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The relative importance of food availability and predation to the juvenile survival of Diacyclops thomasi Bowerman, Joy E. 1983

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c . » THE RELATIVE IMPORTANCE OF FOOD AVAILABILITY AND PREDATION TO THE JUVENILE SURVIVAL OF DIACYCLOPS THOMASI by JOY E.BOWERMAN BSc(Hon) C a r l e t o n U n i v e r s i t y 1980 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA September 1983 0 Joy E.Bowerman, 1983 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of uoc y  The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6 (3/81) ABSTRACT In s i t u e n c l o s u r e experiments were conducted i n P l a c i d Lake, B r i t i s h Columbia to determine the e f f e c t of food a v a i l a b i l i t y and p r e d a t i o n on the n a u p l i a r s u r v i v a l of D. thomasi . These experiments r e v e a l e d that food l i m i t a t i o n due t o c o m p e t i t i o n from the g r a z i n g assemblage, i n t e r s p e c i f i c p r e d a t i o n by D. kenai and i n t r a s p e c i f i c p r e d a t i o n by D. thomasi a d u l t s c o u l d a l l s u b s t a n t i a l l y a f f e c t the s u r v i v a l of D. thomasi n a u p l i i . Subsequent f e e d i n g s t u d i e s showed that D. kenai at lake d e n s i t i e s were capable of p r e y i n g on n a u p l i i at a r a t e of 2 0 % of n a u p l i i per predator per day and t h i s seemed to account f o r most of the m o r t a l i t y of D. thomasi n a u p l i i i n e n c l o s u r e s with D. kenai . When these r e s u l t s were e x t r a p o l a t e d to two o l i g o t r o p h i c montane l a k e s , c a n n i b a l i s m was judged to be the major m o r t a l i t y agent. In the P l a c i d Lake community, the e f f e c t of i n t e r s p e c i f i c p r e d a t i o n and com p e t i t i o n acted only e a r l i e r and l a t e r i n the season r e s p e c t i v e l y , and the magnitude of t h e i r e f f e c t was i n f l u e n c e d by year t o year v a r i a t i o n i n weather. In Eunice Lake, i n which D. thomasi has r e c e n t l y become a community dominant, experimental r e s u l t s suggested that D. kenai had p r e v i o u s l y l i m i t e d D. thomasi i n the l a k e . The i n t r o d u c t i o n of c u t t h r o a t t r o u t and a m i l d winter seemed to be r e s p o n s i b l e f o r the d e c l i n e i n D. kenai and concurrent i n c r e a s e i n D. thomasi . i i i TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v LIST OF FIGURES v i ACKNOWLEDGEMENTS v i i GENERAL INTRODUCTION 1 CHAPTER 1 8 M a t e r i a l s and methods 8 Study area 8 P l a c i d Lake Community 9 Experimental Methods 14 Experimental design 19 A n a l y s i s 20 R e s u l t s 23 P r e c i s i o n of sample data 23 Set up of e n c l o s u r e s 26 Re s u l t s of s u r v i v o r s h i p c a l c u l a t i o n s 28 D i s c u s s i o n 49 CHAPTER 2 55 M a t e r i a l s and methods 55 Experimental methods 55 Experimental design 56 Re s u l t s 62 D i s c u s s i o n 67 GENERAL DISCUSSION 71 i v REFERENCES 7 6 APPENDIX A. Set up of E n c l o s u r e s 84 Zooplankton 84 D i a c y c l o p s thomasi 84 Diaptomus kenai 85 Graz i n g assemblage 86 Diaptomus oregonensis 86 Daphnia rosea 87 Holopedium gibberum 88 N a u p l i i 88 Phytoplankton 89 F e r t i l i z e r 89 Graz i n g assemblage 91 APPENDIX B. E v a l u a t i o n o-f Methods f o r C a l c u l a t i o n of S u r v i v o r s h i p 97 Dura t i o n of n a u p l i a r stages 97 A n a l y s i s of n a u p l i a r s u r v i v a l APPENDIX C. Temperature and P r e c i p i t a t i o n d u r i n g the experimental period(May-June, 1982) 116 APPENDIX D. R e s u l t s of s u r v i v o r s h i p c a l c u l a t i o n s 119 V LIST OF TABLES Table I Some p h y s i c a l , chemical and b i o l o g i c a l c h a r a c t e r i s t i c s of P l a c i d and Eunice Lakes i n the UBC Research For e s t 12 Table II Stocking d e n s i t i e s of zooplankton i n the e n c l o s u r e s 16 Table I I I P r e c i s i o n of zooplankton data 24 Table IV P r e c i s i o n of phytoplankton data 25 Table V Dynamics of zooplankton i n e n c l o s u r e s 27 Table VI The e f f e c t of s u r f a c e s on D. kenai p r e d a t i o n on D i a c y c l o p s n a u p l i i 66 Table VII Predation r a t e s of c a r n i v o r o u s copepods on n a u p l i i 69 Table VIII E f f e c t of f e r t i l i z e r on c h l o r o p h y l l content .... 92 Table IX E f f e c t of f e r t i l i z e r on phytoplankton s i z e c l a s s e s 93 Table X E f f e c t of f e r t i l i z e r and g r azers on t o t a l c h l o r o p h y l l content 95 Table XI E f f e c t of f e r t i l i z e r and grazers on phytoplankton s i z e c l a s s e s 96 Table XII R e s u l t s f o r e s t i m a t i o n of stage d u r a t i o n of immature D i a c y c l o p s thomasi 100 LIST OF FIGURES F i g u r e V S p r i n g zooplankton community composition i n Eunice Lake from 1975-1982 4 F i g u r e 2 L o c a t i o n of study area 10 F i g u r e 3 Experimental design 21 F i g u r e 4 E f f e c t of the g r a z i n g assemblage, f e r t i l i z a t i o n , Diaptomus kenai and D i a c y c l o p s thomasi alone on n a u p l i a r s u r v i v a l ( I n t e g r a t i o n ) 31 F i g u r e 5 E f f e c t of the g r a z i n g assemblage, f e r t i l i z a t i o n , Diaptomus kenai and D i a c y c l o p s thomasi alone on n a u p l i a r s u r v i v a l (Curve f i t t i n g ) 33 F i g u r e 6 E f f e c t of the s u c c e s s i v e a d d i t i o n of m o r t a l i t y agents on n a u p l i a r s u r v i v a l ( I n t e g r a t i o n ) 35 F i g u r e 7 E f f e c t of the s u c c e s s i v e a d d i t i o n of m o r t a l i t y agents on n a u p l i a r s u r v i v a l (Curve f i t t i n g ) 37 F i g u r e 8 Summary of the e f f e c t of a s i n g l e f a c t o r on s u r v i v a l to the end of the n a u p l i a r stages ( I n t e g r a t i o n ) F i g u r e 9 Summary of the e f f e c t of a. s i n g l e f a c t o r on s u r v i v a l to the end of the n a u p l i a r stages (Curve f i t t i n g ) 42 F i g u r e 10 Summary of the e f f e c t of a s i n g l e f a c t o r on s u r v i v a l to the beginning of the t h i r d copepodite stage ( I n t e g r a t i o n ) 44 F i g u r e 11 Summary of the e f f e c t of a s i n g l e f a c t o r on v i i s u r v i v a l to the beginning of the t h i r d copepodite stage ( C u r v e - f i t t i n g ) 46 F i g u r e 12 Design of experimental cages 60 F i g u r e 13 F u n c t i o n a l response of Diaptomus kenai to n a u p l i a r d e n s i t i e s 63 F i g u r e 14 G r a p h i c a l r e p r e s e n t a t i o n of Gehrs and Robertsons method of s u r v i v o r s h i p c a l c u l a t i o n 104 F i g u r e 15 G r a p h i c a l r e p r e s e n t a t i o n of f i t obtained from curve f i t t i n g procedure 111 F i g u r e 16 G r a p h i c a l r e p r e s e n t a t i o n of e r r o r i n s u r v i v o r s h i p curves 113 F i g u r e 17 Temperature and p r e c i p i t a t i o n d u r i n g the experimental p e r i o d (May-June, 1982) 117 F i g u r e 18 S u r v i v o r s h i p c a l c u l a t e d using i n t e g r a t i o n 120 F i g u r e 19 S u r v i v o r s h i p c a l c u l a t e d using curve f i t t i n g 125 v i i i ACKNOWLEDGEMENTS I have been aided i n the course of t h i s r e s e a r c h by a great many people, many of whom are at the I n s t i t u t e of Animal Resource Ecology, which has provided an open forum f o r d i s c u s s i o n and l e a r n i n g . I am indebted to C a r l Walters f o r p r o v i d i n g the o r i g i n a l idea f o r t h i s r e s e a r c h and g i v i n g generous access to h i s data. The i n v a l u a b l e advice of E d i t h Krause and Don Robinson has saved many, many hours of work. Adrienne Peacock helped me to i d e n t i f y those funny l i t t l e animals that are too small f o r anyone e l s e to care about. Wendy H i r d , E d i t h Krause, Don Ludwig, Peter Morrison and B i l l N e i l l e i t h e r s h i v e r e d i n the d r i z z l e or b o i l e d i n the sun f o r many hours s o r t i n g zooplankton. Peter Morrison, Rob Purdy and my Dad d i s c o v e r e d the joy of rowing about f i s h i n g f o r n a u p l i i to feed to Diaptomus kenai. Don Robinson has uncomplainingly graphed and produced numerical data that I always seemed to need ye s t e r d a y . Don Ludwig p r o v i d e d h e l p f u l advice on methods of a n a l y s i s and u n f a i l i n g l y d i s c o u r a g e d or encouraged me - whichever was neccessary. Don Ludwig, Judy Myers and B i l l N e i l l p r ovided many ( i n some cases very many) h e l p f u l comments on e a r l i e r d r a f t s of t h i s manuscript. I wish e s p e c i a l l y to thank B i l l N e i l l who answered qu e s t i o n s u n t i l they must have come out h i s ears and has s k i l l f u l l y guided me through the resea r c h and the red tape. To the thousands of zooplankton, who int r o d u c e d me to a new world of beauty and wonder and who gave t h e i r l i v e s f o r t h i s ix r e s e a r c h , I am profoundly g r a t e f u l . L a s t l y , I wish to thank Trevor Dee, whose generous support of Ma B e l l has s u s t a i n e d me more than anything e l s e . T h i s r e s e a r c h was supported by the people and government of Canada through the N a t u r a l Sciences and E n g i n e e r i n g Research C o u n c i l . $SIGNOFF 1 GENERAL INTRODUCTION Systems ecology, l i k e other systems-oriented d i s c i p l i n e s , i s concerned with the c o n s t r a i n t s which act on systems to keep them w i t h i n c e r t a i n boundaries. U n l i k e most systems i n other d i s c i p l i n e s however, s t o c h a s t i c events p l a y a much l a r g e r r o l e in ecosystems. Because of the complicated nature of ecosystems, the l a r g e i n f l u e n c e of s t o c h a s t i c events and d i f f i c u l t i e s a s s o c i a t e d with q u a n t i f y i n g behaviour, systems theory f o r ecology has not been w e l l developed. H o l l i n g (1973) suggested that because of the above problems, a q u a l i t a t i v e r a t h e r than a r i g o r o u s q u a n t i t a t i v e approach might be more u s e f u l i n studying ecosystems and he began to examine the gen e r a l q u a l i t a t i v e behaviour of some systems. He observed that ecosystems o f t e n p e r s i s t e d i n one c o n f i g u r a t i o n f o r many years and then suddenly appeared to change i n behaviour and community s t r u c t u r e , sometimes even when no l a r g e p e r t u r b a t i o n had oc c u r r e d . He a t t r i b u t e d t h i s phenomenon to the presence of a "domain of a t t r a c t i o n " i n which the system u s u a l l y e x i s t e d . However once the boundary to t h i s domain of a t t r a c t i o n was reached, the system c o u l d q u i c k l y change to another c o n f i g u r a t i o n . T h i s way of viewing systems a l s o e x p l a i n e d why no l a r g e p e r t u r b a t i o n was needed to change the system. A small p e r t u r b a t i o n , a p p l i e d over a number of years might push a system c l o s e r and c l o s e r to the boundary of the domain of a t t r a c t i o n u n t i l only a small a d d i t i o n a l p e r t u r b a t i o n would be needed to push the system i n t o a d i f f e r e n t c o n f i g u r a t i o n . 2 S e v e r a l systems which appear to e x h i b i t t h i s s o r t of behaviour have been examined in d e t a i l . A n a l y s i s of outbreaks of spruce budworm (Ludwig et a l . 1978), t r a n s i t i o n s between grass and woody v e g e t a t i o n i n A f r i c a n s e m i - a r i d g r a z i n g savannas (Walker et a l . 1981) and the c o l l a p s e of many of the Great Lakes f i s h e r i e s (Beeton 1969 i n H o l l i n g 1973) have r e v e a l e d that the sudden changes i n c o n f i g u r a t i o n are the r e s u l t of s u r p a s s i n g the t h r e s h o l d value of some c r i t i c a l v a r i a b l e . In plankton communities, e u t r o p h i c a t i o n has been found to cause sudden a l g a l blooms and s h i f t s i n zooplankton community composition, however, l i t t l e work has been done to examine changes i n zooplankton communities in d e t a i l and determine the causes of these changes. Recently, Eunice Lake in the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t has shown a dramatic sh'ift i n community s t r u c t u r e (Walters unpubl. d a t a ) . In the years 1975 through 1980, the Eunice Lake zooplankton community was dominated by the h e r b i v o r o u s cladoceran Daphnia rosea and the mostly herbivorous c a l a n o i d copepods Diaptomus kenai. In 1981, the community suddenly became dominated by the c a r n i v o r o u s c y c l o p o i d copepod D i a c y c l o p s thomasi and the herbivorous cladoceran Holopedium  gibberum and seems to have continued i n t h i s c o n f i g u r a t i o n u n t i l the present (Figure 1). No l a r g e p e r t u r b a t i o n was a p p l i e d d i r e c t l y to t h i s lake d u r i n g the t r a n s i t i o n p e r i o d although c u t t h r o a t t r o u t were i n t r o d u c e d i n 1974 (Hume 1978) and upstream Gwendoline Lake was f e r t i l i z e d i n 1979 (Hay 1981"). The f e r t i l i z a t i o n of Gwendoline Lake r e s u l t e d i n g r e a t l y i n c r e a s e d biomass of Cladocera i n that year, both i n Gwendoline and Eunice Lakes. However, t h i s i n c r e a s e i n biomass occurred only d u r i n g 3 the year of f e r t i l i z a t i o n and Gwendoline Lake regained i t s former community s t r u c t u r e (Walters unpubl. d a t a ) . In order to gain i n s i g h t i n t o the c o n s t r a i n t s a c t i n g on an o l i g o t r o p h i c montane l a c u s t r i n e community and the mechanisms mediating such s h i f t s i n s t r u c t u r e , t h i s s i t u a t i o n was examined in g r e a t e r d e t a i l . D i a c y c l o p s thomasi changed from being rare to being a community dominant. T h i s r a d i c a l change i n number and importance i n the community suggests that the examination of the p o p u l a t i o n dynamics of t h i s organism may r e v e a l the causes of the s h i f t i n community s t r u c t u r e i n Eunice Lake. Peacock ( N e i l l and Peacock 1980, Peacock 1981) found that the p e r i o d of highest m o r t a l i t y f o r D i a c y c l o p s seemed to occur i n the n a u p l i a r stages, p a r t i c u l a r l y i n the e a r l y n a u p l i a r stages. T h i s p a t t e r n of m o r t a l i t y i n the Copepoda has been found both i n freshwater (Burgis 1971, R i g l e r and Cooley 1974, Gehrs and Robertson.1975, Confer and Cooley 1977, Peacock 1981) and marine systems (Heinle 1966, Parsons et a l . 1969, M u l l i n and Brooks 1970 i n Landry 1978b, Landry 1978a). The u n i f o r m i t y of t h i s p a t t e r n suggests that the major p o r t i o n of the m o r t a l i t y of many s p e c i e s of copepods occurs d u r i n g the n a u p l i a r stages and that the s i g n i f i c a n t f a c t o r s i n f l u e n c i n g p o p u l a t i o n r e g u l a t i o n and t h e r e f o r e the community s t r u c t u r e of communities dominated by copepods may act d u r i n g these e a r l y stages. There are a number of f a c t o r s which h y p o t h e t i c a l l y c o u l d i n f l u e n c e the s u r v i v a l of n a u p l i i i n g e n e r a l . N a u p l i i appear to be e x c e p t i o n a l l y v u l n e r a b l e to food l i m i t a t i o n . In the c a l a n o i d copepod Calmoecia l u c a s i , Green (1975) found that the balance of a s s i m i l a t e d food intake and r e s p i r a t o r y output was negative only 4 F i g u r e 1 S p r i n g zooplankton community composition i n Eunice Lake from 1975-1982. Zooplankton biomasses are averaged from those found from mid-May to e a r l y J u l y . The t o t a l zooplankton biomass i s d i v i d e d i n t o the c a t e g o r i e s shown below. Samples were c o l l e c t e d with a 200 jim net. (Walters (unpubl. data) a l l n a u p l i i and c a l a n o i d copepodites s m a l l c a l a n o i d copepods (Diaptomus leptopus, D. t y r r e l l i ) Diaptomus kenai sma l l C l a d o c e r a (Bosmina l o n g i r o s t r i s , Diaphanosoma brachyurum, Polyphemus p e d i c u l u s ) Daphnia rosea Holopedi . gibberum D i a c y c l o p s thomasi 5 6 f o r the n a u p l i a r stages. T h i s r e s u l t , combined with the o b s e r v a t i o n that a stronger c o r r e l a t i o n between s i z e and metabolic r a t e s e x i s t s f o r c y c l o p o i d and c a l a n o i d n a u p l i i than for a d u l t s (Epp and Lewis 1979,1980) suggests that n a u p l i i are t r e a d i n g a t h i n l i n e between s t a r v a t i o n and s u c c e s s f u l s u r v i v a l to the copepodite stages. L i t t l e i s known about the a c t u a l d i e t and f e e d i n g responses of n a u p l i i i n nature although the s m a l l e r stages of some marine copepods have been found to graze more e f f i c i e n t l y on small algae than the l a r g e r stages ( M u l l i n and Brooks 1970, P a f f e n h o f f e r 1971, Poulet 1977). However i t i s p o s s i b l e that n a u p l i i are poor competitors f o r the food resources and are out-competed by l a r g e r c l a d o c e r a n and copepod g r a z e r s . I t i s a l s o p o s s i b l e that n a u p l i i do not o v e r l a p i n t h e i r . f o o d requirements with l a r g e r grazers and there i s a d e f i c i e n c y of the a p p r o p r i a t e s i z e of food due to other f a c t o r s such as an i n a p p r o p r i a t e n u t r i e n t regime or competition f o r n u t r i e n t s from l a r g e r a l g a e . P r e d a t i o n i s f r e q u e n t l y suggested to play a strong r o l e i n s t r u c t u r i n g zooplankton communities and i n v e r t e b r a t e p r e d a t o r s i n p a r t i c u l a r have been suggested to be major m o r t a l i t y agents f o r s m a l l e r animals (Dodson 1974, Kerfoot 1977, Lane 1978, 1979, Zaret 1980). Much of the work on i n v e r t e b r a t e p r e d a t i o n has i n v o l v e d p r e d a t i o n on c l a d o c e r a n s . However s e v e r a l s t u d i e s which have examined p r e d a t i o n r a t e s on n a u p l i i and e x t r a p o l a t e d t h e i r e f f e c t on community s t r u c t u r e have found that p r e d a t i o n can s u b s t a n t i a l l y a f f e c t n a u p l i a r s u r v i v a l (McQueen 1969, Confer and Cooley 1977, Landry 1978b). I n t r a s p e c i f i c p r e d a t i o n has a l s o been found to be a major m o r t a l i t y f a c t o r f o r copepod 7 n a u p l i i i n s e v e r a l circumstances (McQueen 1969, Landry 1978a). It t h e r e f o r e seems that both food a v a i l a b i l i t y , due e i t h e r to competition or other non-competitive f a c t o r s , and p r e d a t i o n , e i t h e r i n t e r - or i n t r a s p e c i f i c , have the p o t e n t i a l to exert a l a r g e i n f l u e n c e on the s u r v i v a l of n a u p l i i . The r o l e these f a c t o r s p l a y i n i n f l u e n c i n g the n a u p l i a r s u r v i v a l of D. thomasi i n an o l i g o t r o p h i c montane lake was examined i n a set of e n c l o s u r e experiments d e s c r i b e d i n Chapter 1. In these experiments, I hypothesized that n a u p l i i were dying because of a la c k of a v a i l a b l e food. There were two p o s s i b l e e x p l a n a t i o n s f o r t h i s : 1. n a u p l i i u t i l i z e d the same s i z e s of phytoplankton as other g r a z e r s i n the system and were out-competed; 2. n a u p l i i d i d not use the same s i z e s of ' c e l l s as other zooplankton but the abundance of the s i z e s which were used was low. These two hypotheses were t e s t e d by (1) adding the g r a z i n g assemblage to an experimental e n c l o s u r e and (2) adding f e r t i l i z e r to i n c r e a s e the food supply. An a l t e r n a t i v e set of hypotheses was that n a u p l i i were being h e a v i l y a f f e c t e d by p r e d a t i o n . There were two p o s s i b l e sources of p r e d a t i o n : 1. i n t r a s p e c i f i c p r e d a t i o n by D i a c y c l o p s thomasi and 2. i n t e r s p e c i f i c p r e d a t i o n by Diaptomus kenai. These two hypotheses were t e s t e d by (1) adding D i a c y c l o p s thomasi a d u l t s to the e n c l o s u r e s and (2) adding a d u l t Diaptomus kenai to the e n c l o s u r e s . I n t e r s p e c i f i c p r e d a t i o n by Diaptomus kenai on D i a c y c l o p s thomasi n a u p l i i was examined in more d e t a i l i n Chapter 2. 