CDS (II) 2020 General Knowledge · Question 41 of 120

Correct — A, East Australian Coast. The Great Barrier Reef runs down the east coast of Australia off Queensland, in the Coral Sea, and it is the largest coral reef system on Earth by a wide margin — over 2,300 kilometres long, about 344,400 square kilometres in extent, made up of more than 2,900 individual reefs and some 900 islands. It was inscribed on the UNESCO World Heritage list in 1981. Reef-building corals need warm, shallow, clear, sunlit water of normal salinity, and the shallow continental shelf on Australia's tropical east coast supplies exactly that.

• (b)West Australian Coast — Has Ningaloo Reef off the Pilbara coast, which is a genuine World Heritage reef but a fringing reef only a few hundred kilometres long — nothing on the scale of the Great Barrier Reef.
• (c)North Australian Coast — The Arafura and Timor Sea coast is shallow, but it is fed by heavy river sediment and turbid water, which is the wrong setting for a large barrier reef.
• (d)South Australian Coast — Too far from the tropics. Southern Australian waters are temperate and too cool for the reef-building corals that need water above roughly 20 degrees Celsius.

Coral reefs are built by colonial polyps living in partnership with photosynthetic algae called zooxanthellae, which is why reefs need shallow, sunlit water. Three forms are distinguished: fringing reefs attached to the shore, barrier reefs separated from it by a lagoon, and atolls, rings of reef around a lagoon with no central island. Darwin's subsidence theory explains the sequence — a fringing reef around a sinking volcanic island becomes a barrier reef and then an atoll.

Australia appears in all four options, so the item is really asking on which side of the continent tropical reef conditions exist. Queensland's coast lies inside the tropics on a broad shallow shelf; the southern coast is temperate; the northern coast is turbid; the western coast has warm water but a narrow shelf and stronger upwelling. Anchor the reef's condition to the exam and be honest about what has followed: the reef suffered mass bleaching in 1998, 2002, 2006, 2016, 2017, 2020 and 2022, and the 2020 event was under way in the months before this paper was set.

• The Great Barrier Reef lies off Queensland on Australia's east coast, in the Coral Sea.
• It stretches over 2,300 km and covers about 344,400 square kilometres.
• It comprises more than 2,900 individual reefs and about 900 islands.
• It was inscribed as a UNESCO World Heritage site in 1981.
• Recorded mass bleaching events include 1998, 2002, 2006, 2016, 2017, 2020 and 2022.

• Confusing the Great Barrier Reef with Ningaloo Reef on the opposite coast.
• Assuming any warm coastline can host a reef — river sediment and turbidity rule out much of northern Australia.
• Treating bleached coral as dead coral; bleached colonies can recover if the heat stress passes quickly.

As a location pick like this one, as a statements item on reef distribution, or through a related concept such as biorock technology or reef restoration.

• practice — not a real PYQ

  Coral polyps depend for much of their nutrition on a symbiotic association with which one of the following?

  • (a)Nitrogen-fixing bacteria
  • (b)Zooxanthellae
  • (c)Mycorrhizal fungi
  • (d)Diatoms

  Answer(b) Zooxanthellae — photosynthetic algae living inside the polyp's tissues, whose loss under heat stress is what bleaching means.

• practice — not a real PYQ

  A ring-shaped coral reef enclosing a lagoon, with no central island, is called

  • (a)a fringing reef
  • (b)a barrier reef
  • (c)an atoll
  • (d)a patch reef

  Answer(c) an atoll — the final stage in Darwin's sequence, after the volcanic island at the centre has subsided.