Flooding Wetlands Could Be the Next Big Carbon Capture Hack

Arriving at the tidal wetlands of Mungalla Station on the coastline of northern Queensland, ornithologist Simon Kennedy from the not-for-profit BirdLife Australia is greeted by a welcome cacophony. “You start hearing honks and quacks and twitters and noises coming from there,” he says of the area’s diverse and thriving bird populations, “whereas it’s very quiet elsewhere.”

It wasn’t always this way. A decade or so ago, these saltwater wetlands—which cover around a quarter of the 880 hectares that make up Mungalla Station—were a mess of freshwater-sodden pastures, riddled with invasive weeds that were choking the land and waterways. The reason was an earth wall—known as a bund, built more than half a century ago where the estuary of the local river meets the sea—that blocked off incoming seawater to transform the saltwater wetlands into a ponded freshwater pasture for cattle farming. This created the perfect environment for invasive freshwater weeds and drove out much of the native wildlife. By transforming the composition of the land, it likely released a lot of carbon and methane, too.

But in 1999 the Nywaigi—the local Aboriginal people and traditional owners of the land—purchased it. “When we got our country back, we inherited the bad farming practices,” says Jacob Cassady, a Nywaigi man and managing director of the Mungalla Aboriginal Business Corporation, which now owns the station.

Working with scientists and environmental organizations, the Nywaigi people have since transformed Mungalla Station back into traditional saltwater wetlands, creating a haven for native plant and animal life—including huge saltwater crocodiles. This has drawn in birdwatchers and ecotourists, but has had more far-reaching consequences too: improving the health of the nearby Great Barrier Reef as well as capturing carbon, meaning the wetlands could be used in the future to offset emissions elsewhere.

With the impacts of global warming becoming more severe every year, governments—including Australia’s—are waking up to the reality that significant climate action is needed, and that reducing greenhouse gas emissions alone isn’t going to be enough. Drawing carbon out of the air and locking it back into the landscape—what’s known as “carbon sequestration”—has to happen too.

You can do this mechanically, but also through harnessing the processes of nature. Reforestation can lock carbon into the terrestrial landscape, creating “green” carbon stores, while regenerating the plants and wildlife of coastal ecosystems—such as estuaries, mangroves, and saltwater marshes—locks in carbon as well, creating “blue” carbon sinks. These have significant benefits.

When in their natural state, coastal wetland environments like those at Mungalla store carbon for much longer and more efficiently than terrestrial ecosystems, and regenerating them also creates habitats for marine species that are a vital food source for many communities. Re-establishing environments such as mangroves can also serve as a buffer against storms and tsunamis. For the Nywaigi people, returning the wetlands to their original state has also allowed them to reconnect with lands that they share a historic bond with, while also boosting tourism and creating jobs.

The revival of Mungalla is a story of many gains—so much so that it could soon be a template for similar regeneration projects around the coast of Australia.

THE FIRST CHALLENGE in restoring the wetlands was getting rid of the weeds. After the bund was built all those years ago, species such as olive hymenachne and water hyacinth thrived, dominating the newly created freshwater pasture. These were not only invasive, but their thick growth reduced the amount of oxygen in the water, making it difficult for fish and crustaceans to survive, reducing the availability of food for birds.

Given the wetlands’ proximity to the Great Barrier Reef, this was a major concern, says ecologist Lynise Wearne, program director at environmental not-for-profit Greening Australia. Saltwater wetlands “are like the kidneys of the reef,” she says. “They actually help to filter out the water and improve the water quality when they’re functioning at a really high level, before the water would flow out to the reef.”

Saltwater wetlands and estuaries are also vital nurseries for many of the fish species that inhabit the reef. Barramundi and mangrove jack are both commercially important and have a lifecycle that involves the fish migrating to and from the reef, across tidal floodplains to freshwater rivers. Obstacles such as bunds put a stop to that.

Unfortunately for the World Heritage-listed Great Barrier Reef, many of the wetlands that feed into it have suffered a similar fate to Mungalla, their entrances blocked off decades ago by bunds and other obstacles to convert saline wetlands and estuaries into freshwater pastures. There are estimated to be more than a thousand bunds along the Queensland coastline that runs up the eastern side of Australia.

After taking over the management of the land, the Nywaigi people had been trying to eradicate the weeds with herbicides. Not having had any success, in 2012 they tried a different tactic, removing a section of the bund wall to let the salt tides back into the wetlands. The effects were dramatic. The salt water killed the weeds, saltwater plant species such as the blue water lily returned, and with them came the marine wildlife and birds.

“We’d started doing some fish surveys, catching little barramundi and other estuarine fish species and crustaceans that we know move across this landscape,” says Nathan Waltham, an aquatic ecologist at James Cook University’s Centre for Tropical Water and Aquatic Ecosystem Research.

But the success was short-lived. The first three years after the bund was breached were particularly dry. With less fresh water than usual coming down the river that feeds into Mungalla, the saltwater tides were more effective at eradicating the weeds. But when the rains returned in 2015 and 2016, the fresh water flowing downstream overcame the tidal salt waters, the weeds returned, and it looked like all the gains in restoring the salt wetlands would be lost. In an effort to increase the amount of salt water in the wetlands, boreholes were drilled to access subterranean salt water and bring it to the surface.

