Factors controlling the dominance of Planktothrix agardhii and Limnothrix redekei in eutrophic shallow lakes

Lakes in the Scharmutzelsee-region (East-Brandenburg, Germany) were found to be dominated either by Limnothrix redekei or Planktothrix agardhii. These dominance regimes were stable over the whole investigation period from 1993 to 1995. Six lakes (maximum depth between 2 and 12 m) were compared in regard to physical and chemical conditions to find reasons for the superiority of the first or the second species. All investigated lakes are polymictic but show, nevertheless differences in their mixing-behaviour. In lakes dominated by Limnothrix redekei stratification events occur more often than in the lakes dominated by Planktothrix agardhii. The latter are more windexposed due to their greater size and/or their surroundings. In combination with polymixis light and nutrient supply are discussed as the key factors for the species composition. A lower content of soluble reactive phosphorus and lower light supply are considered as possible advantages for Limnothrix redekei over Planktothrix agardhii in three lakes. A lower total nitrogen/total phosphorus ratio may be a reason for the dominance of Planktothrix agardhii in the other three investigated lakes.