Signs warning the public about the possibility of a toxic algal bloom have been erected along the shores of Killarney's largest lake, Lough Leane, one of the jewels of the region's tourism industry.
Although the lake is known to be enriched some years, the last major algal bloom was in 1997.
Mr Mick Boyce, a senior executive engineer with Kerry County Council, said water samples over the past few days had shown high counts of algae and there had been a rapid increase in their growth.
"The dry, warm weather conditions are perfect for algal bloom. We have circumstances that could lead to a bloom," he said.
An algal alert protocol was in place and the council was erring on the side of caution in putting up the warning signs.
It was a general warning, Mr Boyce said, asking people to watch out for the green-blue scum associated with an algal bloom but people were not being told to keep off the water.
A three-year study of the lake has identified phosphorous as the main source of pollution.
Poor practice in slurry-storage and spreading by the farming sector has contributed the single biggest loading. The tourism sector, in its disposal of fats and oils into the sewage system, domestic septic tanks and forestry also contribute.
An estimated 30 tonnes of phosphorous go into the lake each year. Twenty tonnes would guarantee an algal bloom given the right weather conditions, Mr David Linehan, also a senior executive engineer with Kerry County Council, said.
Until the phosphorous entering the lake was reduced, then the lake was at the whim of the weather, he said.
Kerry County Council has appealed to all sectors to redouble their efforts. Householders and the hotel industry should use phosphate-free detergents and farmers have been asked to tidy up farmyard practices.
New draft water-pollution by-laws for the agricultural sector have yet to be adopted by councillors.
Meanwhile the final report of the Lough Leane catchment study is expected in the autumn.