Ocean Acidification

Ocean acidification describes the relative decrease in seawater pH that is caused by oceanic uptake of specific compounds (namely carbon dioxide) from the atmosphere. This article describes the chemistry behind ocean acidification as well as the role of the carbon cycle. Finally, the article provides a sketch of the biological and socioeconomic effects of this process.

Overview

Today, the overwhelming cause of ocean acidification is the absorption of human produced carbon dioxide, although in some coastal regions, nitrogen and sulfur also contribute to this process.¹ The uptake of CO2 by the oceans has lowered the average pH of the oceans by about 0.1 units since the beginning of the industrial revolution. This change represents about a 30 percent increase in the concentration of hydrogen ions, which is a considerable acidification of the oceans. Estimates of future atmospheric and oceanic carbon dioxide concentrations, based on the Intergovernmental Panel on Climate Change (IPCC) CO2 emission scenarios and coupled ocean-atmosphere models, suggest that by the middle of this century atmospheric carbon dioxide levels could reach more than 500 ppm, and near the end of the century they could be over 800 ppm. This would result in an additional surface water pH decrease of approximately 0.3 pH units by 2100.²

Anthropogenic release of carbon dioxide is largely due to combustion of fossil fuels, cement production, agriculture, and deforestation. The concentration of the gas in the atmosphere has been increasing from its recent pre-industrial level of about 280 ppm to about 380 ppm today. The rate of this increase is unprecedented since the peak of the last ice age – for at least 20,000 years. A 2005 report estimates that, over the past 200 years, the oceans have absorbed roughly half of the CO2 emissions produced from fossil fuel use and cement manufacture. Some projects should that is CO2 from human activities is allowed to continue on present trends, this will lead to a decrease in pH of up to 0.5 unites by 2100 in the surface oceans. This is a three fold increase in concentration of hydrogen ions [H+] from pre-industrial times and results in an increase in pH outside the range of natural variation and probably to a level not experience for a least hundreds of thousands of years.³

Chemistry

CO2 exists in three forms in ocean water and varies in proportion based on water temperature, salinity and pressure: aqueous CO2 including carbonic acid (~1%), bicarbonate (~91%), and carbonate ions (~8%).⁴ When CO2 dissolves in water it first reacts with a water molecule to form a weak acid called carbonic acid. However, the excess H+ reacts in solution with carbonate ion resulting in the net effect of increased concentrations of H+, H2CO3 and HCo3-, and a decrease in concentrations of carbonate ions (CO32-). The carbonate ion is critical in formation of carbonate minerals that are commonly used by marine biota to forms shells and skeletons. This formula is reflected in Equation 1.

<--------Mineral Formation
[CaCO3] <---> [Ca2+] + [CO32-] Equation 1

Potential Impacts of Rising Atmospheric Carbon Dioxide on Coral Reef Calcification Rate

Source: http://www.pmel.noaa.gov/co2/OA/Ocea...on%20FINAL.pdf. Author: NOAA. Permission: Publicly Available.

Dissolution ------>

Since the dissolution of CO2 in water decreases free carbonate ions, the reaction moves to the right, and some of the resulting carbonate then reacts with free H+, serving to increase pH and counteract some of the effects of increased CO2 dissolution. Calcium carbonate exists in two main structures: aragonite and calcite. While both forms are plentiful in nature, calcite is less soluble because of its molecular structure.

Carbon cycle

Carbon exists on earth in a number of stores referred to as reservoirs. The exchange of carbon between the biosphere, atmosphere and oceans, all carbon reservoirs, is known as the carbon cycle. The carbon buried in some reservoirs, such as rocks, exchanges with other reservoirs on geologically long timescales. However, the exchange of carbon from the atmosphere to the oceans takes place over a much shorter time period. The evidence is based on the chemistry described above as well as the observation that oceans have become more acidic over the past 200 years, since the start of the industrial revolution, when man’s activities began resulting in the release of carbon into the atmosphere at rates much greater than ever before.

Biological effects

Acidic waters affects species and ecosystems in different ways. Around gas vents, for example, pH varies from 8.2 to 6.6 and can dramatically affect the surrounding biological communities. In water with a mean pH of 7.8, the number of species is 30% lower than at sites with a pH of 8.2 which is considered normal.⁵ Sea grasses and algae, including invasive species, dominate the marine community in more acidic waters, while species which rely on calcium carbonate to build shells are completely absent from highly acidified waters.⁶

Phytoplankton

Unlike terrestrial plants, marine phytoplankton has a mechanism to actively take up and concentrate inorganic carbon, either as carbon dioxide or bicarbonate ions. In almost all of the phytoplankton species examined to date, doubling the present atmospheric CO2 concentration has had only a small direct effect of 10% or less on the rate of photosynthesis. This suggests that even at current CO2 concentrations, the photosynthesis process is already saturated with inorganic carbon. Other studies show that even the elemental composition of marine phytoplankton is relatively unaffected by atmospheric CO2 concentrations. However, these studies were short term and did not provide sufficient time for species to develop adaptations to the changed environment. Nonetheless, it is indirect effects such as the availability of nutrients in a more acidic environment that may have more visible consequences, both to the plants themselves as well as to the communities of organisms that depend on them as primary producers.

