PREDATION BY AN INTRODUCTION CLAM AS THE LIKELY CAUSE OF SUBSTANTIAL DECLINES IN ZOOPLANKTON OF SAN-FRANCISCO BAY

The clam Potamocorbula amurensis was introduced into the San Francisco Bay estuary (California, USA) in 1986 and became abundant in late 1987. Within a year, chlorophyll concentration and the abundance of adults of 3 common estuarine copepod species had declined by 53 to 91%, providing an opportunity to examine mechanisms by which benthic grazing might control the abundance of pelagic populations. Declines in chlorophyll and abundance of the 3 species of copepod coincided approximately with the geographic range of the clam population. The decline in abundance of the copepod Eurytemora affinis was accompanied by a decrease in the ratio of nauplii to adults, but not in the ratio of eggs to females. Therefore, the decline in abundance may be due to elevated mortality of nauplii rather than food limitation of reproductive rate. We argue that direct predation by P. amurensis is the cause of the reduced survival of nauplii, and therefore of the depressed abundance of adults. Experimentally determined clearance rates of P. amurensis on E. affinis nauplii averaged 0.11 l clam-1 d-1. If that clearance rate applied in the field, the clams could remove 8.2% of the nauplii d-1. This removal rate is sufficient to explain the observed rate of population decline. P. amurensis appears to have become well established, and copepod populations of the bay so far have failed to rebound. Thus this invasion may have permanent effects. In a broader sense, predation on zooplankton by soft-bottom benthos may be an important, and heretofore overlooked, source of mortality in shallow waters. Selectivity occurring through differences in escape response and vertical position could make bivalve predation an important influence on biomass and species composition of inshore zooplankton.