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Lake Simcoe Region Conservation Authority

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​​​​​​​Phosphorus Loads Update

2015 - 2017

​Phosphorus loads have been estimated since the 1990s in order to track the total amount of phosphorus entering Lake Simcoe from all sources. Annual phosphorus loads are one of a suite of parameters that are used to help assess the health of the lake and how it’s changing over time. This report provides an update on the three most recently calculated years of 2015 to 2017.

Phosphorus: One of Several Issues in Lake Simcoe

​Lake Simcoe’s health is impacted by several factors, including invasive species and climate change. Phosphorus is an issue that has received intense interest over the past few decades. 

In water bodies such as lakes, a certain amount of phosphorus is necessary, but too much can cause serious issues including excessive plant and algae growth. Not only can excessive plant and algae growth become a nuisance and detract from recreational opportunities, but as plants and algae die and decompose they consume the oxygen in the deep water that cold water fish species like lake trout and lake whitefish need to survive.

An illustration showing fish in water during different seasons. In the late summer,  the  water may become warm with low oxygen.
Lake Simcoe’s coldwater fish live in deeper, colder waters, especially when they are young. During the late summer the water temperature is too warm at the top and the oxygen can become depleted at the bottom of the lake. This forces the young fish into shallower depths where their predators live. So while coldwater fish can reproduce in Lake Simcoe, at times, many young are not able to survive to adulthood​.

Because lake health is dependent on so many factors, monitoring programs on the lake take a holistic approach. This means we look at numerous parameters and how they interact and impact each other. For example, the arrival of the invasive zebra and quagga mussels led to an increase in water clarity. Increased water clarity allows sunlight to penetrate deeper into the water, encouraging additional aquatic plant growth. More plants result in increased plant decay, which consumes oxygen, leaving less oxygen for fish. 

Because phosphorus load, dissolved oxygen and lake phosphorus concentrations, are all interconnected indicators of ecosystem health, they are often assessed together to give a more complete view of lake health.​

Phosphorus Load Update (2015 to 2017) 

This report provides phosphorus loads for the period of June 1, 2015 to May 31, 2018, which is considered three hydrologic years (June 1 to May 31). The phosphorus load for these years is 87 tonnes in 2015, 73 tonnes in 2016 and 131 tonnes in 2017. Phosphorus loads by source from 2000 to 2017 are shown below. A description of the sources of phosphorus is provided in the 2004 to 2007 phosphorus loads report.

Phosphorus loads vary from year to year, due in large part to changes in climate such as wet or dry years. The high load in 2017 can be attributed to a wetter than normal year in which the watershed experienced two large storm events; accounting for 20% of the annual load. 

While we continue to investigate the causes and impact of the high 2017 load, we have seen wet years drive higher loads in the past, as was the case in 2008. Encouragingly, high flow years like 2008 and 2017 don’t appear to be having a significant adverse effect on other key lake health indicators like lake phosphorus concentrations and deep water dissolved oxygen concentrations.​

Phosphorus: Loads Vs Concentrations

It’s important to explain the two ways we report phosphorus: loads and concentrations. A phosphorus load is the total amount of phosphorus entering Lake Simcoe, measured in metric tonnes (1 tonne = 1,000 kilograms), and calculated over the course of a hydrologic year. A hydrologic year is from June 1 of one year to May 31 of the next year. Measuring phosphorus loads requires us to measure how much water enters the lake (flow volume) as well as determine the amount of phosphorus in that water (phosphorus concentration). This is done for all sources of phosphorus to the lake, including precipitation, sewage treatment plants and rivers.

Following its identification as a critical environmental issue for Lake Simcoe in the 1980s, we have been measuring, estimating and tracking phosphorus loads continuously since the 1990s. In 2009, the Lake Simcoe Protection Plan (LSPP) estimated a long-term goal to limit the phosphorus load to 44 tonnes per year. This was identified as the approximate annual load to achieve a self-sustaining cold water fishery. Currently, annual loads are above 44 tonnes, but they are lower than they were in the 1980s when the loads were estimated to be much higher.This graph shows phosphorus load and its correlation with the high tributary flow.

