Understanding the Definition of Plankton

Plankton is a general term for the "floaters," the organisms in the ocean that drift with the currents. This includes zooplankton (animal plankton), phytoplankton (plankton that is capable of photosynthesis), and bacterioplankton (bacteria).

Origin of the Word Plankton

The word plankton comes from the Greek word planktos, which means "wanderer" or "drifter."

Plankton is the plural form. The singular form is plankter.

Can Plankton Move?

Plankton are at the mercy of the wind and the waves, but not all are completely immobile. Some types of plankton can swim, but only weakly or vertically in the water column. And not all plankton are tiny - jellyfish (sea jellies) are considered plankton.

Types of Plankton

Some marine life goes through a planktonic stage (called meroplankton) before they become free-swimming. Once they can swim on their own, they are classified as nekton. Examples of animals that have a meroplankton stage are corals, sea stars (starfish), mussels and lobster.

Holoplankton are organisms that are plankton their entire lives. Examples include diatoms, dinoflagellates, salps, and krill.

Plankton Size Groups

Although most people think of plankton as microscopic animals, there are larger plankton. With their limited swimming capability, jellyfish are often referred to as the largest type of plankton. In addition to being categorized by life stages, plankton can be categorized into different groups based on size.

These groups include:

• Femtoplankton - Organisms under 0.2 micrometers in size, e.g., viruses
• Picoplankton - Organisms 0.2 micrometer to 2 micrometers, e.g., bacteria
• Nanoplankton - Organisms 2-20 micrometers, e.g. phytoplankton and small zooplankton
• Microplankton - Organisms 20-200 micrometers, e.g., phytoplankton and small zooplankton
• Mesoplankton - Organisms 200 micrometers to 2 centimeters, e.g., phytoplankton and zooplankton such as copepods. At this size, the plankton are visible to the naked eye.
• Macroplankton - Organisms 2 centimeters to 20 centimeters, e.g., like ctenophores, salps, and amphipods.
• Megaplankton - Organisms over 20 centimeters, like jellyfish, ctenophores, and amphipods.

The categories for the smallest plankton sizes were needed more recently than some others. It wasn't until the late 1970's that scientists had the equipment available to help them see the great number of planktonic bacteria and viruses in the ocean.

Plankton and the Food Chain

A plankton species' place in the food chain depends on what type of plankton it is. Phytoplankton are autotrophs, so they make their own food and are producers. They are eaten by zooplankton, which are consumers. 

Where Do Plankton Live?

Plankton live in both freshwater and marine environments. Those that live in the ocean are found in both coastal and pelagic zones, and in a range of water temperatures, from tropical to polar waters.

Plankton, As Used in a Sentence

The copepod is a type of zooplankton and is a primary food for right whales.

References and Further Information:

• Australian Museum. What Is Plankton? Accessed October 31, 2015.
• Bigelow Laboratory. Cycling Through the Food Web. Accessed October 31, 2015.
• Microbial Grazers Lab 404 404 404. Marine Biological Laboratory at Woods Hole. Accessed October 31, 2015.