[1] We report the first direct estimates of N 2 fixation rates measured during the spring, 2009 using the 15 N 2 gas tracer technique in the eastern Arabian Sea, which is well known for significant loss of nitrogen due to intense denitrification. Carbon uptake rates are also concurrently estimated using the 13 C tracer technique. The N 2 fixation rates vary from ∼0.1 to 34 mmol N m −2 d −1 after correcting for the isotopic under-equilibrium with dissolved air in the samples. These higher N 2 fixation rates are consistent with higher chlorophyll a and low d 15 N of natural particulate organic nitrogen. Our estimates of N 2 fixation is a useful step toward reducing the uncertainty in the nitrogen budget.

[1] Primary productivity in the sunlit surface layers of tropical oceans is mostly limited by the supply of reactive nitrogen (N r ) through upwelling, N 2 fixation by diazotrophs, riverine flux and atmospheric deposition. The relative importance of these processes varies from region to region. Using recent data on the nitrogen content of aerosols over the ocean and marine new production in parts of the northern Indian Ocean for the period 1994-2006 CE, a quantitative assessment of the contribution of atmospheric deposition to new production in the two biogeochemically different basins of the northern Indian Ocean, viz., the Arabian Sea and the Bay of Bengal, is presented. By suitably converting the measured concentrations of aerosol nitrogen into fluxes and comparing them with 15 N tracer-based direct new and primary production measurements, it is inferred that the contribution of atmospheric deposition to new production in the northern Indian Ocean could at best be $3%. Our estimate of $1.39 Tg N year À1 of N r flux into the northern Indian Ocean through aerosols is a step toward significantly reducing the uncertainty in the global nitrogen budget.Citation: Singh, A., N. Gandhi, and R. Ramesh (2012), Contribution of atmospheric nitrogen deposition to new production in the nitrogen limited photic zone of the northern Indian Ocean,

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