1. Leibniz Institute of Freshwater Ecology and Inland Fisheries, Department of Limnology of Stratified Lakes, Alte Fischerhuette 2, D-16775 Stechlin-Neuglobsow, Germany
2. University of Rostock, Institute of Biological Sciences, Applied Ecology, Albert Einstein Str. 3, D-18051 Rostock, Germany
3. University of Rostock, Institute of Biological Sciences, Ecology, Albert Einstein Str. 3, D-18051 Rostock, Germany