Blue carbon and methane emissions – how coasts and climate are connected

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.

Text: Lisa Bergqvist

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.

Camilla Gustafsson, Tvärminne Zoological Station.

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.

Moderator Gun Rudquist and 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.

Watch a recording of the webinar here:

Answers to questions from the audicence

The audience had many questions to the researchers and not all of them could be addressed during the webinar. Here are some of the remaining questions and the answers from the researchers.

Is the contribution of microbial autotrophs (eg microphytobenthos) for blue carbon quantified? How does it compare relatively to aquatic macrophytes?

Camilla: This is a good example of a knowledge gap that we need to address. We know that microphytobenthos contribute to both production of O2 and respiration of CO2 but they also have a much faster turnover than plants. Hence, their larger role for BC (blue carbon) is not fully understood yet. 

Florian: In addition, the degradation of microphytobenthos is much faster than that of, for example, macroalgae or larger vascular plants. As a result, the carbon captured by microphytobenthos during photosynthesis may be quickly respired before organic matter can be buried in long-term reservoirs like deeper coastal sediment layers or the deep sea.                                                                                              

It would be very interesting to see the function of the meadows of green plants in the Gulf of Bothnia. These cover great areas and are probably important.

Camilla: Yes, more research is needed to be able to quantify BC potential of such habitats. Even though we would not have estimates of their BC potential we know that they provide other functions (e.g. habitat for invertebrates and fish) and services that benefit us humans.

Florian: An exciting new project to follow on this topic is NordSalt – Climate Change Impacts and Biodiversity Interactions in Nordic Salt Marshes.

Is it possible that algal blooms tie carbon dioxide from the atmosphere to the water coloumn?

Camilla: Dissolved inorganic carbon is the main form of carbon used by algal blooms, but the amount of inorganic carbon in its dissolved phase is dependent on atmospheric CO2-concentrations.

Florian: In general, (planktonic) algal blooms are not considered a “Blue Carbon” sink in the classical way. They are efficient in carbon uptake; However, their degradation takes place in the water column, and hardly any of the captured carbon (in the form of organic material) is buried in long-term reservoirs like deep sea sediments. They are too light and do not sink fast enough. This contrasts with some macroalgae that can be transported to the deep sea with much faster sinking rates. 

Given the importance of healthy coastal ecosystems and the the risks of increased methane emissions, how do you consider shoreline protection? Needs to improve?

Florian: I would say it is the other way round: Healthy coastal ecosystems may limit methane emissions and reduce erosion of the coastline. For example, seagrass meadows in shallow waters and low wave energy environments provide optimal conditions for stabilizing sediments, limiting erosion and, thus, enhancing coastal protection. See Potouroglou et al. 2017 and Ondiviela et al. 2014.

To Florian Roth: About the role of T of methane emissions from coastal ecosystems. Is methane oxidation less enhanced by temperature than methanogenesis?

Florian: First results from measurements in 2020 indicate that methane production (methanogenesis) outpaces methane removal with increasing water temperature. However, many other factors may play an even more significant role, for example, the increased availability of organic matter for methane production in summer due to the growth of plants. Also, methane oxidation plays a smaller role in shallow waters than in the deep sea (where almost all methane is oxidized before reaching the surface). This may be because the distance methane has to travel from the sediments to the water surface in shallow waters is very short – hence, less oxidation can occur.

Is there a difference between marine and freshwater species when it comes to CH4-release?

Camilla: When it comes to rooted plants and algae, this is something we do not know yet and will need to explore.

Florian: Rooted freshwater or marine plants can transport oxygen into local sediments, enhancing methane oxidation and, thereby, limiting methane emissions. However, the stems of some aquatic vascular plants can also act as a conduit of methane from deeper sediments into the water column or even straight into the atmosphere. However, our knowledge of the different methane transport pathways by Baltic Sea species is still limited at this stage.  

Could you elaborate on what the GHG aspects are of a bottom turning anoxic?

Florian: Methane formation takes place predominantly in anoxic environments. As such, bottoms turning anoxic are likely to emit more methane than well-oxygenated seafloors. Coastal vegetation releases oxygen into the water column, and their root and rhizomes may transport oxygen into the sediments – therefore, they can help reducing methane emissions.

What can you say about GHG release from bottom trawling etc?

Camilla: A recent study (Sala et al. 2021) published in Nature discusses the effects of fishing activities on CO2 emissions and they estimate that trawling for example, releases substantial amounts of CO2  

Florian: A recent study by colleagues from around the Baltic Sea showed that the physical disturbance by bottom trawling not only suspends particulate matter but that dissolved methane concentrations were elevated in the water for at least 20 h after the trawling activity (Bradshaw et al. 2021).

In your opinion what would be the most important management measure to increase the Baltic carbon sequestration capacity?

Camilla: Reduce nutrient run-off from land and protect marine biodiversity as much as possible.

Florian: I can only agree to that! 

You are stressing that pollution on the sea bottom is more or less invisible resulting in a lack of awareness among the public. But don´t you think that a lot of the threats for the Baltic Sea is well known. Oxygen free or dead bottoms, eutrophication, algae blooming are well known. But we do not make the connections between our activities in society that is ending up in the sea. Isn´t that a very big challenge for all of us?

Camilla: Perhaps the public is aware of the large-scale problems such as anoxic bottoms and algal blooms but there are subtle changes in the coastal areas that might not be as obvious to the public, for example, trash on the seafloor or the reduction in cover and distribution of bladderwrack that happen successively. People might not be aware of the implications that certain activities might have; for example, what happens to the communities living on the seafloor after a dredging event or how long will it take for the seafloor to recover from such an event (perhaps it can be likened to a forest clearcutting, which in turn, is much easier to observe than the seafloor). So yes, it is a big challenge to link the activities in our society to the changes taking place in the sea but through awareness, I think it can be improved.

These seafloor-disturbing activities such as trawling and dredging that were mentioned, that increase ecosystem degradation, are they done in large scale currently, if so where and by whom?

Camilla: For example, it is quite common that people living next to the shoreline or summer house owners in sheltered archipelago areas dredge the immediate sea floor to deepen their waterways. Such dredging activities can, however, be quite detrimental for plant, algal and fish communities in those specific locations (e.g. Eriksson et al. 2004, Sandström et al. 2005). On larger scales, dredging can take place in the vicinity of harbours when navigation channels need to be deepened. If not controlled properly, the plume from these dredging activities can spread over larger areas deteriorating the light climate for plants and algae. 


Are perennial plants more efficient in storing CO2 than annuals? Should the overwintering organs of annual plants, turions, that are found within the sediment also be considered?

Camilla: Some of the CO2 taken up by plants will be stored as carbohydrates (which contain carbon) in the turions that they use as an energy source for the next growth season. However, these turions are part of a more short-term storage of carbon because the stored carbon is used up the following spring and hence, is not a long-term storage of carbon. Obviously, however, healthy plant communities with both annuals and perennials contribute to carbon storage and is a much better alternative than no vegetation.