Ocean Acidification: Causes and Possible Consequences

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The increase in CO2 concentration causes ocean acidification. Photo by Rappersuncle.

New research, published October 23 in Nature online, shows that the oceans are more acidic than expected. The study was performed at the University of Georgia, studying the acidity of coastal waters impacted by nutrient-rich large rivers in the Gulf of Mexico and the East China Sea. Results showed a level of acidity higher than predicted, due to the combined effect of atmospheric and eutrophication-generated carbon dioxide.

Carbon dioxide: an acidic gas

Carbon dioxide (CO2) is a natural gaseous component of the atmosphere; in recent years, however, its concentration increased, due to human activity. CO2, in fact, is a product of the combustion of fossil fuels, such as gas, coal or oil.

There is concern over the CO2 emissions in the atmosphere, as it is a greenhouse gas; it is widely believed, although not everybody agrees, that higher CO2 concentrations could be one of the causes of Global Warming.

CO2, however, causes worries also because of its acidic nature. In fact, it can react with water, to form carbonic acid, according to the reaction:

H2O + CO2  → H2CO3

This acid can then partially dissociate, as shown below:

Ocean Acidification

Part of the CO2 present in the atmosphere gets dissolved in the waters of the ocean; this is due to the relatively high solubility of the gas in water – 1.45 g/L at 25oC. An increase of the CO2 concentration in the atmosphere leads to a corresponding increase in the waters of the oceans.

This phenomenon has reduced the concentration of CO2 in the atmosphere; without this dissolution in the oceans, today the CO2 amount in the atmosphere would be even higher, and hence could have an even bigger impact on the climate. The increase in the CO2 concentration in the oceans, on the other hand, can have a negative impact on its ecosystem, as there is a marked increase in the acidity of the waters.

It was reportedthat today the acidity of the oceans is about 29% higher if compared with the value of 200 years ago.

Effect on corals and other marine species

Oceans already contain some carbonate ion, CO32-; therefore, when CO2 is dissolved in them the following reactions can take place, to form the bicarbonate ion HCO3:

This means that there is less free carbonate in the water, which can have a negative impact on some marine species, such as coral. Their skeletons are made of calcium carbonate, CaCO3; these skeletons are formed and maintained over time from the calcium and the carbonate present in the oceans’ water (calcification).

Corals could be affected by ocean acidification. Photo by Richard Ling

The decrease in the amount of carbonate due to acidification can therefore be a threat to the life of these species, as they become unable to maintain their skeletons.

Eutrophication – a further source of CO2

The release into waters of nitrogen- and phosphorus-based chemicals, such as fertilizers, causes an increased growth of algae and plants. This phenomenon is called eutrophication; the ecosystem of rivers, lakes and coastal sea waters can be severely affected by it.

One of the consequences of eutrophication is a lower concentration of oxygen in the bottom waters (hypoxia); this is because more oxygen is consumed by the plants through their respiration. Hypoxia was widely investigated, as it can cause problems to other organic species living in these eutrophic waters.

Plant respiration, however, also leads to the formation of CO2 and, as reported above, this can increase the acidity of the waters.

The Mississippi river is heavily affected by eutrophication – Image by NASA

Recently-reported worrying data

Professor Cai, and his co-workers from the Department of Marine Science of University of Georgia , recently published a paper in Nature Geoscience about the combined effect of CO2 coming from both ocean acidification and eutrophication.

They studied the waters coming from the costal regions of the Gulf of Mexico and the East China Sea. These two areas receive waters from world’s two largest rivers, the Mississippi and Changjiang respectively, both heavily affected by eutrophication. They monitored these areas during the spring-summer season, from 2006 to 2009.

Their results showed that, due to the combination of these two factors, the level of acidity was higher than predicted.

Synergic effect

Professor Cai said:

“Our study revealed how the atmospheric CO2 and the plants respiration-derived CO2 have a synergistic effect on the acidity of these costal waters. This is because a higher CO2 concentration reduces the buffering capacity of the waters. This means that waters are less capable to neutralize the increased acidity.

If atmospheric CO2 concentration and river eutrophication keep on increasing, the scenario can become quite worrying, as the level of the acidity can have serious consequences on the many marine species and on the ecosystem as a whole.”

This research highlighted a consequence of water eutrophication which was less considered before, as the attention was mainly focused on hypoxia. The acidity of the costal regions should be therefore closely and regularly monitored.

Sources

Environmental Protection Agency (EPA): “Recent Ocean Acidification.” Accessed October 30, 2011.

J.C. Orr et al.: “Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437, 681 (2005). Accessed October 30, 2011.

W.J. Cai et al.: “Acidification of subsurface coastal waters enhanced by eutrophication.Nature Geoscience. Doi:10.1038/ngeo1297 (2011). Accessed October 30, 2011.

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