1. Department of Marine Ecology, National Environmental Research Institute, P.O. Box 358, DK-4000 Roskilde, Denmark
2. European Commission, Joint Research Centre, Institute for Environment and Sustainability, TP 280, I-21020 Ispra (VA), Italy
3. Department of Marine Ecology, Aarhus University, Finlandsgade 14, DK-8200 Aarhus, Denmark