“Uncovering the Unexpected: Cold-Water Bleaching Threatens Deep Coral Reefs in the Eastern Pacific”

The researchers from the Max Planck Institute for Chemistry (MPIC) have made a startling discovery regarding a major coral bleaching and mortality event that occurred on the deep reefs of the Clipperton Atoll in the Eastern Tropical Pacific. This unexpected bleaching event was driven by the displacement of cold water to much shallower depths than usual, leading to significant damage to the coral ecosystems.

Alan Foreman and Nicholas Duprey, postdoctoral researchers from the MPIC, embarked on a scientific expedition to Clipperton Atoll in early 2023 to collect samples from the shallow reefs. While their initial goal was to retrieve coral cores and water samples to study changes in oxygen minimum zones, they were surprised to observe bleaching on the deep coral reefs during one of their dives. This bleaching was attributed to the presence of anomalously cold water, a stark contrast to the warm waters typically found in the region.

With the assistance of their colleagues, the researchers documented the extent of the bleaching by recording the percentage of bleached and dead coral at 32 meters depth and creating a photomosaic of the affected area. Through their analysis, they determined that the cold-water bleaching event was causing significant mortality along the deep reefs, highlighting the threat posed by cold-water intrusions to coral ecosystems.

By compiling a database of past cold- and warm-water bleaching events in the region, the researchers were able to establish a link between cold-water intrusions and changes in easterly wind strength in the Pacific. They found that past cold-water bleaching events coincided with peaks in easterly wind strength, indicating that future intensification of these winds could pose a significant threat to mesophotic coral ecosystems in the Eastern Tropical Pacific.

The researchers also raised concerns about the potential impact of future La Niña events, which are expected to become stronger and more frequent, leading to increased displacement of cold water onto deep and mid-depth reefs. This dual threat of warm-water bleaching from above and cold-water bleaching from below poses a serious challenge to the health and functionality of mesophotic coral ecosystems in the Eastern Pacific.

Overall, the study published in the journal Science of the Total Environment highlights the urgent need to address the threats facing deep coral reefs in the 21st century, emphasizing the importance of understanding and mitigating the impacts of both warm and cold-water bleaching events on these fragile ecosystems.