TROPHIC TRANSFER OF SILVER TO MARINE HERBIVORES : A REVIEW OF RECENT STUDIES

We review recent progress in understanding the trophic transfer of silver (Ag) in marine herbivores, especially mussels that have been extensively used as biomonitors of coastal contamination. A bioenergetic- based kinetic model is invaluable in predicting the trophic transfer and bioaccumulation of Ag in aquatic animals. Critical parameters that need to be quantified in predicting trophic transfer include Ag assimilation efficiency (AE) from ingested food particles, animal feeding rates, and Ag efflux rates. Silver AEs in marine herbivores are generally low (<30%). Assimilation efficiencies from ingested sediments tend to be lower than those from ingested phytoplankton. Various biological and chemical factors, including Ag distribution in phytoplankton cytoplasm, gut passage time, importance of intracellular versus extracellular digestion, and metal desorption at lowered pHs typical of invertebrate guts, all influence Ag assimilation from ingested particles. Many experimental studies show that uptake from the dissolved phase exceeds uptake via ingestion in the overall Ag bioaccumulation in aquatic animals. However, these results are probably not predictive of field situations due to their simplistic experimental conditions in which fluctuations of feeding conditions of animals and physicochemistry of Ag are not considered. In mussels, the kinetic model predicts that either the solute or particulate pathway can dominate Ag overall uptake in nature, and this is dependent on Ag partition coefficients for suspended particles and Ag AE. Silver is the only metal that varies substantially in the importance of different uptake pathways due to its very high particle reactivity and high uptake rate from the dissolved phase. Total suspended solids (TSS) loads can sharply affect Ag bioaccumulation in mussels because high TSS loads can dilute Ag concentrations in both dissolved and particulate phases. Processes affecting Ag trophic transfer and bioaccumulation are discussed.