Why are coral reefs sensitive to changes in ocean chemistry caused by CO2?

• A. Increased CO2 lowers ocean pH, reducing carbonate availability needed for calcium carbonate skeletons, threatening corals.
• B. Ocean warming dissolves calcium carbonate skeletons.
• C. Increased CO2 raises pH, increasing calcification.
• D. Nitrogen runoff directly dissolves coral skeletons.

Answer: (A)

Coral reefs rely on calcium carbonate to build their hard skeletons. When CO2 from the atmosphere dissolves in seawater, it forms carbonic acid, which increases hydrogen ion concentration and lowers pH. This shift reduces the amount of carbonate ions available and lowers the saturation state of calcium carbonate minerals like aragonite. With less carbonate and a lower saturation state, corals have a harder time calcifying, and existing skeletons can even begin to dissolve. That’s why coral reefs are especially vulnerable to changes in ocean chemistry caused by CO2.

Warming emphasizes stress and bleaching but isn’t the direct chemical process dissolving skeletons, and CO2 does not raise pH. Nitrogen runoff affects nutrients and algal growth rather than directly dissolving coral skeletons.