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Abstract Ocean warming and acidification caused by increases of atmospheric carbon dioxide are now thought to be major threats to coral reefs on a global scale. Here we evaluated the environmental conditions and benthic community structures in semi-closed Nikko Bay at the inner reef area in Palau, which has high pCO2 and seawater temperature conditions with high zooxanthellate coral coverage. Nikko Bay is a highly sheltered system with organisms showing low connectivity with surrounding environments, making this bay a unique site for evaluating adaptation and acclimatization responses of organisms to warmed and acidified environments. Seawater pCO2/Ωarag showed strong gradation ranging from 380 to 982 µatm (Ωarag: 1.79–3.66), and benthic coverage, including soft corals and turf algae, changed along with Ωarag while hard coral coverage did not change. In contrast to previous studies, net calcification was maintained in Nikko Bay even under very low mean Ωarag (2.44). Reciprocal transplantation of the dominant coral Porites cylindrica showed that the calcification rate of corals from Nikko Bay did not change when transplanted to a reference site, while calcification of reference site corals decreased when transplanted to Nikko Bay. Corals transplanted out of their origin sites also showed the highest interactive respiration (R) and lower gross photosynthesis (Pg) to respiration (Pg:R), indicating higher energy acquirement of corals at their origin site. The results of this study give important insights about the potential local acclimatization and adaptation capacity of corals to different environmental conditions including pCO2 and temperature.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

An amendment to this paper has been published and can be accessed via a link at the top of the paper. [ABSTRACT FROM AUTHOR]

Coral calcification is affected by the decrease in aragonite saturation state (Ωarag) caused by ocean acidification (OA). However, OA effects are modulated by other environmental factors such as seawater temperature, light intensity and nutrients. Considering that in subtropical coral reefs all these factors vary seasonally, it can be hypothesized that the magnitude of OA effects on coral physiology will also vary seasonally. We evaluated the seasonal coral calcification rate of a subtropical reef-building coral under OA conditions. We approached this aim by culturing Acropora digitifera under three different CO2 partial pressure (pCO2) conditions in three different seasons (summer, autumn and winter) under natural light and temperature conditions. Additionally, to predict future coral net G, the year-round seawater carbonate chemistry was measured on the coast of Okinawa Island, and the annual coral CaCO3 production amount assessed considering seasonal changes in environmental conditions. Coral A. digitifera net calcification (net G) significantly differed among seasons, and summer net G was 1.7 and 2.7 times higher than autumn and winter, respectively. However, the impact of OA did not differ among seasons and the rate of net G decrease per unit Ωarag was 11.1%, 17.4% and 18.7% for summer, autumn and winter, respectively. The regression model indicated that net G of A. digitifera is primarily affected by temperature, secondly by seawater Ωarag, while light intensity was not selected as an explanatory factor, and there was no interactive effect among the factors. The model predicts that the present annual A. digitifera net G in Okinawa Island reef (present mean annual pCO2: 382 μatm and Ωarag: 3.49) is about 0.3 g CaCO3 cm−2 y−1, and it will decrease by 20% with an increase of 500 μatm seawater pCO2 than the present condition. As reefs in high-latitude regions already have marginal positive net G, further decrease in annual CaCO3 production would be detrimental for the reef under conditions of climate change. [ABSTRACT FROM AUTHOR]