Bivalves are important as consumers of pelagic and demersal food resources. With sufficient biomass, they can compete with and, in some cases, outcompete other members of the pelagic food web for food resources such as phytoplankton and zooplankton.  We include bivalves in the San Francisco Estuary (SFE) and Delta in the FLOAT MAST because they have been and continue to be an important member of the food web. Changes in the bivalves over the last 40 years have led us to hypothesize that bivalves may limit food resources for Delta Smelt and other pelagic fish species (Kimmerer and Thompson 2014, Sommer et al 2007).

We will discuss the two bivalves with the largest biomass, the estuarine bivalve Potamocorbula amurensis (hereafter Potamocorbula) and the freshwater bivalve Corbicula fluminea (hereafter Corbicula).  The distribution of these two species together covers the full range of salinities between the Golden Gate and the upstream reaches of the SR and SJR.  The species overlap in the region of X2.  Both species’ distributions expand or contract when the salinity distribution changes and do so in near synchrony with each other at a salinity of approximately 2.   It is this interaction of both bivalves at their threshold levels of salinity within the ecologically sensitive low salinity zone (LSZ) in addition to their distribution in the rest of the system that determines each species effect on the food web.