Coral carbonate pH records show localized stability within Philippine waters

The process of decreasing seawater pH due to increasing atmospheric carbon dioxide known as ocean acidification is a global phenomenon with very strong regional effects. We report the first seasonal and interannual carbonate chemistry variabilities in the Philippines determined using carbonate boron systematics. Coral δ¹¹B-pH_sw from two contrasting site conditions showed more positive trends than basin-wide averages, suggesting that reef waters around the Philippines represented by these sites may be offsetting the general pH decline in global seawater. This pattern is consistent with δ¹¹B-pH_sw studies in neighboring Palau and Taiwan and can be explained by decreases in net ecosystem calcification and/or increases in net ecosystem productivity. Other potential site-specific local drivers of δ¹¹B-pH_sw changes include vent-driven input of nutrient- and CO₂-rich groundwater that alters seawater dissolved inorganic carbon (DIC). The El Niño Southern Oscillation that influences water mass movement along the Philippine Pacific seaboard, and seasonal zonal current reversal in the Verde Island Passage (VIP) that switches source waters within the VIP, may also contribute to the recorded changes in δ¹¹B-pH_sw. Differences between the coral internal fluid chemistry and modelled regional seawater pH trends highlight the importance of localized monitoring of acidification for accurate management strategies of reef habitats.