Water blooms caused by desmids are very rare. In the mid-forest Lubiec peat bog (pH 4.7-6.8) Desmidium grevillei (Kutz. ex Ralfs) de Bary blooms were observed in the period 1986-1996. They occurred at water temperature of 13-18oC. When temperature rose to 20oC, D. grevillei filaments changed colour into olive and broke up into single cells.