Rewilding Under the Waves

The last five years have seen a proliferation of seagrass restoration projects in the UK. Ash Brennan writes about the motivations and complications of these efforts.

A tropical seagrass meadow. Image Credit: Benjamin L. Jones on Unsplash.

I felt a little silly as I waded into the frigid water of Loch Craignish. I was wearing a wetsuit, and my mask and snorkel dangled useless from my hand. The muddy floor of the loch slopes only gradually away from the shore. Soon I was about 50 meters out and still the water was only waist deep, with nothing but silty mud to see at my feet. Was I even in the right place?

Seagrass was my quest that day. This amazing plant is a marine angiosperm (meaning that it flowers) which made the move from the land to the sea some 100 million years ago. The last five years have seen a proliferation of seagrass restoration projects in the UK such as Seawilding in Loch Cragnish, Restoration Forth in the Firth of Forth, Seeding Change Together in Cornwall and Seagrass Ocean Rescue: North Wales. These efforts, and many more worldwide, aim to use this habitat to sequester carbon, support biodiversity and protect against coastal erosion. The UK is ideally situated to capitalise on the benefits seagrass meadows have to offer. It is also an area where individuals can easily get involved, with most projects being volunteer-supported.

Much of the excitement surrounding seagrass has been driven by its excellent carbon sequestration capacity. Seagrass ecosystems bury carbon at a rate that is 30 to 50 times faster than terrestrial forests and, combined with other vegetated marine ecosystems (mangroves and tidal marshes, together labelled blue carbon), contributes 50% of marine carbon burial, despite occupying less than 1 % of the ocean surface.

Worldwide seagrass distribution indicated by green dots. Image Credit: Cullen-Unsworth L, Jones B, Lilley R and Unsworth R on Wikimedia Commons.

It isn’t just conservationists who have taken note of these impressive statistics. Increasingly, governments are incorporating blue carbon into their Nationally Determined Contribution submissions under the Paris Agreement. There has also been increased interest from the business sector for the potential of blue carbon offsetting schemes.

However, some researchers have expressed concern over the disconnect between expectations and realities of blue carbon sequestration. It cannot fully offset current rates of fossil fuel emissions. Additionally, carbon is only sequestered for as long as the habitat survives. For instance, a marine heatwave in Western Australia’s Shark Bay in 2010-2011 resulted in a 22% loss of seagrass and the emission of an estimated 9 million tonnes of carbon dioxide. Seagrass meadows are one of the best natural ways to sequester carbon which we know of, but they are no magic bullet.

Back in Loch Craignish, I finally put my mask on and plunged into the 9℃ water. Gliding along the surface, each breath through my snorkel came faster. Within a minute, I had a headache from the cold. All I could see was the silty bottom. Just as my search began to appear futile, I glimpsed a dead leaf of seagrass resting on the silt. I knew I must be close. The visibility was only a few meters and I kept having to look up above water to orient myself. I swam toward deeper waters, and all of a sudden I was gliding over the top of a swaying meadow. This was no tropical reef, but beauty and biodiversity are there for those who search for it.

A Zostera marina meadow. While seagrass has gained publicity for carbon sequestration, the ecosystem services and habitat it provides are also of great importance. Image credit: Sofia Sadogurska on Wikimedia Commons.

Carbon sequestration isn’t the only benefit of seagrass restoration. It also provides a habitat for many marine organisms. Most UK-based seagrass projects plant the species Zostera marina, commonly known as Eelgrass. This is a subtidal seagrass, living down to a depth of 4 meters in protected waters. These meadows provide an important habitat for invertebrates and fish, and function as a nursery for juvenile fish. In Loch Craignish, seagrass meadows support 68% of biodiversity, despite occupying only 0.4 % of the area.

Broadening the scope to look at the entirety of the UK gives an idea of the potential seagrass restoration holds. As an estimated 92% of seagrass meadows in the UK have been lost in the last century, the potential biomass and biodiversity opportunities for coastal waters are immense. Additionally, species supported by seagrass habitats include commercially important ones such as cod, plaice and herring. This means that restoring seagrass meadows in coastal regions may also provide a measurable economic impact on local communities.

As well as improving fish stocks, seagrass restoration can also reduce coastal erosion. Meadows of seagrass create regions of calmer water where sediment can settle to the sea bed. Increased sedimentation helps to counter erosion. This can create a positive feedback loop, with clearer waters allowing more sunlight, which is essential for seagrass growth, to penetrate the water. Seagrass leaves also create drag on underwater currents and waves, reducing their energy and protecting coasts.

View over Loch Craignish. Much of the suitable coastline in the UK for seagrass restoration is used by multiple groups, for this reason, any projects need community support. Image credit: Gordon Hatton on Wikimedia Commons.

With so many benefits, planting seagrass may seem like a win-win for a place as rich in coastline as the UK. However, restoration is more complex than just transplanting rhizomes (think roots) or spreading seeds. These plants can only thrive if the factors which caused the loss of British seagrass meadows in the first place are reduced. Disease was responsible for much of the loss in the 1930s, but other factors include immediate threats such as boat anchors, dredging and some fishing practices. What we do on the land also has an impact, including agricultural and industrial pollution, and sewage discharge.

For this reason, seagrass restoration projects will always need the cooperation and support of the local community. In Loch Craignish, I spent some of my afternoon volunteering with Seawilding. I was there with perhaps ten others, whilst our task was to process the rhizomes ready for planting. This involves attaching washers to the plants to weigh them down and tying them in groups ready to be embedded in the sea floor. It’s a fiddly task, but one which goes fast with many hands and plenty of conversation. There were a mix of locals and people coming to volunteer for days or weeks at a time. I got the sense that this project will prosper because so many people from such diverse backgrounds care so much about it. It’s only with this broad involvement, as well as clear benefits to both community and environment, that lasting changes can be made.

About the author: Ash is a second-year PhD student in Chemistry and Physics at the University of St Andrews. He has a strong interest in science communication and conservation and dreams of one day combining all these disparate fields. You can find out more or get in touch on his LinkedIn.