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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 IN THE COMMUNITY DYNAMICS OF AN EELGRASS-ASSOCIATED ASSEMBLAGE OF GAMMARID AMPHIPODS by PATRICIA ANNE MILLER B.Sc. (Hons.), University of 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 i n THE FACULTY OP GRADUATE STUDIES ( Z o o l o g y ) We accept t h i s thesis as conforming to the required 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 present ing this thesis in partial fulf i lment of the requ i rements for an advanced deg ree at the University of British C o l u m b i a , I agree that the Library shall make it freely available for reference and study. I further agree that permiss ion for extens ive c o p y i n g of this thesis for scholarly pu rpose s may be granted by the h e a d of my depar tment or by his o r her representatives. It is u n d e r s t o o d that c o p y i n g or publ icat ion of this thesis for f inancial gain shall not b e a l lowed without m y written permiss ion. Depar tment of Z o o l o g y T h e University of British C o l u m b i a 1956 Ma in Mal l Vancouver , C a n a d a V 6 T 1Y3 Date O c t o b e r 15 r 1 9 8 5 i i ABSTRACT The r o l e of density of eelgrass shoots i n regulating d i s t r i b u t i o n and abundance of gammarid amphipods was investigated. Monthly c o l l e c t i o n s of amphipods were made over a one-year period i n a serie s of treatment p l o t s on Roberts Bank, i n southwestern B.C., i n which eelgrass (Zostera marina L.) shoots had been thinned to 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 . This experiment was o r i g i n a l l y designed to test the hypothesis that the abundance and d i v e r s i t y of amphipods would be p o s i t i v e l y r e l a t e d to the density of eelgrass shoots. Due to the rapid recovery of o r i g i n a l shoot d e n s i t i e s within the p l o t s , however, t h i s hypothesis could not be tested. Consequently, the emphasis of the study was r e s t r i c t e d to a consideration of the e f f e c t of the disturbance created during removal of shoots on the d i s t r i b u t i o n of amphipods. Collecti o n s of amphipods were also made i n three areas of d i f f e r e n t natural d e n s i t i e s of Zostera shoots during a three-month period i n summer 1984, to assess further the e f f e c t of shoot density on the d i s t r i b u t i o n and abundance of amphipods. The r o l e of a d d i t i o n a l components of habitat heterogeneity, including d r i f t algae and a second species of seagrass, Zostera japonica, i n modifying the community dynamics of the amphipods was also studied. No r e l a t i o n s h i p between the density of Zostera shoots and the abundance and d i v e r s i t y of amphipods was found. The amphipod community was dominated by Corophium  acherusicum and the d i s t r i b u t i o n of t h i s species, as well as that of the other most frequently c o l l e c t e d species, appeared to be regulated by the seasonality of macrophyte biomass. Peak abundances of amphipods occurred i n the l a t e summer and autumn when large amounts of d r i f t algae and eelgrass d e t r i t u s were present at the sediment surface. This decaying t i i a plant material i s an important source of food for d e t r i t i v o r e s such as gammarids and i t s seasonal abundance was reflected i n the rapid 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., also contributed s i g n i f i c a n t l y to the carrying 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 role 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. i i i TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES v i LIST OF FIGURES i x ACKNOWLEDGEMENTS x i i INTRODUCTION' 1 DESCRIPTION OF STUDY AREA 7 MATERIALS AND METHODS 11 A. The effect of removal of Zostera 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 Design 12 2. Sampling Procedure 16 3. Physical Factors 17 4. Laboratory 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 natural d e n s i t i e s of Zostera shoots and the d i s t r i b u t i o n and abundance of amphipods (The Three-Month Study) 24 1. Design of Study S i t e s . 24 2. Sampling Procedure 25 i v Page 3. Physical Factors 28 4. Laboratory Procedures 28 5. S t a t i s t i c a l A n a l y s i s 29 6. Correlation Matrices 29 C. L i f e Cycles and Size-Frequency D i s t r i b u t i o n s 30 R E S U L T S 3 2 A. The Twelve-Month Study 32 1. Physical Factors 29 2. Growth of Zostera marina 36 3. Composition and seasonality of d r i f t 42 4. E f f e c t of shoot removal on growth of Zostera marina 45 5. E f f e c t of Zostera marina shoot removal on the d i s t r i b u t i o n and abundance of amphipods 45 6. Seasonal d i s t r i b u t i o n of amphipods 50 7. Macrophyte biomass and 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 species of amphipods 67 9. C o r r e l a t i o n matrices and the d i s t r i b u t i o n of species 77 B. The Three-Month Study 99 1. Physical Factors 99 V Page 2. Biofnass and shoot density of Zostera marina 108 3. Biomass and shoot density of Zostera japonica 112 4. Composition and d i s t r i b u t i o n of d r i f t 112 5. Zostera marina shoot density and t h e d i s t r i b u t i o n o f amphipods 121 6. D i s t r i b u t i o n and o v e r a l l abundance of amphipods 121 7. D i s t r i b u t i o n of amphipod species 134 8. Na t u r a l i s t sled samples -.. ....143 9. Cor r e l a t i o n matrices and the d i s t r i b u t i o n of species 147 C. L i f e Cycles and Size-Frequency D i s t r i b u t i o n s 159 1. Size-frequency d i s t r i b u t i o n s 159 2. Reproductive seasonality 167 DISCUSSION 178 REFERENCES 200 APPENDIX 1. Linear regression equations for head length vs t o t a l length for Ampithoe v a l i d a , Anisogammarus pugettensis,, Corophium acherusicum, and C. insidiosum 12.0-9' v i LIST OF TABLES Page 1. Densities of Zostera marina shoots i n treatment p l o t s , before and a f t e r shoot removal 15 2. Relationship between surface area and dry weight of macrophyte species 20 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 of sediments i n treatment p l o t s , June 27, 1984 35 4. Percentage organic content of sediment at study s i t e "T" 37 5. Total numbers of amphipods • m ^ of substrate surface i n treatment plo t s 49 6. Corr 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 with dry weight of Zostera marina rhizomes 61 7. Co r r e l a t i o n c o e f f i c i e n t s for comparisons of monthly mean number of amphipods i n Zostera marina, d r i f t , sediment, and t o t a l quadrat samples, with surface area and dry weight of macrophytes at study s i t e "T" ..' 62 8. Corr e l a t i o n c o e f f i c i e n t s for comparisons of mean number of amphipods i n Zostera marina d r i f t , sediment, and t o t a l quadrat samples with surface area and dry weight of macrophytes at study s i t e "T" 65 9. Relative abundance, d i v e r s i t y , and evenness of species of amphipods at study s i t e "T" 68 10. Co 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 - 1 , c o l l e c t e d at study s i t e "T" and pooled i n a l l months and substrate types 88 11. Cor 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 - 1 at study s i t e "T" 91 12. Co r r e l a t i o n c o e f f i c i e n t s for comparisons of mean numbers of amphipods . species i n Zostera marina, d r i f t , and sediment at study s i t e "T" 96 v i i Page 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 Areas 1, 2, and 3 106 14. Percentage organic content of sediment i n Areas 1, 2, and 3 107 15. Dry weight of Zostera rhizomes i n Areas 1, 2, and 3 I l l 16. Monthly t o t a l number of amphipods • m-2 i n Areas 1, 2, and 3 122 17. Correlation c o e f f i c i e n t s for monthly comparisons of numbers of amphipods i n Zostera marina, d r i f t , sediment, and t o t a l quadrat samples with surface area and dry weight of macrophytes 132 18. Cor r e l a t i o n c o e f f i c i e n t s for comparisons of number of amphipods i n Zostera marina, d r i f t , sediment, and t o t a l quadrat samples with surface area and dry weight of macrophytes 135 19. Relative abundance, d i v e r s i t y , and evenness of species of amphipods i n Areas 1, 2, and 3 136 20. Numbers of amphipods i n n a t u r a l i s t sled tows i n Areas 1, 2, and 3 144 21. Correlation c o e f f i c i e n t s for comparisons of r e l a t i v e numbers of amphipods * s p e c i e s " 1 c o l l e c t e d i n Areas 1, 2, and 3 148 22. Cor r e l a t i o n c o e f f i c i e n t s for comparisons of monthly mean numbers of.amphipods * s p e c i e s - 1 i n Zostera marina, d r i f t , and sediment samples 150 23. Correlation c o e f f i c i e n t s for comparisons of mean numbers of amphipods • s p e c i e s - 1 i n Areas 1, 2, and 3 153 24. Monthly percentages of males, females, and ovigerous females of Corophium acherusicum 171 25. Monthly percentages of males, females, and ovigerous females of Corophium insidiosum 172 26. Monthly percentages of males, females, and ovigerous females of Ampithoe v a l i d a 174 v i i i Page 27. Monthly percentages of males, females, and ovigerous females of Anisogammarus pugettensis 176 i x LIST OF FIGURES Page 1. Map of intercauseway area on Roberts Bank showing study s i t e 8 2. Plan of treatment p l o t s i n Part A: Twelve-Month Study 13 3. N a t u r a l i s t sled 26 4. Monthly a i r and water temperatures 33 5. Monthly surface s a l i n i t i e s -. 33 6. Dry weight of above-ground Zostera marina shoots at study s i t e "T" 38 7. Number of blades per Zostera marina shoot at study s i t e "T" 38 8. Length of longest blade per Zostera marina shoot at study s i t e "T" 40 9. Dry weight of Zostera marina rhizomes • 0.01m-2 at study s i t e "T" : 40 10. Dry weight of d r i f t • 0.01m"2 at study s i t e "T" 43 11. Change i n density of Zostera marina shoots • 0.25m2 , June 1983 - July 1984 i n row 1 46 12. Change i n density of Zostera marina shoots • 0.25m-2, June 1983 - J u l y 1984 i n row 2 46 13. Monthly abundance of amphipods • m - 2 at study s i t e "T" 51 _ o 14. Mean number of amphipods * m of Zostera marina surface at study s i t e "T" 53 15. Mean number of amphipods • IT. of d r i f t surface at study s i t e "T" 55 16. Mean number of amphipods * m of sediment surface at study s i t e "T" 57 17. Mean number of i n d i v i d u a l s i n each dominant amphipod species • m - 2 surface of l i v e Zostera marina at study s i t e "T" 71 X Page 18. 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 - 2 s u r f a c e o f d r i f t a t s tudy s i t e " T " 73 19 . 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 sed iment s u r f a c e a t s t u d y s i t e " T " 75 20 . P r i n c i p a l component o r d i n a t i o n o f dominant amphipod s p e c i e s i n month ly samp les , August 1983 - J u l y 1984 78 21 . Month l y mean p r i n c i p a l component s c o r e s i n each s u b s t r a t e , August 1983 - J u l y 1984 82 22. A r e a 1, 28 J u l y 1984 100 2 3 . A r e a 2, 28 J u l y 1984 102 24 . A r e a 3, 28 J u l y 1984 104 25 . Dry weight o f above - g round Z o s t e r a m a r i n a shoots i n A rea s 1, 2, and 3 109 26. D e n s i t i e s o f Z o s t e r a mar ina shoot s • 0 . 2 5 m - 2 i n A rea s 1, 2, and 3 109 2 7 . D e n s i t y o f Z o s t e r a j a p o n i c a shoo t s ' 0.25m i n A r e a 1 113 28 . Mon th l y mean d r y we ights * 0 . 0 1 m - 2 o f d r i f t i n A r e a s 1, 2, and 3 115 29 . M o n t h l y abundance o f amphipods ' m i n A r e a s 1, 2, and 3 123 30 . Mean number o f amphipods * m - 2 o f Z o s t e r a mar ina s u r f a c e i n A rea s 1, 2, and 3 124 _o 3 1 . Mean number o f amphipods * m o f d r i f t s u r f a c e i n A r e a s 1, 2, and 3 127 32. Mean number o f amphipods * m * o f sed iment s u r f a c e i n A r e a s 1, 2, and 3 129 x i Page 33. Mean number of in d i v i d u a l s i n each dominant amphipod species * m~2 surface of l i v e Zostera marina i n Areas 1, 2, and 3 139 34. Mean number of in d i v i d u a l s i n each dominant amphipod species ' m surface of d r i f t i n Areas 1, 2, and 3 140 35. Mean number of ind i v i d u a l s i n each dominant amphipod species • m~2 of sediment surface i n Areas 1, 2, and 3 141 36. Size-frequency d i s t r i b u t i o n s of Corophium acherusicum 160 37. Size-frequency d i s t r i b u t i o n s of Corophium insidiosum 162 38. Size-frequency d i s t r i b u t i o n s of Ampithoe v a l i d a 165 39. Size-frequency d i s t r i b u t i o n s - o f Anisogammarus pugettensis 168 x i i ACKNOWLEDGMENTS I thank my supervisors, Dr. T. Carefoot and Dr. P.G. Harrison, for th e i r encouragement, c r i t i c i s m , and f i n a n c i a l support. I am g r a t e f u l to many people who donned chest-waders and braved the mudflats to 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. Boyle, D. Christiansen, C. Durance, J . Fisher, S. Guenther, S. Khanna, B. Ko, B. Power, T. Walters, and M. Yip. Many conversations with P. Shaw have improved my appreciation of amphipods, i f not of systematics. Most of a l l , I am indebted to J . McNicol for his generous companionship on a l l midnight sampling expeditions and throughout the study. 1 INTRODUCTION Seagrass meadows are fundamental components of estuarine and foreshore habitats. They are highly productive areas which are a e s t h e t i c a l l y as well as economically important, not just as feeding areas for birds, but also as nursery areas for juvenile f i s h and for crabs. S u p e r f i c i a l l y , a seagrass meadow appears to be a simple, homogeneous environment. Closer scrutiny, however, reveals i t s s t r u c t u r a l complexity. Many studies have shown that the abundance and d i v e r s i t y of organisms i n soft-bottom systems are enhanced by the presence of seagrass (e.g. Orth, 1977; Homziak et a l . , 1982). Seagrass communities c h a r a c t e r i s t i c a l l y include s e s s i l e and motile animals which attach themselves to the blades and stems of plants (epifauna), and animals which are f r e e - l i v i n g at the sediment surface, as well as those which b u i l d tubes on plant roots and rhizomes or burrow i n t o the sediment (infauna)(Kikuchi & Peres, 1977; Jacobs, 1980). In comparison, the faunal community of i n t e r t i d a l unvegetated sand f l a t s , where the sediments are r e a d i l y resuspended by wave energy and t i d a l currents, i s often dominated by-a few species which are able to escape the rigours of the sediment surface only by burrowing r a p i d l y or by l i v i n g i n deep tubes (Orth, 1977; Heck and Orth, 1980a; Summerson and Peterson, 1984). The mosaic of microhabitats i n seagrass systems provides refuges from both physical stress and b i o l o g i c a l i n t e r a c t i o n s for a wide v a r i e t y of macrofauna and i n so doing permits the development of a complex community structure (Heck and Wetstone, 1977; Nelson, 1979a,1980a; Stoner, 1980a). In seagrass beds, the emergent blades of the plants b a f f l e currents while the rhizomes trap 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 substrate, reducing sediment scour (Grady, 1981; Orth, 2 1977; Peterson et al.,1984). During low t i d e , when the seagrass blades and t h e i r attached epiphytes l i e prone on the substrate surface, the fauna that are sheltered beneath them are e f f e c t i v e l y protected against exposure to strong l i g h t and desi c c a t i o n (Kikuchi and Peres, 1977). S i m i l a r l y , when the seagrass meadow i s submerged at high t i d e , the leaf canopy creates a calm underwater space i n which f i s h and invertebrates may f i n d shelter (Kikuchi and Peres, 1977). These quiescent areas enhance sedimentation and prevent passive transport of small f r e e - l i v i n g adult and l a r v a l invertebrates out of the meadow (Orth, 1977). Seagrass plants also influence the outcome 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 predation (Nelson, 1979b, 1980a; Stoner, 1980a) and competition (Coen et a l . , 1981; Robertson & Mann, 1982). Although c e r t a i n f i s h predators may be attracted to seagrass meadows, t h e i r a b i l i t y to detect and capture epibenthic prey may be reduced due. to the physical presence of the plants. As a r e s u l t , the density of invertebrate macrofauna i s often d i r e c t l y r e l a t e d to macrophyte biomass (e.g. Stoner, 1980a; Heck and Orth, 1980b). Due to the high primary p r o d u c t i v i t y and rapid turnover rate of above-ground, biomass which are c h a r a c t e r i s t i c of seagrass systems (Ferguson & Adams, 1979), the p o s s i b i l i t y of d e t r i t a l food shortages i s generally discounted (e.g. Nelson, 1979a), and examples of competitive interactions involving t h i s resource are seldom described. In a study of a newly established Zostera marina L. meadow i n North Carolina, Thayer et a l . (1975) estimated that f ar from competing for food, the macrofauna consumed only 55% of the net production of the eelgrass, phytoplankton and benthic algae i n the system. In addition, Zimmerman et a l . (1979) found s i g n i f i c a n t differences i n both the s i z e and kind of de t r i t u s exploited by three species of gammarid amphipods i n the 3 seagrass beds of F l o r i d a , 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 prevent competition f o r food. Robertson and Mann (1982), however, speculated that gastropod grazers such as L i t t o r i n a neglecta may compete for ephiphytes and d e t r i t u s during the summer when t h e i r population d e n s i t i e s are high. They noted that s n a i l density was d i r e c t l y r e l a t e d to plant surface area. In an e a r l i e r study, Robertson (1981) had found that the population dynamics of another gastropod, L i t t o r i n a s a x i t a l i s , were c l o s e l y associated with the l i m i t e d periphyton food supply growing on Z. marina leaves and that i n t r a s p e c i f i c competition for t h i s resource resulted i n post-recruitment m o r t a l i t i e s . If the conclusions of these two studies are cor r e c t , and food i s l i m i t i n g for the L i t t o r i n a species, 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. Despite the large number of microhabitats i n seagrass meadows, there i s growing evidence that competition for shelter, another p o t e n t i a l l y l i m i t i n g resource, can occur within epifaunal communities of seagrass beds. Coen et a l . (1981), f o r example, concluded that the nonoverlapping microgeographical d i s t r i b u t i o n of two caridean shrimp species was p r i m a r i l y due. to i n t e r s p e c i f i c s p a t i a l competition rather than to microhabitat s e l e c t i o n . In repeated laboratory t r i a l s the dominant shrimp species, Palaemonetes floridanus, was co n s i s t e n t l y able to displace the second species, P. v u l g a r i s , from a s t r u c t u r a l l y complex red a l g a l substrate onto a simpler t u r t l e - g r a s s (Thalassia testudinum) substrate, where i t was more vulnerable to predation. Similar displacement might be expected to occur i n mixed stands of seagrass, i f one macrophyte species provided better shelter f o r macrofauna than d i d another species (Middleton et a l . , 1984). 4 Species composition and plant morphology may be as important as shoot density and biomass i n contributing to the s t r u c t u r a l complexity of seagrass systems (Heck and Orth, 1980a). Middleton et a l . (1984) concluded that differences i n the structure of the f i s h and invertebrate communities i n Zostera c a p r i c o r n i and Posidonia 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 to v a r i a t i o n s i n the s t r u c t u r a l complexity of the canopies created by the two seagrass species. The larger P. a u s t r a l i s plants formed a higher canopy and provided more shelter and attachment s i t e s for macrofauna than d i d the smaller species, which i n turn, attracted more f i s h predators. Stoner (1983) concluded that the r o l e of seagrass biomass i n determining the structure of macrofaunal assemblages was s i g n i f i c a n t only within a plant species. He also found that c e r t a i n species of epifaunal amphipods, in c l u d i n g Cymadusa compta, G r a n d i d i e r e l l a bonnieroides, and Me l i t a elongata, were capable of detecting small differences i n shoot density and that they could 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 plants with high surface area/biomass r a t i o s (Stoner, 1980c). In a study of the Thalassia testudinum beds i n F l o r i d a , Gore et a l . (1981) observed that the abundance and d i v e r s i t y of the macrocrustacean community were enhanced by the presence of d r i f t algae. Other factors which may enhance the s t r u c t u r a l complexity of a seagrass system include the presence of epiphytes and encrusting epifauna, which may provide a d d i t i o n a l microhabitats for associated organisms (Heck and Orth, 1980a). Topographical patchiness and the proximity of adjacent habitats such as c o r a l reefs can also account for v a r i a t i o n s i n the organization of the faunal community (Heck, 1977; Heck and Orth, 1980a). 5 Although there have been several recent studies on the d i s t r i b u t i o n and autecology of Zostera marina and Z. japonica Aschers. & Graebn. i n the P a c i f i c Northwest (Harrison, 1982a, 1984; P h i l l i p s , 1983), there have been few reported studies of the associated faunal communities (Thayer & P h i l l i p s , 1977; Kikuchi, 1980). Exceptions include Swinbank's (1979) study of the soft-bottom communities of Roberts Bank i n the Fraser River estuary, and Levings and Coustalin's (1975) survey of the benthic invertebrate fauna i n t h i s area. Pomeroy and Levings (1980) also included Roberts Bank i n th e i r study of the association and feeding r e l a t i o n s h i p s of the amphipod Eogammarus  confervicolus with various species of benthic algae. Gammaridean amphipods were chosen as the focus of the present study for several reasons: 1) High d e n s i t i e s of these animals have been recorded i n a v a r i e t y of d i f f e r e n t seagrass systems, including the eelgrass meadow on Roberts Bank (Young and Young, 1978; Nelson, 1981; M i l l e r & Hensel, 1982). 2)They are an i n t e g r a l component of the detritus-based food chain (Levings, 1973), and several species are consumed by juvenile salmon which reside i n the eelgrass beds of Roberts Bank during the spring 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 are large enough that t h e i r behaviour i n the f i e l d can be observed. 4)These animals occupy a v a r i e t y of microhabitats i n seagrass systems. Free-l i v i n g epifaunal species l i v e on the surface of the blades, while domiculous species l i v e i n tubes attached to the stems and rhizomes; other species are infaunal (Kikuchi and Peres, 1977). Because epifaunal species are more vulnerable to predation than are infaunal species (Stoner, 1979), I expected to observe di f f e r e n c e s i n temporal and s p a t i a l d i s t r i b u t i o n s of these two groups. Also, c e r t a i n species of 6 epifaunal amphipods have been shown to exhibit strong behavioural a f f i n i t i e s for high d e n s i t i e s of seagrass (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 possible to r e l a t e seasonal patterns of reproduction to those of species abundance and d i v e r s i t y (Nelson, 1980a). The present study had three objectives: 1) to document the seasonal patterns of d i s t r i b u t i o n and abundance of the gammaridean amphipod community i n the eelgrass bed located on Roberts Bank i n the Fraser River estuary; 2) to examine the r e l a t i o n s h i p between eelgrass biomass/shoot density and amphipod 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 habitat complexity, such as d r i f t algae and a second seagrass species, Z. japonica, i n the organization of the amphipod community. The study was divided i n t o two parts. The f i r s t part took place over twelve months from June 1983 - July 1984 and involved the manipulation of Zostera marina shoot d e n s i t i e s i n a single study s i t e . Patterns of 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 to shoot density and to the seasonality of macrophyte biomass. The second part took place from. May - J u l y 1984 and involved an inv 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 three areas of d i f f e r e n t natural shoot density, including a mixed stand of Z^ marina and i t s smaller r e l a t i v e , Z_^  japonica. 