Coral reefs at Higher Level: The same statement as SL. HL questions give a list of conditions or changes and ask which ones allow, or stop, reef formation.
Practise this as you read
- Identify the conditions a reef needs.
- Explain how a change in the sea stops reef growth.
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The big idea: A coral reef is a marine ecosystem built by tiny animals, coral polyps, that lay down skeletons of calcium carbonate.
The points to remember
- Each polyp is a small animal related to jellyfish; thousands live together as a colony.
- Single-celled algae live inside the polyps' tissue and photosynthesise, passing sugars to the coral.
- The coral uses this energy to build a calcium carbonate skeleton; layers of skeleton build up into a reef.
- The reef gives food and shelter to fish, crabs, molluscs and many other species.
Because the algae need light and the skeleton needs the right water chemistry, reefs form only where the abiotic conditions suit both partners.
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The big idea: Reef-building corals need water that is shallow, clear, warm, salty and slightly alkaline: every one of these must be inside their range of tolerance.
The points to remember
- Depth: shallow, less than about 50 m, so light reaches the algae.
- Clarity: clear water; mud from rivers blocks the light.
- Temperature: about 20 to 30 °C; too hot and the corals lose their algae (bleaching).
- Salinity: full-strength seawater, about 32 to 42 parts per thousand.
- pH: about 8.0 to 8.3; a lower pH makes it harder to build calcium carbonate skeletons.
Remember it as: Shallow, clear, warm, salty, alkaline.
Real example: the Red Sea. The Red Sea is saltier than most oceans, about 40 parts per thousand, yet it has long reefs along its coasts: the salt is still within the corals' range, and almost no rivers bring mud into it, so the water is very clear.
Trap: More dissolved carbon dioxide does not help corals: it lowers the pH of seawater, so they build their skeletons more slowly.
How it is asked: Expect a choice of conditions, a list of changes to judge, or 'explain why reefs form only in...'. Name the condition and say how it acts on the coral or its algae.
Explain why coral reefs grow only in shallow, clear water.
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