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The dangers of blue-green algae


In some parts of the country in warm weather, waterway users are faced with a potentially very serious growth in the water - blue-green algae, which can sometimes be toxic. With many people understandably concerned, the British Waterways environment team recieves lots of queries about the dangers and how to spot it. In particular, people want to know how to tell it apart from duckweed, which can also grow out of control in warm weather, but which is not toxic. So, the enviroment team have provided waterscape with the information you need to protect yourself.

  • Blue-green algae occur naturally in freshwater and can start to grow out of control in warm weather. Large growths or 'blooms' of blue-green algae can sometimes produce toxins. Contact with the toxins can cause reactions including itchy eyes, skin irritation and hay fever like symptoms, while ingesting them can cause abdominal pains, vomiting and diarrhoea. Children and pets are particularly at risk.
    • You can tell blue-green algae apart from duckweed, because duckweed is a plant. It has tiny leaves like cress or clover and is only found floating on the surface. Find out more about duckweed.
    • Blue-green algae, on the other hand, are tiny organisms that are actually classed as bacteria. The first sign of a bloom of algae is water being discoloured bright green or blue-green. Then, a scum forms on the surface of the water, which can look like turquoise paint, jelly, grass clippings or floating mats, and can be blue-green, grey-green, greenish-brown or reddish-brown in colour. If toxins are produced, they are most concentrated in the scums.
      Blue-green algae scum
    • It is not very common for blue-green algae blooms to produce toxins and the reasons for it are not fully understood. This means it is impossible to predict where toxicity will occur - the only to find out if a bloom is toxic is to examine the algae in a laboratory under a microscope, which British Waterways unfortunately does not have the capability to do. It is therefore advisable to treat all water with signs of the algae as potentially hazardous and to stick to the following guidelines:

Stay safe

  • Do not allow pets or livestock to swim in or drink the water. It is recommended to keep dogs on leads in affected areas.
    • High risk activities are those where extended skin contact or ingestion of water is likely, such as paddling, swimming, diving, wind-surfing or water-skiing. These activities should be avoided in affected water.
    • Activities involving incidental water contact, such as placid water canoeing (with no capsize drill), rowing, sailing, rafting and dragon boating, carry a medium risk of ingestion or inhalation of splashes of water. For these activities, it is advisable to avoid accumulations of scum, which often happen along downwind shorelines.
    • The risk to boaters is low, but it is best to maintain good personal hygiene, and wash hands after handling ropes, or other equipment which has been in the water.
    • If you do come in to contact with blue-green algae, remove contaminated clothing and wash skin in clean water as soon as possible, especially before eating or drinking. If in any doubt about your wellbeing, seek medical advice.

Last updated: 27/09/2011

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