Blue-green algae are microscopic, plant-like organisms that occur naturally in ponds, rivers, lakes and streams.1 Although often blue-green, they can also be olive-green or red.1 Their scientific name is cyanobacteria, but they are more commonly known as pond scum.
Blue-green algae grow in shallow, warm, slow moving water, but may also live below the surface in deeper cooler water.2 During warm weather, blue-green algae can grow quickly to form a large mass called a bloom.2 In Ontario, blooms most commonly occur during the late summer and early fall.2
Blue-green algae grow best when there are high levels of nutrients in the water, such as phosphorus and nitrogen.2 These nutrients can get into the water by run-off of:
Although many varieties of blue-green algae are harmless, some can produce toxins that are harmful to the health of both humans and animals.2 People can become ill from contact with blue-green algae toxins, therefore take a cautious approach with blue-green algal blooms. View information about blue-green algae (PDF 3.6MB) to learn about the potential health effects associated with blue-green algal toxins.
Possible ways you can come into contact with blue-green algae toxins in water are:
The list below suggests ways to help reduce contact with blue-green algae toxins in areas where blue-green algae is found:2
If drinking water is obtained from a water source that is not properly treated during a blue-green algal bloom, then it is possible that the water may be contaminated with toxins.4 Usually people won’t drink water contaminated with blue-green algal blooms because of its unsightly pea soup appearance and foul smell.4 However, drinking water obtained during an algal bloom should be tested for algal toxins because sometimes it’s hard to tell from its appearance whether the drinking water has been contaminated.4
The Ontario Drinking Water Quality Standards state that the current allowable level of microcystin-LR (a cyanobacterial toxin) is a concentration of 0.0015mg/L or 1.5 parts per billion.2
Drinking Water System Operators will monitor drinking water quality more frequently once a bloom is reported in the area of the water intake.2 Central treatment plants usually have filtration, chlorination, and other treatment systems that are capable of removing the algal cells and toxins.2 For more information on how water systems can be treated, view Ontario's Blue-Green Algae: Information for drinking water system owners and operators (PDF 2.7MB) fact sheet.
For more information on blue-green algae, visit Ontario's Blue-green algae web page.
To speak to a Public Health Inspector on the Environmental Health Team about blue-green algae, please call:
