Nitrogen deficits in the Arabian Sea, implications from a three component mixing analysis

The Arabian Sea plays an important role in the marine nitrogen cycle because in its oxygen-depleted thermocline water mass bio-available fixed nitrogen is reduced to N2 (denitrification). To improve our understanding of processes controlling the oxygen deficiency in the Arabian Sea and the nitrogen cycle, data obtained during the international Joint Global Ocean Flux Study (JGOFS) in the Arabian Sea have been evaluated within this work. The results suggest that particulate and dissolved organic carbon exported from the surface ocean and from the continental shelves act as important sinks for oxygen in the oxygen minimum zone (OMZ), but inputs of oxygen-poor water from the south are needed to sustain the oxygen deficiency in the Arabian Sea. Furthermore our results show inline with the N* approach and nitrogen-to-argon (N2/Ar) ratios that nitrogen deficits are up to ∼50% higher than those resulting from other methods. Excess-nitrogen produced during the remineralization of cyanobacteria seems to contribute ∼10% to the N2 formed during denitrification in the core of the OMZ.