1. Department of Environmental Biology, University of Guelph, Guelph, Ontario, N1G 2W1 Canada, Solvay S.A., rue de Ransbeek 310, Brussels, Belgium B-1120, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, M53 3H6 Canada, and National Water Research Institute, Environment Canada, 867 Lakeshore Road, Burlington, Ontario, L7R 4A6 Canada