UBC Theses and Dissertations

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UBC Theses and Dissertations

Predation interactions between zooplankton and two species of Chaoborus (Diptepa, Chaoboridae) in a small… Fedorenko, Alice Y.R. 1973

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PREDATION INTERACTIONS BETWEEN ZOOPLANKTON AND TWO SPE C I E S OF CHAOBORUS (DIPTERA, CHAOBORIDAE) I N A SMALL COASTAL LAKE by A L I C E Y. R. FEDORENKO B . S c , U n i v e r s i t y o f T o r o n t o , 1970 A THESIS SUBMITTED I N PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e D e p a r t m e n t o f ZOOLOGY We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF B R I T I S H COLUMBIA 1973 In presenting 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 of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the 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 reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada ABSTRACT F e e d i n g h a b i t s o f two c o e x i s t i n g s p e c i e s o f C h a o b o r u s , C . t r i v i t t a t u s a n d C . a m e r j c a n u s , w e r e i n v e s t i g a t e d i n l a k e and i n l a b o r a t o r y e x p e r i m e n t s , a n d by an e x t e n s i v e s u r v e y o f l a r v a l c r o p c o n t e n t s . The f i e l d s t u d y showed t h a t C t r i v i t t a t u s h a s a two y e a r l i f e c y c l e , m i g r a t e s d i e l l y d u r i n g t h e summer s e a s o n down t o 20 m, and i s t h e more a b u n d a n t o f t h e two C h a o b o r u s s p e c i e s . C . a n t e r i c a n u s h a s a one y e a r l i f e c y c l e a n d m i g -r a t e s a t mos t o v e r a d i s t a n c e o f o n l y 5 m. S e a s o n a l a b u n -d a n c e a n d d i s t r i b u t i o n o f mos t p l a n k t o n t y p e s f o u n d i n E u n i c e L a k e w e r e m o n i t o r e d d u r i n g 1971 a n d 1972. Z o o p l a n k -t o n h a d a l o w s t a n d i n g c r o p a n d w e r e m o s t l y f o u n d a b o v e 6 m. Z o o p l a n k t o n s i z e a n d swimming v e l o c i t y a n d s i z e o f l a r v a l h e a d p a r t s w e r e m e a s u r e d i n t h e l a b o r a t o r y , a n d , t o g e t h e r w i t h t h e a b o v e d a t a on v e r t i c a l d i s t r i b u t i o n , w e r e u s e d t o e v a l u a t e t h e a v a i l a b i l i t y o f z o o p l a n k t o n a s p r e y f o r C h a o b o r u s . The l a k e a n d l a b o r a t o r y e x p e r i m e n t s showed t h a t r a t e s o f l a r v a l f e e d i n g a n d d i g e s t i o n i n c r e a s e s i g n i f i c a n t l y w i t h t e m p e r a t u r e . T e m p e r a t u r e , h o w e v e r , d o e s n o t seem t o a f f e c t f e e d i n g r a t e o f t h e 4 t h i n s t a r C . t r i v i t t a t u s l a r v a e . F e e d i n g r a t e s i n c r e a s e w i t h l a r v a l age and v a r y w i t h p r e y t y p e a n d d e n s i t y . F r o m a n a l y s i s o f c h a o b o r i d c r o p c o n t e n t s , d i e t d i f f e r e n c e s w e r e f o u n d among a l l i n s t a r s o f t h e two s p e c i e s , a n d t h e s e w e r e r e l a t e d t o p r e y i i i s i z e , a b u n d a n c e , a n d d i s t r i b u t i o n . The c a l c u l a t e d p e r c e n t o f p r e y s t a n d i n g c r o p t h a t t h e C h a o b o r u s i n E u n i c e L a k e c o u l d p o t e n t i a l l y remove r a n g e d f r o m a minimum o f 3% f o r n a u p l i i t o a maximum o f 20% f o r D i a p t o m u s k e n a i . S e a s o n a l c h a n g e s i n C h o a b o r u s i n s t a r c o m p o s i t i o n a n d i n z o o p l a n k t o n s p e c i e s c o m p o s i t i o n r e s u l t i n r e d u c e d p r e d a t i o n o n a n y s i n g l e z o o p l a n k t o n g r o u p . The o b s e r v e d d i e t o f C h a o b o r u s l a r v a e i n E u n i c e L a k e was shown t o be c l o s e l y r e l a t e d t o t h e m o r p h o l o g y o f C h a o b o r u s a n d t h e i r p r e y a n d t o t h e r e l a t i v e d i s t r i b u t i o n o f p r e d a t o r a nd p r e y . i v TABLE OF CONTENTS Page T I T L E PAGE . . . . . . . . . i ABSTRACT i i TABLE OF CONTENTS i v L I S T OF FIGURES v i i L I S T OF TABLES x i L I S T OF APPENDICES x i i i ACKNOWLEDGEMENTS x i v I INTRODUCTION . 1 I I DESCRIPTION OF THE STUDY AREA 4 1 P h y s i c a l a n d C h e m i c a l F e a t u r e s 4 2 B i o l o g i c a l F e a t u r e s 4 I I I GENERAL MATERIALS AND METHODS 8 1 T e m p e r a t u r e and Oxygen 8 2 P l a n k t o n S a m p l i n g 8 3 P l a n k t o n C o u n t i n g 1° 4 P l a n k t o n W e i g h i n g ... - . r. . 10 5 S i z e M e a s u r e m e n t s 11 6 Swimming V e l o c i t y o f Z o o p l a n k t o n 11 7 C h a o b o r u s G r o w t h E x p e r i m e n t 13 I V BIOLOGY OF ZOOPLANKTON 16 1 R e s u l t s 16 a Body s i z e a n d w e i g h t 16 b S p e e d o f swimming a n d movements 16 c S e a s o n a l c h a n g e s i n s t a n d i n g c r o p a n d i n s p e c i e s c o m p o s i t i o n 19 V P a g e d D i e l d i s t r i b u t i o n a n d m i g r a t i o n . . . . . . 23 e P a t c h i n e s s i n p l a n k t o n d i s t r i b u t i o n . . . . 25 3 Summary 30 V BIOLOGY OF CHAOBORUS LARVAE 31 1 R e s u l t s 31 a Head m o r p h o l o g y o f l a r v a l i n s t a r s 31 b D e v e l o p m e n t o f l a r v a e 31 c S e a s o n a l c h a n g e s i n s t a n d i n g c r o p 36 d S e a s o n a l c h a n g e s i n d i s t r i b u t i o n a n d m i g r a t i o n 3 8 e P o t e n t i a l g r o w t h o f l a r v a e 3 8 2 D i s c u s s i o n 41 3 Summary 46 V I FEEDING OF CHAOBORUS 47 1 M e t h o d s 47 a D i g e s t i o n r a t e s 47 b F e e d i n g r a t e s 47 c T e m p e r a t u r e e f f e c t o n f e e d i n g r a t e s . . . . 49 d L o n g t e r m f e e d i n g e x p e r i m e n t 49 e E f f i c i e n c y o f c o p e p o d i n g e s t i o n 50 f A n a l y s i s o f l a r v a l c r o p s 51 g D i e l f e e d i n g a c t i v i t y 52 2 R e s u l t s 53 a D i g e s t i o n r a t e s 53 b F e e d i n g r a t e s 53 c T e m p e r a t u r e e f f e c t o n f e e d i n g r a t e s . . . . 59 v i P a g e d L o n g t e r m f e e d i n g e x p e r i m e n t 59 e E f f i c i e n c y o f p r e y i n g e s t i o n . . . . . . . 62 f A n a l y s i s o f l a r v a l c r o p s 64 g D i e l f e e d i n g a c t i v i t y 73 3 D i s c u s s i o n 78 4 Summary 8 6 V I I EFFECT OF CHAOBORUS PREDATION ON ZOOPLANKTON . . . 87 1 M e t h o d s 87 a C a l c u l a t i o n o f o v e r a l l p r e d a t i o n i m p a c t o n z o o p l a n k t o n 8 7 b C a l c u l a t i o n o f c o n s u m p t i o n b y C . t r 4° l a r v a e 8 9 2 R e s u l t s 92 a P r e y - n a u p l i i 92 b P r e y - D . t y r e l l i 94 c P r e y - D . k e n a i 94 d P r ey-D i a p h a n o soma 96 e C o n s u m p t i o n b y C . t r 4° l a r v a e 9 6 3 D i s c u s s i o n 98 4 Summary 107 BIBLIOGRAPHY 1 ° 8 APPENDICES H 3 v i i L I S T OF FIGURES FIGURE P a g e 1. C o n t o u r map ( i n m e t e r s ) o f E u n i c e L a k e . S a m p l i n g s t a t i o n s a r e c i r c l e d . P o s i t i o n s o f r a f t i n 1971 a n d 1972 a r e i n d i c a t e d by s q u a r e s 2. I s o t h e r m s i n E u n i c e L a k e , L a t e r a l v i e w (a), and v e n t r a l v i e w (b) , o f a l a r v a l h e a d s h o w i n g t h e t h r e e h e a d p a r a -m e t e r s m e a s u r e d . (MAG.=40X). 12 4. T u b e s u s e d f o r C h a o b o r u s g r o w t h e x p e r i m e n t . 15 5. Means * 1 S.E. o f swimming v e l o c i t i e s o f z o o p l a n k t o n a t d i f f e r e n t t e m p e r a t u r e s . . . . 18 6. S t a n d i n g c r o p o f z o o p l a n k t o n g r o u p s i n 1971 ( d o t t e d l i n e ) a nd i n 1972 ( s o l i d l i n e ) . H o l o p e d i u m w e r e n o t r e c o r d e d i n 1 9 7 1 . 20 7. T o t a l s t a n d i n g c r o p by number ( N ) , a n d by b i o m a s s ( B ) , o f z o o p l a n k t o n i n 1971 ( d o t t e d l i n e ) a n d i n 1972 ( s o l i d l i n e ) . R o t i f e r s a n d C h a o b o r u s w e r e e x c l u d e d . H o l o p e d i u m w e r e n o t c o u n t e d i n 1971 22 8. Noon and m i d n i g h t (MT) v e r t i c a l d i s t r i b u t i o n o f z o o p l a n k t o n t y p e s (on l e f t ) , a n d o f C h a o b o r u s t y p e s (on r i g h t ) , i n E u n i c e L a K e i n 1972. L e t t e r s i n d i c a t e t y p e o f p l a n k t o n : N - n a u p l i i , K - D . k e n a i , B-BQsmina, D - P a p h n i a , D I - D i a p h a n o s o m a , H - H o l o p e d i u m , P - P o l y p h e m u s , C L - c o m b i n e d c l a d o c e r a n t y p e s . Roman n u m e r a l s show i n s t a r s o f C . a m e r i c a n u s , am, (on l e f t ) , a n d o f C . t r i v i t t a t u s , t r , Jon r i g h t ) . 24 9. Means t 1 S.D. o f h e a d l e n g t h , a n t e n n a a n d s e t a l e n g t h , a n d mouth d i a m e t e r o f l a r v a l i n s t a r s ( i n Roman n u m e r a l s ) o f C . a m e r i c a n u s (open c i r c l e s ) a n d C . t r i v i t t a t u s ( s o l i d c i r c l e s ) . F i r s t i n s t a r s o f b o t n s p e c i e s a r e c o m b i n e d ( h a t c h e d c i r c l e s ) . Number o f a n i m a l s m e a s u r e d i s g i v e n i n p a r e n t h e s i s . . 32 v i i i FIGURE Page 10. S u c c e s s i o n o f l a r v a l i n s t a r s o f C. a m e r i c a n u s ( l e f t ) a n d o f C . t r i v i t t a t u s ( r i g h t ) I n 1971 ( d o t t e d l T h e l and i n 1972 ( s o l i d l i n e ) . Y e a r h a t c h e d i s shown f o r 4 t h i n s t a r l a r v a e 33 11. S e a s o n a l c h a n g e s i n l a r v a l i n s t a r l e n g t h i n 1 9 7 1 . Means and 9 5 % c o n f i d e n c e l i m i t s a r e shown f o r e a c h i n s t a r (Roman n u m e r a l s ) o f C . a m e r i c a n u s ( d o t t e d l i n e s ) a n d C . t r i v i t - t a t u s ( s o l i d l i n e s ) . ( f r o m F e d o r e n k o a n d S w i f t d 9 7 2 ) 34 12. S e a s o n a l c h a n g e s i n l a r v a l w e i g h t i n 1971 and 1972. M e a n s , 95% c o n f i d e n c e l i m i t s , a n d t h e y e a r h a t c h e d a r e shown f o r e a c h s p e c i e s . ( f r o m S w i f t 1973) 3 5 13. C u m u l a t i v e s t a n d i n g c r o p by number ( N ) , a n d by b i o m a s s ( B ) , o f C . a m e r i c a n u s a n d C. t r i v i t t a t u s i n 1971 ( d o t t e d l i n e ) a n d 1972 ( s o l i d l i n e ) . A l l i n s t a r 1 l a r v a e w e r e i n c l u d e d w i t h C . t r i v i t t a t u s • 37 14. D i e l v e r t i c a l m i g r a t i o n o f C . a m e r i c a n u s and C . t r i v i t t a t u s l a r v a e on 16-17 S e p t e m b e r , 1 9 7 1 . I n s t a r s o f l a r v a e a n d d e n s i t y p e r 100 l i t e r s a r e a s shown. ( f r o m F e d o r e n k o and S w i f t 1972) 39 15. Mean w e i g h t s a n d u p p e r 95% c o n f i d e n c e l i m i t s o f t h r e e t y p e s o f C h a o b o r u s l a r v a e f r o m t h e t u b e e x p e r i m e n t : d o t t e d b a r s a r e i n i t i a l w e i g h t s i n l a k e a n d t u b e s ; c o a r s e l y h a t c h e d b a r s a r e f i n a l w e i g h t s i n l a k e ; g r e y b a r s a r e f i n a l w e i g h t s i n t u b e s 4 0 16. A v e r a g e t i m e r e q u i r e d t o a c h i e v e 50% and 80% o f c o m p l e t e d i g e s t i o n o f D . k e n a i by C . t r i v i t t a t u s 4 Y l a r v a e a t d i f f e r e n t t e m p e r a t u r e s 55 17. Means t 1 S.D. o f C h a o b o r u s f e e d i n g r a t e s on D . t y r e l l i a t d i f f e r e n t t e m p e r a t u r e s . ( c u r v e s w e r e f i t t e d by e y e) 57 i x FIGURE P a g e 18. Means + I S . D . o f C h a o b o r u s f e e d i n g r a t e s o n D . k e n a i (a-d)., o n D i a p h a n o s o m a ( e ) , a n d o n n a u p l i i ( f ) . S o l i d l i n e s i n d i c a t e f e e d i n g r a t e s ; d o t t e d l i n e s g i v e t h e t o t a l o f e a t e n a n d k i l l e d p r e y . ( c u r v e s w e r e f i t t e d b y e y e ) . . . . . 5 8 19. P r e y consumed b y a C h a o b o r u s l a r v a a t d i f f e r e n t t e m p e r a t u r e s : (a ,b ,c) o n s e c o n d d a y o f e x p e r i m e n t , ( d , e , f ) on f o u r s u c c e s s i v e d a y s . Day number i s g i v e n o n e x t r e m e r i g h t . P r e y a r e P . k e n a i f o r 4 t h i n s t a r l a r v a e , and" D . t y r e l T i f o r 3 r d i n s t a r l a r v a e . G i v e n a r e t h e means - 1 S.E. ( c u r v e s w e r e f i t t e d b y eye) 60 20. D a i l y f o o d r a t i o n o f 9 t y p i c a l l a r v a e o f C.am 4 ( A ) , C . t r 4 Y ( B ) , a n d C . t r 4° ( C ) . H a t c h e d b a r s a r e f u l l y d i g e s t e d D . k e n a i ; c l e a r b a r s a r e p a r t i a l l y d i g e s t e d " D . k e n a i . . . 61 21. S e a s o n a l d i e t o f C . a m e r i c a n u s l a r v a e b y i n s t a r a n d t h e c o m p o s i t i o n o f l a k e p l a n k t o n i n 1972. N ( c l e a r b a r s ) , i s p e r c e n t c o m p o s i t i o n by number; B ( h a t c h e d b a r s ) , i s p e r c e n t c o m p o s i t i o n by b i o m a s s . L e t t e r s r e f e r t o p r e y t y p e s : R - r o t i f e r s , B - B o s m i n a , P - P o l y p h e m u s , D - D a p h n i a , H - H o l o p e d i u m , C - m i s c e l l a n e o u s c l a d o c e r a n s , K - D . k e n a i , T - D . t y r e l l i , O - C h a o b o r u s , X-u n T d e n t i f i a b l e m a t e r i a l • 68 22. S e a s o n a l d i e t o f C . t r i v i t t a t u s l a r v a e b y i n s t a r i n 1972. " F o r e x p l a n a t i o n s e e F i g . 21 69 23. D i e l f e e d i n g p a t t e r n o n D . k e n a i o f C . t r 4° l a r v a e i n 1972. A r r o w s i n d i c a t e p e a k s o f f e e d i n g a c t i v i t y . S a mple s i z e f o r e a c h d a t a p o i n t r a n g e d f r o m a b o u t 100-300 l a r v a e 74 24. D i e l f e e d i n g a c t i v i t y o n B o s m i n a ( s o l i d l i n e ) , a n d o n a l l p r e y t y p e s c o m b i n e d ( d o t t e d l i n e ) . , o f d i f f e r e n t i n s t a r s o f C . a m e r i c a n u s a n d C . t r i v i t t a t u s l a r v a e i n T972~: Sample s i z e f o r e a c h d a t a p o i n t r a n g e d f r o m a b o u t 50-100 C . a m e r i c a n u s l a r v a e and f r o m a b o u t 10 0-2uu c . t r i v i t -t a t u s l a r v a e ~ ~ • ~ • • • • 75 X FIGURE Page 25. The e f f e c t o f C h a o b o r u s p r e d a t i o n o n n a u p l i i ( l e f t ) , a n d on D . t y r e l l i ( r i g h t ) , (ayd) - p e r c e n t o f p r e y s t a n d i n g c r o p r e m o v e d b y C h a o b o r u s i n 1971 ( d o t t e d l i n e ) a n d i n 1972 ( s o l i d l i n e ) ; (b,e) a n d ( c , f ) - p e r c e n t o f t o t a l p r e d a t i o n a t t r i b u t e d t o d i f f e r e n t l a r v a l i n s t a r s i n 1971 a n d 1972 r e s p e c t i v e l y 93 26. The e f f e c t o f C h a o b o r u s p r e d a t i o n o n D . k e n a i ( l e f t ) , a n d o n D i a p h a n o s o m a ( r i g h t ) . F o r e x p l a n a t i o n s e e F i g . 2 5. 95 27. The m e c h a n i s m s f o r p r e d a t o r a v o i d a n c e f r o m May t o November o f 1972 b y f o u r p r e y t y p e s . S - s e a s o n a l s e g r e g a t i o n , V - v e r t i c a l s e g -r e g a t i o n , D-low p r e y d e n s i t y , H - s i z e i n -c o m p a t i b i l i t y . S o l i d l i n e s r e p r e s e n t o p t i m a l p e r i o d s f o r p r e d a t i o n s i n c e no known p r o t e c t i v e m e c h a n i s m i s o p e r a t i n g ; d a s h e d l i n e s i n d i c a t e t h a t t h e m e c h a n i s m i s o n l y p a r t i a l l y e f f e c t i v e a n d some p r e d a -t i o n c a n o c c u r ; d o t t e d l i n e s r e p r e s e n t l e a s t o p t i m a l p e r i o d s f o r p r e d a t i o n b e c a u s e o f t h e a b o v e m e c h a n i s m s o p e r a t i n g . P e r i o d s f r e e o f l i n e s i n d i c a t e t h a t no p r e d a t i o n i s o c c u r r i n g b e c a u s e o f a b s e n c e o r s c a r c i t y o f p r e d a t o r s . Roman n u m e r a l s i n d i c a t e l a r v a l i n s t a r s 99 x i L I S T OF TABLES TABLE P a g e I Mean l e n g t h and d r y w e i g h t o f E u n i c e L a k e z o o p l a n k t o n 1 . 17 I I C o m p a r i s o n o f 4 h o r i z o n t a l C l a r k e - B u m p u s t o w s t a k e n a t n o o n b e t w e e n s t a t i o n s 1 a n d 2. V a l u e s a r e g i v e n i n numbers p e r 100 l i t e r s . S a m p l i n g d e p t h s and d a t e s a r e i n d i c a t e d i n p a r e n t h e s i s 27 I I I C o m p a r i s o n o f t h e t o t a l number o f C h a o b o r u s l a r v a e p e r s t a n d a r d v e r t i c a l n e t h a u l t a k e n a t e a c h o f t h e 5 s t a t i o n s i n E u n i c e L a k e d u r i n g 1972. V a l u e s a r e g i v e n i n numbers p e r 0.62 m 2 '. 28 I V C o m p a r i s o n o f z o o p l a n k t o n and o f C h a o b o r u s b i o m a s s b e t w e e n s t a t i o n 5 a n d s t a t i o n 1 i n 1972. V a l u e s a r e g i v e n i n mg p e r m^ . . . . 29 V D i g e s t i o n r a t e s o f C h a o b o r u s f e e d i n g oh d i f f e r e n t p r e y t y p e s T '. ". T~ 54 V I I n g e s t i o n s u c c e s s o f C h a o b o r u s f e e d i n g o n c o p e p o d s '. ". '. ". 63 V I I L a r v a l m outh s i z e a n d p r e y s i z e 65 V I I I L a r v a l f o o d o v e r l a p i n d i c e s f o r J u l y , A u g u s t , S e p t e m b e r , a n d O c t o b e r c a l c u l a t e d f r o m n u m e r i c a l d i e t p e r c e n t a g e s A, a n d f r o m b i o m a s s d i e t p e r c e n t a g e s B 70 I X A c h i - s q u a r e a n a l y s i s o f t h e n u m e r i c a l d i e t c o m p o s i t i o n o f a l l l a r v a l i n s t a r s c a u g h t o n some d a t e ( a ) , o f two i n s t a r s o f s i m i l a r a g e s c a u g h t o n same d a t e ( b ) , o f t h e same l a r v a l i n s t a r s c a u g h t o n d i f f e r e n t d a t e s ( c ) , a n d o f t h e s e a s o n a l l a k e p l a n k t o n compo-s i t i o n (d) 72 X A c h i - s q u a r e a n a l y s i s o f t h e d i e l f e e d i n g p a t t e r n o f C h a o b o r u s l a r v a e b a s e d o n D . k e n a i a l o n e ( a ) , o n B o s m i n a a l o n e ( b ) , a n d o n a l l p l a n k t o n t y p e s c o m b i n e d ( c ) . x i i TABLE Page X I A s a m p l e c a l c u l a t i o n o f C h a o b o r u s p r e d a -t i o n o n D . t y r e l l i 8 8 X I I S a m p l e c a l c u l a t i o n s o f mean d a i l y consump-t i o n b y C . t r . 4° u s i n g (1) f e e d i n g r a t e s a n d p r e y a b u n d a n c e d a t a , a n d (2) t h e e n e r g y b u d g e t e q u a t i o n gg X I I I C a l c u l a t e d d a i l y f o o d c o n s u m p t i o n o f C . t r _ 4° l a r v a e i n 1971 a n d 1972 9 7 x i i i L I S T OF APPENDICES APPENDIX Page I P r e d a t i o n o n n a u p l i i , 1971 H 4 I I P r e d a t i o n o n n a u p l i i , 1972 115 I I I P r e d a t i o n o n D . t y r e l l i , 1971 116 I V P r e d a t i o n o n D . t y r e l l i , 1972 117 V P r e d a t i o n on D . k e n a i , 1 9 7 1 118 V I P r e d a t i o n o n D . k e n a i , 1972 119 V I I P r e d a t i o n o n D i a p h a n o s o m a , 1971 . . . . . ^ 2 Q V I I I C a l c u l a t i o n o f t h e mean d a i l y c o n s u m p t i o n (C) o f c o p e p o d p r e y b y C . t r 4° i n 1971 and 1 972, u s i n g f e e d i n g r a t e s a n d p r e y a b u n d a n c e d a t a ^21 I X C a l c u l a t i o n s o f t h e mean d a i l y consump-t i o n b y C . t r 4° i n 1971 and 1972 u s i n g t h e energy""Fudget e q u a t i o n X C a l c u l a t i o n o f t h e mean d a i l y consump-t i o n b y C . t r 4° i n 1972 u s i n g t h e c r o p a n a l y s i s d a t a 123 x i v ACKNOWLEDGEMENTS I w o u l d l i k e t o t h a n k t h e s t a f f o f t h e U.B.C. Re-s e a r c h F o r e s t a n d i n p a r t i c u l a r , Mr. J a c k W a l t e r s , f o r t h e i r h e l p a n d i n t e r e s t i n t h i s w o r k . Numerous f r i e n d s h a v e made t h e w o r k a m e m o r a b l e a nd p l e a s a n t e x p e r i e n c e t h r o u g h t h e i r i n t e r e s t a n d p a r t i c i p a -t i o n , p a r t i c u l a r l y R e g i n a C l a r o t t o a n d t h e W a l l i n f a m i l y . K i m Waterman a n d R e g i n a C l a r o t t o a s s i s t e d i n f i e l d a n d i n t h e l a b o r a t o r y . D o l o r e s L a u r i e n t e was v e r y h e l p f u l w i t h t h e c o m p u t e r w o r k . J u d y D a y t o n k i n d l y h e l p e d w i t h t h e t y p i n g . D r s . C. W a l t e r s a n d A. L e w i s r e a d t h e m a n u s c r i p t a n d made v a l u a b l e s u g g e s t i o n s d u r i n g t h e c o u r s e o f t h e s t u d y . T h a n k s a r e due t o D r . I . E f f o r d f o r h i s i n t e r e s t a n d g o o d c o u n s e l l i n g . D r . T.G. N o r t h c o t e , my s u p e r v i s o r , p r o v i d e d g u i d a n c e a n d much e n c o u r a g e m e n t t h r o u g h o u t t h e s t u d y f o r w h i c h I am v e r y g r a t e f u l . H i s c r i t i c i s m a n d s u p p o r t w e r e i n v a l u a b l e d u r i n g t h e p r e p a r a t i o n o f t h e m a n u s c r i p t . I am e s p e c i a l l y g r a t e f u l t o M i c h a e l C. S w i f t , who w i t h z e a l , s k i l l f u l i n g e n u i t y , a n d a know-how t h a t I l a c k e d , e x e c u t e d many o f my r o u g h e r c h o r e s , a n d who was my c h i e f e d i t o r i n f o r m u l a t i n g t h e t h e s i s . F i n a l l y I w o u l d l i k e t o t h a n k t h e N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a f o r t h e i r f i n a n c i a l s u p p o r t t h a t e n a b l e d me t o be i n t h i s p a r t o f C a n a d a a nd meet s o many f i n e p e o p l e . 