Persistent Pollutants and Biomagnification
In the 1950s and 60s, ospreys and bald eagles along the Atlantic coast laid eggs with shells so thin they cracked under the weight of the parents. The pesticide that had been sprayed on mosquitoes and crops was never detected at high levels in the water. So how did it end up devastating birds at the top of the food chain?
Persistent organic pollutants
A pollutant is only a problem if it sticks around. Most natural wastes are broken down by bacteria and fungi within days or weeks. Some synthetic chemicals are not.
Most POPs share two properties that explain their danger:
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Persistent: microbes, sunlight, and water do not break them down quickly.
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Fat-soluble (lipophilic): they dissolve in fat, not in water. So they are stored in body fat and are not easily flushed out in urine.
| Pollutant | Use | Main concern |
|---|---|---|
| DDT | Insecticide (mosquitoes, crops) | Eggshell thinning in birds |
| PCBs | Coolants in electrical equipment, industrial oils | Cancer, developmental harm, immune effects |
| Mercury (a heavy metal, not organic, but also persistent) | Released by coal-burning power plants | Nervous system damage, especially in fetuses and children |
Mercury is a metal, so it is not a true "organic" pollutant. It behaves the same way in food chains (it persists and builds up), so it is studied alongside POPs.
Define persistent organic pollutant* (POP). Give one example, such as DDT or PCBs.
Define persistent organic pollutant (POP). Give one example, such as DDT or PCBs.
Bioaccumulation and biomagnification
These two words sound alike but describe different things.
Why does it climb the chain? Only about 10% of the energy passes up each level, so a predator must eat many prey. A POP is not burned off or excreted like the rest of the food. It stays in fat. So each predator collects the stored POPs from all the prey it eats over its lifetime.
Distinguish bioaccumulation* from biomagnification* in one or two sentences each.
Distinguish bioaccumulation from biomagnification in one or two sentences each.
In an aquatic food chain (algae, small fish, large fish, osprey), which organism would you expect to have the highest concentration of a biomagnifying pollutant? Why?
In an aquatic food chain (algae, small fish, large fish, osprey), which organism would you expect to have the highest concentration of a biomagnifying pollutant? Why?
Calculating biomagnification
To measure biomagnification, compare concentrations with a factor of increase:
$$\text{factor} = \frac{\text{concentration at the higher level}}{\text{concentration at the lower level}}$$Notice that the water itself has a very low concentration. A pollutant at $0.000002\ \text{ppm}$ in water that reaches $2.0\ \text{ppm}$ in a fish has been magnified a million times in total. This is why "low in the water" does not mean "safe."
A pollutant is measured in a lake food chain. | Organism | Concentration (ppm) | |---|---| | Water | 0.000002 | | Phytoplankton | 0.04 | | Zooplankton | 0.23 | | Small…
A pollutant is measured in a lake food chain.
| Organism | Concentration (ppm) |
|---|---|
| Water | 0.000002 |
| Phytoplankton | 0.04 |
| Zooplankton | 0.23 |
| Small fish | 0.5 |
| Large fish | 2.0 |
(a) Calculate how many times more concentrated the pollutant is in large fish than in phytoplankton.
(b) Explain what the data show about biomagnification.
A researcher measures PCB concentrations in a Great Lakes food web. | Trophic level | Organism | PCB (ppm) | |---|---|---| | Producer | Algae | 0.0025 | | Primary…
A researcher measures PCB concentrations in a Great Lakes food web.
| Trophic level | Organism | PCB (ppm) |
|---|---|---|
| Producer | Algae | 0.0025 |
| Primary consumer | Zooplankton | 0.123 |
| Secondary consumer | Smelt | 1.04 |
| Tertiary consumer | Lake trout | 4.83 |
| Top predator | Herring gull eggs | 124 |
(a) Calculate the factor of increase from algae to herring gull eggs.
(b) Explain why the increase occurs at each step, using the properties of PCBs.
(c) Predict how a ban on PCB production would change these values over time, and explain why recovery may be slow.
Case studies: DDT and mercury
DDT was widely sprayed in the mid-1900s, and bald eagles and ospreys later declined. Explain how DDT moved through food chains and how it affected the birds'…
DDT was widely sprayed in the mid-1900s, and bald eagles and ospreys later declined. Explain how DDT moved through food chains and how it affected the birds' reproduction.
Mercury from coal-burning power plants ends up in lakes and builds up in fish. Explain why a health agency might advise pregnant women to limit eating large predatory…
Mercury from coal-burning power plants ends up in lakes and builds up in fish. Explain why a health agency might advise pregnant women to limit eating large predatory fish such as swordfish more than small fish such as sardines.
Managing persistent pollutants
Because POPs last so long, cleanup is slow even after a ban. PCBs were banned in the U.S. in 1979, yet they still show up in Great Lakes fish, since they remain in sediments and in animals' fat and recycle through the food web. Concentrations typically fall gradually over decades, not years.
Strategies include:
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Bans and phase-outs of the chemical. In the U.S., the EPA banned most DDT uses in 1972, and the Toxic Substances Control Act (TSCA) led to the ban on PCB manufacture in 1979. Internationally, treaties such as the Stockholm Convention on POPs aim to phase out these chemicals (the U.S. has signed but not ratified it).
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Safer alternatives: short-lived, targeted pesticides that break down in days.
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Source reduction: capturing mercury emissions at power plants.
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Consumer advisories: guidance on which fish to eat.
When choosing between options, weigh cost against ecological and health risk. A cheaper pesticide that persists and dissolves in fat may cost far more in damage later than a pricier, short-lived one.
A city must choose between two pesticides for mosquito control. Pesticide X is cheap, kills many insects, breaks down slowly, and dissolves in fat. Pesticide Y costs…
A city must choose between two pesticides for mosquito control. Pesticide X is cheap, kills many insects, breaks down slowly, and dissolves in fat. Pesticide Y costs three times as much, targets mosquito larvae, and breaks down in days.
(a) Predict which is more likely to biomagnify, and justify your answer.
(b) Describe two possible ecosystem effects of the choice.
(c) Recommend one option and explain how you weigh cost against ecological risk.
Back to the start: the osprey's eggs cracked because DDT was persistent and fat-soluble, so it accumulated in every organism and magnified up the food chain to the top predators.