The term “planktonic” refers to any organism that drifts or is carried by water currents. The word originates from the Greek planktos, meaning “wanderer” or “drifter.” Plankton are defined by their inability to actively propel themselves against ocean or freshwater currents, separating them from stronger swimmers like fish, which are classified as nekton. This diverse group includes representatives from nearly every biological kingdom, ranging from bacteria and microscopic algae to larger invertebrates such as jellyfish.
The Two Major Types of Plankton
The plankton community is broadly classified into two functional groups: primary producers and consumers. Phytoplankton, meaning “plant-drifters,” are autotrophs that form the base of the food web by generating their own food through photosynthesis. They require sunlight to convert carbon dioxide and water into energy, confining them to the sunlit, or photic, zone near the water’s surface. Prominent examples include diatoms, which are encased in silicate shells, and dinoflagellates.
Zooplankton, or “animal-drifters,” are heterotrophic consumers that feed on phytoplankton or smaller zooplankton. These organisms range from single-celled protozoans to larger crustaceans, such as copepods and krill, and the larval stages of fish and other invertebrates. Zooplankton transfer energy from microscopic producers up to higher trophic levels, supporting the survival of larger marine animals. Although considered drifters, some species can use appendages to perform short vertical movements or escape predators, but they cannot overcome large-scale currents.
Life in the Water Column
Maintaining a position in the water column is challenging because many plankton are denser than seawater and would naturally sink out of the sunlit zone. To counteract gravity, these organisms have developed specialized physical and chemical adaptations to increase buoyancy and drag. For instance, many diatoms possess long spines or form chains, which increases their surface area-to-volume ratio to slow sinking. Other plankton, like some dinoflagellates, use two whip-like tails, or flagella, to create a corkscrew motion that allows them to migrate vertically to find nutrients or light.
Buoyancy is also regulated chemically by storing low-density materials within their cells, such as oil droplets or large vacuoles. Plankton size spans an enormous range, from picoplankton less than two micrometers wide to megaplankton like large jellyfish, which can measure over 200 millimeters. When plankton die or zooplankton excrete waste, their organic matter begins to sink, forming a continuous shower of decaying material. This sinking debris is known as “marine snow,” which delivers organic carbon and nutrients to deep-sea ecosystems below.
Global Impact and Essential Functions
The collective activity of plankton powers global biogeochemical cycles and sustains most aquatic life. Phytoplankton form the base of nearly all marine food webs, serving as the primary food source for zooplankton, which feed small filter feeders, fish, and the largest baleen whales. This constant supply of energy from planktonic producers is necessary to support all higher-order consumers.
Phytoplankton also regulate the Earth’s atmosphere through oxygen production. Like land plants, they use photosynthesis, and their global output generates an estimated 50% to 80% of the oxygen we breathe. Plankton are also involved in the global carbon cycle through the biological carbon pump. This process involves phytoplankton drawing down atmospheric carbon dioxide into the surface ocean during photosynthesis. When the plankton die or are consumed, the carbon-rich material sinks, transporting carbon to the deep ocean where it can be stored for thousands of years.