Baltic Breakfast: Blue carbon and methane emissions – how coasts and climate are connected
2021.08.31: The coastal ecosystems in the Baltic Sea are important for the climate – both by taking up carbon dioxide from the atmosphere and by emitting methane. But are the Baltic Sea coasts net sinks or sources of carbon? At a recent Baltic Breakfast webinar, Camilla Gustafsson and Florian Roth presented their research and launched a new policy brief on the topic.
Camilla Gustafsson and Florian Roth.
The ocean’s ability to capture carbon dioxide from the atmosphere, so called blue carbon, is gaining increasing attention in global climate discussions.
In particular, carbon dioxide is captured through photosynthesis by vegetated ecosystems, such as seagrass meadows, coastal marshes, mangrove forests and macroalgal belts, describes Camilla Gustafsson, researcher at Tvärminne Zoological Station, University of Helsinki.
– This carbon uptake can result in short term storage in plant and algae, but more importantly to a long-term sequestration within the seafloor. Through continuous sedimentation the organic material is buried in the sediments where they can remain for millennia and is then called a carbon sink, she explains.
The blue carbon ecosystems have a substantially larger carbon burial rate than terrestrial ecosystems and a recent study estimated the monetary value of the carbon sequestered by such ecosystems to 190 billion US dollars per year.
Large variability in the Baltic Sea
In the Baltic Sea, the blue carbon ecosystems are represented by seagrass meadows, macroalgal belts and reed beds. The seagrass meadows mainly consist of eelgrass, Zostera marina, but the shallow bays in the brackish parts of the Baltic Sea are often vegetated with other rooted plant of freshwater origin, whose contribution to blue carbon remains unknown.
Kelp and other macroalgae, such as bladderwrack which is commonly found in the Baltic Sea, has recently been classified as blue carbon ecosystems.
– They often grow on hard substrate and don’t necessarily store carbon where they live, but they can be transported by currents to the deep sea, where they sink and can be sequestered, says Camilla Gustafsson.
The archipelago in the Baltic Sea, for example around Tvärminne Zoological Station, shows a strong variability when it comes to substrate, salinity and wave exposure, resulting in a high diversity of habitats. This provides a good research environment for investigations of ocean-atmosphere interactions in different ecosystems, which is an important part of the research performed within the ongoing Baltic Bridge collaboration between Stockholm University and University of Helsinki. This research is important as there are still many unknowns regarding how the ecosystems function as carbon sinks over time and how that functionality is impacted by climate change, says Camilla Gustafsson.
– But in addition to the functions related to carbon cycling, these ecosystems also provide us with many other ecosystem services, such as food security and natural protection against storms and erosion.
Camilla Gustafsson, Tvärminne Zoological Station.
Methane emissions might offset carbon burial
The coastal areas are, however, not only capturing carbon from the atmosphere but also emitting it. This is done mainly in form of methane, a strong greenhouse gas with warming potential 28 times that of carbon dioxide (in a 100-year perspective), that is produced through microbial processes in the sediments.
– One will think that most of the emissions come from anthropogenic sources, such as industries or agriculture, but when it comes to methane a substantial part comes from natural ecosystems, says Florian Roth, researcher at Tvärminne Zoological Station and Stockholm University Baltic Sea Centre.
A recent study has shown that almost 50 percent of all methane emissions come from aquatic ecosystems, and of the emissions from the oceans, a majority comes from shallow coastal areas, areas commonly found in the Baltic Sea.
Does this then contradict the coastal blue carbon concept, Florian Roth asks rhetorically and answers:
– No, but it has to be part of the calculation.
In a study of the carbon fluxes of a bladderwrack habitat, Florian Roth and his colleagues found that the emissions of methane reduced the net carbon uptake with 43 percent, when recalculated to carbon dioxide equivalents.
– This is important to know and it’s not accounted for in many blue carbon assessments, says Florian Roth.
Florian Roth, Stockholm University Baltic Sea Centre and Tvärminne Zoological Station.
Large variability in the Baltic Sea
The quantities of coastal methane emissions are highly variable in both space and time. Most methane is generally emitted during summer, but changes in the habitat structure can have large effects on the emissions.
– In general, the methane emissions are quite low in a healthy and undisturbed system with good habitat structure. If we add stressors like pollution of nutrients or toxins, exploitation by new constructions or bottom disturbing activities, we lose important species and the methane emissions tend to increase, says Florian Roth.
Global studies have shown that the methane emissions are also influenced by temperature and increase with warming, a pattern confirmed by studies in the Baltic Sea. Climate change might therefore result in an accelerating feedback loop, where higher temperature leads to habitat degradation that results in a decreased carbon uptake combined with increasing methane emissions, that further increase global warming.
– We need to be aware of the possible drastic changes that could occur, says Florian Roth.
He emphasizes the importance of working against climate change on the global scale, but in parallel perform local action to preserve the coastal ecosystems.
– Reducing local stressors really helps ecosystem cope also with global stressors.
Some actions that could help nature to do its own restoration is to reduce pollution of hazardous chemicals and nutrients, especially in shallow areas and bays, to prohibit bottom disturbing activities, such as bottom trawling and dredging, and to stop the ongoing exploitation of the coasts. Active restoration of degraded ecosystems could also help.
– We don’t know right now whether the coastal habitats in the Baltic Sea are efficient sinks or if they are sources. It seems like they are still sinks of atmospheric carbon dioxide, but methane emissions have the potential to offset this, especially because the Baltic Sea is already influenced by human stressors.
Are we doing enough to take care of these important ecosystems, asks moderator Gun Rudquist.
– I would say no, we are not doing enough right now. Coastal ecosystems are often overlooked, says Florian Roth.
– Many people don’t go diving or snorkelling and they don’t see what’s underneath the water, while it is so easy to go out in the forest to see changes. Awareness is really important, adds Camilla Gustafsson.
Moderator Gun Rudquist and Camilla Gustafsson, Tvärminne Zoological Station.
See the webinar here
Last updated: 2025-09-18
Source: Östersjöcentrum