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

one hand of a quantitative inventory of the larvae in both the longitudinal and the cross profdes during spring, early summer, and late summer. On the other hand, once a month samples were caught by landing net along the banks of the Danube and its associated small water bodies, and three stationary light traps were erected. The analysis of the catches proved an outbreak of two chironomid species: the Chironomus-plumosus-group and Glyptotendipes paripes. Outbreaks of both of these species were reported elsewhere. An impressive description of the occurrence of great swarms of Chironomus-plumosus on the Baltic haff-coast in July 1935 was reported Johannes Thienemann, it can be found in Thienemann (1954): "All our life is dominated by the "Haff-midge" now. Besides the bird migration, these swarms of midges can be counted among the outstanding natural spectacle to be seen on the "Kurische Nehrung" (the land between the Kurland Lagoon and the Baltic Sea). They do not come every year. Now we had a pause of about six years, and this year the midges appear again in vast numbers. Sometimes the swarms are so dense that you have to force your horses to go through when riding or driving the coach. For hens and ducks the arrival of the midges is the beginning of a great feast, just like for the wild birds, especially the starlings. They all swallow the "Haff-midges", as do the bathers sometimes involuntarily, who then curse and grumble. One can gather bucketfuls of midges for bird food. All through the day the midges sit under leaves or on the walls of houses and stables, but at dusk they rise and begin to swarm. Then one hears a single continuous high-pitched tone in the fine summer evenings. This year the midge outbreak lasted for 9 days, beginning on 15th of July." The "Haff midges" of the Baltic Coast and the chironomids that were observed in masses on River Danube belong probably to the same species from the group of Chironomus-plumosus. The adults show all species characteristics of Chironomus-plumosus, but in the case of larvae of both the "Haff midge" in 1935 and the midges from the Geisling impoundment in 1989 it was found that the four tubuli of the 11th segment are reduced in contrast to those of "original" plumosus larvae. According to the system by Ryser et al. (1983), these Chironomus-plumosus types are classified as Chironomus muratensis/nudiventris. Moreover, Thienemann (1954) reports that chironomids of the genus Glyptotendipes also occur very frequently in the coastal lagoons of the Baltic Sea. As a whole, epidemic occurrence of Glyptotendipes paripes has rarely been observed in Central Europe. However, numerous reports came from the United States where Glyptotendipes paripes had even been controlled with insecticides in some cases (Patterson et al. 1966, Nielsen 1962). Wundsch (in Thienemann 1954) reports densities of Glyptotendipes paripes larvae of some 2,000 individuals per square metre in the river lakes of the Havel. By examining the intestinal contents he found that the larvae of this species do not feed on detritus like Chironomus plumosus, but on fresh, settling phytoplankton. Furthermore, these larvae prefer not too deep but nutrient-rich waters. In the river lakes of the Havel, sandy-gravely bottoms in depths between 3 and 5 m form most of the area of the water area. The average population densities of chironomid larvae that were observed in 1989 in the longitudinal profile of River Danube are shown in Fig. 8. It becomes clear that the focus of occurrence of the larvae is not in the immediate headwater of the weir but lies some 10 km upstream thereof. The maximum with some 80,000 individuals per square metre was observed in July on the bed near the right-hand bank at rkm 2369. The following three figures show the abundance values plotted in dependence on various parameters: They suggest that the chironomid larvae prefer the characteristic bottom substrate of River Danube consisting of sand, gravel, and fine material (Fig. 9). Moreover, the larvae