1. Centre for Ecology & Hydrology; Maclean Building, Crowmarsh Gifford Wallingford Oxfordshire OX10 8BB UK
2. School of Environment, Natural Resources and Geography, Bangor Univ.; Bangor Gwynedd LL57 2UW UK
3. National Center for Water Quality Research; Heidelberg Univ.; 310 E. Market St. Tiffin OH 44883-2462
4. Dep. of Crop, Soil and Environmental Sciences, 115 Plant Science Building; Univ. of Arkansas; Fayetteville AR 72701