Calcifying species

Of all the organisms in the surface oceans, the effects are likely to be most severe for the calcifying species that include mollusks, crustaceans, echinoderms, corals, large calcareous algae, foraminifera and some phytoplankton. Negative effects are expected primarily because of the reduction in availability of the calcium carbonate needed for calcified shells and plates as ocean water becomes more acidic.

Some regions are under a more immediate threat from increasing acidity than others. Scientists expect to see the affects of acidification in the Southern Ocean off Antarctica sooner than in other places in part because carbonate levels decline during the winter months. This has important consequence for calcifying organisms such as marine pteropods and other small plankton, which form the base of the Antarctic food web. Declines in these species may affect fish, penguins, and even some of the whales that travel great distances to feed in these waters.⁷

There is some evidence that corals may be able to withstand warmer water temperatures and associated changes in water chemistry. Several researchers began studying the resiliency of coral reefs in the Pacific Ocean in 2006. They have found live and healthy corals on reefs already as hot as the ocean is likely to get 100 years which have developed symbiotic relationships with different species of algae that can survive the warmer waters.⁸ These heat resistant corals are also more tolerant of increases in ocean acidity, suggesting that some coral has started to adapt to its changing environment.⁹

The broader effects of reduced calcification include a reduced rate of coral reef building which could lead to diminished resiliency from bleaching, disease, and coral death at potentially increased frequency as a result of warmer ocean temperature. Further, reef building rates could decrease to levels insufficient to maintain the reefs themselves. And, marine plankton, which coexists with coral and is a vital food source for many marine species, may decline and could have serious consequences for the marine food web.¹⁰

Other multicellular animals

Larger marine animals that do not breath air take up oxygen and respire CO2 through their gills. Increased concentrations of CO2 and decreased pH could have a major negative effect on this respiratory gas exchange system by acidifying the body tissues and fluids, and affecting the ability of the blood to carry oxygen. Furthermore, in freshwater fish, pH is known to affect the physiology and activation of sperm.¹¹

Socioeconomic effects

There are a number of identified adverse socioeconomic effects of ocean acidification that already been identified by NOAA:¹²

Degraded coral reefs will likely have a negative impact on commercial fisheries. As of May 2008, the U.S. was the third largest seafood consumer in the world with total consumer spending for fish and shellfish at approximately $60 billion per year. Healthy coral reefs are the foundation of many viable fisheries and could have an enormous impact on this industry. Furthermore, coral reefs are a source of revenue for tourism and recreation.
According to NOAA, approximately half of all federally managed fisheries depend on coral reefs and related habitats for a portion of their life cycles yielding an estimated value to U.S. fish stocks over $250 million.
Changes in coral reefs may compromise the protection they provide to coastal communities against storm surges and hurricanes.

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Related Topics
Ocean Acidification

Footnotes

1. Doney, S.C. et al (2007). The impacts of anthropogenic nitrogen and sulfur deposition on ocean acidification and the inorganic carbon system. Proceedings of the National Academy of Science. 104(37): 14580-14585.

2. NOAA Pacific Marine Environmental Laboratory Ocean Acidification - Background

3. "Ocean acidification due to increasing atmospheric carbon dioxide." The Royal Society, June 2005.

4. "Ocean acidification due to increasing atmospheric carbon dioxide." The Royal Society, June 2005.

5. Hall-Spencer, J.M. et al. (2008). Volcanic carbon dioxide vents reveal ecosystem effects of ocean acidification. Nature 454: 96-99. (sub req'd)

6. Hall-Spencer, J.M. and E. Rauer. Champagne Seas – Foretelling the Ocean’s Future? The Journal of Marine Education (2009) 25(1): 11.

7. Doney, S.C. Effects of Climate Change and Ocean Acidification on Living Marine Resources. Written testimony presented to the U.S. Senate Committee on Commerce, Science and Transportation's Subcommittee on Oceans, Atmosphere, Fisheries, and Coast Guard. May 10, 2007.

8. Heat Tolerant Coral Reefs May Resist Climate Change, Terra Daily, 22 May 09. Accessed May 28, 2009.

9. Heat Tolerant Coral Reefs May Resist Climate Change, Terra Daily, 22 May 09. Accessed May 28, 2009.

10. NOAA, State of the Science Fact Sheet - Ocean Acidification , May 2008.

11. Ingerman, R.L. et al (2003). Respiration of steelhead trout sperm: sensitivity to pH and carbon dioxide. Journal of Fish Biology, 62(1), 13-23. (sub req'd)

12. NOAA, State of the Science Fact Sheet - Ocean Acidification, May 2008.

Resources

The Science and Consequences of Ocean Acidification (PDF, 8 page issues brief), Pew Center on Global Climate Change, August 2009. Outlines how carbon dioxide emissions from the burning of fossil fuels are absorbed by oceans, which produces carbonic acid and makes seawater corrosive to some minerals. Examines the threat to fisheries, marine ecosystems, and the global food supply.

Ocean Acidification Network

NOAA Pacific Marine Environmental Laboratory Ocean Acidification page

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