8 CHAPTER 1 N a u p l i a r s u r v i v a l i s suspected to be the major b o t t l e n e c k in the l i f e h i s t o r y of D. thomasi ( N e i l l and Peacock 1980). Food l i m i t a t i o n and p r e d a t i o n appear to have the p o t e n t i a l to act as major m o r t a l i t y agents d u r i n g t h i s developmental p e r i o d . The r e l a t i v e importance of food l i m i t a t i o n , due both to c o m p e t i t i v e and non-competitive f a c t o r s , and p r e d a t i o n , both i n t e r - and i n t r a s p e c i f i c was examined in a set of enclosure experiments. M a t e r i a l s and methods Study area T h i s study was conducted i n P l a c i d Lake i n the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t l o c a t e d approximately 40 km east of Vancouver i n the Coast Range Mountains ( F i g . 2). Lakes i n the Research F o r e s t have been d e s c r i b e d i n d e t a i l elsewhere ( E f f o r d 1967, Northcote and C l a r o t t o 1975, N e i l l 1978), but they are mostly small o l i g o t r o p h i c l a k e s . D i a c y c l o p s thomasi i s abundant i n P l a c i d Lake, although i t i s much l e s s abundant than in nearby Eunice Lake. Consequently i n P l a c i d Lake, other zooplankters would be more l i k e l y to play a major r o l e i n l i m i t i n g the p o p u l a t i o n , thus more c l o s e l y approximating c o n d i t i o n s i n Eunice Lake p r i o r t o 1979. E n c l o s u r e experiments to examine the r e l a t i v e importance of food a v a i l a b i l i t y and p r e d a t i o n on the n a u p l i a r s u r v i v a l of D. thomasi were conducted 9 in Placid Lake in May and June 1982. Due to i t s small size and large watershed in comparison to i t s volume (Table I ) , this lake undergoes r e l a t i v e l y sudden and drastic changes of temperature and water l e v e l . Nevertheless Placid Lake shows a similar zooplankton community with comparable dynamics to other takes in the Research Forest (Walters, unpubl. data). Both Placid and Eunice Lakes support populations of cutthroat trout (Salmo  c l a r k i c l a r k i ) . The population in Placid Lake i s presumed to be native, however the population in Eunice Lake was introduced from nearby Loon Lake in 1974 and 1975. Placid Lake Community In order to aff e c t the naupliar survival of D. thomasi, other zooplankton in Placid Lake would have to overlap at least temporally with the n a u p l i i . Diacyclops i s univoltine with the major burst of production occurring in early spring. It enters the water column from a winter diapause in late A p r i l as stage 4 and 5 copepodites and molts to reproductive adults. These adults produce nauplii during May and June with a peak in production occurring at the end of May. The nauplii occur mainly in the upper 2 meters of the water column. Examination of the seasonal patterns in Pla c i d Lake reveals one species of cyclopoid copepod, two species of calanoid copepods and several species of cladocerans which occur in s i g n i f i c a n t densities at the same time as Diacyclops n a u p l i i . The most common species of Cladocera present are Holopedium gibberum and Daphnia rosea. The cladoceran species Bosmina l o n g i r o s t r i s , Ceriodaphnia  quadrangula and Polyphemus pediculus are also present but occur 1 0 F i g u r e 2 L o c a t i o n of study a r e a . (from N e i l l , 1978) 1 2 Table I Some p h y s i c a l , chemical and b i o l o g i c a l c h a r a c t e r i s t i c s of P l a c i d and Eunice Lakes i n the UBC Research F o r e s t , (modified from N e i l l , 1978) Character i s t i e s -Plac i d Eunice E l e v a t i o n , m 510 480 Drainage area, ha 44 191 Surface area, ha 1.6 18.2 Maximum depth, m 7 42 Mean depth, m 4.3 15.8 Colour, Pt u n i t s . 20-25 1 5 Transparency (Secchi depth, m) 4-4.5 6-10 T o t a l d i s s o l v e d s o l i d s , mg/1 1 7-23 1 6 Maximal e p i l i m n e t i c depth, m Crustacean-zooplankton species; Diaptomus kenai  Diaptomus t y r r e l l i  Diaptomus oregonensis  Diaptomus leptopus  Daphnia rosea  Holopedium gibberum  Diaphanosoma brachyurum  Bosmina l o n g i r o s t r i s  Ceriodaphnia quadrangula  Polyphemus p e d i c u l u s  D i a c y c l o p s thomasi  Tropocyclops p_. p r a s i n u s 4 7 -1981 1981-A A R - A R A - -- - R A A A A A A A A A R A A R - -A A A A R A R A R A i n d i c a t e s abundant R i n d i c a t e s r a r e - i n d i c a t e s absent 13 in low numbers and are u n l i k e l y to s i g n i f i c a n t l y i n f l u e n c e n a u p l i a r s u r v i v a l . Holopedium peaks i n d e n s i t y toward the end of May, o f t e n c o n t r i b u t i n g the m a j o r i t y of the biomass i n the lake at t h i s time. Daphnia i s g e n e r a l l y i n low numbers i n the s p r i n g but begins i n c r e a s i n g i n biomass at the end of May and peaks i n the summer. The two sp e c i e s of c a l a n o i d copepods i n the lake are Diaptomus oregonensis and D. Kenai. D. oregonensis i s a comparatively small copepod (1.25 - 1.5 mm a d u l t length) which occurs i n high d e n s i t i e s from the beginning of October to the end of June, dropping i n d e n s i t y d u r i n g the summer. D. oregonensis produces n a u p l i i i n the s p r i n g s t a r t i n g l a t e r than D. thomasi and probably peaking i n J u l y . Diaptomus  kenai i s a l a r g e copepod (1.8 - 3.0 mm a d u l t length) which occurs i n high d e n s i t i e s in P l a c i d lake only d u r i n g very e a r l y s p r i n g when small numbers occur i n the upper water column. The m a j o r i t y of the D. kenai p o p u l a t i o n remains deeper although i t migrates to the s u r f a c e at n i g h t . D. kenai r e t r e a t s completely to the lower depths of the water column i n e a r l y June and dis a p p e a r s completely d u r i n g the summer. D. thomasi a d u l t s are a l s o present and occur i n the upper water column duri n g the p e r i o d of n a u p l i a r p r o d u c t i o n . These s p e c i e s can be grouped together a c c o r d i n g to t h e i r p o s s i b l e e f f e c t on n a u p l i a r m o r t a l i t y . The Cladocera present are herbivorous except f o r Polyphemus and c o u l d compete with the n a u p l i i f o r food. Some c a l a n o i d copepods have been found to be c a r n i v o r o u s and Lane (1978) suggested that D. oregonensis was a pr e d a t o r . The small s i z e of D. oregonensis i n P l a c i d Lake seems to make t h i s p o s s i b i l i t y u n l i k e l y so i t can a l s o be regarded as 1 4 a competitor. The above animals were grouped together and are h e r e i n a f t e r c o l l e c t i v e l y r e f e r r e d to as the "gra z i n g assemblage". D. kenai has a l s o been c o n s i d e r e d to be s t r i c t l y h e r b i vorous but i t s l a r g e s i z e allows f o r the p o s s i b i l i t y of p r e d a t i o n . Krause (unpubl. data) has found some evidence f o r i t s predatory a b i l i t y . D. thomasi i s known to be c a n n i b a l i s t i c on i t s own n a u p l i i and has been c a l c u l a t e d to be capable of consuming 31% of i t s own y e a r l y p r o d u c t i o n (McQueen, 1969). Experimental Methods In s i t u e n c l o s u r e experiments were conducted from May 25 u n t i l June 16, 1982 to t e s t the 4 hypotheses p o t e n t i a l l y i n v o l v e d i n l i m i t i n g n a u p l i a r s u r v i v a l . The experimental e n c l o s u r e s used were s i m i l a r to those d e s c r i b e d by N e i l l (1981). A f l o a t i n g c o l l a r c o n s i s t i n g of styrofoam and plywood was d i v i d e d i n frames approximately 45 cm square. The frame was then anchored to the bottom with cement blocks at each of the c o r n e r s . 4 m i l c l e a r p o l y e t h y l e n e p l a s t i c bags s l i g h t l y over 1 m deep were suspended from these frames. T h i s arrangement p r o v i d e d e n c l o s u r e s which were approximately 300 1 i n volume. The bags were i n i t i a l l y f i l l e d with lake water which had been pumped through a 54 um net to remove a l l crustaceans while a l l o w i n g grazable seston to pass. The water used was taken from a depth of s e v e r a l meters in the lake to allow f o r an adequate n u t r i e n t supply f o r the d u r a t i o n of the experiment. Since l a r v a e of the phantom midge, Chaoborus, have been found to exert c o n s i d e r a b l e p r e d a t i o n pressure on i n v e r t e b r a t e communities (Lynch 1979, N e i l l and Peacock 1980), 2 m i l , c l e a r p o l y e t h y l e n e 15 p l a s t i c t e n t s were c o n s t r u c t e d over the experimental e n c l o s u r e s to prevent Chaoborus from l a y i n g eggs in the e n c l o s u r e s yet allow maximal l i g h t p e n e t r a t i o n . The t e n t s c o u l d e a s i l y be removed f o r sampling. The ends of the t e n t s were covered by p l a s t i c s c r e e n i n g to allow f o r a i r c i r c u l a t i o n . The e n c l o s u r e s were stocked with lake d e n s i t i e s of crustacean zooplankton, determined by sampling the lake with a Clarke-Bumpus sampler 3 days e a r l i e r . Since the mesh s i z e on the sampling apparatus was too l a r g e to c o l l e c t n a u p l i i , v e r t i c a l hauls were made over the same depth range that the Clarke-Bumpus sample was taken. The number of n a u p l i i was c a l i b r a t e d by assuming that the p r o p o r t i o n of D. oregonensis i n the v e r t i c a l haul was r e p r e s e n t a t i v e of the p r o p o r t i o n of D. oregonensis i n the Clarke-Bumpus sample. The s t a r t i n g d e n s i t i e s of animals i n the e n c l o s u r e s may be found i n Table I I . Since D. kenai occurs deeper in the water column d u r i n g the day and migrates v e r t i c a l l y upwards at n i g h t , i t was stocked at a s l i g h t l y higher d e n s i t y than found d u r i n g the day to average out the d e n s i t y over a 24 h p e r i o d . The animals used i n the study were c o l l e c t e d from the lake on the day the e n c l o s u r e s were set up. They were s o r t e d f o r a d d i t i o n to the e n c l o s u r e s using a combination of s i e v i n g and p i p e t t i n g . N a u p l i i were obtained by f i l t e r i n g out a l l of the l a r g e r animals using a 153 Min s i e v e . D. thomasi and D. kenai a d u l t s were i n d i v i d u a l l y p i p e t t e d out. The g r a z i n g assemblage was c o n s i d e r e d to be those animals which remained a f t e r n a u p l i i and a l l D i a c y c l o p s and D. kenai copepodites were removed. To o b t a i n the d e s i r e d d e n s i t i e s i n the experimental e n c l o s u r e s , 1 6 Table II S t o c k i n g d e n s i t i e s of zooplankton i n the e n c l o s u r e s . Lake Enclosure Number in Den s i t y D e n s i t y E n c l o s u r e ( l - 1 ) ( l - 1 ) D i a c y c l o p s thomasi 1.0 1.0 300 Diaptomus kenai .06 .22 66 Diaptomus oregonensis 3.6 3.5 1050 Holopedium gibberum .6 .6 180 N a u p l i i 3.6 3.5 1050 1 7 D. kenai and D. thomasi were counted i n d i v i d u a l l y . The d e s i r e d d e n s i t i e s of n a u p l i i and the g r a z i n g assemblage were obtained by adding the r e q u i r e d amount of a c a l i b r a t e d c oncentrated zooplankton "soup". To enhance p o t e n t i a l l y l i m i t i n g food abundance for n a u p l i i , some of the enclosures were f e r t i l i z e d with 6 jxg/1 P i n a 30:1 N:P atomic r a t i o using NH„C1 and (NH u) 2HPOi,. F e r t i l i z e r was added to the enc l o s u r e s twice d u r i n g the course of the experiment, once while the bags were being f i l l e d and subsequently a f t e r the experiments had been running 12 days. The amount of f e r t i l i z e r added approximately doubled the amount of phosphorus i n the water." A l a r g e N:P r a t i o and the use of ammoniacal n i t r o g e n was chosen to t r y to i n c r e a s e the growth of e d i b l e algae without s h i f t i n g the a l g a l composition e x c e s s i v e l y and, i n p a r t i c u l a r , without unduly f a v o u r i n g the growth of l a r g e n i t r o g e n - f i x i n g blue green algae ( N e i l l p ers. comm.). A l l of the en c l o s u r e s were sampled at four day i n t e r v a l s f o r zooplankton, s t a r t i n g the day a f t e r set-up. The zooplankton community was sampled by removing the animals from 15 1 c o l l e c t e d from s e v e r a l depths and areas i n each enclosure using a b a t t e r y - o p e r a t e d b i l g e pump. Animals were preserved i n a sucrose-5% formaldehyde s o l u t i o n (Haney and H a l l 1973) and a l l samples were l a t e r counted i_n t o t o • A l l n a u p l i i and immature copepodites were counted under 50X on a b i n o c u l a r d i s s e c t i n g microscope. C y c l o p o i d n a u p l i i and copepodites were i d e n t i f i e d f o l l o w i n g Torke (1974) and staged under 50X. Larger animals were counted under 25X. C h l o r o p h y l l and phytoplankton were a l s o sampled at 4 day 18 i n t e r v a l s . However the f i r s t sample was taken on the day of set-up p r i o r to the a d d i t i o n of animals to the e n c l o s u r e s . A f t e r the f i r s t sampling p e r i o d , sampling was done i n synchrony with zooplankton sampling. C h l o r o p h y l l and phytoplankton were sampled by withdrawing approximately 700 ml of water from each e n c l o s u r e using a p i e c e of hose approximately 3 cm i n diameter and extending 1 m i n t o the e n c l o s u r e . An 80 ml a l i q u o t of the well-mixed sample was f i x e d with Lugol's s o l u t i o n f o r l a t e r enumeration of phytoplankton c e l l s . A 250 ml a l i q u o t was f i l t e r e d through a Whatman GF/C g l a s s f i b r e f i l t e r , f o l d e d and immediately put i n the dark on i c e . A f t e r t r a n s p o r t back to the la b , these samples were s t o r e d at approximately -20 °C f o r 4 to 6 months. C h l o r o p h y l l a measurements were made on the frozen samples -f o l l o w i n g a method m o d i f i e d from S t r i c k l a n d and Parsons (1965) using a Turner Design Model 10 fluorometer. Magnesium carbonate was not added to the samples. C o r r e c t i o n f o r phaeophytin , i n a c t i v e c h l o r o p h y l l , was made by measuring the sample both before and a f t e r s e v e r a l drops of 1% HOC1 was added. C a l c u l a t i o n of the amounts of l i v e c h l o r o p h y l l and phaeophytin was done using the equations: l i v e c h l o r o p h y l l = r(BA-AA)/(r-1) phaeophytin = r(BA - l i v e c h l o r o p h y l l ) Where BA= f l u o r e s c e n c e before a c i d AA = f l u o r e s c e n c e a f t e r a c i d r = conve r s i o n f a c t o r Phytoplankton c e l l counts were made by s e t t l i n g 50 ml of 1 9 preserved samples for 48 hours, t r a n s f e r r i n g the r e s i d u e to a counting chamber and s e t t l i n g f o r an a d d i t i o n a l 24 hours. Phytoplankton c e l l s were then counted under 400X and d i v i d e d i n t o the s i z e c l a s s e s <2 nm, 2-5 /xm, 5-9 nm, 9-13 Mm, 13-18 ixm, >18 /xm, c o l o n i e s and f i l a m e n t s . The minimum of 200 c e l l s or 30 f i e l d s was counted f o r each s i z e c l a s s f o r each sample. T h i s c r i t e r i o n has been found to be the most e f f i c i e n t i n reducing sample v a r i a n c e (D. Robinson, E. Krause p e r s . comm.). Experimental design Four f a c t o r s were hypothesized to s i g n i f i c a n t l y a f f e c t n a u p l i a r s u r v i v a l . These were: (1) food l i m i t a t i o n due to competition from other g r a z e r s , (2) food l i m i t a t i o n due to non-co m p e t i t i v e f a c t o r s , (3) i n t r a s p e c i f i c p r e d a t i o n by D. thomasi a d u l t s and (4) i n t e r s p e c i f i c p r e d a t i o n by D. kenai . These hypotheses were t e s t e d by (1) adding the g r a z i n g assemblage to an experimental e n c l o s u r e , (2) adding f e r t i l i z e r to an en c l o s u r e , (3) adding a d u l t D. thomasi to an encl o s u r e and (4) adding D. kenai to an e n c l o s u r e . Eighteen experimental e n c l o s u r e s were set up with s i x t e e n d i f f e r e n t experimental treatments. The s i x t e e n treatments i n c l u d e d every p o s s i b l e combination of the four f a c t o r s mentioned above. F i g u r e 3 p r o v i d e s a summary of these treatments. N a u p l i i were added to every experimental e n c l o s u r e . The two a d d i t i o n a l experimental e n c l o s u r e s were set up to be r e p l i c a t e s of the treatment of f e r t i l i z e r and n a u p l i i o nly. Since i t was not f e a s i b l e to r e p l i c a t e a l l experimental treatments, these 3 r e p l i c a t e s were used as an estimate of 20 between encl o s u r e v a r i a b i l i t y . Since f e r t i l i z e r i s known to i n c r e a s e v a r i a n c e between bags ( N e i l l , pers. comm.) and these treatments would have few organisms to s t a b i l i z e t h i s e f f e c t , the treatment with f e r t i l i z e r and n a u p l i i only was c o n s i d e r e d a p r i o r i to-- have the g r e a t e s t p o t e n t i a l f o r experimental v a r i a b i l i t y . On the f i n a l day of sampling, three zooplankton samples were taken from each enclosure and 3 c h l o r o p h y l l and phytoplankton samples were taken from 5 e n c l o s u r e s . These r e p l i c a t e s were used to o b t a i n an estimate of sample v a r i a n c e w i t h i n e n c l o s u r e s . A n a l y s i s A n a l y s i s of v a r i a n c e was the most commonly used s t a t i s t i c a l technique and was done e i t h e r using a pocket c a l c u l a t o r or the computer program UBC GENLIN (Grieg and B j e r r i n g 1977). In e i t h e r case, B a r t l e t t ' s t e s t f o r homogeneity of v a r i a n c e was conducted. If the v a r i a n c e was not homogeneous, the data were log-transformed and the t e s t r e - a p p l i e d . In most cases, the lack of homogeneity was not s i g n i f i c a n t once the data were transformed. S i m u l a t i o n analyses were a l l performed on an Apple II microcomputer. Curve f i t t i n g was performed using the package N2SNO found i n UBC CURVE (Moore 1981). Unless the use of a packaged program i s e x p l i c i t l y s t a t e d , I wrote and performed a l l s i m u l a t i o n s and a n a l y s e s . ure 3 Experimental d e s i g n . Four f a c t o r s were used in the experiment to examine t h e i r e f f e c t on D i a c y c l o p s thomasi n a u p l i a r s u r v i v a l : (1) f e r t i l i z a t i o n (2) presence of the g r a z i n g assemblage ( 3 ) presence of Diaptomus kenai ( 4 ) presence of D i a c y c l o p s thomasi. Every p o s s i b l e combination of these 4 f a c t o r s was i n c l u d e d i n the experimental d e s i g n , In t h i s r e p r e s e n t a t i o n , every box r e p r e s e n t s an e n c l o s u r e and the symbols i n i t i n d i c a t e the treatments i t c o n t a i n s . For example, the box i n the upper l e f t hand corner r e p r e s e n t s the e n c l o s u r e which c o n t a i n s f e r t i l i z e r and grazers-. Grazers Diaptomus kenai D i a c y c l o p s thomasi f e r t i l i z e r G K D FERTILIZER P r e s G R A ; Present ent Z E R S Absent A b s G R A ; Present ent J E R S Absent CLOPS Absent F G F G AbS( DIACY' Present F D G F D D G D >ent CLOPS Absent K F G K F K G K Pres DIACY' Present K F D G K F D K D G K D 23 R e s u l t s P r e c i s i o n of sample data C o e f f i c i e n t s of v a r i a t i o n were c a l c u l a t e d from the three r e p l i c a t e zooplankton samples taken from a l l e n c l o s u r e s at the end of the experiment. A f a i r l y c o n s i s t e n t c o e f f i c i e n t of v a r i a t i o n between 40 and 50 % i s shown for a l l zooplankton (Table I I I ) . If samples to which the t e s t e d treatment was not added are excluded, the c o e f f i c e n t of v a r i a t i o n i s c o n s i s t e n t l y lower. T h i s would be expected i f the v a r i a n c e i n c r e a s e d with d e c r e a s i n g abundance. The p r e c i s i o n of the phytoplankton data can be seen i n Table IV. Phytoplankton c e l l counts were probably more v a r i a b l e although t h i s i s d i f f i c u l t to judge s i n c e a l l samples were counted from only one of the e n c l o s u r e s from which r e p l i c a t e s were taken. The c h l o r o p h y l l measurements showed that estimates of l i v e c h l o r o p h y l l were c o n s i d e r a b l y more v a r i a b l e than those for e i t h e r phaeophytin or t o t a l c h l o r o p h y l l . The c o e f f i c i e n t s f o r both the zooplankton and phytoplankton data are l a r g e r than those found by Marmorek (1983). The i n c r e a s e d v a r i a t i o n i s probably due to the small volume of the samples taken. However, the v a r i a t i o n i s s i m i l a r to that r e p o r t e d by Wiebe and H o l l a n d (1968) in t h e i r study of sampling e r r o r . 