Australia: Thirsty mangroves cause unprecedented dieback

James Cook University Science Daily 14 Mar 17;

A James Cook University scientist has discovered why there was an unprecedented dieback of mangroves in the Gulf of Carpentaria in early 2016 -- the plants died of thirst.

Dr Norman Duke, leader of JCU's Mangrove Research hub, headed an investigation into the massive mangrove dieback. The findings were published in the Journal of Marine and Freshwater Research.

The scientists used aerial observations and satellite mapping data of the area dating back to 1972, combined with weather and climate records.

Dr Duke said they found three factors came together to produce the unprecedented dieback of 7400 hectares of mangroves, which stretched for 1000 kilometres along the Gulf coast.

"From 2011 the coastline had experienced below-average rainfalls, and the 2015/16 drought was particularly severe. Secondly the temperatures in the area were at record levels and thirdly some mangroves were left high and dry as the sea level dropped about 20cm during a particularly strong El Nino."

Dr Duke says this was enough to produce what scientists regard as the largest recorded incident of its kind, and the worst instance of likely climate-related dieback of mangroves ever reported.

"Essentially, they died of thirst," he said.

Dr Duke said scientists now know that mangroves, like coral reefs, are vulnerable to changes in climate and extreme weather events.

He said the mangroves of Australia's Gulf region have experienced relatively little anthropogenic impact and are considered the least altered mangrove ecosystems in the world.

"So the relative dominance of climate influences in this region is of critical interest to world observers of environmental responses to climate change."

Dr Duke said the area is sparsely populated, with passing fisherman and scientists conducting unrelated work the first to notice the dieback.

"It took 4-5 months to come to the attention of mangrove tidal wetland specialists and managers. Our response to this event further involves training and equipping Indigenous rangers and local community volunteers to build local partnerships for rigorous and repeated shoreline assessments."

"We cannot afford to be caught out like this again!" said Dr Duke. "The Gulf dieback has been a wakeup call for action on shoreline monitoring. We urgently need a national shoreline monitoring program commensurate with our global standing. We have the specialists, we have the resources, and we know there is interest and concern amongst the Australian public."

To progress this further, Australia's top specialists and managers will review the situation at a dedicated workshop during next week's Australian Mangrove and Saltmarsh Network annual conference in Hobart, hosted by the University of Tasmania and CSIRO.

"The aim of Australia's specialist network is to apply intelligent, innovative and considered responses, as fully expected by the public, to improve and disseminate informed understandings of the changes taking place in high value natural resources such as Australia's coastal tidal wetland habitats," Dr Duke said.

Journal Reference:

Norman C. Duke, John M. Kovacs, Anthony D. Griffiths, Luke Preece, Duncan J. E. Hill, Penny van Oosterzee, Jock Mackenzie, Hailey S. Morning, Damien Burrows. Large-scale dieback of mangroves in Australia. Marine and Freshwater Research, 2017; DOI: 10.1071/MF16322


Gulf of Carpentaria's record mangrove dieback is a case study of extremes
Our investigation shows a combination of extreme temperatures, drought and lower sea levels caused the 2015-16 event
Penny van Oosterzee and Norman Duke for the Conversation
The Guardian 13 Mar 17;

One of the worst instances of mangrove forest dieback recorded globally struck Australia’s Gulf of Carpentaria in the summer of 2015-16. A combination of extreme temperatures, drought and lowered sea levels likely caused this dieback, according to our investigation published in the journal Marine and Freshwater Research.

The dieback, which coincided with the Great Barrier Reef’s worst recorded bleaching event, affected 1,000km of coastline between the Roper river in the Northern Territory and Karumba in Queensland.

The Gulf of Carpentaria is a continuous sweep of wide tidal wetlands fringed by mangroves, meandering estuaries, creeks and beaches. Its size and naturalness makes it globally exceptional.

An apron of broad mudflats and seagrass meadows supports thousands of marine turtles and dugongs. A thriving fishing industry worth at least $30m ultimately depends on mangroves.

