The lakes Veluwemeer and Wolderwijd have long suffered from eutrophication stress. Recently, however, water quality has improved substantially. An overview of the developments in water quality (transparency, PTOT, chlorophyll) is presented for 1969-1996. Dominance of filamentous cyanobacteria in summer is analyzed. P-reduction is the key-factor to reduce cyanobacterial dominance. At PTOT>0.20 mg l−1 cyanobacterial dominance approaches 100%, at 0.05<PTOT<0.20 mg l−1 the dominance decreases rapidly, while at PTOT≤0.05 mg l−1 filamentous cyanobacteria virtually disappear. In the intermediate range for P summer dominance of cyanobacteria may be either high or low. The occurrence of these two states is related to the winter conditions and the global insolation in spring (April-May). Low summer dominances may occur after a severe winter. A severe winter results in a low inoculum of filamentous cyanobacteria, a mild winter in a high inoculum. High cyanobacterial dominance in summer is related to a high inoculum and/or high global insolation in spring. When the inoculum is low, i.e. after a severe winter, cyanobacterial dominance in summer may be either low or high. Low global insolation does not provide an indication for poor conditions for growth.

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