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In News
- Recently, the management authority of Australia’s Great Barrier Reef confirmed that the reef is experiencing a mass coral bleaching event.
Image Courtesy: NOAA
Key Findings
- Repeating Events:
- This is the sixth time that the coral reef system is being hit by a widespread and damaging bleaching event and the fourth time in six years that such an event has occurred.
- Past Occurrences:
- In the past decade, however, mass bleaching occurrences have become more closely spaced in time, with the longest and most damaging bleaching event taking place from 2014 to 2017.
- This started with reefs in Guam in the Western Pacific region getting affected, then affecting the North, South-Pacific, and the Indian Ocean.
- Global temperature in 2017, was the third-highest to ever be recorded.
- In the 2014-17 event, more than three times as many reefs were exposed to bleaching-level heat stress as compared to 1998.
- Threat to Corals:
- If temperatures continue to rise, bleaching events may occur more often and a large proportion of the remaining reef cover in Australia could be lost.
- In Sync with La Nina:
- It’s a first that the current bleaching event has occurred during a La Niña weather pattern, when warm areas in the pacific ocean shift, giving more cloud cover, rain and creating cooler weather conditions over the reef. ?
Coral Reefs
- About:
- 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.
- Symbiotic Relationship:
- Corals share a symbiotic relationship with single-celled algae called zooxanthellae.
- The zooxanthellae also give corals their bright colour.
- The algae provides the coral with food and nutrients, which they make through photosynthesis, using the sun’s light.
- In turn, the corals give the algae a home and key nutrients.
- Corals share a symbiotic relationship with single-celled algae called zooxanthellae.
- Types:
- Hard coral:
- They are also called hermatypic or ‘reef building’ corals extract calcium carbonate (also found in limestone) from the seawater to build hard, white coral exoskeletons.
- Soft coral:
- Soft coral polyps, however, borrow their appearance from plants, attach themselves to such skeletons and older skeletons built by their ancestors.
- They also add their own skeletons to the hard structure over the years and these growing multiplying structures gradually form coral reefs.
- They are the largest living structures on the planet.
- Hard coral:
- Great Barrier Reef:
- Australia’s Great Barrier Reef is the world’s largest reef system stretching across 2,300 km.
- It hosts 400 different types of coral, gives shelter to 1,500 species of fish and 4,000 types of mollusc.
Significance of Corals
- Supports Marine Biodiversity:
- Coral reefs support over 25% of marine biodiversity, including fish, turtles and lobsters; even as they only take up 1% of the seafloor.
- Fishing Industry:
- The marine life supported by reefs further fuels global fishing industries.
- Even giant clams and whales depend on the reefs to live.
- Tourism & Employment:
- Besides, coral reef systems generate $2.7 trillion in annual economic value through goods and service trade and tourism.
- In Australia, the Barrier Reef, in pre-COVID times, generated $4.6 billion annually through tourism and employed over 60,000 people including divers and guides.
- Protection from Storm:
- Coral reefs also provide protection from storm waves.
Coral Bleaching
- Process:
- 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.
- Causes:
- Increased ocean temperature caused by climate change is the leading cause of coral bleaching.
- Storm generated precipitation can rapidly dilute ocean water and runoff can carry pollutants — these can bleach near-shore corals.
- When temperatures are high, high solar irradiance contributes to bleaching in shallow-water corals.
- Exposure to the air during extreme low tides can cause bleaching in shallow corals.
- Mass Bleaching events:
- The first mass bleaching event occurred in 1998 when the El Niño weather pattern caused sea surfaces in the pacific ocean to heat up. This event caused 8% of the world’s coral to die.
- The second event took place in 2002. ?
- Declining Corals:
- A 2021 study by the Global Coral Reef Monitoring Network (GCRMN), which is supported by the United Nations, showed that 14% of the world’s coral on reefs had been lost between 2009 and 2018, with most of the loss attributed to coral bleaching.
Way Ahead
- Dead reefs can revive over time if there are enough fish species that can graze off the weeds that settle on dead corals, but it takes almost a decade for the reef to start setting up again.
- Bleached corals can survive depending on the levels of bleaching and the recovery of sea temperatures to normal levels.
- If heat-pollution subsides in time, over a few weeks, the zooxanthellae can come back to the corals and restart the partnership.
Source: TH
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