Ocean Depth Zones
Understand how sunlight changes with ocean depth by studying the sunlight, twilight, and midnight zones: bright water near the surface, dim water below, and deeper water that receives no sunlight
What a learner can do afterwards
- Name the sunlight, twilight, and midnight zones in order from shallower to deeper
- Explain that sunlight decreases with depth until no sunlight reaches the water
- Explain that seaweed and tiny floating algae need sunlight to make food and grow
1 · Read
You have met many ocean animals. Ocean water can be grouped into depth zones. Here we will study three: sunlight, twilight, and midnight. During daytime, the sunlight zone near the surface is bright. The twilight zone below is dim. No sunlight reaches the midnight zone farther down. Other depth zones lie below the midnight zone.
Some deep-ocean animals give off their own light. A submarine can carry lamps. The words 'no sunlight' tell us about light from the Sun, not light from animals or lamps.
Seaweed and tiny floating algae use sunlight to make food and grow. Algae are living things found in water. The sunlight zone provides enough sunlight. The twilight zone has too little. No sunlight reaches the midnight zone, so seaweed cannot make food from sunlight there. Food made in sunlit water helps feed many ocean animals.
A research submarine travels down during daytime. It starts in bright water, passes through dim water, and reaches water with no sunlight. It has travelled through the sunlight and twilight zones into the midnight zone. Its lamps can light the water around it.
A light meter gives a number for the light it measures. To find a drop in its reading, subtract the ending number from the starting number. In our example, 7 minus 2 is 5, so the reading drops by 5 units.
Sunlight fades with depth. The sunlight zone is bright, the twilight zone is dim, and the midnight zone receives no sunlight.
A daytime dive and The bottom can be shallow or deep were written for this dataset. The linked TED-Ed video Could you survive the real Twilight Zone? is by Philip Renaud and Kenneth Kostel.
More to read
A daytime dive
Imagine lowering a camera into the ocean during daytime. Near the surface, sunlight lights the water. Lower down, the camera enters dim water in the twilight zone.
Farther down, it reaches the midnight zone. Sunlight cannot reach this water. Other depth zones lie below it too.
The camera could carry a lamp. Some deep-ocean animals also give off light. These lights do not come from sunlight reaching that depth.
Animals live in deep water even where seaweed cannot get sunlight to make food.
The bottom can be shallow or deep
The ocean floor is the bottom beneath the water. Close to shore, sunlight may reach it. In a much deeper place, the bottom receives no sunlight.
A depth zone describes a layer of ocean water. The ocean floor does not always lie in the same depth zone.
Picture a crab on a shallow, sunlit bottom and another animal on a deep, dark bottom. Both are on the ocean floor, but the water above them has different depths.
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2 · Watch
Take it off screen
Where it sits
Learn first
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.