"If the seabed is to function as a carbon sink, it must be left undisturbed"


Does protecting the seas save the climate?

Bottom trawling not only has consequences for fish and organisms living on the seabed, but can also affect the circulation of substances in the sea and, by extension, the climate. A large international study has estimated how much carbon that is stored on the bottoms and risks being released when they are trawled. But researchers at the Baltic Sea Centre mean that the calculations are uncertain.

Text: Lisa Bergqvist, Foto Mats Westerbom/Metsähallitus

Protecting sea areas from, among other things, bottom trawl fishing has positive effects on biodiversity, global food production and also on the climate. That is the message of the study Protecting the Global Ocean for Biodiversity, Food and Climate, which was recently published in Nature and which has attracted a great deal of international attention.

The 26 researchers behind the study have used calculations of the amounts of carbon stored on the seabed worldwide, and linked these with satellite data on trawled areas. During bottom trawling, the sediment on the bottoms is swirled up, which may increase the decomposition of organic material and the re-mineralization of sedimentary carbon to carbon dioxide.

 – Exactly how much carbon that is released depends on whether the decomposition increases and, if so, how much. In addition, trawling changes the structure of the bottom and the composition of species living there, which might have a long-term effect on the carbon turnover at the bottoms. It is not inconceivable that trawling can release significant amounts of carbon, says Sofia Wikström, marine ecologist and researcher at the Baltic Sea Centre.

A very uncertain calculation



Marine ecologist and researcher Sofia Wikström, Stockholm University Baltic Sea Centre.

Sofia Wikström researches species and habitats in the Baltic Sea and has studied both marine area protection and the effects of bottom trawling on various ecosystem services.

 – What we already know today is that bottom trawling causes extensive disturbance to animal communities on the bottoms. The risk of trawling damaging the bottom's ability to store carbon dioxide has not been noticed until recent years. This means that there is very little research that shows what actually happens to the carbon storage when a trawl is pulled over the bottom, she says.

The empirical studies that exist show what happens on a small scale when a bottom is being trawled.

 – The authors of this study use the knowledge of how much carbon that is stored in different areas to make a simple calculation of the amount of carbon that could be released from the sediments on a global scale. It is a very uncertain calculation, says Sofia Wikström.

However, bottom trawling also causes direct emissions of carbon dioxide emissions to the air, she points out.

 –These boats use a lot of fuel.

Emissions comparable to those caused by aviation?

4.9 million square kilometers, or 1.3 percent of the total sea surface, are currently used for bottom trawling worldwide. Most carbon dioxide is released during the first year of trawling, according to the authors of Protecting the Global Ocean for Biodiversity, Food and Climate. If all the area that is trawled today would be trawled for the first time, the researchers estimate a release of 1,470 million metric tonnes of carbon dioxin. The following years, the effect is decreasing, to stabilize after nine years at about 40 percent of year one; 580 million tonnes per year on a global scale. These figures have been compared in for example The Guardianwith the annual emissions from air travels; 918 million tonnes.

   The difference, however, is that emissions from aviation are made to the atmosphere, where they directly contribute to the greenhouse effect and affect the climate, while carbon dioxide in the water rather affects the oceans' ability to absorb carbon dioxide from the air, explains Erik Gustafsson, oceanographer and researcher at the Baltic Sea Center. Today, the oceans are estimated to absorb about a quarter of the carbon dioxide released into the atmosphere due to human activities. This has a dampening effect on global warming, but at the same time leads to an acidification of the oceans, with major consequences for ecosystems, as the Baltic Sea Centre has described in a policy brief.

Oceanographer and researcher Erik Gustafsson, Stockholm University Baltic Sea Centre. 

  – The carbon dioxide released during bottom trawling reduces the oceans' ability to absorb additional carbon dioxide from the air. It also contributes to the ocean acidification, but it is difficult to say how large these effects are. There are probably great variations between different locations, says Erik Gustafsson.

   In addition to carbon dioxide, however, nutrients are also released when the bottoms are trawled, he points out. In the long run, this can lead to increased primary production where more carbon dioxide than before is being taken up by phytoplankton.

 – This can partially counteract the effect of the carbon dioxide being released from the sediments. Site-specific studies are required to answer what the net effects are.

Highlights the importance of marine ecosystems

Alf Norkko, who is a professor at Tvärminne Zoological Station, University of Helsinki, thinks that the study brings attention to an important issue.

 – If the seabed is to function as a carbon sink, it must be left undisturbed for hundreds or even thousands of years. Bottom trawling disturbs this important storage.

Within the research collaboration between Baltic Sea Centre and Tvärminne called Baltic Bridge, studies are underway on the flows of greenhouse gases, such as carbon dioxide and methane, between sediment, water and air. Measurements have shown that large amounts of methane were released from the bottom sediments of the Baltic Sea, and on to the air, during the record hot summer of 2018. Elevated sea temperatures, eutrophication and disturbed coastal ecosystems are believed to lead to increasing amounts of greenhouse gases being released from the sea, instead of being absorbed,

 – This study highlights the importance of marine ecosystems, and that much more research is needed to better understand carbon sinks and carbon sources, Alf Norkko says.

Stronger protection has several benefits

Today, only seven percent of the oceans have some form of protection and even less, 2.7 percent, are highly protected. At the UN Conference on Biological Diversity to be held in Kunming, China, later this year, the countries are expected to take a stand on a goal of preserving by 2030 to extend protection to cover at least 30 percent of land and sea (the so-called 30x30 target).

The authors of Protecting the Global Ocean for Biodiversity, Food and Climate provide further arguments for such a goal. In addition to securing the oceans function as carbon sink, increased protection would have major positive effects on biodiversity and lead to increased food production. Protected areas often seem to conflict with the interests of the fishing industry, but if the right areas are protected, catches of fish and shellfish could increase by as much as 8 million tonnes per year compared to today, according to the study, by increasing both the quantity and diversity of species.

 – The worst enemy of successful fisheries is overfishing—not protected areas, lead author Enric Sala in a press release.

He is supported by marine ecologist Henrik Svedäng, researcher in fish and fisheries, at the Baltic Sea Centre.

 – Today's fisheries is a disaster for ecosystems and the profit is declining. The costs for improving the situation by protecting more areas are very small, if any, he says.

Location and coordination are crucial

To have an optimal effect, however, it’s important that the marine protected areas are strategically located and that countries cooperate. A globally coordinated effort is described by the researchers as almost twice as effective as uncoordinated planning at national level.

Several parts of the Baltic Sea, especially the southwestern parts, are identified as high-priority areas, in order to optimize the protection of biodiversity, carbon storage and food provisioning at a global level.

 – Bottom trawl fishing in the Baltic Sea has been concentrated on cod and has since the 1990s mainly taken place in the southern Baltic Sea, says Henrik Svedäng.

 – Also parts of the Skagerrak and Kattegat would be important to protect, as the fish stocks and the biological diversity there are very negatively affected by bottom trawl fishing.