Aragonite saturation and pH variation in a fringing reef are strongly influenced by oceanic conditions

Seasonal variations of the CO2 system in seawater for coral reefs are often controlled by both chemical-biological processes and by regional oceanographic features in combination with the residence time of adjacent water. This study reports the seasonal variability of the CO2 system along a year though dissolved inorganic carbon and pH measurements in Cabo Pulmo, a fringing reef from the Mexican Pacific. Two conditions were found on the reef: summer with the presence of Tropical Surface Water (TSW), characterized by high temperature, lower DIC levels, and a higher aragonite saturation (Ωarag); and winter with Gulf of California Water (GCW), when the opposite occurred. This seasonal variation was due to the horizontal advection of two surface water masses: Gulf of California water during winter and Tropical Surface Water the rest of the year as a result of the ocean circulation at the entrance of the Gulf of California. Therefore, the reef is significantly influenced by open ocean waters, where the depth is less than 20 m. The seasonal variability in Ωarag between months with the presence of GCW and TSW is one of the largest ranges reported in coral reefs (ΔΩarag = 0.5 units). Our results suggest that the reef community cannot modify the aragonite or pH of incoming ocean water. This might be the case in other fringing reefs, where the use of ocean acidification forecast models is suitable.