Blue-green algae (cyanobacteria) are naturally occurring organisms found in water and on land. They may appear green, blue-green or greenish brown and often produce musty, earthy or grassy smells.
Some species can use nitrogen from air and water to create food through photosynthesis. They can control their buoyancy, allowing them to float to the surface in sunny weather.
When blooms happen
Blue-green algae can multiply to such an extent that ‘blooms’ can form.
Blooms are more likely during:
- long periods warm, sunny weather
- calm conditions with little or no wind
During a bloom:
- the water may look cloudy or discoloured
- foam may appear on the shoreline (this can be mistaken for sewage)
- sunlight is blocked, which can slow plant growth
Blooms usually appear between June and November and rarely persist into winter due to colder temperatures, wind and rainfall.
Health risks
Some blue-green algae can produce toxins. These are known as harmful algal blooms.
These toxins can:
- harm wild animals, farm livestock and domestic pets
- cause rashes after skin contact
- cause illness if swallowed
You can't tell if a bloom is toxic just by looking at it, so it's safest to assume it is. People and pets should avoid entering the water during a bloom.
Reporting blue-green algae
Report suspected blooms or pollution to the Environment Agency 27/7 incident line.
Telephone: 0800 80 70 60
The Environment Agency will:
- collect and test samples
- confirm the presence of blue-green algae
- notify landowners and statutory bodies
- arrange for warning signs to be displayed
Once a bloom is confirmed, weekly sampling will continue until 2 clear results are recorded.
Visit the Spot, snap, and report blue-green algae website.
Help prevent blooms
There's no quick fix for algal blooms, but you can help reduce their resistance by:
- regularly emptying holding tanks on boats
- reducing detergent use (choose phosphate-free products)
- limiting fertiliser use on gardens, lawns and farmland
- checking and maintaining septic tanks
Reducing phosphate helps improve water quality but algae can also use nitrogen and nutrients from dirty water and land erosion.
Advice for land owners
Visit Algal blooms: advice for the public and landowners on GOV.UK