Teacher sheet · Ages 15–16 · about 45–50 minutes
Coral reefs: growth, bleaching and ocean chemistry
Lesson page: echoesof.science/school/coral-reefs/ages-15-16/ · Pupil worksheet: echoesof.science/school/coral-reefs/ages-15-16/print/
Learning aims
- Calculate reef growth times from growth rates.
- Use the logarithmic pH scale to compare acidities.
- Explain the coral–zooxanthellae mutualism and how bleaching breaks it.
- Measure and evaluate the effect of acids on calcium carbonate, with uncertainty.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: 'about 30% more acidic' from a pH change of only 0.1: how can that be? |
| 10 min | Read the text; work through the pH and growth calculations. |
| 20 min | Set up the shell mass practical (weigh again next week). |
| 5 min | Cause or effect? game. |
| 5 min | Quiz. |
Materials for the experiment
- Electronic balance (0.01 g)
- Six to nine similar clean shells (clam, mussel or oyster)
- Three beakers or jars with lids
- White vinegar, water and carbonated water
- Measuring cylinder
- pH meter or pH paper
- Paper towels, labels
- Spreadsheet or graph paper
Safety: Wear eye protection. Vinegar is a weak acid: rinse splashes with plenty of water. Do not taste any liquid. Keep the balance dry. Wash hands after handling shells.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| A pH drop from 8.2 to 8.1 means the ocean is now acidic. | pH 8.1 is still basic; 'acidification' means the pH is moving towards the acidic end. |
| pH changes of 0.1 are tiny. | The scale is logarithmic: 0.1 means about 26–30% more hydrogen ions. |
| Bleached corals are dead. | They are alive but have lost their algae; they can recover if the stress is mild. |
| Fast-growing branching corals make reefs grow fast. | Branches break easily as colonies grow, and reefs build up slowly overall. |
Worksheet answers
- Branching: 200 ÷ 10 = 20 years; massive: 200 ÷ 1 = 200 years. Branches are prone to breaking as colonies grow, and growth depends on light, temperature and waves, so the reef as a whole builds up slowly.
- Factor = 10^(8.2 − 7.8) = 10^0.4 ≈ 2.5.
- (11.78 − 12.40) ÷ 12.40 × 100% = −0.62 ÷ 12.40 × 100% = −5.0%.
- Coral and zooxanthellae live in a mutualism: the algae give the coral up to 90% of the food they make. In bleaching the coral expels the algae, so it loses most of its food; if the stress lasts, it starves.
Quiz answers
- A massive coral grows 1.5 cm per year. About how long does it take to grow 60 cm? About 40 years. 60 ÷ 1.5 = 40 years.[6]
- By what factor does acidity increase when pH falls by 0.3? About 2. 10^0.3 ≈ 2, a doubling.[7]
- Why do reef-building corals live mainly in the top 70 m? Their zooxanthellae need light for photosynthesis. The algae provide most of the coral's food using sunlight.[5][2]
- What is the chemical in coral skeletons that acids attack? Calcium carbonate, CaCO₃. Reefs are made of layers of calcium carbonate.[6]
- Which statement about bleaching is correct? A bleached coral is alive but has lost its algae and most of its food. Corals can recover if the stress is mild.[3]
- About what share of human CO₂ emissions does the ocean absorb each year? About a quarter. This CO₂ makes the ocean more acidic.[7]
Going further
Use NOAA's numbers to model a reef: if a reef grows 1 cm per year but storms and erosion remove 0.5 cm per year, how long to gain 1 m? Then discuss how bleaching events every few years would change the answer.
Sources used in this lesson
- [2] Corals tutorial: zooxanthellae… what's that? (NOAA National Ocean Service (education tutorial)): reliability High
- [3] What is coral bleaching? (NOAA National Ocean Service): reliability High
- [4] Shallow coral reef habitat (NOAA Fisheries): reliability High
- [5] What are coral reefs? (NOAA Coral Reef Information System (CoRIS)): reliability High
- [6] What are coral reefs made of? (NOAA National Ocean Service): reliability High
- [7] Disappearing shells (lesson plan) (NOAA Office of National Marine Sanctuaries): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/coral-reefs/sources/
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