What is ocean acidification?

Ocean acidification is the name given to the lowering of pH of the oceans as a result of increasing carbon dioxide (CO2) in the atmosphere. The pH of the ocean is determined by the level of hydrogen protons (H+) in sea water. The lower the pH, the more acidic the ocean.

The carbonate buffer system

For the last 750,000 years the pH of the surface ocean has been relatively stable and slightly alkaline at 8.2 due to the carbonate buffer system. This is a series of reactions, in which dissolved CO2 is converted to bicarbonate using carbonate as a buffer, that has kept the level of H+ protons (and therefore pH) constant.

Effects of increased carbon dioxide

The amount of CO2 entering the surface ocean has increased over the last century and exceeded the natural replenishment rate of carbonate, with the result that the H+ has increased, making the water more acidic. The pH of the ocean is estimated to have decreased by 0.1 pH units (a 25% increase in H+) since the start of the Industrial Revolution. It’s predicted to decrease by a further 0.3 pH units by 2100.

Impacts of ocean acidification on marine life

The impacts of ocean acidification have only recently begun to attract scientific attention. Scientists are particularly concerned about the survival of organisms that have shells composed of carbonate, as it becomes more difficult to grow and maintain carbonate in a more acidic ocean. This will affect a diverse range of organisms including corals, phytoplankton and zooplankton, algae and molluscs such as shellfish.

Possible effects on organisms with carbonate shells are:

  • chronic effects, such as reduced growth and reproduction
  • acute effects, such as high mortality
  • adaptation, where the organisms are able to adjust to the new pH.

It’s currently unclear which of these will occur.

Organisms without carbonate shells may experience:

  • physiological effects, such as hypercapnia, which is CO2-induced acidification of the blood and other body fluids
  • indirect effects, from loss of food or habitat.

Conversely, increased dissolved CO2 may benefit certain groups of phytoplankton and algae, boosting their growth rate and productivity.

Although it is likely that there will be significant changes in productivity, diversity, and distribution of different groups as a result of ocean acidification, we are unable to predict these with our current state of knowledge.

Ocean Acidification from NIWA on Vimeo.

Emiliania huxleyii, at 18000x magnification [Hoe Chang].