Hydrobiologia 478: 171–180, 2002. P.H. Nienhuis & R.D. Gulati (eds), Ecological Restoration of Aquatic and Semi-Aquatic Ecosystems in the Netherlands (NW Europe). © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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Restoration of aquatic macrophyte vegetation in acidified and eutrophicated shallow soft water wetlands in the Netherlands J.G.M. Roelofs1 , E. Brouwer1 & R. Bobbink2 1 Department
of Aquatic Ecology and Environmental Biology, Research Group Environmental Biology, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands Tel.: +31-24-3652340. Fax: +31-24-3652134. E-mail:
[email protected] 2 Landscape Ecology, Faculty of Biology, Utrecht University, P.O. Box 800.84, Utrecht, The Netherlands Key words: soft water, wetlands, acidification, eutrophication, carbon dioxide, alkalinisation, restoration
Abstract Soft water lakes possess a highly characteristic vegetation adapted to limitation of carbon. Based upon hydrology, vegetation and geographic distribution, boreal and Atlantic lake types can be distinguished. Reducing the input of nutrients or liming, or both, the stream or its catchment is generally sufficient to restore typical soft water vegetation of boreal soft water lakes. The vegetation of Atlantic soft water lakes is subject to many anthropogenic degradation processes. Therefore, spontaneous recovery in the near future is not expected and restoration is urgently required. Removal of nutrient-rich, anoxic, organic sediments is a prerequisite for restoration of these lakes. In acidified or acid-sensitive lakes, additional measures against acidification are required. Controlled supply of calcareous, nutrient-poor water is much better than direct liming. The effects of these restoration measures strongly depend on the detrimental effects of processes such as atmospheric deposition, drainage, catchment acidification, eutrophication and reduced colonisation rates.
Introduction The macrophyte vegetation of shallow soft water lakes and wetlands is very different from that of both acidic and alkaline waters. Very characteristic are macrophytes with an extensive root system and short leaves in a rosette, e.g. Isoetes lacustris (quillwort). These isoetids often form dense stands on the lake sediment. The highest macrophyte diversity in these lakes or wetlands can be found in the Atlantic regions of Europe, including the Netherlands. Furthermore, soft water macrophytes occur scattered throughout the world in mountainous areas, especially Isoetes species (Keeley et al., 1983; Gacia et al., 1994). This characteristic vegetation is adapted to the restricted availability of nutrients and carbon dioxide. An important problem for submerged plants is that the diffusion rate of carbon dioxide in water is 105 lower than in the air. Bio-available carbon can be present in the water layer as carbon dioxide (CO2 ) or bicarbonate (HCO− 3 ). Bicarbonate is the main contributor to the alkalinity,
or to the acid-neutralising capacity (ANC) of lakes. In alkaline lakes, vegetation is dominated by plants that can also use HCO− 3 from the surrounding water as a carbon source if this is present in high concentrations. In the Netherlands, we find shallow soft water lakes and pools on the Pleistocene deposits in the southeastern part and in some coastal dunes. Because of the rainwater surplus, most other nutrient concentrations, like nitrogen and carbon (HCO− 3 and CO2 ) were originally low. In acidic lakes, the carbon equilibrium shifts towards dominance of CO2 , enabling the growth of CO2 -utilising, acid-resistant macrophytes, such as Sphagnum species. Generally, the HCO− 3 concentration in soft water lakes is below 200 µM, which is insufficient to sustain net photosynthesis (a.o. Sand-Jensen, 1983; Roelofs et al., 1984; Maberly, 1985a; Wetzel et al., 1985; Madsen et al., 1996). Most soft water macrophytes have developed morphological adaptations for acquiring CO2 . Isoetid macrophytes take up CO2 from the sediments through their extensive root system, where
172 CO2 levels are 10–100 times higher compared with the water layer (Roelofs et al., 1984). Nymphaeid species have floating leaves, which enable them to use CO2 from the atmosphere. Several small species, like Elatine hexandra (Lapierre) DC, obtain their CO2 from the water layer immediately above the sediment, where concentrations are generally higher (Maberley, 1985b). Larger species filling the water column, like Myriophyllum alterniflorum L., often possess finely divided leafs which efficiently take up CO2 (Madsen & Sand-Jensen, 1994). As many soft water macrophytes grow close to the sediment, they depend on clear water. In the water layer of most soft water lakes and of wetlands the availability of nitrogen and phosphorus is low (NO3 and NH4
