The distribution and nutrient status of phytoplankton in the Southern Ocean between 20� and 70� W
The distribution of phytoplankton along transects amounting to about 10,000 nautical miles in the sector of the Southern Ocean between 20° and 70°W was determined during the austral summer of 1978/79. Chlorophyll a concentration was monitored by the continuous measurement of in vivo fluorescence (IVF). Surface samples were collected for the determination of temperature, salinity, chlorophyll a concentration, carbon fixation rate and species of the phytoplankton. Phytoplankton distribution was found to be extremely patchy both locally and regionally. High phytoplankton concentrations were often associated with either hydrographic features, such as upwelling or the presence of sea-ice, or with bathymetric features, such as shelf breaks, submarine mountain ranges or islands. Enrichment experiments, in which the effects of various nutrient additions on the rate of 14C fixation by the natural phytoplankton were compared, and bioassay experiments, in which the growth of Thalassiosira pseudonana (Hustedt) Hasle and Heimdal in enriched water samples was measured, were carried out using water samples collected at various stations throughout the study area. Although these techniques were effective in demonstrating nutrient limitation elsewhere, the results suggest that availability of nitrate, phosphate, silicate, trace metals or vitamins exerts no primary control over phytoplankton abundance south of the Polar Front.