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Science & Nature • Marine Biology

Phytoplankton

Phytoplankton are microscopic, photosynthetic organisms that drift in aquatic environments, including oceans, lakes, and rivers.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 27, 2026|2 min read| Fact-checked

Phytoplankton are microscopic, photosynthetic organisms that drift in aquatic environments, including oceans, lakes, and rivers. They are primary producers in aquatic food webs and are important for oxygen production, carbon cycling, and supporting marine and freshwater ecosystems.

Definition

Phytoplankton are microscopic organisms that float or drift in water and carry out photosynthesis. The term is used for a diverse set of organisms rather than a single biological group.

Ecological role

As primary producers, phytoplankton convert sunlight and dissolved carbon dioxide into organic matter. They support zooplankton, fish, whales such as the blue whale, jellyfish, and other aquatic organisms like shrimp.

Major types

Common phytoplankton groups include diatoms, dinoflagellates, coccolithophores, green algae, and cyanobacteria. Algae are a major component of phytoplankton, and these groups differ in structure and biology but share a photosynthetic lifestyle.

Distribution and growth

Phytoplankton occur in both marine and freshwater habitats, usually in sunlit surface waters. Their growth depends on light and nutrients such as nitrates and phosphates.

Key facts

  • Phytoplankton are the plant-like component of plankton and include both prokaryotic and eukaryotic organisms.
  • They live in the sunlit upper layers of water where photosynthesis is possible.
  • They form the base of most aquatic food webs.
  • They contribute substantially to global oxygen production and carbon cycling.
  • Major phytoplankton groups include diatoms, dinoflagellates, coccolithophores, and cyanobacteria.
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Canadian context

Phytoplankton are important in Canadian waters, including the Atlantic, Pacific, Arctic, and the Great Lakes, because they support fisheries, influence water quality, and play a major role in carbon and oxygen cycling.

Frequently asked questions

What are phytoplankton?
Phytoplankton are microscopic photosynthetic organisms that drift in water and act as primary producers in aquatic ecosystems.
Are phytoplankton plants?
Not necessarily. Many are algae, protists, or bacteria rather than true plants, though they are plant-like in how they make food.
Where do phytoplankton live?
They live in marine and freshwater environments, usually near the sunlit surface where photosynthesis can occur.
Why are phytoplankton important?
They form the base of aquatic food webs, produce oxygen, and help move carbon through the ocean and atmosphere.
What are common types of phytoplankton?
Common types include diatoms, dinoflagellates, coccolithophores, green algae, and cyanobacteria.

References

  1. Encyclopaedia Britannica — https://www.britannica.com/science/phytoplankton
    Supports: Definition, ecological role, major groups, seasonal variation
  2. NASA Earth Observatory — https://science.nasa.gov/earth/earth-observatory/what-are-phytoplankton/
    Supports: Microscopic nature, aquatic habitats, photosynthesis, common kinds, food-web role
  3. NOAA Ocean Service — https://oceanservice.noaa.gov/facts/phyto.html
    Supports: Microscopic marine algae, need for sunlight and nutrients, major classes, food-web role
  4. Woods Hole Oceanographic Institution — https://www.whoi.edu/ocean-learning-hub/ocean-topics/ocean-life/ocean-plants/phytoplankton/
    Supports: Primary producer role, photosynthesis, suspended in water, diversity
  5. Wikipedia — https://en.wikipedia.org/wiki/Phytoplankton
    Supports: Upper sunlit layer habitat, primary production, plankton classification