In brief
Plankton are organisms that drift with the water instead of swimming against it. Phytoplankton are tiny plant-like drifters that make food from sunlight. Zooplankton eat them, fish eat zooplankton, and so on. Energy is lost at each step, so there is much more plankton than fish and much more fish than whales.
What it is
The word comes from a Greek word for “drifter”. Plankton are defined by how they live, not by what they are: a plankton organism cannot swim strongly enough against currents. They range from bacteria to jellyfish.
| Kind | What they do | Examples |
|---|---|---|
| Phytoplankton | Make food from sunlight, water and carbon dioxide by photosynthesis | Diatoms (glass-like shells), dinoflagellates, tiny cyanobacteria |
| Zooplankton | Eat phytoplankton, bacteria or each other | Copepods, krill, small jellyfish |
| Larvae (temporary plankton) | Young of animals that live elsewhere as adults and drift while growing | Larvae of barnacles, crabs, mussels, fish |
| Bacteria and viruses | Break down dead matter and recycle nutrients | Many kinds; too small to see |
Phytoplankton are tiny but extremely numerous. Together they carry out roughly half of all photosynthesis on Earth, and they produce a large share of the oxygen in the air. Copepods, small crustaceans a few millimetres long, are among the most numerous animals on the planet.
How energy moves
A food chain is a single line of who eats whom. A food web is the whole tangled network. Each level in a web is a trophic level, and only part of the energy at one level reaches the next, because organisms use much of it to live and keep warm. A common rule of thumb says about a tenth passes on; real values vary widely, roughly from a few per cent to about a fifth.
| Trophic level | Role | Example |
|---|---|---|
| Producers | Make food from sunlight | Diatoms, other phytoplankton, seagrass, seaweed |
| Primary consumers | Eat producers | Copepods, krill, mussels, sea snails |
| Secondary consumers | Eat primary consumers | Small fish such as herring and anchovy; crabs |
| Higher consumers | Eat smaller fish and other animals | Larger fish, seabirds, seals, whales |
| Decomposers | Break down dead matter and return nutrients | Bacteria and fungi in water and sediment |
Even tiny bacteria are part of the web. Water-column microbes use dissolved matter that leaks from plankton, and in turn are eaten by tiny grazers. This loop of recycling, called the microbial loop, was recognised in the early 1980s and changed how scientists picture the sea.
Each day, many zooplankton and small fish rise toward the surface at night to feed and sink to deeper, darker water by day to avoid being seen. This daily vertical migration is one of the largest movements of animals on Earth.
Why blooms happen
In temperate seas phytoplankton do not grow at a steady rate. In winter there are plenty of nutrients, because storms mix the water, but not enough light. In spring the days lengthen and the surface warms, and the water layers. Phytoplankton then multiply quickly in a spring bloom, until nutrients near the surface run low. Zooplankton and fish larvae time their breeding to meet it.
Places where winds or currents bring cold, nutrient-rich water from the depths to the surface, called upwelling zones, have rich plankton and some of the world’s great fisheries. In some open-ocean regions phytoplankton are limited not by nitrogen but by iron.
Why it matters to a coast
- Shore animals start as plankton. Barnacles, mussels and crabs release larvae into the water, which drift before settling on a shore. A bad year for plankton can mean a thin year for the shore.
- Harmful blooms. Some phytoplankton produce toxins that can poison shellfish and the people who eat them. Very large blooms can die and rot, using up the oxygen in the water.
- Carbon. Dead plankton and waste sink in a slow rain, carrying carbon to the deep sea. This is called the biological pump.
- Climate change. Warming is shifting where plankton live, with many species moving poleward, and can change when blooms happen. Predators that depend on that timing can then miss their meal.
Field exercise
Draw a food web for a shore you know with at least ten organisms, using arrows that point from the food to the eater. Then remove one organism and trace what happens upstream and downstream. Mark which links you are sure about and which are guesses.
Check yourself
Read the question, think of your answer, then open it with a tap or the Enter key.
What does “plankton” mean?
Drifter. Plankton are organisms that cannot swim strongly against currents.
What is the difference between phytoplankton and zooplankton?
Phytoplankton make food from sunlight; zooplankton eat other organisms.
What is a trophic level?
A step in the food web, such as producer or primary consumer.
Why are there fewer top predators than small fish?
Only part of the energy passes from one level to the next, so each level can support less life than the one below.
Why do spring blooms happen in temperate seas?
Days lengthen and the surface warms and layers, so light is enough and the winter’s nutrients are still present near the surface.
What is upwelling?
The rising of cold, nutrient-rich water to the surface, which feeds large amounts of plankton.
Basis of the facts
Where these facts come from. The roughly-half share of photosynthesis is from Field and others (1998); the other figures follow standard texts such as Lalli and Parsons (1997). The ten per cent rule is stated as a rule of thumb with its wide range. The microbial loop is from Azam and others (1983). Plankton shifts under climate change are reviewed in Hays and others (2005). See the sources list.