Primary productivity and nutrients in the Indian sector of the Southern Ocean

Abstract

Hydrography, nutrient and biological productivity data collected from 1981 to 1986 in the Indian sector of the Southern Ocean between 11-degrees to 53-degrees-E longitude are reported. The physical processes are most active in this region, with nutrient rich water upwelling at the Antarctic divergence (AD) at 65-degrees-S and water downwelling at the Antarctic convergence (AC) or subtropical convergence (STC) to the north. Chlorophyll a, primary producitivity and zooplankton estimations suggest that the regions south of AC are more productive than others. Higher productivity at the continental ice-edge than in oceanic waters is accounted for by the stability of the water column brought about by low salinity due to ice melting. Sharp depletion of Si near the AC where NO3 and PO4 maintain high values, as in the higher latitudes, reflects a unique behaviour of this element (Si) as regards to its biological uptake and regenerative mechanisms. Nutrient anomaly studies indicate that N regeneration is sufficiently fast in surface waters while most of the Si incorporated in diatoms and silicoflagellates tend to escape to deeper waters or to bottom sediment. The importance of microbial population is stressed to understand the food chain dynamics of Antarctic ecosystem.