Bioavailability of sediment-bound metals to marine bivalve molluscs: An overview

Coastal marine sediments are commonly enriched in metals, including potentially toxic trace metals, by natural processes and human activities. These sediments have long been regarded as the final repositories of contaminants, but in recent years it has been recognized that they can also serve as potentially important sources of metal contaminants for benthic organisms and benthic food chains in general. The geochemical and biological factors governing the bioavailability of diverse metals (Ag, Cd, Co, Cr, Se, Zn) that are bound to different kinds of marine sediments are reviewed. Particular attention has been paid to those species of marine bivalve mollusks that are used as bioindicators of coastal contamination. Both deposit-feeding and suspension-feeding bivalves can accumulate metals appreciably by assimilating sediment-bound metals that are ingested, although important differences have been recognized between these two feeding modes as well as between metals. The properties of the digestive tracts of deposit and suspension-feeding bivalves that influence metal bioaccumulation from food are also discussed. Through kinetic modeling, the relative importance of ingestion as a route of metal uptake has been compared quantitatively with uptake from the dissolved phase, including from pore water and from overlying water, and has been shown to account for the high concentrations in bivalve tissues for a number of contaminant metals.