RV Electra heads to Finland for a major sampling expedition. Photo: Anna-Karin Landin

Methane is a powerful greenhouse gas, with 28 times more impact on global warming than carbon dioxide over a 100-year period. The gas is formed, among other things, on the seabed when organic matter decomposes in oxygen-free environments, and is therefore particularly abundant in eutrophic systems. So-called fibre banks - large accumulations of wood fibres resulting from industrial discharges from paper mills - can also lead to high methane production, even near the water surface. The potential of lakes to be major sources of methane is relatively well researched, but the dynamics of coastal areas have not been extensively studied.

 – The shallow coastal areas have been somewhat overlooked, but we are now taking a holistic approach and quantifying methane produced and emitted to the atmosphere in these areas. This is very important for understanding the role of the coast in the carbon cycle and for climate change, says Christoph Humborg, Scientific Director of the Baltic Sea Centre and leader of the expedition.

RV Electra leaving Askö Laboratory.

The questions the researchers will try to answer include how much methane is produced in the seabed sediments, how and where there are so-called hot-spots of methane, and how much of the methane produced actually reaches the atmosphere and is relevant to the climate. Some of the methane has time to decompose along the way, explains Christoph Humborg. 

 – In oxygen-free conditions in the sediments of marine environments, some methane is lost through oxidation by sulphate. But the water column also acts as a filter - methane can bubble up from the bottom, but never reach the atmosphere because it breaks down along the way.
However, in shallower water, where methane bubbles reach the air in a few seconds, they don't have time to decompose in the water. 

 – There is a critical depth at which methane no longer reaches the surface, and one of the things we're going to investigate is what that depth is. It's a big advantage that we're doing the sampling on board Electra because she can go very shallow, says Christoph Humborg.

Multiple types of samples

The expedition is part of the new Centre for Coastal Ecosystem and Climate Research, CoastClim, which includes Stockholm University through the Baltic Sea Centre and the Bolin Centre for Climate Research, and the University of Helsinki through Tvärminne Zoological Station and the Institute for Atmospheric and Earth System Research (INAR). About 30 scientists are currently associated with the centre, which conducts research on the oceans and atmosphere to find solutions to mitigate climate change. Around ten of them, with expertise in oceanography, biogeochemistry, biodiversity and microbial processes, among others, are taking part of the expedition.

Chritoph Humborg. Photo: Anna-Karin Landin

 – The first few days will be devoted to intensive sampling of the sediments, using a piston core. In the sediment samples, we can see the whole story - how much methane was formed during different periods and how much has been lost through sulphate reduction, says Christoph Humborg.

- Then we take our WEGAS system with which we can measure the methane and carbon dioxide levels in the water and study it metre by metre in the water column. We will also run transects over the areas with advanced sonar. Now in late summer you can really see the methane bubbles in the water and we want to quantify those as well.

Comparisions between marine and terrestrial carbon

The surveys will take place at two different sites outside Tvärminne zoological station in Hangö.

- One of the sites is very deep and we know that there is a lot of organic material there, including the remains of algae. The other area is outside an old paper mill where there is also a lot of carbon in the sediments, but it is a different type of carbon because it comes from the forest. It's very interesting to compare how much methane is formed in the different types of sediments, says Christoph Humborg.

Overall, the studies are expected to give the scientists a better picture of how much methane is formed and emitted from the Baltic Sea and to what extent it compensates for the ocean's uptake of carbon dioxide.

- This could give us some answers as to whether the Baltic Sea is currently a carbon sink or a carbon source, says Christoph Humborg.

You can follow Electra's journey across the Baltic Sea here. The expedition can also be followed on the Askö Laboratory's Instagram account and on CoastClim's twitter account.

Text: Lisa Bergqvist