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The role of habitat heterogeneity in the community dynamics of an eelgrass-associated assemblage of gammarid… Miller, Patricia Anne 1985

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THE ROLE OF HABITAT HETEROGENEITY EELGRASS-ASSOCIATED  IN THE COMMUNITY DYNAMICS OF AN  ASSEMBLAGE OF GAMMARID  AMPHIPODS by  PATRICIA ANNE MILLER B.Sc.  (Hons.), U n i v e r s i t y o f B r i t i s h Columbia,  1980  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OP GRADUATE STUDIES (Zoology)  We a c c e p t t h i s t h e s i s as conforming t o t h e r e q u i r e d standard  THE UNIVERSITY OF BRITISH COLUMBIA October 1985 ©  P a t r i c i a Anne M i l l e r ,  1985  In  presenting  degree freely  at  this  the  available  copying  of  department publication  thesis  in  University  of  for  this  of  by this  British  reference  thesis  or  partial  for  his thesis  and  The 1956  University Main  Vancouver, V6T  Date  of  or  her  for  British  Columbia  Mall Canada  1Y3  October  15  r  study.  198 5  I  of I  further  purposes  gain  the  shall  requirements  agree  that  agree  may  representatives.  financial  Zoology of  Columbia,  scholarly  permission.  Department  fulfilment  It not  be  that  the  Library  permission  granted  is  by  understood be  for  allowed  an  advanced  shall for  the that  without  make  it  extensive  head  of  my  copying  or  my  written  ii  ABSTRACT The and  r o l e o f d e n s i t y o f e e l g r a s s shoots i n r e g u l a t i n g d i s t r i b u t i o n  abundance of gammarid amphipods was i n v e s t i g a t e d .  Monthly  c o l l e c t i o n s o f amphipods were made over a one-year p e r i o d i n a s e r i e s o f treatment p l o t s on Roberts Bank, i n southwestern B.C., i n which ( Z o s t e r a marina L.) shoots had been t h i n n e d densities. hypothesis  to different  t h a t t h e abundance and d i v e r s i t y o f amphipods would be  of o r i g i n a l  hypothesis  relative  T h i s experiment was o r i g i n a l l y d e s i g n e d t o t e s t t h e  p o s i t i v e l y r e l a t e d t o the d e n s i t y of e e l g r a s s recovery  eelgrass  shoots.  Due t o t h e r a p i d  shoot d e n s i t i e s w i t h i n t h e p l o t s , however, t h i s  c o u l d n o t be t e s t e d .  Consequently, t h e emphasis o f t h e study  was r e s t r i c t e d t o a c o n s i d e r a t i o n of the e f f e c t o f the d i s t u r b a n c e c r e a t e d d u r i n g removal of shoots on the d i s t r i b u t i o n o f amphipods. C o l l e c t i o n s of amphipods were a l s o made i n t h r e e a r e a s of d i f f e r e n t natural d e n s i t i e s of Zostera summer 1984, t o a s s e s s  shoots d u r i n g a three-month p e r i o d i n  f u r t h e r t h e e f f e c t o f shoot d e n s i t y on t h e  d i s t r i b u t i o n and abundance of amphipods. components o f h a b i t a t h e t e r o g e n e i t y , s p e c i e s o f seagrass,  Zostera  The r o l e of a d d i t i o n a l  including d r i f t  japonica, i n modifying  dynamics of t h e amphipods was a l s o s t u d i e d . density of Zostera was found.  a l g a e and a second t h e community  No r e l a t i o n s h i p between t h e  shoots and t h e abundance and d i v e r s i t y o f amphipods  The amphipod community was dominated by Corophium  acherusicum and t h e d i s t r i b u t i o n o f t h i s s p e c i e s , as w e l l as t h a t o f t h e other most f r e q u e n t l y c o l l e c t e d s p e c i e s , appeared t o be r e g u l a t e d by t h e s e a s o n a l i t y o f macrophyte biomass. Peak abundances of amphipods i n t h e l a t e summer and autumn when l a r g e amounts o f d r i f t e e l g r a s s d e t r i t u s were p r e s e n t  t  a t t h e sediment s u r f a c e .  occurred  a l g a e and This  decaying  iia plant m a t e r i a l i s an important source of food f o r d e t r i t i v o r e s such as gammarids and i t s seasonal abundance was r e f l e c t e d i n the r a p i d growth of populations of amphipods.  The f l o a t i n g mats of d r i f t algae, such as  Ulva sp., a l s o contributed s i g n i f i c a n t l y to the c a r r y i n g capacity of the eelgrass meadow by providing s p a t i a l refuges to amphipods which are targets of f i s h and b i r d predation.  The r o l e of habitat heteogeneity i n  determining the d i s t r i b u t i o n of the dominant species of amphipods, with reference to competition and predation, was discussed.  iii TABLE OF CONTENTS  Page  ABSTRACT  i i  LIST OF TABLES  vi  LIST OF FIGURES  ix  ACKNOWLEDGEMENTS  x i i  INTRODUCTION'  1  DESCRIPTION OF STUDY AREA  7  MATERIALS AND METHODS  11  A. The e f f e c t of removal o f Z o s t e r a marina shoots on the d i s t r i b u t i o n and abundance of amphipods (The Twelve-Month Study)  11  1.  Sampling D e s i g n  12  2.  Sampling Procedure  16  3.  Physical Factors  17  4.  L a b o r a t o r y Procedures  18  5.  S t a t i s t i c a l Analysis  21  6.  Correlation Matrices  23  B. The r e l a t i o n s h i p between n a t u r a l of  densities  Z o s t e r a shoots and t h e d i s t r i b u t i o n  and abundance o f amphipods (The Three-Month Study)  24  1.  Design o f Study S i t e s .  24  2.  Sampling Procedure  25  iv Page  3.  Physical Factors  28  4.  Laboratory  28  5.  Statistical Analysis  29  6.  Correlation Matrices  29  Procedures  C. L i f e C y c l e s and  Size-Frequency  Distributions  30  RESULTS  32  A. The  Twelve-Month Study  32  1.  Physical Factors  29  2.  Growth of Z o s t e r a marina  36  3.  Composition  42  4.  E f f e c t of shoot  and  s e a s o n a l i t y of d r i f t removal on growth  of Z o s t e r a m a r i n a 5.  45  E f f e c t of Z o s t e r a marina shoot on the d i s t r i b u t i o n and  removal  abundance  of amphipods  45  6.  Seasonal  50  7.  Macrophyte biomass and  d i s t r i b u t i o n of amphipods the d i s t r i b u t i o n  of amphipods  60  8.  D i s t r i b u t i o n of s p e c i e s of amphipods  67  9.  Correlation matrices  and  d i s t r i b u t i o n of s p e c i e s B. The Three-Month Study 1.  Physical Factors  the 77 99 99  V  Page  2. Biofnass and shoot d e n s i t y of Z o s t e r a marina 3.  108  Biomass and shoot d e n s i t y of Zostera japonica  112  4.  Composition  112  5.  Z o s t e r a marina shoot d e n s i t y and the  6.  and d i s t r i b u t i o n of d r i f t  distribution  121  o f amphipods  D i s t r i b u t i o n and o v e r a l l abundance o f amphipods  121  7.  D i s t r i b u t i o n of amphipod s p e c i e s  134  8.  Naturalist  9.  C o r r e l a t i o n m a t r i c e s and t h e  s l e d samples  -..  d i s t r i b u t i o n of s p e c i e s C. L i f e C y c l e s and  ....143  147  Size-Frequency  Distributions  159  1.  Size-frequency d i s t r i b u t i o n s  159  2.  Reproductive  167  seasonality  DISCUSSION  178  REFERENCES  200  APPENDIX 1. L i n e a r r e g r e s s i o n equations vs t o t a l  f o r head l e n g t h  l e n g t h f o r Ampithoe v a l i d a ,  Anisogammarus pugettensis,, Corophium acherusicum, and C. i n s i d i o s u m  12.0-9'  vi LIST OF TABLES Page 1.  2.  3.  4.  5.  6.  7.  8.  9.  10.  D e n s i t i e s o f Z o s t e r a marina shoots i n treatment p l o t s , b e f o r e and a f t e r shoot removal  15  R e l a t i o n s h i p between s u r f a c e a r e a and d r y weight o f macrophyte s p e c i e s  20  P a r t i c l e - s i z e d i s t r i b u t i o n o f sediments i n treatment p l o t s , June 27, 1984  35  Percentage o r g a n i c content o f sediment a t study s i t e "T"  37  T o t a l numbers of amphipods s u r f a c e i n treatment p l o t s  49  • m ^ of s u b s t r a t e  C o r r e l a t i o n c o e f f i c i e n t s f o r comparisons o f number o f amphipods w i t h d r y weight of Z o s t e r a marina rhizomes  61  C o r r e l a t i o n c o e f f i c i e n t s f o r comparisons o f monthly mean number of amphipods i n Z o s t e r a marina, d r i f t , sediment, and t o t a l quadrat samples, w i t h s u r f a c e a r e a and d r y weight o f macrophytes a t study s i t e "T" ..'  62  C o r r e l a t i o n c o e f f i c i e n t s f o r comparisons o f mean number o f amphipods i n Z o s t e r a marina d r i f t , sediment, and t o t a l q u a d r a t samples with s u r f a c e a r e a and d r y weight of macrophytes a t study s i t e "T"  65  R e l a t i v e abundance, d i v e r s i t y , and evenness o f s p e c i e s o f amphipods a t study s i t e "T"  68  C o r r e l a t i o n c o e f f i c i e n t s f o r comparisons o f r e l a t i v e numbers of amphipods • s p e c i e s , c o l l e c t e d a t study s i t e "T" and p o o l e d i n a l l months and s u b s t r a t e types  88  C o r r e l a t i o n c o e f f i c i e n t s f o r comparisons o f monthly mean numbers of amphipods * s p e c i e s at study s i t e "T"  91  - 1  11.  12.  - 1  C o r r e l a t i o n c o e f f i c i e n t s f o r comparisons o f mean numbers o f amphipods . s p e c i e s i n Z o s t e r a marina, d r i f t , and sediment a t study s i t e "T"  96  vii Page  13.  14.  15.  16.  17.  18.  19.  20.  21.  P a r t i c l e - s i z e d i s t r i b u t i o n of sediments i n Areas 1, 2, and 3  106  Percentage o r g a n i c content of sediment i n Areas 1, 2, and 3  107  Dry weight of Z o s t e r a rhizomes i n Areas 1, 2, and 3 Monthly t o t a l number of amphipods Areas 1, 2, and 3  I l l • m-2  in 122  C o r r e l a t i o n c o e f f i c i e n t s f o r monthly comparisons of numbers of amphipods i n Z o s t e r a marina, d r i f t , sediment, and t o t a l quadrat samples w i t h s u r f a c e a r e a and d r y weight of macrophytes  132  C o r r e l a t i o n c o e f f i c i e n t s f o r comparisons of number of amphipods i n Z o s t e r a marina, d r i f t , sediment, and t o t a l quadrat samples w i t h s u r f a c e a r e a and d r y weight of macrophytes  135  R e l a t i v e abundance, d i v e r s i t y , and evenness of s p e c i e s of amphipods i n Areas 1, 2, and 3  136  Numbers of amphipods i n n a t u r a l i s t i n Areas 1, 2, and 3  144  s l e d tows  C o r r e l a t i o n c o e f f i c i e n t s f o r comparisons of r e l a t i v e numbers of amphipods * s p e c i e s " c o l l e c t e d i n Areas 1, 2, and 3 1  22.  23.  C o r r e l a t i o n c o e f f i c i e n t s f o r comparisons of monthly mean numbers of.amphipods * s p e c i e s i n Z o s t e r a marina, d r i f t , and sediment samples  148  - 1  150  C o r r e l a t i o n c o e f f i c i e n t s f o r comparisons of mean numbers of amphipods • s p e c i e s i n Areas 1, 2, and 3  153  Monthly percentages of males, females, and o v i g e r o u s females of Corophium acherusicum  171  Monthly percentages of males, females, and o v i g e r o u s females of Corophium i n s i d i o s u m  172  Monthly percentages of males, females, and o v i g e r o u s females of Ampithoe v a l i d a  174  - 1  24.  25.  26.  viii Page 27.  Monthly percentages of males, females, and o v i g e r o u s females of Anisogammarus p u g e t t e n s i s  176  ix LIST OF FIGURES  Page 1.  2.  Map o f intercauseway a r e a on Roberts Bank study s i t e  showing 8  Plan o f treatment p l o t s i n P a r t A: Twelve-Month Study  13  3.  Naturalist  26  4.  Monthly a i r and water temperatures  5.  Monthly s u r f a c e s a l i n i t i e s  6.  Dry weight of above-ground shoots a t study s i t e "T"  7.  8.  9.  sled  33 -.  Z o s t e r a marina 38  Number o f b l a d e s p e r Z o s t e r a marina shoot at study s i t e "T"  38  Length o f l o n g e s t b l a d e p e r Z o s t e r a marina shoot a t study s i t e "T"  40  Dry weight of Z o s t e r a marina rhizomes • 0.01m a t study s i t e "T"  40  -2  10.  33  Dry weight of d r i f t s i t e "T"  • 0.01m"  2  :  a t study 43  11. Change i n d e n s i t y of Z o s t e r a marina shoots • 0.25m , June 1983 - J u l y 1984 i n row 1  46  12. Change i n d e n s i t y of Z o s t e r a marina shoots • 0.25m , June 1983 - J u l y 1984 i n row 2  46  13. Monthly abundance of amphipods • m at study s i t e "T"  51  2  -2  -2  _o  14. Mean number of amphipods * m of Zostera marina s u r f a c e a t study s i t e "T"  53  15. Mean number of amphipods • IT. s u r f a c e a t study s i t e "T"  of d r i f t 55  16. Mean number of amphipods * m s u r f a c e a t study s i t e "T"  o f sediment  17. Mean number o f i n d i v i d u a l s i n each dominant amphipod s p e c i e s • m surface of l i v e Z o s t e r a marina a t study s i t e "T"  57  -2  71  X  Page 1 8 . Mean number o f i n d i v i d u a l s i n e a c h d o m i n a n t amphipod s p e c i e s • m s u r f a c e of d r i f t - 2  at  study  site  "T"  1 9 . Mean number o f amphipod  20.  21.  individuals  species  surface at  73  study  ' m  in  each  surface of  site  dominant sediment  "T"  75  P r i n c i p a l component o r d i n a t i o n o f d o m i n a n t amphipod s p e c i e s i n monthly samples, A u g u s t 1983 - J u l y 1984 M o n t h l y mean p r i n c i p a l c o m p o n e n t each s u b s t r a t e ,  August  1983 -  scores  78  in  J u l y 1984  82  22.  A r e a 1,  28 J u l y 1984  100  23.  A r e a 2,  28 J u l y 1984  102  28 J u l y  104  2 4 . A r e a 3, 25.  Dry weight  26.  Densities  shoots  28.  above-ground  i n Areas  i n Areas 27.  of  1984  of 1,  D e n s i t y of in Area 1  1,  2,  Zostera  and 3  109  Zostera marina 2,  shoots  Zostera  marina 31.  32.  - 2  109  japonica shoots  '  0.25m 113  M o n t h l y mean d r y w e i g h t s i n A r e a s 1, 2, a n d 3  Mean number o f  • 0.25m  and 3  * 0.01m  amphipods  surface i n Areas  of  - 2  drift 115  2 9 . M o n t h l y abundance o f amphipods i n A r e a s 1, 2, a n d 3 30.  marina  * m 1,  2,  - 2  ' m 123 of  Zostera  and 3  _o Mean number o f a m p h i p o d s * m of s u r f a c e i n A r e a s 1, 2, a n d 3  drift  Mean number o f  sediment  amphipods  surface i n Areas  1,  2,  * m * of  and 3  124  127  129  xi Page 33. Mean number o f i n d i v i d u a l s i n each dominant amphipod s p e c i e s * m~ s u r f a c e o f l i v e Z o s t e r a marina i n Areas 1, 2, and 3  139  34. Mean number o f i n d i v i d u a l s i n each dominant amphipod s p e c i e s ' m surface of d r i f t i n Areas 1, 2, and 3  140  35. Mean number of i n d i v i d u a l s i n each dominant amphipod s p e c i e s • m~ o f sediment s u r f a c e i n Areas 1, 2, and 3  141  36. S i z e - f r e q u e n c y acherusicum  d i s t r i b u t i o n s of Corophium 160  37. S i z e - f r e q u e n c y insidiosum  d i s t r i b u t i o n s of Corophium  38. S i z e - f r e q u e n c y valida  d i s t r i b u t i o n s o f Ampithoe  39. S i z e - f r e q u e n c y pugettensis  distributions-of  2  2  162  165 Anisogammarus 168  xii  ACKNOWLEDGMENTS  I thank my s u p e r v i s o r s , Dr. T. C a r e f o o t and Dr. P.G. H a r r i s o n , f o r t h e i r encouragement,  c r i t i c i s m , and f i n a n c i a l s u p p o r t .  I am g r a t e f u l t o  many people who donned chest-waders and braved t h e m u d f l a t s t o a s s i s t me i n the f i e l d .  I e s p e c i a l l y thank G. Boyer, B. B o y l e , D. C h r i s t i a n s e n ,  C. Durance, J . F i s h e r , S. Guenther, S. Khanna, B. Ko, B. Power, T. W a l t e r s , and M. Y i p .  Many c o n v e r s a t i o n s w i t h P. Shaw have improved my  a p p r e c i a t i o n o f amphipods,  i f not of s y s t e m a t i c s .  Most o f a l l ,  I am  i n d e b t e d t o J . M c N i c o l f o r h i s generous companionship on a l l midnight sampling e x p e d i t i o n s and throughout t h e study.  1 INTRODUCTION Seagrass meadows a r e fundamental components of e s t u a r i n e and foreshore habitats.  They a r e h i g h l y p r o d u c t i v e a r e a s which a r e  a e s t h e t i c a l l y as w e l l as e c o n o m i c a l l y important, not j u s t as f e e d i n g areas f o r b i r d s , but a l s o as n u r s e r y areas f o r j u v e n i l e f i s h and f o r crabs.  S u p e r f i c i a l l y , a seagrass meadow appears t o be a simple,  homogeneous environment. s t r u c t u r a l complexity.  C l o s e r s c r u t i n y , however, r e v e a l s i t s Many s t u d i e s have shown t h a t the abundance and  d i v e r s i t y of organisms i n s o f t - b o t t o m systems a r e enhanced presence o f seagrass ( e . g . O r t h , 1977; Homziak et a l . , communities  1982). Seagrass  c h a r a c t e r i s t i c a l l y i n c l u d e s e s s i l e and m o t i l e animals which  a t t a c h themselves t o the b l a d e s and stems of p l a n t s animals which a r e f r e e - l i v i n g a t the sediment  ( e p i f a u n a ) , and  s u r f a c e , as w e l l as those  which b u i l d tubes on p l a n t r o o t s and rhizomes or burrow sediment  by the  ( i n f a u n a ) ( K i k u c h i & Peres, 1977;  i n t o the  Jacobs, 1980). In comparison,  the f a u n a l community o f i n t e r t i d a l unvegetated sand f l a t s , where the sediments a r e r e a d i l y resuspended by wave energy and t i d a l c u r r e n t s , i s o f t e n dominated by-a few s p e c i e s which a r e a b l e t o escape the r i g o u r s of the sediment s u r f a c e o n l y by burrowing r a p i d l y or by l i v i n g i n deep tubes ( O r t h , 1977; Heck and O r t h , 1980a; Summerson and P e t e r s o n , 1984). The mosaic of m i c r o h a b i t a t s i n seagrass systems p r o v i d e s r e f u g e s from both p h y s i c a l s t r e s s and b i o l o g i c a l i n t e r a c t i o n s f o r a wide v a r i e t y of macrofauna  and i n so d o i n g p e r m i t s the development  community s t r u c t u r e (Heck and Wetstone, Stoner, 1980a).  of a  complex  1977; Nelson, 1979a,1980a;  In seagrass beds, the emergent  b l a d e s of the p l a n t s  b a f f l e c u r r e n t s w h i l e the rhizomes t r a p n u t r i e n t - r i c h p a r t i c u l a t e matter and s t a b i l i z e the s u b s t r a t e , r e d u c i n g sediment scour (Grady, 1981; Orth,  1977;  Peterson e t a l . , 1 9 8 4 ) .  2 During low t i d e , when t h e seagrass b l a d e s  and t h e i r a t t a c h e d e p i p h y t e s l i e prone on t h e s u b s t r a t e s u r f a c e , t h e fauna that a r e s h e l t e r e d beneath exposure  them a r e e f f e c t i v e l y p r o t e c t e d a g a i n s t  t o s t r o n g l i g h t and d e s i c c a t i o n  ( K i k u c h i and Peres,  1977).  S i m i l a r l y , when the seagrass meadow i s submerged a t h i g h t i d e , t h e l e a f canopy c r e a t e s a calm underwater space i n which f i s h and i n v e r t e b r a t e s may f i n d s h e l t e r  ( K i k u c h i and Peres, 1977).  These q u i e s c e n t areas  enhance sedimentation and prevent p a s s i v e t r a n s p o r t of s m a l l f r e e - l i v i n g a d u l t and l a r v a l i n v e r t e b r a t e s out o f t h e meadow ( O r t h , 1977). Seagrass p l a n t s a l s o i n f l u e n c e t h e outcome of b i o l o g i c a l interactions,  such as p r e d a t i o n (Nelson, 1979b, 1980a; Stoner, 1980a)  and c o m p e t i t i o n (Coen e t a l . ,  1981; Robertson  & Mann, 1982).  Although  c e r t a i n f i s h p r e d a t o r s may be a t t r a c t e d t o seagrass meadows, t h e i r a b i l i t y t o d e t e c t and c a p t u r e e p i b e n t h i c p r e y may be reduced due. t o t h e p h y s i c a l presence of t h e p l a n t s . As a r e s u l t ,  t h e d e n s i t y of  i n v e r t e b r a t e macrofauna i s o f t e n d i r e c t l y r e l a t e d t o macrophyte biomass (e.g. Stoner, 1980a; Heck and Orth, 1980b). Due t o t h e h i g h primary p r o d u c t i v i t y and r a p i d turnover r a t e o f above-ground, biomass which a r e c h a r a c t e r i s t i c o f seagrass systems (Ferguson & Adams, 1979), t h e p o s s i b i l i t y o f d e t r i t a l food shortages i s g e n e r a l l y d i s c o u n t e d (e.g. Nelson, 1979a), and examples o f c o m p e t i t i v e i n t e r a c t i o n s i n v o l v i n g t h i s r e s o u r c e a r e seldom d e s c r i b e d . of a newly e s t a b l i s h e d Z o s t e r a marina Thayer  L. meadow i n North  e t a l . (1975) e s t i m a t e d t h a t f a r from competing  In a study  Carolina,  f o r food, t h e  macrofauna consumed o n l y 55% of t h e n e t p r o d u c t i o n of t h e e e l g r a s s , phytoplankton and b e n t h i c a l g a e i n t h e system.  In a d d i t i o n , Zimmerman  et a l . (1979) found s i g n i f i c a n t d i f f e r e n c e s i n both t h e s i z e and k i n d o f d e t r i t u s e x p l o i t e d by t h r e e s p e c i e s o f gammarid amphipods i n t h e  3 seagrass beds o f F l o r i d a , prevent  i n d i c a t i n g that resource p a r t i t i o n i n g could  competition f o r food.  speculated that gastropod f o r ephiphytes  Robertson  and Mann (1982),  however,  g r a z e r s such as L i t t o r i n a n e g l e c t a may compete  and d e t r i t u s d u r i n g t h e summer when t h e i r p o p u l a t i o n  densities are high.  They noted t h a t s n a i l d e n s i t y was d i r e c t l y  to plant surface area.  In an e a r l i e r  study, Robertson  t h a t the p o p u l a t i o n dynamics of another  gastropod,  related  (1981) had found  Littorina  saxitalis,  were c l o s e l y a s s o c i a t e d with the l i m i t e d p e r i p h y t o n food supply growing on Z. marina l e a v e s and t h a t i n t r a s p e c i f i c c o m p e t i t i o n f o r t h i s resulted i n post-recruitment m o r t a l i t i e s . two  resource  I f t h e c o n c l u s i o n s of these  s t u d i e s a r e c o r r e c t , and food i s l i m i t i n g f o r the L i t t o r i n a s p e c i e s ,  then i t may a l s o be i n short supply f o r other d e t r i t i v o r e s i n seagrass systems. D e s p i t e t h e l a r g e number o f m i c r o h a b i t a t s i n seagrass meadows, t h e r e i s growing e v i d e n c e  that competition f o r s h e l t e r ,  another  p o t e n t i a l l y l i m i t i n g r e s o u r c e , can occur w i t h i n e p i f a u n a l communities of seagrass beds. Coen e t a l . (1981), nonoverlapping  f o r example, concluded  that the  m i c r o g e o g r a p h i c a l d i s t r i b u t i o n of two c a r i d e a n shrimp  s p e c i e s was p r i m a r i l y due. t o i n t e r s p e c i f i c than t o m i c r o h a b i t a t s e l e c t i o n .  s p a t i a l competition rather  In r e p e a t e d l a b o r a t o r y t r i a l s t h e  dominant shrimp s p e c i e s , Palaemonetes f l o r i d a n u s , was c o n s i s t e n t l y a b l e t o d i s p l a c e t h e second s p e c i e s , P. v u l g a r i s , from a s t r u c t u r a l l y complex r e d a l g a l s u b s t r a t e onto a simpler t u r t l e - g r a s s  ( T h a l a s s i a testudinum)  s u b s t r a t e , where i t was more v u l n e r a b l e t o p r e d a t i o n . displacement  might be expected  t o occur  Similar  i n mixed stands o f s e a g r a s s , i f  one macrophyte s p e c i e s p r o v i d e d b e t t e r s h e l t e r f o r macrofauna than d i d another  s p e c i e s ( M i d d l e t o n e t a l . , 1984).  4 S p e c i e s c o m p o s i t i o n and p l a n t morphology may  be as important  shoot d e n s i t y and biomass i n c o n t r i b u t i n g t o the s t r u c t u r a l of  seagrass systems (Heck and O r t h , 1980a).  as  complexity  M i d d l e t o n et a l . (1984)  concluded t h a t d i f f e r e n c e s i n the s t r u c t u r e of t h e f i s h and i n v e r t e b r a t e communities i n Z o s t e r a c a p r i c o r n i and P o s i d o n i a a u s t r a l i s  meadows i n  A u s t r a l i a were r e l a t e d t o v a r i a t i o n s i n the s t r u c t u r a l c o m p l e x i t y of c a n o p i e s c r e a t e d by the two  seagrass s p e c i e s . The  l a r g e r P.  the  australis  p l a n t s formed a h i g h e r canopy and p r o v i d e d more s h e l t e r and  attachment  s i t e s f o r macrofauna than d i d the s m a l l e r s p e c i e s , which i n t u r n , a t t r a c t e d more f i s h p r e d a t o r s .  Stoner  (1983) c o n c l u d e d t h a t  the  r o l e of seagrass biomass i n d e t e r m i n i n g the s t r u c t u r e of macrofaunal  assemblages was  s i g n i f i c a n t only w i t h i n a plant s p e c i e s .  a l s o found t h a t c e r t a i n s p e c i e s of e p i f a u n a l amphipods,  He  including  Cymadusa compta, G r a n d i d i e r e l l a b o n n i e r o i d e s , and M e l i t a e l o n g a t a , were capable of d e t e c t i n g small d i f f e r e n c e s i n shoot d e n s i t y and t h a t c o u l d p r e f e r e n t i a l l y d i s t i n g u i s h p l a n t s with h i g h s u r f a c e ratios  (Stoner,  area/biomass  1980c).  In a study of the T h a l a s s i a testudinum beds i n F l o r i d a , al.  Gore et  (1981) observed t h a t the abundance and d i v e r s i t y of the  macrocrustacean Other  community were enhanced by the presence of d r i f t a l g a e .  f a c t o r s which may  system may  they  enhance the s t r u c t u r a l c o m p l e x i t y o f a  i n c l u d e the presence of e p i p h y t e s and e n c r u s t i n g e p i f a u n a , which  p r o v i d e a d d i t i o n a l m i c r o h a b i t a t s f o r a s s o c i a t e d organisms  O r t h , 1980a).  (Heck and  T o p o g r a p h i c a l p a t c h i n e s s and the p r o x i m i t y o f a d j a c e n t  h a b i t a t s such as c o r a l r e e f s can a l s o account o r g a n i z a t i o n of the f a u n a l community 1980a).  seagrass  f o r v a r i a t i o n s i n the  (Heck, 1977;  Heck and  Orth,  Although  5 t h e r e have been s e v e r a l r e c e n t  d i s t r i b u t i o n and a u t e c o l o g y  s t u d i e s on  of Z o s t e r a marina and  Z.  the  japonica  & Graebn. i n the P a c i f i c Northwest ( H a r r i s o n , 1982a, 1984;  Aschers.  Phillips,  1983), t h e r e have been few r e p o r t e d s t u d i e s of the a s s o c i a t e d f a u n a l communities (Thayer  & Phillips,  1977;  K i k u c h i , 1980).  i n c l u d e Swinbank's (1979) study of the s o f t - b o t t o m Roberts Bank i n the F r a s e r R i v e r e s t u a r y , and  Exceptions  communities of  Levings and C o u s t a l i n ' s  (1975) survey of the b e n t h i c i n v e r t e b r a t e fauna i n t h i s a r e a . and  Levings  (1980) a l s o i n c l u d e d Roberts Bank i n t h e i r  a s s o c i a t i o n and  study of  algae.  Gammaridean amphipods were chosen as the focus of the study f o r s e v e r a l reasons:  i n a v a r i e t y of d i f f e r e n t  & Hensel,  seagrass  have been  systems, i n c l u d i n g the Young, 1978;  Nelson,  1982). 2)They a r e an i n t e g r a l component of  d e t r i t u s - b a s e d food c h a i n ( L e v i n g s , 1973), and consumed by  present  1) High d e n s i t i e s of these animals  e e l g r a s s meadow on Roberts Bank (Young and Miller  the  f e e d i n g r e l a t i o n s h i p s of the amphipod Eogammarus  c o n f e r v i c o l u s with v a r i o u s s p e c i e s of b e n t h i c  recorded  Pomeroy  1981;  the  several species are  j u v e n i l e salmon which r e s i d e i n the e e l g r a s s beds of Roberts  Bank d u r i n g the s p r i n g and  summer (MacDonald, 1984).  3) Amphipods  can  be c o l l e c t e d r e l a t i v e l y e a s i l y i n the i n t e r t i d a l zone and  they a r e l a r g e  enough t h a t t h e i r behaviour  4)These  animals  i n the f i e l d can be observed.  occupy a v a r i e t y of m i c r o h a b i t a t s i n seagrass  systems.  l i v i n g e p i f a u n a l s p e c i e s l i v e on t h e s u r f a c e of the b l a d e s , domiculous s p e c i e s l i v e i n tubes a t t a c h e d t o the stems and other s p e c i e s a r e i n f a u n a l ( K i k u c h i and  Peres,  1977).  Free-  while rhizomes;  Because e p i f a u n a l  s p e c i e s a r e more v u l n e r a b l e t o p r e d a t i o n than a r e i n f a u n a l s p e c i e s (Stoner, 1979), I expected  t o observe d i f f e r e n c e s i n temporal  s p a t i a l d i s t r i b u t i o n s of these two  groups.  and  A l s o , c e r t a i n s p e c i e s of  6 e p i f a u n a l amphipods have been shown t o e x h i b i t a f f i n i t i e s f o r h i g h d e n s i t i e s o f seagrass  strong behavioural  (Stoner,  1980c). 5) Since  female amphipods brood t h e i r o f f s p r i n g i n marsupia u n t i l the f i r s t moult, i t i s p o s s i b l e t o r e l a t e s e a s o n a l p a t t e r n s of r e p r o d u c t i o n t o those of s p e c i e s abundance and d i v e r s i t y The present  (Nelson, 1980a).  study had t h r e e o b j e c t i v e s : 1) t o document the  seasonal p a t t e r n s of d i s t r i b u t i o n and abundance of the gammaridean amphipod community i n the e e l g r a s s bed l o c a t e d on Roberts Bank i n the F r a s e r R i v e r e s t u a r y ; 2) t o examine the r e l a t i o n s h i p between e e l g r a s s biomass/shoot  d e n s i t y and a m p h i p o d d i v e r s i t y / a b u n d a n c e ; 3) t o  i n v e s t i g a t e the r o l e of a d d i t i o n a l components of h a b i t a t such as d r i f t a l g a e and a second seagrass o r g a n i z a t i o n of the amphipod  s p e c i e s , Z. j a p o n i c a , i n the  community.  The study was d i v i d e d i n t o two p a r t s . over  complexity,  The f i r s t p a r t took p l a c e  twelve months from June 1983 - J u l y 1984 and i n v o l v e d the  manipulation  of Z o s t e r a marina shoot d e n s i t i e s i n a s i n g l e study  P a t t e r n s of amphipod d i s t r i b u t i o n and abundance were monitored to  shoot d e n s i t y and t o the s e a s o n a l i t y of macrophyte biomass.  