24 Table III P r e c i s i o n of zooplankton data Organism No. of Mean C o e f f i c i e n t E n c l o s u r e s of V a r i a t i o n Daphnia rosea 18 48.5 8 * 42.6 D i a c y c l o p s thomasi 18 72.1 8 ** 44.6 Diaptomus oregonensis 18 54.0 8 * 46.9 Holopedium gibberurn 18 50.9 8 * 40.8 C a l a n o i d n a u p l i i 18 52.3 copepodites 18 42.7 C y l o p o i d n a u p l i i and 84 *** 48.0 copepodites - c a l c u l a t e d from only e n c l o s u r e s c o n t a i n i n g the g r a z i n g assemblage. - c a l c u l a t e d from only e n c l o s u r e s c o n t a i n i n g D. thomasi. - c a l c u l a t e d over a l l stages. 25 Table IV P r e c i s i o n of phytoplankton data No. of Mean C o e f f i c i e n t E n c l o s u r e s of V a r i a t i o n C h l o r o p h y l l measurements l i v e c h l o r o p h y l l 5 89.1 phaeophytin 5 51.4 t o t a l c h l o r o p h y l l 5 26.0 C e l l counts < 2 Mm 1 84.0 2 - 5 Mm 1 11.4 5 - 9 Mm 1 7 6.9 9 - 13 Mm 1 52.8 13 - 18 Mm 1 68.5 18 - 30 Mm 1 78.7 Mean of s i z e c l a s s e s 62.1 26 Set up of en c l o s u r e s Count data of zooplankton and phytoplankton from the en c l o s u r e s were examined to confirm proper set up of the experimental e n c l o s u r e s . D e t a i l s of the s t a t i s t i c a l a n a l y s i s may be found i n Appendix A. In ge n e r a l , treatment d i f f e r e n c e s were observed f o r a l l treatments a f t e r set up and these d i f f e r e n c e s continued f o r the d u r a t i o n of the experiment. A l l zooplankton treatments appeared to s u f f e r e x t e n s i v e m o r t a l i t y a s s o c i a t e d with set up and were much lower i n d e n s i t y than the lake at the beginning of the experimental p e r i o d . T h i s d i f f e r e n c e i n c r e a s e d d u r i n g the course of the experiment f o r a d u l t D. thomasi and the g r a z i n g assemblage s i n c e d e n s i t i e s i n the lake rose due to animals breaking diapause. The d i f f e r e n c e s decreased f o r Cladocera at the end of the experimental p e r i o d . N a u p l i i were at lake d e n s i t y at the beginning of the experiment. These r e s u l t s are summarized i n Table V. F e r t i l i z e r treatments r e s u l t e d i n s i g n i f i c a n t i n c r e a s e s i n t o t a l c h l o r o p h y l l a a f t e r , the f i r s t sampling p e r i o d . T h i s i n c r e a s e i n t o t a l c h l o r o p h y l l was l a r g e l y made up of i n c r e a s e s in phaeophytin since l i v e c h l o r o p h y l l showed s i g n i f i c a n t i n c r e a s e s only d u r i n g 2 sampling p e r i o d s . Examination of s i z e c l a s s e s of phytoplankton showed that f e r t i l i z a t i o n r e s u l t e d i n s i g n i f i c a n t i n c r e a s e s i n plankton of the 5-18 Min range f o r the second and t h i r d sample p e r i o d s . Phytoplankton data were a l s o examined to determine i f the g r a z i n g assemblage had a s i g n i f i c a n t impact on the food a v a i l a b i l i t y . T h i s a n a l y s i s showed that the g r a z i n g assemblage decreased t o t a l c h l o r o p h y l l a s t a r t i n g i n the t h i r d sampling p e r i o d and c o n t i n u i n g to the end T a b l e V Dynamics of z o o p l a n k t o n i n e n c l o s u r e s Organi sm E f f e c t of Treatment E f f e c t of Time D e n s i t y i n E n c l o s u r e s ( 1 _ 1 ) (Percentage of Lake D e n s i t y ) Begi nn i ng Middle End D i acyc1 ops thomas i 0.75 (75) 0.45 (50) 0.45 (35) D i aptomus kena i 0.07 Diaptomus oregonens i s 0. 72 (20) 0.67 (50) 0.84 (400) Daphn i a r o s e a 0.13 (67) 0.20 (10) 1 .80 (115) H o i o p e d i urn  Qi bberum 0.47 (75) 7 . 40 (115) 4.29 ( 130) N a u p l i i 3 . 58 ( 100) *** - i n d i c a t e s s i g n i f i c a n c e at the l e v e l .001. ns - i n d i c a t e s n o n - s i g n i f i c a n c e . p - i n d i c a t e s t h a t no s t a t i s t i c a l t e s t was done but v i s u a l i n s p e c t i o n a s s u r e d the p r e s e n c e i n e n c l o s u r e s . 28 of the experiment. An i n t e r a c t i o n between f e r t i l i z e r and the g r a z i n g assemblage occurred d u r i n g the l a s t two sampling p e r i o d s . Examination of the abundance of phytoplankton s i z e c l a s s e s on the t h i r d sampling p e r i o d r e v e a l e d that the g r a z i n g assemblage decreased phytoplankton biomass in the 5-13 jum range and an i n t e r a c t i o n between f e r t i l i z e r and g r a z e r s occurred i n the 5-9 nm range. R e s u l t s of s u r v i v o r s h i p c a l c u l a t i o n s Two methods f o r the c a l c u l a t i o n of s u r v i v o r s h i p were examined i n d e t a i l (Appendix B). I n t e g r a t i o n under the p o p u l a t i o n abundance curve i s the method which has been most f r e q u e n t l y used to examine copepod s u r v i v o r s h i p . The other method i n v e s t i g a t e d i n v o l v e d c o n s t r u c t i n g a s i m u l a t i o n model of c o n d i t i o n s i n the e n c l o s u r e s and f i t t i n g the simulated curve to the a c t u a l p o p u l a t i o n abundance curve by m inimizing the sums of squares. These i n v e s t i g a t i o n s r e v e a l e d that both methods had some d i f f i c u l t i e s but both seemed f a i r l y robust and c o n s i s t e n t w i t h i n themselves. S u r v i v o r s h i p was t h e r e f o r e c a l c u l a t e d u s i n g both techniques and r e s u l t s compared. The r e s u l t s from i n t e g r a t i o n under the curve are presented i n Appendix D, F i g u r e 19. Examination of these r e s u l t s on a very general l e v e l i n d i c a t e d that a l l the f a c t o r s t e s t e d had some e f f e c t on n a u p l i a r s u r v i v a l . F e r t i l i z e r i n c r e a s e d s u r v i v a l whereas a l l other f a c t o r s , to a g r e a t e r or l e s s e r extent, decreased s u r v i v a l . R e s u l t s from curve f i t t i n g are shown i n Appendix D, F i g u r e 20. These r e s u l t s a l s o show an e f f e c t of a l l f a c t o r s on n a u p l i a r s u r v i v a l . The general e f f e c t s of these f a c t o r s on 29 o v e r a l l s u r v i v a l to the t h i r d copepodite stage was the same as the r e s u l t s from i n t e g r a t i o n . When the r e s u l t s of the c a l c u l a t i o n of s u r v i v o r s h i p are examined, the shortcomings of the methods of a n a l y s i s and the sampling technique must be c o n s i d e r e d . These e f f e c t s , examined in d e t a i l i n Appendix B, are summarized below: 1. I n t e g r a t i n g under the p o p u l a t i o n curve underestimates s u r v i v o r s h i p and t h i s e f f e c t becomes more n o t i c e a b l e when reproducing a d u l t D. thomasi are present. T h i s technique i s a l s o v u l n e r a b l e to s l i g h t d i f f e r e n c e s i n developmental r a t e s between e n c l o s u r e s . 2. Curve f i t t i n g underestimates s u r v i v o r s h i p when a d u l t D. thomasi are present i n the e n c l o s u r e s . T h i s technique i s v u l n e r a b l e to inadequacies i n the model used f o r g e n e r a t i n g the curve. 3. Sampling e r r o r r e s u l t s i n a 10% v a r i a t i o n i n d a i l y s u r v i v a l r a t e . T h i s v a r i a t i o n r e s u l t e d i n approximately a 25% v a r i a t i o n i n every p o i n t on the s u r v i v a l curve. D i f f e r e n c e s between' e n c l o s u r e s c o n t a i n i n g the same treatment seemed to show an e r r o r of approximately the same magnitude as the sampling e r r o r . Because of the l a r g e v a r i a b i l i t y i n the c a l c u l a t e d curves and the r e l a t i v e s i m i l a r i t y of the r e s u l t s of the s u r v i v o r s h i p c a l c u l a t i o n s , no s t a t i s t i c a l a n a l y s e s were performed. Instead, general trends were observed. The most confidence was p l a c e d i n 3 0 trends which (a) appeared i n the r e s u l t s from both methods of s u r v i v o r s h i p c a l c u l a t i o n , (b) were c o n s i s t e n t i n t h e i r e f f e c t ( i . e . always i n c r e a s e d or decreased s u r v i v a l when added to an enclosure) and (c) d i f f e r e d from other treatments by more than the sample and w i t h i n treatment v a r i a t i o n . The accuracy of the r e s u l t s from the two methods c o u l d not be compared. Although i t was p o s s i b l e to perform s i m u l a t i o n s to t e s t the accuracy of the method of i n t e g r a t i o n under the curve, i t was not p o s s i b l e to do t h i s f o r the c u r v e - f i t t i n g method s i n c e there was no way to estimate the e f f e c t of inadequacies of the model used. The r e s u l t s from both methods showed that a l l f a c t o r s t e s t e d had a negative e f f e c t on n a u p l i a r s u r v i v a l by themselves. ( F i g u r e s 4 and 5 ) . In both cases, f e r t i l i z e r showed a s l i g h t n egative e f f e c t . Adult D. thomasi showed a stronger n e g a t i v e . e f f e c t . D. kenai and g r a z e r s showed s t i l l stronger n e g a t i v e e f f e c t s than the other two f a c t o r s . Adult D. thomasi , D. kenai and g r a z e r s seemed to be a d d i t i v e i n t h e i r e f f e c t i n both methods of a n a l y s i s . An example of t h i s a d d i t i v i t y can be seen i n F i g u r e s 6 and 7 . An e s t i m a t i o n of the o v e r a l l s t r e n g t h of these f a c t o r s i n i n f l u e n c i n g s u r v i v o r s h i p can be made by comparing s u r v i v o r s h i p between treatments which only d i f f e r by one f a c t o r . Summaries of these comparisons are shown i n F i g u r e s 8-11. From these summaries i t can be seen that f e r t i l i z e r has a very mixed e f f e c t u n t i l the beginning of the copepodite phase. The net e f f e c t of t h i s f a c t o r i s p o s i t i v e u s ing the method of i n t e g r a t i o n and s i g h t l y negative using curve f i t t i n g . If the net e f f e c t of 31 F i g u r e 4 E f f e c t of the g r a z i n g assemblage, f e r t i l i z a t i o n , Diaptomus k e n a i and D i a c y c l o p s thomasi a l o n e on n a u p l i a r s u r v i v a l ( I n t e g r a t i o n ) . The d i s t a n c e on the x - a x i s between d e v e l o p m e n t a l s t a g e s i s p r o p o r t i o n a l t o the d u r a t i o n of the s t a g e s . — — N a u p l i i a l o n e D i a c y c l o p s thomasi Diaptomus k e n a i G r a z i n g assemblage — F e r t i l i z e r 3 2 DEVELOPMENTAL STAGE 3 3 F i g u r e 5 E f f e c t of the g r a z i n g assemblage, f e r t i l i z a t i o n , Diaptomus kenai and D i a c y c l o p s thomasi alone on n a u p l i a r s u r v i v a l (Curve f i t t i n g ) . The d i s t a n c e on the x - a x i s between developmental stages i s p r o p o r t i o n a l to the d u r a t i o n of the stages. — — N a u p l i i alone D i a c y c l o p s thomasi Diaptomus kenai -G r a z i n g assemblage F e r t i l i z e r 34 1 0- — I — N4 — i — C 1 C3 DEVELOPMENTAL STAGE 3 5 F i g u r e 6 E f f e c t of the s u c c e s s i v e a d d i t i o n of m o r t a l i t y agents on n a u p l i a r s u r v i v a l ( I n t e g r a t i o n ) . The d i s t a n c e on the x - a x i s between developmental stages i s p r o p o r t i o n a l to the d u r a t i o n of the stages. I^ M^ N a u p l i i alone D i a c y c l o p s thomasi * D i a c y c l o p s thomasi and g r a z e r s > D i a c y c l o p s thomasi, g r a z e r s and Diaptomus kenai DEVELOPMENTAL STAGE 37 F i g u r e 7 E f f e c t of t h e s u c c e s s i v e a d d i t i o n of m o r t a l i t y a gents on n a u p l i a r s u r v i v a l (Curve f i t t i n g ) . The d i s t a n c e on the x - a x i s between d e v e l o p m e n t a l s t a g e s i s p r o p o r t i o n a l t o t h e d u r a t i o n of the s t a g e s . N a u p l i i a l o n e D i a c y c l o p s thomasi D i a c y c l o p s thomasi and g r a z e r s D i a c y c l o p s t h o m a s i , g r a z e r s and Diaptomus k e n a i DEVELOPMENTAL STAGE 39 f e r t i l i z e r i s examined over a l l stages up to the t h i r d copepodite stage, the e f f e c t of f e r t i l i z e r a d d i t i o n s becomes more p o s i t i v e . The most c o n s i s t e n t negative e f f e c t appeared to be that of the g r a z i n g assemblage. Both methods of a n a l y s i s showed the -same p a t t e r n and e f f e c t s d i d not change over the developmental stages. The curve f i t t e d r e s u l t s gave a c o n s i s t e n t l a r g e negative e f f e c t while the i n t e g r a t e d r e s u l t s showed more v a r i a t i o n both i n magnitude and d i r e c t i o n of e f f e c t . D. kenai a l s o showed a c o n s i s t e n t negative net e f f e c t i n both methods although t h i s e f f e c t seemed to i n c r e a s e i n the copepodite stages of the c u r v e - f i t t e d r e s u l t s . D i a c y c l o p s showed i n c o n s i s t e n t e f f e c t s i n the two methods of a n a l y s i s . The c u r v e - f i t t e d r e s u l t s showed a s l i g h t p o s i t i v e e f f e c t over a l l developmental stages, whereas the i n t e g r a t e d r e s u l t s showed a strong negative e f f e c t on a l l developmental stages. The strong negative e f f e c t of c a n n i b a l i s m i n the i n t e g r a t e d r e s u l t s may l a r g e l y be due to the underestimation of s u r v i v a l which i s known to occur with t h i s method. The s l i g h t p o s i t i v e e f f e c t i n the c u r v e - f i t t e d r e s u l t s i s somewhat suspect. Because i t i s known that D. thomasi are c a n n i b a l i s t i c , one would expect a decrease i n s u r v i v o r s h i p i n the e n c l o s u r e with only D. thomasi a d u l t s and n a u p l i i s i n c e there would be nothing e l s e f o r the a d u l t s to eat. A p o s i t i v e e f f e c t of a d u l t D. thomasi on treatments c o n t a i n i n g g r a z e r s c o u l d be e x p l a i n e d i f the a d u l t s reduced competition f o r j u v e n i l e s by p r e y i n g on competitors. However McQueen ( 1 9 6 9 ) found that the p r e d a t i o n r a t e of a d u l t D. thomasi on n a u p l i i was not a f f e c t e d by the presence of 40 F i g u r e 8 Summary of the e f f e c t of a s i n g l e f a c t o r on s u r v i v a l to the end of the n a u p l i a r stages ( I n t e g r a t i o n ) . T h i s t a b l e shows a summary of comparisons between treatments which only d i f f e r by one f a c t o r . i n d i c a t e s a p o s i t i v e e f f e c t l e s s than the sample var i a n c e . i n d i c a t e s .a p o s i t i v e e f f e c t g r e a t e r than the sample v a r i a n c e . i n d i c a t e s a negative e f f e c t l e s s than the sample v a r i a n c e . i n d i c a t e s a negative e f f e c t g r e a t e r than the sample v a r i a n c e . i n d i c a t e s no e f f e c t An example of the i n t e r p r e t a t i o n of t h i s t a b l e i s as f o l l o w s : The upper l e f t hand box shows that f e r t i l i z e r causes a n e g a t i v e e f f e c t l e s s than the sample v a r i a n c e when added to an e n c l o s u r e c o n t a i n i n g n a u p l i i o n l y . A summary of the net e f f e c t of each f a c t o r i s p r o v i d e d at the bottom of each column. S o l i d arrow are c o n s i d e r e d to be worth twice as much as hollow arrows. ? F e r t i l i z e r Diaptomus kenai D D i a c y c l o p s thomasi G Grazers NB Arrows which r e s u l t from comparisons i n v o l v i n g treatment KG (eg. column 4, row 2) must be t r e a t e d w ith c a u t i o n as u n u s u a l l y low numbers of n a u p l i i reduced the p r e c i s i o n of the c a l c u l a t e d s u r v i v o r s h i p i n t h i s treatment. A D D E D T R E A T M E N T F K D G n a u p l i i o n l y o F K D G F K o F D F G K D K G D G F K D F K G F D G K D G 6 , ° 5 ° 8 o 4 o 42 F i g u r e 9 Summary of the e f f e c t of a s i n g l e f a c t o r on s u r v i v a l t o the end of the n a u p l i a r stages (Curve f i t t i n g ) . T h i s t a b l e shows a summary of comparisons between treatments which only d i f f e r by one f a c t o r . i n d i c a t e s a p o s i t i v e e f f e c t l e s s than the sample var i a n c e . i n d i c a t e s a p o s i t i v e e f f e c t g r e a t e r than the sample v a r i a n c e . ^^.