"Science, Faculty of"@en . "Zoology, Department of"@en . "DSpace"@en . "UBCV"@en . "Peacock, Adrienne"@en . "2010-03-29T19:36:46Z"@en . "1981"@en . "Doctor of Philosophy - PhD"@en . "University of British Columbia"@en . "Factors causing the population limitation of two species of cyclopoid copepods, Tropocyclops prasinus and Cyclops bicuspidatus thomasi, and the subsequent impact of these species on the zooplankton community, were investigated in two montane lakes of the University of British Columbia Research Forest. In Placid Lake, C.b.thomasi was abundant and T.prasinus was rare, while in Gwendoline Lake, T.prasinus was the more abundant species. Field and laboratory experiments were conducted to assess the importance of such factors as food, physical-chemical lake characteristics, and predation upon the population dynamics of these species. The feeding appendages of adult T.prasinus and C.b.thomasi were compared and found to be structurally alike, although adult diets were quite dissimilar. T.prasinus was omnivorous in the later instars, eating algae, protozoans and probably dead macrozooplankters while C.b.thomasi was markedly carnivorous in copepodid and adult instars. Seasonal abundance peaks revealed a temporal separation of the naupliar instars of these species in both lakes studied. Consequently, competition between T.prasinus and C.b.thomasi was unlikely. Large scale enclosure experiments in Placid Lake, where C.b.thomasi was abundant, showed that lake densities of C.b.thomasi could limit the abundance of T.prasinus. Survivorship curves and laboratory feeding experiments indicated that the low abundances of T.prasinus was caused by two important factors: high T.prasinus naupliar mortality even in the absence of C.b.thomasi, and C.b.thomasi predation on these T.prasinus instars. When C.b.thomasi was transplanted to enclosures in Gwendoline Lake, where T.prasinus was the more abundant cyclopoid copepod, C.b.thomasi was able to grow and reproduce within the crustacean plankton community and became as abundant as in its home lake. Naupliar instars were the most mortality-prone stages in both lakes. Fertilization of a Gwendoline Lake enclosure increased the survivorship of C.b.thomasi about 22% over that observed in the unfertilized treatment. However, addition of the midge fly larvae common to Gwendoline Lake (C.trivittatus and C.americanus) to the enclosure plankton community resulted in a decrease in the standing crop of C.b.thomasi to a point at which samples became difficult to obtain. With Chaoborus in the enclosure, fertilization did not enhance the abundance of C. b. thomasi but rather, improved the survivorship of Chaoborus first and second instar larvae, and resulted in an acceleration of the C.b.thomasi nauplii decline. The impact of C.b.thomasi on other zooplankton was also examined. C.b.thomasi appeared to dampen the fluctuations in total zooplankton biomass in enclosures without a Chaoborus population. However, in the fertilized treatment without Chaoborus, C.b.thomasi was unable to track prey demographic responses and caused no major compositional changes in the crustacean community at either high or low nutrient levels. Temporal - spatial overlap and predator and prey developmental responses were critical factors determining prey sensitivity to predation by C.b.thomasi."@en . "https://circle.library.ubc.ca/rest/handle/2429/22897?expand=metadata"@en . "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 \u00C2\u00A9 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\u00E2\u0080\u00A2b.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\u00E2\u0080\u00A2b.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 \u00E2\u0080\u00A2. 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 \u00E2\u0080\u00A2. 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 \u00E2\u0080\u00A2. 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 ) \u00E2\u0080\u00A2 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 \u00E2\u0080\u009Eplanktonic 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\u00E2\u0080\u00A2 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\u00C2\u00A32CJ\u00C2\u00A3&l2\u00C2\u00A35 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\u00C2\u00B12\u00C2\u00B0C. 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\u00C2\u00B0C 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\u00C2\u00B12\u00C2\u00B0C, 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\u00C2\u00B0C 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\u00C2\u00B0C 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\u00C2\u00B11\u00C2\u00B0C 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 \u00E2\u0080\u00A2 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\u00C2\u00B11\u00C2\u00B0C. 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\u00C2\u00B11\u00C2\u00B0C 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\u00C2\u00BB). 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 \u00E2\u0080\u00A2g 2. 45 7. 16 0.35 p >0.05 0.05 \u00E2\u0080\u00A2 >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\u00C2\u00B12\u00C2\u00B0C, and weighed on a Cahn e l e c t r o b a l a n c e , ashed a t 600\u00C2\u00B0C 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 \u00E2\u0080\u00A2\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 ^\u00E2\u0080\u00A2I2s_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 !