Wave Erosion and Deposition

Lessons

Senior 6 · Geography

Wave Erosion and Deposition

5 lessons

Lesson 5 of 5

Coral Reefs and the Great Barrier Reef

25 min

A hole drilled to settle an argument

In 1896 the Royal Society of London sent an expedition to Funafuti, an atoll in what is now Tuvalu, to drill down into the coral. Its aim was to test an idea Charles Darwin had published in 1842: that an atoll is a ring of coral growing on top of a sunken volcano. After three seasons the drill reached about 340 m and was still in coral, and the result was judged inconclusive. The answer came only in the early 1950s, when drilling on Enewetak Atoll in the Marshall Islands passed through about 1,300 m of coral limestone before striking volcanic basalt. Darwin had been right. This lesson explains how coral reefs grow, how their three main types form, the problems they face and, as a case study, the largest reef system on Earth.

Definition

Coral polyp: a tiny, soft-bodied marine animal, related to sea anemones, that builds a skeleton of calcium carbonate ( $\mathrm{CaCO_3}$ ). Millions of polyps living in colonies build a reef.

Zooxanthellae: microscopic algae living inside the polyps' tissues. They photosynthesise and pass most of the food they make to the coral, and they give corals their colour.

Coral reef: a ridge or mound of limestone built up by corals and other organisms in shallow, warm sea water.

Coral bleaching: the loss of zooxanthellae from coral under stress, usually from unusually warm water, leaving the white skeleton visible through the clear tissue.

Conditions for coral growth

Because reef-building corals depend on the algae in their tissues, they grow only where those algae can photosynthesise, and where the polyps themselves can survive.

Condition	Requirement	Why
Temperature	Warm: the optimum for most reefs is about 26–27 °C, and few reefs exist where the water falls below 18 °C	Polyps grow and secrete their skeleton fastest in warm water; too hot and they bleach
Light and depth	Clear, shallow water: most growth is in the top few tens of metres, and reefs do not grow deeper than about 150 m	The algae need sunlight to photosynthesise
Clear water	Little sediment	Mud blocks the light and smothers the polyps
Salinity	Normal sea water	Fresh, muddy water near the mouths of large rivers kills coral, so reefs are broken opposite river mouths
Water movement	Waves and currents	Bring oxygen and plankton (food) and carry away waste
Base	A firm, shallow platform	Coral larvae must settle on a hard surface

These conditions confine reefs mainly to tropical seas between about 30° N and 30° S. They extend further from the equator on the western sides of oceans, where warm currents flow away from the equator, and are almost absent from western coasts washed by cold currents, such as Namibia's, where the Benguela Current flows north.

Definition

Fringing reef: a reef attached to the shore, or separated from it by only a narrow, shallow lagoon. Examples: reefs along much of the East African coast and the Red Sea; Ningaloo Reef in Western Australia, about 260 km long, is the largest.

Barrier reef: a reef separated from the coast by a wide, deeper lagoon. Example: the Great Barrier Reef, Australia.

Atoll: a ring or horseshoe of reef, often topped by low sandy islands, enclosing a lagoon with no central island. Examples: the 26 atolls of the Maldives; Aldabra in the Seychelles, the world's second-largest coral atoll.

How reefs form: three theories

Figure 14: Darwin's subsidence theory, from fringing reef to atoll.

Darwin's subsidence theory (1842). A fringing reef grows round a volcanic island. The island slowly sinks as the sea floor beneath it cools and subsides. The coral, which needs shallow water, keeps growing upward on the outer edge of the reef, where food and oxygen are best, so it stays near sea level while the island sinks. A lagoon opens between island and reef, making a barrier reef; when the island disappears completely, only a ring of reef is left: an atoll. The Enewetak drill holes, which found more than 1,000 m of shallow-water coral above volcanic rock, are strong evidence: the coral could only have grown that thick if its base had been sinking.

Daly's glacial control theory (early 1900s). During the ice ages, sea level fell by up to about 120 m and the water cooled, killing many reefs; waves then planed the exposed platforms flat. As the ice melted and the sea rose, corals recolonised the edges of the platforms and grew upward with the rising water. This explains why many lagoons have similar, fairly uniform depths.

