The Changing Health of Coral Reefs

Throughout their entire global range coral reefs are in decline. Coral bleaching, macroalgal overgrowth and coral diseases — responses signaling the declining health of coral reefs — have occurred with increasing frequency and intensity in recent decades. Decreased calcification may also be affecting coral reefs over longer time scales. Declines in coral reef health have been attributed to various natural and anthropogenic processes, but assignment of causality has proved problematic. Coral bleaching has been observed during extreme climate events such as El Niño; furthermore, there are indications that exposure to UV radiation, air, infectious microbes, and elevated temperature plays a role in the dramatic increase of coral bleaching since the mid-1970s. Macroalgal overgrowth is usually ascribed to eutrophi-cation and coral diseases to weakening of the coral host resistance by anthropogenic pollution. An issue precluding a strict anthropogenic cause of coral reef decline is that both overgrowth and coral diseases are known to occur, although less frequently, on reefs remote from human development. While its causes are still being unraveled, the overall decline in coral reef health sends an unambiguous signal that the coral reef system is losing its ability to withstand sudden or persistent environmental changes.