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Functional responses and feeding strategies of fresh-water filter-feeding zooplankton Buckingham, Sandra 1978

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c FUNCTIONAL RESPONSES AND FEEDING STRATEGIES OF FRESH-WATER FILTER-FEEDING ZOOPLA K KTON by SANDRA LYNN BUCKINGHAM B.Sc, Queen's University, 1967 D.E.A., Oniversite de Montpellier (France), 1969 Doct. de 3® Cycle, Universite de Montpellier, 1970 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES {Department cf Zoology) we accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF ERITISE COLUMBIA July, 1S78 Sandra Lynn Buckingham, 1S78 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 a d v a n c e d d e g r e e 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 a n d s t u d y . I f u r t h e r a g r e e 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 D e p a r t m e n t o r by h i s 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 . D e p a r t m e n t n f Z o o l o g y  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 2075 W e s b r o o k P l a c e V a n c o u v e r , C a n a d a V6T 1W5 D a t e F e b r u a r y 20, 1979 ABSTRACT T h i s study examined the f u n c t i o n a l response t o s e s t o n c o n c e n t r a t i o n of f i l t e r - f e e d i n g zooplankton i n s e v e r a l freshwater l a k e s i n B r i t i s h Columbia. A simple plankton biomass model was used to develop the hy p o t h e s i s t h a t zooplankton f e e d i n g s t r a t e g i e s should be of two d i s t i n c t t y p es - one c h a r a c t e r i z e d by an a b i l i t y t o o b t a i n maximal food i n t a k e at high f o o d d e n s i t i e s and the oth e r by an a b i l i t y t o ha r v e s t food e f f i c i e n t l y i n very low food d e n s i t i e s . A r a d i o a c t i v e t r a c e r technique was ; used to measure f e e d i n g r a t e s : zooplankton were allowed t o graze f o r s h o r t p e r i o d s of time on n a t u r a l seston to which s m a l l amounts of 3 2 P - l a b e l l e d yeast c e l l s had been added. A p r e l i m i n a r y s e r i e s of experiments checked f o r p o s s i b l e e f f e c t s of c o n t a i n e r s i z e , r i n s i n g technigue, kind of f o o d , yeast dosage l e v e l , l e n g t h o f g r a z i n g time, and a c c l i m a t i o n time. A t o t a l of 5 3 f u n c t i o n a l responses, r e p r e s e n t i n g 8 s p e c i e s from 6 l a k e s , were measured. F u n c t i o n a l responses were compared by examining two parameters -maximum f e e d i n g r a t e (asymptote of the f u n c t i o n a l r e s p o n s e ) , and maximum f i l t e r i n g r a t e (slope o f the f u n c t i o n a l response near the o r i g i n ) . Some o f the s p e c i e s o c c a s i o n a l l y e x h i b i t e d a sigmoid f u n c t i o n a l response, but i n g e n e r a l most of the responses were of the D i s c o r Michaelis-Henten type. Most of the s p e c i e s s t u d i e d had a very s i m i l a r range of f u n c t i o n a l response i i i parameters, and e s s e n t i a l l y i d e n t i c a l maximum f i l t e r i n g r a t e s -Maximum f i l t e r i n g r a t e s were independent of temperature from 8° to 20° C. Maximum f e e d i n g r a t e s , on the other hand, were f u n c t i o n s of temperature, d i v i d i n g the zooplankton i n t o "Harm" and " c o l d " s p e c i e s v i t a h i g h e s t measured maximum fe e d i n g r a t e s at 20° and 8° C r e s p e c t i v e l y . The s i m i l a r i t y o f f u n c t i o n a l response parameters over the range of s p e c i e s and l a k e s s t u d i e d i n d i c a t e s t h a t there may i n f a c t be a s m a l l number of g e n e r a l feeding s t r a t e g i e s u n d e r l y i n g the observed complexity o f a g n a t i c ecosystems., TABLE OF CONTENTS L i s t ' Of Tables- . ....... ...... ......... ... ........... • * • • . . - . v i L i s t Of F i g u r e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v i i Acknowledge me nts .......... . ... . . . . ............ . . . . . . • - . . . x i 1 .Introduction' . . m . m . . . . . > . . . . . v . . • .'.'I*-" i t .General- ... . . 7 . ' . : . . < . t 1-2 F u n c t i o n a l Response Of F i l t e r Feeding Zooplankton 4 1.3 Importance Of F u n c t i o n a l Responses ...............6 1.4 O u t l i n e Of O b j e c t i v e s ............................ 8 2 T h e o r e t i c a l Background .....,•......;,•>.......•-••v:«W«.»>-Vi.r,.1.Q 2.1 F u n c t i o n a l Response Types ........................10 2.2 S t a b i l i t y Of F u n c t i o n a l Responses ................20 2.3 E v o l u t i o n a r y Adaptation Of F u n c t i o n a l Responses -.26 2. 3. 1 ..Introduction . - .- .:... . . . ............... • . -... .26 2.3.2 Zooplankton P o p u l a t i o n s L i m i t e d By Food .....28 2 .3.3 Zooplankton P o p u l a t i o n s L i m i t e d By Pr e d a t i o n 31 2. 3.4 . P r e d i c t i o n s .........-. .-.. .. .......................33 -2. 3. 5 D i s c u s s i o n «.r..•...-..''»....,;..>;.<••'..•,•••.••«-35 -2.3.6 Evidence From The L i t e r a t u r e -.38 2.4 . C l a s s x f i e at i o n . . . . . ... . .v • •. .... 4 3 3 Ex p e r i menta1 Design ....•.....•••.-.-....-.-.----...>.-.46 3.1 Choice Of Technique ..............................51 3.2 D e s c r i p t i o n Of Methods Used ......................57 V 3 - 3 E f f e c t s Of Experimental C o n d i t i o n s - - - , , , , - - - . , - - - 6 2 4 R e s u l t s 4- 1 T e s t s Of Experimental C o n d i t i o n s . , , 6 4 4 - 1 - 1 S i z e Of Grazing Chambers . . . . . . . . . , . . . . , , . - . . 6 4 4 . 1 . 2 Zooplankton R i n s i n g ...67 4 . 1 . 3 Kind Of Food .,w..v-:..---«..>-...-i/-,-y,,v«-*>v.»-*$*• 4 . 1 . 4 R a d i o a c t i v e l e a s t ...........................75 4 . 1 .5 Length Of Grazing Time ..........,.......-<. , . 7 8 4 . 1 . 6 A c c l i m a t i o n To Experimental; Food •Concentration ....... .. „>.,,, ,•, ................. 79 •• 4 . 2 F u n c t i o n a l Response R e s u l t s , , , , , 8 8 5 D i s c u s s i o n - . . . . . . . . , - , , . i . : . > ; , • • , , , . • , , ^ , . : > 1 2 6 5 .1 C o n s i s t e n c y With Other Data 126 { 5 . 2 Types Of F u n c t i o n a l Response Obtained ............132 5 . 3 Constancy Of F u n c t i o n a l Responses , v - - . 1 3 3 5 . 3 . 1 E f f e c t Of Temperature .......................133 5 . 3 . 2 A c c l i m a t i o n To Food C o n c e n t r a t i o n , . 1 3 9 5 .4 F u n c t i o n a l Response Adaptation , , , , , , , , , 1 4 1 5 . 5 General Remarks ......................... . . . . . . . . . 1 4 3 6 B i b l i o g r a p h y . , ' . , , . . . , , . . . . . , , : . , ; , . - . . , - . . . , , : , - , > , - . - - . - 1 4 6 v i LIST OF TABLES I. C h a r a c t e r i s t i c s o f the l a k e s used i n t h i s I I - Parameter e s t i m a t e s obtained by f i t t i n g e q u a t i o n s (1), (7) and (8) t o the measured I I I . Comparison of f i l t e r i n g r a t e s measured i n t h i s study v i t h those measured by J.F. Haney (1973) ..,-130 v i i LIST OF F1GJJRES 1- C l a s s e s o f f u n c t i o n a l response generated by H o l l i n g * s 2- Types of f u n c t i o n a l response hypothesized f o r f i l t e r -f e e d i n g zooplankton.^.„. . . . . . . . . . . . . . . . . . . . . . . . . . 3 5 3. F u n c t i o n a l responses proposed by Goodman e t a l . {1973) t o e x p l a i n seasonal s u c c e s s i o n of phytoplankton 4. Hap shoving l a k e s from which zooplankton were taken 6. Experimental c o n f i g u r a t i o n . , ........................,58 7. A comparison of Daphnia r o s e a f i l t e r i n g r a t e s measured i n two d i f f e r e n t k i n d s of c o n t a i n e r . ................66 8. R e s u l t s of using a n o n r a d i o a c t i v e " p o s t " feed t o ensure adequate r i n s i n g of r a d i o a c t i v e p a r t i c l e s from f e e d i n g appendages. ........................68 9. Comparison of Daphnia -.-.rosea f i l t e r i n g r a t e s measured with s e s t o n and with C h l o r e l l a . .....................70 10. Comparison of Daphnia pulex f i l t e r i n g r a t e s measured with s e s t o n and with C h l o r e l l a . 72 11. Comparison of Ceriodaphnia sp. f i l t e r i n g r a t e s measured with s e s t o n and with C h l o r e l l a . ,,,,,.,.73 12. Comparison of Holopedium gibberurn f i l t e r i n g r a t e s v i i i measured with s e s t o n and with C h i or e l l a. ............ 74 13. F i l t e r i n g r a t e s o f Daphnia rosea measured with d i f f e r e n t amounts of r a d i o a c t i v e yeast. 76 14. F i l t e r i n g r a t e s of Holopedium qibberum measured with d i f f e r e n t amounts of r a d i o a c t i v e y e a s t . .............77 15. R a d i o a c t i v i t y of animals as a f u n c t i o n of f e e d i n g 16. E f f e c t of h o l d i n g zooplankton i n the l a b o r a t o r y f o r v a r i o u s l e n g t h s of time between c o l l e c t i o n and measurement of f e e d i n g r a t e s . / .......................81 17. E f f e c t of h o l d i n g Holopedium qibberum under d i f f e r e n t f o o d regimes be f o r e measuring f u n c t i o n a l responses. .83 18. E f f e c t of h o l d i n g P i a i> torn us oreqonensis under d i f f e r e n t food regimes b e f o r e measuring f u n c t i o n a l 19- E f f e c t of a c c l i m a t i o n to d i f f e r e n t f o o d c o n c e n t r a t i o n s on measured f i l t e r i n g r a t e s . .,.....,.85 20. R e s u l t s from p r e v i o u s f i g u r e p l o t t e d to show how f e e d i n g r a t e at a g i v e n c o n c e n t r a t i o n v a r i e s with l e n g t h o f time the animal has to a c c l i m a t e to t h a t 21. "Standard" f u n c t i o n a l : responses measured f o r 8 s p e c i e s . Feeding r a t e s are i n u n i t s of ug. food i n g e s t e d (ash f r e e dry weight)/ug. zooplankton (dry weight)/hour. Food c o n c e n t r a t i o n s are expressed as ug/ral ash f r e e dry weight. . . . . , , . . . . , . . . . , . , . . . . . . , . 8 9 22. I n i t i a l s l o p e "a" o f f u n c t i o n a l response vs. maximum f e e d i n g r a t e "V M f o r Daphnia r o s e a . . . . . . . . . . . . . . . . . . 1 1 1 23. I n i t i a l s l ope " a " of f u n c t i o n a l response vs. maximum f e e d i n g r a t e "7" f o r Daphnia pulex. .................112 24. I n i t i a l s l o p e " a " of f u n c t i o n a l response vs. maximum f e e d i n g r a t e "V" f o r Diaptomus k e n a i . . . . . . . . . . . . . . . . 1 1 3 25. I n i t i a l s l ope " a " o f f u n c t i o n a l response vs. maximum f e e d i n g r a t e M V M f o r C e r i o d a p h n i a sp. ............... 114 26. I n i t i a l s l o p e " a" o f f u n c t i o n a l response vs. maximum f e e d i n g r a t e n l n f o r Holopedlum qibberum. .......v...115 27. I n i t i a l slope " a " of f u n c t i o n a l response vs. maximum fe e d i n g r a t e n V n f o r Diaptomus t y r e l l i and Diaptomus 28. Feeding parameters of s p e c i e s taken from OBC Pond and 29. Feeding parameters of s p e c i e s taken from Deer Lake, P l a c i d Lake, and Katherine Lake. / . . . . . . . . . . . . . . . . . . . . 118 30. Feeding parameters of s p e c i e s taken from Eunice Lake, 31. Temperature v a r i a t i o n of maximum feed i n g r a t e s f o r Daphnia r o s e a and Daphnia pulex. . . , . , . , . . . . . , . . . . . . , 1 2 0 32. Temperature v a r i a t i o n o f maximum f e e d i n g r a t e s f o r Holopedium qibberum and Diaptomus k e n a i . . . . . . . . . . . . . 1 2 1 33. Temperature v a r i a t i o n o f maximum feed i n g r a t e s f o r X Dia ptomus t y r e l l i . - • • ,. ,.,•.'..„> . . ... • .. • ....... .122 34. Temperature v a r i a t i o n of maximum f i l t e r i n g r a t e s f o r 35. Temperature v a r i a t i o n of maximum f i l t e r i n g r a t e s f o r 36. Temperature v a r i a t i o n of maximum f i l t e r i n g r a t e s f o r Diaptomus kenai and Holopedium gibberura. ............125 37. Comparison of Daphnia r o s e a f i l t e r i n g r a t e s measured i n t h i s study w i t h those measured by Burns and a i g l e r 38. F i l t e r i n g r a t e s of Daphnia ro s e a and Daphnia pulex as a f u n c t i o n of body l e n g t h . ..........................129 x i ACKNOWLEDGEMENTS Many people helped me i n t h i s endeavour; without t h e i r a s s i s t a n c e I would not have been a b l e t o c a r r y out the study. I am indebted t o my s u p e r v i s o r , Dr. C.S. H o i l i n g , f o r h i s encouragement, a d v i c e and f i n a n c i a l support. Dr. H. Bos s e r t , Dr. H.E. N e i l l , Dr. T.G. Northcote, and Dr. T. Parsons provided v a l u a b l e h e l p and c r i t i c i s m at v a r i o u s times dur i n g t h e st u d y . Ho thanks would be too much f o r P r o f . H.Q. Yorgue. My exp e r i m e n t a l work would have been much more l i m i t e d without the superb h e l p o f Paul S t a r r , who looked a f t e r a myriad of demanding t a s k s - m a i n t a i n i n g yeast c u l t u r e s , r e p l e n i s h i n g c h e m i c a l s o l u t i o n s , weighing plankton samples, s u p e r v i s i n g the b u i l d i n g of the seston c o n c e n t r a t o r , and teac h i n g me how t o use the s c i n t i l l a t i o n c o u n t e r . Paul and Begina C l a r o t t o c o l l e c t e d most of the zooplankton used i n the study. P i e r r e K l e i b e r c o n t r i b u t e d h i s e x p e r t i s e i n h a n d l i n g r a d i o i s o t o p e s . P i n a l l y , I thank my husband, C a r l H a l t e r s , who, more than anyone, has helped me du r i n g t h i s study. He encouraged me to s t a r t i t , r e v i v e d my enthusiasm when t h i n g s went wrong, helped me s o l v e insurmountable problems, and f i n a l l y , he pleaded, c a j o l e d , and threatened me i n t o f i n i s h i n g my t h e s i s . 1 1 IHT80DDCTI0B 1- 1 Gene r a l In r e c e n t years e x t e n s i v e e f f o r t s have been made to understand and p r e d i c t e u t r o p h i c a t i o n processes i n a q u a t i c ecosystems. Host work has been d e s c r i p t i v e and r e d u c t i o n i s t , emphasizing the c o m p l e x i t i e s of separate component i n t e r a c t i o n s and responses to environmental changes. Other r e s e a r c h e r s , more i n t e r e s t e d i n the behaviour of systems as a whole, have coupled a n a l y s i s o f separate processes with e x t e n s i v e use of models i n an attempt not only t o develop a ge n e r a l understanding o f a q u a t i c food c h a i n s but a l s o t o make p r a c t i c a l p r e d i c t i o n s about e u t r o p h i c a t i o n i n s p e c i f i c systems. Such models must n e c e s s a r i l y assume t h a t some reasonably simple s y s t e m a t i c s t r u c t u r e u n d e r l i e s the apparent observed complexity. Of c e n t r a l importance t o these models and t h e i r p r e d i c t i o n s are the f u n c t i o n a l responses d e s c r i b i n g r a t e s o f b i o l o g i c a l t r a n s f e r , such as f e e d i n g and n u t r i e n t uptake, as f u n c t i o n s of r e s o u r c e a v a i l a b i l i t y . Plankton community models are a c u t e l y s e n s i t i v e t o parameters o f fee d i n g r a t e f u n c t i o n s , but i t i s very d i f f i c u l t t o o b t a i n c o n s i s t e n t and r e l i a b l e e s t i m a t e s o f these parameters i n nat u r e . Yet without g u a n t i f i c a t i o n o f such r e l a t i o n s h i p s f o r 2 a range of resource c o n d i t i o n s , the models cannot apply to new s i t u a t i o n s and have no p r e d i c t i v e power. T h i s t h e s i s i s concerned with the f u n c t i o n a l response o f f i l t e r - f e e d i n g zooplankton - s p e c i f i c a l l y , the biomass i n g e s t e d per biomass of zooplankton per u n i t time as a f u n c t i o n of the biomass of food a v a i l a b l e . T h i s i s on a more ge n e r a l l e v e l of i n v e s t i g a t i o n than most c u r r e n t work on zooplankton f e e d i n g , because i t i s not concerned with the makeup o f the i n g e s t e d f o o d . However i t i s p r e c i s e l y t h i s g e n e r a l r e p r e s e n t a t i o n of zooplankton fe e d i n g t h a t i s r e q u i r e d by aost a q u a t i c ecosystem models. The term f u n c t i o n a l response appears t o have been c o i n e d by Solomon (1949) to d e s c r i b e p a r t of the d e n s i t y dependent nature of p r e d a t i o n . F u n c t i o n a l response r e f e r r e d to the change i n instantaneous a t t a c k r a t e of a predator i n response to a change i n prey d e n s i t y . The e x p r e s s i o n has s i n c e been used t o mean more g e n e r a l l y the changes i n an organism's f e e d i n g r a t e as an i n s t a n t a n e o u s f u n c t i o n of changes i n the food d e n s i t y - where feed i n g r a t e may r e f e r t o p r e d a t o r a t t a c k r a t e , h e r b i v o r e g r a z i n g r a t e , or a p l a n t ' s r a t e : of n u t r i e n t uptake. H o i l i n g (1965) has suggested a q u a l i t a t i v e c l a s s i f i c a t i o n o f f u n c t i o n a l responses i n t o f o u r d i s t i n c t t y p e s (Figure 1 ) . H i s component model of predator-prey i n t e r a c t i o n a l s o allowed him t o i d e n t i f y the b i o l o g i c a l c o n d i t i o n s i m p l i c i t i n each 3 P R E Y D E N S I T Y Figure 1. Classes of functional response generated by Holling's predation model. 4 q u a l i t a t i v e type of response. , & type I response r e s u l t s when prey are l o c a t e d by touch. The r a t e of search remains constant u n t i l s a t i a t i o n o c c u r s . In a type II response, the predator d e t e c t s prey a t a d i s t a n c e , and i t s search r a t e decreases c o n t i n u o u s l y as i t s l e v e l o f s a t i a t i o n i n c r e a s e s . I f , i n a d d i t i o n * a l t e r n a t i v e prey are present,, and i f l e a r n i n g by the p r e d a t o r o c c u r s , then a sigmoid, or type I I I response r e s u l t s , with decreased a t t a c k r a t e a t low prey d e n s i t i e s . A type IV response occurs when prey defense (at high prey d e n s i t i e s ) causes a decrease i n a t t a c k r a t e . At t h i s p o i n t a few d e f i n i t i o n s are i n o r d e r . Then I w i l l d e s c r i b e the type o f f u n c t i o n a l response g e n e r a l l y a t t r i b u t e d t o f i l t e r f e e d i n g zooplankton, and d i s c u s s why f u n c t i o n a l responses of zooplankton merit a t t e n t i o n . . 1.2 F u n c t i o n a l Besponse Of F i l t e r Feeding Zooplankton T h i s study i s concerned with zooplankton known as " f l i t e r - f e e d e r s " . , T h i s l a b e l r e f e r s t o the method of food c o l l e c t i o n and i n g e s t i o n employed by the zooplankton f i l t r a t i o n of water by setose appendages to remove p a r t i c u l a t e o r g a n i c matter. The f i l t e r i n g r a t e o f a z o o p l a n k t e r , measured i n u n i t s o f volume/time, i s the volume of water which would have t o be f i l t e r e d c l e a r of i t s food p a r t i c l e s i n a g i v e n 5 time t o p r o v i d e the q u a n t i t y of food eaten by the animal d u r i n g the same time. Since no zo o p l a n k t e r i s 100% e f f i c i e n t at removing p a r t i c u l a t e matter from the water, the volume " f i l t e r e d " a c c o r d i n g to t h i s d e f i n i t i o n i s a c t u a l l y l e s s than the volume of water t h a t passes over t h e f e e d i n g appendages^ Feeding r a t e i s the q u a n t i t y o f food i n g e s t e d by an animal i n a given time. I t i s measured as the product o f the f i l t e r i n g r a t e and the food c o n c e n t r a t i o n of the water. F i l t e r f e e d e r s a r e g e n e r a l l y assumed to have some k i n d o f s a t u r a t i n g f u n c t i o n a l response (e.g. S i g l e r , 1961; HcHahon, 1965; Parsons e t a l . , 1967), but there i s some disagreement as to the exact q u a l i t a t i v e form ( H u i l i n , F u g l i s t e r Stewart, and F u g l i s t e r , 1975). However, f o r most a p p l i e d purposes, ( i . e . f o r use i n a q u a t i c ecosystem models), a type I I response i s used. Probably the most widely used e x p r e s s i o n of t h i s response i s the H i c h a e l i s - H e n t e n e q u a t i o n : where F = r a t e o f food uptake, i n q u a n t i t y of food per u n i t ¥" x ? = •-k • x (D q u a n t i t y o f zooplankton per u n i t time V maximum asymptotic value o f F x d e n s i t y of food k H i c h a e l i s - H e n t e n c o n s t a n t f o r food uptake, i . e . the food d e n s i t y a t which F = 1/2 V. T h i s 6 parameter r e f l e c t s the r e l a t i v e a b i l i t y o f zooplankton to gather s p a r s e l y d i s t r i b u t e d f o o d . I t i s a l s o known as the " h a l f - s a t u r a t i o n c o n s t a n t " . 1.3 Importance Of F u n c t i o n a l Responses Form and magnitude o f f u n c t i o n a l responses are o f i n t e r e s t i n almost any study of p o p u l a t i o n dynamics. F u n c t i o n a l response was re c o g n i z e d as e a r l y as 1949 by Solomon as p l a y i n g an important r o l e i n the " n a t u r a l c o n t r o l " o f animal p o p u l a t i o n s . B o i l i n g (1965) noted t h a t each q u a l i t a t i v e form of response c o n f e r s d i f f e r e n t s t a b i l i t y p r o p e r t i e s on p o p u l a t i o n s and ecosystems. As f a r as f i l t e r -f e e d i n g zooplankton are concerned, i t has been r e c o g n i z e d t h a t the f u n c t i o n a l response of the zooplankton to c o n c e n t r a t i o n of food a v a i l a b l e i s important f o r an understanding of the a q u a t i c food web (Mayzaud and P o u l e t , 1978; Lehman, 1976; Lam and F r o s t , 1976) ^  F u n c t i o n a l responses b e l i e v e d to f o l l o w Michaelis-Menten dynamics (equation 1) have g e n e r a l l y been s t u d i e d by examining the behaviour of i t s two parameters 7 and k. These parameters f r e q u e n t l y have been a t t r i b u t e d almost m y s t i c a l s i g n i f i c a n c e as p o p u l a t i o n c h a r a c t e r i s t i c s , and they have been the o b j e c t 7 of much h y p o t h e s i z i n g (Maclsaac and Dugdale, 1969; Eppley e t a l . , 1969; Goodman e t al. , 1 9 7 3 ; D u g d a l e , 1967; Crowley, 1975). In s p i t e o f the wide use of equ a t i o n (1) to express plankton f e e d i n g and n u t r i e n t uptake r a t e s i n s t u d i e s of a q u a t i c systems and the acknowledged s e n s i t i v i t y o f p r e d i c t i o n s about the plankton community t o the n u m e r i c a l values o f the parameters V and k, t h e r e i s a conspicuous l a c k of e x p e r i m e n t a l data on the n a t u r a l values and ranges of thes e parameters, p a r t i c u l a r l y f o r zooplankton f e e d i n g r a t e s . The a v a i l a b l e data a r e publi s h e d i n such a wide v a r i e t y of forms and u n i t s as t o render them almost completely u s e l e s s f o r a p p l i c a t i o n i n s y s t e m a t i c comparative and modelling s t u d i e s . Furthermore, the experimental techniques and c o n d i t i o n s under which th e s e data were produced were by no means s t a n d a r d i z e d , or even comparable i n terms o f t h e i r e f f e c t s on what was b e i n g measured. Much of the f e e d i n g parameter i n f o r m a t i o n must be gleaned i n d i r e c t l y from m i s c e l l a n e o u s experiments because few r e s e a r c h e r s have e x p l i c i t l y sought t o measure f e e d i n g f u n c t i o n a l r e l a t i o n s h i p s , e s p e c i a l l y under n a t u r a l c o n d i t i o n s . Part of the problem, of c o u r s e , l i e s with the experimental d i f f i c u l t i e s of such an i n v e s t i g a t i o n . 