Part of the contents of one dip of ahand net. The image contains diverse planktonic organisms, ranging fromphotosyntheticcyanobacteria anddiatoms to many different types ofzooplankton, including bothholoplankton (permanent residents of the plankton) andmeroplankton (temporary residents of the plankton, e.g.,fish eggs, crab larvae, worm larvae). 100 μm = one tenth of a mm.
Plankton areorganisms that drift inwater (orair) but are unable to actively propel themselves againstcurrents (orwind).[1][2] Marine plankton include drifting organisms that inhabit thesaltwater ofoceans and thebrackish waters ofestuaries.Freshwater plankton are similar to marine plankton, but are found in lakes and rivers. An individual plankton organism in the plankton is called aplankter.[3] In the ocean plankton provide a crucial source of food, particularly for largerfilter-feeding animals, such asbivalves,sponges,forage fish andbaleen whales.
Plankton includes organisms from species across all the major biological kingdoms, ranging in size from themicroscopic (such asbacteria,archaea,protozoa and microscopicalgae andfungi[4]) to larger organisms (such as jellyfish and ctenophores).[5] This is because plankton are defined by theirecological niche and level ofmotility rather than by anyphylogenetic ortaxonomic classification. The plankton category differentiates organisms from those that can swim against a current, callednekton, and those that live on the deep sea floor, calledbenthos. Organisms that float on or near the water's surface are calledneuston. 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 inmarine ecosystems. They are a diverse group, includingphytoplankton (likediatoms anddinoflagellates) andzooplankton (such asradiolarians,foraminifera and somecopepods), and serve as a foundational component of themarine food web. These largely unseen microscopic plankton driveprimary 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. Theseaeroplankton can includeplant spores,pollen and wind-scatteredseeds. They can also include microorganisms swept into the air from terrestrial dust storms and oceanic plankton swept into the air bysea spray.
Oceanchlorophyll concentration is aproxy for, or an indicator of, the distribution and abundance ofphytoplankton. 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 fromaeroplankton, plankton inhabits oceans, seas, estuaries, rivers, lakes and ponds. Local abundance varies horizontally, vertically and seasonally. The primary cause of this variability is the availability of light. All plankton ecosystems are driven by the input of solar energy (but seechemosynthesis), confiningprimary production to surface waters, and to geographical regions and seasons having abundant light.
A secondary variable is nutrient availability. Theamount anddistribution of plankton depends on available nutrients, thestate of water and a large amount of other plankton.[7] The local distribution of plankton can be affected by wind-drivenLangmuir circulation and thebiological effects of this physical process. Although large areas of thetropical andsub-tropical oceans have abundant light, they experience relatively low primary production because they offer limited nutrients such asnitrate,phosphate andsilicate. This results from large-scaleocean circulation and water columnstratification. 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 andbacterioplankton instead consume organic material sinking from more productive surface waters above. This flux of sinking material, so-calledmarine snow, can be especially high following the termination ofspring blooms.
Despite significantmacronutrient concentrations, some ocean regions are unproductive (so-calledHNLC regions).[8] Themicronutrientiron is deficient in these regions, andadding it can lead to the formation of phytoplanktonalgal 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 have abundant phytoplankton (e.g., the easternAtlantic Ocean, wheretrade winds bring dust from theSahara Desert in northAfrica).
Algae bloom ofEmiliania huxleyi off the southern coast of England
Plankton is mostly made up of planktonicmicroorganisms less than one millimetre across, most visible only through a microscope. Microorganisms have been variously estimated to make up about 70%,[11] or about 90%,[12][13] of the totalocean biomass. Taken together they form themarine microbiome. Over billions of years this microbiome has evolved many life styles and adaptations and come to participate in theglobal cycling of almost all chemical elements.[14]
Microplankton are ecological linchpins in themarine food web. They are crucial to nutrient recycling in the way they act asdecomposers. They are also responsible for nearly allphotosynthesis that occurs in the ocean, as well as the cycling ofcarbon,nitrogen,phosphorus and othernutrients and trace elements.[15] Microplankton sequesters large amounts of carbon and produce much of the world's oxygen.