^ i n d i c a t e s a negative e f f e c t l e s s than the sample v a r i a n c e . , i n d i c a t e s a negative e f f e c t g r e a t e r than the sample v a r i a n c e . • i n d i c a t e s no e f f e c t An example of the i n t e r p r e t a t i o n of t h i s t a b l e i s as f o l l o w s : The upper l e f t hand box shows that f e r t i l i z e r causes a negative e f f e c t l e s s than the sample v a r i a n c e when added to an enclosure c o n t a i n i n g n a u p l i i o n l y . A summary of the net e f f e c t of each f a c t o r i s p r o v i d e d at the bottom of each column. S o l i d arrow are c o n s i d e r e d to.be worth twice as much as hollow arrows. F F e r t i l i z e r K Diaptomus kenai D D i a c y c l o p s thomasi G Grazers A D D E D T R E A T M E N T z LD z> r-< UJ rr LU UJ CO < m F K D G n a u p l i i o n l y O F K — • D -G — F K O — F D F G K D K G D G — — F K D F K G -F D G K D G 3 o 7 <3> 4 >^ 1 1 o 44 F i g u r e 10 Summary of the e f f e c t of a s i n g l e f a c t o r on s u r v i v a l to the beginning of the t h i r d copepodite stage ( I n t e g r a t i o n ) . T h i s t a b l e shows a summary of comparisons between treatments which only d i f f e r by one f a c t o r . i n d i c a t e s a p o s i t i v e e f f e c t l e s s . t h a n the sample v a r i a n c e . i n d i c a t e s a p o s i t i v e e f f e c t g r e a t e r than the sample v a r i a n c e . i n d i c a t e s a n e g a t i v e e f f e c t l e s s than the sample v a r i a n c e . i n d i c a t e s a n e g a t i v e e f f e c t g r e a t e r than the sample v a r i a n c e . i n d i c a t e s no e f f e c t An example of the i n t e r p r e t a t i o n of t h i s t a b l e i s as f o l l o w s : The upper l e f t hand box shows that f e r t i l i z e r causes a negative e f f e c t l e s s than the sample v a r i a n c e when added to an e n c l o s u r e c o n t a i n i n g n a u p l i i ' o n l y . A summary of the net e f f e c t of each f a c t o r i s p r o v i d e d at the bottom of each column. S o l i d arrow are c o n s i d e r e d to be worth twice as much as hollow arrows. F F e r t i l i z e r K Diaptomus kenai D D i a c y c l o p s thomasi G Grazers NB Arrows which r e s u l t from comparisons i n v o l v i n g treatment KG (eg. column 4, row 2) must be t r e a t e d w ith c a u t i o n as u n u s u a l l y low numbers of n a u p l i i reduced the p r e c i s i o n of the c a l c u l a t e d s u r v i v o r s h i p i n t h i s treatment. A D D E D T R E A T M E N T UJ < LU or. UJ LU CO < CO F K D G n a u p l i i o n l y F — K D G F K — F D F G K D i K G D G F K D — F K G F D G K D G 7 o 5 ° 11 ° 4 ^ 46 F i g u r e 11 Summary of the e f f e c t of a s i n g l e f a c t o r on s u r v i v a l t o the beginning of the t h i r d copepodite stage (Curve-f i t t i n g ) . T h i s t a b l e shows a summary of comparisons between treatments which only d i f f e r by one f a c t o r . i n d i c a t e s a p o s i t i v e e f f e c t l e s s than the sample v a r i a n c e . i n d i c a t e s a p o s i t i v e e f f e c t g r e a t e r than the sample v a r i a n c e . r—, i n d i c a t e s a negative e f f e c t l e s s than the sample v a r i a n c e . ^mp- i n d i c a t e s a negative e f f e c t g r e a t e r than the sample v a r i a n c e . i n d i c a t e s no e f f e c t An example of the i n t e r p r e t a t i o n of t h i s t a b l e i s as f o l l o w s : The upper l e f t hand box shows that f e r t i l i z e r causes-a negative e f f e c t l e s s than the sample v a r i a n c e when added t o an e n c l o s u r e c o n t a i n i n g n a u p l i i o n l y . A summary of the net e f f e c t of each f a c t o r i s p r o v i d e d at the bottom of each column. S o l i d arrow are c o n s i d e r e d to. be worth twice as much as hollow arrows. F F e r t i l i z e r K Diaptomus kenai D D i a c y c l o p s thomasi G Grazers A D D E D T R E A T M E N T F K D G n a u p l i i o n l y o F K — D G — — F K <> F D F G K D K G D G F K D F K G F D G K D G 4 >^ 1 0 <z> 3 o 14 o 48 a l t e r n a t e prey. An i n d i r e c t b e n e f i t due to reduced competition a l s o does not e x p l a i n the i n c r e a s e i n s u r v i v a l when a d u l t D. thomasi were added to treatments c o n t a i n i n g D. kenai or f e r t i l i z e r . These o b s e r v a t i o n s i n d i c a t e an e r r o r i n the model l i n g of recruitment i n e n c l o s u r e s c o n t a i n i n g a d u l t D. thomasi . T h i s c o n c l u s i o n i s f u r t h e r supported by o b s e r v a t i o n s made while c a l i b r a t i n g the model. When i n i t i a l c o n d i t i o n s were a d j u s t e d to minimize negative m o r t a l i t y (Appendix B), r e s u l t s remained r e l a t i v e l y c o n s i s t e n t when examined with one f a c t o r h e l d constant ( i . e . examining the r e s u l t s from a l l e n c l o s u r e s c o n t a i n i n g a d u l t D i a c y c l o p s as shown in F i g u r e 19c). However D i a c y c l o p s r e s u l t s as a group seemed to s h i f t down as negative m o r t a l i t y was minimized. These d i f f i c u l t i e s obscure the importance of c a n n i b a l i s m in i n f l u e n c i n g the j u v e n i l e s u r v i v a l of D. thomasi . However i t i s probable that i t f a l l s somewhere between the extremes shown i n the r e s u l t s from i n t e g r a t i o n and c u r v e - f i t t i n g . 49 D i s c u s s i o n In the experimental e n c l o s u r e s , i t appeared that the g r a z i n g assemblage and D. kenai had a l a r g e r and more c o n s i s t e n t e f f e c t on j u v e n i l e s u r v i v a l than other f a c t o r s although d i f f i c u l t i e s i n e s t i m a t i n g s u r v i v o r s h i p i n e n c l o s u r e s c o n t a i n i n g a d u l t D. thomasi make i t s s t a t u s u n c e r t a i n . There i s however an i n d i c a t i o n that t h i s s p e c i e s may a l s o s u b s t a n t i a l l y a f f e c t s u r v i v a l . D. thomasi i s known to be c a r n i v o r o u s and c a n n i b a l i s t i c and to prey p r e f e r e n t i a l l y on n a u p l i i (McQueen 1969). Therefore i t i s probable that the e f f e c t of a d u l t D. thomasi on n a u p l i a r s u r v i v a l was due to c a n n i b a l i s m . D. kenai i s known to be a f i l t e r feeder and capable of feeding on small food p a r t i c l e s (Buckingham 1978, Chapman 1982, Krause unpubl. " d a t a ) . However i t seems u n l i k e l y that i t s e f f e c t on s u r v i v o r s h i p was due e n t i r e l y to competition s i n c e i t s d e n s i t y and biomass were so much lower than that of the g r a z i n g assemblage, which had o v e r a l l a s i m i l a r e f f e c t . T h i s o b s e r v a t i o n suggests that p r e d a t i o n may have been o c c u r r i n g . T h i s p r e d i c t i o n was l a t e r confirmed (Chapter 2) and a p r e d a t i o n r a t e of 20% of the n a u p l i i per predator per l i t r e per day was found. I t t h e r e f o r e seems l i k e l y that the main e f f e c t of D. kenai on n a u p l i a r s u r v i v a l was due to p r e d a t i o n on n a u p l i i . The i n t e r p r e t a t i o n of r e s u l t s r e l a t i n g to food l i m i t a t i o n i s more d i f f i c u l t . The g r a z i n g assemblage seemed to decrease s u r v i v o r s h i p and f e r t i l i z e r appeared in g e n e r a l to i n c r e a s e i t . However f e r t i l i z e r d i d not show c o n s i s t e n t e f f e c t s w i t h i n e i t h e r method of a n a l y s i s . In e n c l o s u r e s c o n t a i n i n g n a u p l i i alone, f e r t i l i z e r d i d not appear to have any e f f e c t . T h i s r e s u l t c o u l d 50 be e x p l a i n e d i f n a u p l i i ate the s m a l l e s t s i z e s of phytoplankton which were not s t i m u l a t e d by the a p p l i c a t i o n of f e r t i l i z e r . Comparison of small phytoplankton between f e r t i l i z e d and u n f e r t i l i z e d e n c l o s u r e s c o n t a i n i n g n a u p l i i alone showed that there was no s i g n i f i c a n t d i f f e r e n c e between them, i n d i c a t i n g that the lack of response on the part of the small phytoplankton was not due to the l a r g e r grazers consuming any i n c r e a s e d p r o d u c t i o n . The g r a z i n g assemblage showed a c o n s i s t e n t net r e d u c t i o n i n n a u p l i a r s u r v i v a l . The probable e x p l a n a t i o n f o r t h i s i s competition f o r food from the l a r g e r cladocerans and copepods. These grazers d i d not show any d e t e c t a b l e e f f e c t on the s m a l l e r a l g a l s i z e c l a s s e s which n a u p l i i are presumed to eat, d e s p i t e feeding s t u d i e s which have shown that these organisms are capable of e a t i n g the smaller s i z e c l a s s e s (Buckingham 1978, Krause unpubl. d a t a ) . Another p o s s i b l e e x p l a n a t i o n f o r reduced s u r v i v a l i n the presence of grazers i s c o m p e t i t i o n from other n a u p l i i . E n c l o s u r e s with grazers a l s o c o n t a i n e d D. oregonensis n a u p l i i . When f e r t i l i z e r was present in e n c l o s u r e s c o n t a i n i n g grazers or a d u l t D. thomasi, more n a u p l i i were a l s o p r e s e n t . If food resources were not i n c r e a s e d (as i n d i c a t e d by examination of the phytoplankton s i z e d a t a ) , the s u r v i v a l of n a u p l i i i n f e r t i l i z e d c o n d i t i o n s would a c t u a l l y decrease due to an i n c r e a s e i n the number of n a u p l i i . T h i s appeared to be the case only in about one q u a r t e r of the e n c l o s u r e s . N a u p l i a r competition would a l s o imply a r e d u c t i o n in food supply, which was not noted. Changes i n the smaller a l g a l c l a s s e s have o c c u r r e d but been undetectable with my phytoplankton sampling program. I t i s t h e r e f o r e d i f f i c u l t to 51 draw any s p e c i f i c c o n c l u s i o n about the source of c o m p e t i t i o n however i t appears that competition does have s u b s t a n t i a l e f f e c t on n a u p l i a r s u r v i v a l . In summary, i t appeared that i n the experimental e n c l o s u r e s i n t e r s p e c i f i c p r e d a t i o n from D. kenai and competition e i t h e r from l a r g e cladoceran g r a z e r s and c a l a n o i d copepods or from other n a u p l i i had the l a r g e s t e f f e c t s on n a u p l i a r s u r v i v a l . Although d i f f i c u l t i e s e x i s t e d i n a s s e s s i n g i t s importance, i n t r a s p e c i f i c p r e d a t i o n by D. thomasi a l s o seemed to have a s u b s t a n t i a l e f f e c t . I t i s not p o s s i b l e to comment on the importance of a shortage of food from non-competitive sources s i n c e the smaller phytoplankton, which n a u p l i i probably eat, were not s t i m u l a t e d by the a p p l i c a t i o n of f e r t i l i z e r . P r e d a t i o n has p r e v i o u s l y been suggested to play a major r o l e i n n a u p l i a r m o r t a l i t y . Confer and Cooley (1977) found that p r e d a t i o n by omnivorous zooplankton c o u l d account f o r most of the n a u p l i a r m o r t a l i t y of Diaptomus minutus. McQueen (1969) suggested that up to 30% of the c a l a n o i d n a u p l i a r p r o d u c t i o n i n Marion Lake c o u l d be eaten by D. thomasi . Landry (1978b) suggested that Labidocera t r i s p i n o s a c o u l d i n f l i c t p o p u l a t i o n m o r t a l i t i e s of 54%, 18% and 15% on Calanus pac i f i c u s , Paracalanus parvus and A c a r t i a tonsa, r e s p e c t i v e l y . Competition has not been p r e v i o u s l y suggested as a major cause of n a u p l i a r m o r t a l i t y , however there has been some i n d i c a t i o n t h at competition may a f f e c t s u r v i v a l . O l e n i c k (1982) working i n Eunice Lake found that competition from Diaptomus  t y r r e l l i seemed to a f f e c t p r i m a r i l y the n a u p l i a r stages of Diaptomus leptopus. N e i l l ( i n p r e s s ) , a l s o working i n the UBC 52 Research F o r e s t , has found that competition from Daphnia rosea can l i m i t p o p u l a t i o n s of r o t i f e r s suggesting that competition with cladocerans may indeed have a s i g n i f i c a n t e f f e c t on smaller organisms i n these l a k e s . In e x t r a p o l a t i n g the r e s u l t s from the e n c l o s u r e s to P l a c i d Lake both the d i f f i c u l t i e s experienced i n s e t t i n g up the en c l o s u r e s and the seasonal dynamics of the P l a c i d Lake community must be c o n s i d e r e d . The c o n d i t i o n i n the en c l o s u r e s ( i . e . low g r a z e r s , high D. kenai and low a d u l t D. thomasi) probably r e f l e c t f a i r l y a c c u r a t e l y c o n d i t i o n s which occur i n e a r l y May, near the beginning of the r e p r o d u c t i v e p e r i o d of D. thomasi . Although d e n s i t i e s of competitors changed g r e a t l y d u r i n g the experimental p e r i o d , the m a j o r i t y of n a u p l i i had passed i n t o the copepodite stage by the time the l a r g e i n c r e a s e s o c c u r r e d and these changes would have had l i t t l e e f f e c t on n a u p l i a r s u r v i v a l . S u r v i v o r s h i p of the young copepodites was not g r e a t l y a f f e c t e d by the change e i t h e r s i n c e they are probably not as v u l n e r a b l e to competition and they s t a r t to become c a r n i v o r o u s . As the season i n the lake, proceeds, D. kenai decrease i n number and t h e r e f o r e i n importance as a f a c t o r i n D. thomasi n a u p l i a r s u r v i v a l . Both g r a z i n g biomass and the number of n a u p l i i i n c r e a s e and t h e r e f o r e the importance of competition to n a u p l i a r s u r v i v a l would i n c r e a s e , presuming that a l g a l p r o d u c t i v i t y does not in c r e a s e p r o p o r t i o n a l l y . The importance of both i n t e r s p e c i f i c p r e d a t i o n and competition depend to some extent on year to year c l i m a t i c v a r i a t i o n . During c o o l e r years when n a u p l i i develop more slowly and D. kenai remain i n the upper water column f o r a longer p e r i o d of 53 time, D. kenai would assume more importance as a m o r t a l i t y agent. During warm s p r i n g s when cladocerans are l i k e l y to r i s e e a r l y i n the year and i n c r e a s e r a p i d l y , competition i s l i k e l y to be more important. Cannibalism would be most important at the beginning and p a r t i c u l a r l y at the end of the r e p r o d u c t i v e p e r i o d when the r a t i o of o l d e r copepodites to n a u p l i i i s the h i g h e s t . However i n t r a s p e c i f i c p r e d a t i o n would continue to have an e f f e c t throughout the r e p r o d u c t i v e p e r i o d . There are other p o t e n t i a l sources of m o r t a l i t y which are present i n P l a c i d Lake but were not i n c l u d e d i n the enclosure experiments. Both Chaoborus and Polyphemus are known to be p r e d a t o r s . Larger Chaoborus t r i v i t t a t u s and C. americanus have been found to be capable of c o n t r o l l i n g community s t r u c t u r e (Lynch 1979, N e i l l and Peacock 1980, N e i l l . 1 9 8 1 ) but do not prey h e a v i l y on n a u p l i i (Fedorenko 1975). Smaller Chaoborus  f l a v i c a n s , which i s the s p e c i e s r e s i d e n t i n P l a c i d Lake, only r i s e s i n the water column at n i g h t . Although t h i s s p e c i e s probably eats smaller organisms, i t would probably s t i l l have a small e f f e c t on n a u p l i i s i n c e i t would only o v e r l a p s p a t i a l l y with the n a u p l i i f o r a short p e r i o d of time each day. N a u p l i i are a l s o extremely slow swimmers ( G e r r i t s o n 1979) and are t h e r e f o r e r e l a t i v e l y i n v u l n e r a b l e to ambush pre d a t o r s due to low encounter r a t e s . The i n c l u s i o n of Chaoborus in the experiment would probably have a f f e c t e d n a u p l i a r s u r v i v a l i n d i r e c t l y by d e c r e a s i n g the abundance of p r e d a t o r s and c o m p e t i t o r s . In a d d i t i o n , s i n c e f i s h prey h e a v i l y on Chaoborus, i t i s o f t e n reduced i n importance i n lakes with f i s h . Polyphemus i s mainly l i t t o r a l i n P l a c i d Lake and would be u n l i k e l y to g r e a t l y a f f e c t 54 n a u p l i a r s u r v i v a l of the l i m n e t i c D. thomasi . From the p a t t e r n of seasonal dynamics observed i n P l a c i d Lake, i t would appear that c a n n i b a l i s m i s the most important m o r t a l i t y agent over the season. T h e r e f o r e , i n the P l a c i d Lake community, i t would - seem that D. thomasi i s l i m i t e d mostly by c a n n i b a l i s m although p r e d a t i o n by D. kenai e a r l y i n the season c o u l d have an e f f e c t . Competition from e i t h e r l a r g e r g r a z e r s or other n a u p l i i 'could have a s u b s t a n t i a l e f f e c t l a t e r i n the season. Cannibalism has been p r e v i o u s l y i m p l i c a t e d as a major m o r t a l i t y agent i n a q u a t i c ecosystems (McQueen 1969, Landry 1978a). However there i s some suggestion that the p o p u l a t i o n of P l a c i d Lake may e x i s t at a l e v e l lower than c o u l d be accounted f o r by c a n n i b a l i s m alone. McQueen (1969) found that 25-30% of the y e a r l y p r o d u c t i o n of D. thomasi c o u l d be c a n n i b a l i z e d by the a d u l t s . However Peacock (1981) found that m o r t a l i t y of n a u p l i i i n P l a c i d Lake was approximately 75-80%. The r e s u l t s from my e n c l o s u r e experiments, although c a l c u l a t e d s u r v i v a l r a t e s were much lower, i n d i c a t e d that c o m p e t i t i o n from other g r a z e r s and p r e d a t i o n by D. kenai c o u l d s i g n i f i c a n t l y a f f e c t s u r v i v a l and these a d d i t i o n a l f a c t o r s may account f o r the d i s c r e p a n c y i n these r e p o r t e d m o r t a l i t y r a t e s . 