1. Department of Basic Sciences and Environment, the Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark, Department of Marine Ecology, National Environmental Research Institute, University of Aarhus, Frederiksborgvej 399, 4000 Roskilde, Denmark, and Department of Science, Systems and Models, Roskilde University, Universitetsvej 1, 4000 Roskilde, Denmark