For decades, researchers thought seagrass at the Virginia Coast Reserve (VCR) LTER was locally extinct, and hypothesized that local conditions no longer allowed seagrass to grow. But in 1999, a local made a startling discovery: a small patch of seagrass thriving in a small bay. A successful restoration project ensued—those seagrass beds now line over 10,000 acres of seafloor.
That restoration project caught the eye of the carbon offset market. Seagrass beds store a lot of carbon, and the new carbon stored from a successful restoration project could be sold as an offset. The VCR project, conducted in partnership with the Nature Conservancy, showed that these projects might have potential in the offset market—but questions remain as to how much carbon these large scale seagrass restoration projects can actually store, and how much revenue they might generate through offsets.
A new paper uses data from the Virginia Coast Reserve’s seagrass restoration initiative to show how different management approaches affect how much carbon is stored in seagrass beds. The study shows that the amount of carbon sequestration or avoided emissions from a seagrass management project varies widely as a result of which restoration or conservation techniques are used. The findings suggest that only certain types of projects can generate enough revenue to make the expensive accreditation process worthwhile—important context as seagrass restoration gains momentum across the globe.
It’s about the data
LTER sites document how ecosystems function by collecting data on carbon cycling, nutrient flows, and other key processes over long timescales. The VCR LTER started collecting these kinds of data across the reserve just before 1990, when today’s seagrass meadows were just barren seafloor. Collection persists to this day, cataloguing the fate of carbon and several key nutrients through the duration of the restoration project and beyond.
Those high quality data proved essential for this project. Melissa Ward, lead author on the paper and an expert in blue carbon, wanted to understand how different types of seagrass restoration might change the amount of carbon a bed stores. from the University of Oxford, her work was designed to help other countries trying to develop domestic carbon credit markets, such as the United Kingdom, understand the potential for seagrass restoration projects to generate revenue by selling carbon offsets.
With real investment in seagrass restoration likely, Ward needed data that would be as accurate as possible. “I wanted to know not just how much carbon a seagrass meadows sequesters, but how much carbon does it gain after you do a restoration project?,” she says.
Ward looked for publicly available datasets that fit the bill. She needed a few key parameters to fully understand the fate of carbon in the system: measurements of how much carbon is stored in seagrass biomass and in the sediment, and also how much carbon is emitted from beds as methane or nitrous oxide. To get an accurate carbon budget, she needed all those measurements to come from a single area before and after restoration.
Ward found one dataset that met her criteria: the Virginia Coast Reserve LTER dataset. Though she knew of the site, she’d never worked with them before. Yet she found that those at the LTER were eager to collaborate. “We could provide the context of the place, the way the data were collected,” and other important information, says Karen McGlathery, lead PI of the VCR LTER and key collaborator on the project. “I was helping provide the foundation for the work that [Ward and her colleagues] were doing. They took that and then did the modeling.”
Developing a model
Ward wanted to compare several different management strategies for restoring seagrass to see how they change potential carbon benefits, and, ultimately, impact the potential revenue generated by carbon credits. She used the VCR LTER’s seagrass data to parameterize a model that compared four restoration strategies.
In the first, seagrass seeds are collected from meadows in the summer and dispersed into new habitat. This intervention is the least labor intensive, but seeds and young shoots are more vulnerable to grazing and other environmental stressors.
The second scenario modeled transplanting adult seagrass shoots into suitable habitat. This strategy, which is the most common strategy used in the restoration project at the Virginia Coast Reserve, germinates seeds onshore and replants them once the plants are less vulnerable.
Some projects also add sediment to offshore areas that would otherwise be too deep for seagrass to survive. While this greatly increases the suitable habitat for seagrass, it also carries it’s own carbon implications, as disturbed sediments release carbon that might otherwise be stored for a long time. Ward’s third scenario modeled infilling nearshore areas then transplanting adult shoots onto the new habitat.
Fourth, Ward wanted to compare restoration initiatives to carbon offsets generated from conserving a meadow that would otherwise be dredged. Much seagrass loss is a result of trawling or other dredging practices; closing a meadow to fishing would preserve the carbon that’s stored in the sediment without the need for any restoration at all. Ward was interested in how a low impact conservation plan might compare to labor intensive restoration.
Ward ran her model under a number of different possible scenarios, trying to understand the range of potential carbon accumulation for each intervention that might vary between different meadows (for example, if a meadow accumulates sediment at a high or low rate) or under different circumstances (for example, the depth at which fisheries dredge and disturb sediment).
Pick your approach
Ward found that interventions had a huge range in the amount of carbon they might store and the amount of revenue they might generate from credits. At the low end, a seeding project might bring in a couple hundred dollars per hectare. A transplant project that includes transporting sediment might generate ten times that amount, but conservation outpaces all the other restoration scenarios, with potential to generate up to $15,000 in credits per hectare. “A meter of seagrass sediment could be 500 years of organic carbon accumulation,” says Ward. “Protecting a meadow and its top sediment from being lost could have the jump on the restoration project by 500 years.”
That wide range means that organizations considering applying for credits need to think hard about their individual project. Accreditation is expensive. A seeding project might not generate enough revenue to justify the cost of accreditation, but a conservation project might. “I think part of the motivation for doing the work is to try to get people to think early about whether or not carbon credits are a feasible or reasonable approach,” says Ward. “Carbon markets often work with projects that are really large, like 50,000 hectares. But there’s not a lot of seagrass meadows needing restoration globally that are that large.”
Collect data now, reap rewards in the future
LTER sites collect data and run experiments to understand how ecosystems function. The true value of those projects might not appear until much later. This project underscores that point. By documenting carbon across the ecosystem thirty years ago, the VCR LTER laid the foundation for them to be the most important player in the seagrass blue carbon story. Yet when those data were first collected, carbon offsets didn’t yet exist.
The VCR LTER is on the precipice of becoming the first seagrass restoration project to receive a carbon credit on the voluntary carbon market. “It’s been a really hard and long process,” says McGlathery. “We’ve been at this for four years now. To do it right, to do it well, you need a lot of data.” Similarly, Ward has plans to propose seagrass restoration as a way to meet California’s climate goals—scoping restoration initiatives through the lessons learned from this work.
Seagrass is poised to become a key player in the blue carbon world. We can thank the VCR LTER’s data for that.
This work was supported by Blue Marine Foundation and the Ofwat Innovation Fund, with partners at Project Seagrass and the Oxford Agile Initiative.
by Gabriel De La Rosa