8 1 . 4 O u t l i n e Of O b j e c t i v e s T h i s study was concerned with s e v e r a l d i f f e r e n t a s p e c t s of the f u n c t i o n a l responses of freshwater f l i t e r - f e e d i n g zooplankton. The o b j e c t i v e s were as f o l l o w s : 1...... To t r y t o measure, under c o n d i t i o n s as c l o s e as p o s s i b l e t o t h o se of f i e l d s i t u a t i o n s , the shapes and magnitudes o f f u n c t i o n a l responses of s e v e r a l f i l t e r f e e d i n g zooplankton s p e c i e s . The zooplankton were obtained from s e v e r a l d i f f e r e n t l a k e s i n the Vancouver r e g i o n . 2 . To see whether these f u n c t i o n a l responses are i n t r i n s i c a l l y s t a b l e ^ or whether they show t r a n s i e n t behaviour s e n s i t i v e t o hunger, changes i n food s u p p l y , temperature, e t c . 3 . To look f o r any evidence of an e v o l u t i o n a r y a d a p t a t i o n o f f u n c t i o n a l response t o food supply. For example, do the f u n c t i o n a l response parameter values r e f l e c t s p e c i e s d i s t r i b u t i o n among e u t r o p h i c and o l i g o t r o p h i c l a k e s ? 4 . To t r y , with i n f o r m a t i o n obtained from o b j e c t i v e s 1 - 3 above, to c l a s s i f y the g r a z e r s i n t o a s m a l l number o f groups c h a r a c t e r i s t i c of t h e i r f e e d i n g behaviour. T h i s would enable one t o determine f u n c t i o n a l ; responses from 9 b i o l o g i c a l and g e o g r a p h i c a l c h a r a c t e r i s t i c s r a t h e r than from c o s t l y experiments f o r each water body. F o l l o w i n g t h i s b r i e f o u t l i n e o f o b j e c t i v e s , I w i l l d e a l with each one i n g r e a t e r d e t a i l , beginning with a d i s c u s s i o n of the d i f f e r e n t forms o f f u n c t i o n a l response. 10 2 THEORETICAL BACKGROUND 2.1 F u n c t i o n a l Response Ty pes I n order to make use of i n f o r m a t i o n on the r e l a t i o n s h i p between food c o n c e n t r a t i o n and the r a t e of i n g e s t i o n by g r a z i n g zooplankton, one must be a b l e t o express the r e l a t i o n s h i p by some ki n d of q u a n t i f i a b l e f u n c t i o n . A number of s p e c i f i c f u n c t i o n s have t r a d i t i o n a l l y been used i n work on p l a n k t o n i c g r a z e r s (see M u l l i n , F u g l i s t e r Stewart, and F u g l i s t e r , 1975) * Although s e v e r a l o f t h e s e models w i l l o f t e n provide e q u a l l y good f i t s t o a g i v e n s e t o f data, the assumptions behind each of them may be q u i t e d i f f e r e n t . The f o l l o w i n g paragraphs d i s c n s s some of the models t h a t have been used t o d e s c r i b e the f u n c t i o n a l responses of f i l t e r f e e d e r s t o prey d e n s i t y . , B o i l i n g ' s c l a s s i f i c a t i o n (see F i g u r e 1) has been used t o group the models a c c o r d i n g t o t h e q u a l i t a t i v e c h a r a c t e r i s t i c s of t h e i r f u n c t i o n a l form. Type I i s a r e c t i l i n e a r model, i n which r a t e of a t t a c k r i s e s l i n e a r l y with i n c r e a s i n g prey c o n c e n t r a t i o n t o some maximum a t t a c k r a t e . T h i s k i n d of response curve a r i s e s from the assumptions t h a t the search f o r prey i s random, t h a t the s e a r c h i n g r a t e i s independent* up t o a t h r e s h o l d , of the d e n s i t y of prey organisms, and t h a t the time spent s e p a r a t e l y 1 1 on prey h a n d l i n g i s n e g l i g i b l e . Such a model c o u l d apply t o f i l t e r - f e e d i n g zooplankton d e t e c t i n g prey by touch, c o l l e c t i n g p a r t i c l e s t h a t a re s e i v e d from a f e e d i n g c u r r e n t . A c o n s t a n t r a t e o f f i l t e r i n g water past the f e e d i n g appendages would mean a constant r a t e of se a r c h , and a r a t e of i n g e s t i o n which i n c r e a s e d i n d i r e c t p r o p o r t i o n t o i n c r e a s e s i n food c o n c e n t r a t i o n o f the water-, I f the f u n c t i o n s o f food g a t h e r i n g ( f i l t e r i n g ) and p r o c e s s i n g were simultaneous, the concept o f h a n d l i n g time would: not apply- At some s a t u r a t i o n p o i n t , i n g e s t i o n would be l i m i t e d by the c a p a b i l i t i e s o f the d i g e s t i v e system. R i g l e r (1961) f i r s t proposed such a model f o r f i l t e r - f e e d i n g zooplankton, based on r e s u l t s o f h i s experiments with Daphnia magna. Subsequently, i t has been used t o d e s c r i b e much of the f e e d i n g data obtained f o r v a r i o u s s p e c i e s of Daphnia (McMahoo and R i g l e r , 1963, 1965; McMahon, 1965; Burns and B i g l e r , 1967). In Burns and R i g l e r (1967) though, the authors f i t t e d f e e d i n g r a t e s of D^ Rosea with a form t h a t r o s e non l i n e a r l y to a co n s t a n t p l a t e a u . Hoi l i n g (1965), through a n a l y s i s o f the components and mechanisms of p r e d a t i o n , a l s o b e l i e v e d a r e c t i l i n e a r model would be s t d e s c r i b e g r a z i n g by herbivorous zooplankton. Mayzaud and P o u l e t have r e c e n t l y proposed a model which i s s i m i l a r i n many r e s p e c t s t o the r e c t i l i n e a r one, but which c o n t a i n s some q u i t e d i f f e r e n t assumptions. The model i s based on evidence of d i g e s t i v e system a c c l i m a t i o n (Mayzaud and 12 P o u l e t , 1978; Mayzaud and Conover, 1976)- Hayzaud and Poulet measured f e e d i n g r a t e s of f i v e s p e c i e s of n e r i t i c marine copepods and found t h a t i n g e s t i o n r a t e and d i g e s t i v e enzyme a c t i v i t y v a r i e d l i n e a r l y with f l u c t u a t i n g p o t e n t i a l food supply when the experiments were done over a season with n a t u r a l seasonal c o n c e n t r a t i o n s o f p a r t i c u l a t e matter.. However s h o r t term experiments, exposing the zooplankton t o a range o f food c o n c e n t r a t i o n s over 18 - 20 hours, y i e l d e d s a t u r a t i o n curves. Furthermore, the apparent a f f i n i t y and the c a p a b i l i t y o f the d i g e s t i v e enzymes v a r i e d a c c o r d i n g t o sea s o n a l changes i n food c o n c e n t r a t i o n and chemic a l composition. Mayzaud*s and P o u l e t * s model o f h e r b i v o r o u s copepod f e e d i n g i s , a c c o r d i n g l y , l i n e a r f o r v a r i a t i o n s of food c o n c e n t r a t i o n o c c u r r i n g over long time p e r i o d s (weeks t o year) and s a t u r a t i n g f o r v a r i a t i o n s over a time s m a l l e r than t h a t needed f o r a c c l i m a t i o n . In a type I I response, f e e d i n g r a t e r i s e s a s y m p t o t i c a l l y , with a d e c r e a s i n g s l o p e , towards a maximum val u e . Such a response can a r i s e i f the time spent h a n d l i n g a prey item i s n e i t h e r n e g l i g i b l e , nor dependent on prey d e n s i t y * nor completely simultaneous with time spent s e a r c h i n g (filtering)» T h i s would be the ca s e f o r example, i f i n g e s t i o n o f a bol u s o f food i n t e r r u p t e d one of the food g a t h e r i n g / p r o c e s s i n g mechanisms, or i f the zoo p l a n k t e r i s capable of s e l e c t i v e f e e d i n g . U n t i l r e c e n t l y , n o n l i n e a r f u n c t i o n s used to 13 r e p r e s e n t t h i s kind of zooplankton f e e d i n g response were almost e x c l u s i v e l y of the I v l e v ( I v l e v , 1961) or H i c h a e l i s -Henten type. Holling»s d i s c e q u a t i o n ( H o l l i n g , 1959) has a l s o been used. I t i s f o r m a l l y i d e n t i c a l t o the H i c h a e l i s - H e n t o n equation but was d e r i v e d d i f f e r e n t l y - ; I t warrants an e x p l i c i t statement here i n the same format as the H i c h a e l i s - H e n t e n model: (1/ Th) x 1/(a Th) • x where F = r a t e of food i n g e s t i o n , measured i n q u a n t i t y of food per q u a n t i t y of zooplankton per u n i t time Th = time spent h a n d l i n g (pursuing, c a p t u r i n g , and eating) each prey x = prey d e n s i t y a = r a t e of s u c c e s s f u l search I f h a n d l i n g time Th i s n e g l i g i b l e t h i s equation s i m p l i f i e s t o a l i n e a r response: F = a x (3) Comparing (2) with (1) we see 14 V = 1 / Th k = 1 / (a Th) (5) a k (6) Although d e r i v e d t o r e p r e s e n t f i s h f e e d i n g on small prey, the I v l e v curve has o f t e n been used as a model f o r zooplankton f i l t e r - f e e d i n g . Parsons , LeBrasseur, and F u l t o n {1967} f i t data on f e e d i n g r a t e s o f marine copepods with an I v l e v c u r v e modified t o i n c l u d e the concept o f a refuge f o r phytoplankton. T h e i r d a t a suggested t h a t i n g e s t i o n of p a r t i c u l a t e matter s t o p s a t some food d e n s i t y g r e a t e r than zero, H c A l l i s t e r (1970) measured n o c t u r n a l and continuous g r a z i n g r a t e s o f marine copepods as a f u n c t i o n of phytoplankton c o n c e n t r a t i o n , and f i t t e d t h e experimental r e s u l t s with the same m o d i f i e d I v l e v c u r v e used by Parsons et a l . (1967). The r e f u g e concept has a c e r t a i n appeal because when i t i s i n c o r p o r a t e d i n t o plankton models i t i m p a r t s s t a b i l i t y t o plankton communities.. However, i t i s c u r i o u s that the only r e p o r t e d evidence o f such t h r e s h o l d f e e d i n g behaviour i s from experiments which measure f e e d i n g r a t e by d i f f e r e n t i a l c e l l count over a f a i r l y long i n c u b a t i o n p e r i o d (Parsons, LeBrasseur, and F u l t o n , 1967; H c A l l i s t e r , 1970). I t may be 15 that such t h r e s h o l d behaviour i s an a r t i f a c t o f the technique. H u l l i n , F u g l i s t e r Stewart and F u g l i s t e r (1975) have examined F r o s t ' s (1972) data on i n g e s t i o n by copepods as a f u n c t i o n of food c o n c e n t r a t i o n i n terms of t h r e e d i f f e r e n t models - r e c t i l i n e a r , I v l e v , and H i c h a e l i s - H e n t e n , the l a t t e r two modified t o allow a p o s i t i v e x - a x i s i n t e r c e p t . S t a t i s t i c a l a n a l y s i s showed no s i g n i f i c a n t d i f f e r e n c e s i n v a r i a n c e between the f i t t e d c u r v e s . , They a l s o found t h a t ; whether or not the data were c o n s i d e r e d to show the e x i s t e n c e of a r e f u g e depended on the type o f model adopted. The r e c t i l i n e a r model i n t e r c e p t e d the o r i g i n but the two n o n l i n e a r models had p o s i t i v e x -axis i n t e r c e p t s . In t h e i r modelling work on marine food c h a i n s i n the Peruvian u p w e l l i n g ecosystem, Walsh and Bass (1971) used a H i c h a e l i s - H e n t e n e x p r e s s i o n to d e s c r i b e t h e dynamics o f zooplankton g r a z i n g on phytoplankton. They chose i t over an I v l e v curve mainly because i t was e a s i e r to a t t a c h b i o l o g i c a l s i g n i f i c a n c e to the parameters. S t e e l e (1974) and Walsh (1975) used m o d i f i c a t i o n s of the d i s c e quation t o d e s c r i b e g r a z i n g by h e r b i v o r o u s zooplankton. Green (1975) found a type I I response f o r feeding r a t e s of the Hew Zealand freshwater copepod Calamoecia l u c a s i but d i d not r e p o r t the f u n c t i o n used to f i t h i s data. Crowley (1973) used H o l l i n g * s d i s c e q u a t i o n as a b a s i s f o r d e r i v i n g a model i n which f e e d i n g r a t e i s l i n e a r below a 16 c e r t a i n l i m i t i n g c o n c e n t r a t i o n , and a s y m p t o t i c a l l y n o n l i n e a r c above. T h i s model assumes h a n d l i n g time i s z e r o a t low prey d e n s i t i e s , and t h a t below the l i m i t i n g prey d e n s i t y , s e a r c h time i s not reduced by e a t i n g and d i g e s t i o n . Crowley thought the l i m i t i n g c o n c e n t r a t i o n probably represented the prey d e n s i t y a t which a f u l l gut i s j u s t maintained - above t h i s p o i n t , d i g e s t i b i l i t y and i n g e s t i o n of the prey impose a han d l i n g time g r e a t e r than zero on food p r o c e s s i n g . A type I I I response has a sigmoid r i s e t o an upper asymptote. The assumptions behind B o i l i n g ' s d e r i v a t i o n of i t are that p r e d a t o r s can l e a r n t o c o n c e n t r a t e on a prey t h a t becomes numerous, and t h a t a l t e r n a t i v e prey a r e p r e s e n t . Type I I I responses have g e n e r a l l y been considered c h a r a c t e r i s t i c o f v e r t e b r a t e p r e d a t o r s . However H a s s e l l et a l . (1977) have obtained and assembled experimental evidence of sigmoid f u n c t i o n a l response f o r a number o f i n v e r t e b r a t e p r e d a t o r s and p a r a s i t o i d s . He c l a i m s a sigmoid response i s l i k e l y whenever the r e i s a t h r e s h o l d prey d e n s i t y below which p r e d a t o r s e a r c h i n g e f f i c i e n c y d e c l i n e s . T h i s t h r e s h o l d would be h i g h e r f o r n o n - p r e f e r r e d prey s p e c i e s . The e x i s t e n c e of sigmoid response c u r v e s f o r f i l t e r -f e e d i n g zooplankton has long been d i s c u s s e d as a s t a b i l i z i n g mechanism i n the dynamics of phytoplankton-rzooplankton p o p u l a t i o n s . I t i s a l s o tempting to s p e c u l a t e t h a t a sigmoid response may r e f l e c t an attempt on the p a r t of the zooplankton 17 to reduce t h e energy c o s t s i n v o l v e d i n s e a r c h i n g f o r food d u r i n g c o n d i t i o n s where r e t u r n s are not s u f f i c i e n t t o maintain normal s e a r c h e f f o r t s . Lehman (1976) has proposed a sigmoid r e l a t i o n between f e e d i n g r a t e and food c o n c e n t r a t i o n f o r i n v e r t e b r a t e f i l t e r -f e e d i n g . I t i s d e r i v e d to p r e d i c t the f e e d i n g behaviour t h a t maximizes the net r a t e of energy gained by a f i l t e r f e e d i n g zooplankton i n a food suspension of known p a r t i c u l a t e c o m p o s i t i o n . The model assumes t h a t f e e d i n g r a t e i s i n f l u e n c e d by the amount of food i n the gut o f the f i l t e r f e e d e r , and t h a t , w i t h i n v a r i o u s e x t e r n a l c o n s t r a i n t s , one g o a l of the z o o p l a n k t e r i s t o o p t i m i z e energy i n t a k e . The p r e d i c t e d r e l a t i o n f o r optimal f i l t e r i n g r a t e s i s one where f i l t e r i n g r a t e i n c r e a s e s with i n c r e a s i n g c o n c e n t r a t i o n of food p a r t i c l e s u n t i l an i n g e s t i o n r a t e i s reached a t which the gut i s completely packed. Then the f i l t e r i n g r a t e s decrease with f u r t h e r i n c r e a s e s i n food c o n c e n t r a t i o n . The c o r r e s p o n d i n g i n g e s t i o n curve agrees with e m p i r i c a l models a t high food d e n s i t i e s , but i s sigmoid f o r low food c o n d i t i o n s . & f u n c t i o n a l response r e l a t i o n s h i p capable of e x p r e s s i n g both d i s c and sigmoid type behaviour with a s i n g l e e q u a t i o n has been p u b l i s h e d by Heal (1977). The equation was d e r i v e d through an analogy with a l l o s t e r i c enzyme k i n e t i c s , and i n f a c t , i s i d e n t i c a l to the H i c h a e l i s - H e n t e n e x p r e s s i o n except f o r the a d d i t i o n of a t h i r d parameter. According t o t h i s 1 8 e q u a t i o n , response behaviour can be e x p l a i n e d by t h r e e parameters - maximal f e e d i n g r a t e , an* a f f i n i t y c o n s t a n t r e l a t e d t o h a n d l i n g time, capture e f f i c i e n c i e s , e t c . , and a t h i r d parameter whose exact b i o l o g i c a l analogue i s not made c l e a r , but whose v a l u e determines whether the cur v e w i l l be sigmoid o r not. S e a l ' s equation i s expressed as n v x F = — , (7) n G • x where F = f e e d i n g r a t e , as p r e v i o u s l y defined V = maximum f e e d i n g r a t e x = food d e n s i t y n = number of prey encounters the predator must have b e f o r e i t i s maximally e f f i c i e n t a t h a n d l i n g t h a t type of prey G = an a f f i n i t y c o n s t a n t , e qual t o the food d e n s i t y a t which f e e d i n g r a t e y i s h a l f i t s maximum value For n = 1, (7) becomes the d i s c eg n a t i o n . , Type IV responses, with lowered a t t a c k r a t e s at high prey d e n s i t i e s , a r e not o f t e n a t t r i b u t e d t o f i l t e r - f e e d i n g zooplankton. I t may be p o s s i b l e however, t h a t a t very high food c o n c e n t r a t i o n s a decrease i n f e e d i n g r a t e c o u l d r e s u l t from a mechanism such as p h y s i c a l c l o g g i n g of the f i l t e r i n g 19 appendages, o r , i n c l a d o c e r a n s , a decrease i n carapace width t o prevent c l o g g i n g ( G l i w i c z , 1977). The F u j i i equation ( F u j i i , H o l l i n g , and Hace i n prep.) i s a g e n e r a l f o r m u l a t i o n o f the p r e d a t i o n process, from which a l l f o u r of the b a s i c types of f e e d i n g curves emerge as s p e c i a l cases. I t was d e r i v e d from H o l l i n g ' s d i s c e q u a t i o n by making the search r a t e a f u n c t i o n of prey d e n s i t y i n s t e a d o f a co n s t a n t . B i o l o g i c a l l y ; t h i s assumption can be made t o d e s c r i b e , f o r example, predator f a c i l i t a t i o n by l e a r n i n g as the number o f prey c o n t a c t s i n c r e a s e s , or an i n h i b i t i o n o f attack r a t e f o r high prey d e n s i t i e s . The F u j i i f u n c t i o n has t h e form 7 x T = ~ , (8) -ex k e • x where V = maximum fe e d i n g r a t e x = food d e n s i t y k >. analogous t o a f f i n i t y c o n s t a n t ; i n v e r s e l y p r o p o r t i o n a l t o the s l o p e dF/dx near x=Q c = a constant To show i t s r e l a t i o n to t h e d i s c e q u a t i o n , (8) can a l s o be expressed as 20 1 F = # ( 9 ) Th • 1 a* x exp(cx) where a(x) = a 1 exp(ex) i s search r a t e as a f u n c t i o n of prey d e n s i t y I f c = 0, (?) reduces to the d i s c e q u a t i o n . Por c e r t a i n p o s i t i v e v a l u e s of c the curve i s sigmoid, and f o r negative c a dome-shaped curve r e s u l t s . 2.2 S t a b i l i t y Of F u n c t i o n a l Responses I n m odelling s t u d i e s the f u n c t i o n a l response of an animal i s t r e a t e d as a more or l e s s i n t r i n s i c a l l y f i x e d c h a r a c t e r i s t i c of t h a t animal. The magnitude of the response may vary somewhat with temperature, b u t g e n e r a l l y f u n c t i o n a l responses a r e treated as s t a b l e responses t o a uniform environment t h a t changes only on a gross s c a l e . However, the environment seen by zooplankton i s v a r i e d and c o n s t a n t l y changing. Temperature c o n d i t i o n s , food c o n c e n t r a t i o n s , l i g h t regimes a l l vary on a time s c a l e o f hours. F u n c t i o n a l responses measured under s t a b l e l a b o r a t o r y c o n d i t o n s may not r e f l e c t the f e e d i n g behaviour of zooplankton 21 i n l a k e s . C e r t a i n l y , i f the foris of the f u n c t i o n a l response i s s e n s i t i v e t o r a t e o f change of food c o n c e n t r a t i o n , then a lake ecosystem would have very d i f f e r e n t s t a b i l i t y p r o p e r t i e s than would be p r e d i c t e d using a s i n g l e c o n s t a n t f u n c t i o n a l response. Although l a b o r a t o r y f e e d i n g measurements a r e co n s i d e r e d t o be responses t o c o n t r o l l e d c o n d i t i o n s , the r e s u l t s may be misleading i f f u n c t i o n a l responses are s e n s i t i v e t o v a r i o u s environmental cues. For example, animals brought i n t o the l a b o r a t o r y may show t r a n s i e n t p h y s i o l o g i c a l changes over s e v e r a l hours or days a f t e r capture i n the f i e l d . A few s t u d i e s have r e c e n t l y appeared on v a r i o u s a s p e c t s of the changeable nature of f u n c t i o n a l responses, Haney and H a l l (1975) d i d s e v e r a l 24 hour s e r i e s of i n s i t u experiments at v a r i o u s depths, f o l l o w i n g Daphnia pulex and D, g a l e a t a i n v e r t i c a l m i g r a t i o n . They found t h a t the Daphnia p o p u l a t i o n e x h i b i t e d bimodal p a t t e r n s o f f i l t e r i n g a c t i v i t y . F i l t e r i n g r a t e s were lowest d u r i n g the day when zooplankton were i n deep water; they i n c r e a s e d d u r i n g a s c e n t a t dark and decreased during descent a t dawn. Peak values of 5—10 times the daytime r a t e s o c c u r r e d twice d u r i n g the n i g h t , separated by a midnight decrease of about h a l f the peak f i l t e r i n g r a t e s . Haney and H a l l e s t imated t h a t about 85% of d a i l y f i l t e r i n g o c c u r r e d between dusk and dawn. Changes i n f e e d i n g a c t i v i t y d i d not seem to be a s s o c i a t e d with changes i n temperature or food l e v e l s . Ambient s e s t o n c o n c e n t r a t i o n s were s l i g h t l y lower 22 d u r i n g the p e r i o d of f i l t e r i n g r a t e i n c r e a s e , but not s u f f i c i e n t l y t o have caused such a l a r g e and abrupt change i n f i l t e r i n g . Ho pronounced d i e ! f i l t e r i n g r a t e p a t t e r n was observed f o r migrating Diaptomus p a l l i d u s and Diaptomus  oreqonensis . I n a more c o n t r o l l e d s e t of l a b o r a t o r y experiments with Daphnia pulex. and with food l e v e l s and temperature h e l d c o n s t a n t * Starkweather ( 1 9 7 5 ) observed f i l t e r i n g r a t e s 2-3 times higher i n dark than i n l i g h t , with i n c r e a s e s and decreases a b r u p t l y f o l l o w i n g the onset of dark and l i g h t r e s p e c t i v e l y , a t higher temperature ( 1 8 ° C ) the e l e v a t e d f i l t e r i n g r a t e was not maintained but peaked midway i n the dark p e r i o d . Starkweather a l s o found evidence t h a t such d i e l changes i n f i l t e r i n g r a t e have an endogenous component. The f i l t e r i n g r a t e f l u c t u a t i o n s p e r s i s t e d f o r Daphnia which were h e l d f o r s e v e r a l days i n c o n t i n u o u s darkness. But s e v e r a l days of continuous l i g h t caused the f l u c t u a t i o n s t o dampen out to the low daytime r a t e . F u n c t i o n a l responses of some f i l t e r -f e e d i n g zooplankton are thus o b v i o u s l y s e n s i t i v e t o d i u r n a l changes i n the environment; perhaps even t o the p o i n t where d i e l v a r i a t i o n has become endogenous. These o b s e r v a t i o n s c a l l i n t o q u e s t i o n some of the i n i t i a l i n t e r p r e t a t i o n s one might tend to make from " s t a n d a r d " f e e d i n g / f i l t e r i n g r a t e measurements. T h i s study d i d not i n c l u d e any experiments on d i e l v a r i a t i o n . I n s t e a d , I t r i e d 23 to minimize any such e f f e c t s by measuring f e e d i n g r a t e s c o n s i s t e n t l y a t the same time of day. Zooplankton were kept i n t e m p e r a t u r e - c o n t r o l l e d i n c u b a t o r s with a 16:8 l i g h t - d a r k c y c l e . Feeding experiments