CHANGES IN AQUATIC VEGETATION IN RHINE FLOODPLAIN STREAMS IN ALSACE IN RELATION TO DISTURBANCE

Changes are described in aquatic vegetation in oligotrophic, groundwat er-fed Rhine floodplain streams in Alsace (eastern France), resulting from disturbance. Disturbance factors include changes in nutrients, ei ther permanent ones - effluent from a waste water treatment plant or t rout hatcheries - or periodic ones: flooding. Regular inputs of high l evels of phosphate and ammonia modified the macrophyte vegetation in t hese streams. The floristic composition, which was characteristic of o ligotrophic waters upstream of the eutrophicated sector, changed to th at of a eutrophic situation as originally found downstream. Periodic d isturbance by floods which normally occur once a year, irregularly eut rophicates the small streams, causing the development of a mixture of eutrophic and oligotrophic species. Six macrophyte communities are dis tinguished, indicating different trophic levels. The aquatic vegetatio n is adapted to the variations of phosphate and ammonia levels. Hence, aquatic macrophytes can be used as bio-indicators of fluctuations in water nutrient levels in relation to the type of disturbance.