Coral Bleaching
When corals face stress by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae zooxanthellae living in their tissues, causing them to turn completely white. This phenomenon is called coral bleaching. The pale white colour is of the translucent tissues of calcium carbonate which are visible due to the loss of pigment producing zooxanthellae. Coral bleaching is known to be responsible for killing approximately 18% of the world’s coral reefs.
THE DIFFERENT CAUSES OF CORAL BLEACHING ARE DISCUSSED AS FOLLOWS:
- Acidification of oceans: changing the pH level will destroy the ecological balance in corals. Corals need optimum concentration of the hydrogen ions for growth, increasing or decreasing the temperature can lead to change in oceanic pH.
- Temperature: Corals need optimum temperature for growth, increasing or decreasing the temperature can lead to coral bleaching. Bleaching events occur during sudden temperature rise or drops accompanying intense down welling and upwelling episodes, (-3 degrees C to –5 degrees C for 5-10 days), seasonal cold air outbreaks. Bleaching is much more frequently reported from elevated sea water temperature. A small positive anomaly of 1-2 degrees C for 5-10 weeks during the summer season will usually induce bleaching.
- Uncontrolled Fishing and Uncontrolled Navigation: these activities destroy the rich biodiversity thus disturbing the ecological balance by fishing nurseries, creating marine debris, using non-selective gears, fishing spawning aggregations, fishing too many big fishes and using big trawlers and mother vessels eroding the ocean base.
- Sub aerial Exposure: The consequent exposure to high or low temperatures, increased solar radiation, desiccation, and sea water dilution by heavy rains could all play a role in zooxanthellae loss, but could also very well lead to coral death.
- Fresh Water Dilution: Rapid dilution of reef waters from storm-generated precipitation and runoff has been demonstrated to cause coral reef bleaching as it tends to reduce salinity (Optimum Salinity – 35 ppm). Generally, such bleaching events are rare and confined to relatively small, near shore areas.
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Xenobiotic: Zooxanthellae loss occurs during exposure of coral to elevated concentrations of various chemical contaminants, such as Copper, herbicides and oil. Because high concentrations of xenobiotic are required to induce zooxanthellae loss, bleaching from such sources is usually extremely localized and / or transitory.
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Storms and rainfall: Storms, rainfall and floods are all natural, regularly occurring events on the Great Barrier Reef and as such, play a role in the ecosystem. However, predictions suggest an increase in the frequency of severe weather events which could lead to more serious physical damage, with less time between events to recover. The most immediate impact is coral breakage, dislocation and degradation from wind and waves. However, the effect doesn’t stop there. Heavy rainfall events lead to flood plumes stretching across the Reef, exposing inshore coral reefs to freshwater inundations. Freshwater run-off reduces salinity levels, sometimes causing bleaching, and brings increased nutrients and sediments, which can lead to disease outbreaks, algae blooms and murky water reducing light reaching corals. The vulnerability of coral reefs to these impacts is dependent on the effect of other climate change impacts. For example, reefs that are weakened by ocean acidification or stressed from high sea surface temperatures will respond poorly to a major storm or flood event.
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Shell and coral collection: Collection of molluscan shells for local trade and export leads to coral bleaching.
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Scientific collections: Overzealous and uncontrolled collection by students and scientists has caused damages to some reef areas.
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Coral diseases: Corals are also affected by various fungal and bacterial diseases. Silt and sedimentation cause asphyxia on polyps and corals die.