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# 8.12: Ocean Zones

Difficulty Level: At Grade Created by: CK-12
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There's a trench in the bottom of the sea. Would you like to visit it?

In 1960, two men in a specially designed submarine called the Trieste descended into a submarine trench called the Challenger Deep (10,910 meters). The depth of this dive remains a record. No craft exists today that can reach that depth. Would you like to go to the bottom of the ocean in that vessel?

### Divisions of the Ocean

Oceanographers divide the ocean into zones both vertically and horizontally.

#### Vertical Divisions

To better understand regions of the ocean, scientists define the water column by depth. They divide the entire ocean into two zones vertically, based on light level. Large lakes are divided into similar regions.

• Sunlight only penetrates the sea surface to a depth of about 200 m, creating the photic zone ("photic" means light). Organisms that photosynthesize depend on sunlight for food and so are restricted to the photic zone. Since tiny photosynthetic organisms, known as phytoplankton, supply nearly all of the energy and nutrients to the rest of the marine food web, most other marine organisms live in or at least visit the photic zone.
• In the aphotic zone there is not enough light for photosynthesis. The aphotic zone makes up the majority of the ocean, but has a relatively small amount of its life, both in diversity of type and in numbers. The aphotic zone is subdivided based on depth (Figure below).

Vertical and horizontal ocean zones.

The average depth of the ocean is 3,790 m, a lot more shallow than the deep trenches but still an incredible depth for sea creatures to live in. What makes it so hard to live at the bottom of the ocean? The three major factors that make the deep ocean hard to inhabit are the absence of light, low temperature, and extremely high pressure.

#### Horizontal Divisions

The seabed is divided into the zones described above, but ocean itself is also divided horizontally by distance from the shore.

• Nearest to the shore lies the intertidal zone (also called the littoral zone), the region between the high and low tidal marks. The hallmark of the intertidal is change: water is in constant motion in the form of waves, tides, and currents. The land is sometimes under water and sometimes exposed.
• The neritic zone is from low tide mark and slopes gradually downward to the edge of the seaward side of the continental shelf. Some sunlight penetrates to the seabed here.
• The oceanic zone is the entire rest of the ocean from the bottom edge of the neritic zone, where sunlight does not reach the bottom. The sea bed and water column are subdivided further, as seen in the figure above.

### Summary

• The most important vertical distinction in the oceans is between the small surface zone that has light, the photic zone, and the entire rest of the ocean without light, the aphotic zone.
• The ocean is divided into horizontal zones based on the depth of water beneath: the intertidal, neritic, and oceanic.
• Why does most of the life in the oceans live in or at least visit the surface?

### Practice

Use this resource to answer the questions that follow.

The Layers of the Ocean

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. Do you think that the line between the photic and aphotic zones is solid and that life is either in one or the other, or do you think the divisions are more gradational? Why?

### Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes

### Vocabulary Language: English Spanish

TermDefinition
aphotic zone The zone in the water column deeper than 200 m where sunlight does not penetrate.
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.

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