Given the ongoing rapid degradation of coral reefs by climate change, our need to understand and evaluate local reef sites that may serve as refugia is increasing in the context of management and conservation. Corals inhabiting the shallow lagoon of Okinawa reefs, located at high latitude in the northwestern Pacific region, are exposed to strong seasonal and diurnal temperature fluctuations that may influence their thermal tolerance. Here, we investigated the physiology of the coral Acropora digitifera from outer and inner reefs that were experimentally exposed to the more stable outer reef temperature and the highly fluctuating inner reef temperature during both the cold winter and hot summer seasons. In addition, both inner and outer reef corals were exposed to the future temperature condition (+ 3 °C) to verify whether the inner reef corals in Okinawa can serve as potential resilience hotspots under global warming. Our results showed that both inner and outer reef corals tolerated the fluctuating conditions in winter, which can reach 16.5 °C for a short period of time, although outer reef corals are rarely exposed to temperatures below 19 °C. Meanwhile, the fluctuating inner reef temperature condition in summer caused a decrease in calcification rate for both outer and inner reef corals compared to the more stable outer reef condition, suggesting that the extreme high temperature during daytime imposes physiological stress. Despite inner reef corals are living in particularly harsh environments with high thermal variability, both outer and inner reef corals were still vulnerable to the elevated temperatures expected under future climate change scenarios. However, our study also indicates that high temperature variability in inner reef environments can enhance coral thermal tolerance, potentially serving as a source of heat stress-tolerant coral populations. These findings emphasize the importance of taking further efforts to conserve inner reef corals at Okinawa, which may represent valuable reservoirs of genetic diversity under the rapidly changing environment.
Kurihara et al. (Thu,) studied this question.