Comparison of native and neophytic aquatic macrophyte developments in a geothermally warmed river and thermally normal channels

Global warming is expected to accelerate the invasive success of non-native plants. Long-term data on aquatic macrophytes confirming this hypothesis, however, are scarce. In this study we analyzed the development of native and neophytic aquatic macrophyte species number and abundance in a geothermally warmed river (Erft, North-western Germany) during a 4-year period and compared it to a dataset from thermally normal channels of a similar climatic region (The Netherlands). Total native species numbers declined in both systems, but only in the warmed river, this trend was accompanied by an increase in the relative plant mass of neophytes. Strong changes became apparent at the species level. In the warmed river, the subtropic neophyte Vallisneria spiralis was assumed to have displaced the formerly dominant and highly competitive native species Sparganium emersum, Potamogeton pectinatus and Ceratophyllum demersum based on negative correlations between changes in their plant mass in 52 river sections. The evergreen V spiralis expanded particularly in winter after dieback of the native species. In thermally normal channels, a small increase in neophyte plant mass (Azolla filiculoides) did not account for losses of native submerged species. We thus conclude that higher water temperatures, especially in winter, potentially accelerate the displacement of native species by warm-water wintergreen neophytes.