How bottom trawling alters carbon flows on the Baltic Sea seafloor
A one-kilometre-long trawl track, two research vessels and hundreds of water samples. Through detailed measurements, researchers have been able to monitor what happens in the water column when the seabed of the Baltic Sea is disturbed. The results provide new insights into the climate impact of bottom trawling and how we can protect the sea’s role as a carbon sink.
In recent years, the soft seabeds of the Baltic Sea have been highlighted as potentially important carbon sinks. Large quantities of organic material are stored here, which slowly decompose and become incorporated into the sediments. This is sometimes referred to as ‘blue carbon’. But how are these carbon sinks affected when the seabed is trawled?
At present, there are few research studies that have carried out measurements at sea to investigate what happens to the ‘blue carbon’ after a fishing trawl has been dragged across the seabed. It is important to fill this knowledge gap in order to better understand how human activities affect the ocean’s role in the climate.
With support from BalticWaters, researchers at Stockholm University, led by Clare Bradshaw, therefore carried out an extensive field experiment at Askö. The aim was to monitor what happens in the water column when sediment is stirred up as a result of bottom trawling – in real time.
“There are few reliable measurements that can improve climate models, which is why it is important to carry out field studies such as ours,” writes Clare in a final report to BalticWaters.
A unique field experiment at Askö
The study was carried out just north of the Askö Laboratory, where a fishing boat dragged a trawl track approximately one kilometre long across the soft seabed. Immediately after trawling, two research vessels carried out measurements to document the changes.
Among other things, the researchers measured how turbid the water became, how oxygen levels changed and how carbon moved through the water column. They also took samples for microbial DNA and RNA to investigate how microscopic life in the sediments was affected.
It is unusual for bottom trawling to be studied in real time in this way. The combination of the various measurements gave the researchers a unique opportunity to follow the entire sequence of events – from the sediment being churned up to how carbon is cycled within the water column.
Clare on board the research vessel with a sediment sampler – a so-called ‘Gemini corer’. Photo: Clare Bradshaw
In bottom trawling, a trawl door is used to keep the trawl net open and close to the seabed as it is hauled along. The door is 3 metres wide and 2 metres high. Photo: Clare Bradshaw
The sediment cloud that set the seabed in motion
The trawling created a distinct sediment cloud that rose approximately ten metres from the seabed. After 24 hours, the cloud had spread up to 500 metres from the trawl track. Microscopic organisms from the top four to five centimetres of the sediment were found in the cloud, which is a sign that the trawling had disturbed the surface layer of the seabed. The trawling also affected the oxygen and carbon levels in the bottom water – oxygen levels decreased and carbon levels increased.
During the trawling itself, a small and brief increase in methane – a highly potent greenhouse gas – was also observed, both in the surface water and just above the water’s surface. However, the increase was short-lived and disappeared within a few hours.
“The increase appeared to come from methane bubbles released from the sediment as the trawl passed by,” explains Clare.
The researchers, however, saw no signs of increased carbon dioxide levels at the water’s surface. This suggests that the increased amount of dissolved carbon near the seabed remained below the thermocline and did not reach the air.
In the stratified water column of the Baltic Sea, the impact of trawling on carbon turnover therefore appears to occur primarily near the seabed – not at the surface.
Why the results are important
The study provides long-awaited field data that can improve models and reduce uncertainty in estimates of the climate impact of bottom trawling. The results show that trawling affects the turnover of carbon and oxygen in the deeper water layers of the Baltic Sea, but that these effects do not necessarily lead to increased carbon dioxide emissions into the atmosphere.
This raises important questions for future marine management.
“It is rare for deeper soft-bottom areas to be protected for environmental reasons – it is more common for areas to be protected for their biodiversity or for a specific threatened species. But perhaps we should start thinking about carbon protection areas?” writes Clare in the final report.
Furthermore, the Baltic Sea’s soft seabeds are not only potential carbon sinks – they are also central to nitrogen and phosphorus cycles, to food webs and to a species-rich fauna. The study shows that these seabeds are affected by trawling in several ways, and that their role in climate and ecosystem processes deserves greater attention.
Towards better climate management of the Baltic Sea
By following a trawl track metre by metre, Clare and her research team have been able to document how oxygen, carbon and microorganisms react when seabeds are disturbed. The results provide new insights into carbon fluxes in the Baltic Sea and may contribute to better models, improved management and stronger protection of soft seabeds. At the same time, further studies are needed in different seabed environments around the Baltic Sea so that the findings can be extrapolated.
“We need more studies on different types of seabed in areas with varying conditions. But even now, our new measurements can help improve estimates of the impact of bottom trawling on the carbon cycle,” explains Clare.
At the same time, the research team is working to build a network of researchers and stakeholders around the Baltic Sea who are working on sediment carbon. The aim is to produce detailed maps of carbon quantities and disturbance risks – an important basis for identifying areas where carbon storage is particularly valuable to protect.
Targeted field studies such as this one contribute new and valuable knowledge towards a better understanding of blue carbon. It is an important step towards understanding the ocean’s role in climate management and mitigation.