Coral bleaching 

#GS3 #Environment 

A recent study has identified new causes for coral bleaching, namely excessive nutrients from fertilisers and storm-water runoff. 

  • Coral reefs are among the most diverse ecosystems in the world. They provide shelter and nourishment to fish and other marine organisms. Vibrant and healthy reefs form when a coral and an algae — zooxanthellae — start a symbiotic relationship. 
  • The coral provides protection and compounds zooxanthellae’s need for photosynthesis. The algae produces carbohydrates and helps remove the coral’s waste. 
  • But when the corals are stressed due to change in temperature, light and nutrients, they expel the algae and turn white. Corals will die if such bleaching extends. 
  • The ARC Centre of Excellence for Coral Reef Studies in Australia, examined skeletal cores of long-living corals in the Red Sea. The Red Sea was chosen as a study area because it is one of the only marine environments where the effects of summertime nutrients and heat stress are independent of each other. 
  • Corals bleach only when high sea surface temperatures couple with high nutrient levels. Nutrient stress on corals is difficult to measure though. Hence, only temperature-based stress was considered to predict coral bleaching. 
  • Incorporating nutrient-supplying ocean currents into coral bleaching forecasts can enhance the predictions, the researchers said. 
  • According to the National Oceanic and Atmospheric Administration(NOAA), the US lost half of its coral reefs in the Caribbean in one year (2005) due to a massive bleaching event. 

What is 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. 
  • Corals can recover if the stress-caused bleaching is not severe. 
  • Coral bleaching has occurred in the Caribbean, Indian, and Pacific oceans on a regular basis. 

Causes of Coral Bleaching? 

  • Rise in Sea Temperature: Most coral species live in waters close to the warmest temperature they can tolerate i.e., a slight increase in ocean temperature can harm corals. El Nino elevates the sea temperature and destroys coral reefs. 
  • Ocean Acidification: Due to rise in carbon dioxide levels, oceans absorb more carbon dioxide. This increases the acidity of ocean water and inhibits the corals ability to create calcareous skeletons, which is essential for their survival. 
  • Solar radiation and ultraviolet radiation: Changes in tropical weather patterns result in less cloud cover and more radiations which induce coral bleaching. 
  • Infectious Diseases: Penetration of bacterium like vibrio shiloi inhibits photosynthesis of zooxanthellae. These bacteria become more potent with elevated sea temperatures. 
  • Chemical Pollution: Increased nutrient concentrations affect corals by promoting phytoplankton growth, which in turn supports increased numbers of organisms that compete with coral for space. 
  • Increased Sedimentation: Land clearing and coastal construction result in high rates of erosion and a higher density of suspended silt particles which can 
  • smother corals when particles settle out (sedimentation), 
  • reducing light availability (turbidity) and 
  • potentially reducing coral photosynthesis and growth. 
  • Human Induced Threats: Over-fishing, pollution from agricultural and industrial runoff, coral mining, development of industrial areas near coral ecosystems also adversely impact corals. 
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