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Ocean Acidification

Carbon uptake from the atmosphere is an acidifying process. The ocean’s capacity to buffer increasing atmospheric carbon dioxide will decline in the future as the oceans surface concentration of carbon dioxide increases. This anticipated change is certain, with potentially severe consequences. Increased carbon storage in the deep ocean leads to the dissolution of calcareous sediments below their saturation depth. (reference)

The surface waters of the oceans are slightly alkaline, with an average pH of about 8.2, although this varies across the oceans by ±0.3 units because of local, regional and seasonal variations. Carbon dioxide plays an important natural role in defining the pH of seawater. When carbon dioxide dissolves in seawater it forms a weak acid, called carbonic acid. Part of this acidity is neutralised by the buffering effect of seawater, but the overall impact is to increase the acidity. This dissolution of carbon dioxide has lowered the average pH of the oceans by about 0.1 units from pre-industrial levels . Such a value may seem small but because of the way pH is defined, this change represents about a 30% increase in the concentration of hydrogen ions, which is a considerable acidification of the oceans. Increasing atmospheric concentration of carbon dioxide will lead to further acidification of the oceans. (reference)

Marine organisms are normally adapted to their environment. However, changes in ocean chemistry, especially rapid modifications such as ocean acidification, could have substantial direct and indirect effects on these organisms and upon the habitats in which they live. Direct effects include the impact of increasing carbon dioxide concentration and acidity, which may affect all stages of the life cycle. Indirect effects include the impact on organisms arising from changes in availability or composition of nutrients as a result of increased acidity.

Many marine photosynthetic organisms and animals, such as corals, make shells and plates out of calcium carbonate. This process of calcification, which for some marine organisms is important to their biology and survival, is impeded progressively as the water becomes acidified. This adverse effect on calcification is one of the most obvious and possibly most serious of the likely environmental impacts of ocean acidification. Any changes in the biological processes in the surface ocean waters will also affect the deeper water of the oceans. This is because organisms and habitats living far from the sunlight at the lower levels of the oceans rely mainly on the products created by life in the surface waters.

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