The locations where Blue-Green algae has been reported this year in Northern Ireland

The locations include Lough Neagh and Castlewellan Lake

There have been 77 reports of Blue-Green algae in Northern Ireland this year at more than 35 locations.

Blue-Green algae continues to be a problem in waterways across NI with dozens reports of the toxic cyanobacteria since March 18, 2026. The majority of reports of have been in Lough Neagh, Castlewellan Lake and Brantry Lough.

In recent years, the algae blooms in Lough Neagh, which provides 40% of NI’s drinking water, have caused great concern with it being labeled an environmental crisis. This year, algae has been reported at 13 different locations around the lough with the most recent being at Kinnego Marina on June 25.

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The cause has been put down to an excess of nutrients from a number of sources, including waste water, septic tanks and agriculture, exacerbated by climate change and the invasive species zebra mussels.

In July 2024, DAERA launched the Lough Neagh Action Plan which aims to address the issues leading to the crisis in the UK and Ireland’s largest freshwater lake.

The locations where Blue-Green Algae has been confirmed in Northern Ireland

• Black Glen Lake – Castlerock
• Brantry Lough
• Castlewellan Lake
• Church Road Reservoir
• Clay Lake
• Copeland Reservoir
• Craigavon Lakes
• Hillsborough Park Lake
• Lough Aghery
• Lough Island Reavy
• Lough Neagh – Antrim Bay
• Lough Neagh – Antrim Boat Club
• Lough Neagh – Ardboe
• Lough Neagh – Ballyronan
• Lough Neagh – Battery Harbour
• Lough Neagh – Deep point
• Lough Neagh – Derryadd
• Lough Neagh – Derrycrow
• Lough Neagh – Kinnego Marina
• Lough Neagh – Oxford Island
• Lough Neagh – Rea’s Wood
• Lough Neagh – Washing Bay
• Lough Neagh – White Horse Flats
• Lower Bann – Bracknamuckley Forest
• Lower Bann – Newferry
• Lower Bann – Portglenone
• Lower Bann – Seatons Marina
• Lower Conlig Reservoir
• Lower Holywood Reservoir
• Portmore Lough
• Race Course Lough, County Fermanagh
• Six Mile Water
• Tunny Cut
• Unnamed pond, Loughmallon Road
• Upper Ballysallagh Reservoir
• Upper Conlig Reservoir
• Upper Lough Macnean

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