The comparison of the latest and previous data on the structure of phytoplankton and chlorophyll a concentration in Lake Charzykowskie indicated long-term changes and fluctuations. The main objective of this study was to assess the phytoplankton communities in the lake with reference to hydrochemical conditions. Detailed hydrobiological studies were carried out in 2014-2015 to explain the improvement in the trophic status of the lake observed in 2008-2009. The research has shown that the phytoplankton community structure does not change significantly during the limnological cycle. The spring/ autumn season is characterized by the highest contribution of diatoms and cryptophytes. Massive development of dinoflagellates was observed during the summer. Relative abundance of cyanobacteria (Aphanizomenon, Dolichospermum and Microcystis) was significantly lower compared to the previous years. In August 2014, cyanobacteria accounted for 100% of the total phytoplankton at site 2. In the following year, cyanobacteria represented only 13% in the summer phytoplankton. The current research has confirmed the favorable trophic changes in the phytoplankton of the lake. Unfortunately, very poor oxygen conditions will continue to affect the trophic status of the lake through mobilization of the internal nutrient supply. The current conditions of the lake require further monitoring of changes in the ecosystem.
