Mystery of Gulf mangrove dieback revealed

Severe mangrove dieback was recorded at Limmen Bight in the Northern Territory and at Karumba in Queensland in 2016, raising fears of threats to shoreline stability and Gulf fisheries.

Now, James Cook University scientists say they have solved the mystery of the death of the 40 million mangrove trees, adding that their discovery could help scientists predict, and possibly prevent, future events.

The research shows that it was a result of an unusually low sea level due to large-scale swings in El Nino - Southern Oscillation events.

Other research found that a 1982 mass dieback also coincided with an unusually extreme drop in sea level during another severe El Nino event, providing the evidence for the cause of both events.

Satellite imagery shows that the mangroves took at least 15 years to recover from the 1982 dieback, and Dr Norm Duke from JCU's TropWATER Research Centre said the mangroves in the latest episode have not recovered seven years on, making it an ongoing coastal catastrophe.

"The key factor responsible for the mass dieback appears to have been the sudden 40-centimetre drop in sea level that lasted for about six months, coinciding with no rainfall, killing vast areas of mangroves," he said. "Essentially, the trees died of thirst."

More than 76 square kilometres of mangrove trees died along 2000 kilometres of coastline, releasing nearly one million tonnes of carbon, and Dr Duke said recovery had been stymied by severe cyclones and flooding.

Author assisting with data analysis and JCU TropWATER researcher Dr Adam Canning said the question remained when or if the mangroves would recover this time, thanks to a collapse and recovery cycle due to repeated pressure from climate change.

The message from researchers now is that targeted action can enhance the resilience of the ecosystems.

Jock Mackenzie, Earthwatch Australia wetlands researcher, said the localised human impacts that degrade mangrove habitats - pollution, altered hydrology, feral animals, weeds, and improper fire management - must be addressed.

He said these impacts were impeding the natural ability of mangroves to adapt to climate change.

"We encourage community groups, Indigenous custodians and catchment management agencies to continue to monitor mangrove shorelines through a combination of satellite monitoring and the MangroveWatch citizen science program, to help identify and prioritise targeted local mangrove management and threat reduction," he said.

Satellite imagery could also be used to help monitor the recovery of mangroves in remote areas and identify key areas under pressure.

It may even be possible to predict future events, which could help prepare for innovative rescue efforts that may include reducing water stress during El Nino events via targeted irrigation.

Dr Duke said mangroves were vital to the ecology and stability of tropical and sub-tropical coastlines and their protection was critical.

"They provide essential habitat for many species and can hold substantially more carbon than tropical forests within the same area," he said.

"These extraordinary trees are normally environmentally resilient, being able to grow in seawater, inter-tidal zones and on coastal salt flats. They are also essential for preventing or reducing shoreline erosion and retreat."

The dieback's exact cause was revealed after a four-year research partnership between James Cook University, Charles Darwin University, and Carpentaria Land Council Aboriginal Corporation Indigenous rangers in the Gulf, funded by the federal government's National Environmental Science Program and the Northern Territory government.

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