People are being urged not to swim in Killarney's largest lake due to high levels of toxic algae.
Algal bloom warning signs were put up on the shore of Lough Leane in Co Kerry this morning.
Dog owners are also being warned not to let their pets in the water.
The lake, which is ringed with historic and natural attractions, from Ross Castle to O'Sullivan's Cascade, is popular with anglers and boat users, and is particularly busy with tourists this year.
Blue- green and brown Algal scum caused by the build up of phosphates was first identified in Lough Leane in the 1980s.
A three-year study was undertaken after a serious bloom in 1997 identified agriculture as the main culprit for the phosphate loading into the lake
Animal slurry tanks, in particular, were deficient or overloaded. Septic tanks attached to dwellings and businesses were also hugely deficient and were contributing to the enrichment of the lake, it found.
In recent years the toxic algae has been linked to deaths and sickness among dogs with a major bloom in 2016 said to have caused a number of dog deaths.
Kerry County Council has put up signs at a number of locations along the shore "to advise members of the public of the potential for an algal bloom in the water and on the shoreline," it said in a statement..
"Members of the public are advised not to bathe in water if there are visible signs of algal blooms in the vicinity. They are also advised to ensure that they keep their dogs under control as such algal accumulations can be harmful on ingestion."
Samples by Kerry County Council staff in recent days confirmed the presence of cyanobacteria (a blue-green algae) indicating the potential for the formation of algae along the shoreline.
The situation is being monitored closely and on an ongoing basis, the council said.
