Seagrass meadows: creating underwater soil

Trapping tiny particles of organic matter to prevent greenhouse gas release
31 August 2021

Interview with 

Leanne Cullen-Unsworth, Project Seagrass & Loveday Trinick, Ocean Conservation Trust


The surface of the ocean.


Kelp forests and seaweed farms aren’t the only way our oceans can trap carbon. Seagrasses are remarkable plants that form underwater grassy meadows, and are able to trap tiny particles of organic matter and form a stable, carbon-rich sediment that prevents greenhouse gases from being released into the atmosphere. Sally Le Page chatted with Leanne Cullen-Unsworth, the director of research at Project Seagrass to find out more about this unassuming ecosystem...

Leanne - They're a plant that lives in the sea and they are a plant that creates this incredible habitat that supports very high biodiversity. So it supports lots of animals.

Sally - What does it look like?

Leanne - It just looks like a terrestrial grass, really, so a grass that you find on land. They flower and they produce seeds in the same way that land grasses do.

Sally - And why are seagrasses so important?

Leanne - They're a really important habitat for marine animals and birds as well use seagrass to forage in. They're also super important for things like carbon sequestration. And at the moment, there's a lot of interest in seagrass meadows to help mitigate the climate issue.

Sally - How do seagrasses store carbon? Because we've just been hearing about kelp, which draws down carbon, but then they can't hold onto it as much. Is it the same with seagrasses?

Leanne - It's not the same with seagrasses. Seagrasses have got the capacity to store large amounts of carbon. They store it within their own leaves, their roots and their rhizomes under the sediment, but they also then pack it down and store it underneath the sediment. So their capacity for storage is really huge - they can keep storing the carbon underground. As long as those habitats are maintained, then that carbon is locked in under the sediment. They're also really important in the nitrogen cycle, so they cycle other nutrients as well. They're very important for coastal protection - they hold the sediment together and stabilise coastlines and help protect our coasts from the coastal erosion. But they are important as well for commercial fisheries - they provide nursery grounds for a lot of the big offshore commercial fish species, cod for example. They're important for so many reasons.

Sally - How are seagrass meadows faring around the world?

Leanne - Seagrasses are struggling. Across their range they face threats from poor water quality, from physical damage, from inappropriate boating activity, anchor dragging, insensitive moorings, propeller damage, bait digging.

Sally - So things that are physically digging up the grass?

Leanne - Yeah. The UK has lost a lot of its seagrass. Over the past 40 years it's lost around 50% of its seagrass meadows. And we know that the figure could be up to around 92% loss over the last 100 years.

Sally - That's huge. Why have we lost so much?

Leanne - It's huge. It's huge. Across Europe in the 1930s, there was a widespread wasting disease that has been blamed basically for a huge wipeout of massive areas of seagrass, but that also coincided with industrialisation and the peak of the worst water quality that we've had. And so it's not totally clear. However, on a positive note, and particularly in the UK where we've lost huge amounts, there's been a lot of effort put into improving water quality around our coasts. And so we are in a position now where we can start to think about restoring large areas of seagrass.

Sspeaking of restoration, a new project called Reducing and Mitigating Erosion and Disturbance Impacts Affecting the Seabed, or ReMEDIES for short, has recently received £2.5 million of funding from the EU and Natural England, to both replant new seagrass meadows along the south coast of England and protect existing meadows by developing and promoting seagrass-friendly mooring systems for boats. But how exactly do you plant a meadow underwater? Sally Le Page spoke with Education Officer Loveday Trinick from one of the project's partners, The Ocean Conservation Trust, to find out just what it takes...

Loveday - It starts by collecting a lot of seeds. The seeds are not the primary way this plant reproduces, it reproduces mainly through its roots, so taking the seed doesn't impact the bed. So we go to these beds and we collect the seeds. This is done by scuba divers.

Sally - What do the seeds look like?

Loveday - They're tiny, they're weeny. They're like a grain of rice.

Sally - I suppose rice is also a species of grass, so that makes sense.

Loveday - Yeah, similar scenario. And then it's the process of bringing all of those seeds back to the National Marine Aquarium, which is where The Ocean Conservation Trust is based. We have a purpose-built lab that was funded through the project to look after those seeds, care for them, nurture them. And then some of those are planted straight into little seeding units, which are made of a hessian bag.

Sally - Why do you have to put the seeds in bags?

Loveday - That's really because they would get eaten.

Sally - So is it like a seed bomb? Those little parcels of clay that you chuck with wildflower seeds, but instead you're using seagrass seeds.

Loveday - It's exactly that. That's entirely what it is.

Sally - And then once you've put all of these seeds in the bags, how do you plant them?

Loveday - It's a really fun trip actually. We take the thousands of bags out on the barge and we've got two big pipes on the back of the boat and we go along very slowly. We just drop the bags down the pipes.

Sally - And how big a project is this?

Loveday - So the goal of the restoration part of ReMEDIES is to restore eight hectares of seagrass meadow.

Sally - How many seed bombs does it take to plant eight hectares of seagrass meadow?

Loveday - Lots. We're collecting 3.2 million seeds this season.

Sally - Wow. Who's counting?

Loveday - One at a time. 1, 2, 3..! And they are growing. We've had monitoring dives on that site and we've seen little plants growing in our seeding bags, which is very exciting. It's lovely to see.


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