Mowing down seagrass meadows will cut loose carbon

Michael Slezak New Scientist 20 May 12;

They may be trickier than trees for environmental protesters to chain themselves to, but it turns out that seagrass ecosystems hold as much carbon per hectare as the world's forests – and are now among its most threatened ecosystems.

In the past century, 29 per cent of seagrass has been destroyed globally", mostly by water pollution, dredging for new developments, and climate change. With seagrass meadows disappearing at an annual rate of about 1.5 per cent, 299 million tonnes of carbon are also released back into the environment each year, according to research published this week in Nature Geoscience (DOI: 10.1038/ngeo1477).

Piecing together old and new data from 946 seagrass meadows around the world, an international team of researchers estimated that seagrass captures 27.4 million tonnes of carbon each year, burying it in the soil below. And unlike forests that hold carbon for about 60 years then release it again, seagrass ecosystems have been capturing and storing carbon since the last ice age.

That means that up to 19.9 billion tonnes of carbon are currently stored within seagrass plants and the top metre of soil beneath them – more than twice the Earth's global emissions from fossil fuels in 2010. If the seagrass dies, all of that could be released into the environment, says marine ecologist and study author James Fourqurean from Florida International University in Miami, US.

"These are scary numbers," says Gary Kendrick, a co-investigator on the project from University of Western Australia at Crawley, Australia. "It would put us very much into the extreme of greenhouse situations very very quickly."

This grim outlook is reinforced in a study published at the same time in Nature Climate Change (DOI: 10.1038/nclimate1533). Gabriel Jorda from the Mediterranean Institute for Advanced Studies in Esporles, Spain, found the warming climate is eradicating the Mediterranean seagrass Posidonia oceanica, which is likely to be extinct before 2050. That is particularly worrying because Posidonia oceanica holds about 10 times as much carbon as most other species.

"It does look like there's going to be a global tipping point for many of these environments," Kendrick says.

Seagrass physiologist Peter Ralph from the University of Technology Sydney in Australia, who was not involved in the research, said continued destruction of seagrass meadows could ultimately "release the genie from the bottle".

"Destroy them and we're going to release a lot of carbon that we have assumed is sequestered and tied up for a very long period of time," says Ralph.

Seagrass stores carbon like trees
The University of Western Australia Science Alert 22 May 12;

Researchers at The University of Western Australia have contributed to the first global analysis of carbon stored in seagrasses which shows they can hold as much carbon as the world's temperate and tropical forests.

The study 'Seagrass Ecosystems as a Globally Significant Carbon Stock,' published in the journal Nature Geoscience provides further evidence of the important role the world's declining seagrass meadows have to play in mitigating climate change.

Results gathered from 3640 observations of 946 distinct seagrass meadows across the globe show that coastal seagrass beds store up to 83,000 metric tons of carbon per square kilometre, mostly in the soils below them. In comparison, a typical land forest stores around 30,000 metric tons per square kilometre.

The research also estimates that, although seagrass meadows occupy less than 0.2 per cent of the world's oceans, they are responsible for more than 10 per cent of all ‘blue carbon' stores buried annually in the ocean and rival carbon stores in the extensive peat deposits of mangroves.

Data sets as deep as one metre were concentrated in Florida Bay, USA; the Spanish coast of the Western Mediterranean; and Shark Bay, Western Australia. The greatest concentration of carbon found was in the Mediterranean where seagrass meadows stored carbon many metres deep. According to the study, seagrass meadows store ninety per cent of their carbon in the soil and continue to build on this indefinitely.

UWA Professors Gary Kendrick and Carlos Duarte contributed to the study led by Dr James Fourqurean, a professor of biology at Florida International University.

"These results show that seagrass meadows are key sites for carbon storage and probably are far more important as carbon dioxide sinks than we realised," Professor Kendrick said.

Seagrasses are among the world's most threatened ecosystems. Around 29 per cent of all historic seagrass meadows have been destroyed, mainly due to dredging and degradation of water quality and a further 1.5 per cent of seagrass meadows are lost each year. The study estimates that emissions from destruction of seagrass meadows can potentially emit up to 25 per cent as much carbon as deforestation on land.

"The good news is if seagrass meadows are restored they can effectively and rapidly reestablish lost carbon sinks and stores as well providing a range of other valuable ecosystem benefits, including water quality protection, and as an important biodiversity habitat," Professor Kendrick said.

Seagrasses Can Store as Much Carbon as Forests
ScienceDaily 23 May 12;

Seagrasses are a vital part of the solution to climate change and, per unit area, seagrass meadows can store up to twice as much carbon as the world's temperate and tropical forests.

So report researchers publishing a paper this week in the journal Nature Geoscience.

The paper, "Seagrass Ecosystems as a Globally Significant Carbon Stock," is the first global analysis of carbon stored in seagrasses.

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 percent of the world's oceans, they are responsible for more than 10 percent 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," said Fourqurean. "We found places where seagrass beds have been storing carbon for thousands of years."

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 ninety percent 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 among the world's most threatened ecosystems. Some 29 percent of all historic seagrass meadows have been destroyed, mainly due to dredging and degradation of water quality. At least 1.5 percent 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 percent 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," said paper 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.

Journal Reference:

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. Seagrass ecosystems as a globally significant carbon stock. Nature Geoscience, 2012; DOI: 10.1038/ngeo1477