Marine microplankton and mesoplankton Part of the contents of one dip of a hand net. The image contains diverse planktonic organisms, ranging from photosynthetic cyanobacteria and diatoms to many different types of zooplankton, including both holoplankton (permanent residents of the plankton) and meroplankton (temporary residents of the plankton, e.g., fish eggs, crab larvae, worm larvae). 100 μm = one tenth of a mm.
Plankton are organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind).[1][2] Marine plankton include drifting organisms that inhabit the saltwater of oceans and the brackish waters of estuaries. Freshwater plankton are similar to marine plankton, but are found in lakes and rivers. An individual plankton organism in the plankton is called a plankter.[3] In the ocean plankton provide a crucial source of food, particularly for larger filter-feeding animals, such as bivalves, sponges, forage fish and baleen whales.
Plankton includes organisms from many species, ranging in size from the microscopic (such as bacteria, archaea, protozoa and microscopic algae and fungi[4]) to larger organisms (such as jellyfish and ctenophores).[5] This is because plankton are defined by their ecological niche and level of motility rather than by any phylogenetic or taxonomic classification. The plankton category differentiates organisms from those that can swim against a current, called nekton, and those that live on the deep sea floor, called benthos. Organisms that float on or near the water's surface are called neuston. Neuston that drift as water currents or wind take them, and lack the swimming ability to counter this, form a special subgroup of plankton. Mostly plankton just drift where currents take them, though some, like jellyfish, swim slowly but not fast enough to generally overcome the influence of currents.
Microscopic plankton, smaller than about one millimetre in size, play crucial roles in marine ecosystems. They are a diverse group, including phytoplankton (like diatoms and dinoflagellates) and zooplankton (such as radiolarians, foraminifera and some copepods), and serve as a foundational component of the marine food web. These largely unseen microscopic plankton drive primary production, support local food webs, cycle nutrients, and influence global biogeochemical processes. Their role is foundational for maintaining the health and balance of marine ecosystems.
Although plankton are usually thought of as inhabiting water, there are also airborne versions that live part of their lives drifting in the atmosphere. These aeroplankton can include plant spores, pollen and wind-scattered seeds. They can also include microorganisms swept into the air from terrestrial dust storms and oceanic plankton swept into the air by sea spray.
Overview[edit] Ocean chlorophyll concentration is a proxy for, or an indicator of, the distribution and abundance of phytoplankton. The intensity of green indicates how abundant the phytoplankton are, while blue indicates where there are few phytoplankton. – NASA Earth Observatory, October 2019.[6]Apart from aeroplankton, plankton inhabits oceans, seas, estuaries, rivers, lakes and ponds. Local abundance varies horizontally, vertically and seasonally. The primary cause of this variability is the ailability of light. All plankton ecosystems are driven by the input of solar energy (but see chemosynthesis), confining primary production to surface waters, and to geographical regions and seasons hing abundant light.
A secondary variable is nutrient ailability. The amount and distribution of plankton depends on ailable nutrients, the state of water and a large amount of other plankton.[7] The local distribution of plankton can be affected by wind-driven Langmuir circulation and the biological effects of this physical process. Although large areas of the tropical and sub-tropical oceans he abundant light, they experience relatively low primary production because they offer limited nutrients such as nitrate, phosphate and silicate. This results from large-scale ocean circulation and water column stratification. In such regions, primary production usually occurs at greater depth, although at a reduced level (because of reduced light).
While plankton are most abundant in surface waters, they live throughout the water column. At depths where no primary production occurs, zooplankton and bacterioplankton instead consume organic material sinking from more productive surface waters above. This flux of sinking material, so-called marine snow, can be especially high following the termination of spring blooms.
Despite significant macronutrient concentrations, some ocean regions are unproductive (so-called HNLC regions).[8] The micronutrient iron is deficient in these regions, and adding it can lead to the formation of phytoplankton algal blooms.[9] Iron primarily reaches the ocean through the deposition of dust on the sea surface. Paradoxically, oceanic areas adjacent to unproductive, arid land thus typically he abundant phytoplankton (e.g., the eastern Atlantic Ocean, where trade winds bring dust from the Sahara Desert in north Africa).
