Different light conditions at the open and shaded sites caused by marginal vegetation affected the structure of Cyanobacteria and algae communities, but had no effect on chlorophyll a content. In the open area Hydrurus foetidus (Villars) Trevisan, Homoeothrix janthina (Bornet et Flahault) Starmach, and diatoms (mainly Achnanthes minutissima Kutz. together with A. biasolettiana Grun. and species from the Gomphonema genus ) were the most numerous. In shadow the abundance of Hydrurus foetidus drastically decreased, whereas the diatom biomass index, Achnanthes minutissima and A. biasolettiana showed a tendency to reduce their number. On the contrary, the abundance of green algae and Cocconeis placentula Ehr. var. euglypta Ehr. increased. Chlorophyll a contents of both sites obtaining the highest values in summer - autumn and the lowest in the winter - spring seasons.