Coral reefs are formed by calcareous organisms, mainly scleractinian corals in tropical and subtropical coasts. Coral reefs play important roles in exclusive economic zone (EEZ) protection by forming reef structures reaching the low tide level and reducing wave energy. Healthy coral reefs have a potential to keep up with sea level rise and maintain reef structures. However, multiple local and global stressors degrade coral reef ecosystems and threaten their ecosystem functions. To predict whether coral reefs can keep up with contemporary see level rise, it is important to estimate reef carbonate production rate and its controlling factors. However, the method for carbonate production rate has not yet been well-established. In this study, we estimated reef carbonate production rate (G) based on both the biological calcification and the carbonate chemistry change in seawater and we then analyzed their controlling factors.
We successfully estimated the topography of our sites by the digital elevation model (DEM) using combined aerial photographs and used the model for the accurate estimation of G. Based on two methods, G was strongly controlled by coral cover, coral taxa, light intensity and water temperature. The thermal optima of G at the reef flat site was higher than the average highest water temperature around Okinawa island, indicating that hard corals at the reef flat site were acclimated to high water temperature. The G based on census-based method was always higher than the G based on the hydrochemical method at the same hard coral coverage, suggesting that the former may be overestimation. These results suggest that the observation of spatial-temporal variation of G is important for the accurate estimation of reef carbonate production rate, in addition to in situ measurement of coral calcification rate and measurement of carbonate dissolution rate. Reef accretion rates at the reef flat site based on two methods were higher than the rate of relative sea level rise around the site in the latest 5 years, indicating that coral reef at our sites has a potential to keep up with sea level rise.

Key Words: carbonate production, isolated island, coral reef ecosystem, carbonate chemistry