were g e n e r a l l y c a r r i e d out the same day as c o l l e c t i o n of the animals, or a t l e a s t by t h e f o l l o w i n g day. The experiments were done under dim f l u o r e s c e n t l i g h t , and because of the l e n g t h of time necessary t o prepare each experiment, the a c t u a l r a d i o a c t i v e f e e d i n g g e n e r a l l y took place between 7 and 8 p.m., j u s t before the onset o f a dark c y c l e . Thus, as l o n g as a l l z o o p l a n k t e r s t e s t e d e x h i b i t e d s i m i l a r d i e l p a t t e r n s , the i n d i v i d u a l experiments were comparable i n t h a t they were done i n the same time i n t e r v a l of the 24 hour c y c l e . Of c o u r s e , p o s s i b l e p a t t e r n s of d i e l v a r i a t i o n should be measured and taken i n t o account b e f o r e e x t r a p o l a t i n g these f e e d i n g r a t e s i n models of l a k e ecosystems-There i s evidence to suggest t h a t the f u n c t i o n a l responses a r e a l s o a f f e c t e d by temperature and by changes i n food regime. S e v e r a l i n v e s t i g a t o r s have rep o r t e d peak v a l u e s f o r maximum f i l t e r i n g r a t e s of s e v e r a l s p e c i e s of Daphnia at around 20-25 °C (Burns, 1969; H a l l , 1964; Burns 6 H i g l e r , 1967; Mcflahon, 1965). However S c h i n d l e r (1968) found the f i l t e r i n g r a t e s of Daphnia magna t o be u n a f f e c t e d by temperature, and Kibby (1971) found t h a t Daphnia r o s e a r e a r e d at 12 PC had peak f i l t e r i n g r a t e s around 12 °C (Burns and 24 B i g l e r (1967) had observed highestmaximum f i l t e r i n g r a t e s f o r D. Rosea a t 20 °C)- Information i s s c a r c e on temperature dependence of f e e d i n g r a t e s f o r t h e o t h e r zooplankton s t u d i e d here. fis p a r t o f t h i s study I measured f u n c t i o n a l responses a t frequent i n t e r v a l s d u r i n g the s p r i n g and summers Experimental temperatures s e r e kept the same as the temperatures of the water from which the animals were c o l l e c t e d . T h i s allowed me t o o b t a i n f u n c t i o n a l responses over a range o f temperatures, but s i n c e I had no c o n t r o l over the changes i n l a k e temperature, I was unable t o get complete r e s u l t s f o r a l l the s p e c i e s s t u d i e d . U n t i l r e c e n t l y , i n v e s t i g a t i o n s i n t o p o s s i b l e e f f e c t s o f p r i o r food c o n d i t i o n s on zooplankton g r a z i n g experiments have been c o n f i n e d to the q u e s t i o n of whether or not s t a r v i n g the animals b e f o r e measuring t h e i r f e e d i n g r a t e s had any e f f e c t on the r e s u l t i n g r a t e s . Byther (1954) observed f e e d i n g r a t e to be u n a f f e c t e d by the amount of food a l r e a d y i n the gut. HacHahon and B i g l e r (1965) on the o t h e r hand, found t h a t the amount o f food i n the gut d i d have an e f f e c t , and t h a t s t a r v e d zooplankton fed more i n one hour than p r e v i o u s l y f e d animals, when both were exposed t o n o n - l i m i t i n g food c o n c e n t r a t i o n s . However i n l i m i t i n g food c o n c e n t r a t i o n s , D. magna seemed u n a f f e c t e d by p r i o r s t a r v a t i o n . H u l l i n (1963)- shoved t h a t the f i l t e r i n g r a t e s of s t a r v e d Calanns hvperboreus decreased f o r 25 many hours a f t e r the s t a r t of f e e d i n g . . I t now appears (Mayzaud and P o u l e t , 1978; Mayzaud and Conover, 1976) t h a t zooplankton f u n c t i o n a l responses may be a f f e c t e d not only by the l e v e l of food c o n c e n t r a t i o n t o which the animals have p r e v i o u s l y been exposed, but a l s o by the r a t e of change of t h a t food c o n c e n t r a t i o n . The mechanism hypothesized f o r t h i s e f f e c t has been o u t l i n e d i n s e c t i o n 2.1. I d i d three experiments to i n v e s t i g a t e t h i s kind of v a r i a b i l i t y and t h e r e l a t e d q u e s t i o n of whether l a b o r a t o r y h o l d i n g time a f f e c t s f e e d i n g measurements. The f i r s t experiment c o n s i s t e d i n h o l d i n g groups of f r e s h l y c o l l e c t e d zooplankton i n the l a b o r a t o r y i n r e g u l a r l y r e f r e s h e d l a k e water f o r v a r i o u s l e n g t h s of time before measuring t h e i r f u n c t i o n a l responses. The second experiment examined e f f e c t s of h o l d i n g zooplankton under d i f f e r e n t f o o d regimes b e f o r e measuring t h e i r f u n c t i o n a l responses. •• The l a s t experiment i n v e s t i g a t e d how measured f u n c t i o n a l responses changed with i n c r e a s i n g l e n g t h of time given f o r a c c l i m a t i o n t o e x p e r i m e n t a l food c o n c e n t r a t i o n s . 26 2.3 E v o l u t i o n a r y a d a p t a t i o n Of F u n c t i o n a l Besponses 2. 3.1 I n t r o d u c t i o n The t h i r d major q u e s t i o n which t h i s study addressed was whether t r a i t s evolved by freshwater zooplankton t o adapt t o d i f f e r e n t environmental c o n d i t i o n s a r e r e f l e c t e d i n f e e d i n g r a t e parameters of the animals. F o r example, do zooplankton from l a k e s d i f f e r i n g i n p r o d u c t i v i t y , o r zooplankton dominating a t s p e c i f i c seasons, d i s p l a y very d i f f e r e n t f e e d i n g r a t e c h a r a c t e r i s t i c s ? (By f e e d i n g r a t e c h a r a c t e r i s t i c s I mean •f e e d i n g f u n c t i o n a l response*, or amount of food i n g e s t e d pec un i t time as a f u n c t i o n of t o t a l a v a i l a b l e food c o n c e n t r a t i o n . T h i s study i s not e x p l i c i t l y concerned with s i z e d i s t r i b u t i o n or composition of food.) T h i s t h i r d q uestion was approached by us i n g a simple model of zooplankton p r o d u c t i o n . I t i n c l u d e s the f o l l o w i n g assumptions: a) Biomass value i s used as an index to the success of any s p e c i e s . b) Biomass change i s determined only by resource a v a i l a b i l i t y and p r e d a t i o n . c) P r e d a t o r s or consumers at a given t r o p h i c l e v e l do not pe r c e i v e d i f f e r e n c e s among types of f o o d organisms a v a i l a b l e . e) F u n c t i o n a l responses f o l l o w H i c h a e l i s - H e n t e n k i n e t i c s . 27 These assumptions l e a d t o the f o l l o w i n g simple model f o r phytoplankton and zooplankton biomass dynamics : dz (Vz) (P) (z) -~ = (Az) (z) - — — - (Hz) (z) — - f(Z,C) (10) dt (kz) • P (Z) where z = biomass o f one s p e c i e s o f f i l t e r - f e e d i n g zooplankton Az = a s s i m i l a t i o n e f f i c i e n c y Vz = maximum g r a z i n g r a t e (per u n i t zooplankton biomass) kz = h a l f s a t u r a t i o n c o n s t a n t of zooplankton f u n c t i o n a l response P = t o t a l phytoplankton o r seston c o n c e n t r a t i o n Mz = zooplankton metabolic 'rate f (Z,C) = p r e d a t i o n r a t e , a f u n c t i o n of c a r n i v o r e biomass (C) and t o t a l biomass (Z) of f i l t e r - f e e d i n g zooplankton dp (VP) (H) (p) — = (Ap) (p) (Hp) (p) - - g(P,Z) (11) dt (kp) • N (P) where p - biomass of one s p e c i e s o f phytoplankton Ap = a s s i m i l a t i o n e f f i c i e n c y Vp = maximum r a t e of n u t r i e n t uptake kp = h a l f s a t u r a t i o n constant of phytoplankton f u n c t i o n a l response 2 8 N = t o t a l a v a i l a b l e n u t r i e n t c o n c e n t r a t i o n Hp = phytoplankton r e s p i r a t i o n r a t e g(P,Z) = g r a z i n g r a t e , a f u n c t i o n o f h e r b i v o r o u s zooplankton biomass (Z) and t o t a l phytoplankton biomass (P) What do these equations say about the k i n d of food uptake curves we might expect under d i f f e r e n t c o n d i t i o n s of t r o p h i c s t r u c t u r e and n u t r i e n t l e v e l ; i n plankton communities? I f a s s i m i l a t i o n e f f i c i e n c i e s , m e t abolic r a t e s , and v u l n e r a b i l i t y t o p r e d a t i o n are s i m i l a r among the competing s p e c i e s , then f u n c t i o n a l response parameters w i l l determine a s p e c i e s * f i t n e s s -2 . 3 . 2 Zooplankton P o p u l a t i o n s L i m i t e d By Food C o n s i d e r f i r s t the case where zooplankton p o p u l a t i o n s i z e i s l i m i t e d by the amount of food (phytoplankton) a v a i l a b l e . T h i s w i l l probably be the case i f there i s no s i g n i f i c a n t predator p r e s e n t . according to simple c o m p e t i t i o n t h e o r y (e.g. Stewart & L e v i n , 1973), the s u c c e s s f u l zooplankton s p e c i e s w i l l be the one which can maintain a p o s i t i v e e q u i l i b r i u m p o p u l a t i o n a t the s m a l l e s t s t a n d i n g s t o c k of food. 29 In o t h e r words the s u c c e s s f u l s p e c i e s i s a b l e t o i n c r e a s e i t s p o p u l a t i o n u n t i l i t reduces the food supply t o an e q u i l i b r i u m l e v e l t o o low t o enable o t h e r consumer s p e c i e s to p e r s i s t -Now i f c a r n i v o r e s are absent from the system, then from equation (10) the e q u i l i b r i u m s t a n d i n g s t o c k , Peg, f o r whichever foo d ( i . e . phytoplankton) s p e c i e s s u r v i v e s i s g i v e n by (kz) (Mz)/(Az) Peg = . — ) (12) (Vz) - (Mz)/(&z) From the preceding paragraph, the p e r s i s t e n t zooplankton s t r a t e g y should be t h e one which l e a d s , independently o f phytoplankton s t r a t e g y , t o the s m a l l e s t p o s i t i v e value of Peq - thus i t i s the one which minimizes equation (12). Bote t h a t the value of t h i s equation i s s o l e l y a f u n c t i o n of the p h y s i o l o g i c a l a t t r i b u t e s of the s u c c e s s f u l zOoplankter. I t does not depend on f r e e n u t r i e n t l e v e l or primary p r o d u c t i v i t y . S ince V and k are not independent v a r i a b l e s , k can be e l i m i n a t e d from equation (12) by s u b s t i t u t i o n of equation ( 6 ) : 1 (Vz) (Hz)/(Az) peq = — — ) (13) a (Vz) - (Hz)/(Az) The parameter w a " , i n a d d i t i o n t o r e p r e s e n t i n g s e a r c h r a t e , i s 30 a l s o e q u a l to the s l o p e of the f u n c t i o n a l response near the o r i g i n (or wherever P << k) . T h e r e f o r e i t i s a l s o e q u i v a l e n t to the maximum f i l t e r i n g r a t e f o r zooplankton. For animals with a giv e n maximum f e e d i n g r a t e V, Peg w i l l be minimized i f n a w i s maximized - i . e . i f the f u n c t i o n a l response has a high i n i t i a l s l o p e . Note a l s o t h a t Peg i s independent o f phytoplankton p r o d u c t i v i t y as measured by ftp, Vp, or kp. H a l t e r s (pers. comm.) has examined the s t a b i l i t y p r o p e r t i e s o f t h i s e q u i l i b r i u m (equation 12). Shen n u t r i e n t input i s very l a r g e , the e q u i l i b r i u m p o i n t becomes unstable and a l i m i t c y c l e a r i s e s ; t o any z o o p l a a k t e r , t h i s world should look l i k e a s e a s o n a l environment. 8hen t h i s c y c l e i s induced, i t c o u l d be p o s s i b l e t o have c o e x i s t e n c e o f the two f e e d i n g s t r a t e g i e s d e s c r i b e d below i n s e c t i o n 2.3.4. I n a h i g h l y p r o d u c t i v e n u t r i e n t r i c h environment, the e q u i l i b r i u m may be s t a b i l i z e d i f the zooplankton have sigmoid f u n c t i o n a l responses. However, an a p p r o p r i a t e model f o r t h i s s i t u a t i o n would i n v o l v e the p o s s i b i l i t y o f c a t a s t r o p h i c jumps i n phytoplankton abundance a s s o c i a t e d with o c c a s i o n a l escapes from the r e g u l a t o r y e f f e c t o f the zooplankton f u n c t i o n a l response. 31 2-3- 3 Zooplankton P o p u l a t i o n s l i m i t e d By. Pred a t i o n when p o p u l a t i o n l e v e l s are l i m i t e d by p r e d a t i o n r a t h e r than food, the s u c c e s s f u l z o o p l a n k t e r should be the one which maintains the highest p r o d u c t i o n r a t e a t i t s own e q u i l i b r i u m p o p u l a t i o n s i z e , independent of c a r n i v o r e s t r a t e g y (although c a r n i v o r e f e e d i n g parameters determine the e q u i l i b r i u m h e r b i v o r e s t a n d i n g s t o c k ) - T h i s w i l l s u s t a i n the h i g h e s t p r e d a t i o n p r e s s u r e , which should d r i v e o t h e r competing z o o p l a n k t e r s t o e x t i n c t i o n . I t i s not uncommon i n s i t u a t i o n s of a s i n g l e predator with s e v e r a l prey s p e c i e s t o f i n d the l e s s p r o d u c t i v e prey p o p u l a t i o n s d i s a p p e a r i n g under the p r e d a t i o n pressure (Crowley, 1975)., Thus, from eguation (10), the s u c c e s s f u l z o o p l a n k t e r s t r a t e g y i n the presence o f c a r n i v o r e s should be t h a t f o r which <Vz) (P) r = (Az) (Hz) (14). (kz) • (P) i s maximized. Hote t h a t i n t h i s case, the p r e f e r r e d s t r a t e g y i s determined not o n l y by p h y s i o l o g i c a l a t t r i b u t e s o f the s u c c e s s f u l z o o p l a n k t e r , but a l s o by the primary production i n the system. Eguation (14) can be s i m p l i f i e d f o r extremes of resource a v a i l a b i l i t y : a) i f P>>k ( i . e . food i s very abundant) t h e n 3 2 r = (Az) (Vz) - (Hz) (13) b) I f P « k ( i . e . food i s very scarce) then r = (Az) (Vz) (P)/k - (Hz) (16) By s u b s t i t u t i o n o f H o l l i n g * s r e l a t i o n between V and k (equation 6 ) , e q u a t i o n (16) becomes where a = s l o p e of the f u n c t i o n a l response over a range of low food c o n c e n t r a t i o n s . The assumption t h a t changes i n f e e d i n g s t r a t e g y r e p r e s e n t the only response t o p r e d a t i o n i s an o v e r s i m p l i f i c a t i o n / s i n c e zooplankton have o t h e r means, such as a change i n body s i z e , f o r r e d u c i n g p r e d a t i o n pressure. T h i s means t h a t zooplankton p o p u l a t i o n s t r u l y l i m i t e d by p r e d a t i o n may be uncommon. However, the arguments of t h i s s e c t i o n should be v a l i d f o r an e q u i l i b r i u m p o p u l a t i o n which i s i n f a c t p r e d a t i o n l i m i t e d . r = (Az) (a) (P) - (Hz) (17) 3 3 2. 3. ft P r e d i c t i o n s The model d i s c u s s e d above l e a d s to the f o l l o w i n g p r e d i c t i o n s about zooplankton f e e d i n g c h a r a c t e r i s t i c s under e q u i l i b r i u m c o n d i t i o n s : a) Where zooplankton p o p u l a t i o n s are l i m i t e d by food one shou l d f i n d f u n c t i o n a l responses with high i n i t i a l s l o p e s - i . e . animals must be good a t c o p i n g with low c o n c e n t r a t i o n s of food. Furthermore, the i n i t i a l s l o p e s h o u l d be independent of the p r o d u c t i v i t y of the food supply ( i . e . i t should be the same i n o l i g o t r o p h i a and e u t r o p h i c l a k e s ) . b) Where zooplankton p o p u l a t i o n s a r e l i m i t e d by p r e d a t i o n p r e s s u r e , one should f i n d a d i f f e r e n t f u n c t i o n a l response i n low food environments ( o l i g o t r o p h i c ) than i n high food environments ( e u t r o p h i c ) I n the former, the f u n c t i o n a l response would be expected t o have a high i n i t i a l s l o p e (a); i n the l a t t e r i t would be expected t o have a h i g h maximum f e e d i n g r a t e (V). There i s nothing i n these arguments t h a t precludes the same animal from having a f u n c t i o n a l response with both these c h a r a c t e r i s t i c s , but i t i s g e n e r a l l y assumed t h a t p h y s i o l o g i c a l c o n s t r a i n t s would not allow i t (Walters and C l a r k , 1973)., I f t h i s i s so, then the two types of f u n c t i o n a l 34 response j u s t d e s c r i b e d would resemble those of F i g u r e 2 . 2. 3- 5 D i s c u s s i o n Any p a r t i c u l a r s e t - of t r a i t s t h a t r e p r e s e n t s an a d a p t a t i o n t o p a r t i c u l a r e c o l o g i c a l c o n d i t i o n s would g e n e r a l l y o n l y be c o n s i d e r e d c o m p e t i t i v e l y advantageous i f those e c o l o g i c a l c o n d i t i o n s p e r s i s t l o n g enough f o r t h e i r impact t o be s i g n i f i c a n t - i . e . organisms can evolve optimal t a c t i c s only f o r environmental c o n d i t i o n s to which t h e y have had time to respond. An e q u i l i b r i u m s i t u a t i o n would thus be i d e a l f o r i d e n t i f y i n g p a r t i c u l a r a d a p t a t i o n s . A major l i m i t a t i o n t o the theory d i s c u s s e d above i s t h a t i t a p p l i e s e x p l i c i t l y only to e q u i l i b r i u m communities. I t may w e l l be t h a t the e q u i l i b r i u m p r e d i c t i o n s of S e c t i o n 2.3-4 w i l l not hold up w e l l i n seasonal environments. But even f o r an environment with c o n s i d e r a b l e seasonal v a r i a t i o n , there may be p e r i o d s d u r i n g which c o n d i t i o n s c o u l d favour one or the other of the two s t r a t e g i e s d e s c r i b e d above, p a r t i c u l a r l y i f r e l a t i v e l y s t a b l e c o n d i t i o n s p e r s i s t over a p e r i o d of time which i s e i t h e r comparable i n l e n g t h t o zooplankton l i f e s p a n or which c o i n c i d e s with a p a r t i c u l a r l y c r i t i c a l stage of l i f e h i s t o r y . For example, suppose we i g n o r e f o r the moment o t h e r a s p e c t s of a d a p t a t i o n . ( s u c h as p r e d a t i o n avoidance mechanisms) and c o n s i d e r the e a r l y s p r i n g (phytoplankton bloom) p e r i o d . , F O O D C O N C E N T R A T I O N Figure 2. Types of functional response hypothesized for f i l t e r - f e e d i n g zooplankton. Response (a) should characterize populations limited by food a v a i l a b i l i t y or inhabiting oligotrophic waters, and (b) should be t y p i c a l of populations inhabiting eutrophic waters and limited by predation pressure. 36 Overwintering or newly h a t c h i n g zooplankton p o p u l a t i o n s a re low, while f o o d r e s o u r c e s are abundant. Such c o n d i t i o n s would tend to f a v o u r forms with the most r a p i d s p e c i f i c growth r a t e - i . e . , high p r o d u c t i o n i n d i v i d u a l s . These would be animals a b l e t o o b t a i n maximal food i n t a k e a t high food d e n s i t i e s . However, l a t e r i n summer* zooplankton l e v e l s a re high and food g e n e r a l l y low. One would expect zooplankton t h a t a re c h a r a c t e r i s t i c of h a b i t u a l l y f o o d l i m i t e d regimes ( i n t h i s case dominant summer s p e c i e s o r generations) t o have evolved some mechanical and p h y s i o l o g i c a l mechanisms t o permit very e f f i c i e n t f o o d c o l l e c t i o n , i . e . high f i l t e r i n g r a t e s a t low seston c o n c e n t r a t i o n . The appearance o f these p a t t e r n s would depend on environmental changes b e i n g s u f f i c i e n t l y slow t h a t p o p u l a t i o n s c o u l d , d u r i n g some p e r i o d , approach e q u i l i b r i u m , and on a d a p t i v e t r a i t s other than f u n c t i o n a l response being o f l e s s e r importance a t that time. They probably a l s o depend on the environmental changes b e i n g f a i r l y p r e d i c t a b l e . ( N a t u r a l v a r i a b i l i t y present i n a zooplankton p o p u l a t i o n would presumably permit recovery from the o c c a s i o n a l unexpected c o n d i t i o n . ) These c o n d i t i o n s may e x i s t i n the OBG F o r e s t L a k e s , from which a m a j o r i t y o f the animals used i n t h i s study were drawn. These l a k e s probably r e p r e s e n t a good example of f a i r l y s t a b l e s e a s o n a l l a k e s . There i s , f o r example, very l i t t l e s e a s o n a l v a r i a t i o n i n gra z e a b l e seston c o n c e n t r a t i o n . , (In f a c t , f o r 37 temperate l a k e s i n g e n e r a l , there i s some s u g g e s t i o n ( G l i w i c z , 1977) t h a t although t o t a l phytoplankton biomass may have a l a r g e s e a s o n a l v a r i a t i o n , the grazeable f r a c t i o n of plankton may remain q u i t e constant.) S e v e r a l c o n t i n u i n g s t u d i e s on the l a k e s o f the UBC F o r e s t ( N e i l l , Northcote, B a i t e r s , pers. comm.) have y i e l d e d c o n s i d e r a b l e i n f o r m a t i o n about s e v e r a l s p e c i e s of zooplankton used i n t h i s study. A n a l y s i s of t h e i r l i f e h i s t o r y p a t t e r n s i n d i c a t e s t h a t some of these s p e c i e s might be s u s c e p t i b l e t o f e e d i n g s t r a t e g y a d a p t a t i o n . For example, t h e r e are about seven g e n e r a t i o n s of Daphnia rosea per year, o f which the summer ones are food l i m i t e d . . The f i r s t few g e n e r a t i o n s are probably not food l i m i t e d . There may be some developmental switch mechanism which would a l l o w the f e e d i n g behaviour o f d i f f e r e n t g e n e r a t i o n s to respond t o changes i n food c o n d i t i o n s . For example, Krepp (1977) found a s i m i l a r s p e c i e s , Daphnia pnlex, to have tremendous p l a s t i c i t y m o r p h o l o g i c a l l y and p h y s i o l o g i c a l l y , w i t h i n a g i v e n c l o n e , i n response t o environmental f a c t o r s . I t would not be unreasonable t h e r e f o r e , t o expect to f i n d s p r i n g g e n e r a t i o n s of a s i n g l e c l o n e e x h i b i t i n g f e e d i n g c u r v e s l i k e curve "B" o f F i g u r e 2, and summer g e n e r a t i o n s with type "A" f e e d i n g c u r v e s . The most c r i t i c a l p e r i o d f o r p o p u l a t i o n s o f the l o n g e r -l i v e d Diaptomus ke n a i seems t o be during n a u p l i a r and copepodite stages i n J u l y and e a r l y August* when food i s 38 s c a r c e . The f e e d i n g c h a r a c t e r i s t i c s o f these animals c o u l d w e l l have adapted t o best accomodate t h i s p e r i o d of f o o d l i m i t a t i o n . The s m a l l e r copepods of the UBC F o r e s t Lakes, Diaptomus oregonensis and Diaptomus t y r e l l i g e n e r a l l y produce t h r e e g e n e r a t i o n s per year - winter and s p r i n g g e n e r a t i o n s which are not food l i m i t e d , and a summer one which i s . The se a s o n a l t i m i n g o f these t h r e e g e n e r a t i o n s i s p r e d i c t a b l e , and i t i s p o s s i b l e t h a t the ge n e r a t i o n s c o u l d be q u i t e d i f f e r e n t p h y s i o l o g i c a l l y . There could even be ge n e t i c d i f f e r e n c e s cued by temperature or photoperiod. In t h i s case i t would not be unreasonable t o expect t o f i n d response "ft" of Figure 2 f o r the summer g e n e r a t i o n , and p o s s i b l y response "B" f o r the o t h e r two. Thus; although the theory which p r e d i c t e d t h e e x i s t e n c e of two types o f f e e d i n g s t r a t e g y was based on an e q u i l i b r i u m model, i t s t i l l may be p o s s i b l e t o f i n d the same two s t r a t e g i e s i n v a r i o u s n o n - e q u i l i b r i u m s i t u a t i o n s . 