site.  relative The  second p a r t took p l a c e from. May - J u l y 1984 and i n v o l v e d an i n v e s t i g a t i o n of amphipod d i s t r i b u t i o n i n t h r e e areas of d i f f e r e n t n a t u r a l shoot d e n s i t y , i n c l u d i n g a mixed stand of Z^ marina and i t s smaller r e l a t i v e ,  Z_^ j a p o n i c a .  7 DESCRIPTION OF  STUDY AREA  F i e l d s t u d i e s were c a r r i e d out i n the intercauseway southern Roberts - J u l y 1984.  Bank i n the F r a s e r R i v e r d e l t a  a r e a of  ( F i g . 1) from June  T h i s a r e a , bounded on the n o r t h by the Westshore  1983  Terminals  c o a l p o r t causeway and on the south by the Tsawwassen f e r r y t e r m i n a l causeway, i s dominated by a l a r g e (= 200 marina).  ha) meadow of e e l g r a s s ( Z o s t e r a  Because t h i s meadow i s one of the most b i o l o g i c a l l y p r o d u c t i v e  h a b i t a t s i n the e s t u a r y and commercial f i s h  i s important  as a n u r s e r y a r e a f o r  s p e c i e s such as P a c i f i c h e r r i n g (Clupea harengus  p a l l a s i ) and coho (Oncorhyncus k i s u t c h ) , chinook chum (CK k e t a )  salmon (Hoos & Packman, 1979)  (CK  tshawytscha),  and  the e e l g r a s s and i t s  a s s o c i a t e d b i o t a have been the s u b j e c t of s e v e r a l r e c e n t s t u d i e s (Levings and C o u s t a l i n , 1975; Harrison,  (BEAK),  1977;  1982a,1984).  The  F r a s e r R i v e r i s the s i x t h l a r g e s t r i v e r i n North America  der Leeden, 1975) each year  Beak Hinton C o n s u l t a n t s  and d e p o s i t s 20 m i l l i o n tons of sediment i n the d e l t a  (BEAK, 1977).  from the south arm  (van  The  intercauseway  of the r i v e r ,  through  a r e a i s l o c a t e d 11 k i l o m e t r e s  which 85% of i t s annual  d i s c h a r g e f l o w s , but i s p r o t e c t e d from l a r g e s e a s o n a l f l u c t u a t i o n s i n s a l i n i t y and  t u r b i d i t y by the c o a l p o r t causeway which d e f l e c t s  r i v e r ' s plume (BEAK, 1977;  Swinbanks, 1979).  the  As a r e s u l t , a g r e a t e r  abundance and d i v e r s i t y of f i s h and i n v e r t e b r a t e fauna occur i n the intercauseway Sturgeon  a r e a than a r e found  i n a d j a c e n t f o r e s h o r e a r e a s , such  Banks (Levings and C o u s t a l i n , 1975;  Gordon and  Levings,  as  1984).  The c o a l p o r t causeway, and more r e c e n t l y a " r i p - r a p " dyke, which was  b u i l t p a r t way  a c r o s s the seaward margin of the e e l g r a s s meadow i n  8  Figure 1A. Location of the intercauseway area on Roberts Bank in the Fraser River estuary, southwestern British Columbia. B. Intercauseway area with seagrass meadow: Zostera marina; Z. japonica; i'iV"'  mixed Z. marina and Z. japonica; unvegetated. Four study sites are shown: T = Treatment Plots (Part A: Twelve-Month Study); 1 = Mixed stand of Zostera species, 2 = Low density of Z. marina  shoots, and  3 =  High density of Z. marina shoots (Part B: Three-Month Study).  9  10 1982, a l s o b l o c k the d i r e c t i o n of ebb t i d e f l o w and as a r e s u l t t h e d u r a t i o n of submergence i n t h e beds has been i n c r e a s e d H a r r i s o n , p e r s o n a l communication,  1984).  (BEAK, 1977;  T h i s e f f e c t i s enhanced  during  the summer when the s e a s o n a l peak i n e e l g r a s s d e n s i t y a l s o r e s u l t s i n the impoundment of water and t h e r e d u c t i o n o f ebb t i d e  flow.  Consequently, even on t h e lowest t i d e s o f t h e y e a r a s h a l l o w l a y e r o f water remains over t h e beds. The c l i m a t e of t h e F r a s e r R i v e r d e l t a i s "mixed" maritime, w i t h c o o l , wet w i n t e r s and warm, d r y summers (Hoos and Packman, 1979).  Mean  annual r a i n f a l l i s 95 cm w i t h peak p r e c i p i t a t i o n i n December (BEAK, 1977).  Average monthly temperatures range from 7.8° - 17.5° C and t h e r e  a r e u s u a l l y fewer than f i f t y days of f r o s t p e r y e a r (BEAK, 1977). most months s u r f a c e s a l i n i t i e s  In  i n the i n t e r c a u s e w a y a r e a range from 21%e  -30%», a l t h o u g h v a l u e s as low as 14%« -15%o have been r e c o r d e d i n August and October (Moody, 1978; Swinbanks,  1979; B i g l e y ,  t u r b i d i t y may a l s o i n c r e a s e when t h e r i v e r  i s i n freshet  and i n c e r t a i n p a r t s of the e e l g r a s s beds, s l i g h t occur d u r i n g t h e summer ( H a r r i s o n ,  1981).  Similarly, (Moody, 1978)  sediment a c c r e t i o n can  1984).  The lowest o f t h e two, d a i l y low t i d e s o c c u r s from  mid-morning  t o mid-afternoon i n the s p r i n g and summer, and near m i d n i g h t d u r i n g t h e f a l l and w i n t e r . The mean t i d a l range i s 3.0m (Canadian Hydrographic S e r v i c e , 1983).  F l o o d c u r r e n t s f l o w northwest, p a r a l l e l t o t h e S t r a i t  of G e o r g i a , w h i l e ebb c u r r e n t s flow out o f t h e i n t e r c a u s e w a y a r e a t o t h e southwest (BEAK, 1977). Swinbanks  (1979) d e s c r i b e d s e v e r a l b i o s e d i m e n t o l o g i c a l zones i n  the intercauseway a r e a o f Roberts Bank i n c l u d i n g , from t h e s h o r e l i n e seaward, a s a l t marsh, an a l g a l mat zone, an upper sand f l a t an e e l g r a s s zone.  zone, and  The upper r e g i o n o f t h e e e l g r a s s zone i s dominated,  11 at l e a s t d u r i n g the s p r i n g and summer, by Z o s t e r a back almost completely  d u r i n g t h e winter,  while  j a p o n i c a , which d i e s  t h e lower r e g i o n i s  dominated year-round by t h e l a r g e r , p e r e n n i a l s p e c i e s , Z. marina. Although the d i s t r i b u t i o n o f both s p e c i e s i s m o d i f i e d  by l o c a l i z e d  i r r e g u l a r i t i e s i n topography such as t i d e p o o l s and channels, Z_^ marina i s u s u a l l y r e s t r i c t e d t o e l e v a t i o n s below mean lower low water ±2.0 m) and Z. j a p o n i c a does not s u r v i v e above mean h i g h e r (MHHW; ±2.7 m) ( H a r r i s o n , 1982a).  (MLLW;  h i g h water  A s i n g l e sampling s t a t i o n was  e s t a b l i s h e d i n a mixed stand o f t h e s e s p e c i e s i n t h e summer of 1984. With t h i s exception  a l l samples  were c o l l e c t e d from t h e lower e e l g r a s s  zone i n areas c o n t a i n i n g o n l y Zj_ marina.  MATERIALS AND METHODS  Part A. The E f f e c t o f Removal of Z o s t e r a marina Shoots on the D i s t r i b u t i o n and Abundance of Amphipods ( h e r e a f t e r r e f e r r e d t o as "The Twelve-Month  Study").  In June 1983, Z o s t e r a marina shoots were t h i n n e d  i n eight  treatment p l o t s t o f o u r d i f f e r e n t r e l a t i v e d e n s i t i e s i n an i n t e r t i d a l study s i t e ("T") i n t h e intercauseway e e l g r a s s meadow ( F i g . 1 ) . Amphipod d i s t r i b u t i o n and abundance were monitored, r e l a t i v e t o t h e s e  12 initial  shoot d e n s i t i e s and t o the s e a s o n a l i t y o f macrophyte  monthly i n t e r v a l s from J u l y 1983  - July  biomass, a t  1984.  1. Sampling Design Environmental v a r i a t i o n s such as e l e v a t i o n , t i d a l  exposure,  sediment t e x t u r e and o r g a n i c c o n t e n t , were m i n i m i z e d by l o c a t i n g the experimental p l o t s i n an a r e a of r e l a t i v e l y u n i f o r m Z. marina shoot d e n s i t y (19 ± 0.4  shoots • 0.25m" ; x ± S.E.; 2  N=212).  The e i g h t , 3x2m  p l o t s were p o s i t i o n e d i n two rows of f o u r , w i t h t h e l o n g a x i s of each row o r i e n t e d i n a d i r e c t i o n p e r p e n d i c u l a r t o the t i d a l c u r r e n t . rows were separated by a d i a g o n a l d i s t a n c e of 12.5m mean e l e v a t i o n was  1.30m  T a r b o t t o n , Swan Wooster  The  ( F i g . 2) a n d ' t h e i r  above c h a r t datum ( s u r v e y d a t a p r o v i d e d by E n g i n e e r i n g L t d . , Vancouver,  M.  1983).  Wooden stakes d r i v e n i n t o the f o u r c o r n e r s o f each 3x2m p l o t were used t o permanently demarcate the study s i t e .  An i n n e r  2xlm was marked i n each p l o t w i t h s m a l l i r o n pegs. "edge e f f e c t s " which may 0.5m  sampling a r e a of  In o r d e r t o minimize  have been c r e a t e d by the wooden stakes, the  p e r i m e t e r of each p l o t was not  sampled.  The t o t a l number of above-ground  Z o s t e r a m a r i n a shoots i n each o  p l o t was  counted u s i n g a hemp g r i d c o n s i s t i n g o f 24, 0.25rrr quadrats  which f a s t e n e d over the f o u r c o r n e r s t a k e s .  Four t r e a t m e n t s , i n c l u d i n g  t h r e e i n which 25, 50, or 75% of t h e shoots were removed, and c o n t r o l , i n which shoot d e n s i t i e s were not changed, a s s i g n e d t o the p l o t s w i t h i n each row  one  were randomly  (see T a b l e 1 ) .  Shoots were  t h i n n e d a c c o r d i n g l y , based on the lowest common d e n s i t y per p l o t i n each row.  Shoots were removed by c u t t i n g through t h e stem a t t h e node  l o c a t e d immediately below the f i r s t  r o o t s . The c u t was made a t t h i s ,  l e v e l i n an attempt t o p r e v e n t f u r t h e r b r a n c h i n g o f t h e p l a n t  (Harrison,  13 F i g u r e 2. Plan o f treatment p l o t s i n P a r t A; Twelve-Month Study. Two rows o f f o u r 3x2m p l o t s were permanently e s t a b l i s h e d a t study s i t e "T". Above-ground  s h o o t s were t h i n n e d t o t h r e e  r e l a t i v e d e n s i t i e s (25,50, and 75% of o r i g i n a l  density)  i n t h r e e p l o t s i n each row. Shoots were not removed from the c o n t r o l p l o t  (100%) i n each row. The p l o t s were  s i t u a t e d 0.5m a p a r t and the two rows were separated by a d i a g o n a l d i s t a n c e of 12.5m. P l a n t and c o r e samples were c o l l e c t e d from a c e n t r a l 2xlm a r e a i n each p l o t t o a v o i d "edge e f f e c t s " , as shown i n t h e diagram i n lower r i g h t of figure.  14  15  T a b l e 1.  Row  D e n s i t i e s o f Z o s t e r a m a r i n a • 0.25 m (x ± S . E . ) , b e f o r e a n d a f t e r t h i n n i n g treatments. Above-ground s h o o t s were t h i n n e d t o 25%, 50%, and 75% r e l a t i v e d e n s i t i e s i n 3 p l o t s i n e a c h row i n J u n e , 1983. Shoots were n o t t h i n n e d i n t h e c o n t r o l ( 1 0 0 % ) p l o t i n e a c h row. -2  Plot  Treatment  PreTreatment Shoot Density  PostTreatment Shoot Density  Mean ± S.E.  Mean ± S.E.  1  Control  18.2 ± 1.3  16.1 ± 1.0  2  25%  19.9 ± 1.1  4.1 ± 0.1  3  75%  16.3 ± 1.4  12.2 ± 0.5  4  50%  16.3 ± 1.0  8.2 ± 0.1  5  Control  17.3 ± 1.0  17.3 ± 1.0  6  50%  20.3 ± 1.4  8.7 ± 0.1  7  75%  22.6 ± 0.9  13.0 ± 0.5  8  25%  20.7 ± 1.5  4.3 ± 0.1  16 p e r s o n a l communication, 1983).  Damaged and  f l o w e r i n g shoots were  removed i n p r e f e r e n c e t o h e a l t h y , v e g e t a t i v e 2.  shoots.  Sampling Procedure Gammaridean amphipods were c o l l e c t e d monthly from f o u r 0.01m  2  quadrats  i n each p l o t from J u l y 1983  February. shoot  In A p r i l  1984,  sampling  - March 1984,  i n one  counts r e v e a l e d t h a t the o r i g i n a l  recovered  and  t h a t t h e r e were no l o n g e r  row  was  with the e x c e p t i o n of d i s c o n t i n u e d when  shoot d e n s i t i e s i n a l l p l o t s had s i g n i f i c a n t d i f f e r e n c e s i n mean  shoot d e n s i t y between most p l o t s (Duncan's M u l t i p l e Range Test  (DMR), a  =0.05).  patterns  of  In order t o p r o v i d e a complete r e c o r d of' the' seasonal  amphipod abundance, c o l l e c t i o n s were c o n t i n u e d  through J u l y  i n the remaining  row  1984.  Samples were randomly c o l l e c t e d w i t h i n the c e n t r a l 2xlm a r e a of each p l o t by p o s i t i o n i n g the c o r n e r s of an aluminum frame g r i d c o n s i s t i n g of 200,  0.01m  2  quadrats  over  the f o u r i n n e r pegs.  Replicate  samples were c o l l e c t e d u s i n g g r i d c o o r d i n a t e s based on a random numbers table.  A l l sample l o c a t i o n s were r e c o r d e d  from b e i n g resampled.  t o prevent  The p l o t s i n each row  the same quadrat  were sampled s e q u e n t i a l l y  to minimize d i s t u r b a n c e . In each p l o t , f o u r sediment c o r e s , two up t o f o u r d r i f t  Z o s t e r a marina shoots,  and  samples c o n s i s t i n g of f r e e - f l o a t i n g a l g a e and/or dead  e e l g r a s s , were c o l l e c t e d .  The number o f d r i f t  samples v a r i e d with  the  season and depended on the biomass of t h e s e p l a n t s i n a g i v e n month. For example, a l g a e were abundant i n s p r i n g and p o s s i b l e to c o l l e c t  four d r i f t  l a t e winter, quadrats c o u l d not be  summer and  samples from each quadrat.  f r e q u e n t l y c o n t a i n e d no d r i f t  collected.  i t was  always  However, i n  algae,, and  samples  17 In order t o i n v e s t i g a t e t h e e f f e c t o f e e l g r a s s on t h e d i s t r i b u t i o n of did  i n f a u n a l amphipods, two o f t h e f o u r quadrats c o l l e c t e d i n each p l o t not c o n t a i n p l a n t s .  When a quadrat which had been chosen t o i n c l u d e  an e e l g r a s s sample d i d not c o n t a i n a p l a n t , t h e f i r s t quadrat i n which a shoot was found i n a predetermined d i r e c t i o n a c r o s s the g r i d was instead.  sampled  The same procedure was f o l l o w e d t o a v o i d p l a n t s i n quadrats  which had been chosen t o c o n t a i n o n l y d r i f t  and sediment.  The c o r i n g d e v i c e was a hand-held, empty c o f f e e can, e n c l o s e d a t one end w i t h s e v e r a l l a y e r s of c h e e s e c l o t h t o p r e v e n t t h e escape o f m o t i l e a n i m a l s . Each c o r e had a diameter o f 10cm and sampled a s u r f a c e a r e a o f 78cm . 2  Samples were c o l l e c t e d as f o l l o w s : 1) above-ground were c u t o f f a t sediment l e v e l and p l a c e d i n s i d e l a b e l l e d bags; 2) a l l d r i f t  e e l g r a s s shoots collecting  m a t e r i a l and p l a n t d e b r i s were scraped from the  sediment s u r f a c e ; and 3) c o r e s were c o l l e c t e d t o a depth o f 10cm.  In  a l l months, sampling was completed w i t h i n t h r e e hours on a s i n g l e day on the  lower o f the two d a i l y low t i d e s .  S p r i n g and summer samples were  c o l l e c t e d near midday, and f a l l and w i n t e r samples were c o l l e c t e d near midnight.  A s i n g l e Coleman l a n t e r n was used t o i l l u m i n a t e the sampling  a r e a d u r i n g the n i g h t 3. P h y s i c a l  collections.  Factors  On each sampling d a t e , a i r and water temperatures were r e c o r d e d and water samples were c o l l e c t e d f o r s a l i n i t y d e t e r m i n a t i o n s . was measured i n p a r t s p e r thousand  Salinity  (%•) u s i n g a Y e l l o w S p r i n g s  Instrument Corp. s a l i n i t y / c o n d u c t i v i t y / t e m p e r a t u r e meter, Model 33. Two r e p l i c a t e c o r e samples, f o r d e t e r m i n a t i o n of sediment o r g a n i c c o n t e n t , were c o l l e c t e d from each e x p e r i m e n t a l p l o t i n October, January,  18 April,  and June, u s i n g a P l e x i g l a s c o r e r w i t h an i n n e r diameter of  4.5cm.  Each c o r e was s e p a r a t e d i n t o two depth f r a c t i o n s  (0-5cm; 5-10cm)  which were a n a l y z e d s e p a r a t e l y . The samples were d r i e d a t 60° C f o r one week, ground t o a u n i f o r m c o n s i s t e n c y w i t h a mortar and p e s t l e and d r i e d f o r another 24 hours.  Subsamples  were d r i e d f o r two hours a t 100° C and  then ashed a t 550° C f o r two h o u r s .  The o r g a n i c c o n t e n t was determined  from the d i f f e r e n c e i n d r y weight b e f o r e and a f t e r  incineration.  In o r d e r t o determine i f t h e s i z e d i s t r i b u t i o n of sediment p a r t i c l e s among p l o t s i n a g i v e n month was the same, t h r e e  replicate  c o r e s were sampled i n f o u r o f the e x p e r i m e n t a l p l o t s i n June 1984. of the c o r e s was i n d i v i d u a l l y bagged  and f r o z e n .  Each  The thawed c o r e s were  l a t e r d r i e d a t 100° C f o r 24-48 hours t o c o n s t a n t weight and ground i n a mortar and p e s t l e .  P o r t i o n s of each, weighing 60-90 g, were s i e v e d on  an E n d i c o t t ' s shaker f o r t e n minutes u s i n g a s e r i e s of f i v e s i e v e s with mesh s i z e s of 0.595, 0.355, 0.180, 0.075, and 0.053 mm.  Graphs of the  cumulative d r y weight p e r c e n t a g e s r e t a i n e d on each s i e v e were drawn and e s t i m a t e s of the median p a r t i c l e diameter and the p e r c e n t a g e of s i l t clay  (<0.060 mm),  v e r y f i n e sand (0.060-0.125 mm),  0.250 mm),  and c o a r s e sand (0.250-0.500 mm)  (Harrison,  1984).  4.  f i n e sand  (0.125-  f r a c t i o n s , were made  L a b o r a t o r y Procedures All  samples were r e t u r n e d t o the l a b o r a t o r y w i t h i n f i v e hours of  c o l l e c t i o n and s t o r e d o v e r n i g h t i n a cold-room a t 0° C, w i t h t h e e x c e p t i o n of p l a n t  samples c o l l e c t e d i n the summer of 1984, which were  19 frozen.  F r e e z i n g d i d not appear t o damage e i t h e r the p l a n t s o r the  a n i m a l s and b o t h c o u l d l a t e r be r e a d i l y Within freshwater  identified.  24 hours a l l p l a n t samples a t 0 ° C were washed with  over a 0.5mm mesh s t a i n l e s s s t e e l s i e v e .  T h i s mesh s i z e was  s m a l l enough t o c o l l e c t a r e p r e s e n t a t i v e sample o f s m a l l s p e c i e s and j u v e n i l e s without Stoner,  1980).  r e s u l t i n g i n a l a r g e i n c r e a s e i n s o r t i n g time (Lewis &  A l l diatoms, h y d r o i d s , and e n c r u s t i n g e p i f a u n a were  c a r e f u l l y scraped  from the Z. marina b l a d e s .  The m a t e r i a l that was r e t a i n e d on the s i e v e was suspended i n seawater, examined with a d i s s e c t i n g microscope, and s o r t e d . A l l gammarid amphipods were counted, i d e n t i f i e d , and removed f o r f i x a t i o n i n 5% b u f f e r e d f o r m a l i n .  They were l a t e r t r a n s f e r r e d t o , and s t o r e d i n , a  s o l u t i o n o f 70% i s o p r o p a n o l and 5% g l y c e r o l . The  number o f b l a d e s , mean b l a d e l e n g t h , and l e n g t h o f the  b l a d e per Z o s t e r a marina p l a n t , were r e c o r d e d . i n t o i t s component s p e c i e s . (to  constant The  longest  The d r i f t was separated  Each sample was d r i e d a t 60° C f o r 48 hours  weight).  s u r f a c e a r e a o f t h e e e l g r a s s and a l l d r i f t  was e s t i m a t e d by  c o n s t r u c t i n g r e g r e s s i o n l i n e s based on the measured dimensions and weights o f subsamples o f each p l a n t s p e c i e s e x c e p t i o n o f the Z o s t e r a marina sheath, were c o n s i d e r e d  t o be laminar,  a v a i l a b l e f o r settlement  (Table 2 ) .  dry  With the  a l l p o r t i o n s of these  plants  t h a t i s , t o have o n l y two s u r f a c e s  o r attachment o f a n i m a l s .  The s u r f a c e a r e a o f  the e e l g r a s s sheath was determined by c o n s i d e r i n g t h i s p o r t i o n of t h e plant as a three-dimensional  r e c t a n g l e and m u l t i p l y i n g the product o f  the l e n g t h , w i d t h , and h e i g h t by a f a c t o r o f two. Core samples were washed i n t o a 0.5mm mesh s i e v e and was  preserved  a l l residue  i n 7% b u f f e r e d f o r m a l i n and Rose Bengal, a v i t a l  stain.  20  T a b l e 2.  R e l a t i o n s h i p between s u r f a c e a r e a (cm ) and d r y w e i g h t (g) o f m a c r o p h y t e s p e c i e s c a l c u l a t e d from l i n e a r r e g r e s s i o n equations. R = regression coefficient (p < 0 . 0 5 ) . 2  Surface a r e a (cm ) /Dry Weight (g)  R  Zostera marina (live)  457  0.971  Zostera marina (drift)  537  0.965  Zostera japonica (May & J u n e )  263  0.839  Zostera japonica (July)  295  0.907  Enteromorpha sp.  282  0.7 67  Laminaria saccharina  575  0.994  1288  0.953  Macrophyte Species  Ulva sp.  2  21 These s i e v e d samples were l a t e r  suspended  i n f r e s h water, s o r t e d under a  d i s s e c t i n g microscope, and a l l amphipods were counted, i d e n t i f i e d , p r e s e r v e d i n a s o l u t i o n of 70% i s o p r o p a n o l and' 5%' g l y c e r o l .  and  In o r d e r t o  a s s e s s the e f f e c t of rhizome d e n s i t y on the d i s t r i b u t i o n of amphipods i n the sediment, a l l rhizomes p r e s e n t i n t h e c o r e s were d r i e d a t 60° C f o r 48 hours and weighed  ( f o r samples c o l l e c t e d from November 1983  - July  1984). 5. S t a t i s t i c a l  Analysis  The number of amphipods and amphipod s p e c i e s c o l l e c t e d i n each e e l g r a s s , d r i f t , and c o r e sample-was r e c o r d e d .  To a l l o w t h e comparison  of animal d e n s i t i e s between the two types of p l a n t  samples,  abundances  were s t a n d a r d i z e d with r e f e r e n c e t o s u r f a c e a r e a . The t o t a l number of i n d i v i d u a l s c o l l e c t e d i n each 0.01m quadrat was a l s o determined by 2  p o o l i n g a b s o l u t e counts from each of the a s s o c i a t e d p l a n t and c o r e samples. Amphipods were i d e n t i f i e d w i t h r e f e r e n c e t o t h e f o l l o w i n g authorities:  (Crawford, 1937;  B o u s f i e l d , 1973,  Barnard, 1954,1969; M i l l s ,  1979; O t t e , 1975;  1961;  Smith & C a r l t o n , 1975; Conlan &  B o u s f i e l d , 1982; Conlan, 1983). S p e c i e s d i v e r s i t y was  c a l c u l a t e d u s i n g the Shannon-Wiener  d i v e r s i t y index:  H'  =  - Z  P i  In P i  where p^ i s the p r o p o r t i o n of i n d i v i d u a l s i n t h e i t h s p e c i e s r e l a t i v e t o the t o t a l number o f i n d i v i d u a l s i n the and  evenness:  sample,  22 J'  =  H' H'  where H' max  = In S, and  community ( P i e i o u ,  max  S i s the t o t a l number of s p e c i e s i n the  1975).  A Model 1, two-way a n a l y s i s of v a r i a n c e (ANOVA) was the h y p o t h e s i s  t h a t t h e r e was  abundance * 0.01m 1974).  no s i g n i f i c a n t d i f f e r e n c e i n mean amphipod  between months and  -2  used t o t e s t  shoot d e n s i t y treatments  Model 1, three-way a n a l y s e s of v a r i a n c e were used to t e s t  s i m i l a r hypotheses r e l a t i n g t o amphipod d e n s i t y , d i v e r s i t y , and between a l l months, treatments, and  sediment) (Zar, 1974).  and  s u b s t r a t e types  and  tested using  i f h e t e r o s c e d a s t i c i t y was  square r o o t or l o g t r a n s f o r m a t i o n s were performed on the d a t a . a p o s t e r i o r i Student-Newman-Keuls (SNK) comparison t e s t was  or a Duncan's (DMR)  present, An  multiple  Other comparisons, f o r example  the r e l a t i o n s h i p between rhizome biomass and amphipods, were made with a Students' was  the abundance of i n f a u n a l  t - T e s t (Zar, 1974). A p r o b a b i l i t y  used i n a l l s t a t i s t i c a l  Amphipod d e n s i t y was biomass and  the  used t o d i f f e r e n t i a t e means which the ANOVA  i n d i c a t e d were s i g n i f i c a n t l y d i f f e r e n t .  l e v e l of P < 0.05  evenness  (eelgrass, d r i f t ,  Homogeneity of v a r i a n c e was  Fmax-Test ( S o k a l and R o l f , 1973)  tests.  a l s o examined as a f u n c t i o n of macrophyte  surface area using c o r r e l a t i o n a n a l y s i s .  i n d i v i d u a l s c o l l e c t e d i n each e e l g r a s s , d r i f t , two  (Zar,  The number of  and core sample i n the  c o n t r o l p l o t s i n each month were compared with t o t a l e e l g r a s s  with t o t a l d r i f t , biomass, and  surface area.  A l l d a t a were  and  transformed  u s i n g square r o o t s . T h i s t r a n s f o r m a t i o n i s recommended f o r use i n r e g r e s s i o n a n a l y s i s i n s i t u a t i o n s i n which the data a r e i n the form of counts, p a r t i c u l a r l y s m a l l counts (Zar, 1984).  23 P r i n c i p a l components a n a l y s i s  (PCA) was a l s o used t o look f o r  changes i n community s t r u c t u r e , over time and i n t h e t h r e e d i f f e r r e n t substrate  types. This  type o f a n a l y s i s  summarizes m u l t i d i m e n s i o n a l d a t a  by p r o j e c t i n g i t i n t o a low-dimensional o r d i n a t i o n s i m i l a r species  space i n which  a r e c l o s e t o g e t h e r and d i s s i m i l a r s p e c i e s  (Gauch, 1982; P i e l o u ,  1984).  The v a r i a b l e s of t h e c o r r e l a t i o n m a t r i x  used i n t h i s a n a l y s i s , r e p r e s e n t i n g i n each sample, were s t a n d a r d i z e d  numbers of i n d i v i d u a l s * s p e c i e s  species by  - 1  t o permit the comparison of d a t a  c o l l e c t e d i n both t h e p l a n t and sediment s u b s t r a t e s Standardization  a r e f a r apart  ( P i e l o u , 1984).  a l s o reduces t h e h e t e r o g e n e i t y o f v a r i a n c e s among  ( P i e l o u , 1984). The v a r i a b i l i t y of the d a t a was reduced  including only  t h e most f r e q u e n t l y  c o l l e c t e d species  In order t o determine t h e temporal p a t t e r n s  of  further  i n the a n a l y s i s .  species-associations,  component scores o f t h e p r i n c i p a l axes, i d e n t i f i a b l e on t h e b a s i s o f substrate 6.  type, were p l o t t e d through time (Pimental, 1979).  Correlation  Matrices  C o r r e l a t i o n m a t r i c e s , based on the abundance of i n d i v i d u a l s i n each s p e c i e s  per sample, were c o n s t r u c t e d  of h i e r a r c h i c a l c o m p e t i t i v e i n t e r a c t i o n s of a n a l y s i s ,  (Nelson, 1979a).  In t h i s type  s i g n i f i c a n t n e g a t i v e c o r r e l a t i o n s between s p e c i e s  i n d i c a t e t h a t one s p e c i e s included  t o i n v e s t i g a t e the- p o s s i b i l i t y  i s d i s p l a c i n g another.  The f i r s t  may  matrix  t h e mean t o t a l number o f amphipods c o l l e c t e d i n each  species,  p o o l e d from a l l c o l l e c t i o n s w h i l e the second and t h i r d m a t r i c e s only  the mean abundances o f t h e f i v e most f r e q u e n t l y  on each c o l l e c t i o n d a t e , and s u b s t r a t e  type,  collected  respectively.  included species  24  P a r t B.  The R e l a t i o n s h i p Between N a t u r a l Shoot D e n s i t i e s and t h e  D i s t r i b u t i o n and Abundance o f Amphipods ( h e r e a f t e r r e f e r r e d t o as "The Three-Month  Study").  Samples were c o l l e c t e d i n t h r e e areas o f d i f f e r e n t n a t u r a l d e n s i t i e s of Z o s t e r a marina shoots 1984  a t monthly i n t e r v a l s from May - J u l y  i n order t o i n v e s t i g a t e f u r t h e r t h e r e l a t i o n s h i p between t h e  d i s t r i b u t i o n and abundance o f amphipods and t h e h e t e r o g e n e i t y o f t h e seagrass h a b i t a t . 1.  Design o f Study S i t e s Three, 10x5m sampling  of obvious  areas were chosen i n A p r i l  1984 on t h e b a s i s  d i f f e r e n c e s i n the degree o f Z o s t e r a marina cover, and were •  permanently marked with wooden s t a k e s .  The f i r s t  s t a t i o n was l o c a t e d i n  the upper zone of e e l g r a s s d e s c r i b e d by Swinbanks (1979) i n a r e g i o n o f sparse Z. marina cover, 1). The  (approximately  t h r e e shoots  * 0.25m  The annual s p e c i e s , Z. j a p o n i c a , was a l s o p r e s e n t  ) (Fig.  i n this  area.  second a r e a was l o c a t e d i n a r e g i o n of moderate Z. marina cover (9.7  ± 3.8 shoots first  area  ' 0.25m~  ( F i g . 1).  2  (x ± S . E . ) ) , a p p r o x i m a t e l y  Although  200m seaward o f t h e  t h e e e l g r a s s was d i s t r i b u t e d  relatively  e v e n l y through t h i s a r e a t h e r e were s m a l l unvegetated p a t c h e s which may have been caused by t o p o g r a p h i c a l i r r e g u l a r i t i e s o r l o c a l i z e d  patterns  of water movement.  The t h i r d  density  (28.4  • 0.25m- ) was l o c a t e d w i t h i n 10m o f t h e p l o t s  ± 15.2 shoots  d e s c r i b e d i n Part A ( F i ' g . l ) .  2  s t a t i o n , w i t h t h e h i g h e s t shoot  25 2.  Sampling  Procedure  In May, June, and J u l y 1984, amphipods were c o l l e c t e d i n t h r e e randomly  l o c a t e d 0.25m quadrats i n each of t h e t h r e e a r e a s . 2  quadrat sampling c o n s i s t e d o f two sediment marina above-ground  shoots, and two d r i f t  Each  c o r e s , two a s s o c i a t e d Z o s t e r a samples,  randomly  located  •p  w i t h i n the quadrat u s i n g a g r i d o f 24, 0.01m  quadrats.  With a few  e x c e p t i o n s , t h e sampling p r o c e d u r e was t h e same as t h a t d e s c r i b e d i n P a r t A: The Twelve-Month Study. P r i o r t o t h e removal of p l a n t and c o r e samples,  a l l Z o s t e r a marina  and Z. j a p o n i c a shoots w i t h i n t h e quadrat were counted. of  Twenty p l a n t s  each s p e c i e s i n Area 1, and o f Z. marina i n Areas 2 and 3, were  c o l l e c t e d each month on the p e r i m e t e r o f each a r e a f o r the c a l c u l a t i o n of  mean s u r f a c e a r e a and mean d r y weight per p l a n t .  