LAUSR

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.