Abstract

Mercury has unique physico-chemical properties that give it a characteristic behavior in aquatic systems. The fate of mercury depends on the chemical form it takes, which determines its distribution between the water column, sediments, and biota, as well as its toxicity to aquatic organisms. In aquatic ecosystems, it is mainly in suboxic sediment conditions that mercury is likely to be methylated (Parks et al., 2013). Methylmercury (CH₃Hg⁺) is the most toxic form of mercury because it easily interacts with biological barriers to bioaccumulate and biomagnify along the trophic chain (Mason et al., 1995). As a result, methylmercury represents more than 80% of the total mercury concentration in the flesh of predator fish at the end of the trophic chain (Boudou and Ribeyre, 1997). Consequently, the European regulatory context requires the monitoring of mercury in surface waters to be carried out by sampling high trophic level fish ^[1]. This biosurveillance method has its limitations: the costs involved in fishing are significant, and it is impossible to sample all rivers due to the lack of sufficient high trophic level fish. The "gammarus cage" tool, developed by the RiverLy research unit (Besse et al., 2013), was adopted by French regulations in 2017 ^[2] as a complement to fish sampling. This active biosurveillance tool, easy to use and inexpensive to set up, allows more frequent surveys across nearly all rivers in France without impacting the ecosystem.

In this context, it is essential to better understand the fate of mercury and methylmercury in Gammarus fossarum (amphipod), both to address the lack of literature on the bioaccumulation of mercury species by low trophic level organisms and to determine how to apply the gammarus cage tool effectively for mercury biosurveillance.

Measuring methylmercury remains an analytical challenge that few laboratories in France can perform with precision. Methylmercury analysis in biological matrices is typically done by isotopic dilution (Clémens et al., 2011). The measurement is performed by gas chromatography coupled with inductively coupled plasma mass spectrometry (GC-ICPMS). The development and validation of the analytical method, which has been refined over several years by the LAMA team, forms the foundation of this project. An experiment involving the exposure of cage-dwelling Gammarus fossarum at 28 sites (rivers characterized by a strong gradient of HgT contamination observed in fish or amphipods) will measure the variability of mercury and methylmercury concentrations in the amphipod. Complementary work on the kinetics of methylmercury accumulation and depuration at both individual and sub-individual scales (organs) will further clarify the results and provide a finer understanding of the fate of mercury and methylmercury in Gammarus fossarum.

The operational goal of this project is to better understand the links between the different indicators available in mercury surveillance networks (mercury concentrations in water, sediment, biota, and methylmercury concentrations), in order to form the most representative image possible of mercury contamination in French rivers.

^[1] Directive Norme de Qualité Environnementale (2008/105/CE) : https://eur-lex.europa.eu/legal-content/FR/TXT/PDF/?uri=CELEX:02008L0105-20130913
^[2] Note technique de 2017 : https://www.legifrance.gouv.fr/download/pdf/circ?id=42938

References

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Funding

AQUA/INRAE & OFB