Variable responses to chronic and acute elevated temperature of three coral species from reefs with distinct thermal regimes

Ocean warming and marine heatwaves caused by climate change are adversely affecting the health, structure, and function of coral reefs globally. As seawater temperature continues to rise, and with the intensity and duration of heat stress events expected to increase, it is pertinent to understand the environmental drivers of variability in coral thermal tolerance under both chronic and acute warming. We assessed the performance of three species of corals, Acropora nana, Pocillopora acuta, and Porites lutea, sourced from reefs with distinct thermal regimes (variable vs. stable) in southern Taiwan, when exposed to chronic long-term temperature conditions (12 weeks at 26 degrees C or 30 degrees C) and then, using a subset of corals, subsequently exposed to acute short-term elevated temperature conditions (8 days at 32 degrees C). Under chronic temperature exposure we found clear species-specific responses, and some differences between reef sites and temperature treatments. Despite being perceived as relatively thermally sensitive, A. nana had the most stable maximum quantum yield (F-v/F-m), consistent growth, and the highest survival among the study species. Survival of P. acuta under chronic warming showed the clearest reef site effect and was lower in coral nubbins sourced from the thermally stable reef than the thermally variable reef. Porites lutea showed the greatest decline in F-v/F-m and lowest overall growth under chronic temperature exposure. All coral species, regardless of source reef site or previous chronic temperature exposure, experienced substantial bleaching (50-100% of all nubbins) upon acute exposure to elevated temperature. We found that, overall, corals were able to tolerate chronic warming above seasonal mean temperatures, but that bleaching occurred after acute exposure to a temperature similar to 2 degrees C above their mean summer maxima.