1 I INTRODUCTION C h a o b o r u s a r e d i p t e r a n s , w i t h p r e d a c e o u s , p l a n k t o n i c l a r v a e t h a t emerge a s n o n - b i t i n g f l i e s . The l a r v a e a r e common i n l a k e s a n d p o n d s a l l o v e r t h e w o r l d , a r e f r e -q u e n t l y a n i m p o r t a n t f o o d i t e m o f f i s h ( S i k o r o w a 1 9 7 3 ) , and h a v e b e e n shown t o be i m p o r t a n t c o m p o n e n t s o f c e r -t a i n a q u a t i c c o m m u n i t i e s ( W e l c h 1968; S p r u l e s 1 9 7 0 ) . C h a o b o r u s l a r v a e a r e g o o d e x p e r i m e n t a l a n i m a l s b e c a u s e t h e y a r e e a s y t o h a n d l e a n d s u r v i v e w e l l u n d e r l a b o r a t o r y c o n d i t i o n s . C h a o b o r u s l a r v a e a r e ambush p r e d a t o r s t h a t r e s t s e m i -t r a n s p a r e n t a n d m o t i o n l e s s i n t h e w a t e r c o l u m n a n d "make a s t a b a t a n y t h i n g " (G. S p r u l e s 1 9 7 2 , p e r s o n a l c o m m u n i c a t i o n ) . The s t r i k e s a r e p r o b a b l y t r i g g e r e d by v i b r a t i o n s p r o d u c e d by t h e p r e y ( H o r r i d g e 19 6 6 ) . C h a o b o r u s h a v e s p e c i a l i z e d h e a d a p p e n d a g e s f o r c a p t u r i n g a n d i n g e s t i n g t h e i r p r e y . The r e m a i n s o f t h e p r e y , d i g e s t e d i n t h e c r o p , a r e r e g u r -g i t a t e d t h r o u g h l a r v a l m o u t h . The f u n c t i o n a l m o r p h o l o g y o f C h a o b o r u s m o u t h - p a r t s i s g i v e n b y M o n t s h a d s k y ( 1 9 4 5 ) . P u c a t (1965) d e s c r i b e s t h e l a r v a l d i g e s t i v e m e c h a n i s m . O b s e r v a t i o n s o n t h e f e e d i n g e c o l o g y o f C h a o b o r u s l a r v a e h a v e b e e n m o s t l y q u a l i t a t i v e a nd l i m i t e d i n s c o p e . P arma (1971) l i s t s a s p o t e n t i a l l a r v a l f o o d , a l l p l a n k t o n i c c r u s t a c e a n s a n d r o t i f e r s , s e v e r a l b e n t h i c s p e c i e s , a n d e v e n some a l g a e — i n f a c t a n y t h i n g t h a t i s o f a r e a s o n a b l e s i z e f o r s w a l l o w i n g . The f e w i n v e s t i g a t o r s t h a t h a v e 2 l o o k e d a t t h e f o o d o f t h e i n d i v i d u a l l a r v a l i n s t a r s ( D e o n i e r 1 9 4 3 ; M a i n 1953) r e p o r t t h a t d i f f e r e n c e s do e x i s t among t h e i n s t a r s o f l a r v a e ; t h e f o o d o f 1 s t a n d 2nd i n s t a r s i s m a i n l y t h e s m a l l z o o p l a n k t e r s s u c h a s r o t i f e r s a n d n a u p l i i , a n d t h e f o o d o f t h e o l d e r l a r v a e c o n s i s t s m a i n l y o f t h e l a r g e r c o p e p o d s a n d c l a d o c e r a n s . The d i e t s o f a l l f o u r l a r v a l i n s t a r s o f C . f l a v i c a n s a n d C . c r y s t a l l i n u s h a v e b e e n a n a l y z e d q u a n t i t a t i v e l y b y S i k o r o w a ( 1 9 7 3 ) , who a l s o f o u n d d i f f e r e n c e s i n f o o d among t h e l a r v a l i n s t a r s . S i k o r o w a , h o w e v e r , h a s p o o l e d t h e c r o p c o n t e n t s o f a n i m a l s c o l l e c t e d f r o m v a r -i o u s l o c a t i o n s a t d i f f e r e n t t i m e s o f t h e y e a r . Few o t h e r q u a n t i t a t i v e s t u d i e s h a v e b e e n d o n e . R o t h 1 s r a d i o -i s o t o p e a p p r o a c h (1971) was awkward a n d t i m e - c o n s u m i n g . H i s r e s u l t s w e r e o b t a i n e d i n d i r e c t l y a n d a r e t h e r e f o r e somewhat q u e s t i o n a b l e . K a j a k a n d R a n k e - R y b i c k a (1970) c o m p a r e d p r e y numbers b e f o r e a n d a f t e r i n s i t u i n c u b a t i o n s w i t h C h a o b o r u s . H o w e v e r , t h e i r m e t h o d i s n o t a l w a y s p r a c t i c a l o r r e a l i s t i c a s i t s a c c u r a c y r e l i e s o n t h e r e b e i n g a n u n n a t u r a l l y h i g h d e n s i t y o f o r g a n i s m s . I n b o t h o f t h e a b o v e e x a m p l e s , t h e a u t h o r s d i d n o t d i f f e r e n t i a t e b e t w e e n l a r v a l i n s t a r s . S w u s t e e t a l ^ ( i n p r e s s ) u s e d a l a b o r a t o r y e x p e r i m e n t a l s e t up w i t h 7-day s t a r v e d l a r v a e and t h r e e p r e y t y p e s t o s t u d y s e l e c t i v e p r e d a t i o n b y t h e 4 t h i n s t a r l a r v a e o f C . f l a v i c a n s . The p r e s e n t w o r k was c o n d u c t e d i n a s m a l l m o u n t a i n l a k e o n t h e w e s t e r n c o a s t o f B r i t i s h C o l u m b i a . T h i s 3 l a k e i s c o n v e n i e n t l y c l o s e t o t h e i n s t i t u t e a l l o w i n g f o r an e a s y t r a n s p o r t o f a n i m a l s t o t h e l a b o r a t o r y . The o b j e c t o f t h e s t u d y was t h r e e f o l d : (1) t o d e t e r m i n e t h e l i f e h i s t o r i e s o f t h e two C h a o b o r u s s p e c i e s and t h e i r p r e y f o u n d i n E u n i c e L a k e a s r e l a t e d t o t h e l a r v a l f e e d i n g ; (2) t o d e t e r m i n e t h e n a t u r a l c o m p o s i t i o n o f t h e l a r v a l d i e t b y s p e c i e s a n d by i n s t a r a n d t h e e f f e c t o f t e m p e r a t u r e , p r e y t y p e , a n d p r e y d e n s i t y o n l a r v a l f e e d i n g ; a n d (3) t o e v a l u a t e t h e i m p a c t o f l a r v a l p r e d a t i o n on t h e z o o p l a n k t o n p o p u l a t i o n s i n E u n i c e L a k e , a n d t o r e l a t e t h e n a t u r e a n d p a t t e r n o f t h i s p r e d a t i o n t o p r e y a v a i l a b i l i t y . The w o r k i s p r e s e n t e d i n f o u r s e c t i o n s . S e c t i o n s one a n d two d e a l w i t h t h e e c o l o g y o f E u n i c e L a k e z o o -p l a n k t o n and o f C h a o b o r u s r e s p e c t i v e l y . S e c t i o n t h r e e d i s c u s s e s l a r v a l f e e d i n g , a n d s e c t i o n f o u r d e s c r i b e s t h e i m p a c t o f l a r v a l f e e d i n g on t h e p r e y p o p u l a t i o n s i n t h e l a k e . 4 I I DESCRIPTION OF THE STUDY AREA I P h y s i c a l a n d C h e m i c a l F e a t u r e s The f i e l d s t u d y was c o n d u c t e d i n E u n i c e L a k e , a s m a l l o l i g o t r o p h i c l a k e i n t h e U.B.C. R e s e a r c h F o r e s t n e a r Haney ( F i g . 1 ) . I t h a s a n e l e v a t i o n o f 480 m, mean and maximum d e p t h s o f 15.8 a n d 42.0 m r e s p e c t i v e l y , a n d a s u r f a c e a r e a o f 18.2 h a . I t s d r a i n a g e a r e a i s a p p r o x -i m a t e l y 318 h a . I n 1971 t h e mean d a i l y d i s c h a r g e i n t o t h e l a k e was a b o u t 22,200 m^ g i v i n g a r e t e n t i o n t i m e o f 370 d a y s . The l a k e i s u s u a l l y i c e c o v e r e d f r o m m i d -December t o A p r i l o r May a n d h a s a w e 1 1 - d e v e l o p e d t h e r -m o c l i n e i n t h e summer a t a b o u t 4-6 m ( F i g . 2 ) . Oxygen r e a d i n g s w e r e h i g h down t o 25 m w h e r e o v e r 50% s a t u r a t i o n was r e c o r d e d . The w a t e r h a d a b r o w n h u m i c c o l o u r a n d a S e c c h i d e p t h o f 5-6 m r e c o r d e d d u r i n g 1 9 72. 2 B i o l o g i c a l F e a t u r e s The d o m i n a n t z o o p l a n k t e r s i n t h e summer w e r e two c o p e -p o d s p e c i e s - D i a p t o m u s t y r e l l i a n d D . k e n a i , f i v e s p e c i e s o f c l a d o c e r a n s - B o s m i n a l o n g i r o s t r i s , D a p h n i a r o s e a , D i a p h a n o s o m a b r a c h y u r u m , P o l y p h e m u s p e d i c u l u s , a n d H o l o p e -d i u m g i b b e r u r n , a n d two s p e c i e s o f C h a o b o r u s — C. airier i c a n u s and C . t r i v i t t a t u s . R o t i f e r s s u c h a s K e r a t e l l a a n d C o n o c h i l o i d e s w e r e p r e s e n t b u t t h e i r a b u n d a n c e n o t r e c o r d e d . T h e r e w e r e no f i s h i n t h e l a k e . I n t h e w i n t e r , a l l p l a n k t o n e x c e p t C h a o b o r u s became d r a s t i c a l l y r e d u c e d . I n t h e t e x t t h a t f o l l o w s , C . a m e r i c a n u s a n d C . t r i v i t t a t u s a r e r e f e r r e d F i g . l Contour map (in meters) of Eunice Lake. Sampling stat i o n s are c i r c l e d . Positions of r a f t i n 1971 and 1972 are i n d i c a t e d by squares. 6 F i g . 2 I s o t h e r m s i n E u n i c e Lake. 7 t o a s C. am 1-4, a n d C. t r 1-4, t h e numbers i n d i c a t i n g t h e d e v e l o p m e n t a l s t a g e o r i n s t a r o f t h e l a r v a , 4 t h i n s t a r b e i n g t h e o l d e s t . F o u r t h i n s t a r C . t r i v i t t a t u s w i l l be y c a l l e d C. t r 4 i f t h e y a r e i n t h e 1 s t y e a r o f t h e i r l i f e c y c l e ("young" l a r v a e ) a n d C. t r 4° i f t h e y a r e i n t h e 2nd y e a r o f t h e i r l i f e c y c l e ( " o l d " l a r v a e ) . 8 I I I GENERAL MATERIALS AND METHODS I T e m p e r a t u r e a n d Oxygen T e m p e r a t u r e was r e c o r d e d f r o m May t h r o u g h November o f 1971 a n d 1 9 7 2 , u s u a l l y o n c e a week, down t o 20 m a t 1 m i n t e r v a l s u s i n g a Y e l l o w S p r i n g s I n s t r u m e n t s t h e r m i s t o r . The a c c u r a c y o f t h e r e a d i n g s was +1°C. Oxygen p r o f i l e s w e r e t a k e n i n J u n e and S e p t e m b e r o f 1967 and i n A u g u s t and S e p t e m b e r o f 1969 w i t h a F r e s h w a t e r B i o l o g i c a l A s s o c -i a t i o n P r o b e (T.G. N o r t h c o t e , u n p u b l i s h e d d a t a ) , a n d i n J u l y o f 1971 a n d i n A u g u s t a n d O c t o b e r o f 1972 u s i n g a H a ch k i t w i t h a n a c c u r a c y o f + 1 ppm. 2 P l a n k t o n S a m p l i n g To f o l l o w t h e d e v e l o p m e n t o f l a r v a e b y i n s t a r , w e e k l y v e r t i c a l s a m p l e s w e r e t a k e n f r o m May t o November i n 1971 a n d 1972. I n 1 9 7 1 , h a u l s w e r e made b e t w e e n s t a t i o n s 1 and 2 ( F i g . 1 ) , by g r a d u a l l y l o w e r i n g t h e n e t down t o 25 m f r o m a s l o w l y m o v i n g b o a t . A 30 cm s q u a r e n e t w i t h a r e c t a n g u l a r mesh (0.65 x 0. 20 mm) was u s e d . I n 1 9 7 2 , v e r t i c a l h a u l s w e r e t a k e n , one a t e a c h o f t h e 5 s t a t i o n s ( F i g . 1) f r o m 0-15 m, o r 0-20 m, d e p e n d i n g o n s t a t i o n d e p t h . N e t s w i t h 69 o r 89 cm d i a m e t e r , 0.5 mm s q u a r e mesh a n d a No. 20 ( 0 . 0 8 mm mesh) b u c k e t w e r e u s e d d u r i n g t h e 1972 s a m p l i n g . A C l a r k e - B u m p u s s a m p l e r w i t h a No. 20 n y l o n n e t was u s e d t o d e t e r m i n e s e a s o n a l c h a n g e s i n a b u n d a n c e , 9 d e n s i t y , a n d v e r t i c a l d i s t r i b u t i o n o f z o o p l a n k t o n and o f C h a o b o r u s i n E u n i c e L a k e . H o r i z o n t a l t o w s , t a k e n b e t w e e n s t a t i o n s 1 a n d 2 ( F i g . 1) a t s e t s a m p l i n g d e p t h s ( s u r f a c e and e v e r y s e c o n d m e t e r f r o m 1 t o 21 m) f o r 1-3 m i n u t e s , w e r e r e p e a t e d a t b i w e e k l y i n t e r v a l s a t noon and m i d n i g h t f r o m May t h r o u g h November o f 1971 a n d 1972. I n 1 9 7 1 , m i d n i g h t s a m p l e s w e r e t a k e n o n l y e v e r y f o u r t h week. On 29-30 A u g u s t a n d 16-17 S e p t e m b e r 1 9 7 1 , s a m p l i n g was done e v e r y t h r e e h o u r s t o s t u d y t h e d i e l m i g r a t i o n o f p l a n k t o n . I n e a c h tow t h e n e t was o p e n f o r 1.5-3 m i n u t e s , f i l t e r i n g 0.5-1 c u b i c m e t e r s o f w a t e r . A c a l i b r a t i o n f a c t o r o f 4.2 l i t e r s p e r r e v o l u t i o n ( d e t e r m i n e d b y t h e I n s t i t u t e o f O c e a n o g r a p h y , U.B.C.) was u s e d t o c a l c u l a t e t h e v o l u m e s a m p l e d . The s a m p l e s w e r e p r e s e r v e d i n 4% F o r m a l i n b u f f e r e d w i t h b o r a x . The r e p l i c a b i l i t y o f C l a r k e - B u m p u s s a m p l e s was t e s t e d by t a k i n g f o u r t o w s i n c l o s e s u c c e s s i o n a t s e v e r a l d e p t h s on J u l y 20 o f 1971 f o r z o o p l a n k t o n and o n S e p t e m b e r 24 o f 19 72 f o r C h a o b o r u s . To d e t e r m i n e g e n e r a l d i f f e r e n c e s i n t h e a b u n d a n c e o f z o o p l a n k t o n a t t h e two e x t r e m e e n d s o f t h e l a k e , s t a t i o n 1, r e p r e s e n t i n g t h e "open" p a r t o f t h e l a k e a n d s t a t i o n 5, r e p r e s e n t i n g t h e n a r r o w "arm" o f t h e l a k e ( F i g . 1 ) , w e r e s a m p l e d i n 1972 o n 8 o c c a s i o n s . The C l a r k e - B u m p u s s a m p l e r was u s e d o n f o u r o f t h e s e o c c a s i o n s ; t h e o t h e r s a m p l e s w e r e c o l l e c t e d w i t h v e r t i c a l h a u l s . The d e p t h s a m p l e d v a r i e d f r o m 0-7 t o 0-20 m b u t was s i m i l a r f o r b o t h s t a t i o n s o n a n y one d a y . 10 3 P l a n k t o n C o u n t i n g C h a o b o r u s l a r v a e w e r e r e m o v e d f r o m t h e s a m p l e s a nd c o u n t e d by s p e c i e s a n d i n s t a r . The r e m a i n i n g p l a n k t o n w e r e c o u n t e d u s i n g a d i s s e c t i n g m i c r o s c o p e . S a m p l e s w i t h h i g h d e n s i t i e s o f a p a r t i c u l a r s p e c i e s w e r e s u b -d i v i d e d i n t o 6 o r 36 p a r t s a n d one s u c h p a r t was c o u n t e d . T h i s was r e p e a t e d 3 t i m e s t o o b t a i n t h e mean v a l u e f o r t h a t s p e c i e s . The f o l l o w i n g e i g h t p l a n k t o n c a t e g o r i e s w e r e e s t a b l i s h e d : n a u p l i i , D i a p t o m u s t y r e l l i , D i a p t o m u s  k e n a i , B o s m i n a , P o l y p h e m u s , P a p h n i a , D i a p h a n o s o m a , and H o l o p e d i u m . D . t y r e l l i i n c l u d e d t h e s m a l l c o p e p o d i d s < 1.75 mm i n l e n g t h ; P . k e n a i i n c l u d e d t h e l a r g e c o p e p o d i d s ^ 1 . 7 5 mm. T h e s e s i z e c l a s s e s w e r e f o u n d t o be m e a n i n g -f u l f o r d i s t i n g u i s h i n g b e t w e e n t h e two c o p e p o d s p e c i e s . P a p h n i a f r o m 1971 w e r e s e p a r a t e d i n t o s m a l l a n d l a r g e i n d i v i d u a l s o n t h e same b a s i s . 4 P l a n k t o n W e i g h i n g C h a o b o r u s c o l l e c t e d f o r w e i g h i n g w e r e i m m e d i a t e l y s e p a r a t e d f r o m o t h e r z o o p l a n k t o n t o p r e v e n t f e e d i n g . F r e s h l y c o l l e c t e d z o o p l a n k t o n w e r e d r i e d a t 100° o r 60°C f o r 2 d a y s a nd w e i g h e d on a Cahn Gram e l e c t r o b a l a n c e . U s u a l l y 5 r e p l i c a t e s o f e a c h s a m p l e w e r e w e i g h e d ; r e p l i c a t e s c o n t a i n e d 50 o r more a n i m a l s d e p e n d i n g o n t h e s i z e o f p l a n k t o n . 11 5 S i z e M e a s u r e m e n t s A l l m e a s u r e m e n t s w e r e made u s i n g a c a l i b r a t e d o c u l a r m i c r o m e t e r i n a d i s s e c t i n g m i c r o s c o p e , a t 20 X o r 30 X m a g n i f i c a t i o n . Maximum l e n g t h , w i d t h , a nd t h i c k n e s s o f e a c h p r e y t y p e w e r e d e t e r m i n e d , e x c l u d i n g t h e t a i l s p i n e o f c l a d o c e r a n s a n d t h e c a u d a l s e t a e o f c o p e p o d s . Head l e n g t h s , a n t e n n a a n d l o n g e s t s e t a l e n g t h s , a n d mouth s i z e s ( e x c e p t 1 s t i n s t a r ) o f a l l l a r v a l i n s t a r s o f C. a m e r i c a n u s a n d o f C . t r i v i 1 1 a t u s , w e r e m e a s u r e d a s shown i n F i g . 3. The m a x i m a l gape o f t h e l a r v a l mouth was t a k e n a s t h e w i d t h o f t h e h e a d c a p s u l e j u s t b e h i n d t h e e y e s , a s s e e n f r o m a v e n t r a l v i e w . C l o s e e x a m i n a t i o n o f t h e mouth r e g i o n o f l a r v a s w a l l o w i n g l a r g e p r e y showed t h i s t o b e a m e a n i n g f u l m e a s u r e m e n t . 6 Swimming V e l o c i t y o f Z o o p l a n k t o n Swimming v e l o c i t i e s a t v a r i o u s t e m p e r a t u r e s w e r e m e a s u r e d d u r i n g 1972 f o r s i x t y p e s o f z o o p l a n k t o n f r o m E u n i c e L a k e - a d u l t s a n d j u v e n i l e s o f D i a p t o m u s k e n a i , D . t y r e l l i , B o s m i n a , D i a p h a n o s o m a , a n d P o l y p h e m u s . The t e m p e r a t u r e s u s e d w e r e t h o s e t h e p l a n k t e r s commonly e n -c o u n t e r e d i n t h e l a k e . A l l z o o p l a n k t o n w e r e t e s t e d w i t h i n t h e f i r s t f e w d a y s a f t e r c o l l e c t i o n . P l a n k t o n w e r e a c c l i m a t i z e d t o t h e e x p e r i m e n t a l t e m p e r a t u r e s f o r s e v e r a l h o u r s u n d e r i n c a n d e s c e n t room l i g h t i n g . F i v e t o t e n i n d i v i d u a l s w e r e t h e n p l a c e d w i t h a p i p e t t e i n t o a f l a t t r a y (30 cm X 45cm X 2cm) w i t h a g r i d d e d f l o o r 12 M O U T H M O U T H D I A M E T E R Fig.3 L a t e r a l view (a), and v e n t r a l view (b) , of a l a r v a l head showing the three head parameters measured. (MAG.=40X). 13 ( l / 2 c m x l/2cm) c o n t a i n i n g w a t e r a t t h e e x p e r i m e n t a l tem-p e r a t u r e , 1 cm d e e p . The p l a n k t e r s w e r e a l l o w e d t o r e -c o v e r a few m i n u t e s b e f o r e t h e i r v e l o c i t y was m e a s u r e d . A s i n g l e i n d i v i d u a l was o b s e r v e d c o n t i n u o u s l y f o r 1-3 m i n u t e s a n d t h e number o f s q u a r e s c r o s s e d i n t h a t t i m e was r e c o r d e d . A n i m a l s w h i c h c o l l i d e d w i t h t h e w a l l s o f t h e t r a y d u r i n g t h e o b s e r v a t i o n p e r i o d o r d i d n o t swim f r e e l y w e r e n o t m e a s u r e d . E a c h e x p e r i m e n t i n c l u d e d 10-15 d i f f e r e n t a n i m a l s . To d e t e r m i n e t h e l e a p i n g v e l o c i t y o f D . k e n a i , t h e c o p e p o d s w e r e p l a c e d i n t o a s h a l l o w p e t r i d i s h a n d f i l m e d a t a s p e e d o f 32 f r a m e s / s e c . D u r i n g t h e 90 s e c o n d s o f f i l m i n g , t h e D . k e n a i w e r e s t i m u l a t e d t o e s c a p e w i t h a p r o b e . To c a l c u l a t e t h e mean v e l o c i t y o f e a c h l e a p , d i s t a n c e s c o v e r e d b y e a c h l e a p i n a s e q u e n c e o f f r a m e s w e r e a d d e d a n d a v e r a g e d . The e x p e r i m e n t a l d e s i g n was s u c h t h a t t h e movements o f p l a n k t o n w e r e r e s t r i c t e d t o a h o r i z o n t a l p l a n e . D . k e n a i i n p a r t i c u l a r , w e r e o b s e r v e d t o c r u i s e f a s t e r i n s h a l l o w v e s s e l s t h a n i n d e e p o n e s . The v a l u e s o b t a i n e d h e r e may n o t t h e r e f o r e be r e p r e s e n t a t i v e o f t h e n a t u r a l s i t u a t i o n . 7 C h a o b o r u s G r o w t h E x p e r i m e n t I n t e s t i n g t h e e f f e c t o f e x c e s s f o o d o n t h e g r o w t h o f C h a o b o r u s l a r v a e , t h r e e c l e a r , 10 m i l p o l y e t h y l e n e t u b e s , 1 m i n d i a m e t e r , a n d 30 m l o n g , w e r e f i x e d t o a 14 r a f t i n t h e d e e p p a r t o f t h e l a k e ( F i g . 4 ) . O x y g e n , m e a s u r e d j u s t b e f o r e t h e e n d o f t h e e x p e r i m e n t , was n e a r f u l l s a t u r a t i o n b o t h i n t h e l a k e a n d i n t h e t u b e s w i t h 7 0% s a t u r a t i o n a t 20 m. The e n c l o s u r e s w e r e f i l l e d a n d l e f t u n d i s t u r b e d f o r one month p r i o r t o t h e s t a r t o f t h e e x p e r i m e n t w h i c h r a n s e v e n w e e k s (2 A u g u s t - 1 1 O c t o b e r , 1 9 7 2 ) . T h r e e t i m e s t h e n a t u r a l d e n s i t y o f am 3, t r 3, and 4° l a r v a e , a n d t e n t i m e s t h e n a t u r a l d e n s i t y o f z o o p l a n k -t o n w e r e a d d e d o n A u g u s t 2. I n i t i a l d r y w e i g h t s o f a l l t h r e e C h a o b o r u s g r o u p s w e r e m e a s u r e d a t t h a t t i m e . T w i c e w e e k l y , a b o u t 20 l i t r e s o f c o n c e n t r a t e d E u n i c e L a k e 2 z o o p l a n k t o n , c o l l e c t e d w i t h a n e t o f 0.5 mm mesh s i z e , w e r e a d d e d t o e a c h t u b e . C h a o b o r u s a n d H o l o p e d i u r n w e r e e x c l u d e d f r o m t h i s m i x t u r e . G r o w t h o f l a r v a e was d e f i n e d a s t h e d i f f e r e n c e b e t w e e n i n i t i a l a n d f i n a l d r y w e i g h t s . W e i g h t s o f l a r v a e f r o m t h e t h r e e t u b e s w e r e p o o l e d s i n c e no s i g n i f i c a n t d i f f e r e n c e s w e r e f o u n d among i n d i v i d u a l t u b e s . P i g m e n t a t i o n o f t h e h e a d and l e n g t h o f t h e p r e m an-d i b u l a r a p p e n d a g e s w e r e t h e c h a r a c t e r i s t i c s u s e d t o d i f f e r -e n t i a t e b e t w e e n t £ 4 V a n d t r 4° l a r v a e f r o m t h e t u b e s . O l d l a r v a e h a d s t r o n g l y p i g m e n t e d h e a d s a n d u s u a l l y b r o k e n p r e m a n d i b u l a r a p p e n d a g e s ; y o u n g l a r v a e w e r e l e s s p i g m e n t e d and h a d l o n g e r , u n b r o k e n p r e m a n d i b u l a r a p p e n d a g e s . 16 I V BIOLOGY OF ZOOPLANKTON 1 R e s u l t s a Body s i z e a n d w e i g h t A w i d e s i z e r a n g e o f p o t e n t i a l z o o p l a n k t o n p r e y was a v a i l a b l e t o C h a o b o r u s l a r v a e i n E u n i c e L a k e ( T a b l e 1 ) . P r e y i n d i v i d u a l s o f 1 mm i n l e n g t h or l e s s a n d n o t more t h a n 6 ug i n w e i g h t i n c l u d e d r o t i f e r s , n a u p l i i , B o s m i n a , P o l y p h e m u s , D i a p h a n o s o m a , and D i a p t o m u s t y r e l l i . The n e x t l a r g e r s i z e c l a s s ( a p p r o x . 1-2 mm) i n c l u d e d H o l o p e -d i u m , D a p h n i a , D i a p t o m u s k e n a i j u v e n i l e s a nd some D . k e n a i a d u l t s . A d u l t D . k e n a i , w i t h a mean l e n g t h o f 2.3 mm and a mean d r y w e i g h t o f 42.7 u g , w e r e t h e l a r g e s t z o o p l a n k t e r s i n E u n i c e L a k e , e x c e p t f o r a d u l t H o l o p e d i u m w h i c h h a v e a l a r g e g e l a t i n o u s e n v e l o p e up t o 4 mm i n d i a m e t e r . b S p e e d o f swimming a n d movements R e s p o n s e t o t e m p e r a t u r e a n d t h e m a g n i t u d e o f swimming s p e e d s o f p l a n k t o n w e r e f o u n d t o b e s i z e a nd s p e c i e s s p e c i f i c ( F i g . 5 ) . Swimming s p e e d s o f a d u l t D . k e n a i w e r e n e a r l y d o u b l e t h o s e o f i t s j u v e n i l e s . I n c r e a s e i n tempe-r a t u r e c a u s e d a s t r i k i n g i n c r e a s e i n t h e swimming s p e e d o f P o l y p h e m u s , b u t h a d v i r t u a l l y no e f f e c t on o t h e r p l a n k t o n t y p e s . D i a p t o m u s t y r e l l i a n d D i a p h a n o s o m a h a d t h e s l o w e s t s p e e d o f t h e p l a n k t o n t e s t e d . B o s m i n a and P o l y p h e m u s swam f a s t e r , t h e l a t t e r b e c o m i n g v e r y a c t i v e a t t e m p e r a t u r e s > 1 5 ° C . D . k e n a i w e r e t h e f a s t e s t o f t h e z o o p l a n k t o n a t TABLE I Mean length and dry weight of Eunice Lake zooplankton. Length (mm) Dry weight (ug) K e r a t e l l a sp. 0.10 Calanoid n a u p l i i 0.35 Bosmina l o n g i r o s t r i s 0.29 Polyphemus pediculus 0.59 Diaphanosoma brachyurum 0.64 Holopedium gibberum (small) 0.64 Diaptomus t y r e l l i 1.08 Holopedium gibberum (large) (without gelatinous envelope) 1.28 (with gelatinous envelope) 3.98 Paphnia rosea (small) 1.29 Paphnia rosea (large) 1.93 Calanoid copepodids (small) 1.38 Calonoid copepodids (large) 2.0 Diaptomus kenai 2.33 0.005* 0.01* 1.90** 3.07 5.0* 5.9 30.6 12.5 f 23.0 f 6.8 f t 28.0 42.7 * H a l l e t a l . (1970). ** from mean volume estimates of four models m u l t i p l i e d by a s p e c i f i c g r a v i t y of 1.025 (H a l l e t a l . 1970). T Baudouin and Ravera (1972). t t from formula W = 0.055 h'73{ Edmonds on and Winberg 1971). 18 Fig.5 Means t 1 S.E. of swimming v e l o c i t i e s of zooplankton at d i f f e r e n t temperatures. 