Australia: Century of die-offs takes toll on reef-building coral in central Great Barrier Reef

Bianca Nogrady ABC News 12 Sep 17;

Key points
* Little is known about the past coral mortality before the advent of long-term monitoring in the 1980s.
Analysis of dead corals in central Great Barrier Reef links mortality to a series of events occurring in the 1920s and 1960s and again in the 1980s and 1990s.
* Study suggests loss of resilience of hard corals, which once dominated the reef.
* Chemical dating of the Great Barrier Reef's sensitive branching corals reveals a history of massive die-offs over the past century, particularly in the past 30 years, from which these corals have not recovered.

The study, published today in the Proceedings of the National Academy of Science, suggests the die-offs happened in conjunction with significant natural and anthropogenic events, such as large-scale weather patterns and changes in land use since European settlement.

The findings could also mean the effects of more recent bleaching events in 2016 and 2017 have been underestimated because of the long-lasting impacts of earlier die-offs, said lead researcher Tara Clark, a marine palaeoecologist at the University of Queensland and Griffith University.

Dr Clark said growing evidence suggested the Great Barrier Reef is in decline, but information about disruption events and the how the reef responds to stress only goes back around 30 years.

To understand how the reef had changed over a longer period of time, Dr Clark and colleagues dated samples of dead Acropora branching corals from a number of sites around the Palm Islands in the central Great Barrier Reef.

The technique — which is used to estimate when the coral died — allowed researchers to look back over a century of coral activity.

While earlier studies suggested branching corals were common in many parts of the reef, Dr Clark said their team's surveys showed these reef builders now make up less than 5 per cent of the coral population at the sites studied.

"There were some skeletons we picked up that were huge — the branches were as thick as your arm, these were massive branching corals so that's a very different shift in terms of community structure at a lot of these places," she said.

One site in particular — Pelorus Island — experienced a huge die-off of branching coral between the 1920s and 1950s.

Dr Clark said there weren't many environmental records from that time, but the die-off may have been caused by a combination of a shift towards a wetter weather pattern and an increase in sugar cane plantations and grazing in the area.

"What we suspect is there was potentially a lot of land clearing going on, you get this shift in the Pacific Decadal Oscillation, with huge rainfall and a huge amount of sediment being delivered out the Herbert River, just north of Pelorus Island," she said.

"Because Pelorus is the closest reef to the Herbert River, it's possible that these plumes came out and wiped them out then, and they've not been able to recover."

Instead, the reef is now dominated by algae, and small stony coral known as Pavona.

At the other four sites, the die-offs were more recent, taking place between 1970 and 2000.

In particular, there were peaks in coral death that corresponded to mass bleaching events that occurred with warmer sea surface temperatures around 1983, 1987, 1994 and 1998.

Dr Clark said what was most worrying was that at many of these 'death assemblages' of dead coral, there was very little sign of living Acropora, suggesting that these more sensitive coral species have not bounced back.

"I think it's cause for concern because if there was nothing [living] there, then the impact of these larger bleaching events that happened more recently [in 2016 and 2017] has been underrepresented and underestimated," she said.

She said long-term monitoring programs are critical to understanding of how the state of coral has changed over time.

"Being able to use these methods to look at disturbance and recovery and work out which reefs are vulnerable, which haven't come back from some of these disturbances, is really important," she said.


Coral loss on Palm Islands long precedes 2016 mass bleaching on Great Barrier Reef
UNIVERSITY OF QUEENSLAND EurekAlert 11 Sep 17

Extensive loss of branching corals and changes in coral community structure in Australia's Palm Islands region over the past century has been revealed in a new study.

Dr Tara Clark of The University of Queensland Radiogenic Isotope Facility in the School of Earth and Environmental Sciences said these corals were highly sensitive to environmental change.

She said the area in the central region of the Great Barrier Reef warranted close monitoring to avoid irreversible changes in ecosystem health.

"Hard coral cover on the Great Barrier Reef is on a trajectory of decline," Dr Clark said.

"Yet, little is known about past coral mortality before long-term monitoring began around the 1980s to give us a long-term picture of what has happened since European colonisation of the coast."

Dr Clark said the limited baseline information of ecological dynamics before the 1980s made it difficult to understand recent ecosystem trends.

"Our study demonstrates the use of high-resolution uranium-thorium dating, modern and palaeoecological techniques to improve our understanding of coral mortality and recovery dynamics over much broader time scales," she said.

"At a regional scale we found a loss of resilience in ecologically important branching Acropora corals - formerly dominant key framework builders - with recovery severely lagging behind predictions."

Dr Clark said the study found the timing of Acropora coral death to have occurred simultaneously among reefs in the Palm Islands, coinciding with major disturbance events, such as bleaching and flooding, in the 1920s to 1960s and again in the 1980s to 1990s.

"Surveys conducted in 2014 revealed low Acropora cover - less than five per cent - across all sites, with very little evidence of recovery for up to 60 years at some sites, thus there was little left in this region for the 2016 bleaching event to kill." she said.

Together with previous research, the indication was the recent condition of the inshore Great Barrier Reef was a "shifted baseline" - already degraded before long-term monitoring took place. Taking together all such previous undocumented loss, the true picture is probably far worse than that depicted in previous reports of the 2016 mortality event.

This in turn strongly supported the importance of robust management action to reduce human impacts on reefs, especially efforts to reduce sediment and nutrient delivery to reef waters, in order to buy time for the reefs to recover before the next major disturbance event.

"The findings of this study will also prove valuable to reef managers by providing a reliable baseline for ongoing monitoring and identifying reefs at risk for deterioration, especially for those where modern observations are lacking," Dr Clark said.

School of Earth and Environmental Sciences project leader Professor Jian-xin Zhao said Dr Clark should be commended for her contributions to protect Australia's national icon, the Great Barrier Reef.

"Most of the work was done during her PhD studies or as part of a National Environmental Research Program project when she was a postdoctoral fellow," Professor Zhao said.

"This paper, together with two other papers published in Scientific Reports and Nature Communications in the past year, were published when Tara relied on casual administration and technical roles for a living, so this work is really an amazing achievement."

The study involved researchers from UQ's School of Biological Sciences,

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Australian Institute of Marine Science, and the ARC Centre of Excellence for Coral Reef Studies, and was published in Proceedings of the National Academy of Sciences of the United States of America (PNAS) (doi:10.1073/pnas.1705351114).