2.3.6 Evidence From The L i t e r a t u r e S p e c u l a t i n g about e c o l o g i c a l consequences o f d i f f e r e n t i a l l y advantageous f u n c t i o n a l responses i s a t a n t a l i z i n g a c t i v i t y . Host d i s c u s s i o n has focussed on the e v o l u t i o n a r y s i g n i f i c a n c e of the H i c h a e l i s - H e n t e n parameters 7 and k. T h i s emphasis may be m i s l e a d i n g however, f o r t h e r e i s 3 9 c o n s i d e r a b l e evidence t h a t V and k are not independent. Thus the observed p a t t e r n s o f H i c h a e l i s - H e n t e n parameters may be due to c o r r e l a t i o n , not e v o l u t i o n . I t i s n e v e r t h e l e s s a s u b j e c t which has generated some i n t e r e s t i n g s t u d i e s , e s p e c i a l l y i n t h e f i e l d of phytoplankton dynamics. Dugdale (1967) has c o n s i d e r e d s t r a t e g i e s f o r growth o r n u t r i e n t uptake of marine phytoplankton that are s i m i l a r t o t h o s e d i s c u s s e d i n s e c t i o n 2 . 3 - 4 . He s p e c u l a t e d t h a t an e v o l u t i o n a r y t r a d e o f f between H i c h a e l i s - H e n t e n 1 parameters k and V should l e a d to low v a l u e s o f k (and probably a l s o V) v f o r s p e c i e s c h a r a c t e r i s t i c of low p r o d u c t i v i t y t r o p i c a l r e g i o n s , while plankton with high V and h i g h f c v a l u e s should p r e v a i l i n n u t r i e n t - r i c h a r e a s . T h i s was i n f a c t observed by Baclsaac and Dugdale (1969) i n a s e r i e s of n u t r i e n t uptake experiments done at v a r i o u s s t a t i o n s i n the B e r i n g Sea and P a c i f i c Ocean. Hutrient-rpoor waters were c h a r a c t e r i z e d by phytoplankton with low k and low V v a l u e s , w h i l e e a t r o p h i c s t a t i o n s a l l showed higher k and V v a l u e s . T h i s would seem t o i n d i c a t e t h a t phytoplankton p o p u l a t i o n s of o l i g o t r o p h i c areas a r e adapted t o low ambient n u t r i e n t c o n c e n t r a t i o n s . Parsons and Takahashi (1973) have proposed a mechanism 1 Host s t u d i e s of phytoplankton dynamics use H i c h a e l i s - H e n t e n r e l a t i o n s to d e s c r i b e growth and n u t r i e n t uptake as a f u n c t i o n of n u t r i e n t c o n c e n t r a t i o n . 40 which c o u l d enhance such a t r a d e o f f . On the b a s i s of c o n s i d e r a b l e evidence t h a t t h e r e are p h y s i o l o g i c a l d i f f e r e n c e s between l a r g e and s m a l l - c e l l e d phytoplankton s p e c i e s , i n terms of both maximum growth r a t e and M i c h a e l i s c o n s t a n t f o r n u t r i e n t uptake, they c o n s t r u c t e d a model o f phytoplankton dynamics t h a t shows s t r o n g environmental c o n t r o l of c e l l s i z e . In p a r t i c u l a r , i t p r e d i c t s t h a t l a r g e c e l l e d s p e c i e s (with l a r g e 7 , l a r g e k) w i l l dominate the plankton o f c o a s t a l environments and areas o f s t r o n g upwelling c u r r e n t s , while s m a l l c e l l e d s p e c i e s (with s m a l l V, s m a l l k) w i l l be c h a r a c t e r i s t i c of s t a b l e seas. Eppley and Thomas (1969) and Eppley e t a l . (1969) measured n u t r i e n t uptake c h a r a c t e r i s t i c s f o r a l a r g e range o f marine phytoplankton and found evidence to suggest t h a t s p e c i e s d i s t r i b u t i o n among marine environments o f d i f f e r e n t n u t r i e n t regimes may be c o n t r o l l e d by d i f f e r e n c e s i n k v a l u e s among the s p e c i e s . Goodman e t a l . (1973 ) proposed the f u n c t i o n a l responses shown i n F i g u r e 3 as a mechanism u n d e r l y i n g seasonal s u c c e s s i o n of phytoplankton s p e c i e s . I n v t h i s case; i f c o m p e t i t i o n were t o depend on phosphate alone, one would expect green alg a e to dominate under e a r l y s p r i n g (high n u t r i e n t ) c o n d i t i o n s , then diatoms a l i t t l e l a t e r as n u t r i e n t s begin t o be depleted and f i n a l l y blue-greens d u r i n g summer s t r a t i f i c a t i o n . As f a r as work on zooplankton i s concerned, few s t u d i e s 41 Figure 3. Functional responses proposed by Goodman et a l . (1973) to explain seasonal succession of phytoplankton species. 4 2 of t h i s nature have been done. Mac Arthur's and Wilson's (1967) arguments about " r and K s e l e c t i o n " c ould be a p p l i e d t o zooplankton f e e d i n g h a b i t s . They suggest t h a t i n an uncrowded environment, the most f i t genotypes w i l l be those who h a r v e s t t h e most food, even i f they do so w a s t e f u l l y . P r o d u c t i v i t y (high i n t r i n i s i c r a t e o f i n c r e a s e ) i s favoured i n t h i s s i t u a t i o n . However, i n crowded h a b i t a t s wastefulness i s a l i a b i l i t y , and e v o l u t i o n should favour e f f i c i e n c y o f co n v e r s i o n of food i n t o o f f s p r i n g . , The most f i t genotypes here would be those which c o u l d reproduce at the lowest f o o d l e v e l - a l e v e l which would be too low f o r high p r o d u c t i v i t y animals t o reproduce. H a l t e r s (1975) has proposed t h a t t h e u s e o f such e v o l u t i o n a r y arguments can be a v a l u a b l e t o o l f o r i d e n t i f y i n g system parameters a t many l e v e l s o f d e t a i l and f o r g e n e r a t i n g t e s t a b l e hypotheses. He has i l l u s t r a t e d t h i s with an example of n u t r i e n t uptake s t r a t e g i e s as they r e l a t e to s e a s o n a l s u c c e s s i o n of phytoplankton. G l i w i c z (1977) has hypothesized t h a t d i f f e r e n t s t r a t e g i e s should e x i s t i n food l i m i t e d zooplankton i n l a k e s of d i f f e r e n t p r o d u c t i v i t y , because of d i f f e r e n c e s i n food p a r t i c l e s i z e . He has observed, among v a r i o u s Cladoceran s p e c i e s i n e u t r o p h i c l a k e s , d i f f e r e n c e s i n the upper s i z e l i m i t of p a r t i c l e s grazed. T h i s d i f f e r e n c e i s a p p a r e n t l y caused by d i f f e r e n c e s i n the a n t e r i o r width o f the carapace c r e v i c e through which 43 water and food p a r t i c l e s are sucked d u r i n g the f i l t r a t i o n process- He argues t h a t narrow c r e v i c e d s p e c i e s , with lower f e e d i n g r a t e s , should be dominant i n e u t r o p h i c l a k e s i n summer because the narrow gaps between carapace edges prevent l a r g e algae from e n t e r i n g and c l o g g i n g the f i l t e r i n g appendages. In o l i g o t r o p h i c l a k e s , the summer zooplankton should be wide c r e v i c e d forms, because t h e r e i s no net plankton i n t e r f e r e n c e , and because they can remove a l a r g e r amount of food from the water t h a t passes through the f i l t e r i n g chamber. 2-4 C l a s s i f i c a t i o n I have o u t l i n e d three o b j e c t i v e s f o r t h i s study - to measure f u n c t i o n a l responses of many s p e c i e s o f f i l t e r - f e e d i n g zooplankton, to determine how v a r i a b l e o r con s t a n t these f u n c t i o n a l responses tend to be, and t o look f o r any evidence of a d a p t a t i o n of the responses t o environmental c o n d i t i o n s . The r e s u l t s o f these endeavours might make i t p o s s i b l e to c l a s s i f y the zooplankton i n t o a s m a l l number of groups a c c o r d i n g to t h e i r f u n c t i o n a l responses. In s p i t e of the r i c h v a r i e t y of morp h o l o g i c a l forms and o v e r a l l l i f e h i s t o r y s t r a t e g i e s t h a t are e x h i b i t e d by h e r b i v o r o u s f r e s h w a t e r zooplankton, i t i s p o s s i b l e t h a t o n l y : r e l a t i v e l y few f u n c t i o n a l response forms and parameter values a c t u a l l y occur 4 4 i n nature because t h e r e are few d i s t i n c t s e t s of environmental c o n d i t i o n s t h a t p e r s i s t long enough to be a d a p t i v e l y s i g n i f i c a n t . C l a s s i f i c a t i o n of zooplankton by f u n c t i o n a l response t y p e s should s i m p l i f y understanding and p r e d i c t i o n of seasonal s u c c e s s i o n and a q u a t i c community responses to p e r t u r b a t i o n s such as changes i n n u t r i e n t l o a d i n g or temperature, or the i n t r o d u c t i o n of predaceous o r c o m p e t i t i v e organisms t o the community. A comprehensive s e t of e x p e r i m e n t a l l y determined f u n c t i o n a l responses would a l s o y i e l d i n f o r m a t i o n : about how g e n e r a l we can be i n a p p l y i n g experimental data to a q u a t i c models. Parameter values f o r the b i o l o g i c a l f u n c t i o n a l responses i n v i r t u a l l y a l l freshwater and marine mod e l l i n g work are drawn from a s m a l l s e t of i s o l a t e d experiments. These experiments have y i e l d e d parameter values f o r only a few s p e c i e s under a very l i m i t e d s e t of c o n d i t i o n s , yet these few r e s u l t s have been i n c o r p o r a t e d i n models c o v e r i n g c o n d i t i o n s f a r o u t s i d e the range of the o r i g i n a l experiments (DiToro e t al.,1970; Chen and O r i o b , 1972)- The p h y s i c a l components of these models o f t e n show a s o p h i s t i c a t i o n l i m i t e d o n l y by computer power, but the b i o l o g i c a l f u n c t i o n s of the system are u s u a l l y q u a n t i f i e d by educated guesswork, made necessary by the l a c k of measured parameter v a l u e s . As f a r as f e e d i n g f u n c t i o n a l r e l a t i o n s h i p s are concerned, the guesswork i s most confused i n t h e r e g i o n o f low food c o n c e n t r a t i o n s , p r e c i s e l y 4 5 where the exact form of the r e l a t i o n s h i p has i t s g r e a t e s t e f f e c t on the q u a l i t a t i v e and q u a n t i t a t i v e behaviour of the whole system (see Lehman, 1976)., Q u a n t i t a t i v e models are being i n c r e a s i n g l y used as a i d s to manage a q u a t i c systems - f o r example, to p r e d i c t r e s u l t s of change i n n u t r i e n t or s i l t l o a d i n g , water f l o w regime, e t c . (DiToro e t . a l - ( 1 9 7 0 ) - Great Lakes, Delaware a . ; Leendertse 6 G r i t t o n ( 1 9 7 1 ) - Jamaica Bay; Chen 6 Orlob ( 1 9 7 2 ) - San F r a n s i s c o Bay, Lake Washington). Unless response curve parameters a r e g e o g r a p h i c a l l y and s p e c i f i c a l l y s i m i l a r , they must be redetermined f o r each new system t o be modelled; t h i s i s an almost i m p o s s i b l e experimental endeavour., I t would r e p r e s e n t an enormous s i m p l i f i c a t i o n o f e f f o r t i f zooplankton f u n c t i o n a l responses c o u l d be determined from a few 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 alone. From my experiments, I hoped to o b t a i n a t l e a s t an i n i t i a l i n d i c a t i o n of the g e n e r a l i t y o f zooplankton f e e d i n g parameters by comparing f e e d i n g curves f o r the same s p e c i e s over a range of d i f f e r e n t l a k e s and by comparing these r e s u l t s with those r e p o r t e d i n the l i t e r a t u r e . , 46 3 EXPERIMENTAL DESIGN In order to o b t a i n the spectrum of f e e d i n g parameter estimates necessary, f o r the o b j e c t i v e s o u t l i n e d above, I decided t o c o n s i d e r a range o f e x p e r i m e n t a l c o n d i t i o n s d e f i n e d by t h r e e f a c t o r s : 1. s p e c i e s of zooplankton 2 . l e v e l o f l a k e p r o d u c t i v i t y 3. time of year ( t h i s determines water temperature and amount of food present i n the seston) S p e c i f i c a l l y , I proposed t o use ; i d e n t i c a l ; e x p e r i m e n t a l procedures t o t r y to measure g r a z i n g f u n c t i o n a l responses o f the major p a r t i c l e f e e d e r s from a s e r i e s of l a k e s of d i f f e r e n t p r o d u c t i v i t i e s , and t o t r y to maintain experimental c o n d i t i o n s as c l o s e as p o s s i b l e to i n s i t u c o n d i t i o n s . The experiments were repeated a t i n t e r v a l s from e a r l y s p r i n g through f a l l . F o r t u n a t e l y there are near Vancouver a number of a c c e s s i b l e l a k e s o f d i f f e r e n t l e v e l s of p r o d u c t i v i t y , r a n g i n g from deep and o l i g o t r o p h i c to shallow and e u t r o p h i c . F i g u r e s 4 and 5 show the geographic l o c a t i o n s of the water bodies used i n t h i s study. T a b l e I summarizes t h e i r c h a r a c t e r i s t i c s . ; The major h e r b i v o r o u s zooplankton a v a i l a b l e from these l a k e s were: 1.., Daphnia pulex 47 Figure 4. Map showing lakes from which zooplankton were taken for t h i s study. Table I. C h a r a c t e r i s t i c s of the lakes used i n t h i s study. Lake Index of Product-i v i t y * Max. zoo biomass mg/m^ dry Mean depth m. Mean area acres Zooplankton s t u d i e d Eunice • • • • 1 2500 15.8 4 5.5 Daphnia rosea Diaptomus t y r e l l i Diaptomus kenai Holopedium qibberum Diaphanosoma brachyurum Kat h e r i n e 1 2500 7.5. 51.7 Daphnia rosea \ . P l a c i d i • 2 3500 4.3 4.0 Daphnia rosea Diaptomus oreqonensis Holopedium gibberum Pond, UBC Campus • 1 0.2 . 0.01 • Ceriodaphnia sp. Daphnia pulex Loon .4 30.0 2375 Ceriodaphnia sp. Daphnia rosea Deer .5 . 10000 3.5 87. Daphnia pulex Lab c u l t u r e from e u t r o p h i c Rock L-. 5 (reared f o r about 1 year i n v ery e u t r o p h i c laboratory aquarium) . | Daphnia pulex . * 1.= very o l i g o t r o p h i c * 5 = very e u t r o p h i c 50 2. Daphnia rosea 3. C e r i o d a p h n i a sp. 4 . Holopedium qibberum 5. Diaptomus kenai 6. Diaptomus oregonensis 7. Diaptomus t y r e l l i 8. Diaphanosoma braohyurum Each s p e c i e s was present i n more than one of the l a k e s i n v e s t i g a t e d . As p a r t of another p r o j e c t a t the I n s t i t u t e of Animal Resource Ecology (Drs. C.J. f a l t e r s and T.G. H o r t h c o t e ) , the f o u r upper l a k e s i n the UBC f o r e s t ( P l a c i d , E u n i c e , Gwendoline, and Katherine) a r e r e g u l a r l y sampled f o r zooplankton and phytoplankton. In a d d i t i o n , Dr. H.E. H e i l l has been c o n d u c t i n g a s e r i e s of e n c l o s u r e experiments on P l a c i d and Gwendolines Thus there i s a c o n s i d e r a b l e a r r a y o f a d d i t i o n a l data on the zooplankton p o p u l a t i o n s used f o r most of my study. 51 3. 1 Choice Of Technique Methods f o r measuring f i l t e r i n g and f e e d i n g r a t e s o f herbivorous zooplankton are s e l l summarized by E i g l e r (1971) . V i r t u a l l y a l l c u r r e n t work on measurement of zooplankton f i l t e r i n g / f e e d i n g r a t e s i s done with v a r i a t i o n s of the f o l l o w i n g t h r e e b a s i c methods: 1. A l l o w i n g zooplankton to graze f o r a s h o r t time on r a d i o a c t i v e l y l a b e l l e d f o od. T y p i c a l l y , **C or 3 2 P i s used. T h i s works w e l l when the food i s u n i a l g a l , but f o r experiments with n a t u r a l s e s t o n , the r e s u l t s are b i a s e d and p o s s i b l y u n i n t e r p r e t a b l e because of d i f f e r e n c e s i n uptake of the l a b e l among the v a r i o u s components of the s e s t o n . , 2. Adding low c o n c e n t r a t i o n s of " P - l a b e l l e d t r a c e r c e l l s ( g e n e r a l l y yeast) t o n a t u r a l seston and a l l o w i n g zooplankton to graze f o r a short time i n the assemblage. The r e s u l t s a r e o b v i o u s l y dependent on the s i z e of t h e t r a c e r c e l l , and may be s e v e r e l y m i s l e a d i n g i f t h e l a b e l l e d c e l l s i z e i s not r e p r e s e n t a t i v e of the range of s i z e s i n the se s t o n . 3. A l l o w i n g zooplankton to feed f o r long p e r i o d s o f time i n a volume of c u l t u r e d a l g a e or n a t u r a l s e s t o n , and measuring the change i n c e l l c o n c e n t r a t i o n a f t e r the f e e d i n g p e r i o d . T h i s 52 technique i s e x t e n s i v e l y used i n marine s t u d i e s s i n c e the C o u l t e r Counter a l l o w s r e l a t i v e l y easy de t e r m i n a t i o n of food c o n c e n t r a t i o n s - . However, because of the l o n g f e e d i n g time necessary, the r e s u l t s can be e a s i l y b i a s e d by f e c a l p e l l e t p r o d u c t i o n , and by the v a r i o u s e f f e c t s o f zooplankton d i g e s t i o n on s e s t o n dynamics. For example, v i a b l e gut passage by c e r t a i n t y p e s o f a l g a e , n u t r i e n t uptake by a l g a e i n the gut, and breakdown of l a r g e a l g a l c o l o n i e s i n t o s m a l l ( v i a b l e ) c e l l s or c o l o n i e s d u r i n g gut passage, have been observed by P o r t e r (1975). A l s o t h e r e i s p o s s i b l y an e f f e c t of a l g a l f e r t i l i z a t i o n by zooplankton e x c r e t i o n . The • c o n t r o l * v e s s e l s f o r t h i s type of experiment are r a t h e r i n e f f e c t i v e because i t seems t h a t phytoplankton dynamics are a l t e r e d by the presence of zooplankton. O 1Connors e t . a l . (1976) have i l l u s t r a t e d t h i s w e l l with a s e r i e s o f experiments showing how zooplankton can modify p a r t i c l e s i z e d i s t r i b u t i o n s , o f t e n c a u s i n g l a r g e i n c r e a s e s i n s m a l l p a r t i c l e s . F i n a l l y , the c e l l count method assumes that the f i l t e r i n g r a t e remains n e a r l y c o n s t a n t d u r i n g the course of the experiment. T h i s means t h a t the food c o n c e n t r a t i o n should change as l i t t l e as p o s s i b l e over the c o u r s e 53 of the experiment, but with long i n c u b a t i o n times i t may i n f a c t decrease c o n s i d e r a b l y . R i g l e r (1971) p o i n t e d out the experimental d i f f i c u l t i e s i m p l i e d by t h i s assumption, but he seems to have been l a r g e l y i g n o r e d . The method has b e e n < w i d e l y m i s u s e d by experimenters who combine very high zooplankton d e n s i t i e s i n s m a l l volumes with long i n c u b a t i o n p e r i o d s (Hargrave S Seen, 1970; Buikema, 1973; Kryutchkova & Sladecek, 1969; F r o s t ; 1975). For my f e e d i n g experiments I had o r i g i n a l l y d e c i d e d t o use an i n s i t u v e r s i o n of the t h i r d method above. To t e s t the technique, I conducted a s e r i e s of p r e l i m i n a r y experiments i n which c l o s e d g r a z i n g b o t t l e s were mounted on a c o n t i n u o u s l y r o t a t i n g wheel suspended below the s u r f a c e of the l a k e . Seston c o n c e n t r a t i o n s were e v a l u a t e d l a t e r i n the l a b o r a t o r y by c o u n t i n g preserved samples under a microscope. S e v e r a l months of experiments y i e l d e d r e s u l t s i n which c e l l c o n c e n t r a t i o n s of grazed b o t t l e s were f r e q u e n t l y higher than c o n t r o l b o t t l e counts. S i m i l a r r e s u l t s have been observed by Beers and Z a r e t (1975) and by G l i w i c z (1975). P o r t e r (1975) has i n v e s t i g a t e d t h i s phenomenon and proposed s e v e r a l hypotheses t o e x p l a i n i t , a l l o f which imply severe problems i n the i n t e r p r e t a t i o n of long i n c u b a t i o n c e l l - c o u n t f e e d i n g experiments. A second s e r i o u s disadvantage to t h i s method was the 54 d i f f i c u l t y of q u i c k l y measuring the c e l l c o n c e n t r a t i o n s of the food samples. Manual c e l l c o u n t i n g proved too t e d i o u s and time consuming f o r the l a r g e number of f e e d i n g experiments planned, and I was r e l u c t a n t t o use a C o u l t e r Counter because of t h e p o s s i b i l i t y t h a t some of the freshwater c e l l s might be damaged by s a l i n e suspension. Because of these problems, I abandoned the c e l l - c o u n t method i n f a v o u r of a t r a c e r t e chnigue i n which zooplankton were allowed to graze f o r a s h o r t time i n l a k e water to which had been added a s u f f i c i e n t l y s m a l l amount o f r a d i o a c t i v e (32p) y e a s t c e l l s t h a t the nature of t h e s e s t o n was a l t e r e d very l i t t l e . T h i s technigue has been used w i t h : c o n s i d e r a b l e success i n much of the r e p o r t e d work on f e e d i n g r a t e s o f freshwater zooplankton (Burns, 1969; Burns 6 B i g l e r , 1967; Crowley, 1973; Haney, 1971)w. I t s main disadvantage i s t h a t i t assumes a l l food p a r t i c l e s are f i l t e r e d w i t h the same e f f i c i e n c y as the l a b e l l e d p a r t i c l e s . F i l t e r i n g e f f i c i e n c i e s f o r d i f f e r e n t s i z e s and types o f f o o d have been s t u d i e d by many workers, and although no comprehensive r u l e has yet emerged from the r e s u l t s , i t seems t h a t i n g e n e r a l zooplankton graze most e f f e c t i v e l y on a s i z e range t h a t depends on the s i z e o f the animal. F r o s t (1972) measured the i n g e s t i o n r a t e s of Calanus p a c i f i c u s as a f u n c t i o n of food c o n c e n t r a t i o n f o r a range of food s i z e . Haximum i n g e s t i o n r a t e s ( i . e . r a t e s f o r high food 5 5 c o n c e n t r a t i o n ) were i d e n t i c a l f o r p a r t i c l e s i z e s ranging from 11 u to 9 4 u diameter, a t lower food c o n c e n t r a t i o n s however, i n g e s t i o n r a t e s were s l i g h t l y higher f o r l a r g e r p a r t i c l e s . P o u l e t ( 1 9 7 4 ) fonnd Pseudocalanus minutus t o be a very o p p o r t u n i s t i c f e e d e r , with an a b i l i t y t o adapt t o s e a s o n a l v a r i a t i o n s i n food p a r t i c l e s i z e spectrum by s h i f t i n g i t s g r a z i n g pressure from . one s i z e range to another - i . e . . t h e animals a t e whatever s i z e was most abundant i n terms of biomass. another marine copepod, a c a r t i a t o n s a, was observed by Wilson ( 1 9 7 3 ) to feed n o n s e l e c t i v e l y over the most abundant s i z e s and at the same time t o i n g e s t the l a r g e s t p a r t i c l e s i n g r e a t e r p r o p o r t i o n than t h e i r a v a i l a b i l i t y . Host freshwater s t u d i e s i n d i c a t e that zooplankton body s i z e determines food s e l e c t i v i t y , e s p e c i a l l y f o r s m a l l animals, and t h a t l a r g e zooplankton can handle a l a r g e range of s i z e s e q u a l l y w e l l . Lampert ( 1 9 7 4 ) found t h a t Daphnia pulex l a r g e r than 1 . 5 mm i n l e n g t h i n g e s t e d b a c t e r i a ( 1 . 5 u) and l a r g e r a l g a e (28 u) without d i f f e r e n t i a t i o n , and t h a t Daphnia s m a l l e r than 1 . 5 mm p r e f e r e n t i a l l y a t e the s m a l l b a c t e r i a . S i m i l a r r e s u l t s were obtained by Bogdan and HcNaught ( 1 9 7 5 ) f o r Daphnia q a l e a t a and Diaptomus minutus. The l a r g e r •Daphnia i n g e s t e d nannoplankton ( <22 u diam.) and net plankton ( >22 u diam) e q u a l l y w e l l , whereas the s m a l l e r Diaptomus i n g e s t e d nannoplankton p r e f e r e n t i a l l y . HcHahon and R i g l e r ( 1 9 6 5 ) found the f i l t e r i n g r a t e of l a r g e Daphnia magna to be independent of 56 s i z e of food c e l l s over a range of 0 . 