The role of phytoplankton in the ecological assessment of the Southern Bug River middle reaches (Ukraine)

An environmental assessment of the Southern Bug River was performed by using statistical approaches to evaluating major environmental variables such as phytoplankton community composition and hydrochemical data for bio-indication. Chlorophyta (44.9 %) and Bacillariophyta (22.0 %) algae prevail in species number among 287 infraspecies taxa of nine taxonomical divisions. Comparison of the middle reaches of the Southern Bug River with its upper reaches revealed a decreasing number of euglenoid species, and showed characteristic features for a phytoplankton community dominated by green algae as well as diatoms similar to other large European rivers. Bio-indications show that the studied part of the river had low-alkaline and fresh, slow streaming and middleoxygenated waters, and represented Classes I-III of water quality reflecting oligo- to mesotrophic states as found in the upper part of the river. The species richness in the middle stations decreased in species number, i.e., a loss of mostly euglenoids, and dominance of green algae and diatoms typical for the Dnipro, Danube, and other large regional rivers. Fluctuation of abundance and bio-volume variables was very similar (Pearson correlation: 0.748). Studied parts of the river can be divided into three different sections as a result of statistical comparison based on hydrochemistry and phytoplankton community similarity: from Pyatnychany to Sabariv, from Sabariv to Dzhulynka, and from Dzhulynka to Pervomaysk. Species richness and the Shannon index fluctuated similarly with opposite trend lines of the Saprobity Index. The central section of the studied part of the river was under nitrate impact as Redundancy analysis (RDA) revealed. Mean cell volume increases down the river where the phytoplankton community is enriched in the large-celled algal species, attesting to the health of the ecosystem improving in the same direction. The ability of the ecosystem to self-purify was found to be high and increases downstream in spite of the anthropogenic impact in middle stations. Our studies allowed us to recommend three stations, Sabariv, Dzhulynka and Pervomaysk, for subsequent monitoring.