Plankton are organisms that live suspended in water and whose movement is largely governed by currents rather than sustained swimming against them. They occur in the ocean and freshwater environments, and range from microscopic organisms to large jellyfish. The term describes an ecological way of life, not a single evolutionary or taxonomic group. Plankton support aquatic food webs, recycle nutrients, and influence the movement of carbon through the environment. Their name derives from a Greek word meaning “wanderer” or “drifter.” (oceanservice.noaa.gov)
Major groups
Phytoplankton are the photosynthetic component of plankton. They use photosynthesis to convert inorganic carbon into organic matter, releasing oxygen. Although often described as microscopic plants, they include both eukaryotic algae and photosynthetic bacteria, particularly cyanobacteria. Diatoms and dinoflagellates are prominent algal groups. Because photosynthesis requires light, phytoplankton grow chiefly in the sunlit upper waters, whose depth varies with water clarity. (whoi.edu)
Zooplankton comprise animal plankton and heterotrophic protists that obtain food from other organisms or organic material. Familiar examples include copepods, krill, jellyfish, and the larvae of fish and marine invertebrates. Some graze on phytoplankton; others consume smaller zooplankton. They therefore occupy several feeding positions rather than forming one uniform group of herbivores. Many are capable of swimming, but currents still strongly determine their horizontal transport. (oceanservice.noaa.gov)
Microbial plankton also include heterotrophic bacteria and archaea. Their activities recycle organic matter and nutrients throughout the water column. The traditional separation between photosynthetic and consumer plankton is not absolute: some protists combine feeding with photosynthesis. For example, the ciliate Mesodinium rubrum can retain photosynthetic machinery from algal prey. Such organisms illustrate mixotrophy, the combination of different nutritional modes. (whoi.edu)
Life cycles and size
Plankton are classified partly by how much of their life cycle they spend drifting. Holoplankton remain planktonic throughout their lives, whereas meroplankton are planktonic only during particular developmental stages. Many marine invertebrates have drifting larvae that later settle to the seabed. Fish eggs and larvae are collectively termed ichthyoplankton; their distribution can reveal spawning locations and the transport of young fish toward or away from nursery areas. (st.nmfs.noaa.gov)
Size provides another useful classification. Plankton communities contain extremely small microbial cells, larger single-celled organisms, small crustaceans, and conspicuous gelatinous animals. Consequently, “plankton” is not synonymous with “microscopic.” These size differences also affect sampling: some photosynthetic bacteria are too small for conventional plankton nets, while larger organisms require nets with different mesh sizes or direct imaging methods. No single collecting technique captures the whole community equally well. (whoi.edu)
Distribution and seasonal dynamics
Plankton abundance reflects the interaction of light, nutrient supply, temperature, circulation, and consumption by other organisms. Phytoplankton require nutrients such as nitrogen, phosphorus, and iron as well as sufficient illumination. Upwelling, which brings deeper, nutrient-rich water toward the surface, supports productive communities along many coasts and in parts of the equatorial ocean. Conversely, strongly layered waters can restrict nutrient renewal in the surface layer. (science.nasa.gov)
A plankton bloom is a rapid increase in abundance under favorable conditions. Blooms often follow seasonal changes in light, mixing, and nutrient availability. Their timing matters to consumers: larval fish survival can depend on whether suitable planktonic food is available when the larvae begin feeding. Changes in bloom timing can therefore propagate through an aquatic ecosystem without affecting every organism in the same way. (st.nmfs.noaa.gov)
Many zooplankton undertake diel vertical migration, moving toward the surface at night to feed and descending during daylight, when exposure to visually hunting predators is greater. These daily movements connect surface and deeper communities and transport carbon through feeding, waste production, and other biological processes. (oceanservice.noaa.gov)
Ecological and biogeochemical roles
Photosynthetic plankton account for approximately half of global primary production, the formation of organic matter from inorganic sources. This production supports zooplankton and, through successive feeding relationships, fish, seabirds, and marine mammals. Planktonic microbes also transform and recycle elements, making nutrients available for further growth rather than allowing all organic material to accumulate as waste. (st.nmfs.noaa.gov)
Plankton participate in the carbon cycle through the biological carbon pump. Phytoplankton incorporate dissolved carbon dioxide into living tissue. Much of this carbon is consumed and recycled in upper waters, but some sinks as dead organisms, fecal pellets, and aggregates known as marine snow. Carbon reaching sufficiently deep water can remain separated from the atmosphere for centuries or longer. Carbon fixation at the surface is therefore distinct from long-term carbon storage. (whoi.edu)
Environmental change and observation
Nutrient enrichment can contribute to excessive plankton growth. Some blooms deplete oxygen, while certain species produce toxins; these are termed harmful algal blooms. They are not an inevitable feature of every bloom. Plankton communities also respond to changes in salinity, acidity, and temperature. Research on climate change examines shifts in their abundance, composition, seasonal timing, and interactions with consumers. (whoi.edu)
Scientists study plankton using nets, water samples, microscopy, underwater imaging, and flow cytometry. Depth-specific sampling reveals how communities vary vertically. Satellite remote sensing measures ocean color to estimate chlorophyll concentrations, providing broad observations of phytoplankton patterns. These estimates complement rather than replace direct sampling, which supplies information about organism identity, abundance, and size. (prod-01-alb-www-oe.woc.noaa.gov)