\u00E2\u0080\u00A2 P\u00C2\u00A3i*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\u00C2\u00A3\u00C2\u00A7.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 \u00E2\u0080\u00A2 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 \u00E2\u0080\u00A2 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\u00C2\u00B0\u00E2\u0080\u00A2 8 7 r 2=0.85 B. PR = 0.05x\u00C2\u00B0 \u00E2\u0080\u00A2 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\u00C2\u00B0-53 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\u00E2\u0080\u00A2b.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 \u00E2\u0080\u0094 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 \u00C2\u00B0 o O n i\u00C2\u00ABi - \u00C2\u00B0 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 (\u00C2\u00B11SE), 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\u00C2\u00B10.1, 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 \u00C2\u00AB\u00E2\u0080\u00A2 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\u00C2\u00B0 ' 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\u00C2\u00B12\u00C2\u00B0C) 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, \u00E2\u0080\u00A2 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 \u00E2\u0080\u00A2 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 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 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\u00C2\u00A3tomus o r ^ g o n e n s i s fi3\u00C2\u00A3]}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 ) \u00C2\u00AB\u00E2\u0080\u00A2 Key t o s y m b o l s : A \u00E2\u0080\u00A2= a b u n d a n t R = r a r e a b s e n t 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 \u00E2\u0080\u00A2 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 \u00E2\u0080\u00A2 ' 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. /\u00C2\u00A35 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\u00E2\u0080\u00A2package 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* \u00E2\u0080\u00A2 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 \u00E2\u0080\u00A2' 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 \u00E2\u0080\u00A2a 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\u00C2\u00B10.21 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\u00C2\u00B10.17 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\u00C2\u00B10.06 (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\u00C2\u00B10.80 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\u00C2\u00B10.49 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\u00C2\u00B10.14; ' 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\u00C2\u00B1o.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\u00C2\u00B10.17 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\u00C2\u00B10.23 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\u00C2\u00B10.16 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\u00C2\u00B10.79 and 13% w i t h 1.2\u00C2\u00B10.11 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 \u00E2\u0080\u00A2 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\u00C2\u00B10.01) 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 \u00E2\u0080\u00A2Gwendoline 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. \u00E2\u0080\u00A2 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 ' \u00E2\u0080\u00A2treatment. 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 \u00E2\u0080\u00A2 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 - \u00C2\u00A3 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) \u00E2\u0080\u00A2 w (p -*\u00E2\u0080\u00A2 ' 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 \u00C2\u00B0 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 \u00C2\u00B0 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 \u00C2\u00B0 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 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 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 * \u00E2\u0080\u00A2 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 \u00E2\u0080\u00A2 o r i g i n a l l x = 6 . 1 x 1 0 3 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 o r i g i n a l l x = 5 . 1 x 103 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 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 \u00E2\u0080\u00A2 o r i g i n a l l x = 6 . 4 x 1 0 ' \u00E2\u0080\u00A2 ' 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 \u00E2\u0080\u00A23 c u c u O C \u00E2\u0080\u00A2H a in -= * i H U \u00C2\u00AB o ^ -J \u00C2\u00BB -3 a u t a, o c ci 01 Q. T) o \u00C2\u00AB w u in \"3\" <-* o -J U Q) \u00C2\u00AB O l-i \u00E2\u0080\u0094 H T' K \u00E2\u0080\u0094 \u00C2\u00AB Q) W Q. U U U -H O U *H r-i O. ^ \u00E2\u0080\u00A2-t -a M ...\u00C2\u00BB w u a. = 01 T \u00C2\u00AB di M U -H r; U U) c w a ,o oi o \u00E2\u0080\u0094 o o 01 01 y 264 "@en . "Thesis/Dissertation"@en . "10.14288/1.0095281"@en . "eng"@en . "Zoology"@en . "Vancouver : University of British Columbia Library"@en . "University of British Columbia"@en . "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en . "Graduate"@en . "Responses of two coexisting cyclopoid copepods to experimental manipulations of food and predators"@en . "Text"@en . "http://hdl.handle.net/2429/22897"@en .