Ocean acidification is the name given to the process by which the increase in carbon dioxide in the atmosphere, from the burning of fossil fuels, has caused more carbon dioxide to dissolve into the oceans and reduced the pH of the oceans, making them more acidic.
Scientific records show that ocean pH has fallen from 8.2 to around 8.1, and though the oceans remain alkaline, a small change can have an enormous impact on marine life.
The research done on this issue is quite recent, as for a long time the scientific community believed that the oceans played a useful role in reducing the effects of increasing carbon dioxide emissions into the atmosphere, reducing the impact of global warming. The realisation that this could have a negative impact on marine life through ocean acidification is fairly new.
Oceans are part of the carbon cycle. Carbon moves between the atmosphere, the oceans, the biosphere and geosphere. The oceans contain a significant proportion of stored carbon; only the lithosphere, the uppermost crust of the Earth’s surface, contains more. On land the carbon cycle process involves photosynthesis, which is the biological component of the carbon cycle, and the process by which carbon dioxide in the atmosphere is transferred to land, through photosynthesis, into plant biomass.
In the ocean the process is more complex. Carbon dioxide is absorbed organically by phytoplankton through the process of photosynthesis. Phytoplankton inhabit the upper layers of the ocean where there is access to sunlight. Phytoplankton are very important in the carbon cycle and account for about half of all photosynthesised carbon dioxide, and are the source, therefore, of about half of all oxygen production. They feed on minerals, and are an important food item for fish, crustaceans, molluscs and aquatic plants.
Carbon dioxide is also absorbed into the oceans from the atmosphere and converted into dissolved inorganic carbon. It is through this process that the acidification of oceans is occuring, as carbon dioxide dissolves in sea water and creates carbonic acid, also forming bicarbonate ions and hydronium ions, which reduces pH, and increases acidity. At the same time, as the level of saturation of carbon dioxide increases, so the quantity of carbonite ions decreases.
The role of the oceans as a carbon sink, one that stores carbon, rather than a source that adds carbon dioxide to the atmosphere, is important. Carbon dioxide dissolves more easily in cold sea water, whereas phytoplankton grow better in warmer oceans, and through photosynthesis work as a carbon sink. The community that feeds off phytoplankton is, through respiration, a carbon source, releasing carbon dioxide into the atmosphere.
The impact on marine life of ocean acidification is therefore of great importance, not just because of the role of oceans in the carbon cycle, but also because of the role of oceans as a food source.
In recent years research into the effects of ocean acidification on oceanic calcifiying organisms, which include coral reefs, shellfish, molluscs, crustaceans and coccolithophores, has been carried out. These organisms are an important part of the food chain. Calcium carbonate is the main component of sea shells, the carbonate ion plays a role in this process. Acidification decreases the carbonate ions and reduces calcium carbonate and shell formation, and may, if the sea water does not reach saturated levels of carbonate ions, cause shells to dissolve.
Calcium carbonate exists in two different crystal formations: aragonite and calcite. It is believed that the aragonite formations, being less stable than the calcite formations, may be more vulnerable to increasing acidity. The risk is greater below a certain depth, known as the saturation horizon. In some areas, such as the California coast, vertical movement of water is common, and such upwellings can significantly alter pH.
Research is now in the process of revealing the effects of ocean acidification on marine life.
Marine snails, known as pteropods, have shells of aragonite formations, and in 2008 a science cruise involving researchers from a number of organisations, found severe dissolution of the shells of these snails in the waters of the Southern Ocean.
Oysters along the US Pacific coast, an area of sea prone to upwellings because of changing seasonal wind patterns, have, for a number of years, been affected by ocean acidification, as the upwellings reduce pH. The larvae have failed to grow, being unable to form their first shells. Their shells are also of aragonite formation.
Coral reefs are also affected by ocean acidification. Coral reefs are an important marine habitat, made from calcium carbonate, which is secreted by corals. They are home to a quarter of all different marine species, including fish, molluscs and numerous other species. Their main habitat is shallow tropical waters, but there are also cold water and deep water corals.
The risk to coral reefs from ocean acidification is enormous, and they are also at risk from pollution from agricultural run-off, water pollution, and some types of fishing. Their calcium carbonite formation is also aragonite, so they are at risk of slow-down in growth, and potentially dissolving. Currently signs of erosion are evident, and evidence of bleaching is common. This will have an enormous impact on marine life. Their deterioration will also put at risk small islands where coral reefs have a role in sea defence.
Research continues into the effects of ocean acidification; however, a report published in 2009 by the European Project on Ocean Acidification, consisting of 27 different organisations, revealed that research has shown that the seas will become noisier; as the acidity increases, sound travels further. This will affect the ability of whales and dolphins to navigate and communicate.
Species of algae will also be affected; for example, algae living on the shells of coccolithopores, which are a species of phytoplankton with shells, will not survive if the shells do not form, or dissolve. Coccolithophores are the leading calcite produces in the oceans, affecting the production of limestone.
A research experiment in 2007 on brittle stars, which are echinoderms, and also calcifiying organisms, showed increased calcification, but at the expense of muscle growth and larvae production. Fish such as herring feed on the larvae.
Ocean acidification is likely to have an enormous effect on the ocean food chain, particularly on shellfish, unable to build their skeletons or shells. Research is now being done on the economic impacts, including to commercial fisheries. It will be interesting, as further evidence emerges and it becomes clearer that ocean acidification will affect the food chain, how this will change attitudes to global warming.