The research carried out on the Dymer river, a small watercourse in the eastern part of the Olsztyńskie Lakeland, aimed to determine the quantitative and qualitative changes in the phyto- and zooplankton caused by the operation of a small hydropower station. Taxonomic identification was done by using common methods and keys. The vegetation of the reservoir created by damming the river water was studied on the basis of phytosociological records, and the plankton was investigated on the basis of research made at three dates of the vegetation season at sites located upstream and downstream of the dam. In the water reservoir, numerous macrophyte associations of well-developed structure were found to occur. The reservoir itself is also attractive for plankton communities. Among algal groups, Bacillariophyceae account for the largest proportion of the whole algal plankton community. Due to their intensive development and distinct domination, the proportion of the other groups of algae is small. The species compositions and taxonomic structures of the phytoplankton living upstream and downstream of the dam are rather similar, differences concern only the forms in which some species occur. Namely, the colonies of Stephanodiscus downstream of the water turbine are disintegrated, and the coenobia of Pediastrum are deformed and fragmented. The damage is mechanical in nature and is done when water flows through the turbine. Upstream of the dam, in shallow, fertile and sunlit areas, large patches of Enteromorpha intestinalis f. maxima have developed. The zooplankton is dominated by Rotatoria. Among others, some rare, thermophilous species of Rotatoria, typical of warm, shallow, extremely fertile water bodies, occur in the water reservoir. The size of Protozoa community in the water environment downstream of the dam is higher than upstream of it.

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