Ocean Acidification

Carbon has a complex life cycle: its released by living organisms as part of respiration, converted into sugar by plants and algae, and absorbed by the oceans where it is used in the marine food web – only to be given off in respiration again! Unfortunately, human activities during the past two centuries have begun to tip natural carbon balances between the atmosphere and the marine environment.

Acidic processes

Increased carbon dioxide (CO₂) levels can result in ocean acidification, a decrease in pH in the oceans. Currently, its thought that the oceans are taking in approximately 24 million tons of CO₂ each day [1, 2] and that the oceans have already absorbed more than 400 billion metric tons of carbon from the atmosphere due to human activities [1]. With CO₂ levels continuing to rise, its predicted that surface waters in the oceans will experience almost a tripling of dissolved CO₂ , leading to extreme chemical changes.

When CO₂ is absorbed in the oceans, it undergoes several reactions to form carbonic acid, making the water more acidic. Since 1751, the average pH of ocean surface waters has gone from 8.25 to 8.14 [3]. The Intergovernmental Panel on Climate Change's worst case scenario predicted that the pH of ocean waters could decrease to 7.85 by 2100 [1, 3]. This pH is much too low for most marine organisms to survive and reproduce in.

Map showing saturation levels predicted for the world's oceans for the year 2099. Areas below 0 will be below the level at which organisms can absorb carbonate into their shells and structures. Used with permission from Macmillan Publishers Ltd: Nature Publishing Group, Orr, J., et al., Nature 437, 2005 [4].

Acid does more than burn

Ocean acidification will have major effects on marine animals. In environments with low pH, invertebrates and fish show decreased metabolism, reduced reproduction and increased mortality [1, 5]. Clams, perch and sturgeon exhibit a decrease in reproductive success when living in even slightly acidic environments. Lower than normal pH levels also lead to reduced egg size and a delay in hatching in some fish species [5]. This can be bad news for species whose numbers may already be low due to other threats.

Shellfish at great risk

Organisms that incorporate calcium (Ca) and carbonate (CO₃) into their shells and tissues, including corals, echinoderms (like sea stars), shellfish and calcareous algae, are especially at risk from ocean acidification [1]. Normally, ocean waters are saturated with calcium and carbonate, allowing organisms to easily absorb calcium carbonate (CaCO₃) into their shells and bodily structures. This is what makes their shells hard and effective defense barriers. However, with elevated levels of CO₂ lowering the pH, there is less carbonate available in the water, making it difficult for organisms to form new structures [2].

Once the amount of carbonate drops below a critical level, animals can no longer absorb the nutrients they require, preventing them from forming shells. Furthermore, at low levels of pH the carbonate in already formed shells starts to dissolve, causing shells to disintegrate. This phenomenon has already been observed in some plankton species [6].

Plight of the plankton

Different species of coccolithophores that have been exposed to differing level of acidity. The top row shows normal sea water pH levels, the bottom row shows development in lower than normal pH levels. Scale bars are 1 µm. Adapted by permission from Macmillan Publishers Ltd: Nature Publishing Group, Riebesell, U., et al., Nature 407, 2000 [7].

While there are many organisms that rely on calcium carbonate, plankton are the major shell producers in the oceans. These small drifters carry out approximately 90% of biological carbonate production [1]. Coccolithophores are an important group of plankton species in both temperate and tropical regions, and have been shown to have incomplete shell production in waters with lower than normal pH [7]. This trend has been observed in a number of plankton species [6].

Fewer plankton will lead to less food for animals across the food chain such as marine mammals, seabirds, fish and invertebrates. Clearly, oceanic animals need to live in environments where their ability to form shells is not compromised by low pH. If carbon emissions continue at present levels we will see the pH of the oceans an average 0.5 units lower by 2100 [8]. Although this seems small and unimportant, small modifications to oceanic pH can lead to much bigger and potentially catastrophic changes in the marine environment [9].

Acidic Oceans lesson plan

1. Cicerone, R., et al., The ocean in a high CO₂ world. Earth and Ocean Science, 2004. 85 (37): p. 351-353.

2. Dybas, C.L., On a collision course: oceans plankton and climate change. BioScience, 2006. 56 (8).

3. Jacobson, M., Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry. Journal of Geophysical Research-Atmospheres, 2005. 110 (D7).

4. Orr, J., et al., Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 2005. 437: p. 681-686.

5. Portner, H.O., M. Langenbuch, and A. Reipschlager, Biological impacts of elevated ocean CO₂ concentrations: lessons from animal physiology and earth history. Journal of Oceanography, 2004. 60 : p. 705-718.

6. Kolbert, E., The darkening sea , in The New Yorker . 2006. p. 66.

7. Riebesell, U., et al., Reduced calcification of marine plankton in response to increased atmospheric CO₂ . Nature, 2000. 407 : p. 364-368.

8.Ocean acidification due to increasing atmospheric carbon dioxide. 2005, The Royal Society.

9. Caldeira, K. and M.E. Wickett, Anthropogenic carbon and ocean pH. Nature, 2003. 425 : p. 365.