55 CHAPTER 2 R e s u l t s from enclosure experiments (Chapter 1) i n d i c a t e d that D. kenai c o u l d s u b s t a n t i a l l y i n f l u e n c e the s u r v i v a l of D. thomasi n a u p l i i . The proposed mechanism f o r t h i s i n f l u e n c e was p r e d a t i o n . Because D. kenai has p r e v i o u s l y been assumed to be t o t a l l y h e rbivorous, c o n f i r m a t i o n of i t s predatory a b i l i t y was r e q u i r e d to s u b s t a n t i a t e t h i s h y p o t h e s i s . M a t e r i a l s and methods Experimental methods Experiments on the p r e d a t i o n of D. kenai on n a u p l i i were conducted both i n October, 1982 and May, 1.983. A l l animals f o r the experiments were obtained from the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t . In October, the D. kenai used were c o l l e c t e d from Gwendoline Lake. N a u p l i i of D. thomasi were c o l l e c t e d from Eunice Lake and water f o r the experiments was a l s o taken from Eunice Lake. In May, a l l the animals and water were c o l l e c t e d from P l a c i d Lake. A l l f eeding experiments were conducted i n a c o n t r o l l e d environment chamber set to 8 °C with a 16:8 h l i g h t : d a r k c y c l e to simulate s p r i n g l i g h t c o n d i t i o n s unless otherwise s p e c i f i e d . On the day of c o l l e c t i o n , 3.5 1 lake water was f i l t e r e d through a 54 jum si e v e i n t o i d e n t i c a l 4 1 purple p l a s t i c c o n t a i n e r s . D. kenai were p i p e t t e d i n t o the experimental c o n t a i n e r s immediately a f t e r f i l t e r i n g was completed and allowed to e q u i l i b r a t e f o r 24 h. On the f o l l o w i n g day n a u p l i i were i n d i v i d u a l l y p i p e t t e d i n t o small f o r m a l i n - f r e e j a r s of f i l t e r e d 56 lake water. C y c l o p o i d n a u p l i i were always used unless otherwise s p e c i f i e d . The contents of a l l the j a r s were then added to the experimental c o n t a i n e r s and the j a r s r i n s e d with f i l t e r e d lake water w i t h i n a t o t a l of about 15 minutes. The c o n t a i n e r s were then l e f t 24 ± 2 hours . At the end of t h i s time a l l the D. kenai were removed i n t o small j a r s w i t h i n 15 minutes using a 406 Min s i e v e . The water from each experimental c o n t a i n e r was f i l t e r e d through a 54 ixm s i e v e u s i n g a siphon and the remaining animals c o l l e c t e d , added to the a p p r o p r i a t e j a r c o n t a i n i n g D. kenai and then immediately f i x e d u sing concentrated f o r m a l i n . These samples were l a t e r counted under 50 X on a b i n o c u l a r d i s s e c t i n g microscope to determine the remaining number of n a u p l i i . The gut contents of some D. kenai were examined by d i s s e c t i n g out the gut i n a drop of water under a d i s s e c t i n g microscope. The gut was squashed under a c o v e r s l i p and the contents examined. Experimental design Since the experiments were designed to provide i n f o r m a t i o n about p o s s i b l e i n t e r a c t i o n s i n lake c o n d i t i o n s , a l l experiments were conducted with d e n s i t i e s of animals which were n a t u r a l l y found at lake c o n d i t i o n s . A d u r a t i o n of 24 h was chosen s i n c e a p r e l i m i n a r y s e r i e s of experiments i n d i c a t e d that a s i g n i f i c a n t decrease d i d occur when D. kenai were added to experimental c o n t a i n e r s but that the d u r a t i o n of the experiment (24-96 h) d i d not have a s i g n i f i c a n t e f f e c t on the number eaten. T h i s lack of e f f e c t i s probably due to keeping D. kenai i n r e l a t i v e l y small c o n t a i n e r s s i n c e there has been some evidence that the feeding 57 response may change with h o l d i n g time f o r animals from the Research F o r e s t lakes (Buckingham, 1978). No time s h o r t e r than 24 h was used s i n c e t h i s d u r a t i o n allowed the c o n t a i n e r s to go though one complete l i g h t c y c l e . Experiments were always s t a r t e d between 1600 and 1800. A set of experiments was conducted i n October, 1982 to determine the f u n c t i o n a l response of D. kenai . Prey d e n s i t i e s of 5, 10, 15, 25 and 50 n a u p l i i / c o n t a i n e r (125, 250, 375, 625, and 1250 per 100 1) were chosen. A d e n s i t y of 375/ 100 1 corresponds approximately to the n a u p l i a r d e n s i t y at the time the f i e l d experiments were set up. A d e n s i t y of 1250/100 1 corresponds to the hig h e s t d e n s i t y found i n the enclosure experiments. The predator d e n s i t y chosen was 1 per c o n t a i n e r (25/100 1) which was- the approximate d e n s i t y used i n the enclos u r e experiments. Each treatment was r e p l i c a t e d 4 times. Four c o n t a i n e r s were set up c o n t a i n i n g 15 n a u p l i i only to serve as c o n t r o l s f o r background m o r t a l i t y and counting e r r o r . A f t e r I found that D. kenai d i d , i n f a c t , prey on n a u p l i i , I was concerned that they might t r a p t h e i r prey a g a i n s t a s u r f a c e . Neomysis have been observed to do t h i s (Johnston 1981, Johnston and Lasenby 1982) and D. kenai has been observed to repeat e d l y come up and h i t the s u r f a c e of the lake i n calm water ( N e i l l , p e r s . comm.). I f D. kenai trapped t h e i r prey a g a i n s t a s u r f a c e , t h e i r e f f e c t on the m o r t a l i t y of n a u p l i i i n small e n c l o s u r e s would be a r t i f i c i a l l y i n f l a t e d . In order to determine whether t r a p p i n g occurred, an experiment was de v i s e d using cages made of NITEX n e t t i n g ( F i g . 12). The cages were designed to e l i m i n a t e as much as p o s s i b l e the area a v a i l a b l e to 58 the D. kenai f o r entrapment by a l l o w i n g an escape area f o r n a u p l i i . These cages were c o n s t r u c t e d out of 47 1 nm mesh which was l a r g e enough to be e a s i l y permeable to n a u p l i i but small enough that D. kenai were unable to pass through. T h i s net s i z e was t e s t e d beforehand and I found that while D. kenai c o u l d not pass though, both D. thomasi and D. oregonensis a d u l t s c o u l d and t h e r e f o r e I assumed that the smaller n a u p l i i c o u l d a l s o . Although the cages were c o n s t r u c t e d to minimally decrease the volume a v a i l a b l e to the D. kenai , some r e d u c t i o n i n volume occur r e d (1-1.5 1). To c o n t r o l f o r t h i s decrease i n volume, s i m i l a r - s i z e d cages were c o n s t r u c t e d of 54 Mm mesh. N a u p l i i were added to the cages i n d e n s i t i e s such that at the beginning of the experiment, the average number a v a i l a b l e to D. kenai was the same i n cages of both l a r g e and small mesh, assuming an even d i s t r i b u t i o n of n a u p l i i . I t was a l s o p o s s i b l e that D. kenai trapped a g a i n s t a water ' s u r f a c e and not aga i n s t c o n t a i n e r s i d e s . In t h i s case enc l o s u r e experiments would not overestimate the impact of p r e d a t i o n . In order to t e s t t h i s h y p o t h e s i s , both l a r g e and small mesh cages were d i v i d e d i n t o two groups: one completely submerged and one where the s u r f a c e of the water was j u s t below the top of the cage. Twenty experimental c o n t a i n e r s were set up. F i v e c o n t a i n e r s had l a r g e mesh cages completely submerged, 5 c o n t a i n e r s had small mesh cages completely submerged, 5 c o n t a i n e r s had-large mesh cages with a water s u r f a c e and the remaining 5 had small mesh cages with a water s u r f a c e . In each of these 4 treatments, 1 c o n t a i n e r c o n t a i n e d no D. kenai and was used as a c o n t r o l . Twenty-five n a u p l i i were added to c o n t a i n e r s 5 9 with small mesh cages and 3 5 n a u p l i i were added to c o n t a i n e r s with l a r g e mesh cages to c o r r e c t f o r the d i f f e r e n c e i n volume a v a i l a b l e to n a u p l i i the two types of cages. A l l animals were added to the i n s i d e of the cages r e g a r d l e s s of treatment. A comparison was done between the feeding r a t e of D. kenai on c a l a n o i d and c y c l o p o i d n a u p l i i . In t h i s experiment 16 experimental c o n t a i n e r s were set up: 8 with c a l a n o i d n a u p l i i and 8 with c y c l o p o i d n a u p l i i . D. kenai were not added to 3 c o n t a i n e r s of each. type. Twenty-five n a u p l i i were added to each c o n t a i n e r . A comparison was a l s o done on the e f f e c t of temperature on the p r e d a t i o n r a t e of D. kenai . T h i s experiment was set up i n a s i m i l a r manner to the pr e v i o u s one with only c y c l o p o i d n a u p l i i being used. One group was h e l d at 8 °C while the other group was h e l d at 16 °C. 60 F i g u r e 12 Design of experimental cages. V e I c r o I . _ l 1 8 c m 62 R e s u l t s The determination of the f u n c t i o n a l response of D. kenai showed a h i g h l y s i g n i f i c a n t l i n e a r r e g r e s s i o n (F=93.678, p < .01) with an equation of Y= (0.198 ± 0.062(SE))X - 0.514. An a n a l y s i s of v a r i a n c e showed that the l i n e a r r e g r e s s i o n accounted for 72.8% of the t o t a l v a r i a t i o n . The c a l c u l a t e d r e g r e s s i o n equation i s shown i n F i g u r e 13. There appeared to be no l o s s e s in c o n t r o l s a s s o c i a t e d with n a u p l i a r m o r t a l i t y or counting e r r o r . The l i n e a r response of prey disappearance to prey d e n s i t y suggested that i t was i n f a c t p r e d a t i o n that caused a d e c l i n e i n the prey number. To c o n f i r m t h i s , the gut contents of ca. 25 D. kenai were examined and animal remains were found i n 8% of the guts although the m a j o r i t y of the gut contents were phytoplankton. When the r e s u l t s of the experiments designed to examine the e f f e c t of s u r f a c e s on p r e d a t i o n r a t e were examined, there was a l o s s i n c o n t r o l s a s s o c i a t e d with n a u p l i a r m o r t a l i t y , counting and h a n d l i n g e r r o r . In order to d i s t i n g u i s h whether there were any a d d i t i o n a l l o s s e s a s s o c i a t e d with p r e d a t i o n , an a n a l y s i s of v a r i a n c e was performed on a p p r o p r i a t e s e t s of c o n t r o l s and experimental treatments. If these proved, s i g n i f i c a n t , the r e s u l t s of each experimental c o n t a i n e r were s u b t r a c t e d from the mean of the a p p r o p r i a t e c o n t r o l s and any f u r t h e r a n a l y s i s was performed on the r e s u l t a n t data ( number e a t e n ) . These r e s u l t s are shown i n Table VI. An a n a l y s i s of v a r i a n c e showed no s i g n i f i c a n t d i f f e r e n c e i n p r e d a t i o n between D. kenai contained i n small mesh cages and those c o n t a i n e d i n l a r g e mesh cages. 63 F i g u r e 13 F u n c t i o n a l response of Diaptomus kenai to n a u p l i a r d e n s i t i e s . Number Eaten ( / 4 I ) 6 5 T h i s r e s u l t g i v e s no i n d i c a t i o n that D. kenai use s u r f a c e s to t r a p n a u p l i i and thus the p r e d a t i o n e f f e c t observed i n the in  s i t u experiments was probably not an a r t i f a c t of e n c l o s u r e . A comparison of p r e d a t i o n r a t e s of caged D. kenai with those of uncaged D. kenai conducted at s i m i l a r d e n s i t i e s a l s o showed no s i g n i f i c a n t d i f f e r e n c e , suggesting that the r e d u c t i o n i n c o n t a i n e r volume d i d not a f f e c t the feeding r a t e s of D. kenai . Comparisons between p r e d a t i o n r a t e s on c y c l o p o i d and c a l a n o i d n a u p l i i and between d i f f e r e n t temperatures were made i n a s i m i l a r manner to those d e s c r i b e d above ( c o n t r o l s and treatments compared and treatments s u b t r a c t e d from mean of c o n t r o l s ) These r e s u l t s showed no s i g n i f i c a n t d i f f e r e n c e i n pr e d a t i o n r a t e s e i t h e r on c a l a n o i d vs. c y c l o p o i d n a u p l i i (F = .29, P < .75) or between d i f f e r e n t temperatures (F = 1.23, P < .50). A comparison of r e s u l t s obtained i n the s p r i n g to those obtained i n the f a l l a l s o showed no d i f f e r e n c e (F = 1.29, P < .50). A comparison was a l s o made between p r e d a t i o n r a t e s f o r females and those f o r males. To inc r e a s e sample s i z e and allow comparison between d i f f e r e n t prey d e n s i t i e s , these r e s u l t s are expressed as p r o p o r t i o n of n a u p l i i eaten per predator per day. Since the f u n c t i o n a l response was determined to be l i n e a r , t h i s procedure d i d not introduce b i a s . The mean p r o p o r t i o n of n a u p l i i consumed by females was 0.225 ± 0.105(SE, n=16) and the mean f o r males was 0.254 ± 0.137(SE, n=32). T h i s comparison showed no d i f f e r e n c e i n fe e d i n g r a t e s between males and females (F=.224, P > .75). T a b l e VI The e f f e c t of s u r f a c e s on D. kenai p r e d a t i o n on D i a c y c l o p s n a u p l i i . A cage wi 471 mesh r e p r e s e n t s the l e a s t s u r f a c e a v a i l a b l e f o r entrapment. Numbers r e p r e s e n t t number of n a u p l i i e a t e n i n 24 h. 47 1 f,m 471 j,m 54 m^ 54 »m no cage +water s u r f a c e +water s u r f a c e 15.5 6.5 9 9 2.7 8.5 6.5 5 2 7.7 10.5 4.5 5 9 8.7 5.5 6.5 4 5 2.7 10.7 Mean 8.0 6.0 6.0 6.3 6.5 6 7 D i s c u s s i o n D. kenai has been p r e v i o u s l y regarded as t o t a l l y h e r b i vorous i n the c o a s t a l montane lakes around Vancouver (Olenick 1982), and Krause ( i n prep.) has found that they are capable of e a t i n g a wide range of s i z e c l a s s e s of phytoplankton. A recent study of the mouthparts of D. kenai (Chapman 1982) from the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t showed that D. kenai has mouthparts t y p i c a l of those found f o r other Diaptomus s p e c i e s . Chapman suggested that D. kenai i s l i k e l y to be u s u a l l y a f i l t e r - f e e d e r and capable of i n g e s t i n g very small p a r t i c l e s due to the d e n s i t y of the l a b r a l and l a b i a l s etae. She a l s o suggested that D. kenai may be incapable of c a p t u r i n g very l a r g e a l g a l c e l l s and stronger bodied animals s i n c e i t l a c k s the robust" armature of the ma x i l i p e d s found i n D. shoshone but conceded they may be capable of c a p t u r i n g some l a r g e r a l g a l c e l l s and s o f t - b o d i e d animals. N e v e r t h e l e s s , there has been some i n d i c a t i o n that D. kenai may be omnivorous. G e r r i t s o n (1980) has suggested that D. kenai may be a predator although he provides no evidence f o r h i s c l a s s i f i c a t i o n and he does not l i s t i t as an omnivore. A s e r i e s of experiments by E. Krause (unpubl. data) showed that D. kenai seemed to produce extremely l a r g e m o r t a l i t y r a t e s of c a l a n o i d n a u p l i i over a r e l a t i v e l y short p e r i o d of time. T h i s study has confirmed these sug g e s t i o n s . T h e . f u n c t i o n a l response of D. kenai p r e d a t i o n was found to be l i n e a r with a slope of 0.20. These experiments were done at extremely low d e n s i t i e s when compared with other f e e d i n g s t u d i e s (Table VII) which may account f o r the l i n e a r response. However, s i n c e t h i s study was set up with d e n s i t i e s that were 68 found in the lake and in the e n c l o s u r e s , t h i s type of response probably occurs n a t u r a l l y . Other s t u d i e s have a l s o found a l i n e a r response when d e n s i t i e s were low. Peacock (1981) found a l i n e a r response f o r D. thomasi feeding on T. pr a s i n u s copepodites and the m a j o r i t y of feeding s t u d i e s on Mesocyclops  edax done by Jamieson (1980b) showed a l i n e a r response. Ambler and F r o s t (1974) found that at low d e n s i t i e s , the feeding response of the marine c a l a n o i d copepod Tortanus discaudatus d i d not d i f f e r s i g n i f i c a n t l y from a s t r a i g h t l i n e . The p r e d a t i o n r a t e found i n t h i s study was approximately 0.20 at lake d e n s i t i e s expressed as the p r o p o r t i o n of n a u p l i i eaten per predator per day. T h i s r a t e i s w e l l w i t h i n the fe e d i n g r a t e s recorded f o r other s t u d i e s . (See Table V I I ) . I t i s higher than p r e d a t i o n r a t e s recorded f o r c y c l o p o i d predators on n a u p l i i but c o n s i s t e n t with the lower end of the feeding r a t e s recorded f o r c a l a n o i d p r e d a t o r s . Most fee d i n g s t u d i e s with c a l a n o i d p r e d a t o r s on n a u p l i i have been done with marine zooplankton however the two p r e d a t i o n r a t e s recorded f o r freshwater c a l a n o i d copepods, E p i s c h u r a l a c u s t r i s and D. kenai , are c o n s i s t e n t with these v a l u e s . In t h i s study, there was no i n d i c a t i o n that temperature i n f l u e n c e d the p r e d a t i o n r a t e s on n a u p l i i . T h i s r e s u l t i s s u r p r i s i n g s i n c e Buckingham (1978) c o n s i d e r e d D. kenai a " c o l d water" species and found that t h e i r f i l t e r i n g r a t e s on seston d e c l i n e d as temperature i n c r e a s e d above 8 °C. Cal a n o i d and c y c l o p o i d n a u p l i i a l s o seemed to be taken at the same r a t e . Lonsdale et a l . ( 1979) found that A c a r t i a tonsa showed a much lower p r e d a t i o n r a t e on i t s own n a u p l i i while T a b l e VII A comparison of p r e d a t i o n r a t e s of c y c l o p o i d and c a l a n o i d copepods on copepod n a u p l i i . A l l p r e d a t i o n r a t e s are e x p r e s s e d as p r o p o r t i o n of n a u p l i i eaten per p r e d a t o r per day. Where the re s p o n s e was l i n e a r , the range of v a l u e s over which the r a t e was s t u d i e d i s shown. I f the response was n o n - l i n e a r , the r a t e and d e n s i t y shown are those of maximum s1 ope. P r e d a t o r A c a r t i a t o n s a Prey ( N a u p I i i ) A c a r t i a c l aus i i A. c 1 aus i-i A. t o n s a (N1-3) (N4-6) Prey Dens i t y (1-') 500 280 280 • i thona c o l c a r v a (N1-3) 280 (N4-6) 280 S c o t t o l a n a c a n a d e n s i s (N1-3) 280 (N4-6) 280 Predat i on Rate . 38 .06 .03 .09 . 17 16 12 Reference Landry, 1978a Lonsdale et a l . 1979 * D i a c y c l o p s  thomas i D i aptomus kena i D. thomasi D i a c y c l o p s thomas i 180 1-1.5 .05 . 20 McQueen 1969 t h i s study Epi s c h u r a  1 a c u s t r i s Mesocyc1 ops  edax T o r t a n u s d i s c a u d a t u s D i aptomus s i c i1 i s 60 c a l a n o i d n a u p l i i 6-50 Ca1 anus pac i f i cus N3 20 N5 50 . 28-.55 .06-.09 . 55 . 78 Confer and Blades 1975 * Jamieson 1980 Ambler and F r o s t 1974 * - r a t e s measured at one d e n s i t y o n l y 70 Jamieson (1980b) found that M. edax p r e f e r r e d c a l a n o i d to c y c l o p o i d n a u p l i i . The s i m i l a r i t y of p r e d a t i o n r a t e s at d i f f e r e n t temperatures and d i f f e r e n t prey types found i n t h i s study may be due to my i n a b i l i t y to d i s t i n g u i s h between treatments because of the l a r g e v a r i a b i l i t y i n the r e s u l t s . T h i s v a r i a b i l i t y i s common i n many s t u d i e s of crustacean feeding (Buckingham 1978, Landry 1978, Jamieson 1980b, Peacock 1981) and may r e f l e c t p h y s i o l o g i c a l d i f f e r e n c e s among i n d i v i d u a l animals. The m a j o r i t y of s t u d i e s on copepod p r e d a t i o n have found that the p r e d a t i o n r a t e i s higher on smaller animals (McQueen 1969, Confer 1971, Ambler and F r o s t 1974, Wong 1980, Peacock 1981). From the po i n t of view of the predator, the higher capture rate of small prey may be a r e s u l t of r e q u i r i n g more i n d i v i d u a l s to make up t h e i r d a i l y r a t i o n . From t h i s standpoint i t may be u s e f u l to look at p r e d a t i o n r a t e s as the p r o p o r t i o n of dry weight consumed. When viewed i n t h i s way, the l a r g e p r e d a t i o n r a t e s of c a l a n o i d copepods i n Table VII are e a s i l y e x p l a i n a b l e s i n c e most of the c a l a n o i d copepods l i s t e d there are l a r g e r than the c y c l o p o i d copepods. However when the dynamics of the prey p o p u l a t i o n are being examined, these c o n s i d e r a t i o n s are i n t e r e s t i n g but i r r e l e v a n t . From the prey's standpoint, i t makes very l i t t l e d i f f e r e n c e why the predator a c t s as i t does. The s o l e c o n s i d e r a t i o n i s that as an i n d i v i d u a l becomes l a r g e r , i t has a sm a l l e r chance of being eaten. 