UPSTREAM-DOWNSTREAM MOVEMENT OF MACROFAUNA (WITH SPECIAL REFERENCE TOOLIGOCHAETES) 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-do wnstream movements of the macrofauna were studied at one station, with an oligochaete density of 50 000 ind. m(-2), and a chironomid density about 20 000 ind. m(-2). In the drift the proportions of these two gr oups 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 o rder 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 migrati ng were dipterans (7.3%). The Chironomidae and Ephemeroptera had simil ar 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 suffici ently compensated for the decrease in their number associated with the drift or predation.