Increased marine production of N2O due to intensifying anoxia on the Indian continental shelf

Eutrophication of surface waters and hypoxia in bottom waters has been incr easing in many coastal areas(1-4), leading to very large depletions of mari ne life in the affected regions(4). These areas of high surface productivit y and low bottom-water oxygen concentration are caused by increasing runoff of nutrients from land. Although the local ecological and socio-economic e ffects have received much attention(2-4), the potential contribution of inc reasing hypoxia to global-change phenomena is unknown. Here we report the i ntensification of one of the largest low-oxygen zones in the ocean, which d evelops naturally over the western Indian continental shelf during late sum mer and autumn. We also report the highest accumulations yet observed of hy drogen sulphide (H2S) and nitrous oxide (N2O) in open coastal waters. Incre ased N2O production is probably caused by the addition of anthropogenic nit rate and its subsequent denitrification, which is favoured by hypoxic condi tions. We suggest that a global expansion of hypoxic zones may lead to an i ncrease in marine production and emission of N2O, which, as a potent greenh ouse gas, could contribute significantly to the accumulation of radiatively active trace gases in the atmosphere(5).