The ocean, which acts as a buffer against the atmospheric CO2, absorbs this gas until surface seawaters reach the equilibrium with atmospheric levels. In fact, the ocean is one of the biggest reservoir of carbon of our planet. That is why little changes in this reservoir can make a big impact on the planet´s chemistry. For example, consider the release back of CO2 into the atmosphere. When atmospheric CO2 reacts with oceanic waters, it is forming carbonic acid (H2CO3). It is an weak, unstable acid and therefore dissociates into bicarbonate (HCO3–) and protons (H+). The bicarbonate in its turn, further dissociates into carbonate ions (CO3–) and more protons. A higher proton concentration translates into a decrease in pH because pH and Proton concentration have a negative-logarithmic relationship (pH = -log[H+])
So, in surface seawater with pH of 8.1 (more or less how the ocean is now), approximately 90% of the inorganic carbon is bicarbonate ion, 9% is carbonate ion, and only 1% is dissolved CO2. With more CO2, meaning more acidic conditions, the equilibrium of the carbonate system (all the carbon forms in which CO2 dissociates in water) will go towards more CO2, HCO3– and H+ and less CO3–. The decrease in the lastis expected to be about 50% by the end of this century. These changes are illustrated for surface waters in the right sided figure (from Wolf-Gladrow et al. 1999).
It is important to realize that not only the various carbon forms can assume to change. Once carbonate chemistry is affected, the distribution of chemical charges in the ocean is modified and compounds able to donate or receive protons will be affected. The following Figure (modified from Zeebe and Wolf-Gladrow 2001) shows part of them and the direction of the change along with pH variations:
Some of these are important to organisms, for example the nutrient phosphate (PO4) is essential to phytoplankton… but we will see more of the effects of ocean acidification on the marine biota in another page of this blog.