Summer phytoplankton of the Pavlodar (Republic of Kazakhstan) - Omsk (Russian Federation) section of the Irtysh river

For the first time in the twenty-first century, species richness, abundance and structure of summer phytoplankton of the Pavlodar (Kazakhstan) - Omsk (Russia) section of the transboundary Irtysh River is described. Phytoplankton abundance for the investigated area of the river is high, the number varies within the range of 8.6-26.6 million cells/liter (an average is 15.12 ± 1.72 million cells/l), biomass - from 1.59 to 4.39 g/m3 (an average is 3.10 ± 0.24 g/m3). According to value of phytoplankton biomass, the investigated section of the river belongs to the eutrophic waters category and to the 3rd class of water quality - “satisfactory cleanliness”. High values of phytoplankton abundance are caused by vegetation of small-cell cyanobacteria species, forming from 32.03 to 76.65% of phytoplankton total number. The contribution of diatoms and green algae to the formation of the total phytoplankton abundance is approximately the same (21.06-23.22%). Phytoplankton biomass is mainly composed of diatoms, its contribution on average is 72.20 ± 4.24% (47.42-87.26%). The graphs of phytoplankton abundance and biomass for the investigated section of the river have the appearance of broken curves with sharp changes of the indices from the hydrobiological point to the point on the background of a general increase of the phytoplankton abundance downstream. The maximum values of the phytoplankton abundance are recorded near the Cherlak settlement, the minimum values - upstream of the Pavlodar city. The minimum biomass is registered near the Terenkol village, the maximum one - near the Chernoreckoe village. The uneven development of the Irtysh phytoplankton reflects a high level of anthropogenic impact on its waters. The dominant complex includes cyanobacteria ( Аphanocapsa delicatissima, A. holsatica ), indicators of anthropogenic eutrophication ( Stephanodiscus hantzschii, Fragilaria crotonensis ), regional indicators of “dirtyˮ ( Stephanodiscus neoastraea ) and “very dirtyˮ ( Asterionella formosa ) water. The high level of development of cyanobacteria and the indicator species points out a significant level of eutrophication and contamination of the Irtysh River water.