Nitrate deficits by nitrification and denitrification processes in the Indian Ocean

The three-end-member mixing model of Li and Peng [Latitudinal change of remineralization ratios in the oceans and its implication for nutrient cycles. Global Biogeochemical Cycles 16, 1130–1145] was applied to the World Ocean Circulation Experiment (WOCE) data from Indian Ocean to obtain additional estimates on the remineralization ratios (P⧹N⧹Corg⧹‐O2)(P⧹N⧹Corg⧹‐O2) of organic matter in the oxygenated regions. The results show systematic changes of the remineralization ratios with latitude and depth in the Indian Ocean. The average remineralization ratios for Indian warm water masses (potential temperature θ>∼10∘C) are P⧹N⧹Corg⧹‐O2=1⧹(15.6±0.7)⧹(110±9)⧹(159±8)P⧹N⧹Corg⧹‐O2=1⧹(15.6±0.7)⧹(110±9)⧹(159±8). These are comparable to the traditional Redfield ratios (P⧹N⧹Corg⧹‐O2=1⧹16⧹106⧹138)(P⧹N⧹Corg⧹‐O2=1⧹16⧹106⧹138), and are in good agreement with Anderson's [On the hydrogen and oxygen content of marine phytoplankton. Deep-Sea Research I 42, 1675–1680.] values of P⧹N⧹Corg⧹‐O2=1⧹16⧹106⧹150P⧹N⧹Corg⧹‐O2=1⧹16⧹106⧹150 within the given uncertainties. Separation of nitrate deficits resulting from aerobic partial nitrification (dN) and anaerobic denitrification (dN″) processes using empirical equations is shown to be useful and consistent with other observations. The dN maximum coincides with the phosphate and nitrate maximums, lies within the oxycline below the oxygen minimum zone, and is in contact with the continental slope sediments. The dN″ maximum lies within the oxygen minimum zone with O2<∼2 μmol/kg, is in contact with shelf or upper slope sediments, and is always associated with a secondary nitrite maximum in the water column. The spatial extent of dN is much larger than that of dN″. The low N/P remineralization ratio (<15) for deep waters (θ<∼10 °C) and the dN maximum in the lower oxycline can be best explained by the partial conversion of organic nitrogen into N2, N2O, and NO by yet unidentified bacteria during oxidation of organic matter. These bacteria may have evolved in a low oxygen and high nitrate environment to utilize both oxygen and nitrate as terminal electron acceptors during oxidation of organic matter (i.e. the partial nitrification hypothesis). Direct proof is urgently needed.