1. KWR Watercycle Research Institute; P.O. Box 1072 3430 BB Nieuwegein The Netherlands
2. Ecosystem Management Research Group, Department of Biology; University of Antwerp; Universiteitsplein 1-C 2610 Antwerpen-Wilrijk Belgium
3. Department of Systems Ecology; Institute of Ecological Science, VU University; de Boelelaan 1085, 1081 HV Amsterdam The Netherlands