A l l Z. j a p o n i c a  o  encountered i n s i d e the O.Olirr quadrats i n Area 1 were c o l l e c t e d w i t h t h e drift  component of the sample. In  o r d e r t o ensure t h a t a l l amphipod s p e c i e s p r e s e n t were  c o l l e c t e d i n t h e quadrat samples, were c o l l e c t e d  two " n a t u r a l i s t  sled"  ( F i g . 3) samples  j u s t o u t s i d e each a r e a i n each month, by towing t h e s l e d ,  which c a p t u r e d b e n t h i c as w e l l as demersal fauna, i n t o the c u r r e n t f o r a d i s t a n c e of 2m.  These tows were made a t low t i d e a t the same time as  the  quadrat samples  were b e i n g c o l l e c t e d . A f t e r each tow the c o n t e n t s o f  the  n e t were washed i n t o a 0.5mm mesh s i e v e w i t h f i l t e r e d  the  m a t e r i a l r e t a i n e d on the s i e v e was p l a c e d i n a c o l l e c t i n g j a r .  seawater and  9  Shoot counts were made i n a t l e a s t t h r e e 0.25m the  quadrats i n t h e path o f  tow and t h e s e counts were p o o l e d w i t h t h o s e made w i t h i n t h e sampling  a r e a s f o r t h e d e t e r m i n a t i o n o f mean monthly  shoot d e n s i t y .  Because  Z o s t e r a j a p o n i c a shoots were so s p a r s e l y d i s t r i b u t e d i n May i n Area 1,  Figure 3. Naturalist Sled.  27  28 they d i d not o c c u r i n sample quadrats and consequently were counted i n June and J u l y o n l y . 3.  Physical Factors A i r and water temperatures  were r e c o r d e d , and water samples were  c o l l e c t e d f o r s a l i n i t y measurements, on each sampling d a t e . determine  whether or not t h e r e was  In order t o  any v a r i a t i o n i n the depth  submergence between areas d u r i n g low t i d e , water depth was  of  measured  synchronously i n each of the t h r e e areas on J u l y 27 d u r i n g s l a c k water on one of the lowest  t i d e s of the y e a r .  A sediment c o r e f o r the d e t e r m i n a t i o n of percentage c o l l e c t e d i n each 0.25m Month Study.  as d e s c r i b e d i n P a r t A: The  was  Twelvestudy  f o r the d e t e r m i n a t i o n of the s i z e d i s t r i b u t i o n of  sediment p a r t i c l e s .  Laboratory  quadrat,  Three r e p l i c a t e c o r e s were sampled i n each of the  areas i n June 1984  4.  2  organics  These c o r e s were a n a l y z e d as d e s c r i b e d i n Part A.  Procedures  A l l core and p l a n t samples were p r o c e s s e d and a n a l y z e d d e s c r i b e d i n P a r t A.  W i t h i n f i v e hours of c o l l e c t i o n , the  s l e d " samples were washed i n t o a 0.5mm s i e v e and  as  "naturalist  s i e v e c o n t e n t s were  p r e s e r v e d i n 7% b u f f e r e d f o r m a l i n and Rose Bengal. Amphipods from a l l c o l l e c t i o n s were i d e n t i f i e d , counted, the e x c e p t i o n of the " n a t u r a l i s t  and  s l e d " samples, head l e n g t h and  with  life-  stage d a t a were r e c o r d e d . The  s u r f a c e areas of l i v i n g and dead Z o s t e r a marina,  and  live  Z.  j a p o n i c a , U l v a sp., and Enteromorpha sp. were a l s o c a l c u l a t e d i n the same manner as d e s c r i b e d i n Part A and a r e shown i n T a b l e 2. t h a t the r e l a t i o n s h i p between macrophyte biomass and  To  surface area  ensure was  c o n s i s t e n t w i t h i n a s p e c i e s between months, the s l o p e s of the r e g r e s s i o n  29 l i n e s were compared (Zar, 1974).  I f s i g n i f i c a n t d i f f e r e n c e s i n the  slopes o c c u r r e d , as they d i d f o r Z.  j a p o n i c a i n June and  July,  i n d i v i d u a l monthly s u r f a c e a r e a v a l u e s were used i n t h e s e determinations. 5.  S t a t i s t i c a l Analysis As i n P a r t A: The Twelve-Month Study, the t o t a l number of  i n d i v i d u a l s and  s p e c i e s of amphipods i n each sample were r e c o r d e d  amphipod d e n s i t i e s were determined r e l a t i v e t o macrophyte o r s u r f a c e a r e a . A Model 1, two-way a n a l y s i s of v a r i a n c e was the h y p o t h e s i s  t h a t t h e r e was  abundance • 0.01m shoot d e n s i t y .  -2  no  between months and  used to t e s t  Three-way a n a l y s e s  a r e a s of d i f f e r e n t Z o s t e r a marina  of v a r i a n c e were used t o t e s t f o r  evenness ( J ' ) between months, shoot d e n s i t i e s , and  variances.  sediment  s i g n i f i c a n t d i f f e r e n c e i n mean amphipod  d i f f e r e n c e s among means of amphipod d e n s i t y , d i v e r s i t y  Square r o o t and  and  log transformations  (H'),  substrate  and types.  were used t o c o r r e c t h e t e r o s c e d a s t i c  M u l t i p l e comparison t e s t s  (Student-Newman-Keul's or  Duncan's) were used t o d i f f e r e n t i a t e means which the ANOVA i n d i c a t e d were s i g n i f i c a n t l y d i f f e r e n t . P < 0.05  probability  A l l s i g n i f i c a n c e was  the  level.  C o r r e l a t i o n a n a l y s i s was  used t o examine amphipod d e n s i t y as a  f u n c t i o n of macrophyte biomass and 6.  determined at  s u r f a c e area, as d e s c r i b e d i n Part  A.  C o r r e l a t i o n Matrices As d e s c r i b e d i n Part A: The  Twelve-Month Study,  correlation  m a t r i c e s , based on the abundance o f i n d i v i d u a l s i n each amphipod s p e c i e s per  sample, were c o n s t r u c t e d t o i n v e s t i g a t e i n t e r s p e c i f i c  The  f i r s t matrix  interactions.  i n c l u d e d the mean t o t a l number of amphipods c o l l e c t e d  i n each s p e c i e s , per  sample, p o o l e d  from a l l c o l l e c t i o n s .  The  second  30 and t h i r d m a t r i c e s i n c l u d e d the monthly mean abundances of the f o u r dominant s p e c i e s per sample, c o l l e c t e d i n each o f the t h r e e study a r e a s (shoot d e n s i t i e s ) and s u b s t r a t e ' t y p e s , r e s p e c t i v e l y .  P a r t C.  L i f e C y c l e s and S i z e - F r e q u e n c y D i s t r i b u t i o n s  In o r d e r t o o b t a i n i n f o r m a t i o n on t h e s i z e frequency d i s t r i b u t i o n and seasonal r e p r o d u c t i v e p a t t e r n s , a l l amphipods c o l l e c t e d i n two treatment p l o t s per month ( P a r t A) were examined.  Where sample numbers  were s m a l l ( l e s s than 50 a n i m a l s ) , a l l i n d i v i d u a l s were examined and d e s c r i b e d . In l a r g e r  samples a t l e a s t two  respect to l e f t - r i g h t s p l i t t e r and counted. was  splits, A X  2  subsamples,  were removed u s i n g a Folsom  Plankton  t e s t f o r homogeneity (a =0.05)(Zar,1974)  used t o compare p r e l i m i n a r y samples and  the s p l i t t e r was  balanced with  unbiased and b a l a n c e d .  subsamples t o ensure  that  The counts were found t o be  homogeneous and the number of animals i n each sample was  estimated using  the e q u a t i o n :  N- =• 2  T  Z X /n i  = 2  T  X  where T i s the number of s p l i t s made, n i s the number of subsamples, and X^ i s the number of organisms counted i n the i - t h subsample ( G r i f f i t h s All  et a l . ,  1984).  s i z e s r e f e r r e d t o i n the study r e p r e s e n t head l e n g t h which  was  measured from t h e a n t e r i o r t i p of the rostrum t o the j u n c t i o n of the first  t h o r a c i c segment.  l e n g t h because  Head l e n g t h i s a good index of t o t a l body  i t i s easy t o measure and i s independent  (Nelson, 1980c).  of body f l e x i o n  The r e l a t i o n s h i p between head l e n g t h and body l e n g t h ,  measured from the t i p of the rostrum t o t h e d i s t a l end of the t e l s o n ,  31 was examined for a subsample of individuals for each species using linear regression analysis.  These data are presented i n Appendix 1.  For each species, f i v e l i f e stages were i d e n t i f i e d : 1) juveniles or  immatures: individuals i n which sex could not be determined;  2)  nonreproductive females: females without oostegites (brood plates), or without setae on their oostegites; 3) reproductive females: females with setae on their oostegites; 4) ovigerous females: females with eggs or newly hatched juveniles within their marsupia; 5) males: i d e n t i f i e d by their d i f f e r e n t i a l gnathopod or antennal morphology. Length frequency - l i f e stage histograms were constructed for the four most abundant amphipod species by pooling the t o t a l individuals i n a l l samples i n a given month.  32  RESULTS  Part A. The Twelve-Month Study  1. P h y s i c a l  Factors  A i r temperatures r e c o r d e d a t low t i d e ranged from a low of -2° C i n l a t e December t o a h i g h of 24° C i n J u l y 1984. below  Temperatures  remained  10° C from November - March and above 13° C from A p r i l - October  (Fig.4).  Water temperatures were u s u a l l y s l i g h t l y h i g h e r than a i r  temperatures, p a r t i c u l a r l y d u r i n g the summer months when low  tide  c o i n c i d e d w i t h the h o t t e s t p a r t of the day and t h e shallow l a y e r of water t h a t remained on the beds was warmed f o r s e v e r a l hours by the sun. Water temperatures ranged from <1°C i n December t o 26°C i n J u l y During the t h i r d week of December 1983,  temperatures f e l l  (Fig.4). t o below  f r e e z i n g f o r s e v e r a l c o n s e c u t i v e days and on the evening of December 21 the water i n the intercauseway a r e a was  covered by a t h i n sheet of i c e ,  which became d i s l o d g e d as the t i d e t u r n e d and f l o a t e d out of the bay on the ebb t i d e i n l a r g e floes..  The sediment was  f r o z e n t o a, depth of 2 cm  i n the e e l g r a s s beds. Lowest monthly s u r f a c e s a l i n i t i e s were r e c o r d e d i n August (16% ) and i n June  (20% ) d u r i n g the time of the F r a s e r R i v e r  1983  freshet  ( F i g . 5 ) . The h i g h e s t s a l i n i t y was r e c o r d e d i n December (34% ).  With  these e x c e p t i o n s , s u r f a c e s a l i n i t i e s remained between 23 - 27% , s i m i l a r to the v a l u e s found by Swinbanks (1979) over most of the intercauseway tidal  flat. The sediments of the study a r e a were c l a s s i f i e d as f i n e , t o v e r y  fine,  sands w i t h a mean g r a i n diameter r a n g i n g from 0.12-0.15 mm  (Table  33 F i g u r e 4. Monthly a i r and water temperatures (° C) a t study s i t e Dates on the  a b s c i s s a i n the Figure  "T".  (and,in a l l Figures  i n Part A: Twelve-Month Study) c o r r e s p o n d t o J u l y 11, August 6, September 4, October 25, November 23, December 21,1983 and January 20, March 1, A p r i l 27, and J u l y 27,  F i g u r e 5. Monthly s a l i n i t y  15,  June  1984.  (% ) a t s i t e "T".  c o l l e c t e d a t low  17, May  tide.  Water samples were  34  35  T a b l e 3• P a r t i c l e - s i z e d i s t r i b u t i o n o f s e d i m e n t s c o l l e c t e d f r o m t r e a t m e n t p l o t s (June 27, 1 9 8 4 ) . V a l u e s a r e mean p e r c e n t a g e d r y w e i g h t i n e a c h s i z e c l a s s (n=3). VFS = v e r y f i n e s a n d FS = f i n e s a n d .  Size Class (mm)  Control Plot 1 (VFS)  25% Plot 2 (FS)  75% Plot 3 (FS)  50% Plot 4 (FS)  > 0.595  < 1  < 1  < 1  < 1  0.355-0.595  1  1  0.180-0.355  85  79  82  86  0.075-0.180  8  17  12  8  0.053-0.075  1  < 1  < 1  < 1  < 0.053  4  2  4  4  1  1  36  3).  S i l t - c l a y f r a c t i o n s made up 8.0-11.0 p e r c e n t o f t h e sample d r y  weight. The mean o r g a n i c content o f t h e sediments i n t h e treatment p l o t s ranged from 2.1-1.4 % d r y weight i n the 0-5 cm c o r e f r a c t i o n and from 1.4-1.1 % i n t h e 5-10 cm c o r e f r a c t i o n , d e c r e a s i n g s i g n i f i c a n t l y October t o A p r i l  from  (ANOVA, p<0.05; SNK, a=0.05) ( T a b l e 4 ) . 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 i n the o r g a n i c content o f t h e sediment  during  A p r i l and June, and among a l l months i n samples c o l l e c t e d from t h e e i g h t plots  (ANOVA, p>0.05).  The p e r c e n t a g e o f o r g a n i c m a t e r i a l i n t h e 0-5 cm  f r a c t i o n was g r e a t e r than i t was i n the 5-10 cm f r a c t i o n i n a l l months. 2.  Growth o f Z o s t e r a marina The annual growth c y c l e of Z o s t e r a marina was r e f l e c t e d i n t h e  seasonal t r e n d i n mean d r y weight c o l l e c t e d per O.Olirr quadrat i n t h e ' treatment p l o t s from J u l y 1983 - J u l y 1984 ( F i g . 6 ) . peaked  Plant  biomass  from J u l y - October and d e c l i n e d i n November, i n d i c a t i n g t h a t t h e  shoots had begun t h e i r annual d i e - b a c k ( F i g . 6 ) .  The growth o f new  shoots, which o r i g i n a t e s from t h e rhizome, began i n l a t e January and was r e f l e c t e d i n t h e i n c r e a s e i n the number o f b l a d e s per p l a n t from January - March, a t a time when shoot d r y weight and t h e mean l e n g t h of t h e l o n g e s t b l a d e p e r shoot were a t t h e i r minimum v a l u e s ( F i g s . 6, 7 and 8 ) . As t h e young shoots c o n t i n u e d to. grow through t h e s p r i n g , t h e d r y weight, b l a d e number and mean b l a d e l e n g t h a l s o i n c r e a s e d .  Mean d r y  weight per Z. marina shoot i n J u l y 1983 was not s i g n i f i c a n t l y  different  than i t was i n J u l y 1984 (ANOVA, p^0.05). The mean d r y weight of Z o s t e r a marina rhizomes per c o r e was s i g n i f i c a n t l y g r e a t e r i n June than i t was i n January  (Fig. 9).  37  T a b l e 4.  Core Fraction  0-5 cm  5-10 cm  P e r c e n t a g e o r g a n i c c o n t e n t (x ± S.E.) i n . t h e sediment core f r a c t i o n s collected i n t r e a t m e n t p l o t s f r o m O c t o b e r , 1983 - J u n e , 1984. D i f f e r e n c e s were s i g n i f i c a n t among a l l months e x c e p t A p r i l and June f o r t h e 0-5 cm f r a c t i o n . J a n u a r y was a l s o s i g n i f i c a n t l y d i f f e r e n t f r o m June i n t h e 5-10 cm f r a c t i o n . (ANOVA, p < 0.05; SNK, a = 0.05) .  October n=64  January n=48  April n=24  June n=24  ± 0. 1  1.5 ±< 0. 1  1 .4 ±< 0. 1  1. 4 ±< 0 .1  1.4 +< 0. 1  1.2 ±< 0. 1  1 .1 +< 0. 1  1. 1 ±< 0 .1  2.1  38 F i g u r e 6. Dry weight of above-ground Z o s t e r a marina shoots (x ± i n monthly samples a t study s i t e "T". (n = 16, J u l y - March; 8, A p r i l  - July  S.E.) 1983  1984).  F i g u r e 7. Number o f b l a d e s per Z o s t e r a marina shoot (x ±= S.E.) i n monthly samples a t study s i t e "T". March; 8, A p r i l  - July  1984).  (n = 16, J u l y 1983  -  40 F i g u r e 8. Length o f l o n g e s t b l a d e per Z o s t e r a marina shoot (x ± i n monthly samples a t study s i t e "T". - March;  8, A p r i l  - July  (n = 16, J u l y  April;  13, May;  1983  1984).  F i g u r e 9. Dry weight of Z o s t e r a marina rhizomes * 0.01m at study s i t e "T".  S.E.)  -z  (x ±  (n = 26, January; 24, March;  14, June and  July).  12,  S.E.)  4 1  42  However, s i n c e the March samples c o n t a i n e d the lowest rhizome dryweights  and the average d r y weight per c o r e was. s i g n i f i c a n t l y g r e a t e r i n  A p r i l than i t was  i n May,  growth c o u l d be i d e n t i f i e d 3. Composition  no d e f i n i t e s e a s o n a l t r e n d i n below-ground (ANOVA, p>0.05;SNK, a=0.05) ( F i g . 9 ) .  and S e a s o n a l i t y of  Drift  The most abundant components of the d r i f t sp. and fragments of Z o s t e r a marina b l a d e s s m a l l amount of decomposing e e l g r a s s was throughout  the year..  o c c u r r e d i n May, and  were f r e e - f l o a t i n g U l v a  (Fig.lOA & B).  p r e s e n t a t the sediment s u r f a c e  However., peak, accumulations  of t h i s , d e b r i s  June, and J u l y when the t u r n o v e r r a t e s f o r l i v e  l e a v e s were the h i g h e s t ( H a r r i s o n , 1984)  (Fig.lOA).  fragments were g e n e r a l l y e n c r u s t e d w i t h f i l a m e n t o u s and diatoms,  A  and meiofauna such as nematodes were common.  h y d r o i d s and e p i p h y t i c a l g a e such as Smithora  The  shoots  blade  s i n g l e pennate Occasionally,  naiadum were a l s o a t t a c h e d  t o the blade s u r f a c e s . D r i f t U l v a sp. f i r s t appeared  i n the intercauseway  study s i t e i n l a t e March and i t s biomass peaked i n May  a r e a and  the  ( F i g . 10B).  During t h i s time the l a r g e b r i g h t green f r o n d s were so d e n s e l y d i s t r i b u t e d throughout  the study a r e a t h a t t h e y p r o v i d e d a  canopy over the sediment, among the e e l g r a s s p l a n t s . d e t e r i o r a t e i n l a t e June.  The a l g a e began t o  By J u l y the c o l l e c t i o n of d r i f t  c o n s i s t e d of many d i s c o l o u r e d , decomposing b l a d e fragments. was  c o l l e c t e d from November t o A p r i l Other a l g a l  continuous  algae No U l v a sp.  ( F i g . 10B).  s p e c i e s o c c u r r i n g i n the low i n t e r t i d a l a r e a and  samples i n c l u d e d Enteromorpha sp. and L a m i n a r i a s a c c h a r i n a .  drift  The  f l o a t i n g mats of Enteromorpha sp. were not o b s e r v e d as f r e q u e n t l y as  was  43 F i g u r e 10A.  Dry weight of dead Z o s t e r a marina* (x ± S.E.)  in drift  0.01m  2  samples at, study s i t e  "T". (N = 16).  10B.  Dry weight of U l v a sp. ' O.Olnf^ in d r i f t (N = 16).  samples a t study s i t e  (x + "T".  S.E.)  44  45  Ulva  sp. i n the study area, and  April,  June, and  July d r i f t  saccharina, often attached cockle  (Clinocardium  o n l y small amounts were c o l l e c t e d i n the  samples.  In A p r i l and May,  young L.  by t h e i r h o l d f a s t s t o the v a l v e s of empty  n u t t a l l i ) s h e l l s , were c o l l e c t e d i n the  drift  samples. 4.  E f f e c t of Shoot Removal on the Growth of Z o s t e r a  marina  Because I assumed t h a t t h i n n i n g the Z o s t e r a marina by c u t t i n g through the stem at sediment l e v e l would slow shoot p r o d u c t i o n i n any  case,  winter,  few  new  occurred  the shoots i n the e i g h t treatment p l o t s u n t i l  F i g u r e s 11 and  between June 1983  12 and  show the i n c r e a s e i n shoot d e n s i t y which June 1984  in a l l plots.  appears t h a t t h i n n i n g the e e l g r a s s i n t h i s manner may stimulated, 1984,  r a t h e r than r e t a r d e d ,  the p r o d u c t i o n  o n l y P l o t 2 (25% treatment) and  regained  their original  than i t had  been b e f o r e  It therefore  have a c t u a l l y  of new  shoots.  P l o t 7 (75% treatment) had  shoot d e n s i t i e s ( F i g s . 11 and  shoot d e n s i t y i n the study a r e a was 1984  that,  shoots would be generated through the autumn and  I d i d not re-count  A p r i l 1984.  and  not  12).  By June not  Overall  s i g n i f i c a n t l y d i f f e r e n t i n June  the shoots were t h i n n e d  i n June  1983  (ANOVA, p>0.05).  5.  E f f e c t of Z o s t e r a marina Shoot Removal on the D i s t r i b u t i o n and  Abundance of Amphipods Without monthly data on assess  shoot d e n s i t y i t was  not p o s s i b l e t o  the r e l a t i o n s h i p between the d i s t r i b u t i o n of amphipods  e e l g r a s s shoot d e n s i t y i n t h i s p a r t of the study. shoots were removed and  and  However, s i n c e  t h i s c o n s t i t u t e d a form of " d i s t u r b a n c e " ,  impact on the a s s o c i a t e d b i o t a c o u l d not be i g n o r e d .  the the  Consequently,  46 F i g u r e 11. Change i n d e n s i t y of Z o s t e r a marina shoots  ' 0.25m"  2  (x ±. S.E), June 1983 - J u l y  1984 i n row 1 a t study s i t e "T".  (N = 2 4 ) .  The study s i t e was demarcated and shoots were counted on June 8, 1983. Shoots were t h i n n e d on June 13 t o d i f f e r e n t r e l a t i v e d e n s i t i e s • 0.25m" . 2  F i g u r e 12.  Change i n d e n s i t y of Z o s t e r a marina shoots  • 0.25m  -2  (x ± S.E), June 1983 - J u l y  1984 i n row 2 a t study s i t e "T". (N = 2 4 ) . The study s i t e was demarcated and shoots were counted on June 8, 1983. Shoots were t h i n n e d on June 13 t o d i f f e r e n t r e l a t i v e d e n s i t i e s • 0.25m" . 2  47  48  comparisons  of the d i s t r i b u t i o n and abundance of amphipods were made  among the e i g h t p l o t s , based on the p e r c e n t a g e of shoots t h a t were removed i n each treatment.  For example, t h e 25% treatment p l o t s , i n  which t h r e e - q u a r t e r s of the shoots were removed, were c o n s i d e r e d t o have undergone a g r e a t e r d i s t u r b a n c e than were the 75% treatment p l o t s , i n which o n l y one-quarter of the shoots were removed. A two-way a n a l y s i s of v a r i a n c e was used t o compare mean amphipod density  * 0.01m  -2  between months and t r e a t m e n t s . No  significant  d i f f e r e n c e s i n mean amphipod abundance were d e t e c t e d among the f o u r types of treatment p l o t s when d a t a from a l l twelve months were p o o l e d (ANOVA, a=0.14). amphipod numbers  Although s i g n i f i c a n t d i f f e r e n c e s d i d occur i n mean * 0.01m  (ANOVA, p<0.01; DMR,  between treatment p l o t s i n c e r t a i n  months  *=0.05), t h i s v a r i a t i o n was r e l a t e d more t o  f l u c t u a t i o n s i n abundance over time than i t was t o t r e a t m e n t - e f f e c t s . In f a c t , no c o n s i s t e n t p a t t e r n r e l a t i n g these f l u c t u a t i o n s t o treatment e f f e c t s was apparent  (Table 5 ) .  S i g n i f i c a n t d i f f e r e n c e s i n mean amphipod abundance per u n i t and sediment  plant  s u r f a c e a r e a were found u s i n g a three-way a n a l y s i s o f  v a r i a n c e (months x treatments x s u b s t r a t e type) ( T a b l e 5 ) . samples, more animals were c o l l e c t e d  • m  A  of sediment  In t h e c o r e  surface i n the  25% p l o t s than i n the other treatment p l o t s , and the lowest mean abundances were r e c o r d e d i n the c o n t r o l p l o t s . drift  In comparison,  i n the  s u b s t r a t e , a l t h o u g h s i g n i f i c a n t d i f f e r e n c e s i n amphipod abundance  o c c u r r e d between treatments (ANOVA, p<0.0008), t h e r e was no apparent r e l a t i o n s h i p between the d e n s i t y of amphipods and the e x t e n t of shoot removal, as t h e r e was i n the sediment per u n i t s u r f a c e a r e a i n t h e d r i f t  (Table 5).  The h i g h e s t numbers  samples were r e c o r d e d i n t h e 50%  49  T a b l e 5.  T o t a l numbers o f amphipods • m of s u b s t r a t e s u r f a c e (x ± 95% C.L.) i n t r e a t m e n t p l o t s , 1983-84. * v a l u e s i n d i c a t e means a r e s i g n i f i c a n t l y d i f f e r e n t among t r e a t m e n t s w i t h i n a s u b s t r a t e (ANOVA, p < 0.05; DMR, a = 0.05) . - 2  Substrate  Zostera marina  x + C.L. X  25%  241  (n=36)  115-412  1196  (n=31)  75%  50%  127  (n=36)  71-198  2228  74  (n=36)  39-119  (n=36) 793*  Control  171  (n=40)  105-254  (n=44) 1441  (n=35)  Drift ±C.L.  707-1899  X  7213*(n=69)  +C.L.  4826-10078  1262-3468 5396  488-1173  (n=67) 5075  597-2650  (n=66) 3841*(n=73)  Sediment 3763-7322  3523-6908  2498-5472  50  plots  (DMR, a=0.05), but t h e r e were no s i g n i f i c a n t d i f f e r e n c e s i n  abundance between t h e o t h e r t r e a t m e n t s .  F i n a l l y , t h e r e was no  s i g n i f i c a n t d i f f e r e n c e i n t h e d e n s i t y o f amphipods on t h e s u b s t r a t e i n any of t h e p l o t s In  marina  (DMR,a=0.05) ( T a b l e 5 ) .  summary, t h e d i s t u r b a n c e caused by t h e removal o f t h e above-  ground Z o s t e r a marina shoots had no apparent e f f e c t on e i t h e r t h e o v e r a l l amphipod abundance • 0.01m"  2  or t h e d e n s i t i e s o f a n i m a l s  c o l l e c t e d on t h e two macrophyte s u b s t r a t e s .  However, i n t h e sediment,  s i g n i f i c a n t l y more amphipods were c o l l e c t e d i n t h e treatment p l o t s  which  e x p e r i e n c e d a g r e a t e r r e l a t i v e d i s t u r b a n c e than were c o l l e c t e d i n t h e controls.  6.  Seasonal D i s t r i b u t i o n o f Amphipods As p r e v i o u s l y mentioned,  monthly abundance o f amphipods study p e r i o d of  s i g n i f i c a n t d i f f e r e n c e s i n t h e mean * 0.01m" o c c u r r e d over t h e twelve-month 2  (ANOVA, p<0.0001; DMR, a=0.05)(Fig. 13).  The t o t a l number  animals per quadrat doubled from September - October when t h e peak i n  amphipod abundance was r e c o r d e d .  From October - November numbers  d e c l i n e d f o u r - f o l d and c o n t i n u e d t o f a l l  through t h e w i n t e r and s p r i n g ,  r e a c h i n g minimum l e v e l s i n March and A p r i l  ( F i g . 13).  In May t h e  abundance o f amphipods began t o i n c r e a s e and by J u l y mean d e n s i t i e s o f 174 i n d i v i d u a l s  • 0.01m"  2  were r e c o r d e d .  F i g u r e s 14, 15, and 16 i l l u s t r a t e t h e o v e r a l l monthly mean abundance o f amphipods * m~  c o l l e c t e d i n t h e e e l g r a s s and d r i f t  samples, and i n t h e sediment  cores, r e s p e c t i v e l y .  2  the  Since the m a j o r i t y of  animals were c o l l e c t e d i n t h e sediment and d r i f t ,  these two s u b s t r a t e s were most r e f l e c t i v e o f o v e r a l l  abundances i n densities.  51 F i g u r e 13.  Monthly abundance of amphipods  (x 100)  (x ± S.E.) a t s t u d y s i t e "T". abundances of amphipods  *  m"  2  Absolute  were p o o l e d from a l l  s u b s t r a t e samples w i t h i n 0.01m (N = 75, August; 71, September,  -2  quadrats. 68, October;  57, November; 51, December; 54, January; 40, March; 37, A p r i l and May; and 39, J u l y  1984).  40, June;  53 F i g u r e 14.  Mean number o f amphipods * m~  2  o f s u r f a c e of  Z o s t e r a marina (x ± S.E.) a t study s i t e "T". (N = 16, August - January; 20, March; and 8, A p r i l  - July  1984).  55 F i g u r e 15.  Mean number of amphipods drift  (x ± S.E.)  • m  -2  of s u r f a c e of  a t study s i t e "T". (N = 27,  August; 23, September;  20, October;  9, November; 3, December; 7, January; 0, March;  13, A p r i l  and 15, J u l y  1984).  and May;  16, June;  57 F i g u r e 16.  Mean number o f amphipods * m surface  -2  o f sediment  (x. ± S.E.), a t study s i t e "T".  (N = 32, August - January; 20, March; and 16, A p r i l  - July  1984).  59  Amphipods were most abundant i n the sediment i n the l a t e autumn e a r l y winter,  p a r t i c u l a r l y i n October and  d e c l i n e d i n abundance over the winter and A p r i l and equivalent There was present  May.  i n d i v i d u a l s per  square meter of  t h a t a complete annual c y c l e had  (DMR,a=0.05)(Fig. The  t o a mean  sediment  and  i n late July  1984,  been o b s e r v e d  16).  seasonal  p a t t e r n of amphipod abundance i n the d r i f t  s i m i l a r t o t h a t observed i n the  sediment.  However, i n the  d e c l i n e d d r a m a t i c a l l y as the d r i f t  Abundances remained at low  than i n  disappeared,  March, when no amphipods were c o l l e c t e d on t h i s s u b s t r a t e  biomass t h a t o c c u r r e d  d e n s i t y of amphipods i n the d r i f t  i n August 1983  s i g n i f i c a n t l y d i f f e r e n t (DMR,  and  until  ( F i g . 15).'  l e v e l s throughout the s p r i n g and  months, d e s p i t e the peak i n d r i f t  was  drift,  abundances per u n i t s u r f a c e a r e a peaked i n November r a t h e r October, and  surface.  s i g n i f i c a n t d i f f e r e n c e between the number of amphipods  i n the sediment i n e a r l y August 1983  suggesting  They  reached minimum d e n s i t i e s i n  From June - J u l y t h e i r numbers i n c r e a s e d  t o 1790 no  November ( F i g . 16).  and  summer  i n May.  J u l y 1984  The was  not  a=0.05).  Amphipods were more abundant per  unit surface area  i n the  drift,  than on the Z o s t e r a marina, i n a l l months except August, A p r i l , May, June ( F i g s . 14 and  15)(DMR, a=0.05).  Mean amphipod d e n s i t i e s on  e e l g r a s s ranged from a low of 18 i n d i v i d u a l s * m~  2  J u l y 1984  t o 458  • nf  2  s u r f a c e of p l a n t i n A p r i l  and  the  s u r f a c e of p l a n t i n (Fig. l | ) .  60  7.  Macrophyte  Biomass and t h e D i s t r i b u t i o n  Although  t h e amphipods  numbers  as  overall  a b u n d a n c e was  eelgrass for  high  as  biomass  were not  Amphipods  c o l l e c t e d on t h e Z o s t e r a m a r i n a  t h e y were on t h e d r i f t  per  of  and i n  correlated significantly sample  quadrat  (r=0.771;  the  sediment,  w i t h mean  • 0.01m  were r e c o r d e d i n O c t o b e r ; minimum v a l u e s  - 2  recorded  i n March  correlation  content of  of  by t h e p r e s e n c e o r  6).  However,  quadrats  found,  sediment  on t h e  of  t-Test,  collected in i n November  the cores  significant and  (Table  the  4)  1983 a n d i n  (Table  7);  indicating  indicating  biomass that  eelgrass  biomass  i n June,  significant  an o p p o s i t e  t h e one y e a r  trend  than  a n d t h e mean b i o m a s s The o v e r a l l sample  was  of  drift  abundance of significantly  within or  sample  by t h e biomass  J u n e 1984  July  of  1984  significant  amphipods  (Table  7).  in  In  the  November  were lower  in  t h e y were i n  quadrats  with  positive 7).  