The term “plankton” refers to the diverse collection of organisms that live suspended in a body of water and are carried along by currents. These organisms, which are often microscopic, inhabit both marine and freshwater environments across the planet. Plankton are defined not by their size or biological makeup, but by their inability to move effectively against the flow of water around them. This unique, drifting lifestyle makes them a fundamental component of aquatic ecosystems, supporting life on a massive scale.
Defining Plankton
While many plankton are microscopic, ranging from bacteria to single-celled algae, the classification also includes much larger organisms such as jellyfish and the larval stages of certain fish and crustaceans. The size of an organism does not matter as much as its ability to overcome the force of the water flowing around it.
This method of classification distinguishes plankton from nekton, which are free-swimming organisms like fish, whales, and squid that can actively propel themselves against currents. Although some plankton can move vertically in the water column, perhaps to avoid predators or find nutrients, their horizontal position remains largely determined by the prevailing tides and currents. Plankton are found throughout the world’s oceans, estuaries, and even in lakes and rivers, existing as a vast, drifting community.
The Two Major Categories of Plankton
The plankton community is broadly separated into two functional groups based on how they obtain energy: phytoplankton and zooplankton.
Phytoplankton
Phytoplankton are the plant-like producers of the aquatic world, performing photosynthesis to convert sunlight and dissolved nutrients into chemical energy. Like terrestrial plants, they draw carbon dioxide from the water and release oxygen as a byproduct, effectively forming the base of the food web. This group includes organisms like diatoms, which are encased in intricate silica shells, and dinoflagellates, which use whip-like flagella for minor movement. Since they require sunlight for photosynthesis, phytoplankton are primarily found in the upper, sunlit layer of the water column, known as the photic zone.
Zooplankton
Zooplankton represent the animal-like consumers, relying on other organisms for their energy, a process known as heterotrophy. They function as the primary grazers in the water, feeding on phytoplankton or preying on smaller zooplankton. The group includes permanent planktonic residents, called holoplankton, such as tiny crustaceans like copepods, which are among the most abundant multicellular animals on Earth. Zooplankton also includes meroplankton, which are the temporary larval stages of animals that will eventually become larger, free-swimming nekton or bottom-dwelling organisms, such as the larvae of crabs, clams, and fish.
Plankton’s Essential Role in Global Systems
Beyond their individual biological functions, plankton collectively exert an enormous influence on the planet’s atmosphere and its biodiversity. Their combined actions are fundamental to supporting life both within and outside of the water.
Phytoplankton are responsible for producing a significant portion of the Earth’s atmospheric oxygen through photosynthesis. Estimates suggest that these ocean-dwelling organisms generate between 50% and 80% of the oxygen present in the air we breathe. This massive-scale gas exchange means that every second breath a person takes can be traced back to the activity of these microscopic drifters in the oceans.
As the primary producers, phytoplankton form the first trophic level in the marine food web, initiating the flow of energy that sustains the entire ecosystem. Zooplankton consume the phytoplankton, and in turn, are themselves consumed by larger organisms. This transfer of energy extends from small forage fish and shellfish to some of the largest creatures on the planet. Massive filter feeders, such as baleen whales, depend almost entirely on consuming vast quantities of krill and copepods, which are types of zooplankton. The health and abundance of plankton populations are therefore directly linked to the health of global fisheries and the stability of marine biodiversity.