This paper presents the results of research on phytoplankton in an oxbow lake on the Vistula River. Originally, the reservoir was natural; however, at the beginning of the twentieth century, the basin of the oxbow lake was dredged and broadened. In 1934, the harbor basin became a protected spawning ground for pikeperch, and as such is a unique site in Poland. The research presented herein was conducted during the growing season of 2008, when 249 taxa of algae were identified in the

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... , mainly Chlorophyta and Bacillariophyceae. The average biomass of the oxbow lake during the entire research period was 33 mg dm -3 . The largest biomass was recorded in August at more than 90 mg dm -3 . During the entire growing season, the dinoflagellates Peridinium willei (Huitfeld-Kaas) played a significant role in the phytoplankton. In the summer, a mass bloom of blue-green algae occurred in the phytoplankton, with the following dominant species: Aphanizomenon flos-aquae (L.) Ralfs ex Born. et Flah.; Dolichospermum planctonicum (Brunnthaler) comb.n.; Synechocystis aquatilis (Sauvageau). Port Drewny was classified as hypertrophic based on the results of the phytoplankton research and the physicochemical conditions. The dominance of blue-green algae, including potentially toxic species, could be dangerous for Port Drzewny as a protected spawning ground for pikeperch.