As higher levels of CO2 are absorbed into the oceans, fewer carbonate ions (CO32- ) are available for organisms such as corals, clams, sea urchins and plankton to produce their calcium carbonate (CaCO3) shells and skeletons. With increasing ocean acidity, the rates at which reef-building corals produce their skeletons decreases, and if the carbonate concentration falls too low, the shells may start to dissolve. By the middle of the century, coral reefs may erode faster than they can be built.
These impacts will have serious consequences for people that depend on the reefs for their livelihoods, whether from subsistence use or from larger scale tourism, coastal protection and commercial fisheries. Studies show that the consequences become successively worse as CO2 concentration increases, and unmanageable for CO2 concentations above 500 ppm.1
A lower pH means it will become harder for some plankton to maintain their protective shells. Plankton, which are tiny floating plants and animals, form the base of the ocean food chain and produce over half the oxygen in the air we breathe. Many types of plankton, for example, cocoolithophores and foraminifera, form calcium carbonate shells and will be affected by acidification.2 Shell weights of foraminifera are currently 30-35% lower than shells of their counterparts from thousands of years ago, suggesting that acidification may already be negatively affecting these species.3 These organisms play important roles in ocean-atmosphere interactions. However, further research is needed to predict the effects that may arise from their decline.
A lower pH is not only trouble for corals and plankton, but is likely to affect a wide range of marine organisms. For instance, it may decrease the survival rates of young fish.4 Scientists predict that mussel and oyster calcification could decrease significantly by the end of the 21st century.5 And crustose coralline algae, a major component of marine habitats from polar to tropical regions, have calcium carbonate components. These algae help provide the foundation for the growth of corals and if affected by ocean acidification, coral reefs may be compromised.
Pteropods, small marine planktonic snails, are another crucial link in the ocean food chain. They are an important source of food for juvenile North Pacific salmon, giant whales and tiny krill. However, scientists have determined that under the condition of increasing ocean acidification, pteropod shells will weaken, thus compromising the health of these organisms. This has the potential to create significant changes in the biodiversity of the oceans.6
Calcium carbonate comes in a variety of forms. Aragonite is used in coral formation, calcite composes the shells of a variety of plankton such as coccolithophores and magnesium-calcite is the basis of coralline algae. Each of these varieties dissolve with varying levels of carbonate concentration. The level in the seawater below which calcium carbonate dissolves is known as the saturation horizon and each form of calcium carbonate has its own. However, since the Industrial Revolution, the saturation horizon for all forms of calcium carbonate has been reduced.7