A cascade of impacts consisting of rising sea levels, heatwaves, and back-to-back tropical cyclones has created 400km of dead and badly damaged mangroves in the Gulf of Carpentaria, a scientific monitoring trip has discovered.

The mountains of Veneto (Italy) after the Vaia cyclone
Photo by Massimo Rivenci / Unsplash

Prof Norman Duke, of James Cook University, spent ten days on observing 2,000km of coastline from a helicopter and conducting land-based checks at 32 estuaries along the coastline from Weipa, Queensland to Cape Barrow in the Northern Territory.

In 2015, the remote area experienced what is thought to be the worst mass dieback of mangroves ever recorded. The cause, Duke said, was a mix of extreme heat, a temporary drop in sea level at the time caused by air pressure, and drought.

Duke and colleagues returned for a second follow-up monitoring trip to seek devastating impacts of 2 cyclones had built a 400km stretch of dead and damaged mangroves.

In December 2018, Cyclone Owen battered the gulf’s coast and then, only 3 months later, the category 4 Cyclone Trevor struck just to the south of Owen’s impact area.

Duke told Guardian Australia that they were getting those compounding effects that they just had not expected. The mangroves hit in the 2015 dieback were already vulnerable because they were just recovering. Now you had all that dead wood that became like projectiles. It had been a shock to him to see the damage.

For hundreds of kilometers, Duke says what would be a landscape of lush green mangroves has become dull grey trees with striped foliage.

“Mangrove forests are the only continuous forests in this area. These are very successful trees and they usually form a solid green canopy that’s very distinctive. Now we see canopy damage – a grey color where it would normally be green.”

He aforementioned new growth was being smothered and broken by the dead branches stacked up behind them and the shoreline in several places was also in retreat, caused partly by increasing sea levels which, he said, were growing faster than the global average.

How the gulf can sustain that I don’t know,” he said, concerns about the impact of rising sea levels.

According to Duke, it was “abundantly clear” that the estuaries that flow into the gulf had bigger levels of erosion than he had previously seen.

“There will almost certainly be impacts on fisheries and the ability of mangroves to protect the coasts from erosion,” he said.

First reports expected the area damaged in 2015 to be about 1,000km in length, but Duke says subsequent visits put that number at more like 2,000km. Approximately 8,000 hectares (20,000 acres) of mangroves had been altered.

This week, a major study found that Australia was a global hotspot for the carbon stored in mangroves, seagrasses, and tidal marshes, yet warned degradation of the ecosystems was already releasing around 3m tonnes of CO2 a year.

The researches quantified for the first time the major role that “vegetated coastal ecosystems” play in absorbing and locking away carbon.

Duke said he did not know if human-caused climate change was a factor in what he had seen. However, he added that what would you expect with climate change, with more severe storms and higher temperatures? What did that look like?

He believed that example in the gulf was a clear presentation since that area was so remote. There were no other factors that we could attribute to it.

Duke is heading a National Environmental Science Program project to study the after-effects of the vast 2015 mangrove dieback.

He stated local traditional owner rangers he had been working with, and who sponsored the monitoring trips, needed support to maintain their work.
He told rangers were doing essential work in patrolling the beaches, and attempting to look after turtle nesting sites that were being attacked by feral pigs likewise.