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

Orion Norzagaray1, Jose Martin Martin Hernandez-Ayon2, Luis Eduardo Calderon Aguilera3, Hector Reyes-Bonilla4, Ruben Castro5 and Armando Trasviña3, (1)Universidad Autonoma de Baja California, Facultad de Ciencias Marinas, Ensenada, Mexico, (2)Universidad Autónoma de Baja California, Instituto de Investigaciones Oceanológicas, Ensenada, Mexico, (3)Ensenada Center for Scientific Research and Higher Education, Mexico, (4)Universidad Autónoma de Baja California Sur, Mexico, (5)UABC-Fac. de Ciencias Marinas, Ensenada, Mexico
Abstract:
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 (< Ωar) 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 (<Ωar), related to GCW, and two months with TSW; the highest pH values (> Ωar) were from May to June. During winter-spring (mostly-TSW) were present the most optimal conditions for coral calcification (>Ωar). 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 Ωar (<2.6), low temperature (<22oC), 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.