Long-term macrophyte mapping documents a continuously shift from native to non-native aquatic plant dominance in the thermally abnormal River Erft (North Rhine-Westphalia, Germany)

Macrophyte mappings of a 37 km stretch of the thermally abnormal Erft River in 2003, 2005, 2007, 2011 and 2013 documented the presence of nine alien (Azolla filiculoides, Egeria densa, Eichhornia crassipes, Hydrocotyle ranunculoides, Hygrophila polysperma, Lemna minuta, Myriophyllum aquaticum, Pistia stratiotes and Vallisneria spiralis) in addition to 15 native species. During the study period, the number of native plant species decreased significantly in the river sections, while the number of alien species remained constant. The relative plant mass (RPM) of native species decreased from 67 to 33%, while that of the alien species increased. Only two of the nine reported alien species (V. spiralis and P. stratiotes) increased their abundances within the study period and became the most dominant species in the Erft River, while the other introduced species did not show mass development. The highest decline in RPM have been found for native Sparganium emersum and alien E. densa. Summarizing, the evergreen alien V. spiralis and recently P. stratiotes were the most successful invader within this thermally abnormal river since 2003, most likely profiting from the high winter temperatures >10 °C and displacing native vegetation, particularly formerly widespread native S. emersum.