Current State of Planktonic and Benthic Communities in the Mouth Part of the Northern Dvina River

The Northern Dvina River, one of the largest watercourses in the European northeast of Russia, is exposed to a multifactorial anthropogenic impact, which leads to changes in the structure of aquatic biocenoses: changes in the species diversity and abundance and biomass of organisms and a general reduction of aquatic biological resources. This paper presents the results of an integrated study of invertebrate zooplanktonic and zoobenthic communities in the mouth part of the Northern Dvina River. It has been established that zooplanktonic and zoobenthic communities are characterized by quite a high taxonomic diversity. According to the fishery classification, the level of zooplankton development in the lower reaches of the river allows one to consider this area oligotrophic for plankton-feeding fishes. The average values of the abundance and biomass of zoobenthos make it possible to classify the studied part of the river as eutrophic for benthophages. In terms of zooplankton indicators, the quality of the river waters had a clearly defined seasonal dynamics: it varied from class II (weekly polluted waters) during the spring flood period to class I (conditionally clean waters) in other seasons. Two areas, the Korabelny branch and Maimaks channel, were the only exceptions; here, the load on the communities of aquatic organisms was also observed in the summer–autumn low water period. The predominance of eurybiontic species in the zooplankton probably indicates the anthropogenic pressure on the river ecosystem. On the whole, the ecological state of the studied part of the watercourse can be assessed as satisfactory in terms of the zooplankton indicators.