Effects of plant cutting and dredging on habitat conditions in streams

Abstract

The effect of plant cutting and dredging on habitat conditions was studied in two macrophyte-rich streams of the Swiss Plateau (Chriesbach and 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 reaches where plants were removed by cutting or dredging. Plant biomass and physical parameters were measured during a 1.5 to 4 month period before and a 9 month period after plant removal. Apart from the accumulation of fine sediments, habitat parameters and their spatial heterogeneity changed with plant biomass in both streams. Plant removal decreased stream-bed roughness and depth and increased current velocity. Dredging and cutting reduced plant biomass by 90-94 % and 84-87 %, respectively, but these differences in plant biomass were too small to result in significant differences in depth and current velocity. Plants and habitat conditions exhibited different recovery trajectories in the two streams. In the Chriesbach, where plants had been removed before flowering, macrophytes recovered within the same growing season and recovery was faster in the cut than in the dredged reach. In the Muhlibach, where macrophytes had been removed after flowering, plants did not recover until the following spring.