Tiger Webb ABC News 26 Oct 15;
Seagrass meadows provide carbon capture and storage at a rate up to a hundred times greater than rainforests. So why aren't they being used as an economic resource? The Science Show investigates.
Paul Lavery is a professor of marine ecology at Edith Cowan University, and he is excited about seagrass.
Despite the name, seagrasses aren't actually grasses—they evolved from terrestrial plants and have more in common, evolutionarily speaking, with lilies. Seagrass often gets mistaken for seaweed, though only the former produces fruits and flowers, and the latter lacks the specialised plant tissue that more easily allows seagrass to absorb nutrients from sediment on the sea floor.
Seagrasses get a lot done simply by showing up. The leaves and stems of seagrass slow water currents close to the seabed, reducing erosion and improving water quality. Seagrass provide shelter from predators and a safe spawning point for fish. Some species of fish spend their entire life cycles in seagrass meadows. Seagrass is also an important source of food. Even dead seagrass—washed ashore and known as wrack—provides habitat and nutrients for crabs.
Lavery has another reason to be excited about seagrass habitats: seagrass meadows are phenomenally productive carbon sinks.
'They pack a huge punch in terms of the amount of carbon they can lock away,' says Lavery, who believes the carbon storage capacity of the soil beneath seagrasses to be, per square metre, between ten and one hundred times that of terrestrial systems. In fact, the amount of carbon dioxide stored in aquatic ecosystems is so great that biologists had to give it its own label: blue carbon.
'Blue carbon is the term that we use to talk about carbon captured by marine vegetated ecosystems,' Lavery says. 'These are salt marshes, mangroves, seagrass ecosystems mainly, we use that to differentiate from carbon captured by terrestrial ecosystems—rainforests, the arboreal forest of the northern hemisphere—which we refer to as green carbon.'
Australia's coastline offers the largest and most diverse seagrass assemblage in the world. 'We are blessed,' says Lavery, 'it's fantastic to work here.'
'But we also have some of the best examples of destroying seagrass meadows in the world.'
Mining and coastal development throw up sediment, blocking the light needed for seagrass to photosynthesise. Run-off in rainier seasons can result in 'nutrient-laden water', which sounds as though it would be healthy for aquatic plants but unfortunately isn't. Oh, and there's the pressures brought about by climate change and trawling.
But, says Lavery, the biggest problem facing Australian seagrasses has to be dredging. As the Australian economy became more dependent on exports, port numbers increased and their size expanded. And that has necessarily involved a lot of sediment dredging.
'If you took all the of the sediment that was going to be dredged,' Lavery mimes with his hands, 'And put those into one cubic metre packing boxes side by side, you could go around the equator three and a half times.'
And that's a problem for blue carbon, because seagrass meadows can only effectively sequester carbon when it's locked away permanently. When deep sediments are disturbed—by dredging or natural causes such as cyclones or flooding—the carbon stored within the sediment can be released. 'You throw it up into this oxygenated and nutrient-rich surface water, and potentially what happens is that bacteria turn it back into carbon dioxide,' Lavery says. 'And that goes back into the atmosphere.'
Biodiversity isn't often seen as sexy, and it can be difficult for policy makers to get het up about erosion. Efforts to engage citizen science in the cause of Australian meadows, laudable though they are, do effectively amount to watching (sea)grass grow. But what if Australia's seagrass meadows were viewed as an economic resource? Enter Lavery and his team, who worked on a very conservative estimate of the amount of carbon the meadows stored and worked out its dollar value, based on prices paid at the global carbon trading market at the time.
'We came up with a value of many billions of dollars,' he says.
Concerns have been raised over the cost and efficiency of carbon capture and storage as a means of carbon emission abatement in Australia. According to Lavery, many of the problems with carbon capture and storage could be cheaply and efficiently solved by the use of seagrass meadows.
Unfortunately for Lavery, he has seen a gradual shift away from the use of seagrass in global carbon trading schemes. 'Which is a great pity, I think, because there's potentially an economic bonanza there, which would also have the win-win situation of conserving and looking after these really important seagrass ecosystems.'
So, what does he see as the way forward? 'I think two things need to happen: scientists need to be able to provide very reliable information on how much carbon is stored and where its stored—and what we desperately need is economists to come in from the other side and make it apparent to decision makers about how this carbon can be factored into trading systems.
