85 ACTA ZOOLOGICA BULGARICA Acta zool. bulg., 67 (1), 2015: 85-96 Aquatic Ecology Research Article Introduction Reservoirs provide numerous ecological services worldwide, e.g., drinking water supply, lood allevi- ation, power generation, water abstraction for agri- culture, aquacultures, sport ishing, recreation (Prus et al. 1999, FitzPatrick, Bree 2001, Jackson et al. 2001). The importance of reservoirs in Bulgaria is unequivocal: while there are about 400 natural lakes, the number of reservoirs in the country is about 2200 (koPralev 2001). Studies on the struc- ture and functioning of reservoir ecosystems are, therefore, essential from both scientiic and water management perspectives. It is known that zooplankton plays a major role in lentic water bodies as it is linking the energy trans- fer between primary producers, e.g. phytoplankton, and secondary consumers, e.g. ish (DoDDs 2002, likens 2010, Moss 2010). Furthermore, studies on lakes and reservoirs recognise the within-system spa- tial heterogeneity in the distribution and relationships between zooplankton and ish as an important ecolog- ical issue (kuBecka et al. 1998, HorPPila et al. 2000, seDa, Devetter 2000, seDa et al. 2000, WinFielD et al. 2002, v asek et al. 2003, 2006). The existing knowledge of zooplankton communities in Bulgarian reservoirs, however, concerns mostly its composition and dynamics, as well as the importance of various environmental factors for zooplankton communi- ties (e.g. naiDenoW 1964a,b,c, 1976, 1977a,b, 1981, 1993, naiDenov , Baev 1987, kozuHarov 1995a,b, kozuHarov et al. 2007, 2013, PanDourski 2007). Additionally, the European perch (Perca luvia- tilis Linnaues, 1758) is one of the most frequently Zooplankton and its Contribution to the Diet of Europe- an Perch (Perca luviatilis) in Two Lowland Reservoirs in Northwestern Bulgaria Vesela Evtimova 1 , Ivan Pandourksi 1 , Teodora Trichkova 1 1 Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, 2 Gagarin Str., Soia 1113, Bulgaria; E-mail: vesela.evtimova@gmail.com Abstract: This study presents results on zooplankton composition of two lowland reservoirs in the Danube River Basin and gives insight into the trophic relationship between zooplankton and European perch (Perca lu- viatilis Linnaues, 1758). We found signiicant within-reservoir differences in the univariate and multivari- ate biological metrics between spring and autumn samples. Overall zooplankton densities and biomass were higher in spring, both being dominated by Cladocera, while zooplankton diversity was greater in autumn with Rotifera dominating the densities and Copepoda dominating the biomass. In addition, we examined the foregut content of 297 perch specimens. It consisted exclusively of crustacean zooplank- ters, small ish and benthic invertebrates. The zooplankton of the perch foregut content was dominated by cyclopoids in spring and by cladocerans in autumn. This mismatch between the relative abundance of zooplankton in the perch foregut content and in the reservoirs demonstrates the selective feeding of Perca luviatilis. Given that zooplankton plays a major role in standing water bodies, i.e. linking the energy transfer between primary producers and secondary consumers, our indings may serve as a basis for future research on the within-reservoir trophic relationships, as well as for effective and sustainable management of aquacultures of Bulgarian reservoirs. Keywords: Zooplankton composition, seasonality, perch selective feeding, lowland reservoirs, Danube River Basin