Light
Sunlight is necessary for photosynthesis, the process by which plants convert carbon dioxide (CO2) and water to oxygen (O2) and sugar – food for the plants that later becomes food for animals. Without the sun, plants could not live, and without plants, animals could not live.
As coral polyps are sustained by photosynthetic algae called zooxanthellae, these ecosystems must be exposed to a sufficient amount of sunlight. This confines most corals to shallow waters that are clean and clear.
Temperature
Enzymes are most efficient within a narrow and specific range of temperatures; enzyme degradation can occur at higher temperatures. Therefore, organisms must either maintain an internal temperature or inhabit an environment that will keep the body within a temperature range that supports metabolism.
As for reef-building corals, they cannot tolerate water temperatures below 18°C. Many grow optimally in water temperatures between 23°C to 29°C, but some can tolerate temperatures as high as 40°C for short periods. (In what types of water do corals live?, n.d.)
Water
Corals need saltwater to survive and require a certain balance in the ratio of salt to water. This is why corals don’t live in areas where rivers drain fresh water into the ocean (“estuaries”).
Oxygen Availability
Dissolved oxygen exists in coral reef ecosystems. Both animals and plants use this dissolved oxygen for respiration.
More On Coral Bleaching
When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white.
Warmer water temperatures can result in coral bleaching. When water is too warm, corals will expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white. This is called coral bleaching. When a coral bleaches, it is not dead. Corals may survive a bleaching event, but they are under more stress and are more likely to die.
As humans burn more fossil fuels and massive deforestation continues, the rising global temperatures have especially contributed to the bleaching of corals at the Great Barrier Reef, the largest coral reef ecosystem in the world. The Great Barrier Reef has experienced two mass coral bleaching events in 1998 and 2002. Bleaching was more severe in 2002, when aerial surveys showed that over 50% of reefs experienced some coral bleaching. (Coral bleaching comes to the Great Barrier Reef as record-breaking global temperatures continue, Mar 2016)
Not all bleaching events are due to warm water.
In January 2010, cold water temperatures in the Florida Keys caused a coral bleaching event that resulted in some coral death. Water temperatures dropped 11.1°C lower than the typical temperatures. Researchers are evaluating if this cold-stress event will make corals more susceptible to disease in the same way that warmer waters impact corals. (What is Coral Bleaching, n.d.)