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Blue Carbon Part 2: how do we know that seagrass can help combat climate change?
If you’re familiar with the work of the Ocean Conservation Trust – and particularly our flagship habitat restoration program, Blue Meadows – you’ll know that one of the biggest benefits of conserving seagrass meadows is that they can help us tackle climate change. This is because seagrass meadows (and the sediment upon which they sit) act as highly effective carbon stores. This is part of a concept that’s commonly referred to as Blue Carbon.
For those unfamiliar with Blue Carbon, go back to the first blog post in this series, which explains the term. For those already initiated, read on!
But how do we know this? Through an ever-growing number of research projects conducted by marine biologists, including the Ocean Conservation Trust’s very own Jazz Gardiner. We sat down with Jazz, who now works as a seagrass aquaculture technician for Blue Meadows, to break down her research into this very topic.
How Jazz found her way to seagrass
Back when she was studying for her degree, Jazz was looking for a topic for her dissertation and was initially non-plussed when she was assigned seagrass as a focus.
“If I’m being completely honest, I was kind of randomly given seagrass as a topic. And I was thrown off by it, because this was before I did my internship with Blue Meadows and I didn’t know anything about it,” she said. “I did know it could sequester carbon, but I didn’t know the full extent of that.”
Jazz was initially more interested in how seagrass captures microplastics, but then she started working with Blue Meadows and immediately her focus shifted. She said it was an eye-opening experience.
When Jazz started her internship in 2023, the Blue Meadows team was conducting a study in St. Austell Bay, using vibrocores (essentially a tube that is vibrated into the seabed to collect a column of sediment) to test the carbon levels in sediment samples.
St Austell Bay features Cornwall’s largest known seagrass meadow at over 350 hectares, making it one of the largest within the UK. Nine three-metre-long vibrocores were extracted from the meadow that stretches across the bay, and Jazz’s assignment was to help analyse the samples and process the data.
“I was quantifying carbon and looking at whether the particle size affected the level of carbon in the sediment,” she said. “The Cornwall Wildlife Trust had also mapped the density of the entire meadow. I used that data to analyse how seagrass density affected the carbon levels. It ended up being an investigation into how sediment characteristics and meadow morphology affected the amount of carbon stored.”
What the data illuminated
The result surprised her. Jazz’s data revealed that St Austell’s seagrass meadow stored approximately 160 mega grams of carbon per hectare in the top 100cm of sediment. That, she explained, was “quite a lot more” than a lot of similar studies previously conducted had found.
“It was close enough within the range that it seemed it could be accurate, but it was higher than other seagrass meadows in the southwest,” she said. “That was nice to see, because St Austell is a very big and robust meadow. It suggests that, if we have really healthy seagrass meadows, they can store more carbon than we previously thought.”
She also found that there was not a clear correlation between plant height and level of carbon stored. This finding, while surprising, was also positive; it suggests that even smaller plants were just as effective in carbon sequestration. That said, the sample size for plant height and density was extremely small with only 13 replicates (one for each core coordinate) so Jazz says she couldn’t draw a fully robust conclusion from this.
Additionally, Jazz had assumed that most carbon would be consolidated in the top 100cm of the three-metre sample, because that was typically where the seagrass roots would stretch to.
“I found that the carbon was pretty consistent down the entirety of the three-metre core,” she said.
Although this contradicted Jazz’s original hypothesis, it was once again a positive finding, reflecting how seagrass meadows are able to bury carbon deep within the sediment beneath them. Due to its depth, this stored carbon is less likely to be disturbed and reintroduced into the environment.
The main takeaways, Jazz said, are that a well-developed and thriving seagrass meadow can store more carbon that those that are smaller and struggling, and that seagrass meadows can create a long-term stable carbon store and prevent carbon from re-entering the environment. In short: the healthier the seagrass meadow, the more effective it is at storing carbon.
“It really shows how useful seagrass can be when it’s a large and healthy meadow,” she said. “St. Austell is a good goal for us to be getting other meadows to that state. It’s a really nice case study of just how much carbon can be stored if we achieve that.”
Beyond Carbon
In the third and final part of this blog series, we’ll be looking at why Blue Carbon is only part of the picture, and why the OCT takes a more holistic approach to seagrass and its benefits. Stay tuned for a blog post all about seagrass as a biodiversity powerhouse!
More blog posts
Blue Carbon: is this the Ocean’s secret weapon in fighting climate change?
What do people know about seagrass?
Quite a bit, as it turns out…
