p h i c a l d i s t r i b u t i o n must be a c c e p t e d w i t h c a u t i o n . " 236 In summary, by e x p e r i m e n t a l l y s t u d y i n g the mechanisms o p e r a t i n g on a l o c a l s c a l e , b i o g e o g r a p h i c a l p a t t e r n s may be more e a s i l y i n t e r p r e t e d . T h i s study demonstrates t h a t the n a u p l i a r i n s t a r s of both T . p r a s i n u s and C.b.thomasi a r e v e r y s e n s i t i v e t o p r e d a t i o n and food l i m i t a t i o n . W h i l e the n a u p l i a r s t a g e s c o n t r i b u t e l i t t l e t o s p e c i e s biomass, m o r t a l i t y e x p e r i e n c e d by these l i f e s t a g e s s e t s an upper bound on l a t e r i n s t a r abundances. U n f o r t u n a t e l y , l i t t l e i s known about n a u p l i a r f e e d i n g s t r u c t u r e s or f e e d i n g b e h a v i o u r s of these i n s t a r s . However, i t appears t h a t e x p l a n a t i o n s f o r d i s t r i b u t i o n and abundance p a t t e r n s of c y c l o p o i d copepods r e q u i r e some knowledge of the r e l a t i v e s e n s i t i v i t y of a l l i n s t a r s t o v a r i o u s community i n t e r a c t i o n s . The a b i l i t y of c y c l o p o i d n a u p l i i t o u t i l i z e ' u n p a l a t a b l e ' a l g a l t y p e s such as f i l a m e n t o u s b l u e - g r e e n a l g a e needs f u r t h e r i n v e s t i g a t i o n . T h i s study s u g g e s t s t h a t C.b.thomasi and T . p r a s i n u s n a u p l i i a r e more s e n s i t i v e t o low food d e n s i t i e s than t o changes i n a l g a l t y p e s . P h y t o p l a n k t o n s e a s o n a l s h i f t s t o b l u e - g r e e n a l g a e may fa v o u r c y c l o p o i d copepod s p e c i e s over o t h e r h e r b i v o r e s , such as f i l t e r - f e e d i n g c l a d o c e r a n s , because the l a t t e r appear t o have d i f f i c u l t y u t i l i z i n g t h e s e a l g a l forms ( G l i w i c z , 1975; P o r t e r , 1977). The r e l a t i v e abundance of b o t h C.b.thomasi and T . p r a s i n u s i n P l a c i d Lake and Gwendoline Lake i s m a i n t a i n e d by p r e d a t i o n p r e s s u r e on the v u l n e r a b l e s t a g e s of the l e s s n u m e r i c a l l y i m p o r t a n t s p e c i e s i n each l a k e . However, both Gwendoline Lake and P l a c i d Lake a r e p r o b a b l y m a r g i n a l h a b i t a t s f o r T . p r a s i n u s because m i c r o z o o p l a n k t e r s and a l g a e a r e r e l a t i v e l y low i n 237 abundance. I have suggested t h a t , i n d i l u t e food e n v i r o n m e n t s , a c y c l o p o i d s p e c i e s w i l l need t o be c a r n i v o r o u s i n the l a t e r i n s t a r s t o become a n u m e r i c a l l y s i g n i f i c a n t member of the p l a n k t o n . However, w i t h i n a l a k e , the s p e c i e s which becomes dominant w i l l p r o b a b l y be d e t e r m i n e d by p r e d a t i o n , e i t h e r t h r o u g h i n t r a c y c l o p o i d i n t e r a c t i o n s as suggested by Smyly (1978) or by o t h e r v e r t e b r a t e or i n v e r t e b r a t e p r e d a t o r e f f e c t s ( e . g . Dodson, 1970; H u r l b u r t and M u l l a , 1980). In any c a