7 DESCRIPTION OF STUDY AREA F i e l d studies were c a r r i e d out i n the intercauseway area of southern Roberts Bank i n the Fraser River d e l t a ( F i g . 1) from June 1983 - July 1984. This area, bounded on the north by the Westshore Terminals coalport causeway and on the south by the Tsawwassen f e r r y terminal causeway, i s dominated by a large (= 200 ha) meadow of eelgrass (Zostera  marina). Because t h i s meadow i s one of the most b i o l o g i c a l l y productive habitats i n the estuary and i s important as a nursery area for commercial f i s h species such as P a c i f i c herring (Clupea harengus  p a l l a s i ) and coho (Oncorhyncus kisutch), chinook (CK tshawytscha), and chum (CK keta) salmon (Hoos & Packman, 1979) the eelgrass and i t s associated b i o t a have been the subject of several recent studies (Levings and Coustalin, 1975; Beak Hinton Consultants (BEAK), 1977; Harrison, 1982a,1984). The Fraser River i s the sixth l a r g e s t r i v e r i n North America (van der Leeden, 1975) and deposits 20 m i l l i o n tons of sediment i n the d e l t a each year (BEAK, 1977). The intercauseway area i s located 11 kilometres from the south arm of the r i v e r , through which 85% of i t s annual discharge flows, but i s protected from large seasonal 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 coal port causeway which d e f l e c t s the r i v e r ' s plume (BEAK, 1977; Swinbanks, 1979). As a r e s u l t , a greater abundance and d i v e r s i t y of f i s h and invertebrate fauna occur i n the intercauseway area than are found i n adjacent foreshore areas, such as Sturgeon Banks (Levings and Coustalin, 1975; Gordon and Levings, 1984). The coal port causeway, and more recently a " r i p - r a p " dyke, which was b u i l t part way across the seaward margin of the eelgrass 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; mixed Z. marina and i'iV"' Z. japonica; unvegetated. Four study sites are shown: T = Treatment Plots (Part A: Twelve-Month Study); Mixed stand of Zostera 1 = 2 = and 3 = species, Low density of Z. marina shoots, High density of Z. marina shoots (Part B: Three-Month  Study). 9 10 1982, also block the d i r e c t i o n of ebb t i d e flow and as a r e s u l t the duration of submergence i n the beds has been increased (BEAK, 1977; Harrison, personal communication, 1984). This e f f e c t i s enhanced during the summer when the seasonal peak i n eelgrass density also r e s u l t s i n the impoundment of water and the reduction of ebb t i d e flow. Consequently, even on the lowest tides of the year a shallow layer of water remains over the beds. The climate of the Fraser River d e l t a i s "mixed" maritime, with cool, wet winters and warm, dry summers (Hoos and Packman, 1979). Mean annual r a i n f a l l i s 95 cm with 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 there are usually fewer than f i f t y days of f r o s t per year (BEAK, 1977). In most months surface s a l i n i t i e s i n the intercauseway area range from 21%e -30%», although values as low as 14%« -15%o have been recorded i n August and October (Moody, 1978; Swinbanks, 1979; Bigley, 1981). S i m i l a r l y , t u r b i d i t y may also increase when the r i v e r i s i n freshet (Moody, 1978) and i n c e r t a i n parts of the eelgrass beds, s l i g h t sediment accretion can occur during the summer (Harrison, 1984). The lowest of the two, d a i l y low tid e s occurs from mid-morning to mid-afternoon i n the spring and summer, and near midnight during the f a l l and winter. The mean t i d a l range i s 3.0m (Canadian Hydrographic Service, 1983). Flood currents flow northwest, p a r a l l e l to the S t r a i t of Georgia, while ebb currents flow out of the intercauseway area to the southwest (BEAK, 1977). Swinbanks (1979) described several biosedimentological zones i n the intercauseway area of Roberts Bank inc l u d i n g , from the shoreline seaward, a s a l t marsh, an a l g a l mat zone, an upper sand f l a t zone, and an eelgrass zone. The upper region of the eelgrass zone i s dominated, 11 at least during the spring and summer, by Zostera japonica, which dies back almost completely during the winter, while the lower region i s dominated year-round by the larger, perennial species, Z. marina. Although the d i s t r i b u t i o n of both species i s modified 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 tidepools and channels, Z_^  marina i s usually r e s t r i c t e d to elevations below mean lower low water (MLLW; ±2.0 m) and Z. japonica does not survive above mean higher high water (MHHW; ±2.7 m) (Harrison, 1982a). A single sampling station was established i n a mixed stand of these species i n the summer of 1984. With t h i s exception a l l samples were c o l l e c t e d from the lower eelgrass zone i n areas containing only Zj_ marina. MATERIALS AND METHODS Part A. The Effect of Removal of Zostera marina Shoots on the  D i s t r i b u t i o n and Abundance of Amphipods (hereafter r e f e r r e d to as "The Twelve-Month Study"). In June 1983, Zostera marina shoots were thinned i n eight treatment plots to four 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 the intercauseway eelgrass 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 to these 12 i n i t i a l shoot densities and to the seasonality of macrophyte biomass, at monthly i n t e r v a l s from July 1983 - J u l y 1984. 1. Sampling Design Environmental v a r i a t i o n s such as elevation, t i d a l exposure, sediment texture and organic content, were minimized by l o c a t i n g the experimental plots i n an area of r e l a t i v e l y uniform Z. marina shoot density (19 ± 0.4 shoots • 0.25m"2; x ± S.E.; N=212). The eight, 3x2m plots were positioned i n two rows of four, with the long axis of each row oriented i n a d i r e c t i o n perpendicular to the t i d a l current. The rows were separated by a diagonal distance of 12.5m ( F i g . 2) and'their mean elevation was 1.30m above chart datum (survey data provided by M. Tarbotton, Swan Wooster Engineering Ltd., Vancouver, 1983). Wooden stakes driven into the four corners of each 3x2m plo t were used to permanently demarcate the study s i t e . An inner sampling area of 2xlm was marked i n each pl o t with small i r o n pegs. In order to minimize "edge e f f e c t s " which may have been created by the wooden stakes, the 0.5m perimeter of each pl o t was not sampled. The t o t a l number of above-ground Zostera marina shoots i n each o plot was counted using a hemp g r i d c o n s i s t i n g of 24, 0.25rrr quadrats which fastened over the four corner stakes. Four treatments, including three i n which 25, 50, or 75% of the shoots were removed, and one control, i n which shoot d e n s i t i e s were not changed, were randomly assigned to the plots within each row (see Table 1). Shoots were thinned accordingly, based on the lowest common de 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 the stem at the node located immediately below the f i r s t roots. The cut was made at this, l e v e l i n an attempt to prevent further branching of the plant (Harrison, 13 Figure 2. Plan of treatment p l o t s i n Part A; Twelve-Month Study. Two rows of four 3x2m p l o t s were permanently established at study s i t e "T". Above-ground shoots were thinned to three 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 three p l o t s i n each row. Shoots were not removed from the control p l o t (100%) i n each row. The p l o t s were situated 0.5m apart and the two rows were separated by a diagonal distance of 12.5m. Plant and core samples were c o l l e c t e d from a ce n t r a l 2xlm area i n each p l o t to avoid "edge e f f e c t s " , as shown i n the diagram i n lower r i g h t of f i g u r e . 14 15 T a b l e 1. D e n s i t i e s of Z o s t e r a marina • 0.25 m - 2 (x ± S.E.), b e f o r e and a f t e r t h i n n i n g t r e a t m e n t s . 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 each row i n June, 1983. Shoots were not t h i n n e d i n t h e c o n t r o l (100%) p l o t i n each row. Row P l o t Treatment 1 2 3 4 C o n t r o l 25% 75% 50% 5 6 7 8 C o n t r o l 50% 75% 25% P r e -Treatment Shoot D e n s i t y Mean ± S.E. P o s t -Treatment Shoot D e n s i t y Mean ± S.E. 18.2 ± 1.3 19.9 ± 1.1 16.3 ± 1.4 16.3 ± 1.0 16.1 ± 1.0 4.1 ± 0.1 12.2 ± 0.5 8.2 ± 0.1 17.3 ± 1.0 20.3 ± 1.4 22.6 ± 0.9 20.7 ± 1.5 17.3 ± 1.0 8.7 ± 0.1 13.0 ± 0.5 4.3 ± 0.1 16 personal communication, 1983). Damaged and flowering shoots were removed i n preference to healthy, vegetative shoots. 2. Sampling Procedure Gammaridean amphipods were c o l l e c t e d monthly from four 0.01m2 quadrats i n each p l o t from July 1983 - March 1984, with the exception of February. In A p r i l 1984, sampling i n one row was discontinued when shoot counts revealed that the o r i g i n a l shoot d e n s i t i e s i n a l l p l o t s had recovered and that there were no longer s i g n i f i c a n t differences i n mean shoot density between most plots (Duncan's M u l t i p l e Range Test (DMR), a =0.05). In order to provide a complete record of' the' seasonal patterns of amphipod abundance, c o l l e c t i o n s were continued i n the remaining row through July 1984. Samples were randomly c o l l e c t e d within the c e n t r a l 2xlm area of each pl o t by p o s i t i o n i n g the corners of an aluminum frame g r i d c o n s i s t i n g of 200, 0.01m2 quadrats over the four inner pegs. Replicate samples were c o l l e c t e d using g r i d coordinates based on a random numbers table. A l l sample locations were recorded to prevent the same quadrat from being resampled. The p l o t s i n each row were sampled sequentially to minimize disturbance. In each p l o t , four sediment cores, two Zostera marina shoots, and up to four d r i f t samples consisting of f r e e - f l o a t i n g algae and/or dead eelgrass, were c o l l e c t e d . The number of d r i f t samples varied with the season and depended on the biomass of these plants i n a given month. For example, algae were abundant i n spring and summer and i t was always possible to c o l l e c t four d r i f t samples from each quadrat. However, i n l a t e winter, quadrats frequently contained no d r i f t algae,, and samples could not be c o l l e c t e d . 17 In order to investigate the e f f e c t of eelgrass on the d i s t r i b u t i o n of infaunal amphipods, two of the four quadrats c o l l e c t e d i n each p l o t d i d not contain plants. When a quadrat which had been chosen to include an eelgrass sample d i d not contain a plant, the 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 across the g r i d was sampled instead. The same procedure was followed to avoid plants i n quadrats which had been chosen to contain only d r i f t and sediment. The coring device was a hand-held, empty coffee can, enclosed at one end with several layers of cheesecloth to prevent the escape of motile animals. Each core had a diameter of 10cm and sampled a surface area of 78cm2. Samples were c o l l e c t e d as follows: 1) above-ground eelgrass shoots were cut o f f at sediment l e v e l and placed i n s i d e l a b e l l e d c o l l e c t i n g bags; 2) a l l d r i f t material and plant debris were scraped from the sediment surface; and 3) cores were c o l l e c t e d to a depth of 10cm. In a l l months, sampling was completed within three hours on a single day on the lower of the two d a i l y low t i d e s . Spring and summer samples were c o l l e c t e d near midday, and f a l l and winter samples were c o l l e c t e d near midnight. A single Coleman lantern was used to illum i n a t e the sampling area during the night c o l l e c t i o n s . 3. Physical Factors On each sampling date, a i r and water temperatures were recorded and water samples were c o l l e c t e d for s a l i n i t y determinations. S a l i n i t y was measured i n parts per thousand (%•) using a Yellow Springs Instrument Corp. salinity/conductivity/temperature meter, Model 33. Two r e p l i c a t e core samples, for determination of sediment organic content, were c o l l e c t e d from each experimental p l o t i n October, January, 18 A p r i l , and June, using a Ple x i g l a s corer with an inner diameter of 4.5cm. Each core was separated i n t o two depth f r a c t i o n s (0-5cm; 5-10cm) which were analyzed separately. The samples were d r i e d at 60° C for one week, ground to a uniform consistency with a mortar and pe s t l e and drie d for another 24 hours. Subsamples were d r i e d for two hours at 100° C and then ashed at 550° C for two hours. The organic content was determined from the di f f e r e n c e i n dry weight before and a f t e r i n c i n e r a t i o n . In order to determine i f the size d i s t r i b u t i o n of sediment p a r t i c l e s among plot s i n a given month was the same, three r e p l i c a t e cores were sampled i n four of the experimental plots i n June 1984. Each of the cores was i n d i v i d u a l l y bagged and frozen. The thawed cores were l a t e r d r i e d at 100° C for 24-48 hours to constant weight and ground i n a mortar and pestle. Portions of each, weighing 60-90 g, were sieved on an Endicott's shaker for ten minutes using a series of f i v e sieves with mesh sizes of 0.595, 0.355, 0.180, 0.075, and 0.053 mm. Graphs of the cumulative dry weight percentages retained on each sieve were drawn and estimates of the median p a r t i c l e diameter and the percentage of s i l t -c lay (<0.060 mm), very f i n e sand (0.060-0.125 mm), f i n e sand (0.125-0.250 mm), and coarse sand (0.250-0.500 mm) f r a c t i o n s , were made (Harrison, 1984). 4. Laboratory Procedures A l l samples were returned to the laboratory within f i v e hours of c o l l e c t i o n and stored overnight i n a cold-room at 0° C, with the exception of plant samples c o l l e c t e d i n the summer of 1984, which were 19 frozen. Freezing d i d not appear to damage eit h e r the plants or the animals and both could l a t e r be r e a d i l y i d e n t i f i e d . Within 24 hours a l l plant samples at 0° C were washed with freshwater over a 0.5mm mesh s t a i n l e s s s t e e l sieve. This mesh s i z e was small enough to c o l l e c t a representative sample of small species and juveniles without r e s u l t i n g i n a large increase i n sorting time (Lewis & Stoner, 1980). A l l diatoms, hydroids, and encrusting epifauna were c a r e f u l l y scraped from the Z. marina blades. The material that was retained on the sieve was suspended i n seawater, examined with a d i s s e c t i n g microscope, and sorted. A l l gammarid amphipods were counted, i d e n t i f i e d , and removed for f i x a t i o n i n 5% buffered formalin. They were l a t e r t r a n s f e r r e d to, and stored i n , a sol u t i o n of 70% isopropanol and 5% g l y c e r o l . The number of blades, mean blade length, and length of the longest blade per Zostera marina plant, were recorded. The d r i f t was separated i n t o i t s component species. Each sample was d r i e d at 60° C for 48 hours (to constant weight). The surface area of the eelgrass and a l l d r i f t was estimated by constructing regression l i n e s based on the measured dimensions and dry weights of subsamples of each plant species (Table 2). With the exception of the Zostera marina sheath, a l l portions of these plants were considered to be laminar, that i s , to have only two surfaces a v a i l a b l e for settlement or attachment of animals. The surface area of the eelgrass sheath was determined by considering t h i s portion of the plant as a three-dimensional rectangle and m u l t i p l y i n g the product of the length, width, and height by a factor of two. Core samples were washed i n t o a 0.5mm mesh sieve and a l l residue was preserved i n 7% buffered formalin and Rose Bengal, a v i t a l s t a i n . 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 2) and d r y w e i g h t (g) of macrophyte 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 e q u a t i o n s . R = r e g r e s s i o n c o e f f i c i e n t (p < 0.05). Macrophyte S p e c i e s S u r f a c e a r e a (cm 2) /Dry Weight (g) R Z o s t e r a marina ( l i v e ) 457 0.971 Z o s t e r a marina ( d r i f t ) 537 0.965 Z o s t e r a j a p o n i c a (May & June) 263 0.839 Z o s t e r a j a p o n i c a ( J u l y ) 295 0.907 Enteromorpha sp. 282 0.7 67 L a m i n a r i a s a c c h a r i n a 575 0.994 U l v a sp. 1288 0.953 21 These sieved samples were l a t e r suspended i n fresh water, sorted 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 , and preserved i n a solution of 70% isopropanol and' 5%' g l y c e r o l . In order to assess the e f f e c t of rhizome density on the d i s t r i b u t i o n of amphipods i n the sediment, a l l rhizomes present i n the cores were d r i e d at 60° C for 48 hours and weighed (for 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 species c o l l e c t e d i n each eelgrass, d r i f t , and core sample-was recorded. To allow the comparison of animal d e n s i t i e s between the two types of plant samples, abundances were standardized with reference to surface area. 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 0.01m2 quadrat was a l s o determined by pooling absolute counts from each of the associated plant and core samples. Amphipods were i d e n t i f i e d with reference to the following a u t h o r i t i e s : (Crawford, 1937; Barnard, 1954,1969; M i l l s , 1961; Bousfield, 1973, 1979; Otte, 1975; Smith & Carlton, 1975; Conlan & Bousfield, 1982; Conlan, 1983). Species d i v e r s i t y was calculated using 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 proportion of i n d i v i d u a l s i n the i t h species r e l a t i v e to the t o t a l number of i n d i v i d u a l s i n the sample, and evenness: 22 J' = H' H' max where H' max = In S, and S i s the t o t a l number of species i n the community (Pieiou, 1975). A Model 1, two-way analysis of variance (ANOVA) was used to test the hypothesis that there was no s i g n i f i c a n t difference i n mean amphipod abundance * 0.01m-2 between months and shoot density treatments (Zar, 1974). Model 1, three-way analyses of variance were used to test similar hypotheses r e l a t i n g to amphipod density, d i v e r s i t y , and evenness between a l l months, treatments, and substrate types (eelgrass, d r i f t , and sediment) (Zar, 1974). Homogeneity of variance was tested using the Fmax-Test (Sokal and Rolf, 1973) and i f heteroscedasticity was present, square root or log transformations were performed on the data. An a p o s t e r i o r i Student-Newman-Keuls (SNK) or a Duncan's (DMR) multiple comparison test was used to d i f f e r e n t i a t e means which the ANOVA indicated were s i g n i f i c a n t l y d i f f e r e n t . Other comparisons, for example the r e l a t i o n s h i p between rhizome biomass and the abundance of infaunal amphipods, were made with a Students' t-Test (Zar, 1974). A p r o b a b i l i t y l e v e l of P < 0.05 was used i n a l l s t a t i s t i c a l t e s t s . Amphipod density was also examined as a function of macrophyte biomass and surface area using c o r r e l a t i o n a n a l ysis. The 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 eelgrass, d r i f t , and core sample i n the two control p l o t s i n each month were compared with t o t a l eelgrass and with t o t a l d r i f t , biomass, and surface area. A l l data were transformed using square roots. This transformation i s recommended for use i n regression analysis i n situations i n which the data are i n the form of counts, p a r t i c u l a r l y small counts (Zar, 1984). 23 P r i n c i p a l components analysis (PCA) was also used to look for changes i n community structure, over time and i n the three d i f f e r r e n t substrate types. This type of analysis summarizes multidimensional data by projecting i t i n t o a low-dimensional ordination space i n which similar species are close together and d i s s i m i l a r species are far apart (Gauch, 1982; Pielou, 1984). The variables of the c o r r e l a t i o n matrix used i n t h i s analysis, representing numbers of i n d i v i d u a l s * s p e c i e s - 1 i n each sample, were standardized to permit the comparison of data c o l l e c t e d i n both the plant and sediment substrates (Pielou, 1984). Standardization also reduces the heterogeneity of v a r i a n c e s among species (Pielou, 1984). The v a r i a b i l i t y of the data was reduced further by including only the most frequently c o l l e c t e d species i n the a n a l y s i s . In order to determine the temporal patterns of species-associations, component scores of the p r i n c i p a l axes, i d e n t i f i a b l e on the basis of substrate type, were pl o t t e d through time (Pimental, 1979). 6. Cor r e l a t i o n Matrices Correlation matrices, based on the abundance of i n d i v i d u a l s i n each species per sample, were constructed to investigate the- p o s s i b i l i t y of h i e r a r c h i c a l competitive interactions (Nelson, 1979a). In t h i s type of analysis, s i g n i f i c a n t negative correlations between species may ind i c a t e that one species i s d i s p l a c i n g another. The f i r s t matrix included the mean t o t a l number of amphipods c o l l e c t e d i n each species, pooled from a l l c o l l e c t i o n s while the second and t h i r d matrices included only the mean abundances of the f i v e most frequently c o l l e c t e d species on each c o l l e c t i o n date, and substrate type, r e s p e c t i v e l y . 24 Part B. The Relationship Between Natural Shoot Densities and the  D i s t r i b u t i o n and Abundance of Amphipods (hereafter r e f e r r e d to as "The Three-Month Study"). Samples were c o l l e c t e d i n three areas of d i f f e r e n t natural d e n s i t i e s of Zostera marina shoots at monthly i n t e r v a l s from May - July 1984 i n order to investigate further 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 and abundance of amphipods and the heterogeneity of the seagrass habitat. 1. Design of Study Sites Three, 10x5m sampling areas were chosen i n A p r i l 1984 on the basis of obvious differences i n the degree of Zostera marina cover, and were • permanently marked with wooden stakes. The f i r s t s t a t i o n was located i n the upper zone of eelgrass described by Swinbanks (1979) i n a region of sparse Z. marina cover, (approximately three shoots * 0.25m ) (F i g . 1). The annual species, Z. japonica, was also present i n t h i s area. The second area was located i n a region of moderate Z. marina cover (9.7 ± 3.8 shoots ' 0.25m~2 (x ± S.E.)), approximately 200m seaward of the f i r s t area ( F i g . 1). Although the eelgrass was d i s t r i b u t e d r e l a t i v e l y evenly through t h i s area there were small unvegetated patches which may have been caused by topographical i r r e g u l a r i t i e s or l o c a l i z e d patterns of water movement. The t h i r d s t a t i o n , with the highest shoot density (28.4 ± 15.2 shoots • 0.25m-2) was located within 10m of the pl o t s described i n Part A (Fi'g.l). 25 2. Sampling Procedure In May, June, and July 1984, amphipods were c o l l e c t e d i n three randomly located 0.25m2 quadrats i n each of the three areas. Each quadrat sampling consisted of two sediment cores, two associated Zostera marina above-ground shoots, and two d r i f t samples, randomly located •p within the quadrat using a g r i d of 24, 0.01m quadrats. With a few exceptions, the sampling procedure was the same as that described i n Part A: The Twelve-Month Study. Prior to the removal of plant and core samples, a l l Zostera marina and Z. japonica shoots within the quadrat were counted. Twenty plants of each species i n Area 1, and of Z. marina i n Areas 2 and 3, were c o l l e c t e d each month on the perimeter of each area for the c a l c u l a t i o n of mean surface area and mean dry weight per plant. A l l Z. japonica 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 with the d r i f t component of the sample. In order to ensure that a l l amphipod species present were c o l l e c t e d i n the quadrat samples, two " n a t u r a l i s t sled" ( F i g . 