Toxic algae was discovered in Lough Neagh over the summer, which provides 40% of Northern Ireland's drinking water.
The ecological impact on the lake is currently unknown, but there is criticism that monitoring and managing the lake's health has been inadequate.
The decline in water quality and the ecosystem has been observed for decades, and concerns have been raised about the collapse of the Lough Neagh fly population and the reduced breeding of wading birds.
Scientists are working to understand the impact of the algae and address the root causes of the ecosystem's deterioration.