Planktons arePlanktons are marine and freshwatermarine and freshwater
organisms that, because they are nonmotile ororganisms that, because they are nonmotile or
because they are too small or too weak tobecause they are too small or too weak to
swim against the current, exist in a drifting,swim against the current, exist in a drifting,
floating state.floating state.
The termThe term planktonplankton is a collective name for allis a collective name for all
such organisms and includes certainsuch organisms and includes certain algaealgae,,
bacteriabacteria, protozoans, crustaceans, molluscs,, protozoans, crustaceans, molluscs,
and coelenterates, as well as representativesand coelenterates, as well as representatives
from almost every other phylum of animals.from almost every other phylum of animals.
Plankton is distinguished fromPlankton is distinguished from nektonnekton, which, which
is composed of strong-swimming animals, andis composed of strong-swimming animals, and
fromfrom benthosbenthos, which includes sessile, creeping,, which includes sessile, creeping,
and burrowing organisms on the seafloor.and burrowing organisms on the seafloor.
Large floatingLarge floating seaweedsseaweeds (for(for
example,example, SargassumSargassum, which constitutes the, which constitutes the
Sargasso Sea) and various related multicellularSargasso Sea) and various related multicellular
algae are not considered plankton butalgae are not considered plankton but pleustonpleuston..
Organisms resting or swimming on the surfaceOrganisms resting or swimming on the surface
film of the water are calledfilm of the water are called neustonneuston (e.g., the(e.g., the
Plankton is the productive base of both marinePlankton is the productive base of both marine
and freshwaterand freshwater ecosystemsecosystems, providing food for, providing food for
larger animals and indirectly for humans,larger animals and indirectly for humans,
whose fisheries depend upon plankton.whose fisheries depend upon plankton.
As a human resource, plankton has only begunAs a human resource, plankton has only begun
to be developed and exploited.to be developed and exploited.
The chief food supply of the world, in view ofThe chief food supply of the world, in view of
its high biological productivity and wideits high biological productivity and wide
It has been demonstrated on several occasionsIt has been demonstrated on several occasions
that large-scale cultures ofthat large-scale cultures of algaealgae areare
technically feasible.technically feasible.
The unicellular green algaThe unicellular green alga ChlorellaChlorella has beenhas been
Through ample culture conditions, productionThrough ample culture conditions, production
is directed towardis directed toward proteinprotein content greater thancontent greater than
50 percent.50 percent.
Although this protein has a suitable balance ofAlthough this protein has a suitable balance of
essential amino acids, its low degree ofessential amino acids, its low degree of
digestibility prevents practical use.digestibility prevents practical use.
Phytoplankton may become increasinglyPhytoplankton may become increasingly
important in space travel as a source for foodimportant in space travel as a source for food
and for gas exchange.and for gas exchange.
The carbon dioxide released during respirationThe carbon dioxide released during respiration
of spacecraft personnel would be transformedof spacecraft personnel would be transformed
into organic substances by the algae, while theinto organic substances by the algae, while the
oxygen liberated during this process wouldoxygen liberated during this process would
support human respiration.support human respiration.
Plankton are also often described in terms of size andPlankton are also often described in terms of size and
usually the following divisions are used:usually the following divisions are used:
MegaplanktonMegaplankton > 20 cm> 20 cm metazoansmetazoans; e.g.; e.g. jellyfishjellyfish;;
ctenophoresctenophores;; salpssalps and (and (pyrosomespyrosomes pelagicpelagic TunicataTunicata
);); CephalopodaCephalopoda;; AmphipodaAmphipoda metazoansmetazoans; e.g.; e.g.
MacroplanktonMacroplankton 2→20 cm2→20 cm metazoansmetazoans; e.g.; e.g.
PteropodsPteropods;; ChaetognathsChaetognaths;; EuphausiaceaEuphausiacea ((krillkrill););
MedusaeMedusae;; ctenophoresctenophores;; salpssalps, doliolids and, doliolids and
pyrosomespyrosomes(pelagic(pelagic TunicataTunicata);); CephalopodaCephalopoda;;
JanthinidaeJanthinidae (one family gastropods);(one family gastropods); AmphipodaAmphipoda
metazoansmetazoans; e.g.; e.g. copepodscopepods;;
MesoplanktonMesoplankton 0.2→2 mm0.2→2 mm MedusaeMedusae;;
CladoceraCladocera;; OstracodaOstracoda;; ChaetognathsChaetognaths;;
PteropodsPteropods;; TunicataTunicata;; HeteropodaHeteropoda, large, large
eukaryoticeukaryotic protistsprotists; most; most phytoplanktonphytoplankton;;
MicroplanktonMicroplankton 20→20020→200 µmµm ProtozoaProtozoa
ForaminiferaForaminifera;; tintinnidstintinnids; other; other ciliatesciliates;;
RotiferaRotifera; juvenile; juvenile metazoansmetazoans-- CrustaceaCrustacea ((
copepodcopepod nauplii) smallnauplii) small eukaryoticeukaryotic protistsprotists;;
World distribution of planktonWorld distribution of plankton
Plankton inhabit oceans, seas, lakes, ponds.Plankton inhabit oceans, seas, lakes, ponds.
Local abundance varies horizontally, verticallyLocal abundance varies horizontally, vertically
and seasonally.and seasonally.
The primary cause of this variability is theThe primary cause of this variability is the
availability of light.availability of light.
All plankton ecosystems are driven by theAll plankton ecosystems are driven by the
input of solar energy (input of solar energy (chemosynthesischemosynthesis),),
confining primary production to surfaceconfining primary production to surface
waters, and to geographical regions andwaters, and to geographical regions and
seasons having abundant light.seasons having abundant light.
A secondary variable is nutrient availability.A secondary variable is nutrient availability.
Although large areas of theAlthough large areas of the tropicaltropical andand
sub-tropicalsub-tropical oceans have abundant light, theyoceans have abundant light, they
experience relatively low primary productionexperience relatively low primary production
because they offer limited nutrients such asbecause they offer limited nutrients such as
nitratenitrate,, phosphatephosphate andand silicatesilicate..
This results from large-scaleThis results from large-scale ocean circulationocean circulation
and water columnand water column stratificationstratification..
In such regions, primary production usuallyIn such regions, primary production usually
occurs at greater depth, although at a reducedoccurs at greater depth, although at a reduced
level (because of reduced light).level (because of reduced light).
Despite significantDespite significant macronutrientmacronutrient concentrations,concentrations,
some ocean regions are unproductive.some ocean regions are unproductive.
TheThe micronutrientmicronutrient ironiron is deficient in these regions,is deficient in these regions,
and adding it can lead to the formation ofand adding it can lead to the formation of bloomsblooms ofof
many kinds of phytoplankton.many kinds of phytoplankton.
Iron primarily reaches the ocean through theIron primarily reaches the ocean through the
deposition of dust on the sea surface.deposition of dust on the sea surface.
Paradoxically, oceanic areas adjacent toParadoxically, oceanic areas adjacent to
unproductive,unproductive, aridarid land thus typically have abundantland thus typically have abundant
phytoplankton (e.g., the easternphytoplankton (e.g., the eastern Atlantic OceanAtlantic Ocean,,
wherewhere trade windstrade winds bring dust from thebring dust from the Sahara DesertSahara Desert
in northin north AfricaAfrica).).
While plankton are most abundant in surfaceWhile plankton are most abundant in surface
waters, they live throughout the water column.waters, they live throughout the water column.
At depths where no primary productionAt depths where no primary production
occurs,occurs, zooplanktonzooplankton andