Lake Simcoe water quality: phosphorus, algae and beaches

People buying on Lake Simcoe ask about water quality in two ways. One is personal: can I swim off my dock, and is the water nice to be in? The other is long-term: is the lake getting better or worse, and will that affect what the property is worth to the next buyer? The data for both is public, and most of it comes from the Lake Simcoe Region Conservation Authority (LSRCA) and the health unit.

What is the main water quality problem in Lake Simcoe?

Phosphorus. The Lake Simcoe Protection Plan calls excessive phosphorus the most significant cause of water quality impairment in the lake and its tributaries. Phosphorus feeds plant and algae growth. When that growth dies off and decomposes, it uses up oxygen in the deep water that coldwater fish depend on.

The Plan's target is to get deep-water dissolved oxygen up to 7 milligrams per litre at the end of summer, when it is lowest. Its models estimate that would take cutting phosphorus loads to about 44 tonnes a year.

How much phosphorus is going into the lake now?

The LSRCA's latest update covers three hydrologic years:

Hydrologic year	Phosphorus load
2021	70.0 tonnes
2022	66.7 tonnes
2023	77.3 tonnes
2021–2023 average	71.3 tonnes
2018–2020 average (previous period)	83.1 tonnes

For context, the Plan notes that loads exceeded 100 tonnes a year in the early 1990s. So the long-term direction is down, but the lake is still well short of the 44-tonne goal, and loads swing from year to year with weather. A wet year with big spring runoff carries more phosphorus off the land.

The LSRCA tracks loads from tributaries, atmospheric deposition, septic systems within 100 metres of the shoreline, agricultural polders and sewage treatment plants.

Where does a waterfront owner fit in?

More directly than most people expect. The Plan estimates that septic systems adjacent to Lake Simcoe contribute about 4.4 tonnes of phosphorus a year. That is why Oro-Medonte and Ramara run mandatory septic inspection programs for properties within 100 metres of the lake, on a five-year cycle. rossmanteam.com compares those programs side by side in its septic inspections guide, and the township pages here cover the local details: Shanty Bay, Hawkestone and the Oro shore and Brechin and the Ramara shore.

Other things in an owner's control: keeping a natural vegetated strip along the shore (which is also what the shoreline rules push for), not fertilizing lawns down to the water, and managing runoff from roofs and driveways.

Does Lake Simcoe get blue-green algae?

Yes. The LSRCA reported that in 2024 it detected and monitored the first lake-wide blue-green algae bloom on Lake Simcoe, and listed investigating its causes as 2025 work. Blooms are not a constant feature of the lake, but they are now part of the picture.

If you see what looks like a bloom, Ontario's advice is:

• Assume toxins are present; you can't tell by looking.
• Don't swim in it, drink it or let pets or livestock near it.
• Don't boil it or treat it with chlorine. That doesn't remove the toxins, and home treatment systems may not either.
• Report it to the Spills Action Centre, 24 hours a day, at 1-866-MOE-TIPS (663-8477), or online.

How are beaches tested, and what about my own waterfront?

The Simcoe Muskoka District Health Unit samples designated public beaches across the region, including on Lake Simcoe. Monitoring starts around the third or fourth week of June once the water warms up, and runs through the summer:

• Samples are usually taken weekly, or monthly at beaches with good results the previous summer.
• Results usually come back about 24 hours after sampling.
• A swimming advisory goes up when E. coli exceeds 200 per 100 mL, and comes down once levels drop.

An advisory reflects the sample taken that day, not a permanent judgment on a stretch of shore.

Private beaches and waterfronts are not sampled. If you want to know about the water off a specific property, the health unit says to use a lab on Ontario's list of licensed laboratories. That is a reasonable thing to do in the summer before you buy, or in your first season.

What can a buyer actually check?

Here is a practical list for a Lake Simcoe property:

What to check	Where
Nearby public beach history	Health unit beach water testing page (current season)
Lake and tributary data	LSRCA data portal
Watershed trends	LSRCA Watershed Report Card
Current flood or shoreline hazard status	LSRCA Lake Simcoe Shoreline Status page
Septic inspection status	The township, and the seller's last inspection documents
Drinking water from the lake	Seller's treatment system records and your own lab test

Also look at the shoreline itself. Weed growth, clarity and bottom conditions can differ a lot from one lot to the next, even on the same stretch of shore. That is a comfort and use question more than a health one, so see the frontage in mid to late summer if you can. The price drivers page covers how bottom type and exposure affect value, and Lagoon City covers canal weed harvesting.

Is the lake changing in other ways?

The LSRCA's 2023 Watershed Report Card found the watershed fairly stable, with only slight changes since 2018. One clear long-term change is ice: based on records going back to 1852, Lake Simcoe now has about a month less ice cover on average than in the 1850s. That affects ice fishing seasons and how much ice push shorelines see; see ice fishing and winter.

The Plan also identifies climate change, invasive species and emerging contaminants as growing pressures on water quality.

How Jordan can help

When you're comparing Lake Simcoe properties, Jordan can help you look past the photos at the things that shape your time on the water: the bottom, the exposure, the septic status and how the water supply is treated. He can point you to the right records and suggest when to visit so you see the shore at its most typical.