Bioaccumulation kinetics and exposure pathways of inorganic mercury and methylmercury in a marine fish, the sweetlips Plectorhinchus gibbosus

We experimentally determined the assimilation efficiency (AE) of ingested prey, the uptake-rate constant from the aqueous phase, and elimination-rate constant of both inorganic Hg (Hg[II]) and methylmercury (MeHg) in a marine predatory fish, the sweetlips Plectorhinchus gibbosus, using radiotracer techniques. The AE of Hg(II) and MeHg ranged between 10 and 27% and between 56 and 95%, respectively, for 3 different prey (copepods, silverside, and brine shrimp). The ingestion rate of the fish did not significantly affect the AE of Hg(II). Uptake of both species of Hg proceeded in a linear pattern, and the calculated uptake-rate constant was 0.195 and 4.5151 g^-1 d ^-1 for Hg(II) and MeHg, respectively. Most of the accumulated Hg(II) was distributed in muscle tissues, whereas the accumulated MeHg was distributed evenly between gills and muscle tissues. The calculated elimination-rate constants for MeHg were 0.0103 and 0.0129 d^-1 following dietary and aqueous uptake, respectively, whereas the elimination-rate constant of Hg(II) following dietary uptake (0.0547 d^-1) was 1.9 times higher than the elimination following aqueous uptake (0.0287 d^-1). These experimentally determined values were incorporated into a kinetic model to predict the exposure pathways and the relative contribution of Hg(II) and MeHg to the sweetlips. At the high end of the bioconcentration factor for both species of Hg in the prey, dietary ingestion is likely to be the main channel for their accumulation in the fish. The relative contribution of Hg(II) vs MeHg to the overall Hg bioaccumulation is largely controlled by the relative concentration of MeHg dissolved in seawater. Similar to the results of numerous field studies, the kinetic model predicted a potential trophic transfer factor of <0.6 for Hg(II) and >1 for MeHg under conditions likely to be experienced by the fish in its natural environment.