What Do Plankton Eat? Their Diet Explained

Written by Taiwo Victor
Updated: October 7, 2022
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Despite their microscopic size, plankton are crucial for marine ecosystems. They act as the base of the entire food chain in oceans and freshwater. Plankton are microscopic living organisms that drift around seas. The word plankton itself came from a Greek word that translates to “drifter”, referring to their movement. Unlike corals which stay attached on submerged rocks along ocean floors, or typical sea creatures who swim, plankton just drift in the water, going with the tides and currents.

Plankton act as the foundation of the food chain at sea, which makes them extremely important for ecosystems. They are vital to sustain ocean life. Small sea creatures such as fish and crustaceans survive by feeding on plankton. They are, in turn, eaten by larger predators, and so on. Some humongous sea creatures such as blue whales and whale sharks can feed on plankton directly. Needless to say, plankton play a huge role in the food chain, but what do plankton eat?

What Do Plankton Eat?

What Do Plankton Eat

Plankton, just like plants on land, produce their own food through a process called photosynthesis. In this process, plankton convert sunlight into energy, and use this converted energy to form a sugar called glucose, which they store as a source of nutrients. 

Plankton have two main types, the phytoplankton and the zooplankton. Phytoplankton are in the form of microscopic plants, whereas zooplankton are in the form of microscopic animals. The phytoplankton undergo the photosynthesis process to produce its own food. 

All phytoplankton contain chlorophyll, a natural component found in plants, that converts sunlight into energy, which in turn will be combined with carbon dioxide and water to create a sugar called glucose. The glucose formed will be stored as carbohydrates, and these carbohydrates will serve as nutrients for the phytoplankton. 

Glucose is then converted into energy in a process called cellular respiration, into adenosine triphosphate (ATP), the primary energy-carrying molecule in organisms. 

Like plants on land, while phytoplankton take in carbon dioxide in the process, they release oxygen. As phytoplankton are abundant in the ocean, they are responsible for about half the world’s photosynthesis. This makes them one of the planet’s primary oxygen producers. 

What Nutrients Do Plankton Need?

Apart from the ingredients phytoplankton use for photosynthesis, they also need other nutrients in order to thrive in the ocean. These nutrients include phosphate, calcium, iron, silicon, and nitrate. 

Nitrogen and phosphorus are the most critical for phytoplankton, as they are required for both survival and procreation. There is a shortage of nitrogen in some locations, whereas phosphorus is scarce in other areas. When one or the other is depleted, phytoplankton cannot grow.

Who Eats Plankton?

What Do Plankton Eat - Microscopic View

Zooplankton consumes phytoplankton and starts the marine food cycle.


As mentioned above, plankton are categorized into two main types: the phytoplankton and the zooplankton. As phytoplankton produce their own food, zooplankton rely on them to thrive. Swarms of these tiny animal plankton eat phytoplankton, just like how most smaller land animals feed on plants on land.

The zooplankton will then be eaten by vast numbers of larger marine animals, and those plankton-eaters, mostly fish and crustaceans, will in turn be eaten by larger animals, and so on, creating a healthy, balanced food chain. Some huge animals, however, may feed on zooplankton directly. 

Krill is one of the most popular types of zooplankton as they serve as a common food for many aquatic animals. They can be fed to salmon, flatfish, rockfish, sardines, and sometimes even to seabirds such as shearwaters and auklets. Krill are also a popular food choice for many species of whales, especially baleen whales like blue whales and humpback whales.

Blue whales alone can eat about 6 tons or 12,000 pounds of krill in one feeding session using their baleen!

Krill and shrimp are both plankton consumers, along with small fish and crustaceans that make up most of larger fish and other marine creatures’ diet.

What Kind of Fish Eat Plankton?

Forage fish often prefer plankton as the primary part of their diet. Forage fish are fish who usually travel in groups, known as shoals or schools, but do not hunt for prey on their own. These fish include anchovies, smelts, halfbeaks, herrings, sardines, capelin, butterfish, rockfish, and menhaden. 

How Do Plankton Get Their Food?

What Do Plankton Eat - Zooplankton

The majority of plankton live in the upper levels of the ocean.


As phytoplankton produce their own food, the ingredients they need to create them either exist in phytoplankton themselves or are obtained from algae, bacteria, and microscopic organisms like protozoa and rotifers.

The upper level of the ocean called the photic zone, where bulks of plankton dwell, is where the nutrients plankton require can be found. Apart from sunlight, carbon dioxide, and water that plankton need, algae, bacteria, trash and debris known as detritus, and microscopic animals such as rotifers and protozoa are also necessary nutrients required for plankton to thrive and survive amidst oceans and freshwater. 

The nitrogen gas produced by the Earth’s atmospheric conditions also provide the nitrogen that plankton need to survive.

Can Plankton Be Harmful for the Environment?

While plankton are vital for food chains and in sustaining ecosystems, too much of them can contaminate environments too and can cause serious problems at sea. 

You can tell how much plankton a body of water has by looking at its purity. Plankton concentrations tend to be lower in crystal-clear water than in water that is greener or more brown in hue. However, when the population of plankton increases uncontrollably, certain types of phytoplankton can contaminate the marine environment by releasing harmful toxins that can cause red tides. These conditions can poison fish and other marine life and therefore cause great damage to the ecosystem. 

Since plankton are the foundation of aquatic food chains, these tiny microorganisms are essential to be kept at a balance. However, unsteady sea temperatures and climate change pose threats in keeping the plankton population controlled. 

The photo featured at the top of this post is © Choksawatdikorn/Shutterstock.com

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About the Author

For six years, I have worked as a professional writer and editor for books, blogs, and websites, with a particular focus on animals, tech, and finance. When I'm not working, I enjoy playing video games with friends.

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