Phosphorous levels in Killarney's lower lake, Lough Leane, are higher than they have been for years, leading to fears of a recurrence of potentially toxic algal blooms this summer.
Mr David Lenihan, senior chemist with Kerry County Council, has been monitoring the lake closely for weeks. Yesterday he appealed to all waste producers in the area - farmers, hoteliers, B&Bs, householders and forestry bodies - to minimise the levels of phosphates they use.
Enrichment levels in the lake, creeping up over the past two weeks, are higher than previous years and are now "reaching critical levels", Mr Lenihan said.
"If there are favourable conditions - that is warm, sunny and humid weather - then we can expect a bloom," he said.
Lough Leane's problems first surfaced in 1997 when a major algal bloom caused by blue-green cyanobacteria occurred.
Signs warning the public to avoid contact with the water were placed along the shoreline. Last year, however, the lake remained clear of blooms.
The attempt to bring the lake back to acceptable quality will cost around €17 million.