AP Environmental Science

## Learning Outcomes

- Understand what **eutrophication** and **cultural eutrophication** mean
- Identify the main human sources of excess **nitrogen** and **phosphorus** in water
- Explain how runoff, wastewater, and atmospheric deposition move nutrients into aquatic systems
- Distinguish between **point source** and **nonpoint source** nutrient pollution
- Apply the cause-and-effect chain from nutrient loading to algal blooms and hypoxia
- Use freshwater and coastal examples to support AP Environmental Science answers
- Avoid common exam mistakes, especially confusing causes with effects

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand human causes of eutrophication, with a focus on the following syllabus points:

- Know that eutrophication is driven mainly by excess **nitrogen** and **phosphorus** entering water
- Identify major human nutrient sources, including **agricultural runoff**, **wastewater/sewage**, **stormwater runoff**, **animal waste**, and **septic-system effluent**
- Explain how rain events and snowmelt increase **nutrient loading** from land to water
- Understand that wastewater discharge is usually a **point source**, while agricultural and urban runoff are usually **nonpoint sources**
- Recognise that **atmospheric deposition** of nitrogen from fossil-fuel combustion can also contribute to eutrophication
- Distinguish **natural eutrophication** from rapid, human-driven **cultural eutrophication**
- Understand the common AP distinction that **phosphorus is often limiting in freshwater** and **nitrogen is often limiting in marine systems**
- Explain the full pathway from human nutrient input to algal bloom, decomposition, dissolved oxygen decline, and possible dead zone formation

## 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 eutrophication and cultural eutrophication?
2. Name three human sources of nitrogen or phosphorus that can cause eutrophication.
3. How does agricultural runoff differ from wastewater discharge as a pollution source?
4. Why can heavy rainfall make eutrophication worse?
5. How can air pollution contribute to nutrient pollution in water?

## Introduction

## Introduction

Eutrophication is one of the most tested water-pollution topics in AP Environmental Science because it connects several core ideas at once: nutrient cycling, human land use, water quality, aquatic food web disruption, and environmental solutions. Students often remember the visible part—the algal bloom—but miss the actual exam focus: **what human activities add the nutrients in the first place**.

On the exam, you must be able to separate **cause** from **effect**. Fertilizer runoff, sewage effluent, septic leakage, and urban stormwater are causes. Algal blooms, hypoxia, fish kills, and dead zones are effects. If you keep that distinction clear, many multiple-choice and free-response questions become much easier.

> **Key Term: eutrophication**
> The enrichment of a body of water with nutrients that stimulates excessive growth of algae and aquatic plants.

### Cultural eutrophication and nutrient loading

Natural eutrophication can happen slowly over long time periods as lakes and other waters gradually accumulate nutrients and sediments. AP Environmental Science usually focuses instead on the accelerated, human-driven version.

> **Key Term: cultural eutrophication**
> Rapid eutrophication caused by human activities that add excess nutrients, especially nitrogen and phosphorus, to water.

The key idea is **nutrient loading**. Water bodies are not usually damaged by just any pollution in this topic. They are damaged when too much biologically useful nutrient enters the system.

> **Key Term: nutrient loading**
> The input of nutrients such as nitrogen and phosphorus into a water body.

Why do these nutrients matter so much? Because plant and algal growth is usually limited by whichever nutrient is in shortest supply. Add more of that limiting nutrient, and growth can increase sharply. In AP terms, remember the usual pattern:

- **Freshwater systems:** often limited by **phosphorus**
- **Marine or coastal systems:** often limited by **nitrogen**

A useful memory aid is **“P in ponds, N in oceans.”** It is not perfect in every real case, but it is a strong exam rule.

This is why human nutrient inputs have such large effects. A small-looking change in runoff or wastewater can shift a water body from relatively balanced productivity to excessive algal growth.

> **Test Tip:** If a question asks for a _cause_ of eutrophication, do not answer with "algal bloom" or "low dissolved oxygen." Those are consequences. Look for the nutrient source.

### Agricultural runoff: the biggest nonpoint source

Agriculture is one of the most important human causes of eutrophication because it adds large amounts of both **nitrogen** and **phosphorus** to land, and some of that nutrient leaves the field before crops can use it.

> **Key Term: agricultural runoff**
> Water from rainfall, irrigation, or snowmelt that carries soil, fertilizers, manure, and other materials from farmland into waterways.

There are several ways this happens.

First, **synthetic fertilizers** are applied to increase crop yields. Many fertilizers contain nitrogen and phosphorus in forms plants can use. But crops do not absorb all of it. If fertilizer is overapplied, applied at the wrong time, or left exposed before a storm, nutrients can move off the field.

