EFFECTS OF PLANT CUTTING AND DREDGING ON HABITAT CONDITIONS IN STREAMS

The effect of plant cutting and dredging on habitat conditions was stu died in two macrophyte-rich streams of the Swiss Plateau (Chriesbach a nd Muhlibach). In each stream, habitat conditions (macrophyte biomass, roughness of the stream-bed, current velocity, depth and accumulation of fine sediments) were monitored in a control reach and in impact re aches where plants were removed by cutting or dredging. Plant biomass and physical parameters were measured during a 1.5 to 4 month period b efore and a 9 month period after plant removal. Apart from the accumul ation of fine sediments, habitat parameters and their spatial heteroge neity changed with plant biomass in both streams. Plant removal decrea sed stream-bed roughness and depth and increased current velocity. Dre dging and cutting reduced plant biomass by 90-94 % and 84-87 %, respec tively, but these differences in plant biomass were too small to resul t in significant differences in depth and current velocity. Plants and habitat conditions exhibited different recovery trajectories in the t wo streams. In the Chriesbach, where plants had been removed before fl owering, macrophytes recovered within the same growing season and reco very was faster in the cut than in the dredged reach. In the Muhlibach , where macrophytes had been removed after flowering, plants did not r ecover until the following spring.