AP Environmental Science

## Learning Outcomes

- Distinguish clearly between **bioaccumulation** and **biomagnification**
- Identify the major bioaccumulative substances most likely to appear in AP Environmental Science
- Explain why some pollutants persist and increase at higher trophic levels
- Trace how **mercury**, **DDT**, and **PCBs** move through aquatic food webs
- Understand the ecological and human health effects of these substances
- Apply this knowledge to exam questions about trophic levels, pollution pathways, and top predators

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand major bioaccumulative substances, with a focus on the following syllabus points:

- Know that **bioaccumulation** is the buildup of a pollutant within an individual organism over time
- Identify **biomagnification** as the increase in pollutant concentration at successively higher trophic levels
- Recognise that pollutants most likely to biomagnify are **persistent**, **fat-soluble**, and not easily excreted
- Identify major examples including **mercury** (especially **methylmercury**), **DDT**, and **PCBs**
- Understand how mercury enters aquatic systems and is converted by bacteria into methylmercury
- Explain how **DDT** and its breakdown product **DDE** harmed predatory birds through eggshell thinning
- Understand that **top predator fish**, birds of prey, marine mammals, and humans often face the highest exposure
- Recognise that persistent pollutants can remain in sediments, soils, and organisms long after their use has ended

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between bioaccumulation and biomagnification?
2. Why are fat-soluble pollutants more likely to biomagnify than water-soluble pollutants?
3. How does mercury become methylmercury in aquatic environments?
4. Name two major pollutants that commonly trigger fish consumption advisories.
5. Why did DDT have such serious effects on birds of prey?

## Introduction

Pollution does not affect all chemicals in the same way. Some substances break down quickly, get diluted, or are excreted from organisms before much harm is done. Others remain in the environment for years, move into living tissues, and become more concentrated as they pass through food webs. These are the substances AP Environmental Science wants you to know best.

This topic matters because it links chemistry, ecology, and human health. On the exam, you may be asked to define terms, identify which trophic level has the highest concentration, explain why a substance biomagnifies, or trace a pollutant from water or sediment to fish, birds, or humans.

> **Key Term: bioaccumulation**
> The buildup of a pollutant in the tissues of an individual organism over time.

### Why these substances build up in organisms and food webs

Students often confuse the two core ideas in this topic. **Bioaccumulation** happens within one organism. **Biomagnification** happens across a food chain or food web.

> **Key Term: biomagnification**
> The increase in the concentration of a pollutant at successively higher trophic levels in a food chain or food web.

To see the difference, imagine a fish living for several years in contaminated water. If it slowly stores more mercury in its tissues each month, that is bioaccumulation. If a larger fish then eats many of those smaller fish and ends up with an even higher mercury concentration, that is biomagnification.

Why do only some chemicals do this? Three properties matter most.

First, the substance must be **persistent**. If it breaks down quickly in sunlight, water, or living tissue, it will not remain long enough to move through the food web.

Second, it is usually **fat-soluble**. Fat-soluble substances can cross cell membranes and be stored in fatty tissues. Water-soluble substances are more easily excreted in urine or other wastes.

Third, the organism must absorb the substance faster than it can eliminate it. If uptake is greater than excretion, concentration rises over time.

> **Key Term: persistent organic pollutants (POPs)**
> Organic chemicals that resist breakdown, persist in the environment, and can bioaccumulate and biomagnify in living organisms.

A useful APES pattern is this:

- **Persistent + fat-soluble + toxic = high biomagnification risk**
- **Short-lived + easily excreted = low biomagnification risk**

Aquatic systems are especially important because pollutants can enter water, sediments, plankton, and fish in linked ways. Also, many aquatic food webs have long chains: phytoplankton -> zooplankton -> small fish -> large fish -> top predator. At each step, the predator eats many organisms from the level below, so the pollutant becomes more concentrated even while total available energy decreases.

> **Test Tip:** If an MCQ asks which organism will have the highest concentration of a persistent toxin, choose the **top predator**, not the organism closest to the pollution source.

### Mercury and methylmercury: the major aquatic example

Mercury is one of the most important examples in AP Environmental Science because it shows both a source pathway and a food-web pathway.

