planktonic

Planktonic refers to organisms that live in the water column and are primarily carried by currents rather than actively swimming against them over long distances. The term is derived from the Greek planktikos, meaning drifting. It contrasts with nektonic organisms, which swim with purpose, and benthic organisms, which inhabit the seafloor or lakebed.

Planktonic communities include two broad groups: phytoplankton, which are photosynthetic and form the base of aquatic food webs, and zooplankton, which are animals that feed on other plankton or organic matter. Typical phytoplankton examples include diatoms, coccolithophores, and dinoflagellates. Zooplankton includes copepods, krill, and the larval stages of many invertebrates. Size ranges from microscopic cells to larger gelatinous organisms; some species spend only part of their life cycle in the plankton, while others are planktonic throughout.

Ecologically, planktonic organisms are essential to marine and freshwater ecosystems. They convert inorganic carbon into organic matter, support higher trophic levels, and influence biogeochemical cycles, including the carbon cycle through the biological carbon pump. Their distribution and abundance are influenced by light, nutrients, temperature, and water column stability. Phytoplankton blooms can rapidly alter local ecosystems and biogeochemical fluxes, while shifts in plankton communities can propagate through the food web.

Movement in the plankton is typically governed by currents and turbulence; many planktonic organisms also engage in limited swimming. A notable behavior is diel vertical migration, where some zooplankton move to deeper waters by day and rise to feeding opportunities at night. The term is widely used to describe organisms or life stages that inhabit or pass through the water column as drifters.