Within the plankton, holoplankton spend their entire life cycle as plankton (e.g. most algae, copepods, salps, and some jellyfish). By contrast, meroplankton are only planktic for part of their lives (usually the larval stage), and then graduate to either a nektic (swimming) or benthic (sea floor) existence. Examples of meroplankton include the larvae of sea urchins, starfish, crustaceans, marine worms, and most fish.[10]
Microscopic plankton[edit] MicroplanktonThe coccolithophore Emiliania huxleyi (μm = thousandth of one mm)Algae bloom of Emiliania huxleyi off the southern coast of EnglandPlankton is mostly made up of planktonic microorganisms less than one millimetre across, most invisible to the naked eye and needing a microscope if they are to be visually examined. Microorganisms he been variously estimated to make up about 70%,[11] or about 90%,[12][13] of the total ocean biomass. Taken together they form the marine microbiome. Over billions of years this microbiome has evolved many life styles and adaptations and come to participate in the global cycling of almost all chemical elements.[14]
Microplankton are ecological linchpins in the marine food web. They are crucial to nutrient recycling in the way they act as decomposers. They are also responsible for nearly all photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen, phosphorus and other nutrients and trace elements.[15] Microplankton sequesters large amounts of carbon and produce much of the world's oxygen.
It is estimated marine viruses kill 20% of ocean microplankton biomass every day. Viruses are the main agents responsible for the rapid destruction of harmful algal blooms which often kill other marine life. The number of viruses in the plankton decreases further offshore and deeper into the water, where there are fewer host organisms.
Terminology[edit] Plankton species diversity Diverse assemblages of unicellular and multicellular organisms with different sizes, shapes, feeding strategies, ecological functions, life cycle characteristics, and environmental sensitivities.[16] Courtesy of Christian Sardet/CNRS/Tara expeditionsThe name plankton was coined by German marine biologist Victor Hensen in 1887 from shortening the word halyplankton from Greek ᾰ̔́λς háls "sea" and πλανάω planáō to "drift" or "wander".[17]: 1 While some forms are capable of independent movement and can swim hundreds of meters vertically in a single day (a behior called diel vertical migration), their horizontal position is primarily determined by the surrounding water movement, and plankton typically flow with ocean currents. This is in contrast to nekton organisms, such as fish, squid and marine mammals, which can swim against the ambient flow and control their position in the environment.
The study of plankton is termed planktology and a planktonic individual is referred to as a plankter.[18] The adjective planktonic is widely used in both the scientific and popular literature, and is a generally accepted term. However, from the standpoint of prescriptive grammar, the less-commonly used planktic is more strictly the correct adjective. When deriving English words from their Greek or Latin roots, the gender-specific ending (in this case, "-on" which indicates the word is neuter) is normally dropped, using only the root of the word in the derivation.[19]
By habitat[edit] Aeroplankton[edit] Main article: Aeroplankton Sea spray containing microorganisms in marine plankton can be swept high into the atmosphere and may trel the globe as aeroplankton before falling back to earth.Aeroplankton are tiny lifeforms that float and drift in the air, carried by the current of the wind; they are the atmospheric analogue to oceanic plankton. Most of the living things that make up aeroplankton are very small to microscopic in size, and many can be difficult to identify because of their tiny size. Scientists can collect them for study in traps and sweep nets from aircraft, kites or balloons.[20] Aeroplankton is made up of numerous microbes, including viruses, about 1000 different species of bacteria, around 40,000 varieties of fungi, and hundreds of species of protists, algae, mosses and liverworts that live some part of their life cycle as aeroplankton, often as spores, pollen, and wind-scattered seeds. Additionally, peripatetic microorganisms are swept into the air from terrestrial dust storms, and an even larger amount of airborne marine microorganisms are propelled high into the atmosphere in sea spray. Aeroplankton deposits hundreds of millions of airborne viruses and tens of millions of bacteria every day on every square meter around the planet. This means similar mixes of microscopic plankton taxon can be found in open bodies of water around the world.[21][22][23]
The sea surface microlayer, compared to the sub-surface waters, contains elevated concentration of bacteria and viruses.[24][25] These materials can be transferred from the sea-surface to the atmosphere in the form of wind-generated aqueous aerosols due to their high vapour tension and a process known as volatilisation.[26] When airborne, these microbes can be transported long distances to coastal regions. If they hit land they can he an effect on animal, vegetation and human health.[27] Marine aerosols that contain viruses can trel hundreds of kilometers from their source and remain in liquid form as long as the humidity is high enough (over 70%).[28][29][30] These aerosols are able to remain suspended in the atmosphere for about 31 days.[31] Evidence suggests that bacteria can remain viable after being transported inland through aerosols. Some reached as far as 200 meters at 30 meters above sea level.[32] The process which transfers this material to the atmosphere causes further enrichment in both bacteria and viruses in comparison to either the SML or sub-surface waters (up to three orders of magnitude in some locations).[32]
Freshwater plankton[edit]Freshwater plankton parallel marine plankton (below), but are found inland in the freshwaters of lakes and rivers.
