1. Department of Earth Sciences, Faculty of Geosciences; Utrecht University; PO Box 80.021; 3508 TA; Utrecht; the Netherlands
2. NIOZ Royal Netherlands Institute for Sea Research; Department of Marine Organic Biogeochemistry; PO Box 59; 1790AB; Den Burg; the Netherlands
3. Limnology Unit; Ghent University; K. L. Ledeganckstraat 35; B-9000; Gent; Belgium