SOME CONSEQUENCES OF THE INTRODUCTION OF 2 MACROPHYTE SPECIES, ELODEA-CANADENSIS MICHAUX AND ELODEA-NUTTALLII ST-JOHN, IN CONTINENTAL AQUATIC ECOSYSTEMS - EXAMPLE OF 2 AREAS IN THE NORTHEAST OF FRANCE - ALSACEPLAIN AND NORTHERN VOSGES

The introduction of two species of elodeids, Elodea canadensis Michaux and Nodea nuttallii St. John during the XIX th and XX th centuries ha d a significant effect on the aquatic ecosystems in the North-East of France. These species invaded the streams preventing the full developm ent of the native species. Their distribution is determined by ecologi cal factors (mineralization, nutrient loads) in the calcareous streams of the Alsace Plain and in the soft waters of the Northern Vosges. Th ey are able to grow in the same area and the same trophic range, but N odea nuttallii seems to be less sensitive to ammonium and much more co mpetitive than E. canadensis in the Alsace plain. In this area, it was demonstrated that Elodea nuttallii, which is widely distributed espec ially in eutrophic waters, also exhibits the highest phosphate uptake and accumulation capacity. This is why Elodea nuttallii increasingly r eplaced E. canadensis in streams. These aquatic macrophytes improve wa ter quality. But, if not harvested, they cause stream eutrophication a fter their decomposition.