AP Environmental Science

## Learning Outcomes

- Understand how marine biomes are divided by light, depth, and distance from shore
- Identify the roles of **phytoplankton** and other algae as major marine primary producers
- Explain why marine productivity varies between coastal waters, open ocean, and upwelling regions
- Apply the formula **NPP = GPP - respiration** in a marine context
- Describe how nutrient pollution can cause harmful algal blooms, hypoxia, and dead zones
- Explain the relationship between corals and symbiotic algae and why bleaching occurs

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand marine biomes and the ecological role of algae, with a focus on the following syllabus points:

- Know that marine biomes include coastal and open-ocean systems and are structured by light, depth, and nutrient availability
- Identify the **photic/euphotic zone**, **aphotic zone**, **neritic zone**, **oceanic zone**, and **benthic zone**
- Explain that **phytoplankton** are major marine primary producers and form the base of many marine food webs
- Understand how **upwelling** brings cold, nutrient-rich water to the surface and increases productivity
- Explain why coastal and neritic waters are usually more productive than much of the open ocean
- Use **NPP = GPP - respiration** to interpret marine productivity
- Understand how excess **nitrogen** and **phosphorus** can cause harmful algal blooms, hypoxia, and dead zones
- Know that corals depend on symbiotic algae and that loss of these algae leads to coral bleaching

## 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. Why are phytoplankton so important in marine food webs?
2. How does upwelling affect marine productivity and fisheries?
3. What is the difference between the neritic zone and the oceanic zone?
4. Why can the open ocean have high total productivity but lower productivity per square meter than coastal waters?
5. How can nutrient pollution lead to a marine dead zone?

## Introduction

Marine biomes are the largest aquatic systems on Earth. They cover most of the planet’s surface, influence climate, drive major biogeochemical cycles, and support food webs that range from microscopic plankton to whales. For AP Environmental Science, the most testable idea is not just that oceans are big, but that life in them depends strongly on **light**, **nutrients**, and **water movement**.

Within this topic, algae matter more than students often realize. In many marine systems, especially sunlit surface waters, microscopic algae called phytoplankton are the main producers. They capture solar energy, remove carbon dioxide from water, release oxygen, and support higher trophic levels. At the same time, algae can also become part of environmental problems when nutrient pollution drives harmful blooms.

> **Key Term: marine biome**
> A saltwater aquatic biome that includes coastal waters, the open ocean, coral reefs, estuaries, and deep-ocean habitats.

### Marine zones and why location matters

Marine biomes are divided into zones because conditions are not the same everywhere in the ocean. The exam often tests whether you can sort zones by **light**, **distance from shore**, and **position on the seafloor**.

> **Key Term: photic zone**
> The sunlit upper layer of ocean water where enough light is available for photosynthesis.
>
> **Key Term: aphotic zone**
> The deeper ocean layer where there is not enough light for photosynthesis.

The photic zone is where most marine photosynthesis happens. That matters because photosynthesis is the entry point of energy into most marine food webs. If there is no light, then algae cannot make sugars from carbon dioxide and water, so organisms deeper down must rely on food drifting down from above or, in unusual places, chemosynthesis.

A second distinction is **near shore versus open ocean**.

> **Key Term: neritic zone**
> The shallow marine zone from the low-tide mark to the edge of the continental shelf, usually well lit and productive.
>
> **Key Term: oceanic zone**
> The open ocean beyond the continental shelf, generally deeper and often less productive per unit area.

The neritic zone is usually more productive because it is shallow, receives abundant light, and often gets nutrient inputs from land, wave mixing, and nearby sediments. By contrast, much of the oceanic zone has light at the surface but limited nutrients. Students often assume that “more ocean” means “more productivity everywhere.” That is wrong. A lot of open ocean is biologically sparse compared with coastal water.

Another key term is the bottom habitat.

> **Key Term: benthic zone**
> The bottom or seafloor habitat of an aquatic system.

The benthic zone matters because dead organisms and waste sink downward. This moves nutrients and organic matter from surface waters to deeper zones. That is one reason deep marine areas can contain high nutrient levels even though they do not have photosynthesis.

You may also see **coastal zone** and **euphotic zone** in sources. For APES, treat the euphotic zone as the well-lit upper part of the photic zone, and remember that coastal systems usually support high biodiversity and productivity.

