A number of modelling results suggested thermocline shifts as a consequence of
global climate change in stratifying lakes. Abundance and composition of the
phytoplankton assemblage is strongly affected by the stratification patterns,
therefore, change in the thermocline position might have a substantial effect on
this community or even on the whole lake ecosystem. In this study, thermocline
depths in large mesocosms installed in Lake Stechlin (Germany) were deepened
by 2 meters and phytoplankton changes were analysed by comparing changes to
untreated mesocosms. Higher amounts of SRP were registered in the hypolimnion
of treatment mesocosms than in the controls, and there were no differences in the
epilimnion. Small but significant changes were observed on the phytoplankton
community composition related to the effect of deepening the thermocline;
however, it was weaker than the yearly successional changes. The most
remarkable differences were caused by Planktothrix rubescens and by
chlorophytes. P. rubescens became strongly dominant at the end of the experiment
in the mesocosms, and in the open lake as well. The results of the experiment
cannot clearly support the proliferation of cyanobacteria in general; however, the
deepened thermocline can modify the behaviour of some species, as was observed
in case of P. rubescens
