A Limnological Basis For A Lake Simcoe Phosphorus Loading Objective

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−1 required to achieve end-of-summer volume weighted average DO concentrations of 4, 5, or 6 mg · L−1 in 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−1 could 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−1 in the zone of primary lake trout habitat. Present-day concentrations are in the 3 to 3.5 mg · L−1 range and apparently declined in response to an increase in TP loading from about 32 metric T · yr−1 in 1800 to about 100 metric T · yr−1 in the early 1990s. The proposed TP loading target of 75 metric T · yr−1 is 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−1 or less.’