fourth global mass coral bleaching event has been triggered by extraordinary ocean temperatures, the US National Oceanic and Atmospheric Administration (NOAA) said on April 15. This could have serious consequences for ocean life and millions of people who rely on reefs for food, jobs, and coastal defence. Since mid-March 2023, the average Sea Surface Temperature (SST) has been abnormally high. In March this year, it reached a record monthly high of 21.07 degree Celsius, according to the EU Copernicus Climate Change Service (C3S).

WHAT ARE CORAL REEFS?

  • Corals are marine invertebrates or animals not possessing a spine.
  • Each coral is called a polyp and thousands of such polyps live together to form a colony, which grows when polyps multiply to make copies of themselves.
  • Corals share a symbiotic relationship with single-celled algae called zooxanthellae.
  • The algae provides the coral with food and nutrients, which they make through photosynthesis, using the sun’s light. The zooxanthellae also give corals their bright colour.
  • In turn, the corals give the algae a home and key nutrients.

WHAT IS CORAL BLEACHING?

  • Coral Bleaching happens when corals experience stress in their environment due to changes in temperature, pollution or high levels of ocean acidity.
  • Under stressed conditions, the zooxanthellae or food-producing algae living inside coral polyps start producing reactive oxygen species, which are not beneficial to the corals.
  • So, the corals expel the colour-giving zooxanthellae from their polyps, which exposes their pale white exoskeleton, giving the corals a bleached appearance.
  • This also ends the symbiotic relationship that helps the corals to survive and grow.
Fourth Global Mass Coral Bleaching explained for UPSC

CAUSES OF CORAL BLEACHING

  • Change in Ocean Temperature: Increased Ocean temperature caused by climate change is the leading cause of coral bleaching.
  • Runoff and Pollution: Storm generated precipitation can rapidly dilute ocean water and runoff can carry pollutants, which can bleach near shore corals.
  • Overexposure to sunlight: When temperatures are high, high solar irradiance contributes to bleaching in shallow water corals.
  • Extremely low tides: Exposure to the air during extremely low tides can cause bleaching in shallow corals.