What Are Plankton?

The word plankton comes from Greek words that mean drifter or wanderer. A living thing is plankton if tides and currents carry it and it cannot swim against those forces. Some species remain plankton for their whole lives, but others drift only when they are young. Later, those animals grow large enough to swim against the current. Plankton are usually microscopic and often shorter than one inch, but some are larger animals such as certain crustaceans and jellyfish.

Scientists group plankton by size, by type, and by how long they drift. The most basic division separates phytoplankton, which are plants, from zooplankton, which are animals. Phytoplankton are microscopic plants that play a major role in the ocean food web. Like plants on land, they use photosynthesis to turn sunlight into energy they can use. They absorb carbon dioxide and release oxygen, and they live near the surface because they need sunlight.

Zooplankton include tiny animals such as krill, sea snails, and worms that live in open water. They also include the young of larger animals and weak swimmers such as jellyfish. Most zooplankton eat phytoplankton and later become food for larger animals or for one another. Krill may be the best-known zooplankton and make up a large share of the food eaten by humpback, right, and blue whales. During the day, zooplankton usually stay in deeper water to avoid predators. After dark they rise to feed on phytoplankton near the surface. This movement is viewed as the largest migration on Earth, and so many animals take part that it can be detected from space.

Plankton are vital to ocean ecosystems and react strongly to changes in temperature, saltiness, pH, and nutrients. When some nutrients become too abundant, harmful algal blooms such as red tides can form. Because many zooplankton eat phytoplankton, changes in phytoplankton can quickly change zooplankton numbers and then affect other species along the food chain. Researchers are studying how climate change influences plankton, including when their numbers rise or fall. They also study how the shells of small animals called copepods grow harder and how these effects spread through ecosystems.

What might happen to other ocean animals if phytoplankton numbers change?