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10.6: Fresh Water Ecosystems

Difficulty Level: At Grade Created by: CK-12
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Why did people used to rush to fill in swamps?

People didn't know the value of wetlands. Many are in locations that might be desirable for people to live, like near a shoreline. Mosquitoes, which no one seems to like, breed there. But wetlands serve a number of valuable purposes. They are breeding grounds for many organisms and they protect inland areas from storms. Now wetlands are protected.

Freshwater Ecosystems

Organisms that live in lakes, ponds, streams, springs or wetlands are part of freshwater ecosystems. These ecosystems vary by temperature, pressure (in lakes), the amount of light that penetrates and the type of vegetation that lives there.

Lake Ecosystems

Limnology is the study of bodies of fresh water and the organisms that live there. A lake has zones just like the ocean. The ecosystem of a lake is divided into three distinct zones (Figure below):

1. The surface (littoral) zone is the sloped area closest to the edge of the water.

2. The open-water zone (also called the photic or limnetic zone) has abundant sunlight.

3. The deep-water zone (also called the aphotic or profundal zone) has little or no sunlight.

There are several life zones found within a lake:

  • In the littoral zone, sunlight promotes plant growth, which provides food and shelter to animals such as snails, insects, and fish.
  • In the open-water zone, other plants and fish, such as bass and trout, live.
  • The deep-water zone does not have photosynthesis since there is no sunlight. Most deep-water organisms are scavengers, such as crabs and catfish that feed on dead organisms that fall to the bottom of the lake. Fungi and bacteria aid in the decomposition in the deep zone.

Though different creatures live in the oceans, ocean waters also have these same divisions based on sunlight with similar types of creatures that live in each of the zones.

Diagram of the littoral, open-water, and deep-water zones in a lake

The three primary zones of a lake are the littoral, open-water, and deep-water zones.


Wetlands are lands that are wet for significant periods of time. They are common where water and land meet. Wetlands can be large flat areas or relatively small and steep areas.

Wetlands are rich and unique ecosystems with many species that rely on both the land and the water for survival. Only specialized plants are able to grow in these conditions. Wetlands tend have a great deal of biological diversity. Wetland ecosystems can also be fragile systems that are sensitive to the amount and quality of water present within them.


Marshes are shallow wetlands around lakes, streams, or the ocean where grasses and reeds are common, but trees are not (Figure below). Frogs, turtles, muskrats, and many varieties of birds are at home in marshes.

A salt marsh on Cape Cod in Massachusetts

A salt marsh on Cape Cod in Massachusetts.


A swamp is a wetland with lush trees and vines found in low-lying areas beside slow-moving rivers (Figure below). Like marshes, they are frequently or always inundated with water. Since the water in a swamp moves slowly, oxygen in the water is often scarce. Swamp plants and animals must be adapted for these low-oxygen conditions. Like marshes, swamps can be fresh water, salt water, or a mixture of both.

A swamp is characterized by trees in still water

A swamp is characterized by trees in still water.

Ecological Role of Wetlands

As mentioned above, wetlands are home to many different species of organisms. Although they make up only 5% of the area of the United States, wetlands contain more than 30% of the plant types. Many endangered species live in wetlands, so wetlands are protected from human use.

Wetlands also play a key biological role by removing pollutants from water. For example, they can trap and use fertilizer that has washed off a farmer’s field, and therefore they prevent that fertilizer from contaminating another body of water. Since wetlands naturally purify water, preserving wetlands also helps to maintain clean supplies of water.


  • The conditions that affect lake ecosystems are similar to those that affect marine ecosystems, such as light penetration, temperature and water depth.
  • Wetlands are lands that are wet for a significant portion of the year.
  • Wetlands are extremely important as an ecosystem and as a filter for pollutants.

Making Connections

Explore More

Use this resource to answer the questions that follow.

https://www.youtube.com/watch?v=-VP5WNZK6JM Stop at 10:40

  1. What are the three main kinds of water biomes?
  2. What are the two types of freshwater biomes?
  3. How does the geology of a region affect the ecology of a river?
  4. What grows in slow-moving rivers that won't grow in fast rivers?
  5. Why is there plenty of oxygen in a moving river?
  6. Why don't most lakes become saline?
  7. What are bogs?
  8. How are swamps and marshes different from bogs? How are marshes different from swamp?
  9. How do living things last through a winter in a lake if the top of the lake freezes?
  10. What happens to the lake when the ice melts in the spring?
  11. Why is the lake temperature stratified in the summer?
  12. How much circulation is in the lake in autumn?
  13. What brings nutrients into a lake?
  14. What is a young lake with little food called
  15. Is a lake that has a lot of plants at the top teeming with life?


  1. Describe how ecological zones in lakes are similar to ecological zones in oceans.
  2. For many decades, people drained wetlands. Was this a good idea or a bad idea? Why?
  3. How are marshes different from swamps? How are they the same?




The study of freshwater bodies and the organisms that live in them.


A shallow wetland with grasses and reeds, but there no trees. Water may be fresh, saline, or brackish.


A low-lying wetland where water moves very slowly and oxygen levels are low.


Lands that are wet a large amount of the time.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
May 18, 2016
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