Scottish blue carbon action plan

2. Blue carbon - overview

Blue carbon refers to the carbon captured and stored in marine and coastal habitats through natural processes. This includes the capture of carbon dioxide (CO₂) when it is taken up by plants from the atmosphere by photosynthesis and transformed into organic carbon and the storage of organic carbon over climate-relevant timescales (more than 100 years).

Long-term storage takes place through a range of burial processes, which often form deep sediment stores of organic carbon. For example, in a saltmarsh, organic carbon largely in the form of plant matter becomes incorporated into the underlying soil through root systems or is buried over time as old plant matter is incorporated and new growth takes place; whilst within seafloor or shelf sea sediments, stores form through the gradual build-up of organic carbon as it settles through the water column from both marine and terrestrial sources. Some of these natural carbon stores have built up over thousands of years or longer, so protecting or managing vulnerable stores to help prevent the release of carbon into the atmosphere is an important part of avoiding increased emissions that worsen climate change.

In this action plan, the primary focus is climate change mitigation delivered by the accumulation of organic carbon and its long-term storage in blue carbon habitats through natural processes. However, it is important to note that carbon also occurs naturally in other forms and that all these forms of carbon are relevant to the functioning of healthy marine habitats.

Organic carbon is found in all marine life, including fish, crustaceans, mammals, and plankton. As part of the natural carbon cycle, this carbon can be released when these organisms die and decompose. Whilst this carbon is important within wider nutrient cycles that support animal and plant life in the ocean, and the overall carbon held by the marine carbon system can be enhanced by the sustainable management of Scotland’s seas, such carbon is outside the scope of this action plan as it does not contribute to long-term (>100 years) storage.

Inorganic carbon is also formed by marine organisms, such as native oysters, mussels, maerl, and other biogenic reefs as they form their shells or skeletons. This process, known as calcification, involves converting carbon, calcium, and other elements dissolved in seawater into minerals, such as calcium carbonate. However, the process of calcification itself releases CO₂. It is because of this release of CO₂ during shell or coral formation that these habitats are not considered appropriate to contribute to climate change mitigation.^[3] So while calcifying habitats were included in earlier SBCF and NatureScot work on blue carbon, they are not considered within this plan.

However, it should be noted that the hardened surfaces of shell and coral form complex 3D physical structures which can also help to trap and bury small amounts of organic carbon from the water column. Recent research has shown that maerl (and other coralline algae) can recycle around 40% of the CO₂ produced during calcification.^[4] These calcifying habitats and organisms remain important as part of the carbon cycle, as well as for biodiversity, and will continue to be afforded protections in marine policy.

Blue carbon habitats are widely distributed across Scottish waters. Maps of known distribution for each habitat are provided in section 4. Saltmarsh is a coastal habitat, while seagrass and kelp and other seaweeds are found in coastal and nearshore waters. These habitats rely on photosynthesis for growth and need relatively high levels of sunlight. Marine sediments are found across Scotland’s seafloor and are formed by long-term natural accumulation processes. The distribution of organic carbon within marine sediments is not uniform - hotspots have been identified where there is a higher density and quality of carbon, particularly in Scottish sea lochs as well as other depositional areas.

Blue carbon habitats in Scotland, as elsewhere, face a growing number of pressures.^[5] For example, the widespread conversion of Scotland’s saltmarshes to agricultural and development land, especially during the 18th and 19th centuries, together with more recent acceleration of sea-level rise, places this important coastal habitat under extreme pressures. In some regions, such as the Forth Estuary, about half of the intertidal area is estimated to have been lost over the last 400 years.

Seagrass has also experienced declines over the last century, in part due to a wasting disease (called Labyrinthula) as well as declines in water quality, land claim and activities that physically disturb the sediment or change water flow regimes.

While policy measures, including designations as protected features and recognition as Priority Marine Features (PMFs), have helped to limit some of the direct impacts of human pressures, the ongoing impacts of climate change are adding new pressures and risks for these habitats. These climate related impacts include increased storm intensity and frequency, rising sea temperatures, marine heatwave events, and ocean acidification. Rising sea-levels may result in coastal squeeze in some locations and also place an added pressure on saltmarsh habitats.^[6]

Organic carbon in seabed sediments is generally more resilient to the impacts of climate change but may be vulnerable to disturbance by human activities such as bottom trawling and infrastructure developments on the seabed. However, key evidence gaps remain, including on how much of this resuspended organic matter from marine sediments is released into the atmosphere (see section 4 for more details).

Blue carbon habitats are not currently included in the UK greenhouse gas inventory (GHGI) and so are not formally accounted for in reporting on progress to net zero. The International Panel on Climate Change (IPCC) has developed guidance^[7] that provides the framework for the inclusion of saltmarsh and seagrass in national inventories, which are sometimes referred to as ‘actionable’ blue carbon habitats for this reason. The Scottish Government supports inclusion of coastal habitats in the inventory and is working with the UK Government and other Devolved Governments, through the UKBCEP, towards their inclusion; decisions on the inventory are reserved to the UK Government. The evidence base for saltmarsh is closest to enabling potential inclusion in the inventory, and further work is focused towards refining and standardising measures including greenhouse gas flux measures. For seagrass, there are key evidence gaps that still need to be addressed, particularly around the amount of carbon taken up and stored in these habitats around Scotland and potential contributions of restoration efforts.

There is currently no IPCC guidance for the inclusion of kelp and other seaweeds or seafloor sediments into national greenhouse gas inventories. Whilst in part this reflects that the main blue carbon focus of the IPCC has, to date, been on emissions and removals from coastal wetlands, there are also significant evidence gaps that limit understanding of how these habitats may contribute to climate change mitigation efforts. For this reason, they are sometimes known as ‘emerging’ blue carbon habitats.

In light of these considerations, the focus of this action plan is on the role that blue carbon habitats play in the capture and long-term storage of organic carbon. The priority for these habitats is to ensure that stored carbon is not released where this can be avoided, and to recognise that for some habitats (i.e. saltmarsh and seagrass) restoration and enhancement may also be able to make a small contribution to climate change mitigation.