Spatial and temporal variability in chlorophyll-a and diatom distribution in the south-east Indian Ocean

Cross frontal changes in chlorophyll-a (chl-a) concentration and diatom (> 20 mum) community species composition between South Africa and the Prince E dward Islands and in the mid-Indian Ocean were investigated during four cru ises of the Marion Island Oceanographic Survey (MIOS I-IV) conducted in Apr il/May between 1996 and 1999. Enhanced chi-a concentrations were recorded a t stations occupied in the vicinity of the Subtropical Convergence (STC) so uth of Africa and in the vicinity of the Subantarctic Front (SAF) in 1998 a nd 1999. At these stations microphytoplankton (> 20 mum) comprised a substa ntial contribution (up to 15 %) of total chi-a. Outside these regions, howe ver, total chi-a was almost entirely dominated by nano- and picophytoplankt on. In the mid-Indian ocean no biological enhancement was associated with t he STC. Species richness was highest in the region south of Africa (up to 4 4 species), particularly in the waters north of the STC. The high number of species recorded here can be related to the complex oceanographic regime. In the mid-Indian Ocean, species richness was consistently low, < 8 species . Numerical analyses :indicated that south of Africa distinct diatom assemb lages, separated by the major frontal systems, were associated with differe nt water masses. South of Africa, the STC represented a strong biogeographi c boundary to the distribution of diatom species. Within this region, a uni que diatom assemblage was associated with the STC proper. In the mid-Indian Ocean, the STC did not act as a boundary to the distribution of diatoms, w hich is likely, the result of a reduced intensity of the front which allows for species to penetrate the front. During all four cruises, the SAF repre sented an important biogeographic boundary to the distribution of diatoms.