Australia: Erosion threatens weakened mangrove ecosystem in Gulf of Carpentaria

Lucy Murray ABC News 16 Apr 17;

Mangrove ecologists and Indigenous rangers have expressed dire concern a second surge of mangrove dieback could further damage ecosystems in the northern Australia's Gulf of Carpentaria.

There are already more than 7,000 hectares of dead mangroves in the Gulf of Carpentaria.

For locals like ranger Robert Logan, this grey landscape was a shock to the system.

"[I] grew up in this country all my life and it's always been green and in a few months 'bang' it's all gone," Mr Logan said.
As the trees rot, researchers warn of a second surge of dieback.

Norm Duke, the head of James Cook University's Mangrove Hub, said an erosion event was likely to occur.

He said the excess soil released by the dead mangroves would choke some of the surviving trees.

"Based on experience in other places, as the mangroves deteriorate, they are no longer able to hold the soil together," Dr Duke said.

"We expect that those sentiments will pile up somewhere else.

"They will take out plants that are recovering like seedlings and equally, the surviving mature trees are vulnerable to that type of erosion as well."

Mangroves died of thirst

The first wave of dieback occurred in late 2015.

The Gulf Country, like most of Queensland, has been drought declared.

There has been little rainfall and high temperatures over the past few years, so the mangroves were already stressed.

But the final straw was a lack of sea water.

Dr Duke said scientists thought it was due to moisture stress.

"Delivered as low rainfall and high temperatures, and at the corresponding worst possible time, at the end of the dry season, sea levels dropped for a month," he said.

He said it was believed the El-Nino weather event caused tides receded by 20 centimetres for more than a month.

"That is the hypothesis that will be tested and worked on in the years to come," Dr Duke said.

Monitoring mangroves from afar

When the first mass death of mangroves occurred, researchers did not find out about it until five months later.

Because of the remote location, the deaths were not immediately reported.

This meant scientists could not forensically test the soil.

"Without the evidence of what the condition of the site was at the time of the dieback, you miss out on information about the before scenario that changes quickly after the trees die," Dr Duke said.

"There may or may not have been something we could have done."

If there was to be another occurrence of dieback, Dr Duke said he wanted to be on the front foot. He has teamed up with local Indigenous rangers, who act as his eyes on the ground.

Two rangers sit on a boat, one is operating a camera, while the other steers the boat.
PHOTO: When a rangers spots an interesting landmark, they click the GPS tracker. (ABC North West Queensland: Lucy Murray)
The rangers from the Carpentaria Land Council steer a boat close to the shoreline of the Norman River filming the mangroves and marking important landmarks on GPS trackers.

"[On the camera] we're trying to get the water level, the mangroves and a bit of sky," ranger Freddy Pascoe said.

The footage is sent back to the researchers on the east coast, more than 1,000km away.

Mr Logan said the film was then sent to JCU.

"Those guys can analyse them and do what they have to do," Mr Logan said.

"Just looking after the country I guess, best we can."

Unanswered questions remain

The rangers said they were more than happy to help monitor the situation.

Supervising ranger Paul Richardson said the area where the mangroves died was an important ecosystem.

"It's heartbreaking, because mangroves are a habitat for a lot of ocean creatures, fish grabs prawns — it's basically a nursery for them," Mr Richardson said.
"It is important that we do monitoring it now, because where that particular dieback of mangrove is, that sits in our flyway, that is a big concern for us.

"Migratory shore birds use that coastal line for roosting sites, and for feeding sites — mangroves — they act as shade for shells and the things they eat, so if there aren't mangroves they're going to cook."

Both Dr Duke and Mr Richardson want to do more testing to make sure there is not another factor in the widespread deaths.

"We would also like to see by doing the monitoring, will these mangroves grow back, or will another species take over?" Mr Richardson said.

Some of the trees that died were more than 200 years old, so recovery would take some time.

But the rangers said they would be watching and doing what they can.