Algal and macrophyte communities were studied in five river reservoirs of the hydroelectric power plant chain on the Lower Sava River. The aim of the thesis was to find out how environmental factors influenced algal and macrophyte communities. Fieldwork was done between March and August 2021. Sampling sites were selected in the reservoirs of the five hydropower plants, where physical and chemical parameters of water were measured. Phytoplankton and periphyton samples were taken in winter and summer, macrophytes were identified only in summer. Nitrate and orthophosphate concentrations were measured in the laboratory. The species composition of algal communities in phytoplankton and phytobenthos varied between seasons. We found out that temperature of the water was the environmental factor that influenced the taxonomic composition of phytoplanktonic and phytobenthic algal and diatom communities the most. The communities were also more similar between the seasons than between the sampling sites. In total, 51 algal taxa and 81 species of diatoms algae at all sites were recorded. In phytoplanktonic and phytobenthic communities Diatoma ehrenbergii and Cyclotella meneghiniana were the dominant species in summer and the genera Achnanthidium, Cocconeis, Diatoma, Navicula and Nitzschia were the most abundant in winter. According to the Shannon-Wiener diversity index, we found out that the species diversity of diatoms was the same in winter and summer. A total of 15 species of macrophytes were identified in five locations, species composition varied downstream. The species Myriophyllum spicatum was present in all locations.
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