19 a l l t e m p e r a t u r e s e x c e p t f o r P o l y p h e m u s a t h i g h t e m p e r a -t u r e s . Swimming b e h a v i o r o f z o o p l a n k t o n was a l s o f o u n d t o be s p e c i e s s p e c i f i c . B o s m i n a a n d P o l y p h e m u s " s c u r r i e d a b o u t " w i t h s h o r t j e r k y l e a p s . B o t h s p e c i e s h a v e b e e n n o t e d f o r a g g r e g a t i n g i n t o c l u s t e r s (Dumont 1967; B u t o r i n a 1 9 7 1 ) , w h i c h may t e n d t o a t t r a c t b u t a l s o c o n f u s e t h e i r p r e d a t o r s . By c o n t r a s t , D i a p h a n o s o m a a n d t h e c o p e p o d s d i d n o t a g g r e -g a t e a n d moved s l o w l y and s m o o t h l y i n a n e a r h o r i z o n t a l p o s i t i o n , r e m a i n i n g m o t i o n l e s s i n t h e w a t e r c o l u m n f o r l e n g t h y i n t e r v a l s . T h e i r p a t h s w e r e l e s s t o r t u o u s t h a n t h o s e o f B o s m i n a and P o l y p h e m u s ; t h i s a l l o w e d D . k e n a i t o c o v e r a g r e a t e r a r e a p e r u n i t o f t i m e . U n l i k e B o s m i n a a n d P o l y p h e m u s w h i c h d i d n o t r e a c t t o d i s t u r b a n c e f r o m a p r o b e , D i a p h a n o s o m a and t h e d i a p t o m i d s d a r t e d away f r o m i t w i t h r a p i d l e a p s . D . k e n a i c o v e r e d 3-10 cm w i t h e a c h l e a p a t a mean v e l o c i t y o f 147 c m / s e c . D i a p h a n o s o m a , i f d i s t u r b e d , may jump a d i s t a n c e o f 3-6 cm (V. K o r i n e k 1 9 7 2 , p e r s o n a l c o m m u n i c a t i o n ) . c S e a s o n a l c h a n g e s i n s t a n d i n g c r o p a n d i n s p e c i e s c o m p o s i t i o n S t a n d i n g c r o p o f z o o p l a n k t o n was e x p r e s s e d , w h e r e p o s s i b l e , a s t h e a v e r a g e o f n o o n a n d m i d n i g h t v a l u e s . Z o o p l a n k t o n w e r e m o s t a b u n d a n t i n t h e l a k e b e t w e e n May and November ( F i g . 6 ) . The s t a n d i n g c r o p s o f P a p h n i a , D i a p h a n o s o m a , a n d P . k e n a i , w e r e much l o w e r i n 1972 . 20 D. TYRELLI O X DC LU D. 0C LU CO 1 J r-j i A i s i oi*~i5^-i o 30 20r 10 HOLOPEDIUM 20 10 J I A I S 1 O 1 N~l 0 k DIAPHANOSOMA 10 BOSMINA 10 N I 0 POLYPHEMUS Fig.6 Standing crop of zooplankton groups i n 1971 (dotted l i n e ) and i n 1972 ( s o l i d l i n e ) . Holopedium were not recorded i n 1971. 21 t h a n i n 19.71. H o l o p e d l u r n was n o t c o u n t e d i n 1971 b u t was a b u n d a n t i n t h a t y e a r . An e x t r e m e l y h e a v y r a i n f a l l w h i c h o c c u r r e d o n J u l y 11-12 i n 1972 (14.7 cm o f r a i n i n 48 h o u r s ) , s h a r p l y r a i s e d t h e l a k e l e v e l a n d p r o b a b l y f l u s h e d o u t much o f t h e e p i l i m n e t i c w a t e r . T h i s may ha v e c a u s e d t h e d e c l i n e o f H o l o p e d i u m a n d o f P a p h n i a i n J u l y o f t h a t y e a r a n d m o d i f i e d t h e s e a s o n a l c h a n g e i n s t a n d -i n g c r o p s o f t h e o t h e r c l a d o c e r a n s . P a p h n i a w e r e r e c o r d e d i n c o n s i d e r a b l e numbers t h r o u g h o u t t h e summer s e a s o n o f 1967 , 1 9 6 9 , a n d 1970 (T.G. N o r t h c o t e , u n p u b l i s h e d d a t a ) a n d i n 1 9 7 1 , b u t d i s a p p e a r e d e n t i r e l y f r o m t h e l a k e a f t e r t h e J u l y " f l o o d " i n 197 2 . They w e r e r e l a t i v e l y s c a r c e i n 1973 (C. W a l t e r s 1 9 7 3 , p e r s o n a l c o m m u n i c a t i o n ) . The o v e r a l l s t a n d i n g c r o p o f z o o p l a n k t o n by number an d b y b i o m a s s d i f f e r e d i n t h e two y e a r s b y a f a c t o r o f 3-4. P e a k v a l u e s o f 90,000 i n d i v i d u a l s / m 2 a n d 900 mg/m 2 w e r e r e c o r d e d i n 1971 b u t o n l y 30,000 i n d i v i d u a l s / m 2 a n d 200 mg/m 2 w e r e r e c o r d e d i n 1 9 7 2 . ( F i g . 7 ) . P e n s i t y o f c o p e -p o d s a n d c l a d o c e r a n s o f a l l s t a g e s was u s u a l l y l e s s t h a n 1 0 / l i t e r , d u r i n g b o t h y e a r s ( F i g . 8 ) . On t h i s b a s i s , E u n i c e L a k e , when c o m p a r e d t o o t h e r l a k e s ( A l l a n 1972; P o d s o n 1 9 7 2 ) , h a s a l o w z o o p l a n k t o n s t a n d i n g c r o p . I n t h e s e a s o n a l c o m p a r i s o n o f s p e c i e s c o m p o s i t i o n , t h e l a k e p l a n k t o n w e r e s u b d i v i d e d i n t o s e v e n c a t e g o r i e s t o p a r a l l e l t h e g r o u p i n g o f p r e y f o r t h e s t u d y o f l a r v a l f o o d . R o t i f e r s , n a u p l i i , a n d C h a o b o r u s w e r e n o t i n c l u d e d . A t any o ne t i m e , 2-4 z o o p l a n k t o n s p e c i e s c o n t r i b u t e d o v e r T o t a l standing crop by number (N), and by biomass (B), of zooplankton i n 1971 (dotted l i n e ) and i n 1972 ( s o l i d l i n e ) . R o t i f e r s and Chaoborus were excluded. Holopedium were not counted i n 1971. 23 80% o f t h e t o t a l s t a n d i n g c r o p o f p l a n k t o n b o t h b y num-b e r a n d b y b i o m a s s ( F i g . 2 1 ) . D . t y r e H i w e r e n u m e r i -c a l l y t h e m o s t a b u n d a n t s p e c i e s a t a l l t i m e s . I n J u l y a n d A u g u s t , B o s m i n a w e r e t h e s e c o n d m o s t a b u n d a n t s p e c i e s . E x c e p t i n t h e f a l l w h e n H o i o p e d i u m became v e r y n u m e r o u s , D . k e n a i a n d D . t y r e l l i c o m p r i s e d t h e b u l k ( 7 0%-90%) o f t h e p l a n k t o n b i o m a s s i n t h e l a k e . d D i e l d i s t r i b u t i o n a n d m i g r a t i o n The v e r t i c a l d i s t r i b u t i o n o f E u n i c e L a k e z o o p l a n k -t o n was v e r y s i m i l a r i n 1971 a n d 1 9 7 2 , a n d w i l l be d i s -c u s s e d j o i n t l y f o r t h e two y e a r s . M o n t h l y d i s t r i b u t i o n o f p l a n k t o n i n 1972 i s shown i n F i g . 8. T h r o u g h o u t t h e summer t h e h i g h e s t c o n c e n t r a t i o n o f o r g a n i s m s a t m i d d a y a n d m i d n i g h t was f o u n d a b o v e t h e t h e r m o c l i n e (0-6 m). N a u p l i i a n d D . t y r e l l i d i d n o t m i g r a t e v e r t i c a l l y ; B o s m i n a , D i a p h a n o s o m a , a n d H o l o p e d i u m moved s l i g h t l y up-w a r d a t n i g h t . O n l y t h e a d u l t D a p h n i a m i g r a t e d u p w a r d a t n i g h t t o a n y g r e a t e x t e n t . D u r i n g t h e d a y t h e y w e r e f o u n d down t o 15 m, b u t a t n i g h t t h e y moved t o t h e u p p e r -m o s t 5 m ( 1 9 7 1 d a t a ) . P o l y p h e m u s showed a s l i g h t r e v e r s e v e r t i c a l m i g r a t i o n a n d D . k e n a i a s t r o n g r e v e r s e v e r t i c a l m i g r a t i o n . D . k e n a i u s u a l l y c o n c e n t r a t e d a t a d e p t h o f 0-7 m d u r i n g t h e d a y , became d i f f u s e l y s t r a t i f i e d down t o 20 m f r o m m i d n i g h t t o 6 A.M., a n d r e t u r n e d t o t h e s u r f a c e l a y e r s by l a t e m o r n i n g . 24 ZOOPLANKTON CHAOBORUS NOON MT NOON MT NUMBER OF PLANKTON PER 100 LITERS Fig.8 Noon and midnight (MT) v e r t i c a l d i s t r i b u t i o n of zooplankton types (on l e f t ) , and of Chaoborus types (on right), i n Eunice Lake i n 1972. Letters i n d i c a t e type of plankton: N - n a u p l i i , K-D.kenai, B-Bosmina, D-Daphnia, DI-Diaphanosoma, H-Holopedium, P-Polyphemus, CL-combined cladoceran types. Roman numerals show i n s t a r s of C.americanus,' am,(on l e f t ) , and of C . t r i v i t -t a t u s , t r , ( o n r i g h t ) . 25 e P a t c h i n e s s i n p l a n k t o n d i s t r i b u t i o n A c h i - s q u a r e a n a l y s i s t o t e s t f o r r e p l i c a b i l i t y o f C l a r k e - B u m p u s s a m p l e s t a k e n a t t h e same d e p t h , showed t h a t s a m p l e s o f C h a o b o r u s l a r v a e a n d o f P o l y p h e m u s w e r e o b t a i n e d r a n d o m l y ; c h i - s q u a r e v a l u e s f o r P . t y r e l l i , D . k e n a i , a n d P a p h n i a i n d i c a t e d t h a t t h e s e w e r e s a m p l e d n o n - r a n d o m l y a t 0.99 p r o b a b i l i t y l e v e l ( T a b l e I I . ) W h i l e t h i s s a m p l i n g t e c h n i q u e p r o d u c e d r e p l i c a b l e r e s u l t s f o r C h a o b o r u s l a r v a e , a l a r g e r s a m p l e s i z e w i t h a t o w i n g t i m e > 2 m i n u t e s , may be n e c e s s a r y f o r a c c u r a t e m o n i t o r -i n g o f o t h e r z o o p l a n k t o n p o p u l a t i o n s . The w e e k l y C h a o b o r u s s e r i e s o f f i v e r e p l i c a t e d h a u l s c o l l e c t e d i n 1972 w e r e u s e d t o t e s t f o r h o r i z o n t a l p a t c h i -n e s s i n t h e l a r v a e . A c h i - s q u a r e a n a l y s i s made on e a c h o f t h e 10 s e r i e s o f v e r t i c a l h a u l s c o l l e c t e d t h r o u g h o u t t h e summer showed t h a t t h e p r o b a b i l i t y o f r a n d o m d i s t r i b -u t i o n o f l a r v a e was l e s s t h a n 1% o n a l l s a m p l i n g d a t e s a n d no s e a s o n a l t r e n d was e v i d e n t ( T a b l e I I I ) . S u c h h o r i z o n -t a l p a t c h i n e s s i n C h a o b o r u s l a r v a e h a s a l s o b e e n n o t e d by T e r a g u c h i a n d N o r t h c o t e , 1966. F i e l d o b s e r v a t i o n s d u r i n g 1971 a n d 1972 showed t h a t z o o p l a n k t o n , b u t n o t C h a o b o r u s , w e r e c o n s i s t e n t l y more a b u n d a n t i n t h e s h a l l o w e r , n a r r o w "arm" o f t h e l a k e , t h a n i n i t s d e e p e r , "open" p o r t i o n . S i m u l t a n e o u s s a m p l i n g o f t h e s e two a r e a s showed t h a t t h e b i o m a s s o f z o o p l a n k t o n p e r u n i t a r e a a t t h e s h a l l o w e n d ( s t a t i o n 5) e x c e e d e d t h e 26 b i o m a s s a t t h e deep e n d ( s t a t i o n 1) b y up t o 7 f o l d i n a l l c a s e s b u t one ( T a b l e I V ) . I n c o n t r a s t , b i o m a s s o f C h a o b o r u s l a r v a e a t t h e two s t a t i o n s was s i m i l a r . TABLE II Comparison of 4 h o r i z o n t a l Clarke-Bumpus tows taken at noon between s t a t i o n s 1 and 2. Values are given i n numbers per 100 l i t e r s . Sampling depths and dates are i n d i c a t e d i n pare n t h e s i s . Tow No. i. t y r e l l i 7 3 m ) \20-7-71j ZOOPLANKTON l_. kenai Paphnia f 3 m \ ( 3 m ) Polyphemus / 3 m \ \20-7-71j CHAOBORUS C. am 4 C. t r 4 C. t r 4_ ( 2 + 3 m] (10 + 11 m) (10 + 11 ml / \24-9-72/ ^24-9-72 / \24-9-72 / I r 41" + 11 •9C. 14 24 t r 4" + 15 -9-72 1 186.9 ' 240.9 212.5 2.8 2.3 3.3 5.1 3.9 2 203.0 222.9 293.2 2.9 2.0 4.2 5.0 2.6 3 217.1 192.1 259.1 2.9 3.1 2.7 4.6 4.9 4 • 154.6 267.0 154.6 3.3 2.2 3.1 3.7 3.1 761.6 922.7 919.4 11.9 9.6 13.2 18.4 14.5 X 190.4 230.7 229.8 3.0 2.4 3.3 4.6 3.6 147,182.3 215,836.2 222,143.1 35.3 23.7 44.7 85.5 55.8 x2 11.4 12.9 47.2 0.1 0.28 0.37 0.27 0.85 P 0.01-0.001 0.01-0.001 <0.001 >0.99* 0.99-0.95* 0.95-0.05* 0.99-0.95* 0.95-0.05 * good r e p l i c a b i l i t y of tows No TABLE I I I Comparison of the t o t a l number of Chaoborus l a r v a e per standard v e r t i c a l net h a u l taken a t 2 each o f the 5 s t a t i o n s i n Eunice Lake d u r i n g 1972. Values are gi v e n i n numbers per 0.62 m. S t a t i o n No. J u l y 5 J u l y 19 Aug. 2 Aug. 6 Sept. 6 Sept. 21 Oct. 4 Oct. 22 Nov. 4 Nov. 18 1 111 108 94 63 81 122 74 60 83 87 2 86 , 63 53 81 72 7 71 94 157 39 3 85 ' 151 74 110 58 98 86 83 66 126 4 27 124 78 139 37 73 127 90 62 437 5 68 .316 255 174 70 134 50 301 79 191 £x 377. 762. 554.0 567.0 318. 434. 408. 628. 447. 880. X 75.4 152. 4 110.8 113.4 63. 6 86. 8 81.6 125. 6 89.7 176. 0 32,295. 153 ,666. 88 ,230. 72,227. 21 ,378. 47,822. 36 ,542. 90 ,601. 45 ,979. 252 ,416. x 2 51.32 246. 31 242.30 69.92 18. 13 116. 94 39.82 93. 35 67.31 554. 18 p <.001 001 -c .001 < .001 01 001 < .001 001 < .001 001 CO TABLE IV Comparison of zooplankton and of Chaoborus biomass between station 5 and station 1 i n 1972. Values are given i n mg per m. ; Date Time Station 5 ZOOPLANKTON Station 1 Station 1 {Station 5 Station 5 CHAOBORUS Station 1 Station 1 :Station 5 7 June * Noon 53.0 53.5 1:1 19.7 65.4 3.3:1 22 Aug.t Noon 130.5 108.5 1:1.2 86.3 97.2 1.1:1 22 Aug.t Midnight 126.8 84.8 1:1.5 184.9 392.0 , 2.1:1 12 Nov.* Noon 42.8 7.8 1:5.3 155.1 132.5 .1:1.2 18 Nov.* Noon 29.0 4.3 1:6.7 254.9 115.8 1:2.2 18 Nov.t Noon 57.6' 21.3 1:2.7 156.4 51.8 1:3 18 Nov t Midnight 79.2 20.2 1:3.9 346.6 199.6 1:1.7 26 Nov.* Noon 24.3 9.0 1 1:2.7 174.0 254.3 1.5:1 1 * samples collected with v e r t i c a l hauls t samples collected with a Clarke-Bumpus sampler to 30 3 Summary 1 E u n i c e L a k e i s o l i g o t r o p h i c w i t h a l o w s t a n d i n g c r o p o f z o o p l a n k t o n . 2 M o s t o f t h e p l a n k t o n w e r e f o u n d a b o v e a d e p t h o f 6 m a t a l l t i m e s . E x c e p t i o n s w e r e a d u l t D a p h n i a a n d a d u l t D . k e n a i w h i c h s t a y e d w e l l b e l o w t h a t d e p t h d u r i n g t h e d a y a n d n i g h t r e s p e c t i v e l y . 3 The s p e c i e s c o m p o s i t i o n o f z o o p l a n k t o n v a r i e d c o n -s i d e r a b l y w i t h s e a s o n , a n d p l a n k t o n a b u n d a n c e d i f f e r e d s i g n i f i c a n t l y b e t w e e n t h e two y e a r s o f s t u d y . 4 The h o r i z o n t a l d i s t r i b u t i o n o f C h a o b o r u s a n d o f i t s p r e y was p a t c h y a n d n o t n e c e s s a r i l y c o i n c i d e n t w i t h e a c h o t h e r . 5 D i f f e r e n c e s among z o o p l a n k t o n s p e c i e s w e r e f o u n d i n b o d y s i z e , swimming s p e e d , a n d t y p e o f movement. 6 A l l o f t h e a b o v e f a c t o r s may a f f e c t C h a o b o r u s f e e d i n g b y d e t e r m i n i n g t h e p h y s i c a l a v a i l a b i l i t y o f p r e y a n d t h e n a t u r e o f s i z e a n d s p e c i e s s e l e c -t i v e p r e d a t i o n by t h e l a r v a e . 31 V BIOLOGY OF CHAOBORUS LARVAE 1 R e s u l t s a Head m o r p h o l o g y o f l a r v a l i n s t a r s The t h r e e C h a o b o r u s h e a d m e a s u r e m e n t s ( h e a d l e n g t h , a n t e n n a a n d s e t a l e n g t h , a n d mouth d i a m e t e r ) , w e r e i n s t a r a n d s p e c i e s s p e c i f i c , a n d t h e s i z e v a r i a t i o n was more p r o -n o u n c e d among t h e o l d e r l a r v a e t h a n among t h e y o u n g e r o n e s ( F i g . 9 ) . A l l 3 i n s t a r s o f C . a m e r i c a n u s h a d s m a l l e r h e a d d i m e n s i o n s t h a n t h e c o r r e s p o n d i n g i n s t a r s o f C . t r i v -i t t a t u s . T h e r e was no o v e r l a p among t h e i n s t a r s . S i z e s i m i l a r i t y b e t w e e n a n y t w o i n s t a r s was g r e a t e r b e -t w e e n s p e c i e s t h a n w i t h i n s p e c i e s : e g . C a m 3 w e r e more s i m i l a r t o C . t r 2 and C . t r 3 t h a n t o C.am 2 a n d C.am 4. b D e v e l o p m e n t o f l a r v a e The l a r v a l d e v e l o p m e n t p a t t e r n h a s a l r e a d y b e e n d e s -c r i b e d ( F e d o r e n k o a n d S w i f t 1 9 7 2 ) . I n s t a r s u c c e s s i o n i s shown f o r 1971 a n d 1972 i n F i g . 10. The l a r v a e o f b o t h s p e c i e s h a d a s h o r t e g g - l a y i n g p e r i o d i n l a t e s p r i n g a n d e a r l y summer, m o l t e d s y n c h r o n o u s l y t h r o u g h t h e i r f o u r i n -s t a r s f r o m J u n e t o A u g u s t , a nd g r e w t o n e a r a d u l t l e n g t h d u r i n g t h i s t i m e . C . a m e r i c a n u s h a d a one y e a r g e n e r a t i o n t i m e , t h e o v e r w i n t e r i n g l a r v a e e m e r g i n g t h e f o l l o w i n g s p r i n g t o l a y t h e i r e g g s . C . t r i v i t t a t u s l a r v a e w i t h a g e n e r a t i o n t i m e o f two y e a r s e m e r g e d a y e a r l a t e r . A t any one t i m e , t h e r e f o r e , E u n i c e L a k e was i n h a b i t e d by 32 2.2 2.0 1.8 1.6 I 1.4 - 1.2 n | 1.0 LU .4 HEAD ANTENNA AND SETA MOUTH F i g . 9 Means t 1 S.D. o f head l e n g t h , a n t e n n a and s e t a l e n g t h , and mouth d i a m e t e r o f l a r v a l i n s t a r s ( i n Roman numer a l s ) o f C.americanus (open c i r c l e s ) and C . t r i v i t t a t u s ( s o l i d c i r c l e s ) . F i r s t i n s t a r s o f b o t h s p e c i e s a r e combined ( h a t c h e d c i r c l e s ) . Number o f a n i m a l s measured i s g i v e n i n p a r e n t h e s i s . o\° OF LARVAE vQ M O W -J o MO o • (D rt H < a rt rt m rt c cn in cn H-O 3 CD vQ 3 cn ft p) in 3" D> rt o 3" CD vo O Cu -J rh M H- | 0 cn rt CD rh l O H 3 CD ft H 3" Cu CD 3 rh & ft 3 P 05 g trt-e-r ee 18 2-I MAY | JUNE I JULY I AUG I SEPT I OCT I NOV T Fig.11 Seasonal changes i n l a r v a l i n s t a r length i n 1971. Means and 95% confidence l i m i t s are shown f o r each i n s t a r (Roman numerals) of C.americanus (dotted l i n e s ) and C . t r i v i t t a t u s ( s o l i d l i n e s ) . co Tfrom Fedorenko and Swift 1972). 1971 1972 1973 Fig.12 Seasonal changes i n l a r v a l weight i n 1971 and 1972. Means, 95% confidence l i m i t s , and the year hatched are shown f o r each species, (from Swift 1973).• • \ 36 C . a m e r i c a n u s a n d C . t r i v i t t a t u s l a r v a e h a t c h e d i n t h a t y e a r a n d by C . t r i v i t t a t u s l a r v a e i n t h e i r s e c o n d y e a r . The one week d i f f e r e n c e i n i n s t a r s u c c e s s i o n b e t w e e n 1971 and 1972 may be a t t r i b u t e d t o t h e r e l a t i v e l y warmer s p r i n g i n 1972 when p u p a t i o n a n d e g g - l a y i n g p r o b a b l y o c -c u r r e d e a r l i e r t h a n i n 1 9 7 1 . R a t e o f i n c r e a s e i n l e n g t h was s i m i l a r f o r b o t h C h a o b o r u s s p e c i e s ( F i g . 1 1 ) . G r o w t h i n w e i g h t i s shown i n F i g . 12. C . a m e r i c a n u s g a i n e d m o s t o f t h e i r w e i g h t b e f o r e t h e w i n t e r . C. t r i v -i t t a t u s l a r v a e g a i n e d o v e r 2/3 o f t h e f i n a l w e i g h t i n t h e i r s e c o n d y e a r . c S e a s o n a l c h a n g e s i n s t a n d i n g c r o p The maximum a b u n d a n c e o f C h a o b o r u s i n E u n i c e L a k e was a b o u t 1,200 and 1,000 l a r v a e / m 2 i n 1971 a n d 1972 r e s p e c t i v e l y ; t h e maximum b i o m a s s e s w e r e 700 a n d 500 mg/m r e s p e c t i v e l y ( F i g . 1 3 ) . T h i s was a b o u t h a l f t h e b i o m a s s o f a l l t h e p l a n k t o n i n E u n i c e L a k e . S e a s o n a l f l u c t u a t i o n s i n t h e s t a n d i n g c r o p r e f l e c t e d t o some e x t e n t t h e l o s s o f t h e e m e r g i n g 4 t h i n s t a r l a r v a e i n t h e l a t e s p r i n g , a n d t h e r e c r u i t m e n t o f t h e n e w l y h a t c h e d l a r v a e i n t h e summer. O t h e r f l u c t u a t i o n s w e r e p r o b a b l y due t o p a t c h i n e s s i n l a r v a l d i s t r i b u t i o n . I n 1971 a n d 1972 C . a m e r i c a n u s made up o n l y some 1 5 % o f t h e t o t a l C h a o b o r u s p o p u l a t i o n ; C . t r i v i t t a t u s was t h e d o m i n a n t C h a o b o r u s s p e c i e s d u r i n g t h e s t u d y . Fig.13 Cumulative standing crop by number (N), and by biomass (B), of C.americanus and C.trivittatus in 1971 (dotted line) and 1972 Fsolid l i n e ) . A l l instar 1 larvae were included with C. t r i v i t t a t u s . 38 d S e a s o n a l c h a n g e s i n d i s t r i b u t i o n a n d m i g r a t i o n S e a s o n a l d i s t r i b u t i o n a n d d i e l m i g r a t i o n o f C h a o b o r u s l a r v a e i n E u n i c e L a k e d u r i n g 1969 a n d 1971 h a v e b e e n p r e s e n t e d p r e v i o u s l y ( F e d o r e n k o a n d S w i f t 1 9 7 2 ) . V e r t i c a l d i s t r i b u t i o n o f l a r v a e a g r e e d w e l l i n t h e y e a r s s a m p l e d a s i s shown f o r 1971 ( F i g . 14) and f o r 19 72 ( F i g . 8 ) . C . t r i v i t t a t u s l a r v a e i n h a b i t e d d e e p e r l a y e r s t h a n t h e C . a m e r I c a n u s l a r v a e , a n d a l l l a r v a e moved d e e p e r a s t h e y m a t u r e d . 1 C . a m e r i c a n u s u n d e r w e n t a l e s s m a r k e d d i e l v e r t i c a l m i g r a t i o n t h a n d i d C . t r i v i t t a t u s . A l l f o u r i n s t a r s o f C . a m e r i c a n u s a nd 1 s t , 2 n d , a n d y o u n g 3 r d i n s t a r s o f C . t r i v i t t a t u s l a r v a e w e r e f o u n d a b o v e t h e t h e r -m o c l i n e a t a l l t i m e s o f t h e d a y . O l d e r 3 r d i n s t a r a n d 4 t h i n s t a r C . t r i v i t t a t u s l a r v a e s t a y e d b e l o w t h e t h e r m o c l i n e , m o v i n g up o n l y a t n i g h t . As a r e s u l t , c o n t i n u o u s s p a t i a l o v e r l a p among l a r v a e was l i m i t e d t o 1 s t , 2 n d , a n d 3 r d i n s t a r s o f b o t h s p e c i e s a n d 4 t h i n s t a r C . a m e r i c a n u s when-e v e r a n y o f them o c c u r r e d i n t h e l a k e . 0«tr 4° l a r v a e o v e r l a p p e d w i t h t h e o t h e r i n s t a r s o n l y a t n i g h t . T h i s p a t t e r n became d i s r u p t e d i n t h e l a t e f a l l a n d w i n t e r when b o t h s p e c i e s moved d e e p e r , o v e r l a p p i n g e x t e n s i v e l y . e P o t e n t i a l g r o w t h o f l a r v a e I n s i t u i n c u b a t i o n a t h i g h f o o d l e v e l s h a d a m a r k e d e f f e c t o n t h e g r o w t h o f e a c h o f t h e t h r e e l a r v a l t y p e s ( F i g . 1 5 ) . G r o w t h i n t h e l a k e was f a s t e s t f o r C . a m e r i c a n u s TIME - HOURS 10 iSr 2* 1200 1800 Pion P40p 030Q 0 6 n n 0900 o I T"C C. americanus IV 10 15 CO DC UJ 20[- C. trivittatus III LU LU 10 15 20> C. trivittatus IV y 10 15 201 C. trivittatatus IV° 14 D i e l v e r t i c a l migration of C.americanus and C . t r i v i t t a t u s larvae on 16-17 September, 1971. Instars of larvae and density per 100 l i t e r s are as shown, (from Fedorenko and Swift 1972). 40 2.0 r 1.8 1.6 < 1.4 > DC < _ i 1.2 GC 111 D_ O X o LU >-CC Q 1.0 .8 .6 0 in-ivy tr ni-iyy tr iv° F i g . 1 5 Mean w e i g h t s and upper 95% c o n f i d e n c e l i m i t s o f t h r e e t y p e s o f Chaoborus l a r v a e f r o m t h e t u b e e x p e r i m e n t : d o t t e d b a r s a r e i n i t i a l w e i g h t s i n l a k e and t u b e s ; c o a r s e l y h a t c h e d b a r s a r e f i n a l w e i g h t s i n l a k e ; g r e y b a r s a r e f i n a l w e i g h t s i n t u b e s . 41 and s l o w e s t f o r y o u n g C . t r i v i t t a t u s l a r v a e . W e i g h t g a i n i n t h e t u b e s was g r e a t e s t f o r t h e y o u n g C . t r i v i t t a t u s a n d s m a l l e s t f o r t h e o l d C . t r i v i t t a t u s , The r a t i o s o f w e i g h t g a i n o f t u b e l a r v a e t o w e i g h t g a i n o f l a k e l a r v a e f o r am y, t r y , a n d t r o, w e r e 2.6, 6.2 a n d 1.9 r e s p e c -t i v e l y . Due t o t h e d i f f e r e n c e s i n t h e r a t e o f g r o w t h , t h e f i n a l w e i g h t s o f a l l t h e " t u b e l a r v a e " w e r e s i m i l a r . 