1. Institute of Plant Sciences and Oeschger Centre for Climate Change Research; University of Bern; Altenbergrain 21 Bern CH-3013 Switzerland
2. Department of Geology; Makerere University; P.O. Box 7062 Kampala Uganda
3. Limnology Unit; Department of Biology; Ghent University; K. L. Ledeganckstraat 35 Gent B-9000 Belgium