plankton

Plankton is a collective term for drifting organisms that inhabit aquatic environments. Most plankton drift with currents and are unable to swim fast enough to move against them. The term covers a wide range of life forms, from microscopic cells to some larger, weakly swimming species.

Plankton are commonly divided into phytoplankton, which are photosynthetic, and zooplankton, which are heterotrophic. Phytoplankton include diatoms, dinoflagellates, and cyanobacteria; zooplankton include copepods, krill, jellyfish larvae, and the larval stages of many larger animals. Bacterioplankton refers to bacteria that live in the plankton, while meroplankton are organisms that are planktonic only for part of their life cycle.

Ecologically, plankton form the base of aquatic food webs. Phytoplankton are primary producers that convert light energy into organic matter, supporting a wide range of organisms. Zooplankton feed on phytoplankton and on smaller zooplankton, transferring energy through the system. Plankton drive global biogeochemical cycles, contributing to oxygen production and carbon sequestration via the biological pump. Blooms can occur when nutrients and light are plentiful.

Distribution and abundance vary with nutrients, light, temperature, and currents. Plankton are found from freshwaters to open oceans, in coastal zones and the deep sea. Size ranges span from less than a micrometer for some bacteria to several centimeters for large copepods and jellyfish within the plankton community. In marine systems, phytoplankton are the most abundant organisms by biomass.

Human activities influence plankton communities and the processes they support. Nutrient pollution, warming oceans, acidification, and overfishing can alter community composition and the timing of blooms. Because plankton respond quickly to environmental changes, they are commonly used as indicators of ecosystem health and climate change.