Phosphorus loads by source from the 2000 to 2017 hydrologic years are presented along with the LSPP long-term phosphorus goal of 44 tonnes per year and annual tributary flow volume.​

Lake Phosphorus Concentrations

Lake phosphorus concentration is different from a phosphorus load, but the two are related. Phosphorus concentration is the amount of phosphorus in a unit of water and is expressed as micrograms per litre (µg/L). While phosphorus load is the amount of phosphorus entering the lake, phosphorus concentration is the amount available for plants and algae to use in their growth cycle. Phosphorus concentrations in Lake Simcoe are measured year-round but the highest concentrations in lake water are usually in spring. Spring is when the water column of the lake is fully mixed and aquatic plants have not yet started using phosphorus to grow, so spring phosphorus concentrations should tell us the maximum concentration of phosphorus in Lake Simcoe within a given year. 

From 2013 to 2017 the average lake-wide spring total phosphorus concentration was 7.5 µg/L, which is below the 1980-1984 average of 15.6 µg/L.  The good news is that we are observing lower phosphorus concentrations. 

This graph shows the gradual decline in spring phosphorus load concentrations from early 1980’s to 2017.Spring lake phosphorus concentrations from 1980 to 2017 are presented and show that concentrations have decreased since the 1980s.

Dissolved Oxygen in Lake Simcoe

Dissolved oxygen concentrations are another indicator we use to understand the lake’s health. A minimum amount of oxygen in the deep water is critical to sustain healthy fish populations. The Lake Simcoe Protection Plan sets a target of 7mg/L, which is the optimal late summer, deepwater dissolved oxygen concentration required to sustain sensitive lake trout and whitefish.  

In the early 1980s the end-of-summer dissolved oxygen concentrations in the deep water of Lake Simcoe averaged 3.3 mg/L. Since then we have seen encouraging improvements, with dissolved oxygen concentrations averaging 6.2 mg/L between 2013-2017, and with two years (2005 and 2014) being above the 7 mg/L target set by the LSPP.

This graph shows the upward trend of deepwater dissolved oxygen levels from early 1980’s to 2017 with 2 years above the target.Deepwater dissolved oxygen concentrations from 1980 to 2017 are presented along with the Lake Simcoe Protection Plan target of 7 mg/L.  ​

​The Lake’s Health is Improving, But New Challenges are Emerging

While phosphorus loads remain above the long-term goal, we are encouraged that dissolved oxygen and phosphorus concentrations are showing signs of improvement. However, not all aspects of lake health are improving. For example, we are finding new invasive species, such as the aquatic plant starry stonewort, and starting to see the effects of climate change​. Climate change is leading to more frequent and intense rain events and multiple winter snow melts are resulting in greater amounts of runoff flowing into Lake Simcoe. All of this extra water adds more phosphorus to the lake. In 2008, 2013 and 2017 we observed higher than normal river flow volumes, which led to higher phosphorus loads.

Efforts are underway to better understand the factors driving improvements in the overall health of the lake despite the variation we see in phosphorus loads. Factors that will be investigated include changes in phosphorus, hydrology, the impact of invasive mussels and changes in zooplankton populations. 

Moving Forward

Lake Simcoe is a complex ecosystem with many factors impacting its health; factors that interact in ways we can’t always predict. Understanding the lake’s response to these new and dynamic stressors, now, and into the future, will require continued work and ongoing monitoring.

We are working on programs, such as our Lake Simcoe Phosphorus Offsetting Program, which aims to achieve zero net phosphorus for all new large development in the watershed. We’re 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. In addition, we continue to research new and innovative technologies aimed at reducing phosphorus loading.

Residents can help contribute to lower phosphorus levels in the lake by taking individual actions on their properties. Every little bit helps.