s e , i t i s c l e a r t h a t b i o t i c i n t e r a c t i o n s can be c r i t i c a l l y i m portant i n e x p l a i n i n g r e g i o n a l i n t e r - l a k e d i f f e r e n c e s i n c y c l o p o i d copepod s p e c i e s ' abundance. W i t h i n the c r u s t a c e a n community, C.b.thomasi (and Chaoborus i n a low n u t r i e n t environment) appears t o enhance the n u m e r i c a l importance of t h e l a r g e g r a z e r s i n the l a k e , D.rosea and D . l e p t o p u s , p o s s i b l y by r e d u c i n g the abundance of c o m p e t i t o r s . U n l i k e Chaoborus, C.b.thomasi does ' not seem c a p a b l e of c a u s i n g p r e y e x t i n c t i o n s . The reason f o r t h i s d i f f e r e n c e p r o b a b l y l i e s i n the s m a l l f u n c t i o n a l and n u m e r i c a l reponse of C.b.thomasi r e l a t i v e t o i t s p r e y . The prey can escape p r e d a t o r impact b o t h by swamping C.b.thomasi n u m e r i c a l l y t h r o u g h h i g h r a t e s of r e p r o d u c t i o n and by i n c r e a s i n g i n s i z e beyond t h a t which C.b.thomasi can a t t a c k s u c c e s s f u l l y . By c r o p p i n g s m a l l z o o p l a n k t e r s , c y c l o p o i d copepods may a c t as community ' s t a b i l i z e r s ' p r e v e n t i n g l a r g e f l u c t u a t i o n s i n community biomass. T h i s r o l e i s p r o b a b l y most im p o r t a n t i n low n u t r i e n t environments where food i s s c a r c e most of the y e a r . 238 LITERATURE CITED A l l e n , J . D a v i d . 1976. L i f e h i s t o r y p a t t e r n s of z o o p l a n k t o n . Amer. Nat. 110: 165-180. Anderson, R. S. 1970a. P r e d a t o r - p r e y r e l a t i o n s h i p s and p r e d a t i o n r a t e s f o r c r u s t a c e a n z o o p l a n k t e r s from some l a k e s i n Western Canada. Can. J . Z o o l . 48: 1229-1240. 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F i s h p r e d a t i o n on Bosmina l o n q i r o s t r i s : b o d y - s i z e s e l e c t i o n v e r s u s v i s i b i l i t y s e l e c t i o n . E c o l o g y 56: 232-237. APPENDIX A An " i n d e x o f a s s i railatioTi" was c a l c u l a t e d a c c o r d i n g t o t h e f o r m u l a ( S o r o k i n , 1 9 6 8 ) : Ca/C = R. C r . 100. 24 i n % W. t w h e r e Ca = a m o u n t o f f o o d a s s i m i l a t e d C = mean c a r b o n c o n t e n t o f t h e c o n s u m e r s R = r a d i o a c t i v i t y ( d i s i n t e g r a t i o n s p e r m i n u t e ) i n t h e b o d i e s o f t h e c o n s u m e r s C r = c o n t e n t o f o r g a n i c c a r b o n i n a d e f i n i t e p o r t i o n o f l a b e l l e d f o o d , o r u g C / r t i s i n t e q r a t i o n s p e r . m i n u t e . H = mean c o n t e n t o f o r g a n i c c a r b o n i n t h e c o n s u m e r s t =. d u r a t i o n o f f e e d i n g w i t h l a b e l l e d f o o d ( i n h o u r s ) A P P E N D I X B T h e p r i n c i p l e e m p l o y e d i n t h i s a n a l y s i s i s t o a n a l y s e d i f f e r e n c e s i n p r o p o r t i o n o f o v i g e r o u s f e m a l e s L e t w e c - n t r e a t m e n t s o n e a c h s a m p l i n g d a t e t h e r e b y e l i m i n a t i n g t h e u p s a n d d o w n s . A s t h