Summary
“Ocean acidification due to increased atmospheric carbon dioxide,” a report compiled by a team of European researchers and published by The Royal Society of London, illustrates the causes as well as the implications of increasing atmospheric carbon dioxide levels upon oceanic ecosystems. In summary, it discusses the sources of worldwide spikes in CO2 levels, sheds light on the effects of rapid pH changes upon fragile coral reef systems, provides calculations on the state of the oceans in our near future, and offers recommendations for this foreboding epidemic.
Corals
A diverse number of biological organisms on Earth produce shells, plates, and skeletons composed of the mineral calcium carbonate (CaCO3). These “calcareous” organisms rely on calcification to precipitate shells from aqueous carbonate ions (CO32-) and calcium ions (Ca2+) present in the water column. The products of this synthesis are the naturally-occurring CaCO3 minerals calcite or aragonite.
Modern corals, called scleractinids, are both solitary and reef-building animals which build skeletons composed of aragonite (Levin 344). While aragonite is chemically identical to calcite, it is a more soluble and less stable carbonate mineral. Coral reef systems are unique biological organisms, acting as vital anchors to their ecosystems. Typically occurring in well-lit, warm shallow waters, normally high calcification rates in these environments allow corals to develop scaffolding and frameworks which provide shelter and food to a host of marine organisms such as fish, bivalves, sponges, and algae (Levin 417; Royal Society).
Corals require water saturated in carbonate (CO32-) and calcium (Ca2+) ions in order to begin the calcification process. However, a deficiency in aqueous CO32- ions in the world’s seawater is increasingly becoming observed as a result of ocean acidification. The calcium carbonate which forms the skeletons of corals is also one of the geologic minerals most susceptible to chemical weathering, namely from exposure to acids. It is for these reasons that coral reefs constitute one of the most sensitive and fragile ecosystems in the world.
Acidification
The combustion of fossil fuels by electric plants, factories, industrial complexes, and automobiles produces enormous amounts of gaseous CO2, sulfur and nitrous oxides, and other pollution; other contributors to CO2 emissions stem from deforestation, agriculture, and cement production (Campbell 55; Royal Society). Coal-burning electric power plants produce more of these oxides than any other single source (Campbell 55). These pollutants are carried away by winds and precipitated as acidic rainfall or are absorbed from the atmosphere by the world’s largest bodies of water, the oceans.
Surface waters of the world’s oceans are naturally slightly alkaline, with an average pH of about 8.2 (Royal Society). But the pH of the oceans is predictably becoming more acidic. Carbon dioxide, when dissolved in water, forms weak carbonic acid (H2CO3). Increasing levels of atmospheric CO2 result in higher amounts of H2CO3 forming in seawater, which, as explained later, lowers the pH of the water. Products involved with the dissociation of H2CO3 and increased oceanic acidity pose an extremely uncertain situation for the survival of carbonate-depending organisms.
Implications
Over the past 200 years, the world’s oceans have absorbed approximately half of the CO2 produced by industrial processes (Royal Society). Calculations comparing pre-industrial pH levels to those seen today indicate that the saturation of CO2 in oceans has led to a drop in the pH of surface seawater by 0.1 units (Royal Society). This is by no means any small number— such a decrease in pH is equivalent to a 30% increase in the concentration of hydrogen ions, a main indicator of acidity.
The implications of a more acidic ocean are far-reaching and worldwide. Projections based on current trends in global CO2 emissions show that by the year 2100, atmospheric CO2 concentrations may reach levels exceeding 1000 ppm, higher than any level experienced by the Earth in several million years. Accordingly, the average pH of the oceans could fall by as much as 0.5 pH units (Royal Society). While the acidification of seawater affects innumerous aspects of biological and geological systems, my focus is on its impact upon the carbonate reef systems in particular.
Carbonates and acid
Calcium carbonate (CaCO3), the mineral composing the skeletons of corals, dissociates when exposed to an aqueous solution of carbonic acid (H2CO3) (which, as previously mentioned, is the result of CO2 combining with H20). The dissolution of CaCO3 yields calcium (Ca2+) and bicarbonate (HCO3-) ions. The net ionic equation for this reaction is:
CaCO3 (aq) + H+ (aq) + HCO3- (aq) Ca2+ (aq) + 2HCO3- (aq)
(Plummer 122).
