1. Department of Theoretical Biology, Vrije Universiteit of Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands, Delfland Waterboard, Phoenixstraat 32, 2611 AL Delft, Department of Aquatic Ecology and Ecotoxicology, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Kruislaan 320, 1098 SM Amsterdam, The Netherlands, and Department of Biology, Laboratory for Ecophysiology, Biochemistry and Toxicology, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium