Galway County Council has said that people and animals should avoid an algal bloom which is currently visible at Annaghkeen Pier, Headford.
The bloom, which is an overgrowth of algae, can be seen as blue or green streaks or a slimy scum, and can produce toxins which can be dangerous.
People are being asked not to swim in waters where algal blooms are visible and to prevent animals from drinking from the water where the phenomena are present.
Should someone come into contact with the algal bloom, they should shower or wash immediately to remove the algal matter.
All clothing and equipment should also be cleaned thoroughly.