echoesof.science · Ages 15–16
Coral reefs: growth, bleaching and ocean chemistry
Name: Date:
Scan to open the experiment,
the game and the quiz
echoesof.science/school/coral-reefs/ages-15-16/
Read
Reef-building (stony) corals are colonies of polyps 1–3 mm across that secrete skeletons of calcium carbonate, CaCO₃. Colonies grow on the limestone left by earlier generations, so a reef is a record of thousands of years of growth. More than 6,000 coral species are known.
Growth is slow: massive corals add 0.5–2 cm per year (up to about 4.5 cm in good light, steady temperature and moderate waves); branching corals can add up to about 10 cm per year. At 1 cm per year, a 1 m high massive coral is about 100 years old.
Corals and zooxanthellae are a mutualism: the algae photosynthesise using the coral's carbon dioxide and wastes and transfer as much as 90% of the organic material they make to the coral. Because the algae need light, reef corals live mostly in the top 70 m, in clear water at 23–29 °C, roughly between 30° N and 30° S.
Experiment: Measuring how fast acid dissolves shells
- Measure 150 ml of each liquid (vinegar, carbonated water, tap water) into labelled jars and record their pH.
- Dry each shell with a paper towel, weigh it to 0.01 g and record its starting mass.
- Put two or three shells in each jar and close the lids.
- After one week, remove the shells, rinse them, dry them carefully in the same way and weigh them again.
- Calculate the percentage change in mass for each shell: (final − initial) ÷ initial × 100%. Find the mean and range for each liquid.
- Plot mean % mass change against pH. Discuss: what are the main sources of uncertainty (drying, balance resolution, shell differences)? How would you improve the test?
| Liquid (pH) | Mean initial mass (g) | Mean final mass (g) | Mean % change | Range |
|---|---|---|---|---|
| Vinegar | Prediction | |||
| Carbonated water | Prediction | |||
| Tap water | Prediction |
Questions
A branching coral grows 10 cm per year and a massive coral 1 cm per year. How long does each take to grow 2 m? Why might a reef of branching corals still not grow faster overall?
Ocean pH falls from 8.2 to 7.8. Calculate the factor by which the hydrogen ion concentration increases. Show your working.
A shell has a mass of 12.40 g before and 11.78 g after one week in vinegar. Calculate the percentage change in mass.
Explain why coral bleaching can cause a coral to starve, using the word 'mutualism'.
Sources for every fact on this sheet: echoesof.science/school/coral-reefs/sources/