Dinitrogen fixation rates in the bay of bengal during summer monsoon

Biological dinitrogen (N<inf>2</inf>) fixation exerts an important control on oceanic primary production by providing bioavailable form of nitrogen (such as ammonium) to photosynthetic microorganisms. N<inf>2</inf> fixation is dominant in nutrient poor and warm surface waters. The Bay of Bengal is one such region where no measurements of phototrophic N<inf>2</inf> 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 N<inf>2</inf> fixation. We commenced the first N<inf>2</inf> fixation study in the photic zone of the Bay of Bengal using¹⁵ N<inf>2</inf> 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. N<inf>2</inf> fixation rates varied from 4 to 75 μmol N m⁻² d⁻¹ . The contribution of N<inf>2</inf> fixation to primary production was negligible (<1%). However, the upper bound of observed N<inf>2</inf> 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.