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).
Plankton are the diverse collection oforganisms that drift inwater (orair) but are unable to actively propel themselves againstcurrents (orwind).[1][2] The individual organisms constituting plankton are calledplankters.[3] In the ocean, they provide a crucial source of food to many small and large aquatic organisms, such asbivalves,fish, andbaleen whales.
Marine plankton includebacteria,archaea,algae,protozoa, microscopicfungi,[4] and drifting or floatinganimals that inhabit thesaltwater ofoceans and thebrackish waters ofestuaries.Freshwater plankton are similar to marine plankton, but are found in lakes and rivers. Mostly, plankton just drift where currents take them, though some, likejellyfish, swim slowly but not fast enough to generally overcome the influence of currents.
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 includeplant spores,pollen and wind-scatteredseeds. They may also include microorganisms swept into the air from terrestrial dust storms and oceanic plankton swept into the air bysea spray.
Though many planktonicspecies aremicroscopic in size,plankton includes organisms over a wide range of sizes, including large organisms such as jellyfish.[5] This is because plankton are defined by theirecological niche and level ofmotility rather than by anyphylogenetic ortaxonomic classification. The "plankton" category differentiates these organisms from those that float on the water's surface, calledneuston, those that can swim against a current, callednekton, and those that live on the deep sea floor, calledbenthos.
Plankton (organisms that drift with water currents) can be contrasted withnekton (organisms that swim against water currents),neuston (organisms that live at the ocean surface) andbenthos (organisms that live at the ocean floor).
The nameplankton was coined by German marine biologistVictor Hensen in 1887 from shortening the wordhalyplankton fromGreekᾰ̔́λςháls "sea" andπλανάωplanáō to "drift" or "wander".[6]: 1 While some forms are capable of independent movement and can swim hundreds of meters vertically in a single day (a behavior calleddiel vertical migration), their horizontal position is primarily determined by the surrounding water movement, and plankton typically flow withocean 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.[9] 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.[10]
Bacterioplankton includebacteria andarchaea, which play an important role in remineralising organic material down the water column (note that prokaryotic phytoplankton are also bacterioplankton).
Virioplankton areviruses. Viruses are more abundant in the plankton than bacteria and archaea, though much smaller.[12][13]
Mixotrophs. Plankton have traditionally been categorized asproducer,consumer, and recycler groups, but some plankton are able to benefit from more than just one trophic level. In this mixed trophic strategy—known as mixotrophy—organisms act as both producers and consumers, either at the same time or switching between modes of nutrition in response to ambient conditions. This makes it possible to use photosynthesis for growth when nutrients and light are abundant, but switch to eating phytoplankton, zooplankton or each other when growing conditions are poor. Mixotrophs are divided into two groups; constitutive mixotrophs (CMs) which are able to perform photosynthesis on their own, and non-constitutive mixotrophs (NCMs) 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).[14]
Recognition of the importance of mixotrophy as an ecological strategy is increasing,[15] as well as the wider role this may play in marinebiogeochemistry.[16] Studies have shown that mixotrophs are much more important for marine ecology than previously assumed and comprise more than half of all microscopic plankton.[17][18] Their presence acts as a buffer that prevents the collapse of ecosystems during times with little to no light.[19]
Diverse assemblages consist ofunicellular andmulticellular organisms with different sizes, shapes, feeding strategies, ecological functions, life cycle characteristics, and environmental sensitivities.[20]
However, some of these terms may be used with very different boundaries, especially on the larger end. 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.
The microplankton and smaller groups aremicroorganisms and operate at lowReynolds numbers, where the viscosity of water is more important than its mass or inertia.[22]
Marine plankton includesmarine bacteria and archaea,algae,protozoa and drifting or floating animals that inhabit the saltwater of oceans and the brackish waters of estuaries.
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.[24] 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.
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]
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]
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.[36] Gelatinous zooplankton are often transparent.[37] 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.[38]
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 food supply. Both eggs and larvae are themselves eaten by larger animals.[39][40] 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.
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.[44] 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.[45] 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]
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.[46]
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.[47] This can occur by directturbulence or by disruption of the substrate and subsequent entrainment in the water column.[47][48] 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.[49] Tychoplankton are sometimes calledaccidental plankton.
World concentrations of surface ocean chlorophyll as viewed by satellite during the northern spring, averaged from 1998 to 2004. Chlorophyll is a marker for the distribution and abundance of phytoplankton.
Apart from aeroplankton, plankton inhabits oceans, seas, 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. 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).
Despite significantmacronutrient concentrations, some ocean regions are unproductive (so-calledHNLC regions).[50] Themicronutrientiron is deficient in these regions, andadding it can lead to the formation of phytoplanktonalgal blooms.[51] 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).
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.
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.[54][55][56][57] 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.[58]
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.[61] The rest is produced via photosynthesis on land byplants.[61] Furthermore, phytoplankton photosynthesis has controlled the atmosphericCO 2/O 2 balance since the earlyPrecambrian Eon.[62]
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.[63] 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.[64][65] There is also evidence that diet composition can impact nutrient release, with carnivorous diets releasing moredissolved organic carbon (DOC) and ammonium than omnivorous diets.[66]
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.[67] Additionally, changes in the mortality of phytoplankton due to rates of zooplankton grazing may be significant.
Marine phytoplankton cycling throughout water column
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.[69]
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 .[53]
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.[70] 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.[71]
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.[72] 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.[73]
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