The UK has become the first nation to map and estimate the amount of carbon stored in its seabed habitats, including in Marine Protected Areas (MPAs).

The Blue Carbon Mapping Project, completed by the Scottish Association for Marine Science (SAMS) on behalf of WWF, The Wildlife Trusts and the RSPB, reveals that 244 million tonnes of organic carbon are stored in just the top 10cm of UK seabed habitats, with 98% stored in seabed sediments such as mud and silt.

Mudflats in Dumfries & Galloway (Mark Hamblin)

UK seabed habitats could capture up to 13 million tonnes of organic carbon every year – almost three times the amount sequestered by the UK’s forests – 4.8 million tonnes. Seas around the UK and Isle of Man cover nearly 885,000 square kilometres – over three times the size of the UK’s land mass, whereas forests cover a much smaller area (32,500 km2).

Blue carbon, as defined in the report, is the term used for carbon captured and/or stored by the world’s ocean and coastal ecosystems. Marine ecosystems capture carbon and lock it away in seabed sediments, especially in vegetated coastal habitats like saltmarsh and seagrass. Saltmarsh and seagrass beds both capture and store carbon whereas seaweeds and kelp forests capture carbon, a proportion of which is then eroded and transported elsewhere as detritus and subsequently buried in seabed sediments.

The seabed is host to habitats that capture and store carbon. They include seabed sediments (made of mud, silt and sand), vegetated habitats (seagrass meadows, saltmarshes, kelp forests and intertidal seaweeds), maerl beds and biogenic reefs, such as mussel beds and honeycomb worm reefs.

The research analysed the storage capacity of just the top 10cm of sediment. Some sediments are hundreds of metres thick and contain millennia’s worth of carbon, so the total carbon stored will be far greater.

The Blue Carbon Mapping Project highlights how physical disturbances to the seabed, including from human activity such as bottom trawling, as well as moorings and offshore developments, pose threats to blue carbon stores. Disturbing seabed habitats can release large amounts of carbon into the atmosphere, worsening climate change.

Wrinkled swimming crab on maerl (Paul Naylor)

Calls for increased protection of blue carbon stores

WWF, The Wildlife Trusts, and the RSPB are calling on governments across the UK to strengthen protections for blue carbon stores by minimising the impacts of human activities on the seabed. Most MPAs were not designated to protect blue carbon, and failing to protect these areas from disturbance could threaten climate and biodiversity goals – including net-zero and protecting 30% of seas by 2030.

WWF, The Wildlife Trusts & RSPB call for:

Better management of MPAs:

• Ensure that all MPAs are protected from destructive activities that damage blue carbon habitats.
• Account for both carbon and biodiversity in designating new protected areas.

Improved strategic planning of activities in UK seas:

• Consider blue carbon in UK marine plans.
• Undertake blue carbon impact assessments.
• Support a just transition for fishing industries away from activities that damage the seabed.

More investment and research on protecting blue carbon:

• Allocate funding to restore habitats including seagrass beds and saltmarshes.
• Support research and monitoring of blue carbon dynamics.
• Add seagrass and saltmarsh to the Greenhouse Gas Registry to track and monitor emissions.

Helford Seagrass (Paul Naylor)

‘Mud is the star’

Tom Brook, blue carbon specialist at WWF-UK, said: “This project reveals how critical our seas are in regulating the climate and underscores the urgent need to protect and restore our seabed habitats. While saltmarshes and kelp forests punch above their weight in terms of capturing carbon, the mud really is the star here – accumulating and storing vast amounts on the seabed.” 

Kirsten Carter, head of UK marine policy at the RSPB, said: “Accelerating efforts on land to tackle the nature and climate emergency is critical, but we must not underestimate the role of UK seas. This report is a gamechanger for our knowledge of the marine environment and a huge asset for decision-makers. Now we need them to act on its findings.”

A spokesperson for Defra said: “Protecting our oceans, and the precious species that depend on them, is fundamental to a healthy and thriving environment. We will ratify a new Global Oceans Treaty to protect marine life, enforce bottom trawling restrictions in our Marine Protected Areas and ensure all catch limits are set sustainably.”