Major Coral bleaching event in early 2019 in Mo'orea, French Polynesia | Photo: © Steve Doo, Leibniz Centre for Tropical Marine Research (ZMT)

27/11/2020 | Increasing coral bleaching events worldwide are causing growing concerns among scientists. An international consortium of researchers with participation of the Leibniz Centre for Tropical Marine Research (ZMT) has now developed the first common framework to better compare research results on coral bleaching and thus accelerate scientific knowledge. The recommendations have just been published in the journal Ecological Applications.

Coral bleaching is a significant threat for reefs worldwide. The warming of the oceans as a result of climate change can cause corals to cast-off their algae symbionts in a stress reaction. Without their symbionts, the corals lose their coloration and the underlying white skeleton appears. Corals can survive a bleaching event but being bleached puts them at higher risk for disease and death.

If corals die, this can have catastrophic ecological and economic consequences: Coral reefs protect coastlines from erosion, offer a boost to tourism in coastal regions, and are an essential habitat to more than 25% of the world’s marine species. As the world's atmosphere and the oceans warm up due to climate change, scientists are counting more and more coral bleaching events worldwide.

“Reefs are in crisis,” says Andréa Grottoli, Professor of Earth Sciences at Ohio State University and lead author of the new paper on the common guidelines for coral bleaching research.  “And as scientists, we have a responsibility to do our jobs as quickly, cost-effectively, professionally and as well as we can.”

An international team of researchers led by Grottoli has now presented a common framework to better compare research results on coral bleaching. “We need standardised ways of exchanging and using the collected data,” emphasises co-author Dr. Henry C. Wu from the Leibniz Centre for Tropical Marine Research (ZMT) in Bremen.

The biogeochemist is part of the "Coral Bleaching Research Coordination Network" initiated by Grottoli, which met in May 2019 in the state of Ohio. In a workshop funded by the U.S. National Science Foundation, the consortium of 27 scientists representing 21 institutions around the world developed the new common framework.

Their recommendations include guidelines for experiments that help scientists understand what happens when corals are exposed to changes in light or temperature over a short period of time, a moderate period, and long periods. The guidelines include a compendium of the most common methods used for recording and reporting physical and biological parameters in a coral bleaching experiments.

“In order to keep each other informed about the results of our studies and experiments and to be able to quickly compare the reported variables, we need common reference points in experiments for parameters such as light intensity, salinity, alkalinity or temperature,” explains Wu.

Until now, such a reference framework has not existed because the scientific field that seeks to understand the causes of and solutions for coral bleaching is relatively young. The first reported bleaching occurred in 1971 in Hawaii; the first wide-spread bleaching event was reported in Panama and was connected with the 1982-83 El Niño.

But experiments to understand coral bleaching didn’t really start in earnest until the 1990s – and a companion paper by many of the same authors found that two-thirds of the scientific papers about coral bleaching have been published in the last 10 years.

In the last decade, especially the Great Barrier Reef off Australia or the Indonesian Seas in the Coral Triangle suffered from increased water temperatures, which led to large-scale coral bleaching.

Scientists are still trying to understand why some coral species seem more vulnerable to bleaching than others, Grottoli says. Corals can survive bleaching events – but under what conditions? “Adopting a common framework for experiments around coral bleaching would make us more efficient as a discipline,” she says.

Henry C. Wu from ZMT stresses the urgency of the situation: “Due to climate change and the vulnerability of corals, we need to move faster. As we say in the opening of our paper ‘Coral bleaching is the single largest global threat to coral reefs worldwide’.”

Grottoli, A., Toonen, R., van Woesik, R., Vega Thurber, R., Warner, M., McLachlan, R., Price, J., Bahr, K., Baums, I., Castillo, K., Coffroth, M., Cunning, R., Dobson, K., Donahue, M., Hench, J., Iglesias‐Prieto, R., Kemp, D., Kenkel, C., Kline, D., Kuffner, I., Matthews, J., Mayfield, A., Padilla‐Gamino, J., Palumbi, S., Voolstra, C., Weis, V. und Wu, H. (2020, online first), Increasing comparability among coral bleaching experiments. Ecological Applications. DOI: 10.1002/eap.2262