> **Test Tip:** If a multiple-choice question asks where marine photosynthesis is highest, look first for a sunlit, shallow, nutrient-rich option such as the **neritic** or **coastal** zone, not the deep ocean.

### Why algae are the foundation of marine productivity

In terrestrial habitatss, students usually picture grasses, shrubs, or trees as producers. In marine systems, the equivalent role is often played by algae, especially phytoplankton.

> **Key Term: phytoplankton**
> Microscopic photosynthetic organisms that drift in sunlit aquatic waters and act as major primary producers.

Phytoplankton are small, but their ecological role is huge. They take in carbon dioxide and water, use sunlight to build carbohydrates, and release oxygen. That means they do three jobs at once:

- they provide **food energy** for marine food webss
- they contribute to **oxygen production**
- they remove **carbon dioxide** from surface waters

This is why marine food chains often begin with algae rather than rooted plants. A simple chain might be:

phytoplankton → zooplankton → small fish → larger fish

A recent AP-style chain could look like:

algae → zooplankton → macroinvertebrates → yellow perch → rainbow trout

The key idea is that if phytoplankton productivity falls, every higher trophic level is affected. Fish populations are not just controlled by fishing pressure or predation. They are also limited from below by how much energy enters the system through primary producers.

Marine algae are not all microscopic. Larger algae also matter in coastal settings. Coral reefs, for example, depend on symbiotic photosynthetic algae.

> **Key Term: zooxanthellae**
> Photosynthetic algae that live in coral tissues and provide energy to the coral through photosynthesis.

Corals and zooxanthellae show how important algae are even in highly visible animal-dominated systems. Corals are animals, but much of their energy supply comes from these algae. The algae photosynthesize and transfer much of the resulting organic material to the coral. In return, the coral provides shelter and access to sunlight in shallow, clear water.

This is why coral reefs are highly productive despite often existing in nutrient-poor tropical waters. The system is efficient because nutrients are tightly recycled and the coral-algae partnership is strong.

> **Test Tip:** Do not say “algae are harmful” as a general rule. Most algae, especially phytoplankton, are essential producers. Only some blooms become harmful.

### What controls marine primary productivity

Marine productivity depends mainly on the interaction between **light** and **nutrients**. You need both. Light without nutrients gives limited growth. Nutrients without light do not support photosynthesis.

> **Key Term: primary productivity**
> The rate at which producers convert solar energy into chemical energy by photosynthesis.

Many students remember that oceans have high productivity overall because they cover so much of Earth. That is true in total area terms. But productivity **per square meter** is often much higher in estuaries and coastal waters than in the open ocean.

Why? Because coastal areas often combine:

- shallow water
- strong light penetration
- nutrient input from land or sediments
- mixing from tides and waves

By contrast, open-ocean surface waters can be strongly stratified. Warm surface water sits above colder deep water, and the layers do not mix well. If nutrients stay trapped below the photic zone, surface phytoplankton may be nutrient-limited.

This is where **upwelling** becomes one of the most important ideas in marine ecology.

> **Key Term: upwelling**
> The movement of deep, cold, nutrient-rich water to the ocean surface.

Upwelling links deep nutrient stores to surface photosynthesis. When winds and current patterns move surface water away, deeper water rises to replace it. That deep water contains nutrients such as nitrates and phosphates released from decomposing organic matter below.

The cause-and-effect chain is highly testable:

upwelling → more nutrients in photic zone → more phytoplankton growth → more zooplankton → more fish → productive fishery

This is why upwelling regions are often associated with rich fisheries. The nutrients allow algae to grow quickly, and that growth supports the rest of the food web.

The reverse is also important. If strong stratification reduces mixing, or if an El Niño event weakens upwelling in some regions, surface productivity may drop because nutrients do not reach the sunlit layer as easily.

### Marine productivity calculations and biomass patterns

AP Environmental Science sometimes tests productivity with formulas and data. The essential relationship is:

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

The formula is:

NPP = GPP - respiration

In marine systems, this means the useful energy available for growth and for higher trophic levels is what remains after algae use some of their captured energy for their own life processes.

Here is a worked example.

