Crowds have attended a public meeting to express concern at a huge bloom of toxic blue-green algae in Lough Neagh.
They packed into an auditorium at the Seamus Heaney HomePlace centre in Bellaghy, Co Derry close to the northern shores of the lough.
The event was organised by the Save Our Shores group, set up last month in response to the outbreak of algae.
Other waterways in Northern Ireland have also been affected.
Lough Neagh is Ireland's largest fresh water lake, and supplies 40% of Northern Ireland's drinking water.
It also supports a major eel fishing industry.
The meeting started with the reading of Seamus Heaney's poem 'A Lough Neagh Sequence'.
Among the speakers was Mary O'Hagan, co-founder of the Save Our Shores group.
Ms O'Hagan said, that as an open water swimmer, she and others had been growing increasingly concerned about the water in the lough.
The algae growth is linked to excess nutrients in the water.
Nitrogen and phosphorus from agricultural fertiliser running off fields is believed to be a major contributory factor.
The spread of the invasive Zebra mussel species is also understood to have played a role, as they have made the water clearer, allowing more sunlight to penetrate, stimulating more algal photosynthesis.
Climate change is another factor cited, with the temperature of the lough having risen by 1C in the last two decades.
The deaths of birds and dogs have been linked to the recent algae blooms and anglers are being urged not to eat anything they catch.
A number of water-based businesses on Lough Neagh have also had to stop operating.
NI Water has insisted that its intensive treatment processes mean there is no health risk associated with drinking water sourced from the lough.
Civil servants at Stormont are attempting to coordinate efforts to take action, but critics claims the current lack of devolved government in Northern Ireland is hindering the response.
Earlier this week, the archbishops of Armagh criticised the response to the situation as "too slow", urging a task force to be formed.
Archbishops John McDowell and Eamon Martin said the lough is facing an "environmental disaster".