Murray's antecedent platform theory (1880). Reefs grow on any existing submarine platform, such as a bank of sediment built up to shallow depth, and the lagoon is hollowed out by solution of the dead coral in its centre.

Geographers now combine the first two: Darwin's subsidence explains the thick coral caps of oceanic atolls, while the rise and fall of sea level in the ice ages, as Daly argued, has shaped their present surfaces. Murray's theory fits some reefs on continental shelves but cannot explain more than 1,000 m of shallow-water coral beneath an atoll.

Worked example 1: could coral keep up with the sinking?

Problem. At Enewetak, about 1,300 m of coral lies on the volcano. Suppose (illustrative figure) it accumulated over 50 million years. Find the mean rate of subsidence in millimetres per year, and compare it with the vertical growth of healthy reefs, which is between 1 and 25 cm a year.

Solution.

\text{rate} = \frac{1300\,\mathrm{m}}{5.0 \times 10^{7}\,\mathrm{yr}} = 2.6 \times 10^{-5}\,\mathrm{m\,yr^{-1}} = 0.026\,\mathrm{mm\,yr^{-1}}

Even the slowest healthy reef ( $1\,\mathrm{cm\,yr^{-1}} = 10\,\mathrm{mm\,yr^{-1}}$ ) can grow about $\frac{10}{0.026} \approx 380$ times faster than the island sank. Subsidence is so slow that coral keeps up easily; the real limit on upward growth is sea level itself, because coral cannot grow above the low-tide level.

Worked example 2: can a damaged reef keep up with the sea?

Problem. Sea level is rising at 3.7 mm a year. A healthy reef flat builds upward at a net 4.5 mm a year; after repeated bleaching (illustrative figures) its net growth falls to 1.0 mm a year. How does the depth of water over the reef flat change in 50 years in each case, and why does it matter to the coast behind?

Solution. Healthy: $4.5 - 3.7 = +0.8\,\mathrm{mm\,yr^{-1}}$ : the reef keeps up, and the water over it gets $0.8 \times 50 = 40\,\mathrm{mm}$ shallower. Damaged: $1.0 - 3.7 = -2.7\,\mathrm{mm\,yr^{-1}}$ : the water deepens by $2.7 \times 50 = 135\,\mathrm{mm}$ . Deeper water lets bigger waves cross the reef (lesson 1: breaking depth $\approx 1.3H$ ), so more wave energy reaches the beach and the shore erodes faster.

Problems facing coral reefs

Threat	How it harms reefs
Marine heatwaves and bleaching	Water a degree or two above the normal summer maximum for several weeks makes corals expel their algae; if the heat lasts, they starve and die
Ocean acidification	Carbon dioxide dissolving in the sea lowers its pH and makes it harder for corals to build their skeletons
Sediment and nutrient pollution	Soil eroded from cleared land smothers coral; fertiliser runoff feeds algae that overgrow it
Overfishing and destructive fishing	Removing grazing fish lets algae spread; blast (dynamite) fishing shatters the reef
Coral mining	Coral rock is quarried for building stone and burnt for lime
Tourism and shipping	Anchors, trampling, boat groundings and sewage from resorts
Predators and storms	Outbreaks of the coral-eating crown-of-thorns starfish; cyclones break reefs apart

Reefs matter far beyond their size: they cover less than 0.1% of the ocean floor, yet they are home to at least a quarter of all marine species. They support fisheries and tourism, protect coasts by breaking waves offshore, and supply sand to tropical beaches.

Case study: the Great Barrier Reef

The Great Barrier Reef lies in the Coral Sea off the coast of Queensland, north-eastern Australia. It is the world's largest coral reef system: more than 2,900 individual reefs and 900 islands, stretching over 2,300 km and covering about 344,400 km². It was made a World Heritage Site in 1981, and much of it is protected within the Great Barrier Reef Marine Park.

Formation: it is a barrier reef on a continental shelf, not round a volcano. As sea level rose after the last ice age (about 60 m below today's level some 13,000 years ago), corals colonised the drowned coastal plain; the living reef began to grow on older reef platforms about 9,000 years ago.
Importance: great biodiversity, fisheries, and tourism worth more than 3 billion Australian dollars a year to the region.
Threats: mass bleaching (the first major event in 1998; in 2016 bleaching killed an estimated 29–50% of its coral), runoff of sediment and fertiliser from farms in Queensland, outbreaks of crown-of-thorns starfish, cyclones and shipping. Studies have found that it lost more than half of its coral cover between 1985 and 2012, and again between 1995 and 2017.
Management: a zoned marine park, with no-take zones where fishing is banned; plans to cut farm runoff; control of starfish outbreaks; and limits on shipping and dumping. None of these can prevent bleaching, which depends on global ocean temperatures.

