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

Abstract

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 fontanum which 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.