Increased algal growth has been identified at a number of locations around Lough Leane in Killarney.
Kerry County Council is advising members of the public not to bathe or paddle in water if there are visible signs of algal growth in the vicinity.
This growth can be identified by a green, blue or brown colour in the water or along the shoreline.
People are also advised to keep their pets under control as this algal growth can be harmful if ingested by small animals, particularly dogs.
Water from the lake should not be used for cooking, drinking or washing.
The council has erected advisory signage along the shoreline and is continuing to monitor the situation.