Deterioration of atlantic soft water macrophyte communities by acidification, eutrophication and alkalinisation

This review presents an overview of the most important succession patterns and underlying processes associated with the deterioration of soft water macrophyte communities in atlantic and boreo-atlantic regions. As acidification, eutrophication and alkalinisation are the dominant processes, this paper will focus on the successional changes caused by these processes. Observed differences in the succession between soft waters in different areas in Europe are discussed. Most attention has been given to vegetation changes related to acidification, as most research has been carried out on this topic. In acidifying formerly slightly calcareous aquatic systems, a transition takes place from isoetid and other soft water plant communities to vegetation dominated by submerged Sphagnum mosses and Juncus bulbosus forma fluitans. Regional surveys show that this transition is not very common in acidified lakes in Norway, Finland, Canada, and North America. However, the opposite seems to be true for some southern Swedish lakes, some Scottish lakes, lakes in the West European lowlands (The Netherlands and Germany) and also recently for some southern Norwegian lakes. The calcium bicarbonate content of the sediment seems to be an important explanatory variable, as well as the availability and form of inorganic nitrogen. In West European lakes, isoetids disappear during the acidification process. Elsewhere, although Sphagnum mosses and J. bulbosus develop, isoetids continue to coexist. In this paper, it is hypothesized that the succession seen in soft waters outside the West European lowlands has not yet reached the final stage in which isoetids disappear. Thus, the most dramatic deterioration of soft water macrophyte communities has occurred in the West European lowlands, where the number of isoetid sites has decreased dramatically. The West European systems are substantially smaller and shallower than those elsewhere. The morphology of soft water systems is important in determining the way eutrophication becomes manifest. In larger and deeper systems, planktonic algal growth causes a higher turbidity, whilst in smaller and shallower systems lemnids and ricciellids may expand. For a better understanding of the successional differences between soft waters on an interregional scale, further research and cooperation is needed, in which vegetation, physico-chemical and atmospheric deposition data are integrated. (C) 2002 Elsevier Science B.V. All rights reserved.