Geoplankton[edit] See also: Geoplankton A gastrotrich can lay resilient eggs capable of surviving years in a dry environment. Scale bar: 20 μm.Many animals live in terrestrial environments by thriving in transient often microscopic bodies of water and moisture, these include rotifers and gastrotrichs which lay resilient eggs capable of surviving years in dry environments, and some of which can go dormant themselves. Nematodes are usually microscopic with this lifestyle. Water bears, despite only hing lifespans of a few months, famously can enter suspended animation during dry or hostile conditions and survive for decades. This allows them to be ubiquitous in terrestrial environments despite needing water to grow and reproduce. Many microscopic crustacean groups like copepods and amphipods (of which sandhoppers are members) and seed shrimp are known to go dormant when dry and live in transient bodies of water too[33]
Marine plankton[edit] Plankton (organisms that drift with water currents) can be contrasted with nekton (organisms that can swim against water currents) and benthos (organisms that live at the ocean floor). There are also neuston (organisms that live at the ocean surface). Neuston that cannot swim against currents or the wind are a special subset of plankton.Marine plankton includes marine protists (algae and protozoa), drifting and floating animals (particularly microanimals), marine prokaryotes (bacteria and archaea), and marine viruses that inhabit the saltwater of oceans and the brackish waters of estuaries.
At the ocean surface[edit] Further information: Neuston
Plankton are also found at the ocean surface. Organisms that live at or just below the air-sea interface are called neuston. They float either on the water's surface (epineuston) or swim in the top few centimeters (hyponeuston). Many neuston qualify to be categorised as part of the broader plankton community, because they drift largely as currents or wind dictate, lacking strong enough swimming ability to counter them.[34][35][36]
Neustonic animals are primarily adapted to float upside-down on the ocean surface, similar to an inverted benthos,[37] and form a unique subset of the zooplankton community, which plays a pivotal role in the functioning of marine ecosystems.[38] Neustonic zooplankton are partially responsible for the active energy flux between superficial and deep layers of the ocean.[39][40][41]Neustonic plankton is also a food source for marine zooplankton and fish migrating from the deep layers and seabirds.[34]
In deep ocean[edit] The ocean conveyor belt carries warm surface waters (red) northward near the surface and cold deep waters (blue) southward. Diverse and flourishing microbial ecosystems he been found deep in the belt.[42][43]
In 2025, researchers discovered microbial communities inhabiting the ocean conveyor belt, even at great depths in the ocean.[42][43] Ocean currents are generated by surface wind and storms down to about 500 m (1,600 ft) below the surface. But the erage depth of the ocean goes far below to 3.7 km (2.3 mi).[44] At these greater depths, currents are driven by differences in water density, which in turn are controlled by differences in water temperature and salinity. This mechanism results in a circulation which behes like a conveyor belt, carrying water and microorganisms to great depths and around the world.[42]
Water samples were taken along the full depth of the water column in the South Pacific Ocean, from Easter Island to Antarctica. They found marked increases in microbial diversity about 300 m (1,000 ft) deep, in a layer they call the prokaryotic phylocline. This zone, similar to the pycnocline, represents a shift from less diverse surface waters to abundant microbial ecosystems in the deep ocean. For instance, a group they called the Antarctic Bottom Water contains microbes suited to cold and high pressure, while another group they called the Ancient Water Group, located in slowly circulating water isolated from the surface for over a millennium, contains microbes with genes adapted to low oxygen.[42][43]
By taxon[edit]Plankton contains representatives from all major divisions of life. By taxon, it can be divided into the following broad groups:[45][46]
