Abstract We conducted an environmental assessment for the first time in the upper reaches of the Southern Bug River utilizing data on phytoplankton communities and hydrochemistry. Chlorophyta (40%), Bacillariophyta (19%), and Euglenophyta (18%) were among nine identified divisions of common phytoplankters which included 281 taxa. The results of a statistical comparison of species richness showed that the investigated part of the river can be divided into two parts for follow-up monitoring. Class II-III waters reflect a trophic status equivalent to an oligo- to meso-trophic state. Using algal indicators as well as environmental variables and statistical methods we identified major factors impacting the algal communities, such as phosphate and ammonia.