An examination of the factors influencing the bioaccumulation of methylmercury at the base of the estuarine food web.

Estuarine systems have received ongoing mercury (Hg) inputs from both point sources and regional contamination and have high legacy Hg in sediments. This is an environmental concern given that coastal seafood is an important vector for human exposure to methylmercury (MeHg). The base of the food chain represents the most important trophic steps for MeHg bioaccumulation. The magnitude of the uptake by phytoplankton, and their consumers, is influenced by many factors, in addition to sediment and water MeHg concentrations, that impact MeHg assimilation into phytoplankton and the trophic transfer to higher trophic levels, both benthic and pelagic. For forage fish, such as mummichogs (Fundulus heteroclitus), abiotic and biotic (bioenergetic) factors can influence their MeHg content, and diet is also important as they feed both on benthic and pelagic prey. Given that the importance of sediment MeHg versus pelagic MeHg sources has been debated, we updated a phytoplankton bioaccumulation model, and coupled this with a bioaccumulation model for MeHg concentration in mummichog tissue to examine the controlling factors for sites, from Maine to Maryland, USA, ranging widely in their Hg concentrations and other variables. The study highlighted the importance of DOC in modulating uptake into the pelagic food web, but also demonstrated the importance of diet in controlling mummichog MeHg. Finally, the relative importance of MeHg source - sediment or water column - was correlated with the level of Hg contamination. Sediment-derived MeHg was a more important source for highly Hg contaminated systems. As water column and sediment MeHg are not strongly correlated for the studied ecosystems, their importance as a source of MeHg to mummichogs varies with location. The study highlights the differences across ecosystems in MeHg bioaccumulation pathways, and that uptake into phytoplankton is an important variable controlling forage fish concentration.