After monitoring the trophic state of most of the greater pre-alpine lakes in Bavaria (Melzer et al., 1986; Melzer and Hu¨ nerfeld, 1990; Melzer, 1992) there is a need to focus on the eutrophication and nutrient loads of smaller lakes (Raeder et al., 1998). The occurrence of certain macrophyte species coincides with the nutrient charge of a lake or river (Kohler, 1976; Melzer, 1976; Barko, 1980; Carbi- ener et al., 1990). Up to now the nutrient chargeof lakes could be characterized by the macrophyte index (Melzer, 1988a; Melzer et al., 1988; Melzer, 1993). The effect of changes in the catchment area,
e.g. the treatment of the waste water, could only be quantified after several years of adaption (Melzer et al., 1986; Melzer, personalcommunication, 1992)
The diatom index has been used in several studies to describe the trophic status of lakes (e.g. Jochum, 1994; Jacobsen, 1998; Mayr, 1998; Staab, 1998) and to characterize aquatic laboratory sys- tems (Goos and Raeder, 1999). However, short- term changes (fluctuations, successions) were rarely considered (Seele et al., 1998).