Ocean acidification

In order to explain the ocean acidification, first we need to take a look on the carbonate system and how does it work in the ocean. As you can see from figure 1, CO₂ is exchanged between atmosphere and ocean from gaseous to aqueous form. The CO₂ dissolved, exists in three inorganic forms: as free carbon dioxide, CO₂ (aq.), as bicarbonate, HCO₃^-1, and as carbonate ion, CO₃^-2.

1. Carbonate system in the Ocean. Source: 2001, CO 2 IN SEAWATER:EQUILIBRIUM, KINETICS,ISOTOPES, Richard E. Zeebe and and Dieter Wolf-Gladrow, page 3.

 The relative proportions of CO₂, HCO₃^-1 and CO₃^-2 control the pH in the Seawater (see figure 2). At typical seawater conditions, bicarbonate is the dominant species, followed by carbonate ion, whereas dissolved dioxide is present only in small concentration. The pH of the seawater at the moment is about 8.1.

2. Carbonate system: Bjerrum plot, graphical representation of equilibrium relationships. The circle and the diamond indicate the constants of carbonic acid. Source: 2001, CO 2 IN SEAWATER:EQUILIBRIUM, KINETICS,ISOTOPES, Richard E. Zeebe and and Dieter Wolf-Gladrow, page 5. 

Typically, the ocean soaks up a third part of the atmospheric CO₂. However, these conditions are changing. The increasing CO₂ concentration in the atmosphere increases the amount of CO₂ that is being dissolved by the ocean, leading to a lower pH which means acidification of the ocean.

These changes in the equilibrium reduce the carbonate ion, CO₃^-2, which is the form of carbon that many organisms need to make shells and skeletons, coccolithophores, for instance. Ocean acidification is not only a problem for the animals which use carbonate in their skeletons and shells, but for the rest of them.

Some animals have a wide pH range of tolerance, for those animals will be easy to adapt to future changes on pH. But most of them have an optimum pH range which is quite limited, and a low tolerance to pH changes. If the pH falls below the tolerance range, that will lead to death due to respiratory or osmosis regulatory failure.

Sources:
2001, CO 2 IN SEAWATER:EQUILIBRIUM, KINETICS,ISOTOPES, Richard E. Zeebe and and Dieter Wolf-Gladrow
NASA Climate change - Ocean acidification  http://www.nasa.gov/topics/earth/features/climate_acidocean.html