In July, 2016, hydrochemical and microbiological studies were performed on the water area of Boguchanskoe Reservoir. Like in other reservoirs on the Angara River, we noticed in Boguchanskoe Reservoir one increase of biogenic elements concentrations with the maximum in near-bottom layers: mineral phosphorus up to 88 µg PO₄^3-/dm³, nitrate nitrogen up to 469 µg NO^3-/dm³, ammonia nitrogen up to 46 NH₄⁺ µg /dm³. Waters mineralization reached 135–154 mg/dm³ and was close to the forecasted one – 136±10 mg/dm³. Compared with the Angara River, a slight increase in the concentrations of mineral nitrogen and phosphorus was noted in the reservoir; poor oxygen conditions were recorded over the marshy areas and in low-flow bays. In addition, peat bogs can influence the water quality in the future, since the process of thawing and flooding in the Siberian reservoirs is of a long-term nature. Considerable oscillations of total of microorganisms abundance (TMA) are registered – 1520–9887 thousand cells/ml, as well as of organotrophic bacteria – 26˗3516 cells/ml. In the water column of right shore stations, common coliphormic bacteria (CCB) were revealed – 4–8 CFU in 100 ml, thermotolerant coliphormic bacteria (TCB) up to 4 CFU in 100 ml, total microbial abundance (TMA) up to 606 CFU per 100 ml, enterococci up to 24 CFU in 100 ml; this may result from fecal pollution incoming with households wastewaters. By content of organotrophic bacteria and by ratio of TMA to the abundance of organotrophic bacteria, water quality is characterized by pollution degree as conventionally clean, while by TMA content, the water in different parts of Boguchanskoe Reservoir is characterized from conventionally clean to pollute. In the middle part of the reservoir, oil-oxidizing microorganisms were detected, which is probably due to the flooding of old oil wells in the Kova River valley. The number of bacteria varied from 5 to 623 CFU/ml. The number of oil-oxidizing bacteria in the studied areas is comparable with the values observed in the areas of natural oil output in Lake Baikal. By content of standardized chemical compounds (water mineralization, major ions concentrations, nitrate nitrogen, BOD5), the water in the Boguchanskoe Reservoir generally corresponds to norms of non-centralized water supply. By results of hydrochemical and microbiological studies, the Boguchanskoe Reservoir during the period of studies is characterized as an oligo- mesotrophic reservoir.

Zemskaya Tamara Ivanovna, Doctor of Sciences (Biology), Head of Laboratory, Limnological Institute SB RAS, 3, Ulan-Batorskaya st., Irkutsk, 664033, Russian Federation, e-mail: tzema@lin.irk.ru 

Zakharenko Aleksandra Sergeevna, Postgraduate, Limnological Institute SB RAS, 3, Ulan-Batorskaya st., Irkutsk, 664033, Russian Federation, e-mail: zakharenko@lin.irk.ru 

Sorokovikova Larisa Mikhailovna, Candidate of Science (Geography), Senior Research Scientist, Limnological Institute SB RAS, 3, Ulan-Batorskaya st., Irkutsk, 664033, Russian Federation, e-mail: lara@lin.irk.ru 

Sez`ko Natalya Petrovna, Engineer, Limnological Institute SB RAS, 3, Ulan-Batorskaya st., Irkutsk, 664033, Russian Federation, e-mail: se-nat@lin.irk.ru 

Zhuchenko Natalya Al`bertovna, Research Scientist, Limnological Institute SB RAS, 3, Ulan-Batorskaya st., Irkutsk, 664033, Russian Federation, e-mail: zhna@lin.irk.ru 

Bashenkhaeva Nadezhda Viktorovna, Engineer, Limnological Institute SB RAS, 3, Ulan-Batorskaya st., Irkutsk, 664033, Russian Federation, e-mail: nvb@lin.irk.ru 

Minaev Viktor Vasil’evich, Principal Specialist, Limnological Institute SB RAS, 3, Ulan-Batorskaya st., Irkutsk, 664033, Russian Federation, e-mail: minaev@lin.irk.ru

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