Mercury in seston increases downstream along cascade hydroelectric reservoirs in the Amazon

Mercury (Hg) is among the major pollutants affecting tropical ecosystems. Hydroelectric reservoirs can have high Hg concentrations due to soil flooding and also downstream from the dam due to hypolimnion export. These processes contribute to Hg bioaccumulation in different aquatic organisms. We investigated the dynamics of total mercury in different biotic and abiotic compartments across four in-cascade reservoirs in the Brazilian Amazon. We sampled suspended particulate matter (SPM), phytoplankton, and zooplankton within the reservoirs and immediately downstream during two hydrological periods in 2022 and 2023. Hg concentrations followed the pattern SPM < phytoplankton < zooplankton, suggesting Hg biomagnification at the sampled sites. All three compartments exhibited concentrations 1.6 to 2.5 times higher during the wet season. In this period, Hg concentrations in all compartments increased along the cascade of hydroelectric reservoirs from upstream to downstream: Sinop HPP < Colíder HPP < Teles Pires HPP < São Manoel HPP, suggesting a cumulative effect. Conversely, during the dry season, concentrations did not differ spatially. Therefore, we suggested seasonal and longitudinal variation can play an important role in food web accumulation of mercury.