Aragonite saturation states at cold-water coral reefs structured by Lophelia pertusa in the northern Gulf of Mexico

Ocean acidification, the reduction in pH and calcium carbonate saturation states of seawater, is likely to exhibit its most immediate effects on cold-water corals in deep waters with the shoaling of the aragonite saturation horizon. However, empirical data describing the carbonate chemistry at cold-water coral reefs are very rare. Regions of the upper slope of the Northern Gulf of Mexico harbor several deep-water reefs structured by the scleractinian Lophelia pertusa. We collected discreet water samples at a range of depths in the Gulf of Mexico, including eight Lophelia reefs, and measured total alkalinity and pH to calculate the aragonite saturation state (Omega(arag)). The deep waters of the Gulf of Mexico (>300 m depth) were at aragonite saturation states between 0.98 and 1.69. L. pertusa was present at sites with Omega(arag) between 1.25 and 1.69, and carbonate ion concentrations between 92 and 123 mu mol kg(-1). These data provide a critical baseline for detecting future changes in carbonate chemistry in the water column (i.e., aragonite saturation horizon shoaling), as well as at the sites of well-developed cold-water coral structures threatened by ongoing ocean acidification.