Inorganic nitrogen and nitrate reduction in the Arabian Sea

Abstract

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 NO₃-N occurs from south to north at intermediate depths. This is associated with nitrate minima in Northern Arabian Sea. Two maxima in NO₂-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 ( &lt; 0.1 ml.l^-1). It is quite pronounced with nitrite reaching as high as 7 <i>µ</i>g-at.l^-1. The secondary maximum is not observed south of 12ᵒN lat. Neither 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 NO₃-N and PO₄-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 <i>µ</i>g-at.l^-1 are found around 20ᵒN lat. denitrification in Arabian Sea may possibly extend up to 11 or 12ᵒN lat.