J. P. Klein

Changes in aquatic vegetation in Rhine floodplain streams in Alsace in relation to disturbance

Abstract. Changes are described in aquatic vegetation in oligotrophic, groundwater-fed Rhine floodplain streams in Alsace (eastern France), resulting from disturbance. Disturbance factors include changes in nutrients, either permanent ones - effluent from a waste water treatment plant or trout hatcheries - or periodic ones: flooding. Regular inputs of high levels of phosphate and ammonia modified the macrophyte vegetation in these streams. The floristic composition, which was characteristic of oligotrophic waters upstream of the eutrophicated sector, changed to that of a eutrophic situation as originally found downstream. Periodic disturbance by floods which normally occur once a year, irregularly eutrophicates the small streams, causing the development of a mixture of eutrophic and oligotrophic species. Six macrophyte communities are distinguished, indicating different trophic levels. The aquatic vegetation 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.

Aquatic bryophyte assemblages along a gradient of regulation in the river Rhine

A study of the zonation of the aquatic bryophyte assemblages from the Anterior Rhine to the Middle Rhine is presented. The typology obtained differs from the hydrogeomorphological classification because the correction of the river has led to the homogenization of habitats. Since the straightening of the river in the course of the 19th century, the embankments have created more stable habitats on which mountain species formerly characteristic of the rapids of the High Rhine appeared. During the 20th century, regulation, the construction of hydroelectric powerstations even the canalization of the river in Alsace, have caused a great change in water level. On the Upper Rhine, the regularisation of the water level allowed the appearance and the spread of strict hydrophytes such as Octodiceras fontanumwhich is now common between Rastatt and Bonn. Conversely, species characteristic of variable water levels decreased or disappeared. The industrial development along the river has caused water quality to severely decrease downstream of the industrial areas, hence the decrease or the disappearance of their primary flora in favour of pollution-tolerant assemblages. The former side channels of the Upper Rhine in France that have been disconnected from the main river and fed by groundwater since canalization, possess a bryophyte flora very similar to that of good quality waters in the upstream part of the High Rhine or in the Alpine Rhine. This flora is the last regional relic of a flora that has disappeared from the main channel due to water pollution. These refugia should consequently be preserved from the flooding of alluvial forests, part of the flood management program.

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

Abstract 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 oligotrophic 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.

Spatio-temporal aquatic vegetation patterns in former channels in relation to their isolation from the river Rhine (Eastern France)

Summary The degree of isolation between the Rhine and its annexes is related to the distribution of aquatic macrophytes (bryophytes and phanerogames). Three fluvial sectors characterised by different trophic levels and isolation were analysed. Floristic composition and distribution of aquatic plant systems have allowed us to determine the nature of the exchanges between superficial and ground waters. The implications of this relationship between aquatic vegetation and water origin are discussed in the context of restoration work on ecosystems of the river Rhine.

Bryophytic vegetation in riparian forests: their use in the ecological assessment of the connectivity between the Rhine and its floodplain (Alsace, France)

In the context of riparian wetlands restoration, the cryptogam floras of different alluvial areas of the Rhine were analysed in relation to their location within the hydrographic network. Three study sites were chosen for their particular hydrographic characteristics. The first has been totally disconnected from the Rhine since 1970 and is fed only by groundwater, the second is partly connected, and the third is flooded almost every year. Bryophytic vegetation is already very differentiated between the sites. Connection to the Rhine appears to provide eutrophic conditions that influence waters and soils and generates geomorphological processes such as silt settling and thus influences moss communities. Disconnection is characterized by oligotrophization processes that influence the composition of the flora and which lead to drying-up of the forest climate which is detrimental to the epiphytic cryptogams. The data obtained specify the ecological typology that will be considered in the management plan of the Rhine nature reserves. They provide indicators of the impacts of planned flooding projects which may enable optimization between the influence of the river and conservation of the phreatic streams which currently provide important refuges for many taxa which occurred previously in the high quality waters of the Rhine at the turn of the century.