For the third year running toxic blue-green algae blooms that look like pea soup and smell like rotten eggs have covered much of Lough Neagh, the largest lake in the UK.

But this summer, the thick green veneer - so widespread it is visible from space - has been worse than ever, according to locals living near the Northern Ireland landmark. 

"The lake is dying," Mary O'Hagan, an open water swimmer, said at Ballyronan, on its western shore, as ducks struggled over slick green-coated stones.

What's causing it?

The algae growth - fuelled by industrial, agricultural and sewage pollution, as well as climate change, according to experts - has ravaged fishing and watersports, and prompted concerns about drinking water safety. 

Signs banning bathing dot the lake's 125-kilometer long shoreline, including at Ballyronan, 54km west of the Northern Irish capital Belfast. 

Nutrient-rich fertilizer and slurry run-off from farms supplying mega-firms like chicken processor Moy Park are being blamed for contributing to the pollution. Untreated sewage spills and septic tank effluent are also suspected.

Moy Park deny polluting the lake and say the poultry sector is "highly regulated with strict limits set for wastewater quality" at all its local sites.

The algae growth is a "complex issue not specifically linked to any one sector," a spokesperson said.

'Heartbreaking'

Lough Neagh was O'Hagan's "training ground" during the Covid years, but suffering from chronic health problems, she now relies on local swimming pools.

"Swimming here with its spectacular sunrises helped me when I was in a bad place in my life. It's heartbreaking to see it now." O'Hagan has joined a campaign group, 'Save Lough Neagh', and at a recent protest urged Northern Ireland's regional government to act faster.

The invasive zebra mussel species, a recent arrival in the lake's waters, filter water but any benefit is far outweighed by the molluscs also stimulating algal photosynthesis, worsening the green water effect.

Meanwhile the algae has decimated the Lough Neagh fly, a staple for fish and birds.

Europe's largest eel fishery - also on the lough - suspended operations this year due to quality concerns.

'Doctor Sludge'

According to Gavin Knox, whose paddleboard small business also fell victim to the sludge, the foul smell can reach miles inland.

The 48-year-old launched his venture in 2022 to help people safely have fun on the water.

"Doing business became impossible," he said. "Even if there are safe places to paddle, nobody wants to do it when the fish are dying and the birds are covered in green slime." 

Last July the regional government launched an action plan to kickstart a fix. But less than half the planned measures have been delivered.

The remaining steps have "delivery timelines extending into 2026 and beyond," said a government statement.   

With around 40 percent of Northern Ireland's drinking water supplied by Lough Neagh, the risk of a health emergency might force swifter action by authorities, said Les Gornall, a local slurry expert whose nickname is 'Doctor Sludge'.

"If Belfast suddenly cannot guarantee a clean water supply, then there would be a property and tourism crash," he predicted. "Maybe that prospect will jolt them into fixing the lake."