Angeli Mehta reports on the growing drive by governments and companies to conserve the vast carbon sequestration potential in seagrass meadows and mangroves

Over the next few weeks, staff at the Ocean Conservation Trust will find out just how many of the 18,000 seagrass seeds and seedlings they’ve planted off the south coast of England have taken root. The scientists’ painstaking efforts are part of a three-year EU project to restore 8 hectares of seagrass meadows and to find a means of allowing recreational activities such as boating to continue without destroying them.

Seagrass are fragile and easily disturbed, says Mark Parry, who is leading the project. And as in many other parts of the world, the UK’s shores have lost up to 90% of their seagrass meadows.

But they have a carbon sequestration capacity that is two to four times as great as a mature tropical forest, and restoring 80,000 hectares could lock up an estimated 3% of the country’s emissions.

The benefits would also extend far beyond climate-change mitigation. “Seagrasses are a barometer of coastal health; the canary in the cage, if you like,” says Parry.

Scientists estimate that over 3 trillion tonnes of carbon may be stored in the seabed

They improve water quality by removing nitrogen and provide a nursery habitat for young fish, making them one of nature’s most valuable ecosystems.

The importance of protecting seagrass and other coastal habitats will be high on the agenda at the biodiversity summit in Kunming, China, in October this year, amid a growing recognition of their potential role in tackling the climate crisis as well as biodiversity loss.

Scientists estimate that over 3 trillion tonnes of carbon may be stored in the seabed. Globally, some 20bn tonnes of carbon is stored in the top one metre of seagrass sediment alone. Salt marshes, mangroves and kelp forests also create massive carbon sinks.

The security of half of those stores depends on the decisions made by national governments because they lie within their economic exclusion zones.

Ocean Conservation Trust has planted 18,000 seagrass seeds off England’s south coast. (Credit: Ocean Conservation Trust)

In Europe, so-called marine protected areas (MPAs) are actually very poorly protected. Last year, a review carried out for the UK government recommended the creation of “highly marine protected areas” within existing protection zones, to allow ecosystems to fully recover to a natural state.

WWF puts the value of each well-managed MPA at more than £5m annually, through increased biodiversity, tourism and recreation. Plans have yet to be agreed, but the UK has committed to using its presidency of COP26 to lead the charge on action to restore the marine environment to health across the planet.

Environmental groups approve of the ambition, but want to see the UK do more to preserve its own blue carbon. Under its shallow coastal waters, the UK has a carbon stock of some 205m tonnes – more than held by all its forests.

This isn't sending someone to the moon. It's just regenerating coastal habitats

Chris Tuckett, director of programmes at the Marine Conservation Society, says some countries do include blue carbon as part of their nationally determined contributions, although the UK does not.

One challenge is that existing frameworks to encourage investment in carbon sequestration are geared towards terrestrial ecosystems. Tuckett says the science to enable accounting for carbon in different marine-based habitats is still evolving.

“For salt marsh and seagrass the figures are fairly solid, in terms of how much carbon can be captured, but there's less knowledge about seabed sediments further out and whether if you stir up those sediments, you end up with carbon emitted into the air or whether it stays in the water [leading to increased acidification]. But you do know that you're disturbing the carbon store, so that’s a bad thing to do.”

Salt marshes act as carbon sinks as well as providing biodiverse habitats. (Credit: Chris Laurence Travel/Shutterstock)

She says government needs to be clear about what can be included in carbon accounting, and what can not. “It needs to be done well, so there are genuine net gains that are properly accounted for.”

Parry of the Ocean Conservation Trust says he is asked about seagrass restoration every week by businesses that are keen to offset carbon emissions. “To do this at scale we need some alternative approaches, [including] mechanisation. This isn't sending someone to the moon. It's just regenerating coastal habitats.” But it is more expensive than land-based projects. Parry estimates a carbon price of around £100 a tonne would be needed to make it viable – that’s more than double the price of carbon in Europe’s ETS.

Now Verra, which manages carbon offset standards and verifies projects, has announced its first blue carbon project. Conservation International and its partners in Colombia have been funded by Apple to restore 11,000 hectares of mangroves on Colombia’s northern coast, which is expected to sequester 1 million tonnes of CO2 over the project’s lifespan.

They will use the carbon value to create sustainable financing for the region and secure livelihoods of the 1,2000 people who depend in the mangroves. As well as sequestering carbon, mangroves provide natural coastal defences. Recent research suggests that flood damage would cost $65m more every year, and 15 million more people would be flooded, if it were not for these seawater-loving trees.

Let's embrace the sea and collaborate with it as a space for climate solutions and economic development

Another ambitious project on five continents aims to provide solid evidence of the carbon sequestration potential of seaweed in the sediment below seaweed farms.

