Describe the three types of coral reefs and explain why reefs are sensitive to environmental changes

Fringing, barrier and atoll reefs described, with a description of how a fringing reef is close to the coast in shallow water zones, barrier reefs develop through subsidence with a separation between the coast and land. Atoll reefs are reef islands with a central lagoon.
Corals require defined limits of temperature and salinity and any change in these factors will lead to a bleaching response where the coral polyp will expel their photosynthetic zooxanthellae symbionts and become paler in colour. This means the coral organism is no longer provided with the nutrients that are the result of photosynthesis of these symbiotic algae, and is more vulnerable to other changes, such as physical damage.