Use of Hg, C and N stable isotopes as tracers of mercury sources and transfer through temperate coastal marine food webs.

Mercury (Hg) is a natural, toxic element whose cycle is disrupted by human activities. Marine organisms bioaccumulate and bioamplify this element throughout their food webs. Since Hg levels in some high trophic level, coastal species are nearing sanitary and health thresholds, better understanding of biogeochemical processes and mechanisms of Hg transfers is important.

The purpose of this work was to better discriminate the sources and transfers of French coastal Hg and its behavior in the first levels of coastal food webs using stable isotopes. Indeed, stable isotopic signatures of metals is very promising to trace processes and their transfers between biogeochemical reservoirs. Hg isotopes undergo mass dependent or independent fractionation (MDF and MIF, respectively), thereby potentially enabling to track biological processes (MDF), and transfer between geochemical reservoirs (MIF). The coupling of Hg isotopes, with those of carbon (C) and nitrogen (N) in biota that are used for the study of the structure and functioning of trophic networks, will clarify this contaminant's bioaccumulation processes.

The coupling of C, N, and Hg stable isotopes enabled us to investigate i) temporal and spatial isotopic variations at the regional scale on bivalve samples from the French mussel-watch network (ROCCH), and ii) the trophic transfer of Hg within the food-web of coastal model ecosystems from highly human-impacted coastal areas of France.

Spatially, Hg isotopes show a regional signature (Atlantic Ocean vs. Mediterranean Sea), which may come from different trophic regimes as shown by N and C stable isotopes. Temporal δ²⁰²Hg variations are observed near major estuaries, with a constant increase of δ²⁰²Hg since 1990 (-0.5 to 0.25‰), potentially indicating a change in Hg sources reaching the coastal zone. Isotopes of C and N confirm that mussels and oysters feed on similar trophic resources. Nevertheless, Hg isotopes in our model food-web (Mediterranean coast) exhibit different signatures between bivalves, plankton, and surficial sediment, consistent with different sources of Hg, or different extents of MeHg.

Financial support from Ifremer, Région Pays de la Loire, AERMC and EC2CO/Dril through the Pollusols, Scottti, and Trococo projects.