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

Inacio A Pestana; Marcelo G Almeida; Wanderley R Bastos; Cristina M M Souza

doi:10.1016/j.scitotenv.2018.11.388

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

Sci Total Environ. 2019 Mar 15:656:388-399. doi: 10.1016/j.scitotenv.2018.11.388. Epub 2018 Nov 27.

Authors

Inacio A Pestana¹, Marcelo G Almeida², Wanderley R Bastos³, Cristina M M Souza²

Affiliations

¹ Programa de Pós-Graduação em Ecologia e Recursos Naturais, Laboratório de Ciências Ambientais, Centro de Biociências e Biotecnologia, Universidade Estadual do Norte Fluminense Darcy Ribeiro, Av. Alberto Lamego, 2000 - Parque Califórnia, CEP: 28013-602 Campos dos Goytacazes, Rio de Janeiro, Brazil. Electronic address: inacio@uenf.br.
² Programa de Pós-Graduação em Ecologia e Recursos Naturais, Laboratório de Ciências Ambientais, Centro de Biociências e Biotecnologia, Universidade Estadual do Norte Fluminense Darcy Ribeiro, Av. Alberto Lamego, 2000 - Parque Califórnia, CEP: 28013-602 Campos dos Goytacazes, Rio de Janeiro, Brazil.
³ Programa de Pós-Graduação em Desenvolvimento Regional e Meio Ambiente, Laboratório de Biogeoquímica Ambiental Wolfgang C. Pfeiffer, Universidade Federal de Rondônia, Av. Pres. Dutra, 2967 - Olaria, CEP: 76801-059 Porto Velho, Rondônia, Brazil.

PMID: 30513429
DOI: 10.1016/j.scitotenv.2018.11.388

Abstract

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 δ¹³C 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 (δ¹³C 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.

Keywords: Amazon; Floodplain; Hg; Methylation; Organic matter; Stable isotopes.