Researchers from the Max Planck Institute for Chemistry (MPIC) recently published a paper detailing their observations of a major coral bleaching and mortality event that occurred on the deep reefs of the Clipperton Atoll, a remote coral island in the Eastern Tropical Pacific. This bleaching, which was highly unexpected, is driven by the upwards displacement of cold water to much shallower depths than normal. The researchers link the presence of this anomalously shallow, cold water to changes in easterly wind strength in the Pacific, and demonstrate that the variability in wind strength can explain other past observed bleaching events of shallow reefs in the Eastern Tropical 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 the researchers to reconstruct changes in the size of oxygen minimum zones in the Eastern Pacific over the 20th century.
The researchers conducted surveys and collected data to quantify the extent of the bleaching event that was occurring 30 meters underwater. They documented the extensive bleaching present around Clipperton by recording the percentage of bleached and dead coral at 32 meters depth and photographing the bleaching in high resolution. These observations combined with temperature measurements of the upper 300 meters of the water column showed that the deep corals were bleaching due to cold shock, leading to substantial mortality along the deep reefs.
Assisted by other researchers, the team compiled a database of published cold- and warm-water bleaching events that occurred in the shallow reefs of the region to better understand the role of cold-water intrusions in governing reef ecosystem functionality. They found that the timing of cold-water bleaching events coincided with past maxima in easterly wind strength, suggesting a link between anomalously cold water and oscillations in easterly wind strength in the Eastern Tropical Pacific. This link implies that any future intensification of easterly winds in the Pacific may pose a significant threat to its mesophotic coral ecosystems.
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, potentially displacing unusually cold water onto Eastern Tropical Pacific deep and mid-depth reefs following warm thermal stress from the surface. The researchers fear that the impact of cold-water bleaching on deep coral reefs could be long-lasting and significantly impact the health and functionality of a given deep-water reef.
In conclusion, the researchers highlight that 21st century mesophotic coral ecosystems in the Eastern Pacific are facing a two-pronged challenge: warm-water bleaching from above and cold-water bleaching from below. With the potential for both extreme La Niña events and increased warm thermal stress, the health and functionality of deep-water reefs in the region may be severely impacted, posing a major threat to these ecosystems. The researchers emphasize the importance of continued monitoring and research to better understand and protect these vulnerable coral reefs in the face of changing oceanic conditions.
