Context

The world’s southernmost coral reef - Lord Howe Island – approx. 600 kilometres offshore from Sydney - has been hit by bleaching due to increase in temperature but escape severe bleaching that damaged the Great Barrier Reef in 2016 and 2017.

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• Rising sea temperature from climate change is affecting even the most isolated ecosystems and also has been held responsible for the recent adverse phenomenon (affected reef is 600 kilometres from the mainland).
• Deeper-water corals in the marine park, contains species not found anywhere else and like the Barrier Reef is a World Heritage site, were still “looking quite healthy” having mostly escaped the bleaching.
• Increasing baseline temperatures caused by climate change, and local factors such as elevated temperatures in the area has been held responsible for the bleaching.

Coral Bleaching

• It occurs when abnormal environmental conditions, such as warmer sea temperatures, cause coral polyps to expel algae (zooxanthellae) living in their tissues, causing the coral to turn completely white.
• Normally, coral polyps live in an endosymbiotic relationship with this algae crucial for the health of the coral and the reef as the algae provides up to 90% of the coral's energy.
• When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality.
• Corals can recover if the water temperature drops and the algae are able to recolonise the coral reefs

Factors responsible for Coral Bleaching

• Increased water temperature (most commonly due to global warming), or reduced water temperatures
• Oxygen starvation caused by an increase in zooplankton levels
• Increased solar irradiance (photosynthetic active radiation and ultraviolet light)
• Increased sedimentation (due to silt runoff)
• Bacterial infections
• Changes in salinity
• Herbicides
• Extreme low tide and exposure
• Cyanide fishing
• Pollutants such as oxybenzone, butylparaben, octyl methoxycinnamate, or enzacamene: four common sunscreen ingredients that are nonbiodegradable and can wash off of skin
• Ocean acidification due to elevated levels of CO2 caused by air pollution
• Being exposed to Oil or other chemical spills

Consequences

• Coral can survive short-term disturbances, but if the conditions that lead to the expulsion of the zooxanthellae persist, the coral's chances of survival diminish.
• If the coral polyps die of starvation after bleaching, they will decay and the hard coral species will then leave behind their calcium carbonate skeletons, which will be taken over by algae, effectively blocking coral re-growth and eventually, the coral skeletons will erode, causing the reef structure to collapse.

Significance and Apprehensions

• Coral reefs make up less than one percent of Earth's marine environment, but are home to an estimated 25 percent of ocean life, acting as nurseries for many species of fish.
• In the 2012–2040 period, coral reefs are expected to experience more frequent bleaching events and the Intergovernmental Panel on Climate Change (IPCC) sees this as the greatest threat to the world's reef systems.
• Coral reefs worldwide has been lost by 19%, and 60% of the remaining reefs are at immediate risk of being lost.
  
  

  Great Barrier Reef The Great Barrier Reef, off the coast of Queensland in northeastern Australia, is the largest living thing on Earth, and even visible from outer space. The 2,300km-long ecosystem comprises thousands of reefs and hundreds of islands made of over 600 types of hard and soft coral and is home to countless species of colourful fish, molluscs and starfish, plus turtles, dolphins and sharks. According to the United Nations Environment Programme, between 2014 and 2016 the longest recorded global bleaching events killed coral on an unprecedented scale. In 2016, bleaching of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef's coral and in 2017, the bleaching extended into the central region of the reef. The average interval between bleaching events has halved between 1980 and 2016.