Statistical relationships between 1) flushing-corrected total phosphorus (TP) loading rate and lake water TP concentration, 2) lake water TP and chlorophyll concentrations and 3) chlorophyll and temperature-corrected dissolved oxygen (DO) depletion rate have permitted the estimation of annual phosphorus loading rates of 89, 73, or 58 metric tons · yr−1required to achieve end-of-summer volume weighted average DO concentrations of 4, 5, or 6 mg · L−1in the 18-m to bottom zone of Lake Simcoe. These results are in good agreement with an independently developed deterministic model which showed that end-of-summer DO concentrations of 4, 5, or 6 mg · L−1could be achieved by reductions in phosphorus loading to 89, 72, or 55 metric tons · yr−1. These models also hindcast a pre-settlement (circa 1800 AD) end-of-summer DO concentration of about 8 mg · L−1in the zone of primary lake trout habitat. Present-day concentrations are in the 3 to 3.5 mg · L−1range and apparently declined in response to an increase in TP loading from about 32 metric T · yr−1in 1800 to about 100 metric T · yr−1in the early 1990s. The proposed TP loading target of 75 metric T · yr−1is predicted to generate a lake volume-weighted springtime (mixed water column) TP concentration of 9 to 10 μg P · L−1(presently at about 12.7 μg P · L−1). As a water quality objective, this concentration is consistent with the guideline presently advocated by the Ontario Ministry of Environment and Energy which states ‘…a high level of protection against aesthetic deterioration will be provided by a total phosphorus concentration for the ice-free period of 10 μg · L−1or less.’