Seasonal and high-frequency measurements of pH, oxygen and aragonite saturation state in a coral reef: Cabo Pulmo, Mexico.

Cabo Pulmo reef is located in the coastal area within the oxygen minimum zone (OMZ), which has been reported as shallow as 70m, and characterized by CO₂-rich waters (>2200 μmolkg^-1) and low pH (<7.7). To date it is unknown whether the OMZ waters influence these coral reef at any point of the year, or during certain oceanographic episodes, therefore, it is important to know the temporal variability of these parameters. This study presents high frequency data series from November 2013 to June 2014 from a SeapHOX sensor deployed at 15 m depth and 1.5 km from shore. The pH series was calibrated with discrete samples (total carbon and alkalinity measurements). A high-resolution aragonite saturation state (< Ω_ar) series was calculated from pH series and total alkalinity. Discrete and continuous measurements showed the seasonal influence of two water masses, the Gulf of California water during winter (GCW), and the surface Tropical water (TSW) during spring-summer. From December to April the conditions with the lowest pH were found (<Ω_ar), related to GCW, and two months with TSW; the highest pH values (> Ω_ar) were from May to June. During winter-spring (mostly-TSW) were present the most optimal conditions for coral calcification (>Ω_ar). Dissolved oxygen (OD) was always up to 3.4 mlL^-1. However, two events (5-10 days length) arose in winter (February/TSW) and summer (June/GCW) with low pH (<7.9), low Ω_ar (<2.6), low temperature (<22^oC), and low DO (<4 mlL^-1), threshold values reported to cause negative effects on coral calcification and with exposure times on the order of days.