Abstract
Tropical scleractinian corals are particularly vulnerable to global warming as elevated sea surface temperatures (SSTs) disrupt the delicate balance between the coral host and their algal endosymbionts, leading to symbiont expulsion, mass bleaching and mortality. While satellite sensing of SST has proved a reliable predictor of coral bleaching at the regional scale, there are large deviations in bleaching severity and mortality on the local scale that are poorly understood. Here, we show that internal waves play a major role in explaining local coral bleaching and mortality patterns in the Andaman Sea. Despite a severe region-wide SST anomaly in May 2010, frequent upslope intrusions of cold sub-pycnocline waters due to breaking large-amplitude internal waves (LAIW) mitigated coral bleaching and mortality in shallow waters. In LAIW-sheltered waters, by contrast, bleaching-susceptible species suffered severe bleaching and total mortality. These findings suggest that LAIW benefit coral reefs during thermal stress and provide local refugia for bleaching-susceptible corals. LAIW are ubiquitous in tropical stratified waters and their swash zones may thus be important conservation areas for the maintenance of coral diversity in a warming climate. Taking LAIW into account can significantly improve coral bleaching predictions and provide a valuable tool for coral reef conservation and management.
Highlights
Global warming and ocean acidification are recognized as the major threats to coral reefs [1,2]
In large-amplitude internal waves (LAIW)-sheltered waters, by contrast, bleaching-susceptible species suffered severe bleaching and total mortality. These findings suggest that LAIW benefit coral reefs during thermal stress and provide local refugia for bleaching-susceptible corals
A series of global mass bleaching events has led to a marked decline in coral cover and species diversity over recent decades [2,6], and concerns over the projected increase in frequency and intensity of bleaching events with the eventual demise of coral reefs [7] has fostered the search for natural refugia [8,9]
Summary
Global warming and ocean acidification are recognized as the major threats to coral reefs [1,2]. The thermal optimum for most scleractinian corals is very close to their upper thermal temperature limit, and moderate increases in sea surface temperatures (SSTs) of 1–28C can become stressful to corals [3,4]. Reef refugia maintain higher coral cover and species diversity, and are target areas for reef conservation Both extrinsic and intrinsic factors may contribute to coral reef resistance to thermal stress [8,10]. LAIW are strong during periods of maximum thermal stratification and SSTs [15], ubiquitous in the world ocean [19], and observed to reach into many coral reef environments [15,16,20 –28] Their potential role in mitigating thermal stress has not yet been investigated. License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited
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