Potential effects of an invasive bivalve, Nuttallia obscurata, on biogeochemical cycling in the intertidal

The varnish clam, Nuttallia obscurata, is a rapidly spreading invasive species that can reach high densities (i.e. 800 individuals m-2). A three-tiered approach involving a field survey (Tier I), a density manipulation experiment (Tier II) and a microcosm experiment (Tier III) was applied to determine the effects of this invasive bivalve on biogeochemical cycling in the intertidal zone . At natural densities, bivalve distribution was best explained by sediment grain size. High densities of varnish clams did not significantly increase organic matter concentrations; although their ability to deposit feed and bioturbational activities may have prevented accumulations. High densities of varnish clams resulted in significantly higher concentrations of ammonium and percent silt. Nitrogen is a limiting nutrient with ammonium preferentially used by phytoplankton and microphytobenthos. These primary producers form the basis of all marine food webs; thus, changes in the concentration and flux of ammonium may impact ecosystem functioning of the intertidal area.