How do coral reefs respond to changing water quality?

• A. Coral reefs are sensitive to temperature and acidity; heat stress causes bleaching, while excess sediment and pollution reduce calcification and growth.
• B. Only nutrient enrichment benefits coral growth.
• C. Salinity remains constant; pH has no effect.
• D. Sediment has no impact.

Answer: (A)

Understanding how water quality changes affect coral reefs is crucial: corals depend on light, chemistry, and symbiotic algae for energy and building their calcium carbonate skeletons. When water gets warmer, corals experience heat stress that triggers bleaching—their algal partners are expelled, which cuts off a major energy source and can lead to death if stress lasts. At the same time, higher acidity (lower pH) makes calcification harder, so skeletons form more slowly and become weaker.

Sediment and pollution from land runoff further harm reefs. Sediment clouds the water and blocks light, reducing photosynthesis by the symbiotic algae and smothering corals. Pollutants and excess nutrients can fuel algal blooms and overgrowth, outcompeting corals and lowering growth rates.

So coral reefs respond negatively to warming, acidification, sedimentation, and pollution, showing bleaching, reduced growth, and weaker structures. The idea that nutrients always help, that pH has no effect, or that sediment doesn’t matter doesn’t align with how these stressors interact with coral biology.