Phytoplankton vs. Zooplankton: 16 Differences, Examples

Phytoplankton vs Zooplankton
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Phytoplankton Definition

Phytoplankton is a group of free-floating microalgae that drifts with the water current and forms an important part of the ocean, sea, and freshwater ecosystems.

  • The term phytoplankton comes from two Greek words, ‘Phyto’ meaning plants, and ‘plankton’ meaning drifter.
  • Phytoplanktons are autotrophic as, like land vegetation, they are also provided with chlorophyll to make their own food.
  • Most phytoplankton are found floating on the top of water bodies as they require sunlight for their food preparation.
  • Individual phytoplanktons are tiny and cannot be seen with the unaided eye, but when present in masses, they appear as colored patches on the water surface.
  • Phytoplankton makes up about 1% of the total biomass of the world. These organisms are the primary source of food for most marine and freshwater animals.
  • The amount of phytoplankton might change seasonally with the availability of sufficient sunlight, suitable temperature, and other substrates.
  • There are diverse groups within phytoplankton consisting of diatoms, cyanobacteria, dinoflagellates, green algae and
  • These organisms utilize various inorganic minerals during photosynthesis which are then converted into proteins, carbohydrates, and other nutrients for themselves and other animals.
  • Unlike other autotrophs like plants, phytoplankton consists of diverse groups of organisms ranging from archaeal or bacterial prokaryotes to protistan eukaryotes.
  • Phytoplankton accounts for half of all the photosynthetic activity throughout the world and is the primary producer in the freshwater and marine food chains.
  • These are the key food source in aquaculture and mariculture and are even used as a nutritional supplement for various invertebrates in aquaria.
  • However, when the nutrients are available in large quantities, phytoplanktons might grow out of control, resulting in the formation of alga blooms.
  • These blooms might produce harmful and even toxic substances that might cause damage to other ecosystems within the habitat.
  • Based on studies conducted from 2015 to 2019, it was observed that the phytoplankton concentration is decreasing by about 1% every year as a result of global warming.
  • Some examples of phytoplankton include diatoms, green algae, cyanobacteria, and coccolithophores, among others.

Zooplankton Definition

Zooplankton is a group of small and floating organisms that form most of the heterotrophic animals in oceanic environments.

  • The term zooplankton comes from two Greek words ‘zoo’ meaning animals and ‘plankton’ meaning drifter.
  • Zooplanktons form an integral part of food chains in aquatic environments from freshwater to seas and oceans.
  • Because zooplanktons are heterotrophs, they depend on the phytoplankton and other autotrophs for their energy and carbon source.
  • Their movement, along with water allows them to find food and also protect themselves from predators.
  • Zooplanktons include animals of various sizes ranging from small protozoans to large metazoans. Other animals like young starfish and worms might also act as temporary zooplanktons.
  • Zooplanktons, like phytoplanktons, have multiple groups of animals, including radiolarians, foraminiferans, and dinoflagellates, cnidarians, crustaceans, chordates, and molluscs.
  • Most zooplanktons are larval forms of fishes and invertebrates that later undergo metamorphosis to change into full-fledged sea creatures.
  • The distribution of zooplankton is limited due to various factors like predation, competition, and breeding.
  • Additionally, patches of zooplanktons are seen in areas with adequate physical conditions like temperature, water currents, and salinity.
  • The number of zooplankton is also limited by the presence of phytoplankton which, in turn, might be disturbed by various other factors, including their lifecycle.
  • Zooplanktons are an essential part of ocean food chains as they function as a source of food for higher consumers like fishes.
  • These are an important group of animals as they might even function as a conduit for the packaging of organic materials in the biological bumps.
  • They also act rapidly against the increasing number of phytoplankton, resulting in blooms, preventing their harmful effects.
  • Some zooplanktons have also been associated with the removal of toxic materials like mercury from the pollutants in the water.
  • However, zooplanktons also support the survival and transfer of various diseases by housing the pathogenic agents.
  • Bacteria like Vibrio cholerae exist in a symbiotic relationship with crustacean zooplanktons as the exoskeleton of such animals provides carbon and nitrogen necessary for the bacterium.
  • Some examples of zooplanktons include animals like radiolarians, krill, jellyfish, young molluscs, amphipods, among others.

Key Differences (Phytoplankton vs Zooplankton)

