New study: Natural methane emissions offset carbon uptake in coastal seafloor habitats

Coastal vegetated ecosystems can take up and store large amounts of carbon dioxide from the atmosphere, but our understanding remains limited, since simultaneous methane emissions may offset this capacity. Natural methane emissions can, however, reduce the annual carbon sink capacity by about 30%.

In a new study published in Nature Communications, with Florian Roth as first author, researchers from Stockholm University and University of Helsinki in the CoastClim* Centre measured greenhouse gas fluxes between the water surface and atmosphere across key coastal vegetated habitats outside the Askö Laboratory in Trosa over a whole year, using the Water Equilibration Gas Analyzer System, WEGAS.

The measurements confirmed that mixed vegetation and bladderwrack habitats in the coastal zone do take up significant amounts of carbon dioxide from the atmosphere. The fluxes vary over the year, but altogether the uptake of carbon dioxide from the atmosphere over bladderwrack habitats was a net sink of CO2, with up to 0.52 tons co2 per hectare and year taken up in bladder-wrack beds, which can be compared with 0.71 ton CO2 per hectare and year for areas with mixed vegetation.

However, this uptake is offset by methane fluxes from the water to the atmosphere from the very same environments. As methane is a stronger greenhouse gas than carbon dioxide, the net carbon uptake is reduced to 0.38 ton and 0.46 ton CO2-eq. (carbon dioxide equivalents, meaning the global warming potential of the gases converted to that of carbon dioxide) per hectare and year for bladderwrack and mixed vegetation habitats, respectively. In contrast bare sediments served as a net source for CO2 emissions.

The observed methane emissions from bladder-wrack habitats are a bit surprising, since these algae grows on hard substrates and not on soft sediments, where methane is normally produced”, says Christoph Humborg, scientific director of Stockholm University Baltic Sea Centre and co-author of the study.

“But what we found was that these algae form pockets of sediment where methane forming microorganisms, archaea, could be detected. We found these archaea also on floating filamentous algae and organic matter debris associated with dense stands of bladderwrack.”


Healthy coastal ecosystems have climate mitigation potential

Coastal ecosystems can take up and store large amounts of carbon dioxide from the atmosphere, so-called “blue carbon”. Conserving and restoring coastal vegetated habitats have therefore been proposed as an important nature-based solution to mitigate climate change. Well-known blue carbon ecosystems include mangrove, seagrass meadows and salt marshes. However, more recently, it has been suggested that also macroalgae, such as the bladderwrack (Fucus vesiculosus) – common to the Baltic Sea – can take up large amounts of atmospheric carbon. If this carbon is, for example, exported to and sequestered in the deep sea, macroalgae could be one of the most important blue carbon ecosystems globally.

As awareness is rising of the potential of blue carbon habitats and the possibility to include them in climate mitigation strategies, it is increasingly important to quantify the fluxes of greenhouse gases and the net carbon uptake in these environments correctly. Conserving and restoring coastal vegetated habitats could still be important from a climate perspective. Our study shows that these environments are net carbon sinks, just not as large as has sometimes been suggested,

Alf Norkko, the academic leader at Tvärminne Zoological Station, University of Helsinki, and also a co-author, highlights that from a management perspective, it is important to know whether the carbon sink potential, and e.g. methane production, is influenced by their health status. The Baltic Sea and many coastal seas around the world are plagued by eutrophication and rapidly increasing temperatures, which influences the carbon sink capacity. Researchers of the Finnish-Swedish research group in CoastClim are addressing these questions in field campaigns and experimental work at Tvärminne and Askö.


Contact in Finland

For more information in Sweden, please get in contact:

Christoph Humborg, Stockholm University Baltic Sea Centre, e-mail:, phone: +46-8-674 76 68

Florian Roth, Stockholm University Baltic Sea Centre and Tvärminne Zoological Station, University of Helsinki, e-mail:

What is CoastClim?

*The Centre for Coastal Ecosystem and Climate Change Research (CoastClim) evaluates the links between coastal biodiversity, carbon cycling, and climate feedbacks. The data will serve as a foundation for decision-support to improve the use and management of coastal ecosystems with the potential for climate change mitigation.

The development of CoastClim is part of a strategic partnership between the University of Helsinki and Stockholm University that focuses on strengthening collaborative marine ecosystem and climate change research between universities.