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

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.

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.