Benthic hypoxia in anthropogenically-impacted rivers provides positive feedback enhancing the level of bioavailable...

We investigated the effect of hypoxic-anoxic range of dissolved oxygen (DO) on metal release/bioavailability at sediment-water interface (SWI) in the Ganga River. Here, we consider eight sites in the main river stem along 518 km; sixty sites downstream two point sources and two tributary confluences covering 630 km; and an incubation experiment to verify these results. We found higher concentrations of metals and bioavailable fractions at SWI at two locations of main stem and up to 700 m, 1000 m, 400 m and 500 m downstream Assi drain, Wazidpur drain, Ramganga confluence and Varuna confluence respectively where DO at SWI (DO_sw) was <2.0 mgL^-1. The incubation experiment did show higher levels of metal- and P-release and bioavailability under anoxic-hypoxic range of DO. The risk assessment code and eutrophication index indicated high to very high risks of contaminated river sediment and water to aquatic environment at sites with hypoxic-anoxic range of DO_sw. Further, the principal component analyses separated metals and bioavailable fractions opposite to FDAase indicating greater risk at these locations. The study, which forms the first report on benthic hypoxia/anoxia-driven metal release, potential bioavailability and risk to the Ganga River ecosystem will help understanding how human-driven perturbations influence geochemical cycling of metals and ecosystem responses in large rivers.