Blue Green Algae

What are blue-green algae?

Blue green algae on a lochBlue-green algae are tiny organisms which occur naturally in lochs, ponds, reservoirs, rivers and in the sea. They commonly occur during periods of prolonged hot weather but sometimes occur at other times of the year. Waters which have been polluted by agricultural, domestic or industrial discharges are particularly prone to developing blue-green algae.

In still waters the algae can multiply to such an extent that they discolour the water which then appears green, blue-green, greenish brown or dark brown. Sometimes a scum may form on the surface. This scum can appear in different places at different times, but is most commonly found at the water's edge or shore line.

How do blue-green algae affect humans and animals?

Some, but not all the blue-green algae produce toxins or release these into the water. It is not possible to tell which algae do or do not produce toxins simply by their appearance. Laboratory analysis is needed for this. Therefore, it is advisable to regard all algal scums as potentially toxic.

The toxins of blue-green algae can cause deaths of animals which come into contact with algae, either through drinking contaminated water or swallowing quantities of scum, or shoreline matter or crust. Dogs have died after going into the water at the shores of affected lochs.

Canoeists, wind surfers and swimmers who have either swum through algal scum or swallowed it have suffered from skin rashes, eye irritation, vomiting, diarrhoea and pains in muscles and joints. Illnesses can be severe, particularly where affected water has been swallowed.

What should I do about blue-green algae?

Sample blue green algae noticeAvoid all contact with the affected waters and ensure that children and pets are kept away. Farmers should ensure that their animals do not have access to contaminated water. This may require fencing around suspect waters. If you believe you pet may have been exposed to blue green algae then contact your vet as soon as possible to get advice.

If any member of the public finds areas of water affected with blue-green algae they should inform the local Environmental Health Service shown on the back of this leaflet. Environmental Health may then take action to ensure that notices are posted around the affected water to let local residents, visitors or patrons know what the position is. A typical warning notice which may be used is shown here.

What about eating fish from affected waters?

Blue green algaeBlue-green algae and their toxins can adversely affect fish growth and health and, in some circumstances, can cause fish kills.

Fish should not be consumed if dead fish or strange fish behaviour is seen, blue-green algal scum is present or notices are displayed indicating the presence of blue-green algal cell numbers or toxins in unsafe levels.

Anglers and pet owners should not feed the liver or guts from fish to their pets, when fish are caught in waters affected by blue-green algae, as described above.

If you would like any further advice about blue-green algae, please contact your local environmental health service or call 01546 605519. This NHS leaflet on blue green algae gives information about its effects on humans and animals

Other invasive or harmful plants

You can find out information about other invasive or potentially harmful plants in Argyll in the invasive and harmful plants section.