AP Environmental Science

## Learning Outcomes

- Define **net primary productivity (NPP)** and explain why it matters in natures
- Distinguish clearly between **gross primary productivity (GPP)**, **respiration**, and NPP
- Calculate NPP from given data, including light-bottle and dark-bottle investigations
- Explain why NPP is the energy available to herbivores and higher trophic levels
- Compare the main limiting factors on NPP in terrestrial and aquatic environmentss
- Interpret why some biomess have high NPP per unit area while others contribute more total productivity globally
- Avoid common exam mistakes involving biomass, productivity, and habitat comparisons comparisons

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand net primary productivity, with a focus on the following syllabus points:

- Know that **NPP = GPP - respiration by producers**
- Identify **GPP** as the total energy or carbon fixed by photosynthesis before respiratory losses
- Explain that NPP is the rate at which producers store energy as **biomass** available for growth, reproduction, and consumers
- Understand that productivity is a **rate over area and time**, often measured in **g C/m²/yr** or **kcal/m²/yr**
- Compare limiting factors of NPP in **terrestrial** habitatss and **aquatic** habitatss
- Distinguish **productivity per unit area** from **total global productivity**
- Explain why **estuaries** are very productive per square meter, while the **open ocean** contributes greatly overall because of its size
- Understand how nutrient enrichment can increase aquatic productivity and may contribute to **eutrophication**

## 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 gross primary productivity and net primary productivity?
2. Why is NPP described as the energy available to consumers?
3. How do you calculate NPP if GPP is known?
4. Why can the open ocean have low productivity per square meter but high total productivity?
5. Name two factors that limit NPP on land and two that limit NPP in aquatic systems.

## Introduction

Net primary productivity is one of the most useful ideas in AP Environmental Science because it links photosynthesis, food webs, and ecological systems limits. If you understand NPP, you can explain why some habitatss support many consumers, why others support few, and how environmental changes affect energy flow.

In simple terms, producers capture energy, use some of it for their own survival, and store the rest as new biomass. That remaining portion is what matters most to the rest of the environment. It is the starting point for herbivores, then predators, then the whole food web.

> **Key Term: net primary productivity (NPP)**
> The rate at which primary producers store energy as biomass after subtracting the energy they use in respiration.

### How NPP fits into food-web energy flow

Primary productivity begins with producers such as plants, algae, and phytoplankton. These organisms capture energy, usually from sunlight, and convert it into chemical energy through photosynthesis. That total captured energy is called gross primary productivity.

However, producers do not pass all of that captured energy into the food web. They must use some of it themselves for cellular respiration. They need energy to maintain tissues, take up water, repair damage, transport materials, and grow roots, stems, leaves, or reproductive structures.

This is why NPP is always smaller than GPP. The relationship is:

NPP = GPP - R

where **R** is respiration by producers.

> **Key Term: gross primary productivity (GPP)**
> The total rate at which producers capture and fix energy or carbon through photosynthesis before subtracting respiration.
>
> **Key Term: respiration (R)**
> The energy used by producers for metabolism, maintenance, repair, and other life processes.

NPP matters because it represents the biomass gain left over after the producers have paid their own energy costs. If a grassland has low NPP, there is less plant material available for herbivores. If an estuary has high NPP, it can support larger or more active consumer populations.

A common mistake is to think NPP means "all plant energy." It does not. It means the **remaining** plant energy after respiration.

> **Test Tip:** If an APES question asks for the energy "available to consumers" or "available to herbivores," think **NPP**, not GPP.

Another common mistake is confusing NPP with standing biomass. These are not the same. A forest may have high standing biomass because it has many large trees, but productivity asks how much new biomass is produced **per unit time**.

> **Key Term: biomass**
> The mass of living organic material in organisms or habitatss.

### Calculating NPP and using the light-bottle/dark-bottle method

For AP Environmental Science, you must be able to calculate NPP and interpret productivity data. The most direct formula is:

NPP = GPP - R

Suppose an community has:

- GPP = **2,400 kcal/m²/yr**
- Respiration = **1,500 kcal/m²/yr**

Then:

NPP = 2,400 - 1,500 = 900 kcal/m²/yr

That means producers store **900 kcal/m²/yr** as new biomass available for growth and for the rest of the food web.

The logic matters as much as the arithmetic:

1. Producers capture total energy
2. Producers use part of it
3. The remainder is stored as new tissue

In aquatic systems, APES often tests productivity with a light-bottle/dark-bottle investigation using dissolved oxygen. This works because photosynthesis produces oxygen, while respiration consumes oxygen.

The key rules are:

- **Light bottle**: photosynthesis and respiration both happen
- **Dark bottle**: only respiration happens
- **Initial bottle**: starting dissolved oxygen

Use these formulas:

- **NPP = Light - Initial**
- **R = Initial - Dark**
- **GPP = NPP + R**
- or shortcut: **GPP = Light - Dark**

Worked example:

Initial dissolved oxygen = **8 mg/L**  
Light bottle after incubation = **10 mg/L**  
Dark bottle after incubation = **5 mg/L**

Step 1: Calculate NPP  
NPP = Light - Initial = 10 - 8 = 2 mg/L O₂

Step 2: Calculate respiration  
R = Initial - Dark = 8 - 5 = 3 mg/L O₂

Step 3: Calculate GPP  
GPP = NPP + R = 2 + 3 = 5 mg/L O₂

Check with shortcut:  
GPP = Light - Dark = 10 - 5 = 5 mg/L O₂

So the answers are:

- **NPP = 2 mg/L O₂**
- **R = 3 mg/L O₂**
- **GPP = 5 mg/L O₂**

If the incubation lasted one day, you may write the units as **mg/L/day**. If it lasted one week, use **mg/L/week**.

Why does this method work? Because the light bottle shows the **net** result of making oxygen and using oxygen. The dark bottle isolates respiration because no light means no photosynthesis.

> **Test Tip:** In bottle questions, remember the quick pattern: **light change = NPP**, **dark drop = respiration**, **light minus dark = GPP**.

Common student errors in this topic:

- subtracting in the wrong direction
- calling **Light - Dark** NPP instead of GPP
- forgetting that productivity is a **rate**, so units must include time when given
- mixing up oxygen change with biomass amount without reading the question carefully

### What controls NPP in terrestrial and aquatic environmentss

NPP varies because producers need the right environmental conditions to photosynthesize and grow. The exact limiting factors differ between land and water.

On land, the most important controls are usually:

- **water availability**
- **temperature**
- **light**
- **soil nutrient supply**

Warm, wet environments usually support higher NPP because plants can photosynthesize for longer periods and grow more quickly. That is why tropical rain forests tend to have high productivity. By contrast, deserts have intense sunlight but low NPP because water is the main limiting factor. Tundra also has low NPP, but for a different reason: low temperature, short growing season, and often frozen soil.

This distinction is very important. Desert and tundra can both have low NPP, but not for the same cause.

In aquatic systems, major limiting factors are often:

- **light penetration**
- **nutrient availability**
- **depth**
- **turbidity**
- **temperature**
  > **Key Term: limiting factor**
  > A resource or environmental condition that restricts the growth, productivity, or distribution of organisms.

Light matters because photosynthesis can only happen in the photic zone. If water is deep or turbid, light does not reach producers effectively. Nutrients matter because algae and phytoplankton need nitrogen, phosphorus, and other materials for growth.

A useful APES rule of thumb:

- **freshwater** productivity is often limited by **phosphorus**
- **marine** productivity is often limited by **nitrogen**
- some open-ocean areas may also be limited by **iron**

The open ocean is a classic exam trap. It often has enough light near the surface, but nutrient supply is low across much of its area, so productivity per square meter is relatively low.

By contrast, estuaries are nutrient rich and shallow, so they often have very high productivity per unit area. The mixing of freshwater and saltwater, plus nutrient inputs from rivers and tidal action, helps maintain this high output.

Nutrient enrichment can increase productivity, but too much can create major problems. If excess nitrates or phosphates enter a lake or estuary, algal growth may rise sharply. This can seem like "good productivity" at first, but when algae die, decomposers use large amounts of oxygen. The result can be hypoxia and fish kills.

> **Key Term: eutrophication**
> The enrichment of a water body with nutrients, often causing excessive algal growth and reduced dissolved oxygen.

So higher aquatic productivity is not always a sign of environment health. You must ask whether the system is naturally productive or artificially overloaded with nutrients.

### Comparing systemss and avoiding ranking mistakes

APES questions often ask which biomess have the highest or lowest NPP. To answer well, you must notice whether the question asks about **per unit area** or **total productivity**.

Per unit area, environmentss with very high NPP usually include:

- estuaries
- swamps and marshes
- algal beds and coral reefs
- tropical rain forests

These systems combine several advantages: strong light, available water, warm temperatures, and abundant nutrients.

Per unit area, areass with low NPP usually include:

- deserts
- tundra
- open ocean

Again, notice that these low-productivity systems differ in why they are low:

- deserts: lack of water
- tundra: low temperature and short growing season
- open ocean: low nutrient availability in many regions

Now the critical distinction: the **open ocean** can still contribute a huge amount to Earth's total NPP because it covers such a large percentage of the planet's surface.

This means:

- **low per square meter**
- **high total contribution**

Students often miss the phrase **per unit area** and choose open ocean incorrectly on ranking questions.

Another major distinction is between **productivity** and **standing biomass**. A tropical rain forest has high biomass and high productivity. But some marine systems, especially open-ocean phytoplankton communities, can have **low standing biomass** and still maintain considerable productivity because turnover is fast.

Phytoplankton are produced quickly and consumed quickly. At any one moment, there may not be much biomass present, but over time they can still contribute a lot of primary production.

This is why you should not assume:

- high biomass = high productivity
- low biomass = low productivity

In APES, always ask:

1. Is this a **rate** or an **amount**?
2. Is this **per unit area** or **total global output**?
3. What factor is limiting this environment?

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Net primary productivity is the rate at which producers store energy as biomass after respiration is subtracted
- The formula for this relationship is **NPP = GPP - R**
- Gross primary productivity is the total energy or carbon fixed by photosynthesis before respiratory losses
- NPP represents the energy available for producer growth, reproduction, and consumers
- Productivity is a **rate** over area and time, not the same as standing biomass
- In light-bottle and dark-bottle investigations, **NPP = Light - Initial**, **R = Initial - Dark**, and **GPP = Light - Dark**
- Terrestrial NPP is mainly controlled by water, temperature, light, and soil nutrients
- Aquatic NPP is mainly controlled by light penetration, nutrients, depth, turbidity, and temperature
- Freshwater productivity is often limited by phosphorus, while marine productivity is often limited by nitrogen
- Estuaries are among the most productive habitatss per unit area
- The open ocean has low NPP per square meter but high total productivity because of its vast area
- High nutrient input can increase aquatic productivity but may also lead to eutrophication and oxygen depletion

## Key Terms and Concepts

- net primary productivity (NPP)
- gross primary productivity (GPP)
- respiration (R)
- biomass
- limiting factor
- eutrophication


Primary Productivity - Net primary productivity