'NO', a useful tool for the estimation of nitrate deficits in the Arabian Sea

Abstract

The property 'NO', defined as the sum of O₂ and 8.65 NO₃, varies linearly with potential temperature (θ) outside the denitrification zone in the Arabian Sea, but a change in slope of the 'NO'-θ regression line occurs at θ ≈ 15°C. The 'NO'-θ relationships have been utilized to compute the 'original' nitrate concentrations which are combined with the observed data to calculate the nitrate deficits within the denitrification zone. This procedure, which approximately accounts for the 'reserved' nitrate, eliminates the errors associated with the use of the variable ΔAOU:ΔPO₄ ratio in earlier methods. In upper layers, the present method yields deficits close to those deduced from the oxidative ratios and (NO₃)_r-(PO₄)_r relationship. In deep layers (depth > 500 m), however, slightly higher values are obtained with the present method, presumably due to the variability of relationships between 'reserved' nutrients. Distribution of nitrate anomaly (ΔN) along a section running from 15°N, 67°E to 21°N, 63°E closely follows the distribution of nitrite at the secondary maximum. Nitrate anomalies in excess of 8 μg-at. dm^-3 are observed frequently along this section. An intermediate minimum in ΔN is observed at some stations at depths where the Subantarctic Mode Water is encountered. This water mass, relatively rich in oxygen, appears to be a source of limited oxygen supply which probably prevents the intermediate layers from becoming completely anoxic.