Second, **animal manure** is a major source of nutrient pollution. Livestock operations, feedlots, and manure storage areas contain high nutrient concentrations. Rainfall can wash dissolved nutrients and organic waste into nearby streams, ponds, or rivers.

Third, **soil erosion** matters especially for phosphorus. Nitrates are highly water-soluble, so they can dissolve directly into runoff or leach downward. Phosphates are less soluble and often attach to soil particles. That means when topsoil erodes, phosphorus travels with the sediment.

This is an easy place to make a higher-level AP point:

- **Nitrogen** often moves in dissolved form with water
- **Phosphorus** often moves attached to eroded soil

That distinction helps explain why both runoff and erosion control matter.

Rainfall and snowmelt are critical. Nutrients sitting harmlessly on land do not cause eutrophication until water transports them. This is why the timing of fertilizer application matters so much. Fertilizer spread just before heavy rain is much more likely to become nutrient pollution than fertilizer applied when crops are actively growing and weather is dry.

A common exam application is the link between agricultural watersheds and downstream dead zones. Rivers draining agricultural land often carry elevated nutrients to lakes, estuaries, and coastal waters.

A well-known example is the **Gulf of Mexico dead zone**, which forms near the outflow of the Mississippi River. Nutrients from a huge watershed, much of it agricultural, enter Gulf waters and promote large algal blooms. When the algae die, decomposition lowers dissolved oxygen in deeper water.

This shows why eutrophication is not just a local farm problem. Nutrients can travel through whole drainage systems.

> **Key Term: nonpoint source pollution**
> Pollution that comes from many diffuse sources rather than one single, easily identified discharge point.

Agricultural runoff is usually classed as **nonpoint source pollution** because it comes from many fields, feedlots, ditches, and drainage pathways across a broad area.

Students sometimes ask why this matters. It matters because nonpoint sources are harder to regulate and monitor. A wastewater pipe can be measured directly. Thousands of separate runoff pathways across a farming region cannot.

### Wastewater, sewage, and septic systems

Wastewater systems are another major human cause of eutrophication. Unlike most agricultural runoff, these are often easier to identify because they usually involve one discharge location.

> **Key Term: wastewater effluent**
> Treated or untreated water discharged from sewage treatment plants, sewer systems, or similar facilities into the environment.

Wastewater contributes to eutrophication because human waste, household water, detergents, and food residues all contain nutrients. If treatment is incomplete, nitrogen and phosphorus remain in the discharged water.

A sewage treatment plant may remove solids and reduce pathogens, but unless nutrient removal is strong enough, the plant can still release nitrate and phosphate pollution. This is why treatment capacity and treatment level matter. A plant that is overloaded or outdated may not control nutrients effectively.

> **Key Term: point source pollution**
> Pollution released from a single identifiable source, such as a discharge pipe.

Wastewater discharge is usually a **point source** because the nutrient input often enters water through a known outfall or pipe.

There are several wastewater-related causes students should know:

- **Treatment plants with limited nutrient-removal capacity**
- **Sewage discharges**
- **Combined sewer overflows**
- **Storm-driven sewer overflow**
- **Leaking or failing septic systems**
- **Household phosphate sources**, such as some detergents

Combined sewer overflows are especially important after heavy rainfall. In some systems, stormwater and sewage move through the same infrastructure. If rainfall exceeds capacity, untreated or partly treated sewage can overflow into nearby water bodies.

That creates a double problem:

- direct nutrient addition
- increased organic waste for decomposers

Both can worsen eutrophication and lower dissolved oxygen.

Septic systems also matter. A working septic system can reduce immediate waste exposure, but failing, leaking, or poorly maintained systems allow nutrient-rich effluent to enter groundwater or nearby surface water. In areas with many homes on septic systems, the combined effect can be significant even if no single household seems important.

A common AP mistake is to think wastewater matters only in cities. In fact, nutrient pollution from wastewater can affect suburban, rural, freshwater, and coastal systems.

There is also a useful biological link here. Human waste does not just add nutrients. It also adds **biodegradable organic matter**. When bacteria decompose that matter, they consume dissolved oxygen. That helps explain why wastewater inputs can worsen hypoxia even beyond their fertilizer effect.

### Urban runoff, lawns, detergents, and atmospheric deposition

Cities and suburbs also contribute to eutrophication, even where no farms are nearby. Developed land creates fast-moving runoff and often carries nutrients into storm drains and streams.

> **Key Term: stormwater runoff**
> Water from rain or melting snow that flows over land surfaces and carries pollutants into drains and waterways.

