Ocean acidification

What is ocean acidification?

Acidification is defined as an increase in the concentration of H + in a solution or a lowering of a solution's pH. Ocean acidification is therefore the reduction of the pH of the world's oceans.

This can occur when CO₂ dissolves in water and there is a reaction between the H₂O andCO₂ to form carbonic acid (H₂CO₃).

[CO₂] + [H₂O] <=> [H₂CO₃]

This weak acid readily releases a proton (H⁺) and a negatively charged inorganic carbon ion.

[H₂CO₃] <=> [H⁺] + [HCO₃-]

The release of the H⁺ into the water will make it more acidic, that is it will drive the pH down. This increase in H⁺ will also react with the carbonate ion (CO₃^2-) to form HCO₃-

[H⁺] + [CO₃^2-] <=> [HCO₃-]

The overall effect of CO₂ dissolving into water is that the concentrations of H⁺, H₂CO₃ and HCO₃- increase and the concentration of CO₃^2- decreases and the solution is more acidic (i.e. a decrease in pH. The world's oceans readily exchange CO₂ with the atmosphere. As the concentration of CO₂ in the Earths atmosphere increases, so to does the level of CO₂ that the oceans absorb and therefore increasing the concentrations of H⁺ in the ocean making them more acidic.

What is causing ocean acidification?

Carbon dioxide obeys Henry's Law, which states that the concentration of a dissolved gas in a solution is directly proportional to the partial pressure of that gas above the solution. An increase in the concentration of CO₂ in the atmosphere directly leads to an increase in the amounts of CO₂ absorbed by the oceans. Human activity, mostly the burning of fossil fuels and the production of cement, has lead to an increase in average atmospheric CO₂ levels from pre-industrial values of 280 parts per million (ppm) to about 380ppm today (See Enhanced Greenhouse Effect information sheet for more information).

Figure 1. The averaged CO₂ concentrations measured at Cape Grim Tasmania from 1975-2005 [1]. Reprinted with Permission from CSIRO and Bureau of Meteorology.

The rate of increase is far greater than generally occurs naturally and is predicted to continue to rise well into the future [2]. Approximately half of the CO₂ from burning fossil fuels and cement production in the past 200 years has already been absorbed by the oceans. This CO₂ absorption has already led to a decrease in the pH of the oceans of about 0.1 units from pre industrial levels. While this value seems very small, this is mostly an artefact of the way that pH is measured. Put another way this change represents about a 30% increase in the concentration of H⁺ in seawater. More importantly the H⁺ concentration, and the rate at which it is rising, are both still increasing [3].

What is the significance of Ocean Acidification?

Bioavailability of carbonate

Many marine organisms make shells or supporting plates out of calcium carbonate (CaCO₃) in a process called calcification. As water becomes more acidic, the calcification process is inhibited and the growth and/or survival of certain organisms could be affected. As many of these organisms form the primary production of oceans, any change in their life cycle has the potential to impact all marine ecosystems.

Figure 2. Shown are both CO₃^2- concentration (solid lines) and dissolved CO₂ concentration (dashed lines) for average surface waters in the tropical ocean (thick lines), the Southern Ocean (thickest lines) and the global ocean (thin lines). Solid and dashed lines were calculated from the thermodynamic equilibrium approach. For comparison, open symbols are for CO₃^2- concentration from a non-equilibrium, model-data approach versus seawater pCO₂ (open circles) and atmospheric pCO₂ (open squares); symbol thickness corresponds with line thickness, which indicates the regions for area-weighted averages. The nearly flat, thin dotted lines indicate the CO₃^2- concentration for seawater in equilibrium with aragonite (aragonite saturation) and calcite (calcite saturation). Reprinted by permission from Macmillan Publishers Ltd: Nature, Orr et al. 2005 [4]

Figure 3.  a, b, d, e, Emiliania huxleyi ; and c, f, Gephyrocapsa oceanica collected from cultures incubated at [CO₂] ≈ 12 ^µ mol l -1 ( a – c ) and at [CO₂] ≈ 30–33 ^µ mol l -1 ( d – f ), corresponding to p CO₂ levels of about 300 p.p.m.v. and 780–850 p.p.m.v., respectively. Scale bars represent 1 ^µ m. Note the difference in the coccolith structure (including distinct malformations) and in the degree of calcification of cells grown at normal and elevated CO₂ levels. Pictures are selected from a large set of SEM photographs to depict the general trend in coccolith calcification. As the culture medium was super-saturated with respect to calcium carbonate under all experimental conditions, post-formation calcite dissolution is not expected to have occurred. Reprinted by permission from Macmillan Publishers Ltd: Nature, Riebesell et al. 2000 [5]

