Impacts of water pollution
Big idea: Water pollution affects ecosystems through bioaccumulation (toxins build up in organisms) and biomagnification (toxins concentrate up food chains). Top predators — including humans — receive the highest doses.
Bioaccumulation vs biomagnification
Bioaccumulation
- Toxins build up in ONE organism over time
- Organism absorbs toxins faster than it excretes them
- Example: mercury accumulating in a single fish
- Influenced by fat solubility, persistence, and exposure time
Biomagnification
- Toxin concentration increases at higher trophic levels
- Each level contains a higher concentration than the one below
- Example: plankton → fish → seals → top predators
- Top predators have the highest concentrations
Substances that biomagnify
- Heavy metals — mercury, lead, cadmium
- Persistent organic pollutants (POPs) — DDT, PCBs, dioxins
- Microplastics — absorb other toxins and enter food chains
- These substances are fat-soluble, persistent, and not easily excreted
Be able to clearly distinguish between bioaccumulation and biomagnification, and explain why toxin concentrations are highest in top predators using simple food-chain examples.
Seasonal dead zones (hypoxia)
Key idea: A seasonal dead zone is an area of water where dissolved oxygen becomes very low during certain months, often in summer, so many fish and bottom-dwelling animals die or move away.
What does the term mean?
- A dead zone is an area where very few animals can survive because oxygen levels are too low.
- Seasonal means it forms during particular times of year and may improve later.
- It is strongly linked to eutrophication, which is caused by excess nutrients such as nitrogen and phosphorus.
Why do dead zones happen?
Excess nutrients from agriculture and wastewater increase algae growth. When algae die, decomposers break them down and use up dissolved oxygen. Warm summer water also holds less oxygen, making hypoxia more likely.
- More nutrients enter rivers and seas through fertilizer runoff and sewage.
- Algal blooms increase as eutrophication develops.
- Dead algae sink and are decomposed by bacteria.
- Decomposition consumes dissolved oxygen.
- Oxygen falls to hypoxic levels and animals die or migrate.
Exam tip: In data questions, look for the month with the lowest dissolved oxygen, often July or August. Link it to eutrophication: nutrient input → algal bloom → decomposition → oxygen drops → dead zone.
Key numbers students should know
- Hypoxia is commonly defined as dissolved oxygen below about 2 mg/L.
- Fish and many invertebrates avoid hypoxic water or die if they cannot escape.
- Some organisms, such as certain jellyfish, tolerate low oxygen better and may increase.
Mini self-check (fast)
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IB-style question — measuring BOD in a river [3]
A river flows past a dairy farm where manure sometimes washes in.
Describe a method you could use to measure the biochemical oxygen demand (BOD) of the river water. [3]
How to answer it, step by step
- Take samples and the first reading
• Collect water samples from several points along the river.
• Measure the dissolved oxygen straight away using an oxygen probe. - Wait, then re-measure
• Seal the samples and keep them in the dark for 5 days at 20°C.
• Measure dissolved oxygen again; BOD = the drop in oxygen over the 5 days.
Final answer
Name the conditions exactly — dark, 5 days, 20°C — and that BOD is the fall in dissolved oxygen.
IB-style question — biotic index vs chemical tests [2]
A class can check a stream's pollution either by using a biotic index (counting indicator invertebrates) or by directly measuring chemical pollutant levels.
(i) State one advantage of using a biotic index. [1]
(ii) State one advantage of directly measuring chemicals instead. [1]
How to answer it, step by step
- (i) Biotic index advantage
• It shows the real effect of pollution on living things.
• It can reveal past pollution even if the water looks clean now. - (ii) Chemical measurement advantage
• It tells you exactly which pollutant is present and how much.
• This can help trace the source of the pollution.
Final answer
Match each method to its strength: biotic = real ecological impact; chemical = the exact pollutant and amount.