Why seagrass meadows are key to the UK’s climate resilience

28 November 2024

‘Blue carbon’ habitats sequester carbon and protect our coastlines, yet are threatened by development – much work is needed

Seagrass meadows in UK coastal waters are an increasingly recognised asset in climate change mitigation due to their numerous ecosystem services and potential for carbon sequestration.

So-called ‘blue carbon’ habitats are marine ecosystems that sequester carbon dioxide and reduce atmospheric greenhouse gases. Recent research highlights the carbon sequestration capacity of these ecosystems, particularly in coastal areas where they can reduce ocean acidification, support biodiversity and build coastal resilience.

The Ocean Conservation Trust (OCT) is working to understand the lifecycle requirements of seagrasses in temperate waters. Our aim is to identify how large-scale seagrass habitat restoration could help to compensate for habitat loss, as well as understanding the carbon-capturing potential of seagrass.

Wildflower meadows of the sea

Seagrass meadows are submerged flowering plants found in shallow coastal waters. Over 70 species can be found worldwide, with these habitats forming the most widespread coastal ecosystems on the planet. Meadows form over large areas and transform bare sand sediments into rich diverse healthy ecosystems akin to wildflower meadows on land. These coastal habitats provide shelter and homes for some of the ocean’s most beautiful and rare wildlife, including seahorses, and kaleidoscope jellyfish as well as many finned fish species. Within UK waters, Zostera marina meadows form large sub-tidal beds covering areas of over 300 hectares, for example in St Austell and Mounts Bay in Cornwall.

Seagrass meadows at Jennycliff Bay. Credit: Ocean Conservation Trust

They provide habitat for marine life, protect shorelines and serve as significant carbon sinks, with ‘blue carbon’ referring to the carbon captured by marine and coastal ecosystems, including mangroves, salt marshes and seagrasses. Studies published from 2012 in Nature Geoscience suggest that despite globally covering only 0.2 per cent of the ocean floor, seagrass meadows may account for up to 10 per cent of the ocean's carbon storage.

Seagrasses capture and store atmospheric carbon through two primary processes: photosynthesis and the accumulation of other organic carbon transported from outside the ecosystem. Photosynthesis by seagrasses works the same way as in terrestrial plants, involving the absorption and conversion of carbon dioxide or bicarbonate to organic carbon, which is stored in the plant’s leaves and root structure. Unlike terrestrial carbon sinks, however, seagrass ecosystems also filter organic carbon from passing currents and bury this within the sediment, a feature that amplifies their carbon storage capability. In the absence of fire or air, this carbon can remain in the sediments for thousands of years, making seagrasses especially valuable in long-term carbon storage.

Recent work by the OCT and partners, supported by the Environment Agency’s Natural Environment Readiness Fund, found that one seagrass meadow in Plymouth Sound held carbon stocks ranging between 25.74 and 208.91 tonnes of carbon per hectare within the top 30cm, and an average of 91.62 tonnes of carbon over the seagrass meadow (to put this in context, the higher reading within the first 30 cm is equal to the CO2 produced by 47 average UK citizens annually). This carbon content increases with depth, with an average of 195.75 tonnes held to a depth of 50cm.

The accumulation rate for the carbon stock has been estimated to be around 120 years for 30 cm, and sequestration rates therefore appear to vary over the extent of the seagrass meadow, depending on density, local environmental conditions and depth. No measured data exist for carbon sequestration for UK seagrass ecosystems although carbon accumulation rates of the 8,493 ha of mapped UK seagrass meadows have recently estimated to be 2,500 tonnes of carbon per year, or 2,085 average UK per capita emissions. Given that our burning of fossil fuels releases carbon that has accumulated over geological time scales, we should perhaps not be surprised when the rebalance through nature-based solutions requires a long-term vision.

The challenge around carbon credit certification for seagrass

These variations in carbon stock levels create an issue with carbon credit certification for seagrass replanting. With large variations over each seagrass meadow, it is hard to agree an average ecosystems service value that is reliably tradable on credit markets. And if the accountancy of sequestration is not valued correctly, the risk is that we undervalue the effectiveness of seagrass as a climate mitigation tool. Our carbon stock assessment project is now being rolled out at different seagrass meadows around England, with a view to filling this data gap.

While more detail is needed, it is widely acknowledged that seagrass meadows store huge amounts of carbon for very long periods of time. Halting seagrass loss and retaining these carbon stocks should be a priority therefore, if we want to meet our national climate goals. Future research should focus on quantifying the carbon sequestration potential of seagrass meadows across different UK regions to inform regional conservation strategies. Policymakers could also integrate seagrass meadows into carbon accounting frameworks and climate mitigation targets, recognising them as natural assets for carbon management.

Increasing funding for seagrass restoration and conservation initiatives is essential. This includes investing in research to better understand seagrass ecology and resilience to environmental changes. Such efforts would support the UK’s commitment to international treaties like the Paris Agreement, where blue carbon ecosystems are being called on to play a crucial role in achieving emissions targets. The 2022 UK Climate Change Committee Blue Carbon Briefing has made the key recommendation that the Department for Business, Energy and Industrial Strategy (as it was then known) should produce a roadmap to inclusion of saltmarsh and seagrass in the greenhouse gas inventory to contribute towards International agreements.

The role of seagrass in improving water quality

Seagrasses also play a vital role in nutrient cycling, processing and the sequestration of nutrients, including nitrogen fixing by cyanobacteria and heterotrophic bacteria. They filter pollutants from the water, improving water quality and reducing nutrient levels, essential for managing the coastal eutrophication (excess nutrients) caused by pollution in our waterways from combined sewer outflow spills, highway runoff, agricultural pollution and other sources.

Seed collection at Jennycliff Bay. Credit: Ocean Conservation Trust

Valuations of this type of waste treatment service by seagrass are unfortunately limited. Recent studies showed that the loss of Zostera marina in Sweden caused the release of 6.63 tonnes of nitrogen per hectare (this work only assessed the one-time cost of the loss of seagrass nitrogen stock, rather than the ongoing value of the lost treatment capacity of the seagrass meadow). The most respected valuations of seagrass nutrient cycling are from the late 1990s, which valued seagrass habitat as the third most valuable habitat on the planet for ecosystems services and natural capital value.

The good news is that the appreciation of the value of non-market goods and services is growing. Ongoing projects by OCT and our partners are seeking to better understand all the services that seagrass habitats provide to coastal communities – all things considered, we would argue that seagrass is one the UK’s most valuable marine habitats.

Seagrass meadows for coastal resilience

In addition to the carbon and nitrogen storage capabilities of seagrasses, UK seagrass meadows provide ecosystem services that contribute to coastal resilience. By absorbing wave energy, seagrass beds help protect against the impact of storms, which are expected to intensify with climate change. Complex root systems also anchor sediments, reducing coastal erosion and protecting shorelines from storm surges. This natural coastal defence mechanism is important where coastal erosion threatens habitats, properties and infrastructure.

Unfortunately, anthropogenic factors like coastal development, pollution and climate change threaten these ecosystems. Within the UK many of these highly productive ocean habitats sit within Marine Conservation Zones (MCZ) which are designed to safeguard these natural capital assets for generations. Even with this level of marine conservation, however, the extent and health of the ecosystem is deteriorating. The Marine and Coastal Access Act 2009 established the MCZ network and now seeks to recover the health of seagrass habitat through spatial planning, the Water Framework Directive and the Conservation of Habitats and Species Regulations 2017.

UK seagrass meadows are a valuable natural resource, offering significant benefits to biodiversity, coastal protection and climate change mitigation through blue carbon storage. While these ecosystems face numerous challenges, efforts by the OCT, researchers and policymakers can help safeguard and restore seagrass habitats. Recognising the importance of these ecosystems in climate policy, promoting public awareness, and enhancing conservation initiatives such as OCT’s Blue Meadows, are essential for protecting seagrass meadows and ensuring their continued contribution to global carbon sequestration efforts.

By valuing and preserving these habitats, the UK can nurture its natural defences against climate change while fostering a sustainable marine environment for future generations.

Author: Mark Parry is head of ocean habitat restoration at the Ocean Conservation Trust, working for over a decade to understand seagrass loss and recovery.