The Southern Ocean has been identified as being particularly vulnerable to becoming under saturated in calcium carbonate as it already has very low saturation levels [3]. The saturation level of the carbonate minerals is not only dependent on the concentrations of dissolved CO₂ and carbonate but also with water temperature and pressure. The solubility of CaCO₃ increases with decreasing temperature and increasing depth (See Carbonate Buffering). However, with increasing dissolved CO₂ concentrations the depth at which the CaCO₃ minerals will become under saturated will rise, i.e. the depth at which CaCO₃ minerals will begin to dissolve (particularly the more soluble mineral form- Aragonite) will become shallower. This saturation depth is predicted to reach the surface in some areas of the Southern Ocean if CO₂ levels rise to twice their current levels, which could have a significant impact on the marine food webs on Australia 's southern coastline [4].

Figure 4. The change in CO₃3^2- (Δ[CO₃^2-]_A) is the in situ [CO₃^2-] minus that for aragonite-equilibrated sea water at the same salinity, temperature and pressure. Shown are the OCMIP-2 median concentrations in the year 2100 under scenario IS92a: a , surface map; b , Atlantic; and c , Pacific zonal averages. Thick lines indicate the aragonite saturation horizon in 1765 (Preind.; white dashed line), 1994 (white solid line) and 2100 (black solid line for S650; black dashed line for IS92a). Positive Δ[CO₃^2-]_A indicates supersaturation; negative Δ[CO₃^2-]_A indicates undersaturation. Reprinted by permission from Macmillan Publishers Ltd: Nature, Orr et al. 2005 [4]

Speciation of Nutrients

The speciation, or the ionic form, of compounds is dependant on a number of factors such as their concentration, the presence or absence of other ions and pH . For example, in seawater phosphate can be present as PO₄^3-, HP0₄4^2-, H₂PO₄- and H₃3PO₄ depending on what the pH of the solution is. Theoretical speciation diagrams predict that the speciation of nutrients such as phosphate, silicate, iron and ammonia would all be impacted within the range of pH decreases predicted [6]. For example as pH decreases ammonia (NH₃) concentrations would be lowered in preference to the ammonium species (NH₄⁺).

Figure 5. Changes in speciation of phosphate, silicate and ammonia with pH. The red box shows the range that pH is predicted to change within. Reprinted with permission form Reebe and Wolf-Gladrow 2001[6].

The speciation of ions could affect their bioavailability. For example the ratio of soluble to insoluble iron may be increase making iron more available and reducing the growth limiting effect that low soluble iron concentrations have in some areas. The low availability of soluble iron has been shown to limit the growth of phytoplankton in the Southern Ocean and by artificially increasing soluble iron concentrations there is an increase in photosynthetic activity and phytoplankton biomass [7].

Figure 6. The speciation (a) and the measured solubility (b) of Fe(III) in seawater as function of pH. From Millero 1998 [8].

However, changes to metals speciation, could possibly allow for more free dissolved heavy metals and an associated increased toxicity effect.

Changes to biodiversity

Some species will be better suited to higher CO₂ and lower pH and a shift from one set of dominant species to another could have a huge impact on the entire ecosystems. For example animals such as deep sea fish and cephalopods are particularly sensitive to external CO₂. Squid are seen as sensitive due to their energy intensive form of movement. This extremely active use of muscles requires a large supply of oxygen. However, the capacity of blood to carry oxygen can be reduced by high CO₂ levels. At this stage it is unclear what impacts the levels of CO₂ predicted for the next 100 years would have on the life cycles of multicellular organisms [3].

Considerations for measurement and interpretation

Predicting Atmospheric CO₂

While the absorption of CO₂ by seawater is defined Henry's Law, there is no such certainty about the future levels of CO₂ in the atmosphere. For the IPCC Third Assessment report the projections of CO₂ emission are based on a number of scenarios which outline future patterns of economic growth, fossil fuel dependence and technology development [9]. These projections forecast CO₂ reaching levels of between 550 ppm to 850-970 ppm by the year 2100. The lower value assumes a strong uptake of clean and efficient technology across the globe with the higher value resulting from a continued and increased worldwide dependence on fossil fuels. The rate at which atmospheric CO₂ concentrations increase will dependent on economic development, technology advancement and societal pressures along with several environmental processes and all these will need to be accounted for in future models used to predict ocean acidification.

An example of an environmental process in the carbon cycle is calcification, which releases CO₂.

2[HCO₃3^-] + Ca₂₊ <=> [CaCO₃] + [H₂O] + [CO₂]

It is unclear how acidification, and any associated restriction of calcification, might be offset by the decrease in CO₂ released by calcification. Zondervan et al. [10] estimate that with increasing atmospheric CO₂, CO₂ emitted by the oceans could increase from 0.63 Gt of carbon per year in 1850 to 0.85 Gt per year by 2150. However, they assumed a constant rate of calcification which would not be the case if calcification was inhibited by increasing acidity. Also with decrease in the amount of CO₂released during carbonate production, the oceans would have lower dissolved CO₂concentrations and be able to absorb more CO₂from the atmosphere [4].

