Bleached coral at Zenith Reef in November 2016. Photo: ARC Centre of Excellence Coral Reef Studies

19 Apr 2018 (James Cook University) – A new study published online today in Nature shows that corals on the northern Great Barrier Reef experienced a catastrophic die-off following the extended marine heatwave of 2016.

“When corals bleach from a heatwave, they can either survive and regain their colour slowly as the temperature drops, or they can die. Averaged across the whole Great Barrier Reef, we lost 30 per cent of the corals in the nine-month period between March and November 2016,” said Prof Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies (Coral CoE) at James Cook University.

The scientists mapped the geographical pattern of heat exposure from satellites, and measured coral survival along the 2,300-km length of the Great Barrier Reef following the extreme marine heatwave of 2016.

The amount of coral death they measured was closely linked to the amount of bleaching and level of heat exposure, with the northern third of the Great Barrier Reef being the most severely affected. The study found that 29 per cent of the 3,863 reefs comprising the world’s largest reef system lost two-thirds or more of their corals, transforming the ability of these reefs to sustain full ecological functioning.

“The coral die-off has caused radical changes in the mix of coral species on hundreds of individual reefs, where mature and diverse reef communities are being transformed into more degraded systems, with just a few tough species remaining,” said co-author Prof Andrew Baird of Coral CoE at James Cook University.

“As part of a global heat and coral bleaching event spanning 2014-2017, the Great Barrier Reef experienced severe heat stress and bleaching again in 2017, this time affecting the central region of the Great Barrier Reef,” said co-author Dr Mark Eakin of the U.S. National Oceanic and Atmospheric Administration.

“We’re now at a point where we’ve lost close to half of the corals in shallow-water habitats across the northern two-thirds of the Great Barrier Reef due to back-to-back bleaching over two consecutive years,” said Prof Sean Connolly of Coral CoE at James Cook University.

“But, that still leaves a billion or so corals alive, and on average, they are tougher than the ones that died. We need to focus urgently on protecting the glass that’s still half full, by helping these survivors to recover,” said Prof Hughes.

The scientists say these findings reinforce the need for assessing the risk of a wide-scale collapse of reef ecosystems, especially if global action on climate change fails to limit warming to 1.5‒2 °C above pre-industrial levels.

The study is unique because it tests the emerging framework for the International Union for Conservation of Nature (IUCN) Red List of Ecosystems, which seeks to classify vulnerable ecosystems as ‘safe,’ ‘threatened’ or ‘endangered.’

“The Great Barrier Reef is certainly threatened by climate change, but it is not doomed if we deal very quickly with greenhouse gas emissions. Our study shows that coral reefs are already shifting radically in response to unprecedented heatwaves,” said Prof Hughes.

The researchers warn that failure to curb climate change, causing global temperatures to rise far above 2 °C, will radically alter tropical reef ecosystems and undermine the benefits they provide to hundreds of millions of people, mostly in poor, rapidly-developing countries.”

Link to video and images here. Please credit as marked.

Contact

Prof Terry Hughes
Director, ARC Centre of Excellence for Coral Reef Studies
E-mail: terry.hughes@jcu.edu.au

Prof Sean Connolly
ARC Centre of Excellence for Coral Reef Studies at James Cook University
Phone: +61 (0)7 4781 4242 (AEST/UTC +10)
Email: sean.connolly@jcu.edu.au

Prof Andrew Baird
ARC Centre of Excellence for Coral Reef Studies at James Cook University
Email: andrew.baird@jcu.edu.au

C. Mark Eakin, PhD
U.S. National Oceanic & Atmospheric Administration
College Park, MD U.S.A.
Phone: +1 (301) 502 8608  (EST/UTC -5)
Email: mark.eakin@noaa.gov

Catherine Naum
Communications Manager
ARC Centre of Excellence for Coral Reef Studies at James Cook University
Phone: +61 (0)7 4781 6067, +61 (0) 428 785 895 (AEST/UTC +10)
Email: catherine.naum1@jcu.edu.au

Global warming is transforming the Great Barrier Reef


ABSTRACT: Global warming is rapidly emerging as a universal threat to ecological integrity and function, highlighting the urgent need for a better understanding of the impact of heat exposure on the resilience of ecosystems and the people who depend on them. Here we show that in the aftermath of the record-breaking marine heatwave on the Great Barrier Reef in 2016, corals began to die immediately on reefs where the accumulated heat exposure exceeded a critical threshold of degree heating weeks, which was 3–4 °C-weeks. After eight months, an exposure of 6 °C-weeks or more drove an unprecedented, regional-scale shift in the composition of coral assemblages, reflecting markedly divergent responses to heat stress by different taxa. Fast-growing staghorn and tabular corals suffered a catastrophic die-off, transforming the three-dimensionality and ecological functioning of 29% of the 3,863 reefs comprising the world’s largest coral reef system. Our study bridges the gap between the theory and practice of assessing the risk of ecosystem collapse, under the emerging framework for the International Union for Conservation of Nature (IUCN) Red List of Ecosystems, by rigorously defining both the initial and collapsed states, identifying the major driver of change, and establishing quantitative collapse thresholds. The increasing prevalence of post-bleaching mass mortality of corals represents a radical shift in the disturbance regimes of tropical reefs, both adding to and far exceeding the influence of recurrent cyclones and other local pulse events, presenting a fundamental challenge to the long-term future of these iconic ecosystems.

Global warming transforms coral reef assemblages

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