AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/8/8-8)
Unit 8 · Topic 8.8
8.8 Bioaccumulation and Biomagnification
Bioaccumulation is the build-up of a chemical inside one organism over its lifetime. Biomagnification is the increase in concentration of that chemical at each step up a food chain. Together they explain why top predators like eagles, orcas and large tuna carry the highest levels of toxins such as DDT and mercury.
Key terms
- bioaccumulation
- biomagnification
- trophic level
- DDT
- mercury
Bioaccumulation: within one organism
Bioaccumulation happens when an organism takes in a chemical faster than it can break it down or excrete it. The chemical builds up in its tissues over its life. Older and larger individuals usually have higher concentrations than younger ones of the same species.
The chemicals that bioaccumulate are usually fat-soluble and persistent, like the POPs in topic 8.7, along with methylmercury. Water-soluble chemicals tend to be flushed out in urine instead.
Biomagnification: up the food chain
Biomagnification is the increase in a chemical's concentration at each higher trophic level (feeding level). It follows from the 10% rule of energy flow (topic 1.10). A predator has to eat a large mass of prey to get enough energy, because most of the energy in prey is lost as heat. But the fat-soluble chemical in all that prey is not lost; it stays in the predator's fat. So the chemical gets packed into a smaller mass of predator tissue at each step.
The result is that concentrations can rise by many thousands to millions of times from the water to the top predator.
Two classic examples
DDT and birds of prey: DDT sprayed on crops and wetlands washed into water, was absorbed by plankton, then built up through fish to fish-eating birds such as bald eagles, ospreys, brown pelicans and peregrine falcons. A breakdown product of DDT, called DDE, interferes with the way female birds deposit calcium in eggshells. The thin shells cracked under the weight of the parents during incubation, and populations crashed. After the United States banned DDT in 1972, the birds recovered, and the bald eagle was removed from the endangered species list in 2007.
Mercury and fish: burning coal and small-scale gold mining are the largest human sources of mercury. Mercury in the air settles into water, where bacteria in sediments convert it into methylmercury, a form that organisms absorb easily and that builds up in muscle as well as fat. It biomagnifies to high levels in large predatory fish such as swordfish, shark, king mackerel and some tuna. Methylmercury damages the developing nervous system, so pregnant people and young children are advised to limit those fish.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Calculating biomagnification
In a lake, a pesticide is measured at 0.00002 ppm in the water, 0.04 ppm in zooplankton, 0.5 ppm in small fish, 2.0 ppm in large fish and 25 ppm in fish-eating birds. (a) By what factor does the concentration increase from large fish to birds? (b) How many times more concentrated is the pesticide in birds than in the water?
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- Step 1: (a) Factor = concentration at higher level ÷ concentration at lower level = 25 ppm ÷ 2.0 ppm = 12.5.
- Step 2: (b) 25 ppm ÷ 0.00002 ppm = 25 ÷ (2 × 10⁻⁵) = 1,250,000.
- Step 3: So the birds carry the pesticide at about 1.25 million times the concentration in the lake water.
Answer: (a) 12.5 times. (b) 1,250,000 (1.25 × 10⁶) times.
- Example 2
Trap: bioaccumulation versus biomagnification
Identify which process each statement describes: (a) A 12-year-old trout has three times the mercury of a 3-year-old trout from the same lake. (b) Eagles that eat trout have much higher mercury levels than the trout.
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- Step 1: Bioaccumulation is build-up within a single organism over time; biomagnification is the increase from one trophic level to the next.
- Step 2: (a) Same species, different ages: the older fish has had more time to take in mercury. That is bioaccumulation.
- Step 3: (b) A predator has higher levels than its prey: that is biomagnification.
Answer: (a) Bioaccumulation. (b) Biomagnification.
Common mistakes
- Using bioaccumulation and biomagnification as synonyms. One is within an organism over time; the other is across trophic levels.
- Saying DDT killed adult eagles directly. Its main effect was thin eggshells that broke during incubation, causing reproductive failure.
- Explaining biomagnification without the energy link. Predators eat large amounts of prey because energy is lost at each level, but the fat-soluble toxin is retained.
On the exam
- Calculation questions give concentrations at each trophic level; divide higher by lower to find the magnification factor.
- If asked why top predators have the highest levels, mention fat solubility, persistence and the large amount of prey eaten due to energy loss between trophic levels.
Connected topics
Videos
Check yourself
4 questions on 8.8 Bioaccumulation and Biomagnification. Pick an answer to see if you got it, and why.
| Sample | DDT concentration (ppm) |
|---|---|
| Water | 5 × 10⁻⁵ |
| Zooplankton | 0.04 |
| Small fish | 0.5 |
| Large fish | 2.0 |
| Fish-eating birds (osprey) | 25 |
Hypothetical data
The DDT concentration in ospreys is about how many times its concentration in the water?
Which statement best explains why ospreys have the highest DDT concentration?
Which effect on ospreys is most closely linked to these DDT levels?
A regulator is comparing four new pesticides. Which one is most likely to build up in top predators?
0 of 4 answered