It is estimatedmarine viruses kill 20% of ocean microplankton biomass every day. Viruses are the main agents responsible for the rapid destruction of harmfulalgal blooms which often kill othermarine life. The number of viruses in the plankton decreases further offshore and deeper into the water, where there are fewer host organisms.
Diverse assemblages ofunicellular andmulticellular organisms with different sizes, shapes, feeding strategies, ecological functions, life cycle characteristics, and environmental sensitivities.[16]
The nameplankton was coined by German marine biologistVictor Hensen in 1887 from shortening the wordhalyplankton fromGreekἅλςháls "sea" andπλανάομαιplanáomai "(I) drift" or "(I) wander".[17]: 1 [18] Some forms of plankton are capable of independent vertically movement, and can swim hundreds of meters vertically in a single day (a behavior calleddiel vertical migration). However their horizontal position is primarily determined by the surrounding water movement, so plankton typically flow with theocean currents. This is in contrast tonekton organisms, such asfish,squid andmarine mammals, which can swim against the ambient flow and control their position in the environment.
The study of plankton is termedplanktology and a planktonic individual is referred to as a plankter.[19] The adjectiveplanktonic 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 usedplanktic 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.[20]
Sea spray containing microorganisms inmarine plankton can be swept high into the atmosphere and may travel the globe asaeroplankton before falling back to earth.
Aeroplankton are tiny lifeforms that float and drift in the air, carried by thecurrent of thewind; they are theatmosphericanalogue to oceanic plankton. Most of the living things that make up aeroplankton are very small tomicroscopic 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 fromaircraft, kites or balloons.[21] Aeroplankton is made up of numerousmicrobes, includingviruses, about 1000 different species ofbacteria, around 40,000 varieties offungi, and hundreds of species ofprotists,algae,mosses andliverworts that live some part of their life cycle as aeroplankton, often asspores,pollen, and wind-scatteredseeds. 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 planktontaxon can be found in open bodies of water around the world.[22][23][24]
Thesea surface microlayer, compared to the sub-surface waters, contains elevated concentration ofbacteria andviruses.[25][26] These materials can be transferred from the sea-surface to the atmosphere in the form of wind-generated aqueousaerosols due to their high vapour tension and a process known asvolatilisation.[27] When airborne, thesemicrobes can be transported long distances to coastal regions. If they hit land they can have an effect on animal, vegetation and human health.[28] Marine aerosols that contain viruses can travel hundreds of kilometers from their source and remain in liquid form as long as the humidity is high enough (over 70%).[29][30][31] These aerosols are able to remain suspended in the atmosphere for about 31 days.[32] 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.[33] 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).[33]
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 includerotifers andgastrotrichs 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 having 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 likecopepods andamphipods (of whichsandhoppers are members) andseed shrimp are known to go dormant when dry and live in transient bodies of water too[34]
Plankton (organisms that drift with water currents) can be contrasted withnekton (organisms that can swim against water currents) andbenthos (organisms that live at the ocean floor). There are alsoneuston (organisms that live at the ocean surface). Neuston that cannot swim against currents or the wind are a special subset of plankton.
