Long-term Changes in Zooplankton Community in Zhrebchevo Reservoir, Central Bulgaria

The present paper describes the long-term changes in the zooplankton community in Zhrebchevo Reservoir, Central Bulgaria. Zooplankton data from 2009-2011 were compared with those from 1977-1980. Overall 104 zooplankton taxa were recorded; only 30 of them were common for both investigation periods (29% of similarity). The similarity of the dominant species was 26.6%. In 2009-2011, the dominant species were Daphnia pulicaria Forbes, 1893, Bosmina longirostris (Muller, 1785), Diaphanosoma lacustris Korinek, 1981, Eudiaptornus gracilis (Sars, 1863) as well as copepodites and nauplii; rotifers were almost absent. In contrast, in 1977-1980, the rotifers Keratella cochlearis Gosse, 1851, Asplanchna sp., Kellicottia longispina (Kellicott, 1879) dominated the plankton community. The reason for the shift in the zooplankton community was most likely the invasive mussel Dreissena polymorpha (Pallas, 1771), a competitor for the filter feeders and a potential predator on the small zooplankters, such as rotifers.