Coral bleaching: Hawaiian reefs further from pollution were more resistant to 2019 marine heatwave

A study of more than 200 square kilometres of reefs in the Hawaiian islands found that those further from pollution and coastal developments held up better after a 2019 marine heatwave

By Corryn Wetzel

Areas of coral reefs closest to land developments and pollution are less likely to survive when ocean temperature spikes, according to a study which used a novel aerial mapping tool to measure reef health.

After a marine heatwave hit the US state of Hawaii in 2019, ecologist Greg Asner at Arizona State University and his colleagues wanted to know how reefs in the Hawaiian Islands fared. “We’re trying to figure out, how bad is it for these corals? Which corals, in which areas?” says Asner.

Corals are a collection of thousands of tiny animals, called polyps, in a delicate symbiotic partnership with algae. The photosynthesizing algae  produce the coral’s food. When polyps are stressed with unusually warm or acidic water, they will expel their algal partner and turn ghostly white in a process called bleaching. Corals can recover from bleaching, but if stressed for too long, they die.

To get a better understanding of the changing coral coverage, Asner and his colleagues flew a small aeroplane outfitted with a special infrared spectrometer to measure  differences in the spectrum of light emitted by corals. Depending on how the coral molecules stretch, bend and vibrate when exposed to sunlight, the team could determine which parts of the reef were living and which had died. This gave them information on the molecular composition of the corals, to a depth of 16 metres.

“We fly over land and sea, and we measure the molecular composition of things – sometimes it’s water quality, sometimes it’s tropical forest canopy diversity,” says Asner. “In this case, we learned how to convert the molecular information to whether the corals are… alive or dead.”

The team’s analysis of more than 200 square kilometres of reefs around six Hawaiian islands revealed that certain regions were more resilient than others. Corals in some areas were up to 40 per cent more likely to survive than those in neighbouring reefs. The best predictor of coral loss was the health of the reef before a heatwave: areas that started with more live corals experienced fewer losses.

When the researchers compared the extent of living corals before and after the heatwave, they found the islands lost around 26 per cent of their coral cover on average. Areas of the reef nearest to coastal development or sediment runoff were more likely to die.

“It’s a one-two punch that’s killing coral, which is heat, plus pollution,” says Asner.

As marine heat waves become more frequent and severe with climate change, thermal stress will continue to test reefs. Asner is already using the results to help inform conservation efforts in the area, with the goal of reducing damaging pollution where corals have managed to hold on. “They’re surviving, and the state now knows that,” says Asner.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2123331119

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