SURFACE DISTRIBUTION OF MICROPHYTOPLANKTON OF THE SOUTH-WEST INDIAN-OCEAN ALONG A REPEAT TRANSECT BETWEEN CAPE-TOWN AND THE PRINCE-EDWARD-ISLANDS

Surface chlorophyll-a (chl-a) concentrations, microphytoplankton (>20 mu m) species composition and distribution along a repeat transect bet ween Cape Town and the Prince Edward Islands were investigated in earl y austral autumn (April/May) 1996. Samples were collected at approxima tely 30 nautical mile intervals for the analysis of size-fractionated chi-a and the identification and enumeration of microphytoplankton spe cies. Peaks in total chi-a (>1 mu g l(-1)) were recorded at the Subtro pical Convergence (STC), at the Sub-Antarctic Front (SAF) and in the w aters surrounding the Prince Edward Islands. In addition, a minor peak in chi-a concentration was recorded in the continental shelf waters. At stations where elevated chi-a concentrations were recorded, microph ytoplankton generally formed a substantial contribution (similar to 10 %) to total chlorophyll. Outside these regions, total chlorophyll conc entrations were lower (<0.9 mu g l(-1)) and almost entirely dominated by nano-and picophytoplankton, which contributed >95% of the total. Mi crophytoplankton species composition along both transects were dominat ed by chain-forming species of the genera Chaetoceros (mainly C. negle ctum, C. peruvianus and C. constrictus), Nitzschia spp. and Pseudoeuno tia doliolus. Cluster and ordination analysis based on species composi tion identified five distinct microphytoplankton assemblages, which we re closely associated with the different water masses in the region be tween Cape Town and the Prince Edward Islands. The microphytoplankton species composition and biogeographic zones identified during this inv estigation are in general agreement with similar studies conducted in the south-west Indian Ocean during the austral summer; which suggests that there are little seasonal trends in both the microphytoplankton s pecies composition and biogeographic zonation.