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The lipophilic photosynthetic pigments in Limnothrix redekei, Planktothrix agardhii(cyanobacteria), Stephanodiscus minutulus, Synedra acus(diatoms), Scenedesmus acuminatus, and Scenedesmus armatus(chlorophycean) all isolated from an eutrophic lake were quantitatively determined by HPLC. The algae were grown semi‐continuously under nutrient sufficient conditions at 20°C at a 12/12 h light/dark cycle with constant irradiance or with simulated natural light fluctuations as well as at a 6/18 h light/dark cycle with constant irradiance, all at the same daily light exposure.

The nutrient saturated growth and the competition between Limnothrix redekeiand some spring diatoms for phosphate were studied in semi‐continuous cultures at 10°C with a 6/18 h light/dark cycle and at 15 °C with a 12/12 light/dark cycle. Under these conditions in unialgal cultures, the maximum specific growth rate of Limnothrix redekeiwas two to three time lower than that of the diatoms. However, Limnothrix redekeiwas the superior competitor for phosphate under both conditions in mixed cultures. The diatoms in mixed cultures from each species with Limnothrix redekeiwere competitively displaced but at a different rate. This allowed the diatoms to be ranked in terms of their competitive ability for phosphate at 10°C as follows: Synedra acus > Asterionella formosa > Stephanodiscus minutulus > Nitzschia acicularis > Cyclotella meneghiniana > Stephanodiscus neoastrea > Stephanodiscus hantzschii.The competitive ability of Limnothrix redekeiat 15 °C was increased and the rank order of diatoms slightly changed. The hypothesis that cyanobacteria are promoted by phosphate because they are inferior competitors for phosphate is incorrect for Limnothrix redekei.