What is a direct consequence of warming waters for coral reefs?

• A. Bleaching
• B. Increased calcification
• C. Improved habitat complexity
• D. Increased reef growth

Answer: (A)

Warming ocean waters directly stress corals, triggering bleaching. Corals rely on symbiotic algae (zooxanthellae) living in their tissues for most of their energy through photosynthesis. When temperatures rise, these algae become stressed and are expelled from the coral, leaving behind the white skeleton and depriving the coral of its primary energy source. Without enough energy, corals weaken, become more susceptible to disease, and, if heat stress persists, can die. Because bleaching is the immediate, visible response to thermal stress, it is the direct consequence of warming water.

The other ideas—more calcification, improved habitat complexity, or increased reef growth—do not occur under warmer conditions. In fact, higher temperatures often reduce calcification and growth and degrade habitat complexity as corals lose cover or die, leading to simpler reef structure.