Riiven Wild
How Seagrass Meadows Lock Carbon Into Shallow Seabed Mud
In Shark Bay, ribbons of seagrass have spent thousands of years burying carbon in the seabed, and one hot summer showed how quickly that store can come loose.
The Scene
Low tide on the Faure Sill, inside Shark Bay on the coast of Western Australia. The water is warm, shallow and very clear. Below it lies a meadow of seagrass, long green ribbons leaning with the current. Small fish thread between the blades. Fine sediment has settled around the roots, trapped by the leaves as the water slows and drops what it carries. Each blade is taking in light, and each root is holding down mud that has been building, layer by layer, beneath the bay. The meadow here sits on one of the largest stores of seagrass carbon that scientists have measured anywhere on Earth.
The Creature
Seagrasses are not seaweeds. They are flowering plants, relatives of land grasses and lilies, whose ancestors returned to the sea. They have true roots, leaves and flowers, and they pollinate underwater. In Shark Bay the meadows spread across the shallow banks of a World Heritage area, forming some of the most extensive seagrass beds known. A seagrass meadow does two things at once. Above the seabed, its blades photosynthesise, drawing carbon dioxide from the water and turning it into plant tissue. Below the seabed, a dense mat of roots and rhizomes (creeping underground stems that tie neighbouring plants together) anchors the meadow and holds the sediment in place. The leaves also slow passing water, so particles drift down and settle among the stems. Over long stretches of time, dead roots, leaves and trapped organic matter are buried in this sediment. Low in oxygen, the buried layers decompose very slowly. Scientists call this kind of store "blue carbon", carbon held by coastal and marine ecosystems.
How It Works
In a 2012 study, Fourqurean and colleagues measured carbon, nitrogen and phosphorus in subtropical seagrass meadows of Florida Bay and Shark Bay. Across the measured parts of the ecosystem, Shark Bay seagrass sites stored an average of 243.0 megagrams of organic carbon per hectare. Most of that carbon sits not in the living plants but in the sediment beneath them. Arias-Ortiz and colleagues later scaled this up to the whole bay. In their 2018 study, Shark Bay seagrass sediments held an estimated 55 ± 3 teragrams of carbon in the top metre, based on an estimated seagrass extent of 4,300 km². A teragram is a million tonnes. That store stays locked away only while the meadow stands. The roots bind the sediment, and the low oxygen keeps microbes from breaking down the buried organic matter. Remove the plants and the sediment can be stirred up and exposed to oxygen, where microbes turn the stored carbon back into carbon dioxide.
The Ripple
In the summer of 2010 to 2011, a marine heatwave pushed water temperatures in Shark Bay well above normal. Arias-Ortiz and colleagues estimated that the heatwave damaged about 36% of the bay's seagrass meadows. Seagrass is shelter and food for much of the bay: the shallow banks support fish, invertebrates, and grazers such as dugongs and green turtles that feed directly on the meadows. When the blades thinned and died, that habitat thinned with them. The seabed changed too. Without living roots to hold it, sediment that had been undisturbed for a very long time became exposed. The researchers estimated that if 10% to 50% of the sediment carbon stock was exposed after the heatwave, it could have released 2 to 9 teragrams of CO₂ over the following three years. A meadow that had been drawing carbon down became, in places, a source of it.
Why It Matters
Shark Bay shows that a seagrass meadow's carbon store depends on the meadow staying alive. Carbon that took thousands of years to bury can start leaving within a few years once the plants that guard it are gone. One hot summer was enough to set that in motion. The meadows that remain are still trapping sediment and still holding carbon beneath the bay, in water that is warming.
Sources
- Fourqurean et al., Carbon, nitrogen and phosphorus storage in subtropical seagrass meadows: examples from Florida Bay and Shark Bay, 2012 (2012) · study
- Arias-Ortiz et al., A marine heatwave drives massive losses from the world’s largest seagrass carbon stocks, 2018 (2018) · study
- Heatwaves transform world's most efficient carbon sink ... - Nature Asia (2018) · context
- Australian Research Council, Scientists measure the loss of Shark Bay Seagrass, 2020 (2020) · context
- Western Australia's Shark Bay seabeds are a ticking carbon time bomb, says scientist (2024) · context
- Hitzewelle führt zu massiven Kohlenstoffverlusten am Weltnaturerbe · context
