1. Centre for Ecology and Hydrology, Merlewood Research Station, Grange-over-Sands, Cumbria, LA11 6JU, U.K., Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, AB15 8QH, U.K., Department of Biological & Nutritional Science, University of Newcastle, Newcastle Upon Tyne, NE1 7RU, U.K., and Food Standards Agency, Contaminants Division, Ergon House, 17 Smith Square, London, SW1P 3WG, U.K.