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Periphyton community dynamics in a temperate oligotrophic lake Rodriguez, Marco A. 1984

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PERIPHYTON COMMUNITY DYNAMICS IN A TEMPERATE OLIGOTROPHIC LAKE by MARCO A. RODRIGUEZ L i e . B i o l . , U n i v e r s i d a d Simon B o l i v a r , Caracas, 1979 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 October 1984 (c) Marco A. Rodriguez, 1984 In presenting this- 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 -?£>CJU^(^  The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date DE-6 (3/81) i i ABSTRACT M i c r o a l g a l communities growing on P l e x i g l a s s l i d e s were s t u d i e d over a two year p e r i o d . Changes in the t o t a l numbers and r e l a t i v e abundances of taxa of manipulated and unmanipulated s u b s t r a t a were analysed using o r d i n a t i o n and m o d e l l i n g . The sequence of taxa replacements on the s l i d e s was o r d e r l y and repeatable d u r i n g the f i r s t months. The " d i r e c t i o n " of s u c c e s s i o n i n v o l v e d e a r l y appearance of b a c t e r i a and dominance by c o c c o i d greens dur i n g the f i r s t 3-4 weeks, f o l l o w e d by i n c r e a s e s i n Gomphonema, Eun o t i a , Dinobryon, M i c r o c y s t i s , and Merismopedia. Coccoid greens were co-dominant even on the o l d e r s l i d e s . Mature communities underwent a y e a r l y q u a s i -c y c l i c a l sequence of taxa replacements, without reaching s t a t i o n a r i t y i n community s t r u c t u r e . Winter and summer were pe r i o d s of r e l a t i v e s t a b i l i t y , whereas s p r i n g and f a l l were t r a n s i t i o n p e r i o d s ; the s p r i n g and f a l l communities resembled each other. In g e n e r a l , high diatom d e n s i t i e s o c c u r r e d i n winter, whereas c o c c o i d greens, filamentous greens, and diatoms t y p i f i e d summer assemblages. The sequence of changes v a r i e d with depth. The r e l a t i v e abundance of diatoms i n c r e a s e d with depth. Spatio-temporal v a r i a t i o n s i n l i g h t and temperature seem to have s t r o n g l y i n f l u e n c e d p e r i p h y t o n p e r i o d i c i t y and s p a t i a l h e t e r o g e n e i t y . Community age was more important than s e a s o n a l i t y i n determining community s t r u c t u r e . B i o t i c d i f f e r e n c e s between s p a t i a l l y separated s t a t i o n s i n c r e a s e d with community age. Phytoplankton and p e r i p h y t o n d i f f e r e d g r e a t l y i n community s t r u c t u r e ; only two abundant taxa were e s t a b l i s h e d i n both communities. A summer Dinobryon/Epipyxi s pulse began sim u l t a n e o u s l y i n the two communities. Phytoplankton recovered f a s t e r from the p u l s e , suggesting that g e n e r a t i o n times were s h o r t e r , and turnover r a t e s higher, i n phytoplankton than in per iphyton. Communities exposed to e x p e r i m e n t a l l y - i n d u c e d environmental f l u c t u a t i o n s of one- and two-week per i o d s c o u l d not " t r a c k " the f l u c t u a t i o n s . Instead, s u c c e s s i o n was slowed, and the communities remained in a s t a t e intermediate between the two extreme s t a t e s of the f l u c t u a t i o n c y c l e s . The response of communities d i f f e r i n g i n i n i t i a l composition to a Dinobryon/Epipyxi s pulse suggested that development towards m a t u r i t y proceeded along a r a t h e r r i g i d l y determined t r a j e c t o r y . Convergence experiments i n v o l v i n g communities whose s t r u c t u r e d i f f e r e d i n i t i a l l y showed that community s t r u c t u r e at the family/genus l e v e l was r e g u l a t e d . The r a t e of r e g u l a t i o n was commensurate with average c e l l d i v i s i o n r a t e s estimated independently, but r e g u l a t i o n was slow i n comparison to other waterbodies. Community s t r u c t u r e was seemingly r e g u l a t e d towards a s i n g l e , h i g h l y dynamic e q u i l i b r i u m . The estimated c h a r a c t e r i s t i c response time f o r the communities was 12 weeks, i n d i c a t i n g that the communities should t r a c k y e a r l y c y c l i c a l i v e n v i r o n m e n t a l v a r i a t i o n w i t h a l a g . E n v i r o n m e n t a l c h a n g e s o c c u r r i n g o v e r p e r i o d s s h o r t e r t h a n 12 weeks, s u c h as t h o s e seen i n s p r i n g and f a l l , m i g h t have l e f t t h e community f a r from e q u i l i b r i u m f o r e x t e n d e d p e r i o d s of t i m e . The r a t e s of change i n community s t r u c t u r e i n G w e n d o l i n e L a k e were low i n c o m p a r i s o n w i t h o t h e r w a t e r b o d i e s , p r o b a b l y b e c a u s e c e l l d i v i s i o n r a t e s were a l s o low i n c o m p a r i s o n w i t h o t h e r s y s t e m s . P h o s p h o r u s s c a r c i t y m i g h t have been t h e main f a c t o r u n d e r l y i n g t h e s l u g g i s h n e s s o f p e r i p h y t o n r e s p o n s e s . V TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES ix LIST OF FIGURES X ACKNOWLEDGEMENTS x i i i GENERAL INTRODUCTION 1 The i s s u e s 1 A note on methodology 4 Chapter o u t l i n e 6 GENERAL MATERIALS AND METHODS 7 The community 7 The study s i t e ^ 8 Sampling methods '. 11 Data a n a l y s i s 14 CHAPTER 1. SUCCESSION AND SEASONALITY OF GWENDOLINE LAKE PERIPHYTON 16 INTRODUCTION 16 EXPERIMENTAL PROCEDURE 18 RESULTS 19 D e s c r i p t i o n of the community: n a t u r a l h i s t o r y and microscopy o b s e r v a t i o n s 19 Community development 31 T o t a l numbers 31 Community s t r u c t u r e at the d i v i s i o n l e v e l 34 D i v e r s i t y and su c c e s s i o n r a t e s : index-based analyses 38 The f i r s t 20 weeks: o r d i n a t i o n of community t r a j e c t o r i e s 42 I n t e r r e l a t i o n s h i p s between p l a n k t o n i c and p e r i p h y t i c communities 54 S t a t i o n s 1 and 3 i n 1983, and t h e i r r e l a t i o n to the 1982 communities 69 Determination of s i m i l a r i t i e s of taxa responses to environmental v a r i a t i o n : o r d i n a t i o n of taxonomic groups 83 DISCUSSION 89 S u c c e s s i o n a l p a t t e r n s : community development 89 Phytoplankton-periphyton i n t e r r e l a t i o n s h i p s 94 S u c c e s s i o n a l p a t t e r n s : "mature" communities and s e a s o n a l i t y 95 S i m i l a r i t i e s i n the responses of a l g a l groups to environmental v a r i a t i o n 102 A conceptual model of s u c c e s s i o n and seasonal p e r i o d i c i t y i n the periphyton 103 SUMMARY 109 CHAPTER 2. THE EFFECTS OF HISTORICAL ENVIRONMENTAL REGIME AND COMMUNITY AGE ON RECOVERY PROPERTIES 113 EXPERIMENTAL PROCEDURE 116 RESULTS 118 Community development under environmental f l u c t u a t i o n s , and p o s t - f l u c t u a t i o n recovery ..."...118 Changes i n t o t a l d e n s i t y and r e l a t i v e abundances v i i of major taxa 118 Community development i n f l u c t u a t i n g environments 122 Recovery of the communities exposed to f l u c t u a t i n g regimes 128 The e f f e c t of community age on recovery p r o p e r t i e s ..131 T o t a l numbers and r e l a t i v e abundances of major taxa 131 Community s t r u c t u r e 134 DISCUSSION 135 Community development under f l u c t u a t i n g regimes 135 E f f e c t s of pr e v i o u s exposure to f l u c t u a t i n g regimes and of community age on recovery p r o p e r t i e s 137 SUMMARY 140 CHAPTER 3. COMMUNITY STRUCTURE REGULATION IN PERIPHYTON ...142 INTRODUCTION 142 EXPERIMENTAL PROCEDURE 146 RESULTS 147 Convergence of t o t a l c e l l numbers 147 Convergence at the d i v i s i o n l e v e l 150 Convergence at the family/genus l e v e l : a n a l y ses based on s i m i l a r i t y i n d i c e s 155 P a i r w i s e comparisons between c o n t r o l and experimental communities 155 M u l t i p l e simultaneous comparisons using P r i n c i p a l C oordinates A n a l y s i s (PCoA) 164 DISCUSSION 182 v i i i Convergence of t o t a l abundances, and of community s t r u c t u r e at the d i v i s i o n l e v e l 182 Convergence of community s t r u c t u r e at the family/genus l e v e l 183 Performance of p a i r w i s e comparisons v s . o r d i n a t i o n 183 The r e g u l a t i o n of community s t r u c t u r e 183 SUMMARY 191 CHAPTER 4. PERIPHYTON GROWTH DYNAMICS 193 INTRODUCTION 193 The models . 1 95 The data s e t s 1 99 RESULTS 204 DISCUSSION ' 210 SUMMARY 212 REFERENCES 214 APPENDIX 1. DESCRIPTION OF THE "BOOTSTRAP" ANALYSIS 228 APPENDIX 2. TRANSFORMATIONS, STANDARDIZATIONS, AND (DIS)SIMILARITY INDICES 230 Transformations and s t a n d a r d i z a t i o n s 230 In d i c e s 230 LIST OF TABLES Table I. P h y s i c a l - c h e m i c a l c h a r a c t e r i s t i c s of Gwendoline Lake 8 Table I I . Taxonomic grouping used i n t h i s study 20 Table I I I . P r o p o r t i o n s of major taxa at the three s t a t i o n s i n l a t e October 1982 and 1983 38 Table IV. T o t a l c e l l numbers - Comparisons between c o n t r o l and experimental communities 148 Table V. Summary of estimated parameters f o r the two convergence experiments 182 Table VI. Periphyton growth data 200 Table V I I . C h a r a c t e r i s t i c s of the community, waterbody, and sampling procedures f o r the data sets 202 Table V I I I . Parameter estimates and r e s i d u a l sums of squares (RSS) f o r the three models 205 X LIST OF FIGURES F i g u r e 1. L o c a t i o n of study s i t e and sampling s t a t i o n s 9 F i g u r e 2. Micrographs of the community 23 U n i d e n t i f i e d u n i c e l l u l a r a l g a 24 M a t r i x - f o r m i n g b a c t e r i a 24 Attached V o r t i c e l l a 24 Attached filamentous blue-green 24 Secondary epiphytism by diatoms 24 H i g h - d i v e r s i t y a l g a l assemblage 24 L o w - d i v e r s i t y a l g a l assemblage 24 I n h i b i t i o n of growth of neighbouring s p e c i e s 24 D i s s i n t e g r a t i o n of e r e c t - d i a t o m c l u s t e r s 24 F i g u r e 3. Time s e r i e s of IETS t o t a l abundances 32 F i g u r e 4. Time s e r i e s of IETS r e l a t i v e abundances 35 F i g u r e 5. D i v e r s i t y and s u c c e s s i o n r a t e s ....39 F i g u r e 6. O r d i n a t i o n of the f i r s t 20 weeks at s t a t i o n 1 ....43 F i g u r e 7. O r d i n a t i o n of weeks 7-20 at s t a t i o n 1 46 F i g u r e 8. O r d i n a t i o n of the f i r s t 20 weeks at s t a t i o n 2 ....49 F i g u r e 9. O r d i n a t i o n of the f i r s t 20 weeks at s t a t i o n 3 ....51 F i g u r e 10. R e l a t i v e abundances of phytoplankton and SRS ....55 F i g u r e 11. Phytoplankton and SRS o r d i n a t i o n , s t a t i o n 1 58 F i g u r e 12. Phytoplankton and SRS o r d i n a t i o n , s t a t i o n 2 61 F i g u r e 13. Phytoplankton and SRS o r d i n a t i o n , s t a t i o n 3 63 F i g u r e 14. Group 3 a b s o l u t e abundances i n phytoplankton, SRS, and IETS 67 x i F i g u r e 15. O r d i n a t i o n of s t a t i o n s 1 and 3, 1983 70 F i g u r e 16. O r d i n a t i o n of the 1982 and 1983 IETS at s t a t i o n 3 , 75 F i g u r e 17. O r d i n a t i o n of the s t a t i o n 1 IETS s t a r t e d i n 1 982: weeks 7-20 and 42-72 78 Fig u r e 18. O r d i n a t i o n of the 1982 and 1983 IETS at s t a t i o n 3 , 81 Fig u r e 19. O r d i n a t i o n of taxa i n environmental space, 1983 data 87 Fig u r e 20. A g r a p h i c a l model of p e r i p h y t o n community development i n a seasonal environment 104 Fig u r e 21. Time s e r i e s of t o t a l numbers and r e l a t i v e abundances: IA, IB, and II 119 F i g u r e 22. O r d i n a t i o n s of IA, IB, and II and t h e i r r e s p e c t i v e c o n t r o l s 123 F i g u r e 23. O r d i n a t i o n s of experimental communities during recovery 129 Fig u r e 24. T o t a l numbers and r e l a t i v e abundances of major taxa i n B, C, and D 132 F i g u r e 25. T o t a l abundance time s e r i e s : C183, C1N, and T ..148 F i g u r e 26. R e l a t i v e abundances of major taxa i n C183, T, C1N, and C383 151 F i g u r e 27. Pair w i s e comparisons between C1N and C183 using d i s s i m i l a r i t y i n d i c e s 156 F i g u r e 28. P a i r w i s e comparisons between T and C183 using d i s s i m i l a r i t y i n d i c e s 158 Fig u r e 29. Convergence of community s t r u c t u r e between C1N and C183 and between T and C183 168 F i g u r e 30. Community v a r i a t i o n along a x i s I during the convergence experiment 171 F i g u r e 31. Incident l i g h t and water temperature f o r Gwendoline Lake, 1983 174 F i g u r e 32. F i t of the negative e x p o n e n t i a l model to data from the convergence experiments 178 Fig u r e 33. Data sets and b e s t - f i t t i n g curves f o r the c o l o n i z a t i o n model 207 x i i i ACKNOWLEDGEMENTS I am very g r a t e f u l to my s u p e r v i s o r s B i l l N e i l l and C a r l Walters f o r the guidance and support they gave me throughout t h i s p r o j e c t . I b e n e f i t e d g r e a t l y from s i t t i n g i n t h e i r courses and from i n f o r m a l d i s c u s s i o n s with them. In a d d i t i o n , they generously provided f i n a n c i a l a s s i s t a n c e whenever i t was needed. B i l l N e i l l , C a r l Walters, and Don Ludwig made h e l p f u l comments-and e d i t o r i a l suggestions that improved t h i s t h e s i s . Dr. Gary B r a d f i e l d k i n d l y allowed me to use h i s o r d i n a t i o n program and provided h e l p f u l comments and r e f e r e n c e s on m u l t i v a r i a t e methods. I thank Dr. Janet S t e i n f o r h e l p i n g me i d e n t i f y some algae, and Dr. F.J.R. T a y l o r f o r a l l o w i n g me to use h i s photographic equipment. I was f o r t u n a t e to count with the f i e l d a s s i s t a n c e of Harold Waldock, A l i s t a i r B l a c h f o r d , C a r l o s G a l i n d o , and Marie-Claude B a r r e t t e , a l l of whom shared with me the t h r i l l s and c h i l l s of Gwendoline's hypolimnion. My f r i e n d s C a r l o s G a l i n d o , A l i s t a i r B l a c h f o r d , and Juan Merkt taught me a good deal about ecology i n the course of many c o n v e r s a t i o n s . To them, and to Marie-Claude B a r r e t t e , my s i n c e r e thanks f o r t h e i r encouragement. C a r l o s Galindo a l s o p r o v i d e d i n v a l u a b l e h e l p i n the p r e p a r a t i o n of the i l l u s t r a t i o n s . F i n a n c i a l support came from the N a t i o n a l Science C o u n c i l of Venezuela (CONICIT), and from two Teaching A s s i s t a n t s h i p s and a F e l l o w s h i p from the U n i v e r s i t y of B r i t i s h Columbia. 1 GENERAL INTRODUCTION The i ssues "As a matter of f a c t we have a v a r i a b l e approaching a v a r i a b l e r a t h e r than a c o n s t a n t . " H.C. Cowles, back in 1901. The turn of the century saw Henry C. Cowles transform ecology i n t o "a study of dynamics". Cowles sought to study the "laws" that govern v e g e t a t i o n change, and i n so doing he " e s t a b l i s h e d the phenomenon of p l a n t s u c c e s s i o n as fundamental to e c o l o g i c a l thought" (Mcintosh 1982, p. 14). The ideas of Cowles subsequently i n s p i r e d much work on the dynamics of p l a n t communities both d i r e c t l y , and i n d i r e c t l y through h i s i n f l u e n c e on F r e d e r i c k E. Clements (Mcintosh 1982). T h i s work was o r g a n i z e d around the concept of s u c c e s s i o n , i d e a l l y d e p i c t e d as a d i r e c t i o n a l sequence of changes undergone by the community from an i n i t i a l d i s t u r b a n c e u n t i l the attainment of a f i n a l e q u i l i b r i u m s t a t e , the "climax". The focus was t y p i c a l l y on d e s c r i b i n g d i r e c t i o n a l i t y ( o r d e r l y temporal trends i n community p r o p e r t i e s ) and on examining p r o p e r t i e s of the climax such as e x i s t e n c e , uniqueness, and p e r s i s t e n c e (see Whittaker 1953). Some of these i s s u e s have r e s u r f a c e d l a t e r i n other g u i s e s ( e q u i l i b r i u m vs. indeterminacy, m u l t i p l e s t a b l e s t a t e s , and community " t r a c k i n g " of a moving e q u i l i b r i u m ) , imbedded in the more gen e r a l framework of community dynamics. Many of the q u e s t i o n s concerning p a t t e r n and process i n contemporary community ecology are r e l a t e d to e q u i l i b r i u m n o t i o n s . For 2 example, competition theory presumes that communities are at or c l o s e to e q u i l i b r i u m ; f l u c t u a t i o n s i n n a t u r a l communities are acknowledged, o f t e n accompanied by the assumption that i n d i v i d u a l s i n the community are c l o s e l y t r a c k i n g v a r i a t i o n s i n resource l e v e l s (Wiens 1983). E q u i l i b r i a can be s t a t i c (constant i n time) or dynamic (ti m e - v a r y i n g , a l s o commonly r e f e r r e d to as moving e q u i l i b r i a ) . In the case of s t a t i c e q u i l i b r i a , the e q u i l i b r i u m s t a t e i s that towards which the community moves through time, and u l t i m a t e l y a t t a i n s i n the. absence of e x t e r n a l d i s t u r b a n c e . I f the community moves towards a given e q u i l i b r i u m independently of the s t a r t i n g p o i n t , the e q u i l i b r i u m i s a g l o b a l one. I f on the other hand s e v e r a l e q u i l i b r i a e x i s t , and the one that i s reached depends on the s t a r t i n g p o i n t , the e q u i l i b r i a are l o c a l ones (Lewontin 1969). The process of adjustment towards e q u i l i b r i u m i s termed here " r e g u l a t i o n " . In nature, environmental f l u c t u a t i o n s (such as changes i n a b i o t i c v a r i a b l e s , or i n v a s i o n s by a l i e n s p e c i e s ) are bound to cause the e q u i l i b r i u m p o i n t s to change in time. Regulation towards the dynamic e q u i l i b r i u m s t i l l o c c u r s , but how c l o s e l y the e q u i l i b r i u m i s t r a c k e d depends on the speed at which the e q u i l i b r i u m moves and on the c h a r a c t e r i s t i c s of the community ( l i f e h i s t o r i e s of the component s p e c i e s , i n t e r a c t i o n s among s p e c i e s ) . The use of e q u i l i b r i u m - b a s e d approaches i s i n v a l i d here un l e s s t r a c k i n g i s very e f f e c t i v e , t h e r e f o r e , i t i s c r u c i a l to gain some i n s i g h t i n t o the e q u i l i b r i u m s t a t u s (degree of p r o x i m i t y to e q u i l i b r i u m ) of the community. Observed changes in the community can r e f l e c t 3 e i t h e r c l o s e t r a c k i n g of a moving e q u i l i b r i u m or a t r a n s i e n t approach to e q u i l i b r i u m . A major problem i s that i n the f i e l d we can determine, community s t r u c t u r e at any given time, but we cannot measure the e q u i l i b r i u m community s t r u c t u r e at that same time. T h e r e f o r e , i t i s v i r t u a l l y impossible to assess the e q u i l i b r i u m s t a t u s of the community by d i r e c t comparison between the observed and the e q u i l i b r i u m community s t r u c t u r e . In c o n t r a s t , i f the e q u i l i b r i u m was s t a t i c , i t c o u l d be found simply by n o t i c i n g when the community stopped changing. A f u r t h e r c o m p l i c a t i o n i s that environmental f l u c t u a t i o n s occur over many time s c a l e s , and i n general we do not know to which time s c a l e s the organisms can respond. In view of these problems, an i n d i r e c t approach might be necessary and p r o f i t a b l e . I have adopted here an approach that attempts to e l u c i d a t e the e q u i l i b r i u m s t a t u s of a community through an e v a l u a t i o n of the community's c a p a c i t y to respond to environmental change. T h i s study e x p l o r e s some dynamic p a t t e r n s in community s t r u c t u r e ( d e f i n e d here as the d i s t r i b u t i o n of r e l a t i v e abundances among taxa) of freshwater at t a c h e d microalgae (periphyton) i n an o l i g o t r o p h i a l a k e . I f i r s t d e s c r i b e the dynamics of unmanipulated communities. The r e s u l t s obtained by manipulating communities are then used to i l l u s t r a t e some r e g u l a t o r y p r o p e r t i e s and to q u a n t i f y the i n t e n s i t y of r e g u l a t i o n . F i n a l l y , the r a t e s of change i n community s t r u c t u r e are r e l a t e d to estimated growth r a t e s of i n d i v i d u a l organisms. 4 A note on methodology Any e c o l o g i s t c o n f r o n t e d with the task of d e t e c t i n g , o r g a n i z i n g , and e x p l a i n i n g community p a t t e r n s i s bound to encounter a dichotomy sooner or l a t e r which w i l l f o r c e him to make a c h o i c e . Should he adopt a h o l i s t i c ( s y n t h e t i c , "top-bottom") approach to community s t u d i e s , or would he be b e t t e r o f f by embracing a r e d u c t i o n i s t ( a n a l y t i c , "bottom-top") p e r s p e c t i v e ? The r e d u c t i o n i s t approach emphasizes piecewise decomposition and a n a l y s i s of the community, as e x e m p l i f i e d by the study of "elementary processes" (see Mcintosh 1970) such as c o m p e t i t i o n . The u n d e r l y i n g r a t i o n a l e here i s that once the r e l a t i o n s between the p o p u l a t i o n s have been understood and q u a n t i f i e d , i t w i l l be p o s s i b l e to i n t e g r a t e these r e s u l t s and produce mechanistic e x p l a n a t i o n s of whole community p a t t e r n s . In the h o l i s t i c approach, on the other hand, the focus i s on the p a t t e r n s themselves. D e s c r i p t i o n and a n a l y s i s proceed at the whole community l e v e l , with the hope of o b t a i n i n g i n s i g h t s i n t o the processes o p e r a t i n g at lower l e v e l s . Because most s p e c i e s are s t u d i e d s i m u l t a n e o u s l y , the approach can i n d i c a t e which are the main s p e c i e s r e s p o n s i b l e f o r g e n e r a t i n g the p a t t e r n of i n t e r e s t , and suggest which hypotheses may be worthy of f u r t h e r t e s t i n g (perhaps by r e d u c t i o n i s t methods). The i s s u e of which approach i s to be p r e f e r r e d seems important, because the viewpoint one adopts bears on the kind of q u e s t i o n s asked, on the methodology used to address these q u e s t i o n s , and u l t i m a t e l y , on the type of answers o b t a i n e d . However, Mcintosh (1970) has p o i n t e d out that in r e a l i t y the two approaches are complementary. L e v i n s and Lewontin (1982) share t h i s view; they s t a t e that the 5 a l t e r n a t i v e s are not mutually e x c l u s i v e and they propose a " l i b e r a l p l u r a l i s m " ( i n viewpoints about the s t r u c t u r e of nature, the e x p l a n a t i o n of n a t u r a l processes, and the a p p r o p r i a t e methods for research) as a v a l i d o p t i o n . In t h i s study I have f o l l o w e d an approach akin to the h o l i s t i c , s y n t h e t i c approach d e s c r i b e d above. I performed whole community experimental manipulations, and attempted to d e s c r i b e and q u a n t i f y the dynamical responses of the communities to the d i s t u r b a n c e s induced by the m a n i p u l a t i o n s . Periphyton was chosen as a model community because of i t s f a s t dynamics, ease of r e p l i c a t i o n , and amenability, to whole community experimental m a n i p u l a t i o n s . I n c i d e n t a l l y , these advantages are mainly a consequence of the small body s i z e of the organisms; i t i s i n t e r e s t i n g t h a t t h i s same c h a r a c t e r i s t i c h i nders the a p p l i c a t i o n of r e d u c t i o n i s t methods ( s p e c i e s manipulations, ecophysiology of s i n g l e s p e c i e s i n the f i e l d ) to t h i s community. O r d i n a t i o n techniques, which allow complex m u l t i v a r i a t e data to be compressed onto a few i n f o r m a t i v e dimensions, and t h e r e f o r e f a c i l i t a t e the d e t e c t i o n and a n a l y s i s of p a t t e r n , are used h e a v i l y throughout. 6 Chapter o u t l i n e Chapter 1 d e s c r i b e s p e r i p h y t o n s u c c e s s i o n i n Gwendoline Lake, and addresses three q u e s t i o n s . Is succession d i r e c t i o n a l ? Is there an endpoint ( s t a t i o n a r y or otherwise) to succession? Are v a r i a t i o n s i n phytoplankton r e l a t i v e abundances r e f l e c t e d i n per i p h y t o n community s t r u c t u r e ? The e f f e c t of w i t h i n - l a k e s p a t i a l v a r i a b i l i t y on s u c c e s s i o n a l p a t t e r n s i s a l s o d i s c u s s e d , and chapter 1 ends with the p r o p o s a l of a conceptual model of perip h y t o n s u c c e s s i o n i n Gwendoline Lake. Chapter 2 s t u d i e s s u c c e s s i o n under e x p e r i m e n t a l l y - i n d u c e d f l u c t u a t i n g environments, and the e f f e c t s of past environmental regimes and of community age on recovery p r o p e r t i e s (the c a p a c i t y f o r adjustment towards the e q u i l i b r i u m f o l l o w i n g a d i s t u r b a n c e ) . T h i s chapter i l l u s t r a t e s how some notions d e r i v e d from t e r r e s t r i a l s uccessions can be a p p l i e d to these a q u a t i c microsystems. Chapter 3 i s a long-term a n a l y s i s of community s t r u c t u r e r e g u l a t i o n ; the pathways of recovery a f t e r d i s t u r b a n c e are d e s c r i b e d , and the r a t e s of recovery are q u a n t i f i e d u s i n g a simple model. Chapter 3 a l s o p r o v i d e s some s p e c u l a t i o n on the e q u i l i b r i u m s t a t u s of peri p h y t o n communities i n Gwendoline Lake. F i n a l l y , chapter 4 presents a comparison among some methods f o r the a n a l y s i s of periphyton growth. The growth r a t e s d e r i v e d f o r Gwendoline Lake are compared with those from other systems, and are d i s c u s s e d in r e l a t i o n to the r a t e s of change i n community s t r u c t u r e . 7 GENERAL MATERIALS AND METHODS The community " I t i s by st u d y i n g l i t t l e t h i n g s that we a t t a i n the great a r t of having as l i t t l e misery and as much happiness as p o s s i b l e . " Samuel Johnson. A simple and gen e r a l d e f i n i t i o n of peri p h y t o n i s ...that assemblage of organisms growing upon f r e e s u r f a c e s of submerged o b j e c t s i n water, and c o v e r i n g them with a slimy coat. I t i s that s l i p p e r y brown or green l a y e r u s u a l l y found adhering to s u r f a c e s of water p l a n t s , wood, stones or c e r t a i n other o b j e c t s immersed i n water... (Young 1945). Some other names that r e f e r to the same community i n c l u d e Aufwuchs, haptobenthos, and the substratum-dependent terms epiphyton, e p i l i t h o n , e p i p e l o n , epipsammon, and epidendron. B a c t e r i a , f u n g i , and mic r o s c o p i c algae are t y p i c a l components of the community, as w e l l as r o t i f e r s and c i l i a t e s , i n s e c t l a r v a e , nematodes, and s n a i l s . C r a y f i s h , t a d p o l e s , and f i s h can o c c a s i o n a l l y a ct as e x t e r n a l g r a z i n g agents. T h i s study d e a l s e x c l u s i v e l y with the a l g a l component of the community. 8 The study s i t e The experiments were conducted i n Gwendoline Lake, l o c a t e d 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 , about 60 km east of Vancouver ( F i g u r e 1). Gwendoline Lake i s o l i g o t r o p h i c , m i l d l y a c i d i c , oxygen-rich, and r e l a t i v e l y deep. Some of i t s c h a r a c t e r i s t i c s are summarized i n Table I. TABLE I - P h y s i c a l - c h e m i c a l c h a r a c t e r i s t i c s of Gwendoline Lake (from Northcote and C l a r o t t o 1975; transparency values are taken i n the summer of from 1982) 8 weekly readings • E l e v a t i o n , m 522 Drainage area, ha 81 Surface area, ha 13.0 Max. depth, m 27 Mean depth, m 13.4 C o l o r , Pt u n i t s 15 Transparency (Secchi depth, m) 6 .6-9.0 T o t a l d i s s o l v e d s o l i d s , mg/1 18 Maximal e p i l i m n e t i c depth, m 8 Shortreed et a l . (1984) have conducted an e x t e n s i v e study of the p e r i p h y t o n of 21 c o a s t a l l a k e s i n B r i t i s h Columbia' using a r t i f i c i a l s u b s t r a t a . They found that biomass and accumulation r a t e s were among the lowest recorded from temperate o l i g o t r o p h i c l a k e s . Periphyton i n these u l t r a - o l i g o t r o p h i c l a k e s seemed to be l i m i t e d by n i t r o g e n and, e s p e c i a l l y , by phosphorus. Attached algae comprised l e s s than 1% of the t o t a l a l g a l biomass and t o t a l annual carbon metabolism. The same s p e c i e s were common on n a t u r a l and a r t i f i c i a l s u b s t r a t a , although r e l a t i v e abundances F i g u r e 1. L o c a t i o n of study s i t e and sampling s t a t i o n s 11 v a r i e d . Diatoms comprised over 70% of t o t a l biomass, but Mougeotia , a green filamentous algae, c o u l d achieve l a t e summer or f a l l dominance. I know of no pre v i o u s study d e a l i n g s p e c i f i c a l l y with the pe r i p h y t o n of Gwendoline Lake. Three s t a t i o n s were chosen w i t h i n the lake (see F i g u r e 1): lake depths at s t a t i o n s 1, 2, and 3 were 13 m, 3.5 m, and 3.2 m, r e s p e c t i v e l y . S t a t i o n 1 was pl a c e d f a r away from the s h o r e l i n e and bottom ( p o t e n t i a l sources of c o l o n i z e r s ) ; s t a t i o n 2 was pl a c e d near the outflow, on a steep (approx. 45 degree) s l o p e ; s t a t i o n 3 was pl a c e d near the cente r of a r e l a t i v e l y f l a t bottom bed l y i n g between the north-east s i d e of the i s l a n d and the s h o r e l i n e . The i n i t i a l o b j e c t i v e i n choosing these three s t a t i o n s was to maximize the environmental h e t e r o g e n e i t y among the s t a t i o n s , so that d i f f e r e n t communities c o u l d subsequently develop i n them. Sampling methods Periphyton was allowed to grow on P l e x i g l a s s l i d e s (48 sq. cm. c o l o n i z a b l e area) p o s i t i o n e d v e r t i c a l l y on P l e x i g l a s r a c k s . The racks measured e i t h e r 50 x 50 cm or 50 x 30 cm, and were a t t a c h e d to aluminum frames supported by styrofoam f l o a t s . The l a r g e r racks i n i t i a l l y h e l d 60 s l i d e s and the smaller ones h e l d 30. Racks, frames, and f l o a t s were anchored to the lake bottom by polypropylene ropes t i e d to c o n c r e t e " b l o c k s . Whenever i t was r e q u i r e d , racks with s l i d e s were t r a n s f e r r e d underwater between s t a t i o n s i n s i d e a 70 x 70 x 10 cm P l e x i g l a s case. The 1 2 case minimized mechanical d i s t u r b a n c e s d u r i n g the t r a n s f e r s , each of which l a s t e d about 5. minutes. A l l the underwater t r a n s f e r s and sampling were done by SCUBA d i v e r s . S l i d e s f o r mi c r o s c o p i c examination were c o l l e c t e d at random from the racks and p l a c e d i n l a b e l l e d j a r s f i l l e d with lake water (three r e p l i c a t e s l i d e s per c o l l e c t i o n ) . Phytoplankton samples were c o l l e c t e d w i t h i n a few meters of the racks, using three 120 ml g l a s s j a r s . Phytoplankton samples were f i x e d i n the f i e l d with Lugol's s o l u t i o n . Periphyton s l i d e s were kept i n s i d e the j a r s while they were t r a n s p o r t e d to the l a b o r a t o r y ( u s u a l l y w i t h i n 3 hours), where the algae were scraped g e n t l y from both s i d e s of the s l i d e using a razor blade, resuspended i n 100-200 ml of d i s t i l l e d water, homogenized with a t e a s i n g needle, and f i x e d with Lugol's s o l u t i o n . Sporadic m i c r o s c o p i c examinations of the water l e f t i n the j a r s showed that s l o u g h i n g l o s s e s were i n s i g n i f i c a n t . Fresh s l i d e s were observed on the microscope a f t e r s c r a p i n g one s i d e , and were kept wet by repeated a d d i t i o n s of d i s t i l l e d water. No s t r u c t u r a l changes such as c e l l rupture or s w e l l i n g were ever d e t e c t e d d u r i n g these o b s e r v a t i o n s . Three subsamples were taken from each p e r i p h y t o n j a r using a wide-bore p i p e t t e , r e l e a s e d i n t o P l e x i g l a s c o u n t i n g chambers, and allowed to s e t t l e o v e r n i g h t . The volume of the a l i q u o t s was chosen so as to o b t a i n a d e n s i t y of 75-150 c e l l s / s q . mm. of tube bottom area. Phytoplankton samples were conc e n t r a t e d by a l l o w i n g them to s e t t l e . i n g l a s s c y l i n d e r s ; the top 100 ml were then c a r e f u l l y removed by s u c t i o n , and the remaining 20 ml t r a n s f e r r e d to s e t t l i n g chambers. C e l l counts were done using an i n v e r t e d microscope at 250X m a g n i f i c a t i o n . I counted a l l c e l l s f a l l i n g 13 w i t h i n the square f i e l d d e f i n e d by a Whipple o c u l a r micrometer; c e l l s touching the l e f t and top s i d e s of the square were not counted. C o l o n i e s and f i l a m e n t s were counted as one i n d i v i d u a l . Counts were made in ten Whipple f i e l d s s e l e c t e d at random from the chamber bottom, using computer-determined random c o o r d i n a t e s corr e s p o n d i n g to the X-Y t r a n s l a t i o n of the mechanical stage on the microscope. C e l l s p a t i a l d i s t r i b u t i o n i n the chamber bottom was u s u a l l y random or m i l d l y contagious ( v a r i a n c e to mean r a t i o t e s t , E l l i o t 1977). C e l l counts were s t a n d a r d i z e d to u n i t volume, and transformed to c e l l s per sq. cm. of s l i d e s u r f a c e area (periphyton) or c e l l s per l i t e r ( phytoplankton). I chose to take three subsamples per j a r and to. count ten f i e l d s per subsample a f t e r a p r e l i m i n a r y a n a l y s i s of a p e r i p h y t o n suspension of average c e l l d e n s i t y . I took ten subsamples from t h i s suspension, and made counts from 15 f i e l d s f o r each subsample. For each of the ten subsamples, I p l o t t e d estimated c e l l d e n s i t y vs. number of f i e l d s counted, and found that the mean estimated d e n s i t y u s u a l l y s t a b i l i z e d a f t e r c ounting 8 f i e l d s . I a l s o graphed the estimated d e n s i t y vs. the number of subsamples counted, and found that the d e n s i t y estimate s t a b i l i z e d a f t e r c o u n t i n g 3 subsamples. T h i s r e s u l t was almost independent of the number of f i e l d s counted per subsample, suggesting that there was more v a r i a t i o n among subsamples than w i t h i n subsamples. A f t e r a l l the samples had been counted I t e s t e d the e f f e c t i v e n e s s of the counting procedure; the c o e f f i c i e n t of v a r i a t i o n (CV = s t d . dev. X 100 / mean) among subsample d e n s i t i e s was l e s s than 30% i n 75% of the s l i d e s , and l e s s than 50% i n 88% of the s l i d e s . Large CV values (>50%) 1 4 i n v a r i a b l y came from s l i d e s with very low c e l l d e n s i t i e s . O p t i c a l microscope photographs were taken from untreated s l i d e s u s ing a Z e i s s i n v e r t e d microscope at 250X m a g n i f i c a t i o n . S l i d e s used f o r scanning e l e c t r o n microscopy (SEM) were f i x e d i n the f i e l d with a phosphate b u f f e r / g l u t a r a l d e h y d e s o l u t i o n (2.5% g l u t a r a l d e h y d e i n the f i n a l s o l u t i o n ) , dehydrated i n a graded ethanol s e r i e s , c r i t i c a l - p o i n t d r i e d , and g o l d coated f o r subsequent o b s e r v a t i o n under a Stereoscan 250T scanning e l e c t r o n microscope. Data a n a l y s i s P r i n c i p a l Coordinate A n a l y s i s (PCoA) i s an e i g e n v e c t o r o r d i n a t i o n technique developed by Gower (1966). P i e l o u (1977), Greig-Smith (1983), and Legendre and Legendre (1983) provide e x c e l l e n t i n t r o d u c t i o n s to the use of PCoA i n e c o l o g i c a l c o n t e x t s . The technique p r o v i d e s a means of r e p r e s e n t i n g the i n t r i c a t e m u l t i d i m e n s i o n a l p a t t e r n of ( d i s ) s i m i l a r i t y r e l a t i o n s h i p s among many community samples in j u s t a few dimensions i n a way that minimizes the l o s s of i n f o r m a t i o n a s s o c i a t e d with the r e d u c t i o n i n d i m e n s i o n a l i t y . In a d d i t i o n , e i g e n v e c t o r o r d i n a t i o n s can reduce the n o i s e inherent i n e c o l o g i c a l f i e l d data (Gauch 1982a). Under c e r t a i n circumstances PCoA can be c o m p u t a t i o n a l l y more expedient than the better-known p r i n c i p a l components a n a l y s i s (PCA), even when both y i e l d the same r e s u l t s . The main advantage of PCoA over PCA however, i s the f l e x i b i l i t y i t o f f e r s i n the c h o i c e of a 1 5 measure of resemblance between communities ( i f E u c l i d e a n d i s t a n c e i s the chosen measure, PCoA r e v e r t s to PCA). Any metric measure of inter-community d i s s i m i l a r i t y can be used i n PCoA, and the a n a l y s i s can be s u c c e s s f u l even when non-metrics are used (Legendre and Legendre 1983). The end product of a PCoA o r d i n a t i o n i s the best p o s s i b l e E u c l i d e a n r e p r e s e n t a t i o n of inter-community d i s t a n c e s in a reduced space ("best p o s s i b l e " i s d e f i n e d by the mathematical c r i t e r i o n of m i n i m i z a t i o n of the sum of squares of p r o j e c t i o n s of the o b j e c t s onto the s e l e c t e d subspace (Legendre and Legendre 1983)). As i n PCA, axes are ordered a c c o r d i n g to the f r a c t i o n of the t o t a l v a r i a n c e accounted f o r by the a x i s , with a x i s I accounting f o r the l a r g e s t f r a c t i o n . U s u a l l y , the f i r s t few axes capture most of the p a t t e r n i n the data (Gauch 1982a, Greig-Smith 1983). Pa t t e r n s can be i n t e r p r e t e d i n r e l a t i o n to the o r i g i n a l v a r i a b l e s by i n s p e c t i n g the c o r r e l a t i o n s between the v a r i a b l e s and the axes. In t h i s study I used PCoA mainly to d e s c r i b e and s y n t h e t i z e temporal p a t t e r n s i n community s t r u c t u r e . In a d d i t i o n , the choice of d i s s i m i l a r i t y measures o f f e r e d by PCoA was e x p l o i t e d in chapter 3 to examine the s e n s i t i v i t y of some o r d i n a t i o n r e s u l t s to changes in the d i s s i m i l a r i t y index employed. 