Biogeographical structure of the microphytoplankton assemblages in the region of the Subtropical Convergence and across a warm-core eddy during austral winter

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Abstract
The distribution of rnicrophytoplankton in the region of the Subtropical Convergence (STC) and across a warm-core eddy shed from the Aguihas Return Current was investigated along two transects in late austra] winter (June-July) 1993, during the South African Antarctic Marine Eco system Study (SAAMES) III cruise. Sampling was undertaken for the analysis of nutrients, and for the enumeration and identification of microphytoplankton species. Along both transects, chlorophyll con centrations were highest at stations at the southern boundary of the STC and at the periphery of the warm-core eddy. Of the variance associated with chlorophyll concentration, temperature accounted for 65% of the total. Along both transects, a decrease in species richness from north to south was observed. The spatial distribution of the numerically dominant diatom species was similar in both tran sects. The microphytoplankton assemblage was dominated by the subtropical diatom species Chaetoceros constnctus north of the STC, and by Pseudoeunotia doliolus within the eddy and south of the STC. Using cluster and ordination analyses, three significantly different groupings of stations were identified along the 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 biogeograph ical boundary. The predominance of the warm-water species P.doliolus and Planktoniella sol in and around the warm-core eddy south of the STC suggests that eddies are important in the transfer of microphytoplankton across this strong biogeographical boundary.