Ocean Acidification
Refers to a reduction in the pH of the ocean over an extended period of time, caused primarily by uptake of carbon dioxide (CO 2 ) from the atmosphere. When CO 2 is absorbed by seawater, a series of chemical reactions occur resulting in the increased concentration of hydrogen ions, which make building and maintaining shells and other calcium carbonate structures difficult for calcifying organisms such as oysters, clams, sea urchins, shallow water corals, deep sea corals, and calcareous plankton.