TIME: what coral bleaching is and why warm water causes it. Watch for the part played by the algae that live inside the coral.

Warning

Common mistakes

"Coral is a plant" or "coral is a rock". Coral is an animal; the reef is the limestone its colonies build.
"Bleached coral is dead." Bleached coral is starving, not yet dead; it can recover if the water cools in time.
Applying Darwin's theory to every reef. The Great Barrier Reef grew on a continental shelf, not round a sinking volcano.

Tip

Exam technique. For "Explain the formation of an atoll", draw the three-stage sequence (Figure 14), name the theory, and give the evidence (the Enewetak drill holes). For an "evaluate" question, weigh Darwin's subsidence against Daly's glacial control, and say what each explains best.

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Practice

State four conditions needed for the growth of reef-building coral.
Explain why coral reefs are rare on the west coast of southern Africa.
Distinguish between a fringing reef, a barrier reef and an atoll, with an example of each.
With the help of labelled diagrams, explain Darwin's subsidence theory of atoll formation.
A reef cap is 900 m thick and took (illustratively) 30 million years to form. Calculate the mean subsidence rate in millimetres per year.
Sea level rises at 3.7 mm a year and a reef flat grows upward at a net 2.5 mm a year. Find the change in water depth over the reef in 40 years.
Explain three human activities that damage coral reefs.
"Local management cannot save the Great Barrier Reef." Evaluate this statement.

Show answersHide answers

Any four: warm water (optimum 26–27 °C, rarely below 18 °C); clear water; shallow depth with sunlight; normal salinity; wave action bringing oxygen and food; a firm platform.
The cold Benguela Current flows north along that coast, keeping the water too cool for reef-building coral.
Fringing: attached to the shore or separated by a narrow shallow lagoon (East African coast, Ningaloo). Barrier: separated from the coast by a wide, deep lagoon (Great Barrier Reef). Atoll: a ring of reef round a lagoon with no central island (the Maldives, Aldabra).
A fringing reef forms round a volcanic island; the island subsides; coral grows upward on the outer edge to stay near sea level, making a barrier reef with a lagoon; when the island sinks out of sight an atoll remains (diagram as Figure 14).
$\frac{900\,\mathrm{m}}{3.0 \times 10^{7}\,\mathrm{yr}} = 3.0 \times 10^{-5}\,\mathrm{m\,yr^{-1}} = 0.030\,\mathrm{mm\,yr^{-1}}$ .
Net change $= 2.5 - 3.7 = -1.2\,\mathrm{mm\,yr^{-1}}$ , so the water deepens by $1.2 \times 40 = 48\,\mathrm{mm}$ (4.8 cm).
Any three, explained: farming and deforestation (sediment and fertiliser runoff); overfishing and blast fishing; coral mining for lime and building; careless tourism and anchoring; emissions that warm and acidify the oceans.
Local management (no-take zones, cutting farm runoff, starfish control) improves the reef's health and its ability to recover. But bleaching, the biggest threat, is driven by global ocean warming, which only cuts in greenhouse-gas emissions worldwide can reduce. So local management is necessary but not sufficient.

Summary

Reef-building corals need warm, clear, shallow, salty, moving water and a firm base, so reefs are mainly tropical.
The three types are fringing reefs, barrier reefs and atolls; Darwin's subsidence theory, confirmed by drilling at Enewetak, links them as stages.
Bleaching, acidification, pollution, overfishing and mining threaten reefs that support a quarter of marine species.
The Great Barrier Reef, a shelf barrier reef 2,300 km long, has lost much of its coral; local management helps, but warming is a global problem.

1A hole drilled to settle an argument

2Conditions for coral growth

3How reefs form: three theories

4Problems facing coral reefs

5Case study: the Great Barrier Reef

A hole drilled to settle an argument

Conditions for coral growth

How reefs form: three theories

Problems facing coral reefs

Case study: the Great Barrier Reef