Plankton and the base of the food web
0 of 4 hands-on
01 Introduction
The most important living things in the ocean are mostly too small to see. Plankton, the drifting life of the sea, feeds almost everything else. If you want to understand why a reef is busy or why a whale shark turns up in a particular month, start here.
Objectives
- Distinguish phytoplankton from zooplankton
- Describe a food chain and a food web, and what trophic levels are
- Explain why energy is lost at each step of a food chain
02 Watch
Life on the drift
4 min · Video in production
- 1
Show a drop of seawater magnified to reveal diatoms, copepods and fish larvae.
- 2
Show phytoplankton absorbing sunlight, and a copepod eating them.
- 3
Show a small fish eating copepods, a larger fish eating the small fish, and a shark above.
- 4
Show a plankton bloom turning the sea green, and filter feeders arriving to feed.
03 Learn
What is plankton?
Plankton are organisms that drift with the currents instead of swimming against them. The word comes from the Greek for 'wanderer'. They range from single-celled algae to the larvae of fish and crabs, and some jellyfish, which can be very large.
| Group | What it is | Examples |
|---|---|---|
| Phytoplankton | Plant-like plankton that photosynthesise | Diatoms, dinoflagellates, cyanobacteria |
| Zooplankton | Animal-like plankton that eat other plankton | Copepods, krill, jellyfish, tiny shrimp |
| Larvae (meroplankton) | Young stages of animals that live elsewhere as adults | Larvae of fish, crabs, corals, starfish |
Phytoplankton are the main producers of the ocean. They use sunlight, carbon dioxide and nutrients to make food and release oxygen. Zooplankton graze on them and are in turn eaten by fish and many other animals.
Food chains and food webs
A food chain shows who eats whom, in a line. In real life, most animals eat several things and are eaten by several others, so the chains join up into a food web.
- 1.Phytoplankton make food from sunlight (producers).
- 2.Zooplankton eat the phytoplankton (primary consumers).
- 3.Small fish eat the zooplankton (secondary consumers).
- 4.Larger fish eat the small fish (tertiary consumers).
- 5.Top predators such as sharks, tuna and dolphins eat the larger fish.
- Trophic level
- A feeding level in a food chain, counting up from the producers.
- Herbivore
- An animal that eats plants or algae.
- Carnivore
- An animal that eats other animals.
- Apex predator
- A top predator that has few or no natural predators as an adult.
- Filter feeder
- An animal that strains tiny food from the water, such as a sponge, a manta ray or a baleen whale.
Energy and the shape of the web
At each step up a food chain, most of the energy is used by the animal for living, moving and keeping warm, or is lost as heat. Only roughly a tenth is available to the next level. This is why there are always far more small animals than large predators, and why food chains are short.
Take a guess first
A patch of sea holds 10,000 units of energy in its phytoplankton. If about 10% passes up at each step, how many units are available to the third consumer level, three steps up?
Predict first
What happens to the animals higher in the food chain if the phytoplankton below them decline?
Blooms and upwelling
When nutrients are plentiful and light is good, phytoplankton can multiply rapidly in a bloom. This often happens where cold, nutrient-rich deep water rises to the surface, known as upwelling. Blooms draw in zooplankton, fish and larger animals, which is why some places have seasonal visits by whale sharks, manta rays and whales.
Light in the water
Some plankton and many deep animals produce their own light, called bioluminescence. At night in some seas, plankton glow when the water is disturbed, and you may see sparks around your hands and fins.
04 Explore
Try it yourself
Put this food chain in order, from producer to top predator.
Put the food chain in order, starting from the producer.
- 1Small fish
- 2Zooplankton (copepods)
- 3Phytoplankton
- 4Shark
- 5Larger fish
05 Apply
Scenario
A dive guide explains that visibility at a site is poor this week, with green, cloudy water, and apologises. The diver notices several manta rays circling slowly near the surface.
What is the best explanation for what is happening?
06 Check
07 Summary
Key points
- Plankton drift with currents. Phytoplankton are the producers, zooplankton eat them.
- Food chains link into food webs, and every animal depends on the base.
- Only about 10% of energy passes up each level, so predators are few.
- Blooms and upwelling concentrate food and draw in larger animals.
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.