The phenomenon where corals turn completely white is known as coral bleaching. It is a stark visual indicator of stress in coral colonies and a major threat to coral reefs worldwide. Coral bleaching occurs when corals expel the symbiotic algae, called zooxanthellae, that live within their tissues. These algae are crucial for the coral's survival, providing them with up to 90% of their energy through photosynthesis, and also giving corals their vibrant colors. When the zooxanthellae are expelled, the coral's transparent tissue reveals its underlying white skeleton, hence the term 'bleaching'.

Reasons for Coral Bleaching (Stressors):

The primary reason for coral bleaching is environmental stress, which disrupts the delicate symbiotic relationship between the coral host and its zooxanthellae. The most significant stressors include:

1. Elevated Sea Surface Temperatures (SSTs): This is by far the leading cause of mass coral bleaching events globally. When ocean temperatures rise even slightly (typically 1-2°C above the average maximum summer temperature) and persist for an extended period, the photosynthetic machinery of the zooxanthellae becomes damaged. This leads to the production of reactive oxygen species (ROS), which are toxic to the coral host. In response to this stress and the toxicity, the coral expels the algae.

   • Example: Major El Niño events, which cause widespread warming of tropical Pacific waters, have been linked to severe global bleaching events, such as those in 1998, 2010, and the unprecedented event from 2014-2017, which affected over 70% of the world's coral reefs, including large parts of the Great Barrier Reef.

2. Ocean Acidification: The absorption of excess carbon dioxide (CO2) from the atmosphere into the oceans leads to a decrease in seawater pH, a process known as ocean acidification. This makes it harder for corals to build and maintain their calcium carbonate skeletons, weakening them and making them more susceptible to other stressors, including bleaching.

   • Example: While not a direct cause of immediate bleaching like temperature, chronic exposure to acidified waters can weaken corals, making them less resilient to temperature spikes and other stresses, potentially contributing to bleaching susceptibility over time.

3. Pollution: Various forms of pollution can stress corals:

   • Nutrient Pollution: Runoff from agriculture and sewage introduces excess nutrients (nitrogen and phosphorus) into coastal waters. This can lead to algal blooms, which block sunlight, reduce oxygen levels, and can directly harm corals or promote the growth of macroalgae that outcompete corals.
   • Chemical Pollution: Pesticides, herbicides, and industrial chemicals can be toxic to corals and zooxanthellae, impairing their physiological functions and leading to stress and bleaching.
   • Sedimentation: Land-based activities like deforestation, construction, and dredging can increase sediment runoff. Sediment can smother corals, reduce light penetration (hindering photosynthesis), and increase the energy corals expend to clean themselves, making them more vulnerable to bleaching.

4. Extreme Low Tides and Exposure to Air: During unusually low tides, corals in shallow areas can be exposed to direct sunlight and air. This can cause thermal stress, desiccation (drying out), and UV radiation damage, leading to bleaching.

   • Example: In some shallow reef flats, prolonged exposure during extreme low tides can cause localized bleaching and mortality, especially if combined with high air temperatures.

5. Diseases: Coral diseases, often exacerbated by environmental stress, can also lead to tissue loss and a bleached appearance, though this is distinct from the mass expulsion of zooxanthellae.

6. Salinity Changes: Extreme fluctuations in salinity, such as those caused by heavy rainfall (reducing salinity) or prolonged drought (increasing salinity), can stress corals and lead to bleaching.

While corals can sometimes recover from bleaching if the stressor is removed and conditions return to normal, prolonged or severe bleaching events often lead to coral mortality. The frequency and intensity of these events are increasing due to climate change, posing a grave threat to the biodiversity and ecological services provided by coral reefs.