Small but mighty: Study shows S'pore's seagrasses rank among top global carbon stores per hectare

Straits Times

01-06-2025

Seagrass meadows in the Central Indo-Pacific region, which includes Singapore and Malaysia, can lock up an average of 86 metric tonnes of carbon per hectare – equivalent to the annual emissions from 22 cars. PHOTO: NATIONAL PARKS BOARD
SINGAPORE – Singapore's seagrass meadows may be small, but the remaining patches of the ocean's only flowering plant here are punching above their weight when it comes to their ability to store carbon, a new study has found.
Seagrass meadows in the Central Indo-Pacific region, which includes Singapore and Malaysia, can lock up an average of 86 metric tonnes of carbon per hectare – equivalent to the annual emissions from 22 cars. This is more than twice the global average of 38 metric tonnes per hectare, the study found.
The research, which quantified carbon stocks in seagrass ecosystems globally, was done by researchers from academic institutions and organisations across the globe, including the National University of Singapore (NUS) and the Singapore-based International Blue Carbon Institute.
Published in research journal Nature in May, the study identified the lagoons and small deltas of the Indo-Pacific as being among the top five regions globally with the highest organic carbon storage potential, alongside the Mediterranean, Colombia, Florida and South Africa.
Organic carbon refers to carbon that comes from living things.
While seagrass meadows are known to be natural carbon sponges that can soak up planet-warming carbon dioxide, there is still much uncertainty and knowledge gaps about these ecosystems, making their conservation challenging.
The aim of the study, said the researchers, was to provide policymakers with greater understanding about the factors that affect the carbon storage potential of seagrass meadows.
This could help guide the 'development, integrity and reliability of climate change policy, and financing to support seagrass conservation and restoration', they said.
What are seagrasses?
Seagrasses are aquatic plants that take in carbon dioxide through photosynthesis. This carbon is locked up in the plants' roots and underground stems, and the soil they grow in.
Being anchored to the seafloor by their roots, these habitats can also 'trap' carbon-containing particles, such as bits of dead plants or animal faeces, carried by the currents, said Dr Siti Yaakub, senior director of the International Blue Carbon Institute and a co-author of the paper.
As more of these particles eventually settle on the seabed, they get trapped in structures of the seagrass. As these layers accumulate, the carbon gets buried by new layers of sediment and moves deeper into the earth, said Dr Siti.
Burrowing animals in the seafloor, such as clams and worms, also help bring the carbon to deeper layers of the sediment, she added.
These characteristics make seagrass meadows a promising ally in humanity's fight against climate change. A separate study, also published in Nature in 2021, estimated that seagrasses are 40 times more efficient at capturing organic carbon than land forest soils.
However, different environmental conditions and types of seagrass could affect how much carbon is stored at a particular site , resulting in a large variation globally .
For example, Dr Siti said seagrass meadows near big rivers tend to contain a higher carbon content in the seabed due to the discharge of river sediment and carbon-rich organic matter.
Heavy rain can also affect seagrass growth, as the rainfall could stir up sediment and prevent sunlight from reaching the plant.
Similarly, high levels of nutrients in waters can cause algae blooms that reduce water clarity. Nutrient levels can go up when fish feed and waste materials are released from fish farms into the sea.
The study is one of the first to map out the potential of seagrass meadows around the world to act as natural sponges that can soak up planet-warming carbon dioxide, Dr Siti said.
S'pore's underwater carbon banks
Through an analysis of 2,171 soil cores retrieved from seagrass beds around the world, the researchers found that the region's seagrass meadows had one of the world's highest carbon stocks on a per-hectare basis.
'As a region, we do have a slightly higher-than-average carbon stock because there is a lot of seagrass in shallow sheltered coastal waters that are also adjacent to river systems, so there's a good source of allochthonous (external) inputs,' said Dr Siti.
'If we lose all this seagrass with high carbon stocks, we will not be able to recover the carbon that is lost. It will take many decades to restore it.'
Singapore is home to 12 of the 72 known species of seagrass globally, with some of the country's largest meadows found at Chek Jawa on Pulau Ubin, Pulau Semakau, Cyrene Reef and Changi Beach, said Dr Siti.
Seagrass meadows in the Republic also act as shelters and nursery areas for numerous animals like seahorses, and serve as the primary food source for sea turtles and dugongs.
The biggest threats to seagrass meadows in Singapore are coastal development and high sedimentation. Large amounts of sediment in the water can result in low light levels reaching the plant.
A healthy ecosystem should have a balance of both sediment input and output to avoid erosion – which may result in the unearthing of buried carbon or burying of seagrass. But changes to coastlines from land reclamation projects may alter the dynamics of water and sediment movement.
'If you change the coastal environment such that it increases the rate of erosion, and block off sediments coming in from rivers, it becomes a double whammy, because sediments are now being washed away with no supply to replenish these systems,' Dr Siti noted.
Efforts to protect seagrass meadows have been ramped up since 2007, when the National Parks Board (NParks) began working with Seagrass-Watch – the world's largest scientific seagrass assessment and monitoring programme – to monitor the health of seagrass on Singapore's shores.
Singapore in 2024 also launched its first seagrass restoration project, in an effort by NParks and NUS. The project was fully funded by OCBC Bank.
Dr Samantha Lai, deputy director of NParks' National Biodiversity Centre Coastal and Marine branch, said the board has also been working with NUS and NParks volunteers on separate projects that aim to study the health and conservation needs of seagrasses.
Moving forward, Dr Siti said that a better understanding of the seasonal growth cycles of seagrass can help with better planning during dredging and reclamation activities to minimise stress on the seagrass population.
'For example, if there is dredging involved, there should be measures taken to contain the sediment plumes so that they don't spread to areas with seagrass and stress them out even more,' Dr Siti said.
At the global level, the study provided some insight into the carbon storage potential of seagrass meadows in the region, she added.
However, the large variation also shows that countries need to more closely study the quantity of carbon being sequestered in their own meadows. This is especially if they intend to tap the carbon storage abilities of these habitats in their plans to tackle climate change, she said.
'The issue with seagrass is that it's not very well mapped. For example, estimates of seagrass cover in Indonesia range between 300,000ha and three million ha, which is a huge range. This results in big uncertainty in calculating the climate mitigation potential of seagrass with respect to their climate commitments. But there are efforts under way to better understand seagrass cover and map them globally ,' Dr Siti said.
Singapore can contribute to deepening this understanding, said local experts.
Dr Ow Yan Xiang, a seagrass scientist and senior research fellow at the St John's Island National Marine Laboratory who was not involved in the latest research, said: 'This study shows that Singapore has one of the highest data density and sampling efforts for seagrass carbon stocks. Also, our seagrass population is holding steady, despite the region's seagrass declining at an average of 4.7 per cent per year.'
The high data density refers to Singapore contributing a high amount of data, in proportion to its small coastline.
Dr Siti said: 'This places Singapore at the forefront of carbon mitigation efforts because one, we have influence in climate negotiations, and two, we have the resources and expertise to facilitate or help other countries deploy natural climate solutions.'
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