L. Forró szerk.: Miscellanea Zoologica Hungarica 10. 1995 (Budapest, 1995)

Tittizer, T., Leuchs, H.; Banning, M.: The consequences of river impoundments for the macrozoobenthos - demonstrated at the example of the River Danube in Germany

months, the more upstream Danube reach near Kelheim was even rated a quality class DJ to IV (Bayerisches Staatsministerium für Landesentwicklung und Umweltfragen 1985). Faunistic inventories made by the Federal Institute of Hydrology (BfG) during these two decades revealed again and again temporary mass occurrences of the "sewage fungus" Sphaerotilus natans. Meanwhile, the improvements in industrial and municipal wastewater treatment have also gradually improved the water quality, so that the present River Danube in the Geisling impoundment can be characterized by quality class II, i.e. "moderately polluted". Nevertheless, this remains even today the most heavily polluted reach of the River Danube in Bavaria. Moreover, here the mean transport duration of 45 min/km is nearly double compared to the other impoundments (Schmid et al. 1994). The long retention time of the water and its shallowness in many sections are responsible for the presently high trophic status of the river here. The ample supply with nutrients originates especially from industrial discharges in the Kelheim and Regensburg areas. Because of the pollution in River Danube at the time before the impoundment, the prevailing macrozoobenthic community could not be characterized as a natural or near-natural potamal community. At that time generalists dominated; besides the "sewage fungus", e.g. the snail Bithynia tentaculata or the isopod Asellus aquaticus were frequently observed. At that time, a few stenoecious species of running waters had only a small share in the species spectrum. Today, most of them continue to settle widely in the non-impounded reaches of the River Danube. Furthermore, they can be found in the head-zones of the impoundments, sometimes in high numbers. An example is the so-called "Donauisopod" Jaera istri. This isopod was first found in the German reach of River Danube in 1958 directly downstream of Passau (Kothé 1968). Nowadays, Jaera istri inhabits the entire reach of the Federal waterway in the Danube (Fig. 4). Fig. 5 shows its present occurrence in the Geisling impoundment. Larger populations of Jaera istri can be found in the head of the impoundment at Danube rkm 2376 and 2370, while Jaera istri avoids the zones where the fine substrate settles and where the flow velocity is low (Leuchs et al. 1991). Fig. 4. Localities of Jaera istri in the Federal waterway on the reach from rkm 2414.5 to 2202.5