1. Department of Chemistry, The H.C. Ørsted Institute, Copenhagen University, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark, Institute of Freshwater Ecology and Inland Fisheries, Mueggelseedamm 310, D-12587 Berlin-Friedrichshagen, Germany, Department of Policy Analysis, The National Environmental Research Institute, Frederiksborgvej 399, DK-4000 Roskilde, Denmark, and Department of Environment, Technology and Social Studies, Roskilde University, DK-4000 Roskilde, Denmark