Recent samples taken at Loch Ordais, South Bragar have indicated the presence of blue-green algae.
Swallowing the water or algal scum can cause stomach upsets or more serious health effects, warns Comhairle nan Eilean Siar.
Contact with the water or algal scum can cause skin problems. It is a sensible precaution for you, your children and your animals to avoid contact with the scum and the water close to it.
For further information, contact the Environmental Health Team of Comhairle Nan Eilean Siar on 01851 822 694.
As a precautionary measure, notices have been posted next to the loch warning that contact with the algal scum should be avoided.
Adjoining landowners and fishing interests have been advised of the situation as have the Consumer & Environmental Services Section of Comhairle nan Eilean Siar and NHS Western Isles.