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In 1991 Oscillatoria simplicissima replaced Microcystis aeruginosa in the Vaal River, South Africa. This study explored the reason for this succession by looking at the photosynthetic performance of these two cyanoprokaryotes at different temperatures, light intensities and N:P ratios. Microcystis aeruginosa showed higher growth rates and chlorophyll a concentrations than O. simplicissima for most treatments, except the N-deficient treatments. It seems that M. aeruginosa can maintain growth under a range of different conditions and can keep forming mass occurrences, but has high light requirements for photosynthesis and maintenance and can tolerate a much higher light intensity without experiencing photo-inhibition. Oscillatoria simplicissima, on the other hand, is able to harvest available light more efficiently at 25 °C and 15 μmol m-2 s-1, experiences less damage or loss at antenna level and retains more of its electron transport capabilities than Microcystis, giving it a competitive edge over M. aeruginosa under these environmental conditions. [ABSTRACT FROM AUTHOR]