2 D i s c u s s i o n The i n s t a r a n d s p e c i e s s p e c i f i c l e n g t h s o f l a r v a l h e a d s a n d o f a n t e n n a e w i t h s e t a e , a s r e p o r t e d h e r e , h a v e b e e n u s e d f o r l a r v a l i d e n t i f i c a t i o n b y , among o t h e r s , P arma (1971) w o r k i n g o n C f l a v i c a n s , a n d S i k o r o w a (1973) w o r k i n g o n C . p a l l i d u s , C . o b s c u r i p e s , a n d C . c r y s t a l l i n u s . S t a h l (1966a) h a s s u g g e s t e d t h a t t h i s s i z e s p e c i f i c i t y may be r e l a t e d t o t h e f o o d s t h a t t h e d i f f e r e n t l a r v a l t y p e s i n g e s t , a n d t h a t t h e p o s s i b l e d i e t d i f f e r e n c e s may i n t u r n e x p l a i n t h e r e g u l a r c o e x i s t e n c e among t h e C h a o b o r u s s p e c i e s . T h i s p r o p o s i t i o n w i l l be b r i e f l y d i s c u s s e d i n t h e l a t t e r p a r t o f t h e s t u d y . The 2 - y e a r l i f e c y c l e o f C . t r i v i t t a t u s i s u n u s u a l when c o m p a r e d t o t h e 1 - y e a r o r s h o r t e r l i f e c y c l e s o f m o s t o t h e r C h a o b o r u s l a r v a e . T h i s d i f f e r e n c e may b e due i n p a r t t o t h e l o w e r f o o d a n d t e m p e r a t u r e t h a t t h e C . t r i v i t t a t u s e x p e r i e n c e i n E u n i c e L a k e . The s h o r t e r p h o t o p e r i o d o f t h e s e d e e p d w e l l i n g l a r v a e may f u r t h e r 42 a c t t o s l o w down t h e i r m a t u r a t i o n ( S . Parma 1 9 7 1 ) , p e r s o n a l c o m m u n i c a t i o n ) . The s t a n d i n g c r o p o f C h a o b o r u s i n E u n i c e L a k e , a b o u t 2 1,000 i n d i v i d u a l s / m , was l o w c o m p a r e d t o mos t o t h e r l a k e s 2 s t u d i e d . J u d a y (1922) r e c o r d e d 18,000-30,000 l a r v a e / m i n L a k e M e n d o t a ; E g g l e t o n (1931) r e p o r t e d a h i g h o f 71,000 l a r v a e / m i n T h i r d S i s t e r L a k e . The l o w numbers r e p o r t e d h e r e no d o u b t r e f l e c t t h e o l i g o t r o p h i c n a t u r e o f E u n i c e L a k e . The s t r o n g d o m i n a n c e o f one C h a o b o r u s s p e c i e s o v e r o t h e r s ( h e r e , C . t r i v i t t a t u s o v e r C . a m e r i c a n u s ) i s a common o b s e r v a t i o n a n d n e e d n o t i m p l y g r a d u a l e x c l u s i o n o f t h e m i n o r s p e c i e s t h r o u g h c o m p e t i t i o n . S t a h l (1966b) g e n e r a l i z e d , f r o m h i s e x t e n s i v e s u r v e y o f l a k e s w i t h C h a o b o r u s , t h a t i n N o r t h A m e r i c a a l l s p e c i e s f o u n d i n a n a r e a c a n p r o b a b l y c o e x i s t , e v e n t h o u g h some s p e c i e s may be p o o r l y r e p r e s e n t e d . L a r v a e t h a t a r e e n t i r e l y p l a n k t o n i c , a s i n E u n i c e L a k e , a r e r e l a t i v e l y r a r e s i n c e C h a o b o r u s a r e u s u a l l y b e n t h i c i n t h e d a y a n d p l a n k t o n i c o n l y a t n i g h t . E x p l a -n a t i o n f o r t h e p l a n k t o n i c n a t u r e o f l a r v a e i s f a r f r o m s a t i s f a c t o r y . A C h a o b o r u s s p e c i e s t h a t i s e n t i r e l y l i m n e t i c i n one l a k e may be p a r t i a l l y b e n t h i c i n a n o t h e r . I t i s a l s o n o t c l e a r why, o f t h e two C h a o b o r u s s p e c i e s i n E u n i c e L a k e , one i s b a s i c a l l y a n o n - m i g r a t o r y f o r m w h i l e t h e o t h e r i s a s t r o n g m i g r a t o r . T h e o r i e s p r e s e n t e d t o d a t e f o r v e r t i c a l m i g r a t i o n i n c l u d e b o t h e n d o g e n o u s an d e x o g e n o u s r h y t h m i c i t i e s w h e r e f o o d , l i g h t , t e m p e r a t u r e , o x y g e n , a n d p o p u l a t i o n i n t e r a c t i o n s , may a c t a s t h e r e g u l a t i n g 43 o r i n d u c i n g f a c t o r s f o r v e r t i c a l m i g r a t i o n ( T e r a g u c h i a n d N o r t h c o t e 1966; LaRow 1968, 1 9 7 0 ; R u d j a k o v 1 9 7 0 ) . R a t e o f g a i n i n w e i g h t was v e r y d i f f e r e n t f o r t h e two C h a o b o r u s s p e c i e s , b u t b o t h g r e w m o s t r a p i d l y i n t h e summer a n d e a r l y f a l l when t h e z o o p l a n k t o n s t a n d i n g c r o p was a t i t s maximum. T h i s t e m p o r a l c o i n c i d e n c e i n p r o -d u c t i o n o f C h a o b o r u s a n d o f i t s p r e y h a s a l s o b e e n o b s e r -v e d f o r C . f l a v i c a n s b y K a j a k a n d R a n k e - R y b i c k a ( 1 9 7 0 ) , a n d by J o n a s s o n ( 1 9 7 2 ) . I n t h e g r o w t h e x p e r i m e n t , e a c h o f t h e l a r v a l t y p e s i n t h e l a k e e x p e r i e n c e d d i f f e r e n t l e v e l s o f p r e y a b u n d a n c e a n d d i f f e r e n t w a t e r t e m p e r a t u r e s due t o a g e - s p e c i f i c v e r t i c a l d i s t r i b u t i o n p a t t e r n s . A s s u m i n g t h a t t h e m a i n -t e n a n c e c o s t o f C . a m e r i c a n u s was r e l a t i v e l y l o w , t h e y g r e w f a s t e r t h a n t r 4^ o r t r 4° b e c a u s e o f h i g h p r e y l e v e l s a n d a h i g h t u r n o v e r r a t e o f f o o d a t t h e warm tem-p e r a t u r e s f o u n d i n t h e e p i l i m n i o n . T r 4° o u t g r e w t r 4 V p r o b a b l y b e c a u s e o f h i g h e r c o n s u m p t i o n r a t e s . I n t h e t u b e s , t h e c a u s e f o r t h e u n e x p e c t e d l y h i g h g r o w t h o f y o u n g C . t r i v i t t a t u s may h a v e b e e n a r e s t r i c t e d downward m i g r a t i o n o f t h e s e l a r v a e o r a n e x t r a t h i c k p l a n k t o n l a y e r i n t h e t u b e s , b o t h o f w h i c h w o u l d k e e p C . t r i v i t t a t u s i n a f o o d - r i c h z o n e . The c a p a c i t y o f C . a m e r i c a n u s t o grow t o t w i c e t h e i r n o r m a l l y o b s e r v e d p r e p u p a l w e i g h t may i n d i c a t e t h a t , when f o o d i s a b u n d a n t , e x c e s s i v e g r o w t h i s p o s s i b l e i n t h e a b s e n c e o f a p p r o p r i a t e s t i m u l i 44 f o r p u p a t i o n s u c h a s a s p e c i f i c p h o t o p e r i o d ( B r a d s h a w 1 9 7 0 ) . T h i s may a l s o e x p l a i n t h e l a c k o f p u p a t i o n i n t h e t u b e s i n t r 4 V a n d t r 4° l a r v a e . I n E u n i c e L a k e , C h a o b o r u s g a i n e d l e s s w e i g h t t h a n t h e y c o u l d p o t e n t i a l l y g a i n b e c a u s e o f f o o d s h o r t a g e . E v i d e n c e f o r t h i s h y p o t h e s i s came f r o m a n i n s i t u f e e d -i n g e x p e r i m e n t w h e r e C h a o b o r u s i n p l a n k t o n e n r i c h e d t u b e s g r e w much f a s t e r t h a n t h o s e i n t h e l a k e . T h r e e o t h e r i n d e p e n d e n t s t u d i e s f u r t h e r s u p p o r t t h i s h y p o t h e s i s . F i r s t , a s d e t e r m i n e d f r o m f e e d i n g e x p e r i m e n t s , p r e y d e n -s i t i e s i n t h e l a k e w e r e w e l l b e l o w t h e d e n s i t i e s r e q u i r e d f o r maximum l a r v a l f e e d i n g r a t e s . S e c o n d l y , o n l y a b o u t 30% o f t h e 19,000 l a r v a e e x a m i n e d a t v a r i o u s t i m e s d u r i n g 1972 h a d a n y f o o d i n t h e i r c r o p s ( F i g . 2 4 ) , a n d t h o s e w i t h f o o d u s u a l l y c o n t a i n e d o n l y one p r e y i t e m . Y e t w h e n e v e r f r e s h l y c o l l e c t e d l a k e l a r v a e w e r e i n c u b a t e d w i t h a d e n s e m i x t u r e o f p r e y , w i t h i n a n h o u r e a c h o f t h e l a r v a e w o u l d consume s e v e r a l p r e y . A s s u m i n g t h a t t h e a v e r a g e p r e y i s r e t a i n e d i n a c r o p f o r 8 h o u r s ( t h i s i s c l o s e t o t h e v a l u e r e p o r t e d b y M o n t s h a d s k y ( 1 9 4 5 ) , a n d G o l d s p i n k a n d S c o t t ( 1 9 7 1 ) , t h e p e r c e n t a g e o f l a r v a l p o p u l a t i o n i n E u n i c e L a k e t h a t f e e d s d a i l y i s g i v e n b y : t h e mean d a i l y % p o p u l a t i o n w i t h f o o d i n c r o p mean d i g e s t i o n t i m e o f p r e y i n d a y s = 30% / | 4 = 90% 45 T h e r e f o r e , t h e a v e r a g e c h a o b o r i d l a r v a i n t h e l a k e c a n n o t c a t c h more t h a n one p r e y on a n y g i v e n d a y . T h i r d l y , l a b o r a t o r y f e e d i n g e x p e r i m e n t s a t a v a r i e t y o f p r e y d e n -s i t i e s showed a n - i n c r e a s e i n l a r v a l g r o w t h a t h i g h p r e y d e n s i t i e s ( S w i f t 1 9 7 3 ) . 46 3 Summary 1 S i g n i f i c a n t d i f f e r e n c e s among a l l i n s t a r s o f C. a m e r i c a n u s a n d C . t r i v i t t a t u s i n t h e s i z e s o f t h e i r v a r i o u s h e a d p a r a m e t e r s , i n c l u d i n g t h e m o u t h s i z e , s u g g e s t c o r r e s p o n d i n g d i f f e r e n c e s i n t h e i r d i e t s . 2 L a r v a e o f b o t h s p e c i e s m o l t e d s y n c h r o n o u s l y t h r o u g h t h e i r i n s t a r s a n d e l o n g a t e d a t t h e same r a t e b u t h a d a d i f f e r e n t g e n e r a t i o n t i m e a n d a d i f f e r e n t p a t t e r n o f w e i g h t g a i n . 3 S t a n d i n g c r o p o f C h a o b o r u s i n E u n i c e L a k e was v e r y l o w c o m p a r e d t o o t h e r l a k e s s t u d i e d . I n 19 71 a n d 1972 C . t r i v i t t a t u s w e r e p r e s e n t a t much h i g h e r d e n -s i t i e s t h a n C . a m e r i c a n u s . 4 B o t h C h a o b o r u s s p e c i e s i n E u n i c e L a k e w e r e e n t i r e l y p l a n k t o n i c . C . a m e r i c a n u s w e r e b a s i c a l l y n o n -m i g r a t o r y a n d i n h a b i t e d t h e t o p l a y e r o f w a t e r ; o l d e r C . t r i v i t t a t u s w e r e s t r o n g v e r t i c a l m i g r a t o r s . 5 G r o w t h o f t h e l a r v a e was l i m i t e d by l o w p r e y d e n -s i t i e s i n t h e l a k e . 47 V I FEEDING OF" CHAOBORUS 1 M e t h o d s a D i g e s t i o n r a t e s D i g e s t i o n r a t e s o f 3 r d a n d 4 t h i n s t a r l a r v a e o f C . a m e r i c a n u s and C . t r i v i t t a t u s w e r e m e a s u r e d o n s i x prey-t y p e s i n t h e l a b o r a t o r y a t t h e t e m p e r a t u r e s t h a t t h e p r e d a t o r s a n d t h e i r p r e y n o r m a l l y e x p e r i e n c e i n t h e l a k e (5-25°C f o r D . k e n a i ; 17-20°C f o r o t h e r p l a n k t o n ) . L a r v a e , s e p -a r a t e d b y t y p e a n d p r e v i o u s l y s t a r v e d f o r a f e w d a y s , w e r e a l l o w e d t o a c c l i m a t i z e t o t h e e x p e r i m e n t a l t e m p e r a t u r e f o r s e v e r a l h o u r s . They w e r e t h e n g i v e n a l a r g e number o f p r e y o f a p a r t i c u l a r t y p e a n d o b s e r v e d c o n t i n u o u s l y . As s o o n a s a l a r v a f e d , i t was t r a n s f e r r e d i n t o a l a b e l l e d v i a l c o n t a i n i n g w a t e r o f t h e same t e m p e r a t u r e . A t s e t t i m e i n t e r v a l s a f t e r t h e i n g e s t i o n o f p r e y (1,3,5,10, up t o 24 h o u r s ) , 5-10 l a r v a e w e r e r e m o v e d f r o m t h e v i a l s , a n a e s t h e -t i z e d w i t h t h e c h l o r o f o r m : e t h a n o l m i x t u r e , a n d k i l l e d w i t h F o r m a l i n . T h e i r c r o p s w e r e t h e n d i s s e c t e d a n d c r o p c o n t e n t s e x a m i n e d . P r e y w e r e c o n s i d e r e d t o be " f r e s h " i f s t i l l c l e a r l y r e c o g n i z a b l e ; d i g e s t i o n was assumed t o be c o m p l e t e when r e g u r g i t a t i o n o c c u r r e d . I n t h e c a s e o f D . k e n a i , a g r a d a t i o n s y s t e m f r o m 0%-100% was e s t a b l i s h e d b a s e d on t h e d e g r e e o f d i g e s t i o n o f D . k e n a i i n l a r v a l c r o p s . b F e e d i n g r a t e s I n 1 9 7 1 , i n c u b a t i o n e x p e r i m e n t s w e r e c a r r i e d o u t i n t h e l a k e t o d e t e r m i n e t h e e f f e c t o f p r e y d e n s i t y o n t h e 48 f e e d i n g r a t e s o f 2 n d , 3 r d , a n d 4 t h i n s t a r l a r v a e o f C. a m e r i c a n u s a nd C . t r i v i t t a t u s . D . t y r e l l i , D . k e n a i , d i a p t o m i d n a u p l i i , a n d D i a p h a n o s o m a w e r e u s e d a s p r e y . F r e s h l y c a u g h t C h a o b o r u s l a r v a e w e r e i n t r o d u c e d s i n g l y o r i n p a i r s i n t o a t r a n s p a r e n t b a g h o l d i n g a s t a n d a r d v o l u m e o f l a k e w a t e r (0.37-30 l i t e r s ) , a n d c o n t a i n i n g a known number o f p r e y . T h e r e w e r e u s u a l l y s i x r e p l i c a t e s . The b a g s w e r e i n c u b a t e d f r o m a r a f t f o r 24 h o u r s a t t h e d e p t h o f t h e d e s i r e d t e m p e r a t u r e . A t t h e e n d o f t h e i n c u b a t i o n p e r i o d , t h e c o n t e n t s o f e a c h b a g w e r e c a r e f u l l y e m p t i e d i n t o l a r g e w h i t e t r a y s , t h e b a g w a t e r t e m p e r a t u r e m e a s u r e d , and t h e r e m a i n i n g p r e y c o u n t e d . I n t r i a l r u n s , C h a o b o r u s l a r v a e a t e s m a l l c o p e p o d i d s a n d D . t y r e l l i a t s i m i l a r r a t e s . T h i s was a l s o t r u e f o r l a r g e c o p e p o d i d s a n d D . k e n a i . The f e e d i n g r a t e d a t a f o r D. k e n a i a nd D. t y r e l l i c a n t h e r e f o r e a l s o b e a p p l i e d t o t h e a p p r o p r i a t e c o p e p o d i d g r o u p s . To d e t e r m i n e t h e a c c u r a c y o f c o u n t s b e f o r e a nd a f t e r t h e e x p e r i m e n t , a nd t o t e s t f o r p r e y m o r t a l i t y when p r e d a t o r s w e r e a b s e n t , t h r e e o r f o u r c o n t r o l b a g s w i t h p r e y o n l y w e r e u s u a l l y i n c l u d e d w i t h t h e e x p e r i m e n t a l b a g s . I n n e a r l y a l l c a s e s , t h e b e f o r e a nd a f t e r c o u n t s o f t h e c o n t r o l s a g r e e d c l o s e l y . To d e t e r m i n e t h e e f f e c t s o f d i f f e r e n t s i z e d c o n t a i n e r s , o f c h a n g i n g d e p t h s f o r i n c u b a t i o n , a n d o f s h o r t t e r m s t a r v a t i o n o n l a r v a l f e e d i n g i n t e n s i t y , s e v e r a l t r i a l e x p e r i m e n t s w e r e r u n u s i n g v a r i o u s b a g s i z e s , 5 m and 20 m d e p t h s f o r i n c u b a t i o n , a n d f r e s h l y 49 c a u g h t o r one d a y s t a r v e d l a r v a e . T e m p e r a t u r e and d e n s i t y o f p r e y w e r e k e p t c o n s t a n t . c T e m p e r a t u r e e f f e c t on f e e d i n g r a t e s F r e s h l y c a u g h t l a r v a e o f a s p e c i f i c i n s t a r a n d s p e c i e s w e r e p l a c e d s i n g l y o r i n p a i r s i n t o 300 m i s o f w a t e r w i t h 10-20 c o p e p o d p r e y a n d i n c u b a t e d f o r 3 t o 4 d a y s . O n l y t h e 3 r d a n d 4 t h i n s t a r l a r v a e o f C . a m e r i -c a n u s a n d C . t r i v i t t a t u s w e r e u s e d i n t h e e x p e r i m e n t s . F o u r t h i n s t a r l a r v a e w e r e f e d D . k e n a i ; t h i r d i n s t a r l a r v a e w e r e f e d D . t y r e l l i . T e m p e r a t u r e o f t h e w a t e r was 5, .10, 1 5 , o r 2 0°C, a n d t h e l i g h t r e g i m e , 16D/8L. A t t h e e n d o f e a c h d a y , t h e r e p l i c a t e s ( u s u a l l y s i x ) , w e r e exam-i n e d f o r consumed p r e y a n d t h e number o f p r e y r e s t o r e d t o t h e o r i g i n a l d e n s i t y . P r e y w e r e a b u n d a n t a t a l l t i m e s . The e x p e r i m e n t s w e r e p e r f o r m e d o n c e a month i n May, J u l y , A u g u s t , a n d November o f 1972. d L o n g t e r m f e e d i n g e x p e r i m e n t T h r e e g r o u p s , e a c h w i t h s e v e n l a r v a e o f C . t r 4 ° , t r 4 V , a n d am 4, w e r e i n c u b a t e d f o r 25 c o n s e c u t i v e d a y s ( S e p t e m b e r 2 4 - O c t o b e r 18, 1 9 7 1 ) . I n d i v i d u a l l a r v a e k e p t i n 300 m l c o n t a i n e r s w e r e p r o v i d e d d a i l y w i t h 20 a d u l t D . k e n a i a s f o o d . I n c u b a t i o n s w e r e h e l d o u t d o o r s u n d e r n a t u r a l l i g h t c o n d i t i o n s . E a c h d a y , t h e number o f l i v e a n d d e a d c o p e p o d s was r e c o r d e d a n d t h e i r number r e s t o r e d t o t h e o r i g i n a l d e n s i t y . 50 e E f f i c i e n c y o f c o p e p o d i n g e s t i o n I n g e s t i o n e f f i c i e n c y was d e f i n e d a s t h e a b i l i t y o f a l a r v a t o s w a l l o w a c a p t u r e d p r e y . Two a p p r o a c h e s w e r e u s e d t o m e a s u r e t h i s e f f i c i e n c y . The f i r s t was t o mea-s u r e t h e s u c c e s s o f c o p e p o d i n g e s t i o n b y t h e d i f f e r e n t l a r v a l i n s t a r s o f b o t h s p e c i e s b y d e t e r m i n i n g t h e f r a c t i o n o f p r e y e a t e n f r o m t h e t o t a l k i l l e d a n d e a t e n b y t h e p r e d a t o r . U s u a l l y f i v e l a r v a e a nd a s a t u r a t i o n d e n s i t y o f c o p e p o d p r e y , (20 D. k e n a i do" o r D. t y r e l l i ) w e r e p l a c e d i n t o e a c h o f s i x , 350 m l c o n t a i n e r s a nd i n c u b a t e d a t tem-p e r a t u r e s o f 15-17 °C (12 °C i n S e p t e m b e r , 1971) f o r a p e r i o d o f 24 h o u r s . A d i s h o f p r e y w i t h o u t p r e d a t o r s s e r v e d as a c o n t r o l . A t t h e end o f t h e e x p e r i m e n t , t h e r e m a i n i n g p r e y w e r e c o u n t e d a n d t h e number e a t e n a n d k i l l e d was d e t e r m i n e d b y s u b t r a c t i o n . P r e y m o r t a l i t y i n t h e c o n t r o l d i s h e s was n e g l i g i b l e a n d a l l d e a t h s w e r e a t t r i b u t e d t o t h e p r e d a t o r s . The s e c o n d m e t h o d was t o c o m p a r e t h e mouth gape o f a l a r v a t o t h e c r o s s - s e c t i o n a l a r e a o f a p o t e n t i a l p r e y when t h e l a t t e r was " b e s t " o r i e n t e d f o r s w a l l o w i n g b y a l a r v a . I t was assumed t h a t i f t h e gape o f a l a r v a was s m a l l e r t h a n t h e p r e y , i n g e s t i o n w o u l d b e d i f f i c u l t i f n o t i m p o s s i b l e . M e t h o d s f o r m e a s u r i n g t h e s e p a r a m e t e r s w e r e d i s c u s s e d i n e a r l i e r s e c t i o n s . 51 f A n a l y s i s o f l a r v a l c r o p s S e v e r a l h u n d r e d C h a o b o r u s w e r e c o l l e c t e d i n v e r i c a l h a u l s f o r 24 h o u r s , a t 3 h o u r i n t e r v a l s , o n c e a month f r o m J u n e t o O c t o b e r o f 1972. F i r s t i n s t a r l a r v a e w e r e o m i t t e d f r o m t h e s t u d y . C r o p s w e r e p r e p a r e d f o r a n a l y s i s f o l l o w i n g t h e m e t h o d o f S w i f t a n d F e d o r e n k o ( i n p r e s s ) . C r o p s w e r e s o r t e d a n d c o u n t e d a s non-empty ( " f u l l " ) a n d e mpty; t h e l a t t e r w e r e r e j e c t e d . The " f u l l " c r o p s w e r e t e a s e d a p a r t u n d e r a d i s s e c t i n g m i c r o s c o p e a n d t h e i r c o n -t e n t s i d e n t i f i e d a n d e n u m e r a t e d u n d e r h i g h power (40X o r lOOx) w i t h a compound m i c r o s c o p e . P r e y f r o m l a r v a l c r o p s w e r e a s s i g n e d t o 10 c a t e g o r i e s : r o t i f e r s , B o s m i n a , P o l y p h e m u s , D a p h n i a , H o l o p e d i u m , m i s c e l l a -n e o u s c l a d o c e r a n s , D . k e n a i , D . t y r e l l i , C h a o b o r u s , and u n -i d e n t i f i a b l e m a t e r i a l . The " c l a d o c e r a n s " g r o u p i n c l u d e d D i a p h a n o s o m a a n d n o n r e c o g n i z a b l e c l a d o c e r a n s . The " k e n a i " c a t e g o r y i n c l u d e d a d u l t a nd n e a r a d u l t k e n a i , a n d t h e " t y r e l l i " g r o u p c o n t a i n e d a l l t h e o t h e r s m a l l e r c o p e p o d s , m o s t o f w h i c h w e r e t y r e l l i . The s m a l l a n d s o f t - b o d i e d p r e y s u c h a s c o p e p o d n a u p l i i a n d n o n - l o r i c a t e r o t i f e r s ( C o n o c h i l o i d e s ) c o u l d n o t b e d e t e c t e d i n t h e c r o p s b e c a u s e o f r a p i d d i g e s t i o n . I d e n t i f i c a t i o n o f c r o p c o n -t e n t s a s c l a d o c e r a n i n o r i g i n was b a s e d on t h e p r e s e n c e o f t h e f i l t e r i n g a p p e n d a g e s a n d t h e p o s t a b d o m i n a l c l a w ; s p e c i e s i d e n t i f i c a t i o n d e p e n d e d on t h e p o s t a b d o m i n a l c l a w . O t h e r f e a t u r e s u s e d i n p r e y i d e n t i f i c a t i o n w e r e 52 t h e l o n g r o d - l i k e t a i l o f P o l y p h e m u s , r o t i f e r l o r i c a s , t h e c a r a p a c e o f B o s m i n a , a n d t h e numerous s p i n e s a n d s e t a e o n c o p e p o d a p p e n d a g e s . g D i e l f e e d i n g a c t i v i t y F r o m t h e c r o p a n a l y s i s d a t a , t h r e e i n d i c e s w e r e c a l -c u l a t e d f o r u s e i n d e t e r m i n i n g t h e d i e l f e e d i n g a c t i v i t y °f C h a o b o r u s ; t h e p e r c e n t o f l a r v a l c r o p s c o n t a i n i n g B o s m i n a , t h e p e r c e n t c o n t a i n i n g D . k e n a i , and t h e p e r c e n t o f c r o p s c o n t a i n i n g f o o d o f a l l t y p e s . The p e r c e n t a g e o f l a r v a e c o n t a i n i n g a c e r t a i n p r e y c a t e g o r y was t h e n p l o t t e d a g a i n s t t i m e . The p r e y u s e d f o r t h e s t u d y o f d i e l f e e d i n g h a d t o b e r e l a t i v e l y a b u n d a n t a n d e a s i l y r e c o g n i z a b l e f o r a t l e a s t 3 h o u r s a f t e r b e i n g i n g e s t e d . B o t h B o s m i n a a n d D . k e n a i seemed t o be s u i t a b l e f o r t h i s s t u d y . B o s m i n a h a d a d u r a b l e a n d e a s i l y i d e n t i f i a b l e c a r a p a c e and a d i g e s t i o n t i m e o f o n l y 5 h o u r s . D . k e n a i p e r s i s t e d i n t h e c r o p s much l o n g e r s o o n l y t h o s e c o p e p o d s i n r e l a t i v e l y e a r l y s t a g e s o f d i g e s t i o n w e r e c o n s i d e r e d f o r t h e i n d e x . The d e g r e e o f D . k e n a i d i g e s t i o n was an a r b i t r a r y e v a l u a t i o n b a s e d on t h e e x t e n t o f t h e i r d i s i n -t e g r a t i o n . P . t y r e l l i w e r e a b u n d a n t b u t u n s u i t a b l e b e c a u s e o f t h e i r r a p i d l o s s o f i d e n t i t y . O t h e r z o o -p l a n k t o n t y p e s o c c u r r e d e i t h e r i n v e r y l o w numbers o r f o r r e l a t i v e l y s h o r t p e r i o d s . 53 2 R e s u l t s a D i g e s t i o n r a t e s D i g e s t i o n o f c o p e p o d s by 3 r d a n d 4 t h i n s t a r l a r v a e o f b o t h C . a m e r i c a n u s a n d C . t r i v i t t a t u s p r o g r e s s e d a t s i m i l a r r a t e s s o t h a t d a t a f r o m d i f f e r e n t l a r v a l g r o u p s w e r e p o o l e d t o g e t h e r ( T a b l e V ) . D i g e s t i o n r a t e s o f C h a o b o r u s l a r v a e w e r e a f f e c t e d by p r e y s i z e , d e g r e e o f k e r a t i n i z a t i o n o f p r e y e x o s k e l e t o n , a n d w a t e r t e m p e r a t u r e . The s m a l l e r p r e y , D . t y r e l l i , w e r e d i g e s t e d a b o u t t w i c e a s f a s t a s t h e l a r g e r D . k e n a i . M o s t p r e y became d i f f -i c u l t t o r e c o g n i z e w i t h i n 4-5 h o u r s o f i n g e s t i o n b u t t i m e u n t i l f i n a l r e g u r g i t a t i o n was v a r i a b l e . C l a d o c e r a n s , w h i c h l e f t more c o n s p i c u o u s u n d i g e s t e d r e m a i n s t h a n d i d t h e c o p e p o d s , may be l e s s e a s i l y d i g e s t e d due t o t h e i r a p p a r e n t l y t h i c k e r e x o s k e l e t o n . T e m p e r a t u r e h a d a d i r e c t e f f e c t o n 50% a n d 80% d i g e s t i o n o f a d u l t D . k e n a i by t r 4 V ( F i g . 