Winter Convective Mixing Mediating Coupling of N-Gain and -Loss in the Arabian Sea

Marine dinitrogen (N<inf>2</inf>) fixation fuels primary production and thereby influences the Earth's climate. Yet, its geographical distribution and controlling environmental parameters remain debatable. We measured N<inf>2</inf> fixation rates from the two spatially and physicochemically contrasting regions of the Arabian Sea during the winter monsoon: (a) the colder and nutrient-rich waters in the northern region owing to winter convection and (b) the warmer and nutrient-poor waters in the southern region unaffected by winter convection. We found higher N<inf>2</inf> fixation rates at the surface of northern region due to convective mixing driven supply of phosphate (intuitively iron also) from the underlying suboxic waters. N<inf>2</inf> fixation was favored by high nutrient concentrations in the euphotic waters, whereas remained unaffected by nutrient availability in the aphotic waters. We conclude that diazotrophs dwelling in the euphotic zone chose phosphate over fixed nitrogen-poor waters. However, we found that among oligotrophic waters, anticyclonic eddy extremes the barrier of fixed nitrogen supply, and thereby, elevates N<inf>2</inf> fixation. While the Arabian Sea loses about 20%–40% of the global ocean fixed nitrogen, we estimate that N<inf>2</inf> fixation in the Arabian Sea offsets only up to 42% of its fixed nitrogen-loss by denitrification, but this offset could be higher if diazotrophic activity is further examined up to the deeper depths of the Arabian Sea.