Eutrophication of surface waters and hypoxia in bottom waters has been increasing in many coastal areas1,2,3,4, leading to very large depletions of marine life in the affected regions4. These areas of high surface productivity and low bottom-water oxygen concentration are caused by increasing runoff of nutrients from land. Although the local ecological and socio-economic effects have received much attention2,3,4, the potential contribution of increasing hypoxia to global-change phenomena is unknown. Here we report the intensification of one of the largest low-oxygen zones in the ocean, which develops naturally over the western Indian continental shelf during late summer and autumn. We also report the highest accumulations yet observed of hydrogen sulphide (H2S) and nitrous oxide (N2O) in open coastal waters. Increased N2O production is probably caused by the addition of anthropogenic nitrate and its subsequent denitrification, which is favoured by hypoxic conditions. We suggest that a global expansion of hypoxic zones may lead to an increase in marine production and emission of N2O, which, as a potent greenhouse gas, could contribute significantly to the accumulation of radiatively active trace gases in the atmosphere5.
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This work forms a part of the LOICZ-India programme of the Department of Ocean Development (DOD); we thank the Sagar Sampada and Sagar Kanya Management Cells of DOD for the generous allocation of ship time. The custom-made bags used for incubation experiments were given by B. Ward and J. Fernandes, B. Jose and D. Shenoy extended technical assistance at sea. Discussions with K. Banse, L. Codispoti and N. Rabalais greatly improved the content of the paper.
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Anthropogenic nitrogen inputs and impacts on oceanic N2O fluxes in the northern Indian Ocean: The need for an integrated observation and modelling approach
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