Species Richness of Aquatic Macrophytes in Former Channels Connected to a River: A Comparison between Two 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 Rhine. The substrates, through which substantial flows of groundwater occur, are calcareous Rhine gravels, 200 m deep in places. The first sector selected consists of two lateral side arms which are connected to the Rhine and receive highly mineralized bicarbonated waters (conductivity of 500-1000 μS cm-1). The second sector is connected to the river Ill, the main tributary of the Rhine in the Alsace floodplain. This sector receives inputs of more acidic waters and of sediments from the Vosges mountain, and the waters are less highly mineralized (conductivity of 250-600 μS cm-1). The waters of both sectors are classified as eutrophic to hypertrophic. Comparison of the aquatic macrophytes of the flowing waters near the Rhine and the side arms located in the Ill 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 ⩽ 3 strata). Plant community structure and diversity are closely related to habitat variability and in turn to the hydrological conditions and fluvial dynamics. In order to maintain the floristic and phytosociological diversity of these ecosystems, it is necessary first to preserve the fluvial dynamics which create and maintain the variation of habitats.