Mercury cycles between atmosphere, terrestrial and aquatic reservoirs through a complex combination of transport and transformation processes, until it is buried in oceanic or terrestrial sediments. Monomethylmercury is the main species bioaccumulated and bioampli ed in the food chain. Knowledge of the origin (continental, oceanicorlocalsediment)ofthisspeciesforthelittoralenvironnment can still be improved.

This study focuses on ~200 samples of bivalves (mussels: Mytilus edulis and Mytilus galloprovincialis and oysters: Crassostrea gigas
and Isognomon alatus) from ~80 stations along the French coast. The goals of this study were i) to describe Hg levels, speciation and isotopic composition in various bioindicator bivalves from the French coastline, and ii) to identify possible geographic, taxonomic or temporal variations in these properties.

In the bivalves, we observed that the variations in methylmercury concentrations follow linearly those of total mercury (HgT). Preliminary results of isotopic fractionation of HgT in some of the samples do show regional e ects and allow to differentiate English Channel, Atlantic, and Mediterranean coastlines. These differences may come from the various trophic regimes revealed by nC stable isotopes, from different fractionnation by the three biological species of bivalves, from different mercury source isotopic signatures, or both.

At most of the study sites, HgT concentrations have not decreased since 1987, despite regulations to abate or ban mercury used by anthropic activities.

This study is part of the Trococo project, funded by Ifremer, rgion Pays de la loire (Pollusols), and Insu/EC2CO/Dril.