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# Ocean Zones

Introduction: Imagine climbing into a submersible and traveling to the bottom of the deepest parts of the ocean. How deep could you go before the sun no longer passed through the water column? How would animal life change with ocean depth? Oceanographers, scientists who study the ocean, divide the ocean water column into different zones. These zones are determined mainly by depth and amount of sunlight. The diagram shows the five major pelagic zones, or water environments, of the open ocean.

The epipelagic zone is the surface layer of the ocean. It includes the upper 200 meters or so of the ocean. Sunlight penetrates through this region so it is sometimes called the sunlight or photic zone. Here, the sun shines through water and photosynthesis can occur. The epipelagic zone contains only 2 - 3% of the volume of ocean water. However, microscopic phytoplankton thrive in these sunlit ocean waters. They are the base of the ocean food web, helping make this zone home for a wide variety of fish and mammals.

The mesopelagic zone goes from the bottom of the epipelagic zone to about 1,000 meters deep. The amount of sunlight that passes through this layer decreases quickly. This region is also called the twilight or disphotic zone because of this. Photosynthesis cannot occur in the mesopelagic zone leaving animals largely without plants as a food source. Bioluminescent life, able to produce light, appears in this zone. Average water temperatures drop sharply as you descend through the mesopelagic zone. This area of temperature changes from warm to cold waters is called the thermocline.

The bathypelagic zone ranges from the bottom of the mesopelagic zone to about 4,000 meters deep. Sunlight does not reach this region or the zones below it, so they are sometimes referred to as the midnight or aphotic zone. It is both dark and cold here. Water temperatures are a fairly stable at a frigid $4degC$. Fewer animal species are found in the bathypelagic zone then the zones above it. Animals here depend on food sinking from above or on other animals. Squid and angler fish are two familiar species found in this zone.

The abyssopelagic zone extends from the bottom of the bathypelagic zone to around 6,000 meters deep, or to what we often think of as the ocean bottom. It is the largest ocean zone in terms of volume of water. Despite the lack of light and extreme pressures, life still exists in this zone. Species like the giant tube worm that live near deep hydrothermal vents, provide us with insight into the extreme conditions under which life can survive.

The hadalpelagic zone is the deepest of the ocean zones. This zone includes the deepest ocean trenches. Even though we know relatively little about this zone compared to the layers above it, expeditions have confirmed that life still exists at these depths!

Directions for This Lesson: Try the practice questions to see what you know about the pelagic zones of the ocean. Then, apply what you learned in the activity and practice sections.

Activity:
Explore the depths of the ocean as you travel to the Mariana Trench with this interactive diagram.

Practice:
Practice what you have learned by completing the post-lesson worksheet.

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