Powered by solar energy, these bores have had just as much effect as the original breaching of the bund, but they’re not subject to the same climatic vicissitudes. As a result, the saltwater wetlands are flourishing.

“Before the bores, the wetlands were more than half-infested in weeds,” says Kennedy from BirdLife Australia. Now, tens of thousands of native blue water lilies bloom, and bird life is flourishing. Latham’s snipe and the sharp-tailed sandpiper—both species under threat—are thriving in the swamp. One of the most exciting discoveries has been the spotting of a painted snipe, one of Australia’s rarest waterbirds.

Many of these waterbirds are migratory, traveling from as far afield as Siberia and Japan. “It’s like opening up a new hotel for them,” Kennedy says. As a result, Mungalla is listed as a nationally significant location for migratory shorebirds. A knock-on effect is that the revitalized wetlands are now attracting large numbers of tourists, particularly birdwatchers keen for a glimpse of some of the rarer water birds.

For the Nywaigi people, this return of their traditional lands to health has both cultural and economic significance. Cassady says many of the area’s creeks—including Palm Creek, which runs through the Mungalla wetlands—are part of Nywaigi songlines, Aboriginal walking routes across the country that connected important features and locations. “Aboriginal people have looked after this country. We’ve got ancient campsites that are still there on those seabeds,” he says. The local elders still recall a time when the birdlife was so plentiful that the sky was dark with magpie geese.

Local Nywaigi young people are being trained as rangers and participating in programs such as water quality testing on the wetlands, which is providing job opportunities and a vital connection to the country. “Land is so important to Aboriginal people,” Cassady says. “If they haven’t got their land, if they haven’t got their culture, they’re just lost.”

IN ADDITION TO boosting ecotourism, the revitalized wetlands are providing a unique opportunity for the development of a carbon credit business. Restoring Mungalla’s native landscape sequesters carbon dioxide both through reforestation on land and in the soils and sediments of the wetlands and mangroves. That sequestered carbon can be accounted for and then traded on the carbon markets as credits to offset emissions elsewhere.

“By removing that saltwater bund, you bring salt water back in and you change that vegetation to the mangroves and the melaleucas,” Wearne says. Those restored marine ecosystems sequester carbon much more efficiently than what was there before, and much more efficiently than terrestrial ecosystems. However, the challenge here—and for many other blue carbon projects—is how to measure and account for the sequestration of carbon dioxide, as well as all of the co-benefits of restoring the wetlands: improving the reef, providing habitat for commercially important fish species, creating jobs, and honoring Indigenous heritage.

To help with that, the federal government has stepped in. Mungalla is one of five projects to be awarded funding via the newly elected federal government’s Blue Carbon Ecosystem Restoration Grants, which aim to increase investment in blue carbon conservation, restoration, and accounting. The nearly AU$1.78 million (around $1.22 million) investment is aimed at addressing the tricky question of how to properly account for all of Mungalla’s benefits—climate, biodiversity, and social—with a view to bringing its blue carbon credits to market.

The money from the program comes at a much-needed time. While the saltwater bores have been successful, they require ongoing maintenance, and researchers are keen for a more permanent solution. Removing the entire bund wall is one option being considered, but it’s not straightforward. Researchers need to carefully model the effects of removing the entire bund not only on those wetlands, but also the cattle pastures that are still part of the rest of Mungalla Station as well as the cane fields on either side.

Waltham also points out that, leaving aside the invasive weeds, even the engineered freshwater wetlands have biodiversity value that cannot always be discounted. “Although they’re manmade and have been disturbed, they’re the last remaining freshwater habitats that we really have,” he says. Some have become habitats for freshwater turtles and bird species, even some rare and threatened ones.

Climate change is also a rogue element in decisionmaking about how to remediate these wetlands. In the past decade, Queensland and much of Australia’s east coast has experienced a record-breaking drought, followed by record-breaking rains and flooding which have made it difficult to establish any kind of pattern to the ebb and flow of fresh and salt water. Sea level rise is another factor being considered, as modeling up to the year 2100 suggests the saltwater tidal incursions into the wetlands will become more frequent and prolonged.

But while questions remain about the best way to get the right amount of salt water back into these lands, lessons learned from Mungalla will inform the rehabilitation of other similarly strangled wetlands along the Great Barrier Reef coastline. Mungalla is likely to be a test case for other blue carbon projects. A 2020 report estimated that if tidal flow were to be restored through other similar manmade catchments along the reef-facing coast, it could increase the net sequestration of carbon in the area by around 5 million metric tons by 2100—equivalent to taking more than 1 million petrol cars off the road for a year. With sea level rise factored in—which increases the area of these blue carbon ecosystems—their restoration could increase carbon sequestration in the region by 10 to 20 percent.

And with Australia having one of the longest coastlines of any nation, there’s potential for the benefits of Mungalla’s transformation to be replicated elsewhere around the country too. “One day I’d love to see saline water flowing in all the creeks that it was originally flowing in,” Cassady says.

For now, though, with the federal funding for carbon biodiversity accounting in place, the task is to wait and see exactly how the area changes, how much carbon is captured, and how much wildlife and tourism flourish—with this being something that will continually change over time as the wetlands evolve. “It takes years for them to mature,” says Waltham. “Wetlands are so complex and dynamic.”

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