Habitat ecology and long-term development of the macrophyte vegetation of north-west German streams and rivers since the 1950s

Abstract

This PhD thesis deals with the habitat characterization and the long-term development over 60 years of the macrophyte vegetation of north-west German running waters in order to contribute to the knowledge about the ecology of aquatic macrophytes and make use of them as bioindicators. Seventy streams and rivers in the regions Ems-Hunte moraine, Lüneburg Heath, Aller lowlands, northern Harz foothills, base of the Weser-Leine uplands and eastern Holstein were investigated, the emphasis though is on the cross-regional scale. From the analysis of the composition and distribution of the macrophyte communities in relation to river size and physical and chemical water and sediment properties could be concluded that water course depth and current velocity are the most decisive variables for the community composition nowadays, followed by the content of plant-available phosphorus in the sediment. However, it is possible that several decades ago, before the strong intensification of agricultural land-use taking place in the study area since the 1950s, when more pronounced gradients in the nutrient concentrations of running waters existed, chemical variables had a stronger influence on the macrophyte occurrences than today. By means of a semi-permanent plot approach, the vegetation change between the 1950s and 2010 was investigated, revealing a dramatic decline in species diversity (the overall species pool declined by 27.5 % from 51 to 37 hydrophytic species, plot-level richness by 19.4 % from 4.7 to 3.8 species per relevé), accompanied by a profound shift in community composition from the predominance of potamid species to the predominance of lemnid species. Oligotraphent species such as Potamogeton gramineus and P. polygonifolius died out in the study sites and the mesotraphent species Myriophyllum alterniflorum and Ranunculus peltatus declined in their frequency of occurrence by more than 50 %, whereas the eutraphent species Myriophyllum spicatum and Spirodela polyrhiza increased by more than 100 %. In addition, a change in the species traits leaf longevity and specific leaf area (SLA) was found: in the historical macrophyte communities from the 1950s, evergreen species and species with leaves being thin or rich in air-filled lacunae were abundant, while the recent vegetation from 2010 was characterized by summergreen species with a robust structure (low SLA). On the regional scale, the changes in species composition were most profound in the Ems-Hunte moraine country, where drainage of fens and bogs preceded highly intensive land-use especially since the embankment of the Dümmer lake (1953), and less pronounced in the Lüneburg Heath region, where lowland water courses with a natural structure can still be found. On the syntaxonomic level, the application of two different phytosociological classification systems revealed deep changes in the macrophyte community structure over six decades. All relevés from vegetated reaches could be assigned to one of the classes Potamogetonetea, Lemnetea, Phragmitetea or Fontinalietea. While Batrachietalia/Batrachion and Potamogetonetalia/Potamogetonion stands strongly decreased, Nymphaeetalia /Nymphaeion communities gained importance. The observed increase in phytosociologically weakly characterized stands (‘residual communities’) implies losses of highly specialized species. Average similarity was significantly higher in the recent (SBC=0.25) than in the historical (0.22) assemblages, revealing a homogenization of the macrophyte vegetation of north-west German water courses. The most likely causes of the decline in the macrophyte species and community richness and diversity are accelerated eutrophication processes and river regulation measures, that caused uniformization of the running water habitats in the intensively managed cultural landscape, and regular disturbance. Ongoing efforts to reduce the nutrient loads, as well as the enhancement of the habitat heterogeneity by renaturalizing the structure of the water courses and ecologically compatible weed-cutting techniques and dates are necessary to halt and reverse the diversity decline in the macrophyte vegetation of north-west German streams and rivers. Diverse vegetation stands are an essential element in running water ecosystems, whose functionality is relevant, not least for human welfare.