Blue-green algae at Llyn Alaw reservoir, Anglesey
Blue-green algae which can cause a variety of symptoms, including skin rashes and vomiting if swallowed, has been found at an Anglesey reservoir.
Site owner Dŵr Cymru Welsh Water is warning the public about restricted use of the area at the Llyn Alaw reservoir at Llantrisant.
The algae produces toxins which can cause a variety of symptoms.
Environment Agency said the weather can affect algae, which usually declines in cooler windy conditions.
Public Health Wales is advising people that the toxins can cause skin rashes, nausea, vomiting, stomach pains, fever and headache if swallowed.
Occasionally it can cause more serious illness such as liver and brain damage, and children are at greater risk than adults of developing problems because of their comparative lower body weight.
Environment Agency Wales is continuing to monitor the situation .
A spokesperson said if the level of algae increased there is a possibility that the reservoir may have to be closed until it subsides.
"We would not normally expect a new case at this time of year.
"The weather can have an impact on it and usually as it cools down and becomes more windy there is a decline in the algae population."