Composition of zooplankton assemblages along the Zagyva River

The species composition, longitudinal distribution and seasonal dynamics of zooplankton were studied in the Zagyva River, Hungary. A total of 108 taxa was recorded from which 61 were new for the river. Rotatoria was the most abundant group, microcrustaceans were less important, only nauplii and copepodites were represented in similar individual numbers. Frequent species included Anuraeopsis fissa, Pompholyx spp., Keratella cochlearis, Brachionus angularis, Bdelloida sp., Bosmina longirostris. Dominance of cosmopolitan species was observed both in the river and its reservoir, and species characteristic of eutrophic waters were of major importance in the latter. There was a downstream decrease in zooplankton densities, which was explained by modified conditions. The relatively large number of individuals in autumn months, and the characteristic large number of individuals in the upper section contrasted general findings of potamoplankton dynamics. On the basis of the species abundance matrix, three river sections can be distinguished (upper, middle, lower section). Due to waste water discharges - received from the Tarján Stream - we found extremely high number of individuals and the lowest diversity at the sampling site Nagybátony (148 rkm).