Seasonal and local spatial patterns in the upper thermal limits of corals on the inshore Central Great Barrier Reef

Experimental studies of the upper thermal limits of corals from Orpheus Island, an inshore reef in the central Great :Barrier Reef, show that Acropora formosa has a 5-day 50%-bleaching threshold of between 31 and 32 degrees C in summer, only 2 to 3 degrees C higher than local mean summer temperatures (29 degrees C). Summer bleaching thresholds for Pocillopora damicornis and A. elseyi were 1 degrees C higher (between 32 and 33 degrees C). The winter bleaching threshold of Pocillopora damicornis was 1 degrees C lower than its summer threshold, indicating that seasonal acclimatisation may take place. This seasonal difference raises the possibility that at least some corals may be capable of short-term thermal acclimatisation. Neither P. damicornis nor A. elseyi showed habitat-specific (reef flat versus reef slope) differences in bleaching thresholds. Further, colonies of P. damicornis collected from sites 3 km apart also showed no difference in bleaching threshold despite populations of this species responding differently at these two sites during a natural bleaching event. The bleaching thresholds determined in this study are best considered as the maximum tolerable temperatures for local populations of these species because they were determined in the absence of additional stressors (e.g. high light) which often co-occur during natural bleaching events. We consider the 5-day 50% bleaching thresholds determined in these experiments to be fair indicators of upper thermal limits, because > 50% of a sample population died when allowed to recover in situ. We found a delay of up to a month in the bleaching response of corals following thermal stress, a result that has implications for identifying the timing of stressful conditions in natural bleaching events.