Ocean acidification relates to the on-going decrease in ocean pH as a result of the uptake of anthropogenic carbon dioxide (CO2) in the ocean. Surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14 between 1751 and 2004 and may reach 7.85 in 2100.
Anthropogenic CO2 and carbonate chemistry
The partial pressure of CO2 (pCO2) increases in the atmosphere due to anthropogenic (human-caused) inputs of carbon dioxide through fossil fuel burning but also from deforestation and production of cement. It has increased by 32% between 1880 and 2000 (280 vs. 370 µatm) leading to changes in the Earth's climate and in the functioning of terrestrial ecosystems.
Penetration in the ocean
The world’s oceans currently absorb approximately 79 million tons of carbon dioxide (CO2) released into the atmosphere every day. Over the past 250 years, the world's oceans have absorbed about one third of the CO2 released due to anthropogenic activities.
Changes in the carbonate chemistry
The increase of pCO2 in the surface ocean profoundly affects the seawater carbonate system. It lowers the pH, decreases the availability of carbonate (CO32-) ions and lowers the saturation state of the major shell-forming carbonate minerals (Table):
The term "ocean acidification" relates to the decrease in pH and does not imply that the pH of ocean surface waters will become acid (below 7.0) any time soon.
|Carbonate chemistry of surface seawater. Total alkalinity, pCO2, salinity and temperature were fixed and used to derive all other parameters using the seacarb software and the constants of Merbach. pH is expressed on the total scale. TA was held constant at its pre-industrial value from the late 1800s onward.|
|Total alkalinity||mol kg-1||2.356 10-6||2.302 10-6||2.302 10-6||2.302 10-6||2.302 10-6|
|CO2 partial pressure (seawater)||µatm||200||280||360||560||706|
|[CO2]||mol kg-1||7.796 10-6||1.063 10-5||1.326 10-5||2.002 10-5||2.452 10-5|
|[HCO3-]||mol kg-1||1.714 10-3||1.787 10-3||1.851 10-3||1.961 10-3||2.006 10-3|
|[CO32-]||mol kg-1||2.620 10-4||2.105 10-4||1.846 10-4||1.398 10-4||1.216 10-4|
|Dissolved inorganic carbon||mol kg-1||1.984 10-3||2.008 10-3||2.049 10-3||2.121 10-3||2.152 10-3|
|[H+]||-||4.940 10-9||6.577 10-9||8.101 10-9||1.181 10-8||1.446 10-8|
The values shown in the table are global values for ocean surface waters. Changes will be much more pronounced in areas such as the Southern Ocean, which will become undersaturated with respect to aragonite in 2050. Data collected at several time-series stations fully validate the above changes in the carbonate chemistry derived from thermodynamic calculations.
This additional CO2 is already reducing ocean pH and it is also affecting the carbonate chemistry through the reduction of the carbonate ions, aragonite and calcite, which are used by many marine organisms to build their external skeletons and shells. If the current trends in CO2 emissions continue to increase due to human activities, by the end of the century, the pH of surface seawater could decrease by about 0..34 units from pre-industrial times. This change in the chemistry of the oceans is quantifiable and predictable for a given level of atmospheric pCO2.
Biological and biogeochemical consequences
Pteropod Limacina Helicina. Photo courtesy of Russ Hopcroft, University of Alaska, Fairbanks. Although changes in the carbonate chemistry are well known, the biological and biogeochemical consequences are much less well constrained for several reasons. First, very few processes and organisms have been investigated so far (research in this area only began in the late 1990s). Second, most experiments were carried out in the short-term (hours to weeks), effectively neglecting potential acclimation and adaptation by organisms. Third, the interaction between pCO2 and other parameters poised to change, such as temperature, concentration of nutrients and light, are essentially unknown.
It is not anticipated that oceanic primary production will be directly affected by these changes in carbonate chemistry because most primary producers use carbon concentrating mechanisms that rely on bicarbonate rather than CO2. Note, however, that primary production of some species is likely to be stimulated. It is now well established that shell or skeleton growth through calcium carbonate precipitation of most calcifiers will be greatly affected by changging ocean pH. Short-term experiments on corals, benthic macroalgae, planktonic algae and protists, mollusks and sea urchins have shown that calcification decreases by up to 57% at the pCO2 levels expected in 2100. Note, however, that some calcifiers either do not show any response to increasing pCO2 or exhibit a bell-shaped response curve with an optimum rate of calcification at pCO2 values close to current ones and rates that decrease at pCO2 values below and above the current values.
Most modeling studies have aimed at predicting the consequences of the decline in the rate of calcification on the global carbon cycle and its feedback on atmospheric CO2. This is not an easy task because two opposite feedbacks are involved. First, calcification is a source of CO2:
hence a decline in calcification is a negative feedback on atmospheric CO2. The magnitude of this feedback is poorly known because future changes in calcification are poorly understood due to biological variability, very limited data on interaction between pCO2 and other parameters which will also change (e.g., temperature, nutrient and light), and the lack of information on potential remediation by acclimation processes by organisms or selection and evolution of resistant genotypes. Second, the CO2 generated by calcification is a function of pCO2. The calcification equation above is correct in freshwater but the current ration of CO2 generated per mole of CaCO3 precipitated is about 0.6 in 'standard' seawater and will increase as seawater pCO2 will increase. The pteropod, or “sea butterfly”, is a tiny sea creature about the size of a small pea. Pteropods are eaten by organisms ranging in size from tiny krill to whales and are a major food source for North Pacific juvenile salmon. The photos below show what happens to a pteropod’s shell when placed in sea water with pH and carbonate levels projected for the year 2100. The shell slowly dissolves after 45 days. Source: NOAA/ National Geographic Images.
Recent data suggest that elevated CO2 substantially increases nitrogen fixation by the cyanobacterium Trichodesmium, which could fundamentally alter the nitrogen cycle and potentially drive some oceanic regimes towards phosphorus limitation.
Changes in carbonate chemistry can affect many processes based on redox chemistry but such consequences have not been explored yet.
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