L. Forró szerk.: Miscellanea Zoologica Hungarica 9. 1994 (Budapest, 1994)

Csányi, B.: The macrozoobenthon community of the Danube between Rajka and Budapest

Series of hydrobiological studies has been started at the Water Resources Research Centre (VITUKI) in 1981 concerning phytoplankton, Zooplankton and macrozoobenthon communities, to a limited extent fish faunal survey has also been carried out. The basic purpose of the research was to predict the ecological impact of the barrage system. Therefore, knowledge of the original hydrobiological state of the Danubian water bodies using floral and faunal data before the operation of the barrage system was essentially important. The research was accelerated at the end of the 80's resulting in numerous synbiological results. A great amount of data are available about the seasonal chemical water quality of the Danube between Rajka and Budapest measured by the local Water Quality Laboratories (Varga et al. 1989, László & Varga 1990) indicating the pollution effect of the Sewage Treatment Works of Győr arriving from the Mosoni Danube (Várday 1987, Várday & T. Bartalis 1991). The trophic and the saprobic state of the river was also investigated (T. Bartalis 1974, 1978). The continuous increase of the phytoplankton biomass and the organic load were identified since the end of the 70's (T. Bartalis 1987, 1991), pointing out the possible increasing trophity effect of the Austrian reservoirs due to storage (Kiss 1987). Several phytoplankton studies, carried out on the upper stretch of the Hungarian Danube including the water bodies of the Szigetköz floodplain, were dealing with algal floristics (Kiss 1987, T Bartalis 1987, 1991) and community structure (Németh 1989). The spatial heterogeneity of the chlorophyll-a content proportional to the phytoplankton biomass was studied in the Ásványi side-arm (Németh & Skobrák 1985) together with the photosynthetic oxygen production (Csányi et al. 1985). The changing connection between the side-arms and the main river channel in terms of dissolved oxygen budget was measured in order to evaluate the effect of the side-arm system on the natural purification processes in the Danube (T. Dvihally 1987). Studies on the Ciliata fauna (Cs. Bereczky 1987) and the Zooplankton (Gulyás 1989, 1991) were used to characterize the seasonal saprobiological state of the upper stretch of the Hungarian Danube and the side-arm system in different hydrological conditions. The taxon list of potamal molluscs living in the Hungarian Danube contains several aquatic snail and mussel species from the upper stretch having special zoogeographical importance (Bothár 1966, Richnovszky 1970, 1979). However, data on the faunal distribution of molluscs were very seldom. The leech fauna of the Danube and the adjacent region is discussed by Puky (1989). Special notes concerning distribution of benthic taxa along the Danubian river system and differences between neighbouring watersheds due to hydrogeological history are analyzed (Nesemann 1992, Nesemann & Csányi 1993). Predicted ecological effects of the Nagymaros-Bős water barrage system on the ichthyofauna and the benthic community caused by peak power generation technology are summarized by Holcik et al. (1981). Survey of the Szigetköz region describing the community structure of phytoplankton, zooplankton and macrozoobenthon by multivariate methods resulted in a classification of different water bodies (Csányi 1989, Németh 1989, Gulyás et al. 1991). The importance of the Szigetköz in the natural reproduction of fish stock in the process of recolonization of the Danube river downstream was well known (Tóth 1960). The estimated decrease of the fish stock and the average fish catch in the Szigetköz and the River Danube as an essential impact of the barrage system are presented by Jancsó & Tóth (1987). Due to spatial limitations, this paper deals only with the main Danube stretch. Detailed faunal results of the macroscopic invertebrate community (macrozoobenthon) are given as an attempt for the longitudinal classification of the river.