Difference Between Phytoplankton and Zooplankton

Planktons are the small and microscopic organisms drifting or floating in the sea or freshwater consisting chiefly of diatoms, protozoans, small crustaceans and the eggs and larval stages of larger animals. Planktons are divided into two categories. One category is of permanent plankton and the other is called as temporary plankton. Diatoms, radiolarians, foraminifers, krill, dinoflagellates, copepods and sales are permanent planktons, whereas stars, crustaceans, marine worms, sea urchins and mostly fish are temporary members. These are the larval forms, which are also called as meroplankton.

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All the species of the plankton are adapted to certain structural adaptations which enable them to float freely in the water. These adaptations are oil droplets, sheaths, lateral spines, gel-like sheaths, floats filled with gases and flat bodies.

Phytoplankton and zooplankton are also categories of plankton. The basic difference between these two classes is that phytoplankton is small plants, whereas zooplanktons are the small animals. Diatoms and algae are examples of phytoplankton, whereas tiny fish and crustaceans are examples of small animals.

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Comparison Chart

Basis for Comparison Phytoplankton Zooplankton
Definition Phytoplankton is the autotrophic, small plants of the plankton community present in the oceans, seas and freshwaters. Zooplankton are the heterotrophic, small animals
Etymology Greek word “Phyto” means “plants” and “planktos” means “wandering.” Greek word “Zoo” means “animals” and “planktos” means “wandering.”
Mode of nutrition Autotrophs Heterotrophs
Habitat On the surface of the water In the darker and cooler areas of water
Liberation of oxygen Release oxygen in the atmosphere Do not perform such function
Movement Cannot move Move actively (jellyfish)
Color Brown color Different colors
Shapes Cloudy patches Different shapes, mostly translucent
Types Two types: Holoplankton and meroplankton
Ecological importance The basic source of food and check the stability of marine water Help to check the level of toxicity in the marine water
Examples Algae, diatoms Crustaceans and small fishes

What is Phytoplankton?

Phytoplankton is also called microalgae. These are microscopic biotic organisms that are mostly found in water bodies such as oceans, lakes, ponds and rivers. Synechococcus is the example of the common planktonic genus and its density reaches almost 104 to 105 cells per millimeter. Picocyanobacteria is a very small cyanobacterium that represents about 20 to 80% of the total phytoplankton.

Ecological Importance

Phytoplankton play a role as the primary food source for many marine animals. In most marine and freshwater environments, photosynthetic prokaryotes and eukaryotic organisms form the basis of primary production. They dissolved and particulate organic matter is released by the phytoplankton and is further used by the heterotrophic bacteria. A part of this dissolved material is consumed by the predators, which also releases the material and is used eventually by the phytoplankton. But iron and nitrogen can limit these activities in the marine environment.

They are also used as indicators of the health of marine water. The overgrowth of the algae bloom indicates the high level of the presence of toxins. These algae are also known as Red Tides and result in the death of marine animals and fish in that specific water body and thus creating the dead zone.

Dinoflagellates and diatoms are two main classes of phytoplankton, which play a major role in the global carbon cycle. Most importantly, phytoplankton is producing half of the world’s oxygen.

What is Zooplankton?

Zooplankton is a small animal that swims in the water bodies. Zooplankton is further divided into different groups based on their size and developmental stage. Their size ranges from less than 2 µm to 200mm. Almost all the members of kingdom Animalia, such as Protozoa, Cnidarians, Arthropoda, Molluscs, Echinodermata and Chordates are considered zooplankton.

Zooplankton shows a very different feature called vertical migration in which at the night time, they move toward the surface of the water and in the day time, they move down to the deep water. This process protects them from predators, especially diurnal and also supports the phytoplankton to produce their food in the presence of sunlight. This migration depends on the season, size and sex. Zooplankton is also affected by the calcium, pH, aluminum and heavy metals.


Zooplankton is divided into two groups based on their size; meroplankton and holoplankton. This zooplankton is microscopic animals and is usually 1mm long or less than that. Meroplankton are those zooplankton that only spend a portion of their life cycle within the plankton form. Crustaceans, mollusks, Echinodermata and some small fish are examples of meroplankton. Holoplankton is that zooplankton which remains as plankton throughout their life cycle. Polychaetes, larvaceans, copepoda, pteropoda, siphonophores are examples of holoplankton.

Ecological Importance

Zooplankton is ecologically important organisms that are an integral component of the food chain. These organisms serve as an intermediate species in the food chain. They transfer energy from the plankton algae to the large invertebrate predators and fish that eventually feed on them.

Zooplankton is also an indicator of toxicity levels in the marine waters. If there is a sudden change in the level of pollution, acidity, or temperature, the zooplankton will eventually act as a sign of reference.

Key Differences

  1. Phytoplankton is plant-like organisms, whereas zooplankton is animal-like organisms.
  2. Phytoplankton is located on the surface of the water in the sunlight, whereas zooplankton is located in the darker ad cooler places of the water.
  3. Phytoplankton is autotrophs, so makes its food through photosynthesis, whereas zooplankton is heterotrophs and survives on the other life forms in the water.
  4. Phytoplankton is the primary producers in the aquatic food chain, whereas zooplankton is primary or secondary consumers in the marine food chain.
  5. Phytoplankton cannot swim against the ocean currents, whereas zooplankton does have the ability to swim to avoid predators.
  6. Phytoplankton release oxygen in the atmosphere, whereas zooplankton consumes oxygen to release carbon dioxide in the atmosphere.
  7. Phytoplankton has brown color and grows as a group in cloudy patches, whereas zooplankton exists in different colors and shapes (usually as translucent).
  8. Phytoplankton act as indicators of marine health, whereas zooplankton act as indicators of toxicity level in marine water.

Key Similarities

  1. Both phytoplankton and zooplankton belong to the group plankton.
  2. Both phytoplankton and zooplankton are microscopic.
  3. Both phytoplankton and zooplankton found in fresh and marine waters.
  4. Both phytoplankton and zooplankton are sessile organisms.


In conclusion, phytoplankton and zooplankton are categories of plankton organisms found in marine waters. Phytoplankton is microscopic plants, whereas zooplankton is microscopic animals.


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