Extreme variability in coastal methane emissions could have large effects on carbon budgets

Carbon uptake by coastal ecosystems is a hot topic. The so-called ‘blue carbon’ is considered an essential part of nature-based solutions to mitigate climate change and offers opportunities for carbon trading and offset schemes. But new research on methane emissions from coastal ecosystems raises caution – the variability of these emissions is much larger than previously assumed, which implies that upscaled global carbon budgets may be far from realistic.

Florian Roth on a little houseboat where the equipment for measuring greenhouse gas emissions is installed.
Florian Roth spent one year measuring greenhouse gas emissions from the coastal sediments outside Askö Laboratory in Trosa. The measurements showed extremely large variability of methane emissions.

Vegetated coastal ecosystems worldwide take up carbon from the atmosphere, often referred to as ‘blue carbon’. However, a part of this carbon is also emitted from these ecosystems, both as carbon dioxide and in the form of methane – the latter which is a highly potent greenhouse gas formed in sediments when organic material is decomposed. Knowledge about methane fluxes is important to estimate the net carbon uptake of coastal systems, thereby their potential to impact climate change.

However, methane emissions are rarely quantified, and existing studies usually build on a few measurement points in time and space only, which are then extrapolated for use in large-scale carbon budgets.

 – What we did was exactly the opposite. The coastal zone is so dynamic and heterogeneous that we wanted to look at many different areas for long periods closely. We used novel high-resolution measurements to resolve everything that was happening, tells Florian Roth, researcher at the new Centre for Coastal Ecosystem and Climate Research (CoastClim), formed by the universities of Stockholm and Helsinki to connect science on coast and climate issues.


Real-time monitoring revealed variability over hours

Using the Water Equilibration Gas Analyzer System, WEGAS, Florian Roth and his colleagues were able to monitor in real-time the distribution of methane in the water and the emissions from vegetated habitats outside the Askö Laboratory, Trosa, during one full year. What quickly became obvious was the extreme variability of the emissions – both in space and time.

 – We could see differences on metre scales as we moved the equipment around in the bay, says Florian Roth.

Also, researchers, especially in Nordic regions, tend to go out sampling in the summer, at daytime and when the weather is nice, but the new study shows that this could result in highly biased measurements. 

 – The seasonal variability is very strong – the emissions are temperature-dependent and generally larger in the summer. But methane emissions are also extremely variable over a single day, explains Florian Roth.

For these daily fluctuations, the researchers could not find consistent pattern across all sites.

 – But we believe that the variability, which literally takes place within minutes and hours, is driven by physical events, such as wind changes and wave action that cause pressure on the sediments and thereby the release of gases, says Florian Roth.


More high-resolution measurements are needed

To capture the variability of methane emissions, the researchers calculated that around 50 individual samples per day are needed, an amount which is almost impossible to handle by traditional sampling methods. High-resolution devices like the WEGAS system or eddy covariance towers are thus indispensable to resolve uncertainties in coast-climate feedbacks linked to the intrinsically dynamic nature of vegetated coastal ecosystems in the future.  

But in order to accurately estimate the carbon fluxes on a large scale and make reliable global carbon budgets, assessments across more and different types of habitats are also needed, means Florian Roth.

 – Most studies are performed on the classical blue carbon ecosystems, such as seagrasses, mangroves and salt marshes. Sampling in Nordic countries is also rare. With our measurements, we could identify northern temperate coastal habitats with mixed vegetation and macroalgae as understudied but seasonally relevant methane sources. As researchers, we have to make an effort and go out sampling in areas that may seem a little boring at first but are just as important.

Why is this important?

 – Coastal ecosystems are important as they can contribute to mitigating climate change. Due to the large spatial and temporal heterogeneity of coastal environments, high-resolution measurements will improve the reliability of methane estimates and constrain the habitat-specific contribution to regional and global methane budgets, explains Florian Roth.

But there is also money involved, as companies started selling carbon offset certificates. 

– We have to question the current coastal carbon sink estimates because widely unaccounted or wrongly estimated methane emissions from these environments could reduce their value as atmospheric carbon sinks. This makes it necessary that we have good knowledge about the carbon fluxes and use realistic numbers for these calculations.

About the study

  • The study High spatiotemporal variability of methane concentrations challenges estimates of emissions across vegetated coastal ecosystems was conducted by Florian Roth (Stockholm University/University of Helsinki), Xiaole Sun (Stockholm University/Chinese Academy of Sciences), Marc C. Geibel (Stockholm University), John Prytherch (Stockholm University), Volker Brüchert (Stockholm University), Stefano Bonaglia (University of Gothenburg), Elias Broman (Stockholm University), Francisco Nascimento (Stockholm University), Alf Norkko (Stockholm University/University of Helsinki) and Christoph Humborg (Stockholm University/University of Helsinki). 
  • The researchers made continuous measurements of methane concentrations and emissions from contrasting coastal habitats with macroalgae, mixed-vegetation and bare sediments in the bay outside Askö Laboratory in Trosa for one year. Isotopic methods enabled them to trace back habitat- and season-specific production and consumption pathways of the greenhouse gases.
  • Analyses of over 210 000 measurements showed that the distribution of methane in coastal waters is spatially patchy over meter-scales and highly variable in time. In general, most methane was emitted in summer and fall, and emissions from vegetated habitats were higher than those of the non-vegetated sites
  • Based on their extensive data, the team proposes sampling intensities that help resolve the scale and drivers of this variability and reduce uncertainties of emission estimates by up to 70 per cent.
  • The researchers discuss that failing to include spatially and temporally resolved measurements in future studies will result in a continued systematic bias of emission estimates from heterogeneous coastal environments subject to global change.
  • The improved understanding of methane emissions will inform efforts addressing climate change with the net potential of vegetated coastal ecosystems to act as carbon sinks.


See the video where Florian Roth talks about his research:



Text: Lisa Bergqvist