Information About Blue-green Algae: Background, potential impacts to human health and safety of drinking water (part 1)

Note: This information is taken from the Ministry of the Environment factsheet "Information about blue-green algae: Background, potential impacts to human health and safety of drinking water," produced in 2005. To view the factsheet as a pdf file, click here.

What are blue-green algae?

Blue-green algae are primitive microscopic plants that have inhabited the earth for over 2 billion years. The first recognized species were blue-green in colour, hence their scientific name cyanobacteria, but they can
range in colour from olive-green to red. Variants of these algae inhabit a wide variety of environments including fresh water bodies where they are more commonly known as pond scum.

Are fresh water blue-green algae a concern?

Normally blue-green algae are barely visible but when conditions are favourable algae populations can rapidly increase to form a large mass called a bloom. Blue-green algae blooms are natural phenomena that occur in
fresh water lakes, bays and inlets in countries around the world.

Do blue-green algae blooms occur in Ontario?

Blooms of cyanobacteria have been reported in various locations throughout Ontario where they are a seasonal phenomenon. Blooms usually occur during the warmer weather of late summer and early fall and
tend to occur repeatedly in the same water bodies.

What conditions favour algae growth?

Blue-green algae thrive in areas where the water is shallow, slow moving, and warm, but they may also be present below the surface in deeper cooler water. One key factor affecting growth rates is the level of available nutrients such as phosphorus and nitrogen.

Where do these nutrients come from?

Although blooms of cyanobacteria are a naturally occurring phenomenon, human activities can provide the physical and chemical conditions cyanobacteria need to thrive. Storm water runoff, industrial effluent, agricultural runoff and effluent from waste management systems including faulty septic systems lead to the nutrient enrichment of water bodies, thereby promoting the occurrence of algae blooms. Controlling or eliminating the amount of additional nutrients entering the water body can effectively reduce the occurrence and severity of algae blooms.

How can I recognize a blue-green algae bloom?

Dense blue-green algae blooms may make the water look like bluish-green pea soup. When the blooms are very large, they may form solid-looking clumps. Fresh blooms often smell like newly mown grass; older blooms smell like rotting garbage. The nuisance (fouling) factor and the aesthetic (smell) problems caused by blue-green algae blooms are well known.

Should I be concerned about blue-green algae?

It is prudent to be cautious about blue-green algae blooms. Although many varieties of cyanobacteria are relatively harmless, blooms of cyanobacteria may contain algae species with the potential to produce toxins which may be harmful to human health and the health of animals. The first report of harm to animals from blue-green algae came from South Australia in 1878. The most common toxins are called microcystins. These toxins are contained within the algae cell and are released to the water when the cell wall is broken either due to natural death and decay of the cell or by corrosive chemicals or abrasion. Higher levels of toxins may occur during blooms when cyanobacteria cell numbers are high and concentrated in one area. Therefore, as a precaution, any cyanobacteria bloom should be regarded as being potentially toxic. 

Can blue-green algae blooms be treated?

Treating blooms with herbicides, copper sulphate or other algicides is not advisable because these treatments may break open algae cells and release more toxins into the water. This characteristic makes treatment of algae blooms difficult. Therefore, the mitigation of algae blooms is better accomplished through preventive rather than remedial measures.

How can blue-green algae blooms be reduced or prevented?

Blooms of blue-green algae may form when nutrients are readily available in the surface water body. Therefore, taking steps to reduce or prevent additional sources of these nutrients from entering the water can reduce the occurrence of blue-green algae blooms. Preventative steps include, but are not limited to:

• using phosphate-free detergents;
• not using fertilizers and maintaining a naturalized shoreline on lakefront properties;
• taking steps to reduce agricultural run-off such as populating set backs along waterways with plants; and
• ensuring that septic systems do not leak into the water source, have sufficient attenuation area to prevent nutrients in normal septic effluent from reaching the water, or by using alternative septic handling methods.

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