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, igarapé, and Cuniã 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 δ13 C isotopic signature), and physical and chemical influences (pH, dissolved O2 , 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. Igarapé and Cuniã 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 (δ13 C and (C:N)a; r2  = 0.79; p < 0.0001), as well as physical and chemical parameters of the water column (dissolved O2 and pH; r2  = 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 SO4 2- concentrations (r2  = 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 Cuniã Lake, compared to the Madeira River, are due to the physical and chemical differences between these environments.