In the northern and central stretches of the Great Barrier Reef, scientists have recorded the most extensive coral cover seen in 36 years of study, according to a new report from the Australian Institute of Marine Science (AIMS).
Researchers tracked hard coral across 87 reefs along the coast of Queensland from August 2021 to May 2022, finding that coral cover reached 36 percent in the northern third of the reef and 33 percent in the central third, up from 27 percent and 26 percent, respectively, the prior year.
The growth was particularly remarkable given that uncommonly warm waters fueled a mass bleaching event earlier this year, causing overheated corals to eject the colorful algae that live in their branches, risking widespread coral mortality, the report said. The 2022 mass bleaching event was the fourth in seven years and the first ever to strike during the Pacific Ocean’s cooler La Niña phase, though it was less severe than other recent bleaching events, making it possible for parts of the reef to bounce back, scientists say.
“The 2020 and 2022 bleaching events, while extensive, didn’t reach the intensity of the 2016 and 2017 events and, as a result, we have seen less mortality,” Paul Hardisty, CEO of AIMS, said in a statement. “These latest results demonstrate the reef can still recover in periods free of intense disturbances.”
In contrast to the upper stretches of the Great Barrier Reef, the southern third saw coral cover drop from 38 percent to 34 percent year on year. Scientists blamed the decline on an outbreak of crown-of-thorns starfish, which prey on corals. The starfish grow faster and eat more in warmer, more acidic waters, and carbon emissions are both raising ocean temperatures and turning waters more acidic.
In areas where coral cover expanded, it was mostly fast-growing Acropora corals driving the growth, a potentially troubling prospect given that Acropora are particularly vulnerable to strong waves generated by tropical cyclones, highly susceptible to bleaching, and the preferred target of crown-of-thorns starfish.
“This means that large increases in hard coral cover can quickly be negated by disturbances on reefs where Acropora corals predominate,” AIMS monitoring program team leader Mike Emslie said in a statement. “The increasing frequency of warming ocean temperatures and the extent of mass bleaching events highlights the critical threat climate change poses to all reefs, particularly while crown-of-thorns starfish outbreaks and tropical cyclones are also occurring.”