Ocean acidification hotspots: Spatiotemporal dynamics of the seawater CO2 system of eastern Pacific coral reefs

Seawater CO 2 system dynamics were assessed from eastern Pacific reef sites in Panamá over 5 consecutive years (2003‐2008) and twice in the Galápagos Islands (2003 and 2009). The seawater CO 2 system was highly variable in time and space, but was explained by physical forcing from meteorological (seasonal rainfall) and oceanographic (upwelling, tides) processes interacting with diurnal reef metabolism. Galápagos coral reef communities are naturally exposed to the highest ambient partial pressure of CO 2 ( p CO 2 ) and lowest aragonite saturation (Ω arag ) values documented for any coral reef environment to date. During upwelling in the Galápagos, mean p CO 2 2 and mean Ω arag at five different sites ranged from 53.1 to 73.5 Pa and 2.27 to 2.86, respectively. Values of p CO 2 and Ω arag ranged from 21.0 to 48.7 Pa and 2.47 to 4.18, respectively, on the Saboga Reef in the seasonally upwelling Gulf of Panamaá, with the highest p CO 2 and lowest Ω arag values occurring during upwelling. The Uva Reef, in the nonupwelling Gulf of Chiriquí of Pacific Panamá, had mean Ω arag values that were always significantly greater than those at the Saboga Reef. Diurnal changes in the seawater CO 2 system from reef metabolism on the Uva Reef were magnified at low tide and highly significant differences were measured over depths as shallow as 15 m because of the shallow thermocline that is pervasive throughout the eastern Pacific. These naturally high‐CO 2 reefs persist near the Ω arag distributional threshold for coral reefs and are thus expected to be the first and most affected by ocean acidification.