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# 6.21: Ocean Zones

Difficulty Level: Basic Created by: CK-12
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If you go down in the ocean what do you see?

Imagine you're in a submersible going to the bottom of the ocean. The first thing you realize is that it's not light for very long. The next thing is that many of the creatures that live just below where it's light create their own light. This is called bioluminescence. What is the advantage of being able to light up if you live in the dark?

### Ocean Zones

In addition to the amount of salts, other conditions in ocean water vary from place to place. One is the amount of nutrients in the water. Another is the amount of sunlight that reaches the water. These conditions depend mainly on two factors: distance from shore and depth of water. Oceans are divided into zones based on these two factors. The ocean floor makes up another zone. Figure below shows all the ocean zones.

Distance from shore and depth of water define ocean zones. Which zone is on the ocean floor?

#### Zones Based on Distance from Shore

There are three main ocean zones based on distance from shore. They are the intertidal zone, neritic zone, and oceanic zone. Distance from shore influences how many nutrients are in the water. Why? Most nutrients are washed into ocean water from land. Therefore, water closer to shore tends to have more nutrients. Living things need nutrients. So distance from shore also influences how many organisms live in the water.

#### Zones Based on Depth of Water

Scientists refer to the vertical extent of ocean water as the water column. Two main zones based on depth of water are the photic zone and aphotic zone. The photic zone is the top 200 meters of water. The aphotic zone is water deeper than 200 meters. The deeper you go, the darker the water gets. That’s because sunlight cannot penetrate very far under water. Sunlight is needed for photosynthesis. So the depth of water determines whether photosynthesis is possible. There is enough sunlight for photosynthesis only in the photic zone.

Water also gets colder as you go deeper. The weight of the water pressing down from above increases as well. At great depths, life becomes very difficult. The pressure is so great that only specially adapted creatures can live there.

### Vocabulary

• aphotic zone: The zone in the water column deeper than 200 m where sunlight does not penetrate.
• bioluminescence: Light created by organisms from chemical reactions.
• intertidal zone: The part of the ocean closest to the shore, between low and high tide.
• neritic zone: The part of the ocean where the continental shelf gradually slopes seaward. Sunlight can penetrate to the bottom in much of the neritic zone.
• oceanic zone: The open ocean, where sunlight does not reach the seabed.
• photic zone: The upper 200 m of the ocean, where sunlight penetrates.
• water column: A vertical column of ocean water, divided into different zones according to their depth.

### Summary

• The small surface zone that has light is the photic zone. The entire rest of the ocean does not have light and is the aphotic zone.
• The ocean is divided into horizontal zones based on the depth of water beneath: the intertidal, neritic, and oceanic.
• Most of the life in the oceans lives in or at least visits the surface.

### Practice

Use this resource to answer the questions that follow.

The Layers of the Ocean at http://www.youtube.com/watch?v=UEh9cx-b8Og (2:50)

1. Where can 90% of the of the ocean's life be found?
2. What is the twilight zone?
3. What is the dark zone?
4. Why is little life found in the dark zone?
5. What is the abyss?
6. What are trenches?

### Review

1. Why is there so little life at the bottom of the ocean?
2. Compare and contrast the intertidal, neritic, and oceanic zones.
3. What would you expect to find in the photic zone? How would you expect that to be different from the aphotic zone?

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### Vocabulary Language: English

aphotic zone

Zone in the water column deeper than 200 m where sunlight does not penetrate.

bioluminescence

Light created by organisms from chemical reactions.

intertidal zone

Part of the ocean closest to the shore, between low and high tide.

neritic zone

Part of the ocean where the continental shelf gradually slopes seaward; sunlight can penetrate to the bottom in much of the neritic zone.

oceanic zone

Open ocean, where sunlight does not reach the seabed.

photic zone

Upper 200 m of the ocean, where sunlight penetrates.

water column

Vertical column of ocean water, divided into different zones according to their depth.

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Grades:
6 , 7
Date Created:
Jan 04, 2013
Last Modified:
Aug 29, 2016
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