Winter convective mixing mediating coupling of n-gain and -loss in the Arabian sea

Abstract

Marine dinitrogen (N2) fixation fuels primary production and thereby influences the Earth's climate. Yet, its geographical distribution and controlling environmental parameters remain debatable. We measured N2 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 N2 fixation rates at the surface of northern region due to convective mixing driven supply of phosphate (intuitively iron also) from the underlying suboxic waters. N2 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 N2 fixation. While the Arabian Sea loses about 20%–40% of the global ocean fixed nitrogen, we estimate that N2 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.

Key Points Winter convection elevates N2 fixation through the enhanced supply of nutrients from the suboxic waters to the surface waters of the Arabian Sea

Diazotrophy offsets up to 42% of the Arabian Sea's denitrification deficit

Winter convective mixing strengthens the long-held hypothesis of coupling between N-gain and -loss processes

Plain Language Summary Phytoplankton facilitate the removal of greenhouse carbon dioxide gas from the atmosphere. However, phytoplankton's carbon dioxide assimilation ability is largely controlled by the reactive forms of nitrogen (N, such as ammonium, nitrate and nitrite). In contrast, the N-limited environments often create a suitable niche for diazotrophs—organisms genetically proficient to convert dinitrogen (N2) gas to a reactive form of N. Previous research has hypothesized close spatial coupling between N2 fixation and N-loss processes because the latter result in an N-deficit, that is, a conducive condition for diazotrophs. Here, we measured N2 fixation rates in and above the N-loss dominated waters of the Arabian Sea. The highest N2 fixation occurred near the coast of the Arabian Sea. We found relatively high N2 fixation rates above the waters with the highest N-loss owing to convective mixing driven supply of nutrients such as phosphate and probably iron. N2 fixation rates below the sunlit layer were consistent but low. Our study showed that physical processes substantially influence N2 fixation by affecting nutrients supply.