Forests are potent carbon sinks, but also the oceans' seagrasses can store enormous amounts of carbon. A little bay in Denmark stores a record amount of carbon. Here is the secret.

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 Thurobund on the island Thuro 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 Thurobund," says Professor Marianne Holmer, University of Southern Denmark (SDU).

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 Thurobund.

"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.

Thurobund 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 Rohr, University of Southern Denmark and Abo Akademi University in Finland. Co-authors are Marianne Holmer and Christoffer Bostrom from Abo 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 Rohr.

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.

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

"The value in Denmark is 1809 Euro pr. hectar while in Finland it is 281 Euro," says Emilia Rohr.

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.

The worlds' oceans store vast amounts of carbon, especially in coastal areas like mangrove forests, seagrass meadows and salt marshes. This is called blue carbon, and the mechanisms are the same as when land forests store carbon. Seagrass meadows cover only 0.1 - 0.2 pct. of the worlds' seafloor, but they account for storing of up to 18 pct. of all ocean stored carbon. The meadows in the Baltic Ocean store 1.7 - 12 pct. of all seagrass meadow stored carbon in the world.

Seagrass is not a seaweed, but a plant with flowers, leaves and roots just like plants on land. It also produces seeds, which is spread on the seafloor and grow to new plants. There are ca. 60 seagrass species in the world. In Denmark eelgrass (Zostera marina) is the most common. Seagrass needs light and only grows in shallow water. They need min. 10 pct. of the sun's light.