Most Things Biology

1.4 Blue, Then Black

Time To Read

3–4 minutes

Date Last Modified

Place 01 · The Great Barrier Reef · Chapter 4 of 7

Blue, Then Black

Take a red fish to 100 feet and it turns gray. Nothing happened to the fish.

About 15 minutes · 1 video · 1 virtual dive · 1 quick activity

Start here

Seawater eats sunlight, one color at a time

Sunlight entering the ocean does not fade evenly. It gets picked apart, color by color, in a strict order. Red goes first, blue goes last, and by about 200 meters there is almost nothing left. Everything about where a reef can live follows from that.

A scuba diver holding a bright torch in deep, dim blue water.
Photo: Maël BALLAND / Pexels
Watch this

See it move

Scuba Diving International — Color Changes at Depth. The same colors, filmed deeper and deeper.

While you watch, look for

  • Which color disappears first
  • What a diver’s light does to the colors
  • Something you did not expect. Hold onto it.
The short version

Three things going on here

A bright red reef fish swimming beside orange coral.
The order

Red dies first

Red light is mostly gone within the first 5 to 10 meters, then orange, then yellow. Blue travels deepest, which is the real reason the ocean looks blue.

Sunbeams shining down onto a shallow coral reef.
The zone

The sunlit layer

Only the top couple of hundred meters get enough light for photosynthesis. It is a thin, bright skin over an enormous dark ocean.

Deep blue water with almost no light, looking into the depths.
The consequence

Reefs must be shallow

Corals depend on algae that need light. That is why reefs are shallow, and why muddy water can kill a reef without ever touching it.

Photos: Jeffry Surianto, Francesco Ungaro, Diego F. Parra / Pexels

Play with it

Dive the reef with David Attenborough

This is a 360° video. Click and drag inside it, or move your phone, to look in every direction. Pay attention to the colors as you look up toward the surface and down into the deep.

Video: David Attenborough’s Great Barrier Reef Dive, Atlantic Productions, on YouTube.

Two questions to answer while you are in there

  1. Look toward the surface, then down into the deep. What color is everything far away?
  2. Find a red or orange animal. Does it look as red as it would on land?
Try it yourself

Check yourself

Try it

Reef Depth Dive

Click each hotspot to dive down the water column and find out what light is left at every depth.

Try it · interactive

Depth Dial

Dive to 200 meters and watch the reef lose its colors, one at a time.

Open the Depth Dial in its own window (opens in a new tab).

Words you will hear

Four terms, that is all

Attenuation

Light weakening as it travels through water

Photic zone

The sunlit layer at the top

Aphotic zone

Where no useful light reaches

Turbidity

Cloudiness that blocks light

Check your understanding

Practice the whole chapter

Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.

Practice

Practice: Blue, Then Black

Eight questions covering the video, the three cards, the four terms and the interactive above.

If you want more

Go deeper

← Chapter 3: The Warm Belt
Chapter 5: Building With a Dissolved Gas →

Photo credits (Pexels): Pixabay, Maël BALLAND, Jeffry Surianto, Francesco Ungaro, Diego F. Parra.

Hop to:

← The Great Barrier Reef — place overview

All Places
  1. The Great Barrier Reef
  2. Monterey Bay
  3. The Galápagos
  4. The Ridge and the Trench — in development
  5. The Southern Ocean — in development
  6. The North Pacific Garbage Patch — in development
All pages for Place 01 — The Great Barrier Reef