Spatial and environmental effects on hydrophytic macrophyte occurrence in the Upper Rhine floodplain (Germany)

The occurrence of exclusively submerged macrophytes (hydrophytes) was analyzed on a data set of 608 mapped stream segments with a total length of 106 km from the Upper Rhine floodplain. There were three objectives. The first was to test the evidence of plot-sized bias which occurs when structural homogenous river sections are mapped. In the dataset, with a broad range of areas sampled, this bias was small but the mapping procedure nevertheless had some disadvantages for ecological interpretation of hydrophyte data. The second objective was to test for spatial autocorrelation of species composition among consecutive stream sections. Results showed that spatial autocorrelation is an intrinsic and not easily interpretable feature, which might weaken the interpretative strength of pure species–environment relationships in streams. The third objective was to analyse species–environment relationships, by redundancy analysis. The analyses gave satisfying distribution patterns of the 25 most frequent hydrophytes, using a small group of environmental parameters—current, shading, turbidity and maximum depth. They could be assigned into three groups—rheophilic, potamale and related to groundwater influence. The high amount of unexplained variability in species data is due to the large data set where the high plasticity of most hydrophytes becomes apparent and thus resulting in a broad niche overlap. The study gives a representative overview over one of the richest regions in Germany for hydrophytes. Handling editor: S. M. Thomaz