Many hydrobiological investigations of lakes and reservoirs for the most part involve study of certain components of the biocoenosis, and it is often the case that separate groups are examined independently of other components of the biocoenosis and sometimes independently of environmental conditions. A great shortcoming of many investigations is that certain data are lacking, with the result that the complexity of the problem remains inadequately or incompletely explained. In the course of the last decades, increasing attention has been paid to the given problem in the professional literature. In this context, attempts have been made to devise generally applicable models that can be used to explain or predict changes in the plankton community. Application of the PEG model to the Groąnica and Gruľa Reservoirs turned out to be fully justified, even without complete data. Many of the changes observed in the plankton community can be successfully explained in terms of the PEG model.