The effect of grazing by a gastropod, Nassarius obsoletus, on the benthic microbial community of a salt marsh mudflat

Abstract

Removal of Nassarius obsoletus (Nassaridae: Gastropoda) from a salt marsh mudflat dominated by epibenthic microalgae resulted in highly significant increases in algal and total microbial standing stocks as measured by chlorophyll a and ATP respectively. This effect occurred within three days after the removal of the grazer and was maintained for the duration of each experiment (10 and 12 days). Concomitant with an increase in standing stock was an increase in the productivity (mg C fixed m −2 h −1). The assimilation number [mg C (mg Chl a) −1 h −1], however, was not affected by grazer removal. Energy charge ratios, which assessed the growth state of the microbial community, increased in ungrazed areas during the first few days after consumer removal. Disruption of the sediment surface to stimulate bioturbation by the N. obsoletus population had no effect on algal productivity or on the standing stocks of algae and microbes suggesting that the grazer's influence is not mediated through physical disturbance. Simulated grazing using a lens paper removal technique resulted in standing stocks and productivity which were higher than N. obsoletus grazed areas and comparable to ungrazed areas. N. obsoletus grazing rates must have exceeded the 5–10% removal of surface chlorophyll per day measured by the lens paper technique. Natural densities of N. obsoletus reduced both algal biomass and productivity. Although previous studies have documented mechanisms of positive feedback from consumers to producers, in this intertidal mudflat community, grazing appears to act strictly as a negative feedback regulator of benthic algal activity.