Influence of wastewater on zooplankton communities in the Daugava River upstream and downstream of Daugavpils over the last 50 years

During expeditions to the section of the Daugava River from Surozha in Belarus to Dunava in Latvia in 2008, and during expeditions to the Kraslava-Dunava section in 2009, zooplankton were sampled. Quantitative characteristics, the relation of zooplankton taxonomic groups, the saprobity index and species diversity (Shannon-Wiener index) were employed for the analysis of zooplankton community structure in the Daugava River. The results were compared with the Daugava zooplankton findings of the research carried out by Škute in 1962–1968 in similar sections of the river. Both in 1962–1963 and in 2005–2009, the base complex of the Daugava River zooplankton comprised cosmopolitan species; as for the diversity of species, the leading group of organisms was Rotifera. A tendency for decrease in the diversity of species was recorded. Relevant changes were recorded in the structure of taxa. The Daugava River is polluted by Daugavpils’ wastewater; however, the untoward influence of pollution on zooplankton is observed only in years and in seasons with low water level and discharge. Decrease in the amount of zooplankton occurs not only in the dominant species, but also in the decrease or even disappearance of oligosaprobes