Evidence for density-dependent effects of infauna on sediment biogeochemistry and benthic–pelagic coupling in nearshore systems

We report results of a laboratory experiment that examines the effect of density-dependent interactions among infauna on organism–sediment–seawater relationships. The experiment includes a time series documentation of feeding behavior of the depositand suspension-feeding bivalve Macoma balthica, concurrent with measures of sediment biogeochemical processes that are affected by different feeding modes. We hypothesized that feeding behavior and emigration rates might shift with increasing density, and that these shifts would have cascading effects on benthic primary productivity, sediment–seawater exchanges, and porewater concentrations of ammonium and silicate. Macoma individuals were maintained in aquaria at three different densities (46, 230, and 460 individuals m 2) that fall within their natural abundances in the Chesapeake Bay. Individuals fed mostly on suspended material throughout the experiment, resorting to deposit feeding behavior only at the highest densities. Disturbances on the sediment surface during deposit feeding periods were not sufficient to impact benthic primary productivity and the associated interception effect of microalgae on sediment–seawater exchanges, as seen in previous studies. However, the bivalves impacted sediment fluxes directly through bioturbational activity, and these effects showed significant density interactions. For ammonium, fluxes ranged from 1.3 to 3.7 mmolm 2 d 1 and generally did not increase as a function of increasing Macoma density. Rather, highest fluxes generally were observed in intermediate density treatments. For silicate, a different trend was observed. Fluxes tended to parallel density, and ranged from 6.4 to 13.5 mmolm 2 d 1, however, the relationship was not linear. These observations suggest that as infaunal density shifts, so does the balance of ammonium to silicate efflux. Thus, benthic population structure may impact water column processes not only through enhancing the flux, but also through alteration of elemental ratios. Experiments that uncover the strength of these interactions, and models that blend the actions of individuals, populations, and material components, would greatly enhance our understanding of the geochemistry of organism–sediment relations.