1. S. W. Chisholm is in the Department of Civil and Environmental Engineering, MIT, Cambridge, MA 02139, USA.
2. P. Falkowski is in the Department of Geology, Rutgers University, New Brunswick, NJ 08901, USA.
3. J. J. Cullen is in the Department of Oceanography, Dalhousie University, Halifax, NS B3H 4J1, Canada.