planktonic animals (metazoa) : – mostly predators (zooplankton) of smaller plankton. Examples are arrow worms, sea butterfly, ostracods, and salps. There are also planktonic microanimals typically smaller than one mm, such as copepods, water fleas, rotifers, and larval stages of various crustaceans and corals. planktonic protists: – single-celled eukaryote microorganisms, mostly invisible to the naked eye, such as diatoms, dinoflagellates, coccolithophores, foraminifera, radiolarians, and ciliates. Planktonic protists include algae (phytoplankton), protozoa (zooplankton), and many mixoplankton.[47] planktonic fungi: – known also as mycoplankton, play important roles in remineralisation and nutrient cycling.[48] For example, in the mycoloop, parasitic chytrids facilitate the transfer of nutrients from large, inedible phytoplankton to zooplankton. planktonic prokaryotes: – (bacteria and archaea – planktonic bacteria are known also as bacterioplankton) can play important roles as primary producers, or in remineralising organic material like mycoplankton down the water column. Photosynthetic cyanobacteria are important members of the phytoplankton. The unusually small Pelagibacter ubique, perhaps the most abundant bacterium on Earth, makes up about one third of microbial cells in the surface ocean,[49] and plays important roles recycling nutrients in the microbial loop. The Roseobacter clade are significantly connected to phytoplankton. planktonic viruses: – known also as virioplankton, though not always classified as living organisms, are abundant in planktonic communities and influence microbial dynamics. Viruses are small infectious agents that can replicate only inside the living cells of a host organism, because they need the replication machinery of the host to do so.[50] They are more abundant in the plankton than bacteria and archaea, though much smaller.[51][52] Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.[53] In the viral shunt, viruses infect and break down (lyse) bacteria, releasing their nutrients and organic matter back into the water instead of allowing them to be consumed by larger organisms like zooplankton. This "shunts" nutrients away from higher trophic levels, keeping them in the microbial loop for reuse by other microorganisms. This planktonic animal (metazoa) is a female copepod. It has two egg sacs and microalgae attached to its body These are shells of planktonic protists called radiolarians, drawn by Ernst Haeckel (1904) This planktonic bacterium is the cyanobacterium Prochlorococcus, the smallest photosynthetic organism in the world. It contributes up to 20% of the world's oxygen production, more than all tropical rainforests.[54] This planktonic virus (arrowed) is the giant coccolithovirus, Emiliania huxleyi virus 86, infecting an Emiliania huxleyi coccolithophore Plankton sizes by taxonomic groups [55] By size[edit] Part of a series onPlankton By habitat Aeroplankton Geoplankton Marine plankton prokaryotes protists Freshwater plankton By taxon Algae diatoms coccolithophores dinoflagellates Protozoans radiolarians foraminiferans amoebae ciliates Bacteria cyanobacteria Archaea Viruses By size Heterotrophic picoplankton Microalgae Microzooplankton Nanophytoplankton calcareous Photosynthetic picoplankton Picoeukaryote Picoplankton Marine microplankton By trophic mode Phytoplankton Zooplankton Mixoplankton Decomposition microbial loop mycoloop viral shunt Other types Gelatinous zooplankton Holoplankton Ichthyoplankton Meroplankton Pseudoplankton Tychoplankton Blooms Algal bloom Critical depth Cyanobacterial bloom Harmful algal bloom Spring bloom Eutrophication Great Atlantic Sargassum Belt Great Calcite Belt Milky seas effect Related topics Algaculture CLAW hypothesis CPR Diel vertical migration f-ratio Marine primary production Ocean fertilization iron Paradox of the plankton Planktivore Planktology Thin layers NAAMES CategoryvtePlankton are also often described in terms of size. Usually the following divisions are used: [56]
Group Size range (ESD) Examples Megaplankton > 20 cm metazoans; e.g. jellyfish; ctenophores; salps and pyrosomes (pelagic Tunicata); Cephalopoda; Amphipoda Macroplankton 2→20 cm metazoans; e.g. Pteropoda; Chaetognaths; Medusae; ctenophores; salps, doliolids and pyrosomes (pelagic Tunicata); Cephalopoda; Janthina and Recluzia (two genera of gastropods); Amphipoda Mesoplankton 0.2→20 mm metazoans; e.g. copepods; Medusae; Cladocera; Ostracoda; Chaetognaths; Pteropoda; Tunicata Microplankton 20→200 μm large eukaryotic protists; most phytoplankton; Protozoa Foraminifera; tintinnids; other ciliates; Rotifera; juvenile metazoans – Crustacea (copepod nauplii) Nanoplankton 2→20 μm small eukaryotic protists; small diatoms; small flagellates; Pyrrophyta; Chrysophyta; Chlorophyta; Xanthophyta Picoplankton 0.2→2 μm small eukaryotic protists; bacteria; Chrysophyta Femtoplankton < 0.2 μm marine virusesHowever, some of these terms may be used with very different boundaries, especially on the larger end. The term microplankton is sometimes used more broadly to cover plankton that cannot really be seen without using a microscope, say plankton less than about one millimetre across. The existence and importance of nano- and even smaller plankton was only discovered during the 1980s, but they are thought to make up the largest proportion of all plankton in number and diversity. It is the largely unseen microplankton that are the main drivers of the marine food web.
