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Ocean Acidification

As atmospheric concentrations of greenhouse gases increase, so do their respective concentrations in the ocean. Increases in dissolved carbon dioxide (CO2) levels from human activity are a particular concern. When CO2 dissolves in the ocean, it combines with water to form carbonic acid. So more CO2 means more “acidity”. The more acidic the ocean, the harder it is for marine organisms to build their shells. The problem they increasingly face is how to live in an ocean, which is rapidly becoming more acidic. This is already proving too difficult for some.

The Science of Ocean Acidification

Gases in the atmosphere are in constant equilibrium with the world’s oceans (Boxed Text). Increased levels of atmospheric carbon dioxide therefore lead to an increase in dissolved carbon dioxide levels. Currently, the world’s oceans are absorbing about 25 million tonnes of carbon every day1. It is also estimated that about 30% of atmospheric carbon dioxide produced by humans between 1800 and 1994 was absorbed by the oceans (approximately 118,000,000,000 tonnes [118 Gt])2. The remainder was either consumed by land-based organisms, or stayed in the atmosphere, contributing to global warming.

Figure 1
Diagram explaining equilibrium of CO2 in the atmosphere and sea. Hydrogen ions (H+) are the measure of ocean acidity. The ocean also has a high concentration of free calcium ions (Ca2+), needed for calcium carbonate formation.

In the ocean, CO2 is in equilibrium with a number of other carbon species: bicarbonate ions (HCO3-); carbonate ions (CO32-); and carbonic acid (H2CO3) (Fig.1). At the current mean oceanic pH (Boxed Text) of 8.0 (the measure of acidity), bicarbonate ions are the predominant of the three species (Fig.2). Carbon dioxide becomes hydrated in seawater to form carbonic acid. Carbonic acid in turn rapidly dissolves to produce bicarbonate and hydrogen ions, increasing the acidity of the seawater. In addition, as hydrogen ion concentrations increase, some then combine with carbonate ions to form more bicarbonate, reducing the available pool of carbonate. The result of increased carbon dioxide in the ocean is therefore:

1. Increased acidity3
2. Increased bicarbonate ion concentrations
3. Increased dissolved carbon dioxide concentrations
4. Reduced carbonate ion concentrations4

Figure 2
Bjerrum plot showing variation in the proportions of each carbon species as ocean acidity changes. The current oceanic pH level of ~8.0, and the approximate concentrations of each of these carbon species, is indicated.


Why is Ocean Acidification a Concern?

Organisms in our oceans have evolved over many millions of years within an extremely stable environment. As a result of human activity, they are now forced to exist in increasingly acidic water that contains many more hydrogen ions (Boxed Text). Hydrogen ions are highly reactive molecules and therefore extremely harmful. Many organisms do not have the evolutionary flexibility to cope with such a rapid change in ocean acidity and their continued survival comes at a high physiological cost5.

On land, CO2 is absorbed by plants and converted into organic matter. This organic matter passes up through the food chain and is ultimately released back into the atmosphere in the form of carbon dioxide or methane by metabolic processes (Boxed Text). This is known as the carbon cycle. Similar processes occur in the ocean. However, due to ocean chemistry, marine organisms take advantage of another mechanism, the bio- generation of inorganic calcium carbonate (CaCO3).

Calcium carbonate exists in two principal forms (calcite and aragonite) and is used by marine organisms for many purposes such as the formation of defensive shells, teeth, and bones. Corals are predominantly made of aragonite, the more soluble form of calcium carbonate.

The formation of these minerals is dependent on the oceans’ saturation with respect to calcium and carbonate ions. As the levels of carbonate ions drop due to ocean acidification, calcite and aragonite saturation levels also drop6. Since pre-industrial times, there has been a 30% increase in the acidity of the oceans, which corresponds to a drop of only 0.1 of a pH unit7 (Boxed Text). For example we have seen a significant drop in the amount of carbonate molecules compared to dissolved carbon dioxide in the Southern Ocean (Table 1).

Table 1
Historical, current and future carbonate / carbon dioxide ratios in the Southern Ocean.
The Intergovernmental Panel on Climate Change predicts that by 2100, atmospheric CO2 levels may reach 800-1000 parts per million. This would be equivalent to a drop of at least 0.4 pH units, or an increase of 150% in ocean acidity4, 8. It also predicts a rate of increase that is 100 times greater than anything seen for tens of millions of years3! The surface waters of the world have been supersaturated with aragonite for 25 million years3. However, as a result of continued increases in carbon dioxide levels in the atmosphere it is predicted that the Arctic Ocean will be completely undersaturated with aragonite and calcite by 2050 and 2100, respectively7 (Boxed Text). This undersaturation will spread overtime to lower latitudes. Additionally, the lysocline (the depth below which the ocean is undersaturated with aragonite and calcite) is rising7 (Boxed Text). Below this depth, the rate of calcium carbonate dissolution exceeds precipitation rates9. The physiological cost of precipitating calcium carbonate increases as saturation levels drop10-14 because organisms must increase their CaCO3 precipitation rates to compensate for increased dissolution. This can compromise other aspects of their physiology15. Deep cold-water coral and high latitude coral, that already exist in an environment where carbonate levels are close to being undersaturated, will be amongst the first to be severely impacted by ocean acidification. They will battle to maintain their exoskeletons as the rate of dissolution increases.

