The initial and longer term response of coral assemblages to heat exposure. Graphic: Hughes, et al., 2018 / Nature

By Quirin Schiermeier
18 April 2018

(Nature) – Extreme heat in 2016 damaged Australia’s Great Barrier Reef much more substantially than initial surveys indicated, according to ongoing studies that have tracked the health of the coral treasure. The heatwave caused massive bleaching of the corals that captured worldwide attention.

In a paper published on 18 April 2018 in Nature, researchers report that severe bleaching on an unprecedented scale triggered mass death of corals. This drastically changed the species composition of almost one-third of the 3,863 individual reefs that comprise the Great Barrier Reef.

The world’s largest coral reef is unlikely to recover soon. The damage is a harbinger of what a warming future might hold for a wealth of tropical reef ecosystems, says lead study author Terry Hughes, director of the coral-reef centre at James Cook University in Townsville, Australia. “If we fail to curb climate change, and global temperatures rise far above 2 °C [above the pre-industrial level], we will lose the benefits they provide to hundreds of millions of people.”

Hughes and his team of ecologists closely examined the 2,300-kilometre Great Barrier Reef after the 2016 heatwave. Extensive aerial surveys revealed widespread coral bleaching between March and April 2016. This phenomenon occurs when excessive heat kills or expels algae called zooxanthellae, which have a symbiotic relationship with reef-building corals. The algae provide the corals with energy and nutrients from photosynthesis; without them, the corals often die.

But to gauge the full extent of heat damage, Hughes’s team conducted more-comprehensive underwater surveys of coral mortality, both at the peak of the observed bleaching in March and April, and again eight months later.

Many corals — especially those in the northern third of the reef — died immediately from heat stress. Others were killed more slowly, after their algal partners were expelled. The composition of coral assemblages on hundreds of individual reefs changed radically within just a few months of the heatwave. On severely bleached reefs, fast-growing coral species — which have complex shapes that provide important habitats — were replaced by slower-growing groups that shelter fewer sea creatures.

“The study paints a bleak picture of the sheer extent of coral loss on the Great Barrier Reef,” says Nick Graham, a marine ecologist at Lancaster University, UK. Approximately one-third of the world’s coral reefs were affected by bleaching in 2016. On the Great Barrier Reef, less than 10 percent of reefs escaped with no bleaching, compared with more than 40 percent in previous bleaching events.

“It is now critical to understand how governance and local management can maximize recovery between recurrent heatwaves,” Graham says. [more]

Great Barrier Reef saw huge losses from 2016 heatwave

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 percent 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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