Ocean Acidification

Coral reefs are among the most biologically diverse and economically valuable ecosystems on earth. They provide billions of dollars annually in food, jobs, recreation, coastal protection, and other critical ecosystem services (Brander & van Beukering, 2013; Costanza et al., 2014). However, these ecosystems are also among the most vulnerable to ocean acidification (OA). Even under the most optimistic model projections, increasing atmospheric and seawater carbon dioxide concentrations are likely to occur over the next few decades, decreasing seawater pH and reducing the availability of the carbonate ion (CO32-) building blocks that corals and other marine calcifiers use to construct reef habitat (Chan & Connolly, 2013; Jiang et al., 2023). OA threatens the persistence of coral reefs by reducing rates of coral and crustose coralline algae (CCA) calcification and accelerating rates of bioerosion, thereby lowering net production of calcium carbonate (CaCO3) and compromising the structural complexity and integrity of three-dimensional reef habitat (Cornwall et al., 2021; Hill & Hoogenboom, 2022). As a result, many of the ecological, economic, and cultural values offered by coral reefs could be significantly impacted by OA over the next century.

NOAA’s National Coral Reef Monitoring Program (NCRMP) provides a framework for long-term, national-level monitoring of the U.S.-affiliated coral reef areas. Funded jointly by the NOAA Coral Reef Conservation Program and Ocean Acidification Program, NCRMP assesses the status and trends of U.S. coral reef ecosystems and supports the management of the nation’s reefs (NOAA Coral Program, 2021). NCRMP’s long-term monitoring of OA and related coral reef ecosystem responses (NCRMP-OA) evaluates patterns and trends in carbonate chemistry and key ecosystem indicators across gradients of biogeography, oceanographic conditions, habitat types, and human impacts. These data sets are used to inform the efficacy of place-based coral reef management in close collaboration with federal, state, and jurisdictional partners.

To assess the progression of OA and impacts on coral reef ecosystems in the U.S. Pacific Islands, NCRMP-OA monitoring includes the following objectives:

• Conduct carbonate chemistry sampling to monitor spatial variability and temporal change in pH, aragonite saturation state (Ωar), and other carbon system parameters;
• Conduct diel carbonate chemistry water sampling and oceanographic instrument deployments at select sites;
• Conduct census-based carbonate budget assessments to estimate rates of coral reef biological carbonate production and erosion.

This report summarizes the monitoring effort and results from 2021–2023 NCRMP-OA sampling and surveys. Additional NCRMP environmental, benthic, and fish data are not included in this report, but they can be accessed at the links provided in the Data Availability section.

Barkley H. C., 2025. Summary of ocean acidification data collected by the National Coral Reef Monitoring Program in the U.S. Pacific Islands, 2021—2023. Pacific Islands Fisheries Science Center, Administrative Report Series, H-25-09. Report.