Inorganic nitrogen and nitrate reduction in the Arabian Sea
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Concentration of inorganic nitrogen compounds within the euphotic zone is very low and it increases sharply in the thermocline layer. A steady decrease in the NO sub(3)-N occurs from south to north at intermediate depths. This is associated with nitrate minima in Northern Arabian Sea. Two maxima in NO sub(2)-N are observed in northern region. The shallow (primary) nitrite maximum with low nitrite occurs in surface layer near thermocline in conjunction with a relatively high dissolved oxygen. The intermediate (secondary) maximum is invariably associated with very low oxygen ( 0.1 ml.l/1). It is quite pronounced with nitrite reaching as high as 7 mu g-at.l/1. The secondary maximum is not observed south of 12 N lat. Nither the nitrate minima nor the secondary nitrite maximum seem to be associated with Persian Gulf water. It is suggested that these originate as a result of biological reduction of nitrate (denitrification) due to the prevailing reducing conditions associated with a pronounced depletion of dissolved oxygen below the thermocline. The nitrate deficits have been calculated using the relationships of the apparent oxygen utilization (AOU) with NO sub(3)-N and PO sub(4)-P. Results indicate a steady northward increase in nitrate deficits and in the depth range where denitrification occurs. Maximum values reaching up to 16.5 mu-at.l/1 are found around 20 degrees N lat. denitrification in Arabian Sea may possibly extend up to 11 or 12 degrees N lat.
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