correlations  amphipods  • 0.01m  - 2  negative  collected in  correlated with  low  were  correlations  collected in  (Table  this  were  When a b u n d a n c e d a t a  significant  amphipods  the  influenced  abundances  (Table  study p e r i o d ,  were n o t  from January -  between t h e abundance o f  negative,  with high  plant  p>0.05),  w e r e o b s e r v e d b e t w e e n t h e mean number o f  marina  a  biomass  were  " 0.01m  the cores  an e e l g r a s s  a n d Z_^ m a r i n a a b o v e - g r o u n d  pooled over  cores  both  t h e r e was  same d a t e  values  Z_^ m a r i n a  for  amphipods  collected in  (Students'  d i d occur  c o r r e l a t i o n was  biomass  absence  rhizomes  correlations sediment  the  amphipods  i n a n y month  the eelgrass (Table  Similarly,  Maximum  p<0.001).  Densities  quadrat  13 a n d 6 ) .  b e t w e e n t h e mean a b u n d a n c e o f  mean o r g a n i c (r=0.998;  (Fig.  their  monthly  p<0.002).  b o t h mean a m p h i p o d a b u n d a n c e a n d mean a b o v e - g r o u n d  in  the  8). each  Zostera  the d r y weight  of  the  61  T a b l e 6.  C o r r e l a t i o n c o e f f i c i e n t s (R) f o r c o m p a r i s o n o f numbers o f a m p h i p o d s • c o r e " ! w i t h d r y w e i g h t (g) o f Z o s t e r a m a r i n a r h i z o m e s • core -'-, 1984. R was n o t s i g n i f i c a n t l y d i f f e r e n t among months (p > 0 . 0 5 ) . -  January  March  April  May  June  July  n=16  n=16  n=8  n=8  n=8  n=8  0.17  0.22  -0.49  0.43  0.30  -0.24  62  T a b l e 7.  C o r r e l a t i o n c o e f f i c i e n t s (R) f o r c o m p a r i s o n s o f m o n t h l y mean number o f amphipods i n Z o s t e r a m a r i n a , d r i f t , s e d i m e n t , and t o t a l q u a d r a t T o T o T m " ) samples w i t h s u r f a c e a r e a ( c m ) and d r y w e i g h t (g) o f _Z. m a r i n a and d r i f t • 0.01 m" . * values indicate s i g n i f i c a n t R (p < 0.05). 7  2  2  1  =  _Z. m a r i n a  surface  area  2  =  Z_. m a r i n a d r y w e i g h t  3  =  D r i f t surface area  4  =  D r i f t dry weight  =  Insufficient  (cm ) 2  (g) (cm ) 2  (g)  number  of r e p l i c a t e  samples  DATE  Z. marina  Aug./8 3 1  0.905*  -0.499  -0.018  0.030  0.919*  -0.519  0. 010  0.038  2  Drift  Sediment  Total  3  -0.696  0.852*  -0.513  0.335  4  -0.655  0.801*  -0.545  0.269  Sept. 1  0.184  0.219  -0.127  0.073  2  0.197  0.199  -0.114  0.068  3  -0.470  0.762*  -0.076  0.451  4  -0.327  0.903*  0.112  0.646  [cont.]  63  T a b l e 7. DATE Oct. 1 2  [cont.] Z. m a r i n a  Drift  0.815*  0.004  -0.263  -0.041  0.810*  0.002  -0.283  -0.058  Sediment  Total  3  -0.123  0.840*  -0.214  0.609  4  0.159  0.714*  0.046  0.695  Nov. 1 2  0.796*  —  0.801*  -0.770*  -0.552  -0.748*  -0.535  -0.327  -0.296  -0.339  -0.309  3 4 Dec. 1 2  0.465  _  0.471  3 4 Jan. 1  0.715*  -0.567  0.263  0 . 242  2  0.699  -0.533  0.246  0. 227  3  -0.873*  0.652  -0.429  -0.416  4  -0.833*  0.673  -0.425  -0.410  March 1  0.562  _  -0.632  -0.475  2  0.567  -  -0.653  -0.499  -  -  3 4  -  [cont. ]  64  T a b l e 7. DATE  [cont.] Z. m a r i n a  Drift  April 1  0.863  -0.419  -0.246  0.820  2  0.864  -0.419  -0.249  0.822  3  -0.478  0.997*  -0.552  0.002  4  -0.415  0.992*  -0.609  0.071  May 1  0.601  -0.316  -0.080  -0.576  2  0.798  -0.256  -0.242  -0.612  3  0.528  0.501  -0.734  0.194  4  0.535  0.371  -0.620  0.086  0.430  0.640  0.970*  0.795  2  0.435  0.655  0.963*  0.809  3  0.124  -0.365  -0.672  -0.397  4  0.061  -0.456  -0.750  -0.505  0.509  0. 399  -0.240  0.453  2  0.626  0.606  -0.406  0.672  3  -0.171  0.189  -0.228  0.171  4  -0.259  0.303  -0.358  0.277  June 1  July 1  Sediment  Total  65  T a b l e 8.  C o r r e l a t i o n c o e f f i c i e n t s (R) f o r c o m p a r i s o n s o f mean number o f a m p h i p o d s i n Z o s t e r a m a r i n a , d r i f t , sediment, and t o t a l q u a d r a t (0.01m ) samples w i t h s u r f a c e a r e a ( c m ) and d r y w e i g h t (g) o f Z. m a r i n a , d r i f t , and t o t a l m a c r o p h y t e s • 0.01m~ . * v a l u e s i n d i c a t e s i g n i f i c a n t R (p < 0 . 0 5 ) . 2  2  i  Variable  Z. m a r i n a  Drift  Sediment  Total  surface area  0.532*  0.230  0.138  0.310*  dry  0.542*  0.210  0.129  0.289*  Z. m a r i n a  weight  Drift surface area  -0.116  0.220  -0.353*  -0.089  dry  -0.086  0.163  0.315*  -0.101  -0.058  0.247*  -0.340*  -0.056  0.208  -0.287*  -0.038  weight  Total Macrophytes surface area dry  weight  0.031  66  e e l g r a s s i n t h a t sample ( T a b l e 8 ) . months t h i s r e l a t i o n s h i p was and November ( T a b l e 7 ) .  When the d a t a were examined between  found to be  Since previous  s i g n i f i c a n t i n August, October, s t u d i e s have shown t h a t  the  d i s t r i b u t i o n of e e l g r a s s - a s s o c i a t e d amphipods i s r e l a t e d to p l a n t surface area  (Stoner,  1983), and  t h a t these animals can  select plant  s u b s t r a t e s on the b a s i s of the s u r f a c e a r e a of the blades 1980c), the r e l a t i o n s h i p between abundance and  (Stoner,  plant surface area  was  a l s o examined. S i g n i f i c a n t c o r r e l a t i o n s were found i n August, October, November, and  January ( T a b l e  7).  S i m i l a r l y , the number of animals c o l l e c t e d i n the d r i f t c o r r e l a t e d s i g n i f i c a n t l y w i t h both d r i f t August, September, October, and A p r i l  d r y weight and  (Table 7 ) .  was  surface area i n  However, o v e r a l l  abundances were more c l o s e l y r e l a t e d to t o t a l macrophyte (Z. marina d r i f t ) s u r f a c e a r e a w i t h i n a sample quadrat  (Table 8).  and  Data which  d e s c r i b e the r e l a t i o n s h i p between animal abundance and d r i f t  biomass  s u r f a c e a r e a from November - March a r e not i n c l u d e d i n the T a b l e  and  because  the low number of samples c o l l e c t e d i n those months made the a n a l y s i s unreliable. In summary, t h e r e was macrophyte biomass and present  a s i g n i f i c a n t r e l a t i o n s h i p between  s u r f a c e a r e a , and  on both the Z o s t e r a marina and  the d e n s i t y of amphipods  the d r i f t ,  the autumn peak i n a n i m a l abundance (Tables 7 and  particularly 8)  during  ( F i g s . 14 and  15).  Abundances i n the sediment, i n c o n t r a s t , were n e g a t i v e l y r e l a t e d t o e e l g r a s s biomass and  s u r f a c e a r e a d u r i n g t h i s time ( T a b l e 7 ) .  S i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s between macrophyte biomass and  numbers  of amphipods i n the sediment o c c u r r e d o n l y i n June, when p o p u l a t i o n s these animals were c l o s e t o t h e i r annual minima ( T a b l e 7 ) ( F i g . 16).  of  67  8.  D i s t r i b u t i o n of S p e c i e s of Amphipods Nineteen s p e c i e s of amphipods were c o l l e c t e d over the one-year  p e r i o d ; however, the m a j o r i t y of these were r a r e (Table 9 ) .  In most  months, f o u r s p e c i e s , Corophium acherusicum C o s t a , Corophium  insidiosum  Crawford, Ampithoe v a l i d a Smith, and Anisogammarus p u g e t t e n s i s accounted f o r over 90%  of the t o t a l numbers.  Consequently,  diversity  i n a l l months, r a n g i n g  from 0.57  bits  i n June.  The  (H')  was  low  i n d i v i d u a l i n October t o 1.49 months r e p r e s e n t  i n May  and  1.22  times i n which the p o p u l a t i o n s  were c l o s e to t h e i r annual minima ( T a b l e 9) and s p e c i e s were r e l a t i v e l y abundant. the p l a n t samples and i n the d r i f t  i n May  (Table 9).  species  latter  of the dominant when two  Ischyrocerus  other  Ischyrocerus  two  species gammarid  sp. dominated  Pontogeneia r o s t r a t a Gurjanova was  Four of these  i n s i d i o s u m , A. v a l i d a , and ( B o u s f i e l d , 1973).  In A p r i l  Dana  common  s i x s p e c i e s , C. acherusicum,  C.  sp. a r e e p i f a u n a l t u b e - b u i l d e r s  Anisogammarus p u g e t t e n s i s and  P. r o s t r a t a swim  f r e e l y among p l a n t s and o r g a n i c d e b r i s near the sediment s u r f a c e (Chang, 1975;  Stoner,  infaunal.  1980b).  None of the n u m e r i c a l l y dominant s p e c i e s i s  Fewer than 50 i n d i v i d u a l s of the remaining  c o l l e c t e d and  of t h e s e , o n l y Synchelidium  sp. burrow (Smith During  13  species, were  shoemakeri M i l l s and  & C a r l t o n , 1975).  the peak i n amphipod abundance i n October, evenness ( J ' ) ,  which i s a measure of the e q u i t a b i l i t y of s p e c i e s occurrence, than i t was  Orchomene  was  lower  i n a l l o t h e r months ( s i g n i f i c a n t f o r a l l months except  November and December, ANOVA, p<0.05; DMR,  a=0.05).  In these months,  C. acherusicum a c c o u n t e d f o r more than 88% of the t o t a l number of amphipods ( T a b l e  9).  T a b l e 9.  R e l a t i v e abundance, d i v e r s i t y , and evenness o f s p e c i e s of amphipods • 0.01 m i n t r e a t m e n t p l o t s , 1983-84. * v a l u e s i n d i c a t e months i n w h i c h means were s i g n i f i c a n t l y d i f f e r e n t (ANOVA, p < 0.05; DMR, « = 0.05). -2  n  =  T o t a l number c o u n t e d  H' =  Shannon-Wiener D i v e r s i t y  J' =  Shannon-Wiener Evenness  Species  Aug.  Sept.  Oct.  Nov.  Dec.  Jan.  March  April  May  June  July  n=  191  1161  1946  616  383  205  62  73  181  372  1317  lacertosa  -  0.4  0.5  0.6  1.9  A. v a l i d a  19.4  8.2  2.0  0.6  0.8  0.9  Anisogammarus puqettensis Atylus collingii  12.0  2.1  43.0  5.4  2.3  Ampithoe  co  10  C. i n s i d i o s u m Ischyrocerus sp.  2..4  0.8  -  0.5  Corophium acherusicum  7.7  1.6 0.6  1.6  0.3  Calliopius laeviusculus  <0.1  60.8  78.9  6.3  10.4  0.3  -  1.4 2.7  -  -  -  -  -  -  38.4  89.2  88.7  84.4  72.7  1.4  6.0  58.6  78.4  2.7  6.5  6.3  7.8  6.4  4.1  -  22.8  18.4  0.3  1.3  1.6  4.8  12.9  82.2  17.7  0.8  0.5 [cont. ]  Table  9.  [cont.]  Species  Aug.  Sept.  Oct.  Nov.  Dec.  Jan.  March  April  May  June  July  n=  191  1161  1946  616  383  205  62  73  181  372  1317  Orchomene sp.  —  -  -  -  0.3  -  -  -  -  -  -  Paraphoxus spinosus  -  -  -  -  -  -  -  0.6  0.3  -  0.5  -  -  -  -  -  -  -  -  -  -  Photis brevipes  -  -  -  -  -  -  -  -  -  1.9  -  P. oligochaeta  -  -  -  -  -  0.5  3.2  -  0.6  4.8  -  Pontogeneia intermedia  -  -  -  -  -  -  -  -  3.3  -  -  P.  rostrata  -  -  -  0.3  0.6  -  -  8.2  1.6  -  Synchelidum shoemakeri  -  -  -  -  -  -  3.2  -  -  1.1  -  TOTAL SPECIES  7  5  7  6  8  6  6  6  9  11  4  H '  1.13  0.75  0.57*  0.73  0.73  0.71  0.85  0.94  1.49*  1.22*  0.97  J'  0.76  0.53  0.37  0.47  0.52  0.58  0.85  0.70  0.76  0.72  0.70  Parapleustes pugettensis  <0.1  27 .6  70  F i g u r e s 17,  18, and  19 i l l u s t r a t e the monthly mean abundances of  the dominant s p e c i e s c o l l e c t e d i n the Z o s t e r a marina, d r i f t , samples, r e s p e c t i v e l y . C. i n s i d i o s u m ,  Corophium acherusicum, and  and  core  t o a l e s s e r extent,  were common on a l l s u b s t r a t e s i n most months,  although  they were l e s s abundant on the e e l g r a s s than they were i n the d r i f t  or  the sediment. Corophium i n s i d i o s u m peaked i n abundance one month e a r l i e r i n the sediment than d i d C. acherusicum ( F i g . 19). both s p e c i e s d e c l i n e d d r a m a t i c a l l y i n the d r i f t peak, while the numbers i n the 18 and  19).  the d r i f t  The  populations  of  f o l l o w i n g the autumn  sediment d e c l i n e d more g r a d u a l l y  (Figs.  J u v e n i l e Corophium spp. were c o l l e c t e d i n the sediment  i n l a r g e numbers through the autumn and winter  and  but by March  they too had d e c l i n e d i n abundance. Both Corophium s p e c i e s remained i n low numbers throughout March and A p r i l b e f o r e approaching'peak d e n s i t y i n J u l y 1984 S p e c i e s d i v e r s i t y was t h r e e s u b s t r a t e types  not  ( F i g s . 18 and  i n any month (ANOVA, p>0.05).  sediment s u r f a c e . which was 1984 was  t h i s may  19).  have been due  were a l s o c o l l e c t e d i n the  to t h e i r presence at  A good example of t h i s was  (Table 9), p r i m a r i l y i n the d r i f t  Anisogammarus  the pugettensis,  samples ( F i g . 18).  and  This  May species  o f t e n seen swimming among the e e l g r a s s p l a n t s , s h e l t e r i n g w i t h i n sp. b l a d e s , and  S i m i l a r i l y , a number of Ampithoe v a l i d a , and  the  c r a w l i n g on the sediment s u r f a c e . s p e c i e s , i n c l u d i n g Pontogeneia r o s t r a t a ,  Ischyrocerus  the e e l g r a s s . I s c h y r o c e r u s  the  In many i n s t a n c e s ,  c o l l e c t e d i n r e l a t i v e l y l a r g e numbers i n J u l y 1983  f o l d s of U l v a  and  levels  s i g n i f i c a n t l y d i f f e r e n t on e i t h e r of  s p e c i e s which were c o l l e c t e d i n the d r i f t core samples and  i n c r e a s i n g to  sp. were c o l l e c t e d i n both the  sp. was  the o n l y s p e c i e s which  was  drift  71 F i g u r e 17.  Mean number o f i n d i v i d u a l s amphipod s p e c i e s  • m  -2  i n each dominant  surface of l i v e  Zostera marina (x ± S.E.) a t study 1 = Ampithoe v a l i d a ,  site  "T". (N = 4 ) .  2 = Corophium  acherusicum, and 3 = I s c h y r o c e r u s sp.  72  1983-1984  73 F i g u r e 18 A. and  B. Mean number of i n d i v i d u a l s i n each dominant amphipod s u r f a c e of d r i f t "T".  s p e c i e s (x 10) (x ± S.E.)  (N = 7, August; 6,  *  a t study s i t e  December,  January, and March; 7, A p r i l ; and 8, 1984).  1 = Corophium acherusicum, 2 = C. i n s i d i o s u m , and 3 = valida.  2  September;  5, October; 1, November,  June, and J u l y  m~  Ampithoe  May,  74  Mean number of i n d i v i d u a l s  i n each  dominant amphipod s p e c i e s (x 10) s u r f a c e of sediment s u r f a c e (x ± at  study  s i t e "T".  acherusicum, 2 = C.  m~  2  S.E.)  (N = 8 ) . 1 = Corophium insidiosum,  3 = Aropithoe v a l i d a , and pugettensis.  *  4 = Anisogammarus  76  A6  S-4  025  N23  021  J20 1983-1984  Ml  A17  M15  J27  J26  77  c o l l e c t e d at higher d e n s i t i e s per  u n i t s u r f a c e a r e a of p l a n t on  e e l g r a s s than i t was  (Fig..  The  on the d r i f t  17).  r e s u l t s of the p r i n c i p a l component a n a l y s i s a r e shown i n  F i g u r e s 20 and  21.  Monthly mean abundances of t h e f i v e most f r e q u e n t l y  c o l l e c t e d s p e c i e s , Corophium acherusicum, C. i n s i d i o s u m , v a l i d a , Anisogammarus p u g e t t e n s i s , and Corophium spp.  Ischyrocerus  the two  sp., as w e l l as Abundances of  Corophium s p e c i e s were more c l o s e l y  a s s o c i a t e d than were numbers of A. v a l i d a and  and  Ampithoe  j u v e n i l e s , were i n c l u d e d i n t h i s a n a l y s i s .  A. p u g e t t e n s i s and  20).  the  Ischyrocerus  sp. ( F i g .  Given the d i f f e r e n c e s i n the temporal d i s t r i b u t i o n s of A. Ischyrocerus  sp., t h i s p a t t e r n can p r o b a b l y  b a s i s of s u b s t r a t e a s s o c i a t i o n .  Both of these  be e x p l a i n e d on  the  s p e c i e s were c o l l e c t e d  more f r e q u e n t l y i n the e e l g r a s s than were the other the Corophium j u v e n i l e s .  valida  t h r e e s p e c i e s , or  P l o t s of component scores of each p r i n c i p a l  a x i s through time r e f l e c t the dominance of the Corophium s p e c i e s which, as p r e v i o u s l y d e s c r i b e d , were c o l l e c t e d i n l a r g e numbers i n the and  sediment samples ( F i g . 21).  The  s p r i n g and autumn peaks i n  abundance i n the Z. marina s u b s t r a t e may of I s c h y r o c e r u s 9.  be a t t r i b u t e d t o the presence  sp. and A. v a l i d a , r e s p e c t i v e l y ( F i g . 21).  C o r r e l a t i o n M a t r i c e s and Table  drift  the D i s t r i b u t i o n of  Species  10 shows the product-moment c o r r e l a t i o n c o e f f i c i e n t s f o r  " p a i r w i s e " comparisons of the mean r e l a t i v e abundances of a l l amphipod s p e c i e s pooled  from a l l samples c o l l e c t e d from August 1983  With the e x c e p t i o n of the n e g a t i v e  - July  1984.  c o r r e l a t i o n s which were found between  the abundances of both Corophium s p e c i e s and  Ischyrocerus  s i g n i f i c a n t c o r r e l a t i o n s were p o s i t i v e ( T a b l e 10).  sp., a l l  Highest  significant  78 F i g u r e 20A. P r i n c i p a l component amphipod  o r d i n a t i o n of dominant  s p e c i e s i n monthly samples, August  1983 - J u l y 1984 - components  1 and 2.  Values i n parentheses i n d i c a t e p e r c e n t a g e of v a r i a n c e i n d a t a which i s accounted f o r by each component  axis.  1 = Ampithoe v a l i d a , 2 = p u g e t t e n s i s , 3 = Corophium  Anisogammarus acherusicum  4 = C. i n s i d i o s u m , 5 = Corophium spp. j u v e n i l e s , and 6 = I s c h y r o c e r u s sp.  Component 2 (18.0%) -1.0 i O  I  O (Jl  n o  3  U o  CD  cr  o o  ro  o  01  6L  80 F i g u r e 20B. P r i n c i p a l component amphipod  o r d i n a t i o n of dominant  s p e c i e s i n monthly samples, August  1983 - J u l y 1984 - components  2 and 3.  Values i n p a r e n t h e s e s i n d i c a t e p e r c e n t a g e of v a r i a n c e i n d a t a which i s accounted f o r by each component  axis.  1 = Ampithoe v a l i d a , 2 = p u g e t t e n s i s , 3 = Corophium  Anisogammarus acherusicum  4 = C. i n s i d i o s u m , 5 = Corophium  spp.  j u v e n i l e s , and 6 = I s c h y r o c e r u s sp.  Component 3 (16.0%) -i.O  r  R  -0.5  0.0  0.5  1  1  1  1.0 TI  o  10  *  I  o Ul  o o  3  XJ o  3 CD 3 ft  ro  *  o o  Ul  *  a o 0)  o Ul  o 18  82 F i g u r e 2 i A . Monthly p r i n c i p a l component s c o r e s (x ± S.D.)  i n each s u b s t r a t e on  component a x i s 1. Z = Z o s t e r a marina D = Drift S =  Sediment  Values i n p a r e n t h e s e s i n d i c a t e p e r c e n t a g e of t o t a l v a r i a n c e i n d a t a which f o r by component  axis.  i s accounted  83  84 F i g u r e 2IB.  Monthly p r i n c i p a l component s c o r e s (x ± S.D.)  i n each s u b s t r a t e on  component a x i s 2. Z = Z o s t e r a marina D = Drift S =  Sediment  Values i n parentheses i n d i c a t e percentage of t o t a l v a r i a n c e i n d a t a which i s accounted f o r by component  axis.  86 F i g u r e 21C.  Monthly p r i n c i p a l component scores (x ± S.D.)  i n each s u b s t r a t e on  component a x i s 3. Z = Z o s t e r a marina D = Drift S =  Sediment  Values i n parentheses i n d i c a t e p e r c e n t a g e of t o t a l v a r i a n c e i n d a t a which i s accounted f o r by component  axis.  68J03S  3U9U0dU103  U B Q  W  88  T a b l e 10.  C o r r e l a t i o n c o e f f i c i e n t s (R) f o r c o m p a r i s o n o f mean r e l a t i v e numbers o f amphipods • species -'- • 0.01m p o o l e d i n a l l months and s u b s t r a t e t y p e s , 1983-84 (N = 176). * v a l u e s i n d i c a t e s i g n i f i c a n t R (p < 0.05); ** v a l u e s i n d i c a t e s i g n i f i c a n t R (p < 0.01). -  1 = 2 = 3 = 4 = 5 = 6 = 7 = 8 = 9 = 10 = 11 = 12 = 13 = 14 = 15 =  Ampithoe l a c e r t o s a A. v a l i d a Anisogammarus p u g e t t e n s i s Atylus c o l l i n g i i Calliopius laeviusculus Corophium acherusicum C. i n s i d i o s u m I s c h y r o c e r u s sp. Paraphoxus s p i n o s u s Parapleustes pugettensis Photis brevipes Photis oligochaeta Pontogeneia intermedia P_. r o s t r a t a Synchelidium shoemakeri  [cont.]  -2  Table 10. Icont.l  Species  1  2  3  4  5  6  7  8  9  10  11  12  13  14  1  CO  2  -0.036  3  -0.048  0.012  4  -0.014  -0.043  5  -0.010  0.037  0.026 0.282* -0.008  6  0.214** 0.329** 0.159* -0.037  0.028  7  0.284** 0.236** 0.199** 0.029  0.023  8  -0.042  -0.110  -0.090  9  -0.016  -0.051  10  -0.010  -0.001  -0.033  11  0.004  -0.044  12  0.017  -0.066  13  -0.011  -0.034  14  -0.029  -0.051  -0.072  15  -0.022  -0.040  -0.074  0.704**  0.076  -0.024  -0.161* - 0 . 174*  0.312**-0.013  -0.010  -0.056  -0.018  -0.041  -0.008  -0.006  -0.038  -0.051  -0.024  -0.010  -0.027  -0.012  -0.008  -0.019  -0.059  -0.035  -0.014  -0.008  -0.040  -0.017  -0.012  -0.032  -0.036  -0.052  -0.021  -0.012  0.449**-0.009  -0.007  -0.044  -0.059  -0.025  -0.025  -0.018  -0.063  -0.082  -0.018  -0.013  -0.053  0.009  0.655** -0.007  -0.010  -0.014  -0.018  -0.025  -0.025  0.003  0. 255** -0.013  -0.018  -0.028  -0.015  0.469** -0.029 -0.054  0.671**  -0.039  IS  90  c o r r e l a t i o n s were found between the r e l a t i v e abundances of acherusicum and C. i n s i d i o s u m (r=0.794, p<0.01) and P. o l i g o c h a e t a (r=0.671, p<0.01) ( T a b l e 10).  Low,  C.  Photis, b r e v i p e s but  and  significant  p o s i t i v e c o r r e l a t i o n s were a l s o found between the r e l a t i v e abundances of each of the Corophium s p e c i e s and  those of Anisogammarus p u g e t t e n s i s ,  Ampithoe v a l i d a , a n d A.  ( T a b l e 10).  lacertosa  s p e c i e s which were i n f r e q u e n t l y c o l l e c t e d Paraphoxus spinosus, and  The abundances of  (Calliopius laeviuscula,  Pontogeneia i n t e r m e d i a )  s i g n i f i c a n t l y with t h a t of A. p u g e t t e n s i s  were c o r r e l a t e d  ( T a b l e 10).  Significant  p o s i t i v e c o r r e l a t i o n s a l s o o c c u r r e d between Pontogeneia r o s t r a t a Ischyrocerus  three  and  sp., both of which were most numerous i n the s p r i n g .  C o r r e l a t i o n a n a l y s i s was  a l s o used t o i d e n t i f y i n t e r a c t i o n s  between the f i v e most numerous amphipod s p e c i e s i n i n d i v i d u a l months. T a b l e 11 shows c o r r e l a t i o n c o e f f i c i e n t s f o r monthly comparisons of  the  r e l a t i v e abundances of Anisogammarus p u g e t t e n s i s , Ampithoe v a l i d a , Corophium acherusicum C. i n s i d i o s u m , and  Ischyrocerus  sp.  The  relative  abundances of the two Corophium s p e c i e s were p o s i t i v e l y c o r r e l a t e d i n a l l months except  March, A p r i l , May,  acherusicum were c o l l e c t e d and f r e q u e n t l y i n the  and  June, when low numbers of  C. i n s i d i o s u m o c c u r r e d even  C.  less  samples.  S i g n i f i c a n t n e g a t i v e c o r r e l a t i o n s between Corophium acherusicum and  Ischyrocerus  sp. o c c u r r e d i n October and  both of these months I s c h y r o c e r u s samples and C. acherusicum was  was  January ( T a b l e 11).  In  c o l l e c t e d o n l y i n the e e l g r a s s  c o l l e c t e d o n l y i n the d r i f t  and  sediment  samples. The p o s i t i v e c o r r e l a t i o n between the r e l a t i v e abundance of Ischyrocerus  sp. and A. p u g e t t e n s i s i n December was  due  t o the  low  91  Table  11.  Correlation coefficients (R) f o r c o m p a r i s o n s o f m o n t h l y mean n u m b e r s o f a m p h i p o d s • species • 0.01m" , 1983-84. * values i n d i c a t e s i g n i f i c a n t R (p < 0 . 0 5 ) ; * * v a l u e s i n d i c a t e s i g n i f i c a n t R (p < 0 . 0 1 ) . - 1  1 2 3 4 5  August,  1983  Species  = = = = =  2  Ampithoe v a l i d a Anisogammarus p u g e t t e n s i s Corophium acherusicum C_. i n s i d i o s u m Ischyrocerus sp.  n  =  19  1  2  3  4  5  1 2  0.. 4 7 3 *  3  0.. 4 1 1  0.. 8 7 8 * *  4  0.. 178  0.. 4 9 2 *  5  0.. 0 1 3  September  Species  n =  - 0 . .142  0.704** -0.193  -0.155  18  1  2  3  4  5  1 2  0.084  3  -0.622**  0.050  4  -0.633**  0.126  0.879**  0.000  0.000  5  0.000  0.000 [cont.]  92  T a b l e 11.  [cont. ]  October  n = 17  Species  1  2  3  4  5  1 2  -0.134  3  -0.097  0.402  4  0.262  0.350  5  -0.148  -0.050  November Species  0.664** -0.545*  -0.400  n = 13 1  2  3  4  5  1 2  0.000  3  0.629*  0.000  4  0.078  0.000  5  -0.257  0.000  December Species  0.577* -0.427  -0.351  n = 13 1  2  3  4  5  1 2  -0.267  3  0.250  0.039  4  0.295  0.101  5  -0.429  0.774**  0.562* -0.061  0.187 [cont.]  93  T a b l e 11.  [cont.]  J a n u a r y , 1984  n = 13  Species  1  2  3  4  5  1 2  0.000  3  0.605*  0.000  4  0.388  0.000  5  -0.264  0.000  -0.635*  -0.614*  2  3  4  March  0.911**  n = 12  Species  1  5  1 2  0.000  3  0.000  -0.337  4  0.000  0.54.4  -0.493  5  0.000  -0.124  -0.344  April Species  -0.182  n = 12 1  2  3  4  5  1 2  0.000  3  0.000  -0.119  4  0.000  -0.120  -0.027  5  0.000  -0.202  -0.264  -0.266 [cont.]  94  T a b l e 11.  May  [cont.]  n = 20  Species  1  2  3  4  5  1 2  -0.017  3  -0.078  0.443  4  0.000  0.000  0.000  5  -0.057  -0.231  -0.287  June  0.000  n = 20  Species  1  2  3  4  5  1 2  -0.140  3  -0.196  -0.200  4  0.237  -0.183  0..105  5  -0.186  -0.093  -0.. 309  July  -0.244  n = 19  Species  1  2  3  4  1 2  -0.353  3  -0.092  0.554*  4  -0.020  0.631**  0.811**  5  0.000  0.000  0.000  0.000  5  95  abundance of both of these  s p e c i e s c o l l e c t e d i n the sediment i n t h a t  month. Other s i g n i f i c a n t n e g a t i v e c o r r e l a t i o n s o c c u r r e d between the Corophium s p e c i e s and Ampithoe v a l i d a i n September ( T a b l e 11). were p r o b a b l y due  to d i f f e r e n c e s  s p e c i e s . Although  A. v a l i d a was  month, i t was  i n the o v e r a l l abundance of  these  e i t h e r C. acherusicum or  or sediment samples.  c o r r e l a t i o n s between C. acherusicum and and  These  c o l l e c t e d on a l l s u b s t r a t e s i n t h a t  f a r l e s s abundant than was  i n s i d i o s u m i n the d r i f t  C.  Significant positive  A. v a l i d a o c c u r r e d i n November  January, months i n which the s i z e s o f both p o p u l a t i o n s i n the  a r e a were d e c l i n i n g  two  ( T a b l e s 9 and  study  11).  T a b l e 12 shows the product-moment c o r r e l a t i o n c o e f f i c i e n t s f o r comparisons of the r e l a t i v e abundances of the f i v e most numerous amphipod s p e c i e s c o l l e c t e d on each of t h e t h r e e s u b s t r a t e s . . Data were pooled  from a l l months i n t h i s a n a l y s i s .  In the Z. marina samples,  s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s o c c u r r e d o n l y between C.  insidiosum  and A. p u g e t t e n s i s .  correlated  Abundances of A. p u g e t t e n s i s were not  w i t h those of any o'f the o t h e r t h r e e s p e c i e s i n e i t h e r the d r i f t sediment.  In the d r i f t ,  v a l i d a and  the two  Corophium s p e c i e s , w h i l e low but s i g n i f i c a n t ,  acherusicum (Table 12).  The  was  sp. and  r e l a t i v e abundances of t h e two  s p e c i e s were p o s i t i v e l y c o r r e l a t e d  v a l i d a (Table  the  p o s i t i v e c o r r e l a t i o n s o c c u r r e d between A.  n e g a t i v e c o r r e l a t i o n s were found between I s c h y r o c e r u s  t h e i r occurrence  or  C.  Corophium  o n l y i n the sediment samples, where  also positively correlated  with numbers of  A^  12).  In summary, f i v e main, p o i n t s may  be made from t h e r e s u l t s of  the  twelve-month study: 1) Amphipods were most abundant i n t h e autumn and  96  T a b l e 12.  C o r r e l a t i o n c o e f f i c i e n t s (R) o f mean numbers o f a m p h i p o d s c o l l e c t e d i n Zostera marina, sediment samples. * values s i g n i f i c a n t R (p < 0 . 0 5 ) ; ** s i g n i f i c a n t R (p < 0 . 0 1 ) . 1 2 3 4 5  = = = =  -  Ampithoe v a l i d a Anisogammarus p u g e t t e n s i s Corophium a c h e r u s i c u m C. i n s i d i o s u m I s c h y r o c e r u s sp.  Zostera marina Species  f o r comparisons • species ! d r i f t , and indicate values indicate  n = 43 1  2  3  4  5  1 2  0.113  3  0.145  4  -0.044  5  - 0 . 307  - 0 . 121 0. 530** - 0 . 120  0 .204 0 .144  -0.165 [cont.]  97  T a b l e 12.  Drift Species  [cont.]  n = 51 1  2  3  4  5  1 2  -0.154  3  0.532**  -0.003  4  0.378**  0.143  0.271  0.108  -0.286*  2  3  5  -0.116  Sediment  n = 82  Species  1  -0.273  4  1 2  0.115  3  0.489**  0.057  4  0.387**  0.085  0.683  0.068  0.039  5  -0.115  0.021  5  98 t h e i r numbers d e c l i n e d throughout the w i n t e r and s p r i n g . of  This pattern  abundance was p o s i t i v e l y c o r r e l a t e d w i t h the s e a s o n a l i t y of Z o s t e r a  marina biomass.  2) D e s p i t e d e n s i t i e s e q u i v a l e n t t o more than 26,800  amphipods • m ^ i n October, s p e c i e s d i v e r s i t y was  low. Four  species,  Corophium acherusicum, C. i n s i d i o s u m , Ampithoe v a l i d a , and Anisogammarus p u g e t t e n s i s accounted f o r more than 80% of the t o t a l abundance i n most months.  Corophium acherusicum i n d i v i d u a l s a l o n e accounted f o r more than  88% of the t o t a l numbers i n October.  Species d i v e r s i t y  s i g n i f i c a n t l y h i g h e r i n A p r i l and May  when the r e l a t i v e abundance of a  fifth  was  s p e c i e s , I s c h y r o c e r u s sp. i n c r e a s e d . 3) No s i g n i f i c a n t  i n amphipod abundance ' 0.01m  or per u n i t  differences  s u r f a c e a r e a of p l a n t were  found among the treatment p l o t s i n which d i f f e r e n t r e l a t i v e numbers of e e l g r a s s shoots were removed.  There was  a s i g n i f i c a n t trend i n  i n c r e a s i n g abundance of amphipods i n the sediment  i n these p l o t s ,  however, which corresponded t o the r e l a t i v e i n t e n s i t y of the shoot removal  " d i s t u r b a n c e " . 