Algal blooms have been appearing in greater frequency on Lough Guitane - the single biggest source of public water in Co Kerry - over the last number of years, a senior chemist in Kerry County Council's environment department has said.
There is now increasing concern for the protection of the Killarney lake, which supplies more than half of Kerry.
Stringent new by-laws restricting the spread of animal slurries and chemical fertilizers are being proposed, along with a ban on one-off housing by the shores of the lake, to prevent further enrichment.
The concern has come to light as Kerry County Council this week announced a range of measures to tackle the county's drinking-water quality. These include the appointment of separate water technicians to monitor the health of water in the five electoral areas, as well as immediate measures to strengthen security at reservoirs throughout the county.
A programme of hygiene measures has also been brought forward, following a breach in security in which a water reservoir was contaminated by animals.
"There have been algal blooms, an increasing number, over the last few years. Nothing very serious or very lasting but enough to indicate activity or enrichment," Mr David Lenihan, senior executive chemist, said. The blooms have not reached the kind of concentrations which lead to algal alerts, he explained.
He described the council's activity on the lake this week and last week as "routine work". Along with other reservoirs, pipes are being cleaned and disinfected. Divers have also been doing what Mr Lenihan said was "routine" cleaning of sediment.
The environment department, however, is increasingly concerned at the number of one-off houses being built along the lake over the past three years. These have no public sewage system and rely on septic tanks.
"There is concern in the environment department and among Kerry County Council personnel about the deterioration in the lake over the past years," Mr Lenihan said.
Specific controls in the Lough Guitane area, over and above other areas, are being proposed, both on housing development and in the agricultural by-laws which are being proposed to tackle the pollution on Lough Leane.