Mercury is released by human activity, especially from burning fossil fuels and from some mining-related processes. Once in the atmosphere, it can be deposited into lakes, rivers, wetlands, and oceans. This is where the key change happens: bacteria in low-oxygen aquatic sediments can convert inorganic mercury into **methylmercury**.

> **Key Term: methylmercury**
> A highly toxic organic form of mercury produced by bacteria in aquatic environments, which readily bioaccumulates and biomagnifies.

This conversion matters because methylmercury is more biologically available and more dangerous. It enters plankton easily, then moves into fish tissues, especially muscle tissue. Small fish eat contaminated plankton. Larger fish eat many smaller fish. Top predators such as tuna, swordfish, shark, and some freshwater sport fish build up the highest concentrations.

A common AP-style chain is:

- mercury released to environment
- deposited into water
- converted by bacteria to methylmercury
- absorbed by plankton
- eaten by small fish
- eaten by larger fish
- eaten by top predators or humans

This is why fish consumption advisories often focus on mercury. The water itself may contain a low concentration, but the fish tissue concentration can be much higher.

Human health effects are serious because methylmercury damages the nervous system. It is especially dangerous to fetuses, infants, and young children because developing brains are more sensitive. That is why advisories often target pregnant women, women who may become pregnant, nursing mothers, and young children.

There is an important exam distinction here. Mercury pollution does not become especially dangerous in fish simply because mercury is toxic. It becomes especially dangerous because **methylmercury is persistent, bioavailable, and not easily eliminated**.

> **Test Tip:** When explaining mercury in an FRQ, do not stop at “mercury enters water.” Add the crucial middle step: **bacteria convert it to methylmercury**.

### DDT and DDE: the classic bird case study

If mercury is the classic fish example, **DDT** is the classic historical pesticide example.

> **Key Term: DDT**
> A persistent organochlorine pesticide that bioaccumulates in organisms and biomagnifies in food webs.

DDT was widely used because it killed insect pests effectively. The problem was not immediate visible poisoning alone. The deeper problem was persistence. DDT remained in soils and waters for long periods and entered food webs. Because it is fat-soluble, organisms stored it in tissues instead of eliminating it quickly.

As DDT moved from producers and small consumers into larger predators, concentrations rose. Birds of prey such as bald eagles, ospreys, peregrine falcons, and pelicans were heavily affected.

The key damaging compound here is often **DDE**, a breakdown product of DDT.

> **Key Term: DDE**
> A breakdown product of DDT associated with eggshell thinning in predatory birds.

Students often miss the DDT/DDE distinction. DDT is the pesticide originally released into the environment. DDE is the metabolite most strongly linked to the reproductive damage seen in birds.

How did this work? DDE interfered with calcium deposition during eggshell formation. As a result, eggshells became thinner and weaker. Adult birds could crush the eggs during incubation, or the eggs failed before chicks developed fully. So the major effect was not usually that the adult birds died immediately. Instead, their **reproductive success fell sharply**, causing population decline over time.

That cause-and-effect chain is worth learning exactly:

- DDT applied as pesticide
- persists in environment
- enters food web
- biomagnifies into predatory birds
- DDE interferes with calcium deposition
- eggshells thin
- eggs break more easily
- fewer chicks hatch
- bird populations decline

This is a good example of why ecological effects are not always about direct adult mortality. A pollutant can reduce population size by lowering successful reproduction.

### PCBs and dioxins: industrial persistent pollutants

Not all major bioaccumulative substances are pesticides. **PCBs** are a key example of industrial chemicals that behave in similar ways.

> **Key Term: PCBs (polychlorinated biphenyls)**
> Persistent industrial chemicals that accumulate in fatty tissues and biomagnify in aquatic and marine food webs.

PCBs were used in electrical equipment and other industrial applications because of their useful chemical properties. But from an environmental point of view, they share the most dangerous traits with DDT: persistence, fat solubility, and toxicity.

Once released, PCBs can settle into sediments and remain there for long periods. Bottom-dwelling organisms encounter them, small fish eat those organisms, larger fish eat the smaller fish, and marine mammals or humans consuming fish receive even higher concentrations.

PCBs are important for APES because they show that biomagnification is not just an agriculture issue. It can also result from **industrial contamination**.

Another relevant example is **dioxins**.

