AP Environmental Science

## Learning Outcomes

- Distinguish between **lentic** and **lotic** freshwater systems and explain why flow matters
- Identify the main freshwater biomes, including lakes, rivers, streams, wetlands, and groundwater sources
- Explain how watersheds connect land use to freshwater quality and drinking water safety
- Understand lake stratification, the **thermocline**, and seasonal turnover
- Compare **oligotrophic** and **eutrophic** lakes and link nutrient input to oxygen conditions
- Describe how wetlands and aquifers support water supply and environmental services
- Apply these ideas to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand freshwater biomes and drinking water, with a focus on the following syllabus points:

- Know that freshwater biomes are low-salinity systems including lakes, ponds, rivers, streams, wetlands, and groundwater sources
- Identify the difference between **lentic systems** (standing water) and **lotic systems** (flowing water)
- Explain how a **watershed** or drainage basin links runoff, land use, and downstream water quality
- Understand **thermal stratification**, the **thermocline**, and **lake turnover**
- Compare **oligotrophic** and **eutrophic** lakes in terms of nutrients, productivity, clarity, and dissolved oxygen
- Describe the functions services of wetlands, including filtration, flood storage, habitat support, and possible groundwater recharge
- Distinguish **groundwater** from surface water and define **aquifer** and **aquifer recharge**
- Know that the **Safe Drinking Water Act (1974)** regulates public drinking water, not private wells serving fewer than 25 people

## 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 a lentic system and a lotic system?
2. How does runoff from a watershed reduce drinking water quality?
3. What is the thermocline, and why does it matter in lakes?
4. How does a eutrophic lake differ from an oligotrophic lake?
5. Why can groundwater pollution be harder to fix than surface water pollution?

## Introduction

Freshwater biomes matter in AP Environmental Science for two reasons at once. First, they are living systems with their own patterns of energy flow, nutrient cycling, and species interactions. Second, they are one of the main sources of human **drinking water**. That means AP questions often connect ecology to human use: a watershed is not just a landform, but the source area that affects a town's water supply.

A strong answer on this topic must do more than list lakes, rivers, and wetlands. You need to explain how water movement, nutrient levels, oxygen content, and land use shape freshwater habitatss and water quality. This is a classic APES topic because it combines natural processes with human impact.

> **Key Term: freshwater biomes**
> Low-salinity aquatic systems that include lakes, ponds, rivers, streams, wetlands, and groundwater sources.

### Types of freshwater systems and why flow matters

The first distinction to know is between standing water and flowing water. This sounds simple, but it controls oxygen, sediment, nutrient movement, and habitat structure.

> **Key Term: lentic systems**
> Standing freshwater bodies such as lakes and ponds.
>
> **Key Term: lotic systems**
> Flowing freshwater bodies such as rivers and streams.

In **lentic** systems, water stays in one place long enough for layers to form, sediments to settle, and nutrients to build up. Lakes and ponds can therefore become strongly affected by nutrient input, oxygen depletion in deep water, and seasonal mixing.

In **lotic** systems, water is always moving downhill. That movement tends to increase oxygen, especially where water is turbulent, such as in headwaters. Flow also transports sediment and dissolved substances downstream. This means a river is not just a channel of water; it is also a transport system linking the entire watershed to places farther downstream.

The source zone of a stream is usually cold, clear, narrow, and oxygen-rich. As the stream widens and slows in the transition zone, it carries more sediment and nutrients. In the floodplain zone, water is often murkier and warmer because tributaries and runoff have added more material.

This matters ecologically. Trout can survive in cold, oxygen-rich headwaters, but not every downstream zone. Species change as temperature, current speed, turbidity, and dissolved oxygen change.

It matters for drinking water too. Water taken from a fast-moving upland stream may contain fewer suspended sediments than water in a slower downstream section after a storm. But that does not automatically mean it is clean. Upstream land use still matters.

> **Test Tip:** If an AP question asks why a stream has high dissolved oxygen, look first for clues such as cold water, turbulence, or headwaters. Students often focus only on pollution and miss the role of flow.

### Watersheds, runoff, and drinking water sources

To understand freshwater biomes properly, you need to think in terms of the entire drainage area, not just the water body itself.

> **Key Term: watershed**
> The land area from which water drains to a common body of water; also called a drainage basin.

A watershed acts like a collector. Rain falls across hills, fields, forests, suburbs, roads, and farms. Some water infiltrates into soil. Some recharges groundwater. Some flows across the surface as **runoff** into streams, lakes, and reservoirs.

> **Key Term: runoff**
> Water from precipitation or snowmelt that flows over land and can carry sediment, nutrients, pesticides, oil, and other pollutants into water bodies.

This is where APES questions often become cause-and-effect chains:

land clearing -> less plant cover -> more erosion -> more sediment in runoff -> higher turbidity in streams and reservoirs -> lower light penetration and poorer water quality

Or:

fertilizer use -> nitrate and phosphate runoff -> more algal growth -> more decomposition -> lower dissolved oxygen -> fish stress or fish kills

Urban areas create another chain:

more pavement and roofs -> less infiltration -> more runoff -> more pollutant transport and flashier stream flow -> lower recharge and poorer water quality

Freshwater used as **source water** for drinking commonly comes from rivers, streams, lakes, and reservoirs. Groundwater is another major source. Both can be contaminated by watershed activities.

Sediment is a serious issue because it clouds water, increases treatment costs, and can carry attached pollutants, especially phosphorus. Nutrients are serious because they drive eutrophication. Pesticides, oils, heavy metals, and pathogens can also move through the watershed.

> **Key Term: source water**
> Surface water or groundwater used for drinking water supplies.

A common mistake is to think only point sources matter. In freshwater systems, **nonpoint source pollution** is often a bigger long-term challenge because it comes from many diffuse places at once, especially agricultural and urban runoff.

Source protection is often cheaper and more effective than relying only on treatment. If less pollution enters the river or aquifer, less has to be removed later.

> **Test Tip:** In FRQs, trace pollution step by step: land use change -> runoff or infiltration -> water quality change -> ecological or human effect. AP scorers reward clear chains, not isolated facts.

### Lakes, wetlands, and groundwater: how freshwater stores and supplies water

Lakes are major lentic systems, and APES expects you to understand both their zones and their seasonal behavior.

Lakes have biological zones such as the **littoral zone** near shore, the **limnetic zone** of open sunlit water, the **profundal zone** of deeper darker water, and the **benthic zone** at the bottom sediment. But the key AP distinction is often about temperature layers.

> **Key Term: thermal stratification**
> The formation of distinct temperature layers in a lake, usually with warmer surface water above colder deeper water.
>
> **Key Term: thermocline**
> The zone of rapid temperature change between warmer surface water and colder deep water.

In summer, many lakes stratify. Warm surface water is less dense and stays on top. Colder, denser deep water stays below. The **thermocline** acts as a barrier to mixing. That means oxygen added at the surface does not easily reach deeper layers, while nutrients in deep water do not easily rise.

This has major ecological effects. If deep water contains lots of decomposing organic matter, oxygen can fall there because decomposers keep using it. Fish adapted to cold water may then face a trade-off: deep water is cool but oxygen-poor, while surface water is oxygen-rich but too warm.

In spring and fall, temperate lakes often undergo **turnover**.

> **Key Term: lake turnover**
> Seasonal mixing of lake water, usually in spring and fall, that redistributes oxygen and nutrients.

Turnover happens because water density changes with temperature. As surface water cools or warms toward about **4°C**, it can mix more easily with deeper water. Wind then helps circulate the lake. This brings oxygen down and nutrients up.

APES often uses this to test reasoning. If turnover did not happen, deep water in many lakes would become increasingly oxygen-poor, and surface water would stay nutrient-limited.

Lakes also differ by nutrient status.

> **Key Term: oligotrophic lake**
> A lake with low nutrient levels, low primary productivity, and generally clearer, oxygen-rich water.
>
> **Key Term: eutrophic lake**
> A lake with high nutrient levels, high productivity, murkier water, and greater risk of low dissolved oxygen in deeper water.

An **oligotrophic** lake is usually clear, deeper, and relatively low in algae. An **eutrophic** lake has more nutrients, more algae and plant growth, murkier water, and often more oxygen demand from decomposition.

Do not assume eutrophic means "better" because it is more productive. High productivity can create low deep-water oxygen and fish kills, especially after algal die-offs.

Wetlands are another major freshwater biome type and an especially high-yield AP topic.

> **Key Term: wetlands**
> Areas where water saturates or covers the soil for part or all of the year.

Wetlands include marshes, swamps, bogs, floodplains, and seasonal wetlands. They are not all the same. Marshes are dominated by grasses and sedges; swamps by woody vegetation; bogs are acidic and nutrient-poor.

Wetlands provide several environmental services at once:

- they slow floodwater and store it temporarily
- they trap sediment
- they remove some pollutants and excess nutrients
- they provide habitat and nursery areas
- in some settings, they support groundwater recharge

That does not mean wetlands magically remove all pollution. Their filtering ability depends on flow, vegetation, soil, and pollutant load. But compared with a channelized, paved urban area, wetlands usually improve water quality and reduce flood damage.

Groundwater is the less visible but equally important freshwater system.

> **Key Term: groundwater**
> Water stored beneath Earth's surface in pores and fractures of soil, sediment, and rock.
>
> **Key Term: aquifer**
> A permeable underground layer that stores and transmits groundwater.
>
> **Key Term: aquifer recharge**
> The downward movement of water from the surface into the saturated subsurface across the water table.

Groundwater often supplies wells and public water systems. It is usually protected from short-term surface disturbance better than a river is, but once it is contaminated, recovery is very slow. Water can remain in an aquifer for years to centuries or longer, and pollutants are hard to remove.

That is why groundwater contamination is such a major concern. A pollutant spilled on land may infiltrate, move downward, and eventually reach an aquifer. High porosity helps storage, but permeability controls how easily water moves. Students often confuse these.

A final AP distinction: drinking water treatment is not the same as wastewater treatment. Public drinking water is regulated under the **Safe Drinking Water Act (1974)**, which authorizes EPA to regulate contaminants in public systems. It does not generally regulate private wells serving fewer than 25 people.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Freshwater biomes are low-salinity systems including lakes, ponds, rivers, streams, wetlands, and groundwater
- **Lentic systems** are standing waters, while **lotic systems** are flowing waters
- Flow in rivers and streams affects dissolved oxygen, sediment transport, and habitat conditions
- A **watershed** links land use to downstream water quality
- **Runoff** can carry sediment, nutrients, pesticides, oil, and pathogens into source water
- Lakes can develop **thermal stratification** with a **thermocline** separating warm and cold layers
- **Lake turnover** in spring and fall redistributes oxygen and nutrients
- **Oligotrophic** lakes are nutrient-poor and clearer, while **eutrophic** lakes are nutrient-rich and more prone to oxygen depletion
- Wetlands provide filtration, flood storage, habitat support, and can contribute to groundwater recharge
- **Groundwater** is stored in **aquifers**, which are replenished by **aquifer recharge**
- Groundwater pollution is difficult to remove because water moves slowly underground and contaminants persist
- The **Safe Drinking Water Act (1974)** regulates public drinking water supplies, not most small private wells

## Key Terms and Concepts

- freshwater biomes
- lentic systems
- lotic systems
- watershed
- runoff
- source water
- thermal stratification
- thermocline
- lake turnover
- oligotrophic lake
- eutrophic lake
- wetlands
- groundwater
- aquifer
- aquifer recharge


Aquatic Biomes - Freshwater biomes and drinking water