The work is a part of a comprehensive study of the rivers of the south-east of the Arkhangelsk region, carried out with the aim of developing aquaculture in inland waters. Qualitative and quantitative characteristics for the plankton communities state of the Northern Dvina River such as species composition, abundance, and biomass were obtained in the summer period of 2021. For tributaries these characteristics were obtained for the first time. The species diversity of phytoplankton and the saprobiological state of waters were assessed using indices. An assessment of the watercourses trophicity was carried out. 133 taxa of microalgae from eight systematic divisions, and nine zooplankter species of three taxonomic groups have been identified. The basis of the algoflora has been established to be diatoms, green algae, and cyanobacteria. Zooplankton communities were characterized as cladocerne-rotifers, with a significant predominance of cladocerans. The data obtained can be used for ecological monitoring of the region aquatic ecosystems, for assessing the fish productivity of water bodies, and calculating the damage to aquatic biological resources from economic activity.

The work is a part of the comprehensive studies of the rivers and lakes of the south-east of the Arkhangelsk region, carried out in 2021 for the purpose of development of aquaculture of inland water bodies. The data on the species diversity and structure of plankton communities of freshwater bodies belonging to the Vychegda River watershed were obtained. Taxonomic composition of phytoplankton in summer period of 2021 was shown. Species diversity was estimated using Shannon index based on relative abundance of species, Menchinic index to determine species abundance, and Jaccard's similarity coefficient. Pantle-Bucca saprobicity index was calculated to assess pollution of natural waters. Evaluation of watercourses trophicity based on the obtained indicators of phytoplankton and zooplankton was carried out. We have identified 192 taxa of microalgae from 8 divisions and 15 species of zooplankton from three systematic groups. Green and diatom algae were found to be the basis of the algoflora. The zooplankton community was cladoceric-copepodic. New data on the state of the phytoplankton community can serve as an informational and methodological basis for the development of ecological monitoring programs for the aquatic ecosystems of the region. The results of the research can be used to assess the fish productivity of the Arkhangelsk region water bodies and to calculate the damage to aquatic bioresources from economic activities.