9.3: Ocean Zones

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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 (Figure below) based on these two factors. The ocean floor makes up another zone.

Diagram of the vertical and horizontal 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.

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 forms in the oceans live in, or at least visit, the surface.

Review

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

Explore More

Use the resource below to answer the questions that follow.

Where can 90% of the of the ocean's life be found?
Where is the twilight zone? What lives there?
Where is the dark zone? What lives there?
Why is little life found in the dark zone?
What is the abyss? Where is it found?
Why are trenches so deep?