Selenium in sediment and food webs of the Tapajós River basin (Brazilian Amazon) and its relation to mercury

[Display omitted] •First study regarding Se biogeochemistry in the Brazilian Amazon.•Se in sediment might decrease the bioavailability of Hg.•Sediment might be the main source of Se to plankton.•Se concentrations decreased along the food chain.•Se probably reduces Hg bioaccumulation in carnivorous fishes. We investigated Se levels along the Tapajós River basin - which is an important tributary of the Amazon River - and the possible antagonistic effect of Se in Hg availability. This is the first study to investigate Se transfer from abiotic to biotic compartments and along the food chain in aquatic ecosystems of the Amazon basin. Se concentrations were measured in superficial sediment (n = 29), plankton (n = 28) and fishes (n = 121) along two stretches of the Tapajós River basin (Tapup/mi and Taplow), comprising approximately 500 km with different hydrological characteristics. Se concentrations in sediment were significantly higher in the Taplow (345−664 μg kg−1) than in the Tapup/mi (60−424 μg kg−1). The seasonal flooding of the Amazon River probably helps to carry selenium-rich sediment to the Tapajós mouth (Taplow stretch). We suggest that Se in sediment could decrease the bioavailability of Hg resulting in lower MeHg concentrations in fish, as observed in the Taplow (45−934 μg kg−1). Sediment and plankton were positively correlated in relation to their Se concentrations (r = 0.62; p = 0.001) suggesting that sediment can possibly be the main source of Se to plankton. Our data indicate Se uptake by primary consumers, as noted in phytoplankton levels. The decrease of Se concentrations along the food chain was also noteworthy. This work elucidates some aspects of Se biogeochemistry in the Amazon basin and shows its importance regarding Hg cycles in aquatic ecosystems.