1. Department of Theoretical Biology, Vrije Universiteit, de Boelelaan 1085, NL-1081 HV, Amsterdam, The Netherlands
2. Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland
3. Institute for Environmental Research, RWTH Aachen University, Worringerweg 1, D-52064, Aachen, Germany