3) samples were c o l l e c t e d just outside each area i n each month, by towing the sled, which captured benthic as well as demersal fauna, i n t o the current for a distance of 2m. These tows were made at low t i d e at the same time as the quadrat samples were being c o l l e c t e d . A f t e r each tow the contents of the net were washed in t o a 0.5mm mesh sieve with f i l t e r e d seawater and the material retained on the sieve was placed i n a c o l l e c t i n g j a r . 9 Shoot counts were made i n at le a s t three 0.25m quadrats i n the path of the tow and these counts were pooled with those made within the sampling areas for the determination of mean monthly shoot density. Because Zostera japonica shoots were so sparsely d i s t r i b u t e d i n May i n Area 1, Figure 3. Naturalist Sled. 2 7 28 they did not occur i n sample quadrats and consequently were counted i n June and July only. 3. Physical Factors Ai r and water temperatures were recorded, and water samples were co l l e c t e d for s a l i n i t y measurements, on each sampling date. In order to determine whether or not there was any v a r i a t i o n i n the depth of submergence between areas during low t i d e , water depth was measured synchronously i n each of the three areas on July 27 during slack water on one of the lowest tides of the year. A sediment core for the determination of percentage organics was c o l l e c t e d i n each 0.25m2 quadrat, as described i n Part A: The Twelve- Month Study. Three r e p l i c a t e cores were sampled i n each of the study areas i n June 1984 for the determination of the size d i s t r i b u t i o n of sediment p a r t i c l e s . These cores were analyzed as described i n Part A. 4. Laboratory Procedures A l l core and plant samples were processed and analyzed as described i n Part A. Within f i v e hours of c o l l e c t i o n , the " n a t u r a l i s t sled" samples were washed into a 0.5mm sieve and sieve contents were preserved i n 7% buffered formalin 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, and with the exception of the " n a t u r a l i s t sled" samples, head length and l i f e -stage data were recorded. The surface areas of l i v i n g and dead Zostera marina, and l i v e Z. japonica, Ulva sp., and Enteromorpha sp. were also c a l c u l a t e d i n the same manner as described i n Part A and are shown i n Table 2. To ensure that the r e l a t i o n s h i p between macrophyte biomass and surface area was consistent within a species between months, the slopes of the regression 29 l i n e s were compared (Zar, 1974). If s i g n i f i c a n t d ifferences i n the slopes occurred, as they d i d for Z. japonica i n June and July, i n d i v i d u a l monthly surface area values were used i n these determinations. 5. S t a t i s t i c a l Analysis As i n Part A: The Twelve-Month Study, the t o t a l number of individuals and species of amphipods i n each sample were recorded and amphipod d e n s i t i e s were determined r e l a t i v e to macrophyte or sediment surface area. A Model 1, two-way a n a l y s i s of variance was used to test the hypothesis that there was no s i g n i f i c a n t d i f f e r e n c e i n mean amphipod abundance • 0.01m-2 between months and areas of d i f f e r e n t Zostera marina shoot density. Three-way analyses of variance were used to test for differences among means of amphipod density, d i v e r s i t y (H'), and evenness (J') between months, shoot d e n s i t i e s , and substrate types. Square root and log transformations were used to correct heteroscedastic variances. M u l t i p l e comparison t e s t s (Student-Newman-Keul's or Duncan's) were used to d i f f e r e n t i a t e means which the ANOVA indicated were s i g n i f i c a n t l y d i f f e r e n t . A l l s i g n i f i c a n c e was determined at the P < 0.05 p r o b a b i l i t y l e v e l . C o r r e l a t i o n analysis was used to examine amphipod density as a function of macrophyte biomass and surface area, as described i n Part A. 6. Correlation Matrices As described i n Part A: The Twelve-Month Study, c o r r e l a t i o n matrices, based on the abundance of i n d i v i d u a l s i n each amphipod species per sample, were constructed to i n v e s t i g a t e i n t e r s p e c i f i c i n t e r a c t i o n s . The f i r s t matrix included the mean t o t a l number of amphipods c o l l e c t e d i n each species, per sample, pooled from a l l c o l l e c t i o n s . The second 30 and t h i r d matrices included the monthly mean abundances of the four dominant species per sample, c o l l e c t e d i n each of the three study areas (shoot de n s i t i e s ) and substrate' types, r e s p e c t i v e l y . Part C. L i f e Cycles and Size-Frequency D i s t r i b u t i o n s In order to obtain information on the si z e frequency d i s t r i b u t i o n and seasonal reproductive patterns, a l l amphipods c o l l e c t e d i n two treatment p l o t s per month (Part A) were examined. Where sample numbers were small (less than 50 animals), a l l i n d i v i d u a l s were examined and described. In larger samples at least two subsamples, balanced with respect to l e f t - r i g h t s p l i t s , were removed using a Folsom Plankton s p l i t t e r and counted. A X 2 test f o r homogeneity (a =0.05)(Zar,1974) was used to compare preliminary samples and subsamples to ensure that the s p l i t t e r was unbiased and balanced. The counts were found to be homogeneous and the number of animals i n each sample was estimated using the equation: N- =• 2 T Z X i/n = 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 et a l . , 1984). A l l sizes r e f e r r e d to i n the study represent head length which was measured from the anterior t i p of the rostrum to the junction of the f i r s t thoracic segment. Head length i s a good index of t o t a l body length because i t i s easy to measure and i s independent of body f l e x i o n (Nelson, 1980c). The r e l a t i o n s h i p between head length and body length, measured from the t i p of the rostrum to the d i s t a l end of the telson, 31 was examined for a subsample of individuals for each species using linear regression analysis. These data are presented in Appendix 1. For each species, five l i f e stages were identified: 1) juveniles or immatures: individuals in 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: identified by their differential gnathopod or antennal morphology. Length frequency - l i f e stage histograms were constructed for the four most abundant amphipod species by pooling the total individuals in a l l samples in a given month. 32 RESULTS Part A. The Twelve-Month Study 1. Physical Factors A i r temperatures recorded at low t i d e ranged from a low of -2° C i n l a t e December to a high of 24° C i n July 1984. Temperatures remained below 10° C from November - March and above 13° C from A p r i l - October (Fig.4). Water temperatures were usually s l i g h t l y higher than a i r temperatures, p a r t i c u l a r l y during the summer months when low t i d e coincided with the hottest part of the day and the shallow layer of water that remained on the beds was warmed for several hours by the sun. Water temperatures ranged from <1°C i n December to 26°C i n July (Fig.4). During the t h i r d week of December 1983, temperatures f e l l to below freezing for several consecutive days and on the evening of December 21 the water i n the intercauseway area was covered by a th i n sheet of i c e , which became dislodged as the t i d e turned and f l o a t e d out of the bay on the ebb t i d e i n large floes.. The sediment was frozen to a, depth of 2 cm i n the eelgrass beds. Lowest monthly surface s a l i n i t i e s were recorded i n August 1983 (16% ) and i n June (20% ) during the time of the Fraser River freshet (Fig.5). The highest s a l i n i t y was recorded i n December (34% ). With these exceptions, surface s a l i n i t i e s remained between 23 - 27% , similar to the values found by Swinbanks (1979) over most of the intercauseway t i d a l f l a t . The sediments of the study area were c l a s s i f i e d as f i n e , to very fi n e , sands with a mean grain diameter ranging from 0.12-0.15 mm (Table 33 Figure 4. Monthly a i r and water temperatures (° C) at study s i t e "T". Dates on the abscissa i n the Figure (and,in a l l Figures i n Part A: Twelve-Month Study) correspond to July 11, August 6, September 4, October 25, November 23, December 21,1983 and January 20, March 1, A p r i l 17, May 15, June 27, and July 27, 1984. Figure 5. Monthly s a l i n i t y (% ) at s i t e "T". Water samples were co l l e c t e d at low t i d e . 34 35 Table 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 of sediments c o l l e c t e d from treatment p l o t s (June 27, 1984). Values are mean percentage dry weight i n each s i z e c l a s s (n=3). VFS = very f i n e sand FS = f i n e sand. S i z e C l a s s C o n t r o l 25% 75% 50% (mm) P l o t 1 P l o t 2 P l o t 3 P l o t 4 (VFS) (FS) (FS) (FS) > 0.595 < 1 0.355-0.595 1 0.180-0.355 85 0.075-0.180 8 0.053-0.075 1 < 0.053 4 < 1 < 1 < 1 1 1 1 79 82 86 17 12 8 < 1 < 1 < 1 2 4 4 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 percent of the sample dry weight. The mean organic content of the sediments i n the treatment pl o t s ranged from 2.1-1.4 % dry weight i n the 0-5 cm core f r a c t i o n and from 1.4-1.1 % i n the 5-10 cm core f r a c t i o n , decreasing s i g n i f i c a n t l y from October to A p r i l (ANOVA, p<0.05; SNK, a=0.05) (Table 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 organic content of the 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 the eight p l o t s (ANOVA, p>0.05). The percentage of organic material i n the 0-5 cm f r a c t i o n was greater 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 of Zostera marina The annual growth cycle of Zostera marina was r e f l e c t e d i n the seasonal trend i n mean dry weight c o l l e c t e d per O.Olirr quadrat i n the ' treatment pl o t s from J u l y 1983 - Ju l y 1984 (Fig.6). Plant biomass peaked from July - October and declined i n November, i n d i c a t i n g that the shoots had begun t h e i r annual die-back (Fig.6). The growth of new shoots, which o r i g i n a t e s from the rhizome, began i n l a t e January and was r e f l e c t e d i n the increase i n the number of blades per plant from January - March, at a time when shoot dry weight and the mean length of the longest blade per shoot were at t h e i r minimum values (Figs. 6, 7 and 8). As the young shoots continued to. grow through the spring, the dry weight, blade number and mean blade length also increased. Mean dry weight per Z. marina shoot i n July 1983 was not s i g n i f i c a n t l y d i f f e r e n t than i t was i n Ju l y 1984 (ANOVA, p^0.05). The mean dry weight of Zostera marina rhizomes per core was s i g n i f i c a n t l y greater i n June than i t was i n January ( F i g . 9). 37 Table 4. Percentage o r g a n i c content (x ± S.E.) i n . t h e sediment core f r a c t i o n s c o l l e c t e d i n treatment p l o t s from October, 1983 - June, 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 except A p r i l and June f o r the 0-5 cm f r a c t i o n . January 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 from June i n the 5-10 cm f r a c t i o n . (ANOVA, p < 0.05; SNK, a = 0.05) . Core F r a c t i o n October n=64 January n=48 A p r i l n=24 June n=24 0-5 cm 2.1 ± 0. 1 1.5 ±< 0. 1 1 .4 ±< 0. 1 1. 4 ±< 0 .1 5-10 cm 1.4 +< 0. 1 1.2 ±< 0. 1 1 .1 +< 0. 1 1. 1 ±< 0 .1 38 Figure 6. Dry weight of above-ground Zostera marina shoots (x ± S.E.) i n monthly samples at study s i t e "T". (n = 16, July 1983 - March; 8, A p r i l - July 1984). Figure 7. Number of blades per Zostera marina shoot (x ±= S.E.) i n monthly samples at study s i t e "T". (n = 16, July 1983 -March; 8, A p r i l - July 1984). 40 Figure 8. Length of longest blade per Zostera marina shoot (x ± S.E.) i n monthly samples at study s i t e "T". (n = 16, July 1983 - March; 8, A p r i l - July 1984). Figure 9. Dry weight of Zostera marina rhizomes * 0.01m-z (x ± S.E.) at study s i t e "T". (n = 26, January; 24, March; 12, A p r i l ; 13, May; 14, June and J u l y ) . 4 1 42 However, since the March samples contained the lowest rhizome dry-weights and the average dry weight per core was. s i g n i f i c a n t l y greater i n A p r i l than i t was i n May, no d e f i n i t e seasonal trend i n below-ground growth could be i d e n t i f i e d (ANOVA, p>0.05;SNK, a=0.05) ( F i g . 9). 3. Composition and Seasonality of D r i f t The most abundant components of the d r i f t were f r e e - f l o a t i n g Ulva sp. and fragments of Zostera marina blades (Fig.lOA & B). A small amount of decomposing eelgrass was present at the sediment surface throughout the year.. However., peak, accumulations of this, debris occurred i n May, June, and July when the turnover rates for l i v e shoots and leaves were the highest (Harrison, 1984) (Fig.lOA). The blade fragments were generally encrusted with filamentous and s i n g l e pennate diatoms, and meiofauna such as nematodes were common. Occasionally, hydroids and epiphytic algae such as Smithora naiadum were a l s o attached to the blade surfaces. D r i f t Ulva sp. f i r s t appeared i n the intercauseway area and the study s i t e i n l a t e March and i t s biomass peaked i n May ( F i g . 10B). During t h i s time the large bright green fronds were so densely d i s t r i b u t e d throughout the study area that they provided a continuous canopy over the sediment, among the eelgrass plants. The algae began to deteriorate i n l a t e June. By July the c o l l e c t i o n of d r i f t algae consisted of many discoloured, decomposing blade fragments. No Ulva sp. was c o l l e c t e d from November to A p r i l ( F i g . 10B). Other a l g a l species occurring i n the low i n t e r t i d a l area and d r i f t samples included Enteromorpha sp. and Laminaria saccharina. The f l o a t i n g mats of Enteromorpha sp. were not observed as frequently as was 43 Figure 10A. Dry weight of dead Zostera marina* 0.01m2 (x ± S.E.) i n d r i f t samples at, study s i t e "T". (N = 16). 10B. Dry weight of Ulva sp. ' O.Olnf^ (x + S.E.) i n d r i f t samples at study s i t e "T". (N = 16). 44 45 Ulva sp. i n the study area, and only small amounts were c o l l e c t e d i n the A p r i l , June, and July d r i f t samples. In A p r i l and May, young L.  saccharina, often attached by t h e i r holdfasts to the valves of empty cockle (Clinocardium 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 d r i f t samples. 4. E f f e c t of Shoot Removal on the Growth of Zostera marina Because I assumed that thinning the Zostera marina by c u t t i n g through the stem at sediment l e v e l would slow shoot production and that, i n any case, few new shoots would be generated through the autumn and winter, I did not re-count the shoots i n the eight treatment p l o t s u n t i l A p r i l 1984. Figures 11 and 12 show the increase i n shoot density which occurred between June 1983 and June 1984 i n a l l p l o t s . It therefore appears that thinning the eelgrass i n t h i s manner may have a c t u a l l y stimulated, rather than retarded, the production of new shoots. By June 1984, only Plot 2 (25% treatment) and Plot 7 (75% treatment) had not regained t h e i r o r i g i n a l shoot de n s i t i e s (Figs. 11 and 12). Overall shoot density i n the study area was not 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 1984 than i t had been before the shoots were thinned i n June 1983 (ANOVA, p>0.05). 5. E f f e c t of Zostera 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 shoot density i t was not possible to assess 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 and eelgrass shoot density i n t h i s part of the study. However, since the shoots were removed and t h i s constituted a form of "disturbance", the impact on the associated biota could not be ignored. Consequently, 46 Figure 11. Change i n density of Zostera marina shoots ' 0.25m"2 (x ±. S.E), June 1983 - July 1984 i n row 1 at study s i t e "T". (N = 24). The study s i t e was demarcated and shoots were counted on June 8, 1983. Shoots were thinned on June 13 to 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. Figure 12. Change i n density of Zostera marina shoots • 0.25m-2 (x ± S.E), June 1983 - July 1984 i n row 2 at study s i t e "T". (N = 24). The study s i t e was demarcated and shoots were counted on June 8, 1983. Shoots were thinned on June 13 to 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 eight p l o t s , based on the percentage of shoots that were removed i n each treatment. For example, the 25% treatment p l o t s , i n which three-quarters of the shoots were removed, were considered to have undergone a greater disturbance than were the 75% treatment p l o t s , i n which only one-quarter of the shoots were removed. A two-way analysis of variance was used to compare mean amphipod density * 0.01m-2 between months and treatments. No s i g n i f i c a n t differences i n mean amphipod abundance were detected among the four types of treatment plots when data from a l l twelve months were pooled (ANOVA, a=0.14). 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 amphipod numbers * 0.01m between treatment p l o t s i n c e r t a i n months (ANOVA, p<0.01; DMR, *=0.05), t h i s v a r i a t i o n was rel a t e d more to fluctuations i n abundance over time than i t was to treatment-effects. In fa c t , no consistent pattern r e l a t i n g these f l u c t u a t i o n s to treatment e f f e c t s was apparent (Table 5). S i g n i f i c a n t differences i n mean amphipod abundance per unit plant and sediment surface area were found using a three-way analysis of variance (months x treatments x substrate type) (Table 5). In the core samples, more animals were c o l l e c t e d • m A of sediment surface i n the 25% plots than i n the other treatment p l o t s , and the lowest mean abundances were recorded i n the control p l o t s . In comparison, i n the d r i f t substrate, although 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 occurred between treatments (ANOVA, p<0.0008), there was no apparent r e l a t i o n s h i p between the density of amphipods and the extent of shoot removal, as there was i n the sediment (Table 5). The highest numbers per unit surface area i n the d r i f t samples were recorded i n the 50% 49 Table 5. T o t a l numbers of amphipods • m - 2 of s u b s t r a t e s u r f a c e (x ± 95% C.L.) i n treatment p l o t s , 1983-84. * value s i n d i c a t e means are s i g n i f i c a n t l y d i f f e r e n t among treatments w i t h i n a s u b s t r a t e (ANOVA, p < 0.05; DMR, a = 0.05) . Substrate 25% 50% 75% Control Zostera x marina + C.L. 241 (n=36) 115-412 127 (n=36) 71-198 74 (n=36) 39-119 171 (n=40) 105-254 X D r i f t ±C.L. 1196 (n=31) 707-1899 2228 (n=36) 1262-3468 793* (n=44) 488-1173 1441 (n=35) 597-2650 X Sediment +C.L. 7213*(n=69) 4826-10078 5396 (n=67) 3763-7322 5075 (n=66) 3523-6908 3841*(n=73) 2498-5472 50 p l o t s (DMR, a=0.05), but 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 abundance between the other treatments. F i n a l l y , there was no s i g n i f i c a n t d i f f e r e n c e i n the density of amphipods on the marina substrate i n any of the pl o t s (DMR,a=0.05) (Table 5). In summary, the disturbance caused by the removal of the above-ground Zostera marina shoots had no apparent e f f e c t on eit h e r the o v e r a l l amphipod abundance • 0.01m"2 or the de n s i t i e s of animals c o l l e c t e d on the two macrophyte substrates. However, i n the 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 the treatment p l o t s which experienced a greater r e l a t i v e disturbance than were c o l l e c t e d i n the controls. 6. Seasonal D i s t r i b u t i o n of Amphipods As previously mentioned, s i g n i f i c a n t differences i n the mean monthly abundance of amphipods * 0.01m"2 occurred over the twelve-month study period (ANOVA, p<0.0001; DMR, a=0.05)(Fig. 13). The t o t a l number of animals per quadrat doubled from September - October when the peak i n amphipod abundance was recorded. From October - November numbers declined f o u r - f o l d and continued to f a l l through the winter and spring, reaching minimum le v e l s i n March and A p r i l ( F i g . 13). In May the abundance of amphipods began to increase and by July mean d e n s i t i e s of 174 i n d i v i d u a l s • 0.01m"2 were recorded. Figures 14, 15, and 16 i l l u s t r a t e the o v e r a l l monthly mean abundance of amphipods * m~2 c o l l e c t e d i n the eelgrass and d r i f t samples, and i n the sediment cores, r e s p e c t i v e l y . Since the majority of the animals were c o l l e c t e d i n the sediment and d r i f t , abundances i n these two substrates were most r e f l e c t i v e of o v e r a l l d e n s i t i e s . 51 Figure 13. Monthly abundance of amphipods (x 100) * m"2 (x ± S.E.) at study s i t e "T". Absolute abundances of amphipods were pooled from a l l substrate samples within 0.01m-2 quadrats. (N = 75, August; 71, September, 68, October; 57, November; 51, December; 54, January; 40, March; 37, A p r i l and May; 40, June; and 39, July 1984). 53 Figure 14. Mean number of amphipods * m~2 of surface of Zostera marina (x ± S.E.) at study s i t e "T". (N = 16, August - January; 20, March; and 8, A p r i l - Ju l y 1984). 55 Figure 15. Mean number of amphipods • m - 2 of surface of d r i f t (x ± S.E.) at 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 May; 16, June; and 15, July 1984). 57 Figure 16. Mean number of amphipods * m - 2 of sediment surface (x. ± S.E.), at 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 and early winter, p a r t i c u l a r l y i n October and November ( F i g . 16). They declined i n abundance over the winter and reached minimum de n s i t i e s i n A p r i l and May. From June - July t h e i r numbers increased to a mean equivalent to 1790 i n d i v i d u a l s per square meter of sediment surface. There was no s i g n i f i c a n t d i f f e r e n c e between the number of amphipods present i n the sediment i n e a r l y August 1983 and i n l a t e July 1984, suggesting that a complete annual cycle had been observed (DMR,a=0.05)(Fig. 16). The seasonal pattern of amphipod abundance i n the d r i f t was s i m i l a r to that observed i n the sediment. However, i n the d r i f t , abundances per unit surface area peaked i n November rather than i n October, and declined dramatically as the d r i f t disappeared, u n t i l March, when no amphipods were c o l l e c t e d on t h i s substrate ( F i g . 15).' Abundances remained at low l e v e l s throughout the spring and summer months, despite the peak i n d r i f t biomass that occurred i n May. The density of amphipods i n the d r i f t i n August 1983 and July 1984 was not s i g n i f i c a n t l y d i f f e r e n t (DMR, a=0.05). Amphipods were more abundant per unit surface area i n the d r i f t , than on the Zostera marina, i n a l l months except August, A p r i l , May, and June (Figs. 14 and 15)(DMR, a=0.05). Mean amphipod d e n s i t i e s on the eelgrass ranged from a low of 18 i n d i v i d u a l s * m~2 surface of plant i n July 1984 to 458 • n f 2 surface of plant i n A p r i l ( F i g . l | ) . 60 7. Macrophyte Biomass and t h e D i s t r i b u t i o n o f Amphipods A l t h o u g h the amphipods were not c o l l e c t e d on t h e Z o s t e r a mar i na i n numbers as h i g h as t h e y were on t h e d r i f t and i n t h e sed iment , t h e i r o v e r a l l abundance 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 mean month ly e e l g r a s s b iomass per sample q u a d r a t ( r=0 .771; p<0 .002 ) . Maximum v a l u e s f o r b o t h mean amphipod abundance and mean a b o v e - g r o u n d Z_^  ma r i na b iomass • 0 . 0 1 m - 2 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 f o r b o t h were r e c o r d e d i n March ( F i g . 