Aquatic Ecosystems

Objectives

B.4.1 Limiting factors.

4.5.1 Discuss the factors that affect aquatic ecosystems. THINK ABOUT IT We call our planet “Earth,” yet nearly threefourths of Earth’s surface is covered with water. Despite the vital roles aquatic ecosystems play in the biosphere, many of these ecosystems are only partly understood. What’s life like underwater?

Conditions Underwater
What factors affect life in aquatic ecosystems?

Like organisms living on land, underwater organisms are affected by a
Aquatic organisms are affected variety of environmental factors. primarily by the water’s depth, temperature, flow, and amount of dissolved nutrients. Because runoff from land can affect some of these factors, distance from shore also shapes marine communities.

Water Depth Water depth strongly influences aquatic life because sunlight penetrates only a relatively short distance through water, as shown in Figure 4–20. The sunlit region near the surface in which photosynthesis can occur is known as the photic zone. The photic zone may be as deep as 200 meters in tropical seas, but just a few meters deep or less in rivers and swamps. Photosynthetic algae, called phytoplankton, live in the photic zone. Zooplankton—tiny free-floating animals—eat phytoplankton. This is the first step in many aquatic food webs. Below the photic zone is the dark aphotic zone, where photosynthesis cannot occur.
Many aquatic organisms live on, or in, rocks and sediments on the bottoms of lakes, streams, and oceans. These organisms are called the benthos, and their habitat is the benthic zone. Where water is shallow enough for the benthos to be within the photic zone, algae and rooted aquatic plants can grow. When the benthic zone is below the photic zone, chemosynthetic autotrophs are the only primary producers.

Key Questions
What factors affect life in aquatic ecosystems?
What are the major categories of freshwater ecosystems? 4.5.2 Identify the major categories of freshwater ecosystems. 4.5.3 Describe the importance of estuaries.
4.5.4 Describe and compare the distinct ocean zones that make up marine ecosystems.

Why are estuaries so important? How do ecologists usually classify marine ecosystems?

Vocabulary photic zone • aphotic zone • benthos • plankton • wetland • estuary

Taking Notes
Compare/Contrast Table As you read, note the similarities and differences between the major freshwater and marine ecosystems in a compare/ contrast table.

LESSON 4.5

Getting Started

Student Resources
Study Workbooks A/B, 4.5 Worksheets
Spanish Study Workbook, 4.5 Worksheets
Lesson Overview • Lesson Notes
• Activity: Art Review • Assessment: Self-Test,
Lesson Assessment
F
 or corresponding lesson in the
Foundation Edition, see pages 96–99.

Answers
FIGURE 4–20 Sample answer: Photic zones vary in depth depending on how clear the water is. The clearer the water, the deeper sunlight can penetrate.
Figure 4–20 The Photic Zone

Sunlight penetrates only a limited distance into aquatic ecosystems. Whatever the depth of this photic zone, it is the only area in which photosynthesis can occur. Infer Why do you think some photic zones are only a few meters deep and others are as much as 200 meters deep?

Lesson 4.5

• Lesson Overview

• Lesson Notes

INDIANA ACADEMIC STANDARDS
117

For the full text of all standards, see the Course
Overview in the front matter of this book.

Teach for Understanding
ENDURING UNDERSTANDING The existence of life on Earth depends on interactions

among organisms and between organisms and their environment.
GUIDING QUESTION What are the characteristics of aquatic ecosystems?

B.4.1 Explain that the amount of life an environment can support is limited by the available energy, water, oxygen, and minerals, and by the ability of ecosystems to recycle the remains of dead organisms. EVIDENCE of UNDERSTANDING