Blue-green algae (Cyanobacteria)

Cyanobacteria or 'blue-green algae' may be green, blue-green or greenish brown and can produce musty, earthy or grassy odours.

They are a naturally occurring photosynthetic organism. Some species have the ability to use nitrogen from air and water for photosynthesis (to create food), and can be found in most terrestrial and aquatic environments.

Their ability to control their own buoyancy means that during bright sunny weather, the algae migrate to the surface layer of ponds and reservoirs.

During long periods of settled, sunny and warm weather with little or no wind, blue green algae can multiply to such an extent that ‘blooms’ can form.

Blooms can also cause foaming on the shoreline, which can sometimes be confused with sewage pollution.

During a bloom, the water also becomes less clear, blocking sunlight and can slow down plant growth in water.

Once algal numbers are high, the bloom is likely to persist throughout the season, declining only on the onset of winter conditions.

It is unusual for blue-green algal blooms to persist into the winter. The disturbance caused by wind and rainstorms, lower temperatures and faster flushing rates mean that we would normally see blue-green algae from June-November.

Health risk

Bloom and scum forming blue-green algae can produce toxins.

Toxin producing blooms are called Harmful Algal Blooms.

These toxins can be harmful to wild animals, farm livestock and domestic pets.

In humans, they have been known to cause rashes after skin contact and illnesses if swallowed.

Not all blue-green algae blooms and scums are toxic, but you can’t tell just by looking at them, so it’s best to assume they are.

People (and pets), therefore should enter the water at their own discretion.

Reporting blue-green algae

Environmental incidents, such as potential blue green algae blooms or pollution, should be reported to the 24/7 Environment Agency incident line on 0800 80 70 60.

The Environment Agency collects samples when reports are submitted of the presence of blue-green algae on waterbodies, and analyses them to confirm its presence.

Notifications of confirmed blooms are then sent to the landowners and other statutory bodies so that signs can be displayed to warn users of the presence of potentially toxic blue-green algae.

Once there is a confirmed bloom in a particular location, Environment Agency officers sample every week until they have two clear samples in a row.

The Environment Agency has a weekly updated map of suspected blue-green algal bloom locations. 

Preventing the occurrence of blue-green algae

As algal blooms are naturally occurring and require the right conditions to form, there are no quick and easy solutions for reducing their occurrence.

There are many things which residents and users around waterbodies can do to reduce the persistence of blooms, including:

• Regular checking and emptying of holding tanks on boats to prevent pollution
• Reducing detergent use (and using phosphate-free)
• Reducing fertiliser on lawns, gardens and farmland that can ultimately end up in the water
• Checking the condition of septic tank systems to reduce discharge.

Reducing phosphate use helps to improve water quality. It’s important to note, though, that algae isn’t completely reliant on phosphorous and can utilise low levels of both phosphorous and nitrogen. It’s adept at utilising it from dirty water, such as from general land erosion.

Advice for land owners

The Environment Agency gives algal blooms advice to the public and landowners.