Observed denitrification in the northeast Arabian Sea during the winter-spring transition of 2009

The central and northeast Arabian Sea (AS) has an intense and thick oxygen minimum zone (OMZ) and denitrification zone. It is comparable with the strongest OMZ of the north-equatorial Pacific Ocean. Denitrification in the AS is revisited using a set of cruise observations collected during February–March of 2009 by the Centre for Marine Living Resources, India. The region possesses one of the most robust N* depleted water reaching as low as -20 μmol l−1 at depths (~600 m). In AS, the oxygen depletion is mainly due to sluggish circulation, weak lateral and vertical ventilation. The biological respiration in oxygen deficit condition depletes nitrate and further modifies the Redfield ratio at intermediate depths (200-600 m) from 16N:1P to 8N:1P. Further, analysis of phytoplankton groups present at the surface identifies the presence of Trichodesmium erythraeum, a nitrogen fixing species. The denitrification rate is estimated as 25.3 ± 7.0 Tg N yr−1. It is consistent with past estimates, and no dramatic change in denitrification in the AS was noticed. The study also highlights the potential for further research on N2 production from the Arabian Sea Oxygen Minimum Zones in a changing climate.