Comparing the RPM% in different types of canals, two of them are dominated by the hydrophyte Ceratophyllum demersum and the helophyte Phragmites australis (Fig. 2). In the canals with a special function the dominant species were Lemna minor and Carex riparia. In two drainage canals the endangered species Nitellopsis obtusa was dominant. The largest differences of RPM% between the domi-nant and other species were observed in the third group, i.e. in the canals with a special function.