Seagrasses remove the greenhouse gas carbon dioxide from seawater and store its carbon component in the soil – at a higher rate than any forest on land
As plants, seagrasses use photosynthesis. In other words, they absorb the greenhouse gas carbon dioxide from the surrounding water, transform it, and store the carbon in their leaves and roots. There’s nothing particularly remarkable about that. But when some of the plants’ subterranean roots die off, they aren’t broken down by microorganisms on the ocean floor. Due to the lack of oxygen in the sediment, they are almost perfectly preserved and remain intact, sometimes for centuries. The same is true for seagrasses that die and are covered by sediment particles.
Thanks to these two processes, over time, large quantities of organic material accumulate underneath seagrass meadows, and are characterized by high carbon content. In this regard, the meadows are so effective that they can store 30 to 50 times as much carbon per square meter as comparable, forest-covered ecosystems on land. However, since the total worldwide area of seagrass meadows is far smaller than that of forests, they still come in far behind forests on the list of major natural carbon sinks.
Experiments recently conducted on eelgrass (Zostera marina) meadows have shown that, on average, they store 2.7 kilograms of carbon per square meter in the top 25 centimeters of the ocean floor. However, depending on the local conditions, researchers have also recorded values of up to 26.5 kilograms of carbon per square meter. Along Germany’s Baltic Sea coast, the average was 10.7 kilograms per square meter of seagrass meadow. Extrapolated for the top 100 centimeters of ocean floor, the eelgrass meadows could theoretically store between 23.1 and 351.7 metric tons of carbon per hectare.
If our goal is to keep
global warming below two degrees Celsius, these carbon reservoirs need to be
preserved. But in order for that to happen, grass landscapes on the seafloor have
to survive. If the seagrasses were to die, it would essentially “open the gate”
to the subterranean reservoirs. The sediments, no longer protected from the
waves and currents, would then be churned up, rearranged or swept away. In any
event, the deeper sediment layers would be exposed to oxygen, making it child’s
play for marine microorganisms to break down the plant matter in them and
release the carbon as the climate-harmful greenhouse gas carbon dioxide. The consequences
would be not just an intensified acidification of coastal waters, but also
accelerated global warming.
Experts have estimated
that if seagrass meadows around the globe completely died out, it could mean
additional greenhouse-gas emissions of 650 million metric tons of carbon
dioxide per year. By comparison: this additional
amount is only somewhat less than Germany’s total annual carbon dioxide emissions
(703 million metric tons).
How much carbon is absorbed and stored by seagrass meadows varies from species to species and from region to region. As a rule, the largest carbon reservoirs are formed in meadows that are undisturbed by humans, that consist of long seagrass species with dense foliage, and that grow in shallow bays sheltered from the wind and waves – in short, in places that offer ideal living conditions for seagrasses. Moreover, the latest research conducted on restored seagrass meadows off the coast of Virginia, USA, has confirmed that the older seagrass meadows are, the more carbon they store.
Their ability to
create carbon reservoirs makes seagrass meadows an interesting tool when it
comes to the question of how human beings can quickly and effectively reduce the
carbon dioxide concentration in the atmosphere. Accordingly, projects designed
to reintroduce seagrass meadows are now considered not only an investment in
protecting and preserving local marine biodiversity, but also in climate and coastal
protection, producing effects that can be felt far beyond the newly restored plots.
Working on this premise, the SeaStore project partners want to determine how much carbon the seagrass meadows of the southern Baltic Sea store in their reservoirs, and how quickly newly (trans)planted meadows can form new reservoirs. To do so, the participating marine biologists are collecting seagrass and sediment samples from native meadows in Danish and Swedish waters and comparing them on a regular basis with samples taken from two newly planted test meadows near Kiel and Maasholm.
At the same time, the team’s economic experts are preparing a comprehensive cost analysis for seagrass restoration projects and using online surveys to gauge how the German public would view seagrass restoration projects on the Baltic Sea coast. Would coastal communities and tourists support these restoration efforts, or would there be resistance if e.g. certain segments of coastline would need to be temporarily declared “off limits” to protect the newly planted meadows? The answers to these questions will be used to create guidelines on how to plan and implement seagrass reintroduction projects as part of the Seastore project.
Yet one important economic aspect is already becoming clear: anyone who hopes to reintroduce seagrass meadows so as to store carbon below the seafloor and compensate for unavoidable greenhouse-gas emissions in other branches of the economy is likely to be disappointed. On the other hand, if decision-makers and potential investors consider the entire range of services that seagrass meadows provide, and gear their projects toward achieving cleaner water, coastal protection and rebounding biodiversity, not to mention reliable fishing catches, it’s another story entirely, and the restoration of seagrass meadows is sure to be an investment that pays off.
Restoring seagrass meadows means
providing a tremendously important service not just for the ocean, but for all
Yet the success of restoration efforts depends on a complex interplay of factors, all of which are being investigated in the joint project SeaStore.