A comparative study of the hydrophyte flora from the Alpine Rhine to the Middle Rhine. Application to the conservation of the Upper Rhine aquatic ecosystems

A typology of the main channel of the river Rhine according to its aquatic bryophyte and vascular hydrophyte assemblages is presented. The aquatic bryophytes are especially abundant in the main channel, having found stable, rocky habitats with variable water levels in the regulated river, and segregate longitudinally along a gradient of water quality. Conversely, the vascular hydrophytes are restricted to side channels with constant discharges and silt deposits, and segregate laterally along a gradient of connectivity with the main river. The hydrophytes have been affected by water eutrophication which became obvious in the 1960s-1970s. The oligotrophic groundwater-fed side-channels disconnected since the river canalization consequently include a relic reference flora. Important hydraulic works are currently in progress in order to protect the areas located downstream from the canalized Rhine from flooding by retaining the river waters in lateral systems during the discharge peaks and to recreate a functional alluvial floodplain by reconnecting the disconnected side-channels to the main river. The floodpulse caused by the suddent input of surface water in the disconnected brooks will probably wash out most of the hydrophytes and it is very likely that the rare species with their low recolonization strategies will disappear in these conditions. It is highly desirable to preserve from flooding the last oligotrophic brooks with their original hydrophyte assemblages. Those brooks which show a tendency to silt up can be re-dynamized by ecological engineering without disturbing the flowing drains of the watertable. In areas (including flowing drains of the watertable) that have already been designated for flood retention, the hydraulic works should allow, as far as possible, the preservation of the flowing oligotrophy streams by only permitting the input of surface water in the silted-up brooks. The disconnected side-channels could then continue their role as refugia from where the main channel, whose water quality has been improving for a decade, could be recolonized by its primary flora.