Ocean acidification is the ongoing decrease in ocean pH caused mainly by the ocean absorbing extra carbon dioxide from the atmosphere. It matters because seawater chemistry controls how easily corals, shellfish, and some plankton build shells and skeletons. As human activities add more CO2 to the air, the ocean takes up a large share of it, slowing climate change but changing marine habitats.
Even small pH changes can stress organisms because the pH scale is logarithmic.
Understanding Ocean Acidification
When carbon dioxide enters seawater, it does more than make the water slightly more acidic. It reshapes a balance among dissolved carbon dioxide, carbonic acid, bicarbonate, carbonate, and hydrogen ions. Extra hydrogen ions tend to join with carbonate ions.
This leaves fewer carbonate ions available in the water. Carbonate is a key raw material for calcium carbonate, the hard substance used in many marine shells, coral skeletons, sea urchin parts, and tiny drifting organisms called pteropods. The important change is not simply that seawater becomes acidic.
Most ocean water remains alkaline. The problem is that it becomes less favorable for making and keeping calcium carbonate structures.
Shell building depends on a condition called saturation. When seawater contains plenty of carbonate, forming calcium carbonate costs less energy for an organism. In well saturated water, existing shells are more stable too.
As carbonate becomes scarce, animals may need to use more energy to build a shell of the same size or repair damage. Young stages can be especially vulnerable because they are growing quickly and have thin shells.
In some places, water can become low enough in saturation that exposed calcium carbonate begins to dissolve. Different forms of calcium carbonate dissolve at different rates, so some organisms face the effects sooner than others.
The effects do not stop with individual species. Coral reefs create complex shelter for fish, crabs, and many other animals. If corals grow more slowly than waves, storms, erosion, or sea level rise remove reef material, the reef can lose structure over time.
Tiny shell forming plankton are food for larger plankton, fish, seabirds, and whales. A change near the base of a food web can therefore affect species far away from the original chemical change.
Acidification combines with warming, low oxygen, pollution, and overfishing. These pressures can make it harder to identify one single cause, yet together they can reduce survival and recovery.
Students may encounter this process in discussions of fossil fuels, climate change, fisheries, aquaculture, and coastal ecosystems. Oyster and mussel farms can be affected when coastal water has low carbonate levels, particularly where deep water rises to the surface or where runoff adds nutrients. Nutrient pollution can increase plant growth, followed by decay that releases more carbon dioxide into the water.
When reading graphs, pay attention to the time scale, the location, and whether a result shows pH, carbonate availability, or saturation. These measures are related but not identical. It is useful to remember that a lower pH value represents more hydrogen ions, while the biological risk often depends strongly on how much carbonate remains for shell building.
Key Facts
- CO2(g) ⇌ CO2(aq)
- CO2 + H2O ⇌ H2CO3
- H2CO3 ⇌ H+ + HCO3-
- HCO3- ⇌ H+ + CO3^2-
- pH = -log10[H+]
- Ca^2+ + CO3^2- ⇌ CaCO3, the mineral used by many shells and coral skeletons
Vocabulary
- Ocean acidification
- Ocean acidification is the decrease in seawater pH caused by absorption of carbon dioxide from the atmosphere.
- pH
- pH is a logarithmic measure of hydrogen ion concentration, where lower pH means a more acidic solution.
- Carbonic acid
- Carbonic acid is the weak acid formed when dissolved carbon dioxide reacts with water.
- Carbonate ion
- Carbonate ion, CO3^2-, is a dissolved ion needed by many marine organisms to build calcium carbonate shells and skeletons.
- Calcification
- Calcification is the biological process of forming calcium carbonate structures such as coral skeletons and mollusk shells.
Common Mistakes to Avoid
- Saying the ocean becomes acidic, not just more acidic, is misleading because average seawater is still usually above pH 7 but has shifted toward lower pH.
- Treating a pH drop of 0.1 as tiny is wrong because pH is logarithmic, so a 0.1 decrease means about a 26 percent increase in hydrogen ion concentration.
- Assuming acidification only affects corals is wrong because shellfish, plankton, fish behavior, food webs, and local fisheries can also be affected.
- Forgetting carbonate ion availability is a mistake because lower pH reduces CO3^2-, making it harder for organisms to form CaCO3 even if calcium is present.
Practice Questions
- 1 A seawater sample changes from pH 8.2 to pH 8.1. By what factor does the hydrogen ion concentration increase? Use [H+] ratio = 10^(pH_initial - pH_final).
- 2 If the ocean absorbs 30 percent of 40 billion metric tons of human-produced CO2 in a year, how many billion metric tons of CO2 enter the ocean?
- 3 Explain why adding CO2 to seawater can make it harder for corals to build skeletons, even though CO2 itself is not the material in the skeleton.