e p o p u l a t i o n s i n t h e d i f f e r e n t t r e a t m e n t s w e r e n o t t h e s a m e , i t w a s n e c e s s a r y t o w e i g h t t h e p r o p o r t i o n a c c o r d i n g t o t h e s a i f l e s i z e i n e a c h t r e a t m e n t . D i f f e r e n c e s b e t w e e n t h e 2 t r e a t m e n t s i n w h i c h n. o r g _ o n e n s i s w a s p r e s e n t w e r e a n a l y z e d i n t h e f o l l o w i n g w a y : w h e r e t x = p r o p o r t i o n o f o v i g e r o u s f e m a l e s o n e a c h s a m p l i n g d a t e i n t r e a t m e n t x w e i g h t e d a c c o r d i n g t o s a m p l e s i z e n x = n o . o f f e m a l e s w i t h e g g s i n t r e a t m e n t x H x - t o t a l n o . o f f e m a l e s i n t r e a t m e n t x a r c s i n e t r a n s f o r m a t i o n w a s t a k e n o f t a n d t w h e r e t , = t r e a t m e n t w i t h C . b . t h o m a s i tx- t r e a t m e n t w i t h o u t C . b . t h c m a a i T h e n t , - t L = p f o r e a c h s a n p l i n g d a t e u s i n g a t w o - t a i l e d t - t e s t t h e t r e n d s b e t w e e n t h e t w o t r e a t m e n t s w e r e c o m p a r e d (Ho = p = 0 ) . F o r D . o r < * j o n e n n i s , t h e p e r i o d t e s t e d w a s J u l y 7 t o A u g u s t 22, a f t e r w h i c h t i m e t h i s s p e c i e s p r o d u c e d o v i g e r o u s f e m a l e s o n l y i n t h e t r e a t i e n t w i t h o u t C . b - t h o m a s i . T h e J u l y - A u g u s t t i m e i n t e r v a l ho r e p r e s e n t e d t h e p e r i o d w h e n f o o d s h o r t a g e s w e r e m o s t l i k e l y t o o c c u r , a n d w h e n f o o d s h o r t a g e s w e r e m o s t l i k e l y t o h a v e a n e g a t i v e i m p a c t o n t h e r e p r o d u c i n g T . p r a s i n u s ; h o w e v e r , t h e r e v a s n o d i f f e r e n c e b e t w e e n t h e t w o t r e a t m e n t s ( t = O . U , d f = 7, p< 0 . 0 5 ) . 2 - E 2 s e a - a n ^ 1 - l i i i S i n u s w e r e p r e s e n t i n a l l 3 t r e a t m e n t s . T h e s a m e p r i n c i p l e w a s a p p l i e d t o t h e d a t a a n d a n a n a l y s i s o f v a r i a n c e w a s e m p l o y e d . T h e d i f f e r e n c e i n t h e n u m b e r o f o v i g e r o u s f e m a l e s o f D . r o s p a a n d l - £ r a s i n u s b e t w e e n t r e a t m e n t s w e r e a n a l y z e d i n t h e f o l l o w i n g w a y : f o r e a c h t r e a t m e n t , p = n , / N x w h e r e f = p r o p o r t i o n o f o v i g e r o u s f e m a l e s n o . o f f e m a l e s w i t h e g g s N = t o t a l n o . o f f e m a l e s An a r c s i n e t r a n s f o r m a t i o n w a s p e r f o r m e d o n p , p^ a n d p^ w h e r e p^  = t r e a t m e n t w i t h C . b . t h o m a s i \> = t r e a t m e n t w i t h o u t C . b . t h e m a s i b u t w i t h o t h e r P l a c i d L a k e c r u s t a c e a n p l a n k t o n = t r e a t m e n t w i t h T . p r a s i n u s ' a l o n e ' T h e w e i g h t e d a v e r a g e p r o p o r t i o n o f o v i g e r o u s f e m a l e s a c r o s s a l l t r e a t m e n t s (P) w a s t h e n c a l c u l a t e d ; w p + w p + w p I I 2 2 3 3 p = _ _ w w w 4 2 3 w h e r o v_ = ( L a n d c a l c u l a t i o n s w e r e c h e c k e d t o e n s u r e