H2CO3 is a diprotic acid, meaning that it contains two protons which can dissociate (“Carbonic acid”). The first two products are hydrogen (H+) and bicarbonate (HCO3-) ions. The other two products are obtained from the dissociation of the HCO3- ion, yielding hydrogen (H+) and carbonate (CO32-) ions. The reactions look like this:
H2CO3 (aq) H+ (aq) + HCO3- (aq)
HCO3- (aq) H+ (aq) + CO32- (aq)
Interestingly, carbonic acid serves as a pH buffer in the world’s oceans and as well as within our own blood (Campbell 54). Because it is a weak-acid/weak-base pair, its reversible reaction stabilizes the pH of an oceanic system by synthesizing bicarbonate (HCO3-) from CO3- ions in the water when there is an excess of H+ ions is present. Or, a drop in H+ concentration in the system will trigger the dissociation of HCO3- ions to replenish more H+ ions.
CO32- ions produced in the above secondary reaction, as well as CO32- from sedimentary sources, supply the building blocks necessary for calcareous organisms to build their shells and skeletons. However, seawater which is becoming increasingly more acidic is resulting in less and less CO32- ions. As atmospheric carbon dioxide emissions saturate the oceans with more H+ ions, the balance of these dissociation reactions shifts to produce more bicarbonate (HCO3-) ions as the additional H+ ions are taken up by the CO32-. Of a given carbonic acid compound, bicarbonate ions will constitute about 91% of its dissociated products and carbonate ions only 8% (Royal Society). The relative abundance of these forms is approximate and varies somewhat with seawater salinity, temperature, and pressure. The important thing to note is that the reduced concentration and availability of the carbonate ion is a consequence which directly impacts the calcification process of corals and other organisms.
One well-known study spanning nearly 4 years documented the effects of lower carbonate concentrations in a controlled environment which was host to an artificial reef system. The system behaved like a natural coral reef, with factors such as temperature, pH, and the presence of various ions all accounted for. Researchers varied the concentrations of carbonate ions and observed changes in the calcification rates of the reef organisms. Their findings described a direct linear function which showed that lower levels of carbonate ions in the water hindered the rate of calcification of coral. Under these conditions, researchers also compared the rates of calcification to the short-term (days) as well as the long-term (years). The study showed that increased amounts of time and exposure to deficient carbonate levels did not demonstrate any significant change in calcification as compared to shorter time frames, suggesting that reef systems are unable to adapt to diminishing carbonate levels in their environments (Langdon et al.).
The impacts of higher worldwide levels of CO2 on corals includes bleaching due to increased global temperatures, absence due to their inability to precipitate calcium carbonate (particularly in regions of the world where it is already deficient), and eventually the dissolution of their aragonite structures when pH reaches a low enough level. The impact of the disappearance of these reefs would not only be limited to marine animals. The deaths of coral reef systems could mean losses in the billions of dollars to the industries that depend on their existence—fishing and shellfish harvesting operations, recreation, and tourism (Royal Society). Reefs also offer protection to areas below sea level by acting as a buffer from strong waves, shielding coastal regions where human developments are present (Levin 314). Given enough time, the increase in oceanic acidity will almost definitely realize the extinction of many animals which cannot acclimate to it.
Conclusion
Findings presented by the Royal Society of London suggest that the only method for stemming further acidification of the oceans is through the drastic reductions of CO2 emissions worldwide. The levels which have been reached are irreversible and there exists no feasible method for removing CO2 from the water. “Ocean acidification will probably cause significant changes in the whole marine biogeochemical system and the ecosystem services that it provides, to an extent and in ways that cannot at present be foreseen... We conclude on the basis of current evidence that ocean acidification is an inevitable consequence of continued emissions of CO2 into the atmosphere, and the magnitude of this acidification can be predicted with a high level of confidence.”
I agree with the findings of this report and the concerns expressed by its authors. Significant and lasting changes will require the cooperation of world governments in the mandating of carbon dioxide emissions. While predicting the implications of ocean acidification on marine life is unclear, it is likely that certain organisms, such as the calcifying mollusks, coral reef systems, zoo- and phytoplankton, and the organisms which depend on them, will fare worse than others. Whether marine ecosystems will be able to adapt in response to such rapid environmental changes has yet to be seen. In the meantime, it is probably a smart idea to visit these reefs before they are gone.
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