Triggers of Coral Reef Bleaching

There are several environmental conditions that can initiate the process of coral bleaching. These include:

• Temperature. This is generally seen as the principle trigger of bleaching, and is the main consequence of ENSO (Douglas, 2003). Temperatures that are sustained above, or below, the threshold of the coral or zooxanthellae, can lead to bleaching.

• High exposure to solar irradiance. This is another major trigger which is also linked to ENSO. Both the photosynthetically active wavelengths of the spectrum and the ultraviolet band are both linked to bleaching.

• A rapid fall in the nutrient levels of sea water. This can occur after storms and flooding, the ENSO can again be involved in causing such events.

• Subaerial exposure. This occurs when sea levels fall below the corals exposing them to air. Exposure to the atmosphere leads to other triggers in the form of increased irradiance, and increased or decreased temperature.

• High levels of inorganic nutrients. Pollution is the primary cause of this, such as run-off from inland and oil spills. Substances such as copper, oil and cadmium are all potential triggers.

• Salinity. This can affect coral growth but can also trigger bleaching, via both hypo and hyper salinity.

• Falling ocean pH. This makes the process of calcification more difficult within the corals, due to the increasing acidity. This in turn reduces the availability of carbonate ions which are incorporated into their framework. This may not directly lead to bleaching but a weakened framework may leave corals vulnerable to other environmental conditions.

These triggers are not independent of one another, bleaching may occur due to a combination of two or more of these triggers (Porter et al., 1999; Smith & Buddemeier, 1992).