Coral reef structure, zonation and distribution (QCE Marine Science Unit 3)
“Describe the structure and zonation of coral reefs (fringing, barrier, platform, atoll, coral cay) and explain the abiotic factors that control where reef-building corals grow”
Coral reefs form where reef-building corals and their zooxanthellae thrive, which requires warm (about 23 to 29 degrees C), clear, sunlit, normal-salinity, low-nutrient water saturated with aragonite. Reefs are classified as fringing, barrier, platform or atoll, with coral cays as sand islands on top. Across a reef they show clear zonation from the sheltered reef flat to the wave-exposed reef crest and the steep fore-reef slope.
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What this dot point is asking
QCAA wants you to classify the main reef types, describe the zones across a reef cross-section, and explain the abiotic factors that decide where reef-building (hermatypic) corals can grow. You should be able to read a reef profile diagram and link each zone to its conditions, using Great Barrier Reef examples.
Coral reef types
- Fringing reefs grow directly out from a shoreline with little or no lagoon. Many continental islands of the Great Barrier Reef, such as those in the Whitsundays, carry fringing reefs.
- Barrier reefs run parallel to a coast but are separated from it by a wide, deep lagoon. The Great Barrier Reef itself, stretching about 2300 km along the Queensland coast, is the world's largest.
- Platform (patch) reefs are isolated reefs that grow up from the continental shelf, common across the central Great Barrier Reef.
- Atolls are ring-shaped reefs surrounding a central lagoon, formed as a fringing reef around a volcanic island keeps growing while the island subsides. Atolls are rare in Australia but common in the Pacific and Indian Oceans.
- Coral cays are low sand islands built from reef debris on top of a platform reef. Green Island and Heron Island off Queensland are well-known cays.
Reef zonation
A cross-section of a typical reef shows distinct zones, each with its own conditions and dominant organisms.
- Reef flat / lagoon. The sheltered shallow area behind the crest. Calm, sometimes hot and variable in salinity at low tide. Hardy massive corals, seagrass and soft corals dominate.
- Reef crest. The highest, most wave-exposed part, often exposed at low tide. Tough, encrusting and branching corals and coralline algae that resist pounding waves.
- Reef slope / fore-reef. The seaward face dropping into deeper water. Best light and water flow at the top, with the greatest coral diversity; diversity falls with depth as light declines.
- Reef base. Where the slope meets the sea floor, beyond the depth corals can grow.
Hermatypic (reef-building) corals depend on zooxanthellae, symbiotic dinoflagellates living in the coral tissue. The algae photosynthesise and supply the coral with up to 90 per cent of its energy, and in return get shelter and nutrients. Because the algae need light, reef growth is limited to the sunlit upper few tens of metres, which is the single biggest control on reef distribution.
Abiotic factors controlling coral distribution
Reef-building corals have narrow tolerance ranges, which is why reefs occupy a thin band of shallow tropical seas.
- Light. Needed by zooxanthellae, so reefs grow only in clear water, generally above about 40 m depth.
- Temperature. Optimal roughly 23 to 29 degrees C. Below about 18 degrees C corals stop building; sustained warmth above the seasonal maximum causes bleaching (corals expel their zooxanthellae).
- Salinity. Corals need near-normal salinity (about 32 to 38 ppt). Flood plumes from Queensland rivers that lower salinity can kill inshore corals.
- Clarity and low nutrients. Corals need clear, low-nutrient water; sediment smothers polyps and high nutrients trigger algal overgrowth that outcompetes coral.
- Aragonite saturation. Corals build skeletons from aragonite, a form of calcium carbonate. Warm tropical water is saturated with aragonite, which allows rapid skeleton growth. Ocean acidification (Unit 4) lowers this saturation and slows reef building.
- Substrate and water movement. Corals need a hard surface to settle on and good water flow to deliver oxygen and food and remove waste.
Great Barrier Reef in context
The Great Barrier Reef sits in clear, warm, low-nutrient Coral Sea water with abundant hard substrate on the continental shelf, which is why such an enormous reef system developed there. Inshore reefs near river mouths experience sediment and nutrient runoff and so are less diverse than the clear-water outer reefs. This same set of abiotic factors explains both where reefs form and why they are vulnerable to the changes studied in Unit 4.
- Mixing up barrier and fringing reefs
- A barrier reef is separated from the shore by a wide lagoon; a fringing reef grows straight off the shore.
- Forgetting aragonite
- Corals need aragonite-saturated water to build skeletons; this is a key abiotic factor, not just temperature and light.
- Saying corals just photosynthesise
- Corals are animals; their zooxanthellae photosynthesise. The coral also feeds on plankton.
- Treating high nutrients as good for reefs
- Reef corals are adapted to low-nutrient water; high nutrients favour algae that smother coral.
Exam-style questions
Questions in the style of QCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice1 markContrast hermatypic and ahermatypic scleractinian corals.Show worked answer →
For 1 mark, give the key point of difference (a contrast, not just a definition of each).
Hermatypic corals are reef-building and contain symbiotic zooxanthellae, whereas ahermatypic corals are non-reef-building and lack zooxanthellae.
Because hermatypic corals host photosynthetic zooxanthellae, they calcify rapidly and are confined to warm, shallow, well lit water, so they construct reef frameworks. Ahermatypic corals do not depend on light, can live in deep or cold water, and deposit calcium carbonate too slowly to build a reef.
Practice4 marksThe diagram shows the depth and mean sea surface temperature of water around a group of coral reefs (temperatures 21 to 25 degrees C; reefs sit where depth is less than 100 m). Explain how abiotic factors have affected the distribution of coral reefs in this region.Show worked answer →
For 4 marks, link at least two abiotic factors from the map to where reefs do and do not occur.
Light and depth. Reef-building (hermatypic) corals need their zooxanthellae to photosynthesise, so they are restricted to the photic zone. On the map the reefs sit only where the depth is less than 100 m; deeper water (greater than 100 m) has too little light and supports no reef.
Temperature. Reef corals grow best in roughly 23 to 29 degrees C. The reefs cluster in the warmer 24 to 25 degrees C water; the cooler 21 to 22 degrees C areas are unfavourable and lack reefs.
Conclusion. The distribution of reefs is therefore controlled by the combination of shallow, well lit water and warm sea surface temperature, which together allow the coral-zooxanthellae symbiosis and rapid calcification.