Unravelling hydrological mechanisms behind fen deterioration

Abstract

Low-productive fens are wetland ecosystems that have strongly declined in number and size in temperate regions of Western Europe since World War II. This fen type has significant conservation value in many countries because of its high species diversity and the presence of endangered plant species. Along with the growing awareness of the importance of biodiversity in general, an interest has arisen to conserve fens for future generations. For this reason, ambitious efforts have been undertaken in The Netherlands to reverse fen deterioration by active nature and water management. However, the effectiveness of fen restoration schemes has so far been variable, partly because of inadequacy of measures to rewet fen reserves with unpolluted groundwater. A possible cause of this inadequacy is that currently utilized rewetting strategies are based on a biased perception of the hydrological mechanisms behind fen deterioration due to an incomplete knowledge of human impact on groundwater flow since the first water management actions. In this thesis, the hydrological mechanisms behind fen deterioration are studied from a historical perspective in order to identify the hydrological requirements for successful fen restoration in anthropogenically dominated regions. A series of palaeo-groundwater models of the Gooi- and Vechtstreek area (The Netherlands) for the time-frame 0-2000 AD indicates that the volume of groundwater that discharges into the studied fen area increased with intensifying water management, except for a minor decrease since the start of groundwater abstraction by the end of the 19th century. Moreover, the models demonstrate that water management actions underlie the shift of the main groundwater discharge mechanism from regional throughflow to local drain discharge. The importance of throughflow for contiguous fen habitat patches in natural fens is demonstrated for the Biebrza River valley (Poland) by confronting modelled zones of groundwater supply with observed patterns in plant alliances and groundwater composition. Likewise, the impact of drainage networks on the groundwater supply of fens is demonstrated by high-resolution groundwater models and environmental tracers observed in a managed fen area. The results show that drainage networks diffusively intercept exfiltrating groundwater, enhance the infiltration of local precipitation on a landscape scale and interrupt the lateral redistribution of excess water by throughflow. A strategy analysis with a linked habitat suitability and seed dispersal model indicates that the elimination of drainage ditches from the upstream margins of fens, i.e., the natural throughflow zone, provides the best prospects for fen restoration. Using this strategy, both abiotic conditions suitable for fen plants and connectivity between habitat patches can be effectively restored. The results of this thesis suggest that effective hydrological fen restoration requires the elimination of drainage ditches in a spatially coherent way, starting at the upstream fen margins and proceeding in a downstream direction. Only if drainage has been sufficiently impeded to restore natural throughflow mechanisms, additional measures, like raising surface water levels in polders, can further enhance fen restoration, by delaying the reinfiltration of exfiltrated groundwater. Lowest priority should be assigned to measures that enhance regional groundwater flow towards drained fen areas, like closing abstraction wells, because such measures do not contribute to a more efficient supply of exfiltrated groundwater to fens.