East Pacific study highlights severe cold-water bleaching as additional threat to deep reef ecosystems

Researchers at the Max Planck Institute of Chemistry (MPIC) recently published a paper detailing their observations of a major coral bleaching and mortality event occurring on the deep reefs of Clipperton Atoll, an isolated coral island in the Eastern tropical Pacific. This bleaching, very unexpected, is due to the upward movement of cold water towards shallower depths than normal.

The researchers link the presence of this cold, anomalously shallow water to changes in easterly wind strength in the Pacific and demonstrate that variability in wind strength can explain other shallow reef bleaching events observed in the Pacific. passed in the eastern tropical Pacific. Their study, recently published in the journal Total Environmental Sciencehighlights that cold-water bleaching could prove to be a major threat to deep reef ecosystems during the 21st century.

A disturbing surprise in the remote Eastern Pacific

Alan Foreman and Nicholas Duprey, two postdoctoral researchers from Alfredo Martínez-García’s group at the Max Planck Institute for Chemistry in Mainz, led a scientific expedition to Clipperton Atoll in early 2023 to collect samples from the shallow reefs of Clipperton.

This work, carried out aboard the sailboat S/Y Acadia as part of a collaboration with the Rohr Foundation, aimed to recover coral cores and water samples in an area that will allow MPIC researchers to reconstruct the changes in the size of oxygen minimum zones in the Eastern Pacific in the 20th century.

“Clipperton is a truly isolated destination: 800 nautical miles south of Mexico and 1000 nautical miles west of Costa Rica,” explains Nicolas Duprey.

“Although we were in the area collecting more samples, we happened to observe bleaching of deep coral reefs during one of our first dives,” Foreman recalls.

“From the temperature of the surrounding water, we were able to deduce that this was almost certainly due to the presence of cold water,” adds Duprey.

This was indeed a real surprise for the two paleoclimatologists, knowing that Clipperton is located in an area of ​​the Pacific known for its warm waters, with an average surface water temperature of 28°C for most of the year. .

How to quantify a mass bleaching event 30 meters underwater

With on-the-ground assistance from study co-authors Mark Rohr, Rose Dodwell, and Guy Dodwell, the two scientists began documenting the extensive bleaching present around Clipperton, both by recording the percentage of bleached corals and dead at 32 meters depth (~ 70%) and by photographing the bleaching in high resolution.

With the help of Matan Yuval from the University of Haifa, they merged the huge collection of photos into a single image, a so-called photomosaic, for further analysis on dry land. In combination with temperature measurements of the upper 300 meters of the water column, it became clear that deep corals were bleaching due to thermal shock and that this shock was playing an important role in significant mortality along deep reefs.

Assisted by Marielle Dumestre (MPIC), the researchers compiled a database of published cold and warm water bleaching events occurring in the region’s shallow reefs to better elucidate the role of cold water intrusions in the governance of reef ecosystem functionality. The timing of cold-water bleaching events was found to coincide with past maxima in easterly wind strength.

“We quickly realized that this abnormally cold water, so high in the water column, was linked to oscillations in the strength of the easterly wind in the eastern tropical Pacific,” says Foreman.

This link suggests that any future intensification of easterly winds in the Pacific could pose a significant threat to its mesophotic coral ecosystems, as it could lead to severe bleaching via increased exposure to colder deep waters.

Mesophotic coral reefs face a double threat in the 21st century

Recent studies suggest that major La Niña events associated with strong easterly winds will become both stronger and more frequent in the near future. Models also predict that extreme La Niña events will more often follow extreme El Niño events. The researchers hypothesize that, if this becomes a reality, unusually cold water will be moved to deep and mid-deep reefs in the eastern tropical Pacific, as a result of increased warm heat stress from the surface.

The study authors are concerned that, given the severity of bleaching and associated coral mortality observed at Clipperton, the impact of cold water bleaching on deep coral reefs is not transient and that such events could have a significant impact on the health and functionality of a deep coral reef. given the deep water reef.

“Our observations, combined with recent reports of hot water bleaching of mesophotic coral ecosystems in the Red Sea and Indian Ocean, highlight that 21st century mesophotic coral ecosystems in the Eastern Pacific face a dual challenge : hot water bleaching from above, and cold water bleaching from below,” concludes Foreman.