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

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 delta N-15 and delta C-13 to examine whether Hg patterns are due to differences in trophic level. Differences in delta N-15 and delta C-13 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. Delta Hg-199 variation in fish (-0.5 to 0.2 parts per thousand) was higher than the ecosystem baseline defined by a Delta Hg-199 of -0.66 parts per thousand in sediments, and suggested limited aqueous photochemical MeHg degradation. Photochemistry-corrected delta Hg-202 in fish was 0.7 parts per thousand 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.

Automated summary

The study in French Guiana aimed to investigate the impact of gold-mining activities on mercury concentrations and isotopic signatures in freshwater fish consumed by local residents. Analysis of Hg concentrations and isotopic ratios in fish muscles revealed differences in biogeochemical processes between fish from impacted and pristine areas, indicating potential influences of suspended particles on mercury concentrations. The study also highlighted the potential of Hg isotopes in fish as complementary tracers to understand the sources and processes of mercury accumulation in aquatic ecosystems.