The role of habitat heterogeneity in the community dynamics of an eelgrass-associated assemblage of gammarid amphipods

The role of density of eelgrass shoots in regulating distribution and abundance of gammarid amphipods was investigated. Monthly collections of amphipods were made over a one-year period in a series of treatment plots on Roberts Bank, in southwestern B.C., in which eelgrass (Zostera marina L.) shoots had been thinned to different relative densities. This experiment was originally designed to test the hypothesis that the abundance and diversity of amphipods would be positively related to the density of eelgrass shoots. Due to the rapid recovery of original shoot densities within the plots, however, this hypothesis could not be tested. Consequently, the emphasis of the study was restricted to a consideration of the effect of the disturbance created during removal of shoots on the distribution of amphipods. Collections of amphipods were also made in three areas of different natural densities of Zostera shoots during a three-month period in summer 1984, to assess further the effect of shoot density on the distribution and abundance of amphipods. The role of additional components of habitat heterogeneity, including drift algae and a second species of seagrass, Zostera japonica, in modifying the community dynamics of the amphipods was also studied. No relationship between the density of Zostera shoots and the abundance and diversity of amphipods was found. The amphipod community was dominated by Corophium acherusicum and the distribution of this species, as well as that of the other most frequently collected species, appeared to be regulated by the seasonality of macrophyte biomass. Peak abundances of amphipods occurred in the late summer and autumn when large amounts of drift algae and eelgrass detritus were present at the sediment surface. This decaying plant material is an important source of food for detritivores such as gammarids and its seasonal abundance was reflected in the rapid growth of populations of amphipods. The floating mats of drift algae, such as Ulva sp., also contributed significantly to the carrying capacity of the eelgrass meadow by providing spatial refuges to amphipods which are targets of fish and bird predation. The role of habitat heteogeneity in determining the distribution of the dominant species of amphipods, with reference to competition and predation, was discussed.Science, Faculty ofZoology, Department ofGraduat