Influence of environmental forcings on the seasonality of dissolved oxygen and nutrients in the Bay of Bengal

Abstract

Studies on seasonal variability of oxygen and nutrients during three seasons namely SW monsoon, fall intermonsoon and spring intermonsoon indicate influence of physical forcings on the distribution of these hydrochemical properties in the subsurface layer. In the open ocean the Minimum Oxygen Layer (MOL 10mu Mol L-1) during the southwest monsoon and fall intermonsoon is mostly confined to the north of 11N due to the penetration of high salinity water in the deeper waters of the central Bay. During spring intermonsoon MOL is mostly confined to the northern region between 14 to 20N with a narrow band of suboxic waters (5mu Mol L-1) around 19 to 20N. Along the western margin, the MOL occupies a larger area in the intermediate and deeper waters during the SW monsoon and fall intermonsoon with a thick layer of suboxic waters during the SW monsoon which gets reduced and confined to the northern region during fall intermonsoon. The core of suboxic waters seems to disappear during the spring intermonsoon. The displacement of the water mass to shallower depths under the influence of cold core eddies is the major mechanism supplying nutrients to the surface waters whereas stratification due to the immense runoff from major rivers in the north and the associated suspended load addition seems to be inhibiting the biological production through curtailment of light penetration in the northern Bay of Bengal during the southwest monsoon. Pockets of low oxygen contents are not associated with elevation in secondary nitrite levels suggesting that circulation of the water mass under the influence of seasonal currents and gyres and the geochemical processes play a significant role in regenerative processes and regulating the intensity of the MOL in the Bay of Bengal.