Materials and methods
Lakes Licheñskie and Œlesiñskie have different ther-mal regimes. In winter the waters of Lake Licheñskiefroze sporadically and only in the northern stagnant section, and were heated to a maximum temperatureof 30.0
C in summer (Table 1). Ice cover occurred ir-regularly throughout Lake Œlesiñskie, while in sum-mer the maximum temperature was 27.6
C. Thesurface water layers of these lakes were well oxygen-ated, and had similar contents of nitrogen and phos-phorus. However, their water retentions differedsometimes, as did the chlorophyll content in theepilimnion (Pyka et al. 2007, Stawecki et al. 2007). The studies of the plankton were conducted in2004-2005. Samples were collected at 1 m intervalsfrom the 0-5 m layer from March to November in thedeepest part of the lakes. Quantitative analysis of thephytoplankton was performed with an inverted mi-croscope according to the Utermöhl method (1958)and international monitoring norms (Kelly 2004).Organisms, so-called units, were single cells,cenobia, colonies, and threads, and they werecounted in sedimentation chambers (10 ml) using variousmagnifications:largetaxainthewholecham-ber at 100x; medium-sized taxa in 2-4 bands at 200x; nanoplankton in 100 fields at 400x. The bio-mass was calculated by measuring the volume of thecells (Pliñski et al. 1984, Kawecka and Eloranta1994).AJenamedlightmicroscope(CarlZeissJena,Germany) was used to observe the phytoplankton inimmersion oil at magnifications of 200x, 400x, and1000x.