Kinetic modeling of Ag, Cd and Co bioaccumulation in the clam Macoma balthica:: quantifying dietary and dissolved sources

A biokinetic model was used to better understand Ag, Cd and Co concentrations in a population of the clam Macoma balthica in San Francisco Bay. Model parameters included laboratory-derived uptake- and loss-rate constants from food and water and field measurements of metal concentrations in food, overlying water and oxidized pore water. Parameters used in modeling were taken from recent studies of metal influx from dissolved sources and metal assimilation from ingested sediment, and from laboratory experiments and field measurements in this study. Assimilation efficiencies from surface sediments ranged from 12 to 22% for Ag, 6 to 13% for Cd and 8 to 20% for Co. Assimilation efficiencies from phytoplankton were higher than metal assimilation efficiencies from sediment, ranging from 36 to 42% for Ag, 47 to 55% for Cd and 27 to 33% for Co. Influx of dissolved metals from overlying water increased with increasing ambient concentration, with uptake-rate constants for Ag (0.34 1 g(-1) d(-1)) about an order of magnitude higher than for Cd and Co. Influx-rate constants for Ag and Cd from oxidized pore water were comparable to overlying water-rate constants, whereas the rate constant for Co influx from oxidized pore water was 3 times lower than that from overlying water. Efflux rates of all metals out of the clams ranged from 1 to 3% d(-1). To estimate the potentially bioavailable fraction of particle-bound metals, assumed to be the metal bound to particle surfaces, mean metal concentrations in shale were subtracted from metal concentrations in total sediment digestions. Metal accumulation was modeled for clams that were assumed to be surface deposit-feeding and those that were filter-feeding. By adding uptake from food (surface sediment or phytoplankton) and from dissolved sources (oxidized pore water or overlying water), the modelpredicted ranges of concentrations of Ag, Cd and Co in deposit-feeding clams are shown to be directly comparable to tissue concentrations in field-collected clams from San Francisco Bay. Thus, it appears that the parameters experimentally derived for M balthica are applicable to field conditions and that the model can account for the major processes governing metal concentrations in these clams. Further, through modeling, ingested sediment was shown to be a major source for Ag and Cd under all realistic environmental conditions, but Co accumulation was principally from the dissolved phase.