Geochemical influences on assimilation of sediment-bound metals in clams and mussels

A series of experiments was performed to evaluate the extent to which Cd, C o, Ag, Se, Cr, and Zn bound to sediments with different geochemical propert ies could be assimilated by the mussel Mytilus edulis and the clam Macoma b althica. Oxidized and reduced radiolabeled sediments were fed to suspension -feeding animals, the depuration patterns of the individuals we re followed by gamma-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 we re assimilated from anoxic sediment by both animals; Ag, Cd, and Co AEs in M. balthica we re 9-16%, 2-fold lower th an from oxic sed im ent, 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 w ith increased sediment exposure time to the metals with smaller but signifi cant effects also noted for Zn and Se but not Ag. A less pronounced decreas e in AE for M. balthica was evident only after 6 months exposure time. Sequ ential extractions of the oxidized sediments showed a transfer of metals in to more resistant sediment components over time, but the rate did not corre late with a decrease in metal AEs. Comparing the two bivalves, TOC concentr ations 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 resul ts imply that metals associated with sulfides and anoxic sediments are bioa vailable, 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.