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Climate changes

Loss of eelgrass - a severe threat to climate account

Eelgrass is disappearing in many places in the world, and with it disappears nature’s own effective binding of carbon and nitrogen.

By Birgitte Svennevig, , 8/11/2021

A new study shows for the first time that lost eelgrass beds lead to significant emissions of carbon and nitrogen – substances that contribute to climate change and threaten biodiversity.

Normal, healthy eelgrass beds are effective in absorbing and storing both carbon and nitrogen, and when these beds disappear, we must either live with the substances being released, or we must remove them from the globe’s cyclic process ourselves, e.g. through climate compensation.

According to the new study, it will cost a shade under DKK 1,038,000 to compensate for the loss of one hectare of eelgrass bed in the area they have studied. Of this sum, DKK 984,000 goes to compensation for nitrogen, and DKK 53,800 to compensation for carbon.

Take good care of the eelgrass

- Therefore, it makes perfect sense to take good care of the eelgrass beds we have and thus get nature’s own help to absorb and store carbon and nitrogen, says Marianne Holmer, an expert in coastal zone ecology and Professor at the University of Southern Denmark.

Marianne Holmer is one of the authors of the new study, whose other authors are researchers from Stockholm University, Gothenburg University and Åbo Akademi University. The study is published in the scientific journalEcosphere

According to the researchers, this is the first study that shows that the loss of eelgrass beds leads to significant emissions of carbon and nitrogen into the environment.

It makes perfect sense to take good care of the eelgrass beds we have and thus get nature’s own help to absorb and store carbon and nitrogen

Marianne Holmer, Professor

The study has compared two areas in western Sweden: One area has been overgrown with eelgrass beds present throughout the period, while the other has lost its eelgrass beds since the 1980s.

In the area where the beds have disappeared, there is today significantly less carbon and nitrogen in the sediments where eelgrass used to grow. The researchers estimate that a sediment layer of at least 35 cm has eroded away since the 1980s, and during that process, its content of carbon and nitrogen has been released into the environment. - Eelgrass beds can have carbon and nitrogen-storing sediment layers beneath them, which can be several metres thick. This makes them global hotspots for storing carbon and nitrogen, says the study’s head researcher, Per Moksnes, Associate Professor at the University of Gothenburg.

Comparable to fish farms

The researchers estimate that – conservatively calculated – more than 60 tonnes of carbon and 6.6 tonnes of nitrogen are released for each hectare of eelgrass beds that disappears. 60 tonnes of carbon corresponds to 220 tonnes of CO2 and thus to the annual CO2 footprint of 17 average Danes. 6.6 tonnes of nitrogen correspond to the annual average emissions from a fish farm in Sweden. - It is therefore important to take these emissions into account when permitting activities that damage eelgrass beds, he says.

Identical patterns in Denmark and the rest of the world

In the fjords surrounding Marstrand near Gothenburg, almost 10 square kilometres of eelgrass beds have disappeared since the 1980s. This has led to the release of 60,000 tonnes of carbon and 6,600 tonnes of nitrogen over a 20-year period. If the loss is to be compensated, it will cost DKK 1,038 billion.

According to Per Moksnes, during this period, three times more nitrogen have been released from this area into the Skagerrak strait than from all other Swedish watercourses. - It is not only in Sweden that eelgrass beds are disappearing. Seagrasses are in decline in all parts of the world, especially in areas with a high level of construction and tourism activity, and this emphasises the importance of preserving the beds that still exist and making an effort to re-establish lost eelgrass beds, says Marianne Holmer.

Planting new eelgrass

For example, in Denmark and Sweden, work is being done in several places to plant small new eelgrass shoots in fjords and other protected coastal areas that have previously had eelgrass beds.

By establishing new eelgrass beds, nature’s own solutions are used to capture and store carbon instead of resorting to, for example, technological solutions.

Denmark, Finland and Sweden are all members of the

International Union for the Conservation of Nature, which works to use nature’s own methods to solve many of the major problems facing the world, such as climate change, food shortages and the loss of biodiversity.

The blue forests

Eelgrass beds, salt marshes and the like are also called ‘blue forests’ because, like terrestrial forests, they bind large amounts of carbon. Researchers believe restoring salt marshes and eelgrass beds can improve Denmark’s carbon accounting.

Read more in this article from Aktuel Naturvidenskab

Meet the researcher

Marianne Holmer is an expert in coastal ecology. Professor Holmer is the Dean of the Faculty of Science and Director of the Danish Institute for Advanced Study at SDU.

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Editing was completed: 11.08.2021