Lastly, evidence also suggests that as the oceans become more acidified, the absorption of carbon dioxide may be compromised. We can therefore expect a build-up of carbon dioxide in the atmosphere, due to both human production and a reduced capacity of the ocean to draw CO2 down.

References:

  1. Watson, R. T. et al. Climate Change 2001: Summary for Policymakers. IPCC Third Assessment Report Geneva, (2001).
  2. Sabine, C. L. The Global Carbon Cycle: Integrating Humans, Climate, and the Natural World. SCOPE 62, 17–46 (2004).
  3. Caldeira, K. & Wickett, M. E. Oceanography: Anthropogenic carbon and ocean pH. Nature 425, 365-365 (2003).
  4. Brewer, P. G. Ocean chemistry of the fossil fuel CO2 signal: The haline signal of 'business as usual'. Geophys. Res. Lett. 24, 1367-1369 (1997).
  5. Riebesell, U. Climate change: Acid test for marine biodiversity. Nature 454, 46- 47 (2008).
  6. Feely, R. A. et al. Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans. Science 305, 362-366 (2004).
  7. Orr, J. C. et al. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437, 681-686 (2005).
  8. Haugan, P. M. & Drange, H. Effects of CO2 on the ocean environment. Energy Conversion and Management 37, 1019-1022.
  9. Broecker., W. S. Fate of Fossil Fuel Carbon Dioxide and the Global Carbon Budget. Science 26, 409-418 (1979).
  10. Gattuso, J. P., Frankignoulle, M., Bourge, I., Romaine, S. & Buddemeier, R. W. Effect of calcium carbonate saturation of seawater on coral calcification. Global and Planetary Change 18, 37-46 (1998).
  11. Kleypas,J.A.etal. Geochemical Consequences of Increased Atmospheric Carbon Dioxide on Coral Reefs. Science 284, 118-120 (1999).
  12. Langdon, C. et al. Effect of elevated CO2 on the community metabolism of an experimental coral reef. Global Biogeochem. Cycles 17, 1011 (2003).
  13. Riebesell, U. et al. Reduced calcification of marine plankton in response to increased atmospheric CO2. Nature 407, 364-367 (2000).
  14. Zondervan, I., Zeebe, R. E., Rost, B. & Riebesell, U. Decreasing Marine Biogenic Calcification: A Negative Feedback on Rising Atmospheric pCO2. Global Biogeochem. Cycles 15, 507-516 (2001).
  15. Wood, H. L., Widdicombe, S. & Spicer, J. I. The influence of hypercapnia and the infaunal brittlestar Amphiura filiformis on sediment nutrient flux: Will ocean acidification affect nutrient exchange? Biogeosciences 6, 2015-2024 (2009).
Links
• El Niño Southern Oscillation
• Global Impact of Carbon Dioxide
• Ocean Acidification









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Gas Equilibrium
Gas molecules in the atmosphere pass into the ocean by a process known as diffusion. Gases dissolved in the ocean also move into the atmosphere. This is a process that is going on constantly. Eventually the concentration, and then the rates of exchange, of these gas molecules will equal each other and effectively cancel one another out. This is known as equilibrium.

If the concentration of a gas molecule in the atmosphere or the ocean increases, then the concentrations and rates will no longer be in equilibrium with one another. Diffusion will move the system towards equilibrium again.

Due to continued increases in carbon dioxide levels in the atmosphere, diffusion acts to continuously increase amounts of carbon dioxide in the ocean in order to achieve equilibrium.

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pH, Hydrogen Ions and Acidity
pH is the measure of acidity. It is a measure of the concentration of hydrogen ions (H+) in a solution. Oddly, as the numbers of hydrogen ions increase, the pH level drops. To make things more complicated, this is a logarithmic relationship. This means that a drop in pH of 0.1 units is equivalent to a 30% increase in hydrogen ions or acidity level. So the predicted drop of 0.4 pH units by 2100 actually means the oceans will be 150% more acidic!

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Metabolism and the Carbon Cycle
Long chain organic molecules are made by the fixation of carbon dioxide during photosynthesis. Photosynthesis uses high energy ultraviolet radiation from the sun to drive the formation of long chains of carbon atoms. These atoms are joined by high energy covalent bonds. In photosynthetic organisms, these molecules are used for a host of purposes such as building cell walls.

They are the principle source of energy for organisms higher up the food chain. Enzymes produced by these organisms break down the covalent carbon-carbon bonds and in doing so produce energy molecules that the cells use to drive the processes of life. As these carbon-carbon bonds are broken, carbon dioxide and other small carbon-containing molecules such as methane are once again released.

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Lysoclines & Supersaturation
The lysocline is the depth below which the ocean is undersaturated with respect to a particular salt. A saturated or super-saturated solution is a solution that cannot hold anymore of a particular salt. This salt will have a tendency to precipitate. In an undersaturated solution, salts have a tendency to dissolve.

So when the ocean is undersaturated with respect to calcium carbonate, the shells of organisms, made of CaCO3, have a tendency to dissolve. The lower the level of calcium carbonate saturation, the quicker shells dissolve and the harder organisms have to work to maintain them.
As the lysocline in the ocean rises, organisms, at or near to the surface, that rely on calcium carbonate, start to struggle.

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