A long-term study of the Heiligensee (1975-1992). Evidence for effects of climatic change on the dynamics of eutrophied lake ecosystems

Physical, chemical and biological lake features were examined in the hypereutrophic Heiligensee over a 18-yr. period (1975-1992). Time series analyses of most variables showed trends consistent with a further deterioration in trophic status. Significant rising trends were recorded for water temperature, oxygen saturation, chlorophyll a (mean of the 0-4 m water layer) and hypolimnetic SRP and TP, whereas SRP (mean of 0-4 m) and SD transparency showed significant falling trends. Most of these trends began in 1988/89 and winter and spring months were usually the most affected. Between 1989 and 1992 algal mass remained on high levels during winter (9-63 μgl -1 ) compared to earlier years (5-12 μg -1 ). Starting in 1990 diatoms and cryptophytes, usually dominant during winter and spring, were outcompeted by cyanophytes. The recorded changes happened to occur during a five year period of consecutive mild winters between 1988 and 1992. Basically changes have been triggered by a 1-3 weeks earlier onset of thermal stratification and the absence of longer ice cover periods, which obviously affected the amount of available light for overwintering algae. The earlier onset of thermal stratification affected the system in two major respects: (i) Epilimnetic SRSi limitation started early, and higher sedimentary losses for diatoms are assumed. (ii) It led to an altogether prolongation of the summer stagnation period with repercussion on nutrient dynamics. As hypolimnetic phosphorus accumulated at a constant rate in 9 out of 11 years, the prolongation of the stagnation period is one likely explanation for rising internal phosphorus contents by the end of the summer stagnation period. Sixty-nine percent and 46 % of the variation in maximal algal mass in fall can be explained by maximal hypolimnetic SRP content and the duration of the summer stagnation period. Overall, possible implications of thermal stress on stratified eutrophic lake ecosystems are proposed.