Influence of a hydroelectric dam on fish mercury contamination along the Sinnamary River (French Guiana).

The Petit Saut hydroelectric dam and the upstream and downstream areas of the Sinnamary River in French Guiana (Amazon basin) have been studied from 1993 to 2020. The nearly thirty-years-long study of the monitoring of total mercury concentration in fish and the physicochemical survey of the environment made it possible to demonstrate the impact of the flooding of the forest and the role of the hydroelectric dam on the methylation of mercury. Results show that the physicochemical modifications generated by the construction of the dam led to a significant production of methylmercury (MeHg) in the anoxic part of the reservoir and downstream of the river leading to a strong spatio-temporal impact of the dam. Seven species of fishes are studied and their mercury concentrations vary according to many parameters: fish diet, position in the water column, site, lake oxycline level and time.

Impact of nickel mining in New Caledonia on marbled eels Anguilla marmorata.

New Caledonia is particularly affected by nickel open pit mining activities because of the presence of ultramafic soils rich in metals. The particles dispersed by atmospheric transport and soil erosion during the excavation of nickel end up by deposition or leaching in rivers where they may be bioaccumulated by organisms living downstream the mines. Despite alarming freshwater metals concentrations, no study investigated the level of their bioaccumulation in eels, and if high bioaccumulation levels occur, the potential consequences on their health. The aim of this study was to determine how eels Anguilla marmorata are impacted in situ by metals issued from mining activity by measuring: morphometric parameters; metal concentrations in tissues and organs and transcription levels of target genes encoding proteins involved in several metabolic key functions. Among organs, liver was found to be the most affected by mining with average nickel concentrations of 5.14 mg/kg versus 1.63 mg/kg for eels away from mines leading to dysregulation of numerous genes involved in oxidative stress, DNA repair, apoptosis, reproduction and both lipid and mitochondrial metabolisms. This study should allow us to define in an integrated way if metals released by mining activities influence metals bioaccumulation in eels and induce biological effects.

Nickel and associated metals in New Caledonia: Exposure levels and their determinants.

The ultramafic massifs of the New Caledonian archipelago contain about 10% of the world's nickel reserves, which also contain significant but lower amounts of cobalt, chromium, and manganese. Natural erosion of these massifs and mining activities may contribute to the exposure of local populations to these metals through contamination of air, food, and water resources. We conducted a biomonitoring survey to evaluate exposure to these four metals and its main determinants by constructing a stratified sample of 732 adults and children (>3 years old) from visitors to 22 health centers across the archipelago. Urine was collected and analyzed by inductively-coupled plasma mass spectrometry to determine metal concentrations. A face-to-face interview was conducted to document sociodemographic characteristics, lifestyle and dietary habits, and residence-mine distance. Environmental samples (soil, house dust, water, and foodstuffs) were collected from two areas (one with and one without mining activity) to delineate determinants of exposure in more detail. Nickel and chromium were metals with the highest concentrations found in urine, especially in children, at levels exceeding reference values derived from representative national surveys elsewhere throughout the world (for children: 4.7 µg/g creatinine for nickel and 0.50 µg/g creatinine for chromium): 13% of children exceeded the reference value for nickel and 90% for chromium. Large variations were observed by region, age, and sex. In this geological setting, urinary and environmental nickel concentrations appear to be driven mainly by soil content. This is the first archipelago-wide survey of metal exposure in New Caledonia. The potential health consequences of this chronic high exposure need to be assessed.

Simulation of the chemical fate and bioavailability of liquid elemental mercury drops from gold mining in Amazonian freshwater systems.

Elemental mercury (Hg(o)) for gold amalgamation is the main process applied by artisanal gold miners in South America, leading to important discharges into freshwater ecosystems. Through a 28-day experimental approach based on indoor microcosms, we simulated the chemical fate and bioavailability of Hg(o) droplets in the presence or absence of sediment collected from a typical forest creek that is unaffected by gold mining activities. Our results clearly showed significant mercury transfers in the water column in both the dissolved gaseous Hg(o) and oxidized (Hg(II)) forms, with a marked effect of the presence of sediment. After 28 days, Hg total (HgT) concentration in the water column was 25 times higher in sediment-free units (108 +/- 17 vs. 4 +/- 0.4 nM). Methylmercury (MeHg) determinations in the dissolved fraction showed a significant increase only in the presence of sediment after 7 and 14 days. Zebrafish (Danio rerio) were used as indicators for mercury bioavailability. The HgT determinations in four organs revealed significant accumulation levels as early as 7 days exposure, with marked differences in favor of fish collected from the sediment-free units. Significant MeHg increases were observed in the four organs only when sediment was present. Genomic tools applied to estimate sulfate-reducing bacteria communities showed mercury impacts on their diversity and distribution in the different compartments (water, sediment, biofilm, fish gut).

Biofilm and mercury availability as key factors for mercury accumulation in fish (Curimata cyprinoides) from a disturbed Amazonian freshwater system.

The Petit-Saut hydroelectric reservoir was filled in 1994 on the Sinnamary River in French Guiana (Amazonian basin). Flooding of the equatorial rain forest led to anoxia in most of the water column and enhanced mercury methylation in the reservoir hypolimnion. We selected the benthivorous/omnivorous fish species Curimata cyprinoides to investigate total mercury and methylmercury (MeHg) bioavailability and bioaccumulation capacities in the reservoir and downstream in the Sinnamary River. Mercury concentrations in the dorsal skeletal muscle were 10-fold higher in fish from the downstream zone. Stomach contents and stable nitrogen and carbon isotope ratios showed that biofilms and the associated invertebrate communities represented important food sources at the two sites. The delta 13C measurements indicated that biofilms in the flooded forest zone of the reservoir consist of endogenous primary producers; downstream, they are based on exogenous organic matter and microorganisms, mainly from the anoxic layers of the reservoir. Total mercury and MeHg concentrations in the biofilms and associated invertebrates were much higher at the downstream site compared to concentrations at the reservoir. Our results clearly show the importance of MeHg export from the anoxic layers of this tropical reservoir. We conclude that differences between biofilm composition and MeHg concentrations in the ingested food could explain the marked differences observed between mercury levels in fish.