San Francisco Bay has recently been invaded by the suspension-feeding clam Potamocorbula amurensis. Previous work has shown that phytoplankton biomass in the upper estuary is low (2-3 mg Chl a m-3) during seasonal periods of high river flow and short residence time and it is usually high (peak >30 mg Chl a m-3) during the summer-autumn seasons of low river flow and long residence time. However since P. amurensis became widespread and abundant in 1987, the summer phytoplankton biomass maximum has disappeared, presumably because of increased grazing pressure by this newly introduced species. For 1977-1990, mean estimated primary production was only 39 g C m-2 yr-1 during years when bivalve suspension feeders were abundant (>2000 m-2), compared to 106 g C m-2 yr-1 when bivalves were absent or present in low numbers. These observations support the hypothesis that seasonal and interannual fluctuations in estuarine phytoplankton biomass and primary production can be regulated jointly by direct physical effects (eg river-driven transport) and trophic interactions (episodes of enhanced grazing pressure by immigrant populations of benthic suspension feeders). -from Authors