Carbonate Buffering

As CO₂levels in the atmosphere increase and there is a increase in H⁺ and a decrease in CO₃^-2, the depth below the surface of the ocean at which CaCO₃ becomes supersaturated (below which CaCO₃ will become under saturated and therefore dissolves) will become shallower. This would result in some currently mineralised CaCO₃ deposits dissolving and releasing carbonate ions.

[CaCO₃] + [H₂CO₃] <=> [Ca²⁺] + 2[HCO₃^-]

Note that this reaction would utilise the carbonic acid resulting from increased CO₂ absorption and decrease the level of acidification produced. This natural process, called buffering, acts to continuously stabilise the pH of seawater. The work that has been done to model the extent to which ocean acidification will affect the carbonate saturation depth, and how changes to that depth will modify the capacity of seawater to buffer any increases in H⁺ concentrations, suggests that that the process will be very slow and not fast enough to offset the rapid acidification from CO₂ absorption [11].

Temperature and Circulation Effects

Temperature and pressure (water depth) both have an impact on the solubility of CO₂ and CaCO₃ in seawater. Sea surface temperatures (SST) also impact on ocean circulation and mixing which will affect the rate and extent to which any changes to seawater pH will occur. It is unclear at this time how any increases in SST and or changes to oceanic circulation associated with climate change might enhance or ease the overall ocean acidification effect of increased CO₂ in the earth's atmosphere.

Existing information and Data

Little research has been done to investigate the potential impacts of ocean acidification, and even less on these might manifest in estuaries and coastal areas.

Key questions and further research needs    

• The identification of the species and ecosystems most sensitive to increased CO₂
• The interaction between increased CO₂ and other factors such as water temperature, sediment process, changed runoff patterns and carbon cycles and nutrient cycles, speciation of trace metals and particle flocculation .
• Identification of estuaries that would be most sensitive to impacts of ocean acidification

References

1. CSIRO Cape Grim Website
2. IPCC (2001). The Third assessment report of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press: Cambridge , UK , and New York , USA
3. The Royal Society (2005) Ocean Acidification due to increasing atmospheric carbon dioxide. Science Section, The Royal Society, London . www.royalsoc.ac.uk
4. Orr J. C., Fabry V. J., Aumont O., Bopp L., Doney S. C., Feely R. A., Gnanadesikan A., Gruber N., Ishida A., Joss F., Key R. M., Lindsay K., Maier-Reimer E., Matear R., Monfray P., Mouchet A., Najjar R. G., Plattner G. K., Rodgers K. B., Sabine C. L., Sarmiento J. L., Schlitzer R., Slater R. D., Totterdell I. J., Weirig M. F., Yamanaka Y. and Yool A. (2005) Anthropogenic ocean acidification over the twenty first century and its impact on calcifying organisms . Nature 437 , 681-686
5. Riebesell, U., Zondervani, I. , Rost, B., Tortell, P.D., Zeebe, R. and Morel, F.M.M. (2000) Reduced calcification of marine plankton in response to increased atmospheric CO 2 . Nature 407 , 364-367.
6. Zeebe and Wolf-Gladrow (2001)
7. Boyd P. W., Watson A. J., Law C. S., Abraham E. R., Trull T., Murdoch R., Bakker D. C. E., Bowie A. R., Buesseler K. O., Chang H., Charette M., Croot P., Downing K., Frew R., Gall M., Hadfield M., Hall J., Harvey M., Jameson G., LaRoche J., Liddicoat M., Ling R., Maldonado M. T., McKay R. M., Nodder S., Pickmere S., Pridmore R., Rintoul S., Safi K., Sutton P., Strzepek R., Tanneberger K., Turner S., Waite A. and Zeldis J. (2000) A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization . Nature 407 695-702
8. Millero F. J. (1998) The solubility of Fe(III) n seawater . Earth and Planetary Science Letters 154 (1-4) 323-329
9. IPCC (2000). Special Report on Emission Scenarios. A Special Report of Working Group III of the Intergovernmental Panel on Climate Change . Cambridge University Press, Cambridge , UK
10. Zondervan I, Zeebe R E, Rost B & Riebesell U (2001) Decreasing marine biogenic calcification: a negative feedback on rising atmospheric pCO2 . Global Biogeochemical Cycles 15 , 507-516
11. Caldeira K. and Wickett M. J. (2003) Anthropogenic carbon and ocean pH . Nature 425 Page 365

Authors

Heppelwhite, .C., Geoscience Australia