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Coral reefs occur along coastlines in tropical and subtropical regions wherever environmental conditions are suitable for their development.

Factors influencing reef development include:

Light

Corals require light to maintain their symbiotic association with zooxanthellae. Photo courtesy U.S. Geological Survey
Corals require light to maintain their symbiotic association with zooxanthellae. Photo courtesy U.S. Geological Survey

Light is critical in maintaining the symbiotic association between corals and symbiotic algae (zooxanthellae). The intensity of light greatly affects photosynthetic rates of the zooxanthellae, indirectly impacting coral growth and survival. Abundance of corals decreases rapidly with depth due to reduced light levels. In clear tropical waters, corals may live as deep as 150 feet (48 m), with very limited species found beyond that depth.

Temperature

Warm water temperatures are required for coral survival. Photo courtesy NASA
Warm water temperatures are required for coral survival. Photo courtesy NASA

Seawater temperatures can be tolerated between 61-95°F (16-35°C), with optimal coral growth occurring at temperatures of 73-77°F (23-25°C). These temperatures exist throughout most of the tropics with the exception of cool water currents off the west coasts of Africa and Australia. However, where currents move warm water from the tropics towards the north, reefs can survive in subtropical regions as near Bermuda.

Sedimentation

Water clarity is necessary for optimal coral growth. Photo courtesy NASA
Water clarity is necessary for optimal coral growth. Photo courtesy NASA

Reefs can only develop in areas lacking nearby rivers that bring silt and freshwater into marine environments. Excessive sedimentation reduces available light, inhibiting photosynthesis by the symbiotic algae. Silt also settles on the coral surface, blocking feeding and respiration.

Salinity

Corals tolerate a narrow range of salinity. Photo courtesy National Park Service
Corals tolerate a narrow range of salinity. Photo courtesy National Park Service

The amount of dissolved salts in the water is referred to as salinity and is measured in parts per thousand (ppt). Corals can tolerate a narrow range of salinities, between 30 and 40 ppt.


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