Dynamics of Zooplankton Composition in the Lower Northern Dvina River and Some Factors Determining Zooplankton Abundance

Abstract-The long-term dynamics of the taxonomic composition of zooplankton in the lower reaches of the Northern Dvina River and the effect of some hydrochemical factors on its abundance have been studied. It is found that the species list of the zooplankton has included 141 species over 50 years: it consisted of a total of 98 taxa in 1965 and 104 taxa in 2012-2019. The results of analyzing the spatiotemporal abundance distribution of the zooplankton and its taxonomic groups are presented. A significant increase in abundance (on account of copepods) and changes in the taxonomic structure of zooplankton have been revealed in the lower parts of the study water area. In 2019, changes in the structure-forming complex of zooplankton were noted for the first time over the study period. It is found that species diversity reaches high values in waters classified as "heavily polluted" and "dirty," which is evidence for a complex structure of zooplankton communities. The main factor influencing the horizontal distribution of the zooplankton abundance is the dissolved oxygen content of water.