Mercury concentrations along the Amazon estuary and plume: Spatial trends and geochemical processes

Mercury (Hg) contamination in the Amazon is a recurring topic in scientific literature, with high concentrations reported in various environmental compartments, including water in dissolved and particulate fractions, soils, sediments, and fish. Despite the substantial body of research on Hg contamination in the Amazon, studies focusing on the coastal and continental shelf zones remain scarce, and, to our knowledge, no study has specifically evaluated Hg contamination in the waters of these areas. The Amazon coastal region and its continental shelf exhibit complex hydrodynamics influenced by the Amazon River discharge, ocean currents, and extensive mangrove systems, all of which can significantly affect Hg dynamics and transport. This study aims to investigate Hg concentrations in water and sediments in the coastal and oceanic region of the Amazon, identifying spatial trends and the main geochemical processes influencing its transport. The research covered the Amazon River estuary (Amazon Transect), the northern Amazon River plume (North Plume), the Pará River estuary (Pará Transect), an extensive mangrove area (Mangrove Belt), and a region influenced by the North Brazil Current (NBC). Sampling occurred during the high-discharge period of the Amazon River. Results showed that total Hg concentrations in water varied across regions, with the highest levels recorded in the Pará Transect and the lowest in the North Plume. These concentrations were positively correlated with total organic carbon and suspended particulate matter, which were identified as important geochemical supports for Hg transport on the continental shelf. Similarly, Hg concentrations in sediments reflected the patterns observed in unfiltered water. The Mangrove Belt and North Plume were significant zones of Hg deposition, while estuarine areas, such as the Amazon and Pará transects, acted as primary sources of Hg to the marine environment. Isotopic and elemental analyses of organic matter, combined with the observed negative relationship between Hg concentrations and the salinity gradient, suggest that the primary sources of Hg on the continental shelf are Amazonian soils and the resuspension of sediments from the shelf itself. These findings highlight that deforestation in the Amazon, a widely debated issue, extends its impact beyond terrestrial ecosystems, influencing water quality in the tropical Atlantic Ocean and posing risks to coastal and marine biota.