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Abstract
The western Indian continental shelf houses the world's largest naturally-formed coastal low-oxygen zone that develops seasonally during the summer monsoon. We investigated multiple reductive nitrogen transformation pathways and quantified their rates in this system through anaerobic incubations with additions of 15N-labeled substrates during the anoxic period for three consecutive years (2008 to 2010). Addition of 15N labeled ammonium (15NH4+) resulted in low to moderate anaerobic ammonia oxidation (Anammox) rates in about half of our incubations from the oxygen depleted waters. In contrast, incubations with labeled nitrite (15NO2-) led to large production of 30N2 over 29N2 in all incubation experiments, indicating denitrification to be the dominant N-loss pathway. Rates of dissimilatory nitrate/nitrite reduction to ammonia (DNRA) were found to be highly variable and were lower by an order of magnitude than the denitrification rates. Extrapolation of average rates over the sampling periods and volume of anoxic waters showed large nitrogen removal (3.70 to 11.1 Tg yr-1) which is about three times as high as the previously reported estimate (1.3 to 3.8 Tg yr-1). Despite the small area it occupies, this shallow seasonal anoxic zone may account for as much as 20-60% the of the total annual fixed nitrogen loss in the perennial oxygen minimum zone of the Arabian Sea.
Highlights
As in other parts of the North Indian Ocean, hydrography and biogeochemistry over the western Indian continental shelf (WICS) are dominated by seasonal reversals of surface currents associated with the monsoons (Schott and McCreary, 2001; Naqvi et al, 2006a)
Despite moderate oxygen demand there are several factors that contribute to development of intense anoxia in near bottom waters: (1) The upwelled water, derived from the upper part of the mesopelagic oxygen minimum zone of the Arabian Sea is oxygen-depleted to begin with, not anoxic; (2) Due to slow upwelling the upwelled water remains over the shelf for a long time; and (3) The cold, saline upwelled water is capped by a thin (
Our results show the presence of intensely O2-depleted waters underlying a shallow and sharp pycnocline suitable for suboxic/anoxic microbial processes like denitrification, anammox and dissimilatory nitrate/nitrite reduction to ammonium (DNRA)
Summary
As in other parts of the North Indian Ocean, hydrography and biogeochemistry over the western Indian continental shelf (WICS) are dominated by seasonal reversals of surface currents associated with the monsoons (Schott and McCreary, 2001; Naqvi et al, 2006a). In the north (19oN, off Mumbai) the effect of upwelling is felt at least until early December (Naik, 2003; Shankar et al, 2005) This time span far exceeds the duration of the SWM. Overall, reducing conditions over the WICS prevail for at least 3 months (90 days) (Naik, 2003; Naqvi et al, 2006b) These processes operate over a large geographical area, making the WICS the single largest low-oxygen system of its kind in the world, occupying an area of ∼200,000 km (O2 < 20 μM) – an order of magnitude bigger than the area of the famous dead zone of the Gulf of Mexico (area with O2 < 62.5 μM: 22,000 km2 – Rabalais et al, 2010). The shallow anoxic zone in the eastern Arabian Sea is believed to provide significant feedback to the global climate change through unusually high emissions of N2O (nitrous oxide) and DMS (dimethyl sulfide) (Naqvi et al, 2000; Shenoy et al, 2012)
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