Total Hg and methylmercury dynamics in a river-floodplain system in the Western Amazon: Influence of seasonality, organic matter and physical and chemical parameters

Total mercury (Hg) and methylmercury (MeHg) circulation in a connected river-floodplain system composed of two black water (a small forest river, igarape, and Cunia Lake) and a white water body (Madeira River), located in the Madeira River Basin were evaluated during the rising-water, early and late falling-water periods. We assessed organic matter (C and N composition, (C: N) a; and delta C-13 isotopic signature), and physical and chemical influences (pH, dissolved O-2, electric conductivity) in relationship to Hg and MeHg concentrations. Hg and MeHg concentrations in a sediment profile and three aquatic macrophytes (E. crassipes, E. azuera and Oryza sp.) were measured. Igarape and Cunia Lake showed higher Hg and MeHg concentrations (115-709; 10-25 ng g(-1)) in the suspended particulate matter compared (SPM) compared to the Madeira River (Hg: 5-16; MeHg: 0.2-0.3 ng g(-1)), partially independent of seasonality (p = 0.06). Total Hg had higher affinity for the SPM(1.75 times) than for dissolved organic matter. Organic matter characteristics correlated with MeHg concentrations (delta C-13 and (C: N) a; r(2) = 0.79; p < 0.0001), as well as physical and chemical parameters of the water column (dissolved O-2 and pH; r(2) = 0.80; p < 0.0001), demonstrating that physical and chemical changes between the river-floodplain system affect MeHg circulation and production. The inverse correlation of MeHg and SO42- concentrations (r(2) = 0.73; p < 0.0001) suggests the action of sulfate-reducing bacteria. Total Hg and MeHg concentrations as well as % MeHg were detected in the sediment profile (Hg: 24-51; MeHg: 0.6-3.2 ng g(-1); % MeHg: 1.8-6.2) and aquatic macrophytes (Hg: 1-30; MeHg: 0.3-7.5 ng g(-1); % MeHg: 1.6-33.7). We conclude that the highest Hg and MeHg concentrations in Cunia Lake, compared to the Madeira River, are due to the physical and chemical differences between these environments. (C) 2018 Elsevier B.V. All rights reserved.