Mercury Accumulation and Transfer in Hydrothermal Coastal Environment: The Case of the Geothermal Plant of Bouillante.

Geothermal vents can constitute local significant sources of mercury (Hg) in the environment. The geothermal power plant of Bouillante (Guadeloupe, Lesser Antilles) artificially enhances the release of hydrothermal water in shallow areas of the bay. To assess the impact of this release on the Hg transfer in the environment, Hg concentrations were assessed in sediments, sulphur-oxidising bacteria and six animal species (urchin, sponges and fish) with various diets and trophic levels from the Bouillante Bay and a distant Control Site. Concentrations of Hg in all samples from Bouillante were greater than those from the Control Site (2-627 times higher). A comparison with the Hg concentrations reported in the literature for similar sample types reveals that they are abnormally high in most Bouillante samples suggesting a local Hg contamination imputable to the release of Hg hydrothermal water. Rocky pebbles of the shallow discharge channel are covered by a mat of sulphur-oxidising bacteria presenting high concentration of Hg (13 µg g). Through analysis of species with various diet, two pathways of Hg integration into Bouillante's food webs were emphasised. First, greater Hg concentrations in filter feeders (sponges ×3 and bivalves ×10) not consuming bacterial mats and liver/muscle ratios in fishes suggest that discharge waters enhance the exposure of marine organisms to dissolved inorganic Hg (i.e., waterborne pathway). Second, greater Hg concentrations in opportunistic animals such as the fish Acanthurus bahianus (×49) and the sea urchin Diadema antillarum (×50) known to ingest sulphur-bacterial mats from the discharge channel suggest that these chemosynthetic bacteria play a role in the integration (i.e., dietary pathway) and transfer of Hg in local biota, which enlightens the consideration of vent fields as natural sources of Hg.