1. Schofield, C. L. Proc. Conf. Emerging Environmental Problems, Acid Precipitation, 76–87 (US Environmental Protection Agency, 1976).
2. Davis, R. B., Smith, M. O., Bailey, J. H. & Norton, S. A. Verh. int. Verein. theor. angew. Limnol. 20, 532–537 (1978).
3. Johnson, N. M. Science 204, 497–499 (1979).
4. Dickson, W. Verh. int. Verein. theor. angew. Limnol. 20, 851–856 (1978).
5. Cronan, C. S. & Schofield, C. L. Science 204, 304–305 (1979).