Led by Oceans 2050, an organisation co-founded by legendary explorer Jacques Cousteau’s grand-daughter Alexandra Cousteau, it aims to develop a scientific foundation for a methodology for the voluntary carbon offset market. Regenerative seaweed farmers could thus benefit financially from the work they are doing to restore kelp forests and the biodiverse habitats they support.

In the UK, a seaweed-processing business Oceanium has just secured £2m investment, led by WWF and the Green Angel Syndicate, to scale up its operations. Its network of seaweed farmers provides the raw materials for plant-based foods and sustainable packaging.

Bren Smith, founder of non-profit GreenWave, is a pioneer of regenerative ocean farming. He’s trained over 500 farmers on the east coast of the U.S., as well as Alaska and California, and expects to train thousands more. Smith has shown them that it is possible to make a good living from a system that requires no land, no fresh water and no fertilisers. In his model, scallops, mussels, oysters and clams grow in the seaweed beds.

“Let's embrace the sea and collaborate with it as a space for climate solutions and economic development,” he says. “People have been regeneratively farming the ocean for 3,000 years, since the first indigenous communities in the Pacific north-west built clam walls to start clam farming. I think this is the moment that society, and the planet, needs regenerative ocean farming. It's not the answer to everything, but I think it's a piece of the puzzle.”

Smith says regenerative ocean farming is also about climate justice. “Who farms matters. We need to make sure it’s about a just transition for fishermen directly affected by climate change as well as indigenous communities. We need to make sure this isn't the next big white land grab in the ocean.”

Another emerging use for seaweed is to cut the methane emissions from cows’ burps

One of the reasons kelp forests have been dying is because of warming oceans. GreenWave is growing over 100 strains of local kelp to test out their climate resiliency as part of a U.S. Department of Energy project, MARINER, which is aimed at exploring marine biomass as an energy source. In any case, having polyculture farms will nurture biodiversity.

Smith wants to see kelp used in everything from food to bioplastics. Last year, his harvest was sold to a variety of producers including Lolliware, a producer of edible or degradable straws, and to Primitives Biodesign, a MIT spin out making bioplastics with sensory capabilities.

Another emerging use for seaweed is to cut the methane emissions from cows’ burps. Research that’s been carried out in the U.S. and Australia is now beginning to be commercialised.

Studies are assessing the carbon sequestration potential of seaweed. (Credit: GreenWave)

In Vancouver, David Parker’s startup, Ocean Regenerative, has test lines in the water at two sites on a coastline that once was home to vast kelp beds. “To my mind, it was the most potentially impactful enterprise I could be engaged with, because it adds to a lot of the [sustainable development goals] SDGs and provides essential ecosystem services.” In the waters around British Columbia that means getting to grips with the deadly consequence of oceans becoming more acidic as a result of absorbing so much carbon dioxide.

A more acid ocean affects marine organisms’ ability to make and keep their shells.

Research suggests seaweed can have a local impact on ocean acidification by absorbing carbon dioxide, but Parker says a lot ends up back in the water column as the kelp dies. “You've got to draw some of that biomass and put it in the soil,” suggests Parker.

The theory is that adding kelp will keep nitrogen in soil for longer

He plans to target the market for bio-stimulants, to help restore soil carbon and develop stronger root systems. Kelp’s micronutrients, including zinc, copper and iodine, have beneficial effects on both plants and their root systems, making them more tolerant to extremes of drought and the freeze-thaw cycles that climate change brings.

Adding seaweed to soil has another potential benefit: GreenWave is involved in two research projects to measure what proportion of carbon and nutrients remain in the soil, as well as to explore the potential for kelp to trap nitrous oxide, a greenhouse gas that is 300 times as potent as carbon dioxide. As farmers try to sequester carbon through regenerative techniques the carbon builds up until it eventually causes the release of nitrous oxide, undoing some of the farmer’s good work. The theory is that adding kelp will keep the nitrogen in the soil for longer.

There are hopes that countries meeting for this autumn’ s biodiversity summit will reach agreement on the proposed goal of returning 30% of oceans to their natural state by 2030. Valuing blue carbon provides a road map for how they might get there.  

Angeli Mehta is a former BBC current affairs producer, with a research PhD. She now writes about science, and has a particular interest in the environment and sustainability. She also writes the Policy Watch column in Reuters Events Sustainable Business's monthly magazine, The Sustainable Business Review. @AngeliMehta.

Main picture credit: Lewis Jefferies/WWF


This article is part of The Ethical Corporation summer 2021 in-depth briefing on natural capital. Click on the cover to download your digital copy for free.

Ocean Conservation Trust  carbon sequestration  ocean carbon  biodiversity  salt marshes  mangroves  Marine Protected Areas  seagrass  seaweed  GreenWave  Verra  Oceans 2050  ocean regenerative 

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