Algal Blooms

Algal blooms contain a potentially toxic bacterium known as Cyanobacteria (blue-green algae), which can multiply during the warmer months – although blooms sometimes occur at other times of the year.  Blue-green algae occur naturally in lochs, ponds, reservoirs, rivers and the sea. Waters which have been polluted by agricultural, domestic or industrial discharges are also prone to developing blue-green algae.

In still waters the algae can multiply to such an extent that they discolour the water, usually a green or blue-green colour but can also be khaki, blue, black, dark brown or red.  This can then appear as a green or blue-green scum on the surface of the water, most commonly at the water’s edge or shoreline.

The Scottish Environment Protection Agency (SEPA) routinely monitor water bodies throughout the Outer Hebrides for levels of blue-green algae, and inform Environmental Health in instances when their sampling indicates a potential problem.

What Are The Health Risks?

Some, but not all blue-green algae release toxins into the water.  However it is not possible to tell which algae produce toxins simply by their appearance, which is why it is advisable to regard all algal scums – or blooms – as toxic.

Direct contact with an algal bloom can potentially leave people with skin rashes, eye irritation, vomiting, diarrhoea, fever, and pains in muscles and joints.  Occasionally they can cause more serious illness such as liver and brain damage.  Children are at greater risk than adults of developing problems because of their comparative lower body weight.

The toxins which may be produced by blue-green algae can also be poisonous to animals that come into contact with the algae, either through drinking the contaminated water or swallowing quantities of the scum.  It is therefore advisable to keep dogs and livestock away from waters where the blue-green algae have produced a scum, or bloom.

How Can The Health Risks Be Minimised?

Although the majority of waters in the Outer Hebrides remain clean and safe throughout the year, there is some risk of exposure to harmful algal blooms.

Avoiding all contact with affected waters is the safest option.  Ensure children and pets are kept away from potential blooms.  Do not consume water or fish from within the immediate vicinity of an algal bloom, and always wash your hands and fishing gear thoroughly after coming into contact with contaminated water.

Crofters should also ensure that their animals do not have access to affected waters, which may require fencing around suspected waters or consideration given to relocating livestock to suitable grazing elsewhere.

How To Report A Bloom

If you see a potential bloom, please Contact Us or alternatively the local SEPA office in Stornoway on (01851) 706477.

Any report received will be investigated, and where appropriate, sampled for analysis.  Once a bloom has been identified, warning notices will be erected in the immediate area of the bloom, and alerts will be circulated around relevant agencies and to the public.

Monitoring will continue until such time as the bloom has cleared.

If you have any suspicions that the loch or river may be contaminated, please remember the following:

This information is also available in a handy leaflet:
Algal Blooms - How To Identify and Report A Potential Bloom (500.9kB)

Any potential algal bloom can be recorded using the smart phone app, Bloomin’ Algae, which can be downloaded via Google Play or the App store. The app can be used to send photos and locations of possible blooms, and comments can be received on reported blooms after an account has been registered and authorised. The app also:

  • provides rapid notifications
  • speeds up public and animal health alerts and warnings
  • provides verification of harmful algal blooms to SEPA, local authorities and other agencies

Further Resources

Health Protection Scotland (Opens in a new window or downloads a file) (HPS) 

Scottish Government (Opens in a new window or downloads a file) 

Scottish Water (Opens in a new window or downloads a file) monitors drinking water supplies to prevent any harmful effects on health from blue-green algae. 

Scottish Environmental Protection Agency (SEPA) - The Council’s Environmental Health team works closely with SEPA where there are reports of possible blue-green algae growth.  SEPA are the primary Regulator for all pollution related matters associated with the water environment.  Please see the SEPA website (Opens in a new window or downloads a file) for full details.

If you wish to report a serious pollution incident SEPA’s Pollution Hotline can be contacted 24 hours a day on 0800 80 70 60.