Suppose marine phytoplankton in a coastal study area have:

- **GPP = 900 g C/m²/yr**
- **respiration = 250 g C/m²/yr**

Step 1: Write the formula  
NPP = GPP - respiration

Step 2: Substitute the values  
NPP = 900 - 250

Step 3: Calculate  
NPP = 650 g C/m²/yr

So the algae store **650 g C/m²/yr** as biomass available for growth and the food web.

This matters because a higher NPP generally means more energy available for consumers such as zooplankton and fish.

A second marine pattern students often find confusing is the **inverted biomass pyramid**. In many ocean systems, producer biomass at any one moment can be lower than consumer biomass. That sounds impossible until you think about turnover rate. Phytoplankton reproduce very quickly and are eaten very quickly. Even with low standing biomass, they can support more zooplankton biomass over time because they are replaced fast.

That does **not** mean the energy pyramid is inverted. Energy still decreases upward through trophic levels.

Common student errors:

- confusing **GPP** with **NPP**
- forgetting to subtract respiration
- assuming low producer biomass means low productivity
- claiming marine energy pyramids can be inverted

### When algae become a pollution problem

Algae are usually beneficial. Problems begin when human nutrient inputs push growth far beyond normal levels.

> **Key Term: harmful algal bloom (HAB)**
> A rapid growth of algae that harms the environments, animals, or humans through toxins, shading, or oxygen depletion.
>
> **Key Term: eutrophication**
> The enrichment of water with excess nutrients, often causing algal blooms and oxygen depletion.

In marine systems, excess **nitrogen** is especially important, though phosphorus can also contribute. Nutrient sources include fertilizer runoff, sewage, manure, and stormwater.

The classic APES chain is:

fertilizer or sewage runoff → nutrient pollution → algal bloom → algae die → decomposers break them down → dissolved oxygen drops → hypoxia → dead zone

> **Key Term: hypoxia**
> Low dissolved oxygen in water.

This is a key distinction: high algal growth is not always healthy. Natural background productivity supports food webs. Human-driven over-enrichment can create harmful blooms that block sunlight, release toxins, and cause oxygen crashes after decomposition.

A special marine example is **red tide**.

> **Key Term: red tide**
> A marine harmful algal bloom, often caused by dinoflagellates, that may discolor water and produce toxins.

Red tides can kill fish and contaminate shellfish. But students often miss two points:

- not all harmful algal blooms are red
- not all algal blooms are harmful

Another algae-related issue involves coral reefs.

> **Key Term: coral bleaching**
> The loss of symbiotic algae from coral tissues, usually due to stress such as unusually warm water.

When corals are stressed, especially by ocean warming, they expel zooxanthellae. Without those algae, corals lose a major energy source and turn white. Bleaching does not always mean immediate death, but prolonged stress can kill the coral.

So algae appear in marine ecology in two very different ways:

- as essential partners and producers in healthy systems
- as drivers of ecological damage when nutrient pollution or warming disrupts normal balances

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Marine biomes are structured by light, depth, and distance from shore
- The **photic** zone supports photosynthesis, while the **aphotic** zone does not
- The **neritic** zone is usually more productive than the **oceanic** zone because it is shallow, well lit, and better supplied with nutrients
- **Phytoplankton** are major marine primary producers and form the base of many marine food webs
- Algae remove carbon dioxide, release oxygen, and transfer energy to higher trophic levels
- Marine primary productivity depends mainly on light availability and nutrient availability
- **Upwelling** brings deep, nutrient-rich water to the surface and supports productive fisheries
- **NPP = GPP - respiration** describes the useful biomass produced after producer energy use is subtracted
- Marine biomass pyramids can be inverted because phytoplankton have low standing biomass but rapid turnover
- Excess nitrogen and phosphorus can cause **eutrophication** and **harmful algal blooms**
- Decomposition of bloom material can lower dissolved oxygen and create **hypoxia** and dead zones
- Corals depend on symbiotic algae called **zooxanthellae**, and loss of these algae causes **coral bleaching**

## Key Terms and Concepts

- marine biome
- photic zone
- aphotic zone
- neritic zone
- oceanic zone
- benthic zone
- phytoplankton
- zooxanthellae
- primary productivity
- upwelling
- net primary productivity (NPP)
- gross primary productivity (GPP)
- harmful algal bloom (HAB)
- eutrophication
- hypoxia
- red tide
- coral bleaching


Aquatic Biomes - Marine biomes and algae role