Basis for Comparison



DefinitionPhytoplankton is a group of free-floating microalgae that drifts with the water current and forms an important part of the ocean, sea, and freshwater ecosystems.Zooplankton is a group of small and floating organisms that form most of the heterotrophic animals in oceanic environments.
Terms‘Phyto’ refers to ‘plant-like’.‘Zoo’ refers to ‘animal-like’.
Consists ofPhytoplanktons consist of diatoms, cyanobacteria, dinoflagellates, green algae, and coccolithophores.Zooplanktons consist of organisms like radiolarians, foraminiferans, and dinoflagellates, cnidarians, crustaceans, chordates, and molluscs.
NutritionPhytoplanktons are autotrophic and thus, can make their own food with sunlight and chlorophyll.Zooplanktons are heterotrophic depending on the distribution of phytoplankton for their food and energy.
HabitatPhytoplanktons are found mostly floating on the surface of water bodies as they require sunlight for photosynthesis.Zooplanktons remain mostly around the dark and deeper areas of water.
AppearancePhytoplanktons are seen as cloudy green patches on water. Otherwise, they appear brown in color.Zooplanktons are mostly translucent, but their shape, size, and color might differ with the type of the organism.
SizePhytoplanktons are invisible to the unaided eyes and can only be seen as green patches when present in large numbers.Most zooplanktons are large enough to be seen with naked eyes.
PhotosynthesisPhytoplanktons are capable of photosynthesis, being responsible for about half of the photosynthesis performed around the worldZooplanktons are not capable of photosynthesis.
Oxygen releasePhytoplanktons are photosynthetic and thus are extremely important for oxygen release.Zooplanktons only take in oxygen and do not produce it.
EnergyPhytoplanktons obtain their energy via photosynthesis by utilizing inorganic minerals.Zooplanktons obtain their energy by feeding on phytoplankton.
Position in the food chainPhytoplanktons are the producers of the oceanic food chains.Zooplanktons are the primary or secondary consumers of the oceanic food chain.
MovementMost phytoplanktons are not capable of freely moving with the water currents.Zooplanktons are capable of moving with or against water currents against predators or competitors.
MetamorphosisPhytoplanktons do not undergo metamorphosis.Most zooplanktons are larval forms of fishes and invertebrates that eventually metamorphose to form free-swimming creatures.
Vertical migrationPhytoplanktons are not capable of vertical migration.Zooplanktons are capable of vertical migration in water.
FunctionsPhytoplanktons act as food for zooplankton and also as the indicators of the health of the marine environments.Zooplanktons are the indicators of toxic substances present in the ecosystems and also serve as food for higher heterotrophs.
ExamplesSome examples of phytoplankton include diatoms, green algae, cyanobacteria, and coccolithophores, among others.Some examples of zooplanktons include animals like radiolarians, krill, jellyfish, young molluscs, and amphipods, among others.

Examples of phytoplanktons


  • Cyanobacteria are the photosynthetic bacteria mostly found in water that utilize sulfur compounds to make their food via chemosynthesis.
  • Cyanobacteria are also called blue-green algae as they are autotrophs and are responsible for most of the oxygen produced in the marine environments.
  • These form a large group within phytoplankton and are distributed uniformly through all water bodies in the world. Cyanobacteria are found in varying shapes, sizes, and colors as well.
  • Most cyanobacteria are well suited for various aquatic environments as they are more resistant than other phytoplanktons and thus can survive even extreme aquatic habitats.
  • These exist mostly in colonies consisting of unicellular to filamentous colonies distributed randomly through water bodies.
  • Different biomasses are formed in different areas as the colonies separate from each other to inhabit different locations. Some cyanobacteria like Lyngbya might even form blooms.
  • Cyanobacteria, like all phytoplanktons, serve as a food source for zooplankton.
  • Some examples of cyanobacteria found in oceanic habitats include Synechocystis, Oscillatoria, Lyngbya, etc.


  • These are unicellular organisms that appear golden-brown due to the presence of golden-brown plastids.
  • Most dinoflagellates form a symbiotic relationship with by taking in inorganic minerals while providing them with sufficient oxygen.
  • They have a dented cell membrane and show distinct swimming patterns with a rather large nucleus and visible chromosomes.
  • They have two dissimilar flagella protruding from the cell membrane, thus, the name dinoflagellates.
  • Even though they are important as a part of phytoplanktons in producing food and oxygen for the environment, these might be harmful if developed into blooms.
  • Some of them even produce toxic substances harmful to other animals and plants sharing the habitat.
  • Some examples of dinoflagellates include Oxyrrhis marina, Dinophysis acuminate, Symbiodinium, etc.

Examples of zooplanktons


  • Jellyfish is an example of a zooplankton that is capable of both drifting and swimming through oceans.
  • There are hundreds of jellyfishes residing in almost all parts of oceans, all belonging to the group called sea anemones or corals.
  • Jellyfish are transparent and soft-bodied that appears as an umbrella with tentacles around the edges hanging off of them. The structure is called the medusa.
  • The size of jellyfish ranges from being microscopic to some being more than one meter in length.
  • Because ocean food chains are comparatively shorter than those on the land, zooplanktons like jellyfish form a connecting link between the phytoplankton and higher animals.
  • However, the increase in the number of these jellyfish above normal might also be a problem as some large jellyfish can consume the larva of small fishes.


  • Krill form an important part of zooplanktons that are a type of crustacean found populating oceans throughout the world.
  • Krill are found on the surface of the water during the day, but they move towards the deeper areas during nights.
  • These usually form a lower trophic level as primary consumers that form a bridge between the phytoplanktons and secondary or tertiary consumers.
  • Most krill act as a food source for larger marine animals.
  • Some of them are even fished commercially as they can be used as a fooder for aquaculture and mariculture.
  • Like most crustaceans, they have a chitinous exoskeleton which is mostly transparent.
  • Krill are bioluminescent having organs called photophores that can emit light, possibly important for mating and orientation.

References and Sources

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About Author

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Anupama Sapkota

Anupama Sapkota has a bachelor’s degree (B.Sc.) in Microbiology from St. Xavier's College, Kathmandu, Nepal. She is particularly interested in studies regarding antibiotic resistance with a focus on drug discovery.

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