1. Department of Aquatic Ecology and Water Quality Management, Wageningen University and Research,
P.O. Box 47, 6700 AAWageningen, The Netherlands
2. System Science Group/Institute of Mathematics, Osnabrück University, Barbarastr. 12, D-49076Osnabrück, Germany
3. Wageningen Environmental Research, P.O. Box 47, 6700 AAWageningen, The Netherlands