Coral Reefs Case Study
What Is a Coral Reef?
A coral reef is one of the planet's most productive ecosystems. It is built by millions of tiny coral polyps that secrete a hard calcium carbonate skeleton. Inside each polyp lives a microscopic algae called zooxanthellae, which photosynthesises and shares its sugars with the coral in exchange for shelter. This symbiotic relationship is the foundation of every coral reef.
Reefs cover less than 1% of the ocean floor but support an estimated 25% of all marine species — from clownfish and parrotfish to sharks, turtles and giant clams. They form in shallow tropical seas where conditions are perfect: water temperatures between 23 and 29°C, salty water, plenty of sunlight, and clear, low-nutrient conditions that favour coral over algae.
Where Coral Reefs Are
The world's major warm-water coral reefs cluster between 30°N and 30°S. The largest is the Great Barrier Reef off the coast of Queensland, Australia — 2,300 km long, covering 344,000 km2, the world's largest living structure and visible from space. Other significant reefs include the Mesoamerican Barrier Reef from Mexico to Honduras, the Red Sea Reef between Africa and Arabia, the New Caledonia Barrier Reef in the Pacific, the Florida Reef, and the Andros Coral Reef in the Bahamas. Each shares the same conditions but supports a slightly different community of species.
Coral reefs need warm water (23-29°C), salinity, sunlight and clear, low-nutrient conditions. The Great Barrier Reef in Australia is the world's largest, covering 344,000 km2 and supporting 1,500 fish species and 400 coral species.
Reef Nutrient Cycle
Reef ecosystems run on a tight, fast nutrient cycle. Coral polyps catch tiny plankton from the water column at night. Their zooxanthellae photosynthesise during the day, providing sugars and oxygen. When fish or invertebrates die, decomposer bacteria release nutrients quickly back into the water column where they are absorbed by other organisms. Because the surrounding tropical seas are nutrient-poor, almost all the nutrients in a reef stay locked inside the ecosystem — a closed loop that only works because the reef itself is alive and intact.
Case Study: The Great Barrier Reef
The Great Barrier Reef is Australia's most famous natural feature. Its biodiversity is staggering: 1,500 fish species, 400 coral species, 4,000 mollusc species, 240 bird species, plus dugongs, dolphins and humpback whales. Value to humans: the reef supports 64,000 jobs and generates AU$6.4 billion a year in tourism revenue. It protects the Queensland coast from storm waves and provides nurseries for commercial fisheries.
Threats: the Great Barrier Reef has experienced five mass bleaching events between 2016 and 2024 — the most severe in 2024, affecting around 75% of the reef. Bleaching is caused by sea-surface temperatures 1-2°C above normal, often during El Nino years. If the heat doesn't ease, bleached coral dies. Other threats include water pollution from agricultural run-off (sediment from sugar cane farming), crown-of-thorns starfish outbreaks fed by nutrient pollution, and damage from coastal development.
When water becomes too warm, coral polyps expel the zooxanthellae living inside them. The coral turns white — bleached — and immediately starts to starve. If temperatures fall again within weeks, polyps can take new algae back in and recover. If the heat persists, the coral dies. Globally, the planet has lost about half its shallow-water reef cover since 1950, with the rate accelerating as ocean heatwaves become more frequent. Bleaching is the most visible signal of climate change in marine ecosystems.
Australia's Reef 2050 Plan combines several strategies. Marine Protected Areas cover the entire Great Barrier Reef Marine Park (344,000 km2), with no-take zones in 33% of it. Water quality improvements aim to cut sediment and fertiliser run-off from sugar cane farms. Crown-of-thorns starfish culls reduce coral predation. Coral gardening — growing heat-tolerant corals in nurseries and replanting on damaged reefs — is being scaled up. International climate action is the most important: even if warming is held to 1.5°C, scientists project the loss of 70-90% of coral reefs worldwide, and over 99% at 2°C.
The Great Barrier Reef has had five mass bleaching events between 2016 and 2024, with up to 75% of the reef affected in 2024. Climate action below 1.5°C is the only strategy that can save the reef long-term, alongside Marine Protected Areas and water quality improvements.
Putting It Together
Coral reefs concentrate enormous biodiversity into shallow tropical seas, supported by the symbiotic relationship between coral polyps and zooxanthellae algae. The Great Barrier Reef shows how climate change, water pollution and coastal development combine to threaten reefs worldwide. Strong exam answers always cite specific reef statistics and connect bleaching to its global cause — ocean warming.