Mangroves and saltmarsh plants are uniquely adapted to extreme and fickle coastal shoreline ecosystems. They normally cope with salt and daily inundation, having evolved specialised physiological and morphological traits, such as salt excretion and unique breathing roots.

But in early 2016, local tour operators and consultants doing bird surveys alerted authorities to mangroves dying en masse along entire shorelines. They reported skeletonised mangroves over several hundred kilometres, with the trees appearing to have died simultaneously. They sent photos and even tracked down satellite images to confirm their concerns. The NT government supported the first investigative surveys in June 2016.

In the end, the emails from citizen scientists nailed the timing: “looks like it started maybe December 2015”; the severity: “I’ve seen dieback before, but not like this”; and the cause: “guessing it may be the consequence of the four-year drought”.

Our investigation used satellite imagery dating back to 1972 to confirm that the dieback was an unparalleled event. Further aerial helicopter surveys and mapping during 2016, after the dieback, validated the severity of the event extending across the entire gulf. Mangrove dieback has been recorded in Australia in the past but over decades, not months.

We still don’t fully understand what caused the dieback. But we can rule out the usual suspects of chemical or oil spills, or severe storm events. It was also significant that losses occurred simultaneously across a 1,000km front.

There were also a number of tell-tale patterns in the dieback. The worst-impacted locations had more or less complete loss of shoreline-fringing mangroves. This mirrored a general loss of mangroves fringing tidal saltpans and saltmarshes along this semi-arid coast.

Mangroves were unaffected where they kept their feet wet along estuaries and rivers. This, as well as the timing and severity of the event, points to a connection with extreme weather and climate patterns, and particularly the month-long drop of 20cm in local sea levels.

Extreme weather the likely culprit
We believe the dieback is best explained by drought, hot water, hot air and the temporary drop in sea level. Each of these was correlated with the strong 2015-16 El Niño. Let’s take a look at each in turn.

First, the dieback happened at the end of an unusually long period of severe drought conditions, which prevailed for much of 2015 following four years of below-average rainfall. This caused severe moisture stress in mangroves growing alongside saltmarsh and saltpans.

Second, the dieback coincided with hot sea temperatures that also caused coral bleaching along the Great Barrier Reef. While mangroves are known to be relatively heat-tolerant, they have their limits.

The air temperatures recorded at the time of the mangrove dieback, particularly from February to September 2015, were also exceptionally high.

Third, the sea level dropped by up to 20cm at the time of the dieback when the mangroves were both heat- and moisture-stressed. Sea levels commonly drop in the western Pacific (and rise in the eastern Pacific) during strong El Niño years: and the 2015-16 El Niño was the third-strongest recorded.

The mangroves appear to have died of thirst. Mangroves may be hardy plants, but when sea levels drop, reducing inundation, coupled with already heat- and drought-stressed weather conditions, then the plants will die – much like your neglected pot plants.

We don’t yet know what role human-caused climate change played in these particular weather events or El Niño. But the unprecedented extent of the dieback, the confluence of extreme climate events and the coincidence with the bleaching of the Great Barrier Reef mean the role of climate change will be of critical interest in the global response to mangrove decline.

What future for mangroves?
The future for mangroves around the world is mixed. Thanks to climate change, droughts are expected to become hotter and more frequent. If the gulf’s mangroves experience further dieback in the future, this will have serious implications for Australia’s northern fisheries including the prawn fishery, mudcrab and fin fish fisheries. All species are closely associated with healthy mangroves.

We don’t know whether the mangroves will recover or not. But there is now a further risk of shoreline erosion and retreat, particularly if the region is struck by a cyclone – and this may have already begun with recent cyclonic weather and flooding in the gulf. The movement of mangrove sediments will lead to massive releases of carbon uniquely buried among their roots.

Aerial view of severe mangrove dieback near Karumba in Queensland, October 2016.
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Aerial view of severe mangrove dieback near Karumba in Queensland, October 2016. Photograph: Norman Duke/James Cook University
Mangroves are among the most carbon-rich forests in the tropics and semi-tropics and much of this carbon could enter the atmosphere.

Now we urgently need to understand how mangroves died at large and smaller scales (such as river catchments), so we can develop strategies to help them adapt to future change.

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