1. Section of Ecology, Department of Biology, University of Turku20014 Turku, Finland
2. Department of Animal and Plant Sciences, University of SheffieldSheffield S10 2TN, UK
3. EAWAG, Swiss Federal Institute of Aquatic Science and Technology8600 Dübendorf, Switzerland
4. ETH-Zürich, Institute of Integrative Biology (IBZ)8600 Dübendorf, Switzerland