13 and 6 ) . S i m i l a r l y , t h e r e was a s i g n i f i c a n t c o r r e l a t i o n between t he mean abundance o f amphipods " 0.01m and the mean o r g a n i c c o n t e n t o f t h e sediment on t h e same d a t e ( T a b l e 4) ( r=0 .998; p<0.001) . D e n s i t i e s o f amphipods c o l l e c t e d i n t h e c o r e s were no t i n f l u e n c e d by t h e p r e s e n c e o r absence o f an e e l g r a s s p l a n t w i t h i n t h e sample quadra t i n any month ( S t u d e n t s ' t - T e s t , p>0.05) , o r by t h e b iomass o f t he e e l g r a s s rh izomes c o l l e c t e d i n t h e c o r e s f rom J a n u a r y - J u l y 1984 ( T a b l e 6 ) . However, i n November 1983 and i n June 1984 s i g n i f i c a n t c o r r e l a t i o n s d i d o c c u r between t h e abundance o f amphipods i n t h e sed iment and Z_^  mar ina above - g round b iomass ( T a b l e 7 ) . In November t h i s c o r r e l a t i o n was n e g a t i v e , i n d i c a t i n g t h a t abundances were lower i n q u a d r a t s w i t h h i g h e e l g r a s s b iomass than t h e y were i n q u a d r a t s w i t h low b iomass ( T a b l e 7 ) ; i n J u n e , 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 were f o u n d , i n d i c a t i n g an o p p o s i t e t r e n d ( T a b l e 7 ) . When abundance d a t a were p o o l e d o v e r the one y e a r s t udy p e r i o d , 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 were o b s e r v e d between t h e mean number o f amphipods c o l l e c t e d i n t he c o r e s and t h e mean b iomass o f d r i f t • 0 . 0 1 m - 2 ( T a b l e 8). The o v e r a l l abundance o f amphipods c o l l e c t e d i n each Z o s t e r a  mar ina sample was s i g n i f i c a n t l y c o r r e l a t e d w i t h t h e d r y we ight o f t h e 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 comparison of numbers of amphipods • c o r e " ! w i t h d r y we i g h t (g) of Z o s t e r a marina rhizomes • core --'-, 1984. R was not 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 > 0.05). J a n u a r y March A p r i l May June J u l y n=16 n=16 n=8 n=8 n=8 n=8 0.17 0.22 -0.49 0.43 0.30 -0.24 6 2 Table 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 comparisons of monthly mean number of amphipods i n Zos t e r a marina, d r i f t , sediment, and t o t a l quadrat ToToTm 7 " ) samples with s u r f a c e area (cm 2) and dry weight (g) of _Z. marina and d r i f t • 0.01 m"2. * valu 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). 1 = _Z. marina s u r f a c e area (cm 2) 2 = Z_. marina dry weight (g) 3 = D r i f t s u r f a c e area (cm 2) 4 = D r i f t dry weight (g) = I n s u f f i c i e n t number of r e p l i c a t e samples DATE Z. marina D r i f t Sediment Total Aug./8 3 1 0.905* -0.499 -0.018 0.030 2 0.919* -0.519 0. 010 0.038 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.] T a b l e 7. [ c o n t . ] 63 DATE Z. marina D r i f t Sediment T o t a l Oct. 1 0.815* 0.004 -0.263 -0.041 2 0.810* 0.002 -0.283 -0.058 3 -0.123 0.840* -0.214 0.609 4 0.159 0.714* 0.046 0.695 Nov. 1 0.796* — -0.770* -0.552 2 3 4 0.801* -0.748* -0.535 Dec. 1 0.465 _ -0.327 -0.296 2 3 4 0.471 -0.339 -0.309 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 3 4 0.567 - -0.653 -0.499 - - - -[ c o n t . ] T a b l e 7. [ c o n t . ] 64 DATE Z. m a r i n a D r i f t Sediment T o t a l A p r i l 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 June 1 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 J u l y 1 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 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 comparisons of mean number of amphipods 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 2) samples w i t h s u r f a c e a r e a (cm 2) and d r y we i g h t (g) of Z. m a r i n a , d r i f t , and t o t a l macrophytes • 0.01m~ i. * 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 r i a b l e Z. m a r i n a D r i f t Sediment T o t a l Z. m a r i n a s u r f a c e a r e a 0.532* 0.230 0.138 0.310* d r y w e i g h t 0.542* 0.210 0.129 0.289* D r i f t s u r f a c e a r e a -0.116 0.220 -0.353* -0.089 d r y w e i ght -0.086 0.163 0.315* -0.101 T o t a l Macrophytes s u r f a c e a r e a -0.058 0.247* -0.340* -0.056 d r y w e i g h t 0.031 0.208 -0.287* -0.038 66 eelgrass i n that sample (Table 8). When the data were examined between months th i s r e l a t i o n s h i p was found to be s i g n i f i c a n t i n August, October, and November (Table 7). Since previous studies have shown that the d i s t r i b u t i o n of eelgrass-associated amphipods i s r e l a t e d to plant surface area (Stoner, 1983), and that these animals can select plant substrates on the b a s i s of the surface area of the blades (Stoner, 1980c), the r e l a t i o n s h i p between abundance and plant surface area was also 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 (Table 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 was correlated s i g n i f i c a n t l y with both d r i f t dry weight and surface area i n August, September, October, and A p r i l (Table 7). 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 and d r i f t ) surface area within a sample quadrat (Table 8). Data which describe the r e l a t i o n s h i p between animal abundance and d r i f t biomass and surface area from November - March are not included i n the Table because the low number of samples c o l l e c t e d i n those months made the analysis u n r e l i a b l e . In summary, there was 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 macrophyte biomass and surface area, and the density of amphipods present on both the Zostera marina and the d r i f t , p a r t i c u l a r l y during the autumn peak i n animal abundance (Tables 7 and 8) (Figs. 14 and 15). Abundances i n the sediment, i n contrast, were negatively r e l a t e d to eelgrass biomass and surface area during t h i s time (Table 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 occurred only i n June, when populations of these animals were c l o s e to t h e i r annual minima (Table 7 ) ( F i g . 16). 67 8. D i s t r i b u t i o n of Species of Amphipods Nineteen species of amphipods were c o l l e c t e d over the one-year period; however, the majority of these were rare (Table 9). In most months, four species, Corophium acherusicum Costa, Corophium insidiosum Crawford, Ampithoe v a l i d a Smith, and Anisogammarus pugettensis Dana accounted for over 90% of the t o t a l numbers. Consequently, species d i v e r s i t y (H') was low i n a l l months, ranging from 0.57 b i t s i n d i v i d u a l i n October to 1.49 i n May and 1.22 i n June. The l a t t e r two months represent times i n which the populations of the dominant species were close to t h e i r annual minima (Table 9) and when two other gammarid species were r e l a t i v e l y abundant. In A p r i l Ischyrocerus sp. dominated the plant samples and i n May Pontogeneia r o s t r a t a Gurjanova was common i n the d r i f t (Table 9). Four of these six species, C. acherusicum, C. insidiosum, A. v a l i d a , and Ischyrocerus sp. are epifaunal tube-builders (Bousfield, 1973). Anisogammarus pugettensis and P. r o s t r a t a swim f r e e l y among plants and organic debris near the sediment surface (Chang, 1975; Stoner, 1980b). None of the numerically dominant species i s infaunal. Fewer than 50 i n d i v i d u a l s of the remaining 13 species, were c o l l e c t e d and of these, only Synchelidium shoemakeri M i l l s and Orchomene sp. burrow (Smith & Carlton, 1975). During 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 species occurrence, was lower than i t was i n a l l other months ( s i g n i f i c a n t for a l l months except November and December, ANOVA, p<0.05; DMR, a=0.05). In these months, C. acherusicum accounted for more than 88% of the t o t a l number of amphipods (Table 9). Table 9. R e l a t i v e abundance, d i v e r s i t y , and evenness of spe c i e s of amphipods • 0.01 m - 2 i n treatment p l o t s , 1983-84. * values i n d i c a t e months i n which 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). n = T o t a l number counted H' = Shannon-Wiener D i v e r s i t y J ' = Shannon-Wiener Evenness Species Aug. Sept. Oct. Nov. Dec. J an . March A p r i l May June J u l y n= 191 1161 1946 616 383 205 62 73 181 372 1317 co 10 Ampithoe l a c e r t o s a - 0.4 A. v a l i d a 19.4 8.2 Anisogammarus pu q e t t e n s i s 12.0 2.1 A t y l u s  c o l l i n g i i C a l l i o p i u s l a e v i u s c u l u s 0.5 Corophium acherusicum 60.8 78.9 C. i n s i d i o s u m 6.3 10.4 Ischyrocerus sp. 0.5 <0.1 7.7 0.8 2..4 1.6 0.6 0.3 0.5 2.0 0.6 0.6 1.9 0.8 0.9 1.6 1.4 43.0 5.4 2.3 2.7 - 0.3 - - - - - - -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 [cont. ] Tab le 9. [cont . ] Species Aug. Sept. Oct . Nov. Dec. J a n . March A p r i l May June J u l y n= 191 1161 1946 616 383 205 62 73 181 372 1317 Orchomene sp. — - - - 0.3 - - - - - -Paraphoxus spinosus - - <0.1 - - - - - 0.6 0.3 -Parapleustes pugettensis 0.5 - - - - - - - - - -Photis brevipes - - - - - - - - - 1.9 -P. oligochaeta - - - - - 0.5 3.2 - 0.6 4.8 -Pontogeneia intermedia - - - - - - - - 3.3 - -P. r o s t r a t a - - - 0.3 0.6 - - 8.2 27 .6 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 70 Figures 17, 18, and 19 i l l u s t r a t e the monthly mean abundances of the dominant species c o l l e c t e d i n the Zostera marina, d r i f t , and core samples, r e s p e c t i v e l y . Corophium acherusicum, and to a lesser extent, C. insidiosum, were common on a l l substrates i n most months, although they were less abundant on the eelgrass than they were i n the d r i f t or the sediment. Corophium insidiosum 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). The populations of both species declined dramatically i n the d r i f t following the autumn peak, while the numbers i n the sediment declined more gradually (Figs. 18 and 19). Juvenile Corophium spp. were c o l l e c t e d i n the sediment and the d r i f t i n large numbers through the autumn and winter but by March they too had declined i n abundance. Both Corophium species remained i n low numbers throughout March and A p r i l before increasing to levels approaching'peak density i n July 1984 (Figs. 18 and 19). Species d i v e r s i t y was not s i g n i f i c a n t l y d i f f e r e n t on either of the three substrate types i n any month (ANOVA, p>0.05). In many instances, species which were c o l l e c t e d i n the d r i f t were also c o l l e c t e d i n the core samples and t h i s may have been due to t h e i r presence at the sediment surface. A good example of t h i s was Anisogammarus pugettensis, which was c o l l e c t e d i n r e l a t i v e l y large numbers i n July 1983 and May 1984 (Table 9), p r i m a r i l y i n the d r i f t samples (Fig. 18). This species was often seen swimming among the eelgrass plants, s h e l t e r i n g within the folds of Ulva sp. blades, and crawling on the sediment surface. S i m i l a r i l y , a number of species, i n c l u d i n g Pontogeneia ros t r a t a , Ampithoe v a l i d a , and Ischyrocerus sp. were c o l l e c t e d i n both the d r i f t and the eelgrass. Ischyrocerus sp. was the only species which was 71 Figure 17. Mean number of i n d i v i d u a l s i n each dominant amphipod species • m - 2 surface of l i v e Zostera marina (x ± S.E.) at study s i t e "T". (N = 4). 1 = Ampithoe v a l i d a , 2 = Corophium  acherusicum, and 3 = Ischyrocerus sp. 72 1983-1984 73 Figure 18 A. and B. Mean number of i n d i v i d u a l s i n each dominant amphipod species (x 10) * m~2 surface of d r i f t (x ± S.E.) at study s i t e "T". (N = 7, August; 6, September; 5, October; 1, November, December, January, and March; 7, A p r i l ; and 8, May, June, and July 1984). 1 = Corophium acherusicum, 2 = C. insidiosum, and 3 = Ampithoe v a l i d a . 74 Mean number of i n d i v i d u a l s i n each dominant amphipod species (x 10) * m~2 surface of sediment surface (x ± S.E.) at study s i t e "T". (N = 8). 1 = Corophium  acherusicum, 2 = C. insidiosum, 3 = Aropithoe v a l i d a , and 4 = Anisogammarus  pugettensis. 76 A6 S-4 025 N23 021 J20 Ml A17 M15 J27 J26 1983-1984 77 c o l l e c t e d at higher d e n s i t i e s per unit surface area of plant on the eelgrass than i t was on the d r i f t (Fig.. 17). The r e s u l t s of the p r i n c i p a l component analysis are shown i n Figures 20 and 21. Monthly mean abundances of the f i v e most frequently c o l l e c t e d species, Corophium acherusicum, C. insidiosum, Ampithoe  v a l i d a , Anisogammarus pugettensis, and Ischyrocerus sp., as well as Corophium spp. juveniles, were included i n t h i s a n a l y s i s . Abundances of A. pugettensis and the two Corophium species were more c l o s e l y associated than were numbers of A. v a l i d a and Ischyrocerus sp. (Fig. 20). Given the differences i n the temporal d i s t r i b u t i o n s of A. v a l i d a and Ischyrocerus sp., t h i s pattern can probably be explained on the basis of substrate a s s o c i a t i o n . Both of these species were c o l l e c t e d more frequently i n the eelgrass than were the other three species, or the Corophium juveniles. Plots of component scores of each p r i n c i p a l axis through time r e f l e c t the dominance of the Corophium species which, as previously described, were c o l l e c t e d i n large numbers i n the d r i f t and sediment samples ( F i g . 21). The spring and autumn peaks i n abundance i n the Z. marina substrate may be a t t r i b u t e d to the presence of Ischyrocerus 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). 9. Correlation Matrices and the D i s t r i b u t i o n of Species Table 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 for "pairwise" comparisons of the mean r e l a t i v e abundances of a l l amphipod species pooled from a l l samples c o l l e c t e d from August 1983 - J u l y 1984. With the exception of the negative c o r r e l a t i o n s which were found between the abundances of both Corophium species and Ischyrocerus sp., 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 (Table 10). Highest s i g n i f i c a n t 78 Figure 20A. P r i n c i p a l component ordination of dominant amphipod species i n monthly samples, August 1983 - July 1984 - components 1 and 2. Values i n parentheses i n d i c a t e percentage of variance i n data which i s accounted for by each component a x i s . 1 = Ampithoe v a l i d a , 2 = Anisogammarus  pugettensis, 3 = Corophium acherusicum 4 = C. insidiosum, 5 = Corophium spp. juveniles, and 6 = Ischyrocerus sp. Component 2 (18.0%) -1.0 i O I O (Jl n o 3 U o CD cr ro o o o 01 6L 80 Figure 20B. P r i n c i p a l component ordination of dominant amphipod species i n monthly samples, August 1983 - July 1984 - components 2 and 3. Values i n parentheses i n d i c a t e percentage of variance i n data which i s accounted for by each component a x i s . 1 = Ampithoe v a l i d a , 2 = Anisogammarus  pugettensis, 3 = Corophium acherusicum 4 = C. insidiosum, 5 = Corophium spp. juveniles, and 6 = Ischyrocerus sp. Component 3 (16.0%) -i.O -0.5 0.0 0.5 1.0 r R 1 1 1 TI o 10 * I o Ul o o 3 X J o 3 CD 3 ft ro o o Ul * * a o 0) o Ul o 18 82 Figure 2 i A . Monthly p r i n c i p a l component scores (x ± S.D.) i n each substrate on component axis 1. Z = Zostera marina D = D r i f t S = Sediment Values i n parentheses i n d i c a t e percentage of t o t a l variance i n data which i s accounted for by component a x i s . 83 84 Figure 2IB. Monthly p r i n c i p a l component scores (x ± S.D.) i n each substrate on component axis 2. Z = Zostera marina D = D r i f t S = Sediment Values i n parentheses i n d i c a t e percentage of t o t a l variance i n data which i s accounted for by component a x i s . 86 Figure 21C. Monthly p r i n c i p a l component scores (x ± S.D.) i n each substrate on component axis 3. Z = Zostera marina D = D r i f t S = Sediment Values i n parentheses i n d i c a t e percentage of t o t a l variance i n data which i s accounted for by component a x i s . 6 8 J 0 3 S 3U9U0dU103 U B QW 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 comparison of mean r e l a t i v e numbers of amphipods • species --'- • 0.01m - 2 pooled i n a l l months and s u b s t r a t e types, 1983-84 (N = 176). * values i n d i c a t e s i g n i f i c a n t R (p < 0.05); ** values i n d i c a t e s i g n i f i c a n t R (p < 0.01). 1 = Ampithoe l a c e r t o s a 2 = A. v a l i d a 3 = Anisogammarus p u g e t t e n s i s 4 = A t y l u s c o l l i n g i i 5 = C a l l i o p i u s l a e v i u s c u l u s 6 = Corophium acherusicum 7 = C. i n s i d i o s u m 8 = Ischyrocerus sp. 9 = Paraphoxus spinosus 10 = P a r a p l e u s t e s p u g e t t e n s i s 11 = P h o t i s b r e v i p e s 12 = P h o t i s o l i g o c h a e t a 13 = Pontogeneia i n t e r m e d i a 14 = P_. r o s t r a t a 15 = Synchelidium shoemakeri [ c o n t . ] Table 10. Icont.l CO Species 1 2 3 4 5 6 7 8 9 10 11 12 13 14 IS 1 2 -0.036 3 -0.048 0.012 4 -0.014 -0.043 0.026 5 -0.010 0.037 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 0.704** 8 -0.042 -0.110 -0.090 0.076 -0.024 -0.161* - 0 . 174* 9 -0.016 -0.051 0.312**-0.013 -0.010 -0.056 -0.018 -0.041 10 -0.010 -0.001 -0.033 -0.008 -0.006 -0.038 -0.051 -0.024 -0.010 11 0.004 -0.044 -0.027 -0.012 -0.008 -0.019 -0.059 -0.035 -0.014 -0.008 12 0.017 -0.066 -0.040 -0.017 -0.012 -0.032 -0.036 -0.052 -0.021 -0.012 0.671** 13 -0.011 -0.034 0.449**-0.009 -0.007 -0.044 -0.059 -0.025 0.655** -0.007 -0.010 -0.014 14 -0.029 -0.051 -0.072 -0.025 -0.018 -0.063 -0.082 0.469** -0.029 -0.018 -0.025 -0.025 0.003 15 -0.022 -0.040 -0.074 -0.018 -0.013 -0.053 0.009 -0.054 0. 255** -0.013 -0.018 -0.028 -0.015 -0.039 90 co r r e l a t i o n s were found between the r e l a t i v e abundances of C. acherusicum and C. insidiosum (r=0.794, p<0.01) and Photis, brevipes and P. oligochaeta (r=0.671, p<0.01) (Table 10). 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 were a l s o found between the r e l a t i v e abundances of each of the Corophium species and those of Anisogammarus pugettensis, Ampithoe valida,and A. lacertosa (Table 10). The abundances of three species which were infrequently c o l l e c t e d ( C a l l i o p i u s l aeviuscula, Paraphoxus spinosus, and Pontogeneia intermedia) were cor r e l a t e d s i g n i f i c a n t l y with that of A. pugettensis (Table 10). 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 also occurred between Pontogeneia r o s t r a t a and Ischyrocerus sp., both of which were most numerous i n the spring. Correlation analysis was also used to 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 species i n i n d i v i d u a l months. Table 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 or monthly comparisons of the r e l a t i v e abundances of Anisogammarus pugettensis, Ampithoe v a l i d a , Corophium acherusicum C. insidiosum, and Ischyrocerus sp. The r e l a t i v e abundances of the two Corophium species were p o s i t i v e l y c orrelated i n a l l months except March, A p r i l , May, and June, when low numbers of C. acherusicum were c o l l e c t e d and C. insidiosum occurred even less frequently i n the samples. S i g n i f i c a n t negative c o r r e l a t i o n s between Corophium acherusicum and Ischyrocerus sp. occurred i n October and January (Table 11). In both of these months Ischyrocerus was c o l l e c t e d only i n the eelgrass samples and C. acherusicum was c o l l e c t e d only 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. pugettensis i n December was due to the low 91 T a b l e 1 1 . 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 n u m b e r s o f a m p h i p o d s • s p e c i e s - 1 • 0 . 0 1 m " 2 , 1 9 8 3 - 8 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 . 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 = A m p i t h o e v a l i d a 2 = A n i s o g a m m a r u s p u g e t t e n s i s 3 = C o r o p h i u m a c h e r u s i c u m 4 = C_. i n s i d i o s u m 5 = I s c h y r o c e r u s s p . A u g u s t , 1983 n = 19 S p e c i e s 1 2 3 4 5 1 2 0. . 4 7 3 * 3 0. .411 0. . 8 7 8 * * 4 0. . 178 0. . 492* 0 . 7 0 4 * * 5 0. .013 - 0 . .142 - 0 . 1 9 3 - 0 . 1 5 5 S e p t e m b e r n = 18 S p e c i e s 1 2 3 4 5 1 2 0 .084 3 - 0 . 6 2 2 * * 0 . 0 5 0 4 - 0 . 6 3 3 * * 0 . 1 2 6 0 . 8 7 9 * * 5 0 .000 0 .000 0 . 0 0 0 0 . 0 0 0 [ c o n t . ] 92 Table 11. [cont. ] October n = 17 Species 1 2 3 4 5 1 2 3 4 5 -0.134 -0.097 0.262 -0.148 0.402 0.350 -0.050 0.664** -0.545* -0.400 November n = 13 Species 1 2 3 4 5 1 2 3 4 5 0.000 0.629* 0.078 -0.257 0.000 0.000 0.000 0.577* -0.427 -0.351 December n = 13 Species 1 2 3 4 5 1 2 3 4 5 -0.267 0.250 0.295 -0.429 0.039 0.101 0.774** 0.562* -0.061 0.187 [cont.] 93 Table 11. [cont.] January, 1984 n = 13 Species 1 2 3 4 5 1 2 0.000 3 0.605* 0.000 4 0.388 0.000 0.911** 5 -0.264 0.000 -0.635* -0.614* March n = 12 Species 1 2 3 4 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 -0.182 A p r i l n = 12 Species 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 Table 11. [cont.] May 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 0.000 June 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 -0.244 J u l y n = 19 Species 1 2 3 4 5 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 95 abundance of both of these species c o l l e c t e d i n the sediment i n that month. Other s i g n i f i c a n t negative c o r r e l a t i o n s occurred between the two Corophium species and Ampithoe v a l i d a i n September (Table 11). These were probably due to d i f f e r e n c e s i n the o v e r a l l abundance of these species. Although A. v a l i d a was c o l l e c t e d on a l l substrates i n that month, i t was far less abundant than was either C. acherusicum or C. insidiosum i n the d r i f t or sediment 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 between C. acherusicum and A. v a l i d a occurred i n November and January, months i n which the sizes of both populations i n the study area were d e c l i n i n g (Tables 9 and 11). Table 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 for comparisons of the r e l a t i v e abundances of the f i v e most numerous amphipod species c o l l e c t e d on each of the three substrates. . 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 occurred only between C. insidiosum and A. pugettensis. Abundances of A. pugettensis were not correlated with those of any o'f the other three species i n either the d r i f t or the sediment. In the d r i f t , p o s i t i v e c o r r e l a t i o n s occurred between A. v a l i d a and the two Corophium species, while low but s i g n i f i c a n t , negative correlations were found between Ischyrocerus sp. and C. acherusicum (Table 12). The r e l a t i v e abundances of the two Corophium species were p o s i t i v e l y c o r r e l a t e d only i n the sediment samples, where th e i r occurrence was a l s o p o s i t i v e l y c o r r e l a t e d with numbers of A^ v a l i d a (Table 12). In summary, f i v e main, points may be made from the r e s u l t s of the twelve-month study: 1) Amphipods were most abundant i n the 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) f o r comparisons of mean numbers of 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. * 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 = Ampithoe v a l i d a 2 = Anisogammarus p u g e t t e n s i s 3 = Corophium a c h e r u s i c u m 4 - C. i n s i d i o s u m 5 = I s c h y r o c e r u s sp. Z o s t e r a m a r i n a n = 43 S p e c i e s 1 2 3 4 5 1 2 0.113 3 0.145 -0. 121 4 -0.044 0. 530** 0 .204 5 -0. 307 -0. 120 0 .144 -0.165 [ c o n t . ] 97 T a b l e 12. [ c o n t . ] D r i f t n = 51 S p e c i e s 1 2 3 4 5 1 2 -0.154 3 0.532** -0.003 4 0.378** 0.143 0.271 5 -0.116 0.108 -0.286* -0.273 Sediment n = 82 S p e c i e s 1 2 3 4 5 1 2 0.115 3 0.489** 0.057 4 0.387** 0.085 0.683 5 -0.115 0.068 0.039 0.021 98 t h e i r numbers declined throughout the winter and spring. This pattern of abundance was p o s i t i v e l y c o r r e l a t e d with the seasonality of Zostera  marina biomass. 2) Despite d e n s i t i e s equivalent to more than 26,800 amphipods • m ^ i n October, species d i v e r s i t y was low. Four species, Corophium acherusicum, C. insidiosum, Ampithoe v a l i d a , and Anisogammarus  pugettensis accounted for 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 alone accounted for more than 88% of the t o t a l numbers i n October. Species d i v e r s i t y was s i g n i f i c a n t l y higher i n A p r i l and May when the r e l a t i v e abundance of a f i f t h species, Ischyrocerus sp. increased. 3) No s i g n i f i c a n t differences i n amphipod abundance ' 0.01m or per unit surface area of plant were found among the treatment pl 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 eelgrass shoots were removed. There was a s i g n i f i c a n t trend i n increasing abundance of amphipods i n the sediment i n these p l o t s , however, which corresponded to the r e l a t i v e i n t e n s i t y of the shoot removal "disturbance". 4) S i g n i f i c a n t l y higher de n s i t i e s of amphipods were recorded i n the d r i f t per unit surface area than were found on the Zostera 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 plant substrates was correlated s i g n i f i c a n t l y with macrophyte biomass and surface area. This p o s i t i v e r e l a t i o n s h i p was also observed on the two types of plant substrates i n i n d i v i d u a l months i n the autumn. The o v e r a l l abundance of amphipods c o l l e c t e d i n the sediment was correlated negatively with mean biomass of d r i f t per quadrat. Negative c o r r e l a t i o n s between the number of amphipods c o l l e c t e d i n the sediment and eelgrass biomass per quadrat occurred 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 variables were i d e n t i f i e d i n June. 99 Part B. The Three-Month Study These r e s u l t s describe the patterns of d i s t r i b u t i o n and abundance of amphipods i n three areas of d i f f e r e n t natural d e n s i t i e s of eelgrass shoots from May - July 1984. These areas include: 1) Area 1 - a mixed stand of Zostera marina and Z. japonica ( F i g . 22); 2) Area 2 - low density of Z. marina ( F i g . 23); and 3) Area 3 - high density of Z. marina (Fig. 24). 1. Physical Factors Monthly a i r and water temperatures and surface s a l i n i t y measurements are described i n Part A: The Twelve-Month Study. The sediments of the three areas were c l a s s i f i e d as f i n e , to very f i n e , sands with a mean grain diameter of 0.11mm i n Area 2, and 0.14mm i n Areas 1 and 3 (Table 13). The s i l t - c l a y f r a c t i o n accounted for 13 and 14% of the sample dry weight i n Areas 1 and 2, r e s p e c t i v e l y , and for 5% of the dry weight i n Area 3. Synchronized measurements of the depth of submergence i n the three study s i t e s , made at lower low water on 27 Jul y 1984, revealed a maximum diff e r e n c e of 6cm i n elevation between the three areas. Area 1 (13cm) and Area 3 (16cm) were s l i g h t l y lower i n elevation that was Area 2 (10cm). The mean organic content of the sediments i n the 0 - 5cm core f r a c t i o n s ranged from 1.3-2.2% i n a l l areas (Table 14). S i g n i f i c a n t differences were indicated among the areas over the three month study period (ANOVA, p<0.005), but a multiple range t e s t was unable to di s t i n g u i s h between the means (SNK, a =0.05). From an examination of the mean values i n Table 14, i t seems apparent that the d i f f e r e n c e i n 100 Figure 22. Area 1, 28 July 1984. A. The stand of mixed Zostera species. The shoot density of Zostera  japonica increased throughout the summer u n t i l July when the leaves of these plants provided a continuous canopy over the surface of the sediment. 9 B. A 0.25nr quadrat containing both species of Zostera. Note the f i l m of diatoms on the blades of Z. japonica and the comparatively clean blades of the Z. marina, f i v e shoots of which appear i n th i s photograph. 101 102 Figure 23. Area 2, 28 July 1984. A. Low density of Z. marina shoots (14 ± 2 shoots * 0.25m"2;x ± S.E.). Although the density of eelgrass shoots was r e l a t i v e l y uniform i n t h i s area, unvegetated patches d i d occur, created 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. Typ i c a l unvegetated patches are shown i n t h i s photograph. 9 B. A 0.25m quadrat containing Z. marina and Enteromorpha sp. 1 0 3 104 Figure 24. Area 3, 28 July 1984. A. High density Z. marina shoots (23 ± 1 shoots • 0.25m"2; x ± S.E.). B. A 0.25m2 quadrat containing Z. marina and decaying Ulva sp. 105 1 0 6 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 t h e t h r e e 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 Z o s t e r a s h o o ts (June 28, 1984). V a l u e s ar e mean p e r c e n t a g e d r y w e i g h t i n each s i z e c l a s s (n = 3 ) . VFS = Very f i n e sand FS = F i n e sand Area 1 : Mixed s t a n d of Z o s t e r a m a r i n a and _Z_. j a p o n i c a . Area 2 : Low d e n s i t y of Z_. m a r i n a s h o o t s . Area 3 : High d e n s i t y of _Z. mari n a s h o o t s . S i z e A r e a 1 A r e a 2 A r e a 3 C l a s s (mm) (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 1 1 1 < 0.053 7 5 5 T a b l e 14. Percentage o r g a n i c content (x + S.E.) of sediment core f r a c t i o n s i n areas of d i f f e r e n t d e n s i t i e s of Zos t e r a shoots from May - J u l y , 1984. Area 1 : Mixed stand of Zostera marina and _Z. j a p o n i c a . Area 2 : Low d e n s i t y of Z_. marina shoots. Area 3 : High d e n s i t y of Z_. marina shoots. Core F r a c t i o n May June J u l y A r e a 1 A r e a 2 A r e a 3 A r e a 1 A r e a 2 A r e a 3 Ar e a 1 A r e a 2 A r e a 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 organic content among the sediment i n the three areas which was shown by the ANOVA was caused by the lower organic content i n Area 2. The mean organic content of the sediments i n the 5-10cm core f r a c t i o n s ranged from 1.0-1.6% of the sediment dry weight i n the three areas over the summer months (Table 14). M u l t i p l e range tests were also unable to i d e n t i f y s i g n i f i c a n t differences between areas 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, Area 1 sediment had a higher organic content than d i d sediments i n either Area 2 or 3 (Table 14). 2. Biomass and Shoot Density of Zostera marina Figure 25 i l l u s t r a t e s the monthly mean dry weight' of i n d i v i d u a l Zostera marina above-ground shoots from the three study areas. With the exception of Area 3 i n June, when the plants weighed s i g n i f i c a n t l y more than they d i d i n Area 3 i n May, Area 1 i n June, and Area 2 i n July, few differences were found i n dry weight of shoots over the summer months (ANOVA, p<0.025; SNK,a =0.05). In addition, there was no s i g n i f i c a n t d i f f e r e n c e i n mean surface area per shoot between areas or months (ANOVA, p>0.10) despite the fac t that the mean number of blades per shoot ranged from 5 i n Area 3 i n June and July to 9 i n Area 1 i n May. Also, no s i g n i f i c a n t differences were found i n the mean dry weight of eelgrass rhizomes per core between months or areas (ANOVA, p>0.05) (Table 15). Figure 26 i l l u s t r a t e s the monthly mean shoot density of Zostera  marina i n each of the three areas. There were no s i g n i f i c a n t d ifferences i n d e n s i t i e s of shoots during the three-month study period within areas (ANOVA, p>0.05). However, among areas, 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). Overall mean density was 4 + 1 109 Figure 25. Dry weight of above-ground Zostera marina shoots (x ± S.E.) i n monthly samples i n Areas 1, 2, and 3 May - July 1984. (N = 6). Figure 26. Densities of Zostera marina shoots * 0.25m_^ (x ± S.E.) i n Areas 1, 2, and 3, A p r i l - July 1984 (N = 6). Shoot count i n A p r i l i n Area 1 i s an estimate. Differences were s i g n i f i c a n t (p>0.05) among a l l areas i n each month with the exception of May i n Areas 2 and 3. 110 in m o ro A p r i l 17 Hay 15 June 27 J u l y 26 19B4 I l l Table 15. Dry weight (g) of Zostera marina rhizomes • c o r e - 1 (x ± S.E.) i n Areas 1, 2, and 3, May - J u l y , 1984. There was no s i g n i f i c a n t d i f f e r e n c e i n mean dry weight among months, areas, or months x areas (2-way ANOVA, p > 0.25) . May June July Area Area Area 1 2 3 Area Area Area 1 2 3 Area Area Area 1 2 3 0.08 0.28 0.29 ±0.02 ±0.10 ±0.08 0.29 0.35 0.30 ±0.12 ±0.09 ±0.10 0.17 0.20 0.33 ±0.08 ±0.06 ±0.11 112 shoots • 0.25m"2 (x ± S.E.) i n Area 1, 14 ± 2 shoots • 0.25m~2 i n Area 2. and 25 ± 1 shoots' 0.25m2 i n Area 3 ( F i g . 26). The equivalent dry-weights were 9, 47, and 86 g * m~2 i n Areas 1, 2, and 3, respectively. 3. Biomass and Shoot Density of Zostera japonica Figure 27 i l l u s t r a t e s the mean shoot density of Zostera japonica i n Area 1 from May - July 1984. Unlike Z. marina, the above-ground shoots of Z. japonica die back completely during the winter and t h i s species i s l a r g e l y dependent on seed d i s p e r s a l and germination for i t s spring r e c o l o n i z a t i o n (Harrison, personal communication, 1984). In A p r i l 1984, when the three study areas were selected, fewer than 30 Z. japonica shoots • 0.25m~2 were observed within Area 1. By May, the shoot density of t h i s species had r i s e n to approximately 80 shoots • 0.25m-2 and, by the following month, i t s d e n s i t i e s had increased three-f o l d ( F i g . 27). In July, at low t i d e , the leaves of Z. japonica formed an uninterrupted canopy over the sediment surface ( F i g . 22). 4. Composition and D i s t r i b u t i o n of D r i f t Figure 28 i l l u s t r a t e s the monthly mean dry weights of each component of the d r i f t that was c o l l e c t e d i n the three study areas. In Area 1, l i v e Zostera japonica, c o l l e c t e d as d r i f t , was the dominant component of these samples ( F i g . 28A). It was not found i n Areas 2 or 3 (F i g . 28B and C). Here, the d r i f t was dominated by f r e e - f l o a t i n g Ulva sp. While the biomass of t h i s alga peaked i n May i n Area 3 and declined through the rest of the summer, i t was most abundant i n June i n Area 2 ( F i g . 28B). Fragments of eelgrass blades, mats of Enteromorpha sp. and, to a lesser extent, Laminaria saccharina, were a l s o c o l l e c t e d i n these two areas from May - July ( F i g . 28B and C). 113 Figure 27. Density of Zostera japonica shoots • 0.25m' (x ± S.E.) i n Area 1, (N = 6). 115 Figure 28A. Monthly mean dry weights • 0.01m-2 (± S.E.) of major components of d r i f t i n Area 1. (N = 6). U = Ulva sp., Z.m = dead Zostera  marina, and Z.j = l i v e Zostera japonica. 9TT 117 Figure 28B. Monthly mean dry weights ' 0.01m"2 (± S.E.) of major components of d r i f t i n Area 2. (N = 6). E = Enteromorpha sp., U = Ulva sp., Z.m = dead Zostera marina. s 8TI 119 Figure 28C. Monthly mean dry weights • 0.01 m - 2 (±.S.E.)of major components of d r i f t i n Area 3. (N = 6). L = Laminaria saccharina, U = Ulva sp., and Z.m = Zostera marina. 121 5. Zostera marina Shoot Density and the D i s t r i b u t i o n of Amphipods A two-way analysis of variance was used to investigate differences i n t o t a l mean abundances of amphipods • 0.01m-2 among the three months and the three areas of d i f f e r e n t d e n s i t i e s of Zostera marina shoots. The highest mean dens i t i e s occurred i n July using data pooled from a l l areas (SNK, a =0.05) (Table 16). No s i g n i f i c a n t differences i n mean amphipod abundance, however, were found among the three study areas using data pooled from May, June, and July (ANOVA, p>0.05) ( F i g . 29). Although s i g n i f i c a n t differences d i d occur i n mean amphipod number * 0.01m-2 among areas i n cer t a i n months (ANOVA, p<0.05; SNK, a=0.05), with the exception of June there was no apparent pattern to these v a r i a t i o n s . The density 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 the highest numbers of animals c o l l e c t e d i n Area 2 and the 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 recorded 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 Overall Abundance of Amphipods When data from a l l three areas were pooled, s i g n i f i c a n t differences i n abundance between months were revealed (ANOVA, p<0.05; SNK, a=0.05). Amphipod abundances increased t h r e e - f o l d from May - June and from June - July (Table 16). Figures 30, 31, and 32 i l l u s t r a t e the o v e r a l l monthly mean abundances of amphipods within the three study areas and t h e i r d i s t r i b u t i o n on the plants and sediment substrates. A three-way analysis of variance was used to investigate differences i n the number of amphipods per unit area of plant and of sediment surface, among months and areas of d i f f e r e n t d e n s i t i e s of eelgrass shoots. S i g n i f i c a n t increases i n abundance occurred i n both sediment and d r i f t over time; 1 2 2 T a b l e 16. Monthly t o t a l number of amphipods • m~ 2 (x ± 95% C.L.) p o o l e d from a l l samples i n Areas 1, 2, and 3 (n = 1 8 ) . * v a l u e s i n d i c a t e means which 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 = 0.05). May June J u l y X 2240 6980 20187 + C.L. 1508-3320 4503-10789 13672-30169 123 Figure 29. Monthly abundance of amphipods (x 100) • m - 2 (x ± S.E.) i n Areas 1, 2, and 3, (N = 6). Absolute abundances of amphipods were pooled from a l l substrate samples within 0.01m quadrats. 125 Figure 30. Mean number of amphipods • m ^ of surface of Zostera marina (x ± S.E.) i n Areas 1, 2, and 3. (N = 6). 931 127 Figure 31. Mean number of amphipods (xlO) * m - 2 of surface of. d r i f t (x ± S.E.) i n Areas 1, 2, and 3. (N = 6). May 15 June 27 1984 July 26 129 Figure 32. Mean number of amphipods (x 10) • sediment surface (x ± S.E.) i n Areas 1, 2, and 3. (N = 6). Mean Number of Amphipods x 10 /m OCT 131 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 , however, i n amphipod abundance among months i n the Zostera marina samples (ANOVA, p<0.002; DMR,a =0.05). As i n Part A: The Twelve-Month Study, the d r i f t supported-s i g n i f i c a n t l y more amphipods per unit surface area than d i d the eelgrass (ANOVA, p<0.0005; DMR, a =0.05) (Figs. 30 and 31). S i g n i f i c a n t differences i n amphipod density per unit surface area occurred between areas i n both the plant and sediment samples (ANOVA, p<0.05; SNK,a =0.05). For example, more amphipods were c o l l e c t e d i n the d r i f t and i n the sediment i n Areas 1 and 2 than i n Area 3, over the three-month period (Figs. 31 and 32). There was no s i g n i f i c a n t d i f f e r e n c e between the number of amphipods per unit surface area c o l l e c t e d i n the d r i f t i n Area 3, and i n any of the eelgrass samples. C o r r e l a t i o n analyses were used to examine the as s o c i a t i o n between the abundance of amphipods i n the eelgrass, d r i f t , and sediment samples, and macrophyte biomass and surface area, r e s p e c t i v e l y (Table 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 Area 2 i n May and June, and i n a l l three areas i n July (Table 17). In May, the number of amphipods c o l l e c t e d i n the d r i f t i n Area 2 was cor r e l a t e d s i g n i f i c a n t l y with the dry weight of the d r i f t i n each sample (Table 17) (r=0.81.,. p<0.05,). Similar 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 dry weight and surface area of d r i f t , occurred i n the samples c o l l e c t e d i n Areas 2 and 3 i n July (r=0.92, p<0.01 and r=0.85, p<0.01 - dry weight; r=0.89, p<0.05 and r=0.84, p<0.05 - surface area, r e s p e c t i v e l y ) . In contrast, i n Area 1 i n July, the abundance of amphipods i n the d r i f t , p r i m a r i l y composed of Zostera japonica, was more c l o s e l y r e l a t e d to surface area (r=0.95, p<0.01) than i t was to dry weight (Table 17). 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 t o t a l abundance of amphipods per sample quadrat and d r i f t dry weight and surface area i n Area 2 and 3 i n July 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 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 , s e d i m e n t , and t o t a l q u a d r a t ( 0 . 0 1 m - 2 ) samp l e s w i t h s u r f a c e a r e a (cm 2) 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 • 0.01m" 2 (n = 6 ) . * 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). 1 = _Z. m a r i n a s u r f a c e a r e a (cm 2) 2 = _Z. m a r i n a d r y w e i g h t (g) 3 = D r i f t s u r f a c e a r e a (cm ) 4 = D r i f t d r y w e i g h t (g) [cont.] T a b l e 1.7. [ c o n t . ] AREA 1 A R E A 2 A R E A 3 M o n t h Z. m a r i n a D r i f t S e d i m . T o t a l •L. m a r i n a D r i f t S e d i m . T o t a l Z. m a r i n a D r i f t S e d i m . T o t a l May 1 .065 .072 -.480 .242 .457 -.240 -.423 -.099 -.205 .321 -.591 . 325 2 .086 .091 -.464 .258 .445 -.241 .448 -.126 -.179 . 330 -.587 .342 3 .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 June 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 J u l y 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 proportion of animals i n each quadrat that were c o l l e c t e d i n the d r i f t (Table 17). When data from a l l months and study areas were pooled, the abundance of amphipods i n the sediment was negatively c o r r e l a t e d with the d r i f t as well as t o t a l macrophyte, biomass and surface area per quadrat- (Table 18). 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 dry weight and the surface area of Zostera marina per quadrat were observed (Table 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 cores, and eelgrass dry weight and surface area, were found i n June i n the study area described i n Part A: The Twelve-Month Study (Table 7). 7. D i s t r i b u t i o n of Amphipod Species Although fourteen species of amphipods were c o l l e c t e d i n the three-month study, o v e r a l l species d i v e r s i t y was low, as i t was i n Part A, due to the dominance of four species. Thus, Corophium acherusicum, C. insidiosum, Anisogammarus pugettensis, 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 July (Table 19). Overall d i v e r s i t y (H') was s i g n i f i c a n t l y higher i n May and i n June than i t was i n July, due to the presence of large numbers of the f i f t h species, Ischyrocerus sp. (Table 19). Lowest d i v e r s i t y occurred i n May i n Area 1 (H' = 1.13) and i n July i n Area 3 (H = 0.83) (ANOVA, p<0.0001;DMR,a =0.05). Evenness (J')was r e l a t i v e l y high i n a l l samples, i n d i c a t i n g that the rare amphipod species contributed few i n d i v i d u a l s to the t o t a l abundance and the dominant species were equitably d i s t r i b u t e d (Table 19). 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 comparisons of number of amphipods 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 2) samples w i t h s u r f a c e a r e a (cm 2) and d r y we i g h t (g) of Z_. m a r i n a , d r i f t , and t o t a l macrophytes • 0. O l r n - 2 . A l l samples c o l l e c t e d i n Areas 1, 2, and 3, May - J u l y , 1984 (n = 54). * 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 r i a b l e Z. m a r i n a D r i f t Sediment T o t a l Z. m a r i n a : s u r f a c e a r e a 0.219 -0.176 -0.230 -0.169 d r y w e i g h t 0.218 -0.181 -0.227 -0.176 D r i f t : s u r f a c e a r e a -0.167 0.080 -0.489* -0.027 dr y w e i g h t -0.058 0.172 -0.342* 0.088 T o t a l macrophytes: s u r f a c e a r e a -0.137 0.066 -0.506* -0.038 dr y w e i g h t 0.021 0.148 -0.392* 0.072 Table 19. R e l a t i v e abundance, d i v e r s i t y , and evenness of species of amphipods • 0.01m - 2 i n Areas 1, 2, and 3, May - J u l y , 1984. * values i n d i c a t e area x months which 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 ) . n = T o t a l number counted H' = Shannon-Wiener D i v e r s i t y J ' = Shannon-Wiener Evenness May June July Species Area 1 Area 2 Area 3 Area 1 Area 2 Area 3 Area 1 Area 2 Area 3 n = 249 60 145 853 372 176 728 1582 929 Ampithoe l a c e r t o s a - 1.7 0.7 _ _ _ _ _ n . l 1 A. v a l i d a 3.2 1.7 0.7 7.0 1.6 2.3 33.8 1.2 0.1 Anisogammarus pug e t t e n s i s 52.2 21.7 13.8 29.8 9.9 12.5 4.3 38.4 A t y l u s c o l l i n q i i - - 1.4 _ _ _ _ _ _ C a l l i o p i u s l a e r i u s c u l u s 0.4 8.3 0.7 _ _ _ _ _ _ C a l l i o p i u s sp. 1.7 _ _ _ _ _ _ Corophium acherusicum 29.3 26.7 20.0 46.1 57.2 51.1 32.7 43.3 76.9 [cont. ] Tab le 19. [cont . ] Spec ies May June J u l y 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 C. insidiosum 12.4 8.3 6.9 17.1 29.6 13.6 29.0 17.0 22.9 Ishyrocerus sp. 0.4 28.3 33.8 - 0.5 14.8 -Parapleustes pugettensis _ _ _ _ _ n.5 -Photis o l i g o c h a e t a - 0.5 -Pontogeneia intermedia - - 12.4 _ _ _ _ _ _ Pontogeneia r o s t r a t a 2.0 - 9.7 - 0.5 5.1 0.3 Synchelidium shoemakeri - 0.2 - _ _ _ _ _ _ T o t a l Species 7 9 10 4 7 7 5 4 4 D i v e r s i t y (H') 1.13 1.40 1.86* 1.58 1.19 1.30 1.26 1.26 0.83* Evenness ( J ' ) 0.76 0.90 0.85 0.90 0.81 0.77 0.82 0.86 0.83 138 Figures 33, 34, and 35 i l l u s t r a t e the monthly mean abundances of the dominant amphipod species per square meter of eelgrass, d r i f t , and sediment surface, respectively, i n each of the three areas. A comparison of the patterns of species d i s t r i b u t i o n among the areas of d i f f e r e n t eelgrass shoot density i n the Zostera marina and d r i f t samples, reveals notable d i f f e r e n c e s . For example, i n Area 1 i n May, both the eelgrass and the d r i f t samples contained large numbers of the f r e e - l i v i n g species, Anisogammarus pugettensis (Figs. 33 and 34). In June and July, as numbers of t h i s amphipod species declined, d e n s i t i e s of the three tube-dwelling species, Corophium acherusicum, C. insidiosum, and Ampithoe v a l i d a , increased (Figs. 33 and 34). In contrast, i n Area 2, abundances of a l l four species were lower i n June than they were i n either May or July, at le a s t i n the d r i f t ( F i g . 34). The f i f t h species, Ischyrocerus sp., was c o l l e c t e d i n the eelgrass i n th i s area i n May and June, and declined i n abundance from June - July, while the de n s i t i e s of the other four species increased ( F i g . 33). In Area 3, Ischyrocerus sp. was also present on Z. marina i n large numbers i n May and June ( F i g . 33). Overall numbers of the four dominant species were low on both substrates i n t h i s area, despite an increase i n the number of C. acherusicum i n the d r i f t i n Ju l y (Figs. 33 and 34). The pattern of d i s t r i b u t i o n and abundance of each of the dominant amphipod species on the two types of macrophyte substrate v a r i e d among the three areas (Figs. 33 and 34). Although the abundance of A. pugettensis declined i n both Area 1 and 2 from May - June, d e n s i t i e s increased more than f o u r - f o l d i n Area 2 from June - July, while they decreased i n Area 1 (Figs. 33 and 34). Few ind i v i d u a l s of t h i s species were c o l l e c t e d i n Area 3 on the plant substrates throughout the summer (Figs. 33 and 34). 139a Figure 33A. Mean number of i n d i v i d u a l s i n each dominant amphipod species • m~2 surface of l i v e Zostera marina (x ± S.E.) i n Area 1. (N = 6). 1 = Corophium acherusicum, 2 = C. insidiosum, and 4 = Anisogammarus  pugettensis. o CD Area 1 Z o s t e r a marina 139c Figure 33B. Mean number of i n d i v i d u a l s i n each dominant amphipod species • m surface of l i v e Zostera marina (x ± S.E.) i n Area 2. (N = 6). 1 = Corophium acherusicum, 2 = C. insidiosum, 3 = Ampithoe y a l i d a , 4 = Anisogammarus pugettensis, and 5 = Ischyrocerus sp. May 15 June 27 July 26 B. 1984 139e Figure 33C. Mean number of i n d i v i d u a l s i n each dominant amphipod species • m~2 surface of l i v e Zostera marina (x ± S.E.) i n Area 3. (N = 6). 1 = Corophium acherusicum and 5 = Ischyrocerus sp. CM 0) T J O a a E O z c ra 0} O o ID o o in o o O o ro o o cu o o o H Hi May 15 C. June 27 1984 July 26 140a Figure 34A. Mean number of i n d i v i d u a l s i n each dominant amphipod species (x 10) * m - 2 surface of, d r i f t (x ± S.E.) i n Area 1. (N = 6). 1 = Corophium acherusicum, 2 = C. insidiosum, 3 = Ampithoe v a l i d a , and 4 = Anisogammarus pugettensis. 140c Figure 34B. Mean number of i n d i v i d u a l s i n each dominant amphipod species (x 10) • m~2 surface of d r i f t (x ± S.E.) i n Area 2. (N = 6). 1 = Corophium acherusicum, 2 = C. insidiosum, 4 = Anisogammarus  pugettensis, and 5 = Ischyrocerus sp. 140e Figure 34C. Mean number of i n d i v i d u a l s i n each dominant amphipod species (x 10) • m~2 surface of d r i f t (x ± S.E.) i n Area 3 . (N = 6 ) . 