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 Pe ninsula during the summer of 1993 was examined by analysing the phytoplankt on cells undergoing mitosis. The frequency of dividing cells (FDC) of an un identified phototrophic species of Cryptophyceae, which dominated phytoplan kton biomass at the front, varied between 1.2 and 31.6%, with higher percen tages associated with the frontal structure. FDC values increased as phytop lankton biomass increased, suggesting that active growth, rather than passi ve accumulation, was responsible for the enhanced phytoplankton biomass obs erved. The low temperatures (mean +/- SE = 1.11 +/- 0.52 degrees C) that ch aracterized the Antarctic waters sampled imposed an upper limit to the maxi mum FDC reached by the Cryptophyceae, but the relationship between FDC and temperature suggests a clear response of the maximal growth rate of this sp ecies to small changes in temperature. The stabilization of the water colum n, resulting in higher light availability and heating of the surface water in this frontal area, appeared to promote the growth of the phototrophic sp ecies of Cryptophyceae and emphasizes the importance of mesoscale processes as determinants of phytoplankton growth dynamics.