| Summary: | Includes bibliographical references. === Colonization and temporal changes in phytoplankton diversity and biomass in the upwelling plumes off the Cape Peninsula are described for two different communities. A drogue study showed the presence of a fast-developing mixed phytoplankton community with Chaetoceros compressus Laud. and Skeletonema costatum (Grev.) Cleve the dominant species. Monthly transect studies, however, revealed the presence of another community consistently dominated by Nitzschia spp. Species succession did not occur in either phytoplankton community. Factors Likely to affect the development of the phytoplankton communities and cause differences in certain community characteristics, e.g. dominance, biomass and diversity, were attributed to differences in: ( 1) the origins of the source water; (2) the wind speed and direction; and (3) the sampling strategies employed. The factors responsible for the successful colonization of the different species in the phytoplankton community were thought to depend on: (1) the initial variations in species diversity and abundance of the seeded population in newly upwelled waters; (2) the extent to which the upwelled waters mixed with neighbouring waters; and (3) the specific selective adaptations for growth of the individual species. Possible adaptive phytoplanktonic mechanisms in a changing environment, were discussed in terms of cell size, growth, nutrient absorption and buoyancy.
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