Lake Partner sites

4,894in the province’s phosphorus and clarity files

Sites sampled in 2025

1,077phosphorus readings, the latest year in the files

Sites with a status

2,334three or more years of phosphorus

Oligotrophic sites

54%1,262 of 2,334, under 10 µg/L

Lakes with a site status

221 lakesof the 265 tabled below

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What Lake Water Quality Measures

The water quality of an Ontario lake is most often described by two readings: the concentration of total phosphorus in the water, and the depth at which a Secchi disk disappears from view. The province’s Lake Partner Program collects both, and its files hold 4,894 monitoring sites.

Phosphorus

Phosphorus is the nutrient that controls how much algae most Ontario lakes can grow. More phosphorus supports more algae, and more algae clouds the water. Total phosphorus is reported in micrograms per litre (µg/L): millionths of a gram in each litre of water, or parts per billion.

Some lakes are naturally richer in phosphorus than others. It also reaches a lake from shoreline erosion, from runoff off farms and towns, from sewage treatment plants and from septic systems that are not working properly.

Water Clarity and the Secchi Disk

A Secchi disk is a round black and white plate lowered on a marked line until it disappears from view. The depth at which it disappears is the Secchi depth, and a deeper reading means clearer water.

Algae cloud the water, and so do silt and the dissolved organic matter that stains many Shield lakes the colour of tea. Zebra mussels, which filter plankton out of the water, can leave a lake clearer. The province therefore judges a lake’s nutrient status by its phosphorus. Clarity readings record changes that phosphorus alone would miss, such as a zebra mussel invasion or a disturbance in the watershed.

How the Samples Are Taken

The readings are taken, mostly by volunteers, each at a fixed site, usually over the deepest water of a lake or bay. A sample from the surface of the open water is taken by the province to represent the phosphorus of the whole basin around it.

On a lake on the Canadian Shield a volunteer collects one sample in May, at spring turnover, when the lake has mixed from top to bottom after the ice has gone. On a lake off the Shield, samples are collected once or twice a month, May to October. Secchi readings are taken once or twice a month, May to October. The sample is filtered, sealed in two glass tubes and mailed to the Dorset Environmental Science Centre, where the province has analysed Lake Partner phosphorus since 2002. The two tubes are analysed separately, so a contaminated sample shows as a pair that disagrees.

Oligotrophic, Mesotrophic and Eutrophic Lakes

The province sorts lake water into three bands by its total phosphorus, with edges at 10 µg/L and 20 µg/L. A site receives a band once it holds three years of phosphorus readings, and the band is set by the average of those years.

• Oligotrophic, under 10 µg/L: dilute, unproductive water in which nuisance algal blooms are rare.
• Mesotrophic, 10 to 20 µg/L: a broad middle range, from clear, unproductive water near 10 to water that can carry moderate algal blooms near 20.
• Eutrophic, over 20 µg/L: productive water, which can carry persistent nuisance algal blooms.

Across Ontario, 2,334 Lake Partner sites hold a status: 1,262 oligotrophic, 839 mesotrophic and 233 eutrophic. Half of them average less than 9.5 µg/L, and 51 percent average between 4 and 10 µg/L.

Water Quality Across a Single Lake

A large lake is sampled at several sites, and its sites can read differently. Of the 168 lakes tabled below with two or more sites holding a status, 52 have sites in more than one band.

Lake Muskoka has 21 registered sites, nine of them with a status. Their averages run from 5.83 µg/L at site 2465/19, Dudley Bay deep spot, to 10.57 µg/L at site 2465/9, Muskoka Bay-CliffBay. A bay that receives a creek or a town’s runoff can read differently from open water half a kilometer away.

Water Quality by Lake

The tables below cover 265 Ontario lakes, grouped by region. On 221 of them at least one Lake Partner site holds a status, and each is given with the site that has the longest phosphorus record: its band, its long-term phosphorus average, the years it was sampled and the median of its Secchi readings.

Twenty-five lakes have registered sites with fewer than three years of phosphorus readings, and nineteen have no registered site. Each lake’s page tables every site on it that holds a status.

Muskoka Lake Water Quality

Lake Partner sites with a status lie on 22 of the 25 Muskoka lakes below. In all, 63 sites hold one: 42 oligotrophic, sixteen mesotrophic and five eutrophic.

Bass Lake (Muskoka), Butterfly Lake and Dotty Lake have registered sites with fewer than three years of phosphorus readings.

Haliburton Lake Water Quality

Lake Partner sites with a status lie on 33 of the 35 Haliburton lakes below. In all, 64 sites hold one: 60 oligotrophic and four mesotrophic.

Head Lake (Haliburton County) and Sherborne Lake have registered sites with fewer than three years of phosphorus readings.

Parry Sound Lake Water Quality

Lake Partner sites with a status lie on 23 of the 25 Parry Sound lakes below. In all, 84 sites hold one: 65 oligotrophic, eighteen mesotrophic and one eutrophic.

Doe Lake and Pickerel Lake have registered sites with fewer than three years of phosphorus readings.

Kawartha Lakes and Peterborough Water Quality

Lake Partner sites with a status lie on 34 of the 37 Kawartha and Peterborough lakes below. In all, 108 sites hold one: 34 oligotrophic, 64 mesotrophic and ten eutrophic.

Gold Lake and Shadow Lake have registered sites with fewer than three years of phosphorus readings. No Lake Partner site is registered on Little Bald Lake.

Simcoe County and Southern Ontario

Lake Partner sites with a status lie on nine of the thirteen Simcoe County and southern Ontario lakes below. In all, seventeen sites hold one: seven oligotrophic, five mesotrophic and five eutrophic.

MacLean Lake, Puslinch Lake and Tea Lake have registered sites with fewer than three years of phosphorus readings. No Lake Partner site is registered on Conestogo Lake.

Eastern Ontario Lake Water Quality

Lake Partner sites with a status lie on 44 of the 50 eastern Ontario lakes below. In all, 164 sites hold one: 96 oligotrophic, 55 mesotrophic and thirteen eutrophic.

Bark Lake, Centennial Lake, Crotch Lake and Dalhousie Lake have registered sites with fewer than three years of phosphorus readings. No Lake Partner site is registered on Constant Lake or Varty Lake.

Nipissing, Sudbury and Northeastern Ontario

Lake Partner sites with a status lie on 38 of the 48 northeastern Ontario lakes below. In all, 241 sites hold one: 162 oligotrophic, 63 mesotrophic and sixteen eutrophic.

Canoe Lake, Long Lake (Timiskaming District), Night Hawk Lake and Ranger Lake have registered sites with fewer than three years of phosphorus readings. No Lake Partner site is registered on Lady Evelyn Lake, Missinaibi Lake, Mistinikon Lake, Obabika Lake, Lake Opeongo or Red Cedar Lake.

Northwestern Ontario Lake Water Quality

Lake Partner sites with a status lie on eighteen of the 32 northwestern Ontario lakes below. In all, 113 sites hold one: seventeen oligotrophic, 65 mesotrophic and 31 eutrophic.

Abram Lake, Dryberry Lake, Pelican Lake, Red Lake and White Otter Lake have registered sites with fewer than three years of phosphorus readings. No Lake Partner site is registered on Dinorwic Lake, Dog Lake (Thunder Bay District), Kenogamisis Lake, Marmion Lake, Minnitaki Lake, Lake Nipigon, Lac Seul, Sturgeon Lake (Thunder Bay District) or Wildgoose Lake.

Lake Simcoe Water Quality

Lake Simcoe is measured by the province itself. The Ministry of the Environment, Conservation and Parks samples eight lake stations about every two weeks from spring to fall, and has published the record from 1980 as open data.

The province’s Lake Simcoe 15-Year Monitoring Report, published in December 2025 and covering the record to 2022, records that phosphorus in the ice-free season has fallen significantly at all eight lake stations since the 1980s, and that clarity has risen at every station since 1980, appearing to level off or decline in recent years. Chloride has risen at all eight stations.

At station K42, Kempenfelt Bay 42, the Secchi disk disappeared at an average 4.3 meters in 1980 to 1984, at 7.6 meters in 2015 to 2019, and at 6.5 meters in 2020 to 2024. Over 2015 to 2024, ice-free phosphorus averaged 7.8 µg/L at station K45 and 14.1 µg/L at station C1, Cooks Bay 1, the shallowest of the eight.

Mussels, Algae and Blooms on Lake Simcoe

The same report records zebra mussels establishing in the lake in the mid-1990s and quagga mussels spreading through its nearshore and deep water from 2010. Quagga mussels now cover the whole bottom of the lake and feed at cooler temperatures than zebra mussels, which held only the nearshore bottom. Both filter phytoplankton and phosphorus-bearing particles from the water, and the province links them to the clearer water and to a change in how phosphorus cycles through the lake. Starry stonewort, an invasive macroalga, appears to have replaced Eurasian watermilfoil among the lake’s most abundant plants since 2018.

Blue-green algal blooms confirmed on Lake Simcoe from 2018 to 2022 were confined to small, localized areas. A larger bloom was observed in several parts of the lake in 2024, and its toxin measurements were very low or below detection.

The Lake Partner Program has seven registered sites on Lake Simcoe, and two hold a status: site 6982/3, Cedarmont Beach, averaging 12.3 µg/L over 23 years to 2024 and site 6982/7, Strawberry Island, averaging 11.58 µg/L over eight years to 2009.

Blue-Green Algae in Ontario Lakes

Blue-green algae, more correctly called cyanobacteria, are microscopic organisms that occur naturally in lakes, ponds, rivers and streams. When their numbers rise quickly they form a bloom, a dense mass or scum in the water. Some kinds produce toxins, called cyanotoxins, which are harmful to people and animals.

When Blooms Occur

In Ontario, blooms are most common in late summer and early fall, and they occur in other seasons as well. Shallow, slow-moving, warm water suits the organisms best. A bloom that appears on a lake in one year does not always return in the next.

What Causes Blue-Green Algae

The province lists calm water, high temperatures and a rise in nutrients, phosphorus above all, among the conditions that promote a bloom. Phosphorus added by people comes from shoreline and land erosion, farm and urban runoff, sewage treatment plant effluent and septic systems that are not working properly. Invasive species and a changing climate can also raise nutrient levels in ways that favour blooms.

The province’s phosphorus bands describe the same relationship: nuisance blooms are rare in oligotrophic water, moderate blooms can occur near the top of the mesotrophic band, and eutrophic water can carry persistent ones.

What a Bloom Looks Like

A dense bloom colours the water bright green or blue-green, and sometimes yellow, purple, red or brown, and can gather as a scum or a mat at the surface. Several other things resemble one: mats of aquatic plants such as duckweed or milfoil, which show leaves and stems; yellow pollen in spring, which also coats the ground near the shore; and filamentous green algae, which are harmless. Whether a bloom holds toxins cannot be told from its appearance.

How to Report Blue-Green Algae in Ontario

After a report, the ministry works with the local public health unit and its Medical Officer of Health to manage the bloom and to screen samples for toxins. The public health unit issues any advisory and decides whether a beach is posted. A municipal drinking water system that draws from surface water must monitor its source for blooms under its licence.

No list or map of reported blooms is published in Ontario’s open data catalogue. Current advisories are held by each public health unit.

Invasive Species That Change a Lake

Four invasive species change the Ontario lakes they reach: a plant that crowds the shallows, two mussels that filter the water, and a small predator of the plankton. Sightings of each are reported to the Invading Species Hotline at 1-800-563-7711 or through EDDMapS Ontario. The province’s Invading Species Awareness Program is a partnership of the Ministry of Natural Resources and the Ontario Federation of Anglers and Hunters.

Eurasian Milfoil

Eurasian water-milfoil is a submerged aquatic plant, first found in Canada in Lake Erie in 1961. It is now present in the Great Lakes, the St. Lawrence River and many inland lakes of southern and central Ontario, and it spreads from small fragments carried on boats, trailers and fishing gear. It grows into dense mats at the surface that crowd out native plants, draw down oxygen and interfere with swimming, fishing and boating.

Its leaves are finely divided, usually into 12 to 21 pairs of thread-like leaflets, and the native northern water-milfoil usually has fewer than 10 pairs. Ontario regulates Eurasian water-milfoil as a restricted invasive species under the Invasive Species Act, which makes it illegal to bring it into the province, to release or plant it, or to buy or sell it.

Zebra and Quagga Mussels

Zebra and quagga mussels are small invasive mussels that colonize a lake bottom and filter plankton from the water. By removing algae they can leave a lake clearer, which is one reason the province judges nutrient status by phosphorus. On Lake Simcoe the province records both species changing how phosphorus moves through the lake, as described above.

Spiny Water Flea

The spiny water flea is a small predatory zooplankton, 1 to 1.5 centimeters long with a barbed tail spine, first reported in North America in Lake Ontario in 1982. It is recorded in all the Great Lakes and in more than 100 inland lakes in Ontario. It feeds on native zooplankton, whose numbers fall by an average of 30 to 40 percent after an introduction, and so reduces the food of small fish and of young bass, walleye and yellow perch. Its spines collect on fishing line. It spreads on angling gear and in bait buckets, live wells and bilge water, and its eggs survive transport on damp equipment.

Clean, Drain, Dry

Since January 1, 2022, Ontario requires a boater to open or remove drain plugs, and to take reasonable measures to remove plants, animals and algae from the boat, its equipment and its trailer, before moving a boat overland. A boat must also arrive at a launch with none attached.

Where the Records Come From

• The Lake Partner Program. The Ministry of the Environment, Conservation and Parks runs the program through the Dorset Environmental Science Centre, with the Federation of Ontario Cottagers’ Associations as a partner since 1996. Volunteers check the water of about 800 lakes each year, and the province publishes the readings each year as open data. Once flagged and unplaceable readings are set aside, the files hold 44,356 phosphorus readings from 2002 to 2025 and 118,001 Secchi readings.
• Lake Simcoe Monitoring. The ministry’s own sampling of Lake Simcoe at fixed stations, with phosphorus, nitrogen, chlorophyll, clarity, temperature, dissolved oxygen and other measures, published as open data.
• The interpretation guide. The bands, the three-year rule and the sampling methods described here are set out in the province’s Guide to Interpreting Total Phosphorus and Secchi Depth Data from the Lake Partner Program.

Some lake associations run monitoring programs of their own and publish reports for their members.

Common Questions