Where reefs grow: global and Australian distribution, latitudinal and cross-shelf patterns, and the abiotic limits on corals

1. The global belt where reefs grow

Shallow-water coral reefs occupy a narrow tropical and subtropical belt, roughly between 30°N and 30°S, and are concentrated on the western margins of ocean basins, where warm poleward boundary currents carry tropical water to higher latitudes. The largest reef provinces are the Indo-Pacific (centred on the Coral Triangle of Indonesia, the Philippines and Papua New Guinea), the Red Sea and western Indian Ocean, and the much smaller Caribbean–western Atlantic province. Species richness peaks in the Coral Triangle and falls away with distance from it, both east across the Pacific and north–south along continents.

Reefs are absent from some tropical coasts, and that absence is as examinable as their presence. The eastern Pacific and the west coast of Africa have cold upwelling currents (Humboldt, Benguela) that lower temperature and raise nutrients. Large tropical river mouths such as the Amazon and Ganges deliver fresh water and sediment, which lower salinity and light. A good exam answer links each gap to a named abiotic factor rather than saying the water is "not suitable".

When a map question asks you to describe global distribution, give the latitude band, the western-boundary pattern and one named exception. When it asks you to explain, add the mechanism: warm currents raise the minimum winter temperature above the coral limit, so reefs extend further poleward on western margins than on eastern margins at the same latitude.

2. Australia's reefs and the currents that place them

Australia has reefs on both coasts, and the two warm boundary currents explain why they reach so far south. The East Australian Current (EAC) carries Coral Sea water south along Queensland and New South Wales; the Leeuwin Current is unusual in flowing south along the Western Australian coast, against the general pattern of cold eastern-boundary currents.

High-latitude reefs such as Lord Howe and the Abrolhos have fewer coral species, slower growth and a mix of tropical and temperate organisms. Use them as evidence that temperature, not latitude itself, sets the limit: where a warm current raises winter temperature, corals persist further south.

3. Patterns down the latitudinal gradient

Along the Great Barrier Reef, conditions change predictably from north to south, and the syllabus expects you to describe this gradient. The northern GBR (Cape York region) is closer to the Coral Triangle, has warmer average water, a narrow continental shelf and generally higher coral species richness. The southern GBR (Capricorn–Bunker group, Swains) has cooler winters, a wider shelf and generally fewer species, with some temperate influence.

Three processes drive the gradient. First, temperature: average and minimum temperatures fall southward, slowing calcification and excluding some heat-loving species. Second, larval supply: the richest source populations lie to the north, so the number of species able to arrive and recruit declines with distance. Third, light and day length: winter light falls with latitude, lowering photosynthesis by zooxanthellae.

The gradient is not a perfect line. Bleaching exposure, for example, has not followed latitude neatly: in some recent events the northern and central GBR experienced the greatest heat stress, while in others the southern reefs were hit. When data show an exception, describe it explicitly and offer a reason drawn from the data (for example, local cloud cover or a current bringing cooler water) rather than forcing the general rule onto the graph.

Model sentence: "Species richness decreases from the northern to the southern GBR (from about 400 to about 250 hard coral species in the constructed data), because minimum water temperature and larval supply both decline with increasing latitude." Note how the direction, the data and the mechanism all appear.

4. Patterns across the shelf: inshore to outer reefs

Moving offshore from the Queensland coast is a second, steeper gradient. The shelf is commonly divided into inshore, mid-shelf and outer-shelf reefs, and almost every abiotic factor changes along that line.

Inshore reefs are mostly fringing reefs around continental islands, whereas the outer shelf of the northern GBR carries the long ribbon reefs. Inshore reefs are the first to receive flood plumes, so they are the most exposed to run-off, low salinity and the nutrient pulses linked to crown-of-thorns starfish outbreaks.

Exam technique: when given a transect of sites at increasing distance from a river mouth, state the trend in each variable, then infer which reef has the healthiest corals and justify with at least two data points. Past QCAA questions have asked students to infer which reef was least affected by surface run-off; the reef furthest from the river with the lowest turbidity and nutrient readings is usually the defensible answer.

5. Abiotic limits on coral distribution over geological time

The syllabus lists seven abiotic factors that have shaped where reef-building corals can live: dissolved oxygen, light availability, salinity, temperature, substrate, aragonite, and low nitrate and phosphate. Learn a typical range and a mechanism for each.

• Temperature: reef growth is best at about 23–29 °C, with a lower limit near 18 °C. Too cold slows calcification; too hot causes bleaching.
• Light: zooxanthellae need light for photosynthesis, so most reef growth occurs in the top 30–50 m of clear water.
• Salinity: corals tolerate roughly 32–40 ppt; flood water lowers salinity and stresses polyps through osmotic imbalance.
• Dissolved oxygen: needed for respiration of coral tissue at night when zooxanthellae are not producing oxygen; still, warm, eutrophic water can become hypoxic.
• Substrate: larvae need a hard, stable, clean surface; mud and shifting sand prevent settlement.
• Aragonite: corals build skeletons of aragonite, which forms most readily where seawater is strongly supersaturated (Ω above about 3).
• Low nitrate and phosphate: high nutrients favour fast-growing algae and phytoplankton, which shade and outcompete corals; phosphate also interferes with skeleton crystal formation.

Over geological time, these limits explain why the reef belt has expanded and contracted with global climate. In warm periods the belt widened toward the poles; in glacial periods it contracted and many shallow reefs were exposed by falling sea level. When asked to analyse abiotic data against reef distribution, pair each variable with the observed presence or absence of reefs and state which factor is limiting at each site.

6. Answering distribution questions in the exam

Distribution questions in QCAA papers usually supply a map, a table of abiotic readings or a pair of graphs, and ask you to identify, describe, explain or infer. Each command has a different minimum.

1. Identify: name the factor or site only. Do not waste time explaining.
2. Describe: give the pattern with direction and data, for example "coral cover decreases as distance from the river mouth decreases, from 42% at site D to 9% at site A".
3. Explain: give the pattern and the cause-and-effect mechanism, using precise terms such as turbidity, light attenuation and photosynthesis.
4. Infer / predict: state the outcome with a direction ("coral cover will decrease") and justify with data. The 2025 subject report specifically warned that "corals will be affected" earns little; state negatively or positively affected.

Use scientific terms the markers expect: pH not "acidity", temperature not "heat", salinity not "saltiness", turbidity not "dirty water". When two graphs are named, quote data from both; a conclusion justified from only one graph usually loses a mark.

Worked example (constructed data): Reef P has a winter minimum of 17 °C, salinity 35 ppt and clear water; Reef Q has a winter minimum of 22 °C, salinity 29 ppt after floods and turbidity of 18 NTU. Which reef is more likely to support hermatypic corals? A strong answer notes that each reef has one limiting factor and decides which is more severe: Reef P's minimum is below the roughly 18 °C limit for sustained reef growth, while Reef Q's low salinity and high turbidity are seasonal. You could argue either way, but only if the answer weighs both sites with numbers.