Nitrogen dynamics and nitrate stable isotopes indicate nitrogen loss in the Bay of Bengal

Abstract. Oxygen minimum zones (OMZ) play an important role for the global oceanic nitrogen cycle because they account for 20 to 40 % of the global loss of bioavailable nitrogen despite covering only about 1 % of the global ocean volume. The intermediate waters of the Bay of Bengal (BoB) host one of the most pronounced OMZs with near-anoxic conditions. However, it has not yet been recognized as a site with significant nitrate reduction. In this study, we examined the nitrogen cycling processes in the East Equatorial Indian Ocean (EEIO) and the BoB by measuring water column properties, including temperature, salinity, oxygen and nutrient concentrations, as well as nitrate isotope signatures, collected during the SO305 BIOCAT-IIOE2 cruise in April and May 2024. Potential temperature and salinity profiles showed distinct water masses and limited mixing between BoB and the EEIO at 5° N.

Nitrate stable isotope depth profiles varied significantly, driven by water mass distribution below 300 m and in-situ fractionation above 300 m. Phytoplankton uptake acts as a nitrate sink in the surface waters, showing a significant isotopic enrichment and nitrogen deficit. Below, nitrification was observed, primarily through regenerative production using previously assimilated biomass rather than newly fixed nitrogen from N₂ fixation. Within the OMZ of the BoB, we identified a persistent nitrogen deficit and slightly enriched nitrate isotopes between 100 and 300 m, indicating a nitrogen loss, which we attributed to anammox as the dominant nitrogen loss pathway in the BoB.