What is Plankton?

Normally, if I ask a student, “What is plankton?” the first response I get is “The little dude on Spongebob!” Then, after a little thought, a student will say something like “A microscopic, crab-like critter.” I like this answer a little better than the Spongebob one, but its still off. The question is actually harder than students think, because plankton can be small, they can be big, they can be plants, and they can be animals. Plankton are not just one thing, they are a lifestyle, a way an organism lives in its environment. Plankton are drifters, meaning they cannot move against a current. Many types of plankton are microscopic, but jellyfish that are meters long can be considered plankton because they always travel with the current. These organisms can be divided into two main categories: Phytoplankton and Zooplankton.


Plant-like plankton are called phytoplankton. Like all plants, they get energy through photosynthesis. This is the process of using both carbon dioxide and energy from sunlight to form sugar and useable energy. Plants breathe in carbon dioxide and breathe out oxygen as a by-product. Scientists estimate that phytoplankton account for about half of all photosynthetic activity on Earth, therefore producing more than half of the world’s oxygen supply! The majority of phytoplankton come in two flavors: Diatoms and Dinoflagellates

Diatoms and Dinoflagellates

Diatoms are microscopic, pill-box shaped plants that inhabit all the oceans on earth. Diatoms are special in that they are capable of building shells, called frustules, out of glass! Each species has a very unique shape and can be very beautiful under a microscope. The majority of diatoms are unicellular, meaning they exist as only one cell, but some species are colonial and live in large chains of unicellular phytoplankton.

Dinoflagellates are not technically a plant, they are a protist, part of a different kingdom of living things. Even though these organisms are not technically plants, dinoflagellates are capable of photosynthesis, basically pretending to be plants! Dinoflagellates have two whip-like flagella, which they use to propel themselves through the water. Because these flagella are two different lengths, they appear to move in erratic circles.

When nutrients like nitrates and phosphates wash into marine and estuarine systems, phytoplankton absorb the nutrients and are able to reproduce very quickly. When this happens, it is called a bloom. Some algae and phytoplankton blooms can be dangerous. For example, Karenia brevis is a common dinoflagellate that blooms off the Florida coast. When it blooms, millions of them cloud the waters off the coast of Florida, turning the water red, which is why when this happens it is called the Florida Red Tide. Toxins produced by Karenia brevis can kill a lot of marine life, and they can even be sent up into the air and cause respiratory problems for humans.


Animal-like plankton are called zooplankton. These organisms need to consume other organisms, like phytoplankton, to survive. While phytoplankton are the base of the food web, zooplankton are the next level. These animals take energy from phytoplankton and then they themselves become food for larger marine life. Even animals as big as whales can survive primarily on zooplankton, called krill. There are two categories that zooplankton can fall into:

Meroplankton are species that only spend part of their lives as plankton. Most commonly, these are the larval stages of aquatic animals. For example, barnacles are born as a type of zooplankton, drifting through the water until they find a suitable resting spot. They then drive their head into the surface and begin to build a shell around their body. Sweeping their legs through the water barnacles are then able to filter phytoplankton into their mouths inside the shell. Many animals like oysters and mussels behave similarly. In fact, two-thirds of all seafood that we eat spends some of its life as zooplankton in intertidal estuaries! Holoplankton are species that spend their entire lives as plankton. Examples of these are diatoms, dinoflagellates and krill.

Get involved!

The National Oceanic and Atmospheric Administration (NOAA) sponsors the Phytoplankton Monitoring Network. The PMN monitors blooms and types of phytoplankton along the coast of the United States. Volunteers can get trained by PMN employees and learn how to gather and identify different phytoplankton species and then submit their data to NOAA. For ecogeeks, this is great way to do real science!

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Written by Rob Nelson

Rob is an ecologist from the University of Hawaii. He is the co-creator and director of Untamed Science. His goal is to create videos and content that are entertaining, accurate, and educational. When he's not making science content, he races whitewater kayaks and works on Stone Age Man.

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