4) S i g n i f i c a n t l y h i g h e r d e n s i t i e s of amphipods  were r e c o r d e d i n the d r i f t per u n i t  s u r f a c e a r e a than were found on the  Z o s t e r a marina i n a l l months except August, A p r i l , May,  and June. 5)  The  o v e r a l l abundance of amphipods c o l l e c t e d on each of the p l a n t s u b s t r a t e s was  c o r r e l a t e d s i g n i f i c a n t l y w i t h macrophyte biomass and  T h i s p o s i t i v e r e l a t i o n s h i p was  a l s o observed on the two types of p l a n t  s u b s t r a t e s i n i n d i v i d u a l months i n the autumn. amphipods c o l l e c t e d i n the sediment was biomass of d r i f t per quadrat. of  surface area.  The o v e r a l l abundance of  c o r r e l a t e d n e g a t i v e l y w i t h mean  N e g a t i v e c o r r e l a t i o n s between the number  amphipods c o l l e c t e d i n the sediment and e e l g r a s s biomass per quadrat  o c c u r r e d i n November; p o s i t i v e c o r r e l a t i o n s between these two were i d e n t i f i e d i n June.  variables  99 P a r t B.  The Three-Month Study  These r e s u l t s d e s c r i b e the p a t t e r n s o f d i s t r i b u t i o n and abundance of amphipods i n t h r e e areas o f d i f f e r e n t n a t u r a l d e n s i t i e s o f e e l g r a s s shoots from May - J u l y 1984.  These a r e a s i n c l u d e : 1) Area 1 - a mixed  stand o f Z o s t e r a marina and Z. j a p o n i c a ( F i g . 22); 2) A r e a 2 - low d e n s i t y o f Z. marina marina 1.  ( F i g . 23); and 3) Area 3 - h i g h d e n s i t y o f Z.  ( F i g . 24).  Physical Factors Monthly a i r and water temperatures and s u r f a c e  salinity  measurements a r e d e s c r i b e d i n Part A: The Twelve-Month Study. The sediments o f the t h r e e a r e a s were c l a s s i f i e d as f i n e , fine,  to very  sands w i t h a mean g r a i n diameter o f 0.11mm i n A r e a 2, and 0.14mm  i n Areas 1 and 3 ( T a b l e 13).  The s i l t - c l a y f r a c t i o n accounted f o r 13  and 14% o f t h e sample d r y weight i n Areas 1 and 2, r e s p e c t i v e l y , and f o r 5% o f the d r y weight i n Area 3. Synchronized measurements o f the depth o f submergence i n the t h r e e study s i t e s , made a t lower low water on 27 J u l y 1984,  r e v e a l e d a maximum  d i f f e r e n c e o f 6cm i n e l e v a t i o n between t h e t h r e e a r e a s .  A r e a 1 (13cm)  and Area 3 (16cm) were s l i g h t l y lower i n e l e v a t i o n t h a t was A r e a 2 (10cm). The mean o r g a n i c content o f t h e sediments i n t h e 0 - 5cm c o r e f r a c t i o n s ranged from 1.3-2.2% i n a l l areas ( T a b l e 1 4 ) .  Significant  d i f f e r e n c e s were i n d i c a t e d among the a r e a s over t h e t h r e e month study period  (ANOVA, p<0.005), but a m u l t i p l e range t e s t was unable t o  d i s t i n g u i s h between the means (SNK, a =0.05).  From a n examination o f  the mean v a l u e s i n T a b l e 14, i t seems apparent t h a t t h e d i f f e r e n c e i n  100 F i g u r e 22.  A r e a 1, 28 J u l y 1984.  A. The stand o f mixed species.  Zostera  The shoot d e n s i t y o f Z o s t e r a  j a p o n i c a i n c r e a s e d throughout t h e summer u n t i l J u l y when the l e a v e s o f t h e s e p l a n t s provided  a c o n t i n u o u s canopy over t h e  s u r f a c e o f t h e sediment. 9  B. A 0.25nr quadrat c o n t a i n i n g b o t h s p e c i e s o f Z o s t e r a . Note t h e f i l m of diatoms on the b l a d e s  o f Z. j a p o n i c a and t h e  comparatively  c l e a n blades  Z. marina, f i v e t h i s photograph.  of the  shoots o f which appear i n  101  102 F i g u r e 23. A r e a 2, 28 J u l y 1984.  A. Low d e n s i t y o f Z. marina  shoots  (14 ± 2 shoots * 0.25m" ;x ± S.E.). 2  Although t h e d e n s i t y o f e e l g r a s s shoots was r e l a t i v e l y u n i f o r m i n t h i s  area,  unvegetated patches d i d o c c u r , c r e a t e d by localized  i r r e g u l a r i t i e s i n topography.  T y p i c a l unvegetated patches a r e shown i n this  photograph. 9  B. A 0.25m  quadrat c o n t a i n i n g Z. marina and  Enteromorpha sp.  103  104 F i g u r e 24.  Area 3, 28 J u l y 1984.  A. High d e n s i t y Z. marina  shoots  (23 ± 1 shoots • 0.25m" ; x ± S.E.). 2  B. A 0.25m  2  quadrat c o n t a i n i n g Z. marina and  d e c a y i n g U l v a sp.  105  106  T a b l e 13.  P a r t i c l e s i z e d i s t r i b u t i o n of sediments i n the three areas of d i f f e r e n t d e n s i t i e s o f Z o s t e r a s h o o t s ( J u n e 28, 1 9 8 4 ) . Values a r e mean p e r c e n t a g e d r y w e i g h t i n e a c h s i z e c l a s s (n = 3 ) . VFS FS  = =  Very f i n e sand F i n e sand  Area 1 : Mixed stand of Z o s t e r a marina and _Z_. j a p o n i c a . A r e a 2 : Low d e n s i t y o f Z_. m a r i n a s h o o t s . A r e a 3 : H i g h d e n s i t y o f _Z. m a r i n a s h o o t s .  Size Class (mm)  Area 1  Area 2  Area 3  (FS)  (VFS)  (FS)  > 0.595  < 1  < 1  1  0.355-0.595  1  1  1  0.180-0.355  81  73  82  0.075-0.180  10  19  10  0.053-0.075 < 0.053  1 7  1 5  1 5  T a b l e 14.  P e r c e n t a g e o r g a n i c c o n t e n t (x + S.E.) of sediment c o r e f r a c t i o n s i n areas o f d i f f e r e n t d e n s i t i e s of Zostera shoots f r o m May - J u l y , 1984. Area Area Area  Core Fraction  1 : Mixed s t a n d o f Z o s t e r a marina and _Z. j a p o n i c a . 2 : Low d e n s i t y o f Z_. m a r i n a s h o o t s . 3 : H i g h d e n s i t y o f Z_. m a r i n a s h o o t s .  May  June  July  Area 1  Area  2  Area 3  Area  1  Area 2  Area 3  Area 1  Area  2  Area  3  0-5 cm  2.11.1  1.6+.1  2.01.1  2.0+.2  1.31.1  1.8+.1  2.2 + .1  1.5K.1  2.0K.1  5-10 cm  1.6+.2  1.4+.1  1.21.1  1.2+.1  1.0K.1  1.0K.1  1.3K.1  1.0±.l  1.0K.1  108 o r g a n i c c o n t e n t among t h e sediment  i n t h e t h r e e a r e a s which was shown by  the ANOVA was caused by t h e lower o r g a n i c c o n t e n t i n A r e a 2. The mean o r g a n i c content of the sediments i n t h e 5-10cm c o r e f r a c t i o n s ranged from 1.0-1.6% o f the sediment d r y weight i n t h e t h r e e a r e a s over the summer months ( T a b l e 1 4 ) . M u l t i p l e range t e s t s were a l s o unable t o i d e n t i f y s i g n i f i c a n t d i f f e r e n c e s between a r e a s i n t h i s f r a c t i o n , but i t appears that i n t h i s case, A r e a 1 sediment had a h i g h e r o r g a n i c content than d i d sediments i n e i t h e r A r e a 2 o r 3 ( T a b l e 14). 2.  Biomass and Shoot D e n s i t y o f Z o s t e r a marina F i g u r e 25 i l l u s t r a t e s t h e monthly mean d r y weight' o f i n d i v i d u a l  Z o s t e r a marina above-ground  shoots from t h e t h r e e study a r e a s .  With t h e  e x c e p t i o n of Area 3 i n June, when t h e p l a n t s weighed s i g n i f i c a n t l y more than t h e y d i d i n A r e a 3 i n May, Area 1 i n June, and Area 2 i n J u l y , few d i f f e r e n c e s were found i n d r y weight of shoots over t h e summer months (ANOVA, p<0.025; SNK,a =0.05).  In a d d i t i o n , t h e r e was no s i g n i f i c a n t  d i f f e r e n c e i n mean s u r f a c e a r e a p e r shoot between a r e a s o r months (ANOVA, p>0.10) d e s p i t e the f a c t t h a t t h e mean number of b l a d e s per shoot ranged from 5 i n Area 3 i n June and J u l y t o 9 i n Area 1 i n May. A l s o , no s i g n i f i c a n t d i f f e r e n c e s were found i n t h e mean d r y weight of e e l g r a s s rhizomes per c o r e between months or a r e a s (ANOVA, p>0.05) (Table 15). F i g u r e 26 i l l u s t r a t e s t h e monthly mean shoot d e n s i t y o f Z o s t e r a marina i n each of t h e t h r e e a r e a s .  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 i n d e n s i t i e s of shoots d u r i n g t h e three-month w i t h i n a r e a s (ANOVA, p>0.05).  study p e r i o d  However, among a r e a s , shoot d e n s i t i e s  were s i g n i f i c a n t l y d i f f e r e n t i n a l l months except i n Areas 2 and. 3 i n May (ANOVA, p<0.0005; SNK,a =0.05).  O v e r a l l mean d e n s i t y was 4  +  1  109 F i g u r e 25.  Dry weight o f above-ground  Z o s t e r a marina  shoots (x ± S.E.) i n monthly  samples i n Areas  1, 2, and 3 May - J u l y 1984. (N = 6 ) .  F i g u r e 26.  D e n s i t i e s of Z o s t e r a marina shoots * 0.25m ^ _  (x ± S.E.) i n Areas 1, 2, and 3, A p r i l - J u l y 1984 (N = 6 ) .  Shoot count i n A p r i l  i s an e s t i m a t e . D i f f e r e n c e s were  i n Area 1  significant  (p>0.05) among a l l areas i n each month w i t h the  e x c e p t i o n o f May i n Areas 2 and 3.  110  in  m  o  ro  April  17  Hay  15  J u n e 27  19B4  July  26  Ill  T a b l e 15.  D r y w e i g h t (g) o f Z o s t e r a m a r i n a r h i z o m e s • core (x ± S.E.) i n A r e a s 1, 2, and 3, May - J u l y , 1984. T h e r e was no s i g n i f i c a n t d i f f e r e n c e i n mean d r y w e i g h t among months, a r e a s , o r months x a r e a s (2-way ANOVA, p > 0.25) . - 1  May Area 1  0.08 ±0.02  Area 2  0.28 ±0.10  June Area 3  Area 1  0.29 ±0.08  0.29 ±0.12  Area 2  0.35 ±0.09  July Area 3  Area 1  0.30 ±0.10  0.17 ±0.08  Area  Area 3  0.20 ±0.06  0.33 ±0.11  2  112 shoots  • 0.25m"  (x ± S.E.) i n Area 1, 14 ± 2 shoots  2  2. and 25 ± 1 s h o o t s '  0.25m  2  2  i n Area  i n Area 3 ( F i g . 2 6 ) . The e q u i v a l e n t dry-  weights were 9, 47, and 86 g * m~  2  3.  • 0.25m~  i n Areas 1, 2, and 3, r e s p e c t i v e l y .  Biomass and Shoot D e n s i t y o f Z o s t e r a j a p o n i c a F i g u r e 27 i l l u s t r a t e s t h e mean shoot d e n s i t y of Z o s t e r a j a p o n i c a  i n Area 1 from May - J u l y 1984.  U n l i k e Z. marina,  t h e above-ground  shoots o f Z. j a p o n i c a d i e back c o m p l e t e l y d u r i n g t h e w i n t e r and t h i s s p e c i e s i s l a r g e l y dependent on seed d i s p e r s a l and g e r m i n a t i o n f o r i t s spring recolonization April  • 0.25m~ were observed w i t h i n Area 1. 2  shoot d e n s i t y o f t h i s 0.25m  -2  In  than 30 Z.  By May, t h e  s p e c i e s had r i s e n t o a p p r o x i m a t e l y 80 shoots •  and, by t h e f o l l o w i n g month, i t s d e n s i t i e s had i n c r e a s e d t h r e e -  ( F i g . 2 7 ) . In J u l y , a t low t i d e , t h e l e a v e s of Z. j a p o n i c a formed  an u n i n t e r r u p t e d canopy over t h e sediment  4.  1984).  1984, when t h e t h r e e study areas were s e l e c t e d , fewer  j a p o n i c a shoots  fold  ( H a r r i s o n , p e r s o n a l communication,  surface ( F i g . 22).  Composition and D i s t r i b u t i o n o f D r i f t F i g u r e 28 i l l u s t r a t e s t h e monthly mean d r y weights o f each  component o f t h e d r i f t  t h a t was c o l l e c t e d i n the t h r e e study a r e a s .  A r e a 1, l i v e Z o s t e r a j a p o n i c a , c o l l e c t e d as d r i f t , component o f these samples ( F i g . 28A). (Fig. sp.  28B and C ) .  Here, t h e d r i f t  In  was t h e dominant  I t was not found i n Areas 2 or 3  was dominated  by f r e e - f l o a t i n g U l v a  While t h e biomass of t h i s a l g a peaked i n May i n Area 3 and  d e c l i n e d through t h e r e s t o f t h e summer, i t was most abundant i n June i n A r e a 2 ( F i g . 28B). Fragments o f e e l g r a s s b l a d e s , mats of Enteromorpha sp. and, t o a l e s s e r e x t e n t , L a m i n a r i a s a c c h a r i n a , were a l s o i n these two a r e a s from May - J u l y  ( F i g . 28B and C ) .  collected  113 F i g u r e 27.  D e n s i t y of Z o s t e r a (x ± S.E.)  j a p o n i c a shoots  i n Area 1,  (N = 6 ) .  • 0.25m'  115 F i g u r e 28A. Monthly mean d r y weights  • 0.01m  of major components of d r i f t  -2  (±  i n Area  S.E.) 1.  (N = 6 ) . U = U l v a sp., Z.m  = dead  Zostera  marina, and Z.j = l i v e Z o s t e r a  japonica.  9TT  117 F i g u r e 28B. Monthly mean d r y weights  ' 0.01m"  2  of major components of d r i f t  (±  i n Area 2.  (N = 6 ) . E = Enteromorpha Z.m  sp., U = U l v a sp.,  = dead Z o s t e r a marina.  s  S.E.)  8TI  119 F i g u r e 28C.  Monthly mean d r y weights  • 0.01  m  -2  (±.S.E.)of major components of d r i f t Area 3.  (N = 6 ) .  L = Laminaria Z.m  in  s a c c h a r i n a , U = U l v a sp.,  = Z o s t e r a marina.  and  121 5. Z o s t e r a marina  Shoot D e n s i t y and t h e D i s t r i b u t i o n of Amphipods  A two-way a n a l y s i s of v a r i a n c e was used t o i n v e s t i g a t e d i f f e r e n c e s i n t o t a l mean abundances of amphipods • 0.01m  -2  among t h e t h r e e months  and t h e t h r e e areas o f d i f f e r e n t d e n s i t i e s o f Z o s t e r a marina  shoots.  The h i g h e s t mean d e n s i t i e s o c c u r r e d i n J u l y u s i n g d a t a p o o l e d from a l l areas (SNK, a =0.05) ( T a b l e 16).  No s i g n i f i c a n t d i f f e r e n c e s i n mean  amphipod abundance, however, were found among t h e t h r e e study areas u s i n g d a t a pooled from May, June, and J u l y (ANOVA, p>0.05) ( F i g . 2 9 ) . Although 0.01m  -2  s i g n i f i c a n t d i f f e r e n c e s d i d occur i n mean amphipod number * among areas i n c e r t a i n months (ANOVA, p<0.05; SNK, a=0.05), with  the e x c e p t i o n of June t h e r e was no apparent p a t t e r n t o these v a r i a t i o n s . The d e n s i t y of amphipods was s i g n i f i c a n t l y d i f f e r e n t among areas i n June with t h e h i g h e s t numbers of animals c o l l e c t e d i n Area 2 and t h e lowest numbers c o l l e c t e d i n Area 3 (SNK,a=0.05) ( F i g . 29).  Lowest mean  abundance was r e c o r d e d i n May i n Areas 2 and 3 (SNK,a =0.05). 6.  D i s t r i b u t i o n and O v e r a l l Abundance of Amphipods When d a t a from a l l t h r e e areas were p o o l e d ,  significant  d i f f e r e n c e s i n abundance between months were r e v e a l e d SNK,  (ANOVA, p<0.05;  a=0.05). Amphipod abundances i n c r e a s e d t h r e e - f o l d from May - June  and from June - J u l y  (Table 1 6 ) .  F i g u r e s 30, 31, and 32 i l l u s t r a t e t h e o v e r a l l monthly mean abundances of amphipods w i t h i n t h e t h r e e study areas and t h e i r d i s t r i b u t i o n on t h e p l a n t s and sediment  substrates.  A three-way  a n a l y s i s o f v a r i a n c e was used t o i n v e s t i g a t e d i f f e r e n c e s i n t h e number of amphipods p e r u n i t a r e a o f p l a n t and o f sediment  s u r f a c e , among  months and areas of d i f f e r e n t d e n s i t i e s o f e e l g r a s s s h o o t s . i n c r e a s e s i n abundance o c c u r r e d i n both sediment  and d r i f t  Significant over  time;  122  T a b l e 16.  M o n t h l y t o t a l number o f a m p h i p o d s • m~ (x ± 9 5 % C.L.) p o o l e d f r o m a l l s a m p l e s i n A r e a s 1, 2, and 3 (n = 1 8 ) . * v a l u e s i n d i c a t e means w h i c h a r e s i g n i f i c a n t l y d i f f e r e n t (ANOVA, p < 0.05; DMR, oc = 0 . 0 5 ) . 2  May  June  July  X  2240  6980  20187  + C.L.  1508-3320  4503-10789  13672-30169  123 F i g u r e 29.  Monthly abundance o f amphipods (x 100) •  m  -2  (x ± S.E.) i n Areas 1, 2, and 3, (N = 6 ) . A b s o l u t e abundances o f amphipods were p o o l e d from a l l quadrats.  s u b s t r a t e samples w i t h i n  0.01m  125 F i g u r e 30. Mean number of amphipods of Z o s t e r a marina and 3. (N = 6 ) .  • m ^ of s u r f a c e  (x ± S.E.) i n Areas 1, 2,  931  127 F i g u r e 31. Mean number o f amphipods ( x l O ) * m s u r f a c e of. d r i f t and  3. (N = 6 ) .  -2  of  (x ± S.E.) i n A r e a s 1, 2,  May 15  June 27 1984  July 26  129 F i g u r e 32. Mean number o f amphipods (x 10) • sediment s u r f a c e (x ± S.E.) i n Areas 1, 2, and 3. (N = 6 ) .  Mean Number o f Amphipods x 10 /m  OCT  131 t h e r e were no s i g n i f i c a n t d i f f e r e n c e s , however, i n amphipod abundance among months i n the Z o s t e r a marina samples (ANOVA, p<0.002; DMR,a =0.05).  As i n P a r t A: The Twelve-Month Study, the d r i f t  s i g n i f i c a n t l y more amphipods per u n i t (ANOVA, p<0.0005; DMR,  supported-  s u r f a c e a r e a than d i d the e e l g r a s s  a =0.05) ( F i g s . 30 and 31).  S i g n i f i c a n t d i f f e r e n c e s i n amphipod d e n s i t y per u n i t s u r f a c e a r e a o c c u r r e d between areas i n both the p l a n t and sediment samples (ANOVA, p<0.05; SNK,a =0.05). drift  For example, more amphipods were c o l l e c t e d i n the  and i n the sediment i n Areas 1 and 2 than i n Area 3, over the  three-month p e r i o d  ( F i g s . 31 and 32).  There was no  d i f f e r e n c e between the number of amphipods per u n i t c o l l e c t e d i n the d r i f t  significant surface area  i n Area 3, and i n any of t h e e e l g r a s s samples.  C o r r e l a t i o n a n a l y s e s were used t o examine t h e a s s o c i a t i o n between the  abundance of amphipods i n the e e l g r a s s , d r i f t ,  and sediment samples,  and  macrophyte biomass and s u r f a c e a r e a , r e s p e c t i v e l y  ( T a b l e 17).  S i g n i f i c a n t c o r r e l a t i o n s were found i n A r e a 2 i n May and June, and i n all  t h r e e a r e a s i n J u l y ( T a b l e 17).  c o l l e c t e d i n the d r i f t dry  weight o f t h e d r i f t  In May,  the number o f amphipods  i n Area 2 was c o r r e l a t e d s i g n i f i c a n t l y with t h e i n each sample ( T a b l e 17) (r=0.81.,. p<0.05,).  S i m i l a r p o s i t i v e c o r r e l a t i o n s , i n v o l v i n g both d r y weight and s u r f a c e a r e a of d r i f t , July  o c c u r r e d i n the samples c o l l e c t e d i n Areas 2 and 3 i n  (r=0.92, p<0.01 and r=0.85, p<0.01 - d r y weight; r=0.89, p<0.05 and  r=0.84, p<0.05 - s u r f a c e a r e a , r e s p e c t i v e l y ) .  In c o n t r a s t , i n Area 1 i n  J u l y , t h e abundance of amphipods i n t h e d r i f t ,  p r i m a r i l y composed of  Z o s t e r a j a p o n i c a , was more c l o s e l y r e l a t e d t o s u r f a c e a r e a (r=0.95, p<0.01) t h a n i t was t o d r y weight ( T a b l e 1 7 ) .  Significant  correlations  between t h e t o t a l abundance of amphipods per sample quadrat and d r i f t dry  weight and s u r f a c e a r e a i n Area 2 and 3 i n J u l y r e f l e c t  the large  132  Table  17.  C o r r e l a t i o n c o e f f i c i e n t s (R) f o r m o n t h l y c o m p a r i s o n s o f numbers o f amphipods collected i n Z o s t e r a m a r i n a , d r i f t , s e d i m e n t , and t o t a l quadrat (0.01m ) samples w i t h s u r f a c e a r e a ( c m ) a n d d r y w e i g h t ( g ) o f _Z. m a r i n a and d r i f t • 0.01m" (n = 6 ) . * values i n d i c a t e s i g n i f i c a n t R (p < 0 . 0 5 ) . - 2  2  2  1 2 3 4  = = = =  _Z. m a r i n a s u r f a c e a r e a (cm ) _Z. m a r i n a d r y w e i g h t (g) D r i f t s u r f a c e a r e a (cm ) D r i f t dry weight (g) 2  [cont.]  Table  1.7.  [cont.]  AREA  June  July  AREA  2  AREA  3  Total  •L. marina  -.480  .242  .457  -.240  -.423  -.099  -.205  .321  -.591  . 325  .091  -.464  .258  .445  -.241  .448  -.126  -.179  . 330  -.587  .342  .596  .028  .662  -.035  -.018  .797  -.033  .475  .215  .474  .361  .603  4  .508  .196  .629  .051  -.026  .813*  -.092  .435  .214  .468  -.360  .598  1  .045  .074  -.550  .260  . 560  -.043  .888*  .080  .798  -.140  .078  .168  2  .065  .086  -.534  .274  . 585  -.045  .902*  .080  .795  -.157  .069  .155  3  -.168  .078  .080  -.159  .085  .641  -.377  .569  .057  . 104  -.524  .044  4  -.513  -.174  . 255  -.537  . 104  .669  -.370  .598  .127  .007  -.367  .004  Z. marina  Drift  1  .065  .072  2  .086  3  Month  May  1  Sedim.  Drift  Sedim.  Total  Z. marina  Drift  Sedim.  Total  1  .587  .506  -.596  .465  .467  -.185  .476  -.104  .467  -.121  -.079  -.168  2  .588  .522  -.577  .486  .441  -.203  .478  -.122  .465  -.114  -.088  -.162  3  -.138  .952*  -.012  .909*  . 190  .888*  -.410  .910*  -.594  .842*  -.587  .855*  4  -.230  .705  -.233  .639  . 130  .924*  -.495  .949*  -.616  .847*  -.602  .856*  134 p r o p o r t i o n of animals i n each quadrat t h a t were c o l l e c t e d i n the (Table  drift  17). When d a t a from a l l months and  study areas were pooled,  abundance of amphipods i n the sediment was the d r i f t  n e g a t i v e l y c o r r e l a t e d with  as w e l l as t o t a l macrophyte, biomass and  quadrat- ( T a b l e 18).  the  surface area  per  In June i n Area 2, however, s i g n i f i c a n t p o s i t i v e  c o r r e l a t i o n s between the abundance of amphipods c o l l e c t e d i n the sediment samples, and both the d r y weight and  the s u r f a c e a r e a of  Z o s t e r a marina per quadrat were observed ( T a b l e 17).  Similar  r e l a t i o n s h i p s between the abundance of amphipods c o l l e c t e d i n the and  e e l g r a s s d r y weight and  s u r f a c e area, were found i n June i n the  study a r e a d e s c r i b e d i n P a r t A: The 7.  cores,  D i s t r i b u t i o n of Amphipod  Twelve-Month Study (Table  7).  Species  A l t h o u g h f o u r t e e n s p e c i e s of amphipods were c o l l e c t e d i n the three-month study, A, due  o v e r a l l s p e c i e s d i v e r s i t y was  t o the dominance of f o u r s p e c i e s .  low,  as i t was  i n Part  Thus, Corophium acherusicum,  C. i n s i d i o s u m , Anisogammarus p u g e t t e n s i s , and Ampithoe v a l i d a accounted for  more than 80% of the t o t a l numbers i n June and  Overall diversity i t was  (H')  i n J u l y , due  was  s i g n i f i c a n t l y higher  J u l y (Table  i n May  and  sp.  (Table 19).  i n Area 1 (H' = 1.13)  and  i n J u l y i n Area 3 (H = 0.83)  p<0.0001;DMR,a =0.05).  the t o t a l abundance and 19).  fifth  Lowest d i v e r s i t y o c c u r r e d  in  May  (ANOVA,  Evenness (J')was r e l a t i v e l y h i g h i n a l l samples,  i n d i c a t i n g t h a t the r a r e amphipod s p e c i e s c o n t r i b u t e d few  (Table  i n June than  to the presence of l a r g e numbers of the  species, Ischyrocerus  19).  i n d i v i d u a l s to  the dominant s p e c i e s were e q u i t a b l y d i s t r i b u t e d  135  T a b l e 18.  C o r r e l a t i o n c o e f f i c i e n t s (R) f o r c o m p a r i s o n s o f number o f a m p h i p o d s i n Z o s t e r a m a r i n a , d r i f t , s e d i m e n t , and t o t a l q u a d r a t (0.01m ) samples w i t h s u r f a c e a r e a (cm ) and d r y w e i g h t (g) o f Z_. m a r i n a , d r i f t , a n d t o t a l macrophytes • 0.Olrn . A l l samples c o l l e c t e d i n A r e a s 1, 2, a n d 3, May - J u l y , 1984 (n = 5 4 ) . * v a l u e s i n d i c a t e s i g n i f i c a n t R (p < 0.05) . 2  2  - 2  Variable  Z. m a r i n a  Drift  surface area  0.219  -0.176  -0.230  -0.169  dry  0.218  -0.181  -0.227  -0.176  surface area  -0.167  0.080  -0.489*  -0.027  dry  -0.058  0.172  -0.342*  0.088  -0.137  0.066  -0.506*  -0.038  0.021  0.148  -0.392*  0.072  Sediment  Total  Z. m a r i n a :  weight  Drift:  weight  Total macrophytes: surface area dry  weight  Table 19.  R e l a t i v e abundance, d i v e r s i t y , and evenness of s p e c i e s o f amphipods • 0.01m i n Areas 1, 2, and 3, May - J u l y , 1984. * v a l u e s i n d i c a t e a r e a x months w h i c h were s i g n i f i c a n t l y d i f f e r e n t (2-way ANOVA, p < 0.05; DMR, « = 0.05 ) . -2  n = H' = J' =  T o t a l number c o u n t e d Shannon-Wiener D i v e r s i t y Shannon-Wiener Evenness  May Species  n =  1  June  July  Area 1  Area 2  Area 3  Area 1  Area 2  Area 3  Area 1  Area 2  Area 3  249  60  145  853  372  176  728  1582  929  Ampithoe lacertosa  -  1.7  0.7  _  A. v a l i d a  3.2  1.7  0.7  7.0  1.6  2.3  33.8  1.2  52.2  21.7  13.8  29.8  9.9  12.5  4.3  38.4  -  -  1.4  _  _  _  _  _  _  0.4  8.3  0.7  _  _  _  _  _  _  _  _  _  _  n.l 0.1  Anisogammarus pugettensis Atylus collinqii Calliopius laeriusculus Calliopius sp. Corophium acherusicum  1.7 29.3  26.7  _ 20.0  _ 46.1  _ 57.2  _  _  51.1  32.7  _ 43.3  76.9 [cont. ]  Table  19.  [cont.]  May Species  Area 1  Area 2  249  60  12.4  0.4  June Area  3  July  Area 1  Area 2  Area 3  Area 1  Area 2  Area 3  145  853  372  176  728  1582  929  8.3  6.9  17.1  29.6  13.6  29.0  17.0  22.9  28.3  33.8  0.5  14.8  -  n.5  -  C.  insidiosum Ishyrocerus sp.  -  Parapleustes pugettensis  _  _  _  _  _  Photis oligochaeta  -  Pontogeneia intermedia Pontogeneia  -  -  12.4  2.0  -  9.7  Synchelidium shoemakeri  -  0.2  -  Total  7  9  rostrata  Species  Diversity Evenness  (H') (J')  0.5 _ -  _  _  0.5 _  10  -  4  5.1 _  _  _  _  _  _  5  4  4  0.3 _  7  _  7  1.13  1.40  1.86*  1.58  1.19  1.30  1.26  1.26  0.83*  0.76  0.90  0.85  0.90  0.81  0.77  0.82  0.86  0.83  138 F i g u r e s 33, 34, and 35 i l l u s t r a t e t h e monthly mean abundances o f the dominant amphipod s p e c i e s per square meter of e e l g r a s s , d r i f t , and sediment s u r f a c e , r e s p e c t i v e l y , i n each o f t h e t h r e e  areas.  A comparison o f the p a t t e r n s o f s p e c i e s d i s t r i b u t i o n among t h e areas of d i f f e r e n t e e l g r a s s shoot d e n s i t y i n the Z o s t e r a marina and drift  samples, r e v e a l s n o t a b l e d i f f e r e n c e s .  May, both the e e l g r a s s and t h e d r i f t the f r e e - l i v i n g  For example, i n Area 1 i n  samples c o n t a i n e d  s p e c i e s , Anisogammarus p u g e t t e n s i s  l a r g e numbers of  ( F i g s . 33 and 3 4 ) .  In June and J u l y , as numbers o f t h i s amphipod s p e c i e s d e c l i n e d , d e n s i t i e s of the t h r e e t u b e - d w e l l i n g  s p e c i e s , Corophium acherusicum, C.  i n s i d i o s u m , and Ampithoe v a l i d a , i n c r e a s e d  ( F i g s . 33 and 34).  In  c o n t r a s t , i n Area 2, abundances o f a l l f o u r s p e c i e s were lower i n June than they were i n e i t h e r May or J u l y , a t l e a s t i n the d r i f t The  fifth  species, Ischyrocerus  ( F i g . 34).  sp., was c o l l e c t e d i n t h e e e l g r a s s i n  t h i s a r e a i n May and June, and d e c l i n e d i n abundance from June - J u l y , w h i l e t h e d e n s i t i e s o f the other f o u r s p e c i e s i n c r e a s e d Area 3, I s c h y r o c e r u s  sp. was a l s o p r e s e n t  ( F i g . 3 3 ) . In  on Z. marina i n l a r g e numbers  i n May and June ( F i g . 3 3 ) . O v e r a l l numbers of the four dominant  species  were low on both s u b s t r a t e s i n t h i s a r e a , d e s p i t e an i n c r e a s e i n t h e number o f C. acherusicum i n the d r i f t  i n J u l y ( F i g s . 33 and 3 4 ) .  The p a t t e r n o f d i s t r i b u t i o n and abundance o f each o f t h e dominant amphipod s p e c i e s on t h e two types of macrophyte s u b s t r a t e v a r i e d among the t h r e e areas  ( F i g s . 33 and 34).  A l t h o u g h t h e abundance o f A.  p u g e t t e n s i s d e c l i n e d i n both Area 1 and 2 from May - June, d e n s i t i e s i n c r e a s e d more than f o u r - f o l d i n Area 2 from June - J u l y , w h i l e  they  decreased  species  i n Area 1 ( F i g s . 33 and 34).  Few i n d i v i d u a l s o f t h i s  were c o l l e c t e d i n Area 3 on t h e p l a n t s u b s t r a t e s throughout t h e summer ( F i g s . 33 and 3 4 ) .  139a F i g u r e 33A.  Mean number of i n d i v i d u a l s amphipod s p e c i e s  • m~  2  s u r f a c e of  Z o s t e r a marina (x ± S.E.) (N = 6 ) . 1 = Corophium  i n each  dominant live  i n Area 1.  acherusicum,  2 = C. i n s i d i o s u m , and 4 = Anisogammarus pugettensis.  o  CD  Area 1 Zostera  marina  139c F i g u r e 33B.  Mean number o f i n d i v i d u a l s amphipod s p e c i e s  • m  2 = C. i n s i d i o s u m ,  acherusicum,  3 = Ampithoe  sp.  live  i n A r e a 2.  4 = Anisogammarus p u g e t t e n s i s , 5 = Ischyrocerus  dominant  s u r f a c e of  Z o s t e r a marina (x ± S.E.) (N = 6 ) . 1 = Corophium  i n each  yalida, and  May 15  B.  June 27 1984  J u l y 26  139e F i g u r e 33C.  Mean number o f i n d i v i d u a l s amphipod  species  • m~  2  i n each dominant  surface of l i v e  Z o s t e r a marina (x ± S.E.) i n A r e a 3. (N = 6 ) . 1 = Corophium acherusicum and 5 = I s c h y r o c e r u s sp.  O  o ID  o o in CM  0)  o o  TJ O  a a  O  o ro  Hi  E  O  z  o o cu  c ra 0}  o o  o  H May  C.  15  June 27 1984  J u l y 26  140a F i g u r e 34A.  Mean number of i n d i v i d u a l s amphipod drift  species  (x 10)  i n each dominant * m  -2  s u r f a c e of,  (x ± S.E.) i n A r e a 1. (N = 6 ) .  1 = Corophium acherusicum, 2 = C. i n s i d i o s u m , and  3 = Ampithoe  4 = Anisogammarus  valida,  pugettensis.  140c F i g u r e 34B.  Mean number of i n d i v i d u a l s i n each dominant amphipod drift  species  (x ± S.E.)  (x 10) • m~  2  s u r f a c e of  i n Area 2. (N = 6 ) .  1 = Corophium acherusicum, 2 = C. i n s i d i o s u m ,  4 =  Anisogammarus  p u g e t t e n s i s , and 5 = Ischyrocerus  sp.  140e F i g u r e 34C.  Mean number of i n d i v i d u a l s i n each dominant amphipod s p e c i e s drift  (x 10) • m~  2  surface of  (x ± S.E.) i n Area 3 . (N = 6 ) .  1 = Corophium acherusicum, 2 = C. i n s i d i o s u m .  O ID (0  CM E  O O  cn  o in cu  0)  TJ  O  rx JZ  o o cu  a.  E <  a c  o in  o o  (D CU  o m  o  H  May 15  C.  June 27 1984  J u l y 26  141a F i g u r e 35A.  Mean number of i n d i v i d u a l s i n each dominant amphipod  species  sediment s u r f a c e  (x 10)  • m * s u r f a c e of  (x ± S.E.)  i n Area 1.  (N = 6 ) . 1 = Corophium acherusicum, 2 = C. i n s i d i o s u m , and 4 = pugettensis.  Anisogammarus  o o in  Area 1 Sediment  y  y  r May 15  A.  y  y  y  y  y  y  y  y  y^.  y  June 27 1984  July 26  141c F i g u r e 35B.  Mean number o f i n d i v i d u a l s amphipod  s p e c i e s (x 10)  i n each dominant • m~  sediment s u r f a c e (x ± S.E.)  2  s u r f a c e of i n Area 2.  (N = 6 ) . 