Seasonal and interannual changes of the zooplankton of the Neman river

In the zooplankton of the Neman River (a projected receiver of waste waters of the Baltic Nuclear Power Plant) 81 taxons of the species and the subspecies range were identified. Annually rotifers (up to 73,9±3,5 % in the community) were the dominant group of invertebrates in the community presented mainly by such species as Euchlanis dilatata, Keratella cochlearis, K. quadrata. Cladocera dominated by biomass (up to 57,9±4,4 %), among them Daphnia cucullata, Bosmina longirostris and Chydorus sphaericus reached the greatest development. The maximum values of abundance (N) and biomass (B) of zooplankton were recorded in spring (N = 30,419±10,298 thousand examples/m3, B = 0,344±0,136 g/m3), which may be associated with the increased productivity of hydrobiocenosis. It was caused by the flow of biogenic substances into the river during spring floods and the creation of favorable conditions for the formation of temporary zooplankton communities in the floodplain waters. In summer and autumn, when the water level and concentration of nutrients decreases, the quantitative indicators of planktonic invertebrates reduce. In the course of long-term observations the increase in the number and biomass of zooplankton, and the trophicity of the watercourse was revealed. It was connected with the growth of the nutrient load associated with water run-off from agricultural areas and the flow of pollutants in the areas of human settlements. Water in the Neman River corresponded to the β-mesosaprobic zone. The trophic water status varied from spring to autumn from prevailing hypereutrophic conditions to eutrophic and mesotrophic ones. The highest degree of trophicity was registered in the vicinity of such cities as Grodno, Kaunas, Jurbarkas and Neman.