Sixty years of change in the macrophyte vegetation of north‐west German running waters: a community‐ and landscape‐level analysis

Abstract

Long‐term change in the macrophyte assemblages of 70 streams and rivers in four regions of north‐west Germany was examined with a semi‐permanent plot design based on 337 historical vegetation relevés from the 1950s that were repeated in 2010/2011. Plant assemblages were classified with two alternative phytosociological approaches based on character species and dominant species. Over the ∼60 years, the structure and composition of the vegetation changed profoundly in all four regions, even though local differences in vegetation dynamics existed. In general, stands assignable to the alliances Ranunculion (Batrachion) and Potamogetonion (assemblages dominated by submerged rooted plants) decreased greatly in frequency, while Nymphaeion stands (dominated by floating‐leaved rooted plants) increased. Communities weakly characterized by diagnostic species increased, revealing losses of specialist taxa and homogenization of the assemblages; macrophyte community diversity at the landscape level decreased. Eutrophication, hydraulic engineering and regular maintenance works in the water courses are discussed as likely causes of the vegetation change. We conclude that decade‐long human impact on river hydraulics and chemistry has significantly reduced community diversity at the landscape level, profoundly altering the relative abundance of the assemblages.