Upstream-downstream movement of macrofauna (with special reference to oligochaetes) in the River Raba below a reservoir

Oligochaetes are the dominant group of macrofauna in a river reach (1-450 m) below the Dobczyce dam on the River Raba. The Oligochaeta made up from 60.2% to 78.7% of the community. The next most abundant group were the Chironomidae comprising 20.5%-38.8% of the fauna. Upstream-downstream movements of the macrofauna were studied at one station, with an oligochaete density of 50000 indo m-2, and a chironomid density about 20000 indo m -2. In the drift the proportions of these two groups were reversed - Chironomidae represented 59% of the drift fauna, and the oligochaetes - about 40%. On the river bottom and in the drift a similar number of species (12-13) of Naididae were identified. In order to compare the intensity of the movement of the various taxonomic groups, the percentage of animals moving upstream and downstream over 24 h was calculated. The highest percentage of the population migrating were dipterans (7.3%). The Chironomidae and Ephemeroptera had similar proportions migrating (3.2% and 2.6% respectively). The Oligochaeta had the lowest value - only 0.6% of animals were displaced in a 24 h period. The Naididae appear to be strongly attached to the substratum and, under normal environmental conditions, their reproduction sufficiently compensated for the decrease in their number associated with the drift or predation.