An Invisible Watery World

By Amy Hansen
Illustrated by Sabine Deviche

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When you visit a pond or the beach, what kinds of living things do you see in the water? Depending on the environment, you might find fish, frogs, crabs, insects, seaweed, or lily pads. Don’t let your eyes fool you, though… there’s a hidden world in water full of creatures too small to be seen! When you go swimming in a lake or at the ocean, each stroke pushes you past billions upon billions of microscopic creatures called plankton.

It was a German scientist named Viktor Hensen who first gave plankton their name. If you wonder why he used this name, you need to know your Greek and something about how these tiny life forms travel to find the answer.  Planktos in Greek means to drift or wander.  Plankton are too small to swim in water in the same way fish or whales can… they simply drift along.  This is why they came to be known as plankton.

Different types of plankton

There are two main kinds of plankton: phytoplankton, which are also called algae, and zooplankton.

Phytoplankton are like plants. They use energy from sunlight to turn carbon dioxide, a gas in air and water, into sugars they can use to grow. Because they depend on the sun, phytoplankton can only live in the upper parts of a lake or the ocean. In deeper, darker waters, there just isn’t enough light for these creatures to grow and survive.

Zooplankton, the other kind of plankton, are tiny animals. They must eat to stay alive. Some zooplankton graze algae just like cows munch on grass. Some are hunters that catch other zooplankton. And some zooplankton eat detritus—that means they eat dead organisms and poop sinking through the water!

How small are plankton?

Like all life on Earth, plankton come in all sorts of shapes and sizes. The smallest are the bacteria, which are much too small to be seen without a powerful microscope. Most bacteria are only a few micrometers wide at most.

Next are the unicellular phytoplankton and zooplankton. “Unicellular” means their bodies are made up of only one cell, like a cabin with just one room (although sometimes unicellular creatures can form chains with others of their same species). Although they may be ten to 100 times larger than a bacterial cell, you would still need to look through a microscope to see these organisms.

Some plankton, though, are big enough to be seen with the naked eye. Try this the next time you visit a pond or lake: scoop up a glass of water and hold it up to the light. Unless the water is very dirty, you should be able to see small specks swimming around. These specks—the largest no bigger than a few millimeters long—are zooplankton. They’re probably the smallest animals you’ve ever seen!

Copepods are one kind of zooplankton. These tiny creatures are the most abundant organisms on Earth! They’re even outnumber all the insects in the world.

Little creatures with a big impact

Even though plankton are microscopic, our world would be a very different place without them. Plankton can be found in almost any body of water. About 71% of our planet is covered by water, so because plankton are incredibly small, there are a lot of plankton on Earth! In fact, you can find thousands of plankton in a single drop of water.

Like the plants you see around you, phytoplankton give off oxygen gas when they use sunlight to turn carbon dioxide into sugars. Half of the oxygen in our atmosphere was made by phytoplankton. Take a deep breath and think about how 50% of the oxygen you just inhaled was made by microscopic creatures.

Phytoplankton also form the base of aquatic food webs. In other words, all life in the ocean ultimately depends on algae for food. Because algae can use the sun’s energy to transform air into sugars, they provide a rich supply of food for the zooplankton and other creatures that eat them. Those zooplankton are eaten by larger zooplankton, by shellfish, by fish, and by baleen whales, little fish are eaten by birds and bigger fish, and so on throughout the tangled food web. Sea lions, penguins, sharks, killer whales, dolphins… all of these animals ultimately depend on plankton to survive!

This section of Ask A Biologist was funded by NSF Grant Award number 0752592 and 1030345.