23-05-2025
Maerl beds: the irreplaceable fish nurseries under threat from sewage and pollution in Cornwall
Marine experts have warned that Cornwall's ancient coral beds are being damaged by pollution.
Divers have found that the previously "purple and beautiful" maerl beds off St Mawes are now covered in a brown algae caused by sewage and to Cornwall Wildlife Trust, this development is a "massive concern" for the essential ecosystems.
What are maerl beds? Maerl is a purple-pink seaweed that forms a hard layer on the seabed over thousands of years.
This spiky surface is an irreplaceable marine habitat which acts as a nursery for commercial fish and shellfish species, as well as being a vital carbon store.
Maerl beds are fragile structures which can easily be damaged by human activities such as towed fishing gear, anchor damage and commercial extraction. Protections like the EU Special Area of Conservation apply within the Fal estuary, meaning mobile fishing gear and maerl extraction are both now banned.
What has happened to the maerl beds in Falmouth Bay? However, water quality is also a key threat to the habitat, and an organised dive at the recent UK Maerl Forum found that the ancient maerl beds off St Mawes had suffered a serious deterioration. Sewage and agricultural run-off cause raised nutrient levels in the water, which can lead to this algae now covering the beds.
Speaking to ITV News West Country, a Marine Conservation Officer at Cornwall Wildlife Trust, Matt Slater, explained: 'We were all absolutely shocked to find that instead of seeing a beautiful purple carpet of living mearl, it was absolutely smothered in brown algae, and you could tell that there was virtually no light getting through that seaweed."
He added: "We really hope that it's going to survive, but we know from experience that was shared at the Maerl Forum, that in other parts of Europe where this has been witnessed, it has been really bad news."
What can be done?
According to Mr Slater, this deterioration should be "a wake up call that something needs to be done about water quality in the Fal".
He noted: "It's great that there is work going on but we would love to see more of it, and quicker."
Cornwall Wildlife Trust is working with farmers to reduce agricultural run-off and is encouraging sustainable fishing practises.
South West Water has said it is working with the Environment Agency and Natural England to understand the sources and distribution of nutrients and chemicals. In a statement, the utility company added: "We are also planning to reduce the nutrient load at a number of wastewater treatment plants across our region, including at North Fal (St Stephen) and St Dennis, both within the wider Fal/Helford catchments, which will improve the quality of the water in the rivers that flow into the estuary.
"We are reducing levels of phosphate in the treated wastewater that we return to the environment. We are serious about reducing the use of storm overflows and our plan is working, but there is more to do. We will be carrying out improvements at 16 Storm Overflows closest to the Fal Estuary area by 2030."
