What is blue carbon?
Blue carbon is the carbon captured and stored in marine and coastal ecosystems that accumulates over long timescales through natural processes. Blue carbon habitats include saltmarshes, seagrasses, kelp beds, biogenic reefs and geological sedimentary stores, such as seafloor and sea loch sediments.
Saltmarshes are wetland habitats with salt-tolerant plants that cover over 77 square kilometres of Scotland's coastline. Most saltmarsh carbon is stored in the sediments, with an estimated 118.5 grams of carbon buried annually per square meter of Scotland’s saltmarshes. An estimated 367,888 ± 102,278 tonnes of organic carbon is stored in the top 10 cm of Scotland's saltmarsh soils, which is higher than most other terrestrial land cover. Saltmarshes also make our coasts more resilient to sea-level rise and reduce flood risks, provide habitat for various bird, plant and invertebrate species, and are enjoyed for recreation.
Seagrasses are marine flowering plants found in subtidal and intertidal regions of the coastline. Seagrass meadows cover over 21 square kilometres of Scotland’s coastline. It is estimated that Scottish seagrass meadows store 0.17 megatons of carbon in their leaves, roots and rhizomes, and can take up 1,021 tonnes of carbon annually. Most seagrass carbon is stored longer term in the sediments. Seagrass meadows also provide habitat for numerous fish species and help make our coastline more resilient to climate change and erosion, by stabilising sediments and reducing wave impacts.
The majority of Scotland’s blue carbon is stored in seafloor and sea loch sediments, particularly around the Northern Isles and Scotland's western coastline. An estimated 7.64 mega tonnes of carbon is sequestered annually in the top 10 cm of Scotland’s marine sediments, adding to an estimated 2,622 megatons of carbon that is currently stored within the sediments. Scottish fjords are considered as blue carbon 'hotspots', burying an estimated 84,000 tonnes of organic carbon annually. There is more carbon stored in Scotland’s seafloor and sea loch sediments than in all of Scotland’s terrestrial environment. This is mainly because Scotland’s seafloor is around six times greater than Scotland's land area.
Kelp beds are forests of large brown seaweeds that cover much of Scotland’s coastline of over 3740 square kilometres. The carbon that is stored within living kelp is swept away into the marine environment, where it may become a long-term carbon store buried in seafloor sediments. Scottish kelp is estimated to produce approximately 1.4 megatones of carbon annually and stores approximately 0.6 mega tonnes of carbon within its living biomass. Kelp has numerous other benefits too, including supporting over 1800 species of flora and fauna in the UK, providing coastal defence to waves, and finds use in various industries.
Biogenic reefs comprise discrete accumulations of organisms having calcium carbonate structures, like shells, which are found on the seafloor. Biogenic reefs in Scotland include maerl beds, cold-water coral reefs, flame shell beds and horse mussel beds. These reefs are found in shallow waters and in deeper waters, but their full extent in Scottish waters is currently unknown. Biogenic reefs are thought to have some of the highest carbon sequestration rates of all habitats, even compared to terrestrial habitats, but further work is needed to understand carbon flows in these habitats.
Maerl beds are accumulations of living and dead red coralline algae with calcium carbonate skeletons. Maerl beds are one of our most-researched biogenic reefs around Scotland and cover over 31 square kilometres of Scotland's coastline, including along the west coast, Western Isles, Orkney and Shetland. An estimated 541 tonnes of carbon is sequestered by Scottish maerl beds annually, adding to an estimated 2 megatons of carbon already stored within Scottish maerl beds. Maerl beds also support a wide range of plants and animal communities, including commercially important shellfish, such as scallops.
Blue carbon projects
The Scottish Blue Carbon Forum has supported a range of scientific projects to advance our collective knowledge on Scotland's blue carbon. These projects include mapping Scotland’s blue carbon saltmarsh habitats, establishing the relationship between carbon moving from the land to the marine environment and understanding the impacts of human activity on blue carbon stores. These projects are informing new policy actions to protect, restore and enhance blue carbon habitats – all of which will help deliver against Scotland’s climate and biodiversity commitments.