1. Leibniz Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 301, D-12561 Berlin, Germany, Institute of Chemistry, Humboldt University of Berlin, Brook-Taylor-Strasse 2, D-12489 Berlin, Germany, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway, and Institute of Biology, Humboldt University of Berlin, Arboretum, Späthstrasse 80/81, D-12437 Berlin, Germany