BIOGEOGRAPHICAL STRUCTURE OF THE MICROPHYTOPLANKTON ASSEMBLAGES IN THE REGION OF THE SUBTROPICAL CONVERGENCE AND ACROSS A WARM-CORE EDDY DURING AUSTRAL WINTER

The distribution of microphytoplankton in the region of the Subtropica l Convergence (STC) and across a warm-core eddy shed from the Agulhas Return Current was investigated along two transects in late austral wi nter (June-July) 1993, during the South African Antarctic Marine Ecosy stem Study (SAAMES) III cruise. Sampling was undertaken for the analys is of nutrients, and for the enumeration and identification of microph ytoplankton species. Along both transects, chlorophyll concentrations were highest at stations at the southern boundary of the STC and at th e periphery of the warm-core eddy. Of the variance associated with chl orophyll concentration, temperature accounted for 65% of the total. Al ong both transects, a decrease in species richness from north to south was observed. The spatial distribution of the numerically dominant di atom species was similar in both transects. The microphytoplankton ass emblage was dominated by the subtropical diatom species Chaetoceros co nstrictus north of the STC, and by Pseudoeunotia doliolus within the e ddy and south of the STC. Using cluster and ordination analyses, three significantly different groupings of stations were identified along t he combined transects. These coincided with stations located north and south of the STC and with the warm-core eddy proper, confirming that the STC represents a strong biogeographical boundary. The predominance of the warm-water species P.doliolus and Planktoniella sol in and aro und the warm-core eddy south of the STC suggests that eddies are impor tant in the transfer of microphytoplankton across this strong biogeogr aphical boundary.