A warning has been issued to members of the public to remain vigilant when visiting the north coast after a potentially toxic algae was detected.
Visitors to the Portstewart/Castlerock area are being urged to take extra care following confirmation of blue-green algae at Castlerock Beach.
It comes after the Northern Ireland Environment Agency (NIEA) received a report of suspected algae at the beach on Wednesday.
The report was made by Department of Agriculture, Environment and Rural Affairs (DAERA) samplers undertaking bathing water testing in that area.
A water sample has since confirmed the presence of a blue-green algae 'Microcystis sp', which has the potential to produce harmful toxins.
Blue-green algae can be harmful to humans and is highly toxic to animals.
The National Trust and local council, as beach operators in the area, were informed of the results and erected signage warning users of the presence of potentially toxic blue-green algae.
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Further sampling will be undertaken over the weekend to assess the extent of algal blooms in the area, with results expected early next week.
Precautionary red flag warnings (advising against swimming) will remain in place at least until Tuesday.
The blue-green algae in Lough Neagh, confirmed on June 6, has been moving downstream with the natural flow of water out of the lough down the River Bann, reaching the coast.
The natural movement of the tides and wave action will break up the algae, with some potentially washing up on the shore.
Portstewart Strand has been red-flagged by the RNLI and is unsafe for bathing due to blue-green algae. DAERA sampling confirmed the presence of potentially toxic blue-green algae at Castlerock, with suspected blue green algae noted at Portstewart. Please stay out of the water. pic.twitter.com/wTWgZimOF6
— National Trust NI (@NationalTrustNI) July 6, 2023
Blue-green algae have been found in 20 locations across Northern Ireland this summer and can appear along in the water column or along the shoreline.
An NIEA spokesman said: “Algal blooms are naturally occurring events that are often associated with warmer conditions coupled with longer daylight hours, hence their appearance in spring and summer months.
"Although this is not always the case the blooms can be toxic and occur due to a wide range of complex interdependent factors.
“The public should be vigilant, especially in the Portstewart and Castlerock areas, and adhere to any warnings not to enter water or let their pets come into contact with algae along the shoreline – it can be highly toxic to animals.
"We are also asking the public to report any sightings of blue-green algae to our incident hotline on 0800 80 70 60 or email: emergency-pollution@daera-ni.gov.uk.”
