What is ocean acidification and what causes it?

• A. Ocean acidification is a reduction in salinity due to freshwater input.
• B. Ocean acidification is the increase in pH due to dilution.
• C. Ocean acidification is the lowering of seawater pH due to uptake of atmospheric CO2 forming carbonic acid.
• D. Ocean acidification is the depletion of dissolved oxygen due to respiration.

Answer: (C)

Ocean acidification is the lowering of seawater pH caused by the uptake of atmospheric carbon dioxide. When CO2 from the air dissolves in seawater, it reacts with water to form carbonic acid, which then dissociates to bicarbonate and hydrogen ions. The added hydrogen ions raise acidity (lower pH) and reduce the availability of carbonate ions that many marine organisms need to build shells and skeletons. This change in the chemical balance happens specifically because CO2 is entering the ocean and forming carbonic acid.

The other descriptions don’t match acidification. A describes a change in salinity from freshwater input, which is not about pH. B describes an increase in pH due to dilution, which is the opposite of acidification. D describes depletion of dissolved oxygen from respiration, which is about oxygen levels, not pH.