Bivalve grazing causes substantial mortality to an estuarine copepod population

Abstract

The escape responses of planktonic copepods to grazing by introduced clams in the San Francisco Estuary were estimated for several copepod species, and resulting loss rates were compared with overall mortality for one species. Experiments were conducted to determine the escape responses of both nauplii and copepodites to entrainment in the siphons of clams (Potamocorbula amurensis) and artificial siphons. Copepod nauplii escaped both classes of siphon about 75% of the time and copepodites were more capable of escaping than were nauplii. Escape probabilities were combined with field-based data on clam grazing rates to estimate the impact of clam grazing on copepod survival in the low-salinity region of the San Francisco Estuary. Clams removed nauplii of Eurytemora affinis at a rate of ~10%d−1 with a seasonal pattern from 5%d−1 in spring to 20%d−1 in summer–fall and considerable interannual variability. This range of loss rates was similar to the estimated range of total mortality rates for nauplii. Furthermore, the annual estimates of loss rate were negatively correlated with an index of decline in the annual cycle of the copepod population. By combining an understanding of copepod escape capabilities with extensive data on copepod abundance and clam grazing, this paper shows that clam grazing has persisted as a dominant control on the estuarine ecosystem and is implicated in the long-term depression in abundance of food organisms for several fish species that are also in decline.