Biologia 68/6: 1163—1171, 2013 Section Zoology DOI: 10.2478/s11756-013-0263-6 The diversity and longitudinal changes of zooplankton in the lower course of a large, regulated European river (the lower Vistula River, Poland) Pawel Napiórkowski 1 & Teresa Napiórkowska 2 1 Department of Hydrobiology, Faculty of Biology and Environmental Protection, Nicolaus Copernicus University in Toru´ n, Gagarina 9, 87-100 Toru´ n, Poland, e-mail: pnapiork@umk.pl 2 Department of Invertebrate Zoology, Faculty of Biology and Environmental Protection, Nicolaus Copernicus University in Toru´ n, Gagarina 9, 87-100 Toru´ n, Poland Abstract: The diversity and longitudinal variation of zooplankton in the lower Vistula River were analyzed. Samples were taken from 40 stations located along a 272-km long section of the lower river course. During the study the unique technique of taking samples from “the same water” was used. The zooplankton community was dominated by rotifers and nauplii – larval stages of copepods. The most abundant species were: Brachionus angularis, Brachionus calyciflorus and Brachionus budapestiensis. The zooplankton species diversity in the main channel of the lower Vistula River was similar to other large European rivers; however, its abundance was lower. The diversity, abundance and biomass of potamoplankton steadily decreased downstream. This could be related both to scarcity of storage zones for potamoplankton development in the river due to the extensive regulation processes, and changes in hydrological conditions of the main channel (by the straightening of riverbed) where the samples were collected. Key words: zooplankton; river; Rotifera; Crustacea; Vistula Introduction Zooplankton plays an essential role in water ecosystems, including rivers. The planktonic animals take part in the transformation and circulation of organic matter (Ejsmont-Karabin et al. 2004), regulate the biomass of phytoplankton (Lair 2005; Kentzer et al. 2010) and pro- vide food for fish, especially for their larval stages and for fish fry (Pourriot et al. 1997). However, despite the increased interest in large river ecosystems (Tockner et al. 2009), our knowledge of potamoplankton is still in- complete. The lack of research on potamoplankton may have resulted from the opinion that rivers were not suit- able environments for zooplankton. However, potamo- plankton is usually abundant in the main channel of large rivers, although the factors regulating their spa- tial occurrence are not completely understood. Recent investigation has emphasized the importance of water residence time for zooplankton development (Basu & Pick 1996). Water residence time increases with the increasing river length and there is often a tendency for plankton to be more abundant downstream. Gener- ally these downstream increases have been associated with the increases of time available for plankton to de- velop (Viroux 1999; Zimmermann-Timm et al. 2007). So we can say that the “Age of Water” (Baranyi et al. 2002) is very important for zooplankton develop- ment. While zooplankton composition and abundance have been quite well documented in many large rivers of Europe (Lair & Reyes-Marchant 1997; Reckendorfer et al. 1999; Zimmermann-Timm et al. 2007; Rossetti et al. 2009; Vadadi-F¨ ul¨ op et al. 2009, 2010) little is known of these communities in the mid-eastern European rivers such as the Vistula River. The Vistula River is one of the largest rivers of the Baltic Sea basin and its valley is an important ecological corridor in this part of Europe (Bij de Vaate et al. 2002). Unfortunately very few studies have been under- taken in the Vistula River and even these have been restricted to such sections of the river as dam reser- voirs (Starzykowa et al. 1972; Krzanowski 1987; ˙ Zurek & Kasza 2002; ˙ Zurek 2004; Kentzer et al. 2010) or the upper (Bednarz et al. 1988) and middle reaches (Praszkiewicz et al. 1983; Papi´ nska 1990; S lo´ n & Kowal- czewski 1991). To address the dearth of information on zooplank- ton in the Vistula River at a large spatial scale, the present study was carried out to determine species di- versity, abundance and biomass of these organisms in the main channel along a 272-km long section of the lower river course. According to the River Continuum Concept (RCC) (Vanote et al. 1980) physical variables present a con- tinuous gradient in hydrological conditions in an ideal, freely flowing river, which conforms to the biological c 2013 Institute of Zoology, Slovak Academy of Sciences