71 GENERAL DISCUSSION Food l i m i t a t i o n and p r e d a t i o n were c o n s i d e r e d to be p o t e n t i a l l y important i n a f f e c t i n g the n a u p l i a r s u r v i v a l of Di a c y c l o p s thomasi. In s i t u e n c l o s u r e experiments i n P l a c i d Lake using n a t u r a l d e n s i t i e s of zooplankton r e v e a l e d that food l i m i t a t i o n due to com p e t i t i o n , i n t e r s p e c i f i c p r e d a t i o n by D. kenai and i n t r a s p e c i f i c p r e d a t i o n by a d u l t D. thomasi had strong negative e f f e c t s on the s u r v i v o r s h i p of j u v e n i l e D. thomasi . E x t r a p o l a t i o n of these r e s u l t s to P l a c i d Lake, t a k i n g seasonal dynamics i n t o c o n s i d e r a t i o n , suggested that c a n n i b a l i s m was the major m o r t a l i t y agent over the whole season although both i n t e r s p e c i f i c p r e d a t i o n by D. kenai and competition from the g r a z i n g assemblage exerted a s u b s t a n t i a l e f f e c t at the beginning and end of the r e p r o d u c t i v e p e r i o d , r e s p e c t i v e l y . With these r e s u l t s , i t i s now p o s s i b l e to c o n s t r u c t a scenario' f o r the s h i f t i n community s t r u c t u r e i n Eunice Lake ( F i g 1). Cut t h r o a t t r o u t were int r o d u c e d i n t o Eunice Lake i n 1974 and 1975 (Hume 1978) and have s i n c e i n c r e a s e d to the p o i n t where they can now be c o n s i d e r e d to be at t h e i r " c a r r y i n g c a p a c i t y " ( J . Andrew, pers. comm.). Se v e r a l years a f t e r the t r o u t were introduced, D. kenai began to decrease u n t i l they are now r a r e in the l a k e . T h i s d e c l i n e i s probably due to t r o u t p r e d a t i o n s i n c e t r o u t are known to prey on D. kenai , p a r t i c u l a r l y d u r i n g the winter and s p r i n g (Hume 1978). The D. kenai p o p u l a t i o n would be p a r t i c u l a r l y v u l n e r a b l e to p r e d a t i o n s i n c e i t i s u n i v o l t i n e . In 1978, a f t e r D. kenai had d e c l i n e d f o r s e v e r a l years, D. thomasi began to appear i n r e g u l a r zooplankton samples 72 in c o n s i s t e n t but low numbers durin g the l a t e f a l l and e a r l y winter. Before t h e i r d e c l i n e , D. kenai i n Eunice Lake had occurred i n much higher d e n s i t i e s than in P l a c i d Lake and they remained i n the water column for a much longer p e r i o d of time. These o b s e r v a t i o n s , combined with the knowledge that D. kenai are capable of s u b s t a n t i a l l y i n f l u e n c i n g n a u p l i a r s u r v i v a l , suggest that D. kenai had l i m i t e d the p o p u l a t i o n of D. thomasi i n Eunice Lake to the p o i n t where i t was extremely rare i n r e g u l a r zooplankton samples. In 1979, upstream Gwendoline Lake was f e r t i l i z e d and t h i s r e s u l t e d i n an increase i n the biomass of Daphnia rosea i n Eunice Lake d u r i n g that summer. D. thomasi continued to i n c r e a s e i n number in Eunice Lake while D. kenai continued to d e c l i n e . The winter of 1980-1981 was warm and the lake never f r o z e . D. thomasi i n c r e a s e in number u n t i l i t dominated the community. D. kenai disappeared almost e n t i r e l y from the lake and the g r a z i n g assemblage i n c r e a s e d i n d e n s i t y l a t e i n the s p r i n g and was g r e a t l y reduced in number compared to previous y e a r s . One p o s s i b l e e x p l a n a t i o n f o r these events i s as f o l l o w s . The warm winter with open water allowed wind to mix the lake w e l l , r e l e a s i n g i n t o the water column n u t r i e n t s from the sediment. The l e v e l of n u t r i e n t s i n the sediment had probably been i n c r e a s e d by the f e r t i l i z a t i o n which had o c c u r r e d f i f t e e n months p r e v i o u s l y i n an upstream lake and t h e r e f o r e the n u t r i e n t s r e l e a s e d i n t o the water column were probably a l s o i n c r e a s e d . D. thomasi i s known to increase i t s c l u t c h s i z e and lengthen i t s r e p r o d u c t i v e p e r i o d i n response to f e r t i l i z a t i o n (Peacock 1981). Increased n a u p l i a r production combined with 73 i n c r e a s e d s u r v i v a l due to reduced i n t e r s p e c i f i c p r e d a t i o n from D. kenai and reduced competition from other g r a z e r s were probably r e s p o n s i b l e f o r the l a r g e i n c rease i n D. thomasi . C y c l o p o i d copepod pr e d a t o r s have been found to p r e f e r e n t i a l l y a t t a c k s m a l l e r prey (McQueen 1969, Brandl and Fernando 1975,1978, Jamieson 1980b). By the time the Cladocera began t h e i r s p r i n g i n c r e a s e , there may have been enough D i a c y c l o p s copepodites to have a s i g n i f i c a n t impact on the s m a l l e r neonate c l a d o c e r a n s . Although t h i s e f f e c t cannot be confirmed, the idea i s supported by the s h i f t i n the dominant c l a d o c e r a n from Daphnia rosea to the r e l a t i v e l y i n e d i b l e Holopedium gibberum . The combination of a warm winter and c o o l s p r i n g with i n c r e a s e d n u t r i e n t l e v e l s probably hastened what would have been a gradual i n c r e a s e i n D. thomasi i n the l a k e , although i t probably would have e q u i l i b r a t e d at a much lower l e v e l i f the s p r i n g had not been c o o l . The present c o n f i g u r a t i o n appears r e l a t i v e l y s t a b l e and should p e r s i s t . P r i o r to 1979, i t appeared that D. thomasi i n Eunice lake was l i m i t e d by p r e d a t i o n from D. kenai . Some p o s s i b l e evidence fo r p o p u l a t i o n l i m i t a t i o n by D. kenai has a l s o been found i n Lake F i n d l e y , Washington. Pederson and L i t t (1976), i n examining the congeneric occurrence of Diaptomus franc iscanus and D. kenai , found l a r g e year to year v a r i a t i o n i n the abundance of D. f r a n c i s c a n u s n a u p l i i but r e l a t i v e l y l i t t l e v a r i a t i o n i n the abundance of a d u l t s . They o f f e r e d no e x p l a n a t i o n f o r t h i s phenomenon but, a f t e r examining the l i f e c y c l e s of both organisms, concluded that they d i d not compete s i n c e they were temporally separated. However, examination of 74 t h e i r l i f e c y c l e s r e v e a l s that D. kenai a d u l t s and copepodites reach peak d e n s i t i e s d u r i n g the same p e r i o d of time as D. f r a n c i s c a n u s n a u p l i i are p r e s e n t . One e x p l a n a t i o n f o r the v a r i a t i o n i n the abundance of n a u p l i i i s l i m i t a t i o n by p r e d a t i o n from D. kenai . The c u r r e n t p o p u l a t i o n of D. thomasi i n Eunice Lake i s probably almost e n t i r e l y l i m i t e d by c a n n i b a l i s m and may have a c t u a l l y i n c r e a s e d i t s c a r r y i n g c a p a c i t y by the l a r g e i n c i d e n c e of c a n n i b a l i s m . Fox (1975) suggested that low food a v a i l a b i l i t y may i n c r e a s e r a t e s of c a n n i b a l i s m . P o l i s (1981) suggested that c a n n i b a l i s m can i n c r e a s e the p o p u l a t i o n c a r r y i n g c a p a c i t y by making use of immature animals to t r a n s f o r m food resources u n a v a i l a b l e to the a d u l t . The r e q u i r e d c o n d i t i o n s f o r t h i s to occur a r e : (1) immature animals feed on resources i n a c c e s s i b l e to the a d u l t s ; (2) a d u l t s feed on immature animals and thus i n d i r e c t l y i n c o r p o r a t e p r e v i o u s l y u n a v a i l a b l e resources and (3) food i s l i m i t i n g t o the a d u l t p o p u l a t i o n ( P o l i s 1981). These c o n d i t i o n s seem to h o l d i n Eunice Lake. McQueen (1969) found that a d u l t s were completely c a r n i v o r o u s while n a u p l i i are known to be h e r b i v o r o u s . Eunice Lake thus appears to have s h i f t e d from one "domain of a t t r a c t i o n " to another. In the former community c o n f i g u r a t i o n D. thomasi appeared to be l i m i t e d by i n t e r s p e c i f i c p r e d a t i o n by D. kenai . 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Paffenhofer, G.A. 1 9 7 1 . Grazing and ingestion rates of n a u p l i i , copepodids and adults of the marine planktonic copepod Calanus helgolandicus. Mar. B i o l . J J_: 2 6 6 - 2 9 8 . Parsons, T.R., R.J. LeBrasseur, J.D. Fulton and O.D. Kennedy. . 1 9 6 9 . Production studies in the s t r a i t of Georgia. 2 . Secondary production under the Fraser River plume, February to May, 1 9 6 7 . J. Exp. Mar. B i o l . Ecol. 3 : 3 9 - 5 0 . Peacock, A. 1 9 8 1 . Responses of two coexisting cyclopoid copepods to experimental manipulations of food and predators. PhD Thesis, University of B r i t i s h Columbia Pederson, G.L. and A.H. L i t t . 1 9 7 6 . an example of congeneric occurrence of Diaptomus species. Hydrobiologia 5_0: 2 5 5 -2 5 8 . P o l i s , G.A. 1 9 8 1 . The evolution and dynamics of i n t r a s p e c i f i c predation. Ann. Rev. Ecol. Syst. J _ 2 : 2 2 5 - 2 5 1 . 82 Poulet, S.A. 1977. Grazing of marine copepod developmental stages on n a t u r a l l y o c c u r r i n g p a r t i c l e s . J . F i s h . Res. Bd. Can. 34: 2381-2387. R i g l e r , F.H. and J.M. Cooley. 1974. The use of f i e l d data to d e r i v e p o p u l a t i o n s t a t i s t i c s of m u l t i v o l t i n e copepods. Limnol. Oceanogr. J_9: 636-655. S a r v a l a , J . 1979. E f f e c t of temperature on the d u r a t i o n of egg, na u p l i u s and copepodite development of some freshwater benthic copepods. Freshwater Biology 9: 515-535. S e i t z , A. 1979. On the c a l c u l a t i o n of b i r t h - r a t e s and death-r a t e s i n f l u c t u a t i n g p o p u l a t i o n s with continuous r e c r u i t m e n t . Oecologia 4J_: 343-360. Smyly, W.J.P. 1970. Observations on the r a t e of development, l o n g e v i t y and f e c u n d i t y of Acanthocyclops v i r i d i s ( J u r i n e ) (Copepoda:Calanoida) i n r e l a t i o n to type of prey. Crustaceana 18: 21-36. Southwood, T.R.E. 1966. E c o l o g i c a l methods. Methuen and Co. L t d . S p i n d l e r , K.D. 1971. I n v e s t i g a t i o n s on the i n f l u e n c e of e x t e r n a l f a c t o r s on the d u r a t i o n of the embryonic development and on the moulting rhythm of Cyclops v i e i n u s . Oecologia 7_: 342-355. S t r i c k l a n d , J.D.H. and T.R. Parsons. 1965. A manual of sea water a n a l y s i s , 2nd ed. B u l l . F i s h . Res. Bd. Can. 125 203pp. T a y l o r , B.L. and M. S l a t k i n . 1981. E s t i m a t i n g b i r t h and death r a t e s of zooplankton. Limnol. Oceanogr. 26^ : 143-158. Torke, B.C. 1974. An i l l u s t r a t e d guide to the i d e n t i f i c a t i o n of the p l a n k t o n i c Crustacea of Lake Michigan with notes on t h e i r ecology. S p e c i a l Report No. 17, Center f o r Great Lakes S t u d i e s , U n i v e r s i t y of Wisconsin P r e s s . V i j v e r b e r g , J . 1980. E f f e c t of temperature i n l a b o r a t o r y s t u d i e s on development and growth of Cladocera and Copepoda from Tjeukemeer, The Netherlands. Freshwater B i o l o g y J_0: 317 — 340. 83 Walker, B.H., D. Ludwig, C.S. H o l l i n g and R.M. Peterman. 1981. S t a b i l i t y of s e m i - a r i d savanna g r a z i n g systems. J . E c o l . 69: 473-498. Weglenska, T. 1971. The i n f l u e n c e of v a r i o u s c o n c e n t r a t i o n s of n a t u r a l food on the development, f e c u n d i t y and production of p l a n k t o n i c crustacean f i l t r a t o r s . E k o l . P o l . J_9: 427-473. Whitehorse, J.W. and B.G. Lewis. 1973. The e f f e c t of d i e t and d e n s i t y on development, s i z e and egg p r o d u c t i o n i n Cyclops  abyssorum Sars,1863. (Copepoda, Cyclopoida) Crustaceana 25: 225-236. Wiebe, P.H. and W.R. H o l l a n d . 1968. Plankton p a t c h i n e s s : e f f e c t s on repeated net tows. Limnol. Oceanogr. j_3: 315-321. Wong, C.K. 1981. Predatory f e e d i n g behaviour of E p i s c h u r a l a c u s t r i s (Copepoda:Calanoida) and prey defense. Can. J . F i s h . Aqat. S c i . 38: 275-279. Zaret, T.M. 1980. Predation and Freshwater Communities. Yale U n i v e r s i t y Press. New Haven Conn. 187 p. 8 4 APPENDIX A. SET UP OF ENCLOSURES Zooplankton The count data of the en c l o s u r e s were examined to co n f i r m that proper set-up of the experimental c o n d i t i o n s had been achieved. Comparisons were done between those e n c l o s u r e s to which a p a r t i c u l a r treatment had been added (eg. D. thomasi ) and those to which i t had not. Comparisons with the lake were made at the beginning of the experimental p e r i o d between the i n i t i a l average d e n s i t y of a l l e n c l o s u r e s to which the treatment in q u e s t i o n had been added and the s t o c k i n g d e n s i t y . The e f f e c t of enclosure was examined by comparing the behaviour of zooplankton p o p u l a t i o n s i n the upper 2 m of the lake with those in the encl o s u r e which c o n t a i n e d the same c o n d i t i o n s as those of the lake ( i . e . f e r t i l i z e r absent and D. thomasi , D. kenai and the g r a z i n g assemblage p r e s e n t ) . T h i s comparison was made at the middle and end of the experimental p e r i o d . A l l comparisons were made using n u m e r i c a l , d e n s i t i e s s i n c e e n c l o s u r e samples were too small to allow an acc u r a t e measurement of len g t h f o r biomass c a l c u l a t i o n s . These comparisons are summarized i n Table V. D i a c y c l o p s thomasi Analyses of va r i a n c e of the d i f f e r e n c e i n numbers of D. thomasi a d u l t s between treatments c o n t a i n i n g D. thomasi and those without showed an extremely s i g n i f i c a n t d i f f e r e n c e (P << .001 i n a l l but one case, P < .005 in the f i f t h sampling period) 85 that was maintained throughout the experimental p e r i o d . When a 2-way a n a l y s i s of v a r i a n c e was done using treatment and time as the 2 f a c t o r s , the r e s u l t s showed an extremely s i g n i f i c a n t e f f e c t of treatment (P << .001) but i n s i g n i f i c a n t e f f e c t s of e i t h e r time or the i n t e r a c t i o n between treatment and time. T h i s r e s u l t i s to be expected when d e a l i n g with animals with a l i f e c y c l e longer than the experimental p e r i o d . When compared to lake data, the d e n s i t i e s of a d u l t D. thomasi i n e n c l o s u r e s were 25% lower at set-up and t h i s d i f f e r e n c e i n c r e a s e d to 50% as numbers i n the lake rose. There appeared to be m o r t a l i t y a s s o c i a t e d with h a n d l i n g s i n c e the numbers of D. thomasi i n the bags decreased a f t e r the f i r s t sampling p e r i o d and t h e r e a f t e r h e l d constant. Since the i n c r e a s e i n the lake was a s s o c i a t e d with the breaking of diapause and en t r y i n t o the water column of ov e r w i n t e r i n g animals, the i n c r e a s i n g d i f f e r e n c e between the lake and the en c l o s u r e s i s expected. Diaptomus kenai No s t a t i s t i c a l a nalyses were done on the numbers of D. kenai captured s i n c e they o c c u r r e d i n very low d e n s i t i e s i n the e n c l o s u r e s . I f p e r f e c t capture e f f i c i e n c y i s assumed, only 3 D. kenai c o u l d be expected to be caught i n each sample. In f a c t , capture e f f i c i e n c y i s much l e s s than t h i s s i n c e D. kenai are strong swimmers. However, D. kenai was captured at some time d u r i n g the experiment from every e n c l o s u r e to which i t had been added and never captured from those to which i t had not been added. V i s u a l i n s p e c t i o n at each sampling p e r i o d assured 8 6 that D. kenai were s t i l l p r e s e n t . R e p l i c a t e sampling d u r i n g the l a s t sampling p e r i o d gave an estimate of .07 / l compared to .22 / l which were o r i g i n a l l y added. Grazing assemblage • The g r a z i n g biomass of both the lake and the e n c l o s u r e s was almost e n t i r e l y made up of the three s p e c i e s : Diaptomus  oregonensis, Daphnia rosea and Holopedium gibberum . These s p e c i e s were examined in d e t a i l . Bosmina l o n g i r o s t r i s, Ceriodaphnia quadrangula and Polyphemus p e d i c u l u s were found i n f r e q u e n t l y . Bosmina seemed to occur both i n e n c l o s u r e s with and without grazers whereas Ceriodaphnia and Polyphemus occurred more in e n c l o s u r e s with g r a z e r s . The wide-spread, a l b e i t low d e n s i t y of Bosmina i s probably due to i t s small s i z e . Because of i t s s i z e , i t i s more l i k e l y to be p i c k e d up i n a p i p e t t e and remain undetected and loose eggs are more l i k e l y to pass through the n a u p l i u s s i e v e . Diaptomus oregonensis D. oregonensis showed a s i m i l a r p a t t e r n to that of D. thomasi . Analyses of v a r i a n c e showed s i g n i f i c a n t d i f f e r e n c e s (P << .001 except f o r sampling p e r i o d 5 where P < .005) between treatments c o n t a i n i n g the g r a z i n g assemblage and those without f o r the d u r a t i o n of the experiment. A two-way a n a l y s i s of v a r i a n c e showed s i g n i f i c a n t e f f e c t s of both time, treatment (P << .001) and the i n t e r a c t i o n term (P < .01) but the 87 F of the treatment term was 15 X as l a r g e as that of time ( F t r t = 168.16, Ftime = 10.24) i n d i c a t i n g that the treatment e f f e c t was much l a r g e r than that of time. When compared to lake data, D. oregonensis were 80% lower i n d e n s i t y i n the e n c l o s u r e s at the beginning of the experimental p e r i o d and were much more v a r i a b l e i n d e n s i t y . In the middle of the sampling p e r i o d , d e n s i t i e s were approximately 50% of those i n the l a k e . However at the end of the experiment, d e n s i t i e s in the e n c l o s u r e s were higher s i n c e most D. oregonensis i n the lake had moved lower i n the water column and numerical d e n s i t i e s i n the lake had a l s o decreased. Daphnia rosea Daphnia a l s o showed c o n s i s t e n t s i g n i f i c a n t d i f f e r e n c e s between treatments with g r a z e r s and those without g r a z e r s f o r the e n t i r e experimental p e r i o d although the s i g n i f i c a n c e of the d i f f e r e n c e s was i n general not so great (P < .001 i n 3 cases, P < .01 i n 3 c a s e s ) . A two-way a n a l y s i s of v a r i a n c e again showed s i g n i f i c a n t e f f e c t of treatment, time and the i n t e r a c t i o n term although the i n t e r a c t i o n was an order of magnitude smaller than the other two terms ( F t r t = 111.94 (P < .001), Ftime = 30.33 (P < .001), F t x t = 3.42 (P < .01)). The s i g n i f i c a n c e of time i s to be expected s i n c e the e n c l o s u r e s were set up d u r i n g the s p r i n g growth phase. The Daphnia i n the e n c l o s u r e s showed a steady i n c r e a s e i n number d u r i n g the experimental p e r i o d . Those i n the lake a l s o showed an i n c r e a s e although numbers i n the lake peaked d u r i n g the middle of the experimental p e r i o d 8 8 r a t h e r than at the end. The d e n s i t y of Daphnia was 33% lower i n the e n c l o s u r e s a f t e r set-up and the magnitude of t h i s d i f f e r e n c e i n c r e a s e d to 90% with recruitment