- Two new reports showing the extent of carbon storage in the marine sediments around the coasts of Canada and the U.K. are helping to build the case for greater protection of the seabed, as part of efforts to mitigate climate change.
- They also highlight some of the threats to this underwater carbon sink, particularly sediment disturbance caused by bottom trawling, a fishing method.
- A new project funded by the U.K.’s Natural Environment Research Council is set to investigate whether or not continued disturbance poses the risk of turning the seabed into a source of CO₂.
Among marine ecosystems, the capacity of mangroves, seagrass meadows and kelp forests to store carbon is well documented, but less is known about the sediments of the ocean floor.
Two recent “blue carbon” mapping projects carried out in the coastal waters of Canada and the U.K. are changing this. By improving the accuracy of the mapping, the report authors aim to provide policymakers with the evidence they need to make climate change considerations part of seabed conservation.
According to Graham Epstein, a research fellow at the University of Victoria in Canada, carbon stored in the seabed has been “ignored until now because of the lack of reliable, high-resolution maps.”
The study he co-authored used a machine-learning predictive mapping process that analyzed the best available existing data on the composition of the seabed sediment around Canada, along with other environmental data, to create a map of organic carbon stocks.
Covering most Canadian waters shallower than 2,500 meters (8,200 feet), an area totaling 4.5 million square kilometers (1.74 million square miles), the map showed that 10.9 billion metric tons of carbon is stored within the top 30 centimeters (1 foot) of the Canadian seabed.
The research, published in May, highlighted geographical differences, showing that muddy sediment inside deep fjords and inlets was particularly high in carbon compared to shallower areas, where strong currents prevent the sediment from settling.
In the U.K., the Blue Carbon Mapping Project published a report in September showing that 244 million metric tons of carbon is stored in the top 10 cm (4 in) of the U.K. seabed. It estimates that this sediment could capture up to 13 million metric tons of carbon every year, almost three times the amount sequestered by the U.K.’s forests, albeit across a larger area.
The report, produced for conservation nonprofits WWF, Wildlife Trusts and the Royal Society for the Protection of Birds, drew on existing data, including extensive work by the British Geological Survey, to carry out and analyze thousands of scrapes of the seabed.
Before the U.K. report, there was limited coordination of information about how much carbon is stored in U.K. marine habitats, especially seabed sediment, co-author Mike Burrows, a marine ecologist with the nonprofit Scottish Association for Marine Science, tells Mongabay.
“The gloop and the mud doesn’t really get so much focus from a conservation angle, because it’s a harder sell,” he says.
“Saltmarshes and seagrass beds are significant carbon storage hotspots, while kelp beds and especially phytoplankton contribute large amounts of organic carbon [to the seabed] annually. However, the exact fraction of this carbon that is stored in sediments remains uncertain. By consolidating various information sources, we have gained valuable insights into our coastal seabed.”
Carbon dioxide is primarily absorbed by phytoplankton near the ocean surface, which sinks to the bottom of the sea when it dies. Organic carbon compounds within the phytoplankton can be broken down by bacteria and consumed by fish as they drift downward, Epstein says.
What’s left creates a carbon-rich layer of sediment on the seafloor that is constantly replenished from above. Underneath this layer lies tens of meters of sediment containing carbon and other organic compounds that have been locked away for millennia.
“In terms of the carbon that could be released, it’s that top layer that’s really important,” because it both contains the most carbon and is at the greatest risk of disturbance, Epstein says. Threats include dredging, infrastructure developments such as offshore windfarms, and bottom trawling, a common fishing technique that Epstein describes as “the most widespread, pervasive human stressor to the seafloor.”
Fishing gear generally only disturbs the top few centimeters of the seabed, but this is enough to create huge sediment plumes that release carbon back into the water. Some of that carbon may reach the surface and be absorbed back into the atmosphere, but most stays in the ocean, according to Epstein. This can have the dual effect of slowing the uptake of further CO2 from the atmosphere to the ocean, as levels in the water will already be too high, as well as exacerbating ocean acidification, he adds.
Both reports show that coastal areas store the most carbon, due to increased primary production of carbon caused by factors such as more light in shallower water, which boosts photosynthesis, and the influence of carbon runoff from the land.
Although the full impact of the disruption caused by future deep-sea mining for rare earth minerals is unknown, Epstein says that because carbon stored in the deep ocean takes so long to re-reach the surface, “disturbance in the very deep sea is likely to have less direct influence on the atmosphere.”
Being able to pinpoint specific areas where carbon stocks are highest can help policymakers decide which areas are most important to protect, Burrows says. As with Canada, the U.K. report highlights the importance of protecting deep fjords and inlets. It also urges greater protection of the U.K.’s designated marine protected areas, where trawling is rampant, and the blue carbon they contain, arguing that seabeds could help the U.K. meet its commitments to achieve a net-zero-emissions economy by 2050, and to protect at least 30% of its seas by 2030.
The report also calls on the U.K. government to give blue carbon greater consideration in its marine plans and to minimize the impacts of fishing and other activities by carrying out blue carbon impact assessments. Longer term, the report advocates that fishing industries move away from activities that damage the seabed.
Susanna Fuller, conservation lead at Ontario-based nonprofit Oceans North and a co-author of the Canadian study, says she agrees this kind of research can be an important planning tool. While some early estimates of seabed carbon stores have been made on a global scale, the kind of detailed local information the new research provides is more actionable, particularly when it comes to the fishing industry, she says. Industry members “want to be seen as environmental stewards, given that they actually rely 100% on the environment,” she says.
“So much of the work on bottom trawling over the years has been on hard bottom substrates, [and the] impact on habitats, but a lot of the trawling actually takes place in soft sediment ecosystems” that store more carbon, she says. “As the fishing industry will also be required to demonstrate that it’s lowering its carbon footprint, I think there is real value in them understanding [seabed carbon storage].”
A 2021 paper calculated that trawling stirs up as much as 1 metric tons of carbon each year. By adjusting the models it used, a subsequent paper published earlier this year recalculated the amount of carbon, finding that 55-60% will be released back into the atmosphere in nine years, with the rest dissolving in the water and contributing to acidification.
Both papers drew rebuttals from fellow scientists, who argued that the 2021 study had overestimated the amount of carbon stored in the seabed and the role that bottom trawling played in releasing it back into the atmosphere. Marija Sciberras, an assistant professor at the Lyell Centre, Heriot Watt University in Scotland, was among the critics. She tells Mongabay the global estimates used by the study were too broad, and failed to account for local and geographical differences in the seabed, something partially rectified by the two new reports from Canada and the U.K.
Using more detailed local findings instead of global averages will help avoid the flaws of past research, she says. Yet she adds that researchers still need to dig deeper to map the carbon composition of the sediment that lies below the top layer.
Doing so is part of a new 2 million pound ($2.56 million) project funded by the U.K.’s Natural Environment Research Council that has a consortium of researchers, including Sciberras, focusing on how trawling affects seabed carbon storage. A key aspect of the research will be to assess the possibility that disturbance will turn marine sediment into a net source of CO2, and how this resuspended carbon can affect ocean chemistry. It will also look more closely at bioturbation, the role that burrowing animals such as clams and worms play in burying carbon-rich sediment deeper in the seabed.
Banner image: A burrowing anemone in the seabed. Image courtesy of Paul Naylor/The Wildlife Trusts.
How much carbon does ocean trawling put into the atmosphere?
Citations:
Epstein, G., Fuller, S. D., Hingmire, D., Myers, P. G., Peña, A., Pennelly, C., & Baum, J. K. (2024). Predictive mapping of organic carbon stocks in surficial sediments of the Canadian continental margin. Earth System Science Data, 16(5), 2165-2195. doi:10.5194/essd-16-2165-2024
Sala, E., Mayorga, J., Bradley, D., Cabral, R. B., Atwood, T. B., Auber, A., … Lubchenco, J. (2021). Protecting the global ocean for biodiversity, food and climate. Nature, 592, 397-402. doi:10.1038/s41586-021-03371-z
Atwood, T. B., Romanou, A., DeVries, T., Lerner, P. E., Mayorga, J. S., Bradley, D., … Sala, E. (2024). Atmospheric CO2 emissions and ocean acidification from bottom-trawling. Frontiers in Marine Science, 10. doi:10.3389/fmars.2023.1125137
Hiddink, J. G., Van de Velde, S. J., McConnaughey, R. A., De Borger, E., Tiano, J., Kaiser, M. J., … Sciberras, M. (2023). Quantifying the carbon benefits of ending bottom trawling. Nature, 617(7960), E1-E2. doi:10.1038/s41586-023-06014-7
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