1 6 ) . A t 25 °C d i g e s t i o n was n e a r l y c o m p l e t e a f t e r f o u r h o u r s ; a t 5 °C, a t l e a s t a d a y was r e q u i r e d . A l t h o u g h n o t shown, v a r i a b i l i t y i n d i g e s t i o n among t h e d i f f e r e n t l a r v a e was c o n s i d e r a b l e . b F e e d i n g r a t e s F e e d i n g r a t e s o n D . t y r e l l i i n c r e a s e d w i t h i n c r e a s i n g p r e y c o n c e n t r a t i o n s a n d , e x c e p t f o r t r 4 ° , w i t h i n c r e a s i n g t e m p e r a t u r e s ( F i g . 1 7 ) . The maximum r a t e s o f t r 2 a n d t r 3 i n s t a r l a r v a e w e r e 2 p r e y and 6-7 p r e y p e r l a r v a TABLE V D i g e s t i o n rates of Chaoborus feeding on d i f f e r e n t prey types. Prey type Temperature Time to end of Time t o Species and i n s t a r Number of prey °C " f r e s h " appearance r e g u r g i t a t i o n of l a r v a t e s t e d * analyzed D. t y r e l l i 1 7 - 2 0 p . kenai (juvenile) " J). kenai (adult) Bosmina " Polyphemus " Paphnia " data from d i f f e r e n t l a r v a l groups were pooled since no d i f f e r e n c e s among them were observed 1 hr 8 hr am 3, am 4, t r 4° . 140 3 hr 8 hr am 4, t r 4° 51 4 hr 12+ hr am 4, t r 3, t r 4 , t r 4 122 5 hr 5 hr am 4 130 4 hr. 5 hr t r 4° 36 9 hr 12 hr t r 4° 30 55 Fig.16 Average time required to achieve 50% and 80% of complete dig e s t i o n of D.kenai by C . t r i v i t t a t u s 4Y larvae at d i f f e r e n t temperatures. 56 p e r d a y r e s p e c t i v e l y , a t a c o n c e n t r a t i o n o f 5 D . t y r e l l i p e r l i t e r ( F i g . 17 a , b ) . I t i s l i k e l y t h a t t h e y o u n g C. a m e r i c a n u s h a d s i m i l a r f e e d i n g r a t e s . F e e d i n g r a t e s o f 4 t h i n s t a r l a r v a e o f b o t h s p e c i e s i n c r e a s e d up t o a b o u t 20 p r e y p e r l a r v a p e r d a y b u t a t much h i g h e r c o n c e n t r a t i o n s o f 60-80 D . t y r e l l i p e r l i t e r ( F i g . 17 c , d ) . C h a o b o r u s l a r v a e a c h i e v e d maximum f e e d i n g r a t e s on D. k e n a i a t d e n s i t i e s w e l l b e l o w t h o s e r e q u i r e d f o r t h e o t h e r p r e y t y p e s s t u d i e d ( F i g . 1 8 ) . T r 4° l a r v a e w e r e t h e m o s t e f f i c i e n t o f t h e p r e d a t o r s , o b t a i n i n g t h e i r maximum d a i l y r a t i o n o f 2.5-4.0 k e n a i a t p r e y d e n s i t i e s o f 1-2 p e r l i t e r ( F i g . 18 c ) . T r 4 V a n d am 4 l a r v a e h a d maximum c o n s u m p t i o n r a t e s s i m i l a r t o t h o s e o f t r 4° l a r v a e b u t a t h i g h e r d e n s i t i e s o f a b o u t 5 p r e y p e r l i t e r ( F i g . 18 b , d ) . Maximum c o n s u m p t i o n r a t e o f t r 3 l a r v a e was o n l y 1.5 D . k e n a i p e r d a y o r a b o u t h a l f t h a t o f t h e o l d e r l a r v a e ( F i g . 18 a ) . How e v e r , b o t h am 4 a n d t r 3 l a r v a e k i l l e d a p p r o x i m a t e l y 2 0 % a n d 50% o f D . k e n a i r e s p e c -t i v e l y i n u n s u c c e s s f u l c a p t u r e a t t e m p t s . T h i s r e s u l t e d i n a n o v e r a l l r e m o v a l b y am 4 a n d t r _ 3 o f up t o 3 D . k e n a i p e r l a r v a p e r d a y . R a t e s o f f e e d i n g o n D i a p h a n o s o m a b y t r 3 l a r v a e w e r e s i m i l a r t o t h e f e e d i n g r a t e s on D . t y r e l l i by t h e 4 t h i n s t a r l a r v a e ( F i g . 18 e ) . The maximum d a i l y r a t i o n o f a b o u t 30 p r e y p e r l a r v a p e r d a y was r e a c h e d a t a d e n s i t y o f 80 D i a p h a n o s o m a p e r l i t e r . N a u p l i i w e r e e a t e n l e a s t e f f i c i e n t l y o f t h e p r e y F i g . 1 7 Means ± 1 S.D. o f Chaoborus f e e d i n g r a t e s on D . t y r e l l i a t d i f f e r e n t t e m p e r a t u r e s , ( c u r v e s were f i t t e d by e y e ) . 4CH NUMBER OF PREY / L I T E R F i g . 1 8 Means i 1 S.D. o f Chaoborus f e e d i n g r a t e s on D . k e n a i ( a - d ) , on Diaphanosoma ( e ) , and on n a u p l i i ( f ) . S o l i d l i n e s i n d i c a t e f e e d i n g r a t e s ; d o t t e d l i n e s g i v e t h e t o t a l o f e a t e n and k i l l e d p r e y , ( c u r v e s were f i t t e d by e y e ) . 59 s t u d i e d . F e e d i n g r a t e s o f am 2 l a r v a e i n c r e a s e d up t o 20 n a u p l i i p e r l a r v a p e r d a y a t a d e n s i t y o f 160 p r e y p e r l i t e r ( F i g . 18 f ) . c T e m p e r a t u r e e f f e c t o n f e e d i n g r a t e s I n o r d e r t o s t a n d a r d i z e h u n g e r l e v e l a n d a c c l i m a -t i z a t i o n , d a t a f r o m t h e 2nd d a y o f e x p e r i m e n t s w e r e u s e d t o show t h e e f f e c t o f t e m p e r a t u r e o n f e e d i n g r a t e s ; r e s u l t s f r o m a l l f o u r c o n s e c u t i v e d a y s a r e a l s o g i v e n ( F i g . 1 9 ) . F e e d i n g r a t e s i n c r e a s e d d i r e c t l y w i t h i n c r e a s e i n t e m p e r a t u r e i n 4 t h a n d 3 r d i n s t a r s o f C. a m e r i c a n u s l a r v a e t e s t e d i n May a n d J u l y r e s p e c t i v e l y , a n d i n t h e 3 r d i n s t a r l a r v a e o f C . t r i v i t t a t u s t e s t e d i n J u l y ( F i g . 19 a , b , d ) . F e e d i n g r a t e s o f C . t r 4° l a r v a e w e r e l a r g e l y i n d e p e n d e n t o f t e m p e r a t u r e t h r o u g h o u t t h e s e a s o n ( F i g . 19 c , e , f ) , e x c e p t i n J u l y when t h e y showed a p e a k a t 15 °C. F e e d i n g r a t e s o f l a r v a e o f b o t h s p e c i e s c o l l e c t e d i n November showed l i t t l e o r no r e s p o n s e t o t e m p e r a t u r e ( F i g . 19 a , b , c ) . Maximum c o n s u m p t i o n a t d i f f e r e n t t e m p e r a t u r e s was s i m i l a r on f o u r s u c c e s s i v e d a y s f o r C . a m e r i c a n u s l a r v a e ( F i g . 19 d) b u t v a r i e d c o n -s i d e r a b l y f r o m d a y t o d a y f o r C . t r 4° ( F i g . 19 e ) . d L o n g t e r m f e e d i n g e x p e r i m e n t D a i l y f o o d i n t a k e by i n d i v i d u a l l a r v a e was q u i t e i r r e g u l a r i n b o t h C h a o b o r u s s p e c i e s ( F i g . 2 0 ) . I n s p i t e o f t h i s , C . t r 4 V a n d C a m 4 c o n s u m e d , on t h e a v e r a g e , PJ *». PJ >tf H rt D) PJ hj fD 3 ^ - (D tn t r ^ 3" 3 O O fD W O — O rt Cu 3 p) O tn H 3 C 3 i+< ID o tr 3 n> B) 3 cn PJ 3 C 3 tn ro (D Cu I-1- P> cn Oi • P> p) o 3* • PJ H- PJ < 0 O H P CD Hi tr o • 3 o rr ro :-( n O X c < K 3 t i ro ro ro to r-J ro i-i (-• X H- pj s: H- rt 3 H ro H ro <! Hi ro 3 P> ro 0 3 rt h ro -PJ Hi rt OJ hj ^  rt h( h" PJ PJ rt PJIQ <• H-(D 3* ro Hi PJ H- rt N HI • HI ro n ro >< M P pj H h 3 tr w <^ rt p) (D rj •< ro ro 3 rt Hi rt o ro C 3 PJ ro ro|D w K • • c pi X o rt o ro o c H1 3 ro 1^ < pi tn ro ro H- m w 3 H - » Hi < o ro H m D m > H C m o O NUMBER OF D. KENAI EATEN / PREDATOR / DAY O _i rp co J> cn p -t ro co j> oi O -J. ro Co ro o IO - b — p; IO ' 3 * • -O—. c_ >J-4 «^. c_ c . 0-, ,--*>-. IP DJ 3 CD o' 0) c c/> p) -JO.-.-.. o to j*- o Co ' o ° ro A oi co NUMBER OF D. TYRELLI EATEN / PREDATOR / DAY o -» ro co J> co 03 A -* fO 5 09 41 2-2-f 0 TIME — DAYS Fig.20 Da i l y food r a t i o n of 9 t y p i c a l larvae of Cam 4 (A), C.tr 4 V (B) , and C.tr 4o (c). Hatched bars are f u l l y dTgested D.kenai; c l e a r bars are p a r t i a l l y digested D.kenai. 62 s i m i l a r d a i l y f o o d r a t i o n s - 1,6 k e n a i p e r i n d i v i d u a l ; C . t r 4° consumed 3X t h e d a i l y i n t a k e o f t h e y o u n g 4 t h i n s t a r l a r v a e - 4.5 k e n a i p e r i n d i v i d u a l . H o w e v e r , C . t r 4° f r e q u e n t l y l e f t t h e i r p r e y o n l y p a r t i a l l y d i g e s t e d , a n o b s e r v a t i o n n e v e r r e c o r d e d f o r any l a r v a e d u r i n g t h e 24 h o u r f e e d i n g e x p e r i m e n t s i n t h e f i e l d . S u c h i n c o m p l e t e d i g e s t i o n a t h i g h p r e y d e n s i t i e s was a l s o n o t e d f o r a n o t h e r p l a n k t o n p r e d a t o r , P o l y p h e m u s p e d i c u l u s ( B u t o r i n a 1 9 7 1 ) . T h i s phenomenon may be t h e r e s u l t o f l e s s e f f i c i e n t d i g e s t i o n o f p r e y a t c o n t i n u o u s l y h i g h f o o d l e v e l s . e E f f i c i e n c y o f p r e y i n g e s t i o n I n g e s t i o n o f D . t y r e l l i by t h e 2nd i n s t a r l a r v a e o f b o t h s p e c i e s was a b o u t 9 0% s u c c e s s f u l ( T a b l e V I ) . O l d e r l a r v a e , a c c o r d i n g t o f i e l d o b s e r v a t i o n s , w e r e n e a r l y 100% s u c c e s s f u l . M a t u r e D . k e n a i w h i c h a r e a b o u t 10X a s l a r g e a s D ' t y r e l l i w e r e n e v e r s e e n t o be e a t e n by am 2, am 3, o r t r 2. The o l d e r t r 3 l a r v a e ( A u g u s t 2 3 , S e p t e m b e r 1 1 ) , a t e a b o u t 6X more D . k e n a i p e r d a y t h a n d i d t h e y o u n g e r t r 3 l a r v a e ( J u l y 2 1 , A u g u s t 2 ) , p o s s i b l y b e c a u s e o f s t r o n g e r m outh a p p e n d a g e s . W a s t e f u l k i l l i n g o f a b o u t 50% a n d 20% o f t h e c a p t u r e d D . k e n a i c o p e p o d s was o b s e r v e d f o r t r 3 a n d am 4 l a r v a e r e s p e c t i v e l y . The h i g h e s t f e e d i n g r a t e s o n D . k e n a i w i t h o v e r 90% i n g e s t i o n s u c c e s s w e r e n o t e d f o r t r 4 V a n d t r 4°. M e a s u r e m e n t s o f l a r v a l mouth s i z e a nd o f p r e y s i z e TABLE VI Ingestion success of Chaoborus feeding on copepods. Date Predator-prey Number prey Number prey k i l l e d % s u c c e s s f u l combinationf - eaten/larva/24 hr but not eaten/larva/24 hr i n g e s t i o n J u l y 21/72 am 2 - D. t y r e l l i 1.4 0.2 87 J u l y 5/72 t r 2 - D_. t y r e l l i 2.5 0.1 96 Aug. 2/72 am 3 - R' kenai 0* 0.01 0.37 3 J u l y 21/72 t r 3 - D. kenai c? 0.03 0.20 13 Aug. 2/72 t r 3 - D. kenai c? 0.03 0.23 12 Aug. 23/72 t r 3 - D_. kenai c? 0.28 1.44 16 Sept. 11/71 t r 3 - p_. kenaicf 1.0 0.34 75 Aug. 23/72 am 4 - D. kenai <S 0.9 0.38 70 Sept. 11/71 am 4 - D. kenai o" 2.7 0.30 90 Sept. 11/71 t r 4 Y • - p . kenai d" 3.7 0.29 93 Aug. 23/72 t r 4° • - p . , kenai d" 3.7* 0.20 95 Sept. 11/71 t r 4° • - p_. , kenai <f 3.8 0.25 94 + u s u a l l y 30 larvae t e s t e d * value underestimated because of prey shortage 64 i n d i c a t e d t h a t l a r v a l i n s t a r s 2, 3, a n d 4 ( 1 s t i n s t a r was n o t s t u d i e d ) s h o u l d be c a p a b l e o f f r e e l y s w a l l o w i n g m o s t p r e y t y p e s ; h o w e v e r , l a r g e P a p h n i a , l a r g e H o l o p e d i u m a n d a d u l t P . k e n a i , c o u l d be s w a l l o w e d o n l y b y t h e 4 t h i n s t a r l a r v a e ( T a b l e V I I ) . f A n a l y s i s o f l a r v a l c r o p s F r o m t h e a n a l y s i s o f c r o p c o n t e n t s , t h e p r o p o r t i o n t h a t a g i v e n f o o d c a t e g o r y c o n t r i b u t e d t o t h e s p e c i e s ' d i e t was g i v e n b y t h e f o r m u l a : n i = l w h e r e " P j _ j " i s t h e p r o p o r t i o n t h a t a f o o d c a t e g o r y " i " c o n t r i b u t e d t o t h e d i e t o f i n s t a r " j " ; " n " i s t h e t o t a l number o f " j " t y p e l a r v a e a n a l y z e d , a n d " a " i s t h e sum o f a l l t h e f o o d i t e m s f o u n d i n s a m p l e " n " . S i n c e t h e u s u a l number o f p r e y p e r l a r v a l c r o p was 1 o r 2, n u m e r i c a l " P ^ / ' s h o u l d r e f l e c t d i r e c t l y t h e p r o p o r t i o n o f l a r v a e c o n t a i n i n g t h a t p r e y . C o m p o s i t i o n o f d i e t by b i o m a s s was d e t e r m i n e d by m u l t i p l y i n g t h e n u m e r i c a l p r o p o r t i o n o f a f o o d i t e m b y t h e d r y w e i g h t o f t h a t f o o d i t e m , summing up a l l t h e TABLE V I I L a r v a l mouth s i z e and prey s i z e . Mouth gape of Chaoborus l a r v a C r o s s - s e c t i o n a l area of prey i n the (diameter i n mm) best o r i e n t a t i o n f o r being i n g e s t e d (width and h e i g h t i n mm) am 2 0.30 K e r a t e l l a 0.08 x 0.03 am 3 0.43 1 n a u p l i u s 0.12 (diameter) am 4 0.71 Bosmina 0.21 x 0.16 t r 2 0.36 Diaphanosoma . 0.23 x 0.23 t r 3 0.51 Polyphemus 0.26 (diameter) t r 4* 0.85 D. t y r e l l i 0.28 (diameter) t r 40.. 0.89 Daphnia (medium) 0.74 x 0.53 D. kenai 0.62 (diameter) Holopedium 1.01 x 0.68 ( l a r g e , without g e l a t i n o u s envelope) Holopedium 3.17 (diameter) ( l a r g e , w i t h g e l a t i n o u s envelope) 66 c a t e g o r i e s b y w e i g h t , a n d d e t e r m i n i n g t h e p e r c e n t a g e f r o m t h e sum. The w e i g h t p r o p o r t i o n s e r v e d t o s t r e s s t h e i m p o r t a n c e o f t h e l a r g e r p r e y i t e m s . A An o v e r l a p i n d e x , , d e v e l o p e d b y M o r i s i t a ( 1 9 5 9 ) , a n d m o d i f i e d b y H o r n ( 1 9 6 6 ) , h a s b e e n u s e d t o s u m m a r i z e t h e f o o d s i m i l a r i t i e s b e t w e e n p a i r s o f c o e x i s t i n g l a r v a e . The f o r m u l a f o r i s : s 2 Y~ X.Y. z — i l c A= _ i = l i = l w h e r e "S" i s t h e t o t a l number o f f o o d c a t e g o r i e s and"x.T and"Y'^ a r e t h e p r o p o r t i o n s o f t o t a l d i e t o f i n s t a r "x" and o f i n s t a r " Y " r e s p e c t i v e l y t a k e n f r o m a g i v e n c a t e g o r y o f f o o d " i " . The o v e r l a p c o e f f i c i e n t v a r i e s f r o m 0, when t h e s a m p l e s a r e c o m p l e t e l y d i s t i n c t ( c o n t a i n i n g no f o o d c a t e -g o r i e s i n common), t o 1 when t h e s a m p l e s a r e i d e n t i c a l w i t h r e s p e c t t o p r o p o r t i o n a l f o o d c a t e g o r y c o m p o s i t i o n . The c a l c u l a t i o n s w e r e made f r o m p e r c e n t c o m p o s i t i o n o f t h e d i e t b y number a n d by b i o m a s s . S i n c e no s i g n i f i c a n c e t e s t i s a v a i l a b l e f o r , a n a s s u m p t i o n was made t h a t v a l u e s > 0.80 i n d i c a t e s t r o n g o v e r l a p i n t h e d i e t o f t h e two l a r v a l t y p e s , a n d 0 . 6 0 ^ <2^< 0.80 i n d i c a t e a weak o v e r l a p . 67 The d i e t s o f a p a i r o f c o e x i s t i n g l a r v a l i n s t a r s w e r e assumed t o be s i g n i f i c a n t l y d i f f e r e n t i f was l e s s t h a n 0.40. The n u m e r i c a l a n d t h e b i o m a s s p r o p o r t i o n s o f l a r v a l d i e t s ( F i g s . 2 1 , 22), and t h e f o o d o v e r l a p i n d e x among l a r v a l i n s t a r s f o r e a c h month ( T a b l e V I I I ) , a r e p r e s e n t e d t o g e t h e r . I n J u n e , o n l y the' t r 4° l a r v a e w e r e a v a i l a b l e f o r s t u d y . Bosmina,, P o l y p h e m u s , a n d D . t y r e l l i , w e r e t h e i r n u m e r i c a l l y d o m i n a n t p r e y ; D . k e n a i c o n t r i b u t e d m o s t t o t h e i r d i e t by w e i g h t ( 3 2 % ) . I n J u l y , a s i g n i f i c a n t f o o d o v e r l a p was o b s e r v e d b e t w e e n t r 2 a n d t r 3 a n d b e t w e e n t r 2 and am 2. The m a j o r p r e y o f t r 2 a n d t r 3 b y number a n d by b i o m a s s w e r e B o s m i n a and D . t y r e l l i . T o g e t h e r t h e two p r e y t y p e s c o n t r i b u t e d 7 0 - 8 0 % t o t h e n u m e r i c a l d i e t o f t r 2 a n d t r 3 a n d 50-60% t o t h e i r b i o m a s s d i e t . C.am 2 l a r v a e f e d a l m o s t e x c l u s i v e l y o n B o s m i n a . The d i e t s o f t h e 2nd i n s t a r l a r v a e o f b o t h s p e c i e s w e r e s i g n i f i c a n t l y d i f f e r e n t f r o m t h e d i e t o f C . t r 4° w h i c h was c h a r a c t e r i z e d by a much g r e a t e r d i v e r s i t y , a n d an e m p h a s i s on D a p h n i a and D . k e n a i a s p r e y . The l a t t e r made up 67% o f t h e b i o m a s s i n t h e d i e t o f C . t r 4°. I n A u g u s t , f o o d o v e r l a p was o b s e r v e d o n l y i n t h e n u m e r i c a l f o o d p r o p o r t i o n s b e t w e e n t r 3 and t r 4 ° , b o t h o f w h i c h a t e D . t y r e l l i m o s t f r e q u e n t l y . D i e t s o f t r 4° JUNE JULY AUG SEPT OCT 50-Cam II o 50-: B I I : N r n R B P D H C K T O X 50-Q C.arnlll § o. . 50 o CO o 5~ O O Cam IV R B P D H C K T O X ; B j -n N > 0J . — - 1 _ n n rfl r L n n R B P D H C K T O X R B P D H C K T O X B P D H C K T B P D H C K T B P D H C K T B P D H C K T B P D H C K T FOOD CATEGORIES Fig.21 Seasonal d i e t of C.americanus larvae by i n s t a r and the composition of laJce plankton i n 1972. N (clear bars), i s percent composition by number; B(hatched bars), i s percent composition by biomass. Let t e r s r e f e r to prey types: R-r o t i f e r s , B-Bosmina, P-Polyphemus, D-Daphnia, H-Holope-dium, C-miscellaneous cladocerans, K-D.kenaT, T - D . t y r e l l i , ; 0-Chaoborus , X-unidentif i a b l e material". C.tr II 50H 0 50H JUNE B JULY AUG SEPT OCT JL RBPDHCKTOX Ctr o o LL I LL ' o C.trlVv cn O a. O o 50-] B! J 1 I ] N _TL-,— n r-TL _ n n_ RBPDHCKTOX RBPDHCKTOX RBPDHCKTOX ] B : N U l RBPDHCKTOX RBPDHCKTOX 5CH C t r IV 0 0 50-1 N . J r-TU r-f-T - I T n RBPDHCKTOX RBPDHCKTOX RBPDHCKTOX RBPDHCKTOX RBPDHCKTOX FOOD CATEGORIES Fig.22 Seasonal d i e t of C . t r i v i t t a t u s larvae by i n s t a r i n 1972. For explanation see Fig.21. TABLE V I I I L a r v a l food o v e r l a p i n d i c e s f o r J u l y , August, September and October, c a l c u l a t e d from numerical d i e t percentages A, and from biomass d i e t percentages B. JULY SEPTEMBER am 2 am 3 t r 2 t r 3 t r 4° A am 4 t r 3 t r 4 Y t r 4° am 2 - .80 .53 .19 am 2 am 4 • 9 1 .68 .37 am 4 am 3 - - am 3 t r 3 - .80 .55 t r 3 t r 2 - - .90 .39 t r 2 t r 4 Y .71 - .85 t r 4 Y t r 3 - .42 - .44 t r 3 t r 4° .75 .97 t r 4° t r 4° - .17 - .86 t r 4° am 4 t r 3 t r 4 Y t r 4° am 2 am 3 t r 2 t r 3 t r 4° AUGUST OCTOBER JULY SEPTEMBER am 2 am 3 t r 2 t r 3 t r 4° B am 4 t r 3 t r 4 Y t r 4° am 2 - .64 .32 .07 am 2 am 4 .51 .83 .57 am 4 am 3 - - - - am 3 t r 3 - .66 .28 t r 3 t r 2 - - .86 .11 t r 2 t r 4 Y .95 - ' .69 t r 4 Y t r 3 - .51 • - .25 t r 3 t r 4° .95 - .96 t r 4° t r 4° - • .11 - .46 t r 4° am 4 t r 3 t r 4 Y t r 4° am 2 am 3 t r 2 t r 3 t r 4° OCTOBER AUGUST o 7 1 a n d am 3 w e r e s i g n i f i c a n t l y d i f f e r e n t a t t h a t t i m e . C. t r 4° f e d m a i n l y o n D . k e n a i a n d D . t y r e l l i ( 8 7 % o f t h e d i e t by n u m b e r ) . C.am 3 f e d m a i n l y on r o t i f e r s a n d -B o s m i n a ( 8 4 % o f t h e d i e t b y n u m b e r ) . R o t i f e r s w e r e p r o b a b l y a n e g l i g i b l e s o u r c e o f e n e r g y b e c a u s e o f t h e i r v e r y s m a l l s i z e . By S e p t e m b e r , f o o d o v e r l a p among a l l t h e l a r v a e became more i n t e n s e , p a r t i c u l a r l y among am 4, t r 3, a n d t r 4^, a l l o f w h i c h a t e m a i n l y r o t i f e r s , B o s m i n a a n d D. t y r e l l i . H o wever, B o s m i n a w e r e e n c o u n t e r e d a b o u t 3X more t h a n D . t y r e l l i i n t h e am 4 l a r v a e , a n d t h e r e v e r s e was t r u e f o r C . t r 3 l a r v a e . The l o w o v e r l a p i n d e x i n t h e n u m e r i c a l d i e t o f am 4 a n d t r 4° may b e a t t r i b u t e d t o t h e a b s e n c e o f c a n n i b a l i s m i n am 4 a n d t o t h e i r h i g h c o n s u m p t i o n o f r o t i f e r s (5.1% by number) , a s c o m p a r e d t o t h e c o n s i d e r a b l e c a n n i b a l i s m a n d t h e a b s e n c e o f r o t i f e r s i n t h e f o o d o f t r 4° l a r v a e . The m o s t f r e q u e n t l y e a t e n p r e y o f t r 4° w e r e D . t y r e l l i (36%) b u t t h e i r m a j o r s o u r c e o f b i o m a s s w e r e C h a o b o r u s l a r v a e , m o s t l y t r 3, (38%) a n d D . k e n a i ( 3 3 % ) . The a b s e n c e o f t h e s e two p r e y t y p e s i n t h e d i e t o f C . t r 3 p r o d u c e d t h e l o w o v e r l a p i n d e x b e t w e e n them a n d t r 4°. By O c t o b e r , l a r v a e o f b o t h s p e c i e s w e r e i n t h e 4 t h i n s t a r . F o o d o v e r l a p i n d i c e s w e r e g e n e r a l l y h i g h a n d a l l p o t e n t i a l p r e y w e r e b e i n g e a t e n , p a r t i c u l a r l y H o l o p e - d i u m , D . k e n a i , a n d D . t y r e l l i . The c o p e p o d s c o n t r i b u t e d 6 0 - 7 0 % t o t h e f o o d b i o m a s s o f a l l t h r e e l a r v a l t y p e s . B o s m i n a a n d r o t i f e r s w e r e n u m e r i c a l l y i m p o r t a n t i n t h e \ TABLE IX A chi-square a n a l y s i s of the numerical d i e t composition of a l l l a r v a l i n s t a r s caught on same date ( a ) , of two i n s t a r s of s i m i l a r ages caught on same date (b) , of the same l a r v a l i n s t a r s caught on d i f f e r e n t dates ( c ) , and of the seasonal l a k e p l a n k t o n c o m p o s i t i o n (d). 2 Compared dates Compared i n s t a r s d. f . X P (a) (b) (c) (d) J u l y am 2, t r 2, t r 3, t r 4° 27 1268.5 < .001 Aug. am 3, t r 3, t r 4° 12 1015.2 < .001 Sept. am 4, t r 3, t r 4 Y, t r 4° 24 716.3 < .001 Oct. am 4, t r 4 Y, t r 4° 12 261.6 < .001 J u l y t r 2, t r 3 6 9 26.7 < .001 J u l y am 2, t r 2 5 135.0 < .001 Aug. t r 3, t r 4° 6 232.3 •c .001 Sept. am 4, t r - 3 6 54.9 .001 Sept. t r 4 Y , t r 4° 8 174.9 < .001 Sept. t r 3, t r 4 Y 7 143.3 < .001 Oct. t r 4 Y , t r 4° 6 36.6 < .001 J u l y , Aug., Sept. t r 3 16 802.1 < .001 Sept. , Oct. t r 4 Y 7 188.3 < .001 June, J u l y , Aug., Sept., Oct. t r 4° 36 2744.3 < .001 Sept. , Oct. am 4 6 89.1 < .001 June, J u l y , Aug., Sept., Oct. l a k e 24 40,634.0 < .001 to 73 d i e t o f am 4 l a r v a e C47%) b u t n o t i n t h e d i e t s o f t h e t r 4 Y a n d t r 4° l a r v a e ( 1 0 % ) . A c h i - s q u a r e a n a l y s i s showed h i g h l y s i g n i f i c a n t d i f f e r e n c e s (p < .001) i n t h e n u m e r i c a l d i e t o f a l l c o e x i s t i n g i n s t a r s t a k e n t o g e t h e r ( T a b l e I X a ) , b e t w e e n t h e m o s t s i m i l a r p a i r s o f c o e x i s t i n g i n s t a r s b y age a n d b y d i e t ( T a b l e I X b)„ b e t w e e n s e a s o n s w i t h i n a n y l a r v a l i n s t a r ( T a b l e I X c ) , a n d i n t h e c o m p o s i t i o n o f t h e l a k e p l a n k t o n ( T a b l e I X d ) . The m o s t a b u n d a n t z o o p l a n k t o n s p e c i e s f o u n d i n t h e l a k e a t a n y one t i m e w e r e u s u a l l y t h e m o s t f r e q u e n t l y e a t e n p r e y i n a l l l a r v a l t y p e s . An e x c e p t i o n was i n S e p t e m b e r when H o l o p e d i u r n w e r e i n f r e q u e n t l y e a t e n by a l l C h a o b o r u s i n s p i t e o f t h e i r a b u n d a n c e i n t h e l a k e . g D i e l f e e d i n g a c t i v i t y C h a o b o r i d f e e d i n g p e a k s o n D . k e n a i a t d u s k a n d dawn w e r e o b s e r v e d f o r t r 4° o n a l l s a m p l i n g d a t e s , e x c e p t i n O c t o b e r when o n l y a n i g h t p e a k o c c u r r e d ( F i g . 2 3 ) . T h e s e p e a k s c o i n c i d e d w i t h t h e p r e d a t o r - p r e y s p a t i a l o v e r l a p w h i c h was m o s t i n t e n s e a t d u s k when t h e d e s c e n -d i n g D . k e n a i p a s s e d t h e a s c e n d i n g l a r v a e a n d a t dawn when t h i s p a t t e r n was r e v e r s e d . The c o n s i d e r a b l e d a y t i m e o v e r l a p b e t w e e n D . k e n a i a n d C h a o b o r u s i n J u l y a n d A u g u s t ( F i g . 