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Ocean Zones

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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 (10,916 meters). This depth remains a record for a manned descent. The film director, James Cameron, got to 10,898 meters in his one-man vessel, the Deepsea Challenger, in 2012. 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 ).

Diagram of the vertical and horizontal ocean zones

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?

Making Connections

Explore More

Use this resource to answer the questions that follow.

https://www.youtube.com/watch?v=sNPHuUvW2Nk

  1. What is common to all of the benthic zones? What are the benthic subzones from shallow to deep?
  2. What does pelagic mean? What are the pelagic zones from shallow to deep?
  3. Why does the neritic zone have the highest density of productivity, density of life and nutrient levels?
  4. Why does the epipelagic zone have the most productivity?
  5. Why do organisms go between zones?
  6. Where does the deep oceans get nutrients?
  7. What is unique about the littoral zone?
  8. How do conditions vary between the top and bottom of the littoral zone?

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?

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