Cyanobacteria, commonly known as blue-green algae, are naturally occurring bacteria often found in Ontario’s ponds, lakes, rivers, and wetlands. Though they resemble algae, they are actually bacteria that can form surface blooms or benthic mats (clumps resembling vegetation) when temperatures are high, and nutrients are plentiful.
These blooms typically appear during hot weather and may look like spilled paint, scum, foam, or streaks on the water’s surface in colours ranging from blue-green to red, brown, or green. They may smell like fresh-cut grass or have a foul odour, and some produce toxins harmful to humans and animals if ingested.
We monitor our reservoirs for potential cyanobacteria blooms. If you suspect you see a bloom, please report it to conservation area staff by sending photos and details, including the exact location. Click here for a full contact list.
If you see a blue-green algae bloom or notice posted warnings:
- Avoid contact with the water; do not swim or bathe in it.
- If you have already come into contact with the water, wash yourself as soon as possible with clean water
- Do not drink the water.
- Keep children and pets away from the area.
You can also report suspected blooms to the Ministry of Environment, Conservation and Parks (MECP) by calling the Spills Action Centre at 1-866-MOE-TIPS (663-8477) or submit a report online
For more information on blue-green algae, visit the MECP’s blue-green algae webpage.