'As scientists, we're not very good at that.'
Lavery points to forestry systems that provide an economic incentive for conserving productive ecosystems—particularly through United Nations programmes such as REDD+, which provides financial incentives to countries shown to reduce their emissions from deforestation. But for Lavery, the economic argument is the icing on the seagrass cake.
'Here in Western Australia, the Rock Lobster fisheries are the single biggest fishery that we have. They're dependent on those seagrass ecosystems ... elsewhere with mangroves and salt marshes, it's all interconnected.
'The values that seagrass serve are enormous and this is just one more very compelling reason to be looking after these systems.'
Damaged seagrass meadows release ancient carbon
Anna Salleh ABC Science 21 Oct 15;
Human damage to seagrass meadows leads to the release of ancient carbon stores, a study has found.
Australian researchers studied sea grass meadows in Jervis Bay that had been disturbed by seismic testing in the 1960s.
"We found that in the area that had been disturbed, there had been a 72 per cent decline in the amount of organic carbon," marine ecologist Dr Peter Macreadie for Deakin University and the University of Technology, Sydney said.
The findings suggest protecting and restoring seagrass meadows could be an important strategy in mitigating climate change, say the authors in today's issue of the Royal Society journal Proceedings B.
If our findings are common throughout the rest of the world ... then we're talking about a hell of a lot of carbon that's being released.
Dr Peter Macreadie
Seagrass is a flowering plant that forms underwater meadows around coasts on every continent except Antarctica. Seagrass meadows act as a nursery ground for juvenile fish, cycle nutrients, stabilise our coastlines and prevent erosion.
But, they are also very good at sequestering carbon.
"What we've learnt about seagrasses is they can capture and store carbon at a rate of 40 times faster than tropical rainforest," said Dr Macreadie.
He said seagrasses lock away the carbon for much longer than forests on land.
"They store the carbon for millennial timescales," said Dr Macreadie. "It will stay there more or less permanently so long as you don't disturb those ecosystems."
'About 1,000 years' worth of carbon lost'
It had been unclear whether ancient carbon would stay locked in the ground if seagrass meadows were disturbed, or if it would be released again, eventually back into the atmosphere where it could accelerate climate change.
To investigate the issue, Dr Macreadie and his colleagues sampled soil in the areas where the seagrass had been lost and compared it to nearby pristine areas where there had been no loss.
When the researchers dated the sediments they found the sediments were between 1,300 to 3,000 years old.
"About 1,000 years' worth of carbon that had been sequestered had been released and lost as a result of that disturbance," Dr Macreadie said.
He said the findings had implications for the large amounts of seagrass being lost due to coastal development.
"We know that about 50 per cent of the world's seagrasses have disappeared since the 1990s," Dr Macreadie said.
"That's about two football fields of seagrass per hour."
"If our findings are common throughout the rest of the world ... then we're talking about a hell of a lot of carbon that's being released."
How meadows capture carbon
The carbon in the soil of seagrass meadows comes not only from dead seagrass, but also from a host of other life forms such as plankton, as well as sediment from land, all of which are filtered out of the water by the seagrass.
In fact, said Dr Macreadie, previous research has shown, on average globally, about 50 per cent of carbon in seagrass beds comes from the land.
It is believed carbon is able to collect underneath seagrass meadows because of the anoxic environment they create, which prevents aerobic microbes from breaking down the carbon.
In support of this, Dr Macreadie and colleagues also found the microbes in the soil of the Jervis Bay seagrass meadows were different where disturbance had occurred.
"We saw that in the disturbed areas there were much higher abundances of microbes that have the capacity to eat the carbon and break it down," Dr Macreadie said.
There has been growing awareness over the past five years of the importance of seagrasses, salt marshes and mangroves to the carbon cycle.
"They occupy less than 1 per cent of the sea floor but they sequester more than half of the carbon in the ocean, maybe as much as 70 per cent," Dr Macreadie said.
He said the findings suggest protection and replanting of seagrass meadows could prove to be an important strategy to offset carbon emissions.
While the IPCC is yet to officially include seagrass meadows in carbon accounts, he said the Australian Government was making moves to do so.
