In the eastern part of the Naardermeer peatlands (the Nethedands) a regional calcium-rich groundwater flow discharges (here often called the seepage area), whereas in the western part infiltration takes place. The ecological consequence of this hydrological pattern is reflected by the pattern in reedland communities. In the seepage area, which is characterized by Thelypteris-reedlands including many rare and endangered species (Caricion davallianae, Calthion palustris), there is a complex gradient of water types. The lime potential in the peat soil is clearly influenced by the hydrological gradient. In the ombrotrophic (poor fen) part of the gradient (containing species of Caricion curto-nigrae) the lime potentials are low and the groundwater contains low amounts of dissolved ions. The rare and endangered species (Caricion davallianae) are restricted to a small area with high lime potentials which is nourished by regional calcium-rich groundwater. High lime potentials were also measured in eutrophic reedlands influenced by brackish groundwater. Several species which generally occur in wet meadows even show a preference for this brackish environment.In a part of the seepage area succession from rich fen to poor fen and Alnus wood has taken place over a period of 40 years. This development has been caused by the diminishing amount of fresh seepage due to a lowering of the water levels in the surrounding area.The characteristics of poikilotrophic zones (contact zones between water flows) are discussed in relation to their significance for the preservation of endangered marsh species.Abbreviations: EC25 = Electrical conductivity measured at 25 °C.

Floating fens are species-rich succession stages in fen areas in the Netherlands. Many of these fens are deteriorating due to acidification; Sphagnum species and Polytrichum commune build 10-25 cm thick moss carpets, and the species diversity decreases. Earlier experiments in wet ecosystems indicate that successful restoration of circum-neutral and mesotrophic conditions requires a combination of hydrological measures and sod removal. In an acidified fen recharged by rainwater in the nature reserve Ilperveld (The Netherlands), a ditch/trench system was dug for the purpose of creating a run-off channel for acid rainwater in wet periods and to enable circum-neutral surface water to enter the fen in dry periods. Moreover, the sod was removed in part of the fen. Ditch/trench creation or sod cutting had no effect individually, but a combination of the two measures led to a change in the abiotic conditions (higher pH and Ca), and in turn to an increase of species-richness and the reestablishment of a number of characteristic species. Reestablishment of rare vascular plant species and characteristic bryophytes might be a long-term process because of incomplete recovery of site conditions and constraints in seed dispersal.

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The abiotic conditions and fen vegetation in three lowland mires were analysed. Two of these mires are in the Netherlands. They have deteriorated considerably as a result of human pressure. One mire complex is in Poland. Its hydrology is almost undisturbed. The variation in the water composition in the fens was associated with the variation in the amount of regional groundwater discharge originating from the uplands, and to a lesser extent by the infiltration of polluted water pumped into the fens.The 26 vegetation types ranged from poor fen to eutrophic reedland. Most types were typical for only one region. There were clear differences between the three regions. Both the species composition and the water quality for the vegetation types indicated that the fens in the Vecht river plain are eutrophicated, whereas the fens in Northwestern Overijssel suffer from acidification. The Biebrza valley fens are well developed and are mainly fed by fresh calcareous groundwater.Stepwise logistic regression performed on 28 fen species revealed that the hydrochemical variables explained only a part of the variance: the regional variable 'area' explained a considerable amount of the variance for most species. In 27% of the cases it was possible to fit an optimum curve for the species response to relevant hydrochemical variables. Monotonic curves could be fitted in 64% of the cases; mostly they described the response to variables in a way that is supported by ecological literature. It was concluded that the dataset should be constructed differently to enable the calculation of generally applicable standards.