School › Coral reefs › Ages 15–16
Coral reefs: growth, bleaching and ocean chemistry
Calculate how long reefs take to grow, use the pH scale to see what 'about 30% more acidic' means, and measure how fast acids dissolve calcium carbonate.
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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.[5][6]
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.[5][6]
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.[2][5]
Bleaching happens when stressed corals, usually by water that stays too warm, expel their zooxanthellae. They lose most of their food and their colour. Bleached corals can recover if the stress is mild, but prolonged bleaching leads to starvation and death; in 2005 the U.S. lost half of its Caribbean reefs in one bleaching event.[3][2]
The ocean absorbs about a quarter of the CO₂ humans release each year. Dissolved CO₂ releases hydrogen ions, which lower the pH (now about 8.1) and reduces the carbonate ions that corals and shellfish need to build CaCO₃. The pH scale is logarithmic: the hydrogen ion concentration is 10−pH, so a drop of 0.1 multiplies acidity by 100.1 ≈ 1.26 (NOAA rounds the change from 8.2 to 8.1 to 'about 30%'), and a fall to about 7.8 by 2100 would be 100.3 ≈ 2, a doubling.[7]
Reefs cover less than 1% of the ocean but support nearly a quarter of ocean species and protect coasts as natural breakwaters. They face many threats at once: warming, acidification, pollution and sediment, overfishing, disease and physical damage.[4]
Try it: Measuring how fast acid dissolves shells
You need
- 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.
What to do
- 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?
What happens, and why?
Calcium carbonate reacts with acids: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂, so shells lose mass and give off bubbles. The more acidic the liquid (lower pH), the faster the loss. Vinegar is enormously more acidic than sea water (about 8.1), so this models the direction of the chemistry, not the real speed of ocean acidification. In the ocean the main effect is fewer carbonate ions, which makes skeleton-building harder.[7][6]
Play: Cause or effect?
Each card is part of the story of a reef in trouble. Is it a cause (a stress on the reef) or an effect (what happens to the coral)?
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Quiz
Other ages: Ages 6–7 · Ages 10–11