SPECIES RICHNESS OF AQUATIC MACROPHYTES IN FORMER CHANNELS CONNECTED TO A RIVER - A COMPARISON BETWEEN 2 FLUVIAL HYDROSYSTEMS DIFFERING IN THEIR REGIME AND REGULATION

This paper compares the plant species richness and distribution of two flowing water systems located in functional alluvial forests subject to different hydrodynamics. The study areas are located in the Alsace Rhine floodplain and the braided and anastomosed sector of the upper R hine. The substrates, through which substantial flows of groundwater o ccur, are calcareous Rhine gravels, 200 m deep in places. The first se ctor selected consists of two lateral side arms which are connected to the Rhine and receive highly mineralized bicarbonated waters (conduct ivity of 500-1000 mu S cm(-1)). The second sector is connected to the river III, the main tributary of the Rhine in the Alsace floodplain. T his sector receives inputs of more acidic waters and of sediments from the Vosges mountain, and the waters are less highly mineralized (cond uctivity of 250-600 mu S cm(-1)). The waters of both sectors are class ified as eutrophic to hypertrophic. Comparison of the aquatic macrophy tes of the flowing waters near the Rhine and the side arms located in the III forest shows that the former has greater species richness (43 versus 25 species), greater biomass, as estimated by percentage cover, and a more complex structure (4-5 versus less than or equal to 3 stra ta). Plant community structure and diversity are closely related to ha bitat variability and in turn to the hydrological conditions and fluvi al dynamics. In order to maintain the Aoristic and phytosociological d iversity of these ecosystems, it is necessary first to preserve the fl uvial dynamics which create and maintain the variation of habitats.