The invasive Asian clam Potamocorbula amurensis : potential effects on geochemical flux in South San Francisco Bay, California

Estuaries are dynamic and unique habitats which act as transition regions between fresh water and marine environments, and as receiving waters for many anthropogenic compounds. Chemical flux across the sediment/seawater interface, as influenced by sediment geochemistry and biota, can dramatically effect overlying water concentrations of metals and nutrients within an estuarine system. Benthic filter feeders, by mediating processes such as contamination transfer, biomagnification, and bioturbation have a major influence on the transport of materials across the sediment/seawater interface. Of particular concern in South San Francisco Bay, California is the invasive Asian clam Potamocorbula amurensis, which may act as an accelerator of contaminant transfer to higher trophic levels, including organisms consumed by humans. The purpose of this study was to evaluate the benthic community in South San Francisco Bay, with specific focus on P. amurensis and its effect on chemical fluxes. Using benthic flux chambers, the effects of P. amurensis on phytoplankton standing stock, benthic respiration, and particulate carbon/nitrogen removal rates were measured. Other benthic macrofaunal species were also collected, counted, and weighed to compare total respiratory biomass to that of P. amurensis, and to estimate the effects of these organisms on geochemical flux. During a period of low food availability (2-3 ug chl a /L seawater) and low densities (700 individual clams/m²), P. amurensis did not significantly increase fluxes of POC, PON, or chlorophyll a. The clams did, however, contribute to higher benthic respiration rates and higher fluctuations of suspended POC and PON concentrations. It is estimated that low filtration rates are common for much of the year (non-bloom conditions when food availability is low), but should increase dramatically during spring phytoplankton bloom periods when high food availability and quality stimulate filtration. The role of P. amurensis in benthic flux is therefore episodic depending on the time of year and food availability in the water column. As such, removal rates by P. amurensis may be important for accelerating the transport of both carbon and associated anthropogenic contaminants to the sediments in the South San Francisco Bay