1. Department of Animal and Plant Sciences, The University of Sheffield, Sheffield, S10 2TN, U.K., Alterra, Wageningen University and Research Centre, P.O. Box 47, 6700 AA Wageningen, The Netherlands, and Wageningen University, Aquatic Ecology and Water Quality Management Group, Wageningen University and Research Centre, P.O. Box 47, 6700 AA Wageningen, The Netherlands