8) may e x p l a i n t h e a f t e r n o o n f e e d i n g p e a k s o b s e r v e d i n t h o s e m o n t h s . 74 I £ 5" V) DL O DC O DC < 0 10-i 16-17 JULY 7-8 AUGUST 6-7 SEPTEMBER ~I I 1 I I 1 I I I 6-7 OCTOBER 1 — i 1 1 — i 1 1 — i 1000 1600 2200 0400 1000 1300 1900 0100 0700 TIME - HOURS Fig.23 D i e l feeding pattern on D.kenai of C.tr 4° larvae i n 1972. Arrows i n d i c a t e peaks of feeding a c t i v i t y . Sample s i z e for each data point ranged from about 100-300 larvae. JULY AUGUST SEPTEMBER OCTOBER Q o o u. (a) Cam II (e) C tr II (b) Q. am III 1 I I I (c) C. am IV to-la o-(f) C tr III (r4> C. am IV 1000 1600 2200 0400 1000 1300 1900 0100 0700 40' 20-0-(rft C trll 40 20<" (I) C tr IV° 1000 1600 2200 0400 1000 1300 1900 0100 0700 (i) C tr IVY f 1 f — i — i — i — i — i 40 20-W C.tr IV> 1000 1600 2200 0400 1000 1300 1900 0100 0700 (j) C t r IV* (ro Ct r ivo 1000 1600 2200 0400 1000 1300 1900 0100 0700 TIME — H O U R S Fig.24. D i e l feeding a c t i v i t y on Bosmina ( s o l i d line),, and on a l l prey types combined (dotted l i n e ) , of d i f f e r e n t , i n s t a r s c f C.americanus and C . t r i v i t t a t u s larvae i n 1972. Sample s i z e f o r each data point ranged from about 50-100 C.americanus larvae and from about 100-200 C. : t r i v i t t a t u s larvae. — 76 A c h i - s q u a r e a n a l y s i s was u s e d t o t e s t t h e n u l l h y p o t h e s i s t h a t t h e p o i n t s f o r e a c h d i e l s e r i e s v a r i e d r a n d o m l y . The n u l l h y p o t h e s i s was d i s p r o v e d o n l y f o r t h e S e p t e m b e r d a t a ( p < . 0 2 5 , T a b l e X a ) . The number o f l a r v a e c o l l e c t e d i n A u g u s t a n d O c t o b e r w h i c h c o n t a i n e d D . k e n a i , w e r e t o o f e w f o r a n a l y s i s and d i e l s e r i e s h a d t o be c o m b i n e d . N e v e r t h e l e s s , t h e r e i s an o b v i o u s t r e n d i n t h e a p p e a r a n c e o f dawn and d u s k f e e d i n g p e a k s t h r o u g h o u t t h e s e a s o n . The s l o w d i g e s t i o n o f D . k e n a i by t h e c h a o b o r i d s p r o d u c e d o v e r l a p p i n g e f f e c t s b e t w e e n s u c c e s s i v e s a m p l i n g i n t e r v a l s . A s a r e s u l t , t h e p e a k s w e r e r e l a -t i v e l y w i d e a n d t h e i r a c c u r a c y f o r i n d i c a t i n g t h e a c t u a l f e e d i n g t i m e s was somewhat r e d u c e d . The d i e l p r e d a t i o n p a t t e r n o f C . a m e r i c a n u s l a r v a e f e e d i n g o n B o s m i n a was d i f f e r e n t o n e a c h d a t e s a m p l e d b u t w i t h a t e n d e n c y f o r a p e a k a t m i d n i g h t ( F i g . 24 a - d ) . S i n c e t h e s e l a r v a e and p r e y o c c u p i e d i d e n t i c a l d e p t h z o n e s a t a l l t i m e s ( F i g . 8 ) , t h e o b s e r v e d d i e l f e e d i n g f l u c t u a t i o n s w e r e n o t e x p e c t e d . C . t r 2 f e d on B o s m i n a t h r o u g h o u t t h e 24 h o u r p e r i o d ( F i g . 24 e ) . The n i g h t f e e d i n g b y t h e o l d e r C . t r i v i t t a t u s ( F i g . 24 f - i , k ) , was e a s i l y a c c o u n t e d f o r by t h e l a r v a l m i g r a t o r y b e h a v i o u r t h a t a l l o w e d them o n l y a few h o u r s a t n i g h t i n t h e B o s m i n a - r i c h s u r f a c e l a y e r ( F i g . 8 ) . A c h i - s q u a r e a n a l y s i s showed e a c h s e r i e s o f p o i n t s t o h a v e n o n - r a n d o m d i s t r i b u t i o n f o r a l l l a r v a l t y p e s (p < . 0 0 1 , T a b l e X b) e x c e p t t h e t r 4 y w h i c h p r e y e d l i t t l e on B o s m i n a i n O c t o b e r ( F i g . 24 j ) , a n d t h e t r 4° TABLE X A chi-square a n a l y s i s of the d i e l f e e ding p a t t e r n o f Chaoborus l a r v a e based on D. ke n a i alone ( a ) , on Bosmina alone ( b ) , and on a l l plankton types combined ( c ) . Date I n s t a r d. f . X P (a) J u l y t r 4° 8 13.0 .25>p>.l. Aug. t r 4° 2 3.7 .25>p>. 1 Sept. t r 4° 8 18.8 . ,025>p>.01* Oct. t r 4° 3 3.7 „5>p>.25 (b) J u l y am 2 6 37.2 p<.001* J u l y t r 2 8 61.3 p<.001* J u l y t r 3 8 124.5 p^.OOl* J u l y t r 4° 6 87.1 p<.001* Aug. am 3 8 56.1 p<.001* Aug. t r 3 8 43.3 p<.001* Sept. am 4 4 31.1 p<.001* Sept. t r 3 4 34.8 p<.001* Sept. t r 4 Y 4 25.0 p<.001* Oct. am 4 8 44.4 p<.001* Oct. t r 4 Y 4 11.6 .05>p>.01* Oct. t r 4° 7 10.0 p>.05 (c) J u l y am 2 7 30.8 p<.001* J u l y t r 2 8 30.0 p<.001* J u l y t r 3 8 78.8 p<.001* - J u l y t r 4° 8 25.1 p<.001* Aug. am 3 8 41.1 p<.001* Aug. t r 3 8 31.9 p<.001* Aug. t r 4° 8 7.2 p>.05 Sept. am 4 5 46.8 p<.001* Sept. t r 3 8 56.3 p<.001* Sept. t r 4 Y 8 66.4 p<.001* Sept. t r 4° 8 54.2 p<.001* Oct. am 4 8 67.1 p<.001* Oct. t r 4 Y 8 30.7 p<.001* Oct. t r 4° 8 30.5 p<.001* * d i e l s e r i e s vary non-randomly a t 95% l e v e l 78. w h i c h e s s e n t i a l l y d i d n o t p r e y o n B o s m i n a f r o m A u g u s t t o O c t o b e r ( F i g . 24 l , m , n ) . F e e d i n g p e a k s b a s e d o n t h e r a p i d l y d i g e s t e d B o s m i n a p r o b a b l y g a v e an a c c u r a t e i n d i c a t i o n o f t h e i r m o s t i n t e n s e c r o p p i n g t i m e s by C h a o b o r u s . V a r i a t i o n i n e a c h d i e l s e r i e s o f p o i n t s o b t a i n e d f r o m a l l p l a n k t o n t y p e s c o m b i n e d was n o n - r a n d o m (P < . 0 0 1 , T a b l e X c ) f o r a l l l a r v a e e x c e p t t h e t r 4° i n A u g u s t . T h e s e c u r v e s w e r e s m o o t h e r t h a n t h e o n e s o b t a i n e d f o r D . k e n a i a n d B o s m i n a a l o n e ( F i g . 24 a - n ) , a n d p r o b a b l y d i d n o t g i v e t h e a c t u a l f e e d i n g a c t i v i t y o f t h e l a r v a e b e c a u s e o f t h e m a s k i n g e f f e c t o f d i f f e r e n t d i g e s t i o n r a t e s a nd s p a t i a l d i s t r i b u t i o n s o f t h e v a r i o u s p r e y t y p e s . 3 D i s c u s s i o n I n t h e p r e s e n t s t u d y , d i g e s t i o n r a t e s o f l a r v a e w e r e f o u n d t o i n c r e a s e d i r e c t l y w i t h t e m p e r a t u r e . F e e d i n g r a t e s i n c r e a s e d w i t h b o t h t e m p e r a t u r e a n d p r e y d e n s i t y f o r a l l l a r v a e t e s t e d e x c e p t t h e t r 4° w h i c h d i d n o t seem t o r e s p o n d t o t e m p e r a t u r e . P r e y d e n s i t i e s u s e d i n f e e d i n g e x p e r i m e n t s t o a c h i e v e maximum f e e d i n g r a t e s o f t h e l a r v a e w e r e h i g h c o m p a r e d t o t h e l a k e d e n s i t i e s o f z o o p l a n k t o n i n 1971 a n d 1972. I t i s p r o b a b l e t h a t i n E u n i c e L a k e C h a o b o r u s do n o t f e e d a t t h e i r f u l l p o t e n t i a l on a n y o f t h e p r e y t y p e s t e s t e d , i e . D . t y r e l l i , D . k e n a i , D i a p h a n o s o m a , a n d n a u p l i i . A p o s s i b l e e x c e p -t i o n c o u l d be t h e 2nd a n d 3 r d i n s t a r l a r v a e f e e d i n g o n D . t y r e l l i s i n c e t h e r e i s c o n s i d e r a b l e s p a t i a l o v e r l a p 79 b e t w e e n p r e d a t o r a n d p r e y . A l s o , D . k e n a i , a l t h o u g h w e l l b e l o w t h e s a t u r a t i o n d e n s i t y f o r l a r v a l f e e d i n g , a r e p r o b a b l y a v a l u a b l e p r e y f o r t h e 4 t h i n s t a r l a r v a e b e c a u s e o f t h e i r l a r g e s i z e . The o b s e r v e d d i f f e r e n c e s i n t h e e f f e c t o f t e m p e r a -t u r e o n f e e d i n g r a t e s o f C . t r 4 ° , a n d on t h e o t h e r l a r v a e o f b o t h s p e c i e s , may be due t o d i f f e r e n c e s i n t h e t h e r m a l r e g i m e t h a t t h e s e l a r v a e e x p e r i e n c e . The 3-4 d a y f e e d i n g e x p e r i m e n t s w h e r e l a r v a e w e r e p l a c e d i n t o a u n i f o r m t e m p e r a t u r e e n v i r o n m e n t w e r e p r o b a b l y more " u p s e t t i n g " t o t h e m i g r a t o r y C . t r 4° t h a n t o t h e o t h e r n o n - m i g r a t o r y f o r m s . T h i s may e x p l a i n t h e d a i l y v a r i a t i o n i n , a n d t h e l a c k o f t e m p e r a t u r e c o n t r o l o v e r , t h e f e e d i n g r a t e s o f t r 4° l a r v a e a s c o m p a r e d t o t h e o t h e r l a r v a e . The c a u s e o f l a c k o f t e m p e r a t u r e r e g u l a t i o n o n t h e f e e d i n g r a t e s o f t h e l a r v a e o f b o t h s p e c i e s c o l l e c t e d i n November i s unknown. The l a c k o f a n y f e e d i n g p a t t e r n i n i n d i v i d u a l l a r v a e d u r i n g t h e l o n g t e r m f e e d i n g e x p e r i m e n t was s u r p r i s i n g i n v i e w o f c o n s t a n t p r e y a v a i l a b i l i t y , n o r m a l d a y l i g h t f l u c t u a t i o n s , a n d n e a r - n a t u r a l w a t e r t e m p e r a t u r e . T h i s v a r i a b i l i t y d o e s seem, h o w e v e r , t o be r e a l i n s p i t e o f t h e s m a l l e x p e r i m e n t a l c o n t a i n e r s a n d t h e i s o l a t i o n o f l a r v a e f r o m one a n o t h e r . S i n c e t h e e x p e r i m e n t s o n t h e e f f e c t o f p r e y d e n s i t y o n l a r v a l f e e d i n g a l s o showed m a r k e d d i f f e r e n c e s among i n d i v i d u a l s i n t h e i r d a i l y f o o d i n t a k e a t h i g h a n d l o w p r e y d e n s i t i e s , t h e random 80 n a t u r e o f p r e d a t o r - p r e y e n c o u n t e r s c a n n o t a l o n e a c c o u n t f o r t h i s v a r i a b i l i t y . I t i s s u s p e c t e d t h a t , a l t h o u g h f o o d s u p p l y i n E u n i c e L a k e i s l i m i t e d , C h a o b o r u s l a r v a e p r o b a b l y do n o t f e e d w h e n e v e r t h e y e n c o u n t e r a p r e y . T h i s may b e t h e r e s u l t o f d i f f i c u l t y i n c a p t u r i n g t h e p r e y o r o f t h e p h y s i o l o g i c a l s t a t e o f t h e l a r v a e . A n d e r s o n (1970) a n d Monakov (1972) f o u n d t h a t t h e d i e t o f a q u a t i c i n v e r t e b r a t e p r e d a t o r s d e p e n d s p a r t l y on t h e p h y s i c a l a n d b e h a v i o r a l c h a r a c t e r i s t i c s o f p r e -d a t o r a n d p r e y . T h i s was a l s o f o u n d f o r C h a o b o r u s l a r v a e i n t h e p r e s e n t s t u d y . The d i f f e r e n c e s i n move-ment a n d v e l o c i t y o f z o o p l a n k t o n t y p e s p r o b a b l y p l a y e d a m a j o r r o l e i n d e t e r m i n i n g t h e r e l a t i o n s h i p b e t w e e n p r e y d e n s i t y a n d t h e f e e d i n g r a t e s o f l a r v a e . F o r e x a m p l e , t h e v e r y m o b i l e a d u l t D . k e n a i a p p e a r e d t o be much more a c c e s s i b l e t o C h a o b o r u s i n t e r m s o f e n c o u n t e r r a t e s , t h a n w e r e t h e s l o w - m o v i n g D . t y r e l l i a n d D i a p h a n o s o m a . The a b i l i t y o f D . k e n a i t o l e a p t h r o u g h a d i s t a n c e o f up t o 10 cm when d i s t u r b e d , a n d t h e c o m p l e t e a b s e n c e o f s u c h a g i l i t y i n B O s m i n a , p r o b a b l y l e d t o v e r y d i f f e r e n t i n t e r -a c t i o n s b e t w e e n e a c h o f t h e s e p r e y a n d ChaObOrus. The l a r g e s i z e o f D . k e n a i made them i n a c c e s s i b l e t o i n s t a r s 1-3. The s w a l l o w i n g o f " s t r e a m l i n e d " c o p e p o d s r e q u i r e d l e s s t i m e t h a n s w a l l o w i n g o f t h i c k - c u t i c l e d , d i s c - s h a p e d D a p h n i a o f s i m i l a r b u l k ( S w i f t 1 9 7 3 ) . L a r g e D a p h n i a w e re c a p a b l e o f d r a g g i n g t h e i r p r e d a t o r a b o u t when c a p -t u r e d a n d w e r e e v e n t u a l l y r e l e a s e d . T h i s a p p a r e n t 81 " s e l e c t i v i t y " b y C h a o b o r u s o f c o p e p o d s o v e r c l a d o c e r a n s was n o t e d a l s o b y D e o n i e r (194 3 ) , R o t h ( 1 9 7 1 ) , a n d o t h e r s . T h e r e i s d i s a g r e e m e n t i n t h e l i t e r a t u r e on t h e d i e l f e e d i n g p a t t e r n of^ C h a o b o r u s l a r v a e . M o s t o f t h e s p e c i e s s t u d i e d w e r e t y p i c a l l y b e n t h i c b y day and p l a n k t o n i c b y n i g h t . G u t e x a m i n a t i o n o f l a r v a e c o l l e c t e d b y B e r g (1937) f r o m b e n t h i c a n d f r o m p e l a g i c z o n e s , g a v e i n c o n -c l u s i v e r e s u l t s . K a j a k a n d R a n k e - R y b i c k a (1970) a n d R o t h (1971) b e l i e v e t h a t t h e i r l a r v a e a r e m a i n l y n i g h t f e e d e r s . B u t G o l d s p i n k a n d S c o t t (1971) a n d S i k o r o w a ( 1 9 7 3 ) , who h a v e f o u n d s i m i l a r p r o p o r t i o n s o f f e d C h a o b o r u s a t n i g h t and i n t h e d a y t i m e , b e l i e v e t h a t t h e l a r v a e f e e d e q u a l l y w e l l a t a l l t i m e s . H o w e v e r , i t i s p o s s i b l e t h a t b o t h o f t h e s e a u t h o r s h a v e l o o k e d a t s i t u a -t i o n s w h e r e d u r i n g d a y t i m e t h e l a r v a e w e r e n o t a c t u a l l y b e n t h i c b u t p l a n k t o n i c n e a r t h e mud s u r f a c e and w h e r e f o o d was e q u a l l y a b u n d a n t a t a l l d e p t h z o n e s . I n t h e p r e s e n t s t u d y , t h e d i e l f e e d i n g a c t i v i t y o f C . a m e r i c a n u s a n d C • t r i v i t t a t u s h a s b e e n f o u n d t o b e s p e c i f i c f o r e a c h p r e d a t o r - p r e y c o m b i n a t i o n : D . k e n a i w e r e e a t e n m o s t i n t e n s e l y by C . t r 4° a t d u s k and a t dawn; B o s m i n a w e r e e a t e n a t a l l t i m e s o f t h e d a y by t h e y o u n g e r i n s t a r s b u t m a i n l y a t n i g h t by t h e o l d e r C . t r i v i t t a t u s l a r v a e . The i n v e s t i g a t o r s o f t h e d i e l f e e d i n g p a t t e r n o f C h a o b o r u s , c i t e d a b o v e , h a v e l o o k e d a t c r o p s o f f r e s h l y c a u g h t o r o f p r e s e r v e d l a r v a e , and i n m o s t c a s e s h a v e 82 s c o r e d t h e s e a s f u l l o r empty w i t h o u t a n a l y z i n g t h e i r a c t u a l c o n t e n t s . T h e s e d a t a , h o w e v e r , a r e o f q u e s t i o n a b l e v a l u e b e c a u s e o f t h e l a r v a l t e n d e n c y t o r e g u r g i t a t e when p r e s e r v e d . The l a t t e r p r o b l e m h a s b e e n l a r g e l y r e m e d i e d by t h e u s e o f a n a n a e s t h e t i c ( S w i f t a n d F e d o r e n k o , i n p r e s s ) . A n o t h e r p r o b l e m n o t c o n s i d e r e d b y m o s t w o r k e r s i s t h e c o n s i d e r a b l e v a r i a b i l i t y i n d i g e s t i o n r a t e s a n d r e t e n t i o n t i m e s o f t h e v a r i o u s p r e y a n d t h e d i f f e r e n t s p a t i a l d i s t r i b u t i o n s o f t h e p r e y s p e c i e s w i t h r e s p e c t t o t h e p r e d a t o r s . A l l o f t h e s e e f f e c t s w o u l d c o m b i n e t o mask t h e a c t u a l d i e l f e e d i n g p a t t e r n . I t i s t h e r e f o r e i m p o r t a n t t o w o r k w i t h a l l t h e p r e y t y p e s e n c o u n t e r e d o n an i n d i v i d u a l b a s i s , a n d t o know t h e c h a r a c t e r i s t i c s m e n t i o n e d a b o v e f o r e a c h o n e . H y p o t h e s e s o n t h e n a t u r e o f d i e l c y c l i c f e e d i n g o f C h a o b o r u s a r e v e r y f e w . S i n c e C h a o b o r u s c a p t u r e p r e y w e l l i n t o t a l d a r k n e s s (Duhr 1 9 5 5 ) , a n d s i n c e t h e r e i s some e v i d e n c e t h a t t h e y c a n f e e d w h i l e b u r i e d i n t h e mud d u r i n g t h e d a y (Parma 1 9 7 1 ) , c o n t i n u o u s f e e d i n g a r o u n d t h e c l o c k may b e e x p e c t e d . H owever, R o t h (1971) s u g g e s t e d t h a t d a y t i m e f e e d i n g i s u n l i k e l y i f t h e l a r v a e a r e i n t h e d e e p w a t e r s e d i m e n t s w h e r e p r e y a r e s c a r c e . T h i s e x p l a n a t i o n may be v a l i d f o r h i s l a k e (10 m a t t h e d e e p e s t p o i n t ) , a n d f o r some o t h e r l a k e s w h e r e C h a o b o r u s a r e f o u n d a t d e p t h s o f 10 m o r more w h e r e f o o d s u i t a b l e f o r C h a o b o r u s i s l i m i t e d ( N o r t h c o t e 1964; F e d o r e n k o a n d S w i f t 1 9 7 2 ) . R a k u s a - S u s z c z e w s k i (1969) a n d P e a r r e (1973) 83 h a v e o b s e r v e d t h a t t h e f e e d i n g a c t i v i t y o f c h a e t o g -n a t h s was m o s t i n t e n s i v e a t n i g h t a n d was r e l a t e d t o t h e i r m i g r a t i o n . I n t h o s e i n s t a n c e s , a s i n t h e c a s e o f t h e E u n i c e L a k e C h a o b o r u s , a n i n c r e a s e i n t h e l i k e l i h o o d o f p r e d a t o r - p r e y e n c o u n t e r s d u r i n g n i g h t m i g r a t i o n may e x p l a i n t h e o b s e r v e d o c c u r r e n c e o f t h e f e e d i n g p e a k s a t d u s k a nd dawn f o r D . k e n a i a n d a t n i g h t f o r B o s m i n a . T h i s h y p o t h e s i s i n t e g r a t e s t h e i d e a o f p r e y s c a r c i t y i n d e e p w a t e r s m e n t i o n e d a b o v e . The m a j o r d e t e r m i n i n g f a c t o r s o f C h a o b o r u s d i e t , o b t a i n e d f r o m t h e c r o p a n a l y s i s d a t a , a p p e a r t o be p r e y a b u n d a n c e a n d s p a t i a l d i s t r i b u t i o n o f p r e y w i t h r e s p e c t t o t h e p r e d a t o r . T h i s r e l a t i o n s h i p was s u p p o r t e d b y two d i s t i n c t p i e c e s o f e v i d e n c e . F i r s t , t h e o b s e r v a t i o n f r o m t h e c r o p d a t a t h a t g r e a t e r s i m i l a r i t i e s e x i s t i n t h e d i e t s o f l a r v a e w i t h i n a s p e c i e s , i r r e s p e c t i v e o f t h e i r i n s t a r , t h a n i n t h e d i e t s o f t h e same i n s t a r o f two d i f -f e r e n t s p e c i e s , was c o n t r a d i c t o r y t o t h e e x p e c t e d r e l a -t i o n s h i p . A r e v e r s e d e f f e c t was e x p e c t e d b a s e d o n t h e c o m p a r i s o n among i n s t a r s o f t h e s i z e s o f g r a s p i n g a p p e n -d a g e s a nd mouth gape o f C h a o b o r u s l a r v a e . B o s m i n a was t h e m a j o r p r e y , i n t e r m s o f b i o m a s s , o f t h e 2nd a n d 3 r d i n s t a r s o f C . a m e r i c a n u s ; D . t y r e l l i was t h e m a j o r p r e y o f t h e 2nd a n d 3 r d i n s t a r s o f C . t r i v i t t a t u s . S i z e o f t h e p r e d a t o r c o u l d n o t t h e r e f o r e be o f p r i m e i m p o r t a n c e i n d e t e r m i n i n g t h e l a r v a l d i e t . S e c o n d l y , i t was p o s s i b l e t o e x p l a i n l a r v a l d i e t s 84 q u i t e s a t i s f a c t o r i l y u s i n g t h e p r e y a b u n d a n c e , t h e i r d i s t r i b u t i o n r e l a t i v e t o t h e p r e d a t o r , a n d t h e f a c t t h a t l a r g e p r e y c a n n o t be h a n d l e d by t h e y o u n g e r l a r v a e . F o r e x a m p l e , t h e mos t d i v e r s e d i e t was t h a t o f t h e 4 t h i n s t a r l a r v a e , p a r t i c u l a r l y t h e t r 4° b e c a u s e t h e y c o u l d h a n d l e n e a r l y a l l t h e p r e y s i z e s , a n d e n c o u n t e r e d a l l p r e y t y p e s i n t h e i r e x t e n s i v e , d a i l y v e r t i c a l m i g r a t i o n s . The a p p a r e n t p r e f e r e n c e o f 2nd a n d 3 r d i n s t a r s o f C . a m e r i c a n u s f o r B o s m i n a a n d o f 2nd a n d 3 r d i n s t a r s o f C . t r i v i t t a t u s f o r D . t y r e l l i c o u l d e a s i l y be a c c o u n t e d f o r by t h e d i f f e -r e n c e s i n v e r t i c a l d i s t r i b u t i o n o f t h e s e p l a n k t o n t y p e s . I n J u l y a n d A u g u s t , C . a m e r i c a n u s a nd B o s m i n a i n h a b i t e d i d e n t i c a l d e p t h s w h i c h w e r e c o n s i s t e n t l y h i g h e r t h a n t h o s e i n h a b i t e d by t h e y o u n g C . t r i v i t t a t u s a n d D . t y r e l l i . The a p p a r e n t s e l e c t i v i t y o f C.t r _ 4° f o r D . k e n a i may be e x p l a i n e d b y t h e h i g h f e e d i n g e f f i c i e n c y o f t h e s e l a r v a e a t l o w D . k e n a i d e n s i t i e s . D . k e n a i c o u l d n o t be consumed by t h e y o u n g e r l a r v a e b e c a u s e o f s i z e i n c o m p a t i b i l i t y . The d i e t o v e r l a p i n d e x c l e a r l y r e f l e c t e d t h e s e a s o n a l c h a n g e s i n s p a t i a l o v e r l a p among t h e l a r v a l i n s t a r s . L a r v a e t h a t h a d n e a r - i d e n t i c a l v e r t i c a l d i s t r i b u t i o n s when i n t h e e p i l i m n e t i c , f o o d - r i c h z o n e a l s o h a d v e r y s i m i l a r d i e t s . T h i s was t r u e o f am 2 and t r 2 i n J u l y , am 4, t r 3, a n d t r 4^ i n S e p t e m b e r , a nd a l l t h e 4 t h i n s t a r l a r v a e i n O c t o b e r . C o n v e r s e l y , l a r v a e w i t h d i s t i n c t l y d i f f e r e n t v e r t i c a l d i s t r i b u t i o n s h a d l o w d i e t o v e r l a p i n d i c e s w i t h o t h e r i n s t a r s a s d i d t r 4° t h r o u g h o u t 85 m o s t o f t h e summer. F i n a l l y , t h e d i e l f e e d i n g a c t i v i t y o f C h a o b o r u s i n s t a r s o n B o s m i n a a n d o n D . k e n a i was d e t e r m i n e d l a r g e l y b y t h e t i m e o f maximum s p a t i a l o v e r l a p b e t w e e n p r e d a t o r a n d p r e y . T h i s r e l a t i o n s h i p b e t w e e n t h e n a t u r e o f a p r e d a t o r ' s f o o d and t h e p r e y s i z e , a b u n d a n c e , and d i s t r i b u t i o n i n r e l a t i o n t o t h e p r e d a t o r seems t o be a common one p a r t i -c u l a r l y i n t h e c a s e o f o p p o r t u n i s t i c f e e d e r s s u c h a s t h e C h a o b o r u s l a r v a e d e s c r i b e d h e r e , c h i r o n o m i d l a r v a e ( I z v e k o v a 1 9 7 1 ) , f i s h ( G e r a l d 1966) and c h a e t o g n a t h s ( R a k u s a - S u s z c z e w s k i 1 9 6 9 ) . S i n c e t h e n a t u r e o f l a r v a l f o o d was i n d e p e n d e n t o f l a r v a l s i z e e x c e p t w h e r e e x t r e m e l y l a r g e p r e y w e r e i n v o l v e d , S t a h l ' s (1966a) s u g g e s t i o n t h a t s i z e d i f f e r e n c e s i n c o e x i s t i n g l a r v a e may a l s o i m p l y f o o d d i f f e r e n c e s d o e s n o t a p p e a r t o a p p l y t o t h e E u n i c e L a k e C h a o b o r u s p o p u l a t i o n s . 86 4 Summary 1 D i g e s t i o n r a t e s v a r i e d d i r e c t l y w i t h t e m p e r a t u r e and i n v e r s e l y w i t h p r e y s i z e . 2 F e e d i n g r a t e s i n c r e a s e d w i t h t e m p e r a t u r e a nd p r e y d e n s i t y , e x c e p t f o r t r 4° w h e r e t e m p e r a t u r e a p p a r e n t l y h a d no e f f e c t . 3 Maximum d a i l y f o o d r a t i o n i n c r e a s e d d i r e c t l y w i t h l a r v a l a g e . 4 Maximum c o n s u m p t i o n r a t e s g e n e r a l l y c o u l d n o t be r e a l i z e d i n t h e l a k e due t o l o w p r e y d e n s i t y . 