Underwater photo of bleached staghorn coral
The diverse family of staghorn corals (Acroporidae) is among the most susceptible to thermal stress, turning large stands of coral heads into fields of coral rubble. Credit: Tim McClanahan

Understanding the causes of coral bleaching events is an important goal for conservationists across the globe.

Tropical coral reef ecosystem with bleached coral
Different species of coral respond to thermal stress in different ways, as shown in this flowerpot coral reef in Zanzibar, one of the sites studied in the paper. Credit: Tim McClanahan

A new study, carried out by the Wildlife Conservation Society and an international team of researchers, evaluated 26 different environmental factors at 226 sites throughout the Indian and Pacific Oceans during the 2016 El Niño event to determine which factors were the best predictors of coral bleaching.

Some of the variables researchers measured in the study—degree heating weeks (DHWs), temperature bimodality, the number of high spell events, high spell duration, and low spell duration—were related to temperature. Researchers also examined other environmental variables like coral community composition, depth, and habitat type. Finally, they examined the impact of management to determine whether fishing practices affected the risk of coral bleaching.

Researchers found that the best predictors of bleaching were models that combined four factors: mean sea surface temperatures during extreme warm events, temperature bimodality, length of low-temperature spells, and longitude.

Surprising Discoveries

“One of the most important findings [of the study] was that longitude is important…I don’t think that was really appreciated in previous studies,” said lead author and conservation biologist Tim McClanahan of the Wildlife Conservation Society.

Previously, McClanahan said, scientists had focused more on latitude, with corals closer to the equator believed to be more at risk of bleaching than those at higher latitudes. The new research, however, found that longitude was actually much more important. Corals from East Africa to the central Indian Ocean experienced high levels of bleaching, whereas those farther east, in places like Fiji, seemed to fare somewhat better.

Researchers are still uncertain why longitude has such a big impact on coral bleaching, as longitude (and location in general) can be a proxy for many different factors.

Another startling discovery was that one of the most commonly used methods to predict coral bleaching, DHWs, was, in fact, not very accurate.

DHWs are a way to measure heat that exceeds a certain threshold (1°C more than the expected summertime maximum) over time. For example, a DHW of 2°C-weeks could indicate 1 week in which the temperature exceeded the threshold by 2°C or 2 weeks in which the temperature exceeded the threshold by 1°C.

However, in this new study, only 9% of the variance in the observed coral bleaching could be explained by the DHW temperature index. The National Oceanic and Atmospheric Administration states that significant coral bleaching is likely when the DHW value reaches 4°C-weeks or higher. But this prediction method substantially underestimated the extent of coral bleaching—although only 32% of the sites experienced DHW values reaching 4°C-weeks, 56% of the observed sites experienced bleaching.

Coral Bleaching Still a Mystery

Coral bleaching is a complex phenomenon, and although there are many hypotheses, scientists still don’t know for certain why corals bleach in response to stress—why does heat cause corals to expel the symbiotic algae on which they depend for long-term survival?

Even if we don’t know exactly why coral bleaching is happening, identifying areas of coral that are more or less resistant to bleaching is important for informing conservation efforts.

Stanford University marine scientist Stephen Palumbi, who was not involved with the new research, said that some species of corals are more resistant to bleaching than others and even within a species, some individuals are more able to tolerate heat than others.

Even if we don’t know exactly why coral bleaching is happening, both McClanahan and Palumbi said that identifying areas of coral that are more or less resistant to bleaching is important for informing conservation efforts.

“If you have a bleaching-resistant population,” Palumbi said, “you can protect that population from other stresses like development and overfishing and pollution and things like that so that those heat-resistant corals do as well as possible. And you can use those particular corals in creating future heat-resistant nurseries.”

But Palumbi also said that local conservation efforts aren’t enough on their own. Although these strategies can buy us time, they can’t protect corals forever if the world continues to warm. “It all comes back to needing to have zero carbon emissions by about 2050 so that by 2100 things start getting better.”

—Hannah Thomasy (@hannahthomasy), Freelance Science Writer


Thomasy, H. (2019), A key to coral bleaching events? Location, location, location, Eos, 100, https://doi.org/10.1029/2019EO136196. Published on 29 October 2019.

Text © 2019. The authors. CC BY-NC-ND 3.0
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