Relationships between macrophyte vegetation and physical and chemical conditions in northwest German running waters

• We examined macrophyte vegetation and habitat characteristics in lowland streams and rivers of northwest Germany.
• River size and current velocity were important determinants of macrophyte community composition.
• Chemical water and sediment characteristics were less influential for vegetation composition.
• Indicator species for eutrophic as well as for mesotrophic conditions were identified.

The macrophyte vegetation of Central Europe's rivers and streams has markedly impoverished during the last decades as a consequence of man's impact on water chemistry and hydromorphology. Here, we examine relationships between species composition and about 35 physical, chemical and river morphological parameters in 69 water courses (291 sampling plots) in the Pleistocene lowlands of northwest Germany for identifying indicator species and environmental parameters suited for categorizing habitat types, growing conditions and the anthropogenic influence in these profoundly altered aquatic ecosystems. Of the 94 species recorded in total, 31 frequent taxa were analyzed for their affinity to specific environmental conditions. Current velocity and water depth (river size) were the most influential determinants of community composition while chemical factors were of secondary importance. We identified eight macrophyte assemblages, each with their own distinctive indicator species and characteristic combinations of growth forms. Small rhithral, relatively nutrient-poor water courses with high velocity were indicated by Myriophyllum alterniflorum and Ranunculus peltatus, while Elodea nuttallii, Myriophyllum spicatum and Spirodela polyrhiza characterized large, potamal water courses with high trophy. We conclude that macrophytes may serve as valuable bioindicators of habitat conditions even in anthropogenically altered running waters, but the specific effects of physical, chemical, and river morphological factors are difficult to separate.