The distribution of phytoplankton along several transects of the South Atlantic ocean was determined during the austral summer of 1978/79, Chlorophyll a concentration was monitored by the continuous measurement of in situ fluorescence . Surface samples were collected for the measurement of temperature, salinity, chlorophyll a concentration , carbon fixation rate and qualitative estimation of population composition. The results indicated that the distribution of phytoplankton was extremely patchy throughout the area at both the local and regional level. Above average phytoplankton concentrations were often found to be associated with either disti nct hydrographic features,
such as upwelling or the presence of sea ice, or with distinct bathymetric features, such as shelf breaks, islands or submarine mountain ranges.
Nutrient enrichment experiments, where the effects of various
nutrient additions on the rate of 14c fixation were compared within the
natural phytoplankton assemblage , and bioassay experiments, where the
growth rates of Thalassiosira pseudonana in enriched water samples
were studied, were carried out using water collected at various localities throughout the study area. In Liverpool Bay, during the summer
of 1978, similar experiments had shown that under appropriate conditions
a clear indication of nutrient limitation within a phytoplankton population could be obtained . Population samples collected in the South Atlantic did not appear to be nutrient limited. In one bioassay experiment, using water collected to the north of the sub-tropical convergence, the results indicated that low availability of combined nitrogen might be limiting the rate of phytoplankton growth . No
supporting evidence for this was obtained from the corresponding enrichment experiment. In all other bioassay experiments it was combined nitrogen which limited the final cell yield of the cultures.
The results suggest that nutrient availability exerts no primary control over the distribution or growth of Antarctic phytoplankton, other factors such as light availability and differential grazing pressure may be important in this respect.