Increased frequency of dividing cells of a phototrophic species of Cryptophyceae at a frontal structure off the Antarctic Peninsula

The hypothesis that enhanced phytoplankton growth explained the increase in phytoplankton biomass observed at a frontal structure off the Antarctic Peninsula during the summer of 1993 was examined by analysing the phytoplankton cells undergoing mitosis. The frequency of dividing cells (FDC) of an unidentified phototrophic species of Cryptophyceae, which dominated phytoplankton biomass at the front, varied between 1.2 and 31.6%, with higher percentages associated with the frontal structure. FDC values increased as phytoplankton biomass increased, suggesting that active growth, rather than passive accumulation, was responsible for the enhanced phytoplankton biomass observed. The low temperatures (mean ± SE = 1.11 ± 0.52°C) that characterized the Antarctic waters sampled imposed an upper limit to the maximum FDC reached by the Cryptophyceae, but the relationship between FDC and temperature suggests a clear response of the maximal growth rate of this species to small changes in temperature. The stabilization of the water column, resulting in higher light availability and heating of the surface water in this frontal area, appeared to promote the growth of the phototrophic species of Cryptophyceae and emphasizes the importance of mesoscale processes as determinants of phytoplankton growth dynamics.