Effect of seasonality and organism size on mercury (MeHg) bioaccumulation in plankton of the Madeira River basin, Western Amazon.

Mercury (Hg) contamination is a serious environmental problem due to its toxicity and bioaccumulation in aquatic communities. In the Amazon, hydrological fluctuations influence plankton structure and abundance, while gold mining constitutes an important anthropogenic source of Hg, releasing the metal into the water column and facilitating its incorporation by biota. In this study, we assessed total Hg (THg), methylmercury (MeHg), and the MeHg:THg ratio in the planktonic community of the upper Madeira River basin (Brazil) over a 10-year period. Mean concentrations of THg, MeHg, and MeHg:THg were 0.144 ± 0.138 mg kg-1, 0.035 ± 0.049 mg kg-1, and 0.283 ± 0.245, respectively. THg concentrations were higher during the rising-water period, particularly in larger zooplankton, reflecting seasonal hydrological inputs and changes in plankton composition. MeHg concentrations in total plankton were lower during the rising-water period, likely due to dilution effects and reduced bioavailability. When stratified by mesh size, MeHg concentrations were primarily determined by organism size, with larger zooplankton exhibiting higher levels than phytoplankton. The highest MeHg:THg ratios in total plankton and zooplankton were observed during low-water periods, likely driven by lower THg concentrations rather than by increased MeHg production alone. Across all seasons, larger zooplankton consistently exhibited higher MeHg:THg ratios than phytoplankton, indicating size-dependent bioaccumulation patterns. Our results demonstrate that Hg dynamics in plankton are strongly modulated by the hydrological cycle and exacerbated by anthropogenic inputs, emphasizing the importance of long-term monitoring to assess ecological risks in tropical river systems.