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 1 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.

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[1] The carbon budget for the eastern and central Arabian Sea was constructed using results from the Modular Ocean Model and biogeochemical data collected largely under the Indian Joint Global Ocean Flux Study programme. The study region (east of 64°E and between 11°and 21°N) was divided into two vertical boxes; a surface box of the top 100 m that largely undergoes exchanges with atmosphere and exhibits relatively strong seasonal variability and a subsurface box between 100 and 1000 m. Water transport rates in surface layers were maximal (up to 83 Â 10 12 m 3 ) in the southwest monsoon season. Sinking from surface driven by convection (25 Â 10 12 m 3 ) largely supports lateral outflows of water in subsurface layers in the northeast monsoon. Surface waters are renewed 10 times faster (t = 0.8 years) than intermediate waters (t = 8 year). A net supply of 25 Tg C yr À1 is estimated to the upper 1000-m water column of the study area by the physical pump. Photosynthetic activity (234 Tg C yr À1 ) does not seem to support total carbon demands (1203 Tg C yr À1) by bacteria and microzooplankton and mesozooplankton in the surface layers. Carbon demand rate requires organic carbon nearly double that of the total living biomass production rate (644 Tg yr À1 ) suggesting that most of the demand might be met from internal cycling involving zooplankton grazing/excretion activities. Sinking flux (69 Tg C yr À1 ) from surface accounts for about 30% of the total photosynthetic production rate indicating intense remineralization of organic matter in the surface layers of the Arabian Sea. Grazing and excretion of carbon by the microzooplankton and mesozooplankton appear to easily sustain perennial supersaturation of carbon dioxide in surface waters of the Arabian Sea and emission of 32 Tg C yr À1 to the atmosphere.

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