Plankton: drifting life in the sunlit oceans

Major groups and characteristics

Plankton include a diverse range of life forms that are commonly grouped by function or taxonomy. Phytoplankton are the photosynthetic producers that form the base of many aquatic food webs. Typical phytoplankton include eukaryotic algae such as diatoms and coccolithophores (photosynthesis, biochemical processes), along with single-celled groups listed in taxonomic treatments (eukaryote algae, diatoms, dinoflagellates). Cyanobacteria are photosynthetic bacteria that also contribute substantially to primary production in many regions (cyanobacteria).

• Zooplankton: heterotrophic animals and protists that graze on phytoplankton or other zooplankton. Examples include protozoans, tiny crustaceans, jellyfish-like ctenophores and medusae, rotifers, and foraminifera (protozoans, ctenophores, jellyfish, rotifers, foraminifera, small crustacea).
• Reproductive stages and larvae: many larger animals spend planktonic stages as eggs and larvae before settling or growing into nektonic life (eggs, larvae, annelid larvae).
• Mixotrophs and gray areas: some species combine photosynthesis and grazing (mixotrophy), for instance certain dinoflagellates that may behave as producers or consumers depending on conditions (dinoflagellates, heterotrophy).
• Viruses and microbes: viruses are abundant in planktonic communities and play roles in mortality and nutrient recycling, even if they do not fit neatly into the producer/consumer scheme (viruses).

Ecological role and food webs

Because many planktonic organisms are primary producers or primary consumers, they are fundamental to aquatic food chains. Phytoplankton convert sunlight and inorganic nutrients into organic matter that feeds herbivorous zooplankton; those, in turn, are eaten by larger predators. This connection supports a wide range of animals from small fish larvae to large filter feeders such as basking sharks and baleen whales (food chains, fish larvae, larval feeding, basking sharks, blue whales). Many commercially important fish species depend indirectly on plankton because juvenile stages feed on plankton or plankton-fed prey like herrings (herrings).

Distribution, limiting factors and biogeography

The abundance and composition of plankton across the oceans are governed largely by the availability of light and nutrients rather than temperature alone. Regions near continents often receive nutrients delivered by rivers and winds, supporting higher productivity, while vast ocean gyres remain oligotrophic (nutrient-poor) and appear blue and biologically sparse. In some open-ocean areas, the scarcity of specific trace elements such as iron limits phytoplankton growth because iron is a component of essential proteins involved in electron transfer and metabolism (limiting nutrients, iron, iron-sulfur, proteins, electron transfer, metabolic reactions). These nutrient dynamics shape which plankton species dominate and influence seasonal blooms and fisheries productivity.

Study methods, human impacts and notable phenomena

Plankton are studied using nets, optical instruments, microscopy, genetic sequencing, and remote sensing of chlorophyll. Observations reveal how plankton respond to climate variability, nutrient inputs, and human activities. For example, increased nutrient runoff can cause harmful algal blooms, and warming can shift species distributions. Conversely, plankton influence the global carbon cycle: photosynthetic plankton fix carbon dioxide and, through biological processes, can transfer carbon to deeper waters. Understanding these interactions is an active area of research because changes in plankton communities can cascade through marine ecosystems and affect fisheries, biogeochemical cycles, and climate feedbacks.

Because plankton include microscopic bacteria and viruses as well as visible jellylike animals, they represent a broad functional and taxonomic array rather than a single biological clade. Their study crosses disciplines—from taxonomy and ecology to oceanography and climate science—and links microscopic processes to planetary-scale patterns. For accessible introductions and further reading, see general references and curated resources (introduction, ocean biology, sunlit zone). Additional topic-specific materials are available on phytoplankton (photosynthesis basics), plankton sampling (methods), and the roles of microbes and viruses (viral ecology), among many other sources.