1 = Corophium acherusicum, 2 = C. insidiosum. CM E 0) TJ O rx JZ a. E < a c (D CU O ID (0 O O cn o in cu o o cu o in o o o m o H May 15 C. June 27 1984 July 26 141a Figure 35A. Mean number of i n d i v i d u a l s i n each dominant amphipod species (x 10) • m * surface of sediment surface (x ± S.E.) i n Area 1. (N = 6). 1 = Corophium acherusicum, 2 = C. insidiosum, and 4 = Anisogammarus  pugettensis. o o in Area 1 Sediment r y y y y^. y y y y y y y y May 15 A. June 27 1984 July 26 141c Figure 35B. Mean number of i n d i v i d u a l s i n each dominant amphipod species (x 10) • m~2 surface of sediment surface (x ± S.E.) i n Area 2. (N = 6). 1 = Corophium acherusicum, 2 = C. insidiosum, and 4 = Anisogammarus  pugettensis. 14 Id 141e Figure 35C. Mean number of i n d i v i d u a l s i n each dominant amphipod species (x 10) • n f 2 surface of sediment surface (x ± S.E.) i n Area 3. (N = 6). 1 = Corophium acherusicum 2 = C. insidiosum. May 15 C. June 27 1984 J u l y 26 142 In contrast to Anisogammarus pugettensis, the two Corophium species increased i n abundance on both plant substrates i n a l l areas from May - July, with the exception 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 increase u n t i l July (Figs. 33 and 34). Ampithoe v a l i d a was most abundant i n both the eelgrass and the d r i f t i n Area 1 i n July (Figs. 33 and 34). On the other hand, Ischyrocerus sp. was more abundant i n the monospecific eelgrass stands of Area 2 and 3 i n May and June, than i t was either i n July or i n the mixed stand of Zostera species i n Area 1 (Fig. 33). Figure 35 i l l u s t r a t e s the patterns of d i s t r i b u t i o n and abundance of three amphipod species which dominated the sediment samples from May - July. It c l e a r l y demonstrates the s i g n i f i c a n t trend i n decreasing abundance from Area 1 to Area 3 (ANOVA, p<0.0001; DMR,a =0.05). Ov e r r a l l abundances • m - 2 of sediment surface were ten times greater i n Area 1 than they were i n Area 3 (Figs. 32 and 35). Corophium acherusicum and C. insidiosum were the most numerous amphipod species c o l l e c t e d i n the core samples i n a l l three areas ( F i g . 35). Their d e n s i t i e s increased from May - July, as they did on the two plant substrates (Figs. 33, 34, and 35). Densities of Anisogammarus  pugettensis peaked i n June i n the sediment i n Areas 1 and 2 and, as i n the eelgrass and d r i f t , few i n d i v i d u a l s of t h i s species were c o l l e c t e d i n the core samples i n Area 3 ( F i g . 35). While not shown i n Figure 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 three months and i n Areas 2 and 3 i n July. Ischyrocerus sp. was never c o l l e c t e d i n the core samples. 143 8. N a t u r a l i s t Sled Samples In the " n a t u r a l i s t sled" c o l l e c t i o n s , amphipods were relatively-more abundant i n Area 1 i n May and June than i n the other two areas due to the presence of large numbers of Anisogammarus pugettensis (Table 20). Few Corophium spp. were c o l l e c t e d i n May i n any of the study areas but they increased i n abundance i n June, and numerically dominated the samples c o l l e c t e d i n Areas 2 and 3 i n that month (Table 20). They were never as abundant i n Area 1. More species were c o l l e c t e d using the " n a t u r a l i s t sled" i n May than i n either June or July i n a l l three areas (Table 20). Ischyrocerus sp. were most abundant- i n Area 3 i n May and by-June i t s . numbers.had declined even there (Table 20). Pontogeneia r o s t r a t a displayed a s i m i l a r pattern of abundance. Both were almost e n t i r e l y absent i n the July benthic tows. C a l l i o p i u s laeviusculus Barnard and a second undescribed C a l l i o p i u s species, which were infrequently c o l l e c t e d i n either the plant or the core samples, were abundant i n May i n the mixed stand of eelgrass species (Area 1) but were not c o l l e c t e d i n tows i n the remaining months (Table 20). Other species which were c o l l e c t e d more infrequently included Paraphoxus spinosus Holmes, Photis oligochaeta Conlan, and Syncheldium shoemakeri. A s i n g l e Eogammarus o c l a r i was c o l l e c t e d i n May i n Area 1 (Table 20). 144 Table 20. Numbers of amphipods • tow~l (x ± S.E.) c o l l e c t e d i n N a t u r a l i s t s l e d i n th r e e areas of d i f f e r e n t d e n s i t i e s of Zostera shoots (n = 2). Axea 1 Area 2 Area 3 May Ampithoe v a l i d a 1 ± <1 0 0 Anisogammarus p u g e t t e n s i s 138 ± 71 39 ± 33 78 ± 57 C a l l i o p i u s spp. 79 ± 7 8 ± 5 17 ± 2 Corophium spp. 72 ± 62 5 ± 4 14 ± 4 Eogammarus o c l a r i <1 ± <1 0 0 Ischyrocerus sp. 2 ± <1 8 + 8 38 ± 13 Paraphoxus spinosus 0 0 <1 ± <1 Ph o t i s o l i g o c h a e t a 0 <1 ± <1 1 ± <1 Pontogeneia spp. 3 ± 2 17 ± 11 50 ± 23 Synchelidium shoemakeri <1 ± <1 0 <1 ± <1 T o t a l 296 78 199 [cont.] 145 T a b l e 20. [ c o n t . ] A r e a 1 A r e a 2 A r e a 3 June /Ampithoe v a l i d a 9 ± 1 2 ± 2 <1 ± <1 Anisogammarus p u q e t t e n s i s 128 ± 49 30 ± 3 18 ± 7 C a l l i o p i u s spp. 2 + 2 4 + 1 5 + 2 Corophium spp. 92 + 26 50 ± 37 20 ± 6 I s c h y r o c e r u s sp. 0 0 7 ± 1 P o n t o g e n e i a spp. 0 0 31 + 13 S y n c h e l i d i u m shoemakeri 0 2 + 1 <1 ± <1 T o t a l 231 88 82 [ c o n t . ] 146 Table 20. [cont.] Area 1 Area 2 Area 3 J u l y Ampithoe v a l i d a 5 ± 1 3 ± <1 2 + 1 Anisogammarus p u g e t t e n s i s 19 + 4 37 ± 24 4 ± 1 C a l l i o p i u s spp. 0 0 <1 + <1 Corophium spp. 28 + 7 46 ± 7 167 ± 84 Paraphoxus spinosus 0 0 <1 ± <1 Pontogeneia spp. 0 0 <1 ± <1 T o t a l 52 8 6 1 7 4 147 9. Correlation Matrices and the D i s t r i b u t i o n of Species Table 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 for "pair-wise" comparisons of the r e l a t i v e abundances of the amphipod species c o l l e c t e d i n the three study areas i n May, June, and July 1984. Data from a l l three areas and substrate types were pooled i n t h i s table for comparative purposes. With the exception of the negative co r r e l a t i o n s i n abundance which occurred between Corophium acherusicum and Ischyrocerus sp. i n May, 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 (Table 21). The highest s i g n i f i c a n t c o r r e l a t i o n s occurred i n a l l months between the two Corophium species (May, r=0.816, p<0.01; June, r=0.858, p<0.01; July, r=0.882, p<0.01) (Table 21). The abundance of Anisogammarus pugettensis was also c l o s e l y c o r r e l a t e d with that of C. acherusicum and C. insidiosum i n a l l three months (Table 21). Low, but s i g n i f i c a n t , c o r r e l a t i o n s i n abundance were found between Ampithoe  v a l i d a and the two Corophium species i n June (Table 21). Table 22 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 for "pair-wise" combinations of the mean number of i n d i v i d u a l s per species c o l l e c t e d monthly i n each of the study areas. No negative c o r r e l a t i o n s i n abundance occurred i n any of the study areas throughout the summer. The highest 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 occurred, once again, between C. acherusicum and C. insidiosum i n each month and study area (Table 22). In Area 1, the mixed stand of Zostera species, both A. v a l i d a and A. pugettensis were r e l a t i v e l y abundant, and t h i s resulted i n a p o s i t i v e c o r r e l a t i o n between them (Table 22). However, by June t h i s r e l a t i o n s h i p was no longer s i g n i f i c a n t . In Area 2 i n May and Areas 1 and 2 i n June, abundances of A. pugettensis were s i g n i f i c a n t l y correlated with those of the two Corophium species (Table 22). While 148 T a b l e 2 1 . 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 comparisons of mean r e l a t i v e numbers of amphipods • species --'- • 0.01m - 2 pooled from a l l months, areas, and s u b s t r a t e types, May - J u l y , 1984 (n = 162). * values i n d i c a t e s i g n i f i c a n t R (p < 0.05); ** values i n d i c a t e s i g n i f i c a n t R (p < 0.01). 1 = Ampithoe l a c e r t o s a 2 = A. v a l i d a 3 = Anisogammarus p u g e t t e n s i s 4 = A t y l u s c o l l i n g i i 5 = C a l l i o p i u s l a e v i u s c u l u s 6 = Corophium acherusicum 7 = C. i n s i d i o s u m 8 = Ishyrocerus sp. 9 = Pontoqeneia i n t e r m e d i a 10 = P. r o s t r a t a [cont.] Table 2 1 . [cont.] Species 1 2 3 4 5 6 7 8 9 1 0 1 2 -0.066 3 -0.076 0.092 4 -0.011 -0.094 0.156 5 -0.022 -0.041 -0.001 -0.012 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 0.121* -0.018 -0.039 0.140 0.078 -0.058 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 comparisons of mean 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, May J u l y , 1984 (n = 52). * 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 = Ampithoe v a l i d a 2 = Anisogammarus p u g e t t e n s i s 3 = Corophium a c h e r u s i c u m 4 = C. i n s i d i o s u m Area 1: Species 1 2 3 4 1 2 -0.113 3 0.713** 0.010 4 0.751** 0.088 0.850** [cont.] 151 T a b l e 22. [ c o n t . ] A r e a 2: S p e c i e s 1 2 3 4 1 2 0.222 3 0.346* 0.816** 4 0.444** 0.832** 0.969** A r e a 3: S p e c i e s 1 2 3 4 1 2 -0.090 3 -0.093 -0.046 4 -0.092 -0.076 0.984** 152 t h i s r e l ationship was maintained i n July i n Area 1, i t d i d not occur i n that month i n Area 2 (Table 22). In Area 3 (high Z. marina shoot density), abundances of Ampithoe  v a l i d a , as well as Ischyrocerus sp. were p o s i t i v e l y correlated with those of A. pugettensis i n May (Table 22). The p o s i t i v e r e l a t i o n s h i p between the l a t t e r two epifaunal species a l s o occurred i n June but was not observed i n July following the decline i n the Ischyrocerus sp. population (Table 22). No negative c o r r e l a t i o n s were apparent when mean abundances of amphipod species were compared on the basis of t h e i r occurrence on a p a r t i c u l a r substrate type. Table 23 shows the product-moment correlations c o e f f i c i e n t s for these monthly comparisons, pooled over the three study areas. Again, i n a l l substrates with the exception of the eelgrass 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 occurred between the two Corophium species. In May i n Zostera 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 occurred between A. pugettensis and A. va l i d a , and A. v a l i d a and Ischyrocerus sp. (Table 23). No s i g n i f i c a n t correlations were recorded i n June on t h i s substrate, but i n July, the mean abundance, of A. valida.and the,.two Corophium species, were r e l a t e d (Table 23). In the d r i f t samples, the s i g n i f i c a n t p o s i t i v e r e l a t i o n s h i p between the Corophium species and Anisogammarus pugettensis occurred i n a l l months. Abundances of Ampithoe v a l i d a were also p o s i t i v e l y correlated with those of the two Corophium species i n June (Table 23). 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 occurred between Ampithoe v a l i d a and Anisogammarus pugettensis i n May, A. pugettensis and the two Corophium species i n June, and A. pugettensis and C. acherusicum i n July (Table 23). The mean abundances of each of 1 5 3 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 comparisons of mean numbers of amphipods • s p e c i e s - ! c o l l e c t e d i n Zostera marina, d r i f t , and sediment samples, pooled from three areas of d i f f e r e n t d e n s i t i e s of Zostera shoots, May - J u l y , 1984 (n = 18). * values 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 = Ampithoe v a l i d a 2 = Anisogammarus p u g e t t e n s i s 3 = Corophium acherusicum 4 = C_. i n s i d i o s u m 5 = Ischyrocerus sp. 1. Z o s t e r a 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 T a b l e 23. [ c o n t . ] June S p e c i e s 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 J u l y S p e c i e s 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 [ c o n t . ] 155 Table 23. [cont.] 2. D r i f t : May Species 1 2 3 4 5 1 2 0 . 5 8 6 * 3 0 . 2 7 7 0 . 7 2 2 * * 4 0 . 2 6 8 0 . 8 1 5 * * 0 . 8 1 0 * * 5 - 0 . 2 1 7 - 0 . 2 7 3 - 0 . 2 8 7 - 0 . 2 0 5 June Species 1 2 3 4 5 1 2 0 . 4 3 1 3 0 . 6 3 2 * * 0 . 8 8 3 * * 4 0 . 6 8 2 * * 0 . 7 4 8 * * 0 . 9 0 7 * * 5 - 0 . 1 7 0 - 0 . 2 0 7 - 0 . 2 6 3 - 0 . 1 7 4 [cont.] 1 5 6 T a b l e 23. [ c o n t . ] July-S p e c i e s 1 2 3 4 5 1 2 -0.308 3 0.210 0.499* 4 0.131 0.643** 0. 851** 5 0.000 0.000 0. 000 0.000 3. Sediment: May S p e c i e s 1 2 3 4 5 1 2 0.680** 3 0.252 0.361 4 0.121 0.345 0. 819** 5 0.000 0.000 0. 000 0.000 [ c o n t . ] 15 7 T a b l e 23. [ c o n t . ] June S p e c i e s 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 July-S p e c i e s 1 2 3 4 5 1 2 0.424 3 0.272 0.679** 4 0.015 0.432 0.828** 5 0.000 0.000 0.000 0.000 158 the Corophium species were correlated s i g n i f i c a n t l y on t h i s substrate i n a l l three months (Table 23). In summary, f i v e main points may be made from the r e s u l t s of the three-month study: 1) As i n Part A: The Twelve-Month Study, Corophium  acherusicum, C. insidiosum, Ampithoe v a l i d a , and Anisogammarus  pugettensis numerically dominated the samples. Together these species accounted for more than 80% of the t o t a l numbers of amphipods i n each study area from May - July, with the exceptions of Areas 2 and 3 i n May. R e l a t i v e l y large numbers of a f i f t h species, Ischyrocerus sp., were col l e c t e d on the Zostera marina i n those areas i n May. 2) Overall abundances of amphipods increased s i g n i f i c a n t l y from May - July i n a l l three study areas. S i g n i f i c a n t increases i n amphipod abundance occurred i n both sediment and d r i f t over time, but there were no s i g n i f i c a n t differences i n amphipod abundance among months on Z. marina. 3) With the exception of June, there was no s i g n i f i c a n t trend i n the abundance of amphipods associated 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 that were c o l l e c t e d i n the monotypic and mixed stands of Zostera. 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 area 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 either the mixed species stand or i n the area of high density of Z. marina. 4) S i g n i f i c a n t l y more amphipods • m ^ of plant surface were recorded i n the d r i f t than were c o l l e c t e d i n the eelgrass i n Areas 1 and 2. 5) Overall d e n s i t i e s of amphipods i n the sediment were negatively correlated with both d r i f t and t o t a l macrophyte biomass and surface area. In contrast, numbers i n the plant samples were correlated p o s i t i v e l y with either eelgrass or d r i f t biomass and surface area per quadrat, i n most months and areas. 159 Part C. L i f e Cycles and Size-Frequency D i s t r i b u t i o n s As previously described, the c o l l e c t i o n s of amphipods made i n both the twelve-month and the three-month studies were dominated by four species. Figures 36, 37, 38, and 39 i l l u s t r a t e the s i z e frequency d i s t r i b u t i o n s and reproductive seasonality of these amphipods: Corophium  acherusicum, C. insidiosum, Ampithoe v a l i d a , and Anisogammarus  pugettensis, respectively. 1. Size-Frequency D i s t r i b u t i o n s . Figures 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 C. insidiosum, r e s p e c t i v e l y . Both of these species experienced peak d e n s i t i e s i n the l a t e summer and autumn, and decreased i n abundance throughout the winter and spring. (Figs. 36 and 37)7 Corophium acherusicum was c o l l e c t e d i n a l l months except A p r i l , while C. insidiosum was absent i n both March and A p r i l . Since Corophium spp. juveniles could not be accurately i d e n t i f i e d , t h e i r numbers were apportioned between the two Corophium species on the basis of the monthly r e l a t i v e abundance of each adult population, for the purpose of the histograms. With the exception of July 1983 and March 1984, Corophium juveniles were present i n a l l months i n which the adults were c o l l e c t e d ; however, they never contributed more than 14% to the monthly abundance of the combined adult populations. Consequently, i t was d i f f i c u l t to determine when an appreciable recruitment of juveniles occurred. However, since the smallest i n d i v i d u a l s of both species were c o l l e c t e d i n June and July 1984 (0.10-0.13mm head length), 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 is shown in bold numerals to the right of the ordinate axis in each month. I males • G B I B o v i g fern [ rep fem nonrep fem j uveniles 161 H e a d L e n g t h ( m m ) 162 Figure 37. Size-frequency distributions of Corophium insidiosum collected in the intercauseway area of Roberts Bank, August 1983 - July 1984. N is shown in bold numerals to the right of the ordinate axis in each month. males i I I rep fem V//////A ^ nonrep fem — juveniles 163 H e a d L e n g t h ( m m ) 164 males and immature females were c o l l e c t e d from August - November, peak recruitment probably occurred during the summer (Figs. 36 and 37). There was only small v a r i a t i o n i n mean head length between months i n either the Corophium acherusicum or the C. insidiosum pojDulations from J u l y 1983 - July 1984 (Figs. 36 and 37). In the C. acherusicum population, mean head length ranged from 0.30-0.32mm and revealed no consistent trend i n growth between months. However, an increase i n the minimum size was recorded from September-March, i n d i c a t i n g that 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 the winter. Due to the long rostrum of male C. insidiosum, the mean head length of these animals was s l i g h t l y larger than that of the C. acherusicum population and ranged from 0.31-0.36mm ( F i g . 37). But once again, except for an increase i n minimum size from September to November, no consistent trend 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, periods i n which the population was dominated by juveniles ( F i g . 38). 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 probably occurred i n the autumn as well as i n the spring i n t h i s species because although juveniles were c o l l e c t e d throughout the summer, the smallest i n d i v i d u a l s (0.16mm head length) were recorded i n August ( F i g . 38). The largest mean head lengths were recorded during the winter, 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 the maximum and the mean head lengths decreased ( F i g . 38). The minimum mean size (0.43mm) was recorded i n July 1984. As i n the Ampithoe v a l i d a populations, the periods of peak abundance of Anisogammarus pugettensis, which occurred i n July 1983 and 165 Figure 38. Size-frequency distriubtions of Ampithoe valida collected in the intercauseway area of Roberts Bank, August 1983 - July 1984. N is shown in bold numerals to the right of the ordinate axis in each month. males ovig fem rep fem nonrep fem juveniles 166 H e a d L e n g t h ( m m ) 167 May 1984, were months i n which the population was dominated by large numbers of juveniles ( F i g . 39). Minimum head lengths (0.17mm) were recorded i n May and June 1984 and peak recruitment probably occurred at that time. There was an increase i n both minimum and mean head s i z e from July - October 1983 i n d i c a t i n g that these 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 into the autumn. The largest mean head lengths were observed i n September and October. Smallest mean head lengths were recorded i n June and July 1984 ( F i g . 39). Few Anisogammarus  pugettensis were c o l l e c t e d i n the autumn, a time when other amphipod populations were at t h e i r peak; none was c o l l e c t e d from November 1983 -A p r i l 1984. 2. Reproductive Seasonality Reproductive females (with setous brood plate s ) and ovigerous females of both Corophium species were present i n r e l a t i v e l y low numbers throughout the year (Figs. 36 and 37). In the C. acherusicum population, they were most abundant i n September, when they contributed 27% of the t o t a l numbers. No ovigerous females were c o l l e c t e d i n December or January when the number of females with setous brood plates was p r i m a r i l y made up of large post-reproductive i n d i v i d u a l s (Fig.36). Reproductive and ovigerous females were r e l a t i v e l y more abundant i n the Corophium insidiosum population during c e r t a i n months than they were i n the C. acherusicum population. For example, i n July and August 1983, just p r i o r to the September peak i n population density, such accounted for between 25-40% of the t o t a l numbers ( F i g . 37). By November, however, as the population declined, ovigerous females had disappeared and post-reproductive females represented less than 9% of the t o t a l abundance. No reproductive females were c o l l e c t e d i n January, 168 Figure 39. Size-frequency distributions of Anisogammarus  pugettensis collected in the intercauseway area of Roberts Bank, August 1983 - July 1984. N is shown in bold numerals to the right of the ordinate axis in each month. 1 males HBHHi fem I rep fem nonrep fem Er~~ ''---^  juveniles 169 H e a d L e n g t h ( m m ) 170 perhaps due to the small sample s i z e , and ovigerous females d i d not reappear i n the study area u n t i l May (F i g . 37). Nelson (1980b) observed that many gammaridean amphipods, including Corophium acherusicum, mature at smaller sizes i n the summer than they do i n the winter. This seasonal trend was not observed i n the Corophium populations sampled i n the present study. The largest d i f f e r e n c e i n mean head length of reproductive C. acherusicum females occurred between December (0.36mm) and January (0.39mm); reproductive C. insidiosum females ranged i n head length from 0.34mm i n July 1983 to 0.40mm i n July 1984. In both species the minimum female siz e at reproductive maturity, r e f l e c t e d i n head length measurements of 0.22mm i n each case, occurred i n September. The maximum length of reproductive females occurred i n September and November i n C. acherusicum (0.58mm), and i n July 1984 i n C. insidiosum (0.54mm). Mature C. acherusicum females were generally larger than C. insidiosum females i n a l l months except July 1984. Males were r e l a t i v e l y abundant i n both Corophium populations i n most months (Figs. 36 and 37; Tables 24 and 25). They accounted for at le a s t 40% of the adult C. acherusicum population i n a l l months except August 1983 and March and Jul y 1984. In March, the sex r a t i o of males:females declined to 0.45, and i n August and Jul y i t f e l l to 0.59 (Table 24). This 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. insidiosum population, perhaps due to smaller sample s i z e s . It ranged from 0.12 i n December to 1.00 i n August and November during the peak i n abundance of th i s species (Table 25). Ovigerous Ampithoe v a l i d a were present throughout the summer and autumn but were absent i n November, when post-reproductive females accounted for approximately 15% of the population ( F i g . 38). No 171 T a b l e 24. Monthly p e r c e n t a g e s of males, f e m a l e s , and o v i g e r o u s f e m a l e s , and t h e male:female sex r a t i o s i n t o t a l numbers of a d u l t Corophium  ach e r u s i c u m c o l l e c t e d i n a l l samples, 1983-84. Date T o t a l A d u l t s % Male % Female % O v i g . Female Male:Female 11 J u l y 1983 39 51 49 13 1.05 6 Aug. 604 37 63 10 0.59 4 Sept. 1446 44 56 14 0.79 25 Oct. 1725 44 56 7 0.79 23 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 17 A p r i l 1 100 0 0 -15 May 142 56 44 6 1.27 27 June 418 47 53 4 0.89 26 J u l y 1242 37 63 11 0.59 TOTAL 6772 2886 3886 565 0.74 172 T a b l e 25. Monthly p e r c e n t a g e s of males, f e m a l e s , and o v i g e r o u s f e m a l e s , and t h e male:female sex r a t i o s i n t o t a l numbers of a d u l t Corophium  i n s i d i o s u m c o l l e c t e d i n a l l samples, 1983-84. Date T o t a l A d u l t s % Male % Female % O v i g . Female Male:Female 11 J u l y 1983 18 22 78 28 0.28 6 Aug. 161 50 50 20 1.00 4 Sept. 234 40 60 9 0.67 25 Oct. 