1 = Corophium acherusicum, 2 = C. i n s i d i o s u m , and 4 = pugettensis.  Anisogammarus  14 Id  141e F i g u r e 35C.  Mean number of i n d i v i d u a l s  i n each dominant  amphipod s p e c i e s (x 10) • n f sediment  2  surface of  s u r f a c e (x ± S.E.) i n Area 3.  (N = 6 ) . 1 = Corophium 2 = C. i n s i d i o s u m .  acherusicum  May 15  C.  June 27 1984  J u l y 26  142 In c o n t r a s t to Anisogammarus p u g e t t e n s i s ,  the two  Corophium  s p e c i e s i n c r e a s e d i n abundance on both p l a n t s u b s t r a t e s i n a l l areas from May  - J u l y , w i t h the e x c e p t i o n of the d r i f t  i n Area 2 and  the  Z.  marina i n Area 3, where d e n s i t i e s d i d not i n c r e a s e u n t i l J u l y ( F i g s .  33  and  the  34).  drift  Ampithoe v a l i d a was  most abundant i n both the e e l g r a s s and  i n Area 1 i n J u l y ( F i g s . 33 and  Ischyrocerus of Area 2 and  sp. was  34).  On  the other hand,  more abundant i n the m o n o s p e c i f i c  3 i n May  and  mixed stand of Z o s t e r a  June, than i t was  s p e c i e s i n Area 1  eelgrass  stands  e i t h e r i n J u l y or i n the  ( F i g . 33).  F i g u r e 35 i l l u s t r a t e s the p a t t e r n s of d i s t r i b u t i o n a n d  abundance  of t h r e e amphipod s p e c i e s which dominated the sediment samples from - July.  May  I t c l e a r l y demonstrates the s i g n i f i c a n t t r e n d i n d e c r e a s i n g  abundance from Area 1 t o Area 3 (ANOVA, p<0.0001; DMR,a =0.05). O v e r r a l l abundances • m  -2  of sediment s u r f a c e were t e n times g r e a t e r i n  Area 1 than they were i n Area 3 ( F i g s . 32 and Corophium acherusicum and  35).  C. i n s i d i o s u m were the most numerous  amphipod s p e c i e s c o l l e c t e d i n the c o r e samples i n a l l t h r e e areas 35).  T h e i r d e n s i t i e s i n c r e a s e d from May  plant substrates  ( F i g s . 33,  34,  and  - J u l y , as they d i d on the  35).  few  two  D e n s i t i e s of Anisogammarus  p u g e t t e n s i s peaked i n June i n the sediment i n Areas 1 and the e e l g r a s s and d r i f t ,  (Fig.  2 and,  as i n  i n d i v i d u a l s of t h i s s p e c i e s were c o l l e c t e d  i n the c o r e samples i n Area 3 ( F i g . 35).  While not  shown i n F i g u r e  35,  a few Ampithoe v a l i d a were c o l l e c t e d i n the sediment i n Area 1 i n a l l t h r e e months and  i n Areas 2 and  c o l l e c t e d i n the c o r e  samples.  3 i n July.  Ischyrocerus  sp. was  never  143 8.  N a t u r a l i s t S l e d Samples In the " n a t u r a l i s t  s l e d " c o l l e c t i o n s , amphipods were r e l a t i v e l y -  more abundant i n Area 1 i n May  and  June than i n the other  two  t o the presence of l a r g e numbers of Anisogammarus p u g e t t e n s i s 20). but  Few  Corophium spp. were c o l l e c t e d i n May  they i n c r e a s e d i n abundance i n June, and  samples c o l l e c t e d i n Areas 2 and never as abundant i n Area  due  (Table  i n any of the study  areas  n u m e r i c a l l y dominated  3 i n t h a t month (Table 20).  the  They were  1.  More s p e c i e s were c o l l e c t e d u s i n g the " n a t u r a l i s t than i n e i t h e r June or J u l y i n a l l t h r e e areas sp. were most abundant- i n Area 3 i n May d e c l i n e d even t h e r e  areas  (Table 20).  sled"  ( T a b l e 20).  in  May  Ischyrocerus  and by-June i t s . numbers.had  Pontogeneia r o s t r a t a d i s p l a y e d a  s i m i l a r p a t t e r n of abundance. Both were almost e n t i r e l y absent i n the J u l y b e n t h i c tows. undescribed  C a l l i o p i u s l a e v i u s c u l u s Barnard and  C a l l i o p i u s s p e c i e s , which were i n f r e q u e n t l y c o l l e c t e d i n  e i t h e r the p l a n t or the core stand of e e l g r a s s s p e c i e s remaining  samples, were abundant i n May  Other s p e c i e s which were c o l l e c t e d more  i n f r e q u e n t l y i n c l u d e d Paraphoxus spinosus Syncheldium shoemakeri.  c o l l e c t e d i n May  i n the mixed  (Area 1) but were not c o l l e c t e d i n tows i n the  months ( T a b l e 20).  Conlan, and  a second  i n Area 1 ( T a b l e  Holmes, P h o t i s o l i g o c h a e t a  A s i n g l e Eogammarus o c l a r i  20).  was  144  Table  20.  Numbers o f amphipods • t o w ~ l collected i n Naturalist sled areas of d i f f e r e n t d e n s i t i e s s h o o t s (n = 2 ) .  Axea  (x ± S.E.) i n three of Zostera  1  Area  1 ± <1  0  2  Area  3  May Ampithoe valida Anisogammarus pugettensis Calliopius spp.  138 ± 71  79  ± 7  39  ± 33  0 78 ± 57  8 ± 5  17 ± 2  Corophium spp.  72 ± 62  5 ± 4  14 ± 4  Eogammarus oclari  <1 ± <1  0  0  2 ± <1  8 + 8  38 ± 13  Paraphoxus spinosus  0  0  <1 ± <1  Photis oligochaeta  0  <1 ± <1  1 ± <1  3 ± 2  17 ± 11  50 ± 23  <1 ± <1  0  <1 ± <1  296  78  199  Ischyrocerus sp.  Pontogeneia spp. Synchelidium shoemakeri  Total  [cont.]  145  T a b l e 20.  [cont.]  Area 1  Area 2  Area 3  9 ± 1  2 ± 2  <1 ± <1  128 ± 49  30 ± 3  18 ± 7  June /Ampithoe valida Anisogammarus puqettensis Calliopius spp.  2 + 2  Corophium spp.  92 + 26  50 ± 37  20 ± 6  Ischyrocerus sp.  0  0  7 ± 1  Pontogeneia spp.  0  0  31 + 13  Synchelidium shoemakeri  0  2 + 1  <1 ± <1  231  88  82  Total  4 + 1  5 + 2  [cont. ]  146  T a b l e 20.  [cont.]  1  Area 2  Area 3  5 ± 1  3 ± <1  2 + 1  19 + 4  37 ± 24  4 ± 1  0  0  Area  July Ampithoe valida Anisogammarus pugettensis Calliopius spp. Corophium spp.  28  + 7  46  ± 7  <1  + <1  167  ± 84  Paraphoxus spinosus  0  0  <1  ± <1  Pontogeneia spp.  0  0  <1  ± <1  52  86  Total  174  147 9.  C o r r e l a t i o n M a t r i c e s and  the D i s t r i b u t i o n of  Species  T a b l e 21 shows the product-moment c o r r e l a t i o n c o e f f i c i e n t s f o r " p a i r - w i s e " comparisons of the r e l a t i v e abundances of the amphipod s p e c i e s c o l l e c t e d i n the t h r e e study areas i n May, Data from a l l t h r e e a r e a s and f o r comparative p u r p o s e s . correlations and  s u b s t r a t e types were p o o l e d  With the e x c e p t i o n of the  July  1984.  in this  table  negative  i n abundance which o c c u r r e d between Corophium acherusicum  Ischyrocerus  (Table 21).  June, and  sp. i n May,  The h i g h e s t  between the two  a l l s i g n i f i c a n t c o r r e l a t i o n s were p o s i t i v e  s i g n i f i c a n t c o r r e l a t i o n s o c c u r r e d i n a l l months  Corophium s p e c i e s (May,  r=0.816, p<0.01; June, r=0.858,  p<0.01; J u l y , r=0.882, p<0.01) ( T a b l e 21). Anisogammarus p u g e t t e n s i s was  The  abundance of  also closely correlated  with t h a t of  acherusicum and C. i n s i d i o s u m i n a l l t h r e e months ( T a b l e 21). significant, correlations v a l i d a and  the two  T a b l e 22  i n abundance were found  Corophium s p e c i e s i n June ( T a b l e  but  21).  shows the product-moment c o r r e l a t i o n c o e f f i c i e n t s f o r  c o l l e c t e d monthly i n each of the study a r e a s . i n abundance o c c u r r e d i n any of the study a r e a s  No n e g a t i v e  species  correlations  throughout the summer.  h i g h e s t s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s o c c u r r e d , once a g a i n ,  between C. acherusicum and C. i n s i d i o s u m i n each month and  study  ( T a b l e 22). In A r e a 1, the mixed stand of Z o s t e r a s p e c i e s , both v a l i d a and A. p u g e t t e n s i s were r e l a t i v e l y abundant, and a p o s i t i v e c o r r e l a t i o n between them ( T a b l e 22). r e l a t i o n s h i p was and  Low,  between Ampithoe  " p a i r - w i s e " combinations of the mean number of i n d i v i d u a l s per  The  C.  no l o n g e r s i g n i f i c a n t .  area A.  this resulted in  However, by June t h i s  In Area 2 i n May  and Areas 1  2 i n June, abundances of A. p u g e t t e n s i s were s i g n i f i c a n t l y  c o r r e l a t e d with those of the two  Corophium s p e c i e s ( T a b l e 22).  While  148  Table 21.  C o r r e l a t i o n c o e f f i c i e n t s (R) f o r c o m p a r i s o n s o f mean r e l a t i v e numbers o f amphipods • species -'- • 0.01m pooled from a l l months, a r e a s , and s u b s t r a t e t y p e s , May - J u l y , 1984 (n = 1 6 2 ) . * v a l u e s i n d i c a t e s i g n i f i c a n t R (p < 0.05); ** v a l u e s i n d i c a t e s i g n i f i c a n t R (p < 0.01). -  1 2 3 4 5 6 7 8 9 10  = = = = = = = = = =  -2  Ampithoe l a c e r t o s a A. v a l i d a Anisogammarus p u g e t t e n s i s Atylus c o l l i n g i i Calliopius laeviusculus Corophium acherusicum C. i n s i d i o s u m I s h y r o c e r u s sp. Pontoqeneia intermedia P. r o s t r a t a  [cont.]  Table 2 1 . [cont.]  Species  1  2  3  4  5  6  7  8  1 0  9  1  2  -0.066  3  -0.076  0.092  4  -0.011  -0.094  0.156  5  -0.022  -0.041  -0.001  6  -0.110  0.244**  0.550** -0.074  -0.105  7  -0.093  0.194  0.592  -0.058  -0.114  0.869  8  0.085  -0.060  -0.129  -0.025  0.039  -0.275  -0.227  9  -0.002  -0.079  -0.076  0.004  0.014  -0.134  -0.104  0.153  10  -0.013  0.056  -0.018  -0.039  0.140  0.078  -0.058  0.121*  -0.012  0.018  150  Table 22.  C o r r e l a t i o n c o e f f i c i e n t s (R) f o r c o m p a r i s o n s o f mean numbers o f a m p h i p o d s • s p e c i e s ! c o l l e c t e d i n A r e a s 1, 2, a n d 3, May J u l y , 1984 (n = 5 2 ) . * v a l u e s i n d i c a t e s i g n i f i c a n t R (p < 0 . 0 5 ) ; ** v a l u e s i n d i c a t e s i g n i f i c a n t R (p <0.01) . -  1 2 3 4  Area  = = = =  Ampithoe v a l i d a Anisogammarus p u g e t t e n s i s Corophium acherusicum C. i n s i d i o s u m  1:  Species  1  2  3  4  1 2  -0.113  3  0.713**  0.010  4  0.751**  0.088  0.850**  [cont.]  151  Table  Area  22.  [cont.]  2:  Species  1  2  3  4  1 2  0.222  3  0.346*  0.816**  4  0.444**  0.832**  0.969**  1  2  3  Area  3:  Species  1 2  -0.090  3  -0.093  -0.046  4  -0.092  -0.076  0.984**  4  152 t h i s r e l a t i o n s h i p was maintained  i n J u l y i n Area 1, i t d i d not occur i n  t h a t month i n Area 2 ( T a b l e 2 2 ) . In Area 3 ( h i g h Z. marina shoot d e n s i t y ) , abundances of Ampithoe v a l i d a , as w e l l as I s c h y r o c e r u s  sp. were p o s i t i v e l y c o r r e l a t e d with  those of A. p u g e t t e n s i s i n May (Table 22).  The p o s i t i v e  relationship  between the l a t t e r two e p i f a u n a l s p e c i e s a l s o o c c u r r e d i n June but was not observed  i n J u l y f o l l o w i n g t h e d e c l i n e i n t h e I s c h y r o c e r u s sp.  p o p u l a t i o n (Table 2 2 ) . No n e g a t i v e c o r r e l a t i o n s were apparent when mean abundances of amphipod s p e c i e s were compared on the b a s i s o f t h e i r o c c u r r e n c e p a r t i c u l a r s u b s t r a t e type.  on a  T a b l e 23 shows t h e product-moment  c o r r e l a t i o n s c o e f f i c i e n t s f o r these monthly comparisons, p o o l e d over t h e t h r e e study a r e a s .  Again,  i n a l l substrates with the exception of the  e e l g r a s s i n June, s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s o c c u r r e d between t h e two  Corophium s p e c i e s . In May i n Z o s t e r a marina samples, low but  s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s o c c u r r e d between A. p u g e t t e n s i s and A. v a l i d a , and A. v a l i d a and I s c h y r o c e r u s c o r r e l a t i o n s were r e c o r d e d  sp. (Table 23).  i n June on t h i s  No s i g n i f i c a n t  s u b s t r a t e , but i n J u l y , t h e  mean abundance, of A. v a l i d a . a n d the,.two Corophium species, were r e l a t e d (Table 23). In the d r i f t  samples, t h e s i g n i f i c a n t p o s i t i v e  relationship  between the Corophium s p e c i e s and Anisogammarus p u g e t t e n s i s o c c u r r e d i n a l l months.  Abundances o f Ampithoe v a l i d a were a l s o p o s i t i v e l y  c o r r e l a t e d w i t h those of t h e two Corophium s p e c i e s i n June ( T a b l e 2 3 ) . In the sediment, s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s o c c u r r e d between Ampithoe v a l i d a and Anisogammarus p u g e t t e n s i s i n May, A. p u g e t t e n s i s and t h e two Corophium s p e c i e s i n June, and A. p u g e t t e n s i s and C. acherusicum i n J u l y (Table 23).  The mean abundances o f each o f  153  Table  23.  C o r r e l a t i o n c o e f f i c i e n t s (R) f o r c o m p a r i s o n s of mean numbers o f amphipods • s p e c i e s ! c o l l e c t e d i n Z o s t e r a m a r i n a , d r i f t , and sediment samples, pooled from t h r e e areas of d i f f e r e n t d e n s i t i e s o f Z o s t e r a s h o o t s , May - J u l y , 1984 (n = 1 8 ) . * v a l u e s i n d i c a t e s i g n i f i c a n t R (p < 0.05); ** v a l u e s i n d i c a t e s i g n i f i c a n t R (p <0.01). -  1 2 3 4 5  1. Z o s t e r a  = = = = =  Ampithoe v a l i d a Anisogammarus p u g e t t e n s i s Corophium a c h e r u s i c u m C_. i n s i d i o s u m Ischyrocerus sp.  marina:  May  Species  1  2  3  4  5  1 2  0.698**  3  0.000  0.000  4  0.000  0.000  0.000  5  0.487*  0.134  0.000  0.000  [cont.]  154  Table 23. [cont.]  June Species  1  2  3  4  5  1 2  -0.174  3  0.174  0.107  4  -0.035  -0.121  -0.080  5  -0.167  -0.203  0.291  0.020  July Species  1  2  3  4  5  1 2  0.239  3  0.529*  -0.111*  4  0.506*  0.435*  0.515*  5  0.000  0.000  0.000  0.284  [cont.]  155  Table 23.  2.  [cont.]  Drift:  May Species  1  2  3  4  5  1 2  0.586*  3  0.277  0.722**  4  0.268  0.815**  5  -0.217  -0.273  0.810** -0.287  -0.205  June Species  1  2  3  4  5  1 2  0.431  3  0.632**  0.883**  4  0.682**  0.748**  5  -0.170  -0.207  0.907** -0.263  -0.174  [cont.]  156  Table 23. [ c o n t . ]  JulySpecies  1  2  3  4  5  1 2  -0.308  3  0.210  0.499*  4  0.131  0.643**  0. 8 5 1 * *  5  0.000  0.000  0. 000  0.000  3. S e d i m e n t :  May Species  1  2  3  4  5  1 2  0.680**  3  0.252  0.361  4  0.121  0.345  0. 8 1 9 * *  5  0.000  0.000  0. 000  0.000  [cont.]  15 7  Table 23. [cont.]  June Species  1  2  3  4  5  1 2  0.271  3  0.343  0.656**  4  0.126  0.720**  0.779**  5  0.000  0.000  0.000  0.000  JulySpecies  1  2  3  4  1 2  0.424  3  0.272  0.679**  4  0.015  0.432  0.828**  5  0.000  0.000  0.000  0.000  5  158 the Corophium s p e c i e s were c o r r e l a t e d s i g n i f i c a n t l y on t h i s s u b s t r a t e i n all  t h r e e months (Table  23).  In summary, f i v e main p o i n t s may  be made from the r e s u l t s of  the  three-month study: 1) As i n P a r t A: The Twelve-Month Study, Corophium acherusicum, C. i n s i d i o s u m , Ampithoe v a l i d a ,  and  p u g e t t e n s i s n u m e r i c a l l y dominated the samples.  Anisogammarus Together these  species  accounted f o r more than 80% of the t o t a l numbers of amphipods i n each study area from May  - J u l y , w i t h the e x c e p t i o n s  R e l a t i v e l y l a r g e numbers of a f i f t h  of Areas 2 and  species, Ischyrocerus  c o l l e c t e d on the Z o s t e r a marina i n those a r e a s  i n May.  i n both  May.  sp., were  2) O v e r a l l  abundances of amphipods i n c r e a s e d s i g n i f i c a n t l y from May t h r e e study a r e a s .  3 in  - July i n a l l  S i g n i f i c a n t i n c r e a s e s i n amphipod abundance o c c u r r e d  sediment and d r i f t  over  time, but  t h e r e were no  significant  d i f f e r e n c e s i n amphipod abundance among months on Z. marina. 3) With e x c e p t i o n of June, t h e r e was  no  significant  the  t r e n d i n the abundance of  amphipods a s s o c i a t e d with the two d i f f e r e n t d e n s i t i e s of Z. marina, or between the numbers of these animals  t h a t were c o l l e c t e d i n the  monotypic and mixed stands of Z o s t e r a .  In June, s i g n i f i c a n t l y more  amphipods were c o l l e c t e d i n the a r e a of low d e n s i t y of Z. marina, than were c o l l e c t e d i n e i t h e r the mixed s p e c i e s s t a n d o r i n the a r e a of d e n s i t y of Z. marina. 4) S i g n i f i c a n t l y more amphipods s u r f a c e were r e c o r d e d i n Areas 1 and  2.  i n the d r i f t  surface area.  5) O v e r a l l d e n s i t i e s of amphipods i n the sediment and  t o t a l macrophyte biomass  In c o n t r a s t , numbers i n t h e p l a n t samples were  c o r r e l a t e d p o s i t i v e l y with e i t h e r e e l g r a s s o r d r i f t area per quadrat,  • m ^ of p l a n t  than were c o l l e c t e d i n the e e l g r a s s  were n e g a t i v e l y c o r r e l a t e d w i t h both d r i f t and  high  i n most months and  areas.  biomass and  surface  159  P a r t C.  L i f e C y c l e s and  Size-Frequency  Distributions  As p r e v i o u s l y d e s c r i b e d , the c o l l e c t i o n s of amphipods made i n both the twelve-month and species.  the three-month s t u d i e s were dominated by  F i g u r e s 36,  37,  38,  d i s t r i b u t i o n s and r e p r o d u c t i v e acherusicum,  and  39 i l l u s t r a t e the s i z e  four  frequency  s e a s o n a l i t y of these amphipods: Corophium  C. i n s i d i o s u m , Ampithoe v a l i d a , and  Anisogammarus  pugettensis, r e s p e c t i v e l y . 1.  Size-Frequency D i s t r i b u t i o n s . F i g u r e s 36 and  37  show the monthly s i z e - f r e q u e n c y / l i f e - stage  d i s t r i b u t i o n s of Corophium acherusicum and r e s p e c t i v e l y . Both of these l a t e summer and and  a l l months except A p r i l , and A p r i l .  species experienced  autumn, and decreased  s p r i n g . ( F i g s . 36 and  C.  37)7  peak d e n s i t i e s i n the  i n abundance throughout the  Corophium acherusicum was  w h i l e C. i n s i d i o s u m was  S i n c e Corophium spp.  insidiosum,  winter  collected i n  absent i n both March  j u v e n i l e s c o u l d not be a c c u r a t e l y  i d e n t i f i e d , t h e i r numbers were a p p o r t i o n e d  between the two  Corophium  s p e c i e s on the b a s i s of the monthly r e l a t i v e abundance of each a d u l t p o p u l a t i o n , f o r the purpose of the  histograms.  With the e x c e p t i o n of J u l y 1983 j u v e n i l e s were p r e s e n t  and  March 1984,  Corophium  i n a l l months i n which the a d u l t s were c o l l e c t e d ;  however, they never c o n t r i b u t e d more than 14% of the combined a d u l t p o p u l a t i o n s .  t o the monthly abundance  Consequently, i t was  determine when an a p p r e c i a b l e r e c r u i t m e n t  of  difficult  to  juveniles occurred.  However, s i n c e the s m a l l e s t i n d i v i d u a l s of both s p e c i e s were c o l l e c t e d i n June and  J u l y 1984  (0.10-0.13mm head l e n g t h ) , and because many young  160  Figure 36. Size-frequency distributions of Corophium acherusicum collected in the intercauseway area of Roberts Bank, August 1983 - July 1984. N i s shown in bold numerals to the right of the ordinate axis in each month.  I  males  •GBIB  o v i g fern  [  rep fem  nonrep fem j uveniles  161  Head  Length (mm)  162  Figure 37. Size-frequency distributions of Corophium insidiosum  collected i n the intercauseway  area of Roberts Bank, August 1983 - July 1984.  N i s shown i n bold numerals to the  right of the ordinate axis in each month.  males  i  II  ^ V//////A —  rep fem nonrep fem juveniles  163  Head  Length (mm)  164  males and immature females were c o l l e c t e d from August - November, peak r e c r u i t m e n t p r o b a b l y o c c u r r e d d u r i n g t h e summer ( F i g s . 36 and 37). There was o n l y small v a r i a t i o n i n mean head l e n g t h between months i n e i t h e r t h e Corophium acherusicum or t h e C. i n s i d i o s u m pojDulations from J u l y 1983 - J u l y 1984 ( F i g s . 36 and 37).  In t h e C.  acherusicum p o p u l a t i o n , mean head l e n g t h ranged from 0.30-0.32mm and r e v e a l e d no c o n s i s t e n t t r e n d i n growth between months. i n c r e a s e i n the minimum s i z e was r e c o r d e d  However, an  from September-March,  i n d i c a t i n g t h a t i n d i v i d u a l s born i n the summer grew s t e a d i l y through t h e winter.  Due t o the long rostrum  l e n g t h of these animals  of male C. i n s i d i o s u m , t h e mean head  was s l i g h t l y l a r g e r than t h a t of the C.  acherusicum p o p u l a t i o n and ranged from 0.31-0.36mm ( F i g . 3 7 ) . But once a g a i n , except  f o r an i n c r e a s e i n minimum s i z e from September t o  November, no c o n s i s t e n t t r e n d i n growth was  observed.  Ampithoe v a l i d a were most abundant from August - October 1983 and i n J u l y 1984, p e r i o d s i n which t h e p o p u l a t i o n was dominated by j u v e n i l e s ( F i g . 3 8 ) . No A. v a l i d a were c o l l e c t e d from January - A p r i l 1984. Recruitment p r o b a b l y o c c u r r e d i n t h e autumn as w e l l as i n the s p r i n g i n t h i s s p e c i e s because although  j u v e n i l e s were c o l l e c t e d throughout t h e  summer, t h e s m a l l e s t i n d i v i d u a l s August  (0.16mm head l e n g t h ) were r e c o r d e d i n  ( F i g . 3 8 ) . The l a r g e s t mean head l e n g t h s were r e c o r d e d d u r i n g the  w i n t e r , p a r t i c u l a r l y i n November and December.  There was a d i s t i n c t  s h i f t i n s i z e from May - June when both t h e maximum and t h e mean head lengths decreased recorded As  ( F i g . 38). The minimum mean s i z e (0.43mm) was  i n J u l y 1984. i n t h e Ampithoe v a l i d a p o p u l a t i o n s , t h e p e r i o d s o f peak  abundance o f Anisogammarus p u g e t t e n s i s , which o c c u r r e d i n J u l y 1983 and  165  Figure 38. Size-frequency distriubtions of Ampithoe valida collected i n the intercauseway area of Roberts Bank, August 1983 - July 1984. N i s shown in bold numerals to the right of the ordinate axis i n each month.  males ovig fem rep fem nonrep fem juveniles  166  Head  Length (mm)  167  May 1984, were months i n which the p o p u l a t i o n was dominated numbers of j u v e n i l e s  by l a r g e  ( F i g . 39). Minimum head l e n g t h s (0.17mm) were  r e c o r d e d i n May and June 1984 and peak r e c r u i t m e n t p r o b a b l y o c c u r r e d a t t h a t time. There was an i n c r e a s e i n both minimum and mean head s i z e from J u l y - October 1983 i n d i c a t i n g t h a t t h e s e i n d i v i d u a l s grew s t e a d i l y through the l a t e summer and i n t o the autumn. The l a r g e s t mean head l e n g t h s were observed i n September and October.  S m a l l e s t mean head  l e n g t h s were r e c o r d e d i n June and J u l y 1984 ( F i g . 39). Few Anisogammarus p u g e t t e n s i s were c o l l e c t e d i n the autumn, a time when o t h e r amphipod p o p u l a t i o n s were a t t h e i r peak; none was c o l l e c t e d from November April  1983 -  1984. 2.  Reproductive Seasonality  R e p r o d u c t i v e females (with setous brood p l a t e s ) and o v i g e r o u s females of both Corophium s p e c i e s were p r e s e n t i n r e l a t i v e l y low numbers throughout the year  ( F i g s . 36 and 3 7 ) .  In the C. acherusicum  p o p u l a t i o n , they were most abundant i n September, when they c o n t r i b u t e d 27% o f the t o t a l numbers.  No o v i g e r o u s females were c o l l e c t e d i n  December or January when the number of females w i t h setous brood p l a t e s was p r i m a r i l y made up of l a r g e p o s t - r e p r o d u c t i v e i n d i v i d u a l s  (Fig.36).  R e p r o d u c t i v e and ovigerous females were r e l a t i v e l y more abundant i n t h e Corophium i n s i d i o s u m p o p u l a t i o n d u r i n g c e r t a i n months than they were i n the C. acherusicum p o p u l a t i o n . 1983,  For example, i n J u l y and August  j u s t p r i o r t o t h e September peak i n p o p u l a t i o n d e n s i t y ,  accounted f o r between 25-40% of the t o t a l numbers  ( F i g . 37).  such By  November, however, as the p o p u l a t i o n d e c l i n e d , o v i g e r o u s females had d i s a p p e a r e d and p o s t - r e p r o d u c t i v e females r e p r e s e n t e d l e s s than 9% of the t o t a l abundance.  No r e p r o d u c t i v e females were c o l l e c t e d i n January,  168  Figure 39. Size-frequency distributions of Anisogammarus pugettensis collected i n the intercauseway area of Roberts Bank, August 1983 - July 1984.  N i s shown in bold numerals to the  right of the ordinate axis i n each month.  1  HBHHi I  males  fem rep fem nonrep fem  Er~~ ''---^  juveniles  169  Head  Length (mm)  170  perhaps due t o the s m a l l sample s i z e , and o v i g e r o u s  females d i d not  reappear i n the study a r e a u n t i l May ( F i g . 3 7 ) . Nelson (1980b) observed t h a t many gammaridean amphipods,  including  Corophium acherusicum, mature a t s m a l l e r s i z e s i n t h e summer than do  i n the winter.  populations  T h i s seasonal  they  t r e n d was n o t observed i n the Corophium  sampled i n the p r e s e n t  study.  The l a r g e s t d i f f e r e n c e i n  mean head length o f r e p r o d u c t i v e C. acherusicum females o c c u r r e d  between  December (0.36mm) and January (0.39mm); r e p r o d u c t i v e C. i n s i d i o s u m females ranged i n head l e n g t h from 0.34mm i n J u l y 1983 t o 0.40mm i n J u l y 1984.  In both s p e c i e s the minimum female s i z e a t r e p r o d u c t i v e  r e f l e c t e d i n head l e n g t h measurements o f 0.22mm i n each case, i n September.  maturity, occurred  The maximum l e n g t h o f r e p r o d u c t i v e females o c c u r r e d i n  September and November i n C. acherusicum (0.58mm), and i n J u l y 1984 i n C.  insidiosum  (0.54mm).  Mature C. acherusicum females were g e n e r a l l y  l a r g e r than C. i n s i d i o s u m females i n a l l months except J u l y 1984. Males were r e l a t i v e l y abundant i n both Corophium p o p u l a t i o n s i n most months ( F i g s . 36 and 37; T a b l e s least  40% o f the a d u l t  24 and 25).  C. acherusicum p o p u l a t i o n i n a l l  months except August 1983 and March and J u l y 1984. r a t i o o f males:females d e c l i n e d t o 0.45, t o 0.59 (Table 24).  They accounted f o r a t  I n March, t h e sex  and i n August and J u l y i t f e l l  T h i s r e l a t i o n s h i p was more v a r i a b l e i n the C.  i n s i d i o s u m p o p u l a t i o n , perhaps due t o s m a l l e r sample s i z e s . I t ranged from 0.12 i n December t o 1.00 i n August and November d u r i n g the peak i n abundance o f t h i s s p e c i e s  (Table  25).  Ovigerous Ampithoe v a l i d a were p r e s e n t  throughout t h e summer and  autumn but were absent i n November, when p o s t - r e p r o d u c t i v e a c c o u n t e d f o r approximately  15% o f the p o p u l a t i o n  females  ( F i g . 38).  No  171  T a b l e 24.  Date  Monthly percentages of males, females, and o v i g e r o u s f e m a l e s , and t h e m a l e : f e m a l e s e x r a t i o s i n t o t a l numbers o f a d u l t C o r o p h i u m acherusicum c o l l e c t e d i n a l l samples, 1983-84.  Total Adults  % Male  %  Female  % Ovig. Female  Male:Female  11  July 1983  39  51  49  13  1.05  6  Aug.  604  37  63  10  0.59  4  Sept.  1446  44  56  14  0.79  25 O c t .  1725  44  56  7  0.79  Nov.  601  41  59  1  0.69  21 Dec.  314  54  46  0  1.17  20 J a n . 1984  191  42  58  0  0.72  1 March  49  31  69  2  0.45  0  0  -  23  17  April  15  May  1  100  142  56  44  6  1.27  418  47  53  4  0.89  July  1242  37  63  11  0.59  TOTAL  6772  2886  3886  565  0.74  27 J u n e 26  172  T a b l e 25.  Date  Monthly percentages of males, females, and o v i g e r o u s f e m a l e s , and t h e m a l e : f e m a l e sex r a t i o s i n t o t a l numbers o f a d u l t C o r o p h i u m i n s i d i o s u m c o l l e c t e d i n a l l s a m p l e s , 1983-84.  Total Adults  % Male  %  Female  % Ovig. Female  Male:Female  11  July 1983  18  22  78  28  0.28  6  Aug.  161  50  50  20  1.00  4 Sept.  234  40  60  9  0.67  25 O c t .  59  24  76  14  0.32  23  Nov.  46  48  52  0  0.92  21 Dec.  27  11  89  0  0.12  20 J a n . 1984  18  22  78  0  0 . 28  1 March  4  25  75  0  0.33  17  April  3  67  33  0  2.03  15  May  54  37  63  9  0.59  92  28  72  14  0.39  345  45  55  7  0.82  1061  426  635  326  0.67  27 J u n e 26  July  TOTAL  173  r e p r o d u c t i v e o r o v i g e r o u s females were c o l l e c t e d a g a i n u n t i l June 1984. Females o f t h i s s p e c i e s reached r e p r o d u c t i v e m a t u r i t y a t a s m a l l e r mean s i z e i n t h e summer than they d i d i n t h e autumn.  The minimum  r e p r o d u c t i v e s i z e was r e c o r d e d i n J u l y 1984 when females w i t h  setous  brood p l a t e s and with head l e n g t h s o f o n l y 0.42mm were c o l l e c t e d ( F i g . 38).  The l a r g e s t females were c o l l e c t e d i n October and measured up t o  1.32mm i n head l e n g t h (or 15.0mm i n t o t a l body l e n g t h ) . With the e x c e p t i o n of J u l y 1983, when male Ampithoe v a l i d a accounted  f o r 59% o f the a d u l t p o p u l a t i o n , males were c o l l e c t e d  f r e q u e n t l y than were females and the sex r a t i o ranged August t o 0.12 i n October November.  ( T a b l e 26).  less  from 0.52 i n  No males were c o l l e c t e d i n  The most d i s p r o p o r t i o n a t e sex r a t i o  (0.09) was r e c o r d e d i n  June 1983 a t a time when the p o p u l a t i o n was dominated by n o n r e p r o d u c t i v e females. The absence o f Anisogammarus p u g e t t e n s i s from t h e November - A p r i l c o l l e c t i o n s was i n dramatic c o n t r a s t t o t h e s e a s o n a l p a t t e r n of abundance t h a t had been observed i n t h e p r e v i o u s w i n t e r . 