from o v e r w i n t e r i n g eggs in the lake d u r i n g the middle of the experimental p e r i o d but became equal at the end of the experimental p e r i o d . Holopedium gibberurn Holopedium behaved in a f a i r l y s i m i l a r manner to Daphnia There were -consistent d i f f e r e n c e s between treatments with and without grazers d u r i n g the course of the experiment. These were once again l e s s s i g n i f i c a n t than those shown by the copepods (P < .001 i n 4 cases, P <.025 in 1 case, P < .05 i n 1 c a s e ) . A two-way a n a l y s i s of v a r i a n c e showed s i g n i f i c a n t e f f e c t s of treatment, time and the i n t e r a c t i o n term, a l l with P < .001, although the treatment e f f e c t was again the l a r g e s t . The Holopedium showed a peak of abundance at the middle of the experimental p e r i o d . The Holopedium i n the lake a l s o showed t h i s p a t t e r n . D e n s i t i e s i n the lake were 25% higher than those in the e n c l o s u r e s a f t e r set-up but became almost equal at the peak and were a c t u a l l y 30% higher than lake d e n s i t y at the end. Naupli i The d e n s i t i e s of n a u p l i i were examined at the f i r s t sampling p e r i o d to check setup d e n s i t i e s and check f o r d i f f e r e n c e s between treatments. No d i f f e r e n c e s were found between those treatments without a d u l t D. thomasi and those with 8 9 (ones where n a u p l i i c o u l d have t h e o r e t i c a l l y been r e c r u i t e d ) . In c o n t r a s t to the other zooplankton added to the e n c l o s u r e s , the average d e n s i t y of n a u p l i i i n e n c l o s u r e s was not s i g n i f i c a n t l y d i f f e r e n t from the s t o c k i n g d e n s i t y ( i . e . lake d e n s i t y ) (z = -.43, p = .33). Approximately 50% of the n a u p l i i were c y c l o p o i d (D. thomasi) and the r e s t were c a l a n o i d (D. o r e g o n e n s i s ) . Phytoplankton F e r t i l i z e r The e f f e c t of f e r t i l i z e r on the phytoplankton i n the e n c l o s u r e s was estimated by measuring the amount of c h l o r o p h y l l a. T h i s estimate of phytoplankton abundance was supplemented by counts of phytoplankton s i z e c l a s s e s f o r the second two experimental p e r i o d s , s i n c e t h i s time was c o n s i d e r e d to be c r i t i c a l f o r n a u p l i a r growth. The r e s u l t s of the f i r s t sampling p e r i o d showed a l l treatments to be v i r t u a l l y i d e n t i c a l in l i v e c h l o r o p h y l l a, phaeophytin a and t o t a l c h l o r o p h y l l a. T h i s i s e x a c t l y what was expected s i n c e the samples were taken immediately a f t e r the bags had been f i l l e d and before the phytoplankton had a chance to respond to the f e r t i l i z e r . L i v e c h l o r o p h y l l a has p r e v i o u s l y been used as a measure of a v a i l a b l e food f o r zooplankton (Marmorek 1983) s i n c e phaeophytin a can c o n s i s t of degraded or d i g e s t e d c h l o r o p h y l l a (Daley 1973, Daley and Brown 1973). In t h i s experiment however, the d e n s i t y of g r a z e r s was low and i t seems l i k e l y that most of the phaeophytin 90 measured was degraded rather than d i g e s t e d . T h i s i s e s p e c i a l l y true f o r those cases where no gr a z e r s were added to the e n c l o s u r e s . Some of the l a r g e s t i n c r e a s e s i n phaeophytin occurred i n en c l o s u r e s i n which only n a u p l i i were pres e n t . These r e s u l t s suggest that f e r t i l i z a t i o n caused the phytoplankton to grow r a p i d l y d u r i n g the f i r s t few days and then maintain a f a i r l y high biomass but grow at a slow r a t e with the m a j o r i t y of the c e l l s being senescent. Since phaeophytin i n t h i s case was not d i g e s t e d and was a v a i l a b l e as food, t o t a l c h l o r o p h y l l would probably be a b e t t e r measure of food a v a i l a b i l i t y . The g r e a t e s t c o r r e l a t i o n was a l s o found between t o t a l c h l o r o p h y l l and t o t a l a l g a l biomass (r = .76). T h i s c o n c l u s i o n i s supported by the examination of the phytoplankton s i z e s c l a s s e s (Table V I I I ) . Analyses of v a r i a n c e showed a s i g n i f i c a n t d i f f e r e n c e ( P < .005) between f e r t i l i z e d and u n f e r t i l i z e d e n c l o s u r e s i n the 2-5 /xm, 5-9 nm, 9-13 um and 13-18 Mm c l a s s e s i n the second sampling p e r i o d . In the t h i r d sampling p e r i o d , the same a n a l y s i s showed a s i g n i f i c a n t d i f f e r e n c e i n the abundances of the same s i z e c l a s s e s (P < .05 in 2-5 Mm, P < .001 i n others mentioned above ), although there was no s i g n i f i c a n t d i f f e r e n c e i n the l i v e c h l o r o p h y l l . T o t a l c h l o r o p h y l l would not be an e f f e c t i v e measure' of food a v a i l a b i l i t y i f the m a j o r i t y of the in c r e a s e i n phytoplankton biomass i n f e r t i l i z e d e n c l o s u r e s was due to an in c r e a s e i n i n e d i b l e a l g a e . T h i s d i d not seem to be the case. The d i f f e r e n c e s i n l a r g e i n e d i b l e algae between f e r t i l i z e d and u n f e r t i l i z e d e n c l o s u r e s were not s i g n i f i c a n t i n the samples 91 examined (F < 1). V i s u a l i n s p e c t i o n of data from a f e r t i l i z e d and an u n f e r t i l i z e d e n c l o s u r e seemed to i n d i c a t e that t h i s r e s u l t h e l d u n t i l the end of the experimental p e r i o d . Analyses of v a r i a n c e showed s i g n i f i c a n t d i f f e r e n c e s i n l i v e c h l o r o p h y l l in the second (F = 136.4, P << .001) and s i x t h (F = 6.4, P < .025) sampling p e r i o d s . However, there were c o n s i s t e n t and s i g n i f i c a n t (P < .025) d i f f e r e n c e s between the amount of phaeophytin and t o t a l c h l o r o p h y l l present i n f e r t i l i z e d and u n f e r t i l i z e d treatments from the . second sampling p e r i o d on (Table IX). In summary, f e r t i l i z e r s i g n i f i c a n t l y i n c r e a s e d t o t a l c h l o r o p h y l l and phaeophytin from the second sampling p e r i o d on. L i v e c h l o r o p h y l l was s i g n i f i c a n t l y i n c r e a s e d only d u r i n g the second and s i x t h sampling p e r i o d s . Phytoplankton biomass was s i g n i f i c a n t l y i n c r e a s e d i n the second and t h i r d sampling p e r i o d s i n the s i z e range 2-18 fim. . Grazing assemblage The phytoplankton data can a l s o be examined to determine i f g r a z e r s had a s i g n i f i c a n t impact on the food a v a i l a b l e . Since the hypothesis was that the g r a z i n g assemblage might compete with n a u p l i i f o r food, i t i s important to show that there was a d i f f e r e n c e i n a v a i l a b l e food i n e n c l o s u r e s with grazers and those without. When the t o t a l c h l o r o p h y l l content was examined (Table X), two way analyses of v a r i a n c e , which pools a l l treatments c o n t a i n i n g the f a c t o r of i n t e r e s t , showed that i n the second sampling p e r i o d , f e r t i l i z e r i n c r e a s e d the t o t a l c4 cr T a b l e V I I I E f f e c t of f e r t i l i z e r on c h l o r o p h y l l c o n t e n t . C h l o r o p h y l l content i s expressed as „g/l. L i v e C h l o r o p h y l l P h a e ophytin T o t a l C h l o r o p h y l l Sampli ng No No No ir i od Fer t i i 1 i z e r F e r t i 1 i z e r F e r t i 1 i z e r F e r t i 1 i z e r Fer t i l'i z e r F e r t i 1 i z e r 1. 0. .836 0. .844 0 .511 0. .493 1 . 336 1 . 337 2 3 . 367 0. 969 1 .413 0. .484 4 .675 1 . 453 3 1 . 004 0. 538 2 .046 0. .433 2 .962 0. .972 4 0. .910 0. .411 1 . 574 0. , 323 2 . 323 0. .734 5 0. .928 0. . 185 2 . 328 0. .618 3 .035 0. .803 6 2 . 168 O. . 356 2 .678 0, ,606 4 .097 0, .948 9 3 Table IX E f f e c t of f e r t i l i z e r on phytoplankton s i z e c l a s s e s . A l l biomasses are expressed as Mg/1. Sampling P e r i o d Sampling P e r i o d 2 3 No No S i z e c l a s s e s F e r t i l i z e r F e r t i l i z e r F e r t i l i z e r F e r t i l i z e r < 2 y m 7 . 6 9 . 3 6 . 0 7 . 5 2 - 5 Mm 4 . 7 1 5 . 2 4 . 4 1 6 . 1 5 - 9 Mm 4 0 . 6 7 9 . 0 2 7 . 4 8 3 . 3 9 - 13 Mm 4 3 . 3 1 0 6 . 8 2 2 . 9 1 0 7 . 1 13 - 18 Mm 2 3 . 3 6 0 . 9 1 2 . 4 7 8 . 0 > 18 1 2 . 8 2 4 . 1 1 0 . 0 1 3 . 1 94 c h l o r o p h y l l (F = 148.8, P « .001). The e f f e c t of f e r t i l i z e r c o ntinued to be strong u n t i l the end of the experimental p e r i o d (P < .001 except sampling p e r i o d 4 where P < .005), however gr a z e r s s i g n i f i c a n t l y decreased the t o t a l c h l o r o p h y l l d u r i n g the t h i r d sampling p e r i o d (F = 7.6, P < .025) and t h i s e f f e c t a l s o continued to the end of the experiment. During the f i f t h sampling p e r i o d , the i n t e r a c t i o n between f e r t i l i z e r and grazers became s i g n i f i c a n t (F = 8.0, P <.025) and the i n t e r a c t i o n e f f e c t i n c r e a s e d d u r i n g the l a s t sampling p e r i o d (F = 17.4, P< .005). Examination of phytoplankton s i z e c l a s s e s (Table XI) showed that f e r t i l i z e r s i g n i f i c a n t l y i n c r e a s e d s i z e c l a s s e s 5-9 jum (F = 65.7, P < .001) 9-13 um (F = 22.4, P < .001) and 13-18 ixm (F = 11.0, P < .005). The presence of grazers s i g n i f i c a n t l y decreased s i z e c l a s s e s 5-9 Mm (F = 61.1, P < .001), 9-13 Mm (F = 10.9, P < .005) and the i n t e r a c t i o n between f e r t i l i z e r and gr a z e r s had a s i g n i f i c a n t e f f e c t on the s i z e c l a s s e s 5-9 nm (F = 50.5, P < .001) . 95 Table X E f f e c t of- f e r t i l i z e r and grazers on t o t a l c h l o r o p h y l l c ontent. C h l o r o p h y l l i s shown as Mg/1. No F e r t i l i z e r F e r t i l i z e r Sampling or and Pe r i o d ' Grazers F e r t i l i z e r Grazers Grazers 1 1.374 1.394 1.299 1.278 2 1.535 5.197 1.370 4.153 3 1.245 3.402 0.698 2.521 4 1.021 3.129 0.447 1.517 5 0.931 4.138 0.674 1.931 6 0.842 5.870 1.054 2.3231 96 Table XI E f f e c t of f e r t i l i z e r and grazers on phytoplankton s i z e c l a s s e s . A l l biomasses are expressed as Mg/1. No S i z e C l a s s e s F e r t i l i z e r or Grazers F e r t i l i z e r Grazers F e r t i l and Graze: < 2 Mm 5.8 6.3 6.3 8.8 2 - 5 Mm 5.8 17.8 3.0 14.5 5 - 9 Mm 39.0 114.5 15.8 52.0 9 - 13 Mm 37.0 1 45.3 8.8 69.0 13 - 18 Mm 20.5 75.3 4.3 80.8 > 18 Mm 5.0 15.0 15.0 11.3 97 APPENDIX B. EVALUATION OF METHODS FOR CALCULATION OF SURVIVORSHIP Since the a n a l y s i s of s u r v i v o r s h i p was c r i t i c a l to the enclosure experiments, methods of s u r v i v o r s h i p c a l c u l a t i o n were i n v e s t i g a t e d to determine t h e i r behaviour and r e l i a b i l i t y . D uration of n a u p l i a r stages Estimates of the d u r a t i o n of the n a u p l i a r stages are c r i t i c a l to the c a l c u l a t i o n of s u r v i v a l e s t i m a t e s . The d u r a t i o n of stages i s the l i n k between s t a t i c estimates of age composition i n a p o p u l a t i o n and i t s dynamics. The e f f e c t of v a r i o u s f a c t o r s on n a u p l i a r development has been w e l l r e p o r t e d i n the l i t e r a t u r e . Most of these r e p o r t s i n v e s t i g a t e d the e f f e c t of temperature on developmental r a t e s (Auvrey and Dussart 1966, S p i n d l e r 1971, Munro 1974, Landry 1975, George 1976, Jacobs and Bowhuis 1979, S a r v a l a 1979, Jamieson 1980a, V i j v e r b e r g 1980). I n c r e a s i n g temperature o f t e n r a p i d l y i n c r e a s e s the developmental r a t e . There i s some debate i n the l i t e r a t u r e about whether t h i s i n c rease i s p r o p o r t i o n a l throughout the l i f e h i s t o r y (Landry 1975) or whether these i n c r e a s e s are d i s p r o p o r t i o n a t e (Munro 1974). Some a t t e n t i o n has a l s o been given to the q u a l i t y of d i e t (Auvrey and Dussart 1966, Smyly 1970, Whitehorse and Lewis 1973, Jamieson 1980a) and to the e f f e c t of l i g h t (Auvrey and Dussart 1966, S p i n d l e r 1971). I n c r e a s i n g l i g h t i n t e n s i t y and d u r a t i o n appeared to inc r e a s e the developmental r a t e ( S p i n d l e r 1971). I n c r e a s i n g food q u a l i t y a l s o seemed to increase the developmental r a t e . Some s p e c i e s 98 seemed more s e n s i t i v e than others (Auvrey and Dussart 1966) and mixtures of food a l s o seemed to produce b e t t e r r e s u l t s . Food q u a n t i t y has a l s o been found to i n c r e a s e developmental r a t e of copepods. Weglenska (1971) found that i n c r e a s i n g c o n c e n t r a t i o n s of n a t u r a l seston from a e u t r o p h i c lake s i g n i f i c a n t l y i n c r e a s e d the developmental r a t e of Eudiaptomus  g r a c i l o i d e s . Klekowski and Shushkina (1966), working with d i f f e r e n t c o n c e n t r a t i o n s of a s i n g l e food source found that although there was a s l i g h t i n c r e a s e , Mesocyclops a l b i d u s was r e l a t i v e l y i n s e n s i t i v e to d i f f e r e n c e s i n food c o n c e n t r a t i o n over the range t e s t e d . Most of these f a c t o r s are of l i t t l e s i g n i f i c a n c e i n e s t i m a t i n g developmental r a t e s i n my e n c l o s u r e s . The e f f e c t of temperature i s important only when comparing estimated d u r a t i o n s with p r e v i o u s l y r e p o r t e d ones. However, i f q u a n t i t y of food s i g n i f i c a n t l y i n f l u e n c e s development r a t e s , t h i s c o u l d i n f l u e n c e the r e s u l t s of e n c l o s u r e s with f e r t i l i z e r . E s timates of the d u r a t i o n s of the developmental stages of D. thomasi i n the e n c l o s u r e s were i n i t i a l l y made using the method o u t l i n e d by R i g l e r and Cooley (1974). Because copepods reproduce over a d i s c r e t e time i n t e r v a l , graphs of the abundance of each developmental stage vs time show d i s t i n c t peaks i n abundance. The method of R i g l e r and Cooley i n v o l v e s s e t t i n g the d i f f e r e n c e between the peaks to h a l f the d u r a t i o n of the two i n s t a r s i n v o l v e d . The i n i t i a l stage of t h i s c a l c u l a t i o n i n v o l v e d examining j u v e n i l e copepods from those e n c l o s u r e s without a d u l t D i a c y c l o p s to f i n d when these peaks i n abundance 9 9 o c c u r r e d . Because of the u n c e r t a i n t y surrounding the e f f e c t s of food q u a n t i t y on developmental r a t e , e n c l o s u r e s with and without f e r t i l i z e r were examined s e p a r a t e l y . The means of e n c l o s u r e s w i t h i n each of these groups were pooled a f t e r no s i g n i f i c a n t d i f f e r e n c e was found between them (Table X I I ) . The o v e r a l l mean was used f o r f u r t h e r c a l c u l a t i o n s . One of the problems with R i g l e r and Cooley's method i s that one of the v a r i a b l e s must always be estimated s i n c e the number of unknowns i s always one g r e a t e r than the number of equations. When t h i s was done, there was always one d u r a t i o n which ended up very low. The i n i t i a l estimates obtained from t h i s method are shown i n Table X I I . These, of course, f a i l e d when used to c a l c u l a t e s u r v i v o r s h i p . Comparisons with other estimates f o r developmental r a t e s i n c l u d i n g those made by Peacock (1981) f o r D. thomasi and Tropocyclops p r a s i n u s i n the same lake showed that the estimate f o r the d u r a t i o n of stages n1-3 was badly underestimated. In order to o b t a i n a more reasonable value f o r the d u r a t i o n s , a combination of values obtained by Peacock (1981) and those obtained from the e n c l o s u r e s were f i t t e d using s i m u l a t i o n modelling to the r e s u l t s from e n c l o s u r e s c o n t a i n i n g only n a u p l i i so that the peaks corresponded to those found i n the e n c l o s u r e s . These f i t t e d v alues are a l s o shown i n Table XI I . The d i f f e r e n c e between the f i t t e d d u r a t i o n s and the c a l c u l a t e d d u r a t i o n s i s due to the short p e r s i s t e n c e time of each stage and the i n f r e q u e n t sampling r e l a t i v e to the d u r a t i o n s . The d i f f e r e n c e between f i t t e d v alues and p r e v i o u s l y c a l c u l a t e d v a l u e s are probably due to d i f f e r e n c e i n temperature. o 0 T a b l e XII R e s u l t s f o r e s t i m a t i o n of s t a g e d u r a t i o n of immature D. thomasi i n e n c l o s u r e s a) Appearance of Peak i n Abundance Curves Developmental Stage n a u p l i us 1-3 n a u p l i u s 4-6 c o p e p o d i t e 1 c o p e p o d i t e 2 c o p e p o d i t e 3-4 )b C a l c u l a t e d D u r a t i o n s Developmental Stage naup1i us 1-3 naup1i us 4-6 c o p e p o d i t e 1 c o p e p o d i t e 2 copepodi te 3-4 U n f e r t i 1 i zed (day i n exper iment) (± S.E.) 1.0 + 2.0 5.0 ± 5.8 13.3 ± 2.3 17.2 ± 2.3 18.7 ± 2.3 C a l c u l a t e d * * 1 9 3 2 5 F i t t e d 5 7 2 2 F e r t i 1 i zed (day i n exper i ment) (± S.E.) 0.8 ± 1.8 6.8 ± 1.8 11.2 ± 1.8 12.8 ± 3.4 18.0 ± 2.0 D i a c y c l o p s  thomas i * 7 1 1 7 7 7 Pooled (day i n exper i ment) (± S.E.) 0.9 6 12 14.5 18.3 T r o p o c y c l o p s  pras i nus * 3 7 5 5 6 * - from Peacock (1981). ** - c a l c u l a t e d u s i n g R i g l e r and C o o l e y (1974). 101 As the temperature during the experiment v a r i e d f r e q u e n t l y and was f o r a l a r g e p o r t i o n of the time l a t e r i n the experiment much higher (Appendix C) than those at which Peacock's work was done (13 - 18 °C), the r e s u l t s are reasonable. I t i s worthy of note that n e i t h e r my enc l o s u r e s nor pre v i o u s estimates by Peacock (1981) give any i n d i c a t i o n that food q u a n t i t y had a s i g n i f i c a n t e f f e c t on the n a u p l i a r d u r a t i o n of D. thomasi . A n a l y s i s of n a u p l i a r s u r v i v a l Methods to analyse the s u r v i v a l of zooplankton seem to be d i v i d e d i n t o two c a t e g o r i e s : 1. those f o r p o p u l a t i o n s with continuous recruitment ( u s u a l l y Cladocera) and 2. those f o r po p u l a t i o n s which reproduce i n d i s c r e t e c o h o r t s ( u s u a l l y Copepoda). The method used f o r one category i s i n general u n s u i t a b l e f o r the other. Continuous recruitment methods of a n a l y s i s o f t e n assume a s t a b l e age s t r u c t u r e , or at l e a s t one which does not change r a p i d l y (Edmondson 1968, A r g e n t e s i et a l . 1974, S e i t z 1979, T a y l o r and S l a t k i n 1981). Many of the models a l s o assume constant m o r t a l i t y r a t e s throughout the l i f e h i s t o r y of the organism being i n v e s t i g a t e d (Edmondson 1968, T a y l o r and S l a t k i n 1981). On the other hand, most work which has been done with copepods with d i s c r e t e generations has used the technique of i n t e g r a t i n g under the curve of abundance vs time as o u t l i n e d i n Southwood (1966). Both R i g l e r and Cooley (1974) and Gehrs and Robertson (1975) have introduced t h i s technique i n the zooplankton l i t e r a t u r e and s e v e r a l others have s i n c e used i t to estimate s u r v i v o r s h i p (Confer and Cooley 1977, N e i l l and 1 02 Peacock 1980, Peacock 1981). Since in the m a j o r i t y of cases in the e n c l o s u r e experiments, continuous recruitment was not t a k i n g p l a c e and s i n c e continuous recruitment methods of t e n c o ntained untenable assumptions ( i . e . constant m o r t a l i t y r a t e ), the d i s c r e t e generation method of a n a l y s i s was examined. T h i s method i s o u t l i n e d by R i g l e r and Cooley (1974) and Gehrs and Robertson (1975). In t h i s method, the t o t a l number of animals o c c u r r i n g i n each i n s t a r f o r the whole generation i s c a l c u l a t e d using the t r a p e z o i d a l method f o r i n t e g r a t i n g under a curve and c o r r e c t e d f o r the d u r a t i o n of each i n s t a r . The equation f o r t h i s method i s given in Gehrs and Robertson (1975) and i s l i s t e d below. k N = L ((1 +1 )/2)(W /D ) i x=j i , x i,x+1 x,x+1 i Where l=number of i n d i v i d u a l s a l i v e i = i n s t a r d e s i g n a t i o n x = c o l l e c t i o n d e s i g n a t i o n j = f i r s t c o l l e c t i o n p r i o r to the appearance of i n s t a r i k= c o l l e c t i o n f o l l o w i n g l a s t appearance of i n s t a r i W = i n t e r v a l i n days between c o l l e c t i o n x and x,x+1 c o l l e c t i o n x+1 D = d u r a t i o n of i n s t a r i i N = number of i n d i v i d u a l s of i n s t a r i produced i n the i i n t e r v a l x= j to x= k, that i s the number of i n d i v i d u a l s of i n s t a r i produced i n a p a r t i c u l a r g e n e r a t i o n 1 03 A geometric i l l u s t r a t i o n of t h i s method of i n t e g r a t i o n i s shown in F i g 14. There are some inherent problems with t h i s approach which are a s s o c i a t e d with numerical i n t e g r a t i o n . In the case shown i n F i g 14a, t h i s method estimates the area w e l l . The curve i s r e l a t i v e l y g e n t l e and sampling i n t e r v a l short i n comparison with the l e n g t h of time the i n s t a r p e r s i s t s . However i f the sampling i n t e r v a l i s r e l a t i v e l y long i n comparison with the p e r s i s t e n c e time of the i n s t a r or i f the peak i s missed during sampling, the e s t i m a t i o n becomes much poorer (See F i g 14b,c & d ) . The consequences of t h i s behaviour when e v a l u a t i n g the s u r v i v o r s h i p of a p o p u l a t i o n through a number of i n s t a r s can be profound i f the i n s t a r s do not have peaks e x a c t l y i n phase with the sampling times. T h i s e f f e c t i s p a r t i c u l a r l y important i f the p e r s i s t e n c e of each i n s t a r i s s h o r t . The e f f e c t of the d u r a t i o n of the i n s t a r s can e i t h e r damp or accentuate t h i s phenomenon depending on the r e l a t i v e magnitude of the d u r a t i o n and the sampling i n t e r v a l ; . I f the sampling i n t e r v a l i s small r e l a t i v e to the d u r a t i o n , the e r r o r i s damped; when the sampling i n t e r v a l i s l a r g e , the e r r o r i s accentuated. If i n s t a r d u r a t i o n s are of unequal l e n g t h , the chances of sampling e x a c t l y on the peak are d i m i n i s h e d . The e r r o r of the s u r v i v o r s h i p estimate then becomes g r e a t e r due to v a r y i n g d i f f e r e n c e s i n the r e l a t i v e magnitudes of the sampling i n t e r v a l and the d u r a t i o n . When the equation was used with simulated f i e l d c o n d i t i o n s , I found i t to be q u i t e a c c u r a t e . The accuracy of the equation i n c r e a s e d with d e c r e a s i n g sample width (W ), however the x,x+1 1 04 F i g u r e 14 G r a p h i c a l r e p r e s e n t a t i o n of Gehrs and Robertsons method of s u r v i v o r s h i p c a l c u l a t i o n . The t o t a l number in each stage (only one shown here) i s c a l c u l a t e d by n u m e r i c a l l y i n t e g r a t i n g under the curve using contiguous t r a p e z o i d s . Each t r a p e z o i d has an area of (W )(1 x, x+1 x + 1 )/2. In t h i s example, the d u r a t i o n of the stages x+1 i s assumed to be one and thus drop out of the c a l c u l a t i o n . The sampling i n t e r v a l i s represented by W . Lx r e p r e s e n t s the abundance a sampling p e r i o d x,x+1 x. a) I n t e g r a t i o n under a g e n t l e curve with a r e l a t i v e l y short sampling p e r i o d . b) I n t e g r a t i o n under a g e n t l e curve where the peak i s missed. c) -Integration with a long sampling i n t e r v a l . d) I n t e g r a t i o n with a long sampling i n t e r v a l where the peak i s missed. £0 o 1 06 e r r o r was only 1% even when W was twice as long as the s h o r t e s t d u r a t i o n (and s l i g h t l y longer than a l l the o t h e r s ) . The excep t i o n to t h i s was when the W c o i n c i d e d with the len g t h of s e v e r a l s u c c e s s i v e d u r a t i o n s . In t h i s case, the accuracy i n c r e a s e d . The accuracy of the equation dropped d r a m a t i c a l l y as the length of time each stage p e r s i s t e d decreased. I f the f a s t e s t stage p e r s i s t e d i n the water for more than 20 days, sampling at weekly i n t e r v a l s gave an error' of < 1%. However i f the stage p e r s i s t e d only 12 days, the e r r o r was 8% and i f the same stage p e r s i s t e d only 8 days the e r r o r was 190%. T h i s r e s u l t has severe consequences f o r examining the s u r v i v a l of n a u p l i i i n e n c l o s u r e s . Anytime the e f f e c t of c a n n i b a l i s m on n a u p l i i needs to be separated out, n a u p l i i as a group with no a d u l t s present w i l l n e c c e s s a r i l y p e r s i s t i n the water column f o r a s h o r t e r time than i n s i t u a t i o n s where there i s r e c r u i t m e n t . A d d i t i o n a l n a u p l i i cannot be d i s t i n g u i s h e d u n l e s s the a d u l t s do not produce any eggs u n t i l the o r i g i n a l n a u p l i i have developed to the next stage. T h i s means that i f i n t e g r a t i o n i s performed under the e n t i r e curve, the c a l c u l a t i o n of background m o r t a l i t y of other f a c t o r s w i l l be l e s s a c c u r a t e than that f o r m o r t a l i t y a s s o c i a t e d with presence of a d u l t s , i f the complete development of a cohort i s examined. U n f o r t u n a t e l y t h i s means that a much longer time span i s r e q u i r e d f o r the experiment which i n turn means that the development of the cohort where the a d u l t s are present w i l l i n  s i t u be subject to d i f f e r e n t environmental, c o n d i t i o n s and i t w i l l be impossible to separate t h i s e f f e c t from the e f f e c t of 1 07 a d u l t s . Enclosure experiments a l s o have the added problem that the whole area under the curve cannot be i n t e g r a t e d f o r the f i r s t stage because adding n a u p l i i to e n c l o s u r e s e s s e n t i a l l y s e t s the peak at the beginning of the experiment. T h i s problem can be a d j u s t e d by h a l v i n g the developmental r a t e of the f i r s t i n s t a r and assuming an even age d i s t r i b u t i o n w i t h i n the f i r s t i n s t a r . In s i m u l a t i o n s of e n c l o s u r e c o n d i t i o n s , I found that s u r v i v a l r a t e s were c o n s i s t e n t l y underestimated once the d u r a t i o n of the f i r s t i n s t a r was c o r r e c t e d . In s p i t e of i t s lac k of accuracy however, the technique was f a i r l y robust to e r r o r s i n the data. A Monte C a r l o s i m u l a t i o n to determine the e f f e c t of v a r i a t i o n i n data drawn from a log-normal d i s t r i b u t i o n on the c a l c u l a t i o n of s u r v i v a l r a t e s showed that a 40% c o e f f i c i e n t of v a r i a t i o n i n the data was reduced to an 8% v a r i a t i o n i n the d a i l y s u r v i v a l r a t e . The e f f e c t of egg p r o d u c t i o n on the c a l c u l a t e d s u r v i v a l r a t e s was examined by i n t e g r a t i n g under the same region of the curve as was used i n the other s i m u l a t i o n s , i . e . i g n o r i n g l a t e r n a u p l i i p r o d u c t i o n by the a d u l t s . T h i s technique overestimated s u r v i v o r s h i p f o r stages with short d u r a t i o n s and underestimated i t f o r stages with long d u r a t i o n s because of the d i f f e r e n c e i n l e n g t h of the developmental stages. T h i s e f f e c t i s more n o t i c e a b l e when l a r g e numbers of n a u p l i i are produced. In order to understand the reason f o r t h i s , i t i s h e l p f u l to t h i n k of an analogy. Imagine a freeway which goes from the c e n t r e of town to the suburbs. Suppose everyone works i n the center of 108 town and l i v e s i n the suburbs so that d u r i n g the evening rush hour, the number of c a r s g e t t i n g on the freeway i n the suburbs i s n e g l i g i b l e . Now suppose one wants to estimate the number of c a r s g e t t i n g o f f at each e x i t and one decides to do t h i s by f l y i n g over the freeway and t a k i n g an a e r i a l photograph at s e v e r a l d i f f e r e n t p o i n t s . I f the t r a f f i c i s moving at the same speed at a l l p o i n t s along the way, t h i s method w i l l give a good estimate of the " m o r t a l i t y " . However, i f one of the photographs i s taken where an a c c i d e n t has occurred and t r a f f i c i s backed up, the number of c a r s estimated to have l e f t the freeway at the previous e x i t w i l l be underestimated ( i . e . those which stayed on w i l l be overestimated) but subsequent " m o r t a l i t y " . w i l l be. overestimated. If the a e r i a l photograph was taken of a l a r g e area, and the a c c i d e n t s i g n i f i c a n t l y slowed the t r a f f i c these e f f e c t s would i n c r e a s e with the volume of t r a f f i c . Because of the d i f f i c u l t i e s with t h i s method, I i n v e s t i g a t e d another approach which i n v o l v e d m o d e l l i n g the dynamics of immature D. thomasi i n the e n c l o s u r e s and using a l e a s t squares method of curve f i t t i n g to estimate s u r v i v a l parameters. The curves of abundance of immature D. thomasi vs time were f i t to simulated curves using the r o u t i n e N2SNO (Moore 1981) f o r n o n - l i n e a r parameter e s t i m a t i o n . T h i s method uses a numerical approximation of the d e r i v a t i v e and has three methods for determining convergence of the f i t t e d curve. V a r i a b i l i t y convergence was used i n these c a l c u l a t i o n s . T h i s method a l s o has some problems a s s o c i a t e d with the technique. Least squares 109 i s known to be s u s c e p t i b l e to o u t l i e r s (Moore 1981). As with other models, the accuracy of the r e s u l t s w i l l be dependent on the s t r u c t u r e of the model and can a l s o be s e n s i t i v e to the i n i t i a l c o n d i t i o n s . The i n i t i a l c o n d i t i o n s of the model were f i t t e d by examining the simplest c o n d i t i o n s ( i . e . those e n c l o s u r e s with n a u p l i i alone) and a d j u s t i n g the set-up c o n d i t i o n s so that negative m o r t a l i t y was minimized. Negative m o r t a l i t y (spontaneous generation) i s c l e a r l y impossible and the presence of t h i s phenomenon i n d i c a t e s a d e f i c i e n c y in the model. One of the most obvious and e a s i l y a d j u s t a b l e areas f o r t h i s d e f i c i e n c y to occur i s i n the set up c o n d i t i o n s . Although i n i t i a l number in each stage were known, the age ( r a t h e r than stage) d i s t r i b u t i o n was unknown and had to be estimated. I t i s these estimates which were a d j u s t e d to minimize negative m o r t a l i t y . In most cases, these changes d i d not a f f e c t the r e l a t i v e r e s u l t s . A g r a p h i c a l r e p r e s e n t a t i o n of the f i t obtained i s shown i n Figure 15. When the i n i t i a l c o n d i t i o n s were t e s t e d by f i t t i n g model-generated data, I found that the technique i n f a c t , underestimated copepodite s u r v i v a l . The values were not a d j u s t e d f u r t h e r s i n c e adjustment of c o n d i t i o n s so the technique was accurate f o r model-generated data caused negative m o r t a l i t y i n the r e a l data, probably due to some unknown d e f i c i e n c y i n the model. Since i t i s more important that s u r v i v a l r a t e s w i t h i n the experiment be comparable, t h i s u nderestimation should not s i g n i f i c a n t l y a f f e c t the ranking of m o r t a l i t y f a c t o r s . . The p r e c i s i o n of the curve f i t t i n g was a l s o t e s t e d by f i t t i n g c u rves.to model-generated data which had 1 10 been drawn from a log-normal d i s t r i b u t i o n with a c o e f f i c e n t of v a r i a t i o n of 40%. T h i s procedure showed that t h i s v a r i a t i o n was reduced to 8% v a r i a t i o n i n d a i l y s u r v i v a l r a t e . The e f f e c t of a 40% v a r i a t i o n i n the data (assuming a log-normal d i s t r i b u t i o n ) on s u r v i v o r s h i p curves i s shown in F i g 16. There was a l s o a problem with t h i s technique with e n c l o s u r e s c o n t a i n i n g a d u l t D i a c y c l o p s . Since the water temperature f l u c t u a t e d and was c o n s i d e r a b l y warmer towards the end of the experimental p e r i o d , the d u r a t i o n of developmental stages became s h o r t e r . T h i s i s r e f l e c t e d i n the p r o g r e s s i v e l y l a r g e r d i f f e r e n c e between values recorded by Peacock (1981) and values from my e n c l o s u r e s (Table X I I ) . U n f o r t u n a t e l y t h i s i n c r e a s e i n temperature a l s o caused n a u p l i i which hatched l a t e r i n the experiment to develop f a s t e r than those at the-beginning of the experiment - those f o r which the d u r a t i o n s were c a l c u l a t e d . The change in developmental r a t e over time was not i n c o r p o r a t e d i n the model s i n c e I f e l t that the a d d i t i o n a l assumptions which would have been made and the e x t r a parameters needed would not have improved the accuracy. The probable e f f e c t of t h i s i n c r e a s e i n temperature i s underestimation of the s u r v i v a l of the younger n a u p l i a r stages and o v e r e s t i m a t i o n of the s u r v i v a l of the l a t e r n a u p l i a r s t a g es. As with the t r a p e z o i d a l method of i n t e g r a t i o n , t h i s e f f e c t w i l l become more severe as more n a u p l i i are added to the system. T h i s e f f e c t can be more e a s i l y understood by again r e f e r r i n g to the freeway analogy. Consider the same freeway and use the same technique f o r e s t i m a t i o n of " m o r t a l i t y " . T h i s time however, the freeway 1 1 1 F i g u r e 15 G r a p h i c a l r e p r e s e n t a t i o n of f i t o b t a i n e d from curve f i t t i n g procedure. es t i m a t e d abundance a c t u a l abundance 1 1 3 F i g u r e 16 G r a p h i c a l r e p r e s e n t a t i o n of e r r o r i n s u r v i v o r s h i p c u rves. Each curve was generated from a model drawing data from a lognormal random d i s t r i b u t i o n with a c o e f f i c i e n t of v a r i a t i o n of 40% ( v a r i a n c e of .4). The f i g u r e shows the approximate amount of v a r i a t i o n expected i n c a l c u l a t e d s u r v i v o r s h i p curves with the v a r i a t i o n found i n the data. 11-4 DEVELOPMENTAL STAGE 1 1 5 has been updated to a system of express lanes and feeder lanes for a c e r t a i n d i s t a n c e . One a l s o has the a d d i t i o n a l i n f o r m a t i o n of the posted speed l i m i t . I t i s w e l l known that the posted speed l i m i t has very l i t t l e to do with the a c t u a l speed, p a r t i c u l a r l y i n the express l a n e s . Using t h i s technique one w i l l underestimate the number of c a r s going through the express lanes because they are moving f a s t e r . However, when the data are examined at a l a t e r p o i n t where there are no express l a n e s , one w i l l imagine that fewer c a r s l e f t the freeway than a c t u a l l y d i d because some of the c a r s missed while they were i n the express lanes w i l l show up. 1 16 APPENDIX C. TEMPERATURE AND PRECIPITATION DURING THE EXPERIMENTAL PERIOD(MAY-JUNE, 1982) 1 1 7 F i g u r e 17 Temperature and p r e c i p i t a t i o n d u r i n g the e x p e r i m e n t a l p e r i o d (May-June, 1982). Water te m p e r a t u r e of P l a c i d Lake i s marked when i t was t a k e n . S water temperature a t the s u r f a c e . B water temperature 1 m below s u r f a c e . .— d a i l y maximum temp e r a t u r e d a i l y minimum tem p e r a t u r e t o t a l d a i l y p r e c i p i t a t i o n TEMPERATURE (C) PRECIPITATION (mm) APPENDIX D. RESULTS OF SURVIVORSHIP CALCULATIONS 1 2 0 F i g u r e 18 S u r v i v o r s h i p c a l c u l a t e d u s i n g i n t e g r a t i o n . T h i s method c a l c u l a t e s s u r v i v o r s h i p by i n t e g r a t i n g the area under the abundance curve of each i n s t a r . The d i s t a n c e on the x-axis between developmental stages i s p r o p o r t i o n a l to the d u r a t i o n of the s t a g e s . The s u r v i v o r s h i p curves are presented i n four graphs: (a) e n c l o s u r e s c o n t a i n i n g f e r t i l i z e r (b) e n c l o s u r e s c o n t a i n i n g Diaptomus kenai (c) e n c l o s u r e s c o n t a i n i n g D i a c y c l o p s thomasi (d) e n c l o s u r e s c o n t a i n i n g the g r a z i n g assemblage. The s u r v i v o r s h i p of n a u p l i i from the e n c l o s u r e c o n t a i n i n g n a u p l i i alone i s shown i n each group. The treatment i s marked at the end of each curve. — n a u p l i i alone F F e r t i l i z e r K Diaptomus kenai D D i a c y c l o p s thomasi G Grazing assemblage 122 DEVELOPMENTAL STAGE 123 DEVELOPMENTAL STAGE 124 DEVELOPMENTAL STAGE 1 2 5 F i g u r e 19 S u r v i v o r s h i p c a l c u l a t e d u s i n g curve f i t t i n g . T h i s method c a l c u l a t e s s u r v i v o r s h i p by minimizing the sum of squares between the a c t u a l p o p u l a t i o n curves and simulated c u r v e s . The d i s t a n c e on the x - a x i s between developmental stages i s p r o p o r t i o n a l to the d u r a t i o n of the stages. The s u r v i v o r s h i p curves are p r e s e n t e d i n four graphs: (a) e n c l o s u r e s c o n t a i n i n g f e r t i l i z e r (b) e n c l o s u r e s c o n t a i n i n g Diaptomus kenai (c) e n c l o s u r e s c o n t a i n i n g D i a c y c l o p s thomasi (d) e n c l o s u r e s c o n t a i n i n g the g r a z i n g assemblage. The s u r v i v o r s h i p of n a u p l i i from the e n c l o s u r e c o n t a i n i n g n a u p l i i a lone i s shown i n each group. The treatment i s marked at the end of each curve. n a u p l i i alone F F e r t i l i z e r K Diaptomus kenai D D i a c y c l o p s thomasi G Grazing assemblage 126 DEVELOPMENTAL STAGE 127 DEVELOPMENTAL STAGE c) 129 1 0 N4 C1 C3 DEVELOPMENTAL STAGE 

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