Assimilation efficiencies (AEs) and physiological turnover-rate constants (k) of six trace elements (Ag, Am, Cd, Co, Se, Zn)in four marine bivalves (Crassostrea virginica Gmelin, Macoma balthica Linnaeus, Mercenaria mercenaria Linnaeus, and Mytilus edulis Linnaeus) were measured in radiotracer-depuration experiments. Egestion rates of unassimilated elements were highest during the first 24 h of depuration and declined thereafter. Significant egestion of unassimilated Co, however, continued for up to 5d in Macoma balthica, Mercenaria mercenaria and Mytilus edulis. With the exception of the extremely low values for Ag-110m, Cd-109, and Zn-65 in C. virginica, physiological turnover-rate constants (ii) showed no general pattern of variation among elements, bivalve species or food types, and were relatively invariant. Values from less than or equal to 0.001 to 0.1 d(-1) were observed, but excluding those for Co, most values were less than or equal to 0.04 d(-1). In all four species, the AEs of Ag, Am, and Co were generally lower than those of Cd, Se, and Zn. The AEs of Ag, Cd, Se, and Zn in these bivalves are directly related to the proportion of each element in the cytoplasmic fraction of ingested phytoplankton, indicating that >80% of elements in a prey alga's cytoplasm was assimilated. C. virginica, Macoma balthica, and Mercenaria mercenaria assimilated similar to 36% of the Ag and Cd associated with the noncytoplasmic (membrane/organelle) fraction of ingested cells in addition to the cytoplasmic fraction. The ratio of AE:k, which is proportional to the consumer-prey trace-element bioaccumulation factor (concentration in consumer:concentration in prey) was generally greater for Cd, Se, and Zn than for Ag, Am, and Co. This ratio was lowest in Mytilus edulis, suggesting that this bivalve, the most widely employed organism in global biomonitoring, is relatively inefficient at accumulating important elements such as Ag, Cd, and Zn from ingested phytoplankton.