5 D a i l y r a t i o n o f i n d i v i d u a l l a r v a e was v e r y v a r i a b l e d e s p i t e t h e s t e a d y e n v i r o n m e n t a l c o n d i t i o n s i n t h e l a b o r a t o r y . 6 C o m p o s i t i o n o f l a r v a l d i e t d i f f e r e d w i t h i n s t a r a n d w i t h s p e c i e s . 7 D i v e r s i t y o f d i e t i n c r e a s e d w i t h a ge ( i n s t a r ) o f t h e i n d i v i d u a l l a r v a e . 8 L a r g e p r e y s i z e was a d e f i n i t e r e s t r i c t i v e f o r c e l e a d i n g t o " s i z e - s e l e c t i v e " p r e d a t i o n i n t h e y o u n g e r i n s t a r s . 9 D i e l m i g r a t i o n s o f l a r v a e p l a y a n i m p o r t a n t r o l e i n d e t e r m i n i n g l a r v a l d i e t a n d t h e t i m e o f f e e d i n g . 87 V I I EFFECT OF CHAOBORUS PREDATION ON ZOOPLANKTON 1 M e t h o d s a C a l c u l a t i o n o f o v e r a l l p r e d a t i o n i m p a c t o n z o o p l a n k t o n P r e d a t i o n e f f e c t o n n a u p l i i , D. t y r e l l i , - D . k e n a i , a n d D i a p h a n o s o m a , was c a l c u l a t e d f o r e a c h o f t h e d a t e s s a m p l e d i n 1971 a n d 1972. The f r a c t i o n o f t h e s t a n d i n g c r o p o f a p a r t i c u l a r p r e y t y p e r e m o v e d b y a l l C h a o b o r u s i n s t a r s was c a l c u l a t e d f r o m t h e e q u a t i o n : n M i = .-} = !• p . r i o w h e r e "M^" i s t h e t o t a l d a i l y p r e d a t i o n m o r t a l i t y (No/m )' o f p r e y t y p e " i " , "Ch." a n d n P r / ' a r e t h e s t a n d i n g c r o p s (No/m ) o f C h a o b o r u s t y p e " j " a n d o f p r e y t y p e " i " r e s p e c t i v e l y , " n " i s t h e t o t a l number o f C h a o b o r u s t y p e s w h i c h f e e d o n p r e y " i " , a n d " f ^ j " i s t h e d a i l y f e e d i n g r a t e o f e a c h C h a o b o r u s i n s t a r " j " o n p r e y " i " t a k e n f r o m t h e f e e d i n g e x p e r i m e n t s . T h i s r a t e i s c o r r e c t e d t o t h e a p p r o p r i a t e p r e y d e n s i t y a n d l a k e t e m p e r a t u r e . The f e e d i n g r a t e i n c l u d e d b o t h k i l l e d ( u n e a t e n ) a n d e a t e n p r e y . The s t a n d i n g c r o p v a l u e u s e d f o r e a c h p r e d a -t o r a n d p r e y t y p e was t h e mean o f n o o n a n d m i d n i g h t s a m p l e s w h e r e p o s s i b l e . S e c o n d a nd t h i r d i n s t a r l a r v a e w e r e n o t d i f f e r e n t i a t e d t o s p e c i e s . A l l f o u r t h i n s t a r l a r v a e w e r e t r e a t e d s e p a r a t e l y b y s p e c i e s a n d by i n s t a r TABLE XI A sample c a l c u l a t i o n of Chaoborus p r e d a t i o n on D. t y r e l l i . Equation: M. = (1) Date (2) T°C (3) P o t e n t i a l predators V (4) Pr. (No/m2) (5) Ch . 3 2 (No/m ) (6) Prey d e n s i t y (No/1) (7) fH (8) M. . 1 3 2 (No/m ) (9) M i (No/m2) (10) % P r ± 26-7-72 15-19 3 t r 4° 6,517 343 105 2.2 1.2 1.5 0.5 515 53 568 8.7 C o l . 1 = date of sampling 2 = water temperature i n the zone where p r e d a t i o n can occur 3 = p o t e n t i a l l a r v a l predators present i n the lake on t h a t date 4 = s t a n d i n g crop of prey 5 = standing crop of predator types 6 = mean prey d e n s i t y encountered d a i l y by l a r v a l type j 7 = d a i l y consumption of prey by a l a r v a of each type u s i n g d a t a from c o l s . 2 and 6 8 = ( c o l . 5 x c o l . 7) = d a i l y m o r t a l i t y of prey per l a r v a l type 9 = sum of e n t r i e s i n c o l . 8 = t o t a l d a i l y p r e d a t i o n m o r t a l i t y on prey by a l l l a r v a l types 10 = ( c o l . 9 / c ol. 4 x 100%) = percentage of prey standing crop removed by a l l p r e d a t o r types on t h a t day 89 a n d , i n t h e c a s e o f C . t r i v i t a t t u s , b y a g e . The p r e y d e n s i t y u s e d t o i n t e r p o l a t e t h e f e e d i n g r a t e s was t a k e n a s t h e mean o f maximum p r e y d e n s i t i e s a t n o o n a n d m i d -n i g h t i n t h e r e g i o n w h e r e p r e y a n d p r e d a t o r o v e r l a p p e d s p a t i a l l y . B e c a u s e D . k e n a i w e r e s p a t i a l l y s e p a r a t e d f r o m o l d e r C . t r 3 a n d f r o m C . t r _ 4 f o r m o s t o f t h e d a y due t o t h e i r o p p o s i t e m i g r a t i o n p a t t e r n s , t h e mean d a i l y p r e y d e n s i t y was s e t a t one q u a r t e r o f t h e i r maximum noon d e n s i t y . A s a m p l e c a l c u l a t i o n o f C h a o b o r u s p r e d a t i o n o n D . t y r e l l i i s g i v e n i n T a b l e X I . The i n d i v i d u a l c a l c u -l a t i o n s f o r e a c h p r e y t y p e a n d y e a r a r e f o u n d i n A p p e n d i c e s I - V I I . I n E u n i c e L a k e p r e y d e n s i t i e s w e r e w e l l b e l o w t h e d e n s i t i e s r e q u i r e d f o r maximum f e e d i n g b y l a r v a e a s m e a s u r e d i n f e e d i n g e x p e r i m e n t s i n t h e f i e l d . F o r t h e p u r p o s e o f t h e s e c a l c u l a t i o n s , i t i s assumed t h a t t h e p r e s e n c e o f one p r e y p o p u l a t i o n d o e s n o t a f f e c t t h e p r e d a t i o n r a t e on o t h e r p r e y p o p u l a t i o n s . b C a l c u l a t i o n o f c o n s u m p t i o n b y C . t r 4° l a r v a e The mean d a i l y c a l o r i c i n t a k e p e r C . t r 4° l a r v a b e t w e e n J u n e 1 and S e p t e m b e r 30 was c a l c u l a t e d i n t h r e e w a y s : (1) f e e d i n g r a t e s a n d p r e y a b u n d a n c e d a t a , (2) a n e n e r g y b u d g e t e q u a t i o n , a n d (3) a n a n a l y s i s o f l a r v a l c r o p c o n t e n t s . S ample c a l c u l a t i o n s f o r t h e f i r s t two e s t i m a t e s a r e shown i n T a b l e X I I . 90 TABLE X I I Sample c a l c u l a t i o n s of mean d a i l y c o n s u m p t i o n ^ i by C. t r 4° u s i n g (1) f e e d i n g r a t e s and prey abundance d a t a , and (2) the energy budget equation. n (1) C t y r e l l i (kenai) = r i = .1 f i f c i Date D. t y r e l l i D. k e n a i f. f i f c i f. t . t i eg. 1-6-71 8 5. 8 40. 170.1 2.5 20. C/day = = r r — — = 1.42 D_. t y r e l l i / l a r v a / d a y t y r e l l i 8 i ^ o V 1' 8 1 1 1 221.1 C/day = k e n a i 8 . r t . 120 = 1.84 D_. k e n a i / l a r v a / d a y C a l o r i c i n t a k e of copepod r / , t y r e l l i k enai / l a r v a / d a y = <Y * 5^ 2 ug) + (1.84-xT2.7 ug) ] ' ' •* [5.4 x 10 J cal/ug] = 470 x 1 0 - 3 c a l / l a r v a / d a y (2) A x C = P + R = [7.8 ug/larva/day)(5.47 x 10~ 3 cal/ug) + (0.6 u l 02/mg/hr)(24 hr)(4.83 x 10" 3 c a l / m i 0 2 ) J = 112.2 x 10~ 3 c a l / l a r v a / d a y P+R 112.2 x 10~ 3 C = = = 165 x 10~ 3 c a l / l a r v a / d a y A .68 i n 1971 9 1 The w o r k i n g e q u a t i o n f o r t h e f i r s t e s t i m a t e was: n C= Y. f i fci i = l n II w h e r e C i s t h e t o t a l number o f D . t y r e l l i o r o f D . k e n a i consumed p e r l a r v a d u r i n g t h e 120 d a y s f r o m J u n e 1 t o S e p t e m b e r 3 0 , " f ^ " i s t h e c o n s u m p t i o n r a t e p e r t r 4°. d u r i n g t h e i n t e r v a l " i " ( A p p e n d i x V I I I ) , "t^" i s t h e number o f d a y s i n e a c h t i m e i n t e r v a l " i " f o r w h i c h " f ^ " h o l d s , a n d " n " i s t h e t o t a l number o f t h e s e i n t e r v a l s a s d e t e r m i n e d b y t h e number o f s a m p l i n g d a t e s . D a i l y c o n s u m p t i o n o f e a c h c o p e p o d s p e c i e s was c o n v e r t e d i n t o b i o m a s s u s i n g w e i g h t d a t a i n T a b l e 1 and t h e n i n t o c a l o r i e s u s i n g t h e v a l u e o f 5400 c a l o r i e s p e r gm d r y w e i g h t o f c o p e -p o d ( S c h i n d l e r e t a l 1 9 7 1 ) . The s e c o n d e s t i m a t e was b a s e d o n t h e e n e r g y b u d g e t u e q u t i o n : A x C = P + R ( R i c k e r 1968) where"A" i s a s s i m i l a t i o n e f f i c i e n c y , " c i s f o o d c o n s u m e d , " p " i s p r o d u c t i o n ( g r o w t h ) , and'Vis m e t a b o l i c l o s s t h r o u g h r e s p i r a t i o n . A l l v a l u e s e x c e p t C , w e re a v a i l a b l e f r o m S w i f t ( 1 9 7 3 ) : A=68% f o r b o t h c o p e p o d s and c l a d o c e r a n s , P ( J u n e 1 - S e p t e m b e r 30)=7.8 ug p e r l a r v a p e r d a y i n 1971 a n d 5.7 ug p e r l a r v a p e r d a y i n 1 9 7 2 , and R=0.6 u l 92 0^ p e r mg p e r h o u r . R was m e a s u r e d a t 10 C. The assump-t i o n t h a t t h i s i s a l s o t h e mean t e m p e r a t u r e o f t h e h a b i t a t o f C . t r 4° seems t o be r e a s o n a b l e s i n c e t h e s e l a r v a e c o n t i n u o u s l y m i g r a t e b e t w e e n t h e c o l d , d e e p w a t e r s a n d t h e warm, s u r f a c e w a t e r s . "p"and"R" w e r e c o n v e r t e d i n t o c a l o r i e s u s i n g t h e r e s p e c t i v e c o n v e r s i o n f a c t o r s o f 5469 c a l o r i e s p e r gm d r y w e i g h t o f C h a o b o r u s ( S w i f t 1973) and 4.83 c a l o r i e s p e r m l O2 ( H a r g r a v e 1 9 7 1 ) . The t h i r d e s t i m a t e o f l a r v a l c o n s u m p t i o n was b a s e d o n t h e c r o p a n a l y s i s d a t a f r o m 1971 w h e r e 90% o f t h e l a r v a l p o p u l a t i o n was f o u n d t o consume one p r e y p e r d a y . T h i s e s t i m a t e was m a x i m i z e d b y a t t r i b u t i n g t h e 0.9 p r e y p e r l a r v a p e r d a y t o t h e l a r g e s t p r e y a v a i l a b l e i e . D . k e n a i . The c a l o r i c c o n v e r s i o n f a c t o r was t h e same a s i n t h e f i r s t e s t i m a t e . The a b o v e c a l c u l a t i o n s may be f o u n d i n A p p e n d i c e s V I I I , I X , a n d X. 2 R e s u l t s a P r e y - n a u p l i i The mean p e r c e n t a g e o f t h e s t a n d i n g c r o p o f n a u p l i i r e m o v e d b y C h a o b o r u s was 3% i n b o t h 1971 a n d 1972 ( F i g . 25 a ) . M o s t o f t h e p r e d a t i o n o c c u r r e d i n J u l y a n d A u g u s t o f b o t h y e a r s when d i a p t o m i d n a u p l i i a n d C h a o b o r u s l a r v a l i n s t a r s 1-3 w e r e m o s t a b u n d a n t . D u r i n g t h i s t i m e 2nd a n d 3 r d i n s t a r l a r v a e w e r e r e s p o n s i b l e f o r o v e r 80% o f t h e t o t a l p r e d a t i o n o n n a u p l i i ( F i g . 25 b , c ) . P r e d a t i o n r a t e s r a r e l y e x c e e d e d one n a u p l i u s p e r d a y f o r NAUPLI D.TYRELLI 1971 1971 1972 1972 2 100 MONTHS Fig.25 The e f f e c t of Chaoborus predation on n a u p l i i ( l e f t ) , and on D . t y r e l l i ( r i g h t ) . (a,d) - percent of prey standing crop removed by Chaoborus i n 1971 (dotted l i n e ) and i n 1972 ( s o l i d l i n e ) ; (b,e) and (c,f) -percent of t o t a l 'predation a t t r i b u t e d to d i f f e r e n t l a r v a l i n s t a r s i n 1971 and 1972 r e s p e c t i v e l y . 94; t h e 2nd and 3 r d i n s t a r l a r v a e and w e r e e v e n l o w e r f o r t h e 4 t h i n s t a r l a r v a e ( A p p e n d i c e s I , I I ) . b P r e y - D . t y r e l l i An a v e r a g e o f 14% o f t h e s t a n d i n g c r o p o f D . t y r e l l i was r e m o v e d i n 1 9 7 1 a n d 6% i n 1972 ( F i g . 25 d ) . P r e d a t i o n was e s p e c i a l l y h e a v y i n A u g u s t 1971 when o v e r 30% o f t h e D . t y r e l l i p o p u l a t i o n was e a t e n . I n t h e f o l l o w i n g y e a r , c r o p p i n g was u n i f o r m l y l o w - o n l y a b o u t one t h i r d o f t h a t i n 1 9 7 1 . S e c o n d a n d 3 r d i n s t a r l a r v a e c onsumed t h e l a r g e s t p r o p o r t i o n o f t h e t o t a l D . t y r e l l i p o p u l a t i o n e a t e n i n 1 9 7 1 ; i n 1972, 3 r d a n d 4 t h i n s t a r l a r v a e w e r e t h e m o s t i m p o r t a n t p r e d a t o r s ( F i g . 25 e , f ) . The d a i l y i n t a k e o f D . t y r e l l i was h i g h i n 1 9 7 1 , s o m e t i m e s e x c e e d i n g 6 p r e y p e r l a r v a p e r d a y ; i t was l o w i n 1972 w i t h a maximum o f 2.5 p r e y p e r d a y ( A p p e n d i c e s I I I , I V ) . c P r e y - D . k e n a i The mean p e r c e n t a g e s o f t h e D . k e n a i s t a n d i n g c r o p r e m o v e d i n 1971 a n d 1972 w e r e 14% a n d 2 5 % r e s p e c t i v e l y ( F i g . 26 a ) , b u t t h e a b s o l u t e number o f p r e y r e m o v e d i n 1972 was o n l y a b o u t one t h i r d o f t h a t r e m o v e d i n 1971 ( A p p e n d i c e s V, V I ) . F rom May t o J u l y o f b o t h y e a r s , C . t r 4° w e r e t h e m o s t i m p o r t a n t p r e d a t o r s ( F i g . 26 b , c ) . D u r i n g t h i s p e r i o d , C.am 4 l a r v a e , b e c a u s e o f t h e i r l o w numbers a c c o u n t e d f o r l e s s t h a n 30% o f t h e o v e r a l l p r e -d a t i o n o n D . k e n a i . I n l a t e summer a n d i n t h e f a l l , t r 3 1971 1971 1972 lOCh 1972 CO 50- PREDATION NEGLIGIBLE M MONTHS Fig.26 The e f f e c t of Chaoborus predation on D.kenai ( l e f t ) , and on Diaphanosoma ( r i g h t ) . For explanation see Fig.25. 96 l a r v a e a n d a l l t h e 4 t h i n s t a r l a r v a e w e r e i m p o r t a n t p r e d a t o r s on' P . k e n a i . The a v e r a g e d a i l y c o n s u m p t i o n i n 1971 was 1.8 D . k e n a i p e r l a r v a , w i t h a maximum o f 3.0 P . k e n a i ( A p p e n d i x V ) . I n 1972 t h e mean c o n s u m p t i o n was 0.6 P . k e n a i p e r d a y ( A p p e n d i x V I ) . d P r e y - D i a p h a n o s o m a I n 1971 a n a v e r a g e o f 19% o f t h e s t a n d i n g c r o p o f D i a p h a n o soma was r e m o v e d i n J u l y a n d A u g u s t when t h e s e p r e y w e r e m o s t a b u n d a n t ( F i g . 26 d ) . I n 1972 t h e s t a n d i n g c r o p o f t h i s p r e y s p e c i e s was v e r y l o w a n d p r e d a t i o n was n e g l i g i b l e . The peak, p o p u l a t i o n o f D i a p h a n o s o m a i n 1971 c o i n c i d e d w i t h t h e p r e s e n c e o f 2nd a n d 3 r d i n s t a r l a r v a e i n t h e l a k e . T h e s e l a r v a e r e m o v e d o v e r 8 0 % o f t h e t o t a l P i a p h a n o soma e a t e n ( F i g . 26 e ) , a t a r a t e o f a b o u t 3 p r e y p e r l a r v a p e r d a y ( A p p e n d i x V I I ) . e C o n s u m p t i o n b y C . t r 4° l a r v a e The v a l u e s f r o m t h e t h r e e c o n s u m p t i o n e s t i m a t e s r a n g e d f r o m 148-470 c a l o r i e s p e r l a r v a p e r d a y ( T a b l e X I I I ) . The f i r s t e s t i m a t e u s i n g t h e p l a n k t o n a b u n d a n c e d a t a i n d i c a t e d t h a t o v e r one t o t h r e e c o p e p o d p r e y may h a v e b e e n consumed d a i l y b y a C . t r 4° l a r v a ( A p p e n d i x V I I I ) . T h i s was h i g h when c o m p a r e d t o t h e d a i l y c o n s u m p t i o n o f o n l y a f r a c t i o n o f p r e y p e r l a r v a e s t i m a t e d f r o m t h e a n a l y s i s o f l a r v a l c r o p c o n t e n t s ( A p p e n d i x X ) . 9 7 TABLE X I I I C a l c u l a t e d d a i l y food consumption of C t r 4 l a r v a e i n 1971 and 1972. Method C a l o r i e s consumed (10~ 3 c a l / l a r v a / d a y ) 1971 1972 Feeding and plankton abundance data 470 223 Energy budget equation Crop a n a l y s i s data 165 148 207 98 3 D i s c u s s i o n P r e d a t i o n on E u n i c e L a k e z o o p l a n k t o n a p p e a r s t o be r e d u c e d b y f o u r " m e c h a n i s m s " : s e a s o n a l s e g r e g a t i o n (S) - p r e y a n d p r e d a t o r a r e i n t h e same p l a c e i n t h e w a t e r c o l u m n b u t a t d i f f e r e n t t i m e s ; v e r t i c a l s e g r e g a t i o n (V) - p r e y a n d p r e d a t o r a r e c o n c u r r e n t l y i n d i f f e r e n t p l a c e s i n t h e w a t e r c o l u m n ; l o w p r e y d e n s i t y (D) - p r e y a r e b e l o w t h e t h r e s h o l d c o n c e n t r a t i o n a t w h i c h s i g n i -f i c a n t f e e d i n g by l a r v a e c a n o c c u r ; a n d s i z e i n c o m p a t i -b i l i t y (H) - t h e p r e d a t o r i s u n a b l e t o h a n d l e p r e y . P r e d a t o r d e n s i t y was c o n s i d e r e d l o w i f i t was < 2/100 l i t e r s ; p r e y d e n s i t y was c o n s i d e r e d l o w i f t h e f e e d i n g r a t e p e r l a r v a a t t h a t d e n s i t y was < 0.1 p r e y p e r d a y . The ways i n w h i c h p l a n k t o n i c p r e y e s c a p e d p r e d a t i o n w e r e s i m i l a r f o r b o t h y e a r s a n d a r e d i s c u s s e d t o g e t h e r b u t o n l y t h e 1972 d a t a a r e i l l u s t r a t e d g r a p h i c a l l y ( F i g . 2 7 ) . N a u p l i i c o u l d h a v e b e e n e a t e n b y a l l l a r v a l i n s t a r s e x c e p t C . t r 4 V d u r i n g some p a r t o f t h e summer ( F i g . 27 a ) . C . t r 4^ w e r e p r e v e n t e d f r o m f e e d i n g o n n a u p l i i by t h e a b s e n c e o r l o w d e n s i t y o f p r e y . F e e d i n g b y o t h e r i n s t a r l a r v a e was r e d u c e d b y t h e i r s e a s o n a l a n d v e r t i c a l s e g r e -g a t i o n f r o m t h e s e p r e y a n d by l o w p r e y d e n s i t y . Of t h e z o o p l a n k t o n s t u d i e d , D . t y r e l l i w e r e consumed i n g r e a t e s t numbers m a i n l y b e c a u s e o f t h e i r a b u n d a n c e t h r o u g h o u t t h e s e a s o n . M a j o r p r e d a t o r s w e r e t h e 2nd a n d 3 r d i n s t a r l a r v a e o f b o t h C h a o b o r u s s p e c i e s a n d C a m 4, (a) NAUPLII (b) D. TYRELLI " M J J A S O N M J J A S O N i i i i i . i t 1 1 1 1 1 1— V V D S D S D S AM IV s -TJR IVY J_R IV° s" V D (c) D. KENAI (d) DIAPHANOSOMA M J J A S O N M J J A S O N , , , r-II S H + V 1 , 1 AM D ~v~" v b" i v *"s" TR ""b+v". I V Y S D+V D S V+D S V+D D TR IV 0 V+D S Fig.27 The mechanisms f o r predator avoidance from May to November of 1972 by four prey types. S-seasonal segregation, V - v e r t i c a l segregation, D-low prey density, H-size i n c o m p a t i b i l i t y . S o l i d l i n e s represent optimal periods for predation since no known prot e c t i v e mechanism i s operating; dashed l i n e s i n d i c a t e that the mechanism i s only p a r t i a l l y e f f e c t i v e and some predation can occur; dotted l i n e s represent l e a s t optimal periods f o r predation because of the above mechanisms operating. Periods free of l i n e s i n d i c a t e that no predation i s occurring because of absence or s c a r c i t y of predators. Roman numerals i n d i c a t e l a r v a l i n s t a r s . 100 a l l o f w h i c h o v e r l a p p e d w i t h D . t y r e l l i i n t h e r e g i o n o f h i g h p r e y d e n s i t y ( F i g . 27 b ) . I n s t a r 1 l a r v a e c o u l d n o t h a n d l e t h i s c o p e p o d . P r e d a t i o n by C . t r 4 Y and C . t r 4° was r e d u c e d b y v e r t i c a l s e g r e g a t i o n d u r i n g t h e d a y . I n t h e f a l l , D . t y r e l l i d e n s i t i e s w e r e t o o l o w f o r p r e d a t i o n t o o c c u r . U n l i k e t h e D . t y r e l l i , D . k e n a i w e r e n e v e r f u l l y e x p o s e d t o p r e d a t i o n f o r t h e e n t i r e 24 h o u r p e r i o d b u t a l w a y s h a d some s p a t i a l o r s i z e r e f u g e ( F i g . 27 c ) . F i r s t , 2nd a n d y o u n g 3 r d i n s t a r l a r v a e c o u l d n o t h a n d l e t h e s e l a r g e c o p e p o d s . F o u r t h i n s t a r l a r v a e w e r e s p a t i a l l y s e g r e g a t e d f r o m D . k e n a i f o r much o f t h e 24 h o u r p e r i o d due t o t h e i r o p p o s i t e m i g r a t i o n p a t t e r n s . H o w e v e r , i n t h e s p r i n g a n d e a r l y summer, m i g r a t i o n o f t h e C . t r 4° l a r v a e was s t i l l u n d e v e l o p e d a n d t h e i r m a r k e d s p a t i a l o v e r l a p w i t h t h e D . k e n a i r e s u l t e d i n c o n s i d e -r a b l e p r e d a t i o n . The b r i e f o c c u r r e n c e o f D i a p h a n o s o m a i n t h e l a k e , c o m b i n e d w i t h t h e i r p a r t i a l v e r t i c a l s e g r e g a t i o n f r o m t h e l a r v a e , made them a v a i l a b l e t o p r e d a t o r s f o r o n l y a few w eeks ( F i g . 