1. Eawag, Swiss Federal Institute of Aquatic Science and Technology, Seestrasse 79, 6047 Kastanienbaum, Switzerland, Institute for Biogeochemistry and Pollutant Dynamics, ETH, Zurich, Switzerland, IFM-GEOMAR, Leibniz Institute of Marine Sciences at the University of Kiel, Wischhofstrasse 1-3, D-24148 Kiel, Germany, and Israel Oceanographic & Limnological Research, Yigal Allon Kinneret Limnological Laboratory, Midgal, Israel