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 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.

Mesozooplankton community structure in the upper 1,000 m along the western Bay of Bengal during the 2002 fall intermonsoon

“…However, Sardessai et al (2007) and Prasanna reported the persistence of a shallow cyclonic eddy at WB3 and another one that was as deep as 1,000 m at WB1. Those studies also showed that cold-core eddies led to the shoaling of a nitracline into the mixed layer and enhanced Chl a and primary production.…”

“…In the absence of strong upwelling, this layer is usually devoid of essential dissolved nutrients (Sen Gupta and Naqvi 1984;Sardessai et al 2007). The persistent cloud cover and sediment-induced turbidity shallows the euphotic depth causing the region to remain moderately oligotrophic (Prasanna Kumar et al 2002).…”

Seasonal and spatial distribution of particulate organic matter in the Bay of Bengal

“…These results are published elsewhere (e.g. Madhupratap et al, 2003;Prasanna Kumar et al, 2004;Gauns et al, 2005;Sardessai et al, 2007). These results are briefly described here.…”

“…In the Bay of Bengal, three water masses i.e. the Arabian Sea High Salinity water mass (surface to 100 m), the Bay of Bengal sub-surface water mass (100 to 300 m), and the Indian Equatorial Intermediate water mass (> 300 m) have been identified (Gallagher, 1966;Sardessai et al, 2007). Therefore, the advection of the Indian Equatorial Intermediate water mass rich in the organic matter is probably responsible for the high concentrations of POC at 1000 m. This organic matter was characterized with high % POC and low C/N ratio.…”

Role of meso-scale eddies on the variability of biogenic flux in the northern and central Bay of Bengal

“…To further substantiate this, we used available in situ measurements of air-sea CO 2 fluxes near NBBT and CBBT regions carried out under the Bay of Bengal Process Studies (BOBPS) program ] during 2001, which includes both eddy events and eddy-free events. During the above period, several eddies were picked up in the measurements and were studied in detail Sardessai et al, 2007]. Using the data from the above measurements, Sardessai et al [2010] reported cyclonic eddy-induced airsea CO 2 fluxes of À40 mmol m À2 d À1 and À15 to À20 mmol m À2 d À1 during summer and fall intermonsoon, respectively (negative indicates flux from atmosphere to ocean).…”