Nitrogen Cycling in the East Equatorial Indian Ocean and Bay of Bengal: Insights from Nitrate Isotopes and Water Masses

Oxygen minimum zones (OMZ) contribute to 20 to 40 % of global fixed nitrogen loss despite occupying only about 1 % of the ocean. The Bay of Bengal (BoB) contains one of the most pronounced OMZ in intermediate waters worldwide, with oxygen levels near anoxic conditions. Understanding nitrogen cycling in OMZs is critical for comprehending and accurately modeling the global oceanic nitrogen cycle.

In this study, we examined nitrogen cycling in the East Equatorial Indian Ocean (EEIO) and the BoB using water column properties—including temperature, salinity, oxygen, nutrients, and dual stable isotopes of nitrate—collected during a cruise in April/May 2024. Potential temperature and salinity profiles revealed a clear separation between the BoB and the EEIO at 5°N, with distinct water mass distributions and limited mixing between the two regions.

Depth profiles of nitrate stable isotopes displayed notable variations. In waters below 300 m, isotopic signatures were influenced solely by water mass distribution. In contrast, isotope variations in the upper 200 m reflected active on-site fractionation. Surface waters (<100 m) exhibited significant nitrate isotope enrichment and a nitrate deficit, driven by phytoplankton uptake. Below this layer, nitrification was observed, primarily through regenerative production using previously assimilated biomass rather than newly fixed nitrogen from N₂ fixation. A regional decoupling of nitrate dual isotopes, with more enriched δ¹⁸O-NO₃^- in more northern samples of the central BoB, suggested increased nitrite reduction followed by re-oxidation without full assimilation into organic matter in the BoB.

Within the OMZ of the BoB, we identified a persistent nitrate deficit and slightly enriched nitrate isotopes, indicative of nitrogen loss. Given that oxygen concentrations remained slightly above the threshold for significant denitrification in most samples, anammox likely represents the dominant nitrogen loss pathway in the BoB's OMZ.