The Dynamic Coral Reef

Algae encroachment on a hard coral.

When corals become stressed from environmental conditions, they eject so much zooxanthellae that they turn white. Coral "bleaching" has been reported by more than ninety nations in reefs all over the world. Scientists attribute much of bleaching to increased human pressures (e.g., pollution, agricultural runoff, oil slicks, and overfishing) that often make it difficult for reefs to recover from natural stresses.