Plankton are also found at the ocean surface. Organisms that live at or just below the air-sea interface are calledneuston. 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.[35][36][37]
Neustonic animals are primarily adapted to float upside-down on the ocean surface, similar to an inverted benthos,[38] and form a unique subset of the zooplankton community, which plays a pivotal role in the functioning of marine ecosystems.[39] Neustonic zooplankton are partially responsible for the activeenergy flux between superficial and deep layers of the ocean.[40][41][42]Neustonic plankton is also a food source for marine zooplankton and fish migrating from the deep layers and seabirds.[35]
Theocean conveyor belt carries warm surface waters (red) northward near the surface and cold deep waters (blue) southward. Diverse and flourishing microbial ecosystems have been found deep in the belt.[43][44]
In 2025, researchers discovered microbial communities inhabiting theocean conveyor belt, even at great depths in the ocean.[43][44] Ocean currents are generated by surface wind and storms down to about 500 m (1,600 ft) below the surface. But the average depth of the ocean goes far below to 3.7 km (2.3 mi).[45] 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 behaves like a conveyor belt, carrying water and microorganisms to great depths and around the world.[43]
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 theprokaryotic phylocline. This zone, similar to thepycnocline, represents a shift from less diverse surface waters to abundant microbial ecosystems in the deep ocean. For instance, a group they called theAntarctic Bottom Water contains microbes suited to cold and high pressure, while another group they called theAncient Water Group, located in slowly circulating water isolated from the surface for over a millennium, contains microbes with genes adapted to low oxygen.[43][44]
planktonic prokaryotes: – (bacteria andarchaea – planktonic bacteria are known also asbacterioplankton) can play important roles as primary producers, or in remineralising organic material like mycoplankton down the water column.Photosyntheticcyanobacteria are important members of the phytoplankton. The unusually smallPelagibacter ubique, perhaps the most abundant bacterium on Earth, makes up about one third of microbial cells in the surface ocean,[50] and plays important roles recycling nutrients in themicrobial loop. TheRoseobacter clade are significantly connected to phytoplankton.
planktonic viruses: – known also asvirioplankton, though not always classified as living organisms, are abundant in planktonic communities and influence microbial dynamics. Viruses are smallinfectious agents that canreplicate only inside the livingcells of ahostorganism, because they need the replication machinery of the host to do so.[51] They are more abundant in the plankton than bacteria and archaea, though much smaller.[52][53] Viruses can infect all types oflife forms, fromanimals andplants tomicroorganisms, includingbacteria andarchaea.[54] In theviral 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 femalecopepod. It has two egg sacs and microalgae attached to its body
This planktonic bacterium is thecyanobacteriumProchlorococcus, the smallest photosynthetic organism in the world. It contributes up to 20% of the world's oxygen production, more than all tropical rainforests.[55]
However, some of these terms may be used with very different boundaries, especially on the larger end. The termmicroplankton 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 themarine food web.
Microplankton and smaller groups aremicroorganisms that operate at lowReynolds numbers, where the viscosity of water is more important than its mass or inertia.[58]
Trophic mode describes the role of a planktonic organism in thefood 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.[60][61][62][63]
Mixoplankton (from Greekmixis, or mixture) have 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, aremixotrophic.[64] Plankton have traditionally been categorized asproducer,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, includingcoccolithophores anddinoflagellates, are longer included as strictly phytoplankton, as they not only produce their own food throughphototrophy but can also eat other organisms.[65] These organisms are now more correctly termed mixoplankton.[62] This recognition has important consequences for how the functioning of the planktonic food web is viewed.[66]
Mixoplankton can behave both as phytoplankton and zooplankton
