Mercury in the Amazon Basin: A Meta-Analysis & Socio-Ecological Systems Review

Abstract:

Mercury (Hg) in the Amazon arises from natural mineral deposits and as a by-product of legal and illegal gold mining activity. Increased gold mining within the basin is strongly linked to economic demand, human health, and the integrity of freshwater ecosystems. While Hg is a trace metal inherent to the natural environment, sufficiently high concentrations are toxic to plants and animals, especially in its organic form. The fate of Hg in the Amazon basin has been well-documented in studies of air, soil, water, sediment, plants, fish, and humans, though no holistic effort has been made to characterize spatial and temporal trends in Hg exposure across the Amazon basin. This review synthesizes the results of 50+ studies and 26,000+ observations that have measured Hg in the Amazon and presents key findings over 30 years of research. Across several environmental components, this review provides collective insight on Hg exposure and its transfer between social and ecological systems. The majority of studies take place across several tributaries along the Madeira and Tapajós Rivers, in areas historically associated with gold mining. Within fish species, results indicate strong evidence of bioaccumulation with increasing trophic level, with median total Hg concentrations of 0.09, 0.22, and 0.60 μg/kg, in herbivores, omnivores, and carnivores, respectively. This effect is further refined by the influence of seasonality on migration, where concentrations in certain piscivorous species range from 0.45 μg/kg in the wet season to 1.3 μg/kg in the dry season. In the context of human health, we observe that Hg concentrations in commercial fish often exceed established international thresholds in communities who rely on fisheries for nutrition and economic well-being. Furthermore, studies of human impacts, such as land cover change, dams, and mining, call for basin-scale risk assessments preceding infrastructure development, assessment of direct and indirect impacts, and the selection of indicator species to assess risk to human and natural systems. This collection of mercury studies builds a foundation of knowledge regarding Hg cycling in such a complex basin and necessitates future research efforts directed towards seasonal and climatic variability, retention in tropical soils, methylation processes, and risk assessment.