Macrophyte diversity and composition in relation to substratum characteristics in regulated and unregulated Danish streams

1. The objective of the present study was to examine how the physical strea m environment in regulated and unregulated lowland streams affects the dive rsity and distribution of macrophyte communities. We analysed the abundance , distribution and composition of macrophytes, together with physical param eters, in seven regulated and seven unregulated unshaded Danish stream reac hes. 2. Total macrophyte coverage was similar in the regulated and unregulated s treams, but species richness and Shannon diversity were higher in the unreg ulated streams. Overall, we found fifty-two different species in the regula ted stream reaches and sixty-two in the unregulated stream reaches. The spa tial distribution of macrophytes on the stream bottom was more heterogeneou s in the unregulated streams. 3. We found positive correlations between the coverage and diversity of mac rophytes and the coverage of coarse-textured substratum types on the stream bottom, as well as between macrophyte coverage and diversity and substratu m heterogeneity. We also found that the macrophytes were more heterogeneous ly distributed where substratum heterogeneity was greater. 4. The species growing both submerged and emergent were more abundant in th e regulated streams, whereas species growing only submerged were more abund ant in the unregulated streams. Species growing submerged, species growing both submerged and emergent, and species only growing emergent segregated d ifferently in a canonical correspondence analysis ordination. The submerged species were primarily associated with coarser-tortured substrata, whereas species growing both submerged and emergent, and species growing only emer gent were associated with finer-textured substrata. 5. The most abundant species in the regulated streams, Sparganium emersum, accounting for almost one-third of the total macrophyte coverage, was prima rily associated with clay and sandy bottom substrata, whereas the most abun dant species in the unregulated streams, Batrachium peltatum, was primarily associated with gravel and stony substrata.