Mixotrophs can be divided into two groups;constitutive mixotrophs which are able to perform photosynthesis on their own, and non-constitutive mixotrophs which usephagocytosis to engulf phototrophic prey that are either kept alive inside the host cell, which benefits from its photosynthesis, or they digested, except for theplastids, which continue to perform photosynthesis (kleptoplasty).[67]Recognition of the importance of mixotrophy as an ecological strategy is increasing,[68] as well as the wider role this may play in marinebiogeochemistry.[69] Studies have shown that mixoplankton are much more important for marine ecology than previously assumed.[70][71] Their presence acts as a buffer that prevents the collapse of ecosystems during times with little to no light.[72] Mixoplankton have ancient origins and have 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.[73]
Instead of directly building biomass,decomposers break organic nutrients down into inorganic forms which can be recycled (an approach which metabolically can be costly).[63]
Bacteria/Archaea: These minute prokaryotes (typically <0.001mm) return organic nutrients to inorganic forms by breaking downparticulate anddissolved organic matter through the process called themicrobial loop.[74] Some convert ammonium in animal waste to nitrate, while others transform nitrate to nitrogen gas. Viral infections likely destroy many, while others are eaten by protist zooplankton and mixoplankton, which use their nutrients for photosynthesis. However details of their ecology is complex and it is not clear what sustains them.[63]
Viruses: Typically 10 to 100 times smaller than bacteria and also the most abundant (~100 billion per litre), viruses infect other plankton and larger organisms. It is thought they efficiently halt vast plankton blooms within days, by turning biomass into dissolved organic matter that supports bacterial growth through a process called theviral shunt.[75] Being host-specific, they also likely influence thebiological andmicrobial carbon pumps.[63]
Gelatinous zooplankton are fragile animals that live in the water column in the ocean. Their delicate bodies have no hard parts and are easily damaged or destroyed.[77] Gelatinous zooplankton are often transparent.[78] Alljellyfish are gelatinous zooplankton, but not all gelatinous zooplankton are jellyfish. The most commonly encountered organisms includectenophores,medusae,salps, andChaetognatha in coastal waters. However, almost all marine phyla, includingAnnelida,Mollusca andArthropoda, contain gelatinous species, but many of those odd species live in the open ocean and the deep sea and are less available to the casual ocean observer.[79]
Salmon egg hatching into asac fry. In a few days, the sac fry will absorb the yolk sac and start feeding on smaller plankton.
Ichthyoplankton are theeggs andlarvae of fish. They are mostly found in the sunlit zone of thewater column, less than 200 metres deep, which is sometimes called theepipelagic orphotic zone. Ichthyoplankton areplanktonic, meaning they cannot swim effectively under their own power, but must drift with the ocean currents. Fish eggs cannot swim at all, and are unambiguously planktonic. Early stage larvae swim poorly, but later stage larvae swim better and cease to be planktonic as they grow intojuveniles. Fish larvae are part of thezooplankton that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larvae are themselves eaten by larger animals.[80][81] Fish can produce high numbers of eggs which are often released into the open water column. Fish eggs typically have a diameter of about 1 millimetre (0.039 in). The newly hatched young of oviparous fish are calledlarvae. They are usually poorly formed, carry a largeyolk sac (for nourishment), and are very different in appearance from juvenile and adult specimens. The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termedmetamorphosis) to become juveniles. During this transition larvae must switch from their yolk sac to feeding onzooplankton prey, a process which depends on typically inadequate zooplankton density, starving many larvae. In time fish larvae become able to swim against currents, at which point they cease to be plankton and becomejuvenile fish.
Pseudoplankton are organisms that attach themselves to planktonic organisms or other floating objects, such as drifting wood,buoyant shells of organisms such asSpirula, or man-madeflotsam. Examples includegoose barnacles and the bryozoanJellyella. By themselves these animals cannotfloat, which contrasts them with true planktonic organisms, such asVelella and thePortuguese Man o' War, which are buoyant. Pseudoplankton are often found in the guts of filteringzooplankters.[82]
Tychoplankton are organisms, such as free-living or attachedbenthic organisms and other non-planktonic organisms, that are carried into the plankton through a disturbance of their benthic habitat, or by winds and currents.[83] This can occur by directturbulence or by disruption of the substrate and subsequent entrainment in the water column.[83][84] Tychoplankton are, therefore, a primary subdivision for sorting planktonic organisms by duration of lifecycle spent in the plankton, as neither their entire lives nor particular reproductive portions are confined to planktonic existence.[85] Tychoplankton are sometimes calledaccidental plankton.
Meroplankton are a wide variety of aquatic organisms that have both planktonic andbenthic stages in their life cycles. Much of the meroplankton consists oflarval stages of larger organisms.[34] Meroplankton can be contrasted withholoplankton, which are planktonic organisms that stay in thepelagic zone as plankton throughout their entire life cycle.[89] After some time in the plankton, many meroplankton graduate to thenekton or adopt abenthic (oftensessile) lifestyle on theseafloor. The larval stages of benthicinvertebrates make up a significant proportion of planktonic communities.[90] The planktonic larval stage is particularly crucial to many benthic invertebrates in order todisperse their young. Depending on the particular species and the environmental conditions, larval or juvenile-stage meroplankton may remain in the pelagic zone for durations ranging from hours to months.[34]
The microbial loop: Bacteria play central roles in aquatic food webs. The microbial loop refers to a process in aquatic ecosystems where bacteria consumedissolved organic matter (DOM) and are then consumed by larger microorganisms, effectively cycling nutrients and energy within the ecosystem.[94]
The viral shunt: Viruses also play central roles in aquatic food webs. The viral shunt is a process where viruses infect andlyse (burst) host cells, releasing cellular contents (including dissolved organic matter) that can be utilized by other microplankton like bacteria, effectively bypassing the traditional food web pathways. This process plays a significant role in nutrient cycling and carbon flow within aquatic ecosystems.[95]
Thebasking shark usesfilter feeding to strain plankton from the water.The microbial loop: The link between the microbial loop and the aquatic food web.Dissolved organic matter (DOM) becomesparticulate organic matter (POM) as bacteria eat it and grow to form clumps. Small clumps of organic matter are eaten bybacterivores and zooplankton eat both bacterivores and big clumps of organic matter. Zooplankton are then eaten by fish. Dissolved organic matter is leaked or excreted by zooplankton and fish, and the cycle, called the microbial loop, starts over. Blue arrows show the movement of organic matter from the microbial loop to the food web and back.[94]The viral shunt: Phytoplankton live in thephotic zone of the ocean, wherephotosynthesis is possible. During photosynthesis, they assimilate carbon dioxide and release oxygen. For growth, phytoplankton cells depend on nutrients, which enter the ocean by rivers, continental weathering, and glacial ice meltwater on the poles. Phytoplankton release dissolved organic carbon (DOC) into the ocean. Since phytoplankton are the basis of marine food webs, they serve as prey for zooplankton, fish larvae and other heterotrophic organisms. They can also be degraded by bacteria or by viral lysis.[95]
Fungi have a role as well. Themycoloop is a specific aquatic food web pathway where parasiticchytrid fungi infect large, inedible phytoplankton, and theirzoospores (a type of spore) become a food source for zooplankton. In this manner, the chytrid fungi transfer nutrients from otherwise unusable phytoplankton to zooplankton.[96]
Pennate diatom from an Arcticmeltpond, infected with two chytrid-like fungal pathogens (in false-colour red).[97] Scale bar = 10 μm.The mycoloop: Small phytoplankton can be grazed upon by zooplankton, but large phytoplankton are not easy to eat, or are even inedible. Chytrid infections on large phytoplankton can make them more palatabile, as a result of host aggregation (reduced edibility) or mechanistic fragmentation of cells or filaments (increased palatability). First, chytrid parasites extract and repack nutrients and energy from their hosts in form of readily edible zoospores. Second, infected and fragmented hosts including attachedsporangia can also be ingested by grazers.[96]
Theocean food web, showing the central involvement ofmarine microplankton in how the ocean imports nutrients from and then exports them back to the atmosphere and ocean floorThe central role played bypelagic fungi, both parasitic andsaprotrophic in themycoloop, and saprotrophic fungi as active contributors to themicrobial loop. The activity of heterotrophic microbes, including pelagic fungi, has far-reaching global implications for fisheries (i.e., the amount of carbon that will ultimately flow to higher trophic levels) and climate change (i.e., the amount of carbon that will be sequestered in the ocean or respired back to CO2 and the release of other greenhouse gases; e.g., N2O.[98]
Primarily by grazing on phytoplankton, zooplankton providecarbon to the plankticfoodweb, eitherrespiring it to providemetabolic energy, or upon death asbiomass ordetritus. Organic material tends to bedenser thanseawater, so it sinks into open ocean ecosystems away from the coastlines, transporting carbon along with it. This process, called thebiological pump, is one reason that oceans constitute the largestcarbon sink onEarth. However, it has been shown to be influenced by increments of temperature.[99][100][101][102] In 2019, a study indicated that at ongoing rates ofseawater acidification, Antarctic phytoplanktons could become smaller and less effective at storing carbon before the end of the century.[103]
Yearly cycle of the Great Calcite Belt in theSouthern Ocean. The belt appears during the southern hemisphere summer as a lightteal stripe.
TheGreat Calcite Belt is a region in theSouthern Ocean characterized by high concentrations ofcoccolithophores, a type of calcite-producing phytoplankton. It plays a significant role inocean biogeochemistry and the global carbon cycle. Coccolithophores in the belt producecalcium carbonate (calcite orchalk) plates calledcoccoliths. This process, known ascalcification, affects the ocean's carbon cycle by lowering alkalinity and releasing CO2. However, when coccolithophores die, their calcite shells sink, contributing to the biological pump by transporting carbon to the deep ocean, sequestering it for centuries or longer and mitigating atmospheric CO2 levels.[106]
Phytoplankton absorb energy from the Sun and nutrients from the water to produce their own nourishment or energy. In the process ofphotosynthesis, phytoplankton release molecularoxygen (O 2) into the water as a waste byproduct. It is estimated that about 50% of the world's oxygen is produced via phytoplankton photosynthesis.[107] The rest is produced via photosynthesis on land byplants.[107] Furthermore, phytoplankton photosynthesis has controlled the atmosphericCO 2/O 2 balance since the earlyPrecambrian Eon.[108]
Theabsorption efficiency (AE) of plankton is the proportion of food absorbed by the plankton that determines how available the consumed organic materials are in meeting the required physiological demands.[109] Depending on the feeding rate and prey composition, variations in absorption efficiency may lead to variations infecal pellet production, and thus regulates how much organic material is recycled back to the marine environment. Low feeding rates typically lead to high absorption efficiency and small, dense pellets, while high feeding rates typically lead to low absorption efficiency and larger pellets with more organic content. Another contributing factor todissolved organic matter (DOM) release is respiration rate. Physical factors such as oxygen availability, pH, and light conditions may affect overall oxygen consumption and how much carbon is loss from zooplankton in the form of respired CO2. The relative sizes of zooplankton and prey also mediate how much carbon is released viasloppy feeding. Smaller prey are ingested whole, whereas larger prey may be fed on more "sloppily", that is more biomatter is released through inefficient consumption.[110][111] There is also evidence that diet composition can impact nutrient release, with carnivorous diets releasing moredissolved organic carbon (DOC) and ammonium than omnivorous diets.[112]
The growth of phytoplankton populations is dependent on light levels and nutrient availability. The chief factor limiting growth varies from region to region in the world's oceans. On a broad scale, growth of phytoplankton in the oligotrophic tropical and subtropical gyres is generally limited by nutrient supply, while light often limits phytoplankton growth in subarctic gyres. Environmental variability at multiple scales influences the nutrient and light available for phytoplankton, and as these organisms form the base of the marine food web, this variability in phytoplankton growth influences higher trophic levels. For example, at interannual scales phytoplankton levels temporarily plummet duringEl Niño periods, influencing populations of zooplankton, fishes, sea birds, andmarine mammals.
The effects of anthropogenic warming on the global population of phytoplankton is an area of active research. Changes in the vertical stratification of the water column, the rate of temperature-dependent biological reactions, and the atmospheric supply of nutrients are expected to have important impacts on future phytoplankton productivity.[113] Additionally, changes in the mortality of phytoplankton due to rates of zooplankton grazing may be significant.
Zooplankton are the initial prey item for almost allfish larvae as they switch from theiryolk sacs to external feeding. Fish rely on the density and distribution of zooplankton to match that of new larvae, which can otherwise starve. Natural factors (e.g., current variations, temperature changes) and man-made factors (e.g. river dams,ocean acidification, rising temperatures) can strongly affect zooplankton populations, which can in turn strongly affect fish larval survival, and therefore breeding success.
It has been shown that plankton can be patchy in marine environments where there aren't significant fish populations and additionally, where fish are abundant, zooplankton dynamics are influenced by the fish predation rate in their environment. Depending on the predation rate, they could express regular or chaotic behavior.[114]
A negative effect that fish larvae can have on planktonic algal blooms is that the larvae will prolong the blooming event by diminishing available zooplankton numbers; this in turn permits excessive phytoplankton growth allowing the bloom to flourish .[93]
The importance of both phytoplankton and zooplankton is also well-recognized in extensive and semi-intensive pond fish farming. Plankton population-based pond management strategies for fish rearing have been practiced by traditional fish farmers for decades, illustrating the importance of plankton even in man-made environments.
Of all animal fecal matter, it is whale feces that is the 'trophy' in terms of increasing nutrient availability. Phytoplankton are the powerhouse of open ocean primary production and they can acquire many nutrients from whale feces.[115] In the marine food web, phytoplankton are at the base of the food web and are consumed by zooplankton & krill, which are preyed upon by larger and larger marine organisms, including whales, so it can be said that whale feces fuels the entire food web.
Plankton have many direct and indirect effects on humans.
Around 70% of the oxygen in the atmosphere is produced in the oceans fromphytoplankton performing photosynthesis, meaning that the majority of the oxygen available for us and other organisms thatrespire aerobically is produced by plankton.[116]
Plankton also make up the base of the marine food web, providing food for all the trophic levels above. Recent studies have analyzed the marine food web to see if the system runs on atop-down or bottom-up approach. Essentially, this research is focused on understanding whether changes in the food web are driven by nutrients at the bottom of the food web or predators at the top. The general conclusion is that the bottom-up approach seemed to be more predictive of food web behavior.[117] This indicates that plankton have more sway in determining the success of the primary consumer species that prey on them than do the secondary consumers that prey on the primary consumers.
In some cases, plankton act as an intermediatehost for deadly parasites in humans. One such case is that ofcholera, an infection caused by several pathogenic strains ofVibrio cholerae. These species have been shown to have a symbiotic relationship with chitinous zooplankton species likecopepods. These bacteria benefit not only from the food provided by the chiton from the zooplankton, but also from the protection from acidic environments. Once the copepods have been ingested by a human host, the chitinous exterior protects the bacteria from the stomach acids in the stomach and proceed to the intestines. Once there, the bacteria bind with the surface of the small intestine and the host will start developing symptoms, including extreme diarrhea, within five days.[118]
In 2024, theUnited Nations Global Compact's Ocean Stewardship Coalition launched thePlankton Manifesto,[63] collaboratively developed by over 30 international experts.[119] It outlines strategic recommendations to guide global efforts at safeguarding plankton and harnessing their potential to address planetaryclimate change issues, as well aspollution andbiodiversity loss. It emphasizes plankton's critical role as the foundation ofmarine ecosystems, producing about 50% of Earth's oxygen and sequestering 30–50 billion metric tonnes of carbon annually.[63]
Policy integration: Urging governments, United Nations agencies, and businesses to include plankton in climate and biodiversity frameworks, with endorsements sought atCOP29,COP16, and the2025 United Nations Ocean Conference.
Public awareness: Fostering "plankton literacy" through education and interdisciplinary initiatives to highlight their role infood security andecosystem health.
Collaboration: Encouraging cross-sectoral partnerships among academia, industry, and governments to fund research and protect plankton from threats likenutrient pollution andocean warming.
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![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.[55]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aProchlorococcus_marinus_(cropped).jpg)
![The sea walnut ctenophore has a transient anus which forms only when it needs to defecate[59]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aMnemiopsis_leidyi_2.jpg)
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