blue green algae

Blue Green Algae

What are blue-green algae?

Blue-green algae are not really algae, and they aren’t even always blue-green. They are actually a type of bacteria called “Cyanobacteria” that get their energy through photosynthesis, like plants do. Cyanobacteria are known for rapidly reproducing and collecting to form large, highly visible blooms that can appear throughout the water, on the surface of water as a scum, or on the lake bottom as a mat. Some species of cyanobacteria can release toxins into the water when the cells that make up the bloom rupture or die. Some species do not; blooms contain a mix of species, but it is estimated that up to half of cyanobacterial blooms are non-toxic.

What are the health risks?

The risk to humans is primarily from drinking water that has been contaminated with toxins. Fortunately, there have been no human deaths attributed to drinking water containing cyanobacterial toxins, but the toxins may cause headaches, fever, nausea, vomiting, abdominal pain or diarrhea. Long-term consumption of water containing high levels of cyanobacterial toxins may cause neurological or liver problems.

Farm animals and pets are not as discerning about the water they drink, and if allowed, may consume large quantities of heavily contaminated water, resulting in sickness or death.

Some individuals are sensitive to skin contact with blue-green algae, and may develop a mild skin rash or eye irritation even if there is no toxin produced by the bloom. Some individuals will have no reaction. We think that it is an allergic-type reaction to contact with the actual cell walls, not the toxin. At this time, we don’t know:

  • what percentage of the population reacts to skin or eye contact with blue-green algae
  • what constitutes an exposure: how long, how dense, how often?
  • which species can cause a reaction.

What should I do if I think I have spotted a blue-green algae bloom?

If you see an algae bloom that you think may be blue-green algae, report it to the MInistry of the Environment (MOE) by calling 1-866-MOETIPS (663-8477) If it is the first blue-green algae bloom of the season on that lake or in that area, the MInistry will take a sample of the algae and identify whether it contains cyanobacteria. They will also test the toxin level from a dense area of the bloom, but will not return to confirm that the bloom or the toxin levels have dissipated. Subsequent blooms will not be individually identified.

What precautions should I take?

Even experts cannot tell which blooms are harmful just by looking at them, so waterfront residents will have to be cautious anytime that they have any dense algae bloom. Based on practices developed in the Province of Quebec, blooms can be visually assessed and categorized into 3 stages:

What does blue-green algae look like?

Category 1

Blue Green Algae

Category 2

Blue Green Algae

Category 3

Blue Green Algae

Blue Green Algae


How can I treat my lake water for drinking?

At this time there are no systems that are recommended for homeowners to use to treat their toxin-contaminated water. A one-micron filter (backwashed or changed frequently) plus reverse osmosis, distillation, ozonation, chlorination, and/or activated charcoal treatment will reduce the toxin level, but may not reduce it sufficiently to make the water safe to consume. Water softeners, UV lights, and even boiling will not make toxin-contaminated water safe to drink. The Ontario Drinking Water Standards set a limit of 1.5 micrograms of microcystin LR (one of the blue-green algae toxins) per milliliter of water. This standard is base on daily consumption of the water over a lifetime. At this time, we do not know what amount can safely be consumed in one summer, or one day, or one gulp. In the chart above, we have recommended waiting at least a week after a bloom has completely cleared before consuming treated surface water from an area where there was a blue-green algae bloom. Research is needed to accurately identify the safest timespan.

If you use treated lake or river water in your home or cottage, make sure your intake is in deep water, far from shore. If you see a dense algal bloom in the vicinity of your water intake, find another source of water for drinking and cooking, such as municipal water, bottled water or potable water from a deep drilled well.

Can I use contaminated water for bathing and washing?

Not everyone will have a skin reaction to blue-green algae in water. If you know that you are not allergic or sensitive to it, bathing and washing may not present a risk. The smell of a thick algal bloom will probably be enough to discourage most people.

Why here, why now?

Cyanobacteria blooms have been recorded for 800 years, but reports of blooms have increased significantly in the past decade. Warm temperatures, high nutrient levels and shallow, slow moving water all increase the likelihood of a bloom occurring. The subtle effects of global warming are also being seen, as Ontario’s lakes have longer ice-free periods, with blooms now being reported in the late fall and early winter.

Testing Services

Some commercial laboratories in Ontario can perform testing for cyanobacteria toxins. Call ahead for pricing and sampling procedures. A list of licensed, commercial laboratories can be found here.



Ministry of Envrironment, Conservation and Parks

Blue-green Algae: Get To Know Its True Colours 

Factsheet: Blue Green Algae

Factsheet: Info for drinking water operators and home owners