Seasonal variability in oxygen and nutrients in the central and eastern Arabian Sea

Abstract

Extensive observations made in the central and eastern Arabian Sea under JGOFS (India) programme suggest strong seasonal variations in concentrations of oxygen and nutrients in the water column. In the intermediate waters (depth range 150-800 m) oxygen concentrations were lowest during winter (with values near zero at ∼400 m) in relation to those in the other two seasons (intermonsoon and southwest monsoon). This together with nitrate distributions revealed the occurrence of intense reducing conditions in intermediate waters during winter because of the sluggish water movement and high surface productivity. Secondary nitrite, an indicator of occurrence of denitrification was present at oxygen levels < 10 μM. Nitrate deficits reached a maximum of 10 μM in winter, whereas, it was half this value in monsoon. The average deficits suggest increasing reducing conditions in the order monsoon (1.6 μM), intermonsoon (3.7 μM) and winter (4.0 μM).