Shorth-Term Impacts of Weed Cutting on the Physical Habitats in Lowland Rivers: the importance of the initial environmental conditions

We studied the effects of weed cutting at 3 reaches in two Danish lowland rivers with the objectives of examining the response to cutting in rivers with contrasting physical conditions, macrophyte diversity, and assemblage patterns. Physical characteristics and abundance of macrophyte species were registered 3 or 4 times throughout the study period on all reaches. Weed cutting did not affect the total coverage of stone, gravel and sand and substratum homogeneity, and no common response was found among the reaches. This result is likely to reflect both initial differences in the physical environment among the reaches as well as differences in macrophyte coverage and assemblage patterns. Water depth, variability in current velocity and the coverage of stone and sand were affected by coverage independent of assemblage patterns, whereas the river bed substratum homogeneity was affected by coverage, as well as assemblage pattern. The analysis indicated that diverse macrophyte communities with several growth morphologies enhance the spatial variability in substratum characteristics compared to reaches with a less diverse and more homogeneous distribution of species.