Mercury (Hg) is a natural element toxic to all living organisms. Its ocean biogeochemical cycle is dominated by atmospheric deposition, which human activities contribute to disrupt signi cantly, and to a lesser extent by riverine discharge. This element is bioamplifed and bioaccumulated in marine food webs. since mercury concentrations in some coastal animal species of high trophic level approach sanitary thresholds, understanding biogeochemical processes and mechanisms leading to these elevated Hg concentrations becomes important. The stable isotopic geochemistry of metal is indeed a very promising way to trace processes and transfers from one mercury biogeochemical reservoir to another (Sonke and Blum, 2013). The stable isotopes of Hg undergo fractionation that can be either dependent (MDF or independent (MIF) of their mass, thereby potentially enabling to track biological (MDF) and transfer processes between geochemical reservoirs (MIF). Their coupling with stable isotopes of carbon (C) and nitrogen (N) in biota, clarifes the functioning of trophic networks, and the bioaccumulation processes of contaminants such as Hg.

Since the 1970’s, the Rocch, a Mussel Watch-like program, tracks contaminants on the french metropolitan coastline using of bivalves as quantitative indicators of coastal chemical contamination. The aim of the project was to map for the rst time and on a national scale the temporal and spatial isotopic variations of Hg, C and N. This work also attempts to better discriminate the sources and trace the origin of hg in bivalves (e.g., is Hg from the global ocean, associated with fluvial OM, or atmospheric local deposition...).

Along with its companion paper dealing with hg speciation, we will present here mercury isotopic composition data for our entire dataset. The initial results of mercury speciation suggest that total hg is not a limiting factor for hg methylation (briant et al., 2016). Thus, with global change and the predicted temperature rise, the associated increase in bacterial activity and eutrophication of coastal waters could increase MeHg production as proposed by soerensen et al. (2016). speci cally, preliminary hg isotope data show a regional signature (atlantic ocean vs. Mediterranean sea), which may come from di erent trophic regimes as shown by n and C stable isotopes. In contrast to marine-in uenced sites without temporal variations, river-in uenced sites show temporal δ202Hg increase (e.g., -0.5 to +0.25‰), suggesting a decadal change in the origin of hg reaching the ocean.

This study is part of the Trococo project, funded by Ifremer, région Pays de la Loire (Pollusols), and Insu/EC2Co/Dril.