Stopping the underwater carbon emissions time bomb could require widespread seagrass transplantation.

Seagrass is up to 35 times more efficient at sequestering carbon than rainforests, and stores it for millennia in the sediment below.

But over the past century 29 per cent of global seagrass has been destroyed and as a result it has been estimated that it is releasing carbon at a rate similar to the emissions of Australia and the UK combined.


Now an analysis of a seagrass disturbance shows that natural regrowth can help cap the release of ancient stored carbon, but the slow process could do with a helping hand.

Holes in the seabed

In 1969, a series of large circular holes were hammered into seagrass beds near Jervis Bay in Australia, to test the suitability of the area for a nuclear power plant. The seagrass there has never fully recovered.

Now Peter Macreadie from the University of Technology Sydney and his colleagues have looked at the area to see what happens to seagrass after a disturbance. They took a series of samples from the soil and compared areas that were undisturbed, those that were disturbed but the seagrass has grown back and those that were disturbed and remain bare.

In the bare areas, 72 per cent of the carbon stored in the top metre – which had been stored over the past 6000 years – had been released in less than five decades.

That supports a 2012 estimate that destroyed seagrass meadows are releasing more than 1 billion tonnes of CO 2 -equivalent each year.

“In some ways, this is the next step,” says Gary Kendrick, a marine ecologist at the University of Western Australia.

The team also examined the seabed microbes and found that in the unrecovered areas, there were more of the microbes that can break down carbon faster using oxygen.

Put a cap on it

But recovered areas lost about half as much carbon as the unrecovered areas. What’s more, the microbe communities were similar to those in the undisturbed areas, so would be slower at further breaking down the carbon.

“That was a little bit unexpected for us,” says Macreadie. “They seem to be doing very well at capping the carbon and keeping it in the ground.”

“This is important as it reinforces arguments that the protection of seagrass meadows is an effective carbon mitigation strategy,” says Carlos Duarte, an ecologist at King Abdullah University of Science and Technology in Saudi Arabia.

But the recovery of the seagrass in Australia has been slow over the past 50 years – the average spreading, horizontal growth of the roots is about 2.5 centimetres a year.

So the researchers suggest that we may need to be growing and then transplanting seagrass, rather than waiting for recovery via natural processes.

Macreadie says this is something that could be possible in areas that have a price on carbon.

“Restoration projects could then be funded by the big polluters,” he says. “We had a carbon farming initiative on land, where farmers were getting carbon credits by turning agricultural land back into forest. I think we should be turning our attention to these ‘blue carbon’ ecosystems that not only store the carbon more efficiently that terrestrial forests but lock it up over much longer time periods.”

Journal Reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2015.1537

Image credit: Michael Patrick O’Neill/Alamy