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 A p r i l 3 67 33 0 2.03 15 May 54 37 63 9 0.59 27 June 92 28 72 14 0.39 26 J u l y 345 45 55 7 0.82 TOTAL 1061 426 635 326 0.67 173 reproductive or ovigerous females were c o l l e c t e d again u n t i l June 1984. Females of t h i s species reached reproductive maturity at a smaller mean size i n the summer than they d i d i n the autumn. The minimum reproductive size was recorded i n July 1984 when females with setous brood plates and with head lengths of only 0.42mm were c o l l e c t e d ( F i g . 38). The largest females were c o l l e c t e d i n October and measured up to 1.32mm i n head length (or 15.0mm i n t o t a l body length). With the exception of July 1983, when male Ampithoe v a l i d a accounted for 59% of the adult population, males were c o l l e c t e d less frequently than were females and the sex r a t i o ranged from 0.52 i n August to 0.12 i n October (Table 26). No males were c o l l e c t e d i n November. The most disproportionate sex r a t i o (0.09) was recorded i n June 1983 at a time when the population was dominated by nonreproductive females. The absence of Anisogammarus pugettensis from the November - A p r i l c o l l e c t i o n s was i n dramatic contrast to the seasonal pattern of abundance that had been observed i n the previous winter. In December 1982 and January 1983, many precopulating males and females were observed near the study s i t e s , and i n two samples of Zostera marina c o l l e c t e d i n early January, 67% of the adults c o l l e c t e d were ovigerous females. Several of these females were brooding juveniles i n t h e i r marsupia. Ovigerous females were also c o l l e c t e d i n March 1983 in d i c a t i n g that breeding and recruitment can occur year-round i n t h i s species. Year-round recruitment, however, d i d not occur i n 1983 - 1984, at least i n the study s i t e s . Despite the presence of r e l a t i v e l y large numbers of reproductive and ovigerous females i n September 1983, there 174 T a b l e 26. Monthly p e r c e n t a g e s of males, f e m a l e s , and o v i g e r o u s f e m a l e s , and t h e male:female sex r a t i o s i n t o t a l numbers of a d u l t Ampithoe  v a l i d a c o l l e c t e d i n a l l samples, 1983-84. Date T o t a l A d u l t s % Male % Female % O v i g . Female Male:Female 11 J u l y 1983 17 59 41 24 1.44 6 Aug. 84 34 66 24 0.52 4 Sept. 68 28 72 19 0 .39 25 Oct. 79 11 89 9 0.12 23 Nov. 11 0 100 0 -21 Dec. 5 20 80 0 0.25 15 May 1984 6 17 83 0 0.20 27 June 13 8 92 8 0.09 26 J u l y 33 33 66 9 0.50 TOTAL 316 81 235 48 0.34 175 was no corresponding increase i n the abundance of juvenile Anisogammarus  pugettensis c o l l e c t e d i n the ensuing months ( F i g . 39). The October samples were dominated by large post-reproductive adults. By November t h i s species had disappeared from the study s i t e s . Even i n the spring when A. pugettensis juveniles were abundant, few reproductive females were co l l e c t e d , i n d i c a t i n g that the adults were probably present outside of the study area. The minimum reproductive s i z e of Anisogammarus pugettensis females was recorded i n June 1984 when females approximately 0.40mm i n head length bore setous brood plates (Fig. 39). The largest females (1.55 and 1.68mm i n head length) were c o l l e c t e d i n September 1983 and May 1984, respectively ( F i g . 39). Anisogammarus pugettensis females were generally more abundant than were males, with the sex r a t i o ranging from 0.43 i n July 1983 to 0.08 i n September 1983 (Table 27). Males also accounted for only 8% of the adult population i n July 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. insidiosum, Ampithoe v a l i d a , and Anisogammarus pugettensis each has an annual l i f e cycle with peak juvenile recruitment during the spring andsummer. Ovigerous females and juveniles were also observed i n September and October i n the populations of C. insidiosum, A. v a l i d a , and A. pugettensis, and i n November i n that of C. acherusicum. These r e s u l t s i n d i c a t e an extension i n the period of reproductive a c t i v i t y described for these species by Bousfield (1973, 1979). In fa c t , based on preliminary data, A. pugettensis can probably reproduce year-round. Although the mean s i z e of Corophium  acherusicum and C. insidiosum d i d not vary over the one-year period, 176 T a b l e 27. Monthly p e r c e n t a g e s of males, f e m a l e s , and o v i g e r o u s f e m a l e s , and t h e male:female sex r a t i o s i n t o t a l numbers of a d u l t Anisogammarus 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. Date T o t a l A d u l t s % Male % Female % O v i g . Female Male:Female 11 J u l y 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 -15 May 1984 167 28 72 10 0.39 27 June 94 26 74 1 0. 35 26 J u l y 12 8 92 0 0.09 TOTAL 448 111 337 46 0.33 177 there was a decrease i n the mean size of both Ampithoe va l i d a and Anisogammarus pugettensis from the autumn to the spring. This decrease was also reflected i n the mean size 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 unity i n most months, male amphipods were generally less abundant than females. 178 DISCUSSION A primary focus i n marine ecology 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 competition and predation i n determining the d i s t r i b u t i o n and abundance of species. In studies of the rocky i n t e r t i d a l system, competition for food and space have been shown to govern the d i s t r i b u t i o n not only of animals, but of plants (e.g. Dayton, 1971; 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 are a l s o important i n determining the structure of communities i n seagrass systems (e.g. Heck and Thoman, 1981; Robertson and Mann, 1982 ). In ad d i t i o n , the p h y s i o l o g i c a l stress caused by extremes i n physical f a c t o r s such as temperature and s a l i n i t y may a l s o influence the structure of communities i n both rocky shore and seagrass systems. An obje c t i v e of the present study has been to examine the r o l e of habitat heterogeneity i n moderating the e f f e c t s of both b i o l o g i c a l and p h y s i c a l 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 Zostera marina meadow. In heterogeneous environments, the a b i l i t y of species to coexist i s enhanced by the a v a i l a b i l i t y of a wide v a r i e t y of microhabitats. Within these microhabitats, organisms may shelter from the often detrimental influences of both p h y s i c a l factors and b i o l o g i c a l i n t e r a c t i o n s (Pielou, 1975). Dense stands of seagrass, for example, can accommodate more animals and a greater number of species than can sparsely vegetated areas or barren mudflats, by v i r t u e of increased l i v i n g space. It i s not s u r p r i s i n g , therefore, that seagrass shoot density i s often co r r e l a t e d p o s i t i v e l y with d i v e r s i t y and abundance of macrofauna within the seagrass bed (Stoner, 1980a; Heck & Thoman, 1981; Homziak et a l . , 1982). In the present study, however, while the o v e r a l l 1 7 9 abundance o f gammarid amphipods 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 t h e b iomass o f l i v i n g Z o s t e r a m a r i n a , t h e r e was no r e l a t i o n s h i p between s p e c i e s d i v e r s i t y and e e l g r a s s shoot d e n s i t y . D e s p i t e d e n s i t i e s as h i g h as 26 ,820 amphipods • m~ 2 , o n l y 19 s p e c i e s were c o l l e c t e d a n d , o f t h e s e , f o u r a c c o u n t e d f o r more than 80% o f t h e t o t a l abundance i n most months . In c o n t r a s t , N e l s o n (1979a) c o l l e c t e d as many as 30 amphipod s p e c i e s i n a s i m i l a r s t u d y o f a Z o s t e r a m a r i n a meadow i n N o r t h C a r o l i n a , d e s p i t e lower l e v e l s o f abundance (~6000 " m ' ). A l s o , u n l i k e the s i t u a t i o n i n N e l s o n ' s (1979a) s t u d y , t he amphipod community o f R o b e r t s Bank was domina ted by e p i b e n t h i c and e p i f a u n a l s p e c i e s ; none o f t h 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 was i n f a u n a l . S p e c i e s w i t h s i m i l a r r e s o u r c e r e q u i r e m e n t s c an c o e x i s t and rema in p o t e n t i a l , r a t h e r than a c t u a l , c o m p e t i t o r s p r o v i d e d none o f t h e i r p o p u l a t i o n s i s r e s o u r c e - l i m i t e d and t h e y a r e r e g u l a t e d by an e x t e r n a l f a c t o r o t h e r than t h e s h o r t a g e o f t h e sha red r e s o u r c e ( P i e l o u , 1975 ) . Few s t u d i e s s u p p o r t t h e n o t i o n t h a t f o o d i s a l i m i t i n g r e s o u r c e f o r h e r b i v o r e s and d e t r i t i v o r e s such as amphipods i n s e a g r a s s sy s tems . A l t h o u g h Zimmerman e t a l . (1979) d e m o n s t r a t e d t h a t f o u r d i f f e r e n t s p e c i e s o f amphipods i n a s e a g r a s s community i n F l o r i d a a r e c a p a b l e o f p a r t i t i o n i n g t h e i r f o o d r e s o u r c e s i n terms o f b o t h p a r t i c l e s i z e and p l a n t s p e c i e s , t h e y d i s c o u n t e d t h i s f i n d i n g as an a r t i f a c t on t h e b a s i s o f an a p p a r e n t overabundance o f f o o d . O b s e r v a t i o n s wh i ch were made i n the p r e s e n t s t u d y p r o v i d e some s u p p o r t f o r t h e s e c o n c l u s i o n s . A l t h o u g h t h e r e was a l a r g e v a r i a t i o n i n t h e q u a n t i t y o f d i a t o m s a s s o c i a t e d w i t h the two s p e c i e s o f Z o s t e r a d u r i n g t h e summer, t h i s d i d no t appear t o i n f l u e n c e t h e d i s t r i b u t i o n o f amph ipods . In June and J u l y t h e l e a v e s o f Z. j a p o n i c a were c o a t e d w i t h d i a t o m s w h i l e t h o s e o f t he Z. m a r i n a were c o m p a r a t i v e l y c l e a n . S i n c e d i a toms a r e an i m p o r t a n t s o u r c e o f f o o d f o r 180 grazing amphipods, i f food a v a i l a b i l i t y i s a factor i n choice of substrate, 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 Z. japonica than i n the Z. marina during those months. More amphipods were i n fact c o l l e c t e d i n Area 2, i n a monospecific stand of Z. marina, than were found i n Area 1, the mixed stand of Zostera species. This suggests that, at least i n the summer, amphipods do not se l e c t plant substrates on the basis of food a v a i l a b i l i t y . It further suggests that during the period of peak abundance of amphipods i n the summer and autumn, when competition for food would be most l i k e l y to occur, food i s not a l i m i t i n g resource. Competition for the second p o t e n t i a l l y l i m i t i n g resource, space, may on the other hand be important i n st r u c t u r i n g assemblages of amphipods i n seagrass meadows. Since crustaceans, and p a r t i c u l a r l y amphipods, are an important component i n the d i e t of many birds and f i s h , as well as other invertebrates, i t i s usually assumed that t h e i r d i s t r i b u t i o n i n seagrass systems i s regulated by predation (Nelson, 1979b). But, because the i n t e n s i t y of predation i s often regulated by a factor such as plant density, competition for shelter among these prey species might be expected to occur (Heck and Thoman, 1981; Coen et a l . , 1981). Fis h may be the major predators on amphipods at Roberts Bank, p a r t i c u l a r l y during the summer. Several species of f i s h , i n c l u d i n g shiner perch, herring, and sculpins are abundant i n the intercauseway area i n t h i s season (Gordon and Levings, 1984). In addition, juvenile salmon enter the Fraser River estuary i n A p r i l and remain i n the eelgrass beds of Roberts Bank u n t i l August (Macdonald, 1984). During th e i r residency, these f i s h consume several species of amphipods, including Anisogammarus pugettensis, one of the most frequently 181 c o l l e c t e d species i n the present study. Individuals of t h i s species comprised more than 45% of the d i e t of j u v e n i l e chinook c o l l e c t e d by Macdonald (1984) on J u l y 9, 1982. One would expect, therefore, to f i n d higher concentrations of amphipods, and i n p a r t i c u l a r A. pugettensis, i n high shoot-density areas of the eelgrass meadow. No s i g n i f i c a n t d i f f e r e n c e i n amphipod abundance, however, was recorded between areas of d i f f e r e n t d e n s i t i e s of eelgrass shoots i n the summer of 1984. Although c e r t a i n species of epifaunal amphipods are known to exhibit behavourial a f f i n i t i e s for areas of high shoot density (Stoner, 1980c) i t i s possible that, i n t h i s case, the v a r i a t i o n i n shoot number * m A between the two study areas was too small to influence the amphipods. Seagrass grows i n much denser assemblages on the A t l a n t i c coast of North America, i n North Carolina, and i n F l o r i d a , where most of the studies r e l a t i n g abundance and d i v e r s i t y of associated fauna to shoot density have been done, than i t does i n the P a c i f i c Northwest. For example, Marsh (1973) reported Zostera marina d e n s i t i e s as high as 195 shoots ' 0.25m~2 i n the York River i n V i r g i n i a . In contrast, i n my study, maximum mean d e n s i t i e s of less than 100 Z. marina shoots ' m were recorded. Thus the d i f f e r e n c e i n shoot density between seagrass beds on the A t l a n t i c and P a c i f i c coasts may be one explanation for why the r e s u l t s of my study contrast with those of previous studies. It i s possible that even maximum de n s i t i e s of eelgrass shoots on Roberts"Bank (e.g. 310 shoots • m~2 ) (Harrison, 1984) do not approach the threshold l e v e l which i s necessary to decrease the rate of f i s h predation on amphipods. In a laboratory study, Heck and Thoman (1981) found that only a r t i f i c i a l grass d e n s i t i e s as high as 674 shoots ' m fc were e f f e c t i v e i n reducing the rate of k i l l f i s h (Fundulus h e t e r o c l i t u s ) predation on grass shrimp. 182 Zostera japonica grows i n much denser assemblages than does Z. marina and, i f shoot density i s important i n reducing the i n t e n s i t y of predation, then more amphipods should be associated with t h i s species of seagrass. Monthly mean abundances of amphipods i n the mixed stand of Zostera i n the summer of 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 that than they were i n the two monospecific Z. marina study areas despite extremely high Z. japonica shoot d e n s i t i e s (~ 1500 shoots • n f 2 i n July) which occurred there. Nonetheless, there i s circumstancial evidence that predation i s important i n structuring the amphipod community of Roberts Bank. For example, a decrease i n mean body s i z e of amphipods from winter to summer, si m i l a r to that which occurred i n the Anisogammarus pugettensis and Ampithoe v a l i d a populations, has been correlated with summer abundance of s i z e - s e l e c t i v e predators such as f i s h i n a North Carolina seagrass meadow (Nelson, 1980a). In t h i s regard, Macdonald (1984) has shown that large numbers of A. pugettensis are consumed by juvenile chinook salmon during the summer i n the intercauseway area. In contrast, there was no trend i n eit h e r the Corophium acherusicum or C. insidiosum populations towards smaller mean sizes i n the summer. Nelson (1980a) noted a similar s i t u a t i o n i n a C. acherusicum population i n North Carolina and concluded that since s i z e - s e l e c t i v e predation seemed to have no e f f e c t on the s i z e d i s t r i b u t i o n of these animals, they may have been too small a prey item f o r p i n f i s h (Lagodon rhomboides), the p r i n c i p a l predator of amphipods i n that system. S i m i l a r l y , Corophium spp. are not as important a component of the d i e t of juvenile chinook as are other gammarid species on Roberts Bank (Macdonald, 1984). A further i n d i c a t i o n that predation may be important i n influe n c i n g the seasonal d i s t r i b u t i o n of Anisogammarus pugettensis and 183 Ampithoe v a l i d a relates to growth and the timing of reproductive maturity. The minimum s i z e at reproductive maturity i n both populations occurred during the summer and so coincided with maximum de n s i t i e s of juvenile salmon. Warm summer temperatures have been shown to increase the growth rate and moulting frequency of c e r t a i n amphipod species with the r e s u l t that the mean age and size at maturity i s reduced (Nair & Anger, 1979; Nelson, 1980a). This increased growth rate may be an advantage i n that i t shortens the period of time i n which an amphipod i s exposed to predation before i t can produce i t s f i r s t brood (Nelson, 1980a). Predation may also have been responsible for generating the disproportionate sex r a t i o s observed i n the Ampithoe v a l i d a and Anisogammarus pugettensis populations. 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 rates, u t i l i z a t i o n of separate habitats, aggregation, or emigration of one sex (Darnell,1962; Campbell & Meadows, 1974). In a laboratory study of the reproductive behaviour of A. v a l i d a , Borowsky (1983) found that males "c r u i s e " between tubes occupied by females and are consequently more abundant i n the water column. This behaviour could make them more vulnerable to predation than females, which r a r e l y leave t h e i r tubes (Borowsky, 1983). Since male A. v a l i d a accounted for only 8% of the adult population i n June 1984, these i n d i v i d u a l s may have been captured by f i s h predators during the summer. Si m i l a r l y , free-swimming Anisogammarus pugettensis are probably more vulnerable to f i s h predation than are epifaunal tube-dwellers such as Ampithoe v a l i d a . The reproductive behaviour of A. pugettensis may also increase i t s v u l n e r a b i l i t y to predation. The males of t h i s species clasp the females with t h e i r second gnathopods and may carry them i n t h i s p o s i t i o n for up to a week u n t i l the female moults and f e r t i l i z a t i o n 184 can occur (Chang, 1975). Low numbers of both males and reproductive and ovigerous females were recorded i n June and July 1984, months i n which, based on Macdonald's (1984) observations, peak f i s h predation was occurring. Nelson (1980a) found that amphipod species which aggregated were more a t t r a c t i v e to f i s h predators than were s o l i t a r y species. S i m i l a r l y , amphipods engaged i n precopular a c t i v i t y may provide a larger, more vulnerable target for f i s h predators than do those, such as Ampithoe v a l i d a , not ex h i b i t i n g t h i s type of behaviour. A l t e r n a t i v e l y , since the males and reproductive females were generally the largest i n d i v i d u a l s i n the population at any given time, i t may be that s i z e -s e l e c t i v e predation accounted for t h e i r low abundance i n the samples. Aggressive in t e r a c t i o n s r e s u l t i n g i n the displacement of one species by another from a preferred substrate have been described i n seagrass systems for amphipods (Meadows and Reid, 1966; Nagle,1968) and palaemonid shrimp (Coen et a l . , 1981). In the present study, i f i n t e r s p e c i f i c competition for shelter were occurring, i t would l i k e l y involve Anisogammarus pugettensis, which i n contrast to the other frequently c o l l e c t e d species, i s not a tube-dweller. Consequently, i t i s probably more vulnerable than they are to f i s h predation. With t h i s i n mind, c o r r e l a t i o n analyses of species abundances were used to investigate the p o s s i b i l i t y that interference competition was responsible for inf l u e n c i n g patterns of amphipod d i s t r i b u t i o n and abundance i n the'intercauseway eelgrass meadow. In t h i s type of analysis, s i g n i f i c a n t negative c o r r e l a t i o n s i n abundance between species may i n d i c a t e that one species i s d i s p l a c i n g another (Nelson, 1979a). Heck and Orth (1980a) predicted that interference competition leading to displacement would be more important i n areas of low to intermediate plant density than i t would be i n very dense assemblages where the 185 complexity of the habitat would reduce the intensity of predation. No negative correlations in abundance were observed between any of the amphipod species in the present study, in either low or high density areas of eelgrass, or in the stand of mixed Zostera species. It i s possible that the differences in shoot density among the study sites may have been inadequate to properly test this hypothesis, but these densities represent the maximum range available within the intercauseway area. Although far from conclusive in the absence of experimental data, the results of the correlation analyses support my findings of no significant difference in 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 limiting resource for amphipods in the eelgrass meadow. Negative correlations which did occur between some species when the data were analyzed with respect to amphipod abundance in individual months, rather than to their abundance in areas of different shoot density, could be explained in other ways. Fir s t l y , they may have been caused by variations in overall abundance. An example of this can be seen in the September data, when a significant negative correlation occurred between the two Corophium species and Ampithoe valida. Although the abundance of a l l three species increased throughout the autumn, in September the densities of the two Corophium species were more than ten-fold and 70-fold those of A. valida in the d r i f t and sediment, respectively. Secondly, the negative correlations may have been due to different requirements for substrate. An example of this type of situation occurred in October and January when Corophium  acherusicum were collected 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 probably deposit-feeding at the sediment surface i n those months, which would account for i t s r e l a t i v e l y high abundance i n the core samples, while Ischyrocerus sp., a grazer and suspension-feeder, would have been r e s t r i c t e d to the more stable eelgrass substrate. No negative c o r r e l a t i o n s were observed between these two species 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 only a few exceptions, the abundances of c l o s e l y r e l a t e d species such as Corophium acherusicum and C. insidiosum 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 generally be expected to occur between species that were reproductively synchronous and which shared similar substrate requirements. Although the timing of t h e i r peak abundances was one month apart, both Corophium species were present i n highest d e n s i t i e s i n both the d r i f t and the sediment throughout the autumn. The s i z e structure and seasonal patterns of reproductive a c t i v i t y of each population were also s i m i l a r . Few Corophium insidiosum were c o l l e c t e d i n the winter and spring, which res u l t e d i n nonsignificant c o r r e l a t i o n s between the abundances of these two species i n each month during t h i s time. No s i g n i f i c a n t d i f f e r e n c e i n either the o v e r a l l abundance of amphipods or species d i v e r s i t y was found between the four shoot-removal treatments or "disturbances" i n the twelve-month study. Although the rapid recovery of shoot density i n the treatment p l o t s was unexpected, the absence of an observed v a r i a t i o n i n the pattern of r e c o l o n i z a t i o n by the amphipods was not suprising for two reasons. F i r s t l y , since the treatments were performed o r i g i n a l l y to i n v e s t i g a t e the r o l e of shoot density i n s t r u c t u r i n g the associated amphipod community, only monthly c o l l e c t i o n s were made following the "disturbance". Zajac and Whitlatch 187 (1982), however, found that the timing of a disturbance s i g n i f i c a n t l y influenced the recovery of estuarine infaunal communities and that when a disturbance occurred i n the spring or the summer, as i t d i d i n my study, weekly observations were' necessary before changes i n community structure could be accurately i d e n t i f i e d . Depending