Severe cold-water bleaching and mortality of deep-water reef observed in the Eastern Tropical Pacific

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, conducted a scientific expedition to Clipperton Atoll in early 2023 to collect samples from Clipperton's shallow reefs. This work, conducted onboard the sailing yacht S/Y Acadia as part of a collaboration with the Rohr Foundation, was aimed at retrieving coral cores and water samples in an area that will allow MPIC researchers to reconstruct changes in the size of oxygen minimum zones in the Eastern Pacific over the 20th century. "Clipperton is a truly remote destination: 800 nautical miles south of Mexico and 1000 nautical miles west of Costa Rica," explains Nicolas Duprey.

"Although we were in the area to collect other samples, serendipitously we observed bleaching on the deep coral reefs on one of our first dives," recalls Alan. "From the temperature of the surrounding water, we were able to infer that this was almost certainly driven by the presence of cold water," adds Nic. This was indeed quite a surprise to the two paleoclimatologists, considering that Clipperton is located in an area of the Pacific known for its warm water, with an average surface water temperature of 28°C for most of the year.

How to quantify a massive bleaching event 30 meters underwater

With assistance in the field from study coauthors 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 and dead coral at 32 meters depth (~70 percent) and by photographing the bleaching in high resolution. With help from Matan Yuval at the University of Haifa, they merged the huge photo collection into one single image, a so-called a photomosaic, for further analysis back on dry land. When combined with temperature measurements of the upper 300 meters of the water column, it became clear that the deep corals were bleaching due to cold shock, and that this shock played a large part in was leading to substantial mortality along the deep reefs.

Assisted by Marielle Dumestre (MPIC), the researchers compiled a database of published cold- and warm-water bleaching events that occurred in the shallow reefs of the region to better elucidate the role of cold-water intrusions in governing reef ecosystem functionality. It turned out that the timing of cold-water bleaching events coincided with past maxima in easterly wind strength. "We quickly realized that this anomalously cold water so high in the water column was linked to oscillations in easterly wind strength in the Eastern Tropical Pacific," says Alan. This link suggests that any future intensification of easterly winds in the Pacific may be a significant threat for its mesophotic coral ecosystems, as it could drive severe bleaching via increased exposure to colder waters from depth.

Mesophotic coral reefs are facing dual threats 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 follow extreme El Niño events more often. The researchers hypothesize, that, if this becomes a reality, unusually cold water will be displaced onto Eastern Tropical Pacific deep and mid-depth reefs on the heels of increased warm thermal stress from the surface. The authors of the study fear that, given the severity of bleaching and the associated coral mortality seen at Clipperton, the impact of cold-water bleaching on deep coral reefs is not transient, and that such events could strongly impact the health and functionality of a given deep-water reef.

"Our observations, in combination with recent reports of warm-water bleaching of Red Sea and Indian Ocean mesophotic coral ecosystems, highlight that 21st century mesophotic coral ecosystems in the Eastern Pacific face a two-pronged challenge: warm-water bleaching from above, and cold-water bleaching from below," Alan concludes.