Blue-green algae triggers bathing alerts at two NI beaches; Castlerock watches as blooms spread

Blue-green algae prompted bathing alerts at two north coast beaches in Northern Ireland on Monday, with officials saying high levels were observed during routine monitoring and swimming is now prohibited at Downhill Strand and Portstewart Beach. The notices come as authorities warn that further assessments may lead to additional restrictions if algal conditions worsen.

The Department of Agriculture, Environment and Rural Affairs (DAERA) said algae had been observed on parts of Magilligan, Downhill and Portstewart strands during regular monitoring on Monday 22 September 2025. DAERA said monitoring would continue and that advice would be provided to bathing water operators as required. The statement underscored that the situation remains under review and that public health guidance would be updated as new data become available.

The National Trust, which manages Portstewart Strand, indicated that algae had been detected at a red alert level and urged visitors not to swim, to avoid any scum or mats on the shore, and to keep pets away from the water. Such designation signals a heightened risk to bathers and emphasizes the need for precautionary measures along the shoreline.

Meanwhile, Causeway Coast and Glens Council said beach users should remain vigilant for blue-green algae and related conditions. It noted that increased levels had been detected at Castlerock beach, but at the time there was no bathing notice in place for that location yet. The council urged residents and visitors to avoid contact with any suspected mats or scum on the shore, and to keep animals away from the water. It also advised against swimming, fishing, wading, boating, or kayaking if blue-green algae is suspected, and it reminded pet owners to keep dogs on a lead and away from the tide line.

The evolving situation is connected to broader algal activity that has affected other water bodies in the region. Blue-green algae have been present in Lough Neagh throughout the summer, and recent weather conditions have contributed to portions of algal mats traveling along the Lower Bann River toward the North Coast. In August, the first algae-related bathing ban on the coast was imposed at Benone, though the ban was lifted a few days later as conditions changed and monitoring continued.

Nicole Morelli, a member of the Menopausal Mermaids open-water swimming group, was among local swimmers who expressed disappointment at not being able to enter the water. She said, "We had such a great day yesterday, and we were really looking forward to getting back in. But it's better to be safe." Her group and others have been monitoring the situation closely as authorities work to assess water quality and determine when it will be safe to resume bathing and swimming activities.

Officials emphasized that monitoring will continue across affected beaches and that further guidance would be issued to bathing water operators as needed. While Downhill and Portstewart are currently closed to swimming, authorities stressed that the situation could evolve with changing weather and water conditions. In the interim, beachgoers are urged to follow posted notices, heed advisories, and avoid contact with suspected algae or mats.

The broader context of this season’s algal activity underscores the ongoing environmental dynamics affecting coastal bathing water quality. Local authorities said continued vigilance is essential to protect public health while the ecosystem responds to seasonal and weather-driven changes. As scientists and managers track blooms, authorities will provide regular updates to the public and to operators responsible for maintaining safe and compliant bathing waters across the region.