Composition and richness of macrophyte communities in small Danish streams - influence of environmental factors and weed cutting

The community structure and diversity of aquatic macrophytes was studied in 79 small Danish lowland streams to examine effects of regular anthropogenic disturbances and physico-chemical factors for the development of macrophyte communities. In total, 131 species were found of which 65 were found both in the stream and on the stream banks. Berula erecta, species within the genus Callitriche and Sparganium and Glyceria fluitans, dominated the macrophyte communities in the streams. A detrended correspondence analysis indicated that disturbance in terms of weed cutting was the most important factor separating the macrophyte communities. Obligate water plants and tall reed plants, both amphibious and terrestrial species, were more important in the macrophyte community in highly disturbed streams, whereas small amphibious species were more important in less disturbed streams. A direct comparison of diversity indices between streams subjected to regular cuttings and undisturbed streams revealed that regular cuttings significantly reduced species diversity. Also the macrophyte patch complexity, expressing the spatial distribution of macrophytes on the stream bottom, was lower in disturbed streams both near the banks and in the middle of the streams. The DCA analysis also indicated that the riparian community had a significant influence on the in-stream community structure and confirmed the importance of geographical region, water temperature and other physico-chemical parameters for the macrophyte communities in Danish streams.