Ocean acidification reduces the availability of carbonate ions needed by many marine organisms to build shells and skeletons. This can lower survival, growth and reproduction, disrupt food webs, reduce biodiversity and threaten fisheries.
This is core SL and HL content in 4.3 Aquatic food production systems, with a link to 2.3 Biogeochemical cycles. Atmospheric carbon dioxide dissolves in seawater and reacts with water to form carbonic acid:
Carbonic acid dissociates and releases hydrogen ions:
The increasing concentration of lowers seawater pH. Hydrogen ions also react with carbonate ions, reducing the carbonate available for organisms that produce calcium carbonate structures.
A common misconception is that ocean acidification makes seawater strongly acidic. Seawater generally remains alkaline, but its pH decreases, meaning it becomes less alkaline.
| Ecosystem component | Likely effect |
|---|---|
| Corals | Reduced calcification can weaken reef growth and recovery, decreasing habitat availability. |
| Molluscs and some plankton | Shell formation becomes more difficult, especially during vulnerable larval stages. |
| Food webs | Declines in plankton or reef habitat can affect consumers at higher trophic levels. |
| Human societies | Lower fishery and aquaculture productivity can threaten food security, income and coastal livelihoods. |
Impacts vary among species, and acidification acts alongside warming, deoxygenation, pollution and overfishing. For example, Pacific Northwest oyster hatcheries have treated or buffered seawater to reduce damage to oyster larvae, but such technological adaptation may be unaffordable for poorer coastal communities, creating an environmental justice issue.
For an explain question, give a complete cause-and-effect chain: increased atmospheric carbon dioxide more dissolved carbon dioxide lower pH and fewer carbonate ions reduced calcification ecosystem and societal impacts. Avoid merely stating that acidification “kills marine life.”
