Some consequences of the introduction of two macrophyte species, Elodea canadensis Michaux and Elodea nuttallii St.John, in continental aquatic ecosystems: example of two areas in the north-east of France: Alsace plain and northern Vosges

The introduction of two species of elodeids, Elodea canadensis Michaux and Elodea nuttallii St. John during the XIX th and XX th centuries had a significant effect on the aquatic ecosystems in the North-East of France. These species invaded the streams preventing the full development of the native species. Their distribution is determined by ecological factors (mineralization, nutrient loads) in the calcareous streams of the Alsace Plain and in the soft waters of the Northern Vosges. They are able to grow in the same area and the same trophic range, but Elodea nuttallii seems to be less sensitive to ammonium and much more competitive than E. canadensis in the Alsace plain. In this area, it was demonstrated that Elodea nuttallii, which is widely distributed especially in eutrophic waters, also exhibits the highest phosphate uptake and accumulation capacity. This is why Elodea nuttallii increasingly replaced E. canadensis in streams. These aquatic macrophytes improve water quality. But, if not harvested, they cause stream eutrophication after their decomposition.