Effect of ocean warming and acidification on the early life stages of subtropical Acropora spicifera

This study investigated the impacts of acidified seawater ( p CO 2 ~ 900 μatm) and elevated water temperature (+3 °C) on the early life history stages of Acropora spicifera from the subtropical Houtman Abrolhos Islands (28°S) in Western Australia. Settlement rates were unaffected by high temperature (27 °C, ~250 μatm), high p CO 2 (24 °C, ~900 μatm), or a combination of both high temperature and high p CO 2 treatments (27 °C, ~900 μatm). There were also no significant differences in rates of post-settlement survival after 4 weeks of exposure between any of the treatments, with survival ranging from 60 to 70 % regardless of treatment. Similarly, calcification, as determined by the skeletal weight of recruits, was unaffected by an increase in water temperature under both ambient and high p CO 2 conditions. In contrast, high p CO 2 significantly reduced early skeletal development, with mean skeletal weight in the high p CO 2 and combined treatments reduced by 60 and 48 %, respectively, compared to control weights. Elevated temperature appeared to have a partially mitigative effect on calcification under high p CO 2 ; however, this effect was not significant. Our results show that rates of settlement, post-settlement survival, and calcification in subtropical corals are relatively resilient to increases in temperature. This is in marked contrast to the sensitivity to temperature reported for the majority of tropical larvae and recruits in the literature. The subtropical corals in this study appear able to withstand an increase in temperature of 3 °C above ambient, indicating that they may have a wider thermal tolerance range and may not be adversely affected by initial increases in water temperature from subtropical 24 to 27 °C. However, the reduction in skeletal weight with high p CO 2 indicates that early skeletal formation will be highly vulnerable to the changes in ocean p CO 2 expected to occur over the twenty-first century, with implications for their longer-term growth and resilience.