1. Organic Geochemistry Unit, Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, U.K.
2. Merlewood Research Station, Centre for Ecology and Hydrology, Windermere Road, Grange-over-Sands, Cumbria LA11 6JU, U.K.
3. Department of Biology, University of York, P.O. Box 373, York YO10 5WY, U.K.