- Coastal ecosystems take in huge amounts of carbon dioxide from the atmosphere, but researchers are still deciphering how much methane, a far more potent greenhouse gas, they put back into the system.
- Researchers studying seaweed and mixed vegetation habitats in the Baltic Sea found they emit methane equivalent to 28% and 35% respectively of the CO2 that they absorb.
- The findings highlight that more work is needed to understand methane emissions in different coastal areas to get a better accounting of the carbon balance sheet.
Coastal ecosystems are very good at pulling carbon out of the atmosphere. But, as new research in the Baltic Sea shows, we also need to look at what they’re putting back in.
Vegetated habitats along the coast sequester huge amounts of carbon. In fact, half of all the carbon stored in ocean sediment is in three coastal “blue carbon” ecosystems: mangrove forests, seagrass meadows, and salt marshes. Seaweed, or macroalgae, also take in carbon, though it’s unclear how much of that carbon eventually ends up stored in ocean sediment.
But there’s a catch. Marine areas can also give off methane, a far more potent greenhouse gas than carbon dioxide. And most of that marine methane is also coming from coastal areas. The problem is there’s still uncertainty about how much methane is emitted, and from where. That’s made it difficult for researchers and policymakers to know what’s going in and what’s going out — carbon versus methane — in different ecosystems.
Scientists are starting to figure out what that balance looks like in different places. In a recent study in Nature Communications, researchers from Stockholm University and the University of Finland found that habitats of bladderwrack seaweed (Fucus vesiculosus) emit methane that’s equivalent to 28% of the CO2 that they absorb. In mixed vegetation habitats, they found that methane emission amounted to 35% of the CO2 intake.
The study “highlights that concurrent measurements of both gases are needed to make an overall statement about whether these systems are sinks or sources of carbon-based greenhouse gases,” said lead author Florian Roth, a researcher at Stockholm University’s Baltic Sea Centre.
One of the reasons we have an incomplete picture of CO2 and methane cycling in coastal areas is that most previous sampling was either done manually, which is time-consuming, or from large research vessels, which can’t access shallow coastal areas, according to Roth.
To get a large sample size without having to take constant manual measurements, the research team built a unique automated system that took continuous measurements of CO2 and methane in three habitat types common in the Baltic Sea: bladderwrack, mixed vegetation, and bare sediment areas. The system was stationed on a small houseboat in a shallow bay off Askö Island in Sweden.
The team also took sediment and vegetation samples by hand, wading out from shore or working from a small rowboat. In summer the water was warm, Roth said, but in winter they had to don dry suits and brave the ice-covered waters.
“Your fingers get cold, your face gets cold … and the even more difficult part is you only have sun from 9 o’clock to 2:30.”
While the team expected to find methane coming from the mixed vegetation and bare sediment areas, they were surprised at the high methane levels coming from the bladderwrack stands.
Methane is released when specialized microorganisms called methanogens break down organic matter in environments with low oxygen. Typically, those conditions are found in places where dead plant matter accumulates in the soft sediment, such as mixed vegetation and bare sediment habitats. But bladderwrack, a common brown seaweed named for the air-filled “bladders” on its fronds that allow it to float, anchors itself onto rocky areas, not soft sediment, so the team were puzzled by the higher-than-expected methane levels.
When the scientists looked closer, they found those methane-producing microorganisms in small pockets of sediment among the rocks, and on floating filamentous algae and bits of dead plant debris among the bladderwrack.
This is one of the first studies looking at methane versus carbon in seaweed habitats. Even studies on the interplay between different greenhouse gases in blue carbon ecosystems are scant: a review published in Global Biogeochemical Cycles in 2021 found that the high variability of methane and nitrous oxide emissions in blue carbon ecosystems means much more research is needed to understand how these might counterbalance their sink potential.
While these coastal habitats are still taking in carbon, and are important for biodiversity, Roth said the net sink effect of some coastal habitats “may be smaller than currently believed.” This is especially important when monetizing carbon sinks, through the selling of carbon credits, as “wrong numbers can lead to inaccurate claims.”
Carlos Duarte, distinguished professor of marine science at the King Abdullah University of Science and Technology in Saudi Arabia, who was not involved in the bladderwrack research, said the study is an important contribution to understanding what’s happening in the Baltic Sea. He also noted that this inland sea is unique — it has low salinity, high levels of pollution, and low oxygen conditions — so what’s happening there may not be representative of what’s happening in other marine areas.
According to the latest assessment by the Baltic Marine Environment Protection Commission, better known as the Helsinki Commission (HELCOM), 97% of the Baltic Sea is affected by eutrophication, an excess of nutrients in water, with 12% in the most severe category. Caused by nitrogen and phosphorus runoff coming from agricultural fertilizer, wastewater and other sources, eutrophication favors the growth of phytoplankton and fast-growing algae, leading to low-oxygen conditions where methane-producing microorganisms thrive. Management plans to limit nutrient inputs are in place, but it will take some time before levels come down, in part because the Baltic is an inland sea, so whatever goes into the system sticks around.
Roth agreed that the dynamics of CO2 and methane are likely to be different outside the Baltic Sea. That’s why much more research is needed in other areas and ecosystems, he said.
Study co-author Alf Norkko, a professor at the University of Helsinki’s Tvärminne Zoological Station, said human pressures and disturbances can impact biodiversity and therefore carbon cycling. Mitigating eutrophication would generally lower methane emissions and also improve biodiversity.
“So, conservation efforts are exceptionally important,” he said.
Norkko added that to really understand what’s going on in the oceans and along the coast, we need to look more holistically at the complex interactions between biodiversity, carbon cycling and atmosphere.
For example, he said that while we know methane-producing microorganisms thrive on degrading organic matter, there might also be “other unknown mechanisms that involve the actual primary production” of methane, and that more simultaneous measurements of greenhouse gas exchange in different habitats would shed light on the dynamics of uptake and release.
“So much of our understanding has been focused specifically on the classical ‘blue carbon’ habitats [of salt marshes, seagrass and mangroves] rather than trying to understand the complexity of carbon cycling across coastal seascapes,” Norkko said. “Nevertheless, this is important because of the connectivity of the habitats.”
Banner image: Bladderwrack seaweed on the Baltic seafloor. Image courtesy of the Stockholm University Baltic Sea Centre.
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