Context: 

A widespread coral bleaching event spread over two-thirds of the Great Barrier Reef Marine Park was observed by aerial surveys conducted.

Fourth Global Mass Coral Bleaching? – Aerial Surveys Conducted by the Reef Authority

• The survey was conducted jointly by the Reef Authority with the Australian Institute of Marine Science (AIMS).
• Place impacted the most: The bleaching gripped the reef in over 300 inshore, midshelf and offshore reefs, stretching from Cape Melville of Cooktown towards north of Bundaberg in the southern boundary of the Marine Park
  •  The southern GBR was the most affected by coral bleaching.
• Reason: The results from the surveys are consistent with the patterns of heat stress built up over summer with above-average sea surface temperatures observed  across the Marine Park for an extended period of time.
• The current coral bleaching event: It marks the fifth event in the past eight years.
• The  Coral Reef Watch programme has prophesied that the Earth could witness a fourth global mass coral bleaching event, with reefs located in the Indian, Pacific and Atlantic oceans showing severe signs of bleaching. 

Corals: 

• Corals are colonial marine invertebrates of the phylum Cnidaria. 
• Polyp: An individual coral is known as a polyp. A polyp is a sac-like animal and they excretes an exoskeleton near the base. Polyps form a symbiotic relationship with plant like cells called zooxanthellae (unicellular dinoflagellates). 

• Symbiotic Relationship: Coral Polyp can ingest tiny organisms called plankton & other small creatures but still majority of their energy and nutrients they get from the zooxanthellae living within their tissues which also is responsible for giving the corals its color.
  • In return corals provide the zooxanthellae with shelter and protection.

• Conditions For Survival:

• • Shallow water: corals require sunlight and clear shallow water for their growth. They are found typically in water above 165 feet (50 meters).
  • Clear water: That lets sunlight through. They don’t thrive well when the water is opaque.
  • Warm water: Reef-building corals require warm water conditions to survive. Different corals living in different regions can withstand water temperatures in the range of  20–32° C.
  • Pollution-free water: Corals are sensitive to pollution and sediments. Wastewater discharged into the ocean near the reef can contain too many nutrients that cause seaweeds to overgrow the reef
  • Salinity: Corals need saltwater (salinity almost 27 ppt) to survive and require a certain balance in the ratio of salt to water. This is why corals don’t live in areas where rivers drain fresh water into the ocean ie. estuaries

Coral Reefs:

• Coral reefs are formed when thousands of polyps living together in a coral colony secretes calcium carbonate exoskeleton beneath it. Over time, the skeletons of many coral colonies add up to build the structure of a coral reef.
• Coral reefs are also called the  “rainforests of the sea” as they form some of the most diverse ecosystems on Earth.
• Distribution: They are found mostly in tropical and subtropical waters  occurring  between 30º north and 30º south latitudes.
• Example: The Indonesian/Philippines archipelago has the world’s greatest concentration with greatest coral diversity.
  • In India, they are present around A&N, Lakshadweep, Gulf of Kutch and the Gulf of Mannar.
• Role: It acts as a shelter or home for many marine species apart from zooxanthellae such as  fish, invertebrates, algae and microorganisms.
• Types of Coral Reefs:
  • Fringing reef (Shore reefs): It is a coral platform attached to a continental coast or an island, sometimes separated by a narrow, shallow lagoon known as Boat Channel.
  • Barrier reef 
  • Atoll reefs : These are the reefs that are roughly circular and surround a large central lagoon.
    • If a fringing reef forms around a volcanic island that subsides completely below sea level while the coral continues to grow upward, an atoll forms.  

• Benefits of Coral Reef Ecosystems: 

• • Coastal protection: Coral reefs act as a shock absorber and offer protection to coastlines from storms and erosion.
  • Employment opportunities: It provides jobs for local communities boosting the fishing industry.
  • Tourism opportunity: It  offer opportunities for recreational activities like diving, and snorkeling etc
  • Source of food and medicines: They are also are a source of food and new medicines as it attracts fish population and other marine organism
  • These ecosystems are culturally important to indigenous people around the world.

Coral Bleaching:

• Coral bleaching occurs when the coral Polyp expels the marine algae ie. zooxanthellae from its   tissue and breaks the symbiotic relation. This result in them loosing their colour and source of nutrients and energy with coral’s bright white skeleton is revealed. transparent and the
• Coral reef bleaching indicates a stress response which enables them to recover, depending on the intensity of heat stress and its duration, however prolonged or intense heat can prove fatal to corals.
• Past events of mass bleaching:  It started  from 1998 and then reported in 2002, 2016, 2017, 2020, 2022 and the latest in 2024. 
  • As per, National Oceanic and Atmospheric Administration (NOAA) Satellite and Information Service: The third global coral bleaching event between 2014 and 2017 remains the longest, most widespread and probably the most damaging coral bleaching event on record. 
  • It affected reefs in areas such as GBR, Kiribati, Jarvis Island and is considered worse than any global bleaching event.

• Causes:

• • Temperature: The corals can survive only in a specific temperature range and global sea surface temperature is rising because of global warming stressing the corals.
  • Subaerial Exposure: Low tides, sea level drops and tectonic uplift can result in sudden exposure of coral to the atmosphere. Subaerial exposure often results in bleaching and consequent coral death due to sudden changes in atmospheric conditions.
  • Sedimentation: Human Activities such as land clearing and coastal construction can result in high rates of erosion, increasing sediment content in water and disrupting the natural process of photosynthesis.
  • Inorganic Nutrients: Increases in inorganic nutrients such as ammonia and nitrate cause zooxanthellae to multiply  by 2-3 times which can cause secondary adverse effects such as lower coral resistance and increased disease susceptibility.

News Source: DTE