Communities of Primary Producers in the Series of Reservoirs on the Sava River (Slovenia).

Different communities of primary producers were surveyed in a series of five river reservoirs on the Sava River in southeast Slovenia. Seasonal differences of phytoplanktonic and phytobenthic communities were studied in the reservoirs of hydroelectric power plants. Macrophytes were surveyed in the summer, while phytoplankton and periphyton were sampled in the winter and summer of 2021. The taxonomic composition of diatoms was studied in greater detail and additionally analysed. The results showed that the species composition of phytoplankton and phytobenthos differed between seasons. The communities were also more similar between the seasons at the same sampling site than between the sampling sites. Temperature was the parameter that had the greatest impact on the taxonomic composition of phytoplanktonic and phytobenthic communities. In total, 51 algal taxa and 81 species of diatoms were recorded, respectively. Furthermore, 15 species of aquatic macrophytes were found. The abundance of phytoplankton was the highest in the lowest reservoir in the series, in Brežice, where the temperature was also the highest. The number of macrophytes also increased downstream, but their abundances were low, mainly due to coarse substrate on the banks consisting of rocks and boulders. Constructing additional reservoirs on this section of the Sava River could increase the probability of the substantial growth of phytoplankton and cyanobacteria within it. On the other hand, providing patches of finer substrates (gravel, pebbles) would support the abundance of macrophyte species, which could control the concentrations of nutrients in the summer and prevent the blooms more efficiently.