Cyanobacteria are most often the dominant group of organisms in the phytoplankton of eutrophied freshwater lakes. In the first section the distribution of cyanobacterial populations in the Netherlands was described in relation to depth, chlorophyll concentration and average light climate. Long term dominance in shallow lakes was mainly caused by Oscillatoria like organisms. Microcystis was found to be dominant in deeper lakes. Although the dominance of Oscillatoria was closely related with high chlorophyll concentrations, the relation between the Oscillatoria dominance and the average underwater light climate was not clear. A good explanation could not be given. In the second section the dominance of Oscillatoria was followed during the restoration of three lakes. Here it was found that in two shallow lakes of different depth Oscillatoria disappeared from the epilimnion at a Zeu/Zm value of 0.4. This is in good agreement with competition experiments done in the laboratory. The stratified population of Oscillatoria rubescens disappeared from the metalimnion at a Zeu/Zm ratio of 1.2. This fitted also with the physiological information.

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