16 CHAPTER 1. SUCCESSION AND SEASONALITY OF GWENDOLINE LAKE PERIPHYTON INTRODUCTION Analyses of s u c c e s s i o n and community dynamics i n t e r r e s t r i a l v e g e t a t i o n are u s u a l l y c o n s t r a i n e d by the time s c a l e s r e q u i r e d to make r e p r e s e n t a t i v e o b s e r v a t i o n s . I n d i v i d u a l p e r e n n i a l p l a n t s are l o n g - l i v e d when compared with human beings, and t h e r e f o r e understanding of these systems u s u a l l y comes from two sources: e x t r a p o l a t i o n of processes s t u d i e d over r e l a t i v e l y short time p e r i o d s ( u s u a l l y the i n i t i a l stages of s u c c e s s i o n ) , and i n f e r e n c e s about time sequences o b t a i n e d by i n t e g r a t i n g s p a t i a l l y separated s i t e s at d i f f e r e n t stages of development. M i l e s (1979) d i s c u s s e s the drawbacks of these methods. The processes u n d e r l y i n g p e r i p h y t o n dynamics are q u a l i t a t i v e l y s i m i l a r to those o p e r a t i n g i n t e r r e s t r i a l v e g e t a t i o n . In both systems organisms are f i x e d to a substratum and v e r t i c a l l y s t r a t i f i e d , so both communities can e x h i b i t r e l a t i v e l y s t a b l e s p a t i a l p a t t e r n s . Competition f o r l i g h t and n u t r i e n t s , r e c o l o n i z a t i o n (from e x t e r n a l sources, v e g e t a t i v e r e p r o d u c t i o n , and p r o p a g u l e s ) , and disturbance-mediated gap g e n e r a t i o n are probably commonplace processes i n t e r r e s t r i a l v e g e t a t i o n and i n p e r i p h y t o n . Secondary e p i p h y t i s m and g r a z i n g are a l s o found o f t e n in these systems, as w e l l as f u n g i - p l a n t and b a c t e r i a - p l a n t i n t e r a c t i o n s . T h i s s i m i l a r i t y has motivated P a t r i c k and Roberts (1979) and Hudon and Bourget (1981) to 17 compare t h i s a l g a l community to a m i n i a t u r e f o r e s t . The short time s c a l e s over which s u c c e s s i o n a l events occur i n the p e r i p h y t o n , coupled with the community's a m e n a b i l i t y to experimental manipulation, make i t an i d e a l s u b j e c t f o r r e s e a r c h i n v o l v i n g d i r e c t o b s e r v a t i o n s over an extended p e r i o d of time. S t u d i e s of periphyton s u c c e s s i o n u s u a l l y i n v o l v e submerging a r t i f i c i a l s u b s t r a t a and f o l l o w i n g community development f o r a few weeks. That s u c c e s s i o n does occur i s w e l l documented: even though Blum (1956) quotes . two s t u d i e s i n which no s p e c i e s replacements occurred from i n v a s i o n to "climax" c o n d i t i o n s , many authors have reported changes i n r e l a t i v e abundances of sp e c i e s through time (at times with complete replacement of some s p e c i e s by others) (see Wiegert and F r a l e i g h 1972, and the s t u d i e s c i t e d i n the d i s c u s s i o n at the end of t h i s c h a p t e r ) . The main o b j e c t i v e of the r e s e a r c h presented i n t h i s chapter was to c h a r a c t e r i z e p e r i p h y t o n s u c c e s s i o n i n Gwendoline Lake by means of a long-term study. In p a r t i c u l a r , I attempted to determine, f i r s t , i f s u c c e s s i o n was d i r e c t i o n a l ( i n the sense of p r e s e n t i n g c l e a r l y i d e n t i f i a b l e temporal trends i n community s t r u c t u r e ) , second, whether an end-point to su c c e s s i o n was reached w i t h i n the time frame of the study, and t h i r d , what e f f e c t w i t h i n - l a k e s p a t i a l v a r i a b i l i t y had on s u c c e s s i o n a l p a t t e r n s . The r e l a t i o n s h i p between su c c e s s i o n and s e a s o n a l i t y was a l s o e x p l o r e d , and I present here a simple c o n c e p t u a l model that attempts to t i e together these two aspects of community 18 dynamics. S e v e r a l authors have p r e v i o u s l y s t u d i e d how v a r i a t i o n s i n periphyton community s t r u c t u r e (Brown and A u s t i n 1973a, 1973b, H i l l e b r a n d and van D i j k 1979, M i l l i e and Lowe 1983) and i n p r o d u c t i o n (Castenholz 1961, Maciolek and Kennedy 1964), are a f f e c t e d by s p a t i a l h e t e r o g e n e i t y w i t h i n l a k e s . The e f f e c t s of seasonal v a r i a t i o n i n physico-chemical parameters on c o l o n i z a t i o n and s u c c e s s i o n have been d i s c u s s e d by Dickman (1974), Herder-Brower (1975), and Hoagland e t . a l . (1982). Secondary o b j e c t i v e s i n c l u d e d determining whether s i m i l a r i t i e s i n the responses of d i f f e r e n t p e r i p h y t i c a l g a l groups to environmental v a r i a t i o n c o u l d be r e l a t e d to taxonomic or m o r p h o l o g i c a l a f f i n i t i e s among the groups, and r e l a t i n g phytoplankton and periphyton community s t r u c t u r e and dynamics. The r e l a t i o n s h i p between per i p h y t o n and phytoplanktonic algae, which can act both as a source of propagules and as competitors, has been s t u d i e d by Welch e t . a l . (1972), Brown and A u s t i n (1973b), Moss (1981), and Oleksowicz (1982). EXPERIMENTAL PROCEDURE Three s l i d e racks were p l a c e d at s t a t i o n s 1, 2, and 3 on June 10, 1982. Racks 2 and 3 remained at 1.1 m and 1.7 m depth, r e s p e c t i v e l y , u n t i l the t e r m i n a t i o n of the study on October 29, 1983. Rack 1 stayed at 10.1 m u n t i l week 6, when i t was moved up to 4 m because c o l o n i z a t i o n and growth r a t e s were extremely low at the deeper s i t e . In 1982, p e r i p h y t o n development was f o l l o w e d by c o l l e c t i n g s l i d e s from an i n c r e a s i n g exposure-time 19 s e r i e s (IETS): these s l i d e s were immersed i n i t i a l l y on June 10; p r o g r e s s i v e l y o l d e r s l i d e s were then recovered each week d u r i n g the f i r s t 13 weeks, and then once more i n week 20, a f t e r the f a l l o v e r t u r n . Temporal v a r i a t i o n in the i n i t i a l stages of development was monitored d u r i n g the f i r s t 13 weeks using a s e q u e n t i a l r e p l a c e m e n t - s e r i e s (SRS): r e p l i c a t e s l i d e s were exposed f o r a week, recovered, and r e p l a c e d by c l e a n s l i d e s t h at were i n turn recovered a f t e r a week's exposure. Since c o l o n i z a t i o n r a t e s were low i n Gwendoline Lake, the exposure time f o r the replacement s e r i e s was extended to two weeks a f t e r week 5. Phytoplankton at a l l 3 s t a t i o n s was sampled bi-weekly d u r i n g the f i r s t 13 weeks. In 1983 the long-term development of the communities at s t a t i o n s 1 and 3 was f o l l o w e d by c o l l e c t i n g 42, 46, 50, 54, 62, and 72-week o l d s l i d e s , s t a r t i n g A p r i l 2 and ending October 29. A new IETS was s t a r t e d at s t a t i o n 1 on week 46 by submerging a set of c l e a n s l i d e s , which were then sampled on weeks 50, 54, 62, and 72. RESULTS D e s c r i p t i o n of the community: n a t u r a l h i s t o r y and microscopy  o b s e r v a t i o n s Only 4 d i v i s i o n s accounted f o r a l l of the algae encountered in the study: Chlorophyta, B a c i l l a r i o p h y t a , Chrysophyta, and Cyanophyta. Green algae ( i n p a r t i c u l a r u n i c e l l s ) and pennate diatoms predominated i n terms of numbers. Table II gi v e s the taxonomic grouping used i n t h i s study. I used P a t r i c k and 20 Table I I . Taxonomic grouping used i n t h i s study. Dashes i n d i c a t e u n i d e n t i f i e d taxa. 21 Group code Genus Family D i v i s i o n Comments 1 Chlorococcum Chlorococcaceae G ( u n i c e l l u l a r Bulbococcus Dicranochaetaceae G c o c c o i d T e t r a s p o r a Tetrasporaceae G greens) 2 P a l m e l l a Palmellaceae G 3 Dynobr i o n / Ochromonadaceae Ch E p i p y x i s 4 P a l m e l l a Palmellaceae G 5 Dynobrion/ Ochromonadaceae Ch E p i p y x i s 6 - R h i z o c h r y s i d a c e a e Ch -7 - R h i z o c h r y s i d a c e a e Ch 8 Ank i strodesmus Oocystaceae G 9 - Rh-i zochry s i daceae Ch 10 - R h i z o c h r y s i d a c e a e Ch 1 1 Paraphysomonas Synuraceae Ch 12 Gomphonema Gomphonemaceae Di 13 T a b e l l a r i a F r a g i l a r e a c e a e Di 1 4 Eunot i a Eunotiaceae Di 15 E u n o t i a Eunotiaceae Di 16 - Naviculaceae „ Di 1 7 Synedra F r a g i l a r e a c e a e Di 18 Cymbella Cymbellaceae Di 1 9 - - Di (other diatoms) 20 Cosmarium Desmidiaceae G 21 Staurastrum Desmidiaceae G 22 Cosmarium Desmidiaceae G 23 Gyrosigma Naviculaceae Di 24 - Desmidiaceae G 25 Pleurotaenium Desmidiaceae G 26 - Desmidiaceae G (other desmids) 27 Bambusina Desmidiaceae G 28 - U l o t r i c h a s c e a e G 29 Mougeotia Zygnemataceae G 30 S p i r o g y r a Zygnemataceae G 31 Mougeot i a Zygnemataceae G 32 Mougeot i a Zygnemataceae G 33 Bulbochaete Oedogoniaceae G 34 Mougeotia • Zygnemataceae G 35 - Osc i l l a t o r i a c e a e BG 36 - O s c i 1 1 a t o r i a c e a e BG 37 Anabaena Nostocaceae BG 38 Mi c r o s p o r a Microsporaceae G (other f i l a m e n t s ) 39 - Zygnemataceae G 40 M i c r o c y s t i s Chroococcaceae BG 41 Merismopedia Chroococcaceae BG 42 G l o e o c y s t i s Palmellaceae G 43 G l o e o c y s t i s Palmellaceae G 44 - • - G (other c o l o n i e s ) 45 - — G (other greens) G = Greens ( C h l o r o p h y t a ) ; Ch = Chrysophyceans (Chrysophyta) Di = Diatoms ( B a c i l l a r i o p h y t a ) ; BG = Blue-green (Cyanophyta) 22 Reimer (1966, 1975), Smith (1950), B o u r r e l l y (1966), and S t e i n (1975) as r e f e r e n c e s f o r i d e n t i f i c a t i o n purposes, and was a l s o k i n d l y a s s i s t e d by Dr. Janet S t e i n from the UBC Botany Department. Group 1 lumps together u n i c e l l u l a r greens that became i n d i s t i n g u i s h a b l e a f t e r f i x a t i o n with Lugol's s o l u t i o n . W i t hin t h i s group, the organism I c a l l e d Bulbococcus was hard to i d e n t i f y and has perhaps been misnamed. I i n c l u d e i t s p i c t u r e here ( F i g u r e s 2a and 2b) because the al g a was very abundant at a l l stages of community development. Groups 3 and 5 were proble m a t i c because d i s t i n g u i s h i n g Dinobryon from E p i p y x i s i s d i f f i c u l t . E p i p y x i s tends to be i n d i v i d u a l and attached, whereas Dinobryon i s found both i n attached and p l a n k t o n i c forms, u s u a l l y i n c o l o n i e s . Group 3 c o n s i s t e d both of c o l o n i a l and i n d i v i d u a l forms,, whereas only i n d i v i d u a l c e l l s were i n c l u d e d i n group 5. The "other" c a t e g o r i e s are more aggregated than the remaining groups, but never accounted f o r more than a small f r a c t i o n of the t o t a l abundance. Both b a c t e r i a and c i l i a t e protozoans, many of which were attached, l i k e Vort i c e l l a were very common on the s l i d e s ( F i g u r e s 2c, 2d, 2e). Protozoan biomass was not q u a n t i f i e d , but seemed very low compared to th a t of algae. A red Hydra s p e c i e s appeared o c c a s i o n a l l y , and acted as a l i v i n g substratum f o r the c i l i a t e Kerona . Nematodes, d a m s e l f l i e s , and chironomid l a r v a e , a l l of which are common on n a t u r a l s u b s t r a t a in Gwendoline Lake, were onl y r a r e l y found on the s l i d e s . Grazers have been found to be r a r e ( l e s s than 2% of the t o t a l biomass) on lake and marine g l a s s p l a t e s even though they might be abundant on 23 F i g u r e 2. Micrographs of the community a, b: U n i d e n t i f i e d u n i c e l l u l a r a l g a (Bulboccocus?) c, d: M a t r i x - f o r m i n g b a c t e r i a growing upon (scenescent?) a l g a l c e l l s . e: Attached V o r t i c e l l a f : Filamentous blue-green a t t a c h e d to a green f i l a m e n t , g: Diatoms e p i p h y t i c on Bulbochaete h, i : H i g h - d i v e r s i t y a l g a l assemblages. N o t i c e the a v a i l a b i l i t y of attachment s i t e s . j , k: L o w - d i v e r s i t y , high d e n s i t y diatom assemblage. 1, m, n: I n h i b i t i o n of growth of neighbouring s p e c i e s , presumably due to l o c a l i n t e r a c t i o n s . o, p: D i s s i n t e g r a t i o n of e r e c t - d i a t o m c l u s t e r s . 24 25 26 27 28 n a t u r a l s u b s t r a t a (Castenholz 1961). Brown and A u s t i n (1971, 1973) a l s o found that i n v e r t e b r a t e s were not w e l l represented i n v e r t i c a l s l i d e s , but t h i s i s not always the case (Cattaneo and K a l f f 1978). I was once able to observe under the microscope a chironomid l a r v a g r a z i n g on a h i g h - d e n s i t y diatom a s s o c i a t i o n : there were no v i s i b l e e f f e c t s of the animal's a c t i v i t y on the a l g a l community. A gut d i s s e c t i o n showed that the l a r v a had been fee d i n g only on pedunculate diatoms (more than 95% Gomphonema; the r e s t mainly Eunotia ). The remainder of t h i s s e c t i o n p r e s e n t s m i c r o s c o p i c o b s e r v a t i o n s of the s t r u c t u r a l changes undergone by the communities d u r i n g the 17 months of the study, and of some s p a t i a l p a t t e r n s that developed on the s l i d e s . A f t e r exposure fo r one week, scanning e l e c t r o n microscopy (SEM) o b s e r v a t i o n s showed that the s l i d e s were covered with a t h i n white coat of b a c t e r i a ( b a c i l l i and u n i d e n t i f i e d matrix-forming c o l o n i e s ) , and some algae were a l r e a d y present. A f t e r three weeks the white coat was no longer v i s i b l e , and a green t u f t c o n s i s t i n g mainly of S p i r o g y r a and Mougeotia f i l a m e n t s developed u n t i l i t became 2-5 cm t h i c k . Green f i l a m e n t s a t t a c h e d to the polypropylene cords s e c u r i n g the racks c o u l d grow up to a l e n g t h of >50 cm, and were e a s i l y broken or detached, even by m i l d water t u r b u l e n c e . Once the s l i d e s had been underwater f o r more than 5-8 weeks, d i f f e r e n c e s between communities of v a r i o u s s t a t i o n s and ages became d i f f i c u l t to d i s t i n g u i s h j u s t by simple i n s p e c t i o n 29 under the microscope. In most communities u n i c e l l u l a r c o c c o i d greens and diatoms were n u m e r i c a l l y dominant, but many other groups were present, y i e l d i n g a s p e c i e s - r i c h panorama ( F i g u r e s 2f and 2g). In a d d i t i o n , the numerical d e n s i t i e s of c e i l s d i r e c t l y a t t a c h e d to the s u b s t r a t a were low, and f r e e space was p l e n t i f u l even i n communities more than one year o l d . However, the 1983 s p r i n g communities at s t a t i o n 1 were e x c e p t i o n a l i n t h i s r e s p e c t ; they were dominated by a few diatom groups, and numerical d e n s i t i e s of d i r e c t l y attached c e l l s were high (compare the amount of f r e e s l i d e space l e f t i n F i g u r e s 2f, 2g and 2h, 2 i ) . The a l g a l l a y e r was r e l a t i v e l y t h i n and brown/green. Filamentous green algae were v i r t u a l l y absent from these mid-depth s p r i n g communities, and were not found during s p r i n g nor summer at gre a t e r depths. I v e r i f i e d t h i s through m i c r o s c o p i c examination of the periphyton growing on some v e r t i c a l l y suspended p l a s t i c sheets found near s t a t i o n 1. These sheets were remnants from a pr e v i o u s experiment and had been anchored to the bottom of the lake f o r more than three years (W.E. N e i l l p e r s . comm.). Samples cut out from the sheets at a depth of 10m d u r i n g the summer were covered s o l e l y with diatoms and d e b r i s . However, about 2 weeks a f t e r the f a l l o v e r t u r n , fil a m e n t o u s greens were able to c o l o n i z e the po l y p r o p y l e n e ropes and p l a s t i c sheets down to the bottom of the lake at 13 m depth. Green f i l a m e n t s generated three-dimensional s t r u c t u r e in the community and allowed the occurrence of secondary attachment. Mougeotia and e s p e c i a l l y Bulbochaete were c o l o n i z e d by diatoms, chrysophyceans, and blue-greens ( F i g u r e s 2 j , 2k). The f i l a m e n t s a l s o r e t a i n e d t i n y clumps of organic d e b r i s (presumably d e r i v e d 30 from the plankton) that contained many a l g a l c e l l s . D i r e c t o b s e r v a t i o n s of the communities on the s l i d e s u s i n g l i g h t microscopy and SEM showed that the s p a t i a l d i s t r i b u t i o n of the algae was patchy in communities more than 5-8 weeks o l d : bare regions were juxtaposed with s i n g l e - t a x o n "stands" and with heterogeneous group mixtures. Diatoms were d i s t i n c t l y more abundant along the upper and l a t e r a l edges of the s l i d e s , a f i n d i n g t h at might be r e l a t e d to m i c r o s c a l e t u r b u l e n c e and d i f f u s i o n p a t t e r n s . Cattaneo (1978, and r e f e r e n c e s t h e r e i n ) and Sladeckova (pers. comm.) have a l s o r e p o r t e d edge p r e f e r e n c e by diatoms. No evidence of d i r e c t p h y s i c a l i n t e r a c t i o n s such as overgrowth or u n d e r c u t t i n g was found, although f i g u r e s 21, 2m, and 2n show that some s p e c i e s might be able to i n h i b i t growth of other s p e c i e s i n t h e i r immediate neighbourhood. The i n h i b i t i n g s p e c i e s c o u l d be producing a l l e l o p a t h i c compounds or simply reducing n u t r i e n t s i n i t s v i c i n i t y to l e v e l s i n t o l e r a b l e f o r other s p e c i e s . Another i n t e r e s t i n g s p a t i a l p a t t e r n was found i n the 1983 s p r i n g communities at s t a t i o n 1: f i g u r e s 2o and 2p show h i g h - d e n s i t y c l u s t e r s of Gomphonema. These c l u s t e r s seem to d i s i n t e g r a t e from the center outwards, and there i s a change i n c o l o r (from y e l l o w / l i g h t - b r o w n to dark brown) a s s o c i a t e d with the decay and disappearance of i n d i v i d u a l c e l l s . The sequence of d i s i n t e g r a t i o n suggests that a n u t r i e n t g r a d i e n t r e l a t e d to c e l l d e n s i t i e s might be o p e r a t i n g , with the c e n t r a l c e l l s r e c e i v i n g l e s s and l e s s n u t r i e n t s as the clump grows outwards from i t s p e r i p h e r y . 31 Community development  T o t a l numbers F i g u r e s 3a-3c show the course of a l g a l development at s t a t i o n s 1, 2, and 3 f o r the f i r s t 20 weeks. C e l l numbers remained low at s t a t i o n 1 u n t i l week 6, when the s t a t i o n was r a i s e d from 10.1 m to 4 m. From week 6 on, numbers i n c r e a s e d r a p i d l y to about 26300 c e l l s / s q cm i n week 13, and then d e c l i n e d to 16000 c e l l s / s q cm i n week 20. At a l l three s t a t i o n s there seemed to be an i n i t i a l l a g a f t e r which numbers i n c r e a s e d r a p i d l y . The net growth r a t e decreased at s t a t i o n 2, and became negative at s t a t i o n 1, between weeks 13 and 20, but does not seem to have changed much at s t a t i o n 3 d u r i n g the same time p e r i o d . A f t e r 20 weeks, numbers had not s t a b i l i z e d at any of the s t a t i o n s . T h i s r e s u l t c o n t r a s t s with the commonly encountered p a t t e r n of biomass s t a b i l i z a t i o n (or r a t h e r , f l u c t u a t i o n about a constant or slowly v a r y i n g mean) a f t e r 4-12 weeks (see Table VI i n chapter 4). F i g u r e s 3d and 3e show c e l l numbers vs. time at s t a t i o n s 1 and 3 i n 1983. The v a r i a t i o n s i n numbers might r e f l e c t seasonal trends; both s t a t i o n s showed an in c r e a s e i n c e l l numbers through s p r i n g , and summer and f a l l l e v e l s were much higher than those of f a l l 1982, which corresponded to week 20. F i g u r e 3f shows that the i n c r e a s e i n numbers i n the 1983 i n c r e a s i n g exposure-time s e r i e s (IETS) was s i g m o i d a l . Again there was an i n i t i a l l a g ; although the growth r a t e d i m i n i s h e d d u r i n g the l a t e r weeks, a p l a t e a u had not been reached even a f t e r 26 weeks. 32 Fi g u r e 3. Time s e r i e s of IETS t o t a l abundances. Means are connected by s t r a i g h t l i n e s , unconnected dots represent s c a t t e r about the means, a: St.1, 1982. b: St.3, 1982. c: St.3, 1982. d: St.1, 1983. e: St3, 1983. f : IETS s t a r t e d i n 1983. 42 72 4 2 72 30 T I M E ( in w e e k s ) 34 Community s t r u c t u r e a t t h e d i v i s i ' o n l e v e l F i g u r e s 4a-4c show t h e c h a n g e s i n r e l a t i v e a b u n d a n c e s ( b a s e d on c e l l numbers) o f t h e f o u r m a j o r t a x o n o m i c g r o u p s . The a v e r a g e c o e f f i c i e n t of v a r i a t i o n (CV) o f t h e s a m p l e s c o l l e c t e d a t s t a t i o n s 1, 2, and 3 b e f o r e week 6 was h i g h ( 8 5 . 4 % ) , i n p a r t b e c a u s e o f t h e low c e l l d e n s i t i e s t y p i c a l o f t h e s e e a r l y s a m p l e s . The a v e r a g e CV f o r s a m p l e s from weeks 6-20 was 35.3%. At s t a t i o n 1 d i a t o m s and c h r y s o p h y c e a n s were r e l a t i v e l y uncommon b e f o r e week 6, and g r e e n a l g a e p r e d o m i n a t e d . However, a f t e r t h e t r a n s f e r t o t h e s h a l l o w e r s i t e , d i a t o m s and c h y s o p h y c e a n s i n c r e a s e d i n r e l a t i v e a b u n d a n c e . A f t e r 20 weeks d i a t o m s and g r e e n s were t h e most abundant g r o u p s , f o l l o w e d by b l u e - g r e e n s . E x c l u d i n g t h e f i r s t 6 weeks a t s t a t i o n 1, t h e t r e n d a t a l l t h r e e s t a t i o n s c a n be summarized as f o l l o w s : g r e e n a l g a e ( e s p e c i a l l y u n i c e l l u l a r c o c c o i d f o r m s ) were t h e e a r l y d o m i n a n t s . T h e i r r e l a t i v e a bundance d e c r e a s e d as d i a t o m , c h r y s o p h y c e a n , and t o a l e s s e r e x t e n t b l u e - g r e e n r e l a t i v e a b u n d a n c e s i n c r e a s e d t h r o u g h t h e summer. D i a t o m s and b l u e - g r e e n s d i d not change much from l a t e summer t o f a l l , whereas c h r y s o p h y c e a n r e l a t i v e a b u n d a n c e s d r o p p e d d u r i n g t h e same p e r i o d . T h e r e i s a r e m a r k a b l e agreement of p a t t e r n s between s t a t i o n 2 and s t a t i o n 3 a f t e r week 4. D i a t o m s were p r o p o r t i o n a t e l y more abundant a t s t a t i o n 1 by week 20 t h a n a t s t a t i o n s 2 and 3. A f t e r week 5 t h e a b s o l u t e a b u n d a n c e s o f a l l g r o u p s i n c r e a s e d a t t h e t h r e e s t a t i o n s , w i t h t h e e x c e p t i o n of t h e c h r y s o p h y c e a n s , whose a b s o l u t e a b u n d a n c e s were l o w e r on week 20 t h a n on weeks 12 and 13. The o b s e r v e d c h a n g e s i n r e l a t i v e a b u n d a n c e s were t h e r e f o r e m a i n l y due t o d i f f e r e n t i a l r a t e s of i n c r e a s e i n a b s o l u t e a b u n d a n c e s among 35 Figure 4. Time series of IETS r e l a t i v e abundances, a: St.1, 1982. b: St.2, 1982. c: St.3, 1982. d: St.1, 1983. e: St.3, 1983. o wo 37 taxa, and not to i n c r e a s e s i n some taxa and decreases i n o t h e r s . F i g u r e s 4d and 4e show the r e l a t i v e abundances of the four groups at s t a t i o n s 1 and 3 d u r i n g 1983. The average CV f o r these samples was 39.9%. At s t a t i o n 1 chrysophycean and b l u e -green p r o p o r t i o n s v a r i e d l i t t l e from s p r i n g to f a l l ; d u r i n g the s p r i n g diatoms decreased from 75% to 59% and greens i n c r e a s e d from 12% to 29%, and there was h a r d l y any change in o v e r a l l community s t r u c t u r e from l a t e June to l a t e October. R e l a t i v e abundances remained f a i r l y constant at s t a t i o n 3 u n t i l l a t e summer/fall, when blue-greens i n c r e a s e d . I t i s hard to evaluate the s i g n i f i c a n c e of t h i s i n c r e a s e , s i n c e the data p o i n t s are separated by r a t h e r wide time i n t e r v a l s . However, blue-green r e l a t i v e abundances i n c r e a s e d at a l l s t a t i o n s both i n 1982 and 1983 between l a t e summer and f a l l , which suggests that t h i s may be a r e g u l a r seasonal phenomenon. Community s t r u c t u r e on October 27 1982 and October 29 1983 i s summarized f o r the three s t a t i o n s i n Table I I I . A f t e r 20 weeks, the communities at s t a t i o n s 2 and 3 were s i m i l a r i n s p i t e of t h e i r h o r i z o n t a l s p a t i a l s e p a r a t i o n i n the l a k e . A between-years comparison can be made f o r s t a t i o n s 1 and 3 i n 1982 and 1983, s i n c e the data i n the t a b l e above come from v i r t u a l l y i d e n t i c a l calendar days: chrysophycean and blue-green p r o p o r t i o n s were s i m i l a r between years f o r both s t a t i o n s ; at s t a t i o n 1 however, diatoms were much more common in the second year than i n the f i r s t . At s t a t i o n 3, blue-greens were more common i n the second year than i n the f i r s t . These r e s u l t s 38 TABLE I I I - Proportions of major taxa at the three s t a t i o n s i n l a t e October 1982 and 1983. Standard d e v i a t i o n given i n p a r e n t h e s i s . S t a t i o n 1 82 83 S t a t i o n 2 82 83 S t a t i o n 3 82 83 Chrysophyceans 6. , 1 4. ,8 10. ,9 8. .4 5. ,4 (3. •4) (1 . ,1 ) (8. . 1 ) (1 . .1 ) (1 . ,1 ) Greens 42. ,3 28. .0 42. ,8 46. .6 40. ,2 (7. .6) (13. ,9) (4. ,1 ) (4. ,3) (5. .4) Diatoms 37. ,2 53. ,8 18. ,4 19. .9 18. .8 (4. .7) (14. .3) (3. .5) (0. .9) (2. .3) Blue-greens 14. .5 13. .4 27. ,9 25. .2 36. .3 (5. .1 ) (5. .9) (2. ,4) (3. .9) (4. .3) suggest e i t h e r a high year-to-year v a r i a b i l i t y or that 20 weeks of submergence at s t a t i o n 3 (or 20 - 6 = 14 for the 4 m-deep s i t e at s t a t i o n 1) were not enough to allow the development of communities t r u l y r e p r e s e n t a t i v e of those s i t e s . D i v e r s i t y and succession r a t e s ; index-based analyses The number of taxonomic groups (see Table I) counted (a measure r e l a t e d to "richness") i s graphed vs. time i n Figures 5a-5c. On any given date, none of the s l i d e s had more than 34 groups present. At s t a t i o n s 2 and 3, the number of groups rose r a p i d l y i n i t i a l l y and always remained between 15 and 34 a f t e r week 4. At s t a t i o n 1 i t remained below 10 for the f i r s t 5 weeks, but increased q u i c k l y a f t e r the -station was r a i s e d . The 1982 data for s t a t i o n s 1 and 3 seem to show a trend towards increased richness i n summer and f a l l r e l a t i v e to s p r i n g , although the 39 F i g u r e 5. D i v e r s i t y and s u c c e s s i o n r a t e s . a,b,c: Number of taxa counted vs. time, at s t a t i o n s 1, 2, and 3. d,e,f: Jassby-Goldman s u c c e s s i o n r a t e at s t a t i o n s 1, 2, and 3. Succession rate Number of groups 41 August 19 (week 62) v a l u e s at s t a t i o n 1 are c l o s e to s p r i n g ones. O v e r a l l , the three s t a t i o n s are s i m i l a r with respect to group r i c h n e s s , both in a b s o l u t e values and i n temporal t r e n d s . I c a l c u l a t e d H', the Shannon d i v e r s i t y (H' = -£pi*ln(Pi); P i = p r o p o r t i o n of taxon i ) , and E, the evenness (E = H ' / l n ( S ) , where S i s the number of groups in the sample) f o r the three s t a t i o n s , and p l o t t e d these values vs. time. The graphs are not shown here, because they are not p a r t i c u l a r l y i n f o r m a t i v e : both H' and E i n c r e a s e d to a p l a t e a u a f t e r 1 or 2 weeks, and d i d not vary much a f t e r t h a t . Succession r a t e s were c a l c u l a t e d using a measure p r e v i o u s l y employed by Jassby and Goldman (1974, see formula i n Appendix 2). T h i s measure estimates the r a t e of change i n community s t r u c t u r e but does not p r o v i d e i n f o r m a t i o n about the d i r e c t i o n a l i t y of change. I t i s i n s e n s i t i v e to a b s o l u t e abundances, t a k i n g i n t o account only the r e l a t i v e abundance of each s p e c i e s or group (Lewis, 1978). The r e s u l t s are shown i n F i g u r e s 5d-5f. There i s no d i s c e r n i b l e t r e n d i n s u c c e s s i o n r a t e s through time at any of the s t a t i o n s and furthermore, the r a t e s seem to vary independently among s t a t i o n s . No c l e a r r e s u l t s emerge from t h i s a n a l y s i s because the technique employed to study s u c c e s s i o n a l change generates only p a i r w i s e comparisons (between communities at times t and t+1). 42 The f i r s t 20 weeks: o r d i n a t i o n of community t r a j e c t o r i e s PCoA i s a technique that i s w e l l s u i t e d f o r the a n a l y s i s of r a t e and d i r e c t i o n a l i t y of s u c c e s s i o n . I t g i v e s a compressed and simultaneous r e p r e s e n t a t i o n of the d i s t a n c e s between a l l p o i n t s r e p r e s e n t i n g the community t r a j e c t o r y , and i s t h e r e f o r e not r e s t r i c t e d to p a i r w i s e comparisons as s u c c e s s i o n r a t e i n d i c e s a r e . E u c l i d e a n d i s t a n c e (see Appendix 2) was used as a d i s s i m i l a r i t y measure i n the o r d i n a t i o n s throughout t h i s chapter, so the PCoA employed here i s f o r m a l l y e q u i v a l e n t to p r i n c i p a l component a n a l y s i s (PCA). Unless s p e c i f i e d otherwise, the data were s t a n d a r d i z e d by s i t e norm (see Appendix 2) before c a l c u l a t i n g E u c l i d e a n d i s t a n c e s . R e p l i c a t e s (three per sample) are represented i n the o r d i n a t i o n s as apexes of t r i a n g l e s ; the s i z e of a t r i a n g l e t h e r e f o r e p r o v i d e s a measure of the w i t h i n -sample v a r i a b i l i t y . The l i n e s c o nnecting temporal sequences pass by the c e n t r o i d s of the t r i a n g l e s . An o r d i n a t i o n f o r the f i r s t 20 weeks at s t a t i o n 1 i s shown in F i g u r e 6. The f i r s t 6 weeks (before the upwards t r a n s f e r ) are n e a t l y separated along AI from the l a s t 14. During the f i r s t 6 weeks t o t a l abundances were low and c o l o n i z a t i o n was probably more important than growth i n determining community s t r u c t u r e (because l i t t l e or no growth o c c u r r e d in weeks 1-6, see F i g u r e 15); t h i s p e r i o d was c h a r a c t e r i z e d by dominance of groups 1 and 2 ( u n i c e l l u l a r green a l g a e , a p p a r e n t l y very e f f e c t i v e c o l o n i z e r s ) . The r a p i d s h i f t along AI that occurred between weeks 6 and 7 r e f l e c t s a r e d u c t i o n i n the frequency of group 2, and an i n c r e a s e i n groups 1 and 3. Temporal v a r i a t i o n 43 F i g u r e 6. O r d i n a t i o n of the f i r s t 20 weeks at s t a t i o n 1. Axes I and II account f o r 51% and 20% of the t o t a l v a r i a n c e . The c r o s s marks the o r i g i n of the two axes. 44 Axis 1 45 i n community s t r u c t u r e lacked d i r e c t i o n a l i t y d u r i n g the f i r s t 6 weeks, as might be expected i f c o l o n i z a t i o n were h i g h l y v a r i a b l e and growth r a t e s low. Since most of the v a r i a t i o n i n the data set comes from the s e p a r a t i o n between pre- and p o s t - t r a n s f e r communities, Al d i s t i n g u i s h e s c l e a r l y between these two groups, but says l i t t l e about the p o s t - t r a n s f e r one. T h e r e f o r e , a new o r d i n a t i o n was performed only on the p o s t - t r a n s f e r samples, so as to increase the i n t e r p r e t a b i l i t y of the data. F i g u r e 7 shows the r e s u l t . Al showed high p o s i t i v e c o r r e l a t i o n s with s e v e r a l groups: most diatoms, blue-greens, and s e v e r a l green f i l a m e n t s ; group 1 ( u n i c e l l u l a r green c o c c o i d s ) decreased (r=-0.96) along t h i s a x i s . A l l was n e g a t i v e l y c o r r e l a t e d (r=-0.96) with group 3 (Dinobryon/Epipyxis) . Weeks 7-10 showed l i t t l e change with the e x c e p t i o n of week 8, which was a f f e c t e d by a t r a n s i t o r y pulse of group 3. A second pulse of group 3, combined with i n c r e a s e s i n diatoms and blue-greens, accounts f o r the downward displacement of 12-week and 13-week communities. Week 20 showed dominance by group 1 and diatoms, green f i l a m e n t s and blue-greens; group 3 was scarce at t h i s stage. The o r d e r i n g of the samples along Al i n d i c a t e s that as communities aged, group 1 decreased in r e l a t i v e abundance, while Gomphonema , groups 15, 16, 28, 31, 39, and 41 p r o g r e s s i v e l y became more common. T h i s t r a n s i t i o n , however, was not smooth: there were p e r i o d s where l i t t l e change (or even r e v e r s a l s ) occurred, i n d i c a t i n g that both the rate of change and the d i r e c t i o n of the developmental t r a j e c t o r y may respond to short term (two weeks or l e s s ) components of 46 F i g u r e 7. O r d i n a t i o n of weeks 7-20 at s t a t i o n 1. Axes I and II account f o r 47% and 35% of the t o t a l v a r i a n c e . 47 Axis 1 48 environmental v a r i a t i o n . Sporadic events, such as the group 3 p u l s e s , m o d i f i e d the community s t r u c t u r e over short p e r i o d s of time (the group 3 p u l s e s and t h e i r e f f e c t s on community s t r u c t u r e w i l l be analysed l a t e r ) . F i gure 8 shows the o r d i n a t i o n f o r the f i r s t 20 weeks at s i t e 2. AI i s c o r r e l a t e d p o s i t i v e l y (r=0.92) with group 1; the 5 next highest c o r r e l a t i o n s are a l l negative, with groups 3, 12, 40, 41, and 45. A l l i s harder to i n t e r p r e t i n terms of the responses of j u s t a few groups, s i n c e the c o r r e l a t i o n values are g e n e r a l l y low, and more or l e s s evenly d i s t r i b u t e d among many groups. In any case, v a r i a t i o n along t h i s a x i s i s r e l a t i v e l y unpatterned with respect to time. The f i r s t 3 weeks show no t r e n d with r e s p e c t to AI and A l l . However, l a t e r samples are arranged i n an ordered time-sequence along AI. As was the case with s t a t i o n 1, there i s a temporal trend along AI, towards r e d u c t i o n of group 1 and a s s o c i a t e d i n c r e a s e s i n other groups. D i r e c t i o n a l movement along AI ceases a f t e r week 12, and the unpatterned change that f o l l o w e d was due mainly to f l u c t u a t i o n s in the r e l a t i v e abundance of group 3. The o r d i n a t i o n of s t a t i o n 3 samples i s given i n F i g u r e 9. AI i s c o r r e l a t e d n e g a t i v e l y (r=-0.95) with group 1; groups 3,4,12,41, and 45 account f o r the 5 next h i g h e s t c o r r e l a t i o n s ( a l l p o s i t i v e ) . The highest c o r r e l a t i o n s with A l l come from groups 4 and 5 ( p o s i t i v e ) and from groups 3, 15, and 40 ( n e g a t i v e ) . In t h i s case the samples are ordered with respect to time along both AI and A l l . Group 1 achieved c l e a r dominance 49 F i g u r e 8. O r d i n a t i o n of the f i r s t 20 weeks at s t a t i o n 2. Axes I and II account f o r 41% and 16% of the t o t a l var i a n c e . 