Coral Bleaching
What is Coral Bleaching?
- Bleaching is the paling of coral color caused by:
- Decline in zooxanthellae density.
- Reduction in photosynthetic pigments in zooxanthellae.
- During bleaching:
- Corals lose 60–90% of their zooxanthellae.
- Remaining zooxanthellae lose 50–80% of pigments.
- If stress is temporary → corals recover.
- If stress persists → coral death due to lack of symbionts.
Causes of Coral Bleaching
Physiological Causes
- High temperature & irradiance disrupt zooxanthellae enzymes protecting against oxygen toxicity.
- Photosynthesis in zooxanthellae is impaired >30°C, disrupting coral-algae symbiosis.
- Temperature shocks impair adhesion of endodermal cells & expel zooxanthellae.
Ecological Causes of Bleaching
Bleaching is a general stress response & may be triggered by one or more factors:
Major Causes:
Temperature
- Corals live in a narrow temperature range.
- High sea surface temperatures (SST) or sudden drops during upwelling or cold-air outbreaks trigger bleaching.
- Most common cause of widespread bleaching events.
Solar Irradiance
- High light intensity (PAR & UVR) during summer maxima damages zooxanthellae, especially in shallow waters & exposed colonies.
Subaerial Exposure
- Extreme low tides, ENSO-induced sea level drops, or tectonic uplift expose corals to air, causing bleaching.
Sedimentation
- High sediment load may increase bleaching susceptibility, though rarely the sole cause.
Freshwater Dilution
- Storm runoff & precipitation dilute reef waters, occasionally causing bleaching nearshore.
Inorganic Nutrients
- Excess nutrients (ammonia, nitrate) increase zooxanthellae density but reduce coral resistance & increase disease risk (eutrophication).
Xenobiotics
- Pollutants (copper, herbicides, oil) cause localized & transient bleaching at high concentrations.
Epizootics
Some pathogens cause tissue loss & whitening (not true bleaching), e.g., protozoan infections.
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