Abstract
The studies were carried out in six humic lakes (Polesie Region, Eastern Poland) in order to present a role of phytoplankton in relation to physical and chemical parameters characteristic of particular processes. On the basis of physical and chemical properties of waters (SD, colour, pH, EC, Ca2+, DIC, DOC), these lakes are divided into three groups: dystrophic (Lakes Orchowe and Płotycze), humoeutrophic (Lakes Brudzieniec and Pereszpa) and mesoeutrophic (Lakes Moszne and Długie). The occurrence of dystrophication, humoeutrophication or eutrophication processes is clearly reflected in the phytoplankton composition and its abundance, as well as its biomass. In both dystrophic and humoeutrophic lakes, flagellates dominated in the phytoplankton. Among them, the invasive species Gonyostomum semen was very abundant. Mesoeutrophic lakes were characterized by other phytoplankton composition, mainly by non-flagellate species (green algae).