Two models for phosphorus and phytoplankton growth were field verified along a marked gradient in trophic conditions in Green Bay (Lake Michigan): one, the Monod model, relates growth rate to external (dissolved) phosphorus concentration, and the other, the Droop model, describes growth rate as a function of internal (stored) phosphorus levels. The verification provided through a satisfactory fit of model output to field measurements of phosphorus and gross photosynthesis established a conceptual foundation for empirical models relating phosphorus and trophic state parameters. Phosphorus concentrations corresponding to boundary conditions for trophic state categories were developed based on the verified models by defining oligotrophy as the region of linear response by growth rate to increases in phosphorus mesotrophy as the transitional state, and eutrophy as the region where growth rate is saturated, i.e. insensitive to changes in phosphorus concentration. The trophic state classification scheme was applied to several Great Lakes basins to examine their sensitivity to changes in phosphorus levels. The oligotrophic waters of Lake Superior, Huran, and Michigan and northern Green Bay and Georgian Bay have the greatest sensitivity to increases in total phosphorus concentration.