> **Key Term: dioxins**
> Highly toxic persistent compounds, often produced as by-products of combustion and industrial processes, that bioaccumulate in fatty tissues.

Dioxins are different from DDT in source. They are not mainly sprayed as pesticides. They are often unintended by-products of combustion or industrial activity. But once produced, they can persist and move through food webs in similar ways.

For humans, a major exposure route for both PCBs and dioxins is often food, especially animal fats or contaminated fish. That matters because it reminds you that human exposure is frequently indirect. Pollutants do not need to be inhaled or drunk directly to be dangerous. They can move through food webs first.

A useful AP comparison is:

- **DDT** = pesticide
- **PCBs** = industrial chemicals
- **dioxins** = mostly combustion/industrial by-products

All three can be persistent and bioaccumulative.

### How to compare the major substances on the exam

The AP exam usually rewards clear distinctions. A strong answer does not just list chemicals. It shows how they differ.

| Substance               | Main source type                                       | Why it bioaccumulates/biomagnifies                              | Main APES effect                                        |
| ----------------------- | ------------------------------------------------------ | --------------------------------------------------------------- | ------------------------------------------------------- |
| Mercury / methylmercury | fossil fuel combustion, mining, atmospheric deposition | converted to toxic methylmercury; persists in aquatic food webs | neurological damage; high levels in top predator fish   |
| DDT / DDE               | pesticide                                              | persistent and fat-soluble                                      | eggshell thinning and reproductive failure in birds     |
| PCBs                    | industrial chemical                                    | persistent, stored in fatty tissues                             | contamination of fish, birds, and marine mammals        |
| Dioxins                 | combustion and industrial by-products                  | persistent and bioaccumulative                                  | toxic food-web contamination, often through animal fats |

This comparison also helps with a broader distinction the exam may expect: **POPs** versus **PBTs**. A persistent, bioaccumulative, and toxic chemical is often called a PBT. POPs overlap strongly with that idea, but POPs also emphasise the ability to travel long distances through the environment. That is why they can appear far from where they were first released.

The **Stockholm Convention**, adopted in **2001** and entering into force on **17 May 2004**, focused on such persistent pollutants because they are global problems, not just local ones.

### What students often get wrong

The most common errors in this topic are small but costly.

First, students say bioaccumulation when they mean biomagnification. Remember:

- one organism over time = bioaccumulation
- higher trophic levels across the food web = biomagnification

Second, students say a pollutant biomagnifies because it is toxic. Toxicity alone is not enough. It also needs persistence and poor elimination, and usually fat solubility.

Third, students confuse DDT with DDE. DDT is the pesticide. DDE is the breakdown product strongly linked to eggshell thinning.

Fourth, students treat all heavy metals the same way. In APES, the standout example is mercury because of bacterial conversion to methylmercury in aquatic systems.

Fifth, students forget that top predators are most at risk. This includes not only wild predators but also humans eating high-trophic-level fish.

If you can avoid those five mistakes, you will already be stronger than many exam candidates.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Bioaccumulation is the buildup of a pollutant within one organism over time
- Biomagnification is the increase in pollutant concentration at higher trophic levels
- Pollutants most likely to biomagnify are persistent, fat-soluble, and not easily excreted
- Mercury becomes especially dangerous in aquatic systems when bacteria convert it to methylmercury
- Methylmercury biomagnifies in aquatic food webs and reaches high levels in top predator fish
- Human exposure to methylmercury commonly occurs through eating contaminated fish and shellfish
- DDT is a persistent pesticide that biomagnifies through food webs
- DDE, a breakdown product of DDT, is strongly linked to eggshell thinning in birds of prey
- PCBs are persistent industrial chemicals that accumulate in fatty tissues and biomagnify
- Dioxins are highly toxic by-products of combustion and industrial processes that can bioaccumulate
- Top predators and humans consuming high-trophic-level organisms usually have the highest pollutant concentrations
- Fish consumption advisories often focus on mercury, PCBs, dioxins, and DDT-related compounds

## Key Terms and Concepts

- bioaccumulation
- biomagnification
- persistent organic pollutants (POPs)
- methylmercury
- DDT
- DDE
- PCBs (polychlorinated biphenyls)
- dioxins


Bioaccumulation and Biomagnification - Major bioaccumulative substances