1. Institute for Biodiversity and Ecosystem Dynamics; University of Amsterdam; Science Park PO box 94062 1090 GB Amsterdam the Netherlands
2. Department of Aquatic Ecology & Environmental Biology; Institute for Water and Wetland Research; Radboud University Nijmegen; NL-6525 Nijmegen The Netherlands
3. Witteveen+Bos; PO Box 233 7400 AE Deventer the Netherlands