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

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

4). On the other hand, rare species result in the marginal appearence of the given sites (i.e. site 1 at Rajka has three unique overlapping taxa). The water level had important influence on sampling efficiency. Sampling during long term low water4evel resulted in collection of many species. In this case the littoral zone contains always a more rich benthic community compared to the middle river bed. The poor macrozoobenthon community of the deep, middle part of the river is probably caused by the strong current conditions. 3* Si }

5 23 1 /, :T" 34 Z ft*Fig. 4. Ordination plots of sampling sites (1-9) and taxa (1-48) (Correspondence analysis) Table 2 illustrates the 20 taxa collected by dredge sampler from the deep region of the river-bed. Although the number of taxa is less than the half of the littoral fauna, similar picture is recognizable in the sequence of sampling sites based on the results of the cluster analysis (Fig. 5). Site 3 (Medve) with the very poor fauna appears as an outlier according to three similarity functions of the four. Sampling sites from the upper section have the more similar character to each other. One definite group is recognizable on the ordination plot received by correspondence analysis. Rajka, Dunaremete, the overlapping Gönyü and Komárom together with Esztergom are in central position. The sites of Visegrád (No. 7) and the two Budapest sections (Nos 8, 9) have increasing taxon numbers, most probably because of the decreasing velocity of the water current. According to the multivariate analysis based on kick and dredge sample data of the benthic community the upper Hungarian Danube has one identical stretch that is corresponding more or less to the Szigetköz region. According to the analysis, to the downstream sites gradual transition is experienced with increasing number of taxa, most probably due to moderate changing of current conditions along the river.