The public has been warned not to drink from, fish or paddle in Lough Leane, the biggest of the Killarney lakes, after the emergence of algal scum at a number of locations.
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.
Algal scum caused by the build up of phosphates from farm effluent and sewage was first identified in Lough Leane in the 1980s.
Dogs and other small animals are at particular risk, according to a statement issued by Kerry County Council yesterday.
The council said the water had become blue-green, or greenish-brown, at a number of locations on the shore.
Tests were now being arranged.
“While it must be stressed that the risk to public health is extremely low, the presence of algal toxins have been known to cause illness or skin rashes in certain circumstances,” the council said.
Precautionary warning signs were being erected around the lake and all relevant agencies notified.
A serious algal “bloom” in 1997 led to an in-depth study of the 2,000 hectare lake and its water courses.
The study identified agriculture as the main culprit for the phosphate loading into the lake, the name of which derives from ‘léin’ or learning.
Animal slurry tanks, in particular, were deficient or overloaded. Septic tanks attached to dwellings and businesses were also hugely deficient.
Naturally occurring phosphates were also high, the study found.
Prolonged periods of dull, overcast and humid weather, as experienced this summer, are one of the triggers for algal growth in phosphate-laden waters.