Phosphorus Loads Update 2021-2023
Long-term monitoring is a critical part of the overall effort to restore Lake Simcoe’s health. It measures the success of phosphorus reduction actions and the progress towards achieving the 7mg/L for dissolved oxygen target (Lake Simcoe protection plan | Ontario.ca).
Since the 1990s, the Lake Simcoe Region Conservation Authority has been conducting extensive monitoring to track long-term changes in annual phosphorus loading (the total amount of phosphorus entering the lake from all sources) and dissolved oxygen concentrations in Lake Simcoe. The Conservation Authority has calculated annual phosphorus loads to Lake Simcoe since the 1990s. This report provides an update on the most recently calculated phosphorus loads for hydrologic years of 2021, 2022 and 2023.
Summary
This report provides the most recently calculated phosphorus loads for three consecutive hydrologic years 2021-2023; which runs from June 1, 2021, to May 31, 2024. The hydrologic year runs from June 1 of one year to May 31 of the next, and is named for the year in which it started. For example, the 2021 hydrologic year started on June 1, 2021 and ended May 31, 2022.
The calculated loads in these years are:
- 2021 = 70.0 tonnes
- 2022 = 66.7 tonnes
- 2023 = 77.3 tonnes
The three-year average phosphorus load for 2021-2023 is 71.3 tonnes, a decrease from the previous period (83.1 tonnes for 2018-2020). In comparison, the long-term (24-year) average is 81.6 tonnes. While this decrease is encouraging, more needs to be done to achieve the 44 tonne/year goal established in the Lake Simcoe Protection Plan.
The effect of phosphorus loading on Lake Simcoe’s coldwater fishery
Lake Simcoe is the most intensively fished inland lake in Ontario (Ministry of Natural Resources, 2023). It is renowned for its coldwater fishery, which includes lake trout and lake whitefish. Beginning after European settlement, and intensifying after the 1960s, Lake Simcoe experienced a decline in its coldwater fish populations due to increased nutrient inputs and low dissolved oxygen levels in the lake (North et al., 2023). Conserving these coldwater fish populations is not only economically important; it is also critical for maintaining ecosystem health.
Coldwater fish are indicators of lake ecosystem health because of their sensitivity to changes in dissolved oxygen levels, temperature, and water quality. Reductions in coldwater fish populations suggest a more serious decline in overall ecosystem health. Studies of Lake Simcoe’s coldwater fish populations show that many young fish are not surviving to adulthood. This is because in the summer, surface waters are too warm and the cooler bottom waters are too oxygen poor (Winter et al., 2007), forcing young fish to swim in the middle layers where they can be more easily eaten by predators (Fig. 1).
To support year-round coldwater fish survival in Lake Simcoe, the Lake Simcoe Protection Plan set a 7mg/L target for dissolved oxygen measured in deepwater at the end of summer (when dissolved oxygen concentrations reach a minimum). Established in consultation with scientists, this target is estimated to be the optimal amount of dissolved oxygen needed to support a self-sustaining coldwater fishery (Young et al., 2011).
Phosphorus loads and lake dissolved oxygen concentration
To achieve 7mg/L of dissolved oxygen, the amount of phosphorus entering the lake must be reduced. While phosphorus is a vital nutrient for aquatic ecosystems, excessive amounts can lead to overgrowth of aquatic plants and algae. When these organisms die, they are decomposed by microorganisms that consume oxygen in the process — oxygen that coldwater fish species rely on to survive. Reducing phosphorus inputs will help curb excessive plant and algae growth, thereby increasing deep-water oxygen levels, enhancing coldwater fish habitat, and improving overall ecosystem health in Lake Simcoe.
Limiting phosphorus loading to a maximum of 44 tonnes per year will help achieve the 7mg/L dissolved oxygen target. This loading goal, set in the Lake Simcoe Protection Plan, is grounded in traditional lake management principles.
