Carbonate production of Micronesian reefs suppressed by thermal anomalies and Acanthaster as sea-level rises.

Coral reefs are essential to millions of island inhabitants. Yet, coral reefs are threatened by thermal anomalies associated with climate change and by local disturbances that include land-use change, pollution, and the coral-eating sea star Acanthaster solaris. In combination, these disturbances cause coral mortality that reduce the capacity of reefs to produce enough carbonate to keep up with sea-level rise. This study compared the reef-building capacity of shallow-water inner, patch, and outer reefs in the two islands of Pohnpei and Kosrae, Federated States of Micronesia. We identified which reefs were likely to keep up with sea-level rise under different climate-change scenarios, and estimated whether there were differences across habitats in the threshold of percentage coral cover at which net carbonate production becomes negative. We also quantified the influence of A. solaris on carbonate production. Whereas the northwestern outer reefs of Pohnpei and Kosrae had the highest net rates of carbonate production (18.5 and 16.4 kg CaCO3 m-2 yr-1, respectively), the southeastern outer reefs had the lowest rates of carbonate production (1.2-1.3 and 0.7 kg CaCO3 m-2 yr-1, respectively). The patch reefs of Pohnpei had on average higher net carbonate production rates (9.5 kg CaCO3 m-2 yr-1) than the inner reefs of both Pohnpei and Kosrae (7.0 and 7.8 kg CaCO3 m-2 yr-1, respectively). A. solaris were common on Kosrae and caused an average reduction in carbonate production of 0.6 kg CaCO3 m-2 yr-1 on Kosraean reefs. Northern outer reefs are the most likely habitats to keep up with sea-level rise in both Pohnpei and Kosrae. Overall, the inner reefs of Pohnpei and Kosrae need ~ 5.5% more coral cover to generate the same amount of carbonate as outer reefs. Therefore, inner reefs need special protection from land-use change and local pollution to keep pace with sea-level rise under all climate-change scenarios.