Maintaining a phytoplankton bloom in low mixed layer illumination in the Bellinghausen Sea in the Austral Spring, 1992

A phytoplankton bloom was observed between 67.2 degree(s)S and 68.5 degree(s)S which extended over at least 4 degrees of longitude from 84 degree(s) to 88 degree(s)W in the Bellinghausen Sea. The northern boundary of the bloom was associated with a strong eastward flowing surface jet of low salinity water. The controlling factors for phytoplankton growth were explored by examining north to south transects of surface stability, inorganic nutrients, mean mixed layer irradiance and surface currents. The only parameter found to change along with the phytoplankton biomass was the mean mixed layer irradiance, which approached the light limitation thresholds for Antarctic phytoplankton. A hypothesis to explain the presence of the high concentrations of phytoplankton is a positive feedback mechanism in which the phytoplankton grow well in the low light conditions of the mixed layer. The high concentrations of biomass reduce the light sufficiently to allow low light-adapted phytoplankton flourish, in contrast to the phytoplankton in the clearer waters north and south of the bloom.