Temporal variation of mercury levels in fish, soil, and sediments in an Amazon reservoir: insights from 35 years of river impoundment in Pará State, Brazil

The increase of mercury (Hg) concentrations in abiotic and biotic compartments of aquatic ecosystems following the river impoundment for building a hydroelectric reservoir is one of many environmental and social impacts that the construction of hydroelectric plants can trigger. Yet, long-term studies in Amazon reservoirs are still scarce. The present study aimed to understand the effects of dam impoundment in THg concentrations in an Amazon reservoir up to 35 years of its creation. On March 2019 (35th year after filling), samples of fish, soil, and sediments were collected in the Tucuruí reservoir. Total mercury (THg) concentrations were determinate in those samples and compared with data extract from previous studies referring to the 6^th, 16^th and 18^th years after the reservoir filling. Fish from different guilds at the 6th year after filling the Tucuruí reservoir had high THg concentrations, and those decreased in the 16^th and 18^th years, then the concentrations increased again in the 35^th year after filling. For soils and sediments, a decline in THg concentrations was observed. These results differ from previous studies that predicted that Hg concentrations in fish would return to natural concentrations within 30 years in temperate zones and no decline of THg concentrations would be observed for Amazonian reservoirs. The Tocantins river drainage basin has been subjected to multiple anthropic disturbances and land use changes over the past decades, such as the implementation of new hydroelectric plants, deforestation, and fires, which can explain our observations. This study contributes valuable long-term insights into the dynamics of Hg concentrations in an Amazonian reservoir, highlighting the complex interactions between environmental changes and Hg accumulation over decades.