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<p>Marine heatwaves (MHWs) are intensifying and increasingly threatening coral reef persistence by inducing widespread bleaching and mortality. Although depth is often proposed as a natural refuge, its protective role, especially in reducing post-bleaching coral loss, remains debated. This study quantified how depth (5, 12 and 20&#xa0;m) shaped bleaching and post-bleaching mortality and evaluated drivers of depth-mediated protection during the unprecedented severe 2019 MHW in Mo’orea (French Polynesia), across 6 sites. Despite thermal stress levels considered moderate according to global standards (3&#xa0;°C-weeks), the event caused unexpectedly severe impacts, with widespread bleaching resulting in the loss of half the coral cover. Bleaching was only weakly mitigated by depth with heightened protection (&lt; 30% reduction) from 12 to 20&#xa0;m in sensitive taxa such as <i>Acropora</i>, <i>Montipora</i> and <i>Pocillopora</i>, not present at all in others, indicating refuge from bleaching was not universal. Mortality was concentrated in branching <i>Acropora</i> and <i>Pocillopora</i>, while other heavily bleached taxa showed no mortality. Relative total coral cover loss fell 3.5-fold from 45% at 5&#xa0;m to 13% at 20&#xa0;m, driving threefold weaker community shifts, revealing that depth more strongly limited post-bleaching mortality. This effect was partly mediated by differences in pre-bleaching community composition and coral cover, yet ~ 74% of the protective effect remained unexplained, suggesting that additional depth-related environmental or physiological mechanisms, such as light attenuation or colony size, were at play. These findings position deeper reefs as potential natural buffers, even in the absence of effective thermal stratification, and priorities for proactive conservation under accelerating climate change.</p>

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When depth fails to prevent bleaching but limits coral death: insights from the 2019 heatwave in Mo’orea

  • 期刊论文

  • 发表日期：

    2026年3月8日

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  • Claire Boitel,
  • Yann Lacube,
  • Alexandre Mercière,
  • Caroline Dubé,
  • Gilles Siu,
  • Laetitia Hédouin

摘要

Marine heatwaves (MHWs) are intensifying and increasingly threatening coral reef persistence by inducing widespread bleaching and mortality. Although depth is often proposed as a natural refuge, its protective role, especially in reducing post-bleaching coral loss, remains debated. This study quantified how depth (5, 12 and 20 m) shaped bleaching and post-bleaching mortality and evaluated drivers of depth-mediated protection during the unprecedented severe 2019 MHW in Mo’orea (French Polynesia), across 6 sites. Despite thermal stress levels considered moderate according to global standards (3 °C-weeks), the event caused unexpectedly severe impacts, with widespread bleaching resulting in the loss of half the coral cover. Bleaching was only weakly mitigated by depth with heightened protection (< 30% reduction) from 12 to 20 m in sensitive taxa such as Acropora, Montipora and Pocillopora, not present at all in others, indicating refuge from bleaching was not universal. Mortality was concentrated in branching Acropora and Pocillopora, while other heavily bleached taxa showed no mortality. Relative total coral cover loss fell 3.5-fold from 45% at 5 m to 13% at 20 m, driving threefold weaker community shifts, revealing that depth more strongly limited post-bleaching mortality. This effect was partly mediated by differences in pre-bleaching community composition and coral cover, yet ~ 74% of the protective effect remained unexplained, suggesting that additional depth-related environmental or physiological mechanisms, such as light attenuation or colony size, were at play. These findings position deeper reefs as potential natural buffers, even in the absence of effective thermal stratification, and priorities for proactive conservation under accelerating climate change.