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Seagrass: unsung ecological hero, potential economic powerhouse

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Seagrass fronds
Could Australia's abundant seagrass be harnessed as an economic resource?()
Seagrass fronds
Could Australia's abundant seagrass be harnessed as an economic resource?()
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.
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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.

We have some of the best examples of destroying seagrass meadows in the world.

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.'

Seagrass and fish
Some species of fish spend their entire lives in the habitat that seagrasses create.()

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.'

Read more: Damaged seagrass meadows release ancient carbon

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.'

Seagrass and sediment
Dredging and coastal development are threats to the continued survival of seagrass.()

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.'

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Environment