Even coral reefs found at dark, cool depths aren't protected from the effects of climate change. In a new study published in Nature Communications, scientists have found that both shallow and deep reefs show vulnerabilities to temperature changes, even those as far down as 40 meters (131 feet) beneath the surface.
Deep reefs were once considered to be safe from the extreme heat events that cause mass coral bleaching, such as the 2016 heatwave. In the new study, the researchers report that bleaching events impacted even these deep reefs – 40 percent were bleached while 6 percent of colonies had died. This is still significantly less than for shallower reefs where as much as 69 percent of coral was bleached and 12 percent killed.
“It was a shock to see that the impacts extended to these dimly lit reefs, as we were hoping their depth may have provided protection from this devastating event,” said study lead author Pedro Frade in a statement.
Data collected by remotely operated vehicles (ROVs) recorded how temperature conditions varied with depth. They also conducted surveys during the height of the 2016 bleaching event across several sites along the Great Barrier Reef. Overall, major bleaching and mortality affected almost a quarter of corals at deep points, and the impacts on shallower ones were confirmed.
"Unfortunately, this research further stresses the vulnerability of the Great Barrier Reef," said study co-author Ove Hoegh-Guldberg. "We already established that the refuge role of deep reefs is generally restricted by the limited overlap in species with the shallow reef. However, this adds an extra limitation by demonstrating that the deep reefs themselves are also impacted by higher water temperatures."
Coral reefs around the world are under threat thanks to the warming climate, yet most of what we know about coral bleaching events refers to reefs at depths of 15 meters (49 feet) and shallower. Warming temperatures trigger mass coral bleaching events by causing corals to release photosynthetic algae that drain them of their color and, ultimately, kill them.
A report from earlier this year mapped the impact of the 2016 heatwave, chronicling the “mass mortality” that happened where heat exposure was most extreme. Here, 29 percent of the nearly 3,000 reefs that make up the Great Barrier Reef system lost two-thirds or more. In the northern region, as much as half was lost and scientists think much of it is unlikely to recover.
Because new coral isn’t able to grow at a fast enough pace to replace, estimates suggest as much as 90 percent of all coral could be dead in the next three decades.