The world’s oceans store vast amounts of carbon in their sediments, locking it away for centuries. Yet scientists still know relatively little about how different seabed industries disturb this buried carbon, particularly over long timescales. Such knowledge is important to reduce risks to the climate from carbon re-release.
A new study led by researchers with the Convex Seascape Survey reveals the first long-term estimates of seabed carbon disturbance from port dredging and marine aggregate extraction across the Northwest European shelf. Scientists from the University of Exeter and Bangor University combined historical records dating back to the 19th century with modern seabed data.
The historical analysis shows that dredging has been a persistent pressure in some UK ports for nearly two centuries. Sites including Ramsgate and Aberdeen have been dredged regularly since the 1830s, while the spread and development of the port dredging industry led to a 30-fold increase in sedimentary organic carbon disturbance from UK port dredging between the end of the 19th century and the start of the 21st century.
The results provide new evidence on two historically important seabed industries, while also helping place their carbon disturbance in context alongside better-studied pressures such as bottom trawling.
Looking to the past to understand today’s seabed carbon
Most previous research has focused on bottom trawling, which drags heavy fishing gear across the seafloor. Other industries also disturb marine sediments, including dredging to maintain ports and shipping channels, and the extraction of sand and gravel for construction.
To assess their impact, researchers analyzed 19th-century parliamentary papers, port authority records, industry reports and contemporary European seabed datasets. These were combined with thousands of sediment samples showing how much organic carbon is stored in different seabed types.
This approach enabled the team to estimate how much organic carbon has been disturbed by these activities over decades—and, in the case of UK ports, for more than a century.
“This is the first study to use historical archives alongside modern data to quantify how seabed carbon disturbance has changed over time,” said Ellie Maynard, Graduate Research Assistant at the University of Exeter and lead author of the study. “It reveals the long-term scale and persistence of human activities impacting the seabed, showing how these pressures have accumulated over decades.”
Why Some Seabed Industries Disturb Less Carbon
The study found that although port dredging and marine aggregate extraction do disturb seabed carbon, their impacts are far smaller than those associated with bottom trawling.
That difference is largely explained by where these industries operate. Port dredging and aggregate extraction are restricted to smaller, well-defined areas. Marine aggregate extraction is typically concentrated in sandy and gravelly sediments with relatively low organic carbon, whereas ports and harbors are often located in muddier, silty environments, including estuaries and river systems, where organic carbon content can be higher.
In contrast, bottom trawling affects far larger areas of the seabed, frequently disturbing carbon-rich muddy sediments.
Across UK waters, the researchers found that estimated carbon disturbance from dredging and aggregate extraction is roughly three orders of magnitude lower than published estimates for bottom trawling, underlining the importance of considering activity type, spatial footprint and sediment carbon content when assessing seabed carbon risk.
Filling A Major Knowledge Gap in Ocean Carbon Science
The researchers estimate that port dredging across the Northwest European shelf disturbed an average of 2.2 ± 0.9 million tonnes of organic carbon per year between 1995 and 2021, while marine aggregate extraction disturbed an average of 0.4 ± 0.3 million tonnes per year between 1955 and 2022.
Until now, port dredging had not been systematically quantified in terms of its effect on sedimentary carbon stores, despite being a centuries-old activity vital to keeping navigation channels open. Aggregate extraction and the dumping of dredged sediments at sea have been investigated previously.
“These findings address an important knowledge gap,” added Maynard. “While sediment type and spatial footprint were already known to influence carbon risk, the long-term extent and variety of human activities disturbing the seabed had not previously been quantified. By tracing these activities back over decades, we show how much organic carbon has been put at risk over time, although its ultimate fate after disturbance remains uncertain.”
Implications for Marine Management and Conservation
The researchers say the study has implications for how seabed carbon is considered in marine management and conservation strategies.
By identifying which activities disturb the most carbon-rich sediments, the findings could help policymakers prioritize protection of muddy seabed and focus management efforts on the highest-impact activities.
“If we want to minimize disturbance to seabed carbon, we need to focus on the activities that affect the largest areas and the richest carbon deposits,” said Professor Callum Roberts, another author of the study and lead scientist of the Convex Seascape Survey. “This research provides the evidence needed to support smarter, more targeted ocean management.”
The Convex Seascape Survey is a partnership between Blue Marine Foundation, the University of Exeter and Convex Group Limited. The five-year global research program aims to improve understanding of the role of the ocean and continental shelves in the Earth’s carbon cycle, and how they are affected by human activities.
The paper, published in the journal PLOS One, is entitled: “Estimating historic seabed carbon disturbance by port dredging and aggregate extraction in NW Europe.”
