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Algal blooms

Both fresh and sea-water will contain algae.  Under favourable conditions, such as warm, sunny weather, they can grow in huge numbers and form algal blooms.

Blooms may colour the water or form a scum on the surface that may be smelly.

Whilst most blooms are not harmful, some can contain toxins which may cause health problems.

As it is not possible to tell by looking whether an algal bloom is harmful, it is best to avoid swimming where an algal bloom is present. 

Look out for signs along the water’s edge that may advise you not to use the water.

When you may be at risk

You may come in to contact with a bloom when you use a lake, other body of water or sea. This could include

  • Being in, or splashed by, affected water
  • Swallowing water or breathing in water droplets when sailing, boating or fishing
  • If the water is used for drinking water
  • Touching pets, usually dogs, that have been in affected water

What are the health risks

Toxins in the blooms may cause

  • Skin rashes, 
  • Nausea, 
  • Vomiting, 
  • Stomach pains, 
  • Fever,
  • Headaches,
  • Exposure to high levels of toxic algae can cause more serious illness including liver damage.  

Minimising health risks when there are blooms

Not all algal blooms are a health risk, but it is not possible to tell by looking at the bloom. It is best to assume that any bloom is harmful.

  • Avoid contact with the water.
  • Do not swim or take part in other activities where there is a high risk of falling into the water.
  • Do not allow pets or animals in to the water.
  • Rowing and fishing are lower risk, but may be best avoided.
  • Follow the advice on any signs.
  • If you have been in contact with a bloom  make sure you wash your body with clean, fresh water as soon as possible.
  • In general, whenever you have been contact with the sea, river, lake or any other open water, it is a good idea to wash your hands before eating.
  • Seek medical advice if you become ill after contact with affected water.

Blue-green algae

Blue-green algae (cyanobacteria) typically found in freshwater (e.g. lakes, ponds and streams)

Further information can be found in on the Welsh Water website.

Marine (sea) algae

Marine (sea) algae include seaweeds and tiny plants called phytoplankton. The latter can sometimes give rise to blooms which can contain toxins.  The main risk to health from these harmful algal blooms (HABs) is when the toxins enter the foodchain, in particular shellfish. To prevent this there is a regular monitoring programme for marine algal blooms in the UK.

Marine blooms are less noticeable than blue-green algal blooms in freshwater but if you see a bloom avoid contact and follow the advice above.

More information

To report a bloom visit Natural Resources Wales or call their environmental incident hotline 0300 065 3000 (24 hours).