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Lake Simcoe Region Conservation Authority

Lake Simcoe Region Conservation Authority > Watershed Health > Phosphorus

​​​Image of the lake with the shoreling in the distance


Phosphorus has been an issue in Lake Simcoe for many decades. In fact, we began work on addressing phosphorus inputs into the Lake as far ​back as the 1980's when we led the formation of the Lake Simcoe Environmental Management Str​ategy (LSEMS). Since the 1990s, phosphorus loads have been reduced, on average, by more than 25 metric tons each year through source controls and policy.

More needs to be done and more is being done. In 2009, the Province passed the Lake Simcoe Protection Plan (LSPP), and we have been working collaboratively to implement additional policies and practices to reduce phosphorus through the LSPP.

Phosphorus comes from many sources, but one of the most significant sources is stormwater, which picks up phosphorus as it travels across paved and hardened surfaces, especially in densely populated areas.

​What's the Problem?​

​Phosphorus is a naturally occurring element essential for all life. It's found in plants and animals (and humans) and even DNA; the building blocks of all life. In water bodies such as lakes, a certain amount is necessary, but over time too much phosphorus can cause serious issues including excessive weeds, toxic algae and depleted oxygen levels in our rivers, streams and ultimately, Lake Simcoe.

Because it's a naturally occurring element, one source of phosphorus entering the lake is from the weathering of rocks and minerals. And because all life consumes phosphorus, a certain amount is excreted in our waste or released from tissues during plant decomposition.

Phosphorus is also a very useful element and can be found in many of the products we use, for example in common every day products like cola, fertilizers, toothpaste, shampoo, matches and flares. It's also used in pesticides, pyrotechnics, and the production of steel, to name a few industrial uses.

The Phosphorus Lifecycle

The phosphorus in the lake isn't just in the water. It's also in the plants, the animals and the sediments. All of these sources interact, share, store and even lock away phosphorus in what we call the “phosphorus lifecycle".​

Phosphorus in the Water

Phosphorus that enters the lake is used by organisms, settles out into the sediments or can remain dissolved in the waters of the lake. 

Phosphorus Concentrations

Phosphorus concentration is the amount of phosphorus contained in a volume of water (think of how strong or weak you have a cup of tea or coffee). The current (2018-2021) spring, phosphorus concentrations in Lake Simcoe are 7.1 micrograms per litre, which meets the Ontario water quality objective of 10 micrograms per litre.

This current concentration has been reduced from the 1970-80s whe​n the concentration was 15-30 micrograms per litre​. Due to the nature of the lake, and land use in the subwatersheds, some areas (such as Cook's Bay, the Holland River, and Barrie) still have higher phosphorus concentrations than other areas

The decrease in phosphorus concentrations has been achieved through lake management strategies (such as the Lake Simcoe Protection Plan) but other factors can prevent phosphorus from entering the deep water, including water movement due to filtering by invasive mussels​ and a buffer zone of aquatic plants.​​

Phosphorus Loads

We don’t just measure phosphorus in the lake (concentrations), we also measure the amount of phosphorus entering the lake: phosphorus loading. A phosphorus load is made up of the concentration of phosphorus in the water and how much water is entering the lake. Keeping with our cup of tea example, a load would be not just how strong your tea is, but how many cups of tea you drink​.

The phosphorus loads we report for Lake Simcoe are a measurement of the amount of phosphorus entering the lake from all sources on an annual basis.​

Since 2000, the average phosphorus load has been about 83 tonnes per year. Although this is above the Lake Simcoe Protection Plan (LSPP)​ goal of 44 tonnes, it has declined since the 1980-90s when the average was well over 100 tonnes per year. ​

The graph highlights how high tributary flow years such as 2008, 2013, 2017 and 2018 typically result in high phosphorus loads

The graph above Phosphorus loads by source for the 2000 to 2020 hydrologic years are presented along with the Lake Simcoe Protection Plan long-term phosphorus goal of 44 tonnes per year and annual tributary flow volume. ​​

As you can see on the graph, there are years of higher phosphorus loads and years of lower phosphorus loads. This variation is being driven mostly by changes in tributary (or river) flow rate (or how much water is entering Lake Simcoe) which, in turn, is driven by yearly differences in precipitation. 

The years with highest phosphorus loading on the graph (2008 and 2017) also had the highest tributary flows. Both 2008 amd 2017 were very wet years with higher than average precipitation, a mild winter with several snow melt events, and rain during the winter months that fell on frozen ground and ran directly into rivers. All of these factors create perfect conditions for high runoff, high tributary flow, and high phosphorus loading.

Conversely the most recent year 2020 has one of the lowest loads in the last two decades at 58 tonnes. This is largely due to a dryer than normal year with 10 of the 12 months receiving less precipitation than the long-term average for that month. 

One of the major threats to the LSPP, and our efforts to reduce phosphorus loading in Lake Simcoe, is climate change​. With warmer winters, we are receiving more rain. So even though we have reduced the phosphorus concentration in the water, more tributary flow means less change in the phosphorus loading. 

To circle back to our cups of tea example, even though we are drinking weaker tea, we are drinking more cups and getting the same amount of caffeine!

Phosphorus in the Sediment

​Phosphorus is also found in the sediments in the lake. Although plants predominately get their phosphorus from the sediment, not all sediment phosphorus is accessible to plants. As a result of decreasing light penetration, plants are generally not found in water any deeper than 10 metres in Lake Simcoe and therefore sediment at depths greater than 10 metres is considered a phosphorus “sink".

Phosphorus deposits in the sediment are not evenly distributed throughout the lake. In a lake-wide survey, our researchers recorded the locations of high and low concentrations of sediment. In Cook's Bay, for instance, there is a very low concentration of sediment phosphorus because it is consumed by the large amounts of aquatic plants. In Kempenfelt Bay and near Beaverton, sediment phosphorus is much higher because there are fewer plants to consume the phosphorus.

Phosphorus in the Plants

Two of the main users of phosphorus are plants and algae. In Lake Simcoe, quagga mussels have filtered out most of the algae which has resulted in higher water clarity, which allo​ws the sunlight to penetrate deeper into the water, encouraging abundant plant growth.

Aquatic plants get up to 97% of their phosphorus from lake sediments. When they die, some of the phosphorus in their tissues is released back into the water, while some remains bound in the plant tissue that will join the sediment. Where these sediments are at a water depth of less than 10 metres deep, the plant tissue phosphorus may be used to fuel plant growth in subsequent years. If the plant material is transported to waters deeper than 10 metres, the phosphorus will become essentially locked away in the sediments.​

Positive Changes are Happening

The cycle of phosphorus tells us that there are significant reservoirs of phosphorus in Lake Simcoe and that these reservoirs have been caused by many years of excessive loads. While the lake has many natural processes for absorbing excess phosphorus, when these processes get overwhelmed, the result can be excessive plant growth or algal blooms; indicating a system out of balance. 

It has taken many decades for Lake Simcoe to accumulate this much phosphorus and it will take many more to return it to an ecologically sustainable state.

This means that the efforts we are making today may not show immediate results, but positive change is happening. We are now seeing improvements in dissolved oxygen, decreasing phosphorus concentrations and the return of some environmentally sensitive species in the watershed.​

What We're Doing

Continued monitoring - We continue to monitor the phosphorus sources throughout our watershed. Monitoring helps better understand how wet and dry years impact our management plans and how we can adapt to new challenges.

Phosphorus offsetting policy – This is a first-of-its-kind policy​ in Canada and was developed specifically to control phosphorus from new development. The original Phosphorus Offsetting Policy came into effect on January 1, 2018.

Our new Phosphorus Offsetting Policy, approved by the Board of Directors on May 26, 2023, requires that all new development must control post-development phosphorus loadings leaving their property to pre-development levels. This is referred to as a "post to pre" target, which supports the Lake Simcoe Pr​otection Plan's​ objectives.

If the pre-development phosphorus loading can't be met on the site, then developers must pay a fee so that projects to “offset" the phosphorus can be completed elsewhere in the subwatershed. Fees collected for offsets are at a 2.5:1 ratio, meaning, for every 1 kg. of phosphorus that can't be controlled, the property developer must pay for 2.5 kgs. to be controlled elsewhere. Over time, the restoration projects that are implemented with offset fees, will allow a greater reduction of phosphorus overall.

Some of the types of restoration projects to reduce phosphorus can include engineered wetlands and stormwater pond retrofits.

What You Can Do

You can help reduce phosphorus in stormwater run-off by:

  1. All fertilizers have 3 numbers on them. The middle number represents phosphorus, so look for a fertilizer where the middle number is "0".
  2. Purchasing phosphate free soaps and detergents.
  3. When using water outside, direct the excess to the lawn or garden and not through the stormwater system. Capture rain water in cisterns or rain barrels to water plants and lawn during drier periods.
  4. Wash your car at the car wash, where the water will be properly treated. If you must do it yourself, wash your car on the lawn, so that the water soaks into the ground.
  5. Use native plants in your lawn and garden. They've adapted to survive in our climate and therefore don't have the same need for watering or fertilizer. If you fertilize, look for phosphate free fertilizer.
  6. Make a donation to the Lake Simcoe Conservation Foundation . Their work supports many Conservation Authority projects for a cleaner and healthier Lake Simcoe watershed.​