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Time capsules beneath the sea

Protector of shipwrecks and buried treasure, custodian of environmental records and defender of prehistoric sites… seagrass is nature's secret keeper.
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Dr Oscar Serrano led the team which quantified the amount of blue carbon being absorbed by Australian marine ecosystems.

Protector of shipwrecks and buried treasure, custodian of environmental records and defender of prehistoric sites… seagrass is nature's secret keeper.

On 22 July 1841, a violent storm struck the port of Fremantle in Western Australia. The James Matthews, a former slave ship, came away from its anchor, filled with water and sank.

As the boat settled on the ocean floor near Woodman Point, it began to be colonised by seagrass. Over the years, the seagrass grew, depositing sediments that would cocoon the ship from the outside world. It protected the wreck for more than a century, holding off currents, waves and shipworm.

But in the early 1970s, something changed. Particles from a cement plant off Cockburn Sound clouded the water, stopping light from reaching the seagrass meadow. The seagrass began to die, exposing the treasure it had protected for generations. The James Matthews wreck was then discovered in 1973 by an underwater explorers club.

Dr David Gregory, a researcher at the National Museum of Denmark who worked alongside the Western Australian Museum to help save the James Matthews, says the dying seagrass had completely encompassed the ship. "The seagrass literally covered over the wreck, and the hull and all the contents of the ship," he says. "It had encapsulated it into this wonderful time capsule."

The ultimate protector

Seagrass grows in shallow waters all over the world. It's found in climates ranging from tropical to cool and on every continent except Antarctica. The marine plant has very complex root structures that bind sand and stop it eroding, building up layer upon layer of material and effectively elevating the sea floor.

Dr Oscar Serrano, a marine scientist from the Centre for Marine Ecosystems Research at ECU, says seagrass can bury shipwrecks and other cultural heritage under metres of sediments. "This protects the heritage in two ways," he says. "First of all, there's a physical protection, so it's no longer exposed to currents and waves and erosion."

Secondly, Serrano says seagrass offers protection from shipworm and microbes, which would usually degrade organic materials such as wood, bone and leather. Instead, the seagrass sediments seal off shipwrecks from oxygen, cutting microbial activity and slowing decomposition.

Protection can last thousands of years. In Europe, Gregory spends most of his time looking at submerged prehistoric sites, such as a Neolithic fish trap that has been protected by seagrass on and off for 5,000 years.

Gregory's 'wow' moment came when he and Aarhus University senior researcher Dr Dorte Krause‑Jensen overlayed a map of seagrass meadows in Denmark with all the known prehistoric sites. "There was almost a beautiful one-to-one correlation," Gregory says. "Where we've got seagrass preserved, that's where we're finding the best submerged prehistoric sites. And I'm sure it would be the same for the wrecks as well."

Coastlines of clues

In Australia, there are about 125,000 square kilometres of seagrass meadows.

In Australia, Serrano says there are about 125,000 square kilometres of seagrass meadows. That's almost enough to cover Tasmania twice over – and enough to hide countless important cultural sites. He says some seagrass varieties are tiny, about a centimetre in size, while others can grow up to a metre long. The larger seagrasses, Serrano says, are the best at preserving heritage because they typically grow faster and accumulate more sediments.

But shipwrecks aren't the only historical secrets seagrass is holding on to. Serrano's pioneering work discovered that seagrass 'cores', samples of seagrass, can be used as historical archives. The cores are collected by driving two metre-long pipes into the seabed, and can be dated along their length in much the same way as Antarctic ice cores.

Krause-Jensen says studying the contents of seagrass cores can tell us how much carbon or nitrogen there was in the past. The contents also offer clues to historical timelines, such as when lead stopped being added to fuel. "You can see silver concentrations [declining in the sediments] when we [moved] to digital photography," Krause-Jensen says. "There are many traces from humans that are … accumulated in the sediments."

With carbon-14 dating, researchers can look back thousands of years. A seagrass meadow in the Mediterranean Sea, for instance, is thought to be up to 200,000 years old. That dates it to the Ice Age and the dawn of humanity.

Thirsty for carbon

Seagrass is highly efficient at removing carbon dioxide from the atmosphere. The effect is so great that seagrass meadows absorb CO2 about 40 times faster than rainforests. Krause-Jensen says that's because seagrasses are super photosynthesisers. "They suck up carbon from the water and build it into organic matter in their leaves and roots," she says. "This biomass then gradually decays and accumulates in the sediments."

Krause-Jensen says seagrass also acts as an oasis against ocean acidification, a decrease in pH that occurs when atmospheric carbon dioxide dissolves in seawater. "It takes out the CO2 and that increases the pH level locally around it during growth periods," she says. That can help organisms that build their shells from calcium carbonate, such as oysters and clams. In one study, blue mussels were found to increase shell production during the day when seagrasses were photosynthesising, Krause-Jensen says.

Serrano says seagrass enhances biodiversity and offers a habitat for other marine organisms. He says they are important for coastal protection, neutralising wave energy and reducing erosion. And seagrass keeps the water clean by filtering nitrogens, phosphorus and other pollutants. "They also play a key role for fisheries of commercial species such as snapper," Serrano says. "They're actually the place where juveniles grow and feed."

Under threat

Despite its ecological importance, seagrass is under threat. "Around the world and in Australia since the 1950s, we've lost about 25 to 30 per cent of seagrass meadows," Serrano says.

In the past the biggest threat to seagrass was algal blooms caused by the release of nutrients into the water, he explains. Today, the biggest threats are dredging and climate change. "When you have sustained and very high water temperatures for a few weeks, it stresses seagrass and it dies," Serrano says. He points to the 2011-2012 heatwave in Western Australia, which took out 1,000 square metres of seagrass meadows at Shark Bay alone.

In Europe, though, seagrass may be making a comeback. Krause‑Jensen, who is based in Denmark, says marine action plans have set strict limits on dredging activities and the amount of nitrogen and phosphorus that can be released into the ocean. And in some areas, fixed anchoring places have been installed to prevent anchors from small boats disturbing the seagrass meadows. For the first time in decades, Krause-Jensen says the rate of seagrass decline has slowed, and some places in Europe have even begun to see a positive trend.

If a similar turnaround could be brought about in Australia, it could help slow climate change Time capsules beneath the sea and protect the nation's marine biodiversity. It would also help to preserve our cultural heritage – the 7,000 shipwrecks thought to be in Australian waters and Indigenous heritage dating much further back.

Among this cultural heritage is the James Matthews. Part of the wreck was excavated in the late 1970s, and some relics are on display in the Shipwreck Galleries of the WA Maritime Museum. But much of the ship was reburied beneath the sediments. Seagrasses could protect the secrets of the James Matthews once again.

The blue carbon economy

Blue carbon ecosystems, the carbon stored in coastal and marine ecosystems including seagrass, mangrove and salt marshes, play an important role in climate change mitigation and adaptation.

World-first research by ECU and an international team of collaborators has shown Australia's blue carbon ecosystems absorb 20 million tonnes of carbon dioxide each year – around the same as the annual emissions of more than four million cars.

It's the most accurate estimate for any country in the world and opens the door for investment in conserving and restoring these ecosystems.

It means Australia can potentially start counting carbon credits from those efforts towards emissions reductions in the Paris Agreement on climate change.

Dr Serrano says this new research positions Australia as a world leader in the protection and management of blue carbon ecosystems.

"Australia is home to around 10 per cent of the world's blue carbon ecosystems, so there's enormous potential for us to take a lead role in this space," he says.

"This could take the form of replanting seagrass meadows, restoring mangroves by reflooding or by preventing expected losses through environmental management."


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