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Nitrate limits productivity in much of the ocean. Nitrate residence time is a few thousand years, and changes in nitrate loss could influence ocean productivity. Major sinks for nitrate are denitrification and anaerobic ammonia oxidation in the oxygen minimum zones (OMZs). The Bay of Bengal OMZ is anomalous because large amounts of nitrate loss do not occur there, while nitrate is removed in the nearby OMZ of the Arabian Sea. Observations of nitrate and oxygen made over 5 years by 20 profiling floats equipped with chemical sensors in the Bay of Bengal and the Arabian Sea are used to understand why nitrate is removed rapidly in the Arabian Sea but not in the Bay of Bengal. Our results confirm that nitrate is poised for removal in the Bay of Bengal. However, highly variable oxygen concentrations inhibit its loss. Nitrate loss is regulated by physical oceanographic processes that introduce oxygen.

Plain Language Summary Denitrification is a microbial process that removes nitrate, a compound essential for phytoplankton production, from seawater. Denitrification occurs in the absence of oxygen. Most denitrification in the oceanic water column occurs in three oxygen minimum zones, the Eastern Tropical North Pacific, the Eastern Tropical South Pacific, and the Arabian Sea. The Bay of Bengal is a fourth oxygen minimum zone, but little nitrate loss occurs there. Our profiling float observations show that the Bay of Bengal is poised for denitrification to occur, as found in prior studies. However, we find that frequent events raise oxygen concentrations above levels where denitrification occurs. The amount of denitrification, relative to the Arabian Sea, is set by fraction of time that oxygen is elevated. Onset of large-scale denitrification would require a shift in the ocean physics that transport the water with elevated oxygen concentration.