EFFECTS OF PHYSICAL AND BIOLOGICAL DISTURBANCE ON ZOSTERA MARINA L. MACROBENTHOS (NORTH CAROLINA)

A marine macrobenthic community associated with eelgrass (Zostera marina L.) was investigated (1975-1979) at two sites in the Beaufort, North Carolina area. Experimental manipulation in the field was used to examine the response of the community to physical and biological disturbance. Physical disturbance was created by the removal of all plant material along with the top 5 cm of sediment in small areas (0.25 m('2)) in eelgrass meadows. Recovery of eelgrass plants (eelgrass and macroalgae) and animals was then monitored in disturbed and undisturbed plots for 21 months. Eelgrass in disturbed areas recovered to the level of undisturbed areas within 10 months at one site but deeper plots at another site had not fully recovered even after 21 months. Macroalgae occasionally drifted into depressions left during the disturbance at this second site. Macrobenthic invertebrate density and number of taxa were not significantly different in disturbed areas relative to undisturbed areas after 3 months if plant biomass was equivalent. Unlike many studies of disturbances in unvegetated soft sediment, my study did not detect short term increases in faunal density of opportunistic species. Predator exclusion cages (6 mm mesh) and cage controls were placed for 5 separate 3 month periods in eelgrass meadows to prevent predation, a biological disturbance. There was no response of most taxonomic groups, except bivalves and decopod crustaceans, to predator exclusion. Similar response of most fauna, including bivalves, to full cages and cage controls (regardless of response relative to uncaged controls) indicated a possible cage artifact. The response to cage controls and the presence of increased densities of small carnivorous decapods in cages suggest that the results of previous studies using predator exclusion cages in seagrass beds should be interpreted with caution. Other evidence concerning the importance of predation in regulating macrobenthic invertebrate abundance in seagrass beds is also equivocal. Previous reports that amphipod abundance is regulated by predation are not corroborated in this study. Seasonal patterns of predators and amphipod prey as well as relative abundance of amphipods (some amphipod species being more resistant to predation) did not follow patterns expected from the predation hypothesis.