BENTHIC FAUNAL RESPONSES TO VARIATIONS IN PATCH DENSITY AND PATCH SIZE OF A SUSPENSION-FEEDING BIVALVE

Responses of benthic macrofauna and epibenthic predators/sediment dist urbers to controlled density (0, 12, 120, 600 and 1200 individuals m(- 2)) and patch size (0.25, 1.0 and 9.0 m(2)) manipulations of the vener id bivalve Austrovenus stutchburyi (Gray) were examined on an intertid al sandflat. Bivalve density manipulations greater than 600 m(-2) had a mixed influence on macrofauna colonisation; two species were reduced , three enhanced and three were unaffected. There was no clear functio nal group-related pattern as to which species were affected by Austrov enus. The most pronounced influence of high bivalve densities was the reduction in the abundance and alteration of the size-structure of pos t-set (ca. 250-360 mm) tellinid bivalves, Macomona liliana Iredale. Pa tch size manipulations of Austrovenus had no measurable effect on macr ofauna colonisation, probably because patch density (120 bivalve m(-2) ) was too low. While a variety of epibenthic predators were present at the study site, Austrovenus stutchburyi mortality estimates in both e xperiments were relatively low (0.01-0.03% individuals lost day(-1)) a nd were independent of both density and patch size. In contrast, the p roportion of nipped bivalve siphons was relatively high (11-37%) in bo th experiments. While proportions of nipped siphons were similar acros s the range of manipulated densities, siphon browsing was more than 2 times higher in 9.0 m(2) plots than 0.25 m(2) plots. Scale-dependent f oraging of sub-lethal marine benthic predators is previously unreporte d and our results illustrate the necessity for conducting studies on p redator-prey dynamics at ecologically meaningful spatial scales for bo th the predator and prey species. (C) 1997 Elsevier Science B.V.