In the summer seasons of 2017–2020, Cyanophyta (July – August) and Bacillariophyta (June) prevailed in phytoplankton in the mouth of the Kazanka River (left-hand tributary to Kuibyshevskoe storage reservoir). Cyanobacteria were mostly represented by filamentous forms – Aphanizomenon flos-aquae Ralfs ex Bornet & Flahault and Planktothrix agardhii (Gomont) Anagnostidis & Komárek; diatoms were predominantly centric (Stephanodiscus hantzschii Grunow, Stephanodiscus sp., Cyclotella sp.). The dissolved oxygen content in the surface water layer did not fall below standard values for fishery waters (MPCf) and varied within 6.2–17.0 mgO2/l. Easily oxidizable organic matter expressed as BOD5 exceeded MPCf 1.4–3.7‑fold. Correlation analysis revealed a moderate and/or strong relationship between the taxonomic composition and abundance of the phytoplankton community on the one hand and oxygen regime parameters in the river’s surface water layer on the other. Water enrichment with oxygen is mainly associated with the development of green algae (Spearman’s coefficient at 0.69, p < 0.05) and diatoms (0.56); organic matter formation in the water – with the development of cyanobacteria (0.69), dinophytes (0.69), and diatoms (0.74). Meteorological (air temperature, precipitation) and temporal (year, month of observations) factors influence the phytoplankton density index, dissolved oxygen content, COD and BOD5 in the mouth of the Kazanka River (according to the Kruskal-Wallis test, p < 0.05).