1. Eawag, Swiss Federal Institute of Aquatic Science and Technology; Dübendorf Switzerland
2. Institute of Environmental Engineering; ETH Zurich, Swiss Federal Institute of Technology; Zurich Switzerland
3. Now at Department of Global Ecology; Carnegie Institution for Science; Stanford California USA
4. Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Swiss Federal Institute of Technology; Zurich Switzerland