Dinitrogen fixation rates in the Bay of Bengal during summer monsoon

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Biological dinitrogen (N2) fixation exerts an important control on oceanic primary production by providing bioavailable form of nitrogen (such as ammonium) to photosynthetic microorganisms. N2 fixation is dominant in nutrient poor and warm surface waters. The Bay of Bengal is one such region where no measurements of phototrophic N2 fixation rates exist. The surface water of the Bay of Bengal is generally nitrate-poor and warm due to prevailing stratification and thus, could favour N2 fixation. We commenced the first N2 fixation study in the photic zone of the Bay of Bengal using 15N2 gas tracer incubation experiment during summer monsoon 2018. We collected seawater samples from four depths (covering the mixed layer depth of up to 75 m) at eight stations. N2 fixation rates varied from 4 to 75 μmol N m−2 d−1. The contribution of N2 fixation to primary production was negligible (<1%). However, the upper bound of observed N2 fixation rates is higher than the rates measured in other oceanic regimes, such as the Eastern Tropical South Pacific, the Tropical Northwest Atlantic, and the Equatorial and Southern Indian Ocean.