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Exploring the mysterious hidden heatwaves threatening coral reefs

Exploring the mysterious hidden heatwaves threatening coral reefs

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Sea surface temperatures around Moorea

From April to May 2019, coral reefs near the French Polynesian island of Moorea in the central South Pacific suffered from severe and prolonged thermal bleaching. The disaster occurred despite the absence of El Niño conditions that year and intrigued oceanographers around the world. Photo credit: HKUST

From April to May 2019, coral reefs near the French Polynesian island of Moorea in the central South Pacific suffered from severe and prolonged thermal bleaching. The disaster occurred despite the absence of El Niño conditions that year and intrigued oceanographers around the world.

An international research team led by Prof. Alex Wyatt from the Department of Ocean Science at the Hong Kong University of Science and Technology has investigated this surprising and paradoxical coral bleaching event. The unexpected event was related to the passage of anticyclonic gyres that raised sea levels and concentrated hot water over the reef, resulting in an underwater heatwave that was largely hidden at the surface. The results are published today (January 6, 2023) in the journal Nature Communications.

Extensive coral bleaching on the North Shore of Moorea

Extensive coral bleaching occurred on the north coast of Moorea during the 2019 marine heatwave. Photo credit: Peter J Edmunds

Most studies of coral bleaching patterns rely on sea-surface water temperature measurements, which do not provide the full picture of ocean warming threats to marine ecosystems, including tropical coral reefs. These surface measurements, taken over wide areas using satellites, are valuable but fail to detect subsurface warming affecting communities living in waters deeper than the shallowest meters of the ocean.

Prof. Wyatt and colleagues analyzed data collected in Moorea over 15 years from 2005 to 2019, using a rare combination of remotely recorded sea surface temperatures and high-resolution, long-term in situ temperatures and sea level anomalies. The results showed that the passage of anticyclonic gyres in the open ocean past the island raised sea levels and pushed internal waves into deeper water. Internal waves travel along the interface between the warm surface layer of the ocean and the cooler layers below and in a previous study also led by Prof. Wyatt, have shown that they provide frequent cooling of coral reef habitats.

The present research indicates that internal wave cooling was shut down due to anticyclones in early 2019, as well as during some previous heatwaves. This led to unexpected warming over the reef, which in turn led to widespread coral bleaching and subsequent mortality. Unfortunately for the biodiversity of the local reefs, the extensive coral die-off in 2019 offset the recovery of coral communities that had occurred around Moorea over the past decade.

A notable observation in contrast to the 2019 heatwave was that Moorea’s reefs did not experience significant pale mortality in 2016, despite the prevailing Super El Niño that brought warm conditions and decimated many shallow reefs worldwide. The new research shows the importance of collecting temperature data across the full depth range of coral reefs, as the ability to predict coral bleaching can be lost by just focusing on surface conditions. Sea surface temperature data would predict moderate bleaching in both 2016 and 2019 in Moorea.

However, direct observations indicated that only an ecologically insignificant bleaching event occurred in 2016, with warming that was short-lived and confined to shallow depths. The severe and prolonged ocean heatwave of 2019 would have been missed had researchers only had access to sea surface temperature data, and the resulting catastrophic coral bleaching may have been misattributed to causes other than warming.

“The present study underscores the need to consider environmental dynamics across depths relevant to threatened ecosystems, including those resulting from the passage of underwater weather events in the ocean. This type of analysis depends on long-term in situ data measured across ocean depths, but such data is generally lacking,” said Prof Wyatt.

“Our paper provides a valuable mechanistic example for assessing the future of coastal ecosystems in the context of changing ocean dynamics and climate zones.”

Reference: “Hidden heatwaves and severe coral bleaching associated with mesoscale gyres and thermoclinic dynamics,” January 6, 2023, Nature Communications.
DOI: 10.1038/s41467-022-35550-5

This HKUST-led research was conducted in collaboration with a team of scientists from the Scripps Institution of Oceanography at the University of California San Diego, the University of California Santa Barbara, California State University, Northbridge and Florida State University. The data underlying this study were made possible by coupled long-term physical and ecological observations conducted at the Moorea Coral Reef Long-Term Ecological Research (LTER) site. The long-term analyzes performed here and the simultaneous monitoring of physical conditions and biological dynamics across the full depth spectrum of island and coastal marine communities is a model for future research aimed at protecting vulnerable living resources in the ocean.

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