Recent samples taken in Loch nan Ritheanan, Keose indicate the presence of blue-green algae.
The Comhairle have advised the public to avoid contact with the algal scum or water close to it.
They have said that swallowing the water or algal scum can cause stomach upsets or more serious health effects.
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.
For further information, contact the Environmental Health Team of Comhairle Nan Eilean Siar on 01851 822 694.