Ocean acidification and marine ecosystems
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Question
Define ocean acidification.
Answer
The fall in **seawater pH** caused by the ocean absorbing extra atmospheric **CO₂**, which dissolves to form carbonic acid.
Question
Why does dissolving CO₂ lower the ocean's pH?
Answer
Dissolved CO₂ forms **carbonic acid**, which releases **H⁺ ions** — more H⁺ means a lower (more acidic) pH.
Question
What happens to carbonate ions as the ocean acidifies?
Answer
There are **fewer carbonate ions (CO₃²⁻)** available, because the extra H⁺ ions react with them.
Question
Which organisms are most harmed by acidification, and why?
Answer
**Calcifying organisms** — corals, molluscs and some plankton — because they need carbonate ions to build **calcium carbonate** shells and skeletons.
Question
How does acidification affect coral skeletons?
Answer
Corals build their **calcium carbonate** skeletons **more slowly**, and existing skeletons can **dissolve**, so reefs weaken.
Question
How does acidification alter a coral reef ecosystem?
Answer
Weaker reefs provide **less habitat and shelter**, so **biodiversity falls** and fisheries that depend on reefs decline.
Question
Name the main human causes of the extra atmospheric CO₂.
Answer
Burning **fossil fuels** and **deforestation** (which removes a CO₂ sink).
Question
State the acidification chain in order.
Answer
More CO₂ → dissolves → **carbonic acid** → lower **pH** → fewer **carbonate ions** → slower/dissolving **calcium carbonate**.
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Topic 4.12 hub
Climate change
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