Climate change, acidification and aquatic life at Higher Level: Different examples from SL: Lake Tanganyika, the global bleaching of 2023-25, sea butterflies, Bering Sea snow crab and the KOSMOS experiments. At HL, ask why local rules cannot stop a global problem (law), and who pays when a fishery closes (economics).
Practise this as you read
- Link each change to its effect on a population.
- Plan a fair test with a control and repeats.
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Warmer water, stressed ecosystems: Climate change is warming the oceans, lakes and rivers. Each species can live only within a range of temperatures, so warming moves fish, changes food webs and can make some populations collapse.
The points to remember
- Tropical waters: warmer water cuts fish stocks and raises deaths; cold waters may gain fish for a while.
- Fish shift to cooler water, nearer the poles or deeper, changing migration routes.
- Warmth lowers young fish survival and breeding success.
- Warm water holds less oxygen, which kills animals that cannot move away, on the sea bed.
- Coral bleaching destroys reef habitat and the food webs on it.
- Melting ice lowers salinity; currents, upwelling and storms change, moving nutrients and plankton.
Example (HL): Lake Tanganyika, East Africa. As the lake's surface has warmed, its layers mix less, so fewer nutrients rise to the sunlit water. Plankton growth has fallen, and catches of the small sardine-like fish that millions of people eat have declined.
Name the change: Not 'climate change affects fish' or 'global warming changes migration' alone: say what changes (temperature, oxygen, CO2) and what it does to the catch.
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Corals get most of their food from tiny algae that live inside them. When the water stays too warm for weeks, corals push the algae out and turn white: coral bleaching.
The points to remember
- About 1 °C above the usual summer maximum, for several weeks, causes bleaching.
- If the heat ends soon, corals can recover; if it lasts, they starve and die.
- Dead coral is covered by seaweed and slowly breaks down, so reef fish lose food and shelter.
- About a quarter of all marine species depend on reefs: biodiversity falls.
- People lose fish, tourism income and coastal protection from storms.
Example (HL): the global bleaching of 2023-2025. Record ocean heat bleached reefs from Florida to the Great Barrier Reef. By 2025, bleaching-level heat had reached about 84% of the world's reef area, the largest bleaching event ever recorded.
The ocean absorbs about a quarter of the CO2 that people release, and this makes seawater more acidic: ocean acidification.
The points to remember
- CO2 dissolves in seawater and forms carbonic acid, which releases hydrogen ions: the pH falls.
- Surface pH has fallen from about 8.2 to 8.1: about 30% more acidic, because the pH scale is logarithmic.
- Fewer carbonate ions are left, so molluscs, corals and some plankton struggle to build calcium carbonate shells.
- Shells grow thinner or even dissolve; larvae die; weaker coral adds to bleaching damage.
- Losses spread up the food web to fish, and to shellfish farms and fisheries.
Example (HL): sea butterflies. Pteropods build thin shells. Off the US west coast, over half of those sampled near the shore had badly dissolved shells.
Not acid rain: Ocean acidification comes from dissolved CO2. Acid rain, sulfur dioxide and nitrogen oxides are a different problem. The only lasting fix is to cut CO2 emissions worldwide.
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When warming, low oxygen and acidification hit a population at once, it may collapse: fall so far that it cannot recover quickly, and fisheries must close.
The points to remember
- A marine heatwave can kill adults and young, and remove their food.
- Hungry fish need more food in warm water, just when there is less.
- With few young surviving, the stock is not replaced for years.
- People lose fisheries, jobs and food security: about 3 billion people rely on seafood for protein.
Remember it as: Too hot, too little food, too few young.
Example (HL): snow crab in the Bering Sea. After record-warm water in 2018 and 2019, about 10 billion snow crabs disappeared, probably starving as warmth raised their food needs. In 2022 the Alaska snow crab season was cancelled for the first time.
You may be asked to plan an experiment on how acidification affects shelled organisms. Change one thing, measure one thing, keep everything else the same.
The points to remember
- Question: does lower pH reduce the mass of shells?
- Independent variable: pH of the water, for example 8.2, 7.8, 7.4 and 7.0 (CO2 bubbled in or dilute acid added).
- Dependent variable: % change in dry mass of each shell after 7 days.
- Control: temperature, water volume, shell species and size, time; seawater at pH 8.2 as the control group.
- Repeat: at least 3 shells at each pH; calculate the mean.
- Ethics: use empty shells, not live animals; wear eye protection with acid.
Example (HL): KOSMOS experiments. Scientists from Kiel, Germany, close off huge plastic tubes of real seawater in fjords and bays, from Svalbard to Gran Canaria, and add CO2 to some. Comparing tubes shows how whole plankton communities respond to the ocean of 2100.
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How this comes up: Paper 1 asks how climate change could affect future fish catches on an island [3]; Paper 2 asks for four impacts on wild fisheries [4], or the impacts of acidification [7].
Fishing is an important source of food and income in Tonga, a group of Pacific islands surrounded by coral reefs.
Explain three ways in which climate change could impact future fish catches in Tonga.
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