New research has unveiled a serious coral bleaching incident in deep coral reefs that led to significant coral death. Unusual for bleaching events, this case was caused by extremely cold water. Future intensification of El Niño/La Niña occurrences in the Eastern Tropical Pacific, likely due to climate change, could severely threaten the region’s deep reefs.
A team of researchers from the Max Planck Institute for Chemistry (MPIC) has recently shared findings from their study of a significant coral bleaching and mortality event affecting the deep reefs of Clipperton Atoll, a secluded coral island located in the Eastern Tropical Pacific. This unexpected bleaching incident was triggered by the unusual movement of cold water to shallower depths than typically seen. The researchers have connected this atypical shallow cold water to shifts in the strength of easterly winds across the Pacific, showcasing that variations in wind speed may help explain previous bleaching occurrences in shallow reefs within the Eastern Tropical Pacific. Their findings, which were published in the journal Science of the Total Environment, underline the potential risks that cold-water bleaching poses to deep reef ecosystems throughout the 21st century.
A surprising discovery in the secluded Eastern Pacific
In early 2023, Alan Foreman and Nicholas Duprey, postdoctoral researchers affiliated with Alfredo Martínez-García’s team at the Max Planck Institute for Chemistry, embarked on a scientific mission to Clipperton Atoll to gather samples from its shallow reefs. Conducted aboard the yacht S/Y Acadia in partnership with the Rohr Foundation, their goal was to collect coral cores and water samples, which would aid MPIC researchers in reconstructing the historical changes in oxygen minimum zones in the Eastern Pacific during the 20th century. “Clipperton is incredibly remote: 800 nautical miles south of Mexico and 1000 nautical miles west of Costa Rica,” explains Nicolas Duprey.
“Although our primary aim was to gather other samples, we unexpectedly discovered bleaching on the deep coral reefs during one of our initial dives,” recalls Alan. “The surrounding water temperature indicated that this was likely due to cold water,” adds Nic. This revelation was surprising, given that Clipperton is located in a region known for its warm water, averaging 28°C at the surface throughout much of the year.
Measuring a vast bleaching incident 30 meters underwater
With the support of study coauthors Mark Rohr, Rose Dodwell, and Guy Dodwell, the researchers began documenting the extensive bleaching found around Clipperton, noting that around 70 percent of coral at a depth of 32 meters was bleached or dead and capturing high-resolution images of the bleaching. Collaborating with Matan Yuval from the University of Haifa, they created a comprehensive photomosaic from their extensive photo collection for further analysis once back on land. Combined with temperature assessments of the upper 300 meters of the water column, it became clear that cold shock was causing the deep corals to bleach, significantly contributing to mortality among the deep reefs.
Assisted by Marielle Dumestre (MPIC), the team developed a database of documented cold- and warm-water bleaching incidents affecting the shallow reefs in the region to better understand how cold-water intrusions impact reef ecosystems. They found that the occurrences of cold-water bleaching often aligned with peaks in easterly wind strength. “We soon recognized that this unusually cold water near the surface was connected to variations in easterly wind strength in the Eastern Tropical Pacific,” Alan says. This discovery suggests that any future increase in easterly winds could pose a significant risk to mesophotic coral ecosystems, as it may lead to severe bleaching from the influx of cold waters from below.
Mesophotic coral reefs are facing dual threats in the 21st century
Recent investigations indicate that intense La Niña events, which entail strong easterly winds, are likely to become not only stronger but also more frequent in the coming years. Predictions also suggest that extreme La Niña conditions could frequently follow after intense El Niño events. The researchers postulate that if this trend holds true, the Eastern Tropical Pacific’s deep and mid-depth reefs could experience cold water displacements following periods of increased thermal stress from warmer waters above. They are concerned that the significant bleaching and coral mortality observed at Clipperton may indicate that the effects of cold-water bleaching on deep coral reefs are not short-lived and could greatly affect the health and functioning of these ecosystems.
“Our findings, alongside recent accounts of warm-water bleaching in mesophotic coral ecosystems in the Red Sea and Indian Ocean, emphasize that mesophotic coral ecosystems in the Eastern Pacific are facing a combined threat in the 21st century: warm-water bleaching from above and cold-water bleaching from below,” concludes Alan.
