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

1400 t - 800 E I 600 400 200 -•Lœl 2376 2360 Danube-km 2359 INov 87 BSep 88 BSep 89 «Sep 90 BSep 91 «Mai 92 «Aug 92 DD Mai 93 EOkt 93] Fig. 7. Present occurrence of Brachycentrus subnubilus in the Geisling impoundment impoundment weir is noticeable. Even this euryoecious species is not able to inhabite the entire impoundment because of the low flow rate. Another caddisfly, confined to zones with sufficient flow, is Brachycentrus subnubilus, which is rather frequent in River Danube in comparison with other major watercourses. The second and the third pair of legs of the larvae bear long hairs which allow to filter food particles directly from the water. The strong dependence on water current can be seen also in the distribution pattern of Brachycentrus subnubilus in the Geisling impoundment (Fig. 7). Here again a more or less continuous decrease in population density towards the impoundment weir can be observed. Anderwald (1991) made a detailed study on the biology of this caddisfly in the head zone of the impoundment of the Altenwörth hydropower plant in Austria. He found that the velocity of flow has highly significant impact, and the sediment grain-size significant impact on the population density of Brachycentrus subnubilus. Structure and dynamics of development of the macro-zoocoenosis after the damming-up The beginning of damming-up was in October 1985, and the first filling of the impoundment was reached in January 1986. In the spring of this year, the first massive appearance of chironomid populations occurred in the area of the Geisling impoundment. The insects formed vast swarms on the banks of River Danube and flew - depending on the weather and wind direction - into nearby settlements. Because the beginning of the mass occurrence and the first impoundment of the river coincided, a causal relation between these two events was assumed. In early 1989, the Federal Institute of Hydrology (BfG) was commissioned to study the underlying causes of this outbreak. The crucial questions were whether and to which extent the causes can be ascribed to the damming-up. Moreover, proposals on how to mitigate the "insect pest" were demanded. The studies consisted on the