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Coral Reefs, Ecosystem Under Threat
Reef biodiversity, bleaching, and ocean stressors
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Coral reefs are some of the most diverse ecosystems on Earth, supporting fish, invertebrates, algae, and many coastal communities. Although reefs cover less than 1% of the ocean floor, they provide habitat for about a quarter of all marine species. They also protect shorelines from waves, support fisheries, and attract tourism. Today, many reefs are under severe stress from warming oceans, pollution, overfishing, and ocean acidification.
A coral reef is built by tiny animals called coral polyps that produce calcium carbonate skeletons. Many reef-building corals depend on symbiotic algae called zooxanthellae, which use photosynthesis to provide food to the coral. When water becomes too warm or polluted, corals can expel these algae, causing coral bleaching and reducing the coral's energy supply. If stressful conditions continue, bleached corals may die, weakening the reef structure and the food web it supports.
Key Facts
- Coral reefs cover less than 1% of the ocean floor but support about 25% of marine species.
- Photosynthesis by zooxanthellae can provide most of a coral's food energy in clear, shallow water.
- Coral bleaching occurs when corals lose their symbiotic algae or the algae lose their pigments.
- Ocean acidification lowers carbonate ion availability, making it harder for corals to build CaCO3 skeletons.
- pH = -log[H+] describes acidity, so a lower pH means a higher hydrogen ion concentration.
- Net reef growth depends on calcification exceeding erosion: reef growth = CaCO3 added - CaCO3 lost.
Vocabulary
- Coral polyp
- A small marine animal that builds reef structure by secreting a hard calcium carbonate skeleton.
- Zooxanthellae
- Photosynthetic algae that live inside many corals and provide them with energy-rich sugars.
- Coral bleaching
- A stress response in which corals lose their algae or algal pigments and turn pale or white.
- Ocean acidification
- The decrease in ocean pH caused mainly by absorption of carbon dioxide from the atmosphere.
- Biodiversity
- The variety of living organisms in an ecosystem, including different species and their roles.
Common Mistakes to Avoid
- Thinking bleached coral is always dead. Bleached coral is stressed and may recover if conditions improve, but long-lasting stress can kill it.
- Assuming coral reefs are plants. Corals are animals, although many rely on photosynthetic algae living inside their tissues.
- Blaming only one threat for reef decline. Warming, acidification, pollution, sediment, disease, and overfishing often interact and make damage worse.
- Ignoring pH scale size. A drop from pH 8.2 to 8.1 looks small, but because pH is logarithmic it represents a significant increase in hydrogen ion concentration.
Practice Questions
- 1 A reef area contains 120 coral colonies. After a heat wave, 45 colonies are bleached. What percent of the colonies are bleached?
- 2 A coastal community earns $2,400,000 per year from reef tourism. If reef degradation causes a 35% loss in tourism income, how much money is lost per year?
- 3 Explain why reducing overfishing can help a coral reef recover from bleaching, even though overfishing does not directly cause high water temperature.