9 u 3 t o 18,000 u 3 . Barns (1968) measured the maximum p a r t i c l e s i z e i n g e s t e d by d i f f e r e n t s i z e s of s e v e r a l s p e c i e s of Daphnia and Bosmina. and found i t p r o p o r t i o n a l t o zooplankton body l e n g t h . However, she d i d not measure i n g e s t i o n r a t e s . An overview of s i z e s e l e c t i o n was r e c e n t l y p u b l i s h e d by Boyd { 1 9 7 6 ) , He c l a i m s that i t i s not necessary to c r e d i t zooplankton with an a b i l i t y t o scan p a r t i c l e s i z e d i s t r i b u t i o n s or t o a c t i v e l y s e l e c t c e r t a i n p a r t i c l e s i n o r d e r to have p r e f e r e n t i a l i n g e s t i o n . Simply c o n s i d e r i n g the f i l t e r i n g apparatus as a 'leaky s i e v e * w i l l imply a tendency f o r animals t o ' s e l e c t * l a r g e r p a r t i c l e s . In terms o f t h i s same analogy i t i s p o s s i b l e t h a t as the f i l t e r i n g apparatus becomes p a r t l y s a t u r a t e d , s m a l l e r p a r t i c l e s w i l l be c a p t u r e d . T h i s c o u l d e x p l a i n F r o s t ' s (1972) o b s e r v a t i o n s d i s c u s s e d above. G l i w i c z (1977) found t h a t the upper s i z e l i m i t of p a r t i c l e s grazed was, f o r a number o f c l a d o c e r a n s p e c i e s , r e l a t e d t o the width of the gap between carapace margins. The OBC f o r e s t l a k e s , with which I was mainly concerned f o r my experiments, are c h a r a c t e r i z e d by s m a l l forms o f phytoplankton. About 80S of the s e s t o n p a r t i c l e s f a l l i n the 2 - 4 u diameter range. In my experiments then, the t r a c e r c e l l s s h o u l d f a l l i n t h i s same s i z e range, i n o r d e r to a s s u r e t h a t the uptake of r a d i o a c t i v e c e l l s r e f l e c t a c c u r a t e l y the 57 uptake of s e s t o n p a r t i c l e s . Bhgdotorula r u b r i s was chosen as the t r a c e r organism f o r t h i s and other reasons., I n d i v i d u a l c e l l s i z e was about 2 u diameter; as prepared i n l i q u i d medium, the c e l l s tended to e x i s t s i n g l y or with 2 o r 3 c e l l s clumped together, but not i n l a r g e r aggregates. T h i s y e a s t i s very common, present i n the a i r and l o c a l f r e s h waters, and i s e d i b l e by the z o o p l a n k t e r s i n the proposed experiments. In a d d i t i o n * i t i s easy to c u l t u r e and l a b e l with 3 2 P . 3.2 D e s c r i p t i o n Of Methods Used These experiments measured zooplankton . f i l t e r i n g and f e e d i n g r a t e s as f u n c t i o n s of food c o n c e n t r a t i o n under a wide range of c o n d i t i o n s , using 3 2 P l a b e l l e d yeast as a t r a c e r and e i t h e r n a t u r a l s e s t o n or C h l o r e l l a as the main food component. F i g u r e 6 shows a schematic o u t l i n e of the method. The UBC M i c r o b i o l o g y Department s u p p l i e d the o r i g i n a l yeast stock* Bhodoturala r u b r i s . I t was maintained on agar s l a n t s u n t i l about a week before each experiment, when some o f the yeast was suspended i n a peptone-dextrose l i q u i d medium. C a r r i e r - f r e e 3 2 P was added to the medium a few days l a t e r . Immediately before an experiment a few ml of t h i s y e a s t suspension was c e n t r i f u g e d and r i n s e d t h r e e times with d e i o n i z e d d i s t i l l e d water t o remove a l l 3 2 P not i n c o r p o r a t e d 58 CoUccA 2ooplank\on Vvorn lake \ I C o U e c \ l a k e v U x t c r \ CvU jre I vjeast UWledl | Knsc and cVUo^ c l*>i Ui too -"Sort ?.6o - | pto-M. toon tolo ^cirUiUatioi | VtO-U | Measure . rt\(3-voacAv\/vUj S c m V i U a V i o n m5i Calculate ^Icrwi Vales Calculate Ceciti/v^  ca\e.i Figure 6. Experimental configuration. 59 i n t o the yeast c e l l s . Yeast c e l l d e n s i t y was measured with a haemocytometer, then d i l u t e d so t h a t adding 1 ml of 'hot* yeast to a g r a z i n g chamber would g i v e about 2000 yeast c e l l s / m l i n the f e e d i n g medium ( i . e . about 1/3 the weight o f the most d i l u t e s e s t o n c o n c e n t r a t i o n used.) For experiments with n a t u r a l zooplankton p o p u l a t i o n s , the animals and l a r g e q u a n t i t i e s o f l a k e water were c o l l e c t e d e i t h e r t h e day of the experiment o r the n i g h t b e f o r e , and kept at constant temperature i n c o n t r o l l e d environment i n c u b a t o r s . S i n c e t h e v a r i o u s lakewaters g e n e r a l l y had low s e s t o n l e v e l s , I had t o c o n c e n t r a t e the s e s t o n i n order t o o b t a i n a s u f f i c i e n t range of food c o n c e n t r a t i o n s . To a v o i d damage t o phytoplankton c e l l s , I f i r s t t r i e d r e v e r s e f i l t r a t i o n * but i t proved t o be f a r t o o slow to be p r a c t i c a l f o r p r o c e s s i n g the volumes of water r e g u i r e d i n t h e s e experiments; e v e n t u a l l y I adopted a method of c o n t i n u o u s l y s t i r r e d , slow, d i r e c t f i l t r a t i o n with l a r g e membrane f i l t e r s , u s i n g an apparatus based on a design by H o r r i s and Yentsch (1972)-,, S i n c e membrane f i l t e r s c o n t a i n 2-3% o f t h e i r dry weight as d e t e r g e n t (Cahn, 1967), the f i l t e r s were soaked i n warm d i s t i l l e d water f o r a day and f l u s h e d with about two l i t r e s of d i s t i l l e d water immediately b e f o r e use, i n order to reduce any p o s s i b l e t o x i c i t y . ( S p e c i a l l y ordered f i l t e r s without detergent, which serves as a wetting agent, cannot be used t o f i l t e r water.) a p p r o p r i a t e q u a n t i t i e s of s e s t o n c o n c e n t r a t e and l a k e 60 water f i l t r a t e were recombined to g i v e the v a r i o u s food c o n c e n t r a t i o n s d e s i r e d . , For experiments u s i n g C h l o r e l l a i n s t e a d of n a t u r a l s e s t o n , the same procedure was f o l l o w e d t o o b t a i n l a k e f i l t r a t e . Then C h l o r e l l a was c e n t r i f u g e d and resuspended i n the f i l t r a t e . G e n e r a l l y I used f i v e food c o n c e n t r a t i o n s (approximately 0 . 1 , 0.5, 1-0, 2.0, and 5.0 times n a t u r a l lake c o n c e n t r a t i o n s ) , with t h r e e r e p l i c a t e 250 ml beakers per c o n c e n t r a t i o n . Zooplankton were s o r t e d and p l a c e d i n g r a z i n g v e s s e l s (250 ml beakers); i f more than one s p e c i e s or s i z e c l a s s were abundant, I was c a r e f u l t o put the same numbers of each c l a s s i n t o each beaker (each beaker o f t e n c o n t a i n e d s e v e r a l i s p e c i e s or s i z e s of animals) . The number o f animals i n each v e s s e l (5-50) was chosen so t h a t the c o n c e n t r a t i o n o f food would change very l i t t l e over 3-4 hours. The zooplankton were then l e f t i n the i n c u b a t o r t o a c c l i m a t e f o r 2-3 hours at the same food c o n c e n t r a t i o n a t which they would l a t e r be measured. I used two to f o u r r e p l i c a t e beakers f o r each c o n c e n t r a t i o n , and p e r i o d i c a l l y s t i r r e d them t o prevent s e s t o n from s e t t l i n g . For most f e e d i n g experiments I a r b i t r a r i l y chose to a l l o w 2-3 hours f o r the animals to a d j u s t t o t h e i r v a r i o u s food c o n c e n t r a t i o n s , mainly because t h i s was standard p r a c t i c e f o r s i m i l a r work re p o r t e d i n the l i t e r a t u r e . One other common p r a c t i c e i s t o s t a r v e zooplankton f o r 24 hours p r i o r to 61 f e e d i n g experiments, but t h i s d i d not seem a p p r o p r i a t e here, as I was t r y i n g t o measure " u n d i s t u r b e d " f u n c t i o n a l responses. At the end of the a c c l i m a t i o n time, 3 2 P l a b e l l e d y e a s t was s t i r r e d i n t o the beakers c o n t a i n i n g the animals. Zooplankton were allowed t o f e e d i n the •hot* juedium f o r times v a r y i n g from 5 to 15 minutes. The p a r t i c u l a r times were e s t a b l i s h e d beforehand by measuring r a d i o a c t i v e uptake i n the animals as a f u n c t i o n of g r a z i n g time, t o get an e s t i m a t e o f gut passage time. For measurements of f e e d i n g r a t e , I wanted to allow the animals t o feed long enough to become s u f f i c i e n t l y r a d i o a c t i v e f o r good c o u n t i n g s t a t i s t i c s , but not long enough f o r any r a d i o a c t i v e m a t e r i a l to be d e f e c a t e d . Animals were then removed with a f i n e s e i v e and, s t i l l i n the s e i v e , were immediately a n a e s t h e t i z e d with carbonated water. They were then k i l l e d by immersion i n a mixture of a l c o h o l and c h l o r o f o r m d i l u t e d with d i s t i l l e d water and r i n s e d thoroughly with tap water. The carbonated water i s an a n a e s t h e t i c and seems t o prevent d e f e c a t i o n d u r i n g subsequent p r o c e s s i n g (Burns, 1969). Zooplankton from each r e p l i c a t e were then measured, s o r t e d , and counted by s p e c i e s and s i z e c l a s s i n t o separate l i q u i d s c i n t i l l a t i o n v i a l s . C a b o s i l (to keep animals i n suspension) and Bray's s c i n t i l l a t i o n f l u i d were added before v i a l s were counted on a Nuclear Chicago Isocap programmable s c i n t i l l a t i o n counter. A l i q u o t s of seston from each g r a z i n g chamber were f i l t e r e d and r i n s e d f o r 3 2 P 6 2 c o u n t i n g . L a r g e r samples were f i l t e r e d on preashed g l a s s f i b r e f i l t e r s t o o b t a i n ash f r e e dry weight v a l u e s f o r food c o n c e n t r a t i o n s (the food thus measured i s the combined y e a s t and s e s t o n ) . F i l t e r i n g r a t e s , expressed as the volume of water f i l t e r e d per animal per u n i t time, were c a l c u l a t e d from the average r a d i o a c t i v i t y of the zooplankton and the r a d i o a c t i v i t y o f the yeast--seston suspension by the f o l l o w i n g eguation dpm per animal . dpm per . g r a z i n g c o r r e c t e d f o r - ml. of - time i n quench and . s e s t o n . minutes c o n t r o l Feeding r a t e s were then determined by m u l t i p l y i n g f i l t e r i n g r a t e s by the food c o n c e n t r a t i o n i n the f e e d i n g s u s p e n s i o n . m l . f i l t e r e d per animal per minute 3.3 E f f e c t s Of Experimental C o n d i t i o n s a c c o r d i n g t o the c o l l e c t e d warnings of zooplankton r e s e a r c h e r s , i t should be i m p o s s i b l e t o measure zooplankton f e e d i n g responses. For almost any p o s s i b l e e x p e r i m e n t a l f a c t o r t h e r e i s probably someone who has claimed, i n p r i n t or otherwise, t h a t t h i s f a c t o r i s o f utmost importance and must be c o n t r o l l e d w i t h i n c e r t a i n narrow l i m i t s - Of course such 63 c o n t r o l g e n e r a l l y c o n f l i c t s with one or more of the c o n s t r a i n t s imposed on other f a c t o r s . As p a r t of t h i s study t h e r e f o r e , I c a r r i e d out t e s t s on t h e e f f e c t s of a number o f experimental c o n d i t i o n s - s i z e of g r a z i n g v e s s e l , zooplankton r i n s i n g t e c h n i q u e , type of fo o d , dosage of r a d i o a c t i v e y e a s t , l e n g t h o f g r a z i n g time, and a c c l i m a t i o n time. 64 4 R E S U L T S 4.1 T e s t s Of Experimental C o n d i t i o n s 4-1.1 S i z e Of Grazing Chambers The i n f l u e n c e o f v e s s e l s i z e on f e e d i n g behaviour o f marine c a l a n o i d s has been r e p o r t e d by M a r s h a l l and Orr (1955), Cushing (1958), Anraku (1963). I t i s g e n e r a l l y assumed t h a t f r e s h water c l a d o c e r a n s are l e s s a f f e c t e d by confinement than are c a l a n o i d s ( R i g l e r , 1971). S c h i n d l e r (1968) found no e f f e c t o f crowding on the f e e d i n g r a t e s of Daphnia magna. There i s c e r t a i n l y a wide range i n the v e s s e l s i z e s used i n p u b l i s h e d experiments - from 1000 ml and over (Haney, 1971; Burns 6 R i g l e r , 1967; Bogdan 6 McHaught, 1975) t o 500 ml (Crowley, 1973) to 65 ml (McQueen, 1970). The most important c o n s i d e r a t i o n i n s e l e c t i n g v e s s e l s i z e i s , I b e l i e v e , t h a t the r a t i o o f animal d e n s i t y t o volume o f f e e d i n g medium be such t h a t the food c o n c e n t r a t i o n changes very l i t t l e d u r i n g the course of the experiment. T h i s i s e s p e c i a l l y important i n l o n g - i n c u b a t i o n experiments. As a l r e a d y p o i n t e d out, a s u r p r i s i n g number of r e s e a r c h e r s have i n c o r r e c t l y used Gauld's (Gauld, 1951) e g u a t i o n t o c a l c u l a t e f i l t e r i n g r a t e i n experiments f o r which there must have been l a r g e changes i n food c o n c e n t r a t i o n d u r i n g i n c u b a t i o n . They seem to be unaware 65 t h a t t h i s e q u a t i o n i s based on an assumption of c o n s t a n t f i l t e r i n g r a t e d u r i n g the course of the experiment. But f i l t e r i n g r a t e does not g e n e r a l l y remain c o n s t a n t during r a p i d changes i n food c o n c e n t r a t i o n , p a r t i c u l a r l y i n the r e g i o n of low food c o n c e n t r a t i o n . I experimented with a v a r i e t y of c o n f i g u r a t i o n s i n v o l v i n g c o n t a i n e r s of d i f f e r e n t shapes and s i z e s ranging from 100 ml to 500 ml. Zooplankton d e n s i t i e s were kept about the same. G e n e r a l l y , the s i z e of the c o n t a i n e r made l i t t l e d i f f e r e n c e (e.g. F i g u r e 7, which shows r e s u l t s of comparable experiments f o r Daphnia rosea i n 250 ml. beakers and 100 ml. p i p e t t e s ) . Methods which i n v o l v e d f a i r l y v i g o r o u s d i s t u r b a n c e s to the animals seemed t o d r a s t i c a l l y i n h i b i t the measured f i l t e r i n g r a t e s o f diaptomids and Holopedium. probably because t h e y stopped f i l t e r i n g f o r a w h i l e . I had p r e v i o u s l y observed Holopedium r e a c t t o p h y s i c a l d i s t u r b a n c e s by c e a s i n g appendage movements completely f o r up t o 30 seconds. Two c o n f i g u r a t i o n s I t r i e d were thus abandoned because diaptomids and Holopedium seemed t o b a r e l y feed i n them. One c o n s i s t e d of c l o s e d 100 ml, p i p e t t e s modelled a f t e r Crowley's (1973) design and mounted on a s l o w l y r o t a t i n g wheel, while the other used mesh bottomed baskets suspended i n 500 ml beakers, with mesh l i d s t o keep the animals from being trapped on t h e water s u r f a c e . Ordinary 250 ml open beakers were f i n a l l y chosen f o r the experiments because they r e q u i r e d much l e s s water p r o c e s s i n g 66 Figure 7. A comparison of Daphnia.rosea f i l t e r i n g rates measured i n two d i f f e r e n t kinds of container. 67 than the l a r g e r volumes, and minimum zooplankton h a n d l i n g . 4.1.2 Zooplankton R i n s i n g To determine how e f f e c t i v e my method o f zooplankton r i n s i n g was, and whether non i n g e s t e d r a d i o a c t i v e f o o d remained caught i n mouthparts, or on o t h e r p a r t s of the body, I d i d d u p l i c a t e experiments i n which one s e t o f animals was allowed to f e e d i n n o n - r a d i o a c t i v e food f o r 5 minutes a f t e r having grazed i n r a d i o a c t i v e food f o r 5 minutes. (This i s a technigue used by Burns, 1969). The o t h e r group o f animals was g i v e n no p o s t f e e d . A l l zooplankton were r i n s e d a f t e r f e e d i n g . No measurable d i f f e r e n c e was observed between the two treatments (Figure 8 ) . N e v e r t h e l e s s , t o measure p o s s i b l e background counts caused by a d s o r p t i o n * a b s o r p t i o n , inadequate washing, e t c . , I always c a r r i e d a group of f r e s h l y k i l l e d (by immersion i n a l c o h o l and chloroform) zooplankton through the e x p e r i m e n t a l s t e p s . Whenever t h i s background was not n e g l i g i b l e , i t was s u b t r a c t e d from zooplankton r a d i o a c t i v i t y r e a d i n g s . 68 i .o-o > <= 0.5 LU < cr o cc LLI h-a Holopedium gibberum O 5 minute rodioactive feed, followed by 5 minute non-rGdioactive^feed © 5 minute radioactive feed only 0-0 0.5 1.0 F O O D C O N C E N T R A T I O N ( u g / m l ) Figure 8. Results of using a non-radioactive "post" feed to ensure adequate r i n s i n g of radioactive p a r t i c l e s from feeding appendages. 69 4- 3 Kind Of Food As p r e v i o u s l y d i s c u s s e d , any food s e l e c t i o n on the p a r t of f i l t e r f e e d e r s seems t o operate through a s i z e s e l e c t i o n mechanism. Since the process f o r c o n c e n t r a t i n g n a t u r a l l a k e s e s t o n was t e d i o u s and time consuming, I t r i e d t o f i n d an a l t e r n a t i v e food source of an a p p r o p r i a t e c e l l s i z e t h a t c o u l d be e a s i l y c u l t u r e d i n s u f f i c i e n t c o n c e n t r a t i o n t h a t i t c o u l d be d i l u t e d i n f i l t e r e d lakewater t o g i v e the c o n c e n t r a t i o n s needed, thus e l i m i n a t i n g the need f o r mechanical c o n c e n t r a t i o n . To t e s t the s u i t a b i l i t j o f Chlore11a as a p o s s i b l e food substance, I began a s e r i e s of experiments t o compare f i l t e r i n g r a t e s on c u l t u r e d l a b o r a t o r y Chlore11a a g a i n s t r a t e s with l a k e s e s t o n . 3 2 P - l a b e l l e d yeast was the t r a c e r i n both s e t s of experiments. A l l e x p e r i m e n t a l c o n d i t i o n s , i n c l u d i n g time of day, were i d e n t i c a l . D i f f e r e n c e s i n f i l t e r i n g r a t e measurements thus should be due i n some manner t o d i f f e r e n c e s i n the nature of the food, C h l o r e l l a was very easy t o use i n these experiments - i t was simply c e n t r i f u g e d , r i n s e d and resuspended i n l a k e f i l t r a t e . , The use o f lake f i l t r a t e alone i n s t e a d o f f i l t r a t e and s e s t o n c o n c e n t r a t e meant t h a t much l e s s water had to be processed f o r each experiment - a c r i t i c a l f a c t o r s i n c e l a r g e volumes of water had t o be t r a n s p o r t e d from the f i e l d i n any case. The r e s u l t s o f these comparisons are shown i n F i g u r e s 9, D A P H N I A R O S E A FOOD CONCENTRATION (ug/ml dry wt) Figure 9. Comparison of Daphnia rosea f i l t e r i n g rates measured with seston and with C h l o r e l l a . ( O - seston; @ - Chlorella) 71 10, 11 and 12. the most s t r i k i n g f e a t u r e of t h e r e s u l t s was t h a t r e p l i c a t e v a r i a t i o n was g e n e r a l l y much s m a l l e r with C h l o r e l l a than w i t h s e s t o n f o r f o o d , p o s s i b l y because the seston p a r t i c l e s i z e s p e c t r a may not have been i d e n t i c a l from beaker t o beaker. Or perhaps i n d i v i d u a l v a r i a t i o n i n f i l t e r i n g e f f i c i e n c y i s g r e a t e r f o r a non uniform food s u p p l y . G e n e r a l l y the r e s u l t s i n d i c a t e d t h a t only the s m a l l e r animals were a f f e c t e d by d i f f e r e n c e s between pure C h l o r e l l a and n a t u r a l s e s t o n . The two s m a l l e r s i z e s of Daphnia puj.ex f o r example showed higher f i l t e r i n g r a t e s f o r ses t o n than f o r C h l o r e l l a at the same c o n c e n t r a t i o n by weight. I n other words they behaved as though t h a t p a r t o f the ses t o n a c t u a l l y a v a i l a b l e t o them as food was l e s s than i n d i c a t e d by the dry weight c o n c e n t r a t i o n of t h e s e s t o n . The Daphnia pulex f o r these two s i z e s were i n f a c t taken from a very e u t r o p h i c l a k e and i t i s q u i t e p o s s i b l e t h a t the s e s t o n c o n t a i n e d a f a i r amount of m a t e r i a l t oo l a r g e t o be handled as food., For Holopedium and Cer i o d a p h n i a t h e r e were n o t i c e a b l e but not c o n s i s t e n t d i f f e r e n c e s between r e s u l t s obtained with s e s t o n and with C h l o r e l l a . F i l t e r i n g r a t e s measured with C h l o r e l l a were sometimes h i g h e r , sometimes lower than s e s t o n f i l t e r i n g r a t e s . 72 D A P H N I A P U L E X FOOD CONCENTRATION (yug / ml. dry wt) Figure 10. Comparison of Daphnia pulex f i l t e r i n g rates measured with seston and with C h l o r e l l a . ( O - seston; Q - Chlorella) 73 Figure 11. Comparison of Ceriodaphnia f i l t e r i n g rates measured with seston and with C h l o r e l l a . ( O - seston; & - Chlorella) H 0 L Q P E D 1 U M G 1 B B E R U M (5/jg dry wt) 0 5 10 15 20 FOOD CONCENTRATION (yug/ml dry wt) Figure 12. Comparison of Holopedium gibberum f i l t e r i n g rates measured with seston and with C h l o r e l l a . ( O - seston; 9 - Chlorella) 7 5 4. 1. 4 B a d i o a c t i v e Yeast In order t o measure f e e d i n g responses under c o n d i t i o n s as c l o s e l y as p o s s i b l e approximating those found by zooplankton i n t h e i r own environment, I wanted to add a minimum amount l a b e l l e d yeast t o t h e f e e d i n g medium. However, the t r a c e r c o n c e n t r a t i o n s t i l l had to be s u f f i c i e n t t o ensure good counting s t a t i s t i c s . In most! o f the l i t e r a t u r e on r a d i o a c t i v e t r a c e r measurements of zooplankton f e e d i n g , the a u t h o r s have used monocultures f o r f o o d and have l a b e l l e d the e n t i r e food supply (Burns, 1969; HacHahon, 1965; MacHahon fi B i g l e r , 1965; Lampert, 1974). Only a few r e s e a r c h e r s (Crowley, 1973; Haney, 1971) have t r i e d measuring f e e d i n g c u r v e s with n a t u r a l s e s t o n and very s m a l l a d d i t i o n s of r a d i o a c t i v e c e l l s . To determine what l e v e l o f l a b e l l e d yeast I should use, I d i d a few extreme experiments - one with 6000 c e l l s / m l of hot yeast (0.12 ug/ml dry weight) and the other with 600 c e l l s / m l (see F i g u r e s 13, and 14). The f i r s t dosage i s about the same amount by weight as the lowest s e s t o n c o n c e n t r a t i o n used f o r the response c u r v e s . B e s u l t s with the higher yeast c o n c e n t r a t i o n had l e s s v a r i a b i l i t y ; with the low yeast c o n c e n t r a t i o n t h e r a d i o a c t i v i t y of the animals was somewhat low f o r good co u n t i n g s t a t i s t i c s . Otherwise the f i l t e r i n g r a t e s measured with the two: y e a s t c o n c e n t r a t i o n s were s i m i l a r . I chose an i n t e r m e d i a t e yeast dosage l e v e l of 2000 c e l l s / m l 76 _J 0 ^ n r — , , - , i 1 0 I 2 3 4 5 F O O D C O N C E N T R A T I O N ( u g / m l d r y w e i g h t ) Figure 13. F i l t e r i n g rates of Daphnia rosea measured with d i f f e r e n t amounts of radioactive yeast. ( A - 600 c e l l s / m l . ; O - 6000 cells/ml.) 77 5 FOOD CONCENTRATION ( ml A u g / hr) Figure 14. F i l t e r i n g rates of Holopedium gibberum measured with d i f f e r e n t amounts of radioactive yeast. ( A - 600 c e l l s / m l , O - 6000 cells/ml) 78 f o r the remaining experiments. T h i s i s an order of magnitude l e s s than t h e c o n c e n t r a t i o n below which Burns and R i g l e r (1967) observed f i l t e r i n g r a t e s of IK Rosea to remain c o n s t a n t . 4.1.5 Length Of Grazing Time In t h i s type of experiment, i t i s important t h a t the r a d i o a c t i v e g r a z i n g p e r i o d s be terminated before any r a d i o a c t i v e f e c e s a r e e l i m i n a t e d . Since gut passage times can vary with s p e c i e s and body s i z e , I made time s e r i e s measurements of r a d i o a c t i v i t y uptake f o r each new type o f zooplankton to be t e s t e d , i n order t o determine f o r i t an a c c e p t a b l e r a d i o a c t i v e g r a z i n g time p e r i o d . R e p r e s e n t a t i v e r e s u l t s are shown i n F i g u r e 15 . 