27 d ) . I n 1 9 7 2 , D i a p h a n o s o m a d e n s i t i e s w e r e t o o l o w f o r a p p r e c i a b l e f e e d i n g t o o c c u r . P e r h a p s t h e m o s t i m p o r t a n t f a c t o r r e s p o n s i b l e f o r r e d u c i n g p r e d a t i o n p r e s s u r e o n a l l p r e y t y p e s was t h e r a p i d a n d s y n c h r o n o u s t u r n o v e r o f t h e d i f f e r e n t l a r v a l i n s t a r s . T h i s p r o b a b l y r e s u l t e d i n a c o n t i n u o u s s h i f t 1 0 1 o f e m p h a s i s f r o m one p r e y t y p e t o a n o t h e r a l l o w i n g a t l e a s t p a r t i a l r e c o v e r y o f p r e y p o p u l a t i o n s t o t a k e p l a c e . Any one o f t h e a b o v e f o u r " m e c h a n i s m s " c o u l d b e f u l l y e f f e c t i v e f o r d a m p e n i n g o r e v e n s t o p p i n g p r e d a t i o n . Where l o w p r e y d e n s i t y and v e r t i c a l s e g r e g a t i o n o p e r a t e d p a r t i a l l y b u t s i m u l t a n e o u s l y , p r e d a t i o n was a l s o u s u a l l y a b s e n t . The a b o v e m e c h a n i s m s i n v o l v i n g t h e s e a s o n a l a n d s p a t i a l d i s t r i b u t i o n o f p l a n k t o n i c p r e y r e l a t i v e t o t h e p r e d a t o r , a n d t h e h a n d l i n g a b i l i t y o f p r e y by t h e p r e d a t o r , may be v i e w e d a s p o s s i b l e ways e v o l v e d by p r e y t o a v o i d p r e d a t i o n . McQueen (1969) r e p o r t e d s i m i l a r m e c h a n i s m s o p e r a t i n g i n h i s z o o p l a n k t o n c o m m u n i t y w h e r e C y c l o p s was t h e p r e d a t o r . More e m p h a s i s s h o u l d be p l a c e d o n p r e y r e s p o n s e s t o p r e d a t o r s i n t e r m s o f p r e y l i f e h i s t o r i e s , s p a t i a l - t e m p o r a l d i s t r i b u t i o n , a n d p r e y b e h a v i o r . I t i s e v i d e n t f r o m t h e l i t e r a t u r e t h a t C h a o b o r u s , p a r t i c u l a r l y i f a b u n d a n t , may e x e r t c o n s i d e r a b l e p r e d a t i o n p r e s s u r e on a z o o p l a n k t o n c o m m u n i t y . T h i s p r e s s u r e i s w e a k e n e d i f t h e z o o p l a n k t o n h a v e a h i g h t u r n o v e r r a t e ( R o t h 1971; A l l a n 1 9 7 2 ) . W e l c h (1968) c o n c l u d e d f r o m h i s e n e r g e t i c s s t u d y t h a t " t h e c r u s t a c e a n - C h a o b o r u s f o o d c h a i n a c c o u n t e d f o r 37% o f a l l h e r b i v o r e g r o w t h " . S p r u l e s (1970) r e p o r t e d t h a t C h a o b o r u s , by. f e e d i n g s e l e c -t i v e l y o n t h e l a r g e r d i a p t o m i d s p e c i e s , may k e e p t h i s 1 0 2 f o r m r a r e o r e x t i n c t i n p o n d s t h a t t h e l a r v a e i n h a b i t . I n two l a k e s i n P o l a n d , 7% and 1 3 % o f t h e z o o p l a n k t o n p o p u l a t i o n was r e m o v e d d a i l y b y C h a o b o r u s ( K a j a k a n d R a n k e - R y b i c k a 1 9 7 0 ) . I n L e e c h m e e r e , d a i l y l a r v a l c r o p p i n g a c c o u n t e d f o r 10% o f t h e D a p h n i a p o p u l a t i o n and c o n t r i -b u t e d 9 3% t o t h e t o t a l d a p h n i i d m o r t a l i t y (Dodson 1 9 7 2 ) . B a s e d on h i s f i n d i n g , D odson c o n c l u d e d t h a t s m a l l i n v e r t e b r a t e p r e d a t o r s may remove a l a r g e p a r t o f t h e z o o p l a n k t o n p r o d u c t i o n . F u r t h e r e v i d e n c e f o r t h i s s t a t e m e n t was p r o v i d e d b y McQueen (1969) and b y C o n f e r (19 71) w o r k i n g o n a p r e d a t o r y s p e c i e s o f C y c l o p s and M e s o c y c l o p s r e s p e c t i v e l y . T h e s e a u t h o r s f o u n d t h a t C y c l o p s a n d M e s o c y c l o p s e x e r t e d a s i g n i f i c a n t d e p r e s s i v e f o r c e on t h e p r e y p o p u l a t i o n s . Of t h e f o u r p r e y t y p e s o f E u n i c e L a k e z o o p l a n k t o n i n v e s t i g a t e d f o r l o s s e s due t o p r e d a t i o n b y C h a o b o r u s , t h e n a u p l i i s u f f e r e d t h e l e a s t damage. C o n s i d e r a b l e m o r t a l i t y d u r i n g t h e two summer s e a s o n s was r e c o r d e d f o r b o t h D . t y r e l l i and D . k e n a i ( 1 0 % a n d 20% o f t h e i r s t a n d i n g c r o p s r e s p e c t i v e l y ) . S i n c e t h e s e c o p e p o d s p e c i e s p r o b a b l y h a v e no more t h a n two g e n e r a t i o n s a y e a r (A.Y. F e d o r e n k o , u n p u b l i s h e d d a t a ) , t h e l a r v a l e f f e c t o n t o t a l c o p e p o d p r o d u c t i o n was no d o u b t c o n s i d e r a b l y h i g h e r . D i a p h a n o s o m a w h i c h a p p e a r e d s p o r a d i c a l l y d u r i n g t h e two y e a r s o f s t u d y , seems t o h a v e b e e n c o n t r o l l e d n o t s o much by C h a o b o r u s p r e d a t i o n a s b y w a t e r t e m p e r a t u r e . F e e d i n g o n D i a p h a n o - soma was s e v e r e i n 1971 ( 1 9 % o f p o p u l a t i o n r e m o v e d d a i l y 103 i n J u l y a n d A u g u s t ) , b u t n e g l i g i b l e i n 1972 due t o l o w p r e y d e n s i t i e s . Y e t , i n b o t h y e a r s , t h i s c l a d o c e r a n h a s a p p e a r e d when t e m p e r a t u r e o f t h e s u r f a c e w a t e r e x c e e d e d 20° C a n d d i s a p p e a r e d r a p i d l y a s t h e l a k e c o o l e d . B o s m i n a , commonly e n c o u n t e r e d i n l a r v a l c r o p s f r o m 1 9 7 2 , w e r e p r o b a b l y p r e y e d upon h e a v i l y t h r o u g h o u t t h e summer s e a s o n . D a p h n l a may h a v e b e e n c r o p p e d c o n s i -d e r a b l y i n 1971 when t h e y w e r e a r e l a t i v e l y a b u n d a n t s p e c i e s a n d t h e o n l y p r e y t h a t s h a r e d t h e d e e p e r w a t e r w i t h C . t r 4° l a r v a e i n t h e d a y t i m e . I n 1 9 7 2 , t h e l o w p r o p o r t i o n o f l a r v a e w i t h D a p h n i a o r P o l y p h e m u s i n t h e i r c r o p s may i n d i c a t e p o o r f e e d i n g o n t h e s e p r e y p r o b a b l y b e c a u s e o f t h e i r v e r y l o w numbers i n t h a t y e a r . The s i m i l a r i t y o f t h e t h r e e i n d e p e n d e n t e s t i m a t e s o f d a i l y f o o d c o n s u m p t i o n b y C . t r 4° l a r v a e was a g o o d i n d i c a t i o n t h a t t h e d a t a u s e d , a n d t h e c a l c u l a t i o n s w e r e v a l i d . The r e l a t i v e l y h i g h v a l u e o f 470 x 10~3 c a l o r i e s p e r l a r v a p e r d a y o b t a i n e d f r o m t h e 1971 p r e d a t i o n d a t a , may be a n o v e r e s t i m a t e . H o w e v e r , t h e h i g h c a l o r i c i n t a k e i n 1971 r e l a t i v e t o 1 9 7 2 , f o u n d i n two o f t h e e s t i m a t e s , seems t o be r e a l a n d may b e a t t r i b u t e d t o t h e much h i g h e r s t a n d i n g c r o p o f z o o p l a n k t o n p r e y i n 1 9 7 1 t h a n i n 1972. T h i s s u g g e s t i o n i s s u p p o r t e d b y g r o w t h d a t a f o r t h e two y e a r s . £•'*•£ 4 ° i - n 1972 g a i n e d o n l y 7/10 o f t h e w e i g h t g a i n o f C . t r 4° i n 1 9 7 1 . The e f f e c t o f p r e y a b u n d a n c e o n l a r v a l g r o w t h h a s a l s o b e e n demon-s t r a t e d i n t h e t u b e e x p e r i m e n t s w h e r e a n e x c e s s o f f o o d 104 r e s u l t e d i n f a s t e r w e i g h t g a i n . C . t r 4° l a r v a e , b e c a u s e o f t h e i r 16 month l o n g d u r a t i o n t i m e , h i g h c o n s u m p t i o n r a t e s , a n d r e l a t i v e l y h i g h a b u n d a n c e , w e r e p r o b a b l y t h e m a j o r p l a n k t o n p r e d a t o r s i n E u n i c e L a k e . G i v e n t h e mean d a i l y c o n s u m p t i o n , d e t e r m i n e d h e r e a s 185 x 10" c a l o r i e s p e r l a r v a , a n d t h e mean l a r v a l d r y w e i g h t o f 1.2 mg p e r i n d i v i d u a l ( F i g . 1 2 ) , a C . t r 4° l a r v a consumed d a i l y 2.8% o f i t s b o d y w e i g h t . K a j a k a n d R a n k e - R y b i c k a (1970) r e p o r t e d v a l u e s o f 3.6% and 12.5% f o r t h e C . f l a v i c a n s l a r v a e i n two P o l i s h l a k e s . S e v e r a l s o u r c e s o f e r r o r w e r e i n h e r e n t i n t h e c a l -c u l a t e d c o n s u m p t i o n r a t e s f o r t h e f i r s t e s t i m a t e u s i n g t h e f i e l d d a t a : v a r i a t i o n a b o u t t h e p o i n t s i n t h e f e e d i n g r a t e c u r v e s , v a r i a t i o n i n t h e p r e y d e n s i t y e s t i m a t e s i n t h e l a k e due t o l o w number o f s a m p l i n g o c c a s i o n s , n e g l e c t o f p a r t o f t h e p r e y s p e c t r u m due t o l a c k o f d a t a , a n d l a c k o f p l a s t i c i t y i n t h e c a l c u l a t i o n s t o e x p r e s s t h e d y n a m i c n a t u r e o f d a i l y p r e d a t o r - p r e y s p a t i a l i n t e r -a c t i o n s . T h i s l a s t p o i n t i s o f m o s t c o n c e r n b e c a u s e t h e a v e r a g e p r e y d e n s i t y e n c o u n t e r e d d a i l y b y a l a r v a was b a s e d o n two s a m p l i n g t i m e s o n l y - noon a n d m i d n i g h t - d u r i n g e a c h 14 - d a y i n t e r v a l , a n d i n a l l c a s e s , maximum p r e y d e n s i t i e s w e r e u s e d i n t h e r e g i o n o f o v e r -l a p b e t w e e n p r e d a t o r a n d p r e y . T h i s t r e a t m e n t o f d a t a may w e l l h a v e r e s u l t e d i n a n o v e r e s t i m a t i o n o f t h e f i n a l c o n s u m p t i o n v a l u e s . 105 The e n e r g y b u d g e t e q u a t i o n no d o u b t a l s o c o n t a i n e d i n a c c u r a c i e s i n t h e e s t i m a t e s o f i t s p a r a m e t e r s . The a p p l i c a t i o n o f a r e s p i r a t i o n v a l u e m e a s u r e d a t a c o n s t a n t t e m p e r a t u r e o f 10° C t o a v e r t i c a l l y m i g r a t i n g l a r v a , may h a v e p r o d u c e d a n u n d e r e s t i m a t e o f m e t a b o l i c c o s t . The c o n s u m p t i o n e s t i m a t e d e r i v e d f r o m t h e c r o p a n a l y s i s d a t a may h a v e b e e n t o o l o w i f t h e mean d i g e s t i o n t i m e o f p r e y , a ssumed h e r e t o b e 8 h o u r s , was a c t u a l l y s h o r t e r , o r i f t h e p r o p o r t i o n o f l a r v a e w i t h f u l l c r o p s was h i g h e r t h a n t h e o b s e r v e d 30%. M o n t s h a d s k y (1945) r e p o r t e d a d i g e s t i o n t i m e o f 3 - 5 h o u r s f o r one l a r g e D a p h n i a . H o w e v e r , he d i d n o t i n d i c a t e t h e t e m p e r a t u r e o f t h e w a t e r . G o l d s p i n k a n d S c o t t (1971) r e p o r t e d t h a t l a r v a l c r o p c o n t e n t s p a s s e d t o t h e m i d g u t w i t h i n 4 - 5 h o u r s a f t e r p r e y i n g e s t i o n . I n b o t h c a s e s , t h e d i g e s t i o n t i m e was s h o r t e r t h a n t h e one u s e d h e r e . H owever, t h e 8 h o u r v a l u e i s more r e a l i s t i c b e c a u s e i t was d e r i v e d f r o m s e v e r a l i n d e p e n d e n t e s t i m a t e s o f d i g e s t i o n t i m e s u s i n g d i f f e r e n t p r e y t y p e s . I f a n y t h i n g , t h i s v a l u e i s p r o b a b l y t o o l o w f o r C . t r 4° b e c a u s e i t was o b t a i n e d a t c o n s i d e r a b l y h i g h e r t e m p e r a t u r e s (17° - 20° C) t h a n t h o s e t h a t C t r 4° n o r m a l l y e x p e r i e n c e . I m p r o v e d m e t h o d o l o g y l a r g e l y r e m o v e d t h e p r o b l e m o f r e g u r g i t a t i o n a n d t h e v a l u e o f 30% o f l a r v a e w i t h f u l l c r o p s a g r e e s w e l l w i t h t h a t g i v e n b y G o l d s p i n k and S c o t t (1971) f o r C f l a v i c a n s . I f a l l t h e a b o v e d i s c r e p a n c i e s 106 t e n d e d t o u n d e r e s t i m a t e t h e f i n a l c o n s u m p t i o n v a l u e , t h i s may h a v e b e e n o f f s e t t o some e x t e n t b y t h e u s e o f D . k e n a i a s p r e y t h u s m a x i m i z i n g t h e b i o m a s s f r o m t h e o n l y p r e y consumed d a i l y . I n g e n e r a l , s i n c e t h e t h r e e p r e d a t i o n e s t i m a t e s a g r e e d f a i r l y w e l l among t h e m s e l v e s a n d w i t h w h a t l i t t l e t h e r e was a v a i l a b l e f r o m t h e l i t e r a t u r e , t h e p r e -d a t i o n r a t e s r e p o r t e d h e r e a r e p r o b a b l y c l o s e t o t h e a c t u a l v a l u e s . T h e r e f o r e , a s h a s b e e n n o t e d p r e v i o u s l y , C h a o b o r u s i n E u n i c e L a k e do e x e r t c o n s i d e r a b l e p r e d a t i o n p r e s s u r e o n t h e z o o p l a n k t o n s t a n d i n g c r o p . T h i s e f f e c t o n any s i n g l e p r e y g r o u p seems t o be t r a n s i t o r y a n d t o v a r y w i t h s e a s o n a n d w i t h i n s t a r c o m p o s i t i o n o f p r e d a t o r s . .107 4 Summary 1 P r e d a t i o n b y C h a o b o r u s o n E u n i c e L a k e z o o p l a n k t o n seems t o be r e d u c e d b y s e a s o n a l a n d v e r t i c a l s e g r e g a t i o n , l o w p r e y d e n s i t y , a n d s i z e i n c o m p a t i -b i l i t y w i t h t h e p r e d a t o r . 2 C h a o b o r u s r e m o v e d 1 0% and 20% o f s t a n d i n g c r o p o f D . t y r e l l i and" D . k e n a i r e s p e c t i v e l y i n 1971 and 1972. P r e d a t i o n p r e s s u r e on c l a d o c e r a n s seemed t o i n c r e a s e w i t h t h e i r a b u n d a n c e . 3 £«^£ ^° l a r v a e consumed d u r i n g t h e summer s e a s o n _ •) a b o u t 185 x 10 c a l o r i e s p e r l a r v a p e r d a y o r 2.8% o f t h e i r b o d y w e i g h t . 4 C l o s e a g r e e m e n t was f o u n d b e t w e e n t h e e n e r g e t i c s e s t i m a t e o f d a i l y f o o d c o n s u m p t i o n by l a r v a e a n d t h e e s t i m a t e f r o m t h e f i e l d d a t a . 108 BIBLIOGRAPHY A l l a n , D.J. MS 1972. C o m p e t i t i o n a n d t h e r e l a t i v e a b u n d a n c e o f two c l a d o c e r a n s , Ph.D. t h e s i s . U n i v . o f M i c h i g a n . 110 p. A n d e r s o n , S.R. 1970. P r e d a t o r - p r e y r e l a t i o n s h i p s and p r e d a t i o n r a t e s f o r c r u s t a c e a n z o o p l a n k t e r s f r o m some l a k e s i n w e s t e r n C a n a d a . Can. J . Z o o l . 48: 1 2 2 9 - 1 2 4 0 . B a u d o u i n , M.F. a n d 0. R a v e r a . 1972. W e i g h t , s i z e , a n d c h e m i c a l c o m p o s i t i o n o f some f r e s h w a t e r z o o p l a n k -t e r s : D a p h n i a h y a l i n a ( L e y d i g ) . L i m n o l . O c e a n o g r . 17: 645-6"49. B e r g , K. 1937. C o n t r i b u t i o n s t o t h e b i o l o g y o f C o r e t h r a M e i g e n ( C h a o b o r u s L i c h t e n s t e i n ) . B i o l . Mectd. 13: 1-101. B r a d s h a w , W.E. 1970. I n t e r a c t i o n o f f o o d a n d p h o t o -p e r i o d i n t h e t e r m i n a t i o n o f l a r v a l d i a p a u s e i n C h a o b o r u s a m e r l e a n u s ( D i p t e r a : C u l i c i d a e ) . B i o l . B u l l . 139: 4T6-484. B u t o r i n a , L.G. 1971. The i n t e n s i t y o f f e e d i n g o f P o l y p h e m u s p e d i c u i u s ( L . ) i n r e l a t i o n t o f o o d c o n c e n t r a t i o n . A k a d . Nauk SSSR. I n f . B y u l . B i o l . V n u t r . Vod 10: 35-39 ( I n R u s s i a n ) . 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V e r t i c a l d i s t r i b u t i o n a nd m i g r a t i o n o f C h a o b o r u s f l a v i c a n s i n C o r b e t t L a k e , B r i t i s h C o l u m b i a . E T m n o l . O c e a n o g r . 1 1 : 1 6 4 - 1 7 6 . W e l c h , H.E. MS 1968. E n e r g y f l o w t h r o u g h t h e m a j o r m a c r o s c o p i c c o m p o n e n t s o f a n a q u a t i c e c o s y s t e m . P h . D. t h e s i s . U n i v . o f G e o r g i a . 97p. 113 APPENDICES APPENDIX I . PREDATION ON NAUPLII, 1971. (Dates on which p r e d a t i o n c o u l d not o c c u r a r e o m i t t e d . ) DATE T^C POTENTIAL PREY STANDING CHAOBORUS PREY DENSITY CONSUMPTION PREY REMOVED TOTAL PREY i PREY STANDING PREDATORS CROP (No/m2) STANDING CROP (No/-*) /LARVA /DAY PER PREDATOR REMOVED CROP REMOVED (No/m2) TYPE (No/m2) (No/m2) P r. C h . J M. . I J M. I i % P r . 20 May 5-13 am 4 14,109 67 5.5 0.8 54 205 • 1.5 t r 4° 189 5.5 0.8 151 5 J u l y 9-14 1 5,170 408 3.0 0.5 204 230 4.4 t r 4° 51 2.7 0.5 26 20 J u l y 11-25 2 11,959 434 9.0 1.2 521 536 4.5 t r 4° 148 1.0 0. 1 15 3 Aug. . 9-23 2 14,742 64 9.0 1.2 77 1,046 7.1 3 71 1 9.0 \ .2 853 t r 4° 193 4.0 0.6 1 16 17 Aug. 19-21 3 2,642 862 1.0 0. 1 86 98 3.7 am 4 116 1.0 0.1 12 mean M. = 302 3.0? i (20 May - 17 Aug.) APPENDIX I I . PREDATION ON NAUPLII, 1972.. (Dates on which p r e d a t i o n c o u l d not o c c u r a r e o m i t t e d J DATE T°C POTENTIAL PREDATORS PREY STANDING CROP (No/m2) P r , CHAOBORUS STANDING CROP (NoV) Ch. J PREY DENSITY Wo/l) CONSUMPTION /LARVA /DAY f . . PREY REMOVED PER PREDATOR TYPE (No/m2> M. i j TOTAL PREY t PREY STANDING REMOVED CROP REMOVED M; (No/m2) * P r l 3 May 4 am 4 12,008 1 10 1.4 0.2 22 90 0.7 t r <P 215 1.4 0.2 43 t r 4° 126 1.4 0.2 '25 17 May 5-13 am 4 5,606 66 1.2 0.2 13 • 120 2.1 t r 4y 51 1 1.3 0.2 102 t r 4° 26 1.3 0.2 5 14 June 5-19 1 3,677 521 1. 1 0.2 104 206 5.6 am 4 40 I.I 0.2 8 t r > 470 I.I 0.2 94 28 June 7-13 t r 4° 6,197 258 0.9 . 0.1 26 26 0.4 17 J u l y 9-19 2 9,284 395 6.3 0.8 316 538 5.8 3 167 4.0 0.6 100 t r 4° 306 2.8 0.4 122 26 J u l y 16-19 3 5,660 343 2.1 0.4 124 124 2.2 8 August 13-20 3 3,394 352 I.I 0.2 70 70 2.1 22 August 13-19 3 3,025 334 . I.I 0.2 67 74 2.4 am 4 72 1.0 0. 1 7 mean M. = 139 (3 May - 22 August) APPENDIX I I I . PREDATION ON D. TYRELLI. 1971. (Dates on which p r e d a t i o n c o u l d not o c c u r a r e omitted.) DATE T°C POTENTIAL PREDATORS PREY STANDING CROP (No/m2) P r . i CHAOBORUS STANDING CROP (No/m2) C h . J PREY DENSITY (No/V) CONSUMPTION /LARVA /DAY f . . 1J PREY REMOVED PER PREDATOR TYPE (No/m2) M. . I J TOTAL PREY , REMOVED M. I % PREY STANDING CROP REMOVED % P r . 20 May 7-11 am 4 8,433 67 2.3 0.2 13 126 1 .5 t r 4° 189 2.3 . 0.6 113 1 J une 7-15 am 4 20,861 1 10 9.8 5.0 550 2,030 9.7 ^ 4 ° 296 9.8 5.0 1,480 14 June 5-11 am 4 29,218 65 11 .3 3.0 195 613 2.1 t r > 209 7.0 2.0 418 5 J u l y 5-14 - t r 4° 13,670 51 4.3 1 .7 87 87 0.6 20 J u l y 5-25 2 18,177 434 7.5 2.3 998 t r 4° 148 5.0 1 .8 266 1,264 7.0 3 Aug. 5-23 2. 14,293 64 5.1 2.2 141 4,600 32.2 3 71 1 5.1 6.0 4,266 t r 4° 193 3.0 l i O 193 17 Aug. 5-21 3 18,359 662 7.0 . 6.5 5,603 6,289 34.3 am 4 1 16 7.0 . 4.0 464 t r 4° 222 3.5 i :o 222 3 S e p t . 12-17 3 10,371 543 2.0 1.5 815 1,167 1 1 .3 am 4 168 1.5 1.0 168 t r 4* 221 1.5 0.5 1 1 1 t r 4 ° 146 1.5 0.5 73 16 S e p t . 3 9,095 383 2.3 1 .5 575 ' 1,001 ' 1 1 .0 am 4 128 3.3 1.5 192 t r 4Y . 557 1.3 0.3 167 t r 4° 223 1.3 0.3 67 3 O c t . 1 1 3 5,538 176 3.4 3.5 616 1,494 27.0 am 4 89 6.0 1.8 160 t r 4Y 562 3.0 1.0 562 t r 4° 156 3.0 1.0 156 Mean M. = 1,867 I3.7S& i (20 May - 3 O c t . ) APPENDIX IV. PREDATION ON D. TYRELLI. 1972. (Dates on which p r e d a t i o n c o u l d not o c c u r a r e o m i t t e d ) DATE T°C POTENTIAL PREY STANDING CHAOBORUS PREY DENSITY CONSUMPTION PREY REMOVED TOTAL PREY % PREY STANDING PREDATORS CROP (No/m2) STANDING CROP <No/£) /LARVA /DAY PER PREDATOR REMOVED CROP REMOVED (No/m ) TYPE (No/m2) (No/m2) P ri C h . J f . . 1J M. . 1 J K. 1 % P r . 17 May ' 7-12 am 4 8,945 66 2,4 1.0 66 • 603 6.7 t r 4Y 511 2.4 1.0 511 t r 4° 26 2.4 1 .0 26 31 May .7-14 am 4 12,603 80 5.0 2.5 200 904 7.2 t r 4° 469 3.6 1 .5 704 14 June 7-12 am 4 10,680 40 3.1 1.5 ' 60 530 5.0 t r 4° 470 2.6 1 .0 470 28 June 7-12 2 3,769 565 0.9 0. 1 57 . 186 4.9 t r 4° 258 1.0 0.5 129 17 J u l y 17-19 2 5,909 395 2.2 0.4 158 56? 9.5 3 167 2. 1 1.5 251 t r 4° 306 1.0 0.5 153 26 J u l y 15-19 3 1 6,517 343 2.2 1.5 515 568 8.7 tr .4° 105 1 .2 0.5 53 8 Aug. 9-20 3 6,951 352 1 .7 1.0 358 358 5.2 22 Aug. 11-21 3 5,793 334 1.3 0.8 267 483 8.3 am 4 72 1.2 1.0 . 72 t r 4° 288 1 .0 0.5 144 6 S e p t . 11-16 3 18,063 170 3.1 2.5 425 ' 1,458 ' 8. 1 am 4 91 4.0 ' 2.5 228 t r 4* 183 3.1 1.5 275 • - t r 4° 353 3.1 1.5 530 24 S e p t . 1 1-13 am 4 10,759 75 2.2 1 .0 75 372 3.5 t r 4V 203 , 1.7 0.7 142 . t r 4° 309 1.1 0.5 155 7 O c t . 1 1 am 4 9,699 1 13 3.0 1.0 113 501 5.2 t r 4Y 262 1.6 0.8 210 t r 4° . 223 1 .6 0.8 178 21 O c t . 8 am 4 6,761 82 1.3 0.2 16 16 0.2 mean Mj = 545 6.0* (17 May - 21 Oct. ) \~> APPENDIX V. PREDATION ON D. KENAI, 1971. (Dates on which p r e d a t i o n c o u l d not o c c u r a r e omitted.) DATE POTENTIAL PREDATORS PREY STANDING CROP (No/m2) P r , CHAOBORUS STANDING CROP (No/m2) Ch. J PREY DENSITY (no/*) CONSUMPTION /LARVA '/DAY PREY REMOVED PER PREDATOR TYPE (No/m2) M. . i J TOTAL PREY REMOVED (No/m2) M. PREY STANDING CROP REMOVED $ P r , 20 May 9-11 am 4 3,732 67 • 2.0 2.3 154 608 16.3 t r 4 ° - 189 1 .0 2.4 454 1 June 7-15 am 4 10,816 110 3.5 3.0 330 1,070 9.9 t r 4° 296 3.6 2.5 740 14 J u n e , 5-11 am 4 12,689 65 3.6 3.0 195 718 5.7 t r 4° .209 3.7 2.5 '523 5 J u l y . 5-15 . t r 4° 11,399 51 1.3 2.5 128 • 128 1 . 1 20 J u l y 5-25 t r 4° 8,424 148 1 • 1 2.5 370 370 4.4 3 Aug. 5-24 3 7,723 613 I.I 1 .7 1,042 1,621 21.0 t r 4° 193 1 .4 3.0 579 17 Aug. 5-21 3 10,106 799 2.2 2.2 1,758 2,558 25.3 am 4 116 2.1 2.3 267 IT 4° 222 I.I 2.4 533 3 S e p t . 5-17 3 5,865 543 0.7 1 .2 652 1,049 17.9 am 4 168 1.1 - 1.7 286 t r 4y 221 0.6 0.5 , 1 1 1 16 S e p t . 5-15 3 4,550 383 0.5 1 .0 383 947 20.8 am 4 128 1.1' 1 .7 218 t r 4Y 557 0.7 0.5 279 t r 4° 223 0.6 0.3 67 3 O c t . 1 1 3 1,824 176 0.3 0.9 158 425. 23.0 am 4 89 0.6 I.I 98 t r 4Y 562 0.3 0.3 I6S 17 O c t . 9 am 4 1,162 67 0.2 0.2 13 53 4.6 t r 4Y 398 0.1 0.1 40 mean M. = 868 13.6?. (20 May - 17 Oct.) APPENDIX V I . PREDATION ON D. KENAI, 1972. (Dates on which p r e d a t i o n d i d not o c c u r a r e o m l t t e d J DATE T°C POTENTIAL PREY STANDING CHAOBORUS PREY DENSITY CONSUMPTION PREY REMOVED TOTAL PREY % PREY STANDING PREDATORS CROP (No/m?) • STANDING CROP (N o # ) /LARVA /DAY PER PREDATOR REMOVED CROP REMOVED (No/m2) TYPE (No/m2) (No/m2) P r . i Ch . J f . . 'J M. . IJ M. I % P r . 3 May 5 t r 4* 323 215 0.1 0.5 108 171 53.0 t r 4° 126 0.1 0.5 63 17 May 6-9 am 4 831 66 • 0.2 0.3 20 557 67.0 t r 4Y 511 0.2 1.0 51 1 t i 4° 26 0.2 1.0 26 31 May 7-14 am 4 1,061 80 0.2 0.3 24 493 46.5 t r 4° • 469 0.2 1.0 469 -14 June 5-13 am 4 2,285 40 0.4 0.6 24 870 38.1 t r 4° 470 0.45 1.8 846 28 June 5-13 2,935 258 0.25 1.4 361 361 12.0 17 J u l y 5-23 t r 3 2,996 167 0.4 1.0 167 626 20.9 t r 4° 306 0.25 1.5 459 26 J u l y 5-23 t r 3 l f 193 139 0.2 0.5 70 123 10.0 t r 4° 105 0.1 0.5 53 8 Aug. 5-23 t r 3 1,365 260 0. 15 0.5 130 322 23.6 IX 4° 192 0.2 1.0 192 22 Aug. 5-19 3 1,371 334 0. 17 0.5 167 325 23.7 am 4 72 0.15 0.2 14 t r 4° 288 0.08 0.5 144 6 S e p t . 5-18 3 1,103 170 0.1 0.2 34 123 1 1 .2 am 4 91 0. 17 0.2 18 " t r 4° 353 0.08 0.2 71 24 S e p t . 5-10 am 4 761 75 0.13 0.2 15 15 2.0 7 O c t . 5-10 am 4 735 1 13 0. 14 0.2 23 75 10.0 t r 4y 262 0.1 0.2 52 21 O c t . 5-9 am 4 345 82 0.07 0.1 8 20 5.8 117 0.03 0.1 12 mean M. = 314 24.9? (3 May - 21 O c t . ) APPENDIX V I I . PREDATION ON DIAPHANOSOMA. 1971. (Dates on whIch p r e d a t i o n d i d not o c c u r a r e o m i t t e d . ) DATE T°C POTENTIAL PREDATORS PREY STANDING CHAOBORUS CROP (No/m2) STANDING CROP (No/m2) PREY DENSITY (No/4) CONSUMPTION /LARVA /DAY PREY REMOVED PER PREDATOR TYPE (No/m2) TOTAL PREY REMOVED (No/m2) % PREY STANDING CROP REMOVED P r , C h . 1 J fU M. . I J M. I S P r . 20 J u l y 17 2 7,194 435 5.0 3.0 1,305 1,527 21.2 tr 4° 148 2.5 1.5 222 3 August 20 2 33,503 64 9.0 5.0 320 4,454 13.3 3 711 9.0 5.0. <: 3,555 t r 4° 193 5.5 3.0 579 17 August 14 3 17,927 862 6.0 3.0 2,586 3,267 18.2 am 4 1 16 6.0 3.0 348 - t r 4° 222 3.0 1.5 333 3 September 16 3 543 1.0 1.0 543 931 22.1 am 4 168 2.0 1.0 168 t r 4V 220 2.0 1.0 220 mean M = 2,545 18.7? (20 J u l y - 3 Sept.) I—1 O APPENDIX V I I I . C a l c u l a t i o n of t h e mean d a i l y c o n s u m p t i o n , ( C ) , o f copepod prey by C. t r _ . 4° i n 1971 and 1972, u s i n g f e e d i n g r a t e s and prey abundance d a t a . 1971 D. C /day t y r e l I I ( k e n a i ) I V i 120 days 1972 D. kenai D. kena i Date t . i f . i V l f . I V i Date t . i f . I f . t . i i f . I f . t . i i 1 June 8 5.0 40. 2.5 20. 31 May 8 1.5 12 1.0 8. 14 " 16 2.0 32. 2.5 40. 14 June 14 • 1.0 14 1 .8 25.2 5 J u l y 18 1.7 30.6 2.5 4 5 . 28 " 15 0.5 7.5 1 .4 21 . 20 " 15 1 -8 27. 2.5 37.5 17 J u l y 14 0.5 7. 1.5 2 1 . 3 Aug. 13 1 .0 13. 3.0 39. 26 " 12 0.5 6. 0.5 6. 17 " 14 1.0 14. 2.4 33.6 8 Aug. 12 0 0 1 .0 12. 3 S e p t . 16 0.5 8. 0 0 22 " 13 0.5 6.5 0.5 7.5 16 " 20 0.3 6. 0.3 6. 6 S e p t . 16 1.5 24. 0.2 3.2 \ 24 " 15 0.5 7.5 0 0 C /day = 170.I  t y r e l I i 120 r> tA 221 • I C /day = - f ^ g -kena i C /day t y r e l I i 84.5 120 C /day kena i = 1.42 D . t y r e l I i / l a r v a / d a y = 1.84 D . k e n a j / l a r v a / d a y Copepod biomass consumed/Iarva/day = (1.42 x 5.9ug)+(l.84 x 42.7ug) t y r e l I i kenai = 87.1 u g / l a r v a / d a y C a l o r i c i n t a k e / l a r v a / d a y = 8 7 . l u g x 5.4 x I O- 3 c a l = 470 x I 0 ~3 c a l / l a r v a / d a y = 0 . 7 0 D . t y r e l I i / l a r v a / d a y Copepod biomass consumed/Iarva/day = (0.70 x 5.9ug)+(0.87 x 42.7ug) t y r e l I i kenai = 4 1 . 3 u g / l a r v a / d a y C a l o r i c i n t a k e / l a r v a / d a y = 41.3 yg x 5.4 x I O- 3 c a l = 223 x I 0 ~3 c a l / l a r v a / d a y 103.9 120 0.87 D . k e n a i / l a r v a / d a y 1\J APPENDIX IX. C a l c u l a t i o n o f t h e mean d a i l y consumption by C . t r . 4 i n 1971 and 1972 u s i n g t h e energy budget e q u a t i o n . A = 68? R = 0.6 p i 0 2/mg/hr 0°C , = (0.61(4.83 x I0"-5 c a l / u g 02> x 24 h r . = 69.5 x I P "3 c a l / l a r v a / d a y AP(I June - 30 Sept.) = 1.80 mg - 0.86 mg 1 9 7 1 = 0.94 mg 7.8 u g / l a r v a / d a y P/day 940)jg 1971 " 120 days = ( 7 . 8 X 5 . 4 7 x I 0 ~J c a l / u g ) = 42.7 x I P "3 c a l / l a r v a / d a y AP <I June - 30 Sept.) = 1.15 mg - 0.47 mg 1 9 7 2 = 0.68 mg 1971 A x C = P t R = (42.7 + 69.5) X I 0 " 3 c a l / l a r v a / d a y .'. C = 165 x I P "3 c a l / l a r v a / d a y 1972 A x C = P t R = (31.2 + 69.5) x I 0 "3 c a l / l a r v a / d a y .'. C = 148 x I 0 ~3 c a l / l a r v a / d a y %7? - f f j t v s " 5 ' 7 «/'.n « / d . y = ( 5 . 7 X 5 . 4 7 x 10 J c a l / u g ) = 31.2 x I 0 ~3 c a l / l a r v a / d a y APPENDIX X. Calculation of the mean dally consumption by C.tr 4° In 1972 using the crop analysis data. 90? of larval population feeds dally Assume that a £.tr_ 4° larva eats 0.9 p_. kenai/day Caloric Intake/C.tr 4°/day = (0.9 D. kenai)(42.7ug/D. kenal)(5.4 x I0~3 cal/ug dry wt. copepod) = 207' x IP"3 cal/larva/day 

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