Seagrasses cover less than 0.2% of the sea floor, but account for more than 10% of the total carbon absorbed in the oceans. This is an added reason why it is vitally important to preserve the areas of seagrass in the oceans

Seagrasses form expansive meadows beneath the surface of the sea and serve as guardians of a hidden carbon deposit in the mud of the sea floor. The seagrasses and the beds under them store almost as much carbon per square meter as terrestrial forests.

Unfortunately areas of seagrass are now disappearing at a rapid rate. Currently, approx. 7 % of the area covered by sea grass disappears each year. This is the same rate as rain forests are disappearing.

Carbon is released into the atmosphere

A major international research project has calculated for the first time how much carbon lies hidden among the seagrass meadows of the world. From Denmark, Aarhus University and the University of Southern Denmark participated, and researchers were very surprised.

"Carbon has been slowly stored under the seagrass meadows over thousands of years as they have grown in area and height. Carbon from this sink is released as carbon dioxide into the seawater and later into the atmosphere when a seagrass meadow disappears,” explains Dorte Krause-Jensen, Senior Researcher at the Department of Bioscience at Aarhus University.

The researchers have calculated that the amount of carbon dioxide which is lost from seagrass bed attrition each year corresponds to 10% of what is released from terrestrial sources, when forests are felled and agriculture changes.

The seagrasses bind carbon directly when they perform photosynthesis, grow and form leaves and roots. The leaves also act as particle traps which captures numerous carbon-containing particles in the water, which subsequently become part of the mud on the sea floor.

Seagrass meadows can be up to several thousand years in age, and the sea floor beneath them can have risen several metres above the original level. The carbon hidden in the seagrass beds is only slowly converted, because there is very little or no oxygen in the sea floor

Seagrasses are valuable in many respects

"In Denmark it is eelgrass in particular which we find along the coasts, and we still have areas of seagrass, even though there has been a lot of attrition over the last 100 years. Seagrasses serve as a nursery for juvenile fish, and preserve a high degree of species diversity. They retain nutrients, make the water clear and serve as natural coast protection. We now know that they also play an important role in the global carbon budget,” says Dorte Krause-Jensen

The results show that effort should be made to preserve seagrasses, just like rainforests and coral reefs, and that their effects should be included when discussing global climate strategy initiatives.

The results were published in Nature Geoscience on 20 May 2012.