Plankton: Small Organisms with a Big Role in the Ocean

These tiny plants and animals are some of the most important organisms in the sea.

Written By
Erin Spencer

We get it—compared to charismatic megafauna like whales or quirky creatures like frogfish, plankton may not seem like the most interesting organisms in the sea. But these tiny plants and animals are extremely important—they are responsible for much of the air we breathe and the food we eat.

What are plankton, anyway?

Plankton are a group of marine and freshwater organisms that drift along ocean currents because they are too small or weak to swim against them. Their name comes from the Greek word planktos which means “wandering.” The range in size from 2 micrometers (that’s smaller than a human red blood cell!) to more than 20 centimeters. There are two types of plankton: phytoplankton, which are tiny plants, and zooplankton, which are tiny animals.


Phytoplankton are so small that you need a microscope to see them. Types of phytoplankton include dinoflagellates, diatoms, cyanobacteria and green algae. Just like land plants, they contain chlorophyll and need sunlight and nutrients to grow. However, too much of a good thing (like excess nutrients, including those from human-driven nutrient pollution) can contribute to algae blooms, where colonies of phytoplankton grow in large quantities.

Although not all algal blooms are harmful, some contain toxins that can cause serious problems for the ecosystem and the communities who rely on it. Harmful algal blooms (or HABs) produce toxins that can kill fish, shellfish, birds and more, and can make humans very, very sick if ingested. (Read more about the devastating HAB in Florida in 2018).

Zooplankton, on the other hand, are small animals. Some are small enough that you need a microscope to observe them, but others you can see with the naked eye. There are two categories of zooplankton: holoplankton, or animals that spend their entire lives as plankton like copepods and rotifers, and meroplankton, or animals that are only planktonic for a short time like larval fish and crustaceans.


What is plankton’s role in the ecosystem?

Plankton are at the base of the food chain, meaning they are critical in supporting marine and freshwater food webs. Phytoplankton are also primary produces, meaning they use photosynthesis to convert carbon dioxide to oxygen, and are responsible for up to half of the oxygen we breathe. Phytoplankton is eaten by small zooplankton, which are in turn eaten by other zooplankton. Those plankton are eaten by small fish and crustaceans, which in turn are eaten by larger predators, and so on. Large animals can eat plankton directly, too—blue whales can eat up to 4.5 tons of krill, a large zooplankton, every day. Without plankton, entire food webs around the world would likely collapse.

Unfortunately, plankton abundance has been decreasing. Scientists believe phytoplankton numbers have decreased about 40% since 1950. Changes in ocean temperature and ocean acidification threaten plankton health, which in turn has major implications for marine food webs and oxygen production.

By standing up for the smallest organisms in the ocean, we’re standing up for all animals in the ocean. Learn more about Ocean Conservancy’s work to combat the effects of climate change and ocean acidification. And next time you take a deep breath or enjoy some fresh seafood, thank the plankton who helped make that possible!

Our work is focused on solving some of the greatest threats facing our ocean today. We bring people, science and policy together to champion innovative solutions and fight for a sustainable ocean.
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