Life of plankton: what they are
Plankton are small living things that float in water. They are too weak to swim against waves and currents, so they drift. Many are so small that you need a microscope. A few, like some jellyfish, are big.
Plankton live in the top 100 to 200 metres of the sea, where light can reach. They are also in lakes and rivers.
Phytoplankton: the sea's farmers
Phytoplankton have a green pigment (chlorophyll). In sunlight they take in carbon dioxide and water and make sugar. This is photosynthesis. They also give out oxygen. They are the producers of the sea.
They need two things: light and nutrients (like nitrates and phosphates). When both are plenty, they multiply very fast. This is a bloom. Some blooms (called red tide) make poison and can kill fish.
Zooplankton: the small grazers
Zooplankton cannot make food. They eat phytoplankton or other zooplankton, so they are consumers. They also include the baby stages (eggs and larvae) of fish, crabs and other sea animals.
Many zooplankton make a daily trip called vertical migration: up at night to eat, down by day to hide from fish that hunt by sight.
Main plankton you should know
- Diatoms: phytoplankton with a glass-like (silica) shell. Very common in cold, rich water.
- Dinoflagellates: phytoplankton with two whip-like tails. Some glow at night; some cause red tide.
- Copepods: tiny shrimp-like zooplankton, the most numerous animals in the sea.
- Krill: small shrimp-like zooplankton, food of whales and penguins.
- Larvae: baby fish, crabs and starfish that drift for a while.
- Jellyfish: big drifters, also counted as plankton because they cannot swim against currents.
Why plankton matter
1. Phytoplankton make about half of the world's oxygen. 2. They are the base of the marine food web. 3. They take in carbon dioxide and so help control climate. 4. Where plankton is rich, fish catches are rich. 5. Dead plankton sink and become part of the sea-floor mud. Only about 10% of energy passes from one feeding level to the next, so plankton must be very many to feed one big fish.
Try it
In the 3D, move the slider to zero and then to full. Count the green dots. Predict: what happens to the orange dots if the green dots disappear? At home: leave a glass jar of pond water in the sun for a week. The water turns green: phytoplankton are blooming. (Do not drink it.)
Key formulas and definitions
- Key term: plankton = drifters; phyto = plant-like producer; zoo = animal-like consumer
- Food chain: phytoplankton → zooplankton → small fish → big fish
- Photosynthesis: carbon dioxide + water → sugar + oxygen (needs light)
- 10% rule: only about 10% of energy passes to the next level
- Bloom = light + nutrients + calm water
Worked examples
1. A drop of sea water holds a green, glass-shelled cell. Which plankton group is it?
It is green, so it makes food: phytoplankton. The glass (silica) shell tells us it is a diatom.
2. Why do zooplankton sink down by day and come up at night?
Fish hunt by sight. By day, deep dark water hides zooplankton. At night they are safe at the top, where the phytoplankton food is.
3. Phytoplankton in a farm pond hold 1000 units of energy. How much reaches zooplankton, and then small fish (10% rule)?
Zooplankton: 10% of 1000 = 100 units. Small fish: 10% of 100 = 10 units.
Common mistakes
- Thinking all plankton are plants. Zooplankton are animals.
- Thinking plankton are always tiny. Jellyfish are big and still drift, so they count as plankton.
- Thinking plankton live in the deep dark sea. Phytoplankton need light, so they live near the top.
- Thinking every bloom is good. Some blooms (red tide) are poisonous and use up oxygen.