This bay in Scandinavia holds world record in carbon storing

By Birgitte Svennevig, birs@sdu.dk, 1/12/2017

Seagrass plays a bigger role in the Earth’s carbon cycle than most of us think. The underwater meadows of seagrass are capable of storing large amounts of carbon – a talent that draws attention in a time, where decision makers and scientists are searching for ways to bring down the release of CO₂ to the atmosphere.

Efficient meadows of carbon storing seagrass are found in coastal areas in many parts of the world. But according to biologists one particular meadow in Denmark is by far the most efficient.

The meadow is situated in the bay Thurøbund on the island Thurø in the South Funen Archipelago, Denmark.

Special conditions in this bay

- Many seagrass meadows in the world have been investigated. Recently I was part of investigating and measuring carbon storing capabilities of 10 seagrass meadows in the Baltic Sea. No place comes even close to Thurøbund, says Professor Marianne Holmer.

Professor Holmer is head of SDU’s Department of Biology and she is an expert in seagrass ecology and biogeochemistry.

The explanation is found in the special conditions in Thurøbund.

Protected and productive

- It is a very protected bay – and also very productive. So the seagrass thrives and when the plants die they stay in the meadow. They are buried in the sediment and in this process their content of carbon gets stored with them. In Finland the seagrass grows in open coast areas, which means that the dead plants much more often are washed off to sea, taking the carbon with them. Once the carbon has been taken out to the sea it is unsure what happens to it, says Professor Holmer.

Thurøbund stores ca. 27,000 grams of carbon (gC) pr. square meter. This figure has never been measured to be more than 10,000-11,000 gC pr. m² in other parts of the world.

According to the new study Danish seagrass meadows store 3-4 times more carbon than Finnish meadows.

Harsher environment in Finland

The study’s lead author is PhD student Emilia Röhr, SDU and Åbo Akademi University in Finland. Co-authors are Marianne Holmer and Christoffer Boström from Åbo Akademi University. The study has been published in Biogeosciences.

- The Finnish meadows in our study are more exposed than the Danish and they grow in harsher environments where the dead plants do not sink to the bottom, so their carbon content does not get stored in the sediment, explains Emilia Röhr.

It is unknown where the dead seagrass plants go and what happens to them, when they get washed out to the open sea. Maybe their carbon gets stored elsewhere, maybe it ends up as CO₂ in the atmosphere.

The economical value

Due to the carbon storing efficiency of seagrass meadows, a system has been designed to calculate the economical value of their stored carbon.

- The value in Denmark is € 1809 pr. hectar while in Finland it is € 281, says Emilia Röhr.

Other scientists have calculated that the global loss of seagrass equals 1.9 – 13.7 billion US dollars in lost carbon storing.

Fish and shrimps need seagrass

Many countries have a focus on restoring lost seagrass meadows. The meadows are not only good at storing carbon: they are also home for many small and large animals, including commercially important species as shrimps, cod and flatfish.

The plants also function as particle filters, keeping the water clear.

On a global scale Earth has lost estimated 29 pct. of its seagrass meadows since 1879. Danmark has lost 80-90 pct. since the 1930s.