The peculiarities of the species composition of plankton algae, quantitative indices of their development, and seasonal dynamics were studied in the upper and middle sections of the Southern Bug River. Their ecological state was also assessed. It has been found that phytoplankton of these sections is represented respectively by 261 species (281 infraspecific taxa) and 268 species (283 infraspecific taxa) belonging to nine divisions (Cyanoprokaryota, Euglenophyta, Chrysophyta, Xanthophyta, Bacillariophyta, Dinophyta, Cryptophyta, Chlorophyta, and Streptophyta), 14 classes, 29 and 34 orders, 52 and 57 families, and 113 and 131 genera.In summertime the numbers and biomass of phytoplankton of the Southern Bug River vary over a wide range: in the upper section - from 1720 to 58631 thousand cells/L and from 0,690 to 11,646 mg/L, whereas in the middle section - from 3486 to 102556 thousand cells/L and from 0,756 to 20,885 mg/L. Throughout a year mainly Bacillariophyta and Chlorophyta predominate in terms of their numbers and biomass. The contribution of Chrysophyta increases in wintertime, whereas the contribution of Cyanoprokaryota (in terms of their numbers) and Dinophyta and Euglenophyta (in terms of their biomass) - for the most part in summertime. In terms of bioindication characteristics, phytoplankton of the studied sections of the Southern Bug River is characterized by a predominance of the indicators of moderate speed of water flow and temperature regime, indifferent in relation to pH and water salinity, b-mesosaprobic (according to Sladechek) and eurysaprobic organisms (according to Watanabe), and also of autotrophic organisms capable of developing at a rather high concentration of nitrogen-bearing organic compounds. An assessment of the ecological state of the Southern Bug River in terms of the RPI (River Pollution Index) and WESI indices (Aquatic Ecosystem State Index) is indicative of a high capability of the river ecosystem for self-purification. In the studied sections of the river, water quality is mainly of the III class ("water of the satisfactory quality").
