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By Rebecca Blackburn 27 November 2018 6 min read

Image: ANU/Emma Ligtermoet

IMAGINE sitting in your living room in fifty years’ time with your adult grandchildren looking at photos of their trip to the Northern Territory’s Kakadu National Park, on the UNESCO World Heritage List both for its outstanding natural and cultural values.

It might surprise you that the photos you took of Kakadu in 2018 would look dramatically different, because in fifty years nearly half of Kakadu’s iconic freshwater wetlands are predicted to be inundated with salt water.

While governments were discussing the UN Climate Report this October and whether or not climate change impacts could be avoided by limiting global warming to 1.5ºC, CSIRO in partnership with researchers from other institutes, Parks staff and Traditional Owners, have been researching what might happen, in particular, to Kakadu when sea levels rise.

“One of the reasons Kakadu is World Heritage listed and a Ramsar wetland is because its freshwater river floodplains have very high conservation and cultural values. Sea level rise is going to cause dramatic landscape changes in the not too distant future,” says Dr Peter Bayliss, CSIRO Senior Principal Research Scientist and leader of a project to model the potential impacts of sea level rise in the Kakadu region.

“Kakadu is low lying – if you get a one metre sea level rise, a big chunk of the fresh water floodplains will be under saltwater."

The Intergovernmental Panel on Climate Change predicted a 0.70 metre rise by 2070 and, according to Bayliss, this is expected to inundate 42 percent of Kakadu’s freshwater wetlands with seawater. By 2100 a sea level rise of 1.1 m is expected to inundate 65 percent of Kakadu with seawater and, in 114 years, all of Kakadu’s floodplains are predicted to be under saltwater.

All plants and animals, in fact the whole of Kakadu’s freshwater ecosystem, would be affected. While some plants such as water chestnut sedge don’t mind an occasional flush of salt water that comes with king tides, sustained immersion in salt water would kill them, as it would kill other fresh water plants.

Because Kakadu’s river floodplains are low lying they are more affected by tides, storm surges and seasonal floods during the wet.

“You can’t really put up levees and stop saltwater from coming in across vast stretches of Australia’s coastline; adaptation will be more practical and effective,” says Bayliss.

“It will be more or less a permanent transformation. Even if we stopped global warming now and things turned around, it may take centuries for Kakadu’s coastal floodplains to return to its current state,” says Bayliss.

How will salt water affect Kakadu?

While sea levels have been predicted to increase by 1.4 mm per year and by 2030 it will have risen by 0.14 meters, there might be a threshold effect, according to Bayliss.

“When you hit 0.70 metres by 2070, things really kick in, and saltwater will go further upstream and predicted to spread across more floodplain,” he says.

It would result in a dramatic change for Kakadu’s floodplains, moving it from a predominantly freshwater ecosystem to a predominantly marine ecosystem, he says. These changes would affect every level of the ecosystem – from soil bacteria all the way up to plants, animals and their habitats, as described in a recent edition of Marine and Freshwater Research. It would also impact the cultural values and livelihoods of Indigenous people who live in Kakadu.

Image: ANU/Emma Ligtermoet

In accompanying research:

  • Research on soil bacteria by Dr Tiffanie Nelson from the Australian Institute of Marine Science and her colleagues found that changes in salinity would result in significant changes to bacterial communities, and this would have flow on effects on the rest of Kakadu’s aquatic ecosystems.
  • Professor Richard Lucas from the Centre for Ecosystem Science, University of New South Wales and colleagues wrote that sea level rise would change the distribution of mangroves. Six thousand years ago mangroves occupied low lying areas but are now confined to river margins. Sea level rise however, is likely to increase the spread of mangroves.
  • Dr Neil Pettit from the Centre of Excellence in Natural Resource Management, University of Western Australia, and his colleagues found that saltwater incursions would change freshwater habitats into saltwater habitats, which in turn would put pressure on animal populations dependent on freshwater ecosystems such as waterbirds. For example, populations of the iconic magpie goose are already under threat from aquatic weeds and feral pigs on the Park, and any extensive loss of their nesting and feeding habitats will greatly exacerbate all cumulative impacts.
  • Dr Chris Humphrey, Environmental Research Institute of the Supervising Scientist, Department of the Environmental and Energy, and fellow researchers found that sea level rise would affect the survival of other freshwater animals such as fish and invertebrates. They concluded, as did all the scientists in the joint research, that freshwater areas that escape saltwater refugia, and new areas of freshwater wetland created further inland, would become important for the survival of these species, although their creation and extent would be limited due to the location of the escarpment and surrounding hills.

Sea level rise would affect the Traditional Owners of Kakadu by changing the abundance of freshwater plants and animals with high cultural value and, hence, Indigenous livelihoods. In some places freshwater sites might disappear altogether and mangroves and marine species will move in. CSIRO’s Dr Leo Dutra and colleagues concluded that a socio-economic and cultural research program in partnership with Traditional Owners is now needed to identify effective monitoring and adaptation measures.

It’s not all gloom and doom

It’s not the first time Kakadu’s river floodplains have become a predominantly saltwater ecosystem. When the last ice age ended about 15,000 years ago sea levels rose and Kakadu was predominantly a saltwater ecosystem. Sea level decreased and stabilized about 6,000 years ago and Kakadu’s floodplains were predominantly freshwater again. The rock art of the Arnhem Land escarpment reflect these dramatic sea level changes as they depict many saltwater dependent species hunted over millennia, such as barramundi and mullet.

Magela floodplain Image: UWA/Neil Pettit

Bayliss believes a solution is to focus on how to adapt to these changes.

“Many people in the region believe that you have to think positively and adapt to these changes,” says Bayliss.

The breadth and depth of scientific knowledge of aquatic ecosystems in the Kakadu Region gained after decades of focused research is impressive.

“Nevertheless, the over-riding ‘big-picture’ conclusion of our accumulated knowledge is that Kakadu’s wetlands by 2100 will essentially transform to a novel ecosystem state that renders traditional management goals unachievable, and this has far reaching implications at all levels,” says Bayliss.

“The Kakadu story parallels threats facing other regions in the world of high conservation value such as the Great Barrier Reef and Florida’s Everglades, highlighting the need to now address both local and global threats such as climate change simultaneously.”

Bayliss says that a key feature of the Kakadu research is that it was built on team efforts and in collaboration with Kakadu park staff, Traditional Owners and colleagues from many of Australia’s best research organisations. This combined effort spanned decades and has resulted in invaluable and possibly unrivalled long-term research knowledge of tropical wetlands in Australia.

Read more in the special edition of Marine and Freshwater Research.

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