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 ⧹ C org ⧹ ‐ O 2 ) 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 ⧹ C org ⧹ ‐ O 2 = 1 ⧹ ( 15.6 ± 0.7 ) ⧹ ( 110 ± 9 ) ⧹ ( 159 ± 8 ) . These are comparable to the traditional Redfield ratios ( P ⧹ N ⧹ C org ⧹ ‐ O 2 = 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 ⧹ C org ⧹ ‐ O 2 = 1 ⧹ 16 ⧹ 106 ⧹ 150 within the given uncertainties. Separation of nitrate deficits resulting from aerobic partial nitrification (d N) and anaerobic denitrification (d N″) processes using empirical equations is shown to be useful and consistent with other observations. The d N 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 d N″ maximum lies within the oxygen minimum zone with O 2