Ocean acidification refers to a reduction in the oceans pH over an extended period of time. It is primarily caused by the uptake of carbon dioxide (CO2) from the atmosphere. The ocean absorbs about 30 percent of the CO2 that is released in the atmosphere, and as levels of atmospheric CO2 increase, so do the levels in the ocean. When CO2 is absorbed by seawater, a series of chemical reactions occur resulting the seawater becoming more acidic and carbonate ions to be relatively less abundant. The reduction in carbonate ions is scary when you think of how many things rely on them as a building block – like coral skeletons and sea shells!
Decreases in carbonate ions can 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. The changes in chemistry can also affect the ability of certain fish to detect predators, hunt prey and meet mates! In addition to threatening ocean ecosystems, ocean acidification could also impact on the economies which depend on fish and shellfish.
Source: NOAA