The problem with blue carbon: can seagrass be replanted … by hand? | Carbon offsetting

Inside a wooden boathouse on the shore of Loch Craignish, a mass of fronds sit in two 1,000-litre tanks. The fronds are seagrass, and they are filled with seeds. Next to the tanks is a Heath Robinson-style series of pumps. The laboratory may be low-tech, but it is the headquarters of a pioneering, community-led climate experiment.The goal is to restore the loch’s once-thriving seagrass meadows. “We are trying to create a seismic change in the health of this sea loch,” says Danny Renton, of Craignish Restoration of Marine & Coastal Habitats (Cromach) and founder of Seawilding, a charity backed by people living in the surrounding villages in Argyll and Bute, about three hours’ drive from the Cop26 summit in Glasgow.“Seagrass sequestration of carbon is 35 faster than the rainforest. It’s an amazing carbon sink,” Renton says. “It draws carbon dioxide from the water as part of photosynthesis, and traps it in the mud.” Britain has no rainforests to soak up carbon dioxide emissions but it does have roughly 11,000 miles (17,700km) of coastline, dotted with salt marshes and seagrass. Both habitats, alongside tropical mangroves, are the best understood stores of “blue carbon” – the carbon held in marine ecosystems (as opposed to the “green carbon” of terrestrial habitats).locator map of Loch CraignishThese blue carbon ecosystems sequester up to 2% of the UK’s carbon emissions a year, mostly in soil and, if undisturbed, can store it for millennia. In Scotland, blue carbon stores sequester 28.4 MtCO2e (tonnes of carbon dioxide equivalent) a year, about three times more than Scotland’s forests combined.But while rainforests, peatlands and other habitats on land are well known for their importance in the climate crisis, coastal wetlands remain overlooked. The Intergovernmental Panel on Climate Change (IPCC) includes them in national greenhouse gas inventories, but they are also among the most threatened ecosystems on the planet. A study earlier this year concluded that 92% of the UK’s seagrass has been lost in the past two centuries, with 39% disappearing just since the 1980s, thanks to pollution from industry, mining and farming, along with dredging, bottom trawling and coastal development. The few patches of seagrass that remain in Loch Craignish are fragmented.“Loch Craignish is very typical,” says Renton, examining a map of the 10 remaining meadows. “Most lochs have had damage to the seabed by bottom trawling and aquaculture, with pollution, like nitrates, entering the water.”No one was looking at the carbon in marine sediments … they are poorly mapped, poorly understood and not protectedRichard Unsworth, Project Seagrass The loch once had beds of huge native oysters and scallops. Its seagrass meadows provided nurseries for herring and cod. Now, most of that has gone.Indeed, seagrass, which covers about 0.1% of the ocean globally but provides 18% of its carbon storage, is shrinking by 7% each year – equivalent to a football pitch worth vanishing every 30 minutes, according to a 2020 UN report.Renton’s aim is to enhance the meadows by half a hectare this year. It is part of a wider rewilding project that has reintroduced 220,000 oysters to the loch, to provide a habitat for wildlife and act as a water filter to allow seagrass to photosynthesise better. As well as climate sinks, wetlands protect coasts from storm surges and increase biodiversity. Underwater seagrass meadows, for instance, can also harbour up to 40 times more marine life than bare seabeds.Restoring seagrass meadows is labour-intensive and complex work. For Project Seagrass, volunteers swam underwater to collect seeds by hand. Photograph: Lewis JefferiesBut restoring seagrass, even by half a hectare, is labour-intensive and complex work. In August, 40 volunteers in wetsuits and snorkels swam over the remaining meadows to collect fronds full of seeds, which are now maturing in the tanks. After maturity, hundreds of thousands of seeds were filtered through pumps, then placed in tiny hessian bags, 50 in each, and planted, at one-metre intervals on the bed of the loch. The bags, weighted with sand to make them sink, help protect the seed from predators.With the survival rate of seagrass seeds at about 10%, it could take between five and seven years before the new seagrass growth connects with the old growth to form a meadow. “That’s why it’s so complex,” Renton says.With the pressure on countries and companies to reach net zero, blue carbon is gaining international attention. Projects, mainly involving mangroves, are using “blue carbon credits” to fund their work. Renton has been approached by companies keen to fund the Loch Craignish project in exchange for carbon credits.But he says it is “too early” to consider quantifying, let alone verifying, how the restoration work affects carbon storage. “We don’t have a verifiable metric to offset carbon emissions,” he says. “We know carbon sequestration can be extraordinarily effective, but there’s a lot we don’t know.”Estimating how much carbon seagrass restoration can soak up is just one problem. Labour-intensive seagrass restoration is expensive and has a lower success rate compared with mangroves and salt marshes, according to recent research.The survival rate of seagrass seeds is low and it may take up to seven years before new growth at Loch Craignish connects with old patches to form a meadow. Photograph: Lewis JefferiesAlthough Project Seagrass, a marine conservation charity that set up the Loch Craignish laboratory, aims to mechanise the more labour-intensive aspects, including gathering the seeds, the report in the journal Nature concluded that large-scale restoration of seagrass meadows is “feasible” but “remains challenging”.Richard Unsworth, Project Seagrass director and associate professor at Swansea University, acknowledges that the science behind blue carbon is still developing. But he says: “It is an important part of the complex jigsaw to fight climate change.”In July, more than 7,000 marine and climate scientists, human rights experts and others urged world leaders to include legally binding targets for blue carbon in their emission-reduction plans.There is no time to lose, Unsworth says. “Up until a few years ago, no one was bothering to look at the amount of carbon in marine sediments. There is not much data in Europe, let alone the UK,” he says. “They are poorly mapped, poorly understood and not protected.”For now, he is doing what he can. A consortium, including Project Seagrass, WWF and Swansea University, has been planting more than 750,000 seagrass seeds in Dale Bay in Pembrokeshire, with the aim of eventually restoring 3,000 hectares (12 sq miles) of meadows in the UK by 2030. That area of seagrass could suck up the emissions of 3,000 small cars, create a habitat for billions of small animals, support 4,700 more fish than bare sediment, and increase sediment strength tenfold to prevent erosion.“It’s a win-win,” says Unsworth. “It has to be part of a vision to improve nature as a whole.”Tomorrow: the future of blue carbon

Source: https://www.theguardian.com/environment/2021/nov/05/seagrass-meadows-could-turn-tide-of-climate-crisis-aoe