Coral reefs are predicted to be one of the ecosystems most sensitive to ocean acidification. To improve predictions of coral reef response to acidification, we need to better characterize the natural range of variability of pH, partial pressure of carbon dioxide (pCO 2) and calcium carbonate saturation states (). In this study, autonomous sensors for pH and pCO 2 were deployed on Media Luna reef, Puerto Rico over three seasons from 2007 to 2008. High temporal resolution CaCO 3 saturation states were calculated from the in situ data, giving a much more detailed characterization of reef saturation states than previously possible. Reef pH, pCO 2 and aragonite saturation ( Ar) ranged from 7.89 to 8.17 pH units, 176-613 atm and 2.7-4.7, respectively, in the range characteristic of most other previously studied reef ecosystems. The diel pH, pCO 2 and cycles were also large, encompassing about half of the seasonal range of variability. Warming explained about 50% of the seasonal supersaturation in mean pCO 2, with the remaining supersaturation primarily due to net heterotrophy and net CaCO 3 production. Net heterotrophy was likely driven by remineralization of mangrove derived organic carbon which continued into the fall, sustaining high pCO 2 levels until early winter when the pCO 2 returned to offshore values. As a consequence, the reef was a source of CO 2 to the atmosphere during the summer and fall and a sink during winter, resulting in a net annual source of 0.73 1.7 mol m -2 year -1. These results show that reefs are exposed to a wide range of saturation states in their natural environment. Mean Ar levels will drop to 3.0 when atmospheric CO 2 increases to 500 atm and Ar will be less than 3.0 for greater than 70% of the time in the summer. Long duration exposure to these low Ar levels are expected to significantly decrease calcification rates on the reef.