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

S. N. de Sousa, M. Dileep Kumar, S. Sardessai, V. V. S. S. Sarma and
P. V. Shirodkar

National Institute of Oceanography, Dona Paula, Goa 403 004, India

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 denitrirication was present at oxygen levels < 10mM. Nitrate deficits reached a maximum of 10mM in winter, whereas, it was half this value in monsoon. The average dericits suggest increasing reducing conditions in the order monsoon (1.6 mM), intermonsoon (3.7 mM) and winter (4.0 mM).