Time-integrated thermal bleaching thresholds of reefs and their variation on the Great Barrier Reef

Abstract

Knowledge of the critical levels for key environmental variables that are likely to cause bleaching in reef corals is of fundamental importance in conducting risk assessments of potential climate-change effects on coral reefs. Such knowledge can also be used to provide early warning of mass bleaching events. A number of factors have contributed to the difficulty in determining critical levels for coral bleaching. These factors include the fact that multiple stressors may be involved in bleaching, the duration of stress required to elicit a bleaching response varies with temperature, and bleaching triggers are known to be variable in space, time and by species. In this study, I identify sea surface temperature (SST) as the most important parameter for predicting coral bleaching from 4 possible environmental variables collected over 10 to 12 yr from weather stations at 2 locations on the Great Barrier Reef (GBR): temperature, wind speed, solar radiation and barometric pressure. Predicted bleaching-response curves are constructed from high-resolution in situ temperature records and historical observations of coral bleaching for 13 locations. These curves approximate reef-wide stress-response thresholds for bleaching of thermally sensitive (and often dominant) coral species. Distinct spatial trends exist in the thermal sensitivity of coral populations that correspond with position across the shelf and latitude in the case of mid- and outer-shelf reefs. This suggests that considerable thermal adaptation has taken place over small (10s of km) and large (100s to 1000s of km) spatial scales. Bleaching curves for inshore reefs do not correspond with latitude and are more variable, reflecting greater local-scale variability in temperature regimes.