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Biomonitoring is one of the important stages in the consistent study of the sustainable development of aquatic ecosystems and their biological diversity. Against the background of the global impact on the hydrosphere, there is a need to select priority indicators and methods for determining the environmental risk in case of pollution of river ecosystems. Newly arriving substances are involved in complex biogeochemical processes in the water column, migrate as part of suspensions, or settle to the bottom. During the freezing period, a special place is occupied by cryogenic biochemical processes occurring in ice. Microbiological studies of the Amur River near the city of Khabarovsk showed that the water quality diff ered signifi cantly in winter. In the under-ice water and ice sampled off the coast, a diff erent composition of organic matter (OM) has been recorded for many years. After the fl oods (2013, 2019), a high number of sulfate- reducing bacteria was recorded in individual ice layers, which aff ect the dynamics of the transformation of natural and anthropogenic substances. This is due to the fact that in the area of Khabarovsk, during the period of ice cover formation, humifi ed OM spreads along the left bank, coming from the Zeya and Bureya reservoirs during technological discharges of water and from the fl oodplain, and along the right bank – waters contaminated with OM, mainly anthropogenic character. Long-term studies of the potential activity of sulfate- reducing bacteria in the Amur and its channels (Amurskaya, Pemzenskaya) make it possible to predict the formation of hydrogen sulfi de zones and the likelihood of mercury methylation over a wide temperature range. The indicator of resistance of cryomicrobial communities to high concentrations of mercury can be used for a retrospective assessment of the environmental risk of mercury pollution of rivers during the freeze-up period.

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