Plankton facts for kids

Marine microplankton and mesoplankton This picture shows many tiny living things from the ocean. You can see plant-like plankton (like diatoms) and animal-like plankton (like copepods and baby crabs). Some live their whole lives as plankton, while others are just temporary visitors. 100 μm is one tenth of a millimeter

Plankton are tiny living things that drift in water or air. The word "plankton" comes from a Greek word meaning "drifter" or "wanderer." These organisms cannot swim strongly enough to go against ocean currents or wind. Marine plankton live in salty ocean water and slightly salty estuary water. Freshwater plankton live in lakes and rivers. Each individual plankton organism is called a plankter.

Plankton come in all shapes and sizes, from super tiny bacteria and algae that you need a microscope to see, to larger jellyfish. They are defined by how they live and move, not by their family tree. Plankton are different from nekton, which are animals that can swim against currents (like fish). They are also different from benthos, which live on the seafloor. Some plankton, called neuston, float right on the water's surface.

Plankton are a very diverse group. They are usually divided into groups based on how they get their food.

• Phytoplankton are like tiny plants. They make their own food using sunlight, just like plants on land.
• Zooplankton are like tiny animals. They eat other plankton to get their energy.
• Mixoplankton are special because they can do both! They can make their own food and also eat other plankton.
• Decomposers are tiny fungi, bacteria, and viruses. They break down dead organic material, recycling nutrients so other plankton can use them again.

These microscopic plankton are super important for keeping aquatic environments healthy. Phytoplankton produce about half of the Earth's oxygen through photosynthesis. They also help remove carbon dioxide from the air. Together, these tiny organisms are the base of the entire marine food web. They support everything from small fish to giant whales. While most plankton live in water, some, called aeroplankton, drift in the air. These can include plant spores, pollen, and even tiny ocean organisms swept up by sea spray.

What Plankton Need to Thrive

Ocean chlorophyll shows where phytoplankton are abundant. Green areas have lots of phytoplankton, while blue areas have fewer. – NASA Earth Observatory, October 2019.

Plankton live in oceans, seas, estuaries, rivers, and lakes. Their numbers change depending on the location and time of year. The most important thing for plankton growth is sunlight. Almost all plankton ecosystems rely on the sun's energy. This means most plankton live in the surface waters where light is plentiful.

Another important factor is the availability of nutrients. Plankton need nutrients like nitrates and phosphates to grow. Some parts of the ocean, especially in tropical areas, have lots of sunlight but not many nutrients. In these places, plankton are less abundant.

Even though plankton are most common near the surface, they live throughout the entire water column. In deeper waters where there is no sunlight, zooplankton and bacteria eat organic material that sinks down from the surface. This sinking material is sometimes called "marine snow."

Some ocean areas have plenty of nutrients but are still not very productive. This is often because they lack a tiny but crucial nutrient called iron. Adding iron to these areas can cause phytoplankton to bloom. Iron usually reaches the ocean from dust blown off land. So, oceans next to dry, dusty lands often have lots of phytoplankton.

Plankton's Life Journey

Plankton drift with currents. Nekton (like fish) swim against currents. Benthos live on the seafloor. Neuston live at the water's surface.

Plankton can be categorized by how long they spend drifting.

• Holoplankton spend their entire lives as plankton. Examples include most algae, copepods (tiny crustaceans), and some jellyfish.
• Meroplankton are only plankton for part of their lives, usually when they are babies (larvae). After a while, they grow up and either swim actively (become nekton) or settle on the seafloor (become benthos). Many sea urchins, starfish, crabs, and fish start their lives as meroplankton.

Tiny Plankton: Microscopic Wonders

Microplankton

The tiny Emiliania huxleyi (μm = thousandth of one mm)

A large bloom of Emiliania huxleyi off the coast of England

Most plankton are microorganisms, meaning they are smaller than one millimeter and can only be seen with a microscope. These tiny organisms make up a huge part of all life in the ocean. Together, they form the marine microbiome. Over billions of years, this microbiome has learned many ways to live and helps cycle almost all chemical elements on Earth.

Microplankton are key players in the ocean's food web. They are vital for recycling nutrients as decomposers. They also perform almost all the photosynthesis in the ocean and help cycle carbon, nitrogen, and phosphorus. Microplankton store large amounts of carbon and produce much of the world's oxygen.

Tiny viruses in the ocean kill about 20% of microplankton every day. These viruses are important for stopping harmful algal blooms that can harm other marine life.

Plankton by Where They Live

Plankton in the Air: Aeroplankton

Sea spray carries tiny ocean organisms high into the air. They can travel around the world as aeroplankton before falling back to Earth.

Aeroplankton are tiny living things that float and drift in the air, carried by the wind. They are like the ocean's plankton, but in the atmosphere. Most aeroplankton are microscopic. Scientists collect them using special traps and nets on aircraft or balloons. Aeroplankton include viruses, bacteria, fungi, and spores from plants like mosses. They can also be microorganisms swept into the air from dust storms or sea spray. Every day, aeroplankton deposit millions of viruses and bacteria on every square meter of the planet. This means similar types of tiny plankton can be found in water bodies all over the world.

Sea spray can carry these tiny ocean microbes high into the atmosphere. They can travel long distances, even hundreds of kilometers, and stay alive in the air if the humidity is high. When they land, they can affect the health of animals, plants, and even humans.

Plankton in Freshwater

Freshwater plankton are similar to marine plankton but live in lakes and rivers instead of oceans.

Plankton in the Ocean

Plankton at the Ocean Surface

Organisms that live right at or just below the ocean's surface are called neuston. They float on the water (epineuston) or swim in the top few centimeters (hyponeuston). Many neuston are considered plankton because they drift mostly with currents and wind. They don't have strong enough swimming abilities to go against them.

These surface-dwelling animals are important for moving energy between the ocean's surface and deeper layers. They also serve as food for zooplankton, fish, and seabirds.

Plankton in the Deep Ocean

In 2025, scientists found microbial communities living deep in the ocean's currents, known as the ocean conveyor belt. Surface winds create currents down to about 500 meters. But the average ocean depth is much deeper, about 3.7 kilometers. At these greater depths, currents are driven by differences in water temperature and saltiness. This creates a "conveyor belt" that carries water and tiny organisms around the world.

Researchers took water samples from the South Pacific Ocean, from Easter Island to Antarctica. They found a big increase in different types of microbes about 300 meters deep. This area, called the prokaryotic phylocline, shows a shift from less diverse surface waters to rich microbial ecosystems in the deep ocean. For example, some microbes found in the "Antarctic Bottom Water" are adapted to cold and high pressure. Others, in the "Ancient Water Group," live in water that has been isolated from the surface for over a thousand years and are adapted to low oxygen.

The ocean conveyor belt moves warm surface waters (red) and cold deep waters (blue). Many different microbial ecosystems thrive deep within this belt.

Plankton Groups by Type

Plankton include living things from all major groups of life. This is because "plankton" describes their habitat and drifting behavior, not their family tree. Here are some broad groups:

• Planktonic animals (metazoa): These are mostly tiny predators that eat smaller plankton. Examples include arrow worms, sea butterflies, and copepods. Many baby stages of crabs and corals are also zooplankton.
• Planktonic protists: These are single-celled organisms, usually too small to see without a microscope. They include diatoms, dinoflagellates, and foraminifera. Some are phytoplankton, some are zooplankton, and many are mixoplankton.
• Planktonic fungi (mycoplankton): These tiny fungi help recycle nutrients. For example, some fungi help transfer nutrients from large phytoplankton to zooplankton.
• Planktonic prokaryotes (bacterioplankton and archaeoplankton): These are tiny bacteria and archaea. They are important primary producers (making their own food) or help break down organic material. Photosynthetic cyanobacteria are a key type of phytoplankton.
• Planktonic viruses (virioplankton): Viruses are tiny infectious agents that can only reproduce inside living cells. They are very abundant in plankton communities, even more numerous than bacteria. Viruses infect and break down bacteria, releasing nutrients back into the water for other microorganisms to use.

