LIGHT AS A SELECTIVE FACTOR IN THE DISTRIBUTION OF PHYTOPLANKTON SPECIES

Cyanobacteria are most often the dominant group of organisms in the ph ytoplankton of eutrophied freshwater lakes. In the first section the d istribution of cyanobacterial populations in the Netherlands was descr ibed in relation to depth, chlorophyll concentration and average light climate. Long term dominance in shallow lakes was mainly caused by Os cillatoria like organisms. Microcystis was found to be dominant in dee per lakes. Although the dominance of Oscillatoria was closely related with high chlorophyll concentrations, the relation between the Oscilla toria dominance and the average underwater light climate was not clear . A good explanation could not be given. In the second section the dom inance of Oscillatoria was followed during the restoration of three la kes. Here it was found that in two shallow lakes of different depth Os cillatoria disappeared from the epilimnion at a Z(eu)/Z(m) value of 0. 4. This is in good agreement with competition experiments done in the laboratory. The stratified population of Oscillatoria rubescens disapp eared from the metalimnion at a Z(eu)/Z(m) ratio of 1.2. This fitted a lso with the physiological information.