Current improvements in phosphorus modeling, understanding of watershed dynamics, and cumulative impact assessment have contributed to the proposed new water quality objective for Ontario’s Precambrian Shield lakes. Ontario is proposing an allowable increase in phosphorus of 50% above the predevelopment level from anthropogenic sources. Under that proposal, a lake modeled to a predevelopment phosphorus concentration of 4 µg/L would be allowed to increase to 6 µg/L. A cap at 20 µg/L would still be maintained to protect against nuisance algal blooms. The proposed water quality objective for TP is based on a modeled predevelopment phosphorus concentration, which is intended to provide water quality managers with a constant assessment baseline as well as a buffer against incremental loss of water quality and variable water quality measurements.
Ontario chose two primary trophic status indicators as assessment endpoints for setting phosphorus criteria based on protection of aesthetics for recreational use and protection of aquatic life. Water clarity was chosen as an endpoint based on its relationship to aesthetic changes in water quality. Researchers found that a 50% increase in phosphorus concentration produces an average 25% loss of Secchi depth across the range of initial phosphorus.
Ontario also considered biotic impacts of phosphorus enrichment such as the loss of oxygenated hypolimnetic habitat for cold-water species. Ontario used the Molot et al. (1992) model to predict the impact of a 50% increase in phosphorus on DO. The model predicts the hypolimnetic oxygen profile at the critical end-of-summer period, when lakes are warmest and oxygen depletion is near maximum.
Four stratified lake types were modeled, spanning a range from highly sensitive (shallow and small) to least sensitive (deep and large). Responses were expressed as volume-weighted average hypolimnetic oxygen concentration and as the volume of hypolimnion exceeding the Provincial Water Quality Objective of 6 mg/L. On average, a 50% increase in phosphorus protects DO in any lake that is larger than 67 ha, is 28 m or deeper, and has less than 12 µg/L of predevelopment phosphorus. Some portion of the hypolimnion remained at 6 mg/L of DO or better in all such lakes modeled (USEPA 2000a).
References:
Molot, L.A., P.J. Dillon, B.J. Clark, and B.P. Neary. 1992. Predicting end-of-summer oxygen profiles in stratified lakes. Canadian Journal of Fisheries and Aquatic Sciences 49(11):2363–2372.
USEPA. 2000a. Nutrient Criteria Technical Guidance Manual: Lakes and Reservoirs. EPA-822-B-00-001. U.S. Environmental Protection Agency, Office of Water, Office of Science and Technology, Washington, DC. Accessed October 2016. https://nepis.epa.gov/Exe/ZyPDF.cgi/20003COV.PDF?Dockey=20003COV.PDF.