Understanding how animals are exposed to the large repository of metal pollutants in aquatic sediments is complicated and is important in regulatory decisions. Experiments with four types of invertebrates showed that feeding behavior and dietary uptake control bioaccumulation of cadmium, silver, nickel, and zinc. Metal concentrations in animal tissue correlated with metal concentrations extracted from sediments, but not with metal in porewater, across a range of reactive sulfide concentrations, from 0.5 to 30 micromoles per gram. These results contradict the notion that metal bioavailability in sediments is controlled by geochemical equilibration of metals between porewater and reactive sulfides, a proposed basis for regulatory criteria for metals.

A series of experiments was performed to evaluate the extent to which Cd, Co, Ag, Se, Cr, and Zn bound to sediments with different geochemical properties could be assimilated by the mussel Mytilus edulis and the clam Macoma balthica. Oxidized and reduced radiolabeled sediments were fed to suspension-feeding animals, the depuration patterns of the individuals were followed by γ-spectrometry, and the assimilation efficiencies (AEs) of ingested metals were determined. AEs from geochemically diverse sediments typically varied less than 2-fold and ranged from 1% for Cr to 42% for Zn. Metals were assimilated from anoxic sediment by both animals; Ag, Cd, and Co AEs in M. balthica were 9-16%, 2-fold lower than from oxic sediment, but in M. edulis AEs were about two times greater from anoxic sediment for all metals but Ag. For oxic sediment, Cd and Co AEs in M. edulis decreased 3-4-fold with increased sediment exposure time to the metals with smaller but significant effects also noted for Zn and Se but not Ag. A less pronounced decrease in AE for M. balthica was evident only after 6 months exposure time. Sequential extractions of the oxidized sediments showed a transfer of metals into more resistant sediment components over time, but the rate did not correlate with a decrease in metal AEs. Comparing the two bivalves, TOC concentrations had an inconsistent effect on metal AEs. AEs of metals from bacteria-coated glass beads were slightly higher than from humic acid-coated beads, which were comparable with whole-sediment AEs. There was correspondence of AE with desorption of Ag, Cd, Co, and Se (but not Zn) from sediments into pH 5 seawater, measured to simulate the gut pH of these bivalves. The results imply that metals associated with sulfides and anoxic sediments are bioavailable, that the bioavailability of metals from sediments decreases over exposure time, that organic carbon content generally has a small effect on AEs, and that AEs of sediment-bound metals differ among species.

Mussels have been extensively used as biological monitors of coastal contamination. This study measured the assimilation efficiencies (AEs) of Cr(III) and Cr(VI) in the mussel Mytilus edulis from ingested food, uptake from the dissolved phase, and the physiological efflux rates following uptake. A bioenergetic-based kinetic model was then employed to determine the relative contributions of different Cr species and their accumulation pathways to the overall concentration of Cr in mussels. The concentration factors of Cr(III) in four diverse marine phytoplankters ranged between 10 4 and 10 5 , whereas for Cr(VI) they were 2 to 5 × 10 2 . Rapid reduction of Cr(VI) to Cr(III) was observed in coastal sediments. AEs of Cr(III) in mussels from ingested sediments were consistently <1%, whereas the AEs of Cr(VI) from ingested phytoplankton were 1-10%. The uptake rate of Cr(VI) from the dissolved phase was 3 times higher than Cr(III). The efflux rate constant was 0.011 d -1 for mussels following 7 d dissolved uptake of Cr(VI) and 0.010 d -1 following 8 d ingestion of Cr(III)-labeled diatoms. The model predicted concentrations of Cr in mussels in South San Francisco Bay that are directly comparable to measured Cr concentrations. The model predicts that 13-38% of Cr in mussels is from dissolved Cr(VI), whereas the remaining Cr is from ingested Cr(III); dissolved Cr(III) and ingested Cr(VI) contribute little to Cr accumulation in mussels. The AE of Cr and its concentration in seston critically influence Cr concentrations in mussels.