Food chains move energy and matter. They will also, quite happily, move a pollutant — and concentrate it as they go. Add in the ways we reroute energy flows by burning, clearing, paving and farming, and this page is where the whole topic meets the real world.
📚 What you need to know
Bioaccumulation is the build-up of a persistent pollutant inside one organism over time.
Biomagnification is the increase in concentration of that pollutant along a food chain.
Pollutants that do this are persistent and non-biodegradable: DDT, PCBs, mercury.
Top predators end up with the highest concentrations and suffer the worst effects.
Microplastics absorb pollutants like sponges, carrying them into food chains.
Human activities — burning fossil fuels, deforestation, urbanisation and agriculture — change natural energy and matter flows.
Named examples matter: DDT and birds of prey, mercury at Minamata Bay.
Two words that sound the same
Get this distinction right
bioaccumulation = build-up within one organism biomagnification = build-up along the food chain
Bioaccumulation happens because the pollutant cannot be broken down or excreted, so every mouthful adds a little more that never leaves. Biomagnification happens on top of that: a predator eats many contaminated prey, so it takes in all their accumulated pollutant at once. Since so much of the biomass eaten is lost as heat and waste, the pollutant becomes concentrated into a much smaller mass of predator.
The pollutant is not being made. It is simply being collected up and packed into a smaller and smaller mass of living tissue at every level.
Two named examples worth learning
DDT and birds of prey
DDT was sprayed widely as an insecticide in the mid-twentieth century. It washed into rivers and lakes, entered food chains through plankton, and built up in fish. Birds of prey eating those fish accumulated the highest concentrations of all. The effect was thinner eggshells, which broke during incubation, and reproductive failure across whole populations of eagles and falcons.
Mercury at Minamata Bay
Mercury enters the environment from coal-fired power stations and gold mining, and bacteria convert it into highly toxic methylmercury. It accumulates in fish, and concentrates further as larger fish eat smaller ones. In 1956, a factory released methylmercury into waste water entering Minamata Bay in Japan. Local people who ate the fish and shellfish suffered severe mercury poisoning, including paralysis, deaths and birth defects.
The pattern in both cases: a persistent pollutant, an aquatic food chain, and the worst harm at the top — including humans, who often eat large predatory fish such as tuna and swordfish.
Microplastics: a delivery service for pollutants
Microplastics are plastic fragments smaller than 5 mm, from bottles, packaging and synthetic clothing. On their own they are a physical nuisance. The bigger problem is chemical: they act like sponges, absorbing non-biodegradable pollutants such as PCBs, pesticides and heavy metals from the water around them.
Marine animals then eat the particles while feeding, so the pollutants enter the food chain and biomagnify from there. Research on oysters exposed to contaminated microplastics found lower feeding rates, altered growth and reduced reproductive success — harming individuals and, through them, the whole population.
The link to make in an essay is that microplastics do not just add a pollutant — they make an existing pollutant more available to organisms that would otherwise never have eaten it.
Rerouting the flows: four human activities
The fossil fuel card is worth reading twice. More carbon dioxide sounds like good news for plants, and the fuller answer is that the accompanying damage cancels it out.
Worked examples
WORKED EXAMPLE
Distinguish between bioaccumulation and biomagnification.
Bioaccumulation
The build-up of a persistent pollutant within a single organism over its lifetime, because the pollutant cannot be broken down or excreted.
Biomagnification
The increase in concentration of that pollutant from one trophic level to the next along a food chain.
One is within an organism; the other is along a chain“Distinguish” means make the contrast explicit. Use the phrases “within an organism” and “along a food chain”.
WORKED EXAMPLE
Using the lake data above, calculate how many times more concentrated the pesticide is in the osprey than in the water, and explain the increase.
Step 1: calculate45 ÷ 0.005 = 9 0009 000 times more concentratedStep 2: explain the rise
The pesticide is persistent, so it is not broken down or excreted and accumulates in each organism.
Step 3: explain why each level is higher
Each predator eats many prey and takes in all the pollutant they had stored, while most of the biomass eaten is lost as heat and waste. The pollutant is concentrated into a smaller mass of tissue.
The second half is the explanation mark: the pollutant stays but the biomass shrinks.
WORKED EXAMPLE
Explain why top predators suffer the greatest harm from a persistent pollutant.
Step 1: their position
They sit at the highest trophic level, so the pollutant has been concentrated at every step below them.
Step 2: how much they eat
They consume large numbers of contaminated prey over a long lifetime, so bioaccumulation continues for years.
Step 3: the effect
Concentrations reach levels that damage reproduction — thin eggshells in birds of prey exposed to DDT, for example — so populations decline.
Highest concentration, longest exposure, worst effectsAdding a named example turns a general answer into a full-mark one.
💡 Exam tip
Use the word persistent when explaining either process. It is the property that makes the whole thing possible.
Have one named pollutant and one named place ready: DDT and birds of prey, or mercury and Minamata Bay.
For calculations from a ppm diagram, divide the higher by the lower and give the answer as “times more concentrated”.
When discussing carbon dioxide and productivity, mention both the small gain and the larger losses. Balanced answers score higher.
Link back to energy losses: the pollutant stays while biomass is lost, which is why concentration rises.
⚠ Common mix-up
Swapping the two terms. Accumulation is within one organism; magnification is along the chain.
Saying the pollutant multiplies. The amount does not increase — the concentration does, because the biomass shrinks.
Claiming microplastics are only a choking hazard. The chemical role, absorbing pollutants, is the part examiners want.
Saying more carbon dioxide is simply good for plants. Any gain is usually outweighed by heat, drought and pollution.
Forgetting humans are in the food chain. People who eat large predatory fish are exposed to biomagnified mercury.
Giving no named example in a question that clearly asks for one.
That completes 2.2. Up next: Biogeochemical Cycles (Topic 2.3) — how carbon and nitrogen move between stores, and what happens when we speed those transfers up.
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