Ocean acidification is a reduction of the pH of the ocean over time, according to the National Oceanic and Atmospheric Administration. This is predominantly due to uptake of carbon dioxide from the atmosphere.
The ocean absorbs about 30 percent of the carbon dioxide that is released into the atmosphere, so as the atmospheric carbon dioxide increases, so does carbon dioxide in the ocean. This results in a larger concentration of hydrogen ions and a reduction in carbonate ions.
Less carbonate ions means calcifying organisms cannot build and maintain their shells. This is also an issue for non-calcifying organisms because certain fish have a depressed ability to detect prey in acidic waters.