t h a t : w (p - P) + w (p - P) • w (p -*• ' i 3 3 F) = 0 f o r e a c h s a m p l e . i A w e i g h t e d a n a l y s i s o f v a r i a n c e o f t h e d i r e c t i o n s (p - P) w i t h w e i g h t s ( w ^ ) w a s t h e n p e r f o r m e d , w i t h t h e f o l l o w i n g r e s u l t : A P P E N n i x c T h e - d u r a t i o n t i m e c E t h e v a r i o u s i n s t a r s o f T . p r a s i n u s f r o m P l a c i d L a k e , 1 9 7 6 , a t 1 6 t 2 ° C v a s d e t e r m i n e d i n t h e l a b o r a t o r y a s d e s c r i b e d b y S m y l y ( 1 9 7 0 ) e x c e p t t h a t l a k e w a t e r w a s u s e d a n d t h e n a u p l i i w e r e c h e c k e d b y p l a c i n g t h e m i n a v e r y s m a l l d r o p o f w a t e r . I d i d n o t a p p l y a K r o g h c u r v e t o t h e d a t a a s t h e t e m p e r a t u r e i n P l a c i d L a k e o n l y v a r i e d f r o m 1 4 . 5 t o 1 8 . 5 ° C d u r i n g A u g u s t -S e p t e m b e r , t h e p e r i o d when J - p r a s i n u s j u v e n i l e s w e r e d e v e l o p i n g . T h e o n l y e x c e p t i o n t o t h i s g e n e r a l i z a t i o n w a s o n e w e e k i n e a r l y S e p t e m b e r w h e n t h e t e m p e r a t u r e d r o p p e d t o 1 1 . 5 ° C . T h e r e s u l t s f o r 20 a n i m a l s w e r e a s f o l l o w s : S T A R E r | D U R A T I O N ( IN D A Y S ) 1 E G G T C H A T C H | 5 . 1 N - I I I 1 3 . 0 N IV - V I | 6 . e 1 5 - 2 C I C I I 1 5 . 1 c m 1 7 . C C I V | 6 . 2 cv | 8 . 1 A D U L T | 3 8 . 1 4 . T i i e d e v e l o p m e n t t i m e o f t h e v a r i o u s i n s t a r s o f C . e n c l o s u r e s 1977, w a s e s t i m a t e d f r c i . f i e l d p o p u l a t u n f e r t i l i z e d G w e n d o l i n e e n c l o s u r e p o p u l a t i o n s , c o l f e r t i l i z e d G w e n d o l i n e e n c l o s u r e . B o t h e n c l o s u r e s b . t h o m a s i i n P l a c i d L a k e a n d G w e n d o l i n e L a k e i o n d a t a . C o l u : n n A r e f e r s t o P l a c i d L a k e a n d t h e u n a D i n d i c a t e s t h e d e v e l o p m e n t t i m e i n t h e i n G w e n d o l i n e L a k e w e r e C h a o b o r u s - f r e e . S T A G E | D U R A T I O N ( IN E A Y S ) I A 3 EGG I C HA1CII | 5.0 5.0 N - I I I | 7. 0 7.0 NIV - V I | 10. 5 10.5 C I | 7. 0 7.0 C I I | 7. 0 7.0 c m | 7. 0 7.0 C I V | 10.0 9.0 CV | 1U.0 12. 0 T h e a b u n d a n c e o f e a c h i n s t a r f o r . a p a r t i c u l a r y e n e r a t i o n w a s c a l c u l a t e d u s i n g t h e f o l l o w i n g a o d e l ( G e h r s a n d H c b e r t s o n , 1975): T (1 • • 1; /2) (W /P . ) = N i 1 = n u m b e r o f i n d i v i d u a l s a l i v e X - i n s t a r d e s i g n a t i o n x = c o l l e c t i o n d e s i g n a t i o n j = f i r s t c o l L e c t i c n p r i c r t o t h e a p p e a r a n c e o f i n s t a r X (c = c o l l e c t i o n f o l l o w i n g t h e l a s t c o l l e c t i o n i n w h i c h i n s t a r X a p p e a r s D i = d u r a t i o n o f i n s t a r X i n d a y s H = i n t e r v a l i n d a y s b e t w e e n c o l l e c t i o n x a n d c o l l e c t i o n x+1 N = n e t n u m b e r o f i n d i v i d u a l s o f i n s t a r I o b s e r v e d i n t h e i n t e r v a l x = j t o t h e e s t i m a t