4-1.6 A c c l i m a t i o n To Experimental Food C o n c e n t r a t i o n The f i r s t o f these experiments examined the e f f e c t o f h o l d i n g the animals i n the l a b o r a t o r y f o r v a r i o u s l e n g t h s of time between c o l l e c t i o n and measurement of f e e d i n g r a t e s . Groups of f r e s h l y c o l l e c t e d Diaptomns oregonensis were put i n v e s s e l s c o n t a i n i n g r e g u l a r l y r e f r e s h e d lake water and maintained i n c o n t r o l l e d environment chambers., (The l a k e 1.0 T I M E (minutes) Figure 15. Radioactivity of animals as a function of feeding time. 80 water from which the animals were c o l l e c t e d c o n tained about 0.75 ug/ml (ash f r e e dry weight) of sestons) A f t e r i n t e r v a l s r a n g i n g from 1 t o 5 days, animals were taken f o r f u n c t i o n a l response measurements. Immediately p r i o r to these measurements they were allowed 2 hours t o a c c l i m a t e t o the food c o n c e n t r a t i o n at which they would be measured., A l l experiments were done a t the same time of day ( e a r l y e v e n i n g ) , so any endogenous d i e l e f f e c t s should have been minimal. F i g u r e 16 shows the r e s u l t i n g f i l t e r i n g r a t e s , p l o t t e d as f u n c t i o n s of food c o n c e n t r a t i o n . (In t h i s s e c t i o n r e s u l t s w i l l be given as f i l t e r i n g r a t e s i n s t e a d of f e e d i n g r a t e s because some o f the d e t a i l s are more c l e a r l y v i s i b l e i n t h i s format.) I t i s obvious t h a t l a r g e changes i n t h e magnitude and shape o f the f u n c t i o n a l response have o c c u r r e d d u r i n g the i n t e r v a l over which the animals were kept i n the l a b o r a t o r y . F i l t e r i n g r a t e s , e s p e c i a l l y a t low food c o n c e n t r a t i o n s , i n c r e a s e d w i t h the l e n g t h of time i n the l a b . For the lowest food c o n c e n t r a t i o n * f i l t e r i n g r a t e s i n c r e a s e d by a f a c t o r o f more than 10 over 5 days. A second experiment examined e f f e c t s of h o l d i n g zooplankton under d i f f e r e n t food regimes. Animals were h e l d f o r 5 days between c o l l e c t i o n and measurement of f i l t e r i n g r a t e . One group was held i n untreated lake water (seston about 1 ug/ml ash f r e e dry weight), another i n l a k e water c o n c e n t r a t e d t o about 5 times normal seston c o n t e n t , and a 81 0.25 A 0.20-0.15-0.10-U J h-<t rr o z S 0.05-h-0 Diaptomus oregonensis • Holding time = I day o Holding time = 3 days • Holding time = 5 days 0 1 2 3 4 FOOD CONCENTRATION ( ug./ml. dry weight) Figure 16. E f f e c t of holding zooplankton i n the laboratory for various lengths of time between c o l l e c t i o n and measurement of feeding rates. 82 t h i r d group i n very d i l u t e l a k e water. Again, animals were a c c l i m a t e d f o r 2 hours j u s t p r i o r to the experiment, i n l a k e water with the same food c o n c e n t r a t i o n a t which f i l t e r i n g r a t e s were to be measured. F i g u r e s 17 and 18 show the r e s u l t i n g f i l t e r i n g r a t e curves f o r Holopedium qibberum and Diaptomus oregonensis. Animals h e l d i n h i g h food d e n s i t i e s had lower f e e d i n g r a t e s a t low food c o n c e n t r a t i o n s than animals held i n untreated l a k e water., The l a s t of my a c c l i m a t i o n experiments demonstrated how f i l t e r i n g r a t e was a f f e c t e d by a c c l i m a t i o n t o d i f f e r e n t f o o d c o n c e n t r a t i o n s . F i l t e r i n g r a t e s o f Daphnia pnlex were measured a t 5 food c o n c e n t r a t i o n s . , Groups of animals were held a t each c o n c e n t r a t i o n ( i . e . at t h e i r e v e n t u a l c o n c e n t r a t i o n of measurement) f o r up to 48 hours. At v a r i o u s time i n t e r v a l s zooplankton were taken from each group and t h e i r f i l t e r i n g r a t e s measured f o r the corresponding f o o d c o n c e n t r a t i o n . F i g u r e 19 shows f i l t e r i n g r a t e s as a f u n c t i o n of f o o d c o n c e n t r a t i o n f o r f o u r a c c l i m a t i o n times. For p e r i o d s exceeding 21 hours, a c c l i m a t i o n time had no a p p r e c i a b l e e f f e c t on f i l t e r i n g r a t e s a t high food c o n c e n t r a t i o n s . At low c o n c e n t r a t i o n s however* th e r e was a c o n s i d e r a b l e e f f e c t . The f i l t e r i n g r a t e was i n i t i a l l y very h i g h ; i t decreased by about one t h i r d a f t e r 4 hours o f a c c l i m a t i o n , and by about one h a l f a f t e r 21 hours. At t h i s p o i n t t h e r e was a l s o an i n h i b i t i o n of f i l t e r i n g r a t e a t the 83 Iv5 FOOD C O N C E N T R A T I O N ( u g / m l dry weight) Figure 1 7 . E f f e c t of holding zooplankton under d i f f e r e n t food regimes before measuring functional responses. Diaptomus oregonensis 9 untreated lake water (0.9 ug/ml) T i 1 : r-0 1 2 3 4 5 FOOD CONCENTRATION ( u g / m l . dry weight) e 18. E f f e c t of holding zooplankton under d i f f e r e n t food regimes before measuring functional responses. H -iQ C H CD l-i DJ r t 0 O cn fD • D r+ i-( tu rt M -O CO O 3 ro DJ cn C H CD Ml H -I—' rt (D H H -U3 W H i H i n> o rt o Mi DJ o Q y-> H -3 DJ rt H -O rt O & H -Mi Ml fD 1-1 fD r t Mi O O Ch o e c a LLI r -< rr CD z rr 2.0-4 1.5-1.0-4 0.5H Daphnia pulex I hr. accl imation 4 hr. accl imation 21 hr. accl imat ion 4 8 hr. accl imat ion 0 0 8 FOOD C O N C E N T R A T I O N ( u g . / m l . dry weight ) 86 lowest c o n c e n t r a t i o n . The i n h i b i t i o n then disappeared f o r the 48 hour measurement, and f i l t e r i n g r a t e s were s i m i l a r t o the 4 hour curve, except a t high f o o d c o n c e n t r a t i o n s where they were higher than t h e e a r l i e r c u r v e s . F i g u r e 20 uses the same dat a , converted t o f e e d i n g r a t e s (by m u l t i p l y i n g by f o o d c o n c e n t r a t i o n ) and p l o t t e d t o show how fee d i n g r a t e a t a g i v e n c o n c e n t r a t i o n v a r i e s with the l e n g t h o f time the animal has t o a c c l i m a t e t o t h a t c o n c e n t r a t i o n * a l l curves have the same ge n e r a l shape. Feeding r a t e s are high a t f i r s t , then immediately decrease, and e v e n t u a l l y i n c r e a s e with i n c r e a s i n g a c c l i m a t i o n time.. F o r the t h r e e lowest c o n c e n t r a t i o n s , f e e d i n g r a t e s decreased by h a l f over the f i r s t 20 hours and then remained f a i r l y c o n s t a n t except f o r a s l i g h t i n c r e a s e by 48 hours. Feeding r a t e s a t the 2 highest c o n c e n t r a t i o n s however, changed l e s s over t h e f i r s t 20 hours, then more than doubled between 20 and 48 hours, almost a l l e x p e r i m e n t a l and a p p l i e d work on zooplankton f e e d i n g r a t e s assume i m p l i c i t l y t h a t these curves should be f l a t l i n e s . 0.12 Daphnia pulex O —©*— 0.25 ug ml food concentration A t - , , : , r ~ 0 10 20 30 40 A C C L I M A T I O N T I M E (hours ) F i g u r e 20 . Same ex p e r i m e n t as F i g u r e 19 , p l o t t e d t o show how f e e d i n g r a t e a t a g i v e n c o n c e n t r a t i o n v a r i e s w i t h l e n g t h o f time the a n i m a l has t o a c c l i m a t e t o t h a t c o n c e n t r a t i o n . 88 tt. 2 F u n c t i o n a l Response R e s u l t s For purposes of comparing f u n c t i o n a l responses, a s e t o f " s t a n d a r d " experimental c o n d i t i o n s was e s t a b l i s h e d . , a l l such feeding experiments were done as soon as p o s s i b l e a f t e r the zooplankton were c o l l e c t e d ( r a d i o a c t i v e f e e d i n g g e n e r a l l y took p l a c e 24-28 hours a f t e r c o l l e c t i o n ) , using the same p h y s i c a l apparatus and methods (as d e s c r i b e d i n Chapter 3 ) , and with 2-3 hours allowed f o r a c c l i m a t i o n t o experimental f o o d c o n c e n t r a t i o n s . The only t h i n g t h a t was e x p l i c i t l y allowed to vary was temperature, although food composition may a l s o have v a r i e d , s i n c e n a t u r a l l a k e s e s t o n was used. Feeding r a t e s were measured at the temperature o f the water.from which the animals were c o l l e c t e d . R e s u l t s o f t h i s a r r a y o f f u n c t i o n a l response measurements a r e shown i n F i g u r e s 21(a - p ) . They r e p r e s e n t 8 s p e c i e s from 6 l a k e s . A l l s e t s of f e e d i n g r a t e s measured showed a s a t u r a t i n g r e l a t i o n s h i p with i n c r e a s i n g food c o n c e n t r a t i o n , but there was no s i n g l e mathematical e x p r e s i o n which best d e s c r i b e d a l l the r e s u l t s . In t h i s study I t r i e d t o f i t the f u n c t i o n a l response data with a v a r i e t y of curves. A r e c t i l i n e a r model was not c o n s i d e r e d because i t d i d not seem a p p r o p r i a t e f o r the observed f u n c t i o n a l responses. The shapes of the f i l t e r i n g r a t e curves i n d i c a t e d t h a t a sigmoid f e e d i n g response was i n some cases a p p r o p r i a t e , but not a t h r e s h o l d f e e d i n g curve as 89 Figure 21 (a) - (p). "Standard" functional responses measured for several species. Feeding rates are i n units of ug. food ingested (ash free dry weight)/ug. zooplankton (dry weight)/hour. Food concentrations are expressed as ug/ml ash free dry weight. 90 Figure 21(a) 0.2 i : : = • — — — 1 Cn .£ 0. I FOOD CONCENTRATION (jug./ml.) 0 .2 Figure 21(b) \ c o c Q . O O N CT) \ "CJ H— in cu Cn c Cn LU h-cc CD Q UJ LU Lu DAPHNIA ROSEA 15° C. - Loon L. 0 0 .2 0 O DAPHNIA ROSEA o 15 ° C. - Loon L. o o O O DAPHNIA ROSEA 15°-Loon L. O O -—-—"—' O . 0 25 FOOD CONCENTRATION (jug./ml.) Figure 21(c) 0 . 3 DAPHNIA ROSEA o 2 0 ° C. - Placid L. o o 0 0 0 10 2 0 FOOD CONCENTRATION (jug./ml.) DAPHNIA ROSEA 2 0 °C . • - Katherine L. o o ._ . — f t _ . o O O 3 0 9 3 X c o c _CJ C L O O N C n \ "O QJ C n c - C n L U K ft CD 3 L U L U 0.12 0 0.12 Figure 21(d) DAPHNIA PULEX 10° C. - U.B.C. Pond o 8 ortTo o o o o o 0 0.1 2 0 0.1 2 0 DAPHNIA PULEX 10° C. - U.B.C. Pond o Q -o-O 0 j6o® DAPHNIA PULEX 10° C. - U.B.C. Pond O o o o o 0 / DAPHNIA PULEX O %'§' 10° C. - U.B.C. Pond 0 5 FOOD CONCENTRATION (jug./ml.) 10 9 4 9 5 0 .2 Figure 21(f) -c \ c o c Q. 0 O O N 0.2 oS \ "D OJ f— CO OJ CD c D) Lu 0 AT 0 .2 cc CD Q LU LU U_ 0 o o 8 0 DAPHNIA P U L E X Clone 2 13° C. - Rock L. O -©-O o o DAPHNIA P U L E X Clone 2 I3°C. - Rock L. O .D O FOOD CONCENTRATION (jug/ml) DAPHNIA P U L E X Clone 2. 13° C - Rock L. o o ry f • DAPHNIA PULEX Clone 2 oo cl 13° C. - Rock L. O u 8 35 9 6 0.125 Figure 21(g) -c \ b o N CJ) •3 CD CO CU Cn c 0 ^ 0.125 UJ K CC CD Q LU LU L L 0 DAPHNIA PULEX 19° C. - U.B.C. Pond DAPHNIA P U L E X 19° C. O — -V 0 25 FOOD CONCENTRATION (jug/ml) Figure 21(h) 0.3 -O- -o-DAPHN'IA PULEX 20° C. - Deer L \ o c D Q. O O N 0 C n 0.3 \ "O CD (/> <D C n c C n x — UJ <c 0 cc 0.3 CD Q Lu LU U_ 0 0 DAPHNIA P U L E X o 20° C. - Deer L. 0 O DAPHNIA PULEX 20 ° C. - Deer L. / ° 25 FOOD CONCENTRATION (jug/ml.) 98 O 0.2 Lu s. Q Lu Uj 0 0 4 Figure 2 1(i) / o o / 3 DIAPTOMUS TYRELLI I 2 ° C - Eunice L. ~0~ DIAPTOMUS TYRELLI 15° C - Eunice L. DIAPTOMUS T Y R E L L I 15° - Eunice L. 8 O o . o 0 DIAPTOMUS T Y R E L L I 19° C - Eunice L. o 12 6 FOOD CONCENTRATION (jug/ml) 0.25 0 \ c o c 0.25 O O N Cn R \ •o cu CO OJ Cn c 0 Cn UJ h-CC CD Q LU LU U. 0 0.25 0 Figure 2 1(j) o O O DIAPTOMUS KENAI 8° C. - Eunice L . DIAPTOMUS KENAI 8° C. - E u n i c e L . •© @-10 20 o o 0 • o D I A P T O M U S K E N A I 1 2 ° C. - E u n i c e L. 0 10 FOOD CONCENTRATION (/Ug./ml.) 0.075 0 ^ 0 .075 c o c _o "3. o CD CD Co OJ CJ) c Q LU UJ U. 0 0 O i ^ 0.075 Lu h-§ CD 0-0 Figure 21.(k) O DIAPTOMUS KENAI 15° C - Eunice L. DIAPTOMUS KENAI 15° C. - Eunice L. O 5 ~ i — 10 5 10 FOOD CONCEN TRA TION (/ug/ml) T5 o 15 DIAPTOMUS KENAI ' 19°C.-.Eunice L . 0 loo 15 101 o :3 Figure .21.(1) 0 0.3 \ c o c o CL o o N D> \ "O CD CO CD CD c 0 0 .25 0 CERIODAPHNIA SP 15° C. - Loon L. o f 8 0 o CERIODAPHNIA SP 19° C. - U.B.C. Pond CERIODAPHNIA SP 19° C. - U. B.C. Pond O o / 9/ CERIODAPHNIA SP 15° C. - L o o n L. 25 FOOD CONCENTRATION (jug./ml. ) FOOD CONCENTRATION (jug./ml.) 103 Figure 21(n) 0.08 Lu cc \ - 0.04 c o -+— c o CL o o N 0 CDO .08 N •: CD OJ Q Lu Lu LU 3 CO OJ c? 0.04 0 DIAPHAN0S0MA B R A C H Y U R U M 1 5 ° C - Eunice L . o O O O O O D I A P H A N O S O M A B R A C H Y U R U M 1 5 ° C - Eunice L . 1 ©° 0 . 8 FOOD CONCENTRATION (ug/ml) 16 105 Figure 21 (p) 0.2 O . N -c 0 HOLOPEDIUM GIBBERUM o 14° C - Eunice L. o ^ - ^ ^ o c 0.2 -C L O O N C n 0.1 -\ "O CD to CU Oi C o> 0 -=5 0 .2-LU h-cc CD O.J -Q LU LU L L 0 -0 HOLOPEDIUM GIBBERUM 14° C - Eunice L. u 0 7o o HOLOPEDIUM GIBBERUM 15° C - Placid L. 10 20 FOOD CONCENTRATION Lug / ml) 106 suggested by Parsons e t a l . (1967). The t h r e e models c o n s i d e r e d i n t h i s study were the d i s c e q u a t i o n , the He a l model, and the P u j i i f u n c t i o n . The l a t t e r two can be sigmoid or not i n shape, but they both i n c l u d e the d i s c equation as a s p e c i a l case. A n o n - l i n e a r l e a s t squares program UBC BHDX85 from the UBC Computer Centre was used to o b t a i n parameter e s t i m a t e s f o r f i t t i n g each o f equations (1),(7),and (8) t o the measured fe e d i n g c u r v e s . . T h i s program o b t a i n s a weighted l e a s t squares f i t of a user s p e c i f i e d f u n c t i o n t o data values by means of stepwise Gauss-Newton i t e r a t i o n s on the parameters. The f u n c t i o n g i v i n g t h e b e s t f i t to a s e t of data was s e l e c t e d by comparing f i n a l r e s i d u a l mean squares of the t h r e e f u n c t i o n s t r i e d . Each o f equations (1), (7), and (8) provided the best f i t f o r some s e t s of data, but no s i n g l e f u n c t i o n was found t o be c o n s i s t e n t l y best f o r a l l the s e t s of data. In f a c t , not even w e l l d e f i n e d subsets; o f data (e.g. a l l f e e d i n g curves f o r one sp e c i e s ) corresponded to a s i n g l e " b e s t " f u n c t i o n . / Some s p e c i e s f o r example showed sigmoid responses under c e r t a i n c o n d i t i o n s , but not under o t h e r s . Table I I summarizes these r e s u l t s . 70% o f the c u r v e s were be s t d e s c r i b e d by a d i s c or H i c h a e l i s - H e n t e n (type II) response, 30% by some form of type I I I response. Some of the responses b e s t f i t by a Real o r F u j i i f u n c t i o n , had parameter v a l u e s t h a t f o r a l l p r a c t i c a l purposes reduced these equations t o a TABLE fl. Parameter e s t i m a t e s o b t a i n e d by f i t t i n g e quations ( 1 ) , (7) and (8) t o the measured f e e d i n g c u r v e s . V i s i n u n i t s o f ug d r y weight ingested/ug d ry weight of 2 o o P l a n k t o n / h o u r . k, G are i n J ^ o ug dry weight seston/ml. EMS i s e r r o r mean square, blank e n t r i e s i n d i c a t e t h a t the non l i n e a r l e a s t squares procedure d i d not converge. Temp ni=n E a u a t i o n (equ. 1) Real E q u a t i o n (equ. 7) F u j i i Eq nati o n (equ. t i) S p e c i e s ° c V k EMS V G n EMS V k c EMS Daphnia p u l e x 10 0.042 0. 09 .0003 0. 047 0.26 0.4 3 .000328 0.042 0. 093 -0.007 0.000330 ti 10 0.040 0.3 .000274 0. 044 0.40 0.80 .000313 0.040 0.30 -0.004 0.000313 ii 10 0. 037 0.50 . 000059. 0. 04 0.81 0.80 .000064 0.037 0.58 -0.000005 0.000065 10 0.10 1. 62 .000465 0. 092 1.28 1.32 .000497 0.10 1. 62 0. 0008 ' 0.000505 H 13 0.07 0.42 .000125 0. 067 0.19 1.85 .000122 0. 067 0.59 0.73 0.000121 n 13 0.063 0.30 .000189 0.059 0. 012 3 .84 .000193 0. 061 1.4 9 3.79 0. 000175 ti 13 0. 066 0.64 .000535 0. 06 0.036 4.49 .000538 0.080 0. 95 -0.078 0.000563 it 13 0.10 1. 41 .000564 " 13 0.094 0.33 .00193S 0. 087 0. 016 3.66 .002014 0.118 0.57 -0.115 0.001983 ei 13 0.057 0.20 .000210 0.053 0. 011 3.88 .000212 0.059 0.3 0 -0.057 0.000227 II 13 0.068 0.79 .000172 0.065 0.8 7 2.30 . 000131 0.068 0.79 0.0 0.000186 ti 13 0.096 0. 40 .000557 0. 091 .0. 089 2 .69 .000444 0.105 0.48 -0.076 0.000587 it 19 0.186 14.44 .000048 0.186 14.45 0.00002 0.000055 II .19 0.045 0.80 .000031 0. 045 0.80 -0.0000035 0.000036 ti 20 0.2 9 8.87 . 0072 1.71 37.8 0.54 . 0081 0.29 8.87 0.0000001 0.0082 ti 20 0.26 2.81 . 00020 0.195 1. 93 1.59 .00013 0.259 2.81 0.00005 0.000233 ti 20 . 0. 08 0.74 . 0035 0. 084 0. 92 0.76 . 0040 i' ! " 20 0.27 1.79 . 0011 0.238 1.4 4 1.26 .0012 0.27 1.79 0.000002 0. 0012 TABLE I I . (cont'd.) Temp °C D i s c E a u a t i o n (equ. 1) Real Equation (equ. 7) F u j i i E quation (equ. 8) S p e c i e s V k EMS V G n EMS V k c EMS Daphnia r o s e a 14 0.125 2 . 63 000170 0. 157 3.30 0.7 4 .000175 0. 109 2 .50 o . 11 0. 000182 14 0. 048 0.178 .000108 0. 042 0.002 3.30 .000080 0. 043 0.4 9 5. 00 0. 000082 I I 15 0.10 2. 97 . 0028 0. 086 14.44 3. 05 .0029 15 0.064 1.30 . 001.65 0. 065 1.32 0.94 .00193 11 15 0.065 1.73 .000085 0. 065 1.73 0. 000026 0. 000097 il 15 C. 047 0.73 .00137 0. 045 0.75 1.73 .00154 0. 045 1.76 0. 64 0. 00152 I t 15 0.13 2 . 98 .00073 0. 158 3.49 0.75 .00082 0. 132 2.9 9 -0. 00039 0. 00083 I I 20 0. 25 12.24 .00106 0. 25 12.24 -1. 6 X 1 0 " 1 0 0. 00116 » 20 0.24 3.78 . 0033 it 20 0.13 1.56 . 0011 0. 13 1.56 0. 0 0. 0012 Diaphanosoma 15 0. 055 0. 003 .00027 0. 055 0. 086 1.98 .00029 0. 052 1. 047 5. 0 0. 00029 brachyurum 15 0. 025 0 . 97 .00014 0. 025 1.19 2 . 90 .00015 0. 055 2 .68 -0. 137 0. 00015 Diaptomus t y r e l l i 12 0.35 8 .66 .00031 21 .25 681. 0.88 . 00032. 1. ?5 73.5 0. 0028 0. 00034 » 15 0.047 1.17 .. 000061 0. 043 20.24 2 .88 .000063 0. 043 21. 05 0. 57 0. 000064 v • 15 0.0042 0.144 .000001 0. 039 0. 012 4.1 .000001 i\ 19 0.0088 1. 05 . .000014 0. 008 0. 0 J4 66.6 .000017 Diaptomus o r e q o n e n s i s 12 0.29 10.12 .000107 0. 29 10.12 -0. 0000025 0. 000116 12 0. 082 0.55 .000073 0. 085 0.64 1. 87 .000080 0. 073 2.37 1. 80 0. 000083 TABLE I I . (cont'd.) Temp D i s c E q u a t i o n (equ. 1) Real Equation (equ. 7) F u j i i Equation (equ. 8) S p e c i e s °C V k EMS V G n EMS V k . c EMS Holopedium gibberuir. 8 0.386 3.80 .00073 2. 54 25.8 0.48 .00040 0.386 3.80 0.0000035 0.00083 8 0.40 100. 0 0. 58 0.00032 H 12 0.22 1. 98 . 0020 0. 183 1.37 1.21 .00217 0.22 1. 98 -0.00000008 0.0022 II 12 0.205 0.81 .00117 0.166 0.35 1.81 .00115 4.17 21.2 -0.44 0.00102 II 14 0.106 7.14 .00055 0.15 7.14 0.99 .00056 0.119 5.79 0.088 0.00056 II 14 0. 03 0.056 . 0004 2 0.033 0. 001 3.48 .00038 4.17 23.1 -1.38 0. 00022 it 15 0.18 2.08 .00099 0.148 3.33 2. 64 .00102 0.15 5 .39 0.78 0.00097 Diaptomus k e n a i 8 0.287 6.81 .00157 0.287 6 .81 0.000001 1.56 0.0018 0. 0000002 II 8 0. 041 2. 98 .00001 0.023 0.79 2.53 .0000003 0. 023 6.32. II 12 0.074 1.17 .00007 0.063 0.68 1.58 .00007 0. 074 1.17 0.00015 0.00007 II 14 0. 097 6.19 .00007 0.065 10.. 9 2.84 .00003 II 14 0. 016 1.34 .000011 0. 010 0. 012 • 7.3.5 . 000010 0. 016 1.34 0.00066 0.000012 II 19 0.016 0.2 4 .000042 0.015 0. 047 2.55 .000043 0. 015 0.75 2. 92 0. 000043 C e r i p d a p h n i a 15 0.108 0.96 .00723 0.100 0.84 1.47 .00839 • . sp. 15 0.365 6.76 .000778 » 19 0. 053 0.41 .00016 0. 002 0.3 5 1.19 .00018 i» 19 0. 35 4.10 110 d i s c e g u a t i o n . Values f o r maximum f e e d i n g r a t e V were those obtained by f i t t i n g a d i s c e g u a t i o n to the e x p e r i m e n t a l l y determined f u n c t i o n a l responses. E s t i m a t e s o f maximum f i l t e r i n g r a t e " a " were obtained by measuring th e s l o p e of t & e f i t t e d d i s c equations near the o r i g i n . F i g u r e s 22 - 27 g i v e a g r a p h i c a l r e p r e s e n t a t i o n of the f e e d i n g parameters found f o r 6 s p e c i e s of zooplankton. There i s no c l e a r d i v i s i o n of p o i n t s i n t o "high a, low V", o r "high V, low a" groups. F i g u r e s 28, 29 and 30 show the same data grouped by l a k e i n s t e a d o f by s p e c i e s . There i s e s s e n t i a l l y no d i f f e r e n c e from lake t o l a k e i n the parameter " a " . Most b i o l o g i c a l processes can be r e p r e s e n t e d as dome-shaped f u n c t i o n s of temperature. F i g u r e s 31, 32 and 33 show t h e temperature v a r i a t i o n of maximum f e e d i n g r a t e s "Vtt. The temperature range covered (8 - 20 °G) was not g r e a t enough t o d e f i n e any complete domes, but w i t h i n t h a t range, t h r e e s p e c i e s ( Holopedium qibberum, Diaptomus ken a i and Diaptomus t y r e H i ) had maximum f e e d i n g r a t e s t h a t ; d e c l i n e d with i n c r e a s i n g temperature, and two s p e c i e s ( Daphnia pulex and Daphnia rosea) had r a t e s t h a t i n c r e a s e d . F i g u r e s 34, 35 and 36 show maximum f i l t e r i n g r a t e p l o t t e d a g a i n s t temperature; no c l e a r r e l a t i o n s h i p i s apparent. f 0 .3 0.2 1 O DAPHNIA ROSEA 0. 0 O D o o 0 0 o o 0 0.1 0.2 MAXIMUM INGESTION R A T E " v " . ( jug/ / jg / hr) Figure 22. I n i t i a l slope "a" of functional response vs. maximum .feeding rate "V" for Daphnia rosea. c CD H -H CD r o < H co 3 • H -r t 3 p -pj 0) 3 co 3 O t) Hi CD CD CD = Ch (a H - = O Ml pJ Ml r t C CD 3 Q = r t < H -= O '3 Ml fJ O M a CD cu co V o H - CO OJ CD X ;z CD m co H o >• m < tr M A X . F I L T E R I N G R A T E o ( ml / jug / hr) ZTT =1 L U I -< rr (3 cc L U X < 0.2 0. 0 O O 0 O O DIAPTOMUS KENAI O 0. 0.2 O 0.3 M A X I M U M INGESTION R A T E "v" (jug ./jug/hr ) Figure 24. I n i t i a l slope "a" of functional response vs. maximum feeding rate "V" for Diaptomus kenai. \ 0 . 3 CP £ LxJ 0.2 < CC . CD ZZ. cc 0.1 LU 1— _| \L X 0 < CERIODAPHNIA SP O O O O 0 0.1 0.2 MAXIMUM INGESTION R A T E " V " (/jg / jug / hr) 0.3 Figure 25. I n i t i a l slope "a" of functional response vs. maximum feeding rate "V" for Ceriodaphnia. o HOLOPEDIUM GIBBERUM O o ° T 1 : •• i 1 0 0.1 0.2 0.3 0.4 c e 3 5 MAXIMUM INGESTION RATE " V . " (pq / »q / hr) Figure 26. I n i t i a l slope "a" of functional response vs. maximum feeding rate "V" for Hoiopedium  gibberum. 0 . 3 0 . 2 H 0.1 A 0 DIAPTOMUS TYRELLI :0 DIAPTOMUS OREGONENSIS: A A O o A O O 0 0.1 0 .2 0 .3 M A X I M U M I N G E S T I O N R A T E " V " (ug / ug / hr ) Figure 2.7. I n i t i a l slope "a" of functional response vs. maximum feeding rate "V" for Diaptomus  t y r e l l i and Diaptomus oregonensis. 0.5 0.4 0.3 0.2-0. 0 0 0. UBC POND • Daphnia pulex O Ceriodaphnia sp-0.2 O 0.3 0.2 0.1 0 LOON LAKE O Daphnia rosea O Ceriodaphnia sp. O 0.3 0.4 0 0. M A X I M U M F E E D I N G R A T E V 0.2 0.3 O 0.4 Figure 28. Feeding parameters of species taken from UBC Pond and Loon Lake. 0 D E E R L A K E • Daphnia pulex 0.1 0.2 0.3 0.3H 0.21 0.1 0 0 P L A C I D L A K E • Dophnia rosea O Holopedium gibberum A Diaptomus oregonensis 0.1 O O A 0.3 0.21 O.I 0 0.2 0.3 0 K A T H E R I N E L A K E Daphnia pulex O.I 0.2 0.3 M A X I M U M F E E D I N G R A T E V Figure 29. Feeding parameters of species taken from Deer Lake, Placid Lake and Katherine Lake. 0 . 6 H 0 . 5 0 . 4 1 0 . 3 1 0 . 2 O.I 4 0 • EUNICE LAKE • Holopedium gibberum A Diaptomus kenai • Diaptomus tyrelli W O Daphnia rosea A Diaphanosoma brachyurum o • A A • o A . D r i ^ A A * 0.5-^ 0 . 4 0 . 3 0 . 2 0 . 0 Laboratory-reared Daphnia pulex from ROCK LAKE A Clone I , H O Clone 2, H • Clone I, L A Clone 2, L H= reared on high food cone. M I I , II I I L = low o • A A • • 0 O.I 0 . 2 0 . 3 - 0 . 4 0 0 . 0 . 2 0 . 3 0 . 4 MAXIMUM FEEDING R A T E V Figure 30. Feeding parameters of species taken from Eunice Lake and Rock Lake. 120 0.3 T E M P E R A T U R E (°C.) Figure 31. Temperature v a r i a t i o n of maximum feeding rates for Daphnia rosea and Daphnia pulex. 