1. Aquatic Ecology and Water Quality Management Group, Wageningen University, P.O. Box 47, 6700 AA, Wageningen, The Netherlands, IMARES, Haringkade 1, 1976 CP IJmuiden, The Netherlands, Alterra, P.O. Box 47, 6700AA, Wageningen, The Netherlands, and Deltares, PO Box 85467, 3508 AL Utrecht, The Netherlands