1982  and January  In December  1983, many p r e c o p u l a t i n g males and females were  observed near t h e study s i t e s , and i n two samples o f Z o s t e r a marina c o l l e c t e d i n e a r l y January, females. marsupia.  67% o f the a d u l t s c o l l e c t e d were o v i g e r o u s  S e v e r a l o f these females were b r o o d i n g Ovigerous  juveniles i n their  females were a l s o c o l l e c t e d i n March 1983  i n d i c a t i n g t h a t b r e e d i n g and r e c r u i t m e n t c a n occur y e a r - r o u n d  i n this  species. Year-round  r e c r u i t m e n t , however, d i d n o t occur i n 1983 - 1984, a t  l e a s t i n t h e study s i t e s .  Despite the presence o f r e l a t i v e l y  numbers o f r e p r o d u c t i v e and o v i g e r o u s females  large  i n September 1983, t h e r e  174  T a b l e 26.  Date  Monthly percentages of males, females, and o v i g e r o u s f e m a l e s , and t h e m a l e : f e m a l e s e x r a t i o s i n t o t a l numbers o f a d u l t A m p i t h o e v a l i d a c o l l e c t e d i n a l l s a m p l e s , 1983-84.  Total Adults  % Male  %  Female  % Ovig. Female  Male:Female  11  July 1983  17  59  41  24  1.44  6  Aug.  84  34  66  24  0.52  4 Sept.  68  28  72  19  0 .39  25 O c t .  79  11  89  9  0.12  23  Nov.  11  0  100  0  -  21  Dec.  5  20  80  0  0.25  6  17  83  0  0.20  27 J u n e  13  8  92  8  0.09  26  33  33  66  9  0.50  316  81  235  48  0.34  15  May 1984  July  TOTAL  175  was  no corresponding  i n c r e a s e i n the abundance o f j u v e n i l e Anisogammarus  p u g e t t e n s i s c o l l e c t e d i n t h e ensuing months ( F i g . 39).  The October  samples were dominated by l a r g e p o s t - r e p r o d u c t i v e a d u l t s . t h i s s p e c i e s had d i s a p p e a r e d when A. p u g e t t e n s i s  from the study s i t e s .  By November  Even i n t h e s p r i n g  j u v e n i l e s were abundant, few r e p r o d u c t i v e females  were c o l l e c t e d , i n d i c a t i n g t h a t the a d u l t s were p r o b a b l y p r e s e n t of t h e study  area.  The minimum r e p r o d u c t i v e s i z e o f Anisogammarus p u g e t t e n s i s was  recorded i n June 1984 when females approximately  l e n g t h bore setous brood p l a t e s ( F i g . 39). and  outside  females  0.40mm i n head  The l a r g e s t females (1.55  1.68mm i n head l e n g t h ) were c o l l e c t e d i n September 1983 and May  1984,  r e s p e c t i v e l y ( F i g . 39). Anisogammarus p u g e t t e n s i s females were g e n e r a l l y more abundant  than were males, w i t h the sex r a t i o r a n g i n g from 0.43 i n J u l y 1983 t o 0.08  i n September 1983 (Table 27). Males a l s o accounted f o r o n l y 8% of  the a d u l t p o p u l a t i o n i n J u l y 1984,and were absent  i n the  samples  c o l l e c t e d i n October 1983. In summary, Corophium acherusicum, C. i n s i d i o s u m , Ampithoe v a l i d a , and Anisogammarus p u g e t t e n s i s each has an annual l i f e c y c l e w i t h peak j u v e n i l e r e c r u i t m e n t d u r i n g t h e s p r i n g andsummer. Ovigerous females and j u v e n i l e s were a l s o observed of  i n September and October i n the  C. i n s i d i o s u m , A. v a l i d a , and A. p u g e t t e n s i s , and i n November  i n t h a t o f C. acherusicum.  These r e s u l t s i n d i c a t e an e x t e n s i o n i n t h e  p e r i o d of r e p r o d u c t i v e a c t i v i t y d e s c r i b e d f o r these (1973, 1979). probably  populations  s p e c i e s by B o u s f i e l d  In f a c t , based on p r e l i m i n a r y d a t a , A. p u g e t t e n s i s can  reproduce year-round.  Although  the mean s i z e o f Corophium  acherusicum and C. i n s i d i o s u m d i d not v a r y over  the one-year p e r i o d ,  176  T a b l e 27.  Date  Monthly percentages of males, females, and o v i g e r o u s f e m a l e s , and t h e m a l e : f e m a l e s e x r a t i o s i n t o t a l numbers o f a d u l t A n i s o g a m m a r u s p u g e t t e n s i s c o l l e c t e d i n a l l samples, 1983-84.  Total Adults  % Male  % Female  % Ovig. Female  Male:Female  11  July 1983  86  30  70  8  0.43  6  Aug.  46  24  76  22  0.32  4  Sept.  28  7  93  36  0.08  25  Oct.  15  0  100  7  -  167  28  72  10  0.39  15  May 1984  27  June  94  26  74  1  0. 35  26  July  12  8  92  0  0.09  TOTAL  448  111  337  46  0.33  177  there was a decrease i n the mean s i z e of both Ampithoe v a l i d a and Anisogammarus pugettensis from the autumn t o the spring.  This decrease  was a l s o r e f l e c t e d i n the mean s i z e of reproductive females of these two species. With the exception of the C. acherusicum population i n which the sex r a t i o remained near u n i t y i n most months, male amphipods were generally l e s s abundant than females.  178  DISCUSSION A primary f o c u s i n marine e c o l o g y has been the r o l e of b i o l o g i c a l i n t e r a c t i o n s such as c o m p e t i t i o n and p r e d a t i o n i n d e t e r m i n i n g the d i s t r i b u t i o n and abundance o f s p e c i e s . intertidal govern 1971;  system,  In s t u d i e s o f the rocky  c o m p e t i t i o n f o r food and  space have been shown t o  the d i s t r i b u t i o n not o n l y of animals, but of p l a n t s (e.g. Dayton, Menge, 1982).  These types of b i o l o g i c a l i n t e r a c t i o n s a r e a l s o  important i n d e t e r m i n i n g the s t r u c t u r e of communities i n seagrass systems (e.g. Heck and Thoman, 1981;  Robertson  and Mann, 1982  ). In  a d d i t i o n , the p h y s i o l o g i c a l  s t r e s s caused by extremes i n p h y s i c a l  f a c t o r s such as temperature  and  s a l i n i t y may  a l s o i n f l u e n c e the  s t r u c t u r e of communities i n both r o c k y shore and  seagrass systems.  An  o b j e c t i v e of the p r e s e n t study has been t o examine the r o l e of h a b i t a t h e t e r o g e n e i t y i n moderating  the e f f e c t s of both b i o l o g i c a l and  physical  f a c t o r s , and i n i n f l u e n c i n g the community dynamics of an assemblage of gammarid amphipods i n a Z o s t e r a marina meadow. In heterogeneous  environments,  the a b i l i t y o f s p e c i e s t o c o e x i s t  i s enhanced by the a v a i l a b i l i t y o f a wide v a r i e t y of m i c r o h a b i t a t s . W i t h i n these m i c r o h a b i t a t s , organisms  may  s h e l t e r from the o f t e n  d e t r i m e n t a l i n f l u e n c e s of both p h y s i c a l f a c t o r s and interactions  biological  ( P i e l o u , 1975). Dense stands of s e a g r a s s , f o r example,  accommodate more animals and a g r e a t e r number of s p e c i e s than  can  can  s p a r s e l y v e g e t a t e d a r e a s or b a r r e n m u d f l a t s , by v i r t u e of i n c r e a s e d living  space. I t i s not s u r p r i s i n g , t h e r e f o r e , t h a t seagrass  shoot  d e n s i t y i s o f t e n c o r r e l a t e d p o s i t i v e l y w i t h d i v e r s i t y and abundance of macrofauna w i t h i n the seagrass bed  (Stoner, 1980a; Heck & Thoman,  1981;  Homziak et a l . , 1982). In the p r e s e n t study, however, w h i l e the o v e r a l l  179  abundance of biomass of species as  a  living  diversity  similar  lower  study of  dominated by  is  other  Few s t u d i e s herbivores  species  the  their  in  food  resources  provide  t h e r e was  large  variation  influence Z.  some  Zostera  the d i s t r i b u t i o n  in  clean.  Since  the  in  despite  Bank  in  was  most  is  a  community in  terms finding  food.  in  for  the quantity the  in  of as  summer,  amphipods.  In  while  external 1975).  resource  for  systems.  different  F l o r i d a are capable  both p a r t i c l e  size  an a r t i f a c t  on t h e  these of  four  remain  their  (Pielou,  seagrass  that  and  by an  limiting  Observations  support  diatoms  can c o e x i s t  resource  amphipods  this  during of  shared  food  j a p o n i c a were c o a t e d w i t h diatoms  comparatively  none of  months.  situation  p r o v i d e d none o f  demonstrated  a seagrass  study  of  the  such as  (1979)  the present  Roberts  they are regulated  that  overabundance of  species  species;  the  these,  species  Carolina,  unlike  of  high  infaunal.  of  they discounted  an a p p a r e n t  Also,  competitors  shortage  as  i n most  30 a m p h i p o d  North  resource requirements  the notion  amphipods  species,  t h e two  was  between  densities  abundance  many a s  ).  the  were c o l l e c t e d and,  community o f  epifaunal  than a c t u a l ,  Zimmerman e t a l .  a  " m '  the amphipod  similar  support  Despite  the t o t a l  c o l l e c t e d as  correlated with  no r e l a t i o n s h i p  density.  19 s p e c i e s  (~6000  and d e t r i t i v o r e s  of  shoot  r e s o u r c e - l i m i t e d and  than  partitioning plant  study,  with  t h e r e was  Z o s t e r a m a r i n a meadow i n  abundance  rather  populations  Although  a  positively  80% o f  collected species  potential,  factor  (1979a)  e p i b e n t h i c and  Species  was  only  2  more t h a n  of  (1979a)  frequently  • m~ ,  Nelson  levels  Nelson's  of  and e e l g r a s s  accounted for  contrast,  amphipods  Zostera marina,  26,820 amphipods  four In  gammarid  diatoms this  basis  d i d not  of  a r e an i m p o r t a n t  t h e Z.  appear  with to  leaves  marina  source of  in  Although  associated  June and J u l y t h e those  and  w h i c h w e r e made  conclusions.  of  of  were  food  for  180 g r a z i n g amphipods, i f food a v a i l a b i l i t y i s a f a c t o r i n c h o i c e of s u b s t r a t e , more i n d i v i d u a l s should have been c o l l e c t e d i n the j a p o n i c a than i n the Z. marina d u r i n g those months. i n f a c t c o l l e c t e d i n Area 2, i n a m o n o s p e c i f i c were found i n Area  Z.  More amphipods were  stand of Z. marina,  1, the mixed stand of Z o s t e r a s p e c i e s .  This  than  suggests  t h a t , a t l e a s t i n t h e summer, amphipods do not s e l e c t p l a n t s u b s t r a t e s on the b a s i s of f o o d a v a i l a b i l i t y .  I t f u r t h e r suggests  that during  the  p e r i o d of peak abundance of amphipods i n the summer and autumn, when c o m p e t i t i o n f o r f o o d would be most l i k e l y t o o c c u r , food i s not a limiting  resource.  Competition may  f o r the second p o t e n t i a l l y l i m i t i n g r e s o u r c e ,  on the other hand be important  amphipods i n seagrass meadows. amphipods, a r e an important  space,  i n s t r u c t u r i n g assemblages of  S i n c e c r u s t a c e a n s , and  particularly  component i n the d i e t of many b i r d s and  f i s h , as w e l l as o t h e r i n v e r t e b r a t e s , i t i s u s u a l l y assumed t h a t t h e i r d i s t r i b u t i o n i n seagrass  systems i s r e g u l a t e d by p r e d a t i o n  (Nelson,  1979b). But, because the i n t e n s i t y of p r e d a t i o n i s o f t e n r e g u l a t e d by a f a c t o r such as p l a n t d e n s i t y , c o m p e t i t i o n f o r s h e l t e r among these s p e c i e s might be expected  t o occur  (Heck and Thoman, 1981;  prey  Coen e t a l . ,  1981). F i s h may  be the major p r e d a t o r s on amphipods a t Roberts  p a r t i c u l a r l y d u r i n g the summer. S e v e r a l s p e c i e s of f i s h , s h i n e r p e r c h , h e r r i n g , and  s c u l p i n s a r e abundant i n the  a r e a i n t h i s season (Gordon and Levings, 1984).  Bank,  including intercauseway  In a d d i t i o n , j u v e n i l e  salmon enter t h e F r a s e r R i v e r e s t u a r y i n A p r i l and remain i n the e e l g r a s s beds of Roberts  Bank u n t i l August  (Macdonald, 1984). During  t h e i r r e s i d e n c y , t h e s e f i s h consume s e v e r a l s p e c i e s of amphipods, i n c l u d i n g Anisogammarus p u g e t t e n s i s , one  of the most f r e q u e n t l y  181 c o l l e c t e d s p e c i e s i n the p r e s e n t study.  I n d i v i d u a l s of t h i s s p e c i e s  comprised more than 45% of the d i e t o f j u v e n i l e chinook c o l l e c t e d Macdonald (1984) on J u l y 9, 1982.  One  by  would expect, t h e r e f o r e , t o f i n d  h i g h e r c o n c e n t r a t i o n s of amphipods, and  i n p a r t i c u l a r A. p u g e t t e n s i s , i n  h i g h s h o o t - d e n s i t y a r e a s of the e e l g r a s s meadow. d i f f e r e n c e i n amphipod abundance, however, was  No  significant  r e c o r d e d between a r e a s of  d i f f e r e n t d e n s i t i e s of e e l g r a s s shoots i n the summer of 1984.  Although  c e r t a i n s p e c i e s of e p i f a u n a l amphipods a r e known t o e x h i b i t b e h a v o u r i a l a f f i n i t i e s f o r a r e a s of h i g h shoot d e n s i t y  ( S t o n e r , 1980c) i t i s  p o s s i b l e t h a t , i n t h i s case, the v a r i a t i o n i n shoot number between the two  study areas was  * m  A  too s m a l l t o i n f l u e n c e the amphipods.  Seagrass grows i n much denser assemblages on t h e A t l a n t i c c o a s t of North America,  i n North C a r o l i n a , and i n F l o r i d a , where most of t h e  studies  r e l a t i n g abundance and d i v e r s i t y of a s s o c i a t e d fauna t o shoot d e n s i t y have been done, than i t does i n the P a c i f i c Northwest.  For example,  Marsh (1973) r e p o r t e d Z o s t e r a marina d e n s i t i e s as h i g h as 195 0.25m~  2  i n the York R i v e r i n V i r g i n i a .  maximum mean d e n s i t i e s of l e s s than 100 recorded.  In c o n t r a s t , i n my Z. marina  study,  shoots ' m  were  Thus the d i f f e r e n c e i n shoot d e n s i t y between seagrass beds on  the A t l a n t i c and P a c i f i c c o a s t s may r e s u l t s of my  be one e x p l a n a t i o n f o r why  study c o n t r a s t w i t h those o f p r e v i o u s s t u d i e s .  p o s s i b l e that even maximum d e n s i t i e s of e e l g r a s s shoots on (e.g.  shoots '  310 shoots  • m~  2  ) ( H a r r i s o n , 1984)  the It i s  Roberts"Bank  do not approach t h e t h r e s h o l d  l e v e l which i s n e c e s s a r y t o d e c r e a s e t h e r a t e o f f i s h p r e d a t i o n on amphipods. In a l a b o r a t o r y study, Heck and Thoman (1981) found t h a t o n l y a r t i f i c i a l g r a s s d e n s i t i e s as h i g h as 674 i n r e d u c i n g the r a t e o f k i l l f i s h grass  shrimp.  shoots  ' m  fc  were e f f e c t i v e  (Fundulus h e t e r o c l i t u s ) p r e d a t i o n on  182 Z o s t e r a j a p o n i c a grows i n much denser assemblages than does Z. marina and, i f shoot d e n s i t y i s important i n r e d u c i n g t h e i n t e n s i t y of p r e d a t i o n , then more amphipods s h o u l d be a s s o c i a t e d w i t h t h i s s p e c i e s o f s e a g r a s s . Monthly  mean abundances of amphipods i n t h e mixed stand o f  Z o s t e r a i n t h e summer o f 1984, however, 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 a t than they were i n t h e two m o n o s p e c i f i c Z. marina d e s p i t e extremely h i g h Z. j a p o n i c a shoot d e n s i t i e s  study areas  (~ 1500 shoots  • nf  2  i n J u l y ) which o c c u r r e d t h e r e . Nonetheless,  t h e r e i s c i r c u m s t a n c i a l evidence t h a t p r e d a t i o n i s  important i n s t r u c t u r i n g t h e amphipod community o f Roberts Bank. For example, a d e c r e a s e i n mean body s i z e o f amphipods from winter t o summer, s i m i l a r t o that which o c c u r r e d i n t h e Anisogammarus p u g e t t e n s i s and Ampithoe v a l i d a p o p u l a t i o n s , has been c o r r e l a t e d w i t h summer abundance o f s i z e - s e l e c t i v e p r e d a t o r s such as f i s h i n a North seagrass meadow (Nelson, 1980a).  Carolina  In t h i s r e g a r d , Macdonald (1984) has  shown that l a r g e numbers o f A. p u g e t t e n s i s a r e consumed by j u v e n i l e chinook  salmon d u r i n g t h e summer i n t h e intercauseway a r e a .  In  c o n t r a s t , t h e r e was no t r e n d i n e i t h e r t h e Corophium acherusicum  or C.  i n s i d i o s u m p o p u l a t i o n s towards s m a l l e r mean s i z e s i n t h e summer. Nelson (1980a) noted a s i m i l a r s i t u a t i o n i n a C. acherusicum  population i n  North C a r o l i n a and concluded t h a t s i n c e s i z e - s e l e c t i v e p r e d a t i o n seemed t o have no e f f e c t on t h e s i z e d i s t r i b u t i o n o f these a n i m a l s , they may have been t o o s m a l l a p r e y i t e m f o r p i n f i s h  (Lagodon rhomboides), t h e  p r i n c i p a l p r e d a t o r of amphipods i n t h a t system.  S i m i l a r l y , Corophium  spp. a r e n o t as important a component o f t h e d i e t o f j u v e n i l e chinook as a r e o t h e r gammarid s p e c i e s on Roberts Bank (Macdonald,  1984).  A f u r t h e r i n d i c a t i o n t h a t p r e d a t i o n may be important i n i n f l u e n c i n g t h e s e a s o n a l d i s t r i b u t i o n o f Anisogammarus p u g e t t e n s i s and  183 Ampithoe v a l i d a r e l a t e s t o growth and t h e t i m i n g of r e p r o d u c t i v e maturity.  The minimum s i z e a t r e p r o d u c t i v e m a t u r i t y i n both  populations  o c c u r r e d d u r i n g t h e summer and so c o i n c i d e d w i t h maximum d e n s i t i e s o f j u v e n i l e salmon.  Warm summer temperatures have been shown t o i n c r e a s e  the growth r a t e and m o u l t i n g  frequency  of c e r t a i n amphipod s p e c i e s w i t h  the r e s u l t t h a t t h e mean age and s i z e a t m a t u r i t y i s reduced Anger, 1979; Nelson,  1980a).  advantage i n t h a t i t shortens  (Nair &  T h i s i n c r e a s e d growth r a t e may be an t h e p e r i o d o f time i n which an amphipod i s  exposed t o p r e d a t i o n b e f o r e i t can produce i t s f i r s t brood  (Nelson,  1980a). P r e d a t i o n may a l s o have been r e s p o n s i b l e f o r g e n e r a t i n g t h e d i s p r o p o r t i o n a t e sex r a t i o s observed  i n t h e Ampithoe  v a l i d a and  Anisogammarus p u g e t t e n s i s p o p u l a t i o n s . Such skewed sex r a t i o s may r e f l e c t d i f f e r e n t i a l a c t i v i t y r a t e s , u t i l i z a t i o n of s e p a r a t e h a b i t a t s , aggregation, 1974).  o r e m i g r a t i o n o f one sex (Darnell,1962;  Campbell & Meadows,  In a l a b o r a t o r y study o f t h e r e p r o d u c t i v e behaviour  v a l i d a , Borowsky (1983) found  t h a t males " c r u i s e " between tubes  by females and a r e consequently behaviour  of A. occupied  more abundant i n t h e water column.  c o u l d make them more v u l n e r a b l e t o p r e d a t i o n than  which r a r e l y l e a v e t h e i r tubes (Borowsky, 1983).  This  females,  S i n c e male A. v a l i d a  accounted f o r o n l y 8% of t h e a d u l t p o p u l a t i o n i n June 1984, these i n d i v i d u a l s may have been c a p t u r e d  by f i s h p r e d a t o r s d u r i n g the summer.  S i m i l a r l y , free-swimming Anisogammarus p u g e t t e n s i s a r e p r o b a b l y more v u l n e r a b l e t o f i s h p r e d a t i o n than a r e e p i f a u n a l t u b e - d w e l l e r s as Ampithoe v a l i d a .  The r e p r o d u c t i v e behaviour  also increase i t s v u l n e r a b i l i t y to predation.  such  o f A. p u g e t t e n s i s may The males o f t h i s  species  c l a s p t h e females w i t h t h e i r second gnathopods and may c a r r y them i n t h i s p o s i t i o n f o r up t o a week u n t i l t h e female moults and f e r t i l i z a t i o n  184 can occur  (Chang, 1975). Low  o v i g e r o u s females  numbers of both males and r e p r o d u c t i v e and  were r e c o r d e d i n June and J u l y 1984,  months i n which,  based on Macdonald's (1984) o b s e r v a t i o n s , peak f i s h p r e d a t i o n o c c u r r i n g . Nelson  (1980a) found  t h a t amphipod s p e c i e s which  was  aggregated  were more a t t r a c t i v e t o f i s h p r e d a t o r s than were s o l i t a r y s p e c i e s . S i m i l a r l y , amphipods engaged i n p r e c o p u l a r a c t i v i t y may  provide a  l a r g e r , more v u l n e r a b l e t a r g e t f o r f i s h p r e d a t o r s than do those, Ampithoe v a l i d a , not e x h i b i t i n g t h i s type of b e h a v i o u r . s i n c e the males and  r e p r o d u c t i v e females  s e l e c t i v e p r e d a t i o n accounted  largest  be t h a t  f o r t h e i r low abundance i n the  A g g r e s s i v e i n t e r a c t i o n s r e s u l t i n g i n the displacement by another  palaemonid shrimp (Coen et a l . , 1981).  size-  samples. of one  from a p r e f e r r e d s u b s t r a t e have been d e s c r i b e d i n  systems f o r amphipods (Meadows and R e i d , 1966;  as  Alternatively,  were g e n e r a l l y the  i n d i v i d u a l s i n the p o p u l a t i o n a t any g i v e n time, i t may  such  species  seagrass  Nagle,1968) and  In the p r e s e n t s t u d y , i f  i n t e r s p e c i f i c c o m p e t i t i o n f o r s h e l t e r were o c c u r r i n g , i t would  likely  i n v o l v e Anisogammarus p u g e t t e n s i s , which i n c o n t r a s t t o the other f r e q u e n t l y c o l l e c t e d s p e c i e s , i s not a t u b e - d w e l l e r . Consequently,  i t is  p r o b a b l y more v u l n e r a b l e than they a r e t o f i s h p r e d a t i o n . With t h i s i n mind, c o r r e l a t i o n a n a l y s e s of s p e c i e s abundances were used t o i n v e s t i g a t e the p o s s i b i l i t y t h a t i n t e r f e r e n c e c o m p e t i t i o n  was  r e s p o n s i b l e f o r i n f l u e n c i n g p a t t e r n s of amphipod d i s t r i b u t i o n abundance i n the'intercauseway  e e l g r a s s meadow.  and  In t h i s type of  a n a l y s i s , s i g n i f i c a n t n e g a t i v e c o r r e l a t i o n s i n abundance between s p e c i e s may  i n d i c a t e t h a t one  Heck and O r t h displacement  s p e c i e s i s d i s p l a c i n g another  (Nelson,  1979a).  (1980a) p r e d i c t e d t h a t i n t e r f e r e n c e c o m p e t i t i o n l e a d i n g t o would be more important  i n a r e a s of low t o i n t e r m e d i a t e  p l a n t d e n s i t y than i t would be i n v e r y dense assemblages where the  185  complexity of the habitat would reduce the i n t e n s i t y of predation.  No  negative correlations i n abundance were observed between any of the amphipod species i n the present study, i n either low or high density areas of eelgrass, or i n the stand of mixed Zostera species. I t i s possible that the differences i n shoot density among the study sites may have been inadequate to properly test t h i s hypothesis, but these densities represent the maximum range a v a i l a b l e within the intercauseway area. Although f a r from conclusive i n the absence of experimental data, the results of the c o r r e l a t i o n analyses support my findings of no s i g n i f i c a n t difference i n abundance of amphipods between areas of low and high Z. marina density. Therefore, i t does not appear that shelter, at least with reference to Zostera, i s a l i m i t i n g resource for amphipods i n the eelgrass meadow. Negative correlations which d i d occur between some species when the data were analyzed with respect to amphipod abundance i n individual months, rather than to their abundance i n areas of d i f f e r e n t shoot density, could be explained i n other ways. F i r s t l y , caused by variations i n o v e r a l l abundance.  they may have been  An example of t h i s can be  seen i n the September data, when a s i g n i f i c a n t negative c o r r e l a t i o n occurred between the two Corophium species and Ampithoe v a l i d a . Although the abundance of a l l three species increased throughout the autumn, i n September the densities of the two Corophium species were more than ten-fold and 70-fold those of A. v a l i d a i n the d r i f t and sediment, respectively. Secondly, the negative correlations may have been due to d i f f e r e n t requirements f o r substrate. An example of t h i s type of situation occurred i n October and January when Corophium acherusicum were c o l l e c t e d primarily i n the d r i f t and the sediment samples, while Ischyrocerus sp. was found only on the eelgrass.  186 Corophium acherusicum was  p r o b a b l y d e p o s i t - f e e d i n g a t the sediment  s u r f a c e i n those months, which would account f o r i t s r e l a t i v e l y  high  abundance i n the c o r e samples, w h i l e I s c h y r o c e r u s  and  suspension-feeder,  sp., a g r a z e r  would have been r e s t r i c t e d t o the more s t a b l e  e e l g r a s s s u b s t r a t e . No n e g a t i v e c o r r e l a t i o n s were observed two  between  these  s p e c i e s i n March, when both were c o l l e c t e d i n the Z. marina samples  i n approximately  equal abundance.  With o n l y a few  exceptions,  the abundances of c l o s e l y  s p e c i e s such as Corophium acherusicum and  related  C. i n s i d i o s u m were p o s i t i v e l y  c o r r e l a t e d . Such p o s i t i v e c o r r e l a t i o n s would g e n e r a l l y be expected  to  occur between s p e c i e s t h a t were r e p r o d u c t i v e l y synchronous and  which  shared  their  s i m i l a r s u b s t r a t e requirements.  peak abundances was  Although  one month a p a r t , both Corophium s p e c i e s were p r e s e n t  i n h i g h e s t d e n s i t i e s i n both the d r i f t and autumn.  The  the t i m i n g of  s i z e s t r u c t u r e and  t h e sediment throughout  s e a s o n a l p a t t e r n s of  a c t i v i t y of each p o p u l a t i o n were a l s o s i m i l a r . Few were c o l l e c t e d i n the winter and  reproductive  Corophium i n s i d i o s u m  s p r i n g , which r e s u l t e d i n  n o n s i g n i f i c a n t c o r r e l a t i o n s between the abundances o f these two i n each month d u r i n g t h i s No  the  species  time.  s i g n i f i c a n t d i f f e r e n c e i n e i t h e r the o v e r a l l abundance o f  amphipods or s p e c i e s d i v e r s i t y was treatments  found between the f o u r shoot-removal  o r " d i s t u r b a n c e s " i n the twelve-month study.  r a p i d r e c o v e r y of shoot d e n s i t y i n the treatment t h e absence of an observed t h e amphipods was treatments  Although  p l o t s was  the  unexpected,  v a r i a t i o n i n t h e p a t t e r n of r e c o l o n i z a t i o n by  not s u p r i s i n g f o r two  reasons.  Firstly,  s i n c e the  were performed o r i g i n a l l y t o i n v e s t i g a t e the r o l e o f  shoot  d e n s i t y i n s t r u c t u r i n g the a s s o c i a t e d amphipod community, o n l y monthly c o l l e c t i o n s were made f o l l o w i n g the " d i s t u r b a n c e " .  Z a j a c and  Whitlatch  187 (1982), however, found  t h a t the t i m i n g of a d i s t u r b a n c e  significantly  i n f l u e n c e d the r e c o v e r y of e s t u a r i n e i n f a u n a l communities and  t h a t when  a d i s t u r b a n c e o c c u r r e d i n the s p r i n g or the summer, as i t d i d i n my study, weekly o b s e r v a t i o n s were' necessary  b e f o r e changes i n community  s t r u c t u r e c o u l d be a c c u r a t e l y i d e n t i f i e d . Depending on the extent of d i s t u r b a n c e , amphipod p o p u l a t i o n s may o t h e r b e n t h i c i n v e r t e b r a t e s due p o t e n t i a l and 1977;  den  their  Hartog  t o t h e i r r e l a t i v e l y low  1980).  r e c o l o n i z e l o c a l patches (Santos r e c a p t u r e techniques, epifauna,  not r e c o v e r as q u i c k l y as those of  seasonal p a t t e r n s of r e p r o d u c t i o n  _ Jacobs,  reproductive  (Simon & Dauer,  A d u l t amphipods, however, can _. Simon, 1980).  m o t i l e and  a r e a of a seagrass meadow, t h e r e can be a 50% t u r n o v e r  crustacean t h a t i n a 0.56m  2  i n these  Thus the r e c o l o n i z a t i o n of the treatment  have begun w i t h i n hours of the d i s t u r b a n c e and,  rapidly  U s i n g mark-and-  Howard (1975) demonstrated t h a t  such as amphipods, a r e extremely  i n l e s s than one day.  animals plots  of Z o s t e r a marina Secondly, amphipods ' m  6  may  ambient l e v e l s of abundance d e s p i t e the low d e n s i t y  i n some of the  plots.  s i n c e no s i g n i f i c a n t d i f f e r e n c e i n the number of was  found between the t h r e e areas of d i f f e r e n t  d e n s i t i e s i n the summer of 1984, abundance c o u l d be expected d e n s i t i e s of the treatment  d e n s i t i e s of animals  Zostera  no measurable v a r i a t i o n s i n o v e r a l l  i n the more t e m p o r a l l y ephemeral  shoot  plots.  