Hydrographic and productivity characteristics along 45°E longitude in the southwestern Indian Ocean and Southern Ocean during austral summer 2004

During the austral summer 2004, an intensive multidisciplinary survey was carried out in the Indian Ocean sector of the Southern Ocean to study the main hydrographic features and the associated productivity processes. This sector includes circumpolar zones and fronts with distinct hydrographic and trophic regimes, such as the Subtropical Zone (STZ), Subtropical Frontal Zone (STFZ), Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), North Subtropical Front (NSTF), Agulhas Retroflection Front (ARF), South Subtropical Front (SSTF), Subantarctic Front, Surface Polar Front (SPF), and Subsurface Polar Front. Seasonal variations in the solar irradiance and day length, stratification, lack of micronutrients like iron and increased grazing pressure are the major factors that influenced or constrained biological production in this region. Even though broad differences in these controlling factors exist in time and space between the zonal regions, the upper 1000 m of the water column of the main zones, STZ, STFZ, SAZ, PFZ, supported almost identical standing stocks of mesozooplankton, 0.43, 0.47, 0.45 and 0.49 ml m(-3), respectively, during the austral summer. This unexpected similarity can be explained either through the functioning of the microbial loop within STZ, STFZ and SAZ and the multivorous food web ecology within the PFZ. Dominance of ciliates in the microzooplankton community may be one factor resulting in the maintenance of a high mesozooplankton standing stock in SAZ. In contrast to the zones, frontal regions showed wide differences in hydrography and biological characteristics. The SSTF and SPF were far more biologically productive than that of NSTF and ARF.