Abstract
Coral bleaching is the detrimental expulsion of algal symbionts from their cnidarian hosts, and predominantly occurs when corals are exposed to thermal stress. The incidence and severity of bleaching is often spatially heterogeneous within reef-scales (<1 km), and is therefore not predictable using conventional remote sensing products. Here, we systematically assess the relationship between in situ measurements of 20 environmental variables, along with seven remotely sensed SST thermal stress metrics, and 81 observed bleaching events at coral reef locations spanning five major reef regions globally. We find that high-frequency temperature variability (i.e., daily temperature range) was the most influential factor in predicting bleaching prevalence and had a mitigating effect, such that a 1 °C increase in daily temperature range would reduce the odds of more severe bleaching by a factor of 33. Our findings suggest that reefs with greater high-frequency temperature variability may represent particularly important opportunities to conserve coral ecosystems against the major threat posed by warming ocean temperatures.
Highlights
IntroductionConditions”, “Cumulative Thermal Stress”, and “Acute Thermal Stress” were the three explanatory variable categories largely suspected of exacerbating bleaching
Similar to the maximum monthly mean (MMM), cumulative thermal stress is traditionally derived from remotely sensed Stress (In situ): 0.00 (SST) and is among the most common metrics used to predict coral bleaching[25,33]
Consistent with the wellestablished perspective that anomalously high temperatures are the primary cause of coral bleaching[7], among our highest ranked models, bleaching was most exacerbated by greater cumulative and acute thermal stress, and to a lesser degree, by increases in MMM temperature and heating rate[25]
Summary
Conditions”, “Cumulative Thermal Stress”, and “Acute Thermal Stress” were the three explanatory variable categories largely suspected of exacerbating bleaching. “Background Conditions” (Table 1) consisted of the average summertime, or maximum monthly mean (MMM), temperature, but computed from our in situ time series data, as opposed to conventional remotely sensed SST data. The “Acute Thermal Stress” category (Table 1) was included as a safeguard to differentiate sites with temperatures that may not have exceeded MMM+ 1 °C (i.e. no thermal stress) yet still experienced bleaching. Consistent with the wellestablished perspective that anomalously high temperatures are the primary cause of coral bleaching[7], among our highest ranked models, bleaching was most exacerbated by greater cumulative and acute thermal stress, and to a lesser degree, by increases in MMM temperature and heating rate.
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