Calculating the 2021-2023 phosphorus loads to Lake Simcoe
Several components make up the Lake Simcoe phosphorus load: atmospheric deposition directly to the lake (i.e., bits of pollution from the air such as dust, smoke, or chemicals), septic systems within 100 metres of the shoreline, agricultural polders, water pollution control plants, and tributaries.
Phosphorus loads during this reporting period were: 2021 = 70.0 tonnes, 2022 = 66.7 tonnes, and 2023 = 77.3 tonnes (Fig. 2). These phosphorus loads were lower than the previous three-year period (83.2 tonne average from 2018-2020) and represented a return to more typical loading conditions; however, they still exceeded the 44 tonne/year goal established in the Lake Simcoe Protection Plan. For comparison, the 24-year average total phosphorus load was 81.6 tonnes. Total tributary flows, which are a key component of the total phosphorus loads, were also typical throughout this period.
Phosphorus loads and the impacts of climate change
Climate change is altering when and how much phosphorus flows from tributaries. We are already seeing higher phosphorus loads – and higher flows – during winter months due to increased snowmelt/rain-on-snow events, and in summer due to intense rainstorms. For example, melting from March 15 to 16, 2019, contributed about 16% of all tributary phosphorus for the 2018 hydrologic year. Likewise, a single event from Jan 11 to 12, 2020, contributed about 15% of all tributary phosphorus for the 2019 hydrologic year. In comparison, 2020 was much drier with fewer extreme melting and rainfall events.
Conditions during the 2021 to 2023 hydrologic years were more like 2020 than 2018 and 2019, hence phosphorus loads that were more like the 24-year average. In the future, extreme conditions (e.g., flooding events, storms) are expected to become more frequent, which means the final phosphorus loads may also become more variable.
Moving forward
A healthy Lake Simcoe, now and in the future, requires ongoing work and monitoring throughout the watershed. The Conservation Authority and watershed partners are continually working to reduce phosphorus loading in many capacities: the Lake Simcoe Phosphorus Offsetting Program aims to achieve zero net phosphorus loads for all new large development in the watershed; the offsetting program implements a strategic direction in the Phosphorus Reduction Strategy for Lake Simcoe; we are working with municipalities to retrofit stormwater ponds and to introduce Low Impact Development technologies to manage stormwater; our tree planting programs also serve to help reduce phosphorus by preventing soil erosion; and we continue to research emerging technologies aimed at reducing phosphorus loading.
Residents can help reduce their own phosphorus contributions to the lake by taking individual actions on their properties. Every little bit helps. Check out Funding and Expertise for opportunities to help restore the watershed and save money.
References
Ministry of Natural Resources. 2023. Fisheries Management Zone 16 (FMZ 16). Retrieved July 24, 2025, from https://www.ontario.ca/page/fisheries-management-zone-16-fmz-16
North, R.L., Barton, D., Crowe, A.S., Dillon, P.J., Dolson, R.M.L., Evans, D.O., Ginn, B.K., Håkanson, L., Hawryshyn, J., Jarjanazi, H. and King, J.W., 2013. The state of Lake Simcoe (Ontario, Canada): the effects of multiple stressors on phosphorus and oxygen dynamics. Inland Waters, 3(1), pp.51-74.
Winter, J.G., Eimers, M.C., Dillon, P.J., Scott, L.D., Scheider, W.A. and Willox, C.C., 2007. Phosphorus inputs to Lake Simcoe from 1990 to 2003: declines in tributary loads and observations on lake water quality. Journal of Great Lakes Research, 33(2), pp.381-396.
Young, J.D., Winter, J.G. and Molot, L., 2011. A re-evaluation of the empirical relationships connecting dissolved oxygen and phosphorus loading after dreissenid mussel invasion in Lake Simcoe. Journal of Great Lakes Research, 37, pp.7-14.
This project has received funding support from the Government of Ontario. Such support does not indicate endorsement by the Government of Ontario of the contents of this material.
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