Microplankton and smaller groups are microorganisms that operate at low Reynolds numbers, where the viscosity of water is more important than its mass or inertia.[57]
Microplankton Some marine diatoms — a key phytoplankton group Pelagibacter ubique, the most common bacteria in the ocean, plays a major role in global carbon cycles The sea sparkle dinoflagellate glows in the night to produce the milky seas effect Microzooplankton are major grazers of the plankton: two dinoflagellates and a tintinnid ciliate. Macroplankton Macrozooplankton: the amphipod Hyperia macrocephala The sea walnut ctenophore has a transient anus which forms only when it needs to defecate[58] A Janthina janthina snail (with bubble float) cast up onto a beach in Maui Sargassum seaweed drifts with currents using air bladders to stay afloat By trophic mode[edit]Trophic mode describes the role of a planktonic organism in the food web based on how it obtains energy and nutrients to sustain its growth, reproduction, and survival.[1] By trophic mode, plankton can be divided into four broad functional groups: phytoplankton, zooplankton, mixoplankton and decomposers.[59][60][61][62]
Phytoplankton[edit]Phytoplankton (from Greek phyton, or plant) are autotrophic prokaryotic or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthesis. Among the more important groups are the diatoms, cyanobacteria, dinoflagellates, and coccolithophores.
Phytoplankton largely form the base of the marine food web Diatoms are one of the most common types of phytoplankton Fossil diatom frustule from 32 to 40 mya A cyanobacteria species (Cylindrospermum sp) Green algae, Pyramimonas Zooplankton[edit]Zooplankton (from Greek zoon, or animal) are small protozoans or metazoans (e.g. crustaceans and other animals) that feed on other plankton. Some of the eggs and larvae of larger nektonic animals, such as fish, crustaceans, and annelids, are included here.
Larger plankton tend to be zooplankton which eat phytoplankton Herring larva imaged with the remains of the yolk and the long gut visible in the transparent animal Icefish larvae from Antarctica he no haemoglobin Copepod from Antarctica, a translucent ovoid microanimal with two long antennae A krill larva is zooplankton, though an adult (shown) is nekton Mixoplankton[edit]Mixoplankton (from Greek mixis, or mixture) he a mixed trophic strategy. In recent years, there has been a growing recognition that perhaps the majority of plankton can act in both the above modes.
Traditionally, plankton were divided into just the first two broad trophic groups: plant-like phytoplankton which make their own food, usually by photosynthesis, and animal-like zooplankton that eat other plankton. In recent years, there has been a recognition that many plankton, perhaps over half, are mixotrophic.[63] Plankton he traditionally been categorized as producer, consumer, and recycler groups, but some plankton are able to benefit from more than just one trophic level. This mixed trophic strategy means mixoplankton can act as both producers and consumers, either at the same time or switching between modes of nutrition in response to ambient conditions. In this manner, mixoplankton can use photosynthesis for growth when nutrients and light are abundant, but switch to eating phytoplankton, zooplankton or each other when growing conditions are poor.
As a result of these findings, many plankton formally categorized as phytoplankton, including coccolithophores and dinoflagellates, are longer included as strictly phytoplankton, as they not only produce their own food through phototrophy but can also eat other organisms.[64] These organisms are now more correctly termed mixoplankton.[61] This recognition has important consequences for how the functioning of the planktonic food web is viewed.[65]
Mixoplankton can behe both as phytoplankton and zooplankton A single-celled ciliate with green zoochlorellae living inside endosymbiotically Euglena mutabilis, a photosynthetic flagellate The mixotrophic dinoflagellate Karenia brevis causes harmful red tides Acantharian radiolarian hosts Phaeocystis symbionts. White Phaeocystis algal foam washing up on a beachMixotrophs can be divided into two groups; constitutive mixotrophs which are able to perform photosynthesis on their own, and non-constitutive mixotrophs which use phagocytosis to engulf phototrophic prey that are either kept alive inside the host cell, which benefits from its photosynthesis, or they digested, except for the plastids, which continue to perform photosynthesis (kleptoplasty).[66] Recognition of the importance of mixotrophy as an ecological strategy is increasing,[67] as well as the wider role this may play in marine biogeochemistry.[68] Studies he shown that mixoplankton are much more important for marine ecology than previously assumed.[69][70] Their presence acts as a buffer that prevents the collapse of ecosystems during times with little to no light.[71] Mixoplankton he ancient origins and he been recognized by scientists for over a century. However, it is only in recent years that the widespread significance of mixoplankton has been gaining recognition in mainstream marine science.[72]
Decomposers[edit]Instead of directly building biomass, decomposers break organic nutrients down into inorganic forms which can be recycled (an approach which metabolically can be costly).[62]
Fungi: Mostly tiny mycoplankton (microfungi), yeast, or mobile zoospores, that can recycle organic matter through a process called the mycoloop which involves parasiting plankton.[62]
Bacteria/Archaea: These minute prokaryotes (typically