50 Axis I 51 Figure 9. O r d i n a t i o n of the f i r s t 20 weeks at s t a t i o n 3. Axes I and II account f o r 30% and 23% of the t o t a l v a r i a n c e . 52 Axis I 53 only a f t e r week 3, r e l a t i v e l y l a t e i n comparison with s t a t i o n s 1 and 2. At these s t a t i o n s , group 1 was al r e a d y dominant by week 1, and from then on i t s r e l a t i v e abundance d i m i n i s h e d . Community s t r u c t u r e at s t a t i o n 3 changed c o n t i n u a l l y u n t i l l a t e J u l y , when a p e r i o d of s t a b i l i t y l a s t e d from week 6 u n t i l week 10, i n l a t e August. At that p o i n t , a pu l s e of group 3 caused a r a p i d s h i f t i n community s t r u c t u r e , p l a c i n g weeks 12 and 13 to the r i g h t along A l . Group 3 r e l a t i v e abundance decreased from weeks 12 and 13 to week 20, and group 1 r e l a t i v e abundance in c r e a s e d , causing week 20 samples to move to the l e f t . A problem that a r i s e s o f t e n i n the i n t e r p r e t a t i o n of o r d i n a t i o n s i s the presence of an "arch" or "horseshoe" e f f e c t (Greig-Smith 1983). The arch e f f e c t b r i n g s together the extreme samples of communities c o l l e c t e d along a continuous environmental g r a d i e n t , "bending" what should be a l i n e a r o r d e r i n g of the samples. The second and subsequent axes of the o r d i n a t i o n might then be q u a d r a t i c or hi g h e r - o r d e r d i s t o r t i o n s of a x i s I. The q u a d r a t i c d i s t o r t i o n i n the second a x i s i s g e n e r a l l y the most important one, and the a r t i f a c t represented i n t h i s a x i s might d i s p l a c e e c o l o g i c a l l y r e l e v a n t i n f o r m a t i o n towards higher axes (Gauch 1982b). The arch e f f e c t can a r i s e due to n o n - l i n e a r i t i e s in the response of sp e c i e s to u n d e r l y i n g environmental g r a d i e n t s (Gauch 1982b), or to d i s c o n t i n u i t i e s in a data set such that some s i t e s (stands, i n d i v i d u a l s ) have no species i n common (Williamson 1978). Since s i t e 2 and s i t e 3 o r d i n a t i o n s were c l e a r l y U-shaped, I su b j e c t e d both to a PCoA o r d i n a t i o n employing the " s t e p - a c r o s s " method developed by 54 Williamson (1978), and m o d i f i e d f o r q u a n t i t a t i v e data by Dr. G. B r a d f i e l d of the UBC Botany Department. Williamson (1978) has shown that the method can s t r a i g h t e n out the c u r v a t u r e i n both a r t i f i c i a l and r e a l data s e t s . My o b j e c t i v e was to arrange the samples i n an ordered time-sequence along a s i n g l e a x i s i n order to i n c r e a s e the i n t e r p r e t a b i 1 i t y of that a x i s . The r e s u l t of the o r d i n a t i o n s was a compression of one of the "arms" of the U-shaped curves of F i g u r e s 8 and 9. A J-shaped curve r e s u l t e d i n which the e a r l y - s t a g e samples were moved c l o s e r to the 4-7 week ones. The o b j e c t i v e s t a t e d above c o u l d not be f u l f i l l e d , i n d i c a t i n g t h at the cu r v a t u r e i n the o r d i n a t i o n s d i d not r e s u l t from an overabundance of j o i n t absences. I n t e r r e l a t i o n s h i p s between p l a n k t o n i c and p e r i p h y t i c communities The r e l a t i v e abundances of c e l l s of- the four major taxonomic groups i n the phytoplankton are p l o t t e d vs. time i n F i g u r e s 10a-10c. The p a t t e r n i s roughly the same at the three s t a t i o n s . Diatoms were very scarce d u r i n g the 12-week p e r i o d s t u d i e d . Blue-greens were uncommon i n week 1 samples, but were encountered f r e q u e n t l y a f t e r t h a t . Green algae were the o v e r a l l dominants (mainly i n the form of small u n i c e l l s , as i n the p e r i p h y t o n ) , although week 11 provided a conspicuous e x c e p t i o n , due to a chrysophycean p u l s e . Chrysophyceans had been v i r t u a l l y absent at a l l s t a t i o n s d u r i n g the f i r s t 5 weeks; they i n c r e a s e d somewhat d u r i n g week 7 at s t a t i o n 2, and d u r i n g week 9 at s t a t i o n s 1 and 3. T h e i r r e l a t i v e abundances peaked suddenly duri n g week 11, but had a l r e a d y d e c l i n e d to e a r l i e r l e v e l s by week 13. Group 3 was r e s p o n s i b l e f o r the abrupt i n c r e a s e and 55 F i g u r e 10. R e l a t i v e abundances of phytoplankton and SRS. a,b,c: Phytoplankton at s t a t i o n s 1, 2, and 3. d,e,f: SRS at s t a t i o n s 1, 2, and 3. 57 subsequent d e c l i n e . It i s i n s t r u c t i v e to c o n t r a s t these r e s u l t s with those o b t a i n e d f o r the s e q u e n t i a l replacement s e r i e s (SRS) ( F i g u r e s I0d-1uf). Diatoms were present d u r i n g the 12 weeks, and t h e i r c o n t r i b u t i o n to community s t r u c t u r e was comparable to that of blue-greens. Greens were again the dominant group. A chrysophycean peak occurred d u r i n g week 11, but i t was not so ephemeral as i n the phytoplankton: chrysophycean r e l a t i v e abundances had not decreased to e a r l i e r l e v e l s by week 13. The r e l a t i o n s h i p between the community s t r u c t u r e of the phytoplankton and the SRS was f u r t h e r explored using PCoA. The o r d i n a t i o n of phytoplankton and the SRS at s t a t i o n 1 i s shown in" F i g u r e 11. Group 45 i s important i n the phytoplankton o r d i n a t i o n s because s e v e r a l green algae found i n the phytoplankton c o u l d not be p l a c e d i n any of the other 44 c a t e g o r i e s (see Table I ) , and were a l l scored as "other greens". Even though group 45 was never an important component of the p e r i p h y t o n , i t was o v e r a l l the second most common group i n the phytoplankton, and separates the two communities in the o r d i n a t i o n s along A l l . AI i s c o r r e l a t e d with groups 1 (r=0.84) and 3 (r=-0.88). A l l i s c o r r e l a t e d mainly with group 45 (r=0.87). A x i s .V (not shown here) i s c o r r e l a t e d with s e v e r a l diatoms, and separates phytoplankton from SRS samples. Phytoplankton community s t r u c t u r e responds to the group 3 p u l s e in week 11, as shown by the s h i f t to the l o w e r - l e f t corner of the graph. By week 13, the community had moved c l o s e r to the e a r l i e r samples, but was s t i l l separated from them along AI. V a r i a t i o n along AI i s l e s s marked for the SRS, i n d i c a t i n g that the group 3 peak at week 11 was l e s s pronounced i n the SRS than 58 F i g u r e 11. Phytoplankton and SRS o r d i n a t i o n , s t a t i o n 1. Axes I and II account f o r 39% and 23% of the t o t a l v a r i a n c e . Axis I 60 in the phytoplankton (compare F i g u r e s 10a and I0d). In the s t a t i o n 2 o r d i n a t i o n ( F igure 12), AI i s n e g a t i v e l y c o r r e l a t e d with group 3 (r=-0.99) and p o s i t i v e l y c o r r e l a t e d with group 1 (r=0.72). A l l i s c o r r e l a t e d with groups 1 (r=-0.63) and 45 (r=0.92). The group 3 pu l s e at week 11 caused a s h i f t of both phytoplankton and SRS along AI. The s h i f t i s g r e a t e r f o r the phytoplankton than f o r the SRS, i n d i c a t i n g that the former community was more a f f e c t e d by the pulse than the l a t t e r . Group 3 was s t i l l abundant i n the 13-week SRS, cau s i n g the 11-week and 13-week samples to be c l o s e t o g e t h e r . Phytoplankton samples on week 13, on the other hand, c o u l d not be d i s t i n g u i s h e d from the pre-pu l s e samples along any a x i s . F i g u r e 13 shows the s t a t i o n 3 o r d i n a t i o n . AI i s n e g a t i v e l y c o r r e l a t e d with group 3 (r=-0.90) and p o s i t i v e l y with group 1 (r=0.66), and i n t h i s s t a t i o n i t i s a x i s III that i s c o r r e l a t e d with group 45 (r=0.86) and separates the phytoplankton from the SRS. A x i s IV (not shown here) r e f l e c t s a g r a d i e n t of diatom r e l a t i v e abundances, and a l s o separates phytoplankton from SRS. Week 11 phytoplankton i s d i s p l a c e d towards the l e f t , and by week 13 i t has retu r n e d to pr e - p u l s e c o n d i t i o n s along AI. The SRS samples a l s o show a s h i f t towards the l e f t i n week 11, although l e s s pronounced. Week 13 and week 11 samples are c l o s e together, as i n s t a t i o n 2. In s p i t e of the d i f f e r e n c e s i n taxonomic r e s o l u t i o n between the two a n a l y s e s , the r e s u l t s of these 3 o r d i n a t i o n s are 61 F i g u r e 12. Phytoplankton and SRS o r d i n a t i o n , s t a t i o n 2. Axes I and II account f o r 51% and 33% of the t o t a l v a r i a n c e . 62 Axis I 63 F i g u r e 13. Phytoplankton and SRS o r d i n a t i o n , s t a t i o n 3. Axes I and III account f o r 29% and 14% of the t o t a l v a r i a n c e . 64 Axis I 65 g e n e r a l l y i n agreement with those shown in F i g u r e s I0a-I0f. The o r d i n a t i o n s a d d i t i o n a l l y g ive some i n s i g h t i n t o the r e l a t i o n s h i p between the phytoplankton community and the groups that are s u c c e s s f u l as e a r l y c o l o n i z e r s . The s a l i e n t f e a t u r e s of the a n a l y s e s can be summarized as f o l l o w s : 1) Phytoplankton community s t r u c t u r e was not a good i n d i c a t o r of which groups became s u c c e s s f u l e a r l y c o l o n i z e r s on bare s l i d e s . Some groups (the u n i d e n t i f i e d algae i n c l u d e d i n group 45) were underrepresented i n the SRS i n comparison to t h e i r c o n t r i b u t i o n to the phytoplankton, while others (diatoms) were o v e r r e p r e s e n t e d . Small propagules (such as zoospores) of groups that attached to the s l i d e s , and only subsequently grew to f u l l s i z e , c o u l d have generated a m i s l e a d i n g o v e r r e p r e s e n t a t i o n of those groups on the s l i d e s , because they might have been missed when cou n t i n g the plankton samples. However, only the diatoms, which are a l r e a d y f u l l y grown at the time of attachment, were o v e r r e p r e s e n t e d . 2) Phytoplankton and SRS communities were c o o r d i n a t e d with respect to t h e i r responses to events that t r i g g e r e d a group 3 bloom i n the l a k e ; i n p a r t i c u l a r , group 3 reached a peak i n r e l a t i v e abundance around week 11 i n both communities. There were, however, important d i f f e r e n c e s between phytoplankton and SRS responses: f i r s t , the p u l s e caused a more d r a s t i c change in 66 phytoplankton community s t r u c t u r e (as measured by the extent of the l e f t w a r d s h i f t along Al i n the o r d i n a t i o n s ) than i n the SRS; second, phytoplankton recovered from the pulse more completely and f a s t e r than the SRS. Group 3 r e l a t i v e abundances were s t i l l high i n the l a t t e r samples i n week 13. Phytoplankton a p p a r e n t l y had the s h o r t e s t turnover times of the three communities. The attached communities had a longer "memory" of past events; the e f f e c t s of the group 3 pulse were s t i l l e v i d ent i n the IETS many weeks a f t e r the occurrence of the pu l s e i n the phytoplankton and SRS. The a b s o l u t e d e n s i t i e s of group 3 i n the phytoplankton, SRS and IETS are shown i n F i g u r e 14. The key f e a t u r e s i n these graphs a r e : 1) Absolute d e n s i t i e s behaved much i n the same way as r e l a t i v e abundances: a f t e r the peak at week 11, group 3 decreased r a p i d l y i n the phytoplankton, but more slowly (or even increased) i n the SRS and IETS a f t e r week 11. 2) The changes i n group 3 d e n s i t i e s i n the SRS c o u l d i n p r i n c i p l e be e x p l a i n e d s o l e l y i n terms of v a r i a t i o n s i n p a s s i v e settlement onto the s l i d e s , i . e . those r e s u l t i n g from v a r i a t i o n s i n group 3 phytoplankton d e n s i t i e s . A c c o r d i n g to t h i s e x p l a n a t i o n , f l u c t u a t i o n s of group 3 d e n s i t i e s on the s l i d e s would be more the r e s u l t of c o l o n i z a t i o n by p l a n k t o n i c c e l l s than of growth of 67 F i g u r e 14. Group 3 a b s o l u t e abundances i n phytoplankton, SRS, and IETS IO 20 'O 20 T I M E ( i n w e e k s ) S 69 attached c e l l s . 3) Increases i n group 3 c e l l s d u r i n g the peak p e r i o d were t y p i c a l l y an order of magnitude l a r g e r i n the IETS than i n the c o r r e s p o n d i n g SRS. T h i s means that the " c o l o n i z a t i o n o n l y " e x p l a n a t i o n i n 2) above does not hold, f o r the IETS, and i s probably untrue f o r the SRS as w e l l . (Of course, one c o u l d hypothesize that the c o l o n i z a t i o n r a t e by p l a n k t o n i c c e l l s was ten times higher i n the IETS (which a l r e a d y supported a s u b s t a n t i a l number of c e l l s ) than i n the b a r e - s l i d e SRS, but t h i s i s very u n l i k e l y . Group 3 c e l l s were d i r e c t l y a t t a c h e d to the p l a s t i c s l i d e s i n the IETS d u r i n g the p u l s e , when f r e e s l i d e space was scarce and access to the s l i d e s was c u r t a i l e d by the presence of other a l g a e . If anything, c o l o n i z a t i o n from the plankton should have been l e s s than i n the SRS). The order-of-magnitude i n c r e a s e of group 3 i n the IETS was due to the growth of p r e v i o u s l y a t t a c h e d c e l l s that responded to environmental cues in the same way as p l a n k t o n i c ones. Sta t ions J_ and 3 i^ n 1983, and t h e i r r e l a t ion to the 1982  commun i t i e s F i g u r e 15 shows the o r d i n a t i o n of s t a t i o n s 1 and 3 f o r 1983. Al i s c o r r e l a t e d p o s i t i v e l y (r=0.93) with Gomphonema , and n e g a t i v e l y with group 1, M i c r o c y s t i s and Merismopedia. The only group that c o r r e l a t e s w e l l with A l l i s E u n o t i a (group 15). The most v i s i b l e f e a t u r e of t h i s o r d i n a t i o n i s the n e a r l y complete 7 0 F i g u r e 15. O r d i n a t i o n of s t a t i o n s 1 and 3, 1983. Axes I and II account f o r 56% and 15% of the' t o t a l v a r i a n c e . 71 Axis I 72 s e p a r a t i o n of s t a t i o n s 1 and 3 along A l . The reasons f o r the s e p a r a t i o n were that diatoms, i n p a r t i c u l a r E u n o t i a (groups 14 and 15) and Gomphonema were more common in s t a t i o n 1 than i n s t a t i o n 3, whereas the o p p o s i t e was t r u e of group 1, Merismopedia, and M i c r o c y s t i s . An o r d i n a t i o n not presented here showed that i n 1982 the t r a j e c t o r i e s of both communities v a r i e d i n a c o o r d i n a t e d f a s h i o n a f t e r week 6, and ran p a r a l l e l to each other. Even though s t a t i o n 1 was r i c h e r i n group 3 than s t a t i o n 3, both communities moved i n the same g e n e r a l d i r e c t i o n . However, in 1983 Al separated the two communities at a l l times, and furthermore, the d i r e c t i o n of the communities t r a j e c t o r i e s was not c o o r d i n a t e d as i t had been the p r e v i o u s year. S t a t i o n 1 entered s p r i n g with a high diatom r e p r e s e n t a t i o n . Eunot i a r e l a t i v e abundances dropped q u i c k l y between A p r i l 2 and A p r i l 28; t h i s change was f o l l o w e d by a slower r e d u c t i o n of Gomphonema. Community s t r u c t u r e changed l i t t l e between weeks 46 and 62, although Gomphonema r e l a t i v e abundance was dropping s l o w l y . T h i s t r e n d continued in the f a l l between weeks 62 and 72, together with an i n c r e a s e i n the blue-green M i c r o c y s t i s . The p r o x i m i t y and o v e r l a p of the samples from weeks 46-62 i n d i c a t e t h at summer was a p e r i o d of r e l a t i v e s t a b i l i t y i n community s t r u c t u r e i n s t a t i o n 1. V a r i a t i o n s i n Gomphonema and Eunot i a accounted f o r most of the community change observed i n 1983, with M i c r o c y s t i s c o n t r i b u t i n g to a l e s s e r e x t e n t . At s t a t i o n 3, group 1 was n u m e r i c a l l y dominant i n the week 42 samples (about three times more abundant than M i c r o c y s t i s , 73 Merismopedia , and Gomphonema ). Besides these four groups, only Eunotia (group 15) and group 6 were w e l l represented. Gomphonema r e l a t i v e abundances i n c r e a s e d r a p i d l y between weeks 42 and 46, but no f u r t h e r changes i n community s t r u c t u r e were observed along these axes d u r i n g the r e s t of s p r i n g and e a r l y summer. Gomphonema abundances d e c l i n e d between l a t e June and l a t e August, making week 42 and week 62 samples i n d i s t i n g u i s h a b l e from each other i n the f i r s t two axes. Communities from these two weeks c o u l d however be separated along higher axes: Eunotia (group 14) and the Naviculaceae were b e t t e r represented i n week 62 samples, whereas group 6 was more common in the week 42 samples. Gomphonema r e l a t i v e abundances continued to f a l l between weeks 62 and 72; M i c r o c y s t i s increased d u r i n g t h i s p e r i o d , as i n s t a t i o n 1. When an o r d i n a t i o n i n c l u d i n g only s t a t i o n 3 samples was performed, AI and ' A l l p r o v i d e d much the same infor m a t i o n as F i g u r e 15, but the community t r a j e c t o r y along AI and AIII was shaped l i k e an open r i n g (or a doughnut with a m i s s i n g s e c t i o n ) . T h i s suggests the e x i s t e n c e of a y e a r l y c y c l i c a l p a t t e r n i n community s t r u c t u r e , with the missing s e c t i o n of the r i n g r e p r e s e n t i n g the unsampled November 1983 - March 1984 p e r i o d . T h i s o r d i n a t i o n a l s o h i g h l i g h t e d a l a t e s p r i n g / e a r l y summer inc r e a s e i n group 39 and an i n c r e a s e i n group 6, none of which were d e t e c t a b l e i n F i g u r e 15. As i n s t a t i o n 1, summer was a p e r i o d of r e l a t i v e s t a b i l i t y i n s t a t i o n 3 (see the grouping of samples from weeks 46-54 i n F i g u r e 15). At s t a t i o n 1 j u s t a few diatom groups and the c o c c o i d greens accounted f o r most of the 74 t o t a l abundance, whereas at s t a t i o n 3 the d i s t r i b u t i o n of abundances among taxa was more e q u i t a b l e . A c c o r d i n g l y , the sequence of seasonal changes was more complex i n s t a t i o n 3 than in s t a t i o n 1, i n v o l v i n g group 1, Gomphonema , M i c r o c y s t i s , E u n o t i a , the Naviculaceae, and group 6. In c o n t r a s t with s t a t i o n 3, a seasonal c y c l e i n community s t r u c t u r e was not found in s t a t i o n 1 (the t r a j e c t o r y of s t a t i o n 1 i s not " c l o s e d " i n the 1983 o r d i n a t i o n s ) ; perhaps sampling i n t o the November 1983-March 1984 p e r i o d would have allowed the d e t e c t i o n of such a c y c l e . F i g u r e 16 shows an o r d i n a t i o n of the f i r s t 20 weeks of the s t a t i o n 1 IETS s t a r t e d i n 1982, together with the f i r s t 26 weeks of the IETS s t a r t e d i n 1983 at s t a t i o n 1. Only the samples c o l l e c t e d at 4 m are i n c l u d e d . Al i s n e g a t i v e l y c o r r e l a t e d with group 1 (r=-0.75), and p o s i t i v e l y with groups 12 (r=0.82), 15 (r=0.90) and 41 (r=0.83). A l l i s c o r r e l a t e d almost p e r f e c t l y with group 3 (r=0.98). 1982 and 1983 samples are separated along A l l , i n d i c a t i n g that the former were r i c h e r i n group 3 ( e s p e c i a l l y weeks 8, 12, and 13, which corresponded to group 3 p u l s e s at s t a t i o n 1, as mentioned e a r l i e r ) . 1983 samples conformed w e l l to the general p a t t e r n of group replacements that had been i n f e r r e d e a r l i e r : group 1 d e c l i n e d through time, y i e l d i n g to groups 12, 15, and 41. There i s a l s o a good agreement between years, i f d i f f e r e n c e s due to group 3 are momentarily set a s i d e . Samples of s i m i l a r submergence time are matched along A l ; week 14, 1982 and week 16, 1983 o v e r l a p moderately along A l , and completely along A I I I , which i s not shown here. AIII c l e a r l y separates week 14, 1982 and week 16, 75 F i g u r e 16. O r d i n a t i o n of the 1982 and 1983 IETS at s t a t i o n 3. Axes I and II account f o r 38% and 37% of the t o t a l v a r i a n c e . Axis I 77 1983 from week 26, 1983. The correspondence along AI based on submergence times i s a d m i t t e d l y rough, but i t i s obvious that between-year s i m i l a r i t i e s are more r e l a t e d to submergence time than to time of the year, i . e. that c o l o n i z a t i o n and s u c c e s s i o n o v e r r i d e s e a s o n a l i t y , at l e a s t d u r i n g the time p e r i o d under study. S t a t i o n 1 samples f o r the IETS s t a r t e d i n 1982 (weeks 7-20 (1982) and 42-72 (1983)) were o r d i n a t e d , and the r e s u l t i s shown in F i g u r e 17. AI i s c o r r e l a t e d p o s i t i v e l y with group 1 (r=0.92), and n e g a t i v e l y with groups 12 (r=-0.97) and 14 (r=-0.75). A l l c o r r e l a t i o n s are spread among many groups; the h i g h e s t i s with group 15 (r=0.76). 1982 and 1983 samples are separated along AI; the former samples were dominated by diatoms, whereas groups 1 and 3 were dominant i n the l a t t e r . A f t e r remaining f a i r l y i n v a r i a n t from week 7 to week 13, diatom r e l a t i v e abundances i n c r e a s e d between week 13 and week 42, and remained high throughout 1983. As in the l a s t o r d i n a t i o n , submergence time was more important than s e a s o n a l i t y , because the 1982 samples are w e l l separated from t h e i r 1983 c o u n t e r p a r t s : communities of s i m i l a r ages tend to be c l o s e r together than communities from s i m i l a r c alendar dates. These r e s u l t s suggest that diatoms have low growth r a t e s (at l e a s t d u r i n g the summer), but that they can maintain high p o p u l a t i o n l e v e l s a l l year round. Diatom p o p u l a t i o n s might be f u r t h e r away from a h y p o t h e t i c a l e q u i l i b r i u m i n 1982 samples than i n 1983 samples. The l a t t e r communities would be t r a c k i n g a moving e q u i l i b r i u m that v a r i e d s e a s o n a l l y , and that slowed down durin g 78 F i g u r e 17. O r d i n a t i o n of the s t a t i o n 1 IETS s t a r t e d i n 1982: weeks 7-20 and 42-72. Axes I and II account f o r 67% and 14% of the t o t a l v a r i a n c e . 79 Axis I 80 c e r t a i n time p e r i o d s (e.g. l a t e s p r i n g / e a r l y summer 1983; community s t r u c t u r e changed l i t t l e along axes I to III during t h i s 16-week p e r i o d ) . F i g u r e 18 shows the o r d i n a t i o n of 1982 and 1983 samples at s t a t i o n 3. AI i s c o r r e l a t e d with groups 1 (r=-0.95) and 3 (r=0.62). A l l i s c o r r e l a t e d with groups 4 (r=0.70), 5 (r=0.64), and 40 (r=-0.68). AIII (not shown here) i s c o r r e l a t e d with Gomphonema (r=-0.79); along t h i s a x i s , weeks 42, 62, and 72 are grouped with 1982 samples, and separated from weeks 46-54, which were r i c h e r i n Gomphonema . The w i t h i n - y e a r v a r i a t i o n along AI and A l l i s much g r e a t e r f o r 1982 than f o r 1983. In f a c t , these two axes h a r d l y separate the 1983 samples. Seasonal v a r i a t i o n in the 1983 samples i s expressed mainly along AIII (the i n t e r p r e t a t i o n of t h i s v a r i a t i o n was given i n the e x p l a n a t i o n of F i g u r e 15); along AI and A l l though, community s t r u c t u r e was notably i n v a r i a n t i n 1983. 1982 samples reached the 1983 ones along AI and A l l a f t e r 7 weeks and r e s t e d there u n t i l week 10. The community was then s h i f t e d away by the group 3 p u l s e , and weeks 13 and 20 i n d i c a t e a "recovery" from the e f f e c t s of the p u l s e . F i g u r e 15 showed that d u r i n g weeks 46-54 community s t r u c t u r e was constant at s t a t i o n 3, with Gomphonema being more abundant than i n weeks 42, 62, and 72. One c o u l d s p e c u l a t e that the community s t r u c t u r e d u r i n g weeks 46-54 was c l o s e to e q u i l i b r i u m . I f t h i s e q u i l i b r i u m r e c u r r e d i n the lake every summer, and i f s u c c e s s i o n proceded at a f a s t r a t e , one c o u l d expect the 1982 samples to reach the same e q u i l i b r i u m p o i n t . These samples indeed reached the same r e g i o n , but: 81 F i g u r e 18. O r d i n a t i o n of the 1982 and 1983 IETS at s t a t i o n 3. Axes I and II account f o r 24% and 19% of the t o t a l v a r i a n c e . 82 Axis I 83 1) only along Al and A l l ; along AIII they were c l o s e r to the e a r l y s p r i n g and f a l l samples than to the l a t e spring-summer ones that c h a r a c t e r i z e d the e q u i l i b r i u m . T h i s was due to Gomphonema r e l a t i v e abundances being low in the 1982 samples, and i s c o n s i s t e n t with the idea that diatoms have low summer growth r a t e s i n the l a k e . 2) A group 3 pulse d i s p l a c e d the community from the e q u i l i b r i u m r e g i o n ; a p a r t i a l recovery seemed to occur. T h i s pulse was not observed i n 1983, but i f i t i s a seasonal event c h a r a c t e r i s t i c of l a t e summer, i t might have occurred between weeks 62 and 72, and have been missed e n t i r e l y . Determination of s i m i l a r i t i e s of taxa responses to envi ronmental  v a r i a t i o n : o r d i n a t i o n of taxonomic groups O r d i n a t i o n i s not l i m i t e d to mapping s i m i l a r i t i e s between communities. Instead of a r r a n g i n g community samples along s p e c i e s axes, taxonomic groups (or s p e c i e s ) can be p o s i t i o n e d along axes corresponding to community samples ( s i t e s , s t a n d s ) , so that p r o x i m i t y between taxonomic groups i n the graph r e f l e c t s s i m i l a r i t y i n t h e i r responses to environmental v a r i a t i o n . The c h o i c e of data t r a n s f o r m a t i o n s i s c r i t i c a l here, because d i f f e r e n t t r a n s f o r m a t i o n s emphasize d i f f e r e n t aspects of s i m i l a r i t y among groups. For i n s t a n c e , ( A l l e n and Koonce 1973) found that s t a n d a r d i z a t i o n by group norm caused s p e c i e s to be grouped a c c o r d i n g to the t i m i n g of t h e i r y e a r l y maxima, whereas l o g a r i t h m i c t r a n s f o r m a t i o n made them group a c c o r d i n g to t h e i r 84 standing crop and l e n g t h of d u r a t i o n i n the community. A c c o r d i n g l y , I t e s t e d 4 modes of a n a l y s i s i n the group o r d i n a t i o n s : raw data (no t r a n s f o r m a t i o n ) , the l o g a r i t h m i c t r a n s f o r m a t i o n l n ( x + 1 ) , s t a n d a r d i z a t i o n by s i t e norm, and s t a n d a r d i z a t i o n by group norm. These four o p t i o n s ( d e s c r i b e d in Appendix 2) d i f f e r widely i n the weight that each a s s i g n s to v a r i a t i o n s i n t o t a l , as opposed to r e l a t i v e , abundance. The two norm s t a n d a r d i z a t i o n s are i n s e n s i t i v e to v a r i a t i o n s i n t o t a l abundance; s t a n d a r d i z a t i o n by s i t e norm weighs a group at time t by i t s r e l a t i v e abundance i n the community at that time, whereas s t a n d a r d i z a t i o n by group norm weighs a group at time t according to how abundant the group i s at t h a t time i n comparison with i t s abundance at other times. Raw data y i e l d e d o r d i n a t i o n s i n which AI accounted f o r more than 90% of the v a r i a n c e , and re p r e s e n t e d an abundance g r a d i e n t . The few very abundant groups were separated along AI and A l l , and a l l the others were t i g h t l y c l u s t e r e d around the axes' o r i g i n . The s i t u a t i o n improved somewhat with the l o g a r i t h m i c t r a n s f o r m a t i o n , but AI was s t i l l an abundance a x i s , and the very abundant groups were not separated anymore along A l l . The s i t e -s t a n d a r d i z e d o r d i n a t i o n s were a l s o dominated by the most abundant groups. S t a n d a r d i z a t i o n by group norm e l i m i n a t e s d i f f e r e n c e s i n absolute abundances between groups (the t o t a l abundance of each group i s s t a n d a r d i z e d to unity.) so that s i m i l a r i t i e s depend on how each group p a r t i t i o n s t h i s u n i t a r y abundance among the s i t e axes. Groups that peak at the same time and p l a c e , and i n c r e a s e and decrease to g e t h e r , are placed 85 c l o s e to each other i n the o r d i n a t i o n s , even though one of the groups might be much more abundant than the other. Using s t a n d a r d i z a t i o n by group norms, I o r d i n a t e d s e p a r a t e l y the 1982 IETS and the 1983 data s e t s , e x c l u d i n g the new IETS s t a r t e d on A p r i l 2, 1983. I separated the data s e t s because I expected d i f f e r e n c e s i n the behavior of the 1982 and 1983 s e r i e s . The 1982 s e r i e s s t a r t e d o f f from c l e a n s l i d e s , so i n i t i a l abundances were zero. The predominant response f o r a l l groups was t h e r e f o r e an i n c r e a s e i n d e n s i t i e s throughout the 20 weeks, and t h i s t r e n d c o u l d have masked or swamped out the more f i n e - t u n e d responses of the i n d i v i d u a l groups to environmental s i g n a l s . In c o n t r a s t , i n i t i a l d e n s i t i e s were high i n 1983, and v a r i a t i o n s i n d e n s i t y were t h e r e f o r e more l i k e l y to r e f l e c t responses to environmental f l u c t u a t i o n s than i n the 1982 s e r i e s . The added c o n t r a s t a f f o r d e d to the o r d i n a t i o n c o u l d then separate the groups a c c o r d i n g to e c o l o g i c a l l y r e l e v a n t c r i t e r i a . At l e a s t two approaches can be used i n the i n t e r p r e t a t i o n of the o r d i n a t i o n . In the f i r s t , one checks to see i f any c l e a r - c u t groupings have been produced, and i f so, t r i e s to determine which c h a r a c t e r i s t i c s are common to the members of each group. T h i s approach i s seldom f e a s i b l e because c l e a r - c u t groupings r a r e l y occur. The second approach i s to d e f i n e a c l a s s i f i c a t i o n scheme independently from the o r d i n a t i o n , and then see how w e l l the o r d i n a t i o n r e s u l t s agree with the proposed scheme ( s p e c i e s can be c l a s s i f i e d i n terms of t h e i r taxonomy, s i z e s , shapes, or any other i n t e r e s t i n g p r o p e r t i e s ) . Of the s e v e r a l schemes I used (such as grouping by f a m i l i e s , or 86 d i s t i n g u i s h i n g between adpressed, e r e c t , or filamentous l i f e -forms), the d i v i s i o n i n t o 4 major taxonomic groups was the most s u c c e s s f u l . A v i s u a l i n s p e c t i o n of the 1982 s e r i e s o r d i n a t i o n d i d not r e v e a l any p a t t e r n even when the taxonomic scheme was used. The r e s u l t s of the 1983 o r d i n a t i o n are shown i n F i g u r e 19. Group 34 was absent i n 1983, and i s t h e r e f o r e not i n c l u d e d in t h i s o r d i n a t i o n . Even though green and blue-green algae are s c a t t e r e d throughout the graphs, diatoms and chrysophyceans seemed to c l u s t e r together i n the f i r s t 3 dimensions. In order to e v a l u a t e the s i g n i f i c a n c e of the c l u s t e r s , I d e v i s e d a t e s t based on the "bootstrap" method ( D i a c o n i s and E f r o n 1983; see Appendix 1). Given c l u s t e r X, with n members, the t e s t answers the q u e s t i o n "How probable i s a chance occurrence of an n-member c l u s t e r that i s as small or sma l l e r than X?". The p r o b a b i l i t i e s a s s o c i a t e d with each c l u s t e r were: greens, 0.731; chrysophyceans, 0.260; diatoms, 0.086; blue-greens, 0.146. None of the taxa were s i g n i f i c a n t l y c l u s t e r e d , and the t e s t showed that the blue-greens were more c l u s t e r e d than the chrysophyceans, c o n t r a d i c t i n g my i n i t i a l v i s u a l assessment. However, diatoms were c l o s e to the standard 0.05 acceptance l e v e l , and would f a l l below i t i f group 18 (an o u t l y e r , and the r a r e s t of the diatom groups) were excluded from the a n a l y s i s . Groups 12, 14, and 15, which made up the bulk of t o t a l diatom c e l l numbers, were grouped together, showing that t h e i r abundances v a r i e d s i m i l a r l y through time. Among the blue-greens, groups 40 and 41 accounted f o r most of the t o t a l abundance and a l s o behaved s i m i l a r l y through time, so the o v e r a l l response of diatoms and blue-greens was given by j u s t a 8 7 F i g u r e 19. O r d i n a t i o n of taxa i n environmental space, 1983 data. Axes I, I I , and III account f o r 26%,14, and 11% of the t o t a l v a r i a n c e . •4 < o 5> ft 2> 4 • -v o = " o III S|xv 0) c to Q) w o c >* o x: Cl in a w. o •) esn c ys esn ra k_ D x O) 13 o XI II II n 11 •4 • O > es II s|XV 89 few groups. In a d d i t i o n to s i m p l i f y i n g the i n t e r p r e t a t i o n of the o r d i n a t i o n s , t h i s r e s u l t suggests that a u t e c o l o g i c a l s t u d i e s of a few key s p e c i e s c o u l d l e a d to a b e t t e r understanding of the mechanisms u n d e r l y i n g community dynamics. The 1983 o r d i n a t i o n was more s u c c e s s f u l than that of 1982, suggesting that the g e n e r a l i z e d response shown by a l l groups i n 1982 ( i n c r e a s e i n d e n s i t i e s ) was dominant over responses to environmental v a r i a t i o n . The p o s i t i o n i n g of the groups i n the 1982 o r d i n a t i o n would depend mainly on t h e i r i n t r i n s i c growth r a t e s , whereas i n the 1983 o r d i n a t i o n i t would depend more on temporal v a r i a t i o n s in t h e i r c a r r y i n g c a p a c i t i e s . DISCUSSION S u c c e s s i o n a l p a t t e r n s ; community development A general p a t t e r n c h a r a c t e r i z e d p e r i p h y t o n community development in Gwendoline Lake. B a c t e r i a were the f i r s t c o l o n i z e r s , c o v e r i n g the s l i d e s completely a f t e r only one week. Algae were s t i l l very scarce at t h i s p o i n t , but r a p i d l y moved i n , together with f r e e - l i v i n g and attac h e d protozoans. Groups 1 and 2 were co-dominant d u r i n g the f i r s t two weeks, and by weeks 3-4 group 1 was by f a r the most common group. Group 1 r e l a t i v e abundances decreased at l a t e r dates, as other groups ( i n p a r t i c u l a r the diatoms Gomphonema and Eunotia , the chrysophycean Dinobryon , and the blue-greens M i c r o c y s t i s and Merismopedia ) became p r o g r e s s i v e l y more common. Group 1 was never r e p l a c e d by these groups, and was one of the dominants even on the o l d e r s l i d e s . Diatoms were not common du r i n g the f i r s t s i x weeks, but t h e i r 90 r e l a t i v e abundances r a p i d l y i n c r e a s e d a f t e r week 7. T h i s p a t t e r n was fo l l o w e d by the IETS s t a r t e d i n 1982 and by the IETS s t a r t e d i n 1983, r e g a r d l e s s of t h e i r p o s i t i o n in the l a k e . However, some v a r i a b i l i t y among the s e r i e s , caused by the group 3 p u l s e , was superimposed on t h i s sequence. F i r s t , even though s t a t i o n s 1 and 3 had "recovered" from the pulse by week 20, group 3 was s t i l l common at that time i n s t a t i o n 2. Second, no pulse was d e t e c t e d i n the IETS s t a r t e d i n 1983. In 1982 t h i s pulse was s h o r t - l i v e d i n the phytoplankton and periphyton (with the e x c e p t i o n of s t a t i o n 2), and might r e f l e c t the w e l l -documented seasonal p e r i o d i c i t y of Dinobryon i n o l i g o t r o p h i c waters (Lehman 1976). I f so, the pul s e might have been missed in 1983, when no samples were taken between weeks 62 and 72. How does the sequence compare to others p r e v i o u s l y reported? W h i t f o r d and Schumacher (1963) used g l a s s s l i d e s to analyse a l g a l s u c c e s s i o n i n s e v e r a l North C a r o l i n a streams. They found t h a t the i n i t i a l stages were s i m i l a r i n many of the streams, and a b s t r a c t e d the f o l l o w i n g g e n e r a l sequence: the diatoms Gomphonema and Eunotia were u s u a l l y the p i o n e e r s . They were f o l l o w e d by other diatoms and by secondary s p e c i e s that v a r i e d among h a b i t a t s . In Piedmont streams the appearance of a mat of the blue-green Phormidium followed; Synedra and N a v i c u l a were a s s o c i a t e d with the mat. Filamentous chlorophyceae soon invaded. In mountain and c o a s t a l p l a i n r i v e r s (presumably r e p r e s e n t i n g more n u t r i e n t - p o o r c o n d i t i o n s ) , the sequence was i d e n t i c a l , but the Phormidium mat d i d not appear. 