e d n u m b e r o f i n d i v i d u a l s o f i n s t a r I i n a p a r t i c u l a r g e n e r a t i o n . ho S u r v i v o r s h i p o f T . p r a s i n u s i n t h o P l a c i d L a k e e n c l o s u r e s i n 1 .976. W i t h C . b - t h o m a s i * W i t h o u t C . b . t h o m a s i + S t a g e N u m b e r at . t h e B E G I N N I N G C F A R E i n t e r v a l ( l x ) N u m b e r a t t h e B E C I N N T N G O F A G E i n t e r v a l ( l x ) E g g 1000 1 0 0 0 N I I I 2 0 5 . 2 2 9 u . n N V I 8 7 . 5 1 6 1 . 7 C I 1 2 . 7 1 3 2 . 2 C I I 2 1 . 2 1 2 7 . i t C I 1 1 1 7 . 9 1 2 7 . 1 C I V 1 7 . e 1 2 7 . 6 C V 1 1 . 9 1 0 7 . 8 * o r i g i n a l l x = 5 . 1 0 x 1 0 * • o r i g i n a l l x = 5 . 1 6 x 1 0 ' S u r v i v o r s h i p o f C . b . t h o m a s i i n P l a c i d L a k e a n i l t h e G w e n d o l i n e e n c l o s u r e s w i t h o u t t t e m i d g e l a r v a e C h a o b o r u s i n 1 9 7 7 . l a k e * U n f e r t i l i z e d * * F e r t i l i z e d * * * S u m t e r a t t h e N u m b e r a t t h e b e g i n n i n g o f a g e b e g i n n i n g o f a g e S t a g e E g g NIII N V I CI C I I e n i civ cv i n t e r v a l ( l x ) 1 0 0 0 5 1 0 . 5 5 2 2 . 7 19 9 . 1 1 7 1 . 8 1 2 6 . 5 1 1 5 . 9 1 1 6 . 5 i n t e r v a l ( l x ) 1 0 0 0 6 0 9 . 8 1 0 6 . 8 2 3 8 . 3 2 2 7 . 1 22 1 .1 1 6 6 . 3 1 1 9 . 3 N u m b e r a t t h e b e g i n n i n g o f a g e i n t e r v a l ( l x ) 1 0 0 0 7 9 0 . 0 7 3 0 . 3 6 8 0 . 7 1 5 0 . 2 1 1 0 . 1 3 2 0 . 9 3 1 0 . 6 • o r i g i n a l l x = 6 . 1 x 1 0 3 • • o r i g i n a l l x = 5 . 1 x 103 • • • o r i g i n a l l x = 5 . 7 x 1 0 * S u r v i v o r s h i p o f C . b . t h o m a s i i n P l a c i d l a k e a n d t h e G w e n d o l i n e e n c l o s u r e s w i t h o u t t h e m i d g e l a r v a e C h a o b o r u s i n 1 9 7 7 . l a k e * U n f e r t i l i z e d * * F e r t i l i z e d * * * N u m t e r a t t h e N u m b e r a t t h e b e g i n n i n g o f a g e b e g i n n i n g o f a g e S t a g e E g g N I I I N V I C I C I I e n i civ cv i n t e r v a 1 ( l x ) 1 0 0 0 5 0 0 . 5 3 2 2 . 7 1 9 9 . 4 1 7 4 . 8 1 2 6 . 5 14 5 . 9 1 1 6 . 5 i n t e r v a l ( l x ) 1 0 0 0 6 0 9 . 8 4 0 6 . 8 2 3 8 . 3 2 2 7 . 1 22 1 .1 , 1 6 6 . 3 1 1 9 . 3 N u m b e r a t t h e b e g i n n i n g o f a g e i n t e r v a 1 ( l x ) 1 0 0 0 7 9 0 . 0 7 3 0 . 3 6 8 0 . 7 4 5 0 . 2 4 4 0 . '4 3 2 0 . 9 3 4 0 . 6 • o r i g i n a l l x = 6 . 4 x 1 0 ' • ' o r i g i n a l l x = 5 . 1 x 103 * * * o r i g i n a l l x = 5 . 7 x 1 0 ' ho ON ho 263 c u w c •3 c u c u O C •H a in -= * i H U « o ^  -J » -3 a u t a, o c ci 01 Q. T) o « w u in "3" <-* o -J U Q) « O l-i — H T' K — « Q) W Q. U U U -H O U *H r-i O. ^  •-t <C N. O ' CJ. u. X •C O <C O. *J •u O H l~. —t U \ <u — tO Oi ^ f^j »- „ OO'-'-'— o » - o o o o o O O O O O O O O O O O O O O O O O O O O O O O O o o o o o o o o o o O ' - r i r o n m r - i f N i ' - o O O o o o o o o o c - O o c c o o c o o o o o o o o o 0 *- (-1 =r u"l a 3 f l — o <J 0 o o o u o o o o o o o o ^ " t o ^ J o r ^ f O o ^ j - — o 01 (J o IN JZ I £ •3 —( 3 3 C 3 2 •M o ^3 O O -a a G *< 0 *> -a <u 5! G> M ...» w u a. = 01 T « di M U -H r; U U) c w a ,o oi o — o o 01 01 y 264 

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