Urban and suburban nutrient sources include:

- lawn fertilizers
- golf course fertilizers
- landscaping runoff
- pet waste
- leaves and organic debris
- wastewater leaks
- phosphate-containing household products in some cases

Impervious surfaces such as roads, driveways, and parking lots prevent infiltration. Instead of soaking into soil, water runs quickly across the surface, picking up nutrients and carrying them into waterways. This helps explain why eutrophication can increase as urbanization increases.

Residential fertilizer use can be surprisingly important. People often apply lawn fertilizer for appearance, not crop need, so it may be applied in excess or at the wrong time. A heavy rainstorm soon after application can move a lot of nutrient from lawns into ponds and streams.

Golf courses are another common AP example because they are highly managed areass with repeated fertilizer application near drainage systems.

Household phosphates matter as well. While regulations have reduced this source in many places, phosphate-containing detergents and similar products are still a useful AP example of how household behavior can add phosphorus to wastewater.

A final cause that students often forget is **atmospheric deposition**.

> **Key Term: atmospheric deposition**
> The transfer of pollutants from the atmosphere to land or water through rain, snow, dust, or direct settling.

Nitrogen oxides from fossil-fuel combustion and vehicle exhaust can enter the atmosphere, then return to water bodies directly or indirectly through precipitation and settling. This means air pollution can become water pollution.

This is an important systems link:

- burning fossil fuels releases nitrogen compounds
- those compounds enter the atmosphere
- they are deposited into the environments
- nitrogen availability rises
- eutrophication risk increases

So a multiple-choice question could describe automobile exhaust or power plant emissions and still be asking about a cause of eutrophication.

This is especially useful in coastal and regional contexts, where nitrogen deposition may affect wide areas rather than one local water body.

### The cause-and-effect chain you must be able to explain

Although this article focuses on human causes, AP questions often expect you to connect causes to outcomes in sequence. You should be able to explain the full chain clearly.

1. Human activity adds excess **nitrogen** and/or **phosphorus** to land or directly to water.
2. Rainfall, snowmelt, erosion, discharge, or deposition increases **nutrient loading**.
3. Algae and phytoplankton grow rapidly, producing an **algal bloom**.
4. The bloom blocks light and may reduce deeper plant growth.
5. Algae die, and decomposers break them down.
6. Decomposition increases **biological oxygen demand**.
7. Dissolved oxygen falls.
8. **Hypoxia** may develop, creating fish kills or a dead zone.
   > **Key Term: hypoxia**
   > A condition of low dissolved oxygen in water that can stress or kill aquatic organisms.

One subtle point students miss is that the visible bloom is often not the main killer. The biggest damage often comes later, when decomposers use up the oxygen.

Another subtle point is that some phytoplankton are toxic. So eutrophication can also create harmful algal blooms that poison animals or contaminate shellfish, but the core AP mechanism is still nutrient over-enrichment.

A useful case comparison:

- **Lake Erie** is a strong freshwater example, often linked to phosphorus runoff and harmful algal blooms.
- **The Gulf of Mexico** is a strong coastal example, often linked to nitrogen runoff and seasonal hypoxia.
  > **Test Tip:** In an FRQ, write the chain in order. For example: "Fertilizer runoff adds nitrates and phosphates to water, causing algal blooms. When algae die, decomposers increase biological oxygen demand, lowering dissolved oxygen and causing hypoxia." That sequence earns more credit than isolated facts.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Eutrophication is nutrient enrichment that stimulates excessive algal and aquatic plant growth
- Cultural eutrophication is the rapid, human-driven form of eutrophication
- The main nutrients involved are **nitrogen** and **phosphorus**
- Agricultural runoff is a major **nonpoint source** of nutrient pollution
- Fertilizers and animal manure are major agricultural causes of eutrophication
- Nitrates often move dissolved in water, while phosphates often move with eroded soil
- Wastewater effluent is usually a **point source** of nutrient pollution
- Sewer overflows and failing septic systems can add nitrogen and phosphorus to waterways
- Urban stormwater runoff can carry nutrients from lawns, landscaping, and pet waste
- Atmospheric deposition of nitrogen from fossil-fuel combustion can contribute to eutrophication
- Freshwater systems are often phosphorus-limited, while marine systems are often nitrogen-limited
- The main causal chain is nutrient loading to algal bloom to decomposition to oxygen depletion to hypoxia or dead zones

## Key Terms and Concepts

- eutrophication
- cultural eutrophication
- nutrient loading
- agricultural runoff
- nonpoint source pollution
- wastewater effluent
- point source pollution
- stormwater runoff
- atmospheric deposition
- hypoxia


Eutrophication - Human causes of eutrophication