Algal Blooms

Algae are small, mostly microscopic plants that live in virtually all waterbodies. 

There are literally thousands of species of algae that grow in various habitats. They require nutrients and light, and they grow better when it is warm. Algae produce oxygen and convert carbon into organic compounds that feed zooplankton and – in turn – small and larger fish.

image courtesy J. Winter

image courtesy Ontario Ministry of the Environment

Algae are an important part of lake ‘food webs’ and a necessary part of ecosystem integrity.

However, some types of algae are toxic to humans and animals, and algal blooms in Ontario waterways are becoming more prevalent.

What should you do if you spot a suspected blue-green algae bloom on a waterbody in Ontario?

Report sighting of possible blue-green algae blooms to the Ontario Ministry of the Environment, Conservation and Parks who will investigate and may take water samples to confirm if the algae is one of the species that produces harmful toxins. 

Report possible sightings online here or by phoning the Spills Action Centre at 1-866-663-8477.

Additional tips if you suspect a blue-green algal bloom:

  • assume toxins are present
  • avoid using, drinking, bathing or swimming in the water (call your local health unit for swimming advisories)
  • restrict pet and livestock access to the water

Contact your local health unit for information on health risks associated with blue-green algal blooms.

banner: Latest News

July 2023 – the Summer edition of the North American Lake Management Association‘s publication LakeLine is now available, and open-access to everyone. The entire edition is devoted to scientific papers about harmful algae blooms.

July 28, 2023 – in related algae news, York University research suggests more algal blooms are likely in Lake Erie as deep-water oxygen levels continue to drop. This can also lead to mass kill-offs of fish, when there is a temperature inversion that sends deep waters devoid of oxygen to the surface. The researchers note that low oxygen levels at the bottom of the lake can produce a chemical environment where phosphorus is released from the sediments – a primary nutrient for algae! Read the Water Canada article online.

Feb.3, 2023 – Cyanobacterial blooms in Ontario, Canada: continued increase in reports through the 21st century (NALMS Lake Reservoir Management publication)

Learn more about algae: the good, the bad, and the ugly

Here is a video about algae and algal blooms, from FOCA’s scientific colleagues at the IISD-Experimental Lakes Area:


Scroll down for more about algal blooms, blue-green algae, and the process of eutrophication.

Or, jump directly to related fact sheets and articles from FOCA and our partners.

What is an Algal Bloom?

A ‘bloom’ is the excessive growth of one or more species of algae. Blooms are less likely to occur in deep lakes with lower total phosphorus (TP) levels. Conversely, blooms are most likely to occur in shallow lakes or bays that have moderate to high TP. Blooms may:

  • Affect the appearance of water
  • Result in unpleasant tastes or odours
  • Reduce water clarity
  • Colour the lake a vivid green, brown, yellow or red
  • Deplete oxygen levels in the water
  • Produce toxins that are dangerous to humans or animals.

When do algal blooms occur?

A combination of factors are required to result in the formation of a blue-green algal bloom, including:

  • Calm weather
  • Strong sunlight
  • High air and water temperatures
  • Relatively shallow water
  • Sufficiently high levels of nutrients (in the water or sediments).

The conditions favouring algal blooms unsually occur from mid-summer to fall in Ontario. However, climate change has been having an effect on algal blooms, extending their growing season to new lengths. Whereas reports during the late 1990s tended to end by September of the year, the trend ten years later was for sightings well into October, and by 2010, the last report came in early December.

Blue-green Algae: beware

Though more prevalent in the late summer and Fall, under the right conditions (including areas with excess nutrients such as phosphorus and nitrogen, warm temperatures and lots of light) Ontario lakes can present with blue-green algae blooms throughout the ice-off season. Therefore it is important to protect yourselves and others from this toxic algae by understanding what to look for and how to report it.

Cyanobacteria, commonly called blue-green algae, are microscopic, plant-like organisms that occur naturally in ponds, rivers, lakes and streams. Although often blue-green, they can also be olive-green or red. Dense blue-green algae blooms may make the water look bluish-green, or like green pea soup or turquoise paint. Very dense blooms may form solid-looking clumps. Fresh blooms often smell like newly mown grass, while older blooms may smell like rotting garbage.


Does boiling water contaminated with toxins from blue-green algae make it safe to drink?


No! Boiling water can actually release MORE toxins into the water. Use alternate water sources until further notice.

Eutrophication explained:

Eutrophication refers to excessive plant and algal growth due to the increased availability of nutrients such as phosphorus and nitrogen. Other factors such as sunlight and carbon dioxide may also contribute to eutrophication, but the impact of increased nutrient inputs is much greater.

For more on eutrophication and the different types of lakes, see page 2 of FOCA’s publication, A Shoreline Owner’s Guide to Healthy Waterfronts. (image excerpt, below)

Eutrophication occurs naturally over centuries as lakes age and are filled in with sediments.

However, human activities can accelerate the rate and extent of eutrophication. Increased phosphorus inputs from sources such as agricultural fertilizers or partially treated sewage are examples of human-induced eutrophication of freshwaters, also called cultural eutrophication. Depending on the state of eutrophication there can be dramatic consequences for aquatic ecosystems such as fish kills and blue-green algal blooms. This reminds us why the Lake Partner Program and citizen science monitoring is so vital to maintaining the health of Ontario’s lakes.

To learn more about eutrophication and the differences between natural versus human-induced eutrophication visit:

Fact Sheets, Links & Resources:

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Please note: the following is archival material, and some links to third-party resources may no longer be active.

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For more about FOCA’s work with the Lake Partner Program to monitor Ontario’s lakes for phosphorus and other nutrients, visit FOCA’s Lake Partner Program webpage

For more about developments in Great Lakes research, visit FOCA’s Great Lakes webpage.