1. Limnological Station, Department of Plant and Microbial Biology; University of Zurich, Seestrasse 187; Kilchberg CH-8802 Switzerland
2. Biology Centre of the Academy of Sciences of the Czech Republic, v.v.i., Institute of Hydrobiology, Na Sádkách 7; České Budějovice CZ-37005 Czech Republic
3. Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Limnology of Stratified Lakes, Alte Fisherhütte 2, OT Neuglobsow; Stechlin D-16775 Germany
4. Institute for Biochemistry and Biology, Potsdam University, Maulbeerallee 2; Potsdam D-14468 Germany