0 .4 H > 10 12 14 T E M P E R A T U R E ( ° C ) T E M P E R A T U R E ( ° C ) Figure 32. Temperature v a r i a t i o n of maximum feeding rates for Holopedium gibberum and Diaptomus kenai. 122 Figure 33 Temperature v a r i a t i o n of maximum rates for Diaptomus t y r e l l i . feeding 123 U J I -< rr CD zz cr: UJ X < T E M P E R A T U R E (°C) Figure 34. Temperature v a r i a t i o n of maximum f i l t e r i n g rates for Daphnia rosea. 124 0.3 0- o CD 0.2 o r r S - L LU o _ J — C P ^ 3 x < 0. 0 o o DAPHNIA PULEX 8 Q o O 6b ooo 8 O o O n ^ 1 ' 10 20 TEMPERATURE (°C) Figure 35. Temperature v a r i a t i o n of maximum f i l t e r i n g rates for Daphnia pulex. 125 A c o cz _o CL O o N cn =3 0 . 5 H O DIAPTOMUS KENAI A HOLOPEDIUM GIBBERUM U J < 0 . 2 5 - A rr U J x < 0 A O O A A O 0) O 1 0 A ~~r~ 2 0 T E M P E R A T U R E (°C) Figure 36. Temperature v a r i a t i o n of maximum f i l t e r i n g rates for Diaptomus kenai and Holopedium gibberum. ' 126 5 DISCUSSION 5.1 C o n s i s t e n c y With Other Data Before d i s c u s s i n g s p e c i f i c r e s u l t s of ray study, i t i s worthwhile comparing the f e e d i n g r a t e s o b t a i n e d here with r a t e s r e p o r t e d i n the l i t e r a t u r e , i n order to check t h a t my r e s u l t s are a t l e a s t c o n s i s t e n t with o t h e r measurements f o r the same s p e c i e s . However, comparing ray r e s u l t s with those o b t a i n e d by o t h e r s i s a f a i r l y arduous and u n c e r t a i n t a s k , because of the v a r i e t y of t e c h n i q u e s , c o n d i t i o n s and u n i t s employed by d i f f e r e n t workers, a l s o , f e e d i n g r a t e s are o f t e n measured and r e p o r t e d f o r s i n g l e v a l u e s of food c o n c e n t r a t i o n only ( g e n e r a l l y as "maximum f i l t e r i n g r a t e " - i . e . f i l t e r i n g r a t e f o r very low food c o n c e n t r a t i o n s ) . Such a comparison i s nonetheless v a l u a b l e f o r the c r e d i b i l i t y (or l a c k of i t ) t h a t i t may l e n d the r e s u l t s . The f o l l o w i n g paragraphs d i s c u s s the present work i n the l i g h t of a few o t h e r i n v e s t i g a t i o n s t h a t may be c o n s i d e r e d comparable i n t h a t they d e a l with the same s p e c i e s and seem t o use sound t e c h n i q u e s . Burns and B i g l e r (1967) and Burns (1969) have p u b l i s h e d c o n s i d e r a b l e data on the f i l t e r i n g r a t e s of s e v e r a l s p e c i e s o f Daphnia . Host of t h e i r experiments used 3 2 P - l a b e l l e d y e a s t 127 as the only source of food and most measured the maximum f i l t e r i n g r a t e - i . e . the f i l t e r i n g r a t e observed i n very d i l u t e food c o n d i t i o n s . A comparison o f Daphnia r o s e a f i l t e r i n g r a t e s as measured by Burns and B i g l e r (1967) and by me i s shown i n F i g u r e 37- giv e n t h a t the techniques used were not the same, the r e s u l t s a re remarkably s i m i l a r . I f i n f a c t f e e d i n g r a t e response i s an adaptable t r a i t t h e r e i s no reason the r e s u l t s should be i d e n t i c a l , j u s t comparable w i t h i n the p h y s i o l o g i c a l c a p a b i l i t i e s of the animals. F i g u r e 38 shows f i l t e r i n g r a t e s measured i n t h i s study p l o t t e d with Burns and Rigler»s (1967) r e s u l t s on maximum f i l t e r i n g r a t e as a f u n c t i o n of body l e n g t h - Since t h e i r measurements were done with yeast as the only food (at 2-5 x 10* c e l l s / m l ) , the comparison i s s u b j e c t t o t h e d i f f i c u l t y of i n t e r p r e t i n g t h e se s t o n ash f r e e dry weight e q u i v a l e n t o f t h i s yeast c o n c e n t r a t i o n - There i s n e v e r t h e l e s s c o n s i d e r a b l e o v e r l a p p i n g o f p o i n t s , i n d i c a t i n g t h a t my measurements are not i n c o n s i s t e n t . Haney's (1973) i n s i t u study o f ambient f i l t e r i n g r a t e s d u r i n g s e v e r a l seasons i n Heart Lake y i e l d e d v a l u e s comparable to mine (Table I I I ) f o r Daphnia r o s e a . C e r i o d a p h n i a guadrangula, Diaphanosoma brachyurum. Diaptomus oregonensis, and Holopedium qibberum. For 3 of the 5 s p e c i e s , the range of f i l t e r i n g r a t e s measured i n t h i s study i s c o n s i d e r a b l y l a r g e r than t h a t found by Haney, probably because I used a wider 2 . 0 A o T3 LxJ H < rr C D rr LJ f -_J DAPHNIA ROSEA 20° C. This study. Body length 1.28mm. —6— Burns a Rigler (1967). Body length 1.6 mm. — B u r n s & Rigler (1967) resul ts conver ted to rates for 1.28 mm. body length, using their f i l ter ing rate - body length relationship. -332) 10 15 2 0 2 5 F O O D C O N C E N T R A T I O N ( u g / m l dry weight ) F i g u r e 37. Comparison o f Daphnia r o s e a f i l t e r i n g r a t e s measured i n t h i s s t u d y w i t h t h o s e measured by Burns & R i g l e r (1967) . i to CO 129 A 0.6 0.8 1.0 1.2 1.4 1.8 BODY L E N G T H (mm.) Figure 38. F i l t e r i n g rates of Daphnia rosea and Daphnia pulex as a function of body length. Table I I I . Comparison of f i l t e r i n g rates measured i n t h i s study with those measured by J.F. Haney (1973). Species Haney (1973) (ml./ind./day) High Low This study (ml./ind./day) High Low Daphnia rosea Ceriodaphnia quadrangula Diaphanosoma brachyurum Diaptomus oregonensis Holopedium gibberum 20.8 1.7 7.7 0.4 5.7 0. 2.2 0. 12.5 0. 25.6 0.6 4.5 0.05 5.1 0.02 19.4 0.5 119.5 0.1 131 range of food c o n c e n t r a t i o n s , bounded by a r t i f i c i a l l y low and high v a l u e s . My Ceriodaphnia were s m a l l e r than Haney*s and were t e s t e d over a s m a l l e r temperature range. My Diaphanosoma were measured at o n l y one temperature. T h i s c o u l d e x p l a i n the s m a l l e r v a r i a t i o n i n f i l t e r i n g r a t e s found i n t h i s study f o r these two s p e c i e s . F i l t e r i n g r a t e s f o r Daphnia pulex i n d i f f e r e n t combinations and c o n c e n t r a t i o n s of l a k e and tap water, pu b l i s h e d by Crowley (1973), appear t o be almost i d e n t i c a l t o r a t e s measured here f o r D. pulex a t the same temperature. Haney and H a l l (1975) measured daytime f i l t e r i n g r a t e s o f 5 - 15 m l . / i n d i v i d u a l / d a y f o r Daphnia pulex a t temperatures around 20 °C. I found r a t e s of 8 - 33 m l / i n d i v i d u a l / d a y f o r D. pulex of comparable s i z e a t 19 - 20 °C and s i m i l a r food c o n c e n t r a t i o n s , i n d i c a t i n g t h a t t h e s e r a t e s would probably correspond t o the end of the l i g h t c y c l e (my experiments were done in e a r l y e v e n i n g ) . But the p o s s i b l e e x i s t e n c e of d i e l g r a z i n g c y c l e s f o r some s p e c i e s , whose magnitude and t i m i n g have not been q u a n t i f i e d , r a i s e s some doubt as t o how a c c u r a t e l y these f i l t e r i n g r a t e measurements r e f l e c t o v e r a l l g r a z i n g r a t e s . Furthermore, i f d i e l p a t t e r n s vary a g r e a t d e a l among s p e c i e s , then measurements of f i l t e r i n g r a t e such as I have done a r e i n s u f f i c i e n t as a b a s i s f o r comparing fe e d i n g s t r a t e g i e s among s p e c i e s . 132 5.2 Types Of F u n c t i o n a l Response Obtained S e v e r a l of the standar d f i l t e r - f e e d i n g models proved adequate t o d e s c r i b e most of the f u n c t i o n a l responses measured here, but no s i n g l e model c o n s i s t e n t l y d e s c r i b e d t h e response t o d i l u t e f o o d . The f u n c t i o n a l responses measured f o r i n d i v i d u a l s p e c i e s showed c o n s i d e r a b l e q u a n t i t a t i v e and even some q u a l i t a t i v e v a r i a t i o n ( i . e . most of the responses were Type I I , but t h e r e were a few sigmoid responses as w e l l ) . P a r t of t h i s v a r i a b i l i t y seemed to be due t o temperature, but t h e r e was a l s o evidence t o suggest that p a r t of i t may i n d i c a t e a process of a c c l i m a t i o n t o changes i n food s u p p l y . Although the l i t e r a t u r e abounds with f i l t e r - f e e d i n g models, the c h a r a c t e r i s t i c s o f the models i n the r e g i o n o f d i l u t e f o o d c o n c e n t r a t i o n s are g e n e r a l l y much too f i n e - s c a l e d to be compared with experimental r e s u l t s , which seem to be h i g h l y v a r i a b l e f o r low a v a i l a b l e f o o d . S p e c u l a t i o n about the exact form of the f e e d i n g response i n d i l u t e f o o d c o n d i t i o n s i s e x c i t i n g from a t h e o r e t i c a l p o i n t of view, but presen t techniques are not capable of t e s t i n g the th e o r y . T h i s p o i n t o f t e n seems t o be unacknowleged. For example, H u l l i n , F u g l i s t e r Stewart, and F u g l i s t e r (1975) could not s t a t i s t i c a l l y r e j e c t any o f t h r e e d i f f e r e n t f e e d i n g c u r v e s f i t t e d t o f r o s t ' s (1972) data. 133 5.3 Constancy Of F u n c t i o n a l Responses I f t h i s study has y i e l d e d any c l e a r r e s u l t , i t i s s u r e l y t h a t i n d i v i d u a l f u n c t i o n a l responses of f i l t e r - f e e d e r s are v a r i a b l e commodities, changing with temperature, past food c o n d i t i o n s and probably s e v e r a l unknown f a c t o r s . In t h i s study t h e r e was as much v a r i a b i l i t y among f u n c t i o n a l responses of a gi v e n s p e c i e s (at a given temperature and from a g i v e n lake) as t h e r e was among responses between s p e c i e s . The range o f v a r i a t i o n however, was very s i m i l a r f o r most of the s p e c i e s , and pa r t of the v a r i a b i l i t y , namely temperature dependence, may be q u i t e p r e d i c t a b l e and c h a r a c t e r i s t i c of the s p e c i e s . I t a l s o appears t h a t maximum f i l t e r i n g r a t e s (or f e e d i n g r a t e s a t very low food c o n c e n t r a t i o n s ) may be q u i t e c o n s t a n t , independent o f temperature, and the same f o r most o f the s p e c i e s (see F i g u r e s 34-36). 5.3.1 E f f e c t Of Temperature To my knowledge, t h e r e had not appeared i n t h e l i t e r a t u r e any measurements o f temperature v a r i a t i o n o f maximum f e e d i n g r a t e s f o r f i l t e r f e e d i n g zooplankton. The only r e f e r e n c e s t o temperature and f e e d i n g have been o b s e r v a t i o n s , f o r a few s p e c i e s , o f the e f f e c t o f temperature on maximum f i l t e r i n g 134 r a t e - o r , e q u i v a l e n t l y , on f e e d i n g r a t e at Ion food c o n c e n t r a t i o n s . The temperature v a r i a t i o n of the f u n c t i o n a l responses measured i n t h i s study i n d i c a t e d t h a t i t might be p o s s i b l e t o c l a s s i f y the s p e c i e s according to the temperature dependence o f t h e i r f e e d i n g parameters. Highest observed f e e d i n g r a t e s f o r Daphnia r o s e a and Daphnia pulex occurred a t around 20 °C, and f o r Holopedium qibberum. Diaptomus kenai and Diaptomus  t v r e l l i around 8-12 °C (F i g u r e s 31 - 33). However, maximum f i l t e r i n g r a t e s f o r a l l s p e c i e s (with the p o s s i b l e e x c e p t i o n of Daphnia pulex ) seemed t o be q u i t e independent of temperature ( F i g u r e s 3 4 - 3 6 ) . Maximum f i l t e r i n g r a t e s of s e v e r a l s p e c i e s of Daphnia have been observed t o i n c r e a s e with temperature to maxima around 20 - 25 °C (Burns, 1969; H a l l * 1964; Burns and B i g i e r , 1967; HcBahon, 1965). However S c h i n d l e r (1968) found f i l t e r i n g r a t e s o f Daphnia magna t o be u n a f f e c t e d by temperature. Kibby <1971) . a f t e r r e a r i n g Daphnia rosea a t 12 °C f o r s e v e r a l g e n e r a t i o n s , found i t s maximum f i l t e r i n g r a t e to be h i g h e s t around 12 - 14 °C. He concluded t h a t c l a d o c e r a n s tend t o f i l t e r most r a p i d l y under c o n d i t i o n s o f temperature s i m i l a r t o those under which they were c o l l e c t e d or c u l t u r e d . T h i s probably e x p l a i n s the temperature i n v a r i a n c e of the maximum f i l t e r i n g r a t e s measured i n t h i s study, because f e e d i n g experiments were always done a t the 135 temperature of the water from which the animals were c o l l e c t e d . I am not aware o f any p u b l i s h e d measurements on temperature dependence of f e e d i n g r a t e f o r my " c o l d " s p e c i e s , but Holopedium gibberum and D i a p t o m u s o r e g o n e n s i s have been c l a s s i f i e d as c o l d water s p e c i e s l i m i t e d by temperature i n t h e i r g e o g r a p h i c a l d i s t r i b u t i o n (Pennak, 1953). The s e a s o n a l p a t t e r n of zooplankton d e n s i t i e s i n the UBC F o r e s t Lakes a l s o i n d i c a t e s t h a t temperature a d a p t a t i o n o c c u r s ( B a i t e r s , p e r s . comm.; H e i i l , p ers. comm.). / S p e c i f i c a l l y , diaptomids are b e l i e v e d t o be a c t i v e a l l winter. They are the most abundant s p e c i e s i n the e a r l y s p r i n g , a t which time Holopedium p o p u l a t i o n s s t a r t to i n c r e a s e r a p i d l y . Diaptomid and Holopedium p o p u l a t i o n s peak near the b e g i n n i n g of J u l y , then d e c l i n e and remain low through August and September, with a second s m a l l e r peak i n October and November. In o t h e r words, t h e i r p e r i o d s of most r a p i d growth c o i n c i d e w i t h p e r i o d s of r e l a t i v e l y c o o l water. Daphnia on the other hand, do not become numerous u n t i l mid- J u l y ; they i n c r e a s e t o a peak a t the beginning of August and remain abundant u n t i l the end o f October. Daphnia reproduce throughout the season, whereas Holopedium c o n t a i n eggs o n l y . i n s p r i n g and f a l l . During the summer. Hovement p a t t e r n s a l s o i n d i c a t e t h a t Holopedium and Diaptomus ke n a i p r e f e r c o o l e r waters. Holopedium tend t o descend t o c o o l e r waters as the summer temperature i n c r e a s e s - p. k e n a i e x h i b i t pronounced v e r t i c a l 136 m i g r a t i o n s , but as temperatures i n c r e a s e , they descend to g r e a t e r depths d u r i n g the day; i n P l a c i d Lake, which i s very shallow, they a c t u a l l y have a summer diapause i n the bottom mud. An e n c l o s u r e experiment by H e i l l ( pers. comm.) i n Gwendoline Lake f u r t h e r i l l u s t r a t e s Holopedium*s a f f i n i t y f o r c o l d temperatures. Each e n c l o s u r e was a column of l a k e water en c l o s e d by a l a r g e (10,000 1) p l a s t i c bag. In the c o n t r o l bag Holopedium had completely disappeared by the t h i r d week of J u l y , when the water temperature reached 14-15 °C. In a second bag a s m a l l pump kept the water c i r c u l a t i n g and c o o l e r . In t h i s bag Holopedium p e r s i s t e d u n t i l the end of August, by which time the water temperature had f i n a l l y reached 14-15 °C. Of course the temperature dependence o f f e e d i n g r a t e s cannot be expected to r e v e a l a completely unambiguous p i c t u r e o f zooplankton temperature p r e f e r e n c e , s i n c e metabolic r a t e s a l s o vary with temperature, and i t i s the d i f f e r e n c e between the two r a t e f u n c t i o n s t h a t determines the temperature a t which net energy g a i n w i l l be maximal. However, i t seems t h a t f o r some s p e c i e s a t l e a s t the maximum r a t e s f o r f e e d i n g and net energy i n t a k e occur a t about the same temperature (Green, 1975). From a s t a t i s t i c a l v iewpoint, one might be s k e p t i c a l about my o b s e r v a t i o n of temperature independence of maximum f i l t e r i n g r a t e s , because the l a t t e r v a l u e s were obtained from 137 the s l o p e (near the o r i g i n ) of the f i t t e d f u n c t i o n a l responses, and t h e r e was a g r e a t d e a l o f v a r i a b i l i t y i n f e e d i n g r a t e s f o r low food c o n c e n t r a t i o n s . However, i f I assume t h a t the r e s u l t i s v a l i d , i t c e r t a i n l y deserves f u r t h e r d i s c u s s i o n . I have suggested t h a t temperature a c c l i m a t i o n may be r e s p o n s i b l e f o r t h i s r e s u l t , but then why should maximum f e e d i n g r a t e s "V M not a l s o be temperature independent? And although the f e e d i n g measurements were c a r r i e d out a t temperatures s i m i l a r t o those of the water, from which zooplankton were c o l l e c t e d , the zooplankton do not normally remain a t one temperature d u r i n g the course of a day. Lake temperatures vary with depth, and zooplankton do not s t a y a t j u s t one depth, so they cannot have adapted t o j u s t one temperature. Host of the animals {other than D. pulex ) t e s t e d i n t h i s study were taken from the U B C F o r e s t Lakes. I t i s p o s s i b l e t h a t the temperature i n v a r i a n c e o f t h e i r maximum f i l t e r i n g r a t e s r e f l e c t s an adaptation t o t h i s p a r t i c u l a r environment, and t o a d a i l y range i n temperature. I f "V" depends on temperature, but " a " does not, then the f e e d i n g r a t e s a t u r a t e s a t d i f f e r e n t food c o n c e n t r a t i o n s depending on temperature, but f o r low food c o n c e n t r a t i o n s temperature w i l l have no e f f e c t on f e e d i n g r a t e . Now seston l e v e l s i n the OBC F o r e s t Lakes a re almost always very low ( g e n e r a l l y between 0.8 and 1.5 ug/ml, as h - f r e e dry weight).. Some zooplankton are known t o undergo 138 c o n s i d e r a b l e v e r t i c a l m i g r a t i o n on a d a i l y b a s i s , which means tha t d a r i n g most of the year they experience c o n s i d e r a b l e d a i l y temperatare v a r i a t i o n . Surface water temperatures can vary suddenly a s a r e s u l t of heavy r a i n s , f o g , wind, e t c . I t seems g u i t e r easonable t h a t the zooplankton might have adapted to such c o n d i t i o n s by developing a wide temperature t o l e r a n c e over a range o f low food c o n c e n t r a t i o n s . &n a l t e r n a t i v e h y p o t h e s i s might be t h a t under low f o o d c o n d i t i o n s , the hunger l e v e l remains s u f f i c i e n t l y high t h a t t h e f i l t e r i n g process i s always turned on, and always a t i t s maximum r a t e . The value o f t h i s maximum, r a t e would be determined o n l y by the mechanics of muscle movement, e t c . I f zooplankton muscle enzymes had a f a i r l y f l a t response t o temperature over some range, or i f the zooplankton had s e v e r a l d i f f e r e n t muscle enzymes, each with d i f f e r e n t temperature optima, then the animal would be a b l e t o operate with e q u a l motor a b i l i t y over t h i s temperature range. On the other hand, when food i s abundant, the hunger l e v e l would be l i m i t e d more by the r a t e o f d i g e s t i o n , which i n t u r n might be c o n t r o l l e d by the r a t e of p r o d u c t i o n of d i g e s t i v e enzymes. P o s s i b l y s p e c i e s adapted i n v a r i o u s ways to d i f f e r e n t temperature regimes have ev o l v e d d i g e s t i v e enzymes with d i f f e r e n t temperature optima. 139 5.3.2 ftcclimation To Food C o n c e n t r a t i o n V a s t l y d i f f e r e n t f u n c t i o n a l responses were obtained f o r Diaptomus oregonensis simply by v a r y i n g the l e n g t h of time between c o l l e c t i o n of the animals and measurement of f e e d i n g r a t e s (see F i g u r e 16|. One p o s s i b l e e x p l a n a t i o n i s that the zooplankton were g r a d u a l l y d e p l e t i n g t h e i r food i n the h o l d i n g t a n k s . although the l a k e water i n the v e s s e l s was r e g u l a r l y changed, the whole supply of l a k e water used was c o l l e c t e d a t one time and p l a c e , and i t i s p o s s i b l e t h a t i t might have been taken from a patch c o n t a i n i n g r e l a t i v e l y l i t t l e f o o d . I f t h i s was so, then i t c o u l d have been i n c r e a s i n g hunger t h a t caused the i n c r e a s e i n f i l t e r i n g r a t e s . The change i n form o f the f u n c t i o n a l response i s more p u z z l i n g than the change i n magnitude. I n i t i a l l y t here was an i n h i b i t i o n o f f i l t e r i n g r a t e a t low food c o n c e n t r a t i o n s , but a f t e r 5 days i t d i s a p p e a r e d . Such an i n h i b i t i o n i n f i l t e r i n g r a t e means t h a t the c o r r e s p o n d i n g f e e d i n g r a t e curve i s sigmoid. In other words, d u r i n g the 5 days o f t h i s experiment, the f u n c t i o n a l response was sigmoid a t f i r s t s and f i n a l l y became a simple type I I response. H o l d i n g zooplankton a t d i f f e r e n t food regimes f o r s e v e r a l days a l s o produced d i f f e r e n t f e e d i n g curves (see F i g u r e s 17 and 18). Animals held i n high food d e n s i t i e s had lower f i l t e r i n g r a t e s a t low food c o n c e n t r a t i o n s than animals h e l d 140 i n untreated l a k e water. T h i s c o u l d be because they were s a t i a t e d from 5 days of r i c h f e e d i n g . There was a l s o an i n d i c a t i o n of i n h i b i t i o n o f f i l t e r i n g r a t e s a t low c o n c e n t r a t i o n s , p o s s i b l y because the animals were not s u f f i c i e n t l y hungry to expend a l a r g e e f f o r t f o r s m a l l r e t u r n s . Animals h e l d i n o r d i n a r y l a k e water f o r 5 days showed the c l a s s i c f i l t e r i n g r a t e curve t h a t corresponds t o a type I I f u n c t i o n a l response. T h e i r f i l t e r i n g r a t e s were much h i g h e r at low food c o n c e n t r a t i o n s than those of the w e l l f e d zooplankton, and the same or s l i g h t l y lower at high food c o n c e n t r a t i o n s . The r e d u c t i o n a t high c o n c e n t r a t i o n s c o u l d be caused by the f a c t t h a t they were i n a r i c h e r medium than they were used t o and were t h e r e f o r e g e t t i n g greater r e t u r n s per u n i t e f f o r t than they were used t o . The low c o n c e n t r a t i o n s however, kept them c o n s t a n t l y hungry and f i l t e r i n g f a s t e r . The zooplankton h e l d i n d i l u t e l a k e water had much lower f e e d i n g r a t e s than the other groups a t almost a l l food c o n c e n t r a t i o n s . They had probably been s t a r v e d t o o much and were i n very poor c o n d i t i o n . Length of a c c l i m a t i o n t o s p e c i f i c food c o n c e n t r a t i o n s seemed t o a f f e c t f i l t e r i n g r a t e measurements a t t h a t c o n c e n t r a t i o n mainly f o r low food c o n c e n t r a t i o n s (see F i g u r e 19). T h i s i n d i c a t e s t h a t the standard e x p e r i m e n t a l procedure of a l l o w i n g 2 - 3 hours f o r zooplankton to a c c l i m a t e to a 14.1 p a r t i c u l a r food c o n c e n t r a t i o n may be inappropriate.. 5.4 F u n c t i o n a l Response Adaptation T h i s study t r e a t s f e e d i n g r a t e f u n c t i o n a l response as an adaptable t r a i t and t r i e s t o answer the q u e s t i o n of whether the t r a i t has evolved i n o r d e r to enable the organism t o a t t a i n o p t i m a l t a c t i c s f o r d i f f e r e n t s e t s of environmental c o n d i t i o n s - s p e c i f i c a l l y f o r d i f f e r e n t s i t u a t i o n s o f food a v a i l a b i l i t y . In s e c t i o n 2.3 are l i s t e d some p r e d i c t i o n s about the values t h a t f u n c t i o n a l response parameters "V" and " a " c o u l d be expected t o assume under d i f f e r e n t c o n d i t i o n s , i f f e e d i n g s t r a t e g i e s a re important as adaptable t r a i t s . Depending on food and p r e d a t i o n c o n d i t i o n s , one would expect t o f i n d e i t h e r high v a l u e s of " a " , coupled with low va l u e s of rtV", or h i g h "T" with low "a". And except f o r e u t r o p h i c l a k e p o p u l a t i o n s l i m i t e d by p r e d a t i o n p r e s s u r e , the i n i t i a l s l o p e of the f u n c t i o n a l response should be independent of l a k e p r o d u c t i v i t y or p r e d a t i o n l e v e l . A c r o s s - l a k e comparison o f the f u n c t i o n a l responses measured here i n d i c a t e t h a t the food g a t h e r i n g c h a r a c t e r i s t i c s of zooplankton may indeed be adaptable t r a i t s . As p r e d i c t e d i n s e c t i o n 2.3, the i n i t i a l s l o p e s o f the f u n c t i o n a l responses were e s s e n t i a l l y the same f o r 1 4 2 o l i g o t r o p h i a and e u t r o p h i c waters (see F i g u r e s 28 - 30). There was no evidence s u p p o r t i n g the second p r e d i c t i o n , t h a t p r e d a t i o n l i m i t e d f i l t e r - f e e d i n g zooplankton from h i g h l y p r o d u c t i v e l a k e s should have h i g h e r maximum f e e d i n g r a t e s , and probably lower i n i t i a l s l o p e values. T h i s may i n d i c a t e o n l y t h a t the e u t r o p h i c l a k e p o p u l a t i o n s t e s t e d here were not t r u l y l i m i t e d by p r e d a t i o n . In f a c t i t may be d i f f i c u l t t o f i n d such p o p u l a t i o n s s i n c e the animals would tend t o evolve other medians!ms - such as change i n body s i z e or i n t i m i n g of l i f e h i s t o r y events - t o reduce p r e d a t i o n p r e s s u r e . Most o f the s p e c i e s s t u d i e d here had a very s i m i l a r range o f f u n c t i o n a l response parameters, and e s s e n t i a l l y i d e n t i c a l maximum f i l t e r i n g r a t e s "a" (see F i g u r e s 22 - 27). Maximum f i l t e r i n g r a t e s "a" f o r Daphnia r o s e a , Diaptomus t y r e l l i . Diaptomus k e n a i , and Diaptomus oregonensis were v i r t u a l l y i d e n t i c a l over a l a r g e range of M V H values. Parameters f o r Holopedium gibberurn and Daphnia pulex averaged about twice as : high, with much more v a r i a b i l i t y . C e r i o d a p h n i a was comparable t o the l a t t e r two s p e c i e s . The r e s u l t f o r IK Pulex i s somewhat s u r p r i s i n g s i n c e t h i s s p e c i e s tends t o e x i s t i n more e u t r o p h i c waters than does D. ro s e a . In some e n c l o s u r e experiments on an o l i g o t r o p h i c l a k e i n the UBC F o r e s t , H e i l l (unpublished data) has .found t h a t D. rosea outcompete D. pulex even i n the absence of p r e d a t i o n . T h i s again* i n d i c a t e s t h a t D. rosea should have a h i g h e r maximum f i l t e r i n g r a t e than 1.4.3 P.; pa l e x . .. Burns (1969) found Daphnia pulex f i l t e r i n g r a t e s t o be s i g n i f i c a n t l y lower than those of Daphnia r o s e a (Burns and B i g l e r , 1967) , although she c l a i m e d (Burns, 1969) t h a t they were s i m i l a r . I have no e x p l a n a t i o n f o r the c o n t r a r y r e s u l t s found here. 5.5 General Remarks Although a f a i r l y l i m i t e d range o f lake environments was examined, i t appears t h a t measurements of f i l t e r f e e d i n g f u n c t i o n a l response may be q u i t e g e n e r a l i n t h e i r g e o g r a p h i c a l a p p l i c a b i l i t y . However a problem : of time s c a l e makes i t d i f f i c u l t t o completely a s s e s s zooplankton f e e d i n g s t r a t e g i e s with the data gathered i n t h i s study. Zooplankton f u n c t i o n a l responses, as measured by any p r a c t i c a l method except perhaps an e l a b o r a t e type of continuous flow experiment, are i n s t a n t a n e o u s responses. Even long i n c u b a t i o n c e l l count r e s u l t s are i n s t a n t a n e o u s i n t h a t they are measured under constant c o n d i t i o n s , whereas " r e a l " zooplankton are m i g r a t i n g v e r t i c a l l y * encountering d i f f e r e n t temperature regimes, patchy d i s t r i b u t i o n s of food and p r e d a t o r s , and a r e ; undergoing v a r i o u s c i r c a d i a n rythms. The k i n d of f u n c t i o n a l response g e n e r a l l y r e q u i r e d by ecosystem models i s one which i s somehow averaged over a whole p o p u l a t i o n , over depths, temperature 144 p r o f i l e s and patches of d i f f e r e n t food c o n c e n t r a t i o n s encountered d a i l y . To measure such a response would be i m p o s s i b l e , so ins t a n t a n e o u s r a t e s or approximations based on ins t a n t a n e o u s r a t e s are used whenever f u n c t i o n a l responses a r e r e q u i r e d . T h i s i s a j u s t i f i a b l e approximation, but models using i t should be c a r e f u l l y t e s t e d f o r s e n s i t i v i t y t o d i f f e r e n t f u n c t i o n a l - response hypotheses. B e l i e f i n the models* p r e d i c t i o n s should be tempered by c o n s i d e r a t i o n of the hypotheses they c o n t a i n . S e v e r a l i n t e r e s t i n g r e s u l t s have emerged from t h i s study - t h e e f f e c t o f temperature on f u n c t i o n a l response, the hig h f e e d i n g r a t e s obtained a t low temperatures f o r some s p e c i e s , the changes i n f u n c t i o n a l response c o r r e l a t e d with time between zooplankton c o l l e c t i o n and f e e d i n g measurement, the e f f e c t on f u n c t i o n a l response o f p r i o r food c o n d i t i o n s , and the s i m i l a r i t y of feed i n g parameter v a l u e s over the d i f f e r e n t s p e c i e s and l a k e s . Some of these r e s u l t s do not g i v e a c l e a r i n d i c a t i o n of the und e r l y i n g mechanisms, but they do suggest s e v e r a l e x p e r i m e n t a l avenues which might y i e l d v a l u a b l e i n f o r m a t i o n on the f e e d i n g behavior of f i l t e r - f e e d i n g zooplankton. I t would be i n t e r e s t i n g , f o r example, t o do a more comprehensive g e o g r a p h i c a l and s e a s o n a l survey o f f u n c t i o n a l r e s p o n s e s f o r perhaps two s p e c i e s , one with h i g h e s t f e e d i n g r a t e s i n warm waters (e.g. Daphnia r o s e a ) . and the other with h i g h e s t r a t e s i n c o l d waters (e.g. Holopedium ) . 145 A temperature study c o u l d be done t o f i n d out how long i t takes f o r maximum f i l t e r i n g r a t e s t o adapt to changes i n temperature and whether i t i s p o s s i b l e f o r maximum f e e d i n g r a t e s to adapt t o temperature. T h i s study i n d i c a t e d t h a t t h r e e f a c t o r s may cause a change i n the shape of the laboratory-measured f u n c t i o n a l response, namely, the l e n g t h o f time between zooplankton c a p t u r e and measurement of f e e d i n g r a t e , the l e v e l of food c o n c e n t r a t i o n t o which the animals were exposed before the experiment* and p o s s i b l y a l s o the r a t e of change of t h a t food c o n c e n t r a t i o n . In l i g h t o f these r e s u l t s , a study focussed on the i n f l u e n c e of these t h r e e f a c t o r s on f u n c t i o n a l response form would be very r e l e v a n t t o comparative s t u d i e s of f e e d i n g r a t e s and to a p p l i c a t i o n o f these r a t e s t o f i e l d s i t u a t i o n s o r a g n a t i c ecosystem s t u d i e s . 1.46 6 BIBLIOGRAPHY Anraku, M. 1963. Feeding h a b i t s of p l a n k t o n i c copepods (Review) ( i n Japanese). Inform. B u l l , on Planktonology i n Japan 9:10-35. Bogdan, K.G. and D.C. HcNaught. 1975. S e l e c t i v e f e e d i n g by Diaptomus and Daphnia. Verh. I n t e r n a t . V e r e i n . Limnol. 19:2935-2942-Boyd, C H . 1976.. S e l e c t i o n o f p a r t i c l e s i z e s by f i l t e r -f e e d i n g copepods: a p l e a f o r reason. L i m n o l . Oceanogr. 21(1) : 175. Buikema, A-L., J r . 1973. F i l t e r i n g r a t e o f the c l a d o c e r a n Daphnia pulex as a f u n c t i o n of body s i z e , l i g h t and a c c l i m a t i o n . H y d r o b i o l o g i a 4 1 ( 4 ) : 515-527. Burns, C.8. 1968. The r e l a t i o n s h i p between body s i z e o f f i l t e r f e e d i n g Cladocera and the maximum s i z e of p a r t i c l e i n g e s t i o n . Limnol. Oceanogr. 13:675-678. Burns, C.W. 1969. R e l a t i o n between f i l t e r i n g r a t e , temperature, and body s i z e i n f o u r s p e c i e s of Daphnia . 147 Limnal- Oceanogr. 14:693-700. Burns, C M . and F . H. B i g l e r . 1967. Comparison of f i l t e r i n g r a t e s of Daphnia rosea i n l a k e water and i n suspensions o f y e a s t . Limnol. Oceanogr. 12 (3) : 492-502. Cahn, B.D. 1967. Detergents i n membrane f i l t e r s . S c i e n c e , 155:195-196. Chen, Cvf- and 6.T. Orl o b . 1972- E c o l o g i c a l S i m u l a t i o n f o r Aquatic Environments. Beport to O.9.B.B., U.S. Dept. of I n t e r i o r . 156 pp. Crowley, P.H. 1973. F i l t e r i n g r a t e i n h i b i t i o n of Daphnia pulex i n B i n t e r g r e e n l a k e water. Limnol. Oceanogr. 18(3):394-402. Crowley, P.H. 1975. N a t u r a l s e l e c t i o n and the M i c h a e l i s c o n s t a n t . J . Theor. B i o l . 50(2):461-476. Cushing, D-H. 1958. The e f f e c t o f g r a z i n g i n reducing the primary p r o d u c t i o n : a review. Bappt. Proces-Verbaux Beunion, C o n s e i l Perm. I n t e r n . . E x p l o r a t i o n Her, 144: 149-154. 148 Cushing, D.H. 1976. Grazing i n Lake Erken. Limnol. Oceanogr. 21 (3): 349. DiToro, 0., D. J. 0*Connor and B. V. Thomann. 1970. k dynamic model of phytoplankton p o p u l a t i o n s i n n a t u r a l waters. Env. Eng. and S c i . Prog., Manhattan C o l l e g e , Bronx, H.X. Dugdale, B.C., 1967. N u t r i e n t l i m i t a t i o n i n the sea: dynamics, i d e n t i f i c a t i o n and s i g n i f i c a n c e . Limnol. Oceanogr. 12(4) :685. Eppley, B.W., and W.H. Thomas. 1969. Comparison of h a l f -s a t u r a t i o n c o n s t a n t s f o r growth and n i t r a t e uptake o f marine phytoplankton. J . . Phycol. 5:375-379. Eppley, B.W., J.N. Sogers, and J . J . McCarthy. ., 1969. H a l f s a t u r a t i o n c o n s t a n t s f o r uptake of n i t r a t e and ammonia by marine phytoplankton. Limnol. Oceanogr. 14:912-920. F r o s t , B.W. 1972. E f f e c t s of s i z e and c o n c e n t r a t i o n of food p a r t i c l e s on the feed ing behavior of the marine p l a n k t o n i c copepod Calanus p a c i f i c u s . Limnol. Oceanogr. 17(6):805-815. 149 F r o s t , B-H. . 1975. A t h r e s h o l d f e e d i n g behaviour i n Calanus  p a c i f i c u s L i m n o l . Oceanogr. 2 0 ( 2 ) 3 263-F u j i i , K., C.S. H o l l i n g , and P.H. Hace. ( i n prep.) . A simple g e n e r a l i z e d model of a t t a c k by p r e d a t o r s and p a r a s i t e s . Gauld, D.T. 1951. The g r a z i n g r a t e o f p l a n k t o n i c copepods. J . Har. , B i o l . Ass. U.K. 29:695-706. G l i w i c z * Z.H. 1975. E f f e c t o f zooplankton g r a z i n g on p h o t o s y n t h e t i c a c t i v i t y and composition of phytoplankton. Verb. I n t . V e r e i n . Limnol. 19:1490-1497. G l i w i c z , Z.H. 1977. Food s i z e s e l e c t i o n and s e a s o n a l s u c c e s s i o n of f i l t e r f e e d i n g zooplankton i n an e u t r o p h i c l a k e . ,• E k o l . p o l . 25 (2) : 179- 225. Goodman, E.D., B.H. Zeren, D.J. H a l l , and P.H. Crowley. 1973. A t h e o r e t i c a l and experimental approach t o p r e d i c t i o n and c o n t r o l of e u t r o p h i c a t i o n of a l a k e ecosystem. Bes. p r o p o s a l t o Ecosystem A n a l y s i s Program, 8.S.F., wash. Administered through Div. of E n g i n e e r i n g Research, Michigan State Univ. 150 Green, J.D. 1975. Feeding and r e s p i r a t i o n i n the New Zealand copepod Calamoecia 1 o c a s i Brady. , Oecologia 21:345-358. Haney, J . F . 1971. An i n s i t u method f o r the measurement o f zooplankton g r a z i n g r a t e s . Limnol., Oceanogr. 16(6): 970. Haney, J . F . 1973. An i n s i t u examination of the g r a z i n g a c t i v i t i e s of n a t u r a l zooplankton communities. Arch. H y d r o b i o l . 72(1): 87-132. Haney, J.F. and D.J. H a l l . 1975.. D i e l v e r t i c a l m i g r a t i o n and f i l t e r - f e e d i n g a c t i v i t i e s o f Daphnia . A r c h i v f u r H y d r o b i o l . 75(4):413-441. Hargrave, B.T., and G.H. Geen. 1970. E f f e c t s of copepod g r a z i n g on two n a t u r a l phytoplankton p o p u l a t i o n s . J o u r n . F i s h . Bes- Bd. Can. 2 7 ( 8 ) : 1395. H a s s e l l , M.P., J.H. Lawton, J.8. Beddington. 1977- Sigmoid f u n c t i o n a l responses by i n v e r t e b r a t e p r e d a t o r s and p a r a s i t o i d s . J . Anim. E c o l . 46 (1):249-262-H o l l i n g , C.S. 1959. Some c h a r a c t e r i s t i c s of simple types of p r e d a t i o n and p a r a s i t i s m . C a n . , Entom.,, 91:385-398. 151 H o l l i n g , C S . 1965. The f u n c t i o n a l response of p r e d a t o r s to prey d e n s i t y and i t s r o l e i n mimicry and p o p u l a t i o n r e g u l a t i o n . Hem., Ent. Soc. Can. 45: 1-60., I v l e v , 7.S- 1961. Experimental ecology o f the f e e d i n g of f i s h e s . ( t r a n s . D. , Scott) Y a l e Univ. Press. New Haven. Kibby, H.V. 1971. E f f e c t o f temperature on the f e e d i n g b e h a v i o r of Daphnia r o s e a . Limnol. Oceanogr. 16(3) :580-581. Krepp, S.B. 1977. Genotypic and phenotypic v a r i a t i o n i n p o p u l a t i o n s of Daphnia pulex. 19. Sc. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia. Kryutchkova, N.H. and 7. Sladecek. 1969. Q u a n t i t a t i v e r e l a t i o n s o f f e e d i n g and growth of Daphnia pulex obtusa S c o u r f i e l d . H y d r o b i o l o g i a 33: 47-64. Lam, B.K. and B. H. F r o s t . 1976. Model of cope pod f i l t e r i n g response to changes i n s i z e and c o n c e n t r a t i o n of food. Limnol. Oceanogr. 21 (4) :490. 152 Lampert, W. 1974. A method f o r determining food s e l e c t i o n by zooplankton. Limnol. Oceanogr-. 19:995- 998. Leendertse, J . J . and E.C- G r i t t o n . 1971.,, k w a t e r - q u a l i t y s i m u l a t i o n model f o r well mixed e s t u a r i e s and c o a s t a l s e a s : V o l . i i i , Jamaica Bay S i m u l a t i o n . R-709-NYC, The Mew York C i t y Hand I n s t i t u t e . Lehman, J . J . 1976. The f i l t e r - f e e d e r as an o p t i m a l f o r a g e r , and the p r e d i c t e d shapes of fe e d i n g curves. L i m n o l . Oceanogr. 21 (4): 501. H c & l l i s t e r , CD., 1971. Some as p e c t s of n o c t u r n a l and contin u o u s g r a z i n g by p l a n k t o n i c h e r b i v o r e s i n r e l a t i o n t o p roduction s t u d i e s . FfiBC Tech. Report no. 248. Mac&rthur, R.H. And E.O., Wilson. 1967. Theory of I s l a n d Bioqeoqraphy , Pr i n c e t o n U n i v e r s i t y P r e s s , P r i n c e t o n . McIsaac, J . J . and B.C. Dugdale. 1969. The k i n e t i c s o f n i t r a t e and ammonia uptake by n a t u r a l p o p u l a t i o n s o f marine phytoplankton. Deep-Sea Res., 16: 45-57. HcHahon, J.H. 1965. Some p h y s i c a l f a c t o r s i n f l u e n c i n g the fe e d i n g behaviour of Daphnia magna S t r a u s s . Can- J . 153 Z o o l . 43: 603-612. McMahon, J.H. and P.H. , B i g l e r . , 19.63* Mechanisms r e g u l a t i n g the f e e d i n g r a t e o f Daphnia magna S t r a u s s . Can. Journ. Z o o l . 41: 321-332. HcMahon, J.H., and F. H. fiigler- 1965- Feeding r a t e of Daphnia magna St r a u s s i n d i f f e r e n t foods l a b e l l e d with r a d i o a c t i v e phosphorus. Limnol. oceanogr. 10: 105-113-McQueen, D.T. 1970. Grazing r a t e s and food s e l e c t i o n i n Diaptomus oregonensis (Copepoda) from Marion Lake, B.C. Journ. F i s h . , Bes. Bd. Can. 27: 13-20. M a r s h a l l , S.H., and A.P. Orr. 1955., On t h e b i o l o g y of Calanns f i n m a r c h i c n s ?. Food uptake, a s s i m i l a t i o n and e x c r e t i o n i n a d u l t and stage 5 Calanns. J . Mar., B i o l . Ass. O.K. 34:495-529-Mayzaud, P. and B.J. Conover. 1976. I n f l u e n c e o f p o t e n t i a l food supply on the a c t i v i t y o f d i g e s t i v e enzymes of n e r i t i c zooplankton. I n : Proc. 10th Eur. Symp. Mar. B i o l . , Ostend, belgium, Sept. 17-23, 1975. Persoone, G. and E.J. J a s p e r s (Eds)-, O n i v e r s a , H e t t e r e . , 154 Mayzaud, P. and S.A. P o u l e t . 1978. The importance of the time f a c t o r i n the t r o p h i c r e l a t i o n s h i p s between h e r b i v o r o u s copepods and n a t u r a l l y o c c u r r i n g p a r t i c u l a t e matter. Limnol. Oceanogr. 23 (6) : 1144-1154. M o r r i s , I . and C.S. Xentsch. 1972., A new method f o r c o n c e n t r a t i n g phytoplankton by f i l t r a t i o n with c o n t i n u o u s s t i r r i n g . Limnol. Oceanogr. 17(3):490-493. M u l l i n . M.M. 1963. Some f a c t o r s a f f e c t i n g the f e e d i n g of marine copepods of the genus Calanus. L i m n o l . Oceanogr. 8: 239-250. M u l l i n , M.H., E. F u g l i s t e r Stewart and F . J . F u g l i s t e r . 1975. I n g e s t i o n by p l a n k t o n i c g r a z e r s as a f u n c t i o n o f c o n c e n t r a t i o n of f o o d . Limnol. Oceanogr. 20(2):259-262. O'Connors, H.B., L. F. Small, and P.L.,,, Donaghy. 1976. P a r t i c l e - s i z e m o d i f i c a t i o n by two s i z e c l a s s e s of the e s t u a r i n e copepod A c a r t i a c j a n s i . L i m n o l . Oceanogr. 21 ( 2 ) : 300. Parsons, T.R.,and M. Takahashi. 1973. Environmental c o n t r o l of phytoplankton c e l l s i z e . Limnol. oceanogr. 155 18(4):511-515. Parsons, T.B., B.J. LeBrasseur, and J . D. F u l t o n . 1967* Some o b s e r v a t i o n s on the dependence of zooplankton g r a z i n g on the c e l l s i z e and c o n c e n t r a t i o n of phytoplankton blooms. J * Ocean- Soc. Jap. 23: 10-17. Pennak, B.H. 1953* Fresh-Hater I n v e r t e b r a t e s of the P n i t e d  S t a t e s . The Ronald P r e s s Co. P o r t e r , K.G. 1975. V i a b l e gut passage of g e l a t i n o u s green algae i n g e s t e d by Daphnia. Verh. I n t e r n a t . V e r e i n . Limnol. 19:2840-2850. P o t t s , H.T. W., and G. Parry. ., 1964. Osmotic and I o n i c  R e g u l a t i o n i n ftnimaIs. Hen York, H a c M i l l a n Co. P o u l e t . , S.ft. 1974.;, Seasonal g r a z i n g of Psendocalanus minutus on p a r t i c l e s . Marine B i o l . 25(2):109-123. S e a l , L. 1977. The k i n e t i c s of f u n c t i o n a l response. Am. Hat. 111 (978): 289-300. B i g l e r , F.H. 1961. The r e l a t i o n between c o n c e n t r a t i o n of food and f e e d i n g r a t e of Daphnia magna S t r a u s . Can. J . 15.6 Zool- 39:857-868. B i g l e r , P.H. 1971. Zooplankton f e e d i n g r a t e s , Chapt. 6, i n A Manual of Methods f o r the Assessment of Secondary P r o d u c t i v i t y i n Fresh Haters, Ed. by W.T. Edmondson and G.G. Hinberg. B l a c k w e l l , London., Byther, J.H. 1951. I n h i b i t o r y e f f e c t s of phytoplankton upon the f e e d i n g of Daphnia magna n i t h r e f e r e n c e t o growth, r e p r o d u c t i o n , and s u r v i v a l . Ecology 35: 522-533. Byther, J.H., D.H. Henzel, E.H. Huburt, C.J. Lorenzen, N. Corwin. 1971. The p r o d u c t i o n and u t i l i z a t i o n of o r g a n i c matter i n the Peru c o a s t a l c u r r e n t . . I n v e s t . Pesg. 35(1) :43-59. S c h i n d l e r , D. W. 1968.; Feeding, a s s i m i l a t i o n and r e s p i r a t i o n r a t e s of Daphnia magna under v a r i o u s environmental c o n d i t i o n s and t h e i r r e l a t i o n t o p r o d u c t i o n e s t i m a t e s . J . Anim- E c o l . 37:369-385. Solomon, H.E. 1949. The n a t u r a l c o n t r o l of animal p o p u l a t i o n s . J o u r n a l of Animal Ecology 18(1):1-:35. Starkweather, P.L. / 1975. D i e l p a t t e r n s of g r a z i n g i n Daphnia 157 pulex L e y d i g . Vera. Interna*:. V e r e i n . Limnol. 19:2851-2857. S t e e l e , J . 1974. The S t r u c t u r e of Marine Ecosystems. Harvard. Dniv. Press. Stewart, P.H., and fi.fi. L e v i n . 1973.> P a r t i t i o n i n g o f r e s o u r c e s and the outcome o f i n t e r s p e c i f i c c o m p e t i t i o n : a model and some g e n e r a l c o n s i d e r a t i o n s , am. Bat. 107: 171-198. Walsh, J . J . 1975. & s p a t i a l s i m u l a t i o n model of the Peru u p w e l l i n g ecosystem. Deep-sea Bes. 22: 201--236. Halsh, J . J . , and P.B. Bass. 1971. Oceans, a seagoing s i m u l a t i o n program - a u s e r ' s g u i d e t o the U n i v e r s i t y of Washington's IBM s p a t i a l v e r s i o n of COHSYS 1. S p e c i a l Beport no. 48, IBP Opwelling Biome T e c h n i c a l S e r i e s . Dept. Of ocean., 0. Of Hash. H a l t e r s , C.J. 1975. Dynamic models and e v o l u t i o n a r y s t r a t e g i e s . , Proceedings o f the SIHS Conference on Ecosystems, J u l y 1974. Pp.(68-82). H a l t e r s , C J . And H.C. C l a r k - 1973. Determinants of 158 f e e d i n g s t r a t e g y i n plankton communities. Unpublished manuscript. fleers, S.T. and T. Zaret. 1975., Phytoplankton - Zooplankton r e l a t i o n s h i p s i n Gatun Lake, Panama. Verb. I n t e r n a t . V e r e i n . L imnol. 19:1480-1483-Hhite, Handler, and Smith. 1968., P r i n c i p i e s of Biochemi s t r y, 4th ed. HcGraw H i l l . 1149 pp-B i l s o n , D.S. 1973., Food s i z e s e l e c t i o n among copepods. Ecology 54(4) : 909 -914. JOURNAL PUBLICATIONS H o l l i n g , C S . and S. Buckingham. 1976. A b e h a v i o r a l model of p r e d a t o r - p r e y f u n c t i o n a l responses. B e h a v i o r a l Science, 21(3):183. Buckingham, Sandra, C a r l J . Walters, and P i e r r e K l e i b e r . 1975. A procedure f o r e s t i m a t i n g gross p r o d u c t i o n , net p r o d u c t i o n , and a l g a l carbon content u s i n g l ^ C . Verh. I n t e r n a t . V e r e i n . Limnol. 19:32-38. Himamowa, BuBu (pseud.) 1975. The Obergurgl Model: a microcosm of economic growth i n r e l a t i o n to l i m i t e d e c o l o g i c a l r e s o u r c e s . Nature and Resources 2:10-21. (Himamowa i s a pseudonym f o r myself and 6 other authors. V e r t i n s k y , I., S.L. Buckingham, C J . Walters, and G. Zaltman. 1972. Family p l a n n i n g computer s i m u l a t i o n : the Costa R i c a p o p u l a t i o n model. S i m u l a t i o n and Games, 3(2): 123-145. THESES AND NON-REFEREED PUBLICATIONS Walters, C a r l J . and Sandra Buckingham. 1975. A C o n t r o l system f o r i n t r a s e a s o n salmon management. In " I n t e r n a t i o n a l I n s t i t u t e f o r A p p l i e d Systems A n a l y s i s ; Conference Proceedings, Workshop on Salmon Management". CP-75-2. Buckingham, S.L. 1973. A plankton p r o d u c t i o n model f o r the western G u l f of St. Lawrence. In: Volume of Working Papers f o r NATO Conference on M o d e l l i n g of Marine Systems, June, 1973, i n O f i r , P o r t u g a l . Buckingham, S.L. 1970. C o n t r i b u t i o n d l'etude du b r u i t de fond a s s o c i e aux diodes Schottky. T h e s i s (Doctorat de 3 e C y c l e ) , F a c u l t e des S c i e n c e s , U n i v e r s i t e de M o n t p e l l i e r , France. 

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