Accumulation and retention of metals in mussels from food and water: A comparison under field and laboratory conditions

Mussels are used as bioindicator organisms to assess bioavailable contaminant concentrations in coastal waters. This study used radiotracer methodologies to examine the bioaccumulation and efflux rates of six metals (Ag, Am, Cd, inorganic Ca, organic Co [as cobalamine], Pb, and Zn) in the mussel Mytilus galloprovinciallis, in which the dissolved phase and ingested phytoplankton food were compared quantitatively as sources. Mussels maintained in the laboratory in running seawater and caged in the field for up to 4 months generally displayed similar metal absorption efficiencies (AEs) and efflux rates from their soft parts. AEs from food were typically lower than radioisotope retention efficiencies from the dissolved phase and ranged from 4% for Am to about 60% for Pb; values were comparable to those reported for the common blue mussel, Mytilus edulis, Efflux rate constants from mussel soft parts (1-5% daily) were comparable to M. edulis values from short-term depurations; biological half-lives of all metals in soft parts, shells, and whole mussels typically ranged from 10 to 60 days, although exceptions were noted. Metals obtained from food were primarily bound to soft part tissues; metals from the dissolved phase were mostly associated with the mussel shells. Higher AEs and lower efflux rates of cobalamine than of inorganic Co suggest that cobalamine is a more bioavailable form of cobalt for mussels. Mussel fecal pellets were enriched with metals following ingestion, with retention half-times of the metals in the pellets ranging from 18 days for Ag to 107 days for Am. Fecal pellet deposition beneath mussel beds may play an important role in the biogeochemical cycling of these metals in coastal ecosystems, The efflux rate constants and AEs can be used in kinetic models to estimate the relative importance of food and water as source terms for metals in mussels and to predict metal concentrations in mussels under different field conditions. A sensitivity analysis was performed to show that AEs are critical parameters influencing metal concentrations in mussels (particularly for Am, Ag, and Zn), and the results from this study indicate that laboratory-derived estimates of AEs are applicable to field conditions.