Seagrass: The secret underwater ecosystem

Seagrass is the only marine angiosperm (flowering plant) in existence. This means it has roots, shoots and produces flowers and pollen whilst underwater in the sea. It is these properties that make seagrass meadows unique and incredibly important ecosystems. Yet, seagrass meadows are too often overlooked in favour of more charismatic habitats. Whilst they may not look like much on the surface, they provide a myriad of ecosystem services essential to the health of us and our planet (1).

We are currently in a global climate emergency as a result of increasing levels of atmospheric CO2. The pre-industrial global average concentration of CO2 was 260-280 ppm (2). Today, atmospheric CO2 has reached 415ppm. To put that into context, the last time CO2 concentrations were this high, sea levels were 50-65 feet higher than today and trees were growing near the South Pole. The Intergovernmental Panel for Climate Change (IPCC) predicts that CO2 levels will rise to 600ppm by 2060 (3).

Recently, marine ecosystems have become a topic of particular scientific interest for their ability to sequester carbon, known as ‘blue carbon’. Like any other plant, seagrasses are autotrophic meaning they utilise light energy and carbon dioxide to synthesise their own food for growth and reproduction. However, the amount of carbon that seagrass draws in usually exceeds their metabolic needs (4). A large proportion of this excess carbon is transported to the roots and rhizomes where it is stored and will eventually release into the surrounding sediment to be ‘locked in’. Additionally, seagrass attenuates wave energy meaning that suspended organic carbon in the water column settles in seagrass meadows where it is trapped. Unlike trees which eventually die and rot, thus releasing the carbon they have sequestered back into the atmosphere, this type of carbon storage can persist for millennia, making seagrass meadows highly efficient in locking carbon away long-term. Despite the worldwide loss of seagrass which has led to only 0.1% of the ocean floor now being occupied by it, these meadows are responsible for 10-18% of total oceanic carbon storage (5,6).

Figure 1 - Seagrass meadow under the surface of the water in Cambodia. Ben Jones.

As atmospheric CO2 concentrations increase, so too do oceanic CO2 concentrations, leading to a reduction in ocean pH and acidification. According to the IPCC, average ocean pH has decreased by 0.1 units since the start of the Industrial Revolution, this is expected to further decrease to 0.14–0.43 units by the end of the 21st century (7). As seagrass requires CO2 to photosynthesise and grow, these elevated levels of carbon have less of an effect on seagrass compared to calcifying organisms such as coral. Seagrass, therefore, acts as a buffer against ocean acidification by absorbing this CO2 and preventing a decrease in ocean pH. Coral reefs found alongside seagrass beds benefit from this process which protects them from bleaching and increases their calcification rates (8).

However, its ability to efficiently sequester carbon is not the only reason seagrass receives attention. Not only are they key in the fight against climate change, they are also vital in supporting rich levels of biodiversity. Seagrass is important in supporting commercially important species, such as cod, protecting our coastlines from erosion and producing oxygen and cycling nitrogen (9,10,11). As is widely documented in reports such as WWF’s Living Planet Report (2020), our planet is currently facing a biodiversity crisis. Seagrass meadows are highly productive and are thus incredibly diverse ecosystems, supporting a wide range of marine and terrestrial species.

Figure 2 - Green turtle feeding on seagrass (Halodule uninervsis), Abbi Scott via SeagrassSpotter.

What is Project Seagrass?

Project Seagrass is a UK based environmental charity dedicated towards conserving seagrass ecosystems to ensure they are protected now and for future generations. It’s estimated that every hour, 1 hectare of seagrass is lost globally (12). Coastal development, fishing activities, overexploitation and poor water quality are among the most significant factors contributing to widespread losses. Recovery from damage is often a slow process and depends upon the species of seagrass damaged, the extent of the damage and the sediment type (13). Water quality plays a large role in both decline and recovery (14). Whilst there is some interest in seagrass restoration, predominately for its role in blue carbon sequestration, the reality is that most people are unaware it even exists. Project Seagrass’ goal is to make seagrass a familiar habitat to everyone, so it is no longer overlooked but given the spotlight it deserves.

The charity has just completed the first-ever full-scale seagrass restoration project in the UK. In collaboration with WWF, Sky Ocean Rescue and Swansea University, they have planted more than 1 million seagrass seeds off the west coast of Wales, spanning an area of 2 hectares. In the UK, an estimated 90% of seagrass beds have been lost as a result of pollution, agricultural runoff, coastal development, and damage from boat propellers and chain moorings (15). This project is the start of restoring the UK's lost seagrass beds. It will pave the way to a large-scale nationwide restoration of seagrass, and will boost nature-based solutions to both the climate emergency and the biodiversity crisis.

One of the ways people can help seagrass conservation in the UK is through Project Seagrass’ global citizen science programme ‘SeagrassSpotter’. With SeagrassSpotter, anyone can contribute to marine conservation with just a few taps of their phone. If you’re out for a walk, on the coast or going snorkelling and happen across some seagrass then snap a photo and upload it. Every upload contributes to creating a comprehensive picture of seagrass meadows across the globe which might inspire practical conservation efforts that can be used to help protect them.

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