L. Forró szerk.: Miscellanea Zoologica Hungarica 9. 1994 (Budapest, 1994)
Csányi, B.: The macrozoobenthon community of the Danube between Rajka and Budapest
Fig. 4. Ordination plots of sampling sites (1-9) and taxa (1-48) (Correspondence analysis) The effect of the Slovakian water diversion on the benthic fauna The operation of the Slovakian Barrage System started on October 25, 1992 with the final diversion of the main water flow of the Danube to the channel of the hydroelectric power plant. Several macrozoobenthon samplings have been taking place since the October 26 until now, in order to describe the changes of the coexistential pattern of the community due to the impact. Obviously, the most considerable decrease of the water level developed just below the diverting cross dam. Due to the drastic decrease of the present flow (200-250 m 3 /sec), the water level stabilized more than 2.5 m lower, than the ever measured lowest at low flow (approx. 1000 m /sec) conditions. An extended littoral area became dry and resulted in the immediate death of the aquatic macrofauna. Huge amount of Sphaerium corneum, Dikerogammarus villosus, Corophium curvispinum, leeches, insects and other species (even specimens of fish taxa like Cottus gobio and Proterorhynchus marmoratus) were found dead just in the Rajka stretch on the day following the diversion. Instead of several meters of water level change, only few centimeters are occuring as a result of the highly regulated water discharge. Consequently, many species are found now just below the water air interface because the lack of the level fluctuation (i.e. Ancylus fluviatilis, Hydropsyche taxa). Virtually, there are the same species living in the remaining river bed as earlier, but the flooded area and the water-flow decreased considerably. The most harmful impacts are apparent in the formerly active side-arm system of the Szigetköz flood-plain. Long sections of side-arms became completely dry. This time only the deepest channels contain stagnant water bodies but they are isolated from each other. The earlier common high flood conditions (water flow even above 10000 m 3 /sec) have completely disappeared. The whole hydrological cycle has dramatically changed because of the diversion of the main Danube flow to the barrage system. The considerable damage of the aquatic flora and fauna is characteristic in many places. The
