The coral reefs near the French Polynesian island of Moorea in the central South Pacific Ocean were badly damaged in the spring of 2019. Despite the lack of El Nio conditions, the catastrophe occurred.
A research team led by Prof. Alex Wyatt of the Department of Ocean Science at The Hong Kong University of Science and Technology investigated the coral bleaching episode. The event was caused by the passage of anti-cyclonic eddies that elevated sea levels and led to an underwater marine heatwave that was largely hidden from view. The results have been published.
The full picture of threats from ocean heating to marine ecosystems, including tropical coral reefs, can't be seen by using sea- surface measures of water temperatures. It is not possible to detect heating below the surface that affects communities living in waters less than a few meters deep.
A rare combination of high-resolution, long-term in-situ temperatures and sea level anomalies was used to analyze data collected at Moorea over the course of fifteen years. Sea levels were raised due to the passage of anti-cyclonic eddies in the ocean past the island. There are internal waves that travel along the interface between the warm surface layer of the ocean and the cooler layer below.
The research shows that the internal wave cooling was shut down in the early part of the year due to the anti-cyclones. Large-scale coral bleaching and subsequent mortality occurred due to unexpected heating over the reef. The recovery of coral communities around Moorea has been offset by the coral death in 2019.
Despite the El Nio that brought warm conditions and decimated many shallow reefs around the world, the reefs in Moorea did not suffer significant bleaching mortality. The ability to predict coral bleaching can be lost with a focus only on surface conditions, so collecting temperature data across the range of depths is important. Moderate bleaching is predicted by the sea-surface temperature data. There was only a small amount of bleaching in 2016 with heating that was short in duration and limited to shallow depths. If researchers only had access to sea- surface temperature data, the coral bleaching caused by the marine heatwave could have been mistaken for other causes.
The study shows the need to consider environmental dynamics across depths relevant to threats. This kind of analysis depends on long-term, in situ data, but it's not always available. The paper gives a good example of how to assess the future of coastal ecosystems in the context of changing ocean dynamics and climates.
A team of scientists from the University of California San Diego, the University of California Santa Barbara, California State University, Northbridge, and Florida State University collaborated with HKUST on this research. This study was made possible by long-term physical and ecological observations conducted at the Moorea Coral Reef Long-Term Ecological Research site. The long-term analyses and concurrent monitoring of physical conditions and biological dynamics across the full range of depths of island and coastal marine communities is a model for future research that aims to protect vulnerable living resources in the ocean.
There is more information about hidden heatwaves and coral bleaching.
Journal information: Nature Communications
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