A multi-agency investigation needs to be carried out to find the cause of an algal bloom on Lough Derg and to determine if water levels on the river Shannon were a contributory factor.

The call from independent Cllr Seamie Morris follows the recurrence of a bloom caused by blue-green algae on the shoreline. It is concentrated around Dromineer, an important location for leisure activities on the lake.

Cllr Morris said the pollution “is the worst seen [at that location] for a long, long time”.

The water in past blooms was predominantly blue, he said, but in this case was green, as seen with widespread pollution of Lough Neagh.

Having met anglers who regularly fish the river, including groups as far as Limerick, he said there they observed the lake was “unseasonably low despite high rainfall in recent weeks” and this may have caused the bloom. It has resulted in the death of significant numbers of perch and trout.

The ESB, because of its role in electricity generation at Ardnacrusha, is allowed manage weirs and sluices on the Shannon system, which includes adjusting water levels as the need arises.

Tipperary County Council has advised the public to exercise caution and is monitoring the lake with the HSE. Blue green algae can produce harmful toxins affecting aquatic life but also animals, birds and humans. “It is easily recognised when it forms a bright pea-green paint like scum on or close to the shoreline,” it said.

Cllr Morris said he understood under the EU nitrates directive farmers were allowed discharge liquid wastes into the lake in advance of an October deadline, which meant “there was a perfect storm of conditions” for the bloom to occur.

“This is why I have called for a multi-agency investigation … we can’t accept this is the way it is,” he said.

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Cllr Morris said the circumstances had huge implications for the plan to pipe large volumes of water from the Shannon to the Dublin region and was a timely warning of environmental impacts even where levels fall by a small amount. This potential issue with the scheme had not been fully examined, he said.

Lough Derg Anglers Association said it believed an adjustment to the water level “led to significant algal blooms appearing along the shorelines, particularly in Dromineer and Portumna”.

It added: “This situation raises a critical question. Is Lough Derg beginning to resemble Lough Neagh, or will we see a large-scale fish kill similar to the recent event in the Blackwater, which was the largest in the State’s history with no accountability?

“This ongoing issue provides yet another reason why the proposed pipeline to Dublin is ill-advised, as it will only cause significant damage to the local environment and our water.”

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The ESB said water levels in Lough Derg were normal for this time of the year.

The ESB “continuously monitors the water levels throughout the Shannon and continues to operate in accordance with the regulations and guidelines for control of the river Shannon,” a spokesman added.

“Algal bloom can occur naturally on bodies of water during periods of high temperatures and low wind. They can also be caused by nutrification – nutrients running off the land and into the water, particularly following heavy rainfall,” he said.

While some small parts of Lough Derg are designated bathing areas, the EPA said “we are out of the bathing water season, therefore the local authority are not required to notify us with any issue”.

In this instance Tipperary County Council and the HSE would decide on any health risks and put up any advisory notifications that are relevant, a spokeswoman added.