The living ocean
0 of 3 hands-on
01 Introduction
The ocean covers about seven tenths of the planet, and it is home to almost every major group of animals that has ever existed. It is also where life on Earth began. Divers get to visit a thin, brightly lit slice of this world, and the more you understand it, the more you see.
This first lesson gives you the big picture: where the life is, what it needs and how the whole system hangs together.
Objectives
- Describe the ocean as a living system and the main layers of life in it
- Explain why most marine life lives near the surface and the coast
- Describe the three roles in an ecosystem: producers, consumers and decomposers
02 Watch
A planet of water
4 min · Video in production
- 1
Open on Earth from space, with the oceans highlighted, then drop through the sunlit surface into a coral reef.
- 2
Show a vertical slice of the ocean with the sunlit, twilight and dark zones and examples of the animals in each.
- 3
Show a sunbeam hitting plankton, and the plankton feeding a small fish, which is eaten by a larger one.
- 4
End with a diver hovering above a reef, small against the scale of the living system.
03 Learn
A salty world
Seawater is on average about 3.5% salt, which is why it is denser than fresh water and why the physics of buoyancy changes between the sea and a lake. Temperature, light, pressure and salinity differ across the ocean, and every species is adapted to a particular combination of them.
Layers of life
| Zone | Approximate depth | What it is like |
|---|---|---|
| Sunlit (epipelagic) | 0 to about 200 m | Enough light for photosynthesis. Most marine life, and nearly all recreational diving, happens here. |
| Twilight (mesopelagic) | About 200 to 1,000 m | A dim blue glow, too little light for photosynthesis. Many animals here make their own light. |
| Dark (below 1,000 m) | Below about 1,000 m | No sunlight, cold and under great pressure. Life is sparse but surprisingly varied. |
Light is the key. In the sunlit zone, tiny drifting plants make food from sunlight, and nearly everything else in the ocean depends on them, directly or indirectly. This is why the shallow, clear, coastal waters are so crowded with life, while the great open ocean can be strikingly empty.
Predict first
Why is most marine life found near the surface and close to coasts?
Roles in the ecosystem
- Producers
- Organisms that make their own food from light or chemicals, such as phytoplankton, seaweed and seagrass.
- Consumers
- Animals that eat other organisms. Herbivores eat plants and algae, carnivores eat animals and omnivores eat both.
- Decomposers
- Bacteria and other organisms that break down dead material and return nutrients to the water.
The oxygen we breathe
Phytoplankton and other marine producers make a large share of the oxygen in the atmosphere, often quoted as roughly half. Every second breath you take has a link to the ocean, even on land.
Naming life
Scientists group living things in a nested system: kingdom, phylum, class, order, family, genus and species. Each species has a two-part Latin name, for example the green turtle, Chelonia mydas. Common names can vary by region, so the Latin name is how researchers know they are talking about the same animal.
04 Explore
Try it yourself
Match each zone with its description.
Match each zone with what it is like.
- Sunlit zone
- Twilight zone
- Dark zone
- Producers
- Decomposers
05 Apply
Scenario
A friend visiting the coast says: "The sea is a huge blue desert. Surely most of the life is in the deep, where nobody goes."
What is the best response?
06 Check
07 Summary
Key points
- The ocean has a sunlit upper zone, a dim twilight zone and a dark deep.
- Most marine life is in the sunlit zone and near coasts, powered by light.
- Ecosystems have producers, consumers and decomposers.
- Marine producers make a large share of the oxygen we breathe.
Before you apply this in the water
- Have I been trained by a qualified instructor for this?
- Am I medically fit to dive?
- Do I have a buddy and a plan?
- Am I diving within my training and limits?
Answer every question in the Check section to mark this lesson complete.