Practice a College Board-style free response question on Aquatic & Terrestrial Pollution. Write your response, then reveal the model answer to see exactly what earns each point.
A lake ecosystem was historically contaminated by a persistent organic pollutant (POP) used as an agricultural pesticide decades ago. Researchers measured the concentration of this pollutant in tissue samples from organisms at different trophic levels of the lake's food chain.
| Trophic level | Organism | Pollutant concentration (ppm) |
|---|---|---|
| Producer | Algae | 0.003 |
| Primary consumer | Zooplankton | 0.04 |
| Secondary consumer | Small fish | 0.5 |
| Tertiary consumer | Large fish | 4.8 |
| Top predator | Osprey (fish-eating bird) | 25.0 |
The data show that pollutant concentration increases dramatically at each successive trophic level, from 0.003 ppm in algae to 25.0 ppm in the osprey — over an 8,000-fold increase from producer to top predator. This pattern is caused by biomagnification: because the pollutant is a persistent organic pollutant that resists breakdown, each predator accumulates the pollutant from all the contaminated prey it consumes, concentrating it further at each step up the food chain.
The pollutant is still present because it is a persistent organic pollutant (POP) — a class of chemicals chemically structured to resist environmental breakdown by microbial decomposition, sunlight, or other natural degradation processes. This persistence allows the chemical to remain in soil and water and continue cycling through the food chain for decades after its original application, rather than breaking down within a short time like many other pesticides.
The osprey population is likely to experience reproductive harm, such as eggshell thinning and reduced reproductive success (the well-documented effect of DDT-like POPs on birds of prey). The osprey is more affected than organisms lower in the food chain because, as the top predator, it has accumulated the toxin concentrations from every contaminated fish it has consumed over its lifetime — resulting in by far the highest tissue concentration (25.0 ppm) of any organism measured, due to the cumulative effect of biomagnification up the food chain.