Our study reports the first data on mercury (Hg) isotope composition in marine European fish, for seven distinct populations of the European seabass, Dicentrarchus labrax. The use of δHg and ΔHg values in SIBER enabled us to estimate Hg isotopic niches, successfully discriminating several populations. Recursive-partitioning analyses demonstrated the relevance of Hg stable isotopes as discriminating tools. Hg isotopic values also provided insight on Hg contamination sources for biota in coastal environment. The overall narrow range of δHg around Europe was suggested to be related to a global atmospheric contamination while δHg at some sites was linked either to background contamination, or with local contamination sources. ΔHg was related to Hg levels of fish but we also suggest a relation with ecological conditions. Throughout this study, results from the Black Sea population stood out, displaying a Hg cycling similar to fresh water lakes. Our findings bring out the possibility to use Hg isotopes in order to discriminate distinct populations, to explore the Hg cycle on a large scale (Europe) and to distinguish sites contaminated by global versus local Hg source. The interest of using Hg sable isotopes to investigate the whole European Hg cycle is clearly highlighted.

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Seafood has a great ecological and nutritional value for human and wildlife communities. However, accumulation of mercury (Hg) in fish is a concern to animal and human health. There is a crucial need to understand Hg speciation in marine organisms through controlled feeding experiments. This study represents a first assessment of the biological processes that may influence Hg bioaccumulation and dynamics in a marine predatory fish. We conducted a feeding experiment to investigate the dynamics of MeHg and iHg, as well as Hg isotopes in the liver and muscles of captive juvenile seabass (Dicentrarchus labrax). Three groups of juvenile seabass were fed in captivity during 3 weeks of acclimatization and 6 weeks of experiment. Each group was fed with pellets containing environmentally relevant MeHg concentrations (Control, 200 and 500 ng g−1 dw). We monitored the evolution of MeHg and iHg concentrations as well as Hg isotopic values in liver and muscle. We determined Hg dynamics with respect to the contamination level in the fish diet. Muscle δ202Hg and Δ199Hg turnover rates ranged between 33 and 14 days (Low diet) to 5 and 9 days (Mod diet). Liver δ202Hg and Δ199Hg turnover rates ranged between 3 and 7 days (Low diet) to 3 and 2 days (Mod diet), respectively. Hg species concentrations and δ202Hg varied over time between diet groups and tissues, showing the occurrence of internal mass-dependent fractionation (MDF). No significant intra-tissue and temporal Hg mass-independent fractionation (MIF) was observed. The results of our experiment are strongly in favor of the existence of MeHg demethylation in a coastal predatory fish exposed to low to moderate concentrations of environmental Hg. The decrease over time of δ202Hg in muscle of seabass from the most contaminated diet was accompanied by a temporal increase in iHg, pointing to possible Hg detoxification processes occurring in this tissue when dietary Hg exposure is high. The absence of Hg MDF and different turnover between muscle and liver in seabass exposed to 500 ng Hg g−1 confirmed that Hg speciation and bioaccumulation in juvenile fish are controlled by Hg levels and speciation in their diet.