Hydrographic characteristics of the Indian sector of the Southern Ocean

Oceanographic studies in the Indian sector of the Southern Ocean (58-61 deg rees S; and 30-40 degrees E) carried out during December 1995 and March 199 6 indicate a 3-layer structure typical of summer in the oceanic domain sout h of the Antarctic Polar Front. The upper 300 m water column consists of th ree distinctive thermohaline characteristics. The surface layer (50 m) of s ummer surface water formed by seasonal warming with temperatures (0-2 degre es C) and low salinity (< 33.8 PSU), the intermediate layer (50-150 m) of w inter reminiscent water with negative temperatures (0 to -2 degrees C) and moderate salinity (33.8-34.3 PSU) and the deeper layer (below 150 m) of Cir cumpolar Deep Watermass (CDW) with positive temperatures (0-2 degrees C) an d higher salinity (34.3-34.8PSU). The influence of the strong Antarctic Div ergence between the east and west drifts is noticed at 61 degrees S, 34 deg rees E marked by phytoplankton and zooplankton patches. The dissolved oxygen (DO) shows high concentrations (7.6 to >8.0 ml l(-1)) in the upper layer, These higher levels of DO sometimes extend down to the intermediate layer and gradually decrease further downwards. The observed l ow temperature and high DO in the subsurface water (50-75 m) are typical si gnatures of Antarctic Winter Watermass (AWW) formed during winter, Calcium, magnesium and boron show significant spatial variations with higher values of Ca and Mg along 59 degrees S and 60 degrees S relative to 61 degrees S, Ca exhibits large variations (398-421 mg kg(-1) while Mg shows least varia tions with concentrations close to the average value for normal sea water ( 1294 mg kg(-1)). Boron depicts high values along 59 degrees S and 61 degree s S compared to 60 degrees S, with higher values in the subsurface layer (2 5-75 m) which are attributed to biological characteristics and the frontal systems in the region. Phytoplankton cell counts remain high (between 1.92 and 22.9 x 10(4) per li tre) while zooplankton biomass shows variations from 9.8 to 303.62 ml 100 m (-3). A marked drop of both phytoplankton and zooplankton in the region of krill swarm (between 60 degrees 40'-61 degrees 13'S and 33 degrees 34'-34 d egrees 14'E), however, indicates active feeding by the latter on the plankt onic communities.