Spatial pH variability of coral reef flats of Kiritimati Island, Kiribati.

Ocean acidification poses a threat to carbonate-dominated marine systems, such as tropical coral reefs, as it impacts the ability of organisms to calcify. For assessing the susceptibility of coral reef flats to open ocean acidification it is crucial to better understand the dynamics between the carbonate chemistry of open ocean waters flowing onto coral reef flats and the ecological and hydrodynamic processes that locally modify seawater conditions. In this study, variations in seawater pH and temperature were measured along cross-reef flat transects in high resolution (∼0.3 m) and complemented by surveys of the benthic community composition and reef flat bathymetry. Results represent a snapshot in time and suggest that reef flat hydrodynamic processes determine spatial pH modifications, with little influence of variations in benthic community composition. As mean reef flat pH largely equals ocean conditions, ocean acidification has had and will have an unhampered impact on narrow fringing reef flats.