In c o n t r a s t t o the o v e r a l l abundance of amphipods and  treatment  may  g i v e n the l a c k of  s u b s t r a t e p r e f e r e n c e d i s p l a y e d by the dominant Corophium s p e c i e s , have q u i c k l y reached  the  the  c o l l e c t e d on the two p l a n t s u b s t r a t e s i n the  p l o t s , the number of amphipods i n the sediment samples  was  p o s i t i v e l y r e l a t e d t o the r e l a t i v e i n t e n s i t y of the " d i s t u r b a n c e " . For example, more amphipods • m~  2  of sediment s u r f a c e were c o l l e c t e d i n the  188 p l o t s i n which 75% of t h e above-ground shoots had been removed, t h a n were c o l l e c t e d i n t h e c o n t r o l p l o t s . The reworking o f sediments  i n soft-  bottom h a b i t a t s o f t e n s t i m u l a t e s the growth o f m i c r o b i o t a and t h e p r e s e n c e of t h e s e organisms  i n turn a t t r a c t s c e r t a i n species of  d e t r i t i v o r e s . In t h i s r e g a r d , G a l l a g h e r e t a l . (1983) noted  that  s h a l l o w - d w e l l i n g , t u b e - b u i l d i n g , s u r f a c e - d e p o s i t f e e d e r s (such as Corophium s p e c i e s ) , a r e o f t e n the dominant c o l o n i z e r s i n d i s t u r b e d areas.  The i n t e n s i t y o f d i s t u r b a n c e r e l a t i n g t o shoot removal  treatment p l o t s was undoubtedly  i n the  r e l a t e d t o t h e degree t o which t h e  sediments were d i s r u p t e d . In t u r n , t h e amphipods may have been a t t r a c t e d on t h e b a s i s of how e x t e n s i v e l y the sediment  had been reworked  p l o t . S i m i l a r l y , g i v e n t h a t g r e a t e r r e s u s p e n s i o n o f sediments  i n each likely  o c c u r s i n bare o r p a t c h i l y v e g e t a t e d a r e a s than i n d e n s e l y v e g e t a t e d a r e a s , t h e amphipods c o l l e c t e d i n t h e c o r e samples may a c t u a l l y have been responding t o the absence  of e e l g r a s s .  Although t h e p r e s e n c e o r  absence of a Z. marina p l a n t w i t h i n a quadrat d i d not a f f e c t t h e number of amphipods c o l l e c t e d i n c o r e samples,  a n e g a t i v e c o r r e l a t i o n between  the o v e r a l l abundance o f amphipods c o l l e c t e d i n t h e sediment and e e l g r a s s biomass d i d occur over t h e one-year  p e r i o d . Although  these  r e s u l t s c o n t r a s t w i t h those o f Stoner (1983) who found h i g h e r d e n s i t i e s of i n f a u n a l amphipods i n v e g e t a t e d areas than i n nearby  unvegetated  a r e a s i n a seagrass bed i n F l o r i d a , n e g a t i v e c o r r e l a t i o n s between macrophyte biomass and t h e abundance of amphipods c o l l e c t e d i n sediment c o r e s have been p r e v i o u s l y r e p o r t e d .  For example, Stoner  (1980a) found  t h a t t h e i n f a u n a l amphipods, Ampelisca v e r i l l i and A. vadorum, accounted f o r 67.6% o f a l l amphipods c o l l e c t e d i n an unvegetated Apalachee Bay, F l o r i d a .  study s i t e i n  These s p e c i e s made up a maximum o f 10.2% o f t h e  t o t a l amphipod abundance i n a nearby mixed stand of T h a l a s s i a  testudinum  189 . and Syringodium f i l i f o r m e . Both species of Ampelisca are infaunal tubedwellers and the heavy network of rhizomes i n densely vegetated areas may make i t d i f f i c u l t for them to b u i l d their tubes.  This d i f f i c u l t y  would, i n turn, account for their limited d i s t r i b u t i o n i n seagrass meadows.  In contrast, Corophium acherusicum,  the dominant species i n  the present study, builds i t s tubes on substrates such as plants and organic debris at the sediment surface, and i t s movements would not be r e s t r i c t e d by the presence of eelgrass.  Instead, given that greater  resuspension of the sediment probably occurs i n bare patches within the Zostera marina meadow than i n densely vegetated areas, t h i s depositfeeding species may be responding to enhanced feeding conditions. This type of interaction could explain the o v e r a l l inverse r e l a t i o n s h i p between eelgrass biomass and density of shoots, and the number of amphipods collected i n the sediment.  It could also explain the p o s i t i v e  relationship between their abundance and the extent of shoot removal or "disturbance" i n the treatment p l o t s . In the past, the r o l e of d r i f t algae has been largely ignored i n studies which have sought to explain the r o l e of seagrass i n structuring the associated macroinvertebrate community.  For example, Stoner (1983)  " c a r e f u l l y avoided" clumps of d r i f t algae i n his study of tanaids and amphipods i n the seagrass meadows of the Indian River, F l o r i d a . Other studies have shown that d r i f t algae, which are generally more abundant i n seagrass meadows than i n nearby unvegetated areas (Zimmerman & Livingston, 1979), do play an important r o l e i n the organization of seagrass communities (Hooks et a l . , 1979; Kulczycki et a l . , 1981).  As  an example, Jacobs (1980) attributed an increase i n the abundance and d i v e r s i t y of the animal community i n the eelgrass beds of France to a midsummer bloom of Ulva lactuca, Enteromorpha spp., and Ceramium rubrum.  190 In a s l i g h t l y d i f f e r e n t  system, B e l l and Coen (1982) found a p o s i t i v e  r e l a t i o n s h i p between U l v a sp. and w i t h p o l y c h a e t e tube-caps.  the d e n s i t y of meiofauna a s s o c i a t e d  They a t t r i b u t e d t h i s t o the e f f e c t s of  the  a l g a e i n moderating p h y s i c a l s t r e s s and i n p r o v i d i n g a d d i t i o n a l m i c r o h a b i t a t s f o r these With o n l y a few  animals.  exceptions,  amphipods were observed  s i g n i f i c a n t l y h i g h e r d e n s i t i e s of  on the d r i f t  c o n s i s t e d of U l v a sp. and  s u b s t r a t e , which p r i m a r i l y  fragments of e e l g r a s s b l a d e s , than on  l i v i n g Z. marina. Months i n which t h e r e was t h e number of amphipods c o l l e c t e d i n c l u d e d A p r i l and May,  • m~  2  no  the  significant difference in  on the two p l a n t s u b s t r a t e s  when the p o p u l a t i o n s of the dominant s p e c i e s  were a t t h e i r annual minima, and June and August. In the l a t t e r  two  months, the amphipods were p a t c h i l y d i s t r i b u t e d i n the d r i f t and, r e s u l t although  t h e r e were t h r e e - t o n i n e - f o l d d i f f e r e n c e s i n the mean  d e n s i t y of animals  on the two  types of p l a n t s u b s t r a t e , t h e s e  d i f f e r e n c e s were masked by the v a r i a b i l i t y of the d a t a . although  as a  In any  case,  a l l of the dominant s p e c i e s were c o l l e c t e d on the e e l g r a s s as  w e l l as i n the d r i f t , w i t h the e x c e p t i o n of I s c h y r o c e r u s l e s s e r e x t e n t , A. v a l i d a , each was p a t t e r n of d i s t r i b u t i o n was  sp., and t o a  most abundant i n the d r i f t .  a l s o apparent  i n the r e s u l t s of  This  the  p r i n c i p a l components a n a l y s i s i n which r e l a t i v e l y c l o s e a s s o c i a t i o n s between the Corophium s p e c i e s and A. p u g e t t e n s i s were observed. t h e d i f f e r e n c e s i n temporal  d i s t r i b u t i o n of Corophium s p e c i e s and  p u g e t t e n s i s , t h i s a s s o c i a t i o n was  p r o b a b l y due  A.  t o the c o l l e c t i o n s of  l a r g e numbers o f each of t h e s e s p e c i e s i n the d r i f t and samples. I observed  Given  sediment  p a r t i c u l a r l y h i g h d e n s i t i e s of amphipods w i t h i n the  f o l d s of the t h i n , membranous f r o n d s of U l v a sp. d u r i n g the summer. In J u l y i n the main study s i t e  ( T ) , when more than 1100  Corophium  191.;. acherusicum  • m~  of d r i f t  2  s u r f a c e were c o l l e c t e d , mean d e n s i t i e s of  l e s s than 17 amphipods * m  of e e l g r a s s s u r f a c e were r e c o r d e d .  suggests t h a t the supplementary m i c r o h a b i t a t s the d r i f t  This  c r e a t e d by the presence of  a l g a e i n c r e a s e d the c a r r y i n g c a p a c i t y of the e e l g r a s s meadow,  p e r m i t t i n g the c o e x i s t e n c e of a much h i g h e r d e n s i t y of amphipods than would be p o s s i b l e i n the absence of these  substrates.  Refuges a s s o c i a t e d w i t h the f l o a t i n g mats of d r i f t U l v a sp. or Enteromorpha sp., may  algae,  p r o v i d e more e f f e c t i v e s h e l t e r than  those a s s o c i a t e d with the e e l g r a s s and may  permit  the amphipods to  a c h i e v e h i g h d e n s i t i e s i n s p i t e of p o s s i b l y h i g h p r e d a t i o n (Smith,  1972).  Many i n d i v i d u a l s and  such r e f u g e s , and o n l y the "overflow", vulnerable to predation  (Smith,  1972;  species f i n d  the  shelter i n  f o r c e d out by overcrowding, i s P i e l o u , 1975). Although the wide  sp. do not p r o t e c t f a u n a l i n h a b i t a n t s a g a i n s t wave shock  or d e s i c c a t i o n i n the exposed i n t e r t i d a l a r e a they may  pressure  P i e l o u (1975) r e f e r s t o t h i s type of s i t u a t i o n as  "nook-and-cranny" e f f e c t .  t h a l l i of U l v a  such as  (Pomeroy & Levings,  shade animals such as amphipods d u r i n g prolonged  i n s o l a t i o n a t low  1980),  p e r i o d s of  t i d e i n e e l g r a s s beds, as w e l l as reduce t h e i r  a c c e s s i b i l i t y to v i s u a l p r e d a t o r s . Anisogammarus p u g e t t e n s i s was chinook i n June and  Macdonald (1984) found t h a t  the p r i m a r y amphipod t a r g e t of j u v e n i l e  J u l y . If the d r i f t  a l g a e does p r o v i d e  superior  r e f u g e s , h i g h e r d e n s i t i e s of amphipods, p a r t i c u l a r l y t a r g e t s p e c i e s , should be a s s o c i a t e d w i t h these  substrates.  In f a c t , i n J u l y 1984  Area 2, where the d e n s i t i e s of e e l g r a s s shoots was e q u i v a l e n t of more than 1900 collected.  A. p u g e t t e n s i s  In comparison, fewer than 100  species occupied  in  r e l a t i v e l y low,  * m ^ of d r i f t  individuals * m  the Z. marina s u b s t r a t e . T h e r e f o r e ,  the  s u r f a c e were of  this  i t appears t h a t  f l o a t i n g mats of U l v a t h a t dominated the d r i f t d u r i n g the summer  may  192 have p r o v i d e d a more e f f e c t i v e r e f u g e from p r e d a t i o n f o r A.  pugettensis  than d i d the e e l g r a s s . The seagrass  s t r u c t u r a l heterogeneity that d r i f t system may  a l g a e can c o n t r i b u t e to a  a l s o enhance sediment d e p o s i t i o n , thus p r o v i d i n g  a d d i t i o n a l food r e s o u r c e s i n a l o c a l i z e d a r e a  ( H i c k s , 1977). L e v i n t o n  (1985) demonstrated t h a t a d d i t i o n s of d e t r i t u s from U l v a  rotunda  enhanced the s t a n d i n g s t o c k s of b e n t h i c diatoms, which i n t u r n p r o v i d e d an important  n u t r i e n t s o u r c e f o r gastropod  can be an important  grazers. Also, d r i f t  f o o d r e s o u r c e i n i t s own  1980). In the p r e s e n t  right  algae  (Pomeroy and  Levings,  study, the i n c r e a s e i n amphipod d e n s i t i e s which  began i n l a t e summer d i d not i n f a c t c o i n c i d e w i t h the peak i n d r i f t biomass, but r a t h e r w i t h months i n which the a l g a e were d e c a y i n g . suggests  t h a t the d r i f t  may  have been more important  than as a r e f u g e from p r e d a t o r s and time.  environmental  as a food  This  resource  stress during  this  I t s importance i n t h i s r e g a r d undoubtedly i n c r e a s e d through  winter months when the d r i f t e e l g r a s s , an important  was  the  made up e n t i r e l y of decomposing  s o u r c e of food f o r o v e r w i n t e r i n g amphipods (Mann,  1975). In a review of t h e e c o l o g y of seagrass beds, K i k u c h i and (1973) proposed t h a t a r e l a t i o n s h i p might be found between the  Peres  seasonal  changes i n seagrass biomass and the s t r u c t u r e of t h e a s s o c i a t e d i n v e r t e b r a t e community. T h i s statement  c o u l d p r o b a b l y be extended t o  i n c l u d e a l l i n t e r t i d a l m a r i n e macrophytes, a t l e a s t i n n o r t h e r n temperate h a b i t a t s where t h e r e i s a d e f i n i t e s e a s o n a l c y c l e of growth and d e g e n e r a t i o n ,  and  thus c o u l d a p p l y t o d r i f t  e e l g r a s s . In such a system, d e p o s i t - and  a l g a e as w e l l as t o  suspension-feeders  would  dominate the community d u r i n g the season of p l a n t decay when d e t r i t u s i s  193 abundant, whereas m o t i l e h e r b i v o r o u s  fauna would be most numerous i n the  s p r i n g when t h e p l a n t s and t h e i r a s s o c i a t e d e p i p h y t e s were growing. In t h e P a c i f i c Northwest, peak s t a n d i n g crop and shoot d e n s i t y o f Z o s t e r a marina u s u a l l y occur from June - September with a g r a d u a l d e c l i n e i n biomass and shoot number over ( H a r r i s o n , 1982a; P h i l l i p s ,  1983).  t h e summer and autumn  Large f l o a t i n g mats o f d r i f t  algae,  most t y p i c a l l y U l v a sp., o f t e n f l o u r i s h i n t h e summer i n e s t u a r i e s and these a l s o c o n t r i b u t e s i g n i f i c a n t l y t o t h e accumulation e e l g r a s s meadows. D e t r i t u s i s an important  of d e t r i t u s i n  source o f food f o r many  s p e c i e s o f amphipods, i n c l u d i n g most of t h e dominant s p e c i e s i n the present Algae,  collected  study. u n l i k e v a s c u l a r p l a n t s such as Z o s t e r a marina, a r e h i g h i n  n i t r o g e n , s o l u b l e o r g a n i c s , and a s h and as a r e s u l t , they decompose r a p i d l y , p r o v i d i n g d e t r i t i v o r e s such as Corophium s p e c i e s w i t h a f o o d supply t h a t i s r e l a t i v e l y e a s i l y a s s i m i l a t e d (Tenore and R i c e , 1979). In comparison, e e l g r a s s c o n t a i n s i n h i b i t o r y substances  such as p h e n o l i c s  which must l e a c h out b e f o r e i t can be u t i l i z e d as. food by amphipods ( H a r r i s o n , 1982b).  In t h i s r e g a r d , Robertson and Mann (1980) r e p o r t e d  t h a t Gammarus o c e a n i c u s  would not e a t Z. m a r i n a - d e r i v e d  i t had aged f o r 60 d a y s .  detritus  until  The f e e d i n g a c t i v i t i e s o f e p i b e n t h i c amphipods  are instrumental i n i n i t i a t i n g the fragmentation  of e e l g r a s s and i n  s t i m u l a t i n g t h e c o l o n i z a t i o n of d e t r i t u s p a r t i c l e s by m i c r o f l o r a , which, i n t u r n , p r o v i d e t h e amphipods w i t h a n i t r o g e n - r i c h n u t r i e n t ( H a r r i s o n & Mann, 1975).  As a r e s u l t o f i t s slow  source  decomposition,  e e l g r a s s p r o v i d e s a l o n g term', c o n s i s t e n t food supply t h a t i s p a r t i c u l a r l y important (Mann, 1975).  d u r i n g w i n t e r p e r i o d s of low p r i m a r y p r o d u c t i v i t y  The complementary s e a s o n a l i t y o f a l g a - and e e l g r a s s -  d e r i v e d d e t r i t u s may f a c i l i t a t e t h e expansion  o f amphipod p o p u l a t i o n s ,  194 which i n t h e p r e s e n t  study,  i n c r e a s e d i n s i z e from l a t e summer t o  autumn. The  l i f e c y c l e s o f Corophium acherusicum, and t o a l e s s e r  of C. i n s i d i o s u m and Ampithoe v a l i d a , were s y n c h r o n i z e d s e a s o n a l i t y o f macrophyte biomass.  Peak r e c r u i t m e n t  extent,  with the  i n t h e l a t e summer,  coupled w i t h an abundant supply o f food i n t h e autumn, made i t p o s s i b l e f o r these  s p e c i e s t o a c h i e v e much h i g h e r d e n s i t i e s than  Anisogammarus p u g e t t e n s i s or I s c h y r o c e r u s peak r e c r u i t m e n t  either  sp., both of which  experienced  i n the s p r i n g . This seasonal v a r i a t i o n i n s t r u c t u r e of  the e e l g r a s s - a s s o c i a t e d amphipod community supports K i k u c h i and Pere's (1977) p r e d i c t i o n t h a t , i n seagrass  systems, d e p o s i t and suspension  f e e d e r s would be most abundant i n t h e season o f p l a n t decay, w h i l e h e r b i v o r e s would be most numerous d u r i n g t h e s p r i n g . Both Corophium s p e c i e s a r e d i t r o p h i c and can s w i t c h from s u s p e n s i o n - f e e d i n g t o s e l e c t i v e d e p o s i t - f e e d i n g depending on t h e a v a i l a b i l i t y of food ( E n e q u i s t , 1952; N a i r & Anger, 1979).  types  When t h e c o n c e n t r a t i o n of  p a r t i c u l a t e matter i n t h e water column i s h i g h , Corophium remain i n s i d e t h e i r tubes and use t h e i r pleopods t o c r e a t e a f e e d i n g c u r r e n t .  Food  p a r t i c l e s c a r r i e d i n t h i s c u r r e n t a r e d i r e c t e d i n t o t h e tube by t h e antennae and a r e t r a n s p o r t e d t o t h e mouth by t h e gnathopods and maxillipeds  ( M i l l e r , 1984).  When few o r g a n i c p a r t i c l e s a r e suspended i n  the water column, however, they may l e a v e t h e i r tubes and s w i t c h t o s e l e c t i v e d e p o s i t f e e d i n g ( E n e q u i s t , 1952; N a i r and Anger, 1979). a b i l i t y t o s w i t c h f e e d i n g methods enhances t h e c a p a c i t y o f these t o s u r v i v e when c e r t a i n types o f food a r e i n s h o r t supply. species maintained f o r more than  a p r e s e n c e i n t h e study  s i t e year-round  The animals  Corophium and accounted  88% o f t h e o v e r a l l amphipod abundance i n October when t h e  organic content  o f the sediment was a t i t s peak.  195  Similarly, Ampithoe valida , which l i k e the Corophium species was most abundant i n the autumn, i s a l s o capable of exploiting a f a i r l y wide range of food items.  Members of the Ampithoidae use their gnathopods to  capture organic p a r t i c l e s that d r i f t by the openings of their tubes, and graze on filamentous and thin-bladed algae such as Enteromorpha spp. and Ulva spp. (Goodhart,  1939; Conlan, 1982).  They may also resort to  eating the fragments of algae and detritus that constitute the tube (Conlan, 1982).  This a b i l i t y to u t i l i z e a wide range of food types  may  have made i t possible for A. v a l i d a to maintain i t s e l f i n the eelgrass meadow i n most months, and to exploit the increased food supply which occurs i n late summer by producing large numbers of juveniles at that time.  As the population increased i n size through the autumn, so did  the r e l a t i v e abundance of juveniles, from 4 4 % i n August to 73% i n September. Both the seasonal d i s t r i b u t i o n and feeding behaviour of Anisogammarus pugettensis and Ischyrocerus sp. d i f f e r e d from those of Ampithoe valida and the two Corophium species. The spring increase i n the abundance of A. pugettensis and Ischyrocerus sp., both of which are herbivores, coincided with the new growth of the eelgrass plants and t h e i r attached epiphytes. The feeding methods of these species contrast with the detritivorous habits of Ampithoe v a l i d a and the two Corophium species. Anisogammarus pugettensis feeds on a wide variety of algae including Enteromorpha sp., Ulva sp., and diatoms, and may also consume decomposing animal matter (Chang,1975).  According to Chang (1975),  while individuals of this species e a s i l y manipulate fragments of algae and clumps of diatoms, they are poorly equipped to deal with the fine, loose detritus which would constitute the major food source for amphipods during the winter. S i m i l a r l y , Ischyrocerus sp. i s also an  196 a l g a l grazer. As suggested by the bright green digestive tract t h i s species l i k e l y consumes a v a r i e t y of micro- and macroalgae which would be most abundant i n the spring.  As previously mentioned, due to the  presence of i n h i b i t o r y substances, few species of amphipods eat l i v i n g eelgrass. Caprella laeviuscula, a c a p r e l l i d amphipod which grazes on the microalgae on the surface of eelgrass blades (Caine, 1979) was also extremely abundant i n the study area i n the spring. The l i f e cycles of the Corophium species and Ampithoe v a l i d a appeared to be synchronized with the seasonality of eelgrass biomass. Peak recruitment i n these populations occurred i n the l a t e summer and autumn.  An increase i n the abundance of seagrass-associated macrofauna  i n the season of plant decay, similar to that which I observed i n the intercauseway area, has been reported i n previous studies.  For example,  Marsh (1973) noted i n a survey of the epifauna associated with eelgrass i n Chesapeake Bay that the abundance of amphipods increased i n the autumn despite a summer decline i n Z. marina biomass. Mukai (1971) observed a similar increase i n the abundance of amphipods during the decaying season of Sargassum i n Japan. In the present study, o v e r a l l abundances of amphipods peaked i n October, as d i d eelgrass biomass and sediment organic content, a measure of d e t r i t a l food a v a i l a b i l i t y . The autumn peak i n amphipod density was followed by an abrupt decline, primarily due to a decrease i n the abundance of Corophium species. Similar peaks and sharp declines have been observed i n other studies of amphipod populations and have been attributed to a variety of physical factors such as changes i n temperature or s a l i n i t y 1971; Sheader, 1978).  There was no apparent  (Mukai,  relationship  between seasonal fluctuations i n s a l i n i t y and the decline i n the abundance of amphipods i n the present study.  Lowest s a l i n i t i e s occurred  i n August when, i n f a c t , numbers  o f amphipods were i n c r e a s i n g . The  winter d e c l i n e may have been caused i n s t e a d by t h e e f f e c t o f d e c r e a s i n g temperatures on r e p r o d u c t i v e a c t i v i t y , and b i r d p r e d a t i o n , as w e l l as the m o r t a l i t y of p o s t - r e p r o d u c t i v e a d u l t s . In a study i n n o r t h - e a s t e r n England, Sheader (1978) observed p r e c i p i t o u s decrease  a  i n the abundance of e p i b e n t h i c Corophium i n s i d i o s u m  f o l l o w i n g peak d e n s i t i e s i n l a t e summer, s i m i l a r t o t h a t observed i n both Corophium p o p u l a t i o n s i n t h e p r e s e n t  study.  He a t t r i b u t e d  d e c l i n e t o an i n c r e a s e i n brood m o r t a l i t y , and t o a d e c r e a s e a c t i v i t y and i n the number o f a c t i v e l y b r e e d i n g  females.  this  i n feeding  He found  that  mature females e n t e r e d a " r e s t i n g " stage i n November and December when no young were produced.  In the intercauseway  a r e a , no o v i g e r o u s  C.  acherusicum females were c o l l e c t e d i n December or January, w h i l e a l l i n d i v i d u a l s with setose brood p l a t e s appeared t o be p o s t - r e p r o d u c t i v e . T h i s suggests  t h a t t h e females o f t h i s s p e c i e s a l s o e n t e r a r e s t i n g -  stage i n the w i n t e r . females d i s a p p e a r e d u n t i l May.  Although  In the  C. i n s i d i o s u m p o p u l a t i o n ,  from t h e study  ovigerous  s i t e i n November and d i d not reappear  the low w i n t e r abundance of t h i s s p e c i e s i n the  study area was l i k e l y a r e f l e c t i o n o f the s e a s o n a l d e c l i n e i n r e p r o d u c t i v e a c t i v i t y and the death of mature a d u l t s , i t may a l s o have been caused by o f f s h o r e m i g r a t i o n as d e s c r i b e d by B o u s f i e l d (1973) f o r A t l a n t i c c o a s t p o p u l a t i o n s of C. i n s i d i o s u m .  The d e c l i n e i n the  abundance o f both Corophium s p e c i e s i n the study a r e a i n December and January, and t h e i r absence i n March and A p r i l ,  suggests  c o a s t p o p u l a t i o n s of Corophium may have a s i m i l a r  that  Pacific  s e a s o n a l p a t t e r n of  migration. B i r d p r e d a t i o n may a l s o be important  i n r e g u l a t i n g abundances of  amphipods d u r i n g t h e winter and s p r i n g i n t h e intercauseway  area.  198 F l o c k s of d u n l i n ( C a l i d r i s a l p i n a ) o v e r w i n t e r Roberts Bank and During  i n the f o r e s h o r e areas  forage i n the Z o s t e r a marina meadows d u r i n g low  t h i s time they consume l a r g e numbers of Corophium  tide.  spp.,  p a r t i c u l a r l y C. acherusicum (McEwan and  F r y , 1985;  communication, 1985).  f o r the importance of  F u r t h e r evidence  of  McEwan, p e r s o n a l predation  by b i r d s i n the r e g u l a t i o n of abundances of amphipods has been p r o v i d e d by G r a t t o e t a l . (1984) and  H i c k l i n and  Smith (1984). G r a t t o et a l .  (1984) showed t h a t Corophium v o l u t a t o r i s the p r e f e r r e d p r e y semipalmated sandpipers concluded  i n the Bay o f Fundy.  H i c k l i n and  t h a t the behayour of C. v o l u t a t o r d u r i n g ebb  their vulnerability  to b i r d predation.  of  Smith (1984)  tide  increased  They observed t h a t males of  s p e c i e s do not burrow as soon as the t i d e recedes but  instead,  this  crawl  about on the s u r f a c e of the s u b s t r a t e f o r up t o t h i r t y minutes. Thus, they can be e a s i l y l o c a t e d by edge ( H i c k l i n and drift  cover and  Smith, 1984).  seasonal  Roberts Bank may  sandpipers,  which f o r a g e a t the water's  I t i s p o s s i b l e t h a t the absence of  senescence of the e e l g r a s s i n the winter  i n c r e a s e the exposure and  on  thus the v u l n e r a b i l i t y of  acherusicum and C. i n s i d i o s u m t o b i r d p r e d a t i o n ,  C.  f o r as p r e v i o u s l y  mentioned these amphipods a r e most abundant a t the sediment  surface  d u r i n g t h i s time. In summary, no r e l a t i o n s h i p shoots and was  between the d e n s i t y of Z o s t e r a marina  the abundance and d i v e r s i t y of gammarid amphipod  found i n the present  study.  populations  The d i s t r i b u t i o n of the dominant  s p e c i e s seemed t o be r e g u l a t e d i n s t e a d by the s e a s o n a l i t y of macrophyte biomass.  Numbers of amphipods peaked i n the autumn when the p l a n t s were  decaying,  and more animals were a s s o c i a t e d w i t h the d r i f t  and  organic  d e b r i s a t the sediment s u r f a c e than w i t h the e e l g r a s s p l a n t s . p a t t e r n of d i s t r i b u t i o n r e f l e c t s two  structural  f e a t u r e s of the  This Zostera-  199 a s s o c i a t e d amphipod community. Corophium s p e c i e s . of  Firstly,  t h e community i s dominated by  The summer abundance o f these a n i m a l s ,  unlike  those  Ampithoe v a l i d a and Anisogammarus p u g e t t e n s i s , does n o t appear t o be  r e g u l a t e d by s i z e - s e l e c t i v e o r v i s u a l p r e d a t o r s  such as f i s h .  Consequently, i t i s not s u p r i s i n g t h a t they d i s p l a y no p r e f e r e n c e f o r areas o f h i g h shoot d e n s i t y .  Secondly,  t h e maximum d e n s i t y o f Z o s t e r a  shoots on Roberts Bank may not r e p r e s e n t t h e t h r e s h o l d l e v e l  necessary  to p r o t e c t s p e c i e s o f amphipods which a r e t a r g e t s of f i s h p r e d a t o r s . i s t h e f l o a t i n g mats of Ulva sp. and o t h e r d r i f t a l g a e , i n c o n c e r t  It  with  e e l g r a s s , which s i g n i f i c a n t l y c o n t r i b u t e t o t h e h e t e r o g e n e i t y of t h e environment and p r o v i d e e f f e c t i v e s p a t i a l r e f u g e s f o r p r e y as A. p u g e t t e n s i s . winter  likely  The disappearance  s p e c i e s such  of t h i s substrate during the  i n c r e a s e s the g e n e r a l v u l n e r a b i l i t y o f amphipods t o  p r e d a t i o n by s h o r e b i r d s . and e e l g r a s s a l s o permit  The complementary s e a s o n a l i t y o f d r i f t  algae  c e r t a i n p o p u l a t i o n s of amphipods, i n c l u d i n g  those o f Ampithoe v a l i d a , as w e l l as Corophium spp., d e n s i t i e s d u r i n g the autumn.  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Responses o f e s t u a r i n e i n f a u n a t o d i s t u r b a n c e . I . S p a t i a l and temporal v a r i a t i o n of i n i t i a l r e c o l o n i z a t i o n . Mar.Ecol.Prog.Ser.10: 1-14 ZAR, J.H., 1974. B i o s t a t i s t i c a l A n a l y s i s . P r e n t i c e - H a l l , I n c . , 1984. B i o s t a t i s t i c a l A n a l y s i s (2nd e d . ) . P r e n t i c e - H a l l , Inc. ZIMMERMAN, M.S. and R.J. L i v i n g s t o n , 1979. Dominance and d i s t r i b u t i o n of b e n t h i c macrophyte assemblages i n a n o r t h F l o r i d a e s t u a r y (Apalachee Bay, F l o r i d a ) . B u l l . M a r . S c i . 2 9 : 27-40. ZIMMERMAN, R., R. Gibson and J . H a r r i n t o n , 1979. H e r b i v o r y and d e t r i v o r y among gammaridean amphipods from a F l o r i d a seagrass community. M a r . B i o l . 5 4 : 41-48.  209  APPENDIX 1.  Species  R e g r e s s i o n e q u a t i o n s and c o e f f i c i e n t s (R) f o r t h e r e l a t i o n s h i p b e t w e e n h e a d l e n g t h ( x ) and t o t a l l e n g t h ( y ) o f the dominant s p e c i e s o f amphipods. n = t o t a l number m e a s u r e d . The s l o p e of t h e r e g r e s s i o n l i n e f o r Corophium i n s i d i o s u m m a l e s and f e m a l e s was s i g n i f i c a n t l y d i f f e r e n t (p < 0 . 0 5 ) .  Equation  R  n  y = 10.96x - 0.969  0.968  45  y = 1 2 . 2 x - 2.43  0.940  33  y = 1 2 . l x - 1.05  0.892  63  y = 5.66x + 0.403  0.792  19  y = 9.40x - 0.064  0.834  22  Ampithoe valida Anisogammarus pugettensis Corophium acherusicum Corophium insidiosum (male) Corophium insidiosum (female)  

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