Scientists discover cause of catastrophic destruction of Gulf of Carpentaria mangroves

In the summer of 2015-16, one of the most catastrophic mangrove deaths ever worldwide occurred in the Gulf of Carpentaria.

About 40 million mangroves died across more than 2,000 kilometers of coastline, releasing nearly 1 million tons of carbon – equivalent to 1,000 jumbo jets flying back from Sydney to Paris.

After six years of searching for answers, scientists have formally identified what is causing the mass destruction. They hope the discovery will help predict and possibly prevent future events.

Valuable mangroves died of thirst

Areas affected by severe mangrove deaths (shaded gray) along the gulf at the end of 2015. Air surveys (red lines) were carried out in 2016.(Supplied: NC Duke)

Mangrove ecologist and senior researcher at James Cook University (JCU) Norman Duke was behind the discovery.

dr. Duke found that unusually low sea levels, caused by severe El Niño events, resulted in mangrove trees “essentially dying of thirst.”

“The main factor responsible for this catastrophe appears to have been the sudden drop in sea level of 40 centimeters that lasted about six months, coinciding with no rainfall, which killed vast areas of mangroves,” he said.

Author assisting with data analysis and JCU researcher Adam Canning said the study’s evidence that the sea level drop was the cause was found in the discovery of an earlier mass die-off in 1982, observed in satellite images.

Mass die-off of mangroves at Karumba on the Gulf Country coast in Queensland
Hundreds of miles of mangroves along the coast of Karumba have turned ghostly white.(Delivered: James Cook University)

“The 1982 die-off also coincided with an unusually extreme drop in sea level during another very severe El Niño event. We know from satellite data that the mangroves took at least 15 years to recover from that die-off,” said Dr. canning.

“Now they are trapped in a vicious cycle of collapse and recovery due to repeated pressures from climate change – the question remains when and if they will recover.”

Economic consequences

Mangroves are valuable coastal ecosystems that provide coastal buffers against rising sea levels, protection against erosion, abundant carbon sinks, shelter for animals, nurseries and food for marine life.

The destruction of mangroves can lead to loss of fisheries, increased flooding, increased coastal damage from cyclones and increased salinity of coastal soils and water resources.

Defoliated mangroves along a small tidal gully near the Robinson River.
Defoliated mangroves along a small tidal gully near the Robinson River.(Delivered: Dr. Norman Duke)

In the Gulf, mangrove dieback threatens a $30 million fishing industry, said Dr. duke.

“The fishing industry depends on these mangroves, including for red banana shrimp, mud crabs and fin whales,” he said.

“When the 2015-16 El Niño hit, red-legged shrimp fishermen reported their lowest-ever catches.”

dr. Duke said the gulf’s mangroves are unlikely to recover due to the increasing intensity of El Nino events.

“Our research reveals the presence of a previously unrecognized ‘collapse-recovery cycle’ of mangroves along the gulf’s shorelines,” he said.

Composite image of NT mangrove dieback
Images of different parts of the foreshore, taken months apart, show the extent of the mangrove dieback in the NT.(Supplied: NC Duke)
Before and after the death of the mangrove
Before and after the loss of a piece of mangroves in the Gulf of Carpentaria.(Delivered: Dr. Norman Duke)

“The threat of future El Niño-induced sea level drops appears imminent, as evidence suggests a link between climate change and severe El Niño and La Niña events.

“Indeed, El Niños and La Niñas have become more deadly over the past 50 years and the long-term damage they inflict is expected to escalate.

“Under these conditions, the recovery potential of the mangroves is understandably low.

Protecting Future Ecosystems

dr. Duke said closer monitoring is essential to prevent future massive relapses. He said regular aerial surveys were a place to start.

“Tropical mangroves need much more protection and more effective maintenance with regular health checks through dedicated national coastal monitoring,” he said.

Mangrove deaths in the Northern Territory
Mangroves have died along a 2,000 km coastline in the Gulf of Carpentaria.(Supplied: NC Duke)

“Our aerial surveys of more than 10,000 kilometers of Northern Australian coastlines have begun.

“We recorded environmental conditions and causes of shoreline change for northwestern Australia, the eastern Cape York Peninsula, the Torres Strait Islands and, of course, the Gulf of Carpentaria.

“As the climate continues to change, it is vital to closely monitor our changing coastal wetlands and ensure that we are better prepared the next time another El Niño disaster occurs.”

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