Bleaching, upper thermal limits and temperature adaptation in reef corals

The broad-scale ecology of coral bleaching and coral upper thermal limits were investigated to improve the basis for risk assessment and prediction of possible climate change effects on the Great Barrier Reef (GBR). Evidence from the spatial distribution of bleaching on the GBR in 1998, and a temporal study of physical variables at two reefs over a 10 — 12 year period, indicates that temperature is the single most important factor leading to coral bleaching. A novel empirical approach was developed to quantify the bleaching thresholds of reefs in terms of the magnitude and duration of thermal stress. Results from this study provide the first comprehensive synthesis of exposure times and temperatures involved in coral thermal stress, allowing more accurate predictions of the time-temperature thresholds that are likely to lead to bleaching for a variety of reefs throughout the GBR. Bleaching threshold curves indicate that both latitudinal and cross-shelf trends exist in the bleaching thresholds of reefs. These trends correlate with local temperature regimes and suggest that thermal adaptation has taken place over local and regional scales (10's — 1000's km). These bleaching curves provide the basis for the development of an early warning system for coral bleaching using near real-time automatic weather stations. Experimental studies involving temperature manipulation show that three species of corals (Pocillopora damicornis, Acropora elseyi and A. formosa) from Orpheus Island on the inshore central GBR have sharply defined temperature tolerances and, like corals in other reef provinces, live precariously close to their upper thermal limits during most summers. Winter upper thermal limits of P. damicornis were 1°C lower than summer limits, raising the possibility that corals may be capable of short-term thermal acclimation. However, a short-term (10-day) acclimation study of two coral species (P. damicornis and Porites cylindrica) indicated that their thermal tolerance limits could not be extended beyond existing summer limits. Although these results indicate that the potential for development of short-term phenotypic tolerance to increasing temperatures may be limited, they do not preclude the possibility that acclimation may take place over longer time periods. Experimental studies also indicate that the breakdown of symbiosis between corals and zooxanthellae initiated by thermal stress continues for up to six weeks after removal of stress. Heat stress has a longer-term effect on coral health than can be accounted for by existing bleaching models, suggesting that an additional mechanism or process may be involved in coral bleaching. The delay in onset, or full development, of bleaching symptoms also has implications for the timing of bleaching surveys, namely, that to capture the full extent of bleaching, surveys need to be carried out 4-6 weeks after a stress event. An aerial survey method was employed and evaluated to document the spatial extent and intensity of bleaching on the GBR during the severe 1998 event. This method provided a spatially comprehensive, cost-effective and conservative estimate of bleaching over large areas (>2000km). An equivalent overview of bleaching could not have been obtained from visitor-based reports or small-scale in-situ surveys. The aerial survey results, together with historical reports of bleaching covering six separate events since 1980, were used to detect spatial patterns in bleaching on the GBR. These results indicate that inshore reefs are more prone to bleaching than offshore reefs. Coupled with higher rates of temperature warming on the southern GBR compared to the northern GBR over the 20th century, inshore southern reefs may be at greater risk of climate change-related changes in sea temperature than other areas on the GBR.