Pretend you are about to go scuba diving in the ocean. You jump in the water and begin to sink down. As you start surveying the coral reefs around you, something catches your eye. The coral has turned white, and no longer moves with life. This whiteness seems to have spread over a large area of the reef. You no longer see the colorful branches swaying in the current, or the schools of tropical fish swimming through the leaves. This death-ridden reef will never have the same life it once had.
This phenomenon is known as coral reef bleaching. This makes corals unattractive and lifeless. The biodiversity of a reef is important to the ecosystem. There are different organizations that have joined together to stop this from happening, but it will take a long time to repair most of the damage that has already been done. The futures of the reefs are in danger right now. There are many causes of coral reef bleaching.
The biggest concern of oceanographers is the effect global warming is having on the reefs. It is a stress condition that involves a breakdown of the symbiotic relationship between corals and unicellular algae called zooxantheallae. These microscopic plants live within the coral tissue, giving it color and food. One of the first symptoms of bleaching is the loss of color (?Coral Bleaching?). Though a coral does not have color does not necessarily mean it is dead. If there is still tissue on the coral, it still has a chance to live and regain its original appearance (?Global Coral Reef Alliance?). It does not take much to kill coral.
Some corals existed in past geological periods when temperatures were higher that they are today. However, those species disappeared during mass extinctions at the start of the ice ages about 2 million years ago. The ones that survived had the greatest tolerance for cold weather conditions. They have little or no ability to adapt to warmer waters (?Global Coral Reef Alliance?).
Coral lives in a very narrow temperature range. A mere 1-2 degree increase for a period of a few weeks can cause bleaching. In 1978, two men, Coles and Jokiel, ran a laboratory test to see how coral survives in different temperatures. They tested it at 28º, 26º, 24º, and 20º Celsius. Then they exposed the coral to high temperatures to see what the survival rate was. Between the 28º and 26ºC groups there was only a 13% of survival (Rosenberg 411). This means the slightest change in average seawater temperature can alter the entire ecosystem of the coral reefs. Every mass bleaching event was followed by periods when sea surface temperatures were 1ºC or more above the average values in the warmest month (?GCRA?). Temperature variations from an extremely cold winter or blistering hot summer are not the only causes of bleaching.
Prolonged exposure to air, especially during a very low tide, freshwater dilution due to heavy rainfall, intense sunlight causing increased ultraviolet radiation, and pollution are all causes of coral bleaching (Wells et al. 54). All of these causes alter the homeostasis that the reef environment needs to live. Various anthropogenic and natural variations in the reef environment can cause the symbiosis to deteriorate.
Solar irradiance affects shallow water coral during the summer. Both UV radiation and photosynthetically active radiation penetrate the coral, causing it to lose its color. Subaerial exposure occurs during a low tide. El Niño Southern Oscillation (ENSO) caused the sea levels to drop some. Also, tectonic uplifts can induce bleaching. Sedimentation, though uncommon, can cause loading of sediments, which choke off the corals. Freshwater dilution is also rare, but deadly. A storm that generates large amounts of precipitation and runoff can lower the salinity of the reef water, thus killing the corals. Increased inorganic nutrient concentrations such as ammonia and nitrate increase...