on the extent of the disturbance, amphipod populations may not recover as quickly as those of other benthic invertebrates due to t h e i r r e l a t i v e l y low reproductive p o t e n t i a l and t h e i r seasonal patterns of reproduction (Simon & Dauer, 1977; den Hartog _ Jacobs, 1980). Adult amphipods, however, can r a p i d l y recolonize l o c a l patches (Santos _. Simon, 1980). Using mark-and-recapture techniques, Howard (1975) demonstrated that crustacean epifauna, such as amphipods, are extremely motile and that i n a 0.56m2 area of a seagrass meadow, there can be a 50% turnover i n these animals i n l e s s than one day. Thus the recolonization of the treatment plots may have begun within hours of the disturbance and, given the lack of substrate preference displayed by the dominant Corophium species, may have quickly reached ambient l e v e l s of abundance despite the low density of Zostera marina i n some of the p l o t s . Secondly, since no s i g n i f i c a n t difference i n the number of amphipods ' m 6 was found between the three areas of d i f f e r e n t Zostera d e n s i t i e s i n the summer of 1984, no measurable v a r i a t i o n s i n o v e r a l l abundance could be expected i n the more temporally ephemeral shoot d e n s i t i e s of the treatment p l o t s . In contrast to the o v e r a l l abundance of amphipods and the d e n s i t i e s of animals c o l l e c t e d on the two plant substrates i n the treatment 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 to the r e l a t i v e i n t e n s i t y of the "disturbance". For example, more amphipods • m~2 of sediment surface were c o l l e c t e d i n the 188 p l o t s i n which 75% of the above-ground shoots had been removed, than were c o l l e c t e d i n the control p l o t s . The reworking of sediments i n s o f t -bottom habitats often stimulates the growth of microbiota and the presence of these 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 regard, Gallagher et a l . (1983) noted that shallow-dwelling, tube-building, surface-deposit feeders (such as Corophium species), are often the dominant colonizers i n disturbed areas. The i n t e n s i t y of disturbance r e l a t i n g to shoot removal i n the treatment pl o t s was undoubtedly related to the degree to which the sediments were disrupted. In turn, the amphipods may have been a t t r a c t e d on the basis of how extensively the sediment had been reworked i n each p l o t . S i m i l a r l y , given that greater resuspension of sediments l i k e l y occurs i n bare or p a t c h i l y vegetated areas than i n densely vegetated areas, the amphipods c o l l e c t e d i n the core samples may a c t u a l l y have been responding to the absence of eelgrass. Although the presence or absence of a Z. marina plant within a quadrat d i d not a f f e c t the number of amphipods c o l l e c t e d i n core samples, a negative c o r r e l a t i o n between the o v e r a l l abundance of amphipods c o l l e c t e d i n the sediment and eelgrass biomass d i d occur over the one-year period. Although these r e s u l t s contrast with those of Stoner (1983) who found higher d e n s i t i e s of infaunal amphipods i n vegetated areas than i n nearby unvegetated areas i n a seagrass bed i n F l o r i d a , negative c o r r e l a t i o n s between macrophyte biomass and the abundance of amphipods c o l l e c t e d i n sediment cores have been previously reported. For example, Stoner (1980a) found that the infaunal amphipods, Ampelisca v e r i l l i and A. vadorum, accounted f o r 67.6% of a l l amphipods c o l l e c t e d i n an unvegetated study s i t e i n Apalachee Bay, F l o r i d a . These species made up a maximum of 10.2% of the t o t a l amphipod abundance i n a nearby mixed stand of Thalassia testudinum 189 . and Syringodium filiforme. Both species of Ampelisca are infaunal tube-dwellers and the heavy network of rhizomes in densely vegetated areas may make i t d i f f i c u l t for them to build their tubes. This d i f f i c u l t y would, in turn, account for their limited distribution in seagrass meadows. In contrast, Corophium acherusicum, the dominant species in 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 restricted by the presence of eelgrass. Instead, given that greater resuspension of the sediment probably occurs in bare patches within the Zostera marina meadow than i n densely vegetated areas, this deposit-feeding species may be responding to enhanced feeding conditions. This type of interaction could explain the overall inverse relationship between eelgrass biomass and density of shoots, and the number of amphipods collected in the sediment. It could also explain the positive relationship between their abundance and the extent of shoot removal or "disturbance" in the treatment plots. In the past, the role of d r i f t algae has been largely ignored in studies which have sought to explain the role of seagrass in structuring the associated macroinvertebrate community. For example, Stoner (1983) "carefully avoided" clumps of d r i f t algae in his study of tanaids and amphipods in the seagrass meadows of the Indian River, Florida. Other studies have shown that d r i f t algae, which are generally more abundant in seagrass meadows than in nearby unvegetated areas (Zimmerman & Livingston, 1979), do play an important role in the organization of seagrass communities (Hooks et a l . , 1979; Kulczycki et a l . , 1981). As an example, Jacobs (1980) attributed an increase in the abundance and diversity of the animal community in 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 Ulva sp. and the density of meiofauna associated with polychaete tube-caps. They a t t r i b u t e d t h i s to the e f f e c t s of the algae i n moderating ph y s i c a l stress and i n providing a d d i t i o n a l microhabitats for these animals. With only a few exceptions, s i g n i f i c a n t l y higher d e n s i t i e s of amphipods were observed on the d r i f t substrate, which p r i m a r i l y consisted of Ulva sp. and fragments of eelgrass blades, than on the l i v i n g Z. marina. Months i n which there was no s i g n i f i c a n t d i f f e r e n c e i n the number of amphipods c o l l e c t e d • m~2 on the two plant substrates included A p r i l and May, when the populations of the dominant species were at 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, as a r e s u l t although there were three- to n i n e - f o l d differences i n the mean density of animals on the two types of plant substrate, these differences were masked by the v a r i a b i l i t y of the data. In any case, although a l l of the dominant species were c o l l e c t e d on the eelgrass as well as i n the d r i f t , with the exception of Ischyrocerus sp., and to a le s s e r extent, A. v a l i d a , each was most abundant i n the d r i f t . This pattern of d i s t r i b u t i o n was a l s o apparent i n the r e s u l t s of the p r i n c i p a l components analysis i n which r e l a t i v e l y close associations between the Corophium species and A. pugettensis were observed. Given the 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 species and A. pugettensis, t h i s a ssociation was probably due to the c o l l e c t i o n s of large numbers of each of these species i n the d r i f t and sediment samples. I observed p a r t i c u l a r l y high d e n s i t i e s of amphipods within the f o l d s of the t h i n , membranous fronds of Ulva sp. during 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~2 of d r i f t surface were c o l l e c t e d , mean de n s i t i e s of less than 17 amphipods * m of eelgrass surface were recorded. This suggests that the supplementary microhabitats created by the presence of the d r i f t algae increased the carrying capacity of the eelgrass meadow, permitting the coexistence of a much higher density of amphipods than would be possible i n the absence of these substrates. Refuges associated with the f l o a t i n g mats of d r i f t algae, such as Ulva sp. or Enteromorpha sp., may provide more e f f e c t i v e shelter than those associated with the eelgrass and may permit the amphipods to achieve high d e n s i t i e s i n spite of possibly high predation pressure (Smith, 1972). Pielou (1975) ref e r s to t h i s type of s i t u a t i o n as the "nook-and-cranny" e f f e c t . Many i n d i v i d u a l s and species f i n d shelter i n such refuges, and only the "overflow", forced out by overcrowding, i s vulnerable to predation (Smith, 1972; Pielou, 1975). Although the wide t h a l l i of Ulva sp. do not protect faunal inhabitants against 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 area (Pomeroy & Levings, 1980), they may shade animals such as amphipods during prolonged periods of i n s o l a t i o n at low t i d e i n eelgrass beds, as well 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 predators. Macdonald (1984) found that Anisogammarus pugettensis was the primary amphipod target of juvenile chinook i n June and July. If the d r i f t algae does provide superior refuges, higher d e n s i t i e s of amphipods, p a r t i c u l a r l y target species, should be associated with these substrates. In f a c t , i n July 1984 i n Area 2, where the d e n s i t i e s of eelgrass shoots was r e l a t i v e l y low, the equivalent of more than 1900 A. pugettensis * m ^ of d r i f t surface were c o l l e c t e d . In comparison, fewer than 100 i n d i v i d u a l s * m of t h i s species occupied the Z. marina substrate. Therefore, i t appears that f l o a t i n g mats of Ulva that dominated the d r i f t during the summer may 192 have provided a more e f f e c t i v e refuge from predation for A. pugettensis than d i d the eelgrass. The s t r u c t u r a l heterogeneity that d r i f t algae can contribute to a seagrass system may a l s o enhance sediment deposition, thus providing a d d i t i o n a l food resources i n a l o c a l i z e d area (Hicks, 1977). Levinton (1985) demonstrated that additions of d e t r i t u s from Ulva rotunda enhanced the standing stocks of benthic diatoms, which i n turn provided an important nutrient source for gastropod grazers. Also, d r i f t algae can be an important food resource i n i t s own r i g h t (Pomeroy and Levings, 1980). In the present study, the increase i n amphipod den 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 coincide with the peak i n d r i f t biomass, but rather with months i n which the algae were decaying. This suggests that the d r i f t may have been more important as a food resource than as a refuge from predators and environmental stress during t h i s time. Its importance i n t h i s regard undoubtedly increased through the winter months when the d r i f t was made up e n t i r e l y of decomposing eelgrass, an important source of food for overwintering amphipods (Mann, 1975). In a review of the ecology of seagrass beds, Kikuchi and Peres (1973) proposed that a r e l a t i o n s h i p might be found between the seasonal changes i n seagrass biomass and the structure of the associated invertebrate community. This statement could probably be extended to include a l l i n t e r t i d a l marine macrophytes, at l e a s t i n northern temperate habitats where there i s a d e f i n i t e seasonal cycle of growth and degeneration, and thus could apply to d r i f t algae as well as to eelgrass. In such a system, deposit- and suspension-feeders would dominate the community during the season of plant decay when d e t r i t u s i s 193 abundant, whereas motile herbivorous fauna would be most numerous i n the spring when the plants and t h e i r associated epiphytes were growing. In the P a c i f i c Northwest, peak standing crop and shoot density of Zostera marina u s u a l l y occur from June - September with a gradual decline i n biomass and shoot number over the summer and autumn (Harrison, 1982a; P h i l l i p s , 1983). Large f l o a t i n g mats of d r i f t algae, most t y p i c a l l y Ulva sp., often f l o u r i s h i n the summer i n estuaries and these a l s o contribute s i g n i f i c a n t l y to the accumulation of d e t r i t u s i n eelgrass meadows. Detritus i s an important source of food for many species of amphipods, including most of the dominant species c o l l e c t e d i n the present study. Algae, unlike vascular plants such as Zostera marina, are high i n nitrogen, soluble organics, and ash and as a r e s u l t , they decompose rapi d l y , providing d e t r i t i v o r e s such as Corophium species with a food supply that i s r e l a t i v e l y e a s i l y assimilated (Tenore and Rice, 1979). In comparison, eelgrass contains i n h i b i t o r y substances such as phenolics which must leach out before i t can be u t i l i z e d as. food by amphipods (Harrison, 1982b). In t h i s regard, Robertson and Mann (1980) reported that Gammarus oceanicus would not eat Z. marina-derived d e t r i t u s u n t i l i t had aged for 60 days. The feeding a c t i v i t i e s of epibenthic amphipods are instrumental i n i n i t i a t i n g the fragmentation of eelgrass and i n stimulating the 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 micro f l o r a , which, i n turn, provide the amphipods with a nitrogen-rich nutrient source (Harrison & Mann, 1975). As a r e s u l t of i t s slow decomposition, eelgrass provides a long term', consistent food supply that i s p a r t i c u l a r l y important during winter periods of low primary p r o d u c t i v i t y (Mann, 1975). The complementary seasonality of alga- and eelgrass-derived d e t r i t u s may f a c i l i t a t e the expansion of amphipod populations, 194 which i n the present study, increased i n s i z e from l a t e summer to autumn. The l i f e cycles of Corophium acherusicum, and to a lesser extent, of C. insidiosum and Ampithoe v a l i d a , were synchronized with the seasonality of macrophyte biomass. Peak recruitment i n the l a t e summer, coupled with an abundant supply of food i n the autumn, made i t possible for these species to achieve much higher d e n s i t i e s than either Anisogammarus pugettensis or Ischyrocerus sp., both of which experienced peak recruitment i n the spring. This seasonal v a r i a t i o n i n structure of the eelgrass-associated amphipod community supports Kikuchi and Pere's (1977) p r e d i c t i o n that, i n seagrass systems, deposit and suspension feeders would be most abundant i n the season of plant decay, while herbivores would be most numerous during the spring. Both Corophium species are d i t r o p h i c and can switch from suspension-feeding to se l e c t i v e deposit-feeding depending on the a v a i l a b i l i t y of food types (Enequist, 1952; Nair & Anger, 1979). When the concentration of p a r t i c u l a t e matter i n the water column i s high, Corophium remain inside t h e i r tubes and use the i r pleopods to create a feeding current. Food p a r t i c l e s c a r r i e d i n t h i s current are d i r e c t e d i n t o the tube by the antennae and are transported to the mouth by the gnathopods and maxillipeds ( M i l l e r , 1984). When few organic p a r t i c l e s are suspended i n the water column, however, they may leave t h e i r tubes and switch to se l e c t i v e deposit feeding (Enequist, 1952; Nair and Anger, 1979). The a b i l i t y to switch feeding methods enhances the capacity of these animals to survive when c e r t a i n types of food are i n short supply. Corophium species maintained a presence i n the study s i t e year-round and accounted for more than 88% of the o v e r a l l amphipod abundance i n October when the organic content of the sediment was at i t s peak. 195 Similarly, Ampithoe valida, which like the Corophium species was most abundant in the autumn, i s also 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 particles 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 abi 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. valida to maintain i t s e l f in the eelgrass meadow in most months, and to exploit the increased food supply which occurs in late summer by producing large numbers of juveniles at that time. As the population increased in size through the autumn, so did the relative abundance of juveniles, from 44% in August to 73% in September. Both the seasonal distribution and feeding behaviour of Anisogammarus pugettensis and Ischyrocerus sp. differed from those of Ampithoe valida and the two Corophium species. The spring increase in the abundance of A. pugettensis and Ischyrocerus sp., both of which are herbivores, coincided with the new growth of the eelgrass plants and their attached epiphytes. The feeding methods of these species contrast with the detritivorous habits of Ampithoe valida 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 easily 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. Similarly, Ischyrocerus sp. is also an 196 algal grazer. As suggested by the bright green digestive tract this species l i k e l y consumes a variety of micro- and macroalgae which would be most abundant in the spring. As previously mentioned, due to the presence of inhibitory substances, few species of amphipods eat living eelgrass. Caprella laeviuscula, a caprellid amphipod which grazes on the microalgae on the surface of eelgrass blades (Caine, 1979) was also extremely abundant in the study area in the spring. The l i f e cycles of the Corophium species and Ampithoe valida appeared to be synchronized with the seasonality of eelgrass biomass. Peak recruitment in these populations occurred in the late summer and autumn. An increase in the abundance of seagrass-associated macrofauna in the season of plant decay, similar to that which I observed in the intercauseway area, has been reported in previous studies. For example, Marsh (1973) noted in a survey of the epifauna associated with eelgrass in Chesapeake Bay that the abundance of amphipods increased in the autumn despite a summer decline in Z. marina biomass. Mukai (1971) observed a similar increase in the abundance of amphipods during the decaying season of Sargassum in Japan. In the present study, overall abundances of amphipods peaked in October, as did 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 in amphipod density was followed by an abrupt decline, primarily due to a decrease in the abundance of Corophium species. Similar peaks and sharp declines have been observed in other studies of amphipod populations and have been attributed to a variety of physical factors such as changes in temperature or salinity (Mukai, 1971; Sheader, 1978). There was no apparent relationship between seasonal fluctuations in salinity and the decline in the abundance of amphipods in 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 increasing. The winter decline may have been caused instead by the e f f e c t of decreasing temperatures on reproductive a c t i v i t y , and b i r d predation, as well as the mortality of post-reproductive adults. In a study i n north-eastern England, Sheader (1978) observed a precipitous decrease i n the abundance of epibenthic Corophium insidiosum following peak d e n s i t i e s i n l a t e summer, s i m i l a r to that observed i n both Corophium populations i n the present study. He a t t r i b u t e d t h i s d e cline to an increase i n brood mortality, and to a decrease i n feeding a c t i v i t y and i n the number of a c t i v e l y breeding females. He found that mature females entered a " r e s t i n g " stage i n November and December when no young were produced. In the intercauseway area, no ovigerous C. acherusicum females were c o l l e c t e d i n December or January, while a l l i n d i v i d u a l s with setose brood plates appeared to be post-reproductive. This suggests that the females of t h i s species a l s o enter a r e s t i n g -stage i n the winter. In the C. insidiosum population, ovigerous females disappeared from the study s i t e i n November and d i d not reappear u n t i l May. Although the low winter abundance of t h i s species i n the study area was l i k e l y a r e f l e c t i o n of the seasonal d e c l i n e i n reproductive a c t i v i t y and the death of mature adults, i t may also have been caused by offshore migration as described by Bousfield (1973) for A t l a n t i c coast populations of C. insidiosum. The d e c l i n e i n the abundance of both Corophium species i n the study area i n December and January, and t h e i r absence i n March and A p r i l , suggests that P a c i f i c coast populations of Corophium may have a s i m i l a r seasonal pattern of migration. Bird predation may a l s o be important i n reg u l a t i n g abundances of amphipods during the winter and spring i n the intercauseway area. 198 Flocks of dunlin ( C a l i d r i s alpina) overwinter i n the foreshore areas of Roberts Bank and forage i n the Zostera marina meadows during low t i d e . During t h i s time they consume large numbers of Corophium spp., p a r t i c u l a r l y C. acherusicum (McEwan and Fry, 1985; McEwan, personal communication, 1985). Further evidence for the importance of predation by birds i n the regulation of abundances of amphipods has been provided by Gratto et a l . (1984) and H i c k l i n and Smith (1984). Gratto et a l . (1984) showed that Corophium volutator i s the preferred prey of semipalmated sandpipers i n the Bay of Fundy. H i c k l i n and Smith (1984) concluded that the behayour of C. volutator during ebb t i d e increased t h e i r v u l n e r a b i l i t y to b i r d predation. They observed that males of t h i s species do not burrow as soon as the t i d e recedes but instead, crawl about on the surface of the substrate for up to t h i r t y minutes. Thus, they can be e a s i l y located by sandpipers, which forage at the water's edge ( H i c k l i n and Smith, 1984). It i s possible that the absence of d r i f t cover and seasonal senescence of the eelgrass i n the winter on Roberts Bank may increase the exposure and thus the v u l n e r a b i l i t y of C. acherusicum and C. insidiosum to b i r d predation, for as previously mentioned these amphipods are most abundant at the sediment surface during t h i s time. In summary, no r e l a t i o n s h i p between the density of Zostera marina shoots and the abundance and d i v e r s i t y of gammarid amphipod populations was found i n the present study. The d i s t r i b u t i o n of the dominant species seemed to be regulated instead by the seasonality of macrophyte biomass. Numbers of amphipods peaked i n the autumn when the plants were decaying, and more animals were associated with the d r i f t and organic debris at the sediment surface than with the eelgrass plants. This pattern of d i s t r i b u t i o n r e f l e c t s two s t r u c t u r a l features of the Zostera-199 associated amphipod community. F i r s t l y , the community i s dominated by Corophium species. The summer abundance of these animals, unlike those of Ampithoe v a l i d a and Anisogammarus pugettensis, does not appear to be regulated by s i z e - s e l e c t i v e or v i s u a l predators such as f i s h . Consequently, i t i s not suprising that they d i s p l a y no preference for areas of high shoot density. Secondly, the maximum density of Zostera shoots on Roberts Bank may not represent the threshold l e v e l necessary to protect species of amphipods which are targets of f i s h predators. 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ZIMMERMAN, R., R. Gibson and J . Harrinton, 1979. Herbivory and de t r i v o r y among gammaridean amphipods from a F l o r i d a seagrass community. Mar.Biol.54: 41-48. 209 APPENDIX 1. 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 between head l e n g t h (x) and t o t a l l e n g t h (y) of the dominant s p e c i e s of amphipods. n = t o t a l number measured. 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 males and females was s i g n i f i c a n t l y d i f f e r e n t (p < 0.05). Species Equation R n Ampithoe v a l i d a y = 10.96x - 0.969 0.968 45 Anisogammarus p u g e t t e n s i s y = 12.2x - 2.43 0.940 33 Corophium acherusicum y = 1 2 . l x - 1.05 0.892 63 Corophium i n s i d i o s u m y = 5.66x + 0.403 0.792 19 (male) Corophium i n s i d i o s u m y = 9.40x - 0.064 0.834 22 (female) 

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