91 Dickman (1974) s t u d i e d c o l o n i z a t i o n on g l a s s s l i d e s i n lak e s i n Sweden, Spain, and Canada, at d i f f e r e n t times of the year. He proposed the f o l l o w i n g g e n e r a l d e s c r i p t i o n of temperate lake periphyton c o l o n i z a t i o n : the f i r s t three days were t y p i f i e d by b a c t e r i a l c o l o n i e s and small f l a g e l l a t e s . C i l i a t e s became abundant by the end of the f i r s t week, and blue-greens i n c r e a s e d a b r u p t l y d u r i n g the second week. At the same time, small chlorophyceans and small diatom c o l o n i e s began t o appear. Larger diatoms p r o l i f e r a t e d i n the t h i r d week, du r i n g which j u v e n i l e stages of filamentous greens, as w e l l as nematodes, r o t i f e r s , and c i l i a t e s appeared. Green f i l a m e n t s r i v a l e d blue-greens i n abundance at the end of the f o u r t h week. At t h i s time, a l o s s of algae from the s l i d e s was a s s o c i a t e d with metazoan i n c r e a s e s ( o f t e n chironomids); there was a l r e a d y a c l o s e resemblance with the "mature" p e r i p h y t o n i n h a b i t i n g nearby rocks. Hudon and Bourget (1981) s t u d i e d c o l o n i z a t i o n on p l a s t i c panels immersed i n the S a i n t Lawrence e s t u a r y . They det e c t e d three d i s t i n c t phases: F i r s t , only b a c t e r i a s e t t l e d ; the l e n g t h of t h i s phase depended on phy s i c o - c h e m i c a l f a c t o r s . Second, diatoms s e t t l e d and remained grouped i n separate clumps. ' T h i r d , the clumps began to o v e r l a p f o l l o w i n g m u l t i p l i c a t i o n and recruitment of new c e l l s . One sp e c i e s became dominant, and a l l the a v a i l a b l e substratum was covered by diatoms. Throughout the study the sequence of dominance proceeded from f l a t - l y i n g to v e r t i c a l s p e c i e s . 92 Cattaneo and G h i t t o r i (1975) worked with g l a s s s l i d e s i n an I t a l i a n r i v e r . They found that diatoms were s u c c e s s f u l e a r l y c o l o n i z e r s ( u s u a l l y >50% of t o t a l abundance in the f i r s t week). Blue-greens i n c r e a s e d i n frequency u n t i l they dominated the community by week 4. There was l i t t l e change from week 4 to week 5, and at t h i s p o i n t the s l i d e communities c l o s e l y resembled those of stones on the streambed. The s t r u c t u r e of the stone communities remained constant d u r i n g the 6-week study; community s t r u c t u r e on the s l i d e s was determined by age (exposure time) r a t h e r than by date of c o l l e c t i o n . Hoagland e t . a l . (1982) conducted an e x t e n s i v e study of c o l o n i z a t i o n on g l a s s s l i d e s i n two USA r e s e r v o i r s . C o l o n i z a t i o n was followed f o r 4-7 weeks during each season. They too proposed a sequence that r e c u r r e d i n s e v e r a l seasons. In the f i r s t phase, an organic c o a t i n g formed w i t h i n the f i r s t week, and b a c t e r i a were abundant. " O p p o r t u n i s t i c " , f l a t - l y i n g diatoms moved in du r i n g the second phase; p o p u l a t i o n d e n s i t i e s i n c r e a s e d c o n s i d e r a b l y d u r i n g t h i s p e r i o d . In the t h i r d stage, l o n g - s t a l k e d diatoms, l a r g e diatom r o s s e t t e s , and filamentous green algae appeared. Green algae were p o s t u l a t e d to be the most advanced stage of the t h i r d event. The p a r t i c u l a r s u c c e s s i o n a l sequence, as w e l l as the dominant taxa i n o l d e r samples, were v a r i a b l e in these s t u d i e s . In a l l of them however, b a c t e r i a appeared i n the i n i t i a l s tages. Three of the s t u d i e s report a t r a n s i t i o n from sma l l and " h o r i z o n t a l " diatoms to l a r g e r , e r e c t or s t a l k e d ones. Diatoms 93 were always good e a r l y c o l o n i z e r s . Although b a c t e r i a were a l s o the f i r s t occupants of s l i d e s i n Gwendoline Lake, u n i c e l l u l a r greens, rather than diatoms, were the i n i t i a l c o l o n i z e r s . Furthermore, no t r e n d toward i n c r e a s e d s t a t u r e was found i n the l a t t e r group. In three of the s t u d i e s , age was more important than s e a s o n a l i t y i n determining community s t r u c t u r e , i n agreement with my r e s u l t s . However, Herder-Brower (1975) reached the opposite c o n c l u s i o n a f t e r s tudying c o l o n i z a t i o n on g l a s s s l i d e s submerged in d i t c h e s i n the Netherlands. I suggested e a r l i e r i n the R e s u l t s s e c t i o n that diatoms grew very slowly from A p r i l to October, even though they c o u l d maintain high p o p u l a t i o n d e n s i t i e s d u r i n g t h i s p e r i o d . The delayed appearance of t h i s group i n the c o l o n i z a t i o n sequence i n Gwendoline Lake i n comparison with other s i t e s , f u r t h e r suggests the p o s s i b i l i t y of low diatom growth r a t e s . Eppley (1977) l i s t s growth r a t e s f o r 7 s p e c i e s of freshwater diatoms; the r a t e s vary between 1.1 and 2.1 d i v i s i o n s / d a y , with optimal temperatures f o r growth v a r y i n g between 17 C and 26 C. These r a t e s are not i n accordance with the slow growth observed i n Gwendoline Lake, which might i n d i c a t e that phosphorus s u p p l i e s are not high enough to maintain high diatom growth r a t e s . Succession was reasonably d i r e c t i o n a l d u r i n g the f i r s t months and was c h a r a c t e r i z e d by a v a r i a b l e r a t e of change. Even though the process slowed somewhat towards the l a t e r weeks in 94 the s t a t i o n 2 and s t a t i o n 3 IETS (1982) and i n the s t a t i o n 1 IETS in 1983, no end-point was reached. T h i s was due not only to the group 3 p u l s e and other presumably seasonal changes l i k e the blue-green increase i n the f a l l , but a l s o to the c o n t i n u a l r i s e i n diatom r e l a t i v e abundances. Phytoplankton-periphyton i n t e r r e l a t i o n s h i p s Round (1964) has noted that i n many lakes p l a n k t o n i c and p e r i p h y t i c communities are very d i f f e r e n t , s h a r i n g s p e c i e s only on a c a s u a l or i n c i d e n t a l b a s i s . However, Moss (1981) p o i n t s out that most of the lakes examined have been u n f e r t i l e upland ones, and shows that in more f e r t i l e l akes t h i s i n t e r - h a b i t a t s p e c i f i c i t y may be reduced. In e u t r o p h i c Elk Lake, B r i t i s h Columbia, at l e a s t 12 taxa were found to be shared by both communities, f i v e of which dominated the net plankton diatom p o p u l a t i o n s and were at the same time common in the periphyton (Brown and A u s t i n 1973b). In Gwendoline Lake the u n i c e l l u l a r c o c c o i d greens and Dinobryon/Epipyxis (group 3) were the only common groups e s t a b l i s h e d i n both communities. The group 3 pulse that s t a r t e d around week 9 oc c u r r e d simultaneously i n both communities. T h i s pulse was due to the growth of c e l l s that were a l r e a d y e s t a b l i s h e d i n the communities, r a t h e r than to the r e l e a s e of p e r i p h y t i c c e l l s i n t o the plankton or to the settlement of p l a n k t o n i c c e l l s onto p e r i p h y t i c s u b s t r a t a . A s i m i l a r p a r a l l e l i s m of responses of s p e c i e s present i n both communities has been r e p o r t e d f o r Diatoma and Synedra by Moss (1981). The group 3 pulse caused a g r e a t e r s h i f t i n community s t r u c t u r e , but l a s t e d f o r a s h o r t e r p e r i o d , i n phytoplankton 95 than i n p e r i p h y t o n ( F i g u r e s 11-13). Phytoplankton communities appeared to recover f a s t e r from the p u l s e , perhaps because ge n e r a t i o n times are s h o r t e r i n phytoplankton than i n p e r i p h y t o n p o p u l a t i o n s (Olecsowicz 1982). He s t a t e s that a t t a c h e d e p i p h y t i c algae are g e n e r a l l y l a r g e i n comparison with p l a n k t o n i c a l g a e , have longer l i f e c y c l e s , and are l e s s u t i l i z e d by g r a z e r s . Being a t t a c h e d , p e r i p h y t o n communities should a l s o have lower turnover r a t e s than phytoplankton. The a t t a c h e d communities at Gwendoline Lake s t i l l r e t a i n e d the e f f e c t s of the group 3 p u l s e many weeks a f t e r the occurrence of the pulse i n the p l a n k t o n . Attachment might t h e r e f o r e provide a temporal refuge f o r s p e c i e s between p e r i o d s of f a v o r a b l e environmental c o n d i t i o n s , e n a b l i n g them to s u r v i v e d u r i n g the harsh i n t e r i m . When the environmental c o n d i t i o n s i n the open water allow only a short p e r i o d of e f f e c t i v e r e p r o d u c t i o n d u r i n g the year, attachment c o u l d p r o v i d e a means of extending t h i s p e r i o d , thus i n c r e a s i n g t o t a l r e p r o d u c t i v e output. S u c c e s s i o n a l p a t t e r n s : "mature" communities and s e a s o n a l i t y In the s t u d i e s of Cattaneo and G h i t t o r i (1975) and Dickman (1974), the s l i d e communities had reached the composition of nearby rock (presumably mature) communities a f t e r 4 weeks. In Gwendoline Lake, comparisons of d e v e l o p i n g communities (26 weeks and younger) with mature (42 week and o l d e r ) ones d i d not show a s i m i l a r convergence in community s t r u c t u r e . Furthermore, because the s t a t i o n s c o u l d h a r d l y be d i s t i n g u i s h e d on the b a s i s of the f i r s t weeks of community development, long exposure p e r i o d s may be necessary to o b t a i n communities that are 96 c h a r a c t e r i s t i c of a given s i t e . In c o n t r a s t , mature communities at s t a t i o n s 1 and 3 were c l e a r l y d i f f e r e n t i a t e d , and a sequence of changes, presumably r e f l e c t i n g s e a s o n a l i t y , c h a r a c t e r i z e d each s t a t i o n . The same r e l a t i o n s h i p between submergence time and s p a t i a l v a r i a b i l i t y i n community development was found by Brown and A u s t i n (1973), who fol l o w e d periphyton growth on g l a s s s l i d e s i n a e u t r o p h i c lake on Vancouver I s l a n d , B r i t i s h Columbia. Community development was s i m i l a r at the four s t a t i o n s s t u d i e d when 4-week exposure p e r i o d s were used, but l a r g e d i f f e r e n c e s among s t a t i o n s appeared with longer exposure times.' In Gwendoline.Lake, the sequence at s t a t i o n 1 i n v o l v e d a quick r e d u c t i o n of Eunot i a i n the s p r i n g , followed by a slower but continued decrease i n Gomphonema , and a f a l l i n c r e a s e i n M i c r o c y s t i s . U n i c e l l u l a r greens i n c r e a s e d from s p r i n g u n t i l f a l l , although diatoms were always the dominant group. In c o n t r a s t , at s t a t i o n 3 u n i c e l l u l a r greens were always the dominant group. Again community s t r u c t u r e stayed r a t h e r constant during the summer, and changed more r a p i d l y i n s p r i n g and f a l l . High Gomphonema r e l a t i v e abundances separated summer samples from s p r i n g and f a l l ones. Spring samples were r i c h e r than f a l l ones i n a s e c o n d a r i l y e p i p h y t i c chrysophycean (group 6), and poorer i n diatoms and blue-greens ( t h i s l a s t group peaked i n the f a l l samples). There i s a d e a r t h of s t u d i e s of .'seasonal changes i n periphyton community s t r u c t u r e , and many of the a v a i l a b l e ones deal only with diatoms. Jones and Mayer (1983) examined the seasonal and temporal v a r i a b i l i t y of communities growing on 97 Myriophyllum spicatum i n Lake Wingra. They found that the communities followed an o r d e r l y seasonal p r o g r e s s i o n , and that v a r i a t i o n i n community s t r u c t u r e was a s s o c i a t e d with depth. M i c r o c y s t i s peaked i n l a t e summer-early f a l l , c o i n c i d i n g with high p l a n k t o n i c abundances of the same s p e c i e s . Castenholz (1960) a l s o found a r e g u l a r seasonal p a t t e r n of s p e c i e s replacements on g l a s s s l i d e s i n two freshwater l a k e s : diatoms were c h a r a c t e r i s t i c s p r i n g dominants, and were j o i n e d d u r i n g the summer by filamentous greens. The dominant summer diatoms remained through the f a l l , when some of the s p r i n g s p e c i e s returned. In one of the l a k e s , a few s p r i n g s p e c i e s were common in summer, but only at the g r e a t e r depths. The s e a s o n a l i t y of algae i n North C a r o l i n a (USA) streams was s t u d i e d e x t e n s i v e l y by Wh i t f o r d and Schumacher (1963), using the g l a s s - s l i d e method. They found t h a t : 1) Diatoms were dominant d u r i n g the winter, and the e a r l y winter f l o r a was "remarkably s i m i l a r " to that of l a t e winter. 2) During the summer, filamentous greens, c h l o r o c o c c a l e s , and desmids were dominant, and some blue-green s p e c i e s were p r e s e n t . Diatoms were very common i n cl e a r - w a t e r c o a s t a l p l a i n streams. 3) The communities underwent l i t t l e or no change d u r i n g the summer, and in autumn there was a r e v e r s a l of the s p r i n g s u c c e s s i o n ; e a r l y s p r i n g s p e c i e s reappeared i n l a t e f a l l , and l a t e s p r i n g s p e c i e s reappeared i n e a r l y f a l l . O v e r a l l , autumn f l o r a s resembled those of s p r i n g . 98 Kingston e t . a l . (1983) s t u d i e d the benthic diatom assemblages of Lake Michigan, USA, sampling from communities at d i f f e r e n t depths i n each of the four seasons. A f t e r a s s i g n i n g t h e i r samples to "community types" d e f i n e d by c l u s t e r a n a l y s i s , they found that the community types at the deepest sampling depths, where environmental c o n d i t i o n s were most s t a b l e , remained unchanged a l l year round. The amount of change i n community s t r u c t u r e was i n v e r s e l y r e l a t e d to depth, and the authors concluded that community s t r u c t u r e t r a c k e d temperature changes i n the lake with great f i d e l i t y . The community types "responded s e n s i t i v e l y to temporally and s p a t i a l l y dynamic environmental parameters" (Kingston et a l . 1983, p. 1566). The above r e s u l t s , together with those obtained i n t h i s study, suggest that mature communities undergo a y e a r l y , q u a s i -c y c l i c a l , sequence of s p e c i e s replacements. Wehr (1981) reached the same c o n c l u s i o n i n h i s o r d i n a t i o n a n a l y s i s of the seasonal s u c c e s s i o n of at t a c h e d algae i n a B r i t i s h Columbia mountain stream. In Gwendoline Lake winter and summer are p e r i o d s of r e l a t i v e s t a b i l i t y , whereas s p r i n g and f a l l are t r a n s i t i o n p e r i o d s , and the s p e c i e s assemblages corresponding to these l a t t e r p e r i o d s might resemble each other (see the t r a j e c t o r y of s t a t i o n 3 i n F i g u r e 15 and the e x p l a n a t i o n i n the t e x t ) . A l l e n (1978) reached s i m i l a r c o n c l u s i o n s a f t e r performing a m u l t i v a r i a t e a n a l y s i s of the p e r i o d i c i t y of Lake Wingra phytoplankton. The c h a r a c t e r i s t i c s of the c y c l e vary with depth; i n Gwendoline Lake only two groups were r e s p o n s i b l e f o r most of the seasonal v a r i a t i o n at 4 m whereas the p a t t e r n of 99 replacements i n v o l v e d s e v e r a l groups at 1.7 m. A p o s s i b l e e x p l a n a t i o n of t h i s d i f f e r e n c e i s that the higher environmental v a r i a b i l i t y i n the shallower s i t e ( s t a t i o n 3; e.g. compare the temporal sequences of temperature at 1.7 m and at 4 m i n F i g u r e 31) allows the temporary success of more groups at t h i s s i t e . These groups would be responding to r e l a t i v e l y r a p i d v a r i a t i o n s in environmental c o n d i t i o n s . P r e l i m i n a r y o b s e r v a t i o n s from communities at 10m suggest that these communities vary even l e s s than those at 1.7 m and 4 m, but that the f a l l o v e r t u r n might t r i g g e r an episode of r a p i d change at that depth, i n v o l v i n g the appearance of filamentous greens. In Gwendoline Lake high diatom d e n s i t i e s are t y p i c a l of winter assemblages, and c o c c o i d greens, filamentous greens, and diatoms t y p i f y summer assemblages.' The r e l a t i v e abundance of diatoms i n c r e a s e s with depth. Seasonal f l u c t u a t i o n s are w e l l documented for diatoms, which favor low l i g h t and temperature c o n d i t i o n s ( P a t r i c k and Reimer 1966, Smith 1950). More r e c e n t l y , Davis (1976) has found that diatom c e l l s grown at low s i l i c o n c o n c e n t r a t i o n show signs of s i l i c o n d e f i c i e n c y only at high l i g h t i n t e n s i t i e s . W hitford and Schumacher (1963), basing t h e i r a n a l y s i s on four years of f i e l d o b s e r v a t i o n s and on a study of some twenty s p e c i e s i n a growth chamber at s e v e r a l l i g h t i n t e n s i t i e s and temperatures, found that diatoms grew best at low temperature and medium to low l i g h t . Chrysophyceans had low temperature requirements and growth was best under medium to low l i g h t . Chlorophyceans had a medium temperature and high l i g h t requirement. They concluded that w i t h i n a p a r t i c u l a r 1 00 season the f l o r a was more r e l a t e d to "water q u a l i t y " and c u r r e n t speed than to any other f a c t o r s . V a r i a t i o n of f l o r a with season was a s c r i b e d c h i e f l y to changes i n water temperature, but they noted that changes in t o t a l i n c i d e n t l i g h t were important f o r some s p e c i e s . F i f t e e n degrees C e l s i u s was found to be a c r i t i c a l temperature, with most winter and e a r l y s p r i n g s p e c i e s d e c r e a s i n g or d i s a p p e a r i n g above that p o i n t . The f i n d i n g s of these two authors must be i n t e r p r e t e d c a u t i o u s l y ; f o r i n s t a n c e , i t i s now known that diatom responses to l i g h t are p l a s t i c and temperature-dependent (Eppley 1977). F u r t h e r s t u d i e s i n v o l v i n g more s p e c i e s and . i n c l u d i n g the i n t e r a c t i o n of l i g h t and temperature with other f a c t o r s a f f e c t i n g growth w i l l be necessary before s o l i d g e n e r a l i z a t i o n s can be made. Deeper waters i n Gwendoline Lake remain i n w i n t e r - l i k e c o n d i t i o n s d u r i n g most of the year, at l e a s t with regard to l i g h t and temperature (see F i g u r e 31). The a b s o l u t e range of v a r i a t i o n in l i g h t i s presumably a l s o s m a l l e r i n deeper s t a t i o n s than i n shallow ones, because l i g h t p e n e t r a t i o n decays e x p o n e n t i a l l y with depth. As depth decreases, there i s p r o g r e s s i v e l y more exposure to higher l i g h t and temperature, and to wider seasonal f l u c t u a t i o n i n these two f a c t o r s ; presumably water movement ( c u r r e n t s , waves), and n u t r i e n t s a l s o vary with depth. The seasonal response of p e r i p h y t o n at s t a t i o n s 1 and 3 seems to be s t r o n g l y i n f l u e n c e d by s p a t i a l and temporal v a r i a t i o n i n l i g h t and temperature. Seasonal v a r i a t i o n s i n l i g h t and temperature are almost c e r t a i n l y not the only f a c t o r s a f f e c t i n g p e r i p h y t o n p e r i o d i c i t y , and more g e n e r a l l y , community 101 dynamics. Losses by p e e l i n g , s l o u g h i n g , or s c o u r i n g are u n l i k e l y to be of any importance i n Gwendoline Lake a r t i f i c i a l s u b s t r a t a . Competition f o r space per se was never observed; even at the hig h e s t d e n s i t i e s recorded attachment s i t e s were a v a i l a b l e . I n t r a s p e c i f i c and i n t e r s p e c i f i c competition f o r l i g h t and n u t r i e n t s ( e s p e c i a l l y the l a t t e r , i n view of the o l i g o t r o p h i c s t a t u s of the l a k e ) , are b i o t i c i n t e r a c t i o n s that are l i k e l y to a f f e c t markedly the s u c c e s s i o n and s e a s o n a l i t y of pe r i p h y t o n i n the l a k e . Chapter 3 d i s c u s s e s f u r t h e r the d i f f e r e n c e s i n the sequences of seasonal s u c c e s s i o n of both s t a t i o n s , and t h e i r r e l a t i o n s h i p to l i g h t and temperature v a r i a t i o n s . The mechanisms u n d e r l y i n g w i t h i n - l a k e s p a t i a l h e t e r o g e n e i t y i n p e r i p h y t i c communities are not yet understood. Some evidence i s a v a i l a b l e s i g n a l i n g s p a t i a l v a r i a t i o n s i n l i g h t (Maciolek and Kennedy 1964) and temperature (Kingston et a l . 1983) as f a c t o r s c o n t r i b u t i n g to p e r i p h y t i c h e t e r o g e n e i t y . Brown and A u s t i n (1973a) found s i g n i f i c a n t d i f f e r e n c e s among the peri p h y t o n communities growing at four h o r i z o n t a l l y - s e p a r a t e d s t a t i o n s w i t h i n a l a k e . The four s t a t i o n s were i n d i s t i n g u i s h a b l e with r e s p e c t to 15 p h y s i c a l - c h e m i c a l v a r i a b l e s among which l i g h t and temperature were i n c l u d e d . The authors a t t r i b u t e d the b i o t i c d i f f e r e n c e s among s t a t i o n s to "periphyton s p e c i e s i n t e r a c t i o n s " and to s e t t l i n g of c e l l s from the plankton at o v e r t u r n . In Gwendoline Lake the b i o t i c d i f f e r e n c e s between s t a t i o n s 2 and 3 were minor compared with those between s t a t i o n s 1 and 3 (see F i g u r e s 3a-3c, 4a-4c, and Table I I I ) . Although i t seems l i k e l y 1 02 that d e p t h - r e l a t e d l i g h t and temperature d i f f e r e n c e s were p r i m a r i l y r e s p o n s i b l e f o r the b i o t i c d i s s i m i l a r i t i e s , chemical f a c t o r s ( n u t r i e n t s , pH, d i s s o l v e d gases) might have d i f f e r e d among s t a t i o n s . In 1983, l o g g i n g o p e r a t i o n s on the east s i d e of the lake might have i n c r e a s e d n u t r i e n t r u n o f f i n t o the la k e ; t h i s i n c r e a s e i n turn c o u l d have a f f e c t e d s t a t i o n s 1 and 3 d i f f e r e n t i a l l y . S i m i l a r i t i e s i n the responses of a l g a l groups to environmental  v a r i a t i o n P r i n c i p a l components a n a l y s i s i n the l i t e r a t u r e a p p a r e n t l y has not p r o v i d e d i n t e r e s t i n g r e s u l t s concerning the i d e n t i f i c a t i o n of s p e c i e s a s s o c i a t i o n s (Legendre and Legendre 1983). In t h i s study, I generated an o r d i n a t i o n that used s i m i l a r i t i e s i n spatio-temporal sequences of r e l a t i v e abundances as a measure of a f f i n i t y between taxa. The analyses i n which algae were d i s t i n g u i s h e d a c c o r d i n g to l i f e - f o r m or fa m i l y were u n s u c c e s s f u l , s i n c e taxa from the same f a m i l i e s or taxa having the same l i f e - f o r m s d i d not behave s i m i l a r l y . In c o n t r a s t , when algae were grouped by taxonomic d i v i s i o n s some i n t e r e s t i n g f i n d i n g s emerged. The r e s u l t s of the o r d i n a t i o n suggested that at l e a s t f o r the diatoms and blue-greens i n t e n s i v e s t u d i e s of j u s t a few taxa that seem to be c h a r a c t e r i s t i c of the d i v i s i o n (e.g. Gomphonema , M i c r o c y s t i s ) might be u s e f u l i n understanding the mechanisms of community change. The r e s u l t s were dependent on the developmental stage of the community, with o l d e r communities presumably r e f l e c t i n g changes i n a b i o t i c environmental c o n d i t i o n s b e t t e r than younger ones. 1 03 A conceptual model of s u c c e s s i o n and seasonal p e r i o d i c i t y i n the per iphyton A simple conceptual framework w i l l be presented now that attempts to encompass the r e s u l t s reported here and those found i n the l i t e r a t u r e , and to i n t e g r a t e them with some ideas r e l a t e d to s u c c e s s i o n theory. In p a r t i c u l a r I w i l l t r y to e x p l a i n when s e a s o n a l i t y should o v e r r i d e c o l o n i z a t i o n , and why d i f f e r e n c e s between s t a t i o n s i n c r e a s e with community age. F i g u r e 20a i s a s i m p l i f i e d d e t e r m i n i s t i c r e p r e s e n t a t i o n of community development i n a seasonal environment. The axes c o u l d be s p e c i e s abundances ( i n the case of a two-species community), or the p r i n c i p a l axes of an o r d i n a t i o n . Community s t r u c t u r e at any given time i s d e p i c t e d by a p o i n t i n the plane, and the t r a j e c t o r y of the community i s obtained by connecting a time sequence of such p o i n t s . The c l o s e d curve C r e p r e s e n t s seasonal v a r i a t i o n i n the s t r u c t u r e of mature communities, and the p o i n t r e p r e s e n t i n g community s t r u c t u r e takes a year to complete a c y c l e along the c l o s e d curve. Community development i s represented as a t r a j e c t o r y going from the o r i g i n to the c l o s e d curve C, e.g. t r a j e c t o r i e s A and B, which represent two d i f f e r e n t s u c c e s s i o n a l pathways. The community completes i t s development, i . e . becomes mature, upon reaching C. The curve C i s a rough mapping of the y e a r l y v a r i a t i o n i n a " t r u e " e q u i l i b r i u m p o i n t which i s t r a c k e d but perhaps never reached. I f the dynamics of the community are slow enough, the community might not have time to reach the e q u i l i b r i u m even dur i n g the summer and winter p e r i o d s of r e l a t i v e s t a b i l i t y . A l s o , short-term d i s t u r b a n c e s might keep 1 04 F i g u r e 20. A g r a p h i c a l model of p e r i p h y t o n community development in a seasonal environment 106 the community away from e q u i l i b r i u m even i f i t s dynamics are f a s t enough to tra c k seasonal changes. In g e n e r a l , the l a g s e p a r a t i n g a c t u a l community s t r u c t u r e from the e q u i l i b r i u m p o i n t , i . e . the e f f i c i e n c y of the t r a c k i n g , depends on how q u i c k l y the e q u i l i b r i u m moves i n r e l a t i o n to i n t e r n a l community dynamics. I f the i n t e r n a l dynamics are f a s t r e l a t i v e to the ra t e of change of the e q u i l i b r i u m , the t r a c k i n g l a g i s s m a l l , and the community i s u s u a l l y c l o s e to e q u i l i b r i u m . The converse holds when i n t e r n a l dynamics are slow r e l a t i v e to the rate of change of the e q u i l i b r i u m . The i n t e r n a l dynamics are dependent on the growth r a t e s of each s p e c i e s , and on the type and s t r e n g t h of i n t e r a c t i o n s among s p e c i e s . A p r i o r i one c o u l d expect the i n t e r n a l dynamics of Gwendoline Lake communities t o be slow i n comparison to other l a k e s , because i t s n u t r i e n t - p o o r s t a t u s , as w e l l as temperature and i r r a d i a n c e annual regimes t y p i c a l of temperate r e g i o n s , should r e s u l t i n low growth r a t e s of i n d i v i d u a l s p e c i e s . Assume f o r a moment that community dynamics are f a s t compared to seasonal changes. In F i g u r e 20a a community s t a r t e d at time tO q u i c k l y develops along t r a j e c t o r y A and reaches p o i n t a ( t l ) at time t 1 . I t then continues to move along C, reaching a ( t 2 ) , a ( t 3 ) , and so f o r t h . Sometime s h o r t l y before t 3 , another community i s s t a r t e d that develops along t r a j e c t o r y B. Because development has been assumed to be f a s t , by t3 the two communities are i n d i s t i n g u i s h a b l e and remain so t h e r e a f t e r , d e s p i t e t h e i r having begun t h e i r development on d i f f e r e n t calendar dates. The time r e q u i r e d f o r convergence between the 1 07 two communities i s s m a l l , and most or a l l of the samples taken d u r i n g the year w i l l l i e on C, causing s i m i l a r i t y between the two communities to be determined by the date of c o l l e c t i o n , r a t h e r than by community age. Thus, i f development i s f a s t i n r e l a t i o n to seasonal changes, s e a s o n a l i t y i s more important than age in determining community s t r u c t u r e . Assume now that community dynamics are slow. In t h i s case community A has not reached m a t u r i t y by the time community B i s s t a r t e d . The f i r s t sample of community B ( p o i n t b ' ( t 3 ) ) i s c l o s e r to a ' ( t l ) than to a'(.t3); b'(t4) i s more s i m i l a r to a'(t2) than to a ' ( t 4 ) , and so f o r t h , i n d i c a t i n g t h at age i s a more important determinant of community s t r u c t u r e than c o l l e c t i o n date. Age p r o g r e s s i v e l y l o s e s t h i s importance as the communities approach C, and i s t o t a l l y i r r e l e v a n t once both communities have reached m a t u r i t y . When the communities are f a r from e q u i l i b r i u m , age determines what the i n i t i a l c o n d i t i o n s or s t a r t i n g p o i n t of the t r a j e c t o r y w i l l be. As they come c l o s e r to e q u i l i b r i u m , the e f f e c t s of i n i t i a l c o n d i t i o n s (community " h i s t o r y " ) are erased; communities with f a s t dynamics shed h i s t o r i c a l e f f e c t s q u i c k l y , whereas - s l o w l y responding communities are c o n s t r a i n e d f o r a longer time by past events. The same g r a p h i c a l approach can be used to e x p l a i n why d i f f e r e n c e s between s p a t i a l l y separated s t a t i o n s i n c r e a s e with time. F i g u r e 20b i s a r e p r e s e n t a t i o n of the simultaneous development of two communities at s t a t i o n s A and B. Mature communities at each s t a t i o n f o l l o w d i f f e r e n t t r a j e c t o r i e s 108 (denoted by the c l o s e d curves A and B), because l o c a l environmental c o n d i t i o n s are not the same at the two s t a t i o n s . If the f a c t o r s that determine the community t r a j e c t o r i e s at ma t u r i t y have l i t t l e e f f e c t on the i n i t i a l stages of development, the developmental t r a j e c t o r i e s at the two s t a t i o n s can be s i m i l a r at f i r s t , with c o n t r a s t between the s t a t i o n s emerging only as curves A and B are approached. T h i s can occur, fo r i n s t a n c e , whenever the same pioneer s p e c i e s are always the f i r s t occupants of bare s i t e s , regardless, of the u l t i m a t e community s t r u c t u r e achieved at each of the s t a t i o n s . O r d i n a t i o n of mature communities at the two s t a t i o n s would show them c l e a r l y separated, whereas an o r d i n a t i o n of the developing communities might show only a small s e p a r a t i o n along a secondary a x i s , with both communities moving i n the same general d i r e c t i o n along the main a x i s (see dashed r e c t a n g l e s i n F i g u r e 20b). The c o n c e p t u a l model presented here has been h e l p f u l i n the i n t e r p r e t a t i o n of e m p i r i c a l r e s u l t s . In t h i s scheme, mature communities are d e f i n e d , i n theory, as those f o r which the t r a c k i n g l a g has s t a b i l i z e d (although not n e c e s s a r i l y to z e r o ) , and i n p r a c t i c e as those which have been exposed to the environmental c o n d i t i o n s at a given s t a t i o n f o r a long p e r i o d of time ( r e l a t i v e to the l i f e spans of the organisms). There i s i n t h i s sense a p r e d i c t a b l e end-point (or r a t h e r , an absorbing c l o s e d t r a j e c t o r y ) towards which s u c c e s s i o n a l communities proceed. T h i s t r a j e c t o r y v a r i e s on s e v e r a l time s c a l e s (e.g. short term v a r i a t i o n due to u n p r e d i c t a b l e day-to-day f l u c t u a t i o n s , long-term c l i m a t i c changes), but i s dominated by a 109 y e a r l y seasonal c y c l e . Community development i s d e s c r i b e d by the t r a j e c t o r y going from the o r i g i n to the c l o s e d curve. Development i s d i r e c t i o n a l , and can be r e l a t i v e l y independent of the p o s i t i o n of the f i n a l c y c l i c a l t r a j e c t o r y , e s p e c i a l l y d u r i n g the e a r l y s t a g es. Some q u e s t i o n s remain open: How re p e a t a b l e i s the seasonal c y c l e i n r e a l i t y ? What i s the r e l a t i o n s h i p between the time r e q u i r e d to reach m a t u r i t y , and other macroscopic community parameters, such as the time r e q u i r e d to reach biomass, or p r o d u c t i v i t y , s t a b i l i z a t i o n ? How v a r i a b l e i s t h i s time among d i f f e r e n t waterbodies, and what environmental parameters does i t r e l a t e to? How robust i s the i n i t i a l d i r e c t i o n a l i t y to changes in the p o s i t i o n of the c y c l i c a l t r a j e c t o r y (or, to what extent i s the i n i t i a l sequence independent of the f a c t o r s determining the s t r u c t u r e of mature communities)? SUMMARY 1) Succession i n Gwendoline Lake was reasonably d i r e c t i o n a l d u r i n g the f i r s t months and was c h a r a c t e r i z e d by a v a r i a b l e r a t e of change. Although s u c c e s s i o n slowed down du r i n g the l a t e r weeks of t h i s p e r i o d , no end-point was reached. T h i s was due not only to a pulse of Dinobryon/Epipyxi s and to other presumably seasonal changes such as the blue-green i n c r e a s e i n the f a l l , but a l s o to a c o n t i n u a l r i s e i n diatom r e l a t i v e abundances. The d i r e c t i o n of su c c e s s i o n i n v o l v e d an e a r l y appearance of b a c t e r i a and i n i t i a l dominance by u n i c e l l u l a r c o c c o i d greens d u r i n g the f i r s t 3-4 weeks, f o l l o w e d by i n c r e a s e s 1 10 in the diatoms Gomphonema and Eunotia, the chrysophycean Dinobryon, and the blue-greens M i c r o c y s t i s and Merismopedia. The c o c c o i d greens were never r e p l a c e d by the other groups, and were co-dominant even on the o l d e r s l i d e s . In c o n t r a s t with other s t u d i e s , diatoms were not good e a r l y c o l o n i z e r s , and there was no trend towards i n c r e a s e d s t a t u r e w i t h i n t h i s group. 2) Phytoplankton and p e r i p h y t o n d i f f e r e d g r e a t l y i n community s t r u c t u r e ; c o c c o i d greens and Dinobryon/Epipyxis were the only abundant taxa e s t a b l i s h e d i n both communities. A pulse i n Dinobryon/Epipyxis that s t a r t e d around week 9 occurred simultaneously i n the p e r i p h y t o n and i n the phytoplankton. Phytoplankton communities underwent a l a r g e r change i n s t r u c t u r e d u r i n g the p u l s e , but rec o v e r e d f a s t e r than p e r i p h y t o n . Phytoplankton might have recovered f a s t e r than periphyton because of the s h o r t e r g e n e r a t i o n times and f a s t e r turnover r a t e s of the phytoplankton. 3) S p a t i a l l y separated s t a t i o n s w i t h i n the lake c o u l d not be d i s t i n g u i s h e d on the b a s i s of the f i r s t few weeks of community development, whereas mature communities at s t a t i o n s 1 and 3 were c l e a r l y d i f f e r e n t i a t e d . T h i s suggests that long exposure p e r i o d s may be necessary to o b t a i n communities that are c h a r a c t e r i s t i c of a given s i t e . Mature communities underwent a y e a r l y , q u a s i - c y c l i c a l sequence of species replacements. Winter and summer were pe r i o d s of r e l a t i v e s t a b i l i t y , whereas s p r i n g and f a l l were t r a n s i t i o n p e r i o d s , and the f a l l and s p r i n g communities c o u l d resemble each o t h e r . The sequence of changes 111 i n community s t r u c t u r e v a r i e d with depth; more taxa were i n v o l v e d i n the replacement sequence at shallow than at deep s t a t i o n s . In g e n e r a l , high diatom d e n s i t i e s were t y p i c a l of winter assemblages, whereas c o c c o i d greens, filamentous greens, and diatoms t y p i f i e d summer assemblages. The r e l a t i v e abundance of diatoms i n c r e a s e d with depth. S p a t i a l and temporal v a r i a t i o n s i n l i g h t and temperature seem to have s t r o n g l y a f f e c t e d p e r i p h y t o n p e r i o d i c i t y and s p a t i a l h e t e r o g e n e i t y . 4) Within the diatoms and blue-greens, the groups accounting f o r most of the t o t a l abundance of.each d i v i s i o n seemed to e x h i b i t s i m i l a r s p a t i o - t e m p o r a l p a t t e r n s of v a r i a t i o n i n abundance. T h i s suggests that i n t e n s i v e a u t e c o l o g i c a l s t u d i e s of j u s t a few taxa c h a r a c t e r i s t i c of these d i v i s i o n s (e.g. Gomphonema and M i c r o c y s t i s ) might shed some l i g h t on the mechanisms of community change. These r e s u l t s were dependent on the developmental stage of the communities; o l d e r communities presumably r e f l e c t e d changes i n a b i o t i c environmental c o n d i t i o n s b e t t e r than younger ones. 5) Succession and s e a s o n a l i t y i n Gwendoline Lake can be represented using a model which d e p i c t s communities as p o i n t s i n a m u l t i d i m e n s i o n a l s p e c i e s hyperspace. There i s i n t h i s space a c l o s e d curve towards which s u c c e s s i o n a l communities proceed. The curve determines a t r a j e c t o r y which v a r i e s over s e v e r a l time s c a l e s , but which i s dominated by a y e a r l y seasonal c y c l e . Community development, d e s c r i b e d by the t r a j e c t o r y going from the o r i g i n of the axes to the c l o s e d curve, i s d i r e c t i o n a l and 1 1 2 can be r e l a t i v e l y independent of the p o s i t i o n of the f i n a l c y c l i c a l t r a j e c t o r y , e s p e c i a l l y d u r i n g the e a r l y stages. The model shows that i f development i s f a s t i n r e l a t i o n to seasonal changes i n the environment, s e a s o n a l i t y i s more important than age i n determining community s t r u c t u r e . The converse holds when development i s slow compared to environmental s e a s o n a l i t y . The model a l s o shows that b i o t i c d i f f e r e n c e s between s p a t i a l l y separated s t a t i o n s should i n c r e a s e through time under two c o n d i t i o n s . F i r s t , environmental f a c t o r s must d i f f e r between s t a t i o n s ; second, the same pioneer s p e c i e s must always be the f i r s t occupants of bare s i t e s , r e g a r d l e s s of the u l t i m a t e community s t r u c t u r e achieved at each of the s t a t i o n s . 1 1 3 CHAPTER 2. THE EFFECTS OF HISTORICAL ENVIRONMENTAL REGIME AND COMMUNITY AGE ON RECOVERY PROPERTIES A growing uneasiness with e q u i l i b r i u m - b a s e d analyses of p o p u l a t i o n growth equations has l e a d to a p r o l i f e r a t i o n of t h e o r e t i c a l p u b l i c a t i o n s d e a l i n g with the dynamics of p o p u l a t i o n s (Lewontin and Cohen 1969, Levins 1969, Roughgarden 1975, Whittaker and Goodman 1979, Boyce and Daley 1980, Caswell 1982), and communities or ecosystems (May 1973, 1974, Botkin and Sobel 1975, Roughgarden 1975, Yodzis 1978, L e v i n s 1979) i n f l u c t u a t i n g environments. Some aspe c t s that have been d i s c u s s e d in the community- and e c o s y s t e m - l e v e l s t u d i e s i n c l u d e species c o e x i s t e n c e , l i m i t s to niche o v e r l a p , "communication" of s t o c h a s t i c f l u c t u a t i o n between s p e c i e s , and s t a b i l i t y i n the context of environmental s t o c h a s t i c i t y . S t a r t i n g from a more management-oriented p e r s p e c t i v e than the p r e v i o u s s t u d i e s , H o l l i n g (1973) and Peterman (1979) have d e a l t with the e f f e c t s of environmental f l u c t u a t i o n s on community recovery p r o p e r t i e s . In these authors' scheme, the h i s t o r i c a l d i s t u r b a n c e regime experienced by the community m o d i f i e s i t s present s t a t e (community s t r u c t u r e ) and t h e r e f o r e i t s c a p a c i t y to respond to f u r t h e r d i s t u r b a n c e s . Because community s t r u c t u r e i s a l s o a f f e c t e d by age, one cou l d expect d i f f e r i n g responses to d i s t u r b a n c e from communities l y i n g along an age g r a d i e n t . The r e l a t i o n s h i p between community age and s t a b i l i t y has been d i s c u s s e d by Margalef (1968) and Horn (1974), whose use of d i f f e r e n t d e f i n i t i o n s of s t a b i l i t y l e d them to d i a m e t r i c a l l y opposite c o n c l u s i o n s . 1 1 4 Se v e r a l experimental s t u d i e s d e a l i n g with m i c r o s c o p i c algae in f l u c t u a t i n g environments are a v a i l a b l e ; some d e a l with s i n g l e s p e c i e s ' responses (Moser and Brock 1971, Walsh and Legendre 1982) and ot h e r s with whole communities ( O l l a s o n 1977, T u r p i n and H a r r i s o n 1980, Tu r p i n et a l . 1981). Kaufman's (1982) i s the only p e r i p h y t o n study I know of that attempts to r e l a t e d i s t u r b a n c e frequency and community age to community recovery p r o p e r t i e s . The paper however, d e a l s only with aggregated parameters ( t o t a l biomass, d i v e r s i t y , and number of s p e c i e s ) , and not with d e t a i l e d community composition. The e f f e c t of community age on s e n s i t i v i t y to a streptomycin s u l f a t e (an a l g i c i d e ) treatment has been analysed i n a microcosms study by K i n d i g et a l . (1983). The present chapter addresses three main q u e s t i o n s : 1) How does s u c c e s s i o n under e x p e r i m e n t a l l y - i n d u c e d f l u c t u a t i o n s d i f f e r from that under c o n t r o l c o n d i t i o n s ? 2) How do h i s t o r i c a l d i s t u r b a n c e regimes and community age a f f e c t community recovery p r o p e r t i e s ? Recovery from a dis t u r b a n c e must o b v i o u s l y be d e f i n e d by r e f e r e n c e to an undisturbed c o n t r o l . I f the i n i t i a l c o n d i t i o n s (before the dis t u r b a n c e ) are the same f o r the c o n t r o l and the d i s t u r b e d community, and i f the c o n t r o l does not change through time, then recovery occurs when the d i s t u r b e d community r e t u r n s to the pre-dis t u r b a n c e s t a t e (or a l t e r n a t i v e l y , when i t converges to the c o n t r o l ) . However, i f the c o n t r o l i s not s t a t i o n a r y (as o f t e n happens i n e c o l o g i c a l systems) recovery can no longer be d e f i n e d 1 1 5 as r e t u r n to the p r e - d i s t u r b a n c e c o n d i t i o n s , because the und i s t u r b e d c o n t r o l has moved away from those i n i t i a l c o n d i t i o n s . One must a v o i d confounding the e f f e c t s of the di s t u r b a n c e with those of the ( u n s p e c i f i e d ) f a c t o r s causing changes i n the c o n t r o l . T h i s can be achieved by us i n g the a l t e r n a t i v e c r i t e r i o n mentioned above, convergence to the c o n t r o l . These ideas are c e n t r a l to N i c h o l s o n ' s (1957) proposed "convergence experiment" (see Murdoch 1970 f o r an extended d i s c u s s i o n ) . When using t h i s d e sign, the focus i s no longer on whether there i s a r e t u r n to a steady s t a t e (or even to a moving e q u i l i b r i u m ) or not, but ra t h e r c e n t e r s on the speed of convergence as a measure of the s t r e n g t h of r e g u l a t i o n . The u n d e r l y i n g hypothesis i s that i f community s t r u c t u r e . (or whatever p r o p e r t y happens to be of i n t e r e s t ) i s r e g u l a t e d towards an e q u i l i b r i u m determined by b i o t i c i n t e r a c t i o n s and environmental f a c t o r s , there should be convergence between two communities p l a c e d i n the same environment and d i f f e r i n g only i n i n i t i a l community s t r u c t u r e . The magnitude of the i n i t i a l d i f f e r e n c e measures the i n t e n s i t y or amplitude of the d i s t u r b a n c e , and the r a t e of convergence measures the i n t e n s i t y of environmental r e g u l a t i o n . An absence of convergence w i t h i n the p e r i o d of o b s e r v a t i o n would i n d i c a t e one of the f o l l o w i n g , a) The time frame i s too s h o r t . The i n t e r n a l dynamics of the community c o u l d be i n t r i n s i c a l l y slow, or the c o n t r o l might be moving i n the same d i r e c t i o n as the r e c o v e r i n g community, thus extending the time h o r i z o n r e q u i r e d to detec t convergence, b) Within-community b i o t i c i n t e r a c t i o n s may 1 16 generate m u l t i p l e s t a b l e p o i n t s (Lewontin 1969, G i l p i n and Case 1976, Case and Casten 1979). If the d i s t u r b a n c e moved a community towards the v i c i n i t y of a s t a b l e p o i n t d i f f e r e n t from the c o n t r o l ' s , the two communities might never converge. T h i s s i t u a t i o n c o u l d i n p r i n c i p l e be d e t e c t e d using the "convergence experiment" d e s i g n , c) Regulation of community s t r u c t u r e i s weak or n o n - e x i s t e n t ; density-independent f a c t o r s operate most of the time, and an element of randomness pervades the community's behavior. Convergence c o u l d occur here only on a very long time s c a l e compared to the organisms' l i f e spans. 3) How do convergence p r o p e r t i e s depend on the v a r i a b l e being measured? In p a r t i c u l a r , t o t a l c e l l numbers, and community composition at two l e v e l s (major taxa and family/genus) w i l l be analysed here. EXPERIMENTAL PROCEDURE On June 10, 1982, three s l i d e racks were p l a c e d at s t a t i o n s 1, 2, and 3 (at depths of 1.1 m, 1.7 m, and 10 m, r e s p e c t i v e l y ) . The rack at s t a t i o n 1 was moved upwards to 4 m on week 6, because c o l o n i z a t i o n and growth r a t e s were very low at the deeper s i t e . These three racks (which I w i l l c a l l C l , C2, and C3) represented the c o n t r o l or background c o n d i t i o n s a g a i n s t which s u c c e s s i o n i n f l u c t u a t i n g environments and recovery p r o p e r t i e s were s t u d i e d . F l u c t u a t i n g environments were generated by t r a n s f e r r i n g three racks (IA, IB,, and II) between s t a t i o n s , as f o l l o w s : the 1 1 7 three racks were pl a c e d i n s t a t i o n 1 on June 10, 1982; t h e r e a f t e r , - Rack IA a l t e r n a t e d between s t a t i o n s 1 and 2, remaining one week at a time at each s t a t i o n , u n t i l week 9. On week 9, the rack was t r a n s f e r r e d to s t a t i o n 3, and remained there u n t i l the end of the experiment on October 27, 1982 (week 20). Rack IB a l t e r n a t e d between s t a t i o n s 1 and 3 d u r i n g the f i r s t nine weeks, but was otherwise t r e a t e d i d e n t i c a l l y to IA. Rack II a l t e r n a t e d between s t a t i o n s 1 and 3, and was l e f t f o r two weeks at a time at each s t a t i o n . The rack was t r a n s f e r r e d to s t a t i o n 3 on week 10, and remained there u n t i l the end of the experiment. An i n i t i a l working hypothesis was that i f the environments at the three s t a t i o n s d i f f e r e d , then: - IA and IB communities would be exposed to f l u c t u a t i o n s of s i m i l a r p e r i o d (one week), but d i f f e r e n t types (because IA was t r a n s f e r r e d between s t a t i o n s 1 and 2, whereas IB was t r a n s f e r r e d between s t a t i o n s 1 and 3). IB and II communities would be exposed to s i m i l a r types of f l u c t u a t i o n s but with d i f f e r e n t p e r i o d s . The e f f e c t s of environmental f l u c t u a t i o n s on community development were s t u d i e d by comparing the communities on racks IA, IB, and II d u r i n g the f l u c t u a t i o n s with the a p p r o p r i a t e c o n t r o l s ; e.g. the f i r s t nine weeks of development at IA with C1 ' 118 and C2, or the f i r s t ten weeks at II with C1 and C3. The e f f e c t s of f l u c t u a t i o n s on recovery p r o p e r t i e s were s t u d i e d by comparing the p o s t - f l u c t u a t i o n s behavior of IA, IB, and II with that of C3. On week 10, s l i d e racks that had been submerged at s t a t i o n 1 f o r s i x weeks (rack B), two weeks (rack C), and zero weeks (rack D, h o l d i n g c l e a n s l i d e s ) , were p l a c e d at s t a t i o n 3. The e f f e c t of community age on recovery p r o p e r t i e s was s t u d i e d by comparing the speeds with which communities of d i f f e r e n t ages converged to the composition of C3, the c o n t r o l community. Samples were taken weekly from a l l racks between weeks 1 and 13, and a one-time sampling was done on week 20. RESULTS Community development under envi ronmental f l u c t u a t i o n s , and  p o s t - f l u c t u a t i o n recovery Changes i n t o t a l d e n s i t y and r e l a t i v e abundances of major taxa Periphyton abundance ( c e l l s / c m sq) i s p l o t t e d vs. time i n F i g u r e s 21a-21c. No major d i f f e r e n c e s are obvious among the a c c r u a l p a t t e r n s of IA, IB, and I I , e i t h e r d u r i n g or a f t e r the f l u c t u a t i o n s . A comparison between IA, IB, I I , and the c o n t r o l s (see F i g u r e s 3a-3c) shows that d u r i n g the f i r s t f i v e weeks abundance was s l i g h t l y higher i n the " f l u c t u a t i n g " communities than i n s t a t i o n 1, but much lower than i n s t a t i o n s 2 and 3. 1 1 9 F i g u r e 21. Time s e r i e s of t o t a l numbers and r e l a t i v e abundances: IA, IB, and I I . a,b,c: T o t a l abundances of IA, IB, I I . d,e,f: r e l a t i v e abundances of IA, IB, and II . o o CO 121 Growing c o n d i t i o n s at s t a t i o n 1 were poorer than at s t a t i o n s 2 and 3, and the e f f e c t of a l t e r n a t i n g between s t a t i o n 1 and the shallower s t a t i o n s seems to have been a depression of growth r a t e s i n communities IA, IB, and II r e l a t i v e to the shallow s t a t i o n s . A f t e r being permanently t r a n s f e r r e d to s t a t i o n 3 the f l u c t u a t i n g communities' d e n s i t i e s were s i g n i f i c a n t l y lower than those of C3 u n t i l week 20 (as i n d i c a t e d by p a i r w i s e t - t e s t s ( E l l i o t 1977), and by one-way ANOVA , a p r i o r i comparisons (Sokal and Rohlf 1969); a l l t e s t s performed using l o g -transformed d a t a ) . By week 20 the average c e l l d e n s i t i e s of IA, IB, and II were s t i l l lower than C3 d e n s i t y , even though these three communities had been at s t a t i o n 3 s i n c e week 9 (IA and IB) or week 10 ( I I ) . However, s t a t i s t i c a l t e s t s showed that the d i f f e r e n c e s were not s i g n i f i c a n t . The r e l a t i v e abundances of the four major taxa are graphed vs. time i n F i g u r e s 2d-2f. The average CV f o r samples c o l l e c t e d before week 6 i s 82.9%, and the average CV for samples c o l l e c t e d a f t e r week 5 i s 36.6% (the r e d u c t i o n i n v a r i a b i l i t y i n the l a t t e r samples might have been caused by the higher c e l l d e n s i t i e s of o l d e r s l i d e s ) . The time sequence of community composition i s v i r t u a l l y i d e n t i c a l f o r IA, IB, and I I . A comparison of F i g u r e s 4a-4c with F i g u r e s 21d-21f shows that d u r i n g the f l u c t u a t i o n s IA, IB, and II were more s i m i l a r to C1 than to C2 or C3: green algae were more abundant i n C1 and i n the f l u c t u a t i n g communities than i n C2 and C3 u n t i l week 11, whereas blue-greens and diatoms were suppressed i n the former communities d u r i n g the same p e r i o d . A f t e r week 11 no 122 d i f f e r e n c e s were obvious between IA, IB, I I , and C3. U n l i k e t o t a l abundance, community composition at the l e v e l of major taxa d i d not r e t a i n . f o r long the e f f e c t s of the f l u c t u a t i o n s . Community development in f l u c t u a t i n g environments An o r d i n a t i o n of IA, IB, and II showed that d u r i n g the f l u c t u a t i o n s there was an i n i t i a l d i s s i m i l a r i t y among samples from the three communities. T h i s divergence l a s t e d only two to three weeks, and was presumably due to the s t o c h a s t i c i t y of c o l o n i z a t i o n , which l a r g e l y determined community s t r u c t u r e at low d e n s i t i e s . From week 3 to week 9 samples from the three communities were a l l lumped in a small subregion of the o r d i n a t i o n space, i n d i c a t i n g that IA, IB, and II responded s i m i l a r l y to the f l u c t u a t i n g regimes. Thus the a p r i o r i e x p e c t a t i o n s of d i f f e r e n t i a l responses to d i f f e r i n g amplitudes and p e r i o d i c i t i e s of f l u c t u a t i o n s were not f u l f i l l e d . Did communities developing i n f l u c t u a t i n g environments d i f f e r from those exposed to more steady c o n d i t i o n s ? F i g u r e s 22a-22c show the o r d i n a t i o n s of IA, IB, and II and t h e i r r e s p e c t i v e c o n t r o l s . Only the c e n t r o i d s of each-sample are shown in these o r d i n a t i o n s (these are the c e n t r o i d s of the t r i a n g l e s formed by connecting the three r e p l i c a t e s i n each sample with s t r a i g h t l i n e s ) . The p l o t s show that developmental t r a j e c t o r i e s were indeed d i f f e r e n t i n the f l u c t u a t i n g and c o n t r o l communities. Samples from the former remained f o r 2-3 weeks to the r i g h t of AI, and subsequently moved to the bottom-l e f t area, s t a y i n g there u n t i l the end of the f l u c t u a t i o n s . C1 123 F i g u r e 22. O r d i n a t i o n s of IA, IB, and II and t h e i r r e s p e c t i v e c o n t r o l s . Axes I and II account f o r 59% and 11%, 44% and 14%, and 45% and 11%, of the t o t a l v a r i a n c e i n a, b, and c, r e s p e c t i v e l y . 1 2 4 0 n a Ti Li — O m it • O 0 0 B 0 B E B E Qjr • 0 B II s i x v © • 66 0 s s ' 0 0 B © 0 B s w X < O E 0 E 0 0 E 0 .0 + 0 CO x < II S|xy II s jxv 1 25 samples were s c a t t e r e d around the bo t t o m - c e n t e r / r i g h t f o r the f i r s t 6 weeks, but moved r a p i d l y to the l e f t , c l o s e to the f l u c t u a t i n g communities' samples, a f t e r being moved upwards to the 4-meter s i t e . The "shallow s t a t i o n " samples (C2 i n F i g u r e 22a and C3 i n F i g u r e s 22b and 22c) were p l a c e d along the c e n t e r / r i g h t of Al d u r i n g the f i r s t 2-3 weeks, and to the l e f t of the same a x i s a f t e r w a r d s . A f t e r week 3, the v e r t i c a l a x i s separates C1 and the f l u c t u a t i n g communities from the s h a l l o w - s t a t i o n samples. In the three o r d i n a t i o n s Al i s dominated by group 1 (negative c o r r e l a t i o n ) and group 2 ( p o s i t i v e c o r r e l a t i o n ) , the two most s u c c e s s f u l e a r l y c o l o n i z e r s . The s t r o n g e s t c o r r e l a t i o n s with the v e r t i c a l a x i s corresponded to groups 6, 12, and 40; with the exc e p t i o n of groups 1 and 2, a l l groups were almost i n v a r i a b l y p o s i t i v e l y c o r r e l a t e d with t h i s a x i s . T h i s i n f o r m a t i o n , i n c o n j u n c t i o n with d i r e c t examination of r e l a t i v e abundances, showed that samples on the bottom-right of the o r d i n a t i o n s were r i c h i n groups 1 and 2, but poor i n other groups. Samples i n the b o t t o m - l e f t corner were completely dominated by group 1, with a l l other groups r a r e . Samples from the t o p - l e f t r e g i o n had a more balanced d i s t r i b u t i o n of r e l a t i v e abundances, with groups 1, 6, 12, and 40 co-dominating. In the p r e v i o u s chapter I showed that i n a t y p i c a l s u c c e s s i o n a l sequence groups 1 and 2 were e a r l y co-dominants; group 2 became rare l a t e r on, and group 1 r e l a t i v e abundances decreased as other groups ( s p e c i a l l y 3, 12, 14, 15, 40, and 41) became more common. T h e r e f o r e , i n the o r d i n a t i o n s of F i g u r e 22 there seems to be a curved g r a d i e n t r e f l e c t i n g s u c c e s s i o n a l stages, 126 beginning with high group 1 and group 2 r e l a t i v e abundances i n the bottom-right, followed by t o t a l dominance by group 1 i n the b o t t o m - l e f t , and ending to the l e f t of AI with p r o g r e s s i v e l y r i c h e r and more mature communities a s s o c i a t e d with i n c r e a s e s along the v e r t i c a l a x i s . In t h i s l i g h t , the r e s u l t s of the experimental manipulations can be i n t e r p r e t e d q u i t e s t r a i g h t f o r w a r d l y . The s h a l l o w - s t a t i o n communities f o l l o w e d a "normal" developmental t r a j e c t o r y , as d e s c r i b e d above and i n the p r e v i o u s chapter* The deep s t a t i o n (C1) maintained very low p o p u l a t i o n d e n s i t i e s d u r i n g the f i r s t 6 weeks, and c o l o n i z a t i o n by groups 1 and 2 was the main determinant of community s t r u c t u r e . A f t e r C1 was moved upwards, group. 1 r e l a t i v e abundances i n c r e a s e d r a p i d l y , making t h i s group the s o l e dominant. The f l u c t u a t i n g communities followed a t r a j e c t o r y that was intermediate between those of C1 and the shallow s t a t i o n s . By week 4 the composition of communities IA, IB, and II was comparable to that of the shallow s t a t i o n s , but more advanced in the s u c c e s s i o n a l sequence than that of C1. The e f f e c t s of the f l u c t u a t i o n s were twofold: exposure to the f a v o r a b l e c o n d i t i o n s at the shallow s t a t i o n s seems to have a c c e l e r a t e d the development of IA, IB, and II r e l a t i v e to C1; c o n v e r s e l y , exposure of IA, IB, and II to the poor growing c o n d i t i o n s at C1 slowed down t h e i r development r e l a t i v e to the shallow s t a t i o n s . Another i n t e r e s t i n g f e a t u r e h i g h l i g h t e d i n the o r d i n a t i o n s i s that the f l u c t u a t i n g communities d i d not " t r a c k " the 1 27 community composition of the c o n t r o l s . Under the assumption of e f f i c i e n t t r a c k i n g , one would expect that a f t e r s t a y i n g at a given s t a t i o n f o r a short p e r i o d of time the composition of the f l u c t u a t i n g communities would resemble c l o s e l y that of the s t a t i o n ' s c o n t r o l . Thus, f o r example, community IA would have resembled C1 a f t e r the f i r s t week, C2 a f t e r the second, then C1 again a f t e r the t h i r d , and so f o r t h ; community II would have resembled C1 on weeks 1 and 2, C3 on weeks 3 and 4, C1 again on weeks 5 and 6, and so f o r t h . What was observed i n r e a l i t y was that the f l u c t u a t i n g communities were h e l d at an e a r l y developmental stage that was intermediate between the shallow control's and C1 . When C1 was moved upwards on week 6, community composition changed towards a more advanced s u c c e s s i o n a l stage, and "caught up" with the f l u c t u a t i n g communities, as shown by the s h i f t towards the l e f t of 7-week, 8-week, and 9-week C1 samples. One co u l d expect that a f t e r week 6 the f l u c t u a t i n g communities should move i n the d i r e c t i o n of i n c r e a s i n g m a t u r i t y , because exposure to the new s t a t i o n 1 c o n d i t i o n s would not be so d e t r i m e n t a l to t h e i r development as i t had been at the deeper s i t e . T h i s indeed seems to have o c c u r r e d : week 9 samples i n F i g u r e s 22a and 22b, and week 10 samples i n F i g u r e 22c are d i s p l a c e d upwards, moving away from the r e g i o n i n which they had remained f o r the p r e v i o u s 4 (IA and IB) or 5 (II) weeks. 1 28 Recovery of the communities exposed to f l u c t u a t i n g regimes Once the experimental f l u c t u a t i o n s had been terminated and communities IA, IB, and II had been p l a c e d at s t a t i o n 3, I wanted to t e s t f o r convergence between the compositions of these three communities and that of C3. P o s t - f l u c t u a t i o n samples from the four communities were o r d i n a t e d , and the r e s u l t i s shown in F i g u r e 23a. AI i s c o r r e l a t e d p o s i t i v e l y (r=0.98) with group 1, and n e g a t i v e l y with most of the other groups (the next 5 highest c o r r e l a t i o n s are with groups 2, 3, 12, 39, and 40). A l l c o r r e l a t e s n e g a t i v e l y (r=-0.59) with group 3, and p o s i t i v e l y , with groups 40 (r = 0.48) and 41 (r=0.62). Community development i n c r e a s e s along an imaginary a x i s roughly p a r a l l e l to a d i a g o n a l running from t o p - l e f t to bottom-r i g h t . Movement towards the t o p - l e f t along t h i s imaginary a x i s corresponds to r e d u c t i o n s i n group 1 and i n c r e a s e s i n groups 2, 12, 39, 40, and 41. With the ex c e p t i o n of the i n c r e a s e i n group 2, t h i s was shown e a r l i e r to be a t y p i c a l developmental p a t t e r n . Group 2 c o r r e l a t e s p o s i t i v e l y with a time a x i s because i t s r e l a t i v e abundance i n c r e a s e s moderately i n the o l d e r samples, and the f i r s t weeks are not i n c l u d e d i n the o r d i n a t i o n . U s u a l l y the very h i g h r e l a t i v e abundances of group 2 i n the f i r s t two weeks dominate the c o r r e l a t i o n of the group with the time a x i s , c a u s i n g a negative c o r r e l a t i o n to appear. The displacement towards the b o t t o m - l e f t and subsequent r e t u r n d u r i n g weeks 11-13 were caused by the group 3 peak d i s c u s s e d i n the previous chapter. The f l u c t u a t i n g communities were i n d i s t i n g u i s h a b l e from each 1 29 F i g u r e 23. O r d i n a t i o n s of experimental communities d u r i n g recovery. In a, axes I and II account f o r 50% and 15% of the t o t a l v a r i a n c e . In b, axes I and II account f o r 52% and 25% of the t o t a l v a r i a n c e . 1 3 0 II sjxv 3 < "9 = (ST) -II II « II o < i p o ) • , \ + . L I ' - - . , <&-- _ \ •! """" \";:'-^ (§) II sjxy 131 other on week 10, and d e s p i t e some (presumably s t o c h a s t i c ) v a r i a t i o n i n t h e i r t r a j e c t o r i e s , behaved s i m i l a r l y t h e r e a f t e r . The group 3 p u l s e d e f l e c t e d the communities away from t h e i r "normal" developmental t r a j e c t o r y along the imaginary d i a g o n a l a x i s . Community recovery ensued, and by week 20 the f l u c t u a t i n g communities and C3 had returned to t h e i r o r i g i n a l t r a j e c t o r y . The f l u c t u a t i n g communities always lagged behind C3 along the developmental a x i s , and by week 20 had s t i l l not "caught up", d e s p i t e t h e i r having shared the same s i t e f o r 10 weeks. By t h i s time however, the f l u c t u a t i n g communities had reached a p o s i t i o n along the developmental a x i s that was comparable to that occupied by C3 a f t e r 10-12 weeks, suggesting that time-of-r e s i d e n c e at a s i t e determines r a t h e r r i g i d l y the p o s i t i o n occupied along t h i s a b s t r a c t a x i s . The e f f e c t of community age on recovery p r o p e r t i e s T o t a l numbers and r e l a t i v e abundances of major taxa Racks B, C, and D were t r a n s f e r r e d to s t a t i o n 3 when they were 6, 2, and 0 weeks o l d , r e s p e c t i v e l y . Time s e r i e s of B, C, and D t o t a l numbers a f t e r the t r a n s f e r are shown i n F i g u r e s 24a-24c. I n i t i a l c o n d i t i o n s d i f f e r e d among the three r a c k s , as shown by the d i f f e r e n c e s i n abundance on week 10. By week 13, mean c e l l d e n s i t y v a l u e s were s t i l l higher i n B than i n C, and i n C . than in D. By week 20 however, the numbers of B, C, and D were not s i g n i f i c a n t l y d i f f e r e n t (one-way ANOVA , E l l i o t 1977), although they were s t i l l lower than those of C3. The l a r g e abundance d i s p a r i t y between B, C, and D and C3 on 1 32 F i g u r e 24. T o t a l numbers and r e l a t i v e abundances of major taxa i n B, C, and D. a,b,c: T o t a l c e l l numbers of B, C, and D. d,e,f: R e l a t i v e abundances of major taxa i n B, C, and D. The r e l a t i v e abundances f o r the c o n t r o l (C3) are given by the black dots i n d - f . 134 week 10 had not been overcome by week 20, even though the 4 s t a t i o n s had been exposed to the same environmental c o n d i t i o n s fo r 10 weeks. However, t h i s same 10-week p e r i o d s u f f i c e d to e l i m i n a t e the small abundance d i f f e r e n c e s that e x i s t e d among B, C, and D on week 10. F i g u r e s 24d-24f show that community composition at the l e v e l of major taxa d i f f e r e d between B, C, and D and C3 on weeks 10-13, mainly because green algae were much more common in B, C, and D than i n C3. By week 20 however, these d i f f e r e n c e s had l a r g e l y vanished. Community s t r u c t u r e An o r d i n a t i o n of B, C, and D and C3 samples i s shown in F i g u r e 23b. Al i s dominated by groups 1, 2, 12, 15, 39, 40, 41, and 45 (the c o r r e l a t i o n values are 0.98, -0.82, -0.86, -0.72, -0.79, 0.79, -0.83, and -0.78, r e s p e c t i v e l y ) , whereas A H i s s t r o n g l y c o r r e l a t e d with group 3 ( r = 0 . 9 l ) . The second-highest c o r r e l a t i o n with A H i s only -0.41 (with group 15). B, C, and D h a r d l y d i f f e r e d i n i t i a l l y , as shown by the o v e r l a p i n the 9-week and 10-week samples. As i n the post-f l u c t u a t i o n s o r d i n a t i o n of IA, IB, and II and C3 (Figure 23a) there i s an a x i s r e p r e s e n t i n g a developmental g r a d i e n t (more or l e s s p a r a l l e l to Al i n t h i s o r d i n a t i o n ) , along which group 1 dominance decreased i n time while a few other groups became more common. B, C, and D were i n i t i a l l y w e l l separated from C3 along A l . The s e p a r a t i o n s t i l l e x i s t e d on week 20, by which time B, C, and D reached the p o s i t i o n that C3 had occupied on week 10. V a r i a t i o n along A H demonstrates that B, C, and D and C3 responded s i m i l a r l y to the group 3 p u l s e . B, C, and D samples 135 from week 20 o v e r l a p with C3 samples from week 10 along both axes, suggesting that the group 3 pulse acted as a di s t u r b a n c e that t e m p o r a r i l y d e f l e c t e d the communities from t h e i r "normal" developmental t r a j e c t o r i e s . I t a l s o supports the idea, forwarded e a r l i e r , that t i m e - o f - r e s i d e n c e at a given s i t e i s the main determinant of the community's p o s i t i o n along the developmental sequence c h a r a c t e r i s t i c of that s i t e . DISCUSSION Community development under f l u c t u a t i n g regimes The three c o n t r o l communities C1, C2, and C3 underwent development along s i m i l a r t r a j e c t o r i e s , f o l l o w i n g f a i r l y c l o s e l y the " t y p i c a l " developmental sequence d e s c r i b e d _ e a r l i e r . The r a t e s of development, however, were v a s t l y d i f f e r e n t between the shallow (C2, C3) and the deep (C1) c o n t r o l s , presumably because of d e p t h - r e l a t e d d i f f e r e n c e s i n c e l l growth r a t e s . The d i f f e r e n c e s and s i m i l a r i t i e s among the c o n t r o l s were r e f l e c t e d i n the f l u c t u a t i n g communities' behavior. IA and IB responded s i m i l a r l y to the f l u c t u a t i o n s , as would be expected i f r a t e s and d i r e c t i o n s of development were comparable i n C2 and C3. A l s o , the d i f f e r e n c e s between IA, IB, and II and the c o n t r o l s were a product of the d i f f e r e n c e s between the shallow and deep c o n t r o l s . The s i m i l a r i t y i n the responses of IB and II to f l u c t u a t i o n s of the same amplitude but d i f f e r e n t p e r i o d suggests that the i n t e r n a l dynamics of the community were slow r e l a t i v e to the frequency of the f l u c t u a t i o n s . In e f f e c t , the communities seem to have " i n t e g r a t e d " over the f l u c t u a t i o n s , 136 responding to an "average environment" r a t h e r than t r a c k e d c l o s e l y the changing c o n d i t i o n s (the "average environment" was frequency-independent, and t h e r e f o r e s i m i l a r f o r IA, IB, and I I ) . The absence of t r a c k i n g was a l s o manifest i n comparisons between the f l u c t u a t i n g communities and the c o n t r o l s . The shallow and deep c o n t r o l s were p o s i t i o n e d along the same developmental g r a d i e n t , and d i f f e r e d mainly i n t h e i r r a t e s of development i . e . , i n how f a r along the g r a d i e n t they had moved at any given time. Close t r a c k i n g of the c o n t r o l s by IA, IB, and II would have been expressed as "back and f o r t h " displacements along the g r a d i e n t , with the t i m i n g of movement corresponding to that of the experimental t r a n s f e r s . What was observed i n s t e a d was that the f l u c t u a t i n g communities remained in an intermediate p o s i t i o n along the g r a d i e n t . T h i s phenomenon has been e x t e n s i v e l y d i s c u s s e d i n the l i t e r a t u r e , under the names "pulse s t a b i l i t y " (Odum 1971) and "arrestment of s u c c e s s i o n " ( N i e r i n g and Goodwin 1974). Margalef (1968) d e a l s with i t i n a chapter d i s c u s s i n g s u c c e s s i o n and e x p l o i t a t i o n . The b a s i c idea i s that a r e c u r r e n t , more or l e s s p e r i o d i c a l p h y s i c a l p e r t u r b a t i o n can maintain the system at an intermediate developmental stage, and i s e x e m p l i f i e d , a c c o r d i n g to Odum (1971), by ecosystems in which water l e v e l s f l u c t u a t e ( e s t u a r i e s , i n t e r t i d a l zones, freshwater marshes), and by f i r e -c o n t r o l l e d f o r e s t s . The n o t i o n of s u c c e s s i o n being h a l t e d by d i s t u r b a n c e can be t r a c e d even f u r t h e r back, to Clements' (1936) d e f i n i t i o n of " d i s c l i m a x " . The use of the words " s t a b i l i t y " , "arrestment", and 1 37 " d i s c l i m a x " r e v e a l s that the authors had i n mind s i t u a t i o n s i n which d i s t u r b a n c e m o d i f i e d the end-point reached by the community. In the present study however, the c o n t r o l communities were not s t a t i o n a r y , but r a t h e r moved i n time along the developmental g r a d i e n t . Hence, i t i s not safe to assume that the f l u c t u a t i n g communities reached a s t a b l e end-point; i t c o u l d be more l o g i c a l to view both the c o n t r o l and f l u c t u a t i n g communities as changing through time. The f l u c t u a t i n g communities' intermediate p o s i t i o n along the developmental g r a d i e n t would then be the r e s u l t of a (slow) rate of development intermediate between those of the deep and shallow c o n t r o l s . E f f e c t s of p r e v i o u s exposure to f l u c t u a t i n g regimes and of  community age on recovery p r o p e r t i e s Communities IA, IB, and II showed almost i d e n t i c a l responses during the recovery phase, as d i d B, C, and D . Moreover, the r e s u l t s of these two experiments were s i m i l a r enough to merit j o i n t d i s c u s s i o n . Because communities IA, IB, and II on the one hand, and B, C, and D on the other, d i d not d i f f e r among themselves, I cannot d e a l with the e f f e c t s of community age or of f l u c t u a t i n g regimes on recovery p r o p e r t i e s . Instead, I w i l l view both experiments as procedures which p r o v i d e d communities that had an i n i t i a l composition d i f f e r e n t from the c o n t r o l s ' , and focus on the convergence (or l a c k of i t ) between the c o n t r o l s and the experimental communities . An impressive number of d e f i n i t i o n s of " s t a b i l i t y " have 1 38 appeared i n the e c o l o g i c a l l i t e r a t u r e (see Lewontin 1969, Margalef 1969, H o l l i n g 1973, May 1973, Boesch 1974, Orians 1975, Westman 1978, and Pimm 1984). Most of these d e f i n i t i o n s are concerned with p r o p e r t i e s a s s o c i a t e d with s t a b l e e q u i l i b r i u m p o i n t s ; f o r i n s t a n c e , "adjustment s t a b i l i t y " (Margalef 1969; the concept i s e q u i v a l e n t to H o l l i n g ' s (1973) " s t a b i l i t y " , Boesch's (1974) " r e s i l i e n c y " , O r i a n s ' (1975) " e l a s t i c i t y " , and Pimm's (1984) " r e s i l i e n c e " ) , which i s measured by the time r e q u i r e d to r e t u r n to e q u i l i b r i u m f o l l o w i n g a p e r t u r b a t i o n . S t a b i l i t y or recovery n o t i o n s r e q u i r i n g comparisons between a d i s t u r b e d community and a s t a b l e e q u i l i b r i u m p o i n t do not apply to my r e s u l t s , because n e i t h e r the c o n t r o l nor the experimental communities were s t a t i o n a r y i n the course of the study. However, O r i a n s ' d e f i n i t i o n of " t r a j e c t o r y s t a b i l i t y " p r o v i d e s a convenient t h e o r e t i c a l b u t t r e s s f o r some of the f i n d i n g s r e p o r t e d here: " T r a j e c t o r y s t a b i l i t y C i s ! the p r o p e r t y of a system to move towards some f i n a l end-point or zone d e s p i t e d i f f e r e n c e s i n s t a r t i n g p o i n t s . T h i s i s the meaning of s t a b i l i t y d u r i n g p l a n t s u c c e s s i o n where a s i n g l e 'climax' s t a t e may be reached from a v a r i e t y of s t a r t i n g p o i n t s " (Orians 1975, p. 143). H i s Fi g u r e 1-F shows convergence from d i f f e r e n t s t a r t i n g p o i n t s to a common t r a j e c t o r y and subsequent movement towards an e q u i l i b r i u m zone. I i n t e r p r e t the group 3 pulse as a n a t u r a l d i s t u r b a n c e that d e f l e c t e d both the c o n t r o l and the experimental communities from t h e i r "normal" developmental t r a j e c t o r y . Once the c o n d i t i o n s that had favored the pulse disappeared the communities returned to the normal pathway, e x h i b i t i n g thus t r a j e c t o r y s t a b i l i t y . 139 The experimental communities d i d not converge to the c o n t r o l s w i t h i n the ten weeks duri n g which they shared s t a t i o n 3. The r e s u l t s of the previous chapter suggest that both s e t s of communities were moving towards a r e l a t i v e l y more s t a b l e s t a t e of m a t u r i t y ( d e p i c t e d as a c l o s e d curve i n F i g u r e 20a). One c o u l d hypothesize that convergence d i d not occur because both s e t s of communities were s t i l l f a r from maturity., and t h e r e f o r e the c o n t r o l s were moving towards the c l o s e d curve almost as f a s t as the experimental communities . I f t h i s were the case, a more c a r e f u l experimental design that used mature communities as c o n t r o l s , and i n which samples were c o l l e c t e d over a more extended p e r i o d of time, would be r e q u i r e d f o r an adequate t e s t of convergence. Such an experiment i s d e s c r i b e d in the next chapter. Whether the experimental communities recovered or not from the i n i t i a l d i f f e r e n c e s with the c o n t r o l depended on the community property measured. The o r d i n a t i o n a n a l y s e s used a measure of s i m i l a r i t y , O r l o c i ' s (1967) " s t a n d a r d i z e d E u c l i d e a n d i s t a n c e " , t h at i n c o r p o r a t e d a l l 45 groups. At t h i s l e v e l of d e t a i l , recovery had not occurred by week 20. However, both the " f l u c t u a t i n g environment" and the " d i f f e r e n t age" experimental communities showed convergence to the c o n t r o l when s i m i l a r i t y was assessed at the l e v e l of major ta x a . When the analyses i n v o l v e d t o t a l c e l l numbers, the f l u c t u a t i n g communities converged; the 6-week, 2-week, and 0-week o l d communities d i d not converge to the c o n t r o l , but they converged to a common abundance, e l i m i n a t i n g the s m a l l i n i t i a l 1 40 d i f f e r e n c e s that e x i s t e d among them. Convergence i n these cases seemed to depend on the magnitude of the i n i t i a l d i f f e r e n c e i n abundance between the c o n t r o l and the experimental communities. Westman (1978) has a p t l y noted that s e v e r a l measures of f l u c t u a t i o n i n ecosystem s t r u c t u r e can be used (e.g. p o p u l a t i o n d e n s i t i e s of component s p e c i e s , t o t a l biomass, net primary p r o d u c t i v i t y , n u t r i e n t s t o c k s , s p e c i e s r i c h n e s s ) , but that because d i f f e r e n t p r o p e r t i e s of ecosystem s t r u c t u r e and f u n c t i o n need not vary at the same r a t e s , the measure chosen for study may l a r g e l y determine the s t a b i l i t y p r o p e r t i e s observed. My r e s u l t s seem to provide an e m p i r i c a l c o n f i r m a t i o n of h i s statement. SUMMARY 1) The experimental communities exposed to f l u c t u a t i n g environments d i d not t r a c k environmental changes o c c u r r i n g over one- and two-week p e r i o d s . Instead, the communities seemed to " i n t e g r a t e " over the f l u c t u a t i o n s , responding to environmental c o n d i t i o n s that were i n t e r m e d i a t e between the b e t t e r c o n d i t i o n s of the shallow s t a t i o n s and the poorer ones of the deeper s t a t i o n . T h i s r e s u l t e d i n a slowing of the s u c c e s s i o n r a t e , s i m i l a r to the "arrestment of s u c c e s s i o n " and "pulse s t a b i l i t y " phenomena d e s c r i b e d i n the l i t e r a t u r e . The d i r e c t i o n of s u c c e s s i o n however, was steady, and f o l l o w e d the " t y p i c a l " t r a j e c t o r y d e s c r i b e d i n chapter 1. 2) A f t e r the e x p e r i m e n t a l l y - i n d u c e d f l u c t u a t i o n s had ceased, a p u l s e of Dinobryon/Epipyxi s d e f l e c t e d the c o n t r o l and 141 experimental communities from t h e i r "normal" developmental r o u t e s . Once the c o n d i t i o n s that had favored the pulse d isappeared, both sets of_communities re t u r n e d to t h i s normal pathway, e x h i b i t i n g thus " t r a j e c t o r y s t a b i l i t y " sensu Orians (1975). The community s t r u c t u r e of the experimental communities a f t e r 10 weeks of resid e n c e at s t a t i o n 3 was very s i m i l a r to that of the 10-week o l d c o n t r o l , even though the former communities were 20 weeks o l d and had been a f f e c t e d by the Dinobryon/Epipyxis p u l s e . T h i s r e s u l t suggests t h a t , at l e a s t d u r i n g the f i r s t 20 weeks of development, the time of r e s i d e n c e at a given s i t e determines r a t h e r r i g i d l y the p o s i t i o n of the community along the developmental t r a j e c t o r y t y p i c a l of that s i t e . 3) The experimental communities d i d not converge to the community s t r u c t u r e of the c o n t r o l , perhaps because both s e t s of communities were s t i l l f a r from m a t u r i t y , and t h e r e f o r e the c o n t r o l was moving towards m a t u r i t y almost as f a s t as the experimental communities. Whether convergence was de t e c t e d or not depended on the community p r o p e r t y measured: there was no convergence of t o t a l abundances, or of community s t r u c t u r e s at the family/genus l e v e l , whereas community s t r u c t u r e s converged at the taxonomic d i v i s i o n l e v e l . 1 42 CHAPTER 3. COMMUNITY STRUCTURE REGULATION IN PERIPHYTON INTRODUCTION Is community s t r u c t u r e t i g h t l y r e g u l a t e d and u s u a l l y c l o s e to e q u i l i b r i u m , or do density-independent f a c t o r s keep the community f a r from e q u i l i b r i u m most of the time? T h i s q u e s t i o n l i e s at the heart of much of the c u r r e n t d i s c u s s i o n concerning the r o l e of b i o t i c i n t e r a c t i o n s i n s t r u c t u r i n g communities (Wiens 1977,1983; C o n n e l l 1978), j u s t as i t d i d i n the f i f t i e s and s i x t i e s , with r e f e r e n c e to r e g u l a t i o n of p o p u l a t i o n s i z e rather than of community s t r u c t u r e (Tamarin 1978). E m p i r i c a l s t u d i e s of t h i s i s s u e o f t e n i n v o l v e e i t h e r experimental manipulations of the d e n s i t i e s of a s p e c i e s or group of s p e c i e s and a subsequent assessment of the e f f e c t s of t h i s m a n i p u l a t i o n on other s p e c i e s , or i n d i r e c t measurement of the i n t e r a c t i o n c o e f f i c i e n t s i n the community matrix, with the o b j e c t i v e of p r e d i c t i n g community s t r u c t u r e . In the l a t t e r case, agreement between observed community s t r u c t u r e and that p r e d i c t e d using the e q u i l i b r i u m assumptions i m p l i c i t i n alpha matrix a n a l y s e s i s taken as evidence that the n a t u r a l community i s c l o s e to e q u i l i b r i u m , and t h e r e f o r e s t r o n g l y r e g u l a t e d by b i o t i c i n t e r a c t i o n s . Notice that which p a r t i c u l a r model i s used i s not c r u c i a l . Instead of using a matrix of i n t e r a c t i o n c o e f f i c i e n t s d e r i v e d from L o t k a - V o l t e r r a c o m p e t i t i o n equations, one co u l d p r e d i c t the e q u i l i b r i u m community s t r u c t u r e u s i n g , f o r example, a resource competition model, as Tilman (1982) has done. A t h i r d e m p i r i c a l approach i s p o s s i b l e , but seldom used. In an 1 43 i d e a l world i n which one knew what the e q u i l i b r i u m s t a t e was, one c o u l d simply keep t r a c k of how o f t e n , and to what extent, the community was d i s p l a c e d from e q u i l i b r i u m by e x t e r n a l d i s t u r b a n c e s . Under p e r f e c t l y c o n t r o l l e d constant c o n d i t i o n s , the community would always remain at e q u i l i b r i u m . But how can one know where the e q u i l i b r i u m p o i n t l i e s ? Any t h e o r e t i c a l d e f i n i t i o n of an e q u i l i b r i u m s t a t e based on a p r i o r i assumptions (and o b t a i n e d d e d u c t i v e l y t h e r e f o r e ) must be s t r i c t l y model-dependent. I t can be seen i n t u i t i v e l y that the e q u i l i b r i u m of the model should depend s t r o n g l y on, say, the l e v e l of temporal r e s o l u t i o n with which i t d e s c r i b e s environmental v a r i a b l e s and on the number of v a r i a b l e s that are i n c o r p o r a t e d . Two models can y i e l d e x a c t l y the same p r e d i c t i o n s f o r the s t a t e v a r i a b l e (community s t r u c t u r e ) , , even when the p r e d i c t i o n s for the e q u i l i b r i u m s t a t e d i f f e r between them. In the l a b o r a t o r y one o f t e n knows which f a c t o r s are c h i e f l y r e s p o n s i b l e f o r determining the e q u i l i b r i u m l e v e l s , and can t h e r e f o r e produce a model t h a t i n c o r p o r a t e s adequate d e s c r i p t i o n s of the important f a c t o r s , and ignores the r e s t (e.g. chemostat models). In p r a c t i c e , the d e f i n i t i o n of e q u i l i b r i u m c o u l d be o p e r a t i o n a l , with the e q u i l i b r i u m being d e f i n e d as the p o i n t at which a p o p u l a t i o n (or a l t e r n a t i v e l y , a community) s t a b i l i z e s under constant l a b o r a t o r y c o n d i t i o n s . Under f i e l d c o n d i t i o n s however, one u s u a l l y can not even attempt to d e f i n e the e q u i l i b r i u m o p e r a t i o n a l l y , because community s t r u c t u r e seldom remains constant f o r long enough (C o n n e l l and Sousa 1983). Furthermore, the observed changes i n community s t r u c t u r e may be the r e s u l t e i t h e r of c l o s e " t r a c k i n g " of a moving e q u i l i b r i u m , at one 144 extreme of a continuum, or of a t r a n s i e n t approach to a steady e q u i l i b r i u m , at the ot h e r . An i n d i r e c t approach would seem t h e r e f o r e necessary, an approach that allowed one to determine the c a p a c i t y of the community to r e g u l a t e towards the e q u i l i b r i u m . If such i n f o r m a t i o n c o u l d be obtained, and i f one had some idea of the frequency of environmental change, i t would be p o s s i b l e to set l i m i t s on how c l o s e l y the community t r a c k e d environmental change. The r e g u l a t o r y a b i l i t y of the community can be determined using a p r o t o c o l f i r s t proposed by Ni c h o l s o n (1957), the "convergence experiment". In essence, the experiment i n v o l v e s three p o p u l a t i o n s , one u n a l t e r e d , one i n which d e n s i t i e s have been reduced by removal, and a t h i r d i n which d e n s i t i e s have been i n c r e a s e d (perhaps by adding to i t the organisms removed from the second p o p u l a t i o n ) . The i n t e n s i t y of r e g u l a t i o n can be det e c t e d by the r a t e at which the three p o p u l a t i o n s converge, independently of whether the u n d e r l y i n g e q u i l i b r i u m changes i n  the meantime. The same idea has been proposed by Murdoch (1970) and by F r e t w e l l (1972), and i s reminiscent of the ergodic theorems of demography (Arthur 1983), which s t a t e that p o p u l a t i o n s i n i t i a l l y d i f f e r i n g i n age s t r u c t u r e converge to the same age s t r u c t u r e when exposed to the same v a r y i n g environment. Eisenberg (1966) and L u c k i n b i l l and Fenton (1978) have a c t u a l l y conducted convergence experiments in- s n a i l and protozoan p o p u l a t i o n s , and Mook (1981) used a s i m i l a r p r o t o c o l i n h i s study of marine f o u l i n g communities. 1 45 My main o b j e c t i v e s i n t h i s study were 1) to assess the r a t e s of convergence of two experimental communities to a c o n t r o l ( e v a l u a t i n g t h e r e f o r e the s t r e n g t h of community s t r u c t u r e r e g u l a t i o n ) , 2) to determine which taxonomic changes were a s s o c i a t e d with the convergence, i . e . what was the path of recovery, and 3) to p r o v i d e some i n s i g h t i n t o how the i n t e r a c t i o n between the r e g u l a t i o n r a t e and the frequency of environmental change can a f f e c t the e q u i l i b r i u m s t a t u s of a p eriphyton community. I s t u d i e d convergence at three l e v e l s of d e t a i l : t o t a l c e l l d e n s i t i e s , major taxa ( d i v i s i o n s ) , and family/genus. 1 46 EXPERIMENTAL PROCEDURE A l l experiments were s t a r t e d on A p r i l 28, 1983. On that date, a set of c l e a n s l i d e s (which I w i l l c a l l C1N) was p l a c e d at s t a t i o n 1. At the same time, a set of s l i d e s ( h e r e a f t e r r e f e r r e d to as T) that had remained at s t a t i o n 3 f o r more than 45 weeks was t r a n s f e r r e d to s t a t i o n 1. C o n t r o l racks h o l d i n g s l i d e s that had been submerged f o r more than 45 weeks were i n i t i a l l y present at s t a t i o n s 1 and 3 ( h e r e a f t e r r e f e r r e d to as C183 and C383), and remained there u n t i l the end of the experiments, on October 29, 1983. Samples were c o l l e c t e d from C183, C383, and T on the i n i t i a l date (week 1), and subsequently a l l four s e t s of s l i d e s were sampled on weeks 5, 9, 17, and 27. Convergence of C1N and T (the experimental communities) to C183 (the c o n t r o l community) was assessed at three l e v e l s of d e t a i l . For each sampling date, I compared the c o n t r o l and experimental samples with respect to 1) t o t a l c e l l numbers, 2) community s t r u c t u r e at the taxonomic l e v e l of d i v i s i o n s , and 3) community s t r u c t u r e at the family/genus l e v e l . In a d d i t i o n , I performed o r d i n a t i o n analyses that generated simultaneous comparisons (at the family/genus l e v e l ) of the c o n t r o l and experimental communities over a l l sampling dates. The d e n s i t y comparisons i n v o l v e d simple s t a t i s t i c a l t e s t s ; convergence at the l e v e l of major taxa was evaluated g r a p h i c a l l y . The family/genus l e v e l comparisons ( p a i r w i s e and o r d i n a t i o n s ) were done using s e v e r a l well-known d i s s i m i l a r i t y i n d i c e s . 147 RESULTS Convergence of t o t a l c e l l numbers The r e s u l t s of t - t e s t comparisons ( E l l i o t 1977) between the c e l l d e n s i t i e s of C1N and C183, and of T and C183, are shown i n Table IV (log-transformed data, t h r e e r e p l i c a t e s per sample; the average c o e f f i c i e n t of v a r i a t i o n of the samples was 17.7). Time s e r i e s of the c e l l d e n s i t i e s are p l o t t e d i n F i g u r e 25. TABLE IV - T o t a l c e l l numbers. Comparisons between c o n t r o l and experimental' communities. COMPARISON WEEK C1N vs. C183 T vs. C183 1 0.5 > p > 0.4 5 p < 0.0001 0.02 > p > 0.01 9 p < 0.0001 0.02 > p > 0.01 17 0.2 > p > 0 . 1 0.1 > p > 0 . 0 5 27 0.01 > p > 0.001 0.05 > p > 0.01 C e l l d e n s i t i e s were much lower i n C1N than i n C183 on weeks 1, 5, and 9; by week 17 the two communities d i d not d i f f e r s i g n i f i c a n t l y , but i n week 27 numbers were again lower i n C1N than i n C183. Although the l a r g e i n i t i a l d i f f e r e n c e was reduced through time, there was no evidence of t i g h t r e g u l a t i o n towards a common d e n s i t y . Density i n T was s i g n i f i c a n t l y lower than in C183 on weeks 5, 9, and 27; there were no s i g n i f i c a n t d i f f e r e n c e s on weeks 1 and 17. D e s p i t e the i n i t i a l s i m i l a r i t y 1 48 Fi g u r e 25. T o t a l abundance time s e r i e s : C183, C1N, and 149 T I M E (in w e e k s ) 150 between the two communities , there was again no evidence of s t r o n g r e g u l a t i o n ; furthermore, the d i r e c t i o n of d e n s i t y changes was the same f o r the two communities between weeks 1 and 5 and 5 and 9, but d i f f e r e n t between weeks 9 and 17 and 17 and 27. To summarize, there was some evidence for r e g u l a t i o n : C1N numbers slow l y approached those of T and C183, and on week 27 the mean c e l l d e n s i t y of C183 was 31% and 34% higher than those of T and C1N, r e s p e c t i v e l y , not an extreme d i f f e r e n c e . On a f i n e r s c a l e however, the s t a t i s t i c a l t e s t showed that the d e n s i t i e s of both experimental communities were s i g n i f i c a n t l y d i f f e r e n t from c o n t r o l d e n s i t i e s on week 27, even a f t e r 26 weeks of residence at the same l o c a l i t y . Convergence at the d i v i s i o n l e v e l F i g u r e 26 shows the r e l a t i v e abundances of greens, blue-greens, diatoms, and chrysophyceans in C183, C383, C1N, and T. The d i v i s i o n - l e v e l data are not s u i t a b l e f o r o r d i n a t i o n purposes (because there are only four d e s c r i p t o r s ) and are presented here i n the form of histograms; the v e r t i c a l l i n e s a s s o c i a t e d with the bars represent the ranges of the o b s e r v a t i o n s . Because the d e s c r i p t o r s w i t h i n a given community are not n e c e s s a r i l y independent, the o v e r a l l s i m i l a r i t y between two communities cannot be assessed using p a i r w i s e comparisons between the d e s c r i p t o r s of the communities . A c o r r e c t t e s t of s i m i l a r i t y would have to i n c o r p o r a t e the u n d e r l y i n g (perhaps non-independent, and u s u a l l y unknown) d i s t r i b u t i o n s of the d e s c r i p t o r s . Such a t e s t might d e t e c t a s i g n i f i c a n t d i f f e r e n c e 151 F i g u r e 26. R e l a t i v e abundances of major taxa i n C183, C1N, and C383 P e r c e n t a g e o f t o t a l n u m b e r s CD CD 7? ZT o - I r-03 O T 1 cr MnMttCnfi i o CD 7? CO cr « co • U CD o o -l 1 1 1 r LTB- ^ 5-I 'M i i lU ' l l i i 31 CQ Ui lH IM I I IH I l i F CD CD 7? ro CQ HUH E 3 • m O O O CO 00 z CO CO CO CD CD CD o o T 1 1 1 r cr ca Irs-, CQ 3ST 153 between communities even when none of the p a i r w i s e comparisons i n d i c a t e d s i g n i f i c a n t d i f f e r e n c e s . Conversely, a l l the p a i r w i s e comparisons c o u l d d e t e c t s i g n i f i c a n t d i f f e r e n c e s even when the simultaneous t e s t i n d i c a t e d that the communities were not s i g n i f i c a n t l y d i f f e r e n t . T h i s s e c t i o n t h e r e f o r e focuses on i l l u s t r a t i n g major temporal trends i n community s t r u c t u r e at the d i v i s i o n l e v e l , and on p r o v i d i n g a non-rigorous assessment of convergence at t h i s same l e v e l . F i g u r e 26a shows that T and C383 (both of which had remained at s t a t i o n 3 f o r the previous 45.weeks, and c o u l d t h e r e f o r e be thought of as r e p l i c a t e samples) share s i m i l a r f r e q u e n c i e s of major taxa. With the exception of the chrysophyceans, the f r e q u e n c i e s of a l l d i v i s i o n s were more s i m i l a r between T and C383 than between T and C183. Diatom f r e q u e n c i e s were hig h e r , and green f r e q u e n c i e s lower, i n C183 than i n C383. F i g u r e 26b shows that by week 5 T s t i l l resembled C383 more than i t d i d C183. Diatom f r e q u e n c i e s were much lower, and green f r e q u e n c i e s much hi g h e r , i n C1N than i n the other three communities; chrysophycean and blue-green f r e q u e n c i e s were comparable between C1N and C183, C383, and T. The preponderance of greens in C1N was due to the e a r l y p r o l i f e r a t i o n of u n i c e l l u l a r c o c c o i d s , which seem to be very e f f i c i e n t c o l o n i z e r s in Gwendoline Lake. F i g u r e 26c shows that between weeks 5 and 9 diatom f r e q u e n c i e s i n c r e a s e d , and green f r e q u e n c i e s decreased i n C1N . On week 9 T s t i l l resembled C383 more than i t d i d C183. By week 17 the f r e q u e n c i e s of greens, diatoms, and blue-greens were more s i m i l a r between T and C183 than between T and C383 1 54 (Figure 26d). In C1N diatom f r e q u e n c i e s continued to i n c r e a s e , and chlorophycean f r e q u e n c i e s to drop, although t h i s community s t i l l resembled C183 l e s s than T d i d . Blue-green f r e q u e n c i e s i n c r e a s e d i n a l l four communities between weeks 17 and 27 (Figure 26e). During t h i s 10-week p e r i o d the d i f f e r e n c e s i n diatom f r e q u e n c i e s between C1N and C183 were reduced; on week 27 C1N resembled C383 more than i t d i d C183, even though C183 and C1N had remained together at s t a t i o n 1 s i n c e week 1. Throughout the study, blue-green and chlorophycean f r e q u e n c i e s were c o n s i s t e n t l y h igher, and diatom f r e q u e n c i e s lower, i n C383 than in C183. The comparison of C183 and T on week 27 lends i t s e l f to c o n s i d e r a b l e ambiguity: chrysophycean and green f r e q u e n c i e s were almost i d e n t i c a l between both communities; blue-green f r e q u e n c i e s were somewhat higher i n T than in C183, but some ov e r l a p was p r e s e n t . At t h i s l e v e l of a n a l y s i s any c o n c l u s i o n r e g a r d i n g the convergence of C183 and T would probably be premature and unwarranted. However, there was a c l e a r t r e n d towards i n c r e a s i n g s i m i l a r i t y between the two communities from week 1 u n t i l week 27, e s p e c i a l l y in terms of diatom and chlorophycean f r e q u e n c i e s . C1N blue-green and chrysophycean f r e q u e n c i e s were al r e a d y s i m i l a r to those of C183 by week 5. The l a r g e i n i t i a l d i s s i m i l a r i t y in chlorophycean and diatom f r e q u e n c i e s that e x i s t e d between C1N and C183 d i m i n i s h e d g r a d u a l l y d u r i n g the study. However, on week 27 CMN and C183 s t i l l d i f f e r e d i n b l u e -green, green, and diatom mean f r e q u e n c i e s , more so than C183 and T. In p a r t i c u l a r , the l a r g e d i f f e r e n c e i n diatom f r e q u e n c i e s 1 55 (C183: min=43%, max=70%, mean=53%; C1N: min=22%, max=36%, mean=30%) seems to i n d i c a t e that C183 and C1N had not converged a f t e r 26 weeks. Convergence at the family/genus l e v e l : a n a l y s e s based on  s i m i l a r i t y i n d i c e s P a i r w i s e comparisons between c o n t r o l and experimental  communities F i v e i n d i c e s were used i n the d i s s i m i l a r i t y a n a l y s e s : E u c l i d e a n d i s t a n c e (ED), Canberra metric (CM), percentage d i f f e r e n c e or B r a y - C u r t i s d i s s i m i l a r i t y (BC), a complement of the M o r i s i t a s i m i l a r i t y (MS), and Renkonen d i s s i m i l a r i t y (RD). In c a l c u l a t i n g E u c l i d e a n distance., data were e i t h e r l e f t untransformed (raw d a t a ) , transformed to presence-absence ( q u a l i t a t i v e d a t a ) , log-transformed ( l n ( x + 1 ) ) , or st a n d a r d i z e d by s i t e norm. The s t a n d a r d i z a t i o n s and i n d i c e s are d e s c r i b e d i n d e t a i l in Appendix 2. On each sampling date , the c o n t r o l and experimental samples were compared as f o l l o w s : 1) the c e n t r o i d (or average community) was c a l c u l a t e d f o r both samples; 2) the d i s s i m i l a r i t i e s between each of the r e p l i c a t e s w i t h i n a sample and the sample's c e n t r o i d were computed and averaged; t h i s p r o v i d e d an estimate of within-sample v a r i a b i l i t y ; 3) the d i s s i m i l a r i t i e s between the two sample c e n t r o i d s were c a l c u l a t e d , p r o v i d i n g an estimate of between-sample v a r i a b i l i t y . The r e s u l t s are presented i n F i g u r e s 27 (C1N vs. C183) and 28 (T 1 56 F i g u r e 27. P a i r w i s e comparisons between C1N and C183 using d i s s i m i l a r i t y i n d i c e s , a - f : ED (raw d a t a ) , ED (presence-absence), ED ( l o g - t r a n s f o r m e d ) , BC, CM, ED ( s i t e - s t d . ) , MS, and RD. Dissimilarity i — 1 — r o i i—i—i—i—i—r p co o> o o o b o o cn to l I i — i — r • * -i—i—i—i—r I • I • < • . . . ^ . • 1 1 — • • 1 58 F i g u r e 28. P a i r w i s e comparisons between T and C183 using d i s s i m i l a r i t y i n d i c e s , a-1: ED (raw d a t a ) , ED (presence-absence), ED ( l o g - t r a n s f o r m e d ) , BC, CM, CM (25 t a x a ) , CM (10 t a x a ) , CM (6 t a x a ) , CM (3 t a x a ) , ED ( s i t e - s t d . ) , MS, RD. D i s s i m i l a r i t y 1 60 vs. C183). In these F i g u r e s the d i s s i m i l a r i t y between the c e n t r o i d s of c o n t r o l and experimental samples on any given date i s measured by the y - a x i s d i s t a n c e between t r i a n g l e s (downward p o i n t i n g t r i a n g l e s in the upper s e c t i o n represent experimental communities; c o n t r o l communities, the upward p o i n t i n g t r i a n g l e s , are p l a c e d i n the lower s e c t i o n ) . The v e r t i c a l bars represent average within-sample v a r i a b i l i t y . The h o r i z o n t a l l i n e i s prov i d e d f o r r e f e r e n c e ; experimental and c o n t r o l c e n t r o i d s are e q u i d i s t a n t from t h i s l i n e . The most important f e a t u r e i n the graphs i s that the degree of convergence can be assessed (although without s t a t i s t i c a l r i g o r ) by comparing, the d i s t a n c e between c e n t r o i d s with the average within-sample v a r i a b i l i t i e s . The a n a l y s i s u s i n g E u c l i d e a n d i s t a n c e (ED) and untransformed data (Figure 27a) showed no convergence between C1N and C183. When raw data are used, ED i s a f f e c t e d by d i f f e r e n c e s i n ab s o l u t e numbers as we l l as i n r e l a t i v e abundances. I attempted to reduce the i n f l u e n c e of t o t a l c e l l d e n s i t y on community d i s s i m i l a r i t y , u s ing presence-absence and log-transformed data to r e c a l c u l a t e ED. These two tr a n s f o r m a t i o n s y i e l d e d almost i d e n t i c a l r e s u l t s , as shown i n Fi g u r e s 27b and 27c. When presence-absence data are used, the d i s t a n c e between communities i s a f u n c t i o n of the number of groups that are present i n only one of the communities; j o i n t absences or presences do not c o n t r i b u t e . Because most groups were present at a l l times i n a l l communities, the d i s t a n c e between C183 and C1N h a r d l y decreases through time i n F i g u r e 27b, and sample d i s p e r s i o n s o v e r l a p from an e a r l y stage. Much 161 the same groups were present i n C183 and C1N from the s t a r t of the experiment, reducing thus the value of a q u a l i t a t i v e a n a l y s i s . The l o g - t r a n s f o r m a t i o n b r i n g s high abundance values c l o s e together, r a t h e r than b r i n g i n g h i g h values c l o s e r to very small ones ( A l l e n and Koonce 1973). ' The t r a n s f o r m a t i o n t h e r e f o r e homogenized the c o n t r i b u t i o n s of abundant groups to community d i s s i m i l a r i t y . Because the most abundant groups were a l s o those commonly found at a l l times i n a l l communities, they were v i r t u a l l y e l i m i n a t e d from the a n a l y s i s by the l o g -t r a n s f o r m a t i o n . The s i m i l a r i t y between the q u a l i t a t i v e and l o g a r i t h m i c t r a n s f o r m a t i o n s o r i g i n a t e s from the e l i m i n a t i o n of u b i q u i t o u s groups and of j o i n t absences from both a n a l y s e s . Both the Canberra metric and the B r a y - C u r t i s index use q u a n t i t a t i v e data; the r e s u l t s obtained u s i n g these i n d i c e s are shown in F i g u r e s 27d and 27e. In both cases the d i s t a n c e between communities decreases through time, and t h i s d i s t a n c e i s always l a r g e r than the average d i s p e r s i o n s of both samples. The Canberra metric (CM) and the B r a y - C u r t i s index (BC) have an important drawback: both i n d i c e s are s e n s i t i v e to d i f f e r e n c e s in t o t a l abundances. Although the problem i s not so severe as with other i n d i c e s (e.g. ED using raw d a t a ) , the d i s s i m i l a r i t y between two communities having e x a c t l y the same p r o p o r t i o n s of each s p e c i e s (or group), but d i f f e r e n t t o t a l abundances, w i l l not be z e r o . Complete i d e n t i t y (zero d i s s i m i l a r i t y ) occurs only when the a b s o l u t e numbers of each s p e c i e s are i d e n t i c a l i n both communities; intermediate d i s s i m i l a r i t y values are l i k e w i s e a f f e c t e d by d i f f e r e n c e s i n t o t a l d e n s i t i e s . The ED using s i t e -1 62 s t a n d a r d i z e d data, the M o r i s i t a d i s s i m i l a r i t y , and the Renkonen d i s s i m i l a r i t y a v o i d t h i s inconvenience, because they measure only d i f f e r e n c e s in r e l a t i v e abundances. These three i n d i c e s •generated s i m i l a r r e s u l t s ( F i g u r e s 27f-27h): once more the d i s t a n c e between communities i s seen to decrease through time, but none of the c e n t r o i d s ever f a l l s w i t h i n the other's average d i s p e r s i o n . N o t i c e how the d i s p e r s i o n s are reduced in F i g u r e s 27f-27h as compared with F i g u r e s 27d and 27e; t h i s occurs because the s i t e - s t a n d a r d i z e d ED, the MS, and the RD f i l t e r out abundance-related v a r i a b i l i t y . F i g u r e 28a shows the d i s s i m i l a r i t y a n a l y s i s f o r C183 and T u s i n g raw-data ED. A comparison with F i g u r e 25 shows that the a n a l y s i s i s dominated by abundance d i f f e r e n c e s between the two communities, a r e s u l t that renders the raw-data ED v i r t u a l l y u s e l e s s f o r comparisons i n v o l v i n g community s t r u c t u r e i n t h i s study. Once again, the log-transformed and q u a l i t a t i v e data y i e l d e d s i m i l a r r e s u l t s ( F i g u r e s 28b and 28c).. Both the d i s t a n c e between the communities and the o v e r l a p s i n average d i s p e r s i o n s remained remarkably steady through time, c h i e f l y because the common groups were the same i n both communities from the beginning of the experiment. F i g u r e s 28d and 28e, i n which the BC and CM measures are used, show that community d i s s i m i l a r i t y was lower on weeks 17 and 27 than on the three p r e v i o u s d a t e s . The BC a n a l y s i s does not d i s t i n g u i s h between C183 and T on week 17, but by week 27 the two communities are separated a g a i n . In F i g u r e 28e the average d i s p e r s i o n s are high compared to the between-centroid d i s s i m i l a r i t i e s even on week 1, 1 63 when the two communities would be expected to d i f f e r the most. In the CM, a given d i f f e r e n c e between rare groups c o n t r i b u t e s more to the o v e r a l l d i s s i m i l a r i t y than a d i f f e r e n c e of the same s i z e between common groups. Because the sample s i z e of r a r e groups in the counting procedure was s m a l l , and because t h e i r presence or absence on the s l i d e i s o f t e n a matter of chance, i t seems i n a p p r o p r i a t e to give more weight to r a r e groups than to abundant ones. I t h e r e f o r e r e c a l c u l a t e d the CM v a l u e s s e v e r a l times, u s i n g only the 25, 10, 6, or 3 most abundant groups. These groups accounted f o r 99%, 91%, 81%, and 62% r e s p e c t i v e l y , of the t o t a l abundance (many groups were r a r e ; g l o b a l l y , the twenty r a r e s t groups accounted f o r only one percent of the abundance). The index was s e n s i t i v e to these manipulations ( F i g u r e s 28f-"28i) but s t i l l no c o n s i s t e n t t r e n d emerged. The d i s t a n c e between c e n t r o i d s i n c r e a s e d between weeks 17 and 27 i n the BC a n a l y s i s and i n s e v e r a l of the CM a n a l y s e s . A look at F i g u r e 25 suggests that once more t h i s p a t t e r n might have been generated more by abundance v a r i a t i o n s than by d i f f e r e n c e s i n r e l a t i v e abundances. T h i s suggestion was t e s t e d by using the s i t e - s t a n d a r d i z e d ED, MS, and RD (which are independent of d i f f e r e n c e s i n a b s o l u t e abundance), and the r e s u l t s are shown i n F i g u r e s 28J-281. In the three analyses there i s a (not always monotonic) decrease through time i n the d i s s i m i l a r i t y between C183 and T, i n agreement with a p r i o r i i n t u i t i v e e x p e c t a t i o n s . In p a r t i c u l a r , the l a r g e s t d i s s i m i l a r i t y occurs now on week 1, and the i n c r e a s e i n d i s s i m i l a r i t y that p r e v i o u s l y o c c u r r e d between weeks 17 and 27 has been l a r g e l y e l i m i n a t e d . In f a c t , on week 27 the s i t e - s t a n d a r d i z e d ED and MD i n d i c a t e that 164 between-community d i s s i m i l a r i t y was smaller than the w i t h i n -sample average d i s p e r s i o n of C183. A summary of the r e s u l t s of the p a i r w i s e comparisons f o l l o w s . The q u a l i t a t i v e data ED and log-transformed data ED y i e l d e d s i m i l a r r e s u l t s , and gave l i t t l e or no weight to d i f f e r e n c e s i n a b s o l u t e abundance or r e l a t i v e f r e q u e n c i e s . The raw-data ED, BC, CM, s i t e - s t a n d a r d i z e d ED, MS, and RD un i f o r m l y seem to i n d i c a t e that C1N and C183 had not converged by week 27, but were l e s s d i s s i m i l a r than e a r l i e r . The r e s u l t s were not a f f e c t e d by the s e n s i t i v i t y of some of these measures to abundance d i f f e r e n c e s because both t o t a l abundances and r e l a t i v e frequency d i f f e r e n c e s f o l l o w e d s i m i l a r t r e n d s . In c o n t r a s t , while r e l a t i v e frequency d i f f e r e n c e s between C183 and T were p r o g r e s s i v e l y reduced, d i f f e r e n c e s i n a b s o l u t e abundances behaved more e r r a t i c a l l y . T h e r e f o r e , the measures s e n s i t i v e to abs o l u t e abundances d i d not y i e l d the same r e s u l t s as the s i t e -s t a n d a r d i z e d ED, MS, and RD, which were only s e n s i t i v e to r e l a t i v e abundances. When measured by these three i n d i c e s , d i s s i m i l a r i t y between C183 and T decreased through time; by week 27 the MS and ED d i s s i m i l a r i t y between communities was smaller than the c o n t r o l community's average within-sample d i s s i m i l a r i t y . M u l t i p l e simultaneous comparisons using P r i n c i p a l Coordinates  A n a l y s i s (PCoA) The p a i r w i s e comparisons approach adopted i n the pr e v i o u s 165 s e c t i o n has s e v e r a l drawbacks: 1) the d i s s i m i l a r i t y a x i s i s not i n t e r p r e t a b l e ; d i s s i m i l a r i t y may change through time, but one remains ignorant about the nature of the u n d e r l y i n g changes i n community s t r u c t u r e . T h i s means that even though there might be patterned v a r i a t i o n i n the d i s s i m i l a r i t y v alues through time, i t i s hard to determine i f the p a t t e r n corresponds to o r d e r l y changes i n community s t r u c t u r e . From an e c o l o g i c a l p e r s p e c t i v e i t i s important to know which taxa are predominantly r e s p o n s i b l e f o r the observed p a t t e r n , and i n which d i r e c t i o n they are changing ( i . e . what i s the path of r e c o v e r y ? ) . One might a l s o be i n t e r e s t e d i n knowing whether both communities are moving towards convergence or whether one of them remains unchanged while the other converges to i t . 2) P a i r w i s e d i s s i m i l a r i t i e s c o r r e s p o n d i n g to d i f f e r e n t dates are independent of each other. T h i s may introduce noise i n t o the recovery p a t t e r n , because the p a i r w i s e d i s s i m i l a r i t i e s are s e n s i t i v e both to taxa that e x h i b i t a c o o r d i n a t e d (convergence) response through time and to those whose temporal f l u c t u a t i o n s are e s s e n t i a l l y random. 3) D i s p e r s i o n around the c e n t r o i d s i s not n e c e s s a r i l y i s o t r o p i c ( h y p e r s p h e r i c a l ) ; there might' be " p r e f e r r e d " d i r e c t i o n s of v a r i a t i o n . The r e d u c t i o n of t h i s v e c t o r i a l i n f o r m a t i o n to a s c a l a r "average within-sample d i s p e r s i o n " , while necessary to achieve a one-dimensional comparison between communities, i s not n e c e s s a r i l y j u s t i f i e d . T h i s b r i n g s up the more g e n e r a l problem of what c r i t e r i o n should be used to assess s i m i l a r i t y between communities; the p a i r w i s e comparisons are not 1 66 amenable to simple s t a t i s t i c a l treatment because the parent d i s t r i b u t i o n s of the d i s s i m i l a r i t y f u n c t i o n s are unknown. F o r t u n a t e l y , these drawbacks can be l a r g e l y overcome by the use of m u l t i v a r i a t e techniques, i n p a r t i c u l a r PCoA. PCoA i s an e i g e n v e c t o r o r d i n a t i o n i n which intersample d i s s i m i l a r i t i e s are mapped unto a space of n dimensions. Each dimension has a l i n e a r a x i s a s s o c i a t e d with i t , and the axes are ordered a c c o r d i n g to the f r a c t i o n of the t o t a l v a r i a t i o n i n the data set e x p l a i n e d by each a x i s . Often two or three axes account f o r a high f r a c t i o n of the v a r i a t i o n , and Gauch (1982a) has shown that e i g e n v e c t o r o r d i n a t i o n s s e l e c t i v e l y d e f e r noise to the lower axes, while the f i r s t few ones r e t a i n the s i g n i f i c a n t p a t t e r n s . The r e d u c t i o n i n n o i s e i s achieved because 1) many numbers are averaged at v a r i o u s steps i n the computations, thus reducing v a r i a b i l i t y , and 2) the u n c o r r e l a t e d high-dimensional nature of noise tends to impair i t s r e p r e s e n t a t i o n i n fewer dimensions (Gauch 1982a). A low-dimensional r e p r e s e n t a t i o n a l l o w s a l l samples to be p l o t t e d together so that one can v i s u a l i z e s i m u l t a n e o u s l y the extent and d i r e c t i o n of within-community v a r i a t i o n , the d i s s i m i l a r i t y between c o n t r o l and experimental communities at any p a r t i c u l a r date, and the t r a j e c t o r i e s of the communities through time. In a d d i t i o n , the s t r e n g t h of the c o r r e l a t i o n s between the f i r s t few axes and each of the taxa i n d i c a t e s what kind of changes i n community s t r u c t u r e u n d e r l i e the observed changes i n between-community d i s s i m i l a r i t y . PCoA has been p r e v i o u s l y used by Bloom (1980) and by Santos and Bloom (1980) to q u a n t i f y community recovery. 167 Convergence of community s t r u c t u r e between C183 and C1N, and between C183 and T, was assessed using PCoA o r d i n a t i o n s based on s i t e - s t a n d a r d i z e d ED, MS, and RD, the three i n d i c e s t h at were i n s e n s i t i v e . t o v a r i a t i o n i n t o t a l abundances. F i g u r e s 29a-29c show the r e s u l t s f o r C183 and C1N; only Al values have been p l o t t e d a g a i n s t time because the c o n t r o l and the experimental samples almost never d i f f e r e d along lower axes (an a r b i t r a r y constant has been added to the Al v a l u e s ; the h o r i z o n t a l l i n e i n the middle of the graph marks the " t r u e " zero, or o r i g i n , of A l ) . The three i n d i c e s produced very s i m i l a r r e s u l t s . Al accounts f o r 74%, 86%, and 59% of the t o t a l v a r i a n c e i n F i g u r e s 29a, 29b, and 29c, r e s p e c t i v e l y . In F i g u r e 29a the r e l a t i v e abundances of Gomphonema , Eunot i a , and of members of the Naviculaceae ( a l l diatoms) i n c r e a s e along Al ( c o r r e l a t i o n s of 0.99, 0.70, and 0.65, r e s p e c t i v e l y ) , whereas the r e l a t i v e abundances of u n i c e l l u l a r c o c c o i d greens decrease (r=-0.98). The d i s t a n c e s e p a r a t i n g C183 from C1N along AT decreased c o n s i s t e n t l y through time, and the sample ranges almost overlapped by week 27. The recovery of C1N i n v o l v e d mainly a decrease i n c o c c o i d greens (which are t y p i c a l dominants of e a r l i e r s u c c e s s i o n a l stages, as shown in chapter 1) and i n c r e a s e s i n diatom r e l a t i v e abundances. Coccoid greens i n C1N had reached a b s o l u t e d e n s i t i e s comparable to those of C183 by week 17, whereas diatom a b s o l u t e d e n s i t i e s ( e s p e c i a l l y those of Gomphonema and of Eunot i a ) were s t i l l lower than those of C183 even on week 27. The r e s u l t s f o r C183 and T are shown in F i g u r e s 29d-29f; again only Al i s shown, and accounts f o r 50%, 63%, and 36% of 1 68 o F i g u r e 29. Convergence of community s t r u c t u r e between C1N and C183 and between T and C183. a-c: Convergence between C183 and C1N, u s i n g the s i t e - s t d . ED, MS, and RD. d - f : Convergence between C183 and T, using the s i t e -s t d . ED, MS, and RD. AXIS I VALUES 01 • • • 0 01 • • •f • a CD (Ji P cn • • • •1 M i • • * • • • 01 • • • • - I 1  1 • • • • • • • •• • • • • •• • • • • • • o 691 170 the t o t a l v a r i a n c e in F i g u r e s 29d, 29e, and 29f, r e s p e c t i v e l y . Axes II and III never separated the c o n t r o l from the experimental samples, r e g a r d l e s s of which index was used. In F i g u r e 29d, AI i s n e g a t i v e l y c o r r e l a t e d (r=-0.97) with the r e l a t i v e abundances of u n i c e l l u l a r c o c c o i d greens, and p o s i t i v e l y (r=0.94) with Gomphonema r e l a t i v e abundances. The three i n d i c e s y i e l d e d s i m i l a r r e s u l t s . T remained r e l a t i v e l y s t a t i o n a r y along AI and lower axes, while C183 values along AI decreased through time. The s t a t i o n a r i t y of T was somewhat s u r p r i s i n g . C183 had r e s t e d at s t a t i o n 1 f o r over 45 weeks, and thus I o r i g i n a l l y expected t h i s community to be c l o s e r to a h y p o t h e t i c a l e q u i l i b r i u m (which c o u l d have been s t a t i o n a r y or moving) than a r e c e n t l y i n t r o d u c e d community ( l i k e C1N or T) would be. F i g u r e 30a shows that t h i s was not the case: i t was T, the experimental community coming from shallow s t a t i o n 3 that remained s t a t i o n a r y , while C1N and C183 moved towards T (AI accounts f o r 67% of the t o t a l v a r i a n c e and i s c o r r e l a t e d n e g a t i v e l y with u n i c e l l u l a r c o c c o i d greens (r=-0.98), and p o s i t i v e l y with Gomphonema (r = 0.98) and Eunot i a (r = 0.60); d i s s i m i l a r i t y was measured using the s i t e - s t a n d a r d i z e d ED). Fi g u r e 30b, showing the f i r s t a x i s r e s u l t i n g from an o r d i n a t i o n of C183, T, and C383, suggests an e x p l a n a t i o n f o r t h i s phenomenon. The st r o n g e s t c o r r e l a t i o n s correspond to u n i c e l l u l a r c o c c o i d greens, Gomphonema , and M i c r o c y s t i s (0.84, -0.97, and 0.74, r e s p e c t i v e l y ) ; AI accounts f o r 54% of the t o t a l v a r i a n c e . Both the shallow (C383) and the deep (C183) c o n t r o l s had decreases i n Gomphonema r e l a t i v e abundances a s s o c i a t e d with i n c r e a s e s i n c o c c o i d green and M i c r o c y s t i s r e l a t i v e abundances 171 Figu r e 30. Community v a r i a t i o n along a x i s I d u r i n g the convergence experiment, a: C183 (s q u a r e s ) , T ( t r i a n g l e s ) , C1N ( c i r c l e s ) , b: C183 (squa r e s ) , T ( t r i a n g l e s ) , C383 ( c i r c l e s ) . 1.3 0.7 T V • • T • T • t j i i i i 1— 15 30 T I M E ( i n w e e k s ) 1.2, 0.5 i T T • T • ¥ I _ i l I i_ 15 30 TIME (in w e e k s ) 173 through time, while these r e l a t i v e abundances remained r e l a t i v e l y unchanged i n T. I t e n t a t i v e l y i n t e r p r e t these r e s u l t s as an i n d i c a t i o n that the two c o n t r o l s were t r a c k i n g changes i n an e q u i l i b r i u m p o i n t that v a r i e d both s p a t i a l l y (with depth) and temporally, p r i m a r i l y on a seasonal b a s i s . The e q u i l i b r i u m community s t r u c t u r e at any given moment would be determined by the i n t e r a c t i o n between environmental f o r c i n g s (such as temperature and l i g h t ) and b i o t i c components, and t h e r e f o r e s i t e s d i f f e r i n g i n the l e v e l s of these environmental f a c t o r s c o u l d be expected to present d i f f e r e n t e q u i l i b r i u m s t r u c t u r e s . The p a r a l l e l i s m between C183 and C383 responses might i n d i c a t e that both communities were responding to v a r i a t i o n s i n the same environmental f a c t o r s , although the absolute l e v e l s of the f a c t o r s might have been d i f f e r e n t at s t a t i o n s 1 and 3. The s t a t i o n a r i t y of T suggests that t h i s community was c l o s e to the e q u i l i b r i u m of s t a t i o n 1 on week 1, and furthermore, that t h i s e q u i l i b r i u m d i d not change much between s p r i n g and f a l l . C183 . appears to have reached the e q u i l i b r i u m corresponding to summer c o n d i t i o n s at 4 m only towards the end of the summer, on week 17. The l i g h t and temperature data a v a i l a b l e f o r Gwendoline Lake (W.E. N e i l l and C.J. Walters, unpublished data) support t h i s i n t e r p r e t a t i o n (Figure 31). Spring i s a p e r i o d of r a p i d t r a n s i t i o n in environmental c o n d i t i o n s ; t o t a l i r r a d i a n c e i n c r e a s e s r a p i d l y i n March and A p r i l , and temperature r i s e s q u i c k l y d u r i n g May. These changes co u l d cause a r a p i d s h i f t i n the e q u i l i b r i u m p o i n t , which would then vary slowly under the r e l a t i v e l y steady summer c o n d i t i o n s . The observed summer dynamics of C183 and 174 F i g u r e 31. I n c i d e n t l i g h t and water temperature f o r Gwendoline Lake, 1983 1 7 5 W a t e r t e m p e r a t u r e j ° C .1 76 C383 would then be the t r a n s i e n t s induced by the s h i f t s i n the c o r r e s p o n d i n g e q u i l i b r i u m p o i n t , and would represent an adjustment from a "winter type" community to a "summer type" one. On the other hand, the community s t r u c t u r e of T had presumably moved c l o s e to the summer e q u i l i b r i u m of s t a t i o n 1 d u r i n g i t s r e s i d e n c e at s t a t i o n 3, and d i d not vary much du r i n g summer because t h i s e q u i l i b r i u m h a r d l y changed d u r i n g the same p e r i o d . The " a c c e l e r a t i o n " of community change in T r e l a t i v e to C183 was perhaps due more to exposure to stronger l i g h t than to exposure to higher temperatures d u r i n g i t s r e s i d e n c e at s t a t i o n 3, because the temperature regimes of s t a t i o n s 1 and 3 d i f f e r e d l i t t l e d u r i n g the p r e v i o u s winter and up to the date of the t r a n s f e r i n l a t e A p r i l . I t r i e d to q u a n t i f y the r a t e at which C1N and T converged to the c o n t r o l , as f o l l o w s . F i r s t , I c a l c u l a t e d the means and v a r i a n c e s of the AI v a l u e s of c o n t r o l and experimental samples for each date. Log-log scattergrams showed that the v a r i a n c e s were independent of the means. The second step i n v o l v e d comparing experimental and c o n t r o l means f o r each date using t -t e s t s (an F - t e s t was a p p l i e d to each p a i r of samples before a p p l y i n g the t - t e s t , to check f o r e q u a l i t y of v a r i a n c e s ) . F i n a l l y , the t values were p l o t t e d vs. time. The s i x r e s u l t i n g graphs (3 i n d i c e s x 2 experiments) i n d i c a t e d t h a t the t v a l u e s decreased through time, at a d e c r e a s i n g r a t e . I t h e r e f o r e decided to f i t a negative e x p o n e n t i a l f u n c t i o n to the data: T s ( t ) = K*exp(-z*t) (3.1) 177 T s ( t ) i s the value of the t s t a t i s t i c at time t , K (=Ts(0), the i n i t i a l value of Ts)' and z (the instantaneous rate) are parameters t h a t must be e s t i m a t e d . The d i f f e r e n t i a l form of equation (3.1): dTs/dt = -z*Ts (3.2) shows that the r a t e of recovery i s assumed to be a f i x e d f r a c t i o n of the c u r r e n t value of Ts. The use of a p r o b a b i l i t y -r e l a t e d f u n c t i o n i s advantageous because standard s t a t i s t i c a l c r i t e r i a can be used to e v a l u a t e community convergence. Given that AI c o n t a i n s most of the i n f o r m a t i o n that i s u s e f u l i n s e p a r a t i n g c o n t r o l from experimental communities, one can decide that the two communities no longer d i f f e r when the Ts value c a l c u l a t e d u s i n g equation (3.1) f a l l s below some predetermined l e v e l , say, that corresponding to a 5% p r o b a b i l i t y . Of course, the f i r s t a x i s does not n e c e s s a r i l y i n c o r p o r a t e a l l of the r e l e v a n t i n f o r m a t i o n ; the f i t of the equation to the data p o i n t s i s not p e r f e c t , and the estimated parameter v a l u e s are dependent on the d i s s i m i l a r i t y measure used i n the o r d i n a t i o n . T h e r e f o r e , the method y i e l d s only a rough estimate of the time r e q u i r e d f o r convergence, and the " r e j e c t i o n " p r o b a b i l i t y l e v e l must be i n t e r p r e t e d l o o s e l y . Curve f i t t i n g was done u s i n g Schnute's (1982) Micro-Simplex program f o r n o n - l i n e a r parameter e s t i m a t i o n . The s i x data s e t s , together with t h e i r f i t t e d curves and the corresponding parameter v a l u e s are shown i n F i g u r e 32. The h o r i z o n t a l l i n e 1 78 F i g u r e 32. F i t of the negative e x p o n e n t i a l model to data from the convergence experiments. a,b,c: Convergence between C1N and C183, measured by the ED, MS, and RD, r e s p e c t i v e l y . d,e,f: Convergence between T and C183, measured by the ED, MS, and RD, r e s p e c t i v e l y . T I M E ( in w e e k s ) 180 r e p r e s e n t s the t value corresponding to a 5% p r o b a b i l i t y l e v e l i n a o n e - t a i l e d t e s t with four degrees of freedom (a o n e - t a i l e d t e s t was used because the mean AI va l u e s of C183 were always higher than those of C1N and T ) . I compared the r a t e s of convergence obtained for the s i x data sets by performing l i n e a r r e g r e s s i o n s on the log-transformed data and comparing the r e g r e s s i o n c o e f f i c i e n t s (note that these c o e f f i c i e n t s are not e x a c t l y the same as the z parameters d e r i v e d from the n o n - l i n e a r e s t i m a t i o n ; however, the d i f f e r e n c e was so small that i t i s probably j u s t i f i e d to perform the t e s t s on the c o e f f i c i e n t s d e r i v e d through l i n e a r r e g r e s s i o n ) . A t e s t f o r e q u a l i t y of slopes of s e v e r a l r e g r e s s i o n l i n e s (Sokal and Rohlf 1969) showed that the d i f f e r e n c e s among the s i x r e g r e s s i o n c o e f f i c i e n t s were not s i g n i f i c a n t (0.5 > p > 0.25). The estimate of the common slope was zbar = 0.086. S e v e r a l i n t e r e s t i n g p o i n t s emerge from the curve f i t t i n g e x e r c i s e , a summary of which i s presented i n Table V. Regardless of the index employed, the K values were higher f o r C1N and C183 than f o r T and C183, i n d i c a t i n g that the former communities i n i t i a l l y d i f f e r e d more than the l a t t e r . In other words, the d i f f e r e n c e s i n community s t r u c t u r e between e a r l y and advanced developmental stages at s t a t i o n 1 were l a r g e r than the d i f f e r e n c e s between the advanced stages of s t a t i o n 1 and s t a t i o n 3. The t e s t f o r e q u a l i t y of s l o p e s showed that i n s p i t e of the very d i f f e r e n t paths f o l l o w e d by C183 and C1N and by C183 and T during the recovery (see F i g u r e 29 and i t s e x p l a n a t i o n i n the t e x t ) , the instantaneous r a t e s of recovery were s t a t i s t i c a l l y 181 TABLE V - Summary of estimated parameters for the two convergence e x p e r i -ments. Tr i s the time ( i n weeks) r e q u i r e d to c r o s s the 5% proba-b i l i t y l e v e l r epresented by the h o r i z o n t a l l i n e s i n F i g u r e 32, and was c a l c u l a t e d u sing eq. (3.1) C1N and C183 K z Tr S i t e s t d . ED 23.6 0. 074 32.5 MS 29.0 0. 120 21.7 RD 39.2 0. 147 19.8 T and C183 K z Tr S i t e s t d . ED 7.7 0. 100 12.9 MS 5.3 0. 083 11.1 RD 6.9 0. 065 18.1 i n d i s t i n g u i s h a b l e i n the two experiments. Furthermore, the rate e stimates d i d not depend s t r o n g l y on which index was chosen to c a l c u l a t e them. Table V a l s o shows that the estimates of Tr, the time r e q u i r e d to achieve convergence, are h i g h e r f o r C1N than f o r T. For C1N, Tr i s the time to m a t u r i t y d e f i n e d i n chapter 1, and v a r i e s between 5 and 8 months, depending on which index was used i n the computation. 182 DISCUSSION Convergence of t o t a l abundances, and of community s t r u c t u r e at the d i v i s i o n l e v e l R e g u l a t i o n of d e n s i t y seemed to occur only i n a broad sense: by week 27 the d e n s i t i e s of C1N, T, and C183 were roughly comparable, i n s p i t e of the l a r g e v a r i a t i o n s i n d e n s i t y and i n community s t r u c t u r e experienced by the three communities. I t may be that r e g u l a t i o n operates only on a coarse s c a l e , s e t t i n g fuzzy upper and lower bounds on t o t a l abundance, and that w i t h i n those bounds r e g u l a t i o n i s weak, and "density-vague" processes (Strong 1983) or " s t o c h a s t i c boundedness" ( C o n n e l l and Sousa 1983) p r e v a i l . S l o u g h i n g - o f f , p e e l i n g , and s c o u r i n g are f a c t o r s that can make periphyton numbers unstable at high d e n s i t i e s (only s l o u g h i n g was l i k e l y to be of any s i g n i f i c a n c e i n Gwendoline Lake a r t i f i c i a l s u b s t r a t a ) . Density-vague dynamics c o u l d be generated by e p i s o d i c sloughing or detachment i n which the t o t a l f r a c t i o n of c e l l numbers d i s a p p e a r i n g depended only weakly on the c u r r e n t d e n s i t y . The d i v i s i o n - l e v e l a n a l y s i s p r o v i d e d some evidence of r e g u l a t i o n of community s t r u c t u r e . However, the comparisons between communities were weak, i n part because they d i d not in c o r p o r a t e a l l taxa s i m u l t a n e o u s l y . The r e s u l t s obtained at t h i s l e v e l seem vague, e s p e c i a l l y when compared to the r i c h n e s s of d e t a i l and r e s o l u t i o n o f f e r e d by the techniques based on d i s s i m i l a r i t y i n d i c e s , in p a r t i c u l a r by the o r d i n a t i o n s . 183 Convergence of community s t r u c t u r e at the family/qenus l e v e l Performance of p a i r w i s e comparisons vs. o r d i n a t i o n The c o m p u t a t i o n a l l y simple p a i r w i s e comparisons based on d i s s i m i l a r i t y i n d i c e s ( F i g u r e s 27 and 28) were u s e f u l i n e v a l u a t i n g the performances of s e v e r a l i n d i c e s i n terms of t h e i r s e n s i t i v i t y to v a r i a t i o n s i n t o t a l abundances, and i n terms of the r e l a t i v e weights that some i n d i c e s gave to rare and to common groups. T h i s p r e l i m i n a r y s c r e e n i n g allowed me to d i s c a r d those i n d i c e s that e x h i b i t e d u n d e s i r a b l e p r o p e r t i e s , and to o b t a i n a r e f e r e n c e p i c t u r e of the temporal trends of d i s s i m i l a r i t y between the c o n t r o l and the experimental communities. Somewhat s u r p r i s i n g l y , the trends d e t e c t e d by t h i s simple method of a n a l y s i s were very s i m i l a r to those y i e l d e d by the more s o p h i s t i c a t e d o r d i n a t i o n t e chniques. Apparently the p a i r w i s e comparisons were not s e r i o u s l y a f f e c t e d by the drawbacks mentioned i n the r e s u l t s s e c t i o n , i n p a r t i c u l a r p o i n t s 2) (independence of p a i r w i s e d i s s i m i l a r i t i e s corresponding to d i f f e r e n t dates) and 3) ( n o n - i s o t r o p i c d i s p e r s i o n s ) . However, t h i s i s probably j u s t a c o i n c i d e n c e a r i s i n g from the s t r u c t u r e of the p a r t i c u l a r data s e t s a n a l y s e d here, and should not be expected to be a general occurrence. The r e g u l a t i o n of community s t r u c t u r e The p a t t e r n of decrease in d i s s i m i l a r i t i e s between c o n t r o l and experimental communities through time i n d i c a t e s that community s t r u c t u r e was r e g u l a t e d . At f i r s t s i g h t r e g u l a t i o n seemed to operate over time s c a l e s that were long compared to the 1 84 generat i o n times of i n d i v i d u a l organisms. Reported g e n e r a t i o n times are u s u a l l y l e s s than a week i n p l a n k t o n i c algae (Wetzel 1975; Lewis 1978; Lehman et a l . (1975) give 3, 2.5, and 3 c e l l d i v i s i o n s per day as " t y p i c a l " maximum growth r a t e s for greens, blue-greens, and diatoms, r e s p e c t i v e l y ; Eppley (1977) l i s t s v a l u e s of 1.1 to 2.2 f o r seven s p e c i e s of freshwater diatoms). Estimates f o r doubling times of atta c h e d algae are not so r e a d i l y a v a i l a b l e . P a t r i c k (1975) quotes a value of one d i v i s i o n per day f o r diatoms. In a study that i n v o l v e d d i r e c t o b s e r v a t i o n of algae a t t a c h e d to P e t r i p l a t e s i n the l a b , Rosemarin (1982) obtained maximum growth r a t e s of 0.68 and 0.28 doublings per day f o r the filamentous greens S t i g e o c l o n i u m tenue and Cladophora glomerata. S t a l e y (1971) q u a n t i f i e d .in s i t u the growth r a t e s of a c h l o r e l l o i d a lgae, a sp e c i e s of Coleochaete , and a filamentous green algae (perhaps Protoderma ) i n a small l a k e . The algae were a t t a c h e d to g l a s s s l i d e s , and were observed d i r e c t l y u s i n g an immersed microscope. Average d i v i s i o n r a t e s were between 1.3 and 1.8 doublings per day. However, although these data c e r t a i n l y are t y p i c a l f o r e u t r o p h i c systems, they are not n e c e s s a r i l y v a l i d f o r an o l i g o t r o p h i c l a k e . S t u d i e s of zooplankton g r a z i n g on phytoplankton, as w e l l as estimates of phytoplankton turnover r a t e s seem to i n d i c a t e that d o u b l i n g times f o r the phytoplankton of Gwendoline Lake and for neighbouring Eunice Lake are c l o s e to three weeks (W.E. N e i l l , p e r s . comm.). My own estimates of p e r i p h y t o n growth r a t e s i n Gwendoline Lake (see Table VIII i n chapter 4) y i e l d a turnover time of 20 days (0.05 do u b l i n g s / d a y ) . These r e s u l t s suggest that community s t r u c t u r e r e g u l a t i o n proceeds over time 1 85 s c a l e s commensurate with the growth r a t e s of i n d i v i d u a l organisms. The estimated times to recovery i n Gwendoline Lake (see Table V) were 5-8 months f o r a l a r g e d i s t u r b a n c e ( t o t a l denudation, C1N) and about 11-18 weeks f o r a s m a l l e r d i s t u r b a n c e (change i n environmental c o n d i t i o n s , T ) . I c o u l d f i n d i n the l i t e r a t u r e only three s t u d i e s i n which the convergence of a d i s t u r b e d community to a c o n t r o l was f o l l o w e d . Eichenberger (1978) exposed a Melosira-dominated community in a r t i f i c i a l r i v e r s to h i g h c o n c e n t r a t i o n s of a heavy-metal s o l u t i o n f o r two months. As a r e s u l t , community s t r u c t u r e s h i f t e d towards dominance by the filamentous S t i q e o c l o n i u m and Tribonema , and M e l o s i r a disappeared. When the heavy-metal treatment ended, M e l o s i r a began to i n c r e a s e , but community s t r u c t u r e s t i l l had not recovered one month l a t e r , when the experiment was terminated. In a study of the e f f e c t s of a r t i f i c i a l a c i d i f i c a t i o n on periphyton growth, M u l l e r (1980) t r a n s f e r r e d P l e x i g l a s p l a t e s from an experimental e n c l o s u r e h e l d at pH 4 to another e n c l o s u r e h e l d at a pH of 6.5. He found that a f t e r 19 days the d i v e r s i t y and number of s p e c i e s of the t r a n s f e r r e d s l i d e s resembled those of the c o n t r o l . Genter et a l . (1983) e x p l i c i t l y conducted a convergence experiment i n a small stream. They t r a n s f e r r e d a c r y l i c s u b s t r a t a between s i t e s s upporting d i f f e r e n t communities , and found that convergence was achieved a f t e r only two days (the methods of a n a l y s i s and convergence c r i t e r i a were u n f o r t u n a t e l y not s p e c i f i e d ) . The time r e q u i r e d fo r convergence i n the f i r s t of these s t u d i e s c o u l d not be 186 determined, but i n the other two i t was s u b s t a n t i a l l y l e s s than in Gwendoline Lake. Community dynamics i n Gwendoline Lake might be slow because of the c o n s t r a i n t s imposed on i n d i v i d u a l growth r a t e s by the l o c a l s c a r c i t y of n u t r i e n t s , and by the low temperatures and attenuated l i g h t regime c h a r a c t e r i s t i c of temperate zones. A l a c k of convergence of experimental and c o n t r o l communities, or at l e a s t the presence of " p l a t e a u s " i n the recovery t r a j e c t o r i e s , c o u l d have been i n t e r p r e t e d as evidence for the e x i s t e n c e of m u l t i p l e e q u i l i b r i a (Lewontin 1969, G i l p i n and Case 1976, Case and Casten 1979). However, even though the recovery t r a j e c t o r i e s began from p o i n t s that were i n i t i a l l y w e l l separated (and " t e s t e d " t h e r e f o r e d i f f e r e n t subregions of the s t a t e space f o r the e x i s t e n c e of m u l t i p l e e q u i l i b r i a ) , convergence proceeded smoothly i n both cases. Perhaps t h i s apparent g l o b a l s t a b i l i t y i s a consequence of i n t e r s p e c i f i c i n t e r a c t i o n s (such as competition) being very s u s c e p t i b l e to the i n f l u e n c e of e x t e r n a l a b i o t i c agents. M u l t i p l e e q u i l i b r i a would be expected to occur i f the density-dependence induced by strong b i o t i c i n t e r a c t i o n s c o u l d p r o t e c t the i n t e r n a l community dynamics from v a r i a t i o n s i n a b i o t i c f a c t o r s (Emlen 1984). The e q u i l i b r i u m community s t r u c t u r e seems to vary i n space and time (see F i g u r e s 30 and 31 and t h e i r e x p l a n a t i o n i n the t e x t ) . Summer i s ap p a r e n t l y a p e r i o d of environmental s t a b i l i t y , although community s t r u c t u r e i s t r a n s i e n t d u r i n g t h i s time, presumably i n response to the change from winter to summer 187 c o n d i t i o n s that occurs in the s p r i n g . T h i s environmental t r a n s i t i o n i s c h a r a c t e r i z e d by r a p i d i n c r e a s e s i n t o t a l i r r a d i a n c e and temperature. The r a t e s of convergence seemed to depend l i t t l e on which index was used to assess d i s s i m i l a r i t y . In a d d i t i o n , the r a t e s were the same f o r the two recovery t r a j e c t o r i e s , even though the sequences of taxa replacements were d i f f e r e n t i n each case. For example, the convergence of C183 and T was a s s o c i a t e d with r e d u c t i o n s i n the r e l a t i v e and absolute abundances of diatoms, whereas the convergence of C183 and C1N i n v o l v e d ( o b v i o u s l y ) i n c r e a s e s i n the a b s o l u t e abundances of a l l taxa i n C1N, as w e l l as i n c r e a s e s ' i n diatom r e l a t i v e abundances and decreases in c o c c o i d green r e l a t i v e abundances. Perhaps i t i s p o s s i b l e to c h a r a c t e r i z e the community dynamics of Gwendoline Lake periphyton with a s i n g l e parameter, the instantaneous r a t e of recovery. If so, the f i n d i n g might have s i g n i f i c a n t i m p l i c a t i o n s r e g a r d i n g the e q u i l i b r i u m s t a t u s of the community. Assume f o r a moment that community dynamics can be d e s c r i b e d i n one dimension as f o l l o w s : dY/dt = z*(W(t) - Y) (3.3) where z=instantaneous r a t e (a constant) W(t) = the e q u i l i b r i u m s t a t e of community s t r u c t u r e ( v a r i e s i n time with changes in a b i o t i c environmental c o n d i t i o n s ) Y(t) = the c u r r e n t s t a t e of the community 188 The s o l u t i o n of t h i s equation i s (Bourghes and B o r r i e 1981) Y ( t ) = e x p ( - z * t ) * z * V e x p ( z * t ' ) * W ( t ' ) d t ' + Y(0)*exp(-z*t) (3.4) Jo the exact form of which depends on W(t). If two communities, Y1 and Y2, d i f f e r i n g only i n t h e i r i n i t i a l s t a t e s , are brought together to the same s i t e , the d i f f e r e n c e i n t h e i r s t a t e s through time w i l l be given by Y1 - Y2 = Y1(0)*exp(-z*t) + e x p ( - z * t ) * z * Vexp(z*t 1)*W(t 1)dt' - (Y2(0)*exp(-z*t) + e x p ( - z * t ) * z * Vexp(z*t*)*W(t')dt') or, Y1 - Y2 = (Y1(0) - Y2 ( 0 ) ) * e x p ( - z * t ) (3.5) s t a t e d simply, the i n i t i a l d i s s i m i l a r i t y between the two communities decreases e x p o n e n t i a l l y through time, independently of the form taken by the environmental v a r i a b i l i t y . N o t i c e that equation (3.5) i s e q u i v a l e n t to equation (3.1). The same r e s u l t would have been obtained i f a l o g i s t i c model would have been used i n s t e a d of equation (3.2). I f z can be t r e a t e d as a constant, and i f equation (3.5) i s a r e f l e c t i o n of community dynamics that behave a c c o r d i n g to equation (3.3), then the zbar value estimated by f i t t i n g the negative e x p o n e n t i a l to the data can be used to judge the " t r a c k i n g a b i l i t y " of the community. A zbar value of 0.086 i n d i c a t e s t h a t the c h a r a c t e r i s t i c response time (May 1976) of the community i s about 12 weeks. May (1976), s t a r t i n g from a l o g i s t i c r a t h e r than from a negative e x p o n e n t i a l model has shown t h a t when the e q u i l i b r i u m v a r i e s s i n u s o i d a l l y 189 with p e r i o d Tau, the p o p u l a t i o n " t r a c k s " the e q u i l i b r i u m c l o s e l y i f the response time i s much smal l e r than Tau. If the response time i s very l a r g e compared to Tau, the p o p u l a t i o n "averages out" the f l u c t u a t i o n s , and h a r d l y v a r i e s through time. When Tau and the response time are commensurate, p o p u l a t i o n dynamics f a l l between the two extremes, i . e . , the p o p u l a t i o n t r a c k s with a l a g . L u c k i n b i l l and Fenton (1978) have d i s c u s s e d three s i t u a t i o n s t hat can a r i s e when s p e c i e s a d j u s t to v a r i a b l e e q u i l i b r i a : 1) the e q u i l i b r i u m changes l e s s f r e q u e n t l y than the time i t takes f o r the s p e c i e s to r e g u l a t e to i t , i n which case the p o p u l a t i o n should always be c l o s e to e q u i l i b r i u m ; 2) the time between changes i n e q u i l i b r i u m i s equal to that r e q u i r e d f o r the p o p u l a t i o n to r e g u l a t e to the e q u i l i b r i u m ; 3) the e q u i l i b r i u m changes f a s t e r than the p o p u l a t i o n can r e g u l a t e to i t ; i n t h i s case the p o p u l a t i o n i s seldom or never c l o s e to e q u i l i b r i u m . Environmental v a r i a t i o n i n Gwendoline Lake spans s e v e r a l time s c a l e s , perhaps the most conspicuous of which i s r e l a t e d to seasonal v a r i a t i o n . The p e r i o d i c i t y of t h i s v a r i a b i l i t y i s roughly 52 weeks, which i s l a r g e r than the 12-week response time of the community, but of the same order of magnitude. T h i s means that the community can t r a c k seasonal changes of a s i n u s o i d a l nature i n the e q u i l i b r i u m . However, some of the v a r i a t i o n i s not s i n u s o i d a l and smooth, but occurs r a t h e r a b r u p t l y (at l e a s t i n s p r i n g , and perhaps a l s o i n f a l l ) , as shown e a r l i e r . The displacements from e q u i l i b r i u m ensuing from t h i s sudden change might leave the community i n a t r a n s i e n t stage f o r much of the summer and e a r l y f a l l ; p o s s i b l y the same process (abrupt environmental change fo l l o w e d by t r a n s i e n t 190 community dynamics under more or l e s s steady environmental c o n d i t i o n s ) i s repeated a f t e r the f a l l turnover and during the t e m p e r a t u r e - s t a b l e / l o w - l i g h t winter c o n d i t i o n s . The methods d e s c r i b e d i n t h i s chapter p r o v i d e a means of q u a n t i f y i n g community recovery, and of e s t i m a t i n g a "macroscopic" community parameter (zbar) that might be u s e f u l l y compared among l a c u s t r i n e systems. Some evidence f o r the r e g u l a t i o n of community s t r u c t u r e has been forwarded, as w e l l as some s p e c u l a t i o n on the seasonal dependence of the p r o x i m i t y of the community to e q u i l i b r i u m . T h i s p o s t u l a t e d seasonal v a r i a t i o n i n the e q u i l i b r i u m s t a t u s of the community c o u l d p r o v i d e a p a r t i a l answer to the c l a s s i c a l problem, how can so many s p e c i e s c o e x i s t s h a r i n g what a p p a r e n t l y are so few resources? The "paradox of the p e r i p h y t o n " i s i n some ways more p e r p l e x i n g than i t s p l a n k t o n i c c o u n t e r p a r t , formulated by Hutchinson (1961). U s u a l l y d i v e r s i t y i s even higher i n p e r i p h y t o n than i n phytoplankton (Hutchinson 1975), even though at t a c h e d algae are p h y s i c a l l y c l o s e r to t h e i r neighbours (and f o r longer t i m e s ) , than i n the p l a n k t o n . A l s o , p e r i p h y t i c algae are condemned to s i t e f i x i t y , which p r e c l u d e s the p o s s i b i l i t y of c o n t i n u a l l y encountering d i f f e r e n t environmental c o n d i t i o n s as p l a n k t o n i c algae do when they are moved through the water mass. Seasonal v a r i a b i l i t y i n the e q u i l i b r i u m s t a t u s of the community might i n c r e a s e community d i v e r s i t y , but i t s r e l a t i v e importance must be weighed a g a i n s t that of other f a c t o r s having the same e f f e c t , such as c o n t i n u a l i n v a s i o n by c o m p e t i t i v e l y i n f e r i o r s p e c i e s coming from the plankton, and resource p a r t i t i o n i n g 191 through v e r t i c a l m i c r o - s t r a t i f i c a t i o n on the s l i d e s (a candidate f o r an e q u i l i b r i u m h y p o t h e s i s ) . SUMMARY 1) T o t a l periphyton numbers seemed to be r e g u l a t e d only weakly, f o l l o w i n g perhaps " d e n s i t y vague" dynamics. The a n a l y s i s of the r e g u l a t i o n of community s t r u c t u r e at the taxonomic d i v i s i o n l e v e l d i d not y i e l d c o n c l u s i v e r e s u l t s . 2) Community s t r u c t u r e at the family/genus l e v e l was r e g u l a t e d ; the time s c a l e s over which r e g u l a t i o n operated were long compared with p e r i p h y t o n communities i n other waterbodies, but they were commensurate with the estimated c e l l d i v i s i o n r a t e s i n t h i s o l i g o t r o p h i c l a k e . The e q u i l i b r i u m community s t r u c t u r e seemed to be h i g h l y dynamic, v a r y i n g i n space and time. M u l t i p l e e q u i l i b r i a were not de t e c t e d d e s p i t e the v a r i a t i o n i n the s t a r t i n g p o i n t s of the recovery t r a j e c t o r i e s , suggesting that the system i s g l o b a l l y s t a b l e ( r e g u l a t i o n proceeds towards a s i n g l e , probably n o n - s t a t i o n a r y , e q u i l i b r i u m ) . 3) The c a p a c i t y of the community to tra c k environmental v a r i a t i o n was q u a n t i f i e d i n d i r e c t l y by means of convergence experiments, and was independent of the method used f o r q u a n t i f i c a t i o n and of the d e t a i l e d c h a r a c t e r of the taxonomic changes undergone by the r e c o v e r i n g communities. The c h a r a c t e r i s t i c response time of the community was found to be about 12 weeks. Assuming that the e q u i l i b r i u m community s t r u c t u r e v a r i e s with a p e r i o d of about 52 weeks (due to y e a r l y 1 92 seasonal v a r i a t i o n i n a b i o t i c environmental c o n d i t i o n s ) , the per i p h y t o n communities should t r a c k the e q u i l i b r i u m with a l a g . Environmental changes o c c u r r i n g over time s c a l e s s h orter than 12 weeks, such as those seen i n spring-and summer, probably leave the community i n a t r a n s i e n t s t a t e away from e q u i l i b r i u m , perhaps f o r extended time p e r i o d s . 1 93 CHAPTER 4. PERIPHYTON GROWTH DYNAMICS INTRODUCTION One of the questions that ended chapter 1 was How does the time r e q u i r e d to reach m a t u r i t y (a parameter r e l a t e d to d e t a i l e d community s t r u c t u r e ) r e l a t e to the time r e q u i r e d to reach biomass s t a b i l i z a t i o n (a "macroscopic" parameter)? In t h i s chapter I address t h i s q u e s t i o n a f t e r e s t i m a t i n g average c e l l d i v i s i o n r a t e s using simple growth models. I a l s o show that the models can adequately d e s c r i b e the changes i n t o t a l abundance (a high l e v e l community property, given by the aggregated response of many d i f f e r i n g taxa) i n the per i p h y t o n of a v a r i e t y of aqu a t i c s u c c e s s i o n a l systems. Periphyton primary p r o d u c t i v i t y i s u s u a l l y measured e i t h e r by instantaneous methods or by accumulation methods. Instantaneous methods i n c l u d e 14C and oxygen e v o l u t i o n techniques i n which measurements are done over b r i e f time i n t e r v a l s . Incubation of the samples i s done e i t h e r i n the l a b o r a t o r y or near the waterbody ( a f t e r the s u b s t r a t a have been removed), or in s i t u , by p l a c i n g the i n c u b a t i o n chamber d i r e c t l y on c o l o n i z e d s u b s t r a t a . Accumulation methods, on the other hand, i n v o l v e e s t i m a t i n g p r o d u c t i v i t y from the r a t e of biomass accumulation on i n i t i a l l y bare s u b s t r a t a . Castenholz (1961), Sladececk and Sladeckova (1964), and Wetzel (1965) provide good d i s c u s s i o n s of the methods; more recent accounts are given i n W e i t z e l (1979) and i n s e c t i o n V of Wetzel (1983). 1 94 Average p r o d u c t i v i t y r a t e s i n the accumulation method are at times c a l c u l a t e d by d i v i d i n g the accumulated biomass by the exposure time; Sladececk and Sladeckova (1964) have noted that t h i s procedure makes the c a l c u l a t e d average rate dependent on the exposure time. The u n d e r l y i n g reason i s that accumulation i s not l i n e a r , but u s u a l l y takes an e x p o n e n t i a l or ( i f exposure time i s long enough) sigmoid form, and t h e r e f o r e the instantaneous growth rate v a r i e s with time. Kevern et a l . (1966) used a b i l i n e a r r e g r e s s i o n to estimate periphyton growth r a t e s , and d i s c u s s e d the r e l a t i o n s h i p between instantaneous and average growth r a t e s . Although s e v e r a l authors have developed s i m u l a t i o n models of periphyton biomass that i n c l u d e f a c t o r s such as l i g h t , temperature, and g r a z e r s (e.g. M c l n t i r e 1973, Jones 1978), to my knowledge only Admiraal et a l . (1983) have p r e v i o u s l y used a simple model (the l o g i s t i c ) to estimate p e r i p h y t o n growth r a t e s . In t h i s chapter I f i t t e d three simple models of periphyton biomass dynamics to s i x growth curves e x t r a c t e d from the l i t e r a t u r e and to a growth curve f o r Gwendoline Lake. The main o b j e c t i v e s were two. F i r s t , I compared the growth rate estimated f o r Gwendoline Lake to those of other waterbodies, and l i n k e d these r e s u l t s to the r a t e s of community s t r u c t u r e changes measured in p r e v i o u s c h a p t e r s . Second, I compared the performances of the three models, u n d e r l i n i n g the advantages and disadvantages of each of them. 195 The models 1) L o g i s t i c growth. The l o g i s t i c model: dN/dt = r*N*(1 - N/K) (4.1) where N = s t a t e v a r i a b l e (e.g. p o p u l a t i o n s i z e ) r = growth r a t e K = upper l i m i t f o r the s t a t e v a r i a b l e was f i r s t proposed by P.F. V e r h u l s t i n 1845 as a d e s c r i p t o r of human p o p u l a t i o n growth (Hutchinson 1978). Hutchinson (1978) and K i n g s l a n d (1982) provide comprehensive h i s t o r i c a l accounts of the use of the equation i n demography and b i o l o g y . The growth r a t e dN/dt i s a f u n c t i o n of N; a negative (convex upwards) parabola with zero values at N = K and at N = 0, and a maximum at N = K/2. The i n t e g r a l form of the l o g i s t i c : N(t) = K/(1 + b * e x p ( - r * t ) ) (4.2) where b = K/N(0) - 1 shows that i t i s a three-parameter model. If N r e f e r s to p o p u l a t i o n s i z e , then K i s the e q u i l i b r i u m p o p u l a t i o n s i z e , r i s the i n t r i n s i c or maximum instantaneous growth r a t e , and b i s an 196 i n t e g r a t i o n constant that determines the p o p u l a t i o n s i z e at t = t ( 0 ) . 2) C o l o n i z a t i o n superimposed on l o g i s t i c growth. In a s t r i c t sense, the l o g i s t i c model i s u n r e a l i s t i c whenever there i s c o l o n i z a t i o n from sources e x t e r n a l to the system (the growth r a t e would not be zero when the p o p u l a t i o n s i z e was z e r o ) . A s l i g h t m o d i f i c a t i o n of the l o g i s t i c produces the " c o l o n i z a t i o n " model: dN/dt = (c + r*N)*(1 - N/K) (4.3) with n o t a t i o n as i n the l o g i s t i c ; c i s the c o l o n i z a t i o n r a t e , a co n s t a n t . In t h i s model the growth r a t e f u n c t i o n i s a l s o a negative p a r a b o l a , with a zero at N = K, but not at N = 0. The second root of the parabola corresponds to a negative N; at N = 0 the growth r a t e i s c. The maximum growth r a t e i s ach i e v e d at N = (N - c / r ) / 2 . The same model s t r u c t u r e (but o b v i o u s l y with a d i f f e r e n t i n t e r p r e t a t i o n of the parameters) has been used to model chemical r e a c t i o n s ( B a t s c h e l e t 1979) and the spread of t e c h n o l o g i c a l i n n o v a t i o n s (Bourghes and B o r r i e 1981). The i n t e g r a l form of the model i s : N(t) = K - c * a * e x p ( - b * t ) v / 1 + r*a*exp(-b*.t )v (4.4) 1 97 where a = (K - N(0))/(r*N(0) + c) ; b = (r*K + c)/K T h i s i s a four-parameter equation which, d e s p i t e i t s apparent bulk, can be s i m p l i f i e d by s e t t i n g N(0) = 0, because growth s t a r t s from c l e a n s l i d e s . The r e s u l t i n g equation i s : N(t) = K/ 1 + (r*K + c ) / ( c * ( e x p ( ( r * K + c ) * t / K ) - 1 ) ) V (4.5) now a three-parameter model in which N(0) i s r e a l i s t i c a l l y set to zero. 2) I n c l u s i o n of secondary e p i p h y t i s m . Secondary epiphytism, i n which m i c r o s c o p i c algae grow on other algae (mainly green f i l a m e n t s such as Bulbochate , Mougeotia , and Oedogonium , and ' s t a l k e d diatoms such as Cymbella , but a l s o on mesophytes such as Cladophora), occurs o f t e n i n p e r i p h y t o n (Roos 1979, 1983; Stevenson and Stoermer 1982; see a l s o chapter 1 i n t h i s t h e s i s ) . Roos (1979) has r e p o r t e d that 50% of the t o t a l number of diatoms can be s i t u a t e d on s t a l k s and tubes of Cymbella , whereas in a Cladophora-diatom assemblage e p i p h y t i c diatoms accounted f o r as much as 67% of the t o t a l a s h - f r e e weight (Stevenson and Stoermer 1982). One c o u l d expect the growth dynamics of a system that i n c l u d e d secondary epiphytism to d i f f e r from those i n which algae a t t a c h e d only to the primary substratum. In the l a t t e r , substratum s i z e would be constant through time, whereas i n the former new s i t e s f o r attachment would be made a v a i l a b l e as s t a l k e d and filamentous algae c o l o n i z e d the primary substratum. 198 A crude approximation of such a system might be given by: dNs/dt = (Cs + R S * N S ) * ( 1 - Ns/Ks) (4.6) dNe/dt = (Ce + Re*Ne)*(1 - Ne/Ke) (4.7) where Ns = number of c e l l s a t t a c h e d to the primary substratum ( s i i d e s ) , Ne = number of c e l l s a t t a c h e d s e c o n d a r i l y ( e p i p h y t i c ) Rs = i n t r i n s i c growth r a t e of c e l l s a t t a c h e d to s l i d e s (constant) Re = i n t r i n s i c growth r a t e of e p i p h y t i c c e l l s (constant) Ks = c a r r y i n g c a p a c i t y of c e l l s attached to s l i d e s (constant) Ke(t) = c a r r y i n g c a p a c i t y of e p i p h y t i c c e l l s ( v a r i a b l e ; a f u n c t i o n of Ns) Cs = c o l o n i z a t i o n r a t e of c e l l s a ttached to s l i d e s (constant) Ce = c o l o n i z a t i o n r a t e of e p i p h y t i c c e l l s ( c o n s t a n t ) . The t o t a l c e l l numbers are given by Ns + Ne. I assume that the c a r r y i n g c a p a c i t y of the e p i p h y t e s i s l i n e a r l y r e l a t e d to the number of c e l l s a t t a c h e d to the s l i d e s , i . e . Ke(t) = f * N s ( t ) , where f i s a constant. At e q u i l i b r i u m , the t o t a l c e l l numbers (Ns + Ne) i s given by f*K + K, and t h e r e f o r e the f r a c t i o n of the t o t a l c e l l s t h a t are secondary epiphytes i s f*K/(f*K + K) f / ( f + 1). T h i s model has s i x parameters. Further s i m p l i f i c a t i o n i s achieved by assuming that the c o l o n i z a t i o n and growth r a t e s are the same for c e l l s a t t a c h e d to s l i d e s and f o r epiphytes. I attempted to solve 199 a n a l y t i c a l l y the r e s u l t i n g four-parameter model a f t e r i n s e r t i n g the s o l u t i o n of equation (4.6) (given by equation (4.5)) i n t o equation (4.7). T h i s produced a f i r s t order n o n l i n e a r non-homogeneous d i f f e r e n t i a l equation (a R i c c a t i e q u a t i o n ) , which I transformed i n t o a second order l i n e a r homogeneous equation f o l l o w i n g Boyce and DiPrima (1977). I c o u l d not solve t h i s equation, i n which the c o e f f i c i e n t s were e x p o n e n t i a l f u n c t i o n s r a t h e r than c o n s t a n t s , so I decided to i n t e g r a t e i t n u m e r i c a l l y using a f o u r t h - o r d e r Runge-Rutta method (Boyce and DiPrima 1977) . The three models presented here assume that g r a z i n g , p e e l i n g , and sloughing do not a f f e c t s i g n i f i c a n t l y the biomass dynamics. The parameter values are a l s o assumed constant, which may be u n r e a l i s t i c when the measurement p e r i o d spans s e v e r a l months, s p e c i a l l y in s t r o n g l y seasonal environments. The data s e t s I searched the l i t e r a t u r e f o r data on p e r i p h y t o n growth, and found s i x papers c o n t a i n i n g data s e t s i n which biomass or numbers seemed to have s t a b i l i z e d a f t e r an i n i t i a l p e r i o d of i n c r e a s e . Whenever i n f o r m a t i o n regarding growth at s e v e r a l depths was a v a i l a b l e , I used only the data c o r r e s p o n d i n g to the shallowest s t a t i o n . The s i x data s e t s , p l u s a seventh one from Gwendoline Lake are presented i n Table VI; Table VII provides i n f o r m a t i o n about the communities and the c o l l e c t i o n s i t e s and dates. 200 Table VI. Periphyton growth data. Data s e t s I-VII. 201 I. (Gons 1982; estimated from F i g u r e 22) Time (weeks) 4.0 8.6 11.9 16.7 20.7 24.7 Organic matter 2 23 16 15 21 29 (g/m2) I I . (Stockner and Armstrong 1971; estimated from F i g u r e 5) Time (weeks) 2.1 4.3 6.0 8.0 11.0 16.0 21.0 26.0 Organic matter .05 .04 .42 .67 .54 .76 1.23 .69 (mg/cm2) I I I . (Herder-Brower 1975; estimated from F i g u r e 4) 65 Time (weeks) .3 .6 .9 1 . 1 1 .4 1.7 2 Dry weight 25 40 25 20 25 30 (mg/dm2) (continued) 2. ,3 2.6 2.9 3.1 3. 4 3.7 4.0 75 105 175 210 1 65 275 240 (continued) 4. ,6 4.9 5.1 5.4 5. 7 290 340 380 1 40 210 IV. ( E l o r a n t a and Kunnas 1976; estimated from F i g u r e 9a) Time (weeks) 1 2 3 4 jjig c h l . a/cm2 .2 1.2 3 .2 3. 0 V. ( B l i n n et a l . 1980; from Table II) Time (weeks) 1 2 3 4 5 C e l l s x !000/cm2 760 2533 5081 6434 6934 VI. (Wiegert and F r a l e i g h 1972; from Table 2, 1968 data) Time (weeks) .9 1 .3 2.0 2 .4 3.0 Dry organic 65.3 103.5 1 30.0 161 .2 184.9 matter (g/m2) (continued) 3.4 4.0 4.4 5 .4 6.7 213.9 187.1 232.0 214 .5 252.0 VI I . (This study) Time (weeks) 4 8 1 6 26 C e l l s x l000/cm2 6.6 40. 1 84. 1 1 03. 7 4.3 295 202 Table V I I . C h a r a c t e r i s t i c s of the community, waterbody, and sampling procedures f o r the data s e t s Data Substratum Dominant L o c a l i t y Exposure set type forms dates Vert i c a l perspex p l a t e s II H o r i z o n t a l g l a s s s i ides III V ert i c a l g l a s s s l i d e s IV V V e r t i c a l c e l l u l o i d p l a t e s N a t u r a l cleaned verde 1imestone Filamentous Lake Vechten, greens and The Nether-pennate d i a - lan d s ; .5 m toms; i n s e c t depth; meso-la r v a e and e u t r o p h i c molluscs present May 1/ l a t e Oct. Diatoms dominant, f o l l o w e d by f ilamentous greens and blue-greens Lake 240, O n t a r i o ; 1 m depth, o l i g o t r o p h i c M i d - A p r i l / mid-Oct. Not r e p o r t e d Hortus Botanicus d i t c h e s , The Netherlands; cms below the s u r f a c e ; t r o -p h i c s t a t u s not r eported (presumably e u t r o p h i c ) J u l y 7/ Aug. 22 Filamentous greens and diatoms Nearly 90% diatoms Lake Jyvas-j a r v i , F i n -l a n d ; 1 m depth; very e u t r o p h i c Oak Creek, Ar i zona; 20-30 cm/sec c u r r e n t September 26 June/ 30 J u l y VI H o r i z o n t a l wooden boards Blue^greens VII V e r t i c a l P l e x i g l a s "plates Diatoms and greens Experimental trough fed by hot s p r i n g s (43 C) i n Yellowstone Park, USA Gwendoline Lake, B r i t i s h Columbia; 4 m depth; o l i g o -t r o p h i c 18 J u l y / 3 Sept. Late Apr l a t e Oct 204 RESULTS A l l parameter estimates were obtained using Schnute's (1982) Micro Simplex program f o r n o n - l i n e a r parameter e s t i m a t i o n (with a l e a s t - s q u a r e s c r i t e r i o n of goodness of f i t ) . Parameter valu e s and r e s i d u a l sums of squares (RSS) are l i s t e d i n Table V I I I . The r e s i d u a l sum of squares (RSS) i s a measure of how w e l l the model f i t s a p a r t i c u l a r set of data. The epiphytism model d i d not pr o v i d e a b e t t e r f i t to the data than the other two models i n s p i t e of having an a d d i t i o n a l parameter. The c o l o n i z a t i o n model performed s l i g h t l y b e t t e r than the l o g i s t i c f o r some data s e t s , and s l i g h t l y worse f o r o t h e r s ; o v e r a l l the two models seem to f i t e q u a l l y w e l l . For some data s e t s (e.g. VI) the estimate of r depended g r e a t l y on which model was used. The estimates of the t o t a l f i n a l biomass or numbers (K i n the l o g i s t i c and c o l o n i z a t i o n models, K*(f + 1) i n the epiphytism model) were s i m i l a r f o r the three models. Since a l l models performed s i m i l a r l y , I have p l o t t e d the f i t t e d curves and data f o r only one of them, the c o l o n i z a t i o n model, i n F i g u r e 33. These graphs allow one to assess v i s u a l l y the goodness of f i t of the curves to the data. In some of the data s e t s (e.g. I, I I , and I I I ) there are f l u c t u a t i o n s about the steady s t a t e value p r e d i c t e d by the models, perhaps a consequence of v i o l a t i n g the assumptions of no p e e l i n g , s l o u g h i n g , or g r a z i n g mentioned e a r l i e r . These v a r i a t i o n s c o u l d l e a d to underestimating the steady s t a t e value and the growth r a t e that would be obtained 205 Table V I I I . Parameter estimates and r e s i d u a l sums of squares (RSS) f o r the three models MODEL DATA SET I II III IV V VI VII L o g i s t i c r 2.84 .418 1 .95 6.43 1 .52 .948 .303 (.59) ( .09) (-42)(1 .33) (.31) ( .22) ( .06) K 20.8 .879 277 3.10 7033 238 1 03e3 b 84e4 20.5 1 368 61e4 37.0 5.07 22.5 RSS 1 29 .267 47e3 .060 3537 1746 945e5 C o l o n i z a t i o n r 3.04 .374 1 .97 7.06 1 .37 .211 .236 ( .63) ( .08) (.42) (1 .46) ( .28) ( .04) ( .05) K 20.8 .878 276 3.10 7088 252 1 05e3 c 3. 5e-5 .022 2.62 1e-5 358 85.5 2065 RSS 1 29 .260 48e3 .060 6477 1 589 600e5 Epiphytism r 2.64 .275 2.55 4.77 1 .38 . 1 70 .250 (.54) ( .06) (.53) ( .98) ( .28) ( .04) ( .05) K 18.3 .890 104 3.1 2 7077 228 82e3 c 2e-3 .034 1 . 46 5e-3 447 100 1950 f . 1 35 .018 1.61 8e-4 7e-9 .088 .276 f / ( f + D .12 .02 .62 8e-4 7e-9 .08 .22 RSS 129 .257 48e3 .053 6794 1 753 497e5 NOTE: f / ( f +1) i s the f r a c t i o n of t o t a l c e l l s that are e p i p h y t i c , r and c are expressed i n 1/week u n i t s ; see Table VI f o r the u n i t s of K and b. The values i n p a r e n t h e s i s under the estimates f o r r are the growth r a t e s (expressed as doublings/day) c o r r e s -ponding to the given r value, assuming that growth i s u n l i m i t e d . 207 F i g u r e 33. Data s e t s and b e s t - f i t t i n g curves f o r the c o l o n i z a t i o n model, a-g: Data s e t s I to V I I . V e r t i c a l l i n e s in f and g represent one standard d e v i a t i o n above and below the mean val u e . 209 under more s t a b l e c o n d i t i o n s . The biomass v a r i a t i o n s i n l a t e samples from s e t s I and II might be seasonal, i n view of the l e n g t h of the sampling p e r i o d . With the e x c e p t i o n of the f i r s t few days in set I I I , there seem to be no systematic d e v i a t i o n s from the curves (the r e s i d u a l s are d i s t r i b u t e d uniformly about the p r e d i c t e d v a l u e s ) . How d i d growth r a t e s compare among waterbodies, and how d i d they r e l a t e to t r o p h i c s t a t u s ? I t i s hard to assess the n u t r i e n t s t a t u s of running waters, because the amount of p h y s i o l o g i c a l l y a v a i l a b l e n u t r i e n t depends both on the c o n c e n t r a t i o n and on c u r r e n t speed. When comparisons of growth r a t e s were r e s t r i c t e d to standing waters, there was a good agreement between t r o p h i c s t a t u s (Table VII) and the estimated growth r a t e s . The lowest growth r a t e s corresponded to Lake 240, to Gwendoline Lake (both o l i g o t r o p h i c ) , and to the hot s p r i n g at Yellowstone Park. The v a l u e s f o r Lake Vechten, Lake J y v a s j a r v i , and the Hortus Botanicus d i t c h are c l o s e to those found under good growing c o n d i t i o n s (see values c i t e d i n chapter 3). C o l o n i z a t i o n r a t e s estimated using the c o l o n i z a t i o n model can be compared among data sets by e x p r e s s i n g them as a percentage of the f i n a l s t e a d y - s t a t e biomass. The i n v e r s e of t h i s r a t i o , m u l t i p l i e d by 100, g i v e s the number of weeks i t would take to reach the f i n a l biomass i f only u n l i m i t e d c o l o n i z a t i o n (with no growth) o c c u r r e d . The r a t i o v a r i e d from a low of .0002% of the f i n a l t o t a l biomass/week in Lake Vechten to a h i g h of 34%/week in the Yellowstone Park hot s p r i n g ; the value f o r Gwendoline Lake was 2%/week. The estimated e p i p h y t i c f r a c t i o n of the t o t a l 210 c e l l s (or biomass) was high i n the Hortus Botanicus d i t c h (62%) and i n Gwendoline Lake (22%). U n f o r t u n a t e l y there are no independent estimates of t h i s f r a c t i o n with which to compare the r e s u l t s , although the hi g h e s t a b s o l u t e values found are comparable to those c i t e d e a r l i e r i n the p r e s e n t a t i o n of the epiphytism model. DISCUSSION Models can be e v a l u a t e d i n terms of t h e i r r e a l i s m , d e s c r i p t i v e power, and a b i l i t y to generate b i o l o g i c a l i n s i g h t . The e v a l u a t i o n of model r e a l i s m i s c l o s e l y connected to the p o s s i b i l i t y of t e s t i n g the assumptions that go i n t o each model, and t h e r e f o r e the b i o l o g i c a l i n t e r p r e t a b i l i t y of the parameters play s an important r o l e here. F e l l e r (1940), Smith (1952), and P i e l o u (1974) have f o r c e f u l l y argued that the f i t of the i n t e g r a l form of the l o g i s t i c and s i m i l a r equations to the data provides o n l y a weak t e s t of the model's assumptions, because many models can provide e q u a l l y good f i t s . Smith (1952) went on to perform an elegant experiment that t e s t e d an assumption i m p l i c i t i n the d i f f e r e n t i a l form of the l o g i s t i c equation, that of l i n e a r decrease i n per c a p i t a growth rate with i n c r e a s i n g p o p u l a t i o n s i z e , and found that i t d i d not h o l d , even though the i n t e g r a l form of the equation p r o v i d e d a good f i t to the data. It i s very d i f f i c u l t , however, to conduct t h i s kind of experiment under f i e l d c o n d i t i o n s . S t i l l , i f the parameters i n the model are i n t e r p r e t a b l e , they can i n p r i n c i p l e be measured independently, thus c o n s t r a i n i n g the model and s u b j e c t i n g i t to 21 1 a more r i g o r o u s t e s t than would be p o s s i b l e otherwise. The models can be f i t t e d to the data once, c o n s t r a i n i n g them to use the independently measured parameters, and then again l e a v i n g a l l parameters f r e e . If the r e s i d u a l sum of squares i n c r e a s e s c o n s i d e r a b l y when independently estimated parameters are used, then the model s t r u c t u r e i s probably not adequate. In t h i s context the c o l o n i z a t i o n model i s s u p e r i o r to the l o g i s t i c because c, the c o l o n i z a t i o n r a t e , has a p h y s i c a l i n t e r p r e t a t i o n and can be measured d i r e c t l y by examining the e a r l y stages of c o l o n i z a t i o n , d u r i n g which growth and c o l o n i z a t i o n can be assumed to proceed at maximal r a t e s . In c o n t r a s t , the parameter b i n the l o g i s t i c model can only be estimated by curve f i t t i n g . Parameters c and f i n the epiphytism model are a l s o i n t e r p r e t a b l e , and f c o u l d be measured as f o l l o w s : the t o t a l number of c e l l s a t t a c h e d to the primary substratum are estimated, say by s c r a p i n g one s i d e of the s l i d e and making d i r e c t counts on the s l i d e with an i n v e r t e d microscope. The algae on the counted s i d e of the s l i d e are then scraped, resuspended, allowed to s e t t l e , and counted to o b t a i n an estimate of t o t a l abundance, f i s equal to ( t o t a l abundance/number of c e l l s a t t a c h e d d i r e c t l y to the substratum) -1 . The d e s c r i p t i v e power of the models can be compared in terms of t h e i r RSS and of t h e i r complexity (number of parameters). The three models seem to f i t e q u a l l y w e l l to the data (Table V I I I ) , but the l o g i s t i c and the c o l o n i z a t i o n model are more parsimonious than the e p i p h y t i c model, s i n c e they employ one parameter l e s s . 212 The e v a l u a t i o n of the b i o l o g i c a l i n s i g h t p rovided by the models must remain somewhat s u b j e c t i v e . However, the three models seem to have p r o v i d e d a good i n d i c a t o r of lake t r o p h i c s t a t u s (the growth parameter r ) . Furthermore, the r e s u l t s show that doubling times in Gwendoline Lake are low r e l a t i v e to other systems, which accounts, at l e a s t i n p a r t , f o r the s l u g g i s h n e s s of the dynamics of community s t r u c t u r e d i s c u s s e d i n p r e v i o u s c h a p t e r s . The c o l o n i z a t i o n and epiphytism models a l s o suggest that i n some cases the c o l o n i z a t i o n r a t e i s h i g h compared to the growth r a t e , and that a high f r a c t i o n of the t o t a l biomass i s a t t r i b u t a b l e to secondary epiphytism. E m p i r i c a l t e s t s of these suggestions might l e a d to i n c r e a s e d understanding of these systems. SUMMARY 1) Periphyton growth r a t e s compared f a v o r a b l y with those estimated f o r o l i g o t r o p h i c Lake 240, but were roughly 5-20 times lower than those of some e u t r o p h i c waterbodies. The growth r a t e estimates generated by the three models seemed to r e f l e c t the t r o p h i c s t a t u s of the waterbodies. The low c e l l d i v i s i o n r a t e s were i n accordance with the s l u g g i s h n e s s i n community changes d i s c u s s e d i n e a r l i e r c h a p t e r s . 2) The c o l o n i z a t i o n model was found to f i t the data as w e l l as the l o g i s t i c and epiphytism models, while being more r e a l i s t i c than the former and more parsimonious than the l a t t e r . I recommend the use of the c o l o n i z a t i o n model f o r s t u d i e s i n which growth r a t e s are estimated by accumulation methods, e s p e c i a l l y 213 when the r a t e s are to be used f o r comparative purposes. The assumptions of no seasonal changes or l o s s e s by g r a z i n g or sloughing d u r i n g the sampling p e r i o d must be f u l f i l l e d i f accurate estimates are d e s i r e d . 214 REFERENCES Admiraal, W., L.A. Bowman, L. 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DESCRIPTION OF THE "BOOTSTRAP" ANALYSIS The "bootstrap" i s a computer-intensive s t a t i s t i c a l method r e c e n t l y developed by E f r o n (1977). The method can be used to s o l v e s t a t i s t i c a l problems without having to assume that the data have normal (Gaussian) d i s t r i b u t i o n s . Furthermore, i t makes p o s s i b l e the e x p l o r a t i o n of sample s t a t i s t i c s other than those amenable to a n a l y t i c a l m anipulation (such as the mean, the standard d e v i a t i o n , and the c o r r e l a t i o n c o e f f i c i e n t ) ( D i a c o n i s and E f r o n 1983). The method allows one to estimate the s i g n i f i c a n c e of a r e s u l t by r e f e r e n c e to a frequency d i s t r i b u t i o n generated from a s i n g l e i n i t i a l sample. Many simulated random samples of s i z e n are taken from the i n i t i a l sample; the s t a t i s t i c of i n t e r e s t i s computed f o r each of the fake data s e t s , and the s t a t i s t i c s are then assigned to c l a s s e s and p l o t t e d as frequency d i s t r i b u t i o n s . The d i s t r i b u t i o n of the s t a t i s t i c f o r the b o o t s t r a p samples can be t r e a t e d as i f i t had come from " t r u e " samples, and t h e r e f o r e one can estimate, say, c o n f i d e n c e i n t e r v a l s , or the p r o b a b i l i t y of the s t a t i s t i c f a l l i n g above or below some a r b i t r a r y v a l u e . For i n s t a n c e , the accuracy of the s t a t i s t i c c a l c u l a t e d f o r the i n i t i a l sample can be assessed by seeing whether i t f a l l s w i t h i n the c o n f i d e n c e l i m i t s estimated from the simulated d i s t r i b u t i o n . T h e o r e t i c a l work has shown that f o r a wide v a r i e t y of s t a t i s t i c s the confidence i n t e r v a l s a s s o c i a t e d with the b o o t s t r a p d i s t r i b u t i o n and the confidence i n t e r v a l s a s s o c i a t e d with the d i s t r i b u t i o n of r e a l samples u s u a l l y have n e a r l y the same width ( D i a c o n i s and E f r o n 1983). My c a l c u l a t i o n s f o l l o w e d four s t e p s : 1) computation of the s t a t i s t i c f o r the " r e a l " samples. The d i s p e r s i o n of the c l u s t e r s along the f i r s t three axes was estimated f o r each of the four major taxa, as the average of the E u c l i d e a n d i s t a n c e s between the members of the c l u s t e r and the c e n t r o i d of the c l u s t e r . 2) For each major taxa, a c l u s t e r having the same number of members as the major taxa was picked at random; members of the c l u s t e r were taken without replacement from the 44 groups a v a i l a b l e . Sampling was done without replacement because a l l samples had to have the same s i z e . 3) The average d i s p e r s i o n was c a l c u l a t e d f o r each of the fake c l u s t e r s simulated i n 2). Steps 2) and 3) were repeated 1000 times; each d i s p e r s i o n was subsequently a s s i g n e d i n t o one of 20 c l a s s e s , g e n e r a t i n g thus a frequency d i s t r i b u t i o n . The end products of 1), 2), and 3) were a set of four average d i s p e r s i o n s (one f o r each major taxa) c a l c u l a t e d from the r e a l data, and frequency d i s t r i b u t i o n s of the average d i s p e r s i o n s , c a l c u l a t e d from the simulated data. 4) The p r o b a b i l i t y of f i n d i n g by chance an average d i s p e r s i o n as small or smaller than the average d i s p e r s i o n of the r e a l sample was estimated as X(n)/1000, where X(n) was the number of cases i n which the average d i s p e r s i o n of the simulated c l u s t e r s was smaller or equal than the average d i s p e r s i o n of the " r e a l " c l u s t e r s . X was a f u n c t i o n of n, the number of members of the c l u s t e r . I found that when n = 5 (corresponding to blue-green algae) the 229 range of the frequency d i s t r i b u t i o n was l a r g e r than f o r n = 23 (corresponding to green a l g a e ) ; a l s o , the d i s t r i b u t i o n was skewed to the l e f t when n = 5, whereas the skew was small or absent when n = 23. The a n a l y s i s y i e l d e d s i m i l a r r e s u l t s when only 500 i t e r a t i o n s were performed. Because the amount of i n f o r m a t i o n conveyed by an o r d i n a t i o n a x i s i s t h e o r e t i c a l l y r e l a t e d to the amount of v a r i a n c e accounted f o r by the a x i s , I t r i e d weighing the three axes by the f r a c t i o n of the t o t a l v a r i a n c e e x p l a i n e d by each of them, as recommended by Bloom (1981). A f t e r r e s c a l i n g the c o o r d i n a t e v a l u e s by t h i s procedure, I repeated steps 1) - 4) above, and obta i n e d e s s e n t i a l l y the same r e s u l t s as i n the unweighed a n a l y s i s . 230 APPENDIX 2. TRANSFORMATIONS, STANDARDIZATIONS, AND (DIS)SIMILARITY INDICES Transformations and s t a n d a r d i z a t i o n s In what f o l l o w s , y = transformed v a r i a b l e , x = o r i g i n a l v a r i a b l e . L o g a r i t h m i c t r a n s f o r m a t i o n : y = l n ( x + 1) was used, when there were zero v a l u e s i n the data; otherwise y = l n ( x ) . Q u a l i t a t i v e (presence-absence) t r a n s f o r m a t i o n : y = 0 i f x = 0 y = 1 i f x > 0 S t a n d a r d i z a t i o n by s p e c i e s (or taxa) norm: Yik = X i k / ( E X , i k ) V l where n = number of s p e c i e s (or taxa) Xik = o r i g i n a l data score f o r s p e c i e s k at s i t e i Yik = transformed data score f o r s p e c i e s k at s i t e i Xik = sum of squares of X-scores f o r s p e c i e s (=species norm) S t a n d a r d i z a t i o n (by s i t e norm: Yik = X i k / ( j j ^ i k ) * -where m = number of s i t e s X^k, Yik as above (Exfik) = sum of squares of. x-scores f o r s i t e s ( = s i t e norm) In d i c e s E u c l i d e a n d i s t a n c e (ED): ' x w ED(P1i,P2i) = (fc(P1i - P2i) ) x ( L e g e n d r e and Legendre 1983) ED(P1,P2) = d i s t a n c e between p o i n t P1 and P2 i n n-dimensional space P1i = value of the i t h d e s c r i p t o r of p o i n t P1 P2i = value of the i t h d e s c r i p t o r of p o i n t P2 n = number of d e s c r i p t o r s If P l i and P2i represent r e s p e c t i v e l y the abundances of s p e c i e s 1 to n in-communities 1 and 2, ED measures the d i s t a n c e between communities 1 and 2. I f P1i and P2i r e p r e s e n t r e s p e c t i v e l y the abundances of s p e c i e s 1 and 2 i n communities 1 to n, ED measures the d i s t a n c e between s p e c i e s 1 and 2. The d e s c r i p t o r s can be raw scores or transformed v a r i a b l e s . Jassby-Goldman s u c c e s s i o n rate index (JG): JG = (EAP*k)k/At (Jassby and Goldman 1974) Pk(T) = Xk(T)/(£X\ (T) r l= s i t e - s t a n d a r d i z e d score f o r s p e c i e s k at time T APk = Pk(Tn+l) - Pk(Tn) AT = Tn+1 - Tn n = number of s p e c i e s B r a y - C u r t i s (BC): 231 BC =E|xik - X2kj /£(X1k + X2k) (Bray and C u r t i s 1957) X1k = abundance of s p e c i e s k i n community 1 X2k = abundance of s p e c i e s k in community 2 Canberra m e t r i c (CM): CM = m x i k - X2k|/IX1k + X2k)> (Lance and W i l l i a m s 1967) n o t a t i o n as i n the BC M o r i s i t a d i s s i m i l a r i t y (MS): t h i s index i s based on the s i m p l i f i e d M o r i s i t a index (Horn 1966), C x= 2Efcl kX2k)/(lam1 + lam2)S1S2 S i =IXik;. lami =2X 5ik/Si C\= s i m i l a r i t y between communities 1 and 2; the upper v a l u e of t h i s index i s not f i x e d , but i s "about one" when 2 communities are i d e n t i c a l . D i s s i m i l a r i t i e s were c a l c u l a t e d i n the p a i r w i s e comparisons as MS = 1 - C ( a l l the C value s were l e s s than 1), and i n the o r d i n a t i o n s MS was c a l c u l a t e d as 1 - C/Cmax, where Cmax was the l a r g e s t s i m i l a r i t y value e n t e r i n g the o r d i n a t i o n . Renkonen d i s s i m i l a r i t y (RD): RD = 0 . 55^ X1 k/ZXI k - X2k/£X2k| n o t a t i o n as i n the BC T h i s i s the complement of Renkonen's (1938) and Whit t a k e r ' s (1952) i n d i c e s . For e x t e n s i v e d i s c u s s i o n s of the i n d i c e s p resented here see Huhta (1979), Bloom (1981), and Wolda(1981). 

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