Blue-Green Algae Returns to Lough Neagh
Blue-green algae has been recorded in Lough Neagh for the first time this year. It was spotted on the south-west shore off the Washingbay Lough Walk.
The report, submitted to the Bloomin' Algae app, was marked as "verified" on Wednesday. Other verified sightings have been reported at Brantry Lough in County Tyrone and Castlewellan Lake in County Down since the start of 2026.
The stormy, cool, and dull weather Northern Ireland has been experiencing has helped suppress growth of the algae. However, blooms are expected to resurge when the weather improves.
Historical Context and Causes
Algal blooms have been recorded on the lough since the summer of 2023, when the algae was first detected at levels not seen since the 1970s. A high level of nutrients in the water – phosphorus and nitrogen – is the main cause of its growth.
Those nutrients come mainly from agriculture and wastewater treatment, as well as septic tanks, industry, and households.
Ecological Impact and Contributing Factors
The invasive Zebra mussel species has changed the ecology of the Lough. It eats ordinary algae and avoids blue-green algae as it is toxic. That clears the water, allowing sunshine to penetrate deeper. It also removes competition for the nutrients that all algae consume.
Climate change also plays a role, with the temperature of the Lough increasing by at least one degree since 1995.
