Potential effects of anthropogenic nitrogen on northern Indian Ocean nitrous oxide emissions

Lauren M Zamora1, Parvadha Suntharalingam2, Hermann Bange3, Srinivas Bikkina4, Laure Resplandy5, Manmohan Sarin6, Sunke Schmidtko3, Sybil Seitzinger7 and Arvind Singh3, (1)University of Maryland, College Park, Greenbelt, MD, United States, (2)University of East Anglia, Norwich, United Kingdom, (3)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, (4)Stockholm University, Stockholm, Sweden, (5)Scripps Institution of Oceanography, UCSD, La Jolla, NJ, United States, (6)Physical Research Laboratory, Ahmedabad, India, (7)Royal Swedish Academy of Sciences, Stockholm, Sweden
Abstract:
The North Indian Ocean (Arabian Sea + Bay of Bengal) accounts for ~20-30% of the oceanic emissions of the greenhouse gas, nitrous oxide (N2O). The marine N2O cycle in the suboxic and anoxic waters of this region is very sensitive to relatively small shifts in ambient oxygen (O2); as O2 decreases, N2O production is progressively enhanced and subject to non-linear nitrogen (N) cycle dynamics. Thus, small, sustained changes in local O2 levels (e.g., < 5-10 mmol L-1) may result in detectable impacts on N2O emissions from the North Indian Ocean.

Some recent data suggest that O2 may be declining in the already O2-impoverished Arabian Sea. While the reasons for these possible O2 declines are not fully understood, increasing anthropogenic N inputs from atmospheric and riverine sources likely contribute. In this study we bring together a combination of atmospheric deposition models, in situ measurements, and output from the NEWS riverine model to evaluate recent changes in nitrogen nutrient input to the Arabian Sea. We estimate that there has been a twofold increase in N loading from anthropogenic atmospheric deposition and river runoff to the North Indian Ocean during recent decades. To better understand how anthropogenic N increases might affect regional N2O emissions, we also present analysis of historical N2O and O2 measurements from the North Indian Ocean along with estimates of O2 and N2O fluxes from a regional marine biogeochemical model. We find that as in the Arabian Sea, Bay of Bengal O2 is also likely decreasing. However, due to the paucity of data, we are not yet able to estimate the role of anthropogenic N or how these changes might affect Bay of Bengal N2O emissions. While uncertainties are also high in the Arabian Sea, our preliminary results suggest that increases in atmospheric N deposition are enhancing regional N2O production.

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