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Cambridge University Science Magazine

What is Seagrass?

Blanketing coastlines across every continent except Antarctica, 72 species of humble seagrass quietly engineer their environment to benefit coastlines, wildlife, and people around the globe. Interest in their existence has been largely unacknowledged until recent years when their abilities to draw in and store huge amounts of carbon from the atmosphere has attracted large scientific attention. Despite their vast global benefits, seagrasses remain poorly understood. Belonging to the same group of plants as grasses, lilies, and palms (monocotyledons), seagrasses have roots, leaves, and veins like their terrestrial counterparts, and function much in the same way. Like any other grass species, they photosynthesise and produce flowers and seeds. Of the 72 species, most live in depths of 1–3 metres where light levels are high, however one species (Halophila decipiens) is known to grow at depths of up to 58 metres. The number of species comprising a meadow varies greatly depending upon location, with the highest diversity recorded in tropical areas where up to 14 species can grow together.

What Makes Seagrass a Wonder Plant?

For over 10,000 years, seagrasses have been used by humans to fertilise fields, insulate houses, make bandages, thatch roofs, and even weave furniture. Beyond their direct use, seagrass meadows also support commercial fisheries making them one of the most valuable ecosystems on the planet with one hectare estimated to be worth over US$19,000 per year. 'In Australia, one hectare of seagrass produces 200 kg more fish than a hectare of empty seafloor' says Dr. Maria del Mar Palacios, a marine scientist from the Blue Carbon Lab, Deakin University.

Often referred to as ‘lungs of the sea’, seagrasses can generate 10 litres of oxygen per square metre through photosynthesis. They also play important roles in nutrient cycling and water filtration along the coastlines that they occupy. Their roots and blades trap and stabilise sediment, helping to improve water quality and absorb nutrients in runoff from land. The nutrients trapped in the soil by the seagrass can then be taken up and released by the seagrass plants themselves.

How Can Seagrass Help in the Fight Against Climate Change?

Another feature that makes seagrasses wonder-plants is their ability to mitigate the impacts of climate change. They can do this by dissipating wave energy and protecting coastlines from erosion, but one of their greatest weapons in the fight against climate change is their ability to store and sequester carbon. Dr. Palacios continues 'seagrasses, along with mangroves and tidal marshes, are efficient carbon sinks that are able to draw down atmospheric carbon dioxide back into the ground'. Despite only occupying less than 0.2% of the ocean floor, seagrass meadows account for 10% of all blue carbon stores and can capture carbon from the atmosphere up to 35 times faster than terrestrial forests In fact, it has been estimated that just one square metre of seagrass can sequester approximately 83 grams of carbon per year, the same amount emitted by a car travelling approximately 6,212 km.

'We know that seagrass meadows have fast soil carbon burial rates and capacity to lock the carbon in the soil for centuries and millennia,' says Dr. Palacios. However, there is very little understanding of the drivers that determine the ability of seagrass meadows to store carbon and additionally, how these can be affected by climate change. Scientists around the globe are working to address these critical knowledge gaps and further our understanding of seagrass ecosystems. For example, at the Blue Carbon Lab, scientists are working on projects to quantify the economic value of seagrass ecosystem services (including coastal protection, fisheries production, recreation, and carbon storage), to understand how the degradation of seagrass beds by sea urchins can affect blue carbon stocks and to compare the carbon decomposition rates of seagrass meadows across the world (TeaCompositionH2O program).

The potential of seagrasses to sequester blue carbon is gaining international attention as efforts to tackle the climate emergency become increasingly urgent. In 2016, the United Nations Paris Agreement entered into force, requiring the 189 countries that signed up to submit plans — referred to as nationally determined contributions — to tackle climate change. Of these 189 countries, 159 have seagrass on their shores. However, only ten countries included seagrass in their existing nationally determined contributions (NDCs). If contributions from seagrasses were included by all 159 countries, their impact could be significant.

What Are the Threats and Pressures Facing Seagrass Meadows?

At present, it is estimated that seagrass meadows are being lost globally at a rate of 1.5% per year — equating to approximately 2 football fields every hour. Following an assessment from the International Union for the Conservation of Nature (IUCN) Red List in 2011, it was found that nearly 25% of all seagrass species are threatened or near threatened. Both direct and indirect effects of human activities account for the largest losses of seagrass meadows. For example, anchors and propellers from boats can leave scars in the seagrass beds, which not only kills sections of the seagrass, but also fragments the habitat causing increased erosion around the edges. Seagrass meadows are also sensitive to reductions in light levels caused by sediment runoff from agriculture and land development. This sediment runoff often contains nutrients from fertilisers that cause algal blooms, which further blocks off light from the seagrass and reduces its growth. Despite their climate mitigation potential, seagrass meadows are also under great threat from climate change itself. A study published in the journal, Estuarine, Coastal and Shelf Science, has shown that as sea levels rise, it reduces the amount of light accessible to seagrasses. This reduction in light levels affects the photosynthetic capacity of the seagrasses, increasing their vulnerability to diseases and causing widespread loss of deeper seagrass meadows. Disease can lead to severe devastation of seagrass meadows as was seen in the 1930s when approximately 90% of all eel grass growing in North America was lost to a wasting disease.

What Does the Future Look Like for Seagrass?

Restoring seagrass meadows can offer some hope in combating their alarming rate of loss. However, seagrass restoration is difficult and expensive, and it has been found that in order to have a reasonable chance of success, this restoration must be huge in scale. The largest seagrass restoration project — Seagrass Ocean Rescue — is currently underway in the UK through a collaboration of scientists and conservationists at Sky Ocean Rescue, WWF, and Swansea University. Over the summer of 2019, one million seeds were collected from seagrass meadows around the UK and taken to laboratories at Swansea University, where they are currently being prepared for planting. The seeds will be put in hessian bags and planted across a 2 hectare area in Dale Bay, Pembrokeshire, which despite having suitable conditions for seagrass growth, has suffered widespread loss in recent history.

Should this project prove to be a success, it offers hope for future large-scale seagrass restoration projects. This coincides with the UN Decade on Ecosystem Restoration 2021–2030 which aims to draw together political support, scientific research, and financial muscle to scale up restoration of terrestrial, coastal, and marine systems on a global scale. It is evident that seagrasses offer great hope in the fight against climate change and the more we are able to understand these ecosystems, the better the chance we have of conserving, restoring, and maximising their future potential.

Ellie Wilding is a Geography MPhil student at Fitzwilliam College. Artwork by Eva Pillai.