Predation by an introduced clam as the likely cause of substantial declines in zooplankton of San Francisco Bay

The clam Potamocorbula amurensls was introduced into the San Francisco Bay estuary (California, USA) In 1986 and became abundant in late 1987 Wlthin a year, chlorophyll concentrat~on and the abundance of adults of 3 common estuarine copepod species had declined by 53 to 91 %, providing an opportunlty to examine mechanisms by which benthic grazing might control the abundance of pelaglc populations Decllnes in chlorophyll and abundance of the 3 species of copepod coincided a p p r o m a t e l y with the geographic range of the clam population The decllne in abundance of the copepod Eurytemora affinis was accompanied by a decrease in the ratlo of nauplil to adults, but not in the raho of eggs to females Therefore the decline in abundance may be due to elevated mortahty of nauplii rather than food hmitation of reproductive rate We argue that direct predation by P amurensls is the cause of the reduced survlval of nauplii, and therefore of the depressed abundance of adults Expenrnentally determined clearance rates of P arnurensls on E a f f i n ~s nauphi averaged 0 11 1 clam-' d ' If that clearance rate applled in the fleld, the clams could remove 8 2 % of the n a u p h ~ d-' This removal late is sufficient to explain the observed rate of population decline P amurensls 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 a n important and heretofore overlooked, source of mortality In shallow waters Selectivity occurnng through differences in escape response and vertical position could make bivalve predation an lrnportant influence on biomass and species composltlon of inshore zooplankton