Mowing down seagrass meadows will cut loose carbon

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.