What Do Plankton Eat? (Diet & Facts)

When you’re swimming in a lake, sea, or ocean, you might come across some tiny organisms that float on the water’s surface. These organisms are called plankton and are one of the most critical parts of any ecosystem.

The two main categories of plankton are phytoplankton and zooplankton. Each subspecies has its own purpose in the balance of an ecosystem and has very different dietary requirements. In this article, we’ll thoroughly examine the different types of plankton, what they eat, their predators, and their purpose in an ecosystem.

If you want to learn more about what plankton eat, keep on reading!

What Is Plankton?

Plankton is a broad term that refers to a diverse collection of organisms usually found in the water or the air. Its name originates from the Greek word planktos, which means wanderer or drifter, as these organisms can’t swim and instead float or drift on the surface of the water.

Most types of plankton are tiny in size. For some, you even need a microscope to be able to see them. But, the plankton family is so diverse that it includes large-sized organisms such as jellyfish.

Other than their size, plankton are often categorized depending on their habitat. There are marine plankton, freshwater plankton, geoplankton, and aeroplankton. Finally, plankton are divided into groups based on what they eat.

What Do Plankton Eat?

Different categories of plankton rely on various food sources, and each one of them has its own unique nutrient requirements. Here are the trophic groups of plankton, as well as what their diet consists of:


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The name phytoplankton comes from the Greek word “phyto”, which means plant. So, phytoplankton are essentially a type of aquatic plant. Thus, just like all plants, they produce their own food through photosynthesis.

Phytoplankton like cyanobacteria, dinoflagellates, and diatoms are abundant on the surface of oceans and at shallow depths. This allows them easy access to sunlight which is essential for photosynthesis. Through this process, phytoplankton create oxygen and sugar to use for energy.

Phytoplankton also requires additional nutrients both for its survival and its reproduction. The most important nutrients for plankton are nitrogen and phosphorus. There are some areas with a shortage of either nitrogen or phosphorus. While phytoplankton still produce oxygen in these areas, their reproduction is low due to the lack of these nutrients.


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Zooplankton are a large family that includes microscopic animals such as krill or sea snails, young invertebrates, and weak swimmers like jellyfish. While most zooplankton organisms use the ambient water currents to move around, some types can move by themselves.

Zooplankton are heterotrophic organisms. This means they can’t make their own food and rely on eating other organisms to survive. Most zooplankton organisms eat phytoplankton like algae and bacteria as their primary food source.

Other plankton types, like bacterioplankton, mycoplankton, and virioplankton, utilize dead organic matter as a food source. These plankton play a significant part in remineralization and nutrient cycling. 

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How Do Plankton Get Their Food?

Similar to other plants, phytoplankton contain chlorophyll which is the necessary compound for the process of photosynthesis. This also requires carbon dioxide and water. These elements are then converted into oxygen and carbohydrates through a chemical process.

The phytoplankton turns the carbohydrates into a simpler compound called adenosine triphosphate (ATP), which is essentially the energy provider for cells. It also releases oxygen into the atmosphere, providing a big part of the planet’s oxygen.

As for the necessary nutrients they require for their reproduction, phytoplankton usually finds nitrogen and phosphate in the forms of nitrate and phosphate, respectively. These compounds are generally deposited in the oceans by humans or due to atmospheric conditions.

In order to have access to sunlight, phytoplankton need to be close to the surface of the water. This makes them more vulnerable to predators. However, their sheer abundance and ability to reproduce quickly and efficiently help offset this issue.

On the other hand, zooplankton don’t require constant access to sunlight to create their food. So, to avoid predators, most zooplankton usually drift to deeper waters. During the night, they re-emerge to the water’s surface to eat phytoplankton. This migration is so significant in scale that it’s visible from space.

Other types of plankton like bacterioplankton can either be autotrophic or heterotrophic. The autotrophic types, such as cyanobacteria, rely on photosynthesis for their energy, while the heterotrophic ones utilize dead organic matter to create energy.

What Predators Eat Plankton?

All types of plankton belong at the bottom of the food web, meaning they have many predators. The primary predators of phytoplankton are zooplankton, which solely rely on phytoplankton for energy and nutrients. There are also some small fish and corals that directly eat phytoplankton.

Zooplankton is, in turn, consumed by small fish, which are then consumed by larger fish. This essentially makes plankton the foundation of the entire food web. Even large animals, like sharks and whales, directly consume plankton.

In fact, the largest mammals in the world -the blue whales- have a diet that primarily consists of zooplankton. This enormous whale can consume up to 4 tons of plankton per day. Some phytoplankton then eat its feces since it is rich in nutrients, thus completing a small food cycle.

Zooplankton are also the primary food source for baby fish. Once fish larvae start to feed externally, their initial food source is different kinds of zooplankton. Thus, the population and distribution of zooplankton in an area can be a significant factor in the fish population.

In areas with an abundance of zooplankton, fish larvae will thrive, increasing the total fish population. In other areas where there’s a shortage of zooplankton due to natural or human factors, fish larvae won’t survive, and the breeding process of fish will be unsuccessful.

What Is Plankton Used For?

Plankton are the foundation of many ecosystems and essential to several processes.


The importance of planktons to the environment is immeasurable. As mentioned above, phytoplankton produce oxygen through photosynthesis, similar to trees and other plants.

Since there’s such an enormous amount of plankton in the world, the oxygen they produce is significant. In fact, about 70% of the oxygen in the atmosphere is produced by ocean phytoplankton.

The huge amounts of plankton in all bodies of water can also be utilized to identify or predict environmental changes. Since plankton are simple organisms, they are significantly affected by changes in their environment.

Even the slightest alteration in the water or the atmosphere can induce a physiological reaction from plankton. They can either increase their numbers rapidly or show a certain change in their chemistry. These alterations can be an indicator for scientists that there’s a change in the climate, water, or environment.

Carbon Cycle

Plankton also play an important part in maintaining the carbon cycle balance. Through their photosynthesis, phytoplankton absorb carbon dioxide from the atmosphere at a rate even larger than that of huge forests. Some people theorize that increasing the phytoplankton population through iron fertilization can help reduce the effects of global warming.

This carbon is transferred to zooplankton which eat phytoplankton. Zooplankton are then eaten by other fish, which transfers the carbon up the food chain. Some zooplankton also die and sink to the bottom as dead organic matter.

The fish that eat zooplankton are then consumed by larger animals or generate waste which provides new organic material for phytoplankton, thus completing the cycle.

Food Web

The existence of plankton in such high numbers helps facilitate the entire marine food web. Apart from being the energy source for small and large fish, phytoplankton are also the primary food source for many commercial fisheries that help feed millions of humans.

The importance of plankton in the food web is so significant that their dynamics with the fish that consume them are much more critical than other dynamics. Studies have shown that the amount of plankton affects the survival of primary consumers considerably more than the existence of secondary consumers that eat them.

Also, zooplankton tend to increase their production rate if their ecosystem is full of fish that prey on them. Comparatively, if their predators are scarce, their population is reduced; however, if there are less zooplankton, the population of phytoplankton increases, further increasing the effect of algal blooms.

Algal blooms are essentially rapid increases in the population of phytoplankton. These blooms can have a detrimental effect on an ecosystem in several ways. The enormous phytoplankton population can block sunlight or absorb more oxygen and cause asphyxiation to fish. Some phytoplankton species also produce biotoxins that can kill fish or humans that eat contaminated fish.


All in all, while plankton might seem small in size, they’re one of the most important creatures on the planet. They play a massive role in many processes and are necessary to maintain oxygen levels in the atmosphere. They are also the base of most marine food chains, including most fish that reach your table.

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