Concealed Heatwaves And Catastrophic Coral Bleaching Related To Thermocline Dynamics

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The coral reefs of Moorea, a French Polynesian island in the middle of the South Pacific, have been severely and persistently bleached by rising water temperatures. Between April and May of 2019, this happened. This intriguing and seemingly contradictory coral bleaching event that took place back in 2019 has been investigated by a study team comprised of scientists from all over the world and led by Professor Alex Wyatt from The Hong Kong University of Science and Technology.

Anti-cyclonic eddies passing through the area likely contributed to the fast rise in sea levels and the distribution of hot water throughout the reef. This caused a marine heatwave deep below the surface, which was mostly unseen by onlookers above. The results of the research were published in the journal Nature Communications just recently.

Studies on coral bleaching patterns have relied heavily on sea-surface estimates of water temperatures, which have been shown to be inadequate for capturing the full scope of hazards posed by ocean warming to marine ecosystems like tropical coral reefs. Even while satellites are useful for taking surface data over large regions, they cannot detect warming below the surface that threatens people living in water deeper than a few meters.

From 2005 to 2019, Professor Wyatt and his colleagues sought to analyze Moorea’s collected data. A novel mix of long-term in-situ temperatures, sea level anomalies, and remotely detected high-resolution sea-surface temperatures was used for this purpose. Results indicated that when anti-cyclonic eddies in the outer ocean moved past the island, they raised sea levels and pushed internal waves further below the surface.

In a previous research also led by Professor Wyatt, it was shown that internal waves move ahead of the boundary between the warm top layer of the water and the colder layers below, providing regular cooling of coral reef ecosystems. Existing research shows that anti-cyclones interrupted internal wave cooling during the start of 2019 and during a few previous heatwaves.

As a result, the water temperature over the reef increased rapidly, causing widespread coral bleaching and subsequent death. Unfortunately for local reef biodiversity, 2019’s mass coral mortality has reversed the ten-year trend of coral community recovery at Moorea.

When compared to the heatwave of 2019, it is noteworthy that Moorea’s reefs did not suffer significant bleaching mortality in 2016. This occurred despite the fact that a widespread “super El Nio” was to blame for bringing warm weather and causing the destruction of several shallow reefs around the globe.

Since the ability to predict coral bleaching may be lost with a focus on surface conditions alone, the current research highlights the relevance of accumulating temperature data over the range of depths where coral reefs prefer to interact.

Having knowledge of the sea’s surface temperature in 2016 and 2019 would assist predict mild bleaching on Moorea. However, direct observations showed relatively little bleaching in 2016, with warmth lasting just a few days and confined to shallow depths, making it biologically irrelevant.

If researchers hadn’t had access to sea-surface temperature data, they may not have noticed the intense and protracted maritime heatwave in 2019. It’s possible that the repeated catastrophic bleaching of corals has been misattributed to factors other than warming.

The current research underlines the need of taking into account environmental dynamics at different depths that are important to vulnerable ecosystems, such as those caused by the passage of deep ocean weather events. Long-term, in-situ data obtained at various depths in the ocean are required for this kind of research, but these data are seldom available.

This study’s long-term assessments and simultaneous observation of physical variables and biological processes throughout the whole range of depths of island and coastal marine ecosystems provide a template for future studies aiming to anticipate the vulnerability of marine life resources.

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