The algae was found after tests
The Environment Agency Wales has issued advice for people not to touch or drink water from a lake on Anglesey after a toxic algae was discovered.
The blue green algal bloom was found at Llyn Maelog near Rhosneigr, which does not provide drinking water.
But it is potentially harmful to humans and animals, and the agency warns that with current weather conditions it is likely to last for several weeks.
A blue-green algae is also causing problems at Llyn Padarn, Llanberis.
The agency said a sample had been taken from Llyn Maelog on Monday this week and results received late on Tuesday showed that the algae was present "above a warning threshold".
The species present is Aphanizomenon flosaquae, and the bloom can be clearly seen within the lake.
The agency said it was working closely with Anglesey County Council's environmental health section and a notification process was underway.
An agency spokesman said blue green algae could produce toxins that can harm humans and animals if they come into contact with the water.
"Further samples are being taken today for toxicity analysis and the situation will be monitored over the coming weeks by the Environment Agency," he added.
The advice to the public is not to touch the surface scum, and not to drink the water, or let their pets enter the water.
Notices have been put up
In Llanberis warnings have been re-instated about the algae there.
Barry Davies, Gwynedd Council's maritime and country parks officer said: "Unfortunately the algae has returned to all parts of the lake and is not localised to one specific area.
"This recurrence was not unexpected given the present climatic conditions.
"Over the next few days we will set into motion a system which will warn the public of the latest situation at Llyn Padarn - this will involved a daily update board at various locations on the water edge," he added.
Meanwhile the advice is not to participate in any "immersion sports" and to check the information board before taking part in any activity on the lake.