Investigations for the study of phytoplankton on the lower portion of the Southern Bug River were conducted in 2013–2014. According to the results of original studies, species richness in this section of the river is 210 species, represented by 219 taxa (infraspecific taxa). The basis of the phytoplankton taxonomic structure are formed by representatives of divisions Chlorophyta (42.4%), Bacillariophyta (32.9%), Cyanoprokaryota (10.5%) and Euglenophyta (5.7%). Main genera are Desmodesmus, Navicula, Euglena, Nitzschia, Coelastrum, Phacus, Acutodesmus, Monoraphidium, Oocystis and Cyclotella. According to the generalized data (including information from literature (1941–2004)) and results of our investigations (2013–2014), phytoplankton of lower portion of the river is formed by 332 species (365 infr. taxa) from 10 divisions. Basis of species composition constitute Bacillariophyta – 143 species (157 infr. taxa) or 43.1% from total amount of registered taxons of species rank, Chlorophyta – 99 species (111 infr. taxa) or 29.8%, Cyanoprokaryota – 34 species or 10.2% and Euglenophyta (28 species (33 infr. taxa) or 8.4%. Less diverse were such divisions as Charophyta – 11 species 12 infr. taxa or 3.3%, Dinophyta – 8 species 9 infr. taxa or 2.4%), Xanthophyta – 4 species or 1.2%, Chrysophyta – 3 species or 0.9%,Cryptophyta and Raphidophyta by one species or 0.3%. The composition and taxonomic structure of algae identified in the plankton were analyzed in order to determine changes occurring at present and their comparison with the earlier stage of development of the River. Comparison of original and literature data indicates a shift in the dominant set of taxonomic groups from diatom to green algae. Evidently, it is caused by changes in the hydrological regime of the watercourse – the flow rate, the trophic level and the like, as a result of anthropogenic pressure on this stretch of the river channel and a number of other factors. To interesting floristic findings of the lower portion of the river can be attributed Aulacoseira subarctica (O. Müll.) E.Y. Haw., Cyclotella meduanae Germ., Navicula heimansioides Lange-Bert. and Phormidium regelii (Skuja) Anagn. et Komárek.
Keywords: algal flora, phytoplankton, taxonomical structure, Southern Bug River, lower part
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