4. Department of Aquatic Ecology, Swiss Federal Institute of Aquatic Science and Technology (EAWAG) and Institute of Integrative Biology, Eidgenössische Technische Hochschule (ETH) Zurich, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland; and Department of Biological and Environmental Sciences, University of Jyväskylä, 40014 Jyväskylä, Finland