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Public urged to be blue algae aware this summer

Blue green algae graphic

The public are being urged to remain vigilant and take extra care if visiting the north coast over the July holidays, particularly the Portstewart/Castlerock area, following confirmation of Blue-green algae at Castlerock Beach.

Blue-green algae can be harmful to humans and is highly toxic to animals.

The Northern Ireland Environment Agency (NIEA) received a report of suspected algae at Castlerock Beach on Wednesday 5 July from DAERA samplers undertaking Bathing water testing. A water sample has since confirmed the presence of a blue-green algae Microcystis sp which has the potential to produce harmful toxins.

The National Trust and local council, as beach operators in the area, were informed of the results and erected signage warning users in the area of the presence of potentially toxic blue-green algae. It is the responsibility of the landowner to initiate action as it deems necessary.

Further sampling will be undertaken over the weekend to assess the extent of algal blooms in the area, with results expected early next week. DAERA will continue to liaise with bathing water operators who have agreed that the precautionary red flag warnings (advising against swimming) will remain in place at least until Tuesday.

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:”

The blue-green algae in Lough Neagh, confirmed on 6 June, 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.

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.

Key Blue Algae facts you need to be aware of are:

  • What does Blue-Green Algae look like?

  • Why should you be aware of Blue-Green Algae

If you suspect there is blue-green algae:

  • Report it on the Bloomin Algae App

  • If you don’t have Bloomin Algae App – email photos to NIEA

  • What happens when Blue Algae is reported

Please take extra care if visiting or walking your dog along the coast or at a river/ lake this summer.

What is Blue-Green Algae? Blue-green algae is a type of bacteria called cyanobacteria. Cyanobacteria naturally inhabit our freshwater, coastal and marine waters and, like plants, require sunlight, nutrients, and carbon dioxide to grow and reproduce. Cyanobacteria can increase greatly in numbers and form visible ‘algal blooms’ which can lead to poor water quality and potential toxicity. Suitable conditions for ‘blooms’ to occur include abundant sunlight, still or slow-flowing water and sufficient levels of nutrients (nitrogen and phosphorus). Algal blooms can occur throughout the year, but they are most common from May through to September when suitable weather conditions combine with a ready supply of nutrients.

Roles and responsibilities

Landowners/ Operators of lakes or rivers

You must consider how your water is used when assessing risk to people and animals from exposure to blue-green algae.

You are responsible for controlling discharges of water off-site, warning users, putting up signs and restricting access. You are also responsible for deciding when restrictions can be removed. The local council also has these responsibilities for water they own.

Blue-green algal blooms and scums are natural features of some waters. Increasing shade and reducing nutrients in the water can help reduce risk of algal blooms.


Landowners or operators of lakes and rivers are responsible for managing and assessing risks associated with blue-green algae.

NIEA provides a supporting role to landowners and operators, through verifying the presence of blue-green algae through photos submitted to the Bloomin’ Algae App or directly to:, or in some cases through water samples.

It must be highlighted, that collecting and analysing a sample provides only a location specific indication of the presence or absence of algae at that point in time. NIEA will follow internal response procedures for blue-green algae.


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