Abstract
Climate change and consequent coral bleaching are causing the disappearance of reef-building corals worldwide. While bleaching episodes significantly impact shallow waters, little is known about their impact on mesophotic coral communities. We studied the prevalence of coral bleaching two to three months after a heat stress event, along an extreme depth range from 6 to 90 m in French Polynesia. Bayesian modelling showed a decreasing probability of bleaching of all coral genera over depth, with little to no bleaching observed at lower mesophotic depths (greater than or equal to 60 m). We found that depth-generalist corals benefit more from increasing depth than depth-specialists (corals with a narrow depth range). Our data suggest that the reduced prevalence of bleaching with depth, especially from shallow to upper mesophotic depths (40 m), had a stronger relation with the light-irradiance attenuation than temperature. While acknowledging the geographical and temporal variability of the role of mesophotic reefs as spatial refuges during thermal stress, we ought to understand why coral bleaching reduces with depth. Future studies should consider repeated monitoring and detailed ecophysiological and environmental data. Our study demonstrated how increasing depth may offer a level of protection and that lower mesophotic communities could escape the impacts of a thermal bleaching event.
Highlights
Warming sea surface temperature leading to spatially and taxonomically widespread bleaching events is one of the major drivers of the loss of reef-building corals [1]
Relative to the value at 6 m, photosynthetically active radiation (PAR) was about 25% at 40 m and only 4% at 90 m. These light values were consistent with the results obtained from a vertical CTD profile (PAR was less than 20% of the surface light at 40 m and less than 3% at 90 m; Pearson = 0.99, p-value = 4.8 × 10−7) and well fitted with the Beer–Lambert equation
This study shows a decrease in the incidence of coral bleaching across an extreme depth range (i.e. 6– 90 m) in French Polynesia, as measured two to three months after the peak of a heat stress event
Summary
Warming sea surface temperature leading to spatially and taxonomically widespread bleaching events is one of the major drivers of the loss of reef-building corals [1]. The habitat engineers of one of the most diverse marine ecosystems of our planet, live in obligate symbiosis with unicellular dinoflagellates from the family Symbiodiniaceae. This symbiosis readily breaks down under unfavourable environmental conditions, most commonly during anomalously high temperature and light-irradiance exposure events [2,3,4]. With the ever-increasing frequency and severity of bleaching events, shallow corals are disappearing at an alarming rate [1,12,13,14], and the quest to identify thermally tolerant corals and coral reefs is of great interest for conservation and management [15,16]
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