Plant communities and plant diversity in softwater lakes of northern Europe

Numbered in hundreds of thousands, and ranging from <1 ha in area to Europe's largest lake (Lake Ladoga), North European softwater lakes (lime-deficient waterbodies, ranging from low-nutrient, low pH to meso-eutrophic, circumneutral pH) support an important plant component of the biodiversity resources of Europe. Within this region (comprising the British Isles, Scandinavia, and the North European Plain from Brittany to the Baltic States), some 24 softwater lake euhydropnyte community types occur, including stress-tolerant isoetid communities which particularly characterise softwater lake vegetation. A number of rare species are supported, including plants, such as Eriocaulon aquaticum, which are found nowhere else in Europe. In line with the predictions of the hump-back model of diversity-productivity relationships in lakes, the individual plant diversity found in softwater lakes in this region is low (six or fewer euhydrophyte species present, e.g. in acidic, ultraoligotrophic lakes) to moderately high (>20 euhydrophyte species in lakes with, for example, mesotrophic conditions with pH <7.0). Overall, however, a large set of plant species (>100) occurs, forming assemblages which play a vital role in the functioning of softwater lake ecosystems. Lake area, altitude, trophic state and water quality have been found to be good predictors of macrophyte species richness for lakes within this area. Major threats to the survival of softwater lake vegetation include acidification, eutrophication, increased recreational use of lakes, and the effects of lake regulation for hydro-electric schemes. All such pressures tend to reduce the diversity of typical softwater species present in affected lakes. In addition, there is increasing concern about the possible impacts of global CO2 increase for the continued survival of (especially) isoetid vegetation which is strongly adapted to ambient conditions of limited dissolved inorganic carbon availability in softwater lakes. The implications of such global change for the maintenance of softwater lake plant diversity in northern Europe are potentially severe. (C) 2002 Elsevier Science B.V. All rights reserved.