Hg concentrations and stable isotope variations in tropical fish species of a gold-mining-impacted watershed in French Guiana

Laure Laffont (1) , Johanna Menges , Sylvaine Goix (1) , Sophie Gentès , Régine Maury-Brachet (2, 3) , Jeroen Sonke (1) , Alexia Legeay , Patrice Gonzalez (4, 3) , Raphaëlle Rinaldo , Laurence Maurice (1)

1 GET - Géosciences Environnement Toulouse
2 UB - Université de Bordeaux
3 EPOC - Environnements et Paléoenvironnements OCéaniques
4 CNRS - Centre National de la Recherche Scientifique

The aim of the study was to determine if gold-mining activities could impact the mercury (Hg) concentrations and isotopic signatures in freshwater fish consumed by riparian people in French Guiana. Total Hg, MeHg concentrations, and Hg stable isotopes ratios were analyzed in fish muscles from different species belonging to three feeding patterns (herbivorous, periphytophagous, and piscivorous). We compared tributaries impacted by gold-mining activities (Camopi, CR) with a pristine area upstream (Trois-Sauts, TS), along the Oyapock River. We measured δ15N and δ 13C to examine whether Hg patterns are due to differences in trophic level. Differences in δ 15N and δ 13C values between both studied sites were only observed for periphytophagous fish, due to difference of CN baselines, with enriched values at TS. Total Hg concentrations and Hg stable isotope signatures showed that Hg accumulated in fish from both areas has undergone different biogeochemical processes. Δ199Hg variation in fish (−0.5 to 0.2‰) was higher than the ecosystem baseline defined by a Δ199Hg of −0.66‰ in sediments, and suggested limited aqueous photochemical MeHg degradation. Photochemistry-corrected δ202Hg in fish was 0.7‰ higher than the baseline, consistent with biophysical and chemical isotope fractionation in the aquatic environment. While THg concentrations in periphytophagous fish were higher in the gold-mining area, disturbed by inputs of suspended particles, than in TS, the ensemble of Hg isotope shifts in fish is affected by the difference of biotic (methylation/demethylation) and abiotic (photochemistry) processes between both areas and did therefore not allow to resolve the contribution of gold-mining-related liquid Hg(0) in fish tissues. Mercury isotopes of MeHg in fish and lower trophic level organisms can be complementary to light stable isotope tracers.

Mots clés: Mercury; Gold-mining; French Guiana; Fish; Stable isotopes; Methylmercury

Domaines: Sciences de l'environnement

Laure Laffont, Johanna Menges, Sylvaine Goix, Sophie Gentès, Régine Maury-Brachet, et al.. Hg concentrations and stable isotope variations in tropical fish species of a gold-mining-impacted watershed in French Guiana. Environmental Science and Pollution Research, 2021, 28 (43), pp.60609-60621. ⟨10.1007/s11356-021-14858-7⟩. ⟨hal-03434104⟩