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  This tiny animal is a female copepod with two egg sacs.

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  These are shells of tiny protists called radiolarians, drawn by Ernst Haeckel in 1904.

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  This tiny bacterium, Prochlorococcus, is the smallest organism that makes its own food using sunlight. It produces up to 20% of the world's oxygen.

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  This giant coccolithovirus (arrowed) infects a coccolithophore.

Plankton Sizes by Group

Plankton by Size

Scientists also group plankton by their size. Here are some common size categories:

The term microplankton is sometimes used more broadly for any plankton too small to see without a microscope. Nano- and even smaller plankton were only discovered in the 1980s. They make up the largest number and variety of all plankton. These tiny microplankton are the main drivers of the ocean's food web.

• Microplankton
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  Some marine diatoms — a key phytoplankton group.

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  Pelagibacter ubique, the most common bacteria in the ocean, helps with global carbon cycles.

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  The sea sparkle dinoflagellate glows at night.

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  Microzooplankton like these dinoflagellates and a tintinnid ciliate eat other plankton.

• Macroplankton
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  Macrozooplankton: the amphipod Hyperia macrocephala.

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  A Janthina janthina snail (with bubble float) found on a beach.

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  Sargassum seaweed floats with currents using air bladders.

How Plankton Get Food

Plankton are grouped by their "trophic mode," which means how they get energy and nutrients.

Phytoplankton: The Ocean's Plants

Phytoplankton (from Greek phyton, meaning plant) are like tiny plants. They are autotrophs, meaning they make their own food. They live near the water surface where there is enough light for photosynthesis. Important types include diatoms, cyanobacteria, dinoflagellates, and coccolithophores.

• Phytoplankton largely form the base of the marine food web
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  Diatoms are one of the most common types of phytoplankton.

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  Fossil diatom frustule from 32 to 40 million years ago.

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  A cyanobacteria species (Cylindrospermum sp).

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  Green algae, Pyramimonas.

Zooplankton: The Ocean's Animals

Zooplankton (from Greek zoon, meaning animal) are tiny animals or animal-like organisms. They are heterotrophs, meaning they eat other plankton to get food. This group includes small protozoans and tiny animals like crustaceans. The eggs and larvae (baby stages) of larger animals like fish and crabs are also zooplankton.

• Larger plankton tend to be zooplankton which eat smaller phytoplankton
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  Herring larva with its yolk sac and long gut visible.

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  Icefish larvae from Antarctica have no red blood cells.

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  Copepod from Antarctica, a clear, oval microanimal.

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  A krill larva is zooplankton, but an adult (shown) can swim against currents.

Mixoplankton: The Best of Both Worlds

Mixoplankton (from Greek mixis, meaning mixture) have a mixed way of getting food. Many plankton can act as both plant-like producers and animal-like consumers.

Traditionally, plankton were just phytoplankton (producers) or zooplankton (consumers). But scientists now know that many plankton, perhaps more than half, are mixotrophic. This means they can make their own food using sunlight and also eat other organisms. They can switch between these methods depending on conditions. For example, they might use photosynthesis when there's plenty of light and nutrients. But they can start eating other plankton when conditions are poor.

Because of this, many single-celled plankton once called only phytoplankton, like coccolithophores and dinoflagellates, are now correctly called mixoplankton. This new understanding changes how we view the ocean's food web. Mixoplankton are very important for marine ecology. They help prevent ecosystems from collapsing when there is little light.

• Mixoplankton can behave both as phytoplankton and zooplankton
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  A single-celled ciliate with green algae living inside it.

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  Euglena mutabilis, a photosynthetic flagellate.

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  The mixotrophic dinoflagellate Karenia brevis causes harmful red tides.

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  Acantharian radiolarian hosts tiny Phaeocystis organisms.

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  White Phaeocystis algal foam washing up on a beach.

Decomposers: Nature's Recyclers

Decomposers break down dead organic matter into simpler forms. This allows nutrients to be recycled.

• Fungi: Mostly tiny mycoplankton (microfungi) or mobile spores. They recycle organic matter by sometimes acting as parasites on other plankton.
• Bacteria/Archaea: Often called bacterioplankton. These tiny organisms break down organic material. This process, called the microbial loop, recycles nutrients like nitrogen and phosphorus. These nutrients then become available for primary producers like phytoplankton.
• Viruses: These are 10 to 100 times smaller than bacteria. They are also the most abundant, with about 100 billion per liter. Viruses infect other plankton and larger organisms. They can quickly stop large plankton blooms. They do this by turning biomass into dissolved organic matter. This matter then supports bacterial growth through a process called the viral shunt.

Other Interesting Plankton Groups

Gelatinous Zooplankton

Jellyfish are a type of gelatinous zooplankton.

Gelatinous zooplankton are delicate animals that live in the ocean's water column. Their soft bodies have no hard parts and are easily damaged. Many are transparent. All jellyfish are gelatinous zooplankton. Other common examples include ctenophores (comb jellies) and salps. These fragile creatures play an important role in marine ecosystems.

Ichthyoplankton: Baby Fish

Salmon egg hatching into a sac fry. Soon, the sac fry will absorb its yolk sac and start eating smaller plankton.

Ichthyoplankton are the eggs and larvae (baby stages) of fish. They are mostly found in the sunlit upper 200 meters of the ocean. Ichthyoplankton are planktonic, meaning they drift with ocean currents. Fish eggs cannot swim at all. Young larvae swim poorly, but older larvae swim better. As they grow into juveniles, they stop being plankton. Fish larvae are part of the zooplankton and eat smaller plankton. Fish eggs carry their own food supply. Both eggs and larvae are eaten by larger animals.

Mineralized Plankton

• Some planktons are protected with mineralized shells or tests.
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  Diatoms have glass shells (frustules) and produce much of the world's oxygen.

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  The elaborate silica shells of microscopic marine radiolarians can eventually produce opal.

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  Coccolithophores have chalk plates called coccoliths, and produced the Cliffs of Dover.

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  Foraminiferans have calcium carbonate shells and produced the limestone in the Great Pyramids.

Some plankton have hard, mineralized shells or coverings. These shells protect them. For example, diatoms have glass-like shells. Coccolithophores have tiny chalk plates called coccoliths. When these plankton die, their shells sink to the ocean floor. Over millions of years, these shells can form rocks like chalk and limestone.

Why Plankton are Super Important

Plankton and the Food Web

Marine food web: traditional view.

Marine food web: mixoplankton view.

Plankton are the very bottom of the ocean's food chain. They support almost all other marine life. Fish larvae mainly eat zooplankton, which in turn eat phytoplankton.

• The microbial loop: Tiny bacteria play a key role. They eat dissolved organic matter (DOM) and are then eaten by larger microorganisms. This recycles nutrients and energy within the ecosystem.
• The viral shunt: Viruses also play a central role. They infect and burst host cells, releasing cell contents. This dissolved organic matter can then be used by other tiny plankton like bacteria. This process helps cycle nutrients and carbon in the water.
• The mycoloop: Fungi also have a role. Tiny parasitic fungi infect large phytoplankton. Their spores then become food for zooplankton. This helps transfer nutrients from phytoplankton that are hard to eat to zooplankton.

Plankton and Earth's Oxygen

Phytoplankton absorb energy from the sun and nutrients from the water. They use these to make their own food. During this process, called photosynthesis, phytoplankton release oxygen into the water. It is estimated that about 50% of the world's oxygen comes from phytoplankton photosynthesis. The rest is produced by plants on land. Phytoplankton have controlled the balance of carbon dioxide and oxygen in our atmosphere for a very long time.

Plankton and the Carbon Cycle

The ocean food web shows how marine microplankton are central to the ocean's nutrient exchange with the atmosphere and seafloor.

Zooplankton eat phytoplankton, which helps move carbon through the food web. They either breathe it out as carbon dioxide or, when they die, their bodies and waste sink. This sinking organic material carries carbon with it. This process, called the biological pump, is why oceans are the largest carbon sink on Earth. This means oceans store a lot of carbon, helping to remove carbon dioxide from the atmosphere.

Scientists have explored if adding iron to the ocean could increase plankton production. This might help the ocean absorb more carbon dioxide from human activities. However, this idea has potential downsides, like reducing oxygen in deep waters.

The Great Calcite Belt

The Great Calcite Belt is an area in the Southern Ocean with many coccolithophores. These are phytoplankton that produce calcium carbonate (chalk) plates called coccoliths. This process, called calcification, affects the ocean's carbon cycle. When coccolithophores die, their chalk shells sink. This helps transport carbon to the deep ocean, storing it for a long time. This process helps reduce carbon dioxide levels in the atmosphere.

Plankton and Other Life

Fish and Plankton

Zooplankton are the first food for almost all baby fish. Fish rely on having enough zooplankton in the right places for their larvae to survive. Changes in currents, temperature, or human activities can affect zooplankton populations. This, in turn, can greatly impact how many baby fish survive and how successful fish breeding is.

Sometimes, baby fish can actually make algal blooms last longer. They do this by eating zooplankton, which normally eat the phytoplankton that cause the blooms. Fewer zooplankton mean more phytoplankton can grow.

Whales and Plankton

Whale waste is very rich in nutrients. Phytoplankton can get many nutrients from whale feces. Since phytoplankton are the base of the marine food web, whale waste helps fuel the entire food web. This means it supports everything from tiny zooplankton and krill to the large whales themselves.

Humans and Plankton

Plankton have many direct and indirect effects on humans.

About 70% of the oxygen we breathe comes from phytoplankton in the oceans. This means most of the oxygen available for humans and other animals is produced by plankton.

Plankton also form the base of the marine food web. They provide food for all the animals above them. Research shows that changes at the bottom of the food web (like in plankton populations) have a bigger impact on the success of other species than changes at the top.

In some cases, plankton can carry tiny germs that cause illness in humans. For example, some types of bacteria that cause cholera can live with chitinous zooplankton like copepods. When humans consume these copepods, the bacteria can cause illness.

Protecting Plankton: The Plankton Manifesto

In 2024, the United Nations Global Compact's Ocean Stewardship Coalition launched the Plankton Manifesto. This document was created by over 30 international experts. It gives recommendations to protect plankton and use their potential to help with climate change, pollution, and biodiversity loss. The Manifesto highlights that plankton are crucial for marine ecosystems. They produce about 50% of Earth's oxygen and store billions of tons of carbon each year.

Key recommendations include:

• Better research and monitoring: Using new technologies like DNA sequencing, satellite monitoring, and AI to understand plankton better. This will help create a global map of plankton.
• Plankton-based solutions: Finding new ways to use plankton. This could include using them for water purification, bioplastics, fertilizers, and animal feed. These uses would support sustainable industries.
• Policy integration: Asking governments and businesses to include plankton in plans for climate and biodiversity.
• Public awareness: Teaching people about plankton's role in food security and healthy ecosystems.
• Collaboration: Encouraging partnerships between scientists, businesses, and governments. This will help fund research and protect plankton from threats like pollution and ocean warming.

See also

In Spanish: Plancton para niños

• Paradox of the plankton
• Seston
• Veliger