The Facts on Blue-Green Algae

What causes an Algae Bloom?

Blue-green algae

Blue-green algae

Algae are naturally occurring aquatic organisms, similar to plants, which contain chlorophyll and produce their own food. These help form the base of the food chain. An algae bloom is caused when the right growing conditions combine to allow a population of algae to grow rapidly into a very large noticeable colony. This occurs when a number of environmental factors align to create ideal growing conditions. These factors include calm water, warm sunny conditions, shallow warm water and usually high nutrient levels (some cases this does not seem to be necessary). Conditions ideal for a bloom usually occur in late summer and early fall, however due to the effects of climate change, this window may be shifting.

While we cannot control many of these conditions, we can influence one of the factors that affect their growth; that is the nutrients available to them. Property owners living on or near lakes and rivers can help by reducing or eliminating the use of fertilizers heavy with phosphorous, increasing the width and extent of vegetated buffer strips along shorelines and ensuring septic systems are fully functioning.

Algae blooms are a sign that the nutrient levels in the water are higher than normal. By tracking the occurrence of algae blooms it will help us to monitor changes in the health of our waterways.
You can help. If you see an algae bloom on your lake, report it to Citizen Water Watch.

What are the hazards associated with Blue-Green Algae blooms?

Although an aesthetic nuisance, most algae are not toxic. It only poses a potential health risk when it’s the “blue-green” type algae. A blue-green algae bloom can typically be identified because the water looks bluish-green, like green pea soup, or turquoise paint. When the blooms are very dense, they may form solid-looking clumps. Fresh blooms often smell like newly mown grass; older blooms may smell like rotting garbage. (Ontario Ministry of the Environment and Climate Change website, 2014)

Laboratory analysis of water samples is the only way to identify whether a particular algae is the type that produces toxin and if toxins are present in the water. However, it is difficult to sample all parts of an algae bloom so a precautionary approach is best.

The Leeds, Grenville, and Lanark District Health Unit’s website describes the early symptoms from ingesting water containing toxic blue-green algae as “headaches, fever, diarrhea, abdominal pain, nausea or vomiting. Contact may cause rashes and mucous membrane irritation.” Ingestions of a large amount of the toxins is necessary before the liver and brain symptoms occur.

What you should do if you think you see a blue-green bloom?

Blue-green algae bloom

If you suspect a blue-green algae bloom, report it to the Ministry of the Environment and Climate Change’s (MOECC) Spill Action Hotline (1-800-268-6060) as soon as you see it.

You should then use the precautionary approach and refrain from using surface water sources for drinking, washing, or swimming while the bloom assessment and laboratory analysis takes place. This includes restricting your pet’s interactions with surface water for drinking or playing.

The Leeds, Grenville and Lanark District Health Unit advises people using surface water for recreation and drinking, to become familiar with blue-green algae so they can make informed decisions on when to avoid contact with the water. Algae blooms do degrade with time; however, it is not possible to say whether the toxins have completely left the area. Once the toxin is released from the cell, where it goes is dependent on the local characteristics of water movement in the area. The toxin will eventually be diluted into the body of water as any other soluble compound.

Drinking Water
Surface water is never a safe source of drinking water without effective treatment. The toxins released by blue-green algae are not removed by commonly used treatment methods such as boiling, chlorination or ultraviolet light treatment. Consult a water treatment specialist if your drinking water comes from the lake. You may want to choose another source of water for drinking.

Recreational Water Use
Avoid activities that increase your exposure to toxins in the water during an active algae bloom. When deciding whether to resume recreational use of the water (swimming and water sports) after a blue-green algae bloom consider the following factors:
• Faster moving water will dilute and move the toxins out of an area more quickly, further decreasing the risk to health. Each property will have a different water flow pattern, so a local assessment is useful to further assess risk.
• Skin irritation is the first sign that the level of toxins is significant in the water, so if this occurs, it is important not to go in the water. A few more days will help to clear the water if it is moving well, unless another bloom has occurred.
• The water in lakes and rivers always has the potential to be infected with bacteria, viruses and other microbes that can affect health. So it is important not to swallow the water. Young children are more likely to swallow water so it is critical to observe them carefully when in the water.
• Be cautious about eating fish caught in water where a blue-green algae bloom has occurred. Toxins are concentrated in the liver. So avoid consuming the liver, kidneys and other organs of fish caught in an area affected by blue-green algae.

What is Mississippi Valley Conservation Authority Doing?
With funding from the Ontario Trillium Foundation, MVCA, Rideau Valley Conservation Authority and Carleton University are developing an online tracking system to allow lake users to report algae blooms (the MOE Hotline is the first priority for reporting, our tracking system will be used for other purposes), and changes in large aquatic plant beds (aka “the weeds”). With the large number of lakes in our watersheds we are relying on our lake residents and lake users to help us collect this information by reporting what they are seeing on the lakes. This reporting will allow us to identify problem areas and target stewardship actions.

We often hear from our lake residents that “these conditions have never happened like this in the past.” Scientists have found archival evidence to suggest that algae blooms may have occurred periodically many years past in some lakes. Our partners at Carleton University are studying the lake bed sediments to help us assess the historic occurrence of algae on our lakes.

2014 was MVCA’s first year with the project so results are not yet available but we hope that with your participation we will have a useful tool to understand the extent, duration, and number of occurrences of blooms within a year and how that interacts with the extent of weed beds, presence of zebra mussels, and effects of climate change.

The website we have created for this project can be found here:
For more information please contact your local health unit:
City of Ottawa Public Health, Ottawa
Phone: (613) 722-2242
Leeds, Grenville & Lanark Counties Leeds, Grenville & Lanark District Health Unit, Smith Falls
Phone: (613) 283-2740
North Frontenac & Addington Highlands KFL & A Health Unit, Cloyne
Phone:(613) 336-8989
Fax:(613) 336-0522
Central Frontenac KFL & A Health Unit, Sharbot Lake
Phone: (613) 279-2151
Fax:(613) 279-3997

Or go online:

Huynh, M. and N. Serediak. 2006. Algae Identification Field Guide. Agriculture and Agri-Food Canada. 40 pages.