1. Netherlands Institute of Ecology, Centre for Limnology, Rijksstraatweg 6, Nieuwersluis, P.O. Box 1299, 3600 BG Maarssen, The Netherlands, Institute for Environmental Studies, Free University, De Boelelaan 1115, 1081 HV Amsterdam, The Netherlands, Department of Civil & Environmental Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102, and Management Unit of the Mathematical Models of the North Sea, 100 Gulledelle, B-1200 Brussels, Belgium