Using only data from sites for which full inventories exist, we estimate that, globally, seagrass ecosystems could store as much as 19.9 Pg organic carbon; according to a more conservative approach, in which we incorporate more data from surface soils and depth-dependent declines in soil carbon stocks, we estimate that the seagrass carbon pool lies between 4.2 and 8.4 Pg carbon. We estimate that present rates of seagrass loss could result in the release of up to 299 Tg carbon per year, assuming that all of the organic carbon in seagrass biomass and the top metre of soils is remineralized.

—Fourqurean et al.

The results demonstrate that coastal seagrass beds store up to 83,000 metric tons of carbon per square kilometer, mostly in the soils beneath them. As a comparison, a typical terrestrial forest stores about 30,000 metric tons per square kilometer, most of which is in the form of wood.

The research also estimates that, although seagrass meadows occupy less than 0.2% of the world’s oceans, they are responsible for more than 10% of all carbon buried annually in the sea.

“Seagrasses only take up a small percentage of global coastal area, but this assessment shows that they’re a dynamic ecosystem for carbon transformation,” said James Fourqurean, the lead author of the paper and a scientist at Florida International University and the National Science Foundation’s (NSF) Florida Coastal Everglades Long-Term Ecological Research (LTER) site. The Florida Coastal Everglades LTER site is one of 26 such NSF LTER sites around the world in ecosystems from forests to tundra, coral reefs to barrier islands.

Seagrasses have the unique ability to continue to store carbon in their roots and soil in coastal seas. We found places where seagrass beds have been storing carbon for thousands of years.

—James Fourqurean

The research was led by Fourqurean in partnership with scientists at the Spanish High Council for Scientific Investigation, the Oceans Institute at the University of Western Australia, Bangor University in the United Kingdom, the University of Southern Denmark, the Hellenic Center for Marine Research in Greece, Aarhus University in Denmark and the University of Virginia.

Seagrass meadows, the researchers found, store 90% of their carbon in the soil, and continue to build on it for centuries.

In the Mediterranean, the geographic region with the greatest concentration of carbon found in the study, seagrass meadows store carbon in deposits many meters deep.

Seagrasses are also among the world’s most threatened ecosystems. Some 29% of all historic seagrass meadows have been destroyed, mainly due to dredging and degradation of water quality. At least 1.5% of Earth’s seagrass meadows are lost every year.

The study estimates that emissions from destruction of seagrass meadows can potentially emit up to 25% as much carbon as those from terrestrial deforestation.

One remarkable thing about seagrass meadows is that, if restored, they can effectively and rapidly sequester carbon and reestablish lost carbon sinks.

—co-author Karen McGlathery, a scientist at the University of Virginia and NSF’s Virginia Coast Reserve LTER site

The Virginia Coast Reserve and Florida Coastal Everglades LTER sites are known for their extensive seagrass beds.

Seagrasses have long been recognized for their many ecosystem benefits: they filter sediment from the oceans; protect coastlines against floods and storms; and serve as habitats for fish and other marine life.

The new results, say the scientists, emphasize that conserving and restoring seagrass meadows may reduce greenhouse gas emissions and increase carbon stores—while delivering important “ecosystem services” to coastal communities.

The research is part of the Blue Carbon Initiative, a collaborative effort of Conservation International, the International Union for Conservation of Nature, and the Intergovernmental Oceanographic Commission of UNESCO.

Resources

• James W. Fourqurean, Carlos M. Duarte, Hilary Kennedy, Núria Marbà, Marianne Holmer, Miguel Angel Mateo, Eugenia T. Apostolaki, Gary A. Kendrick, Dorte Krause-Jensen, Karen J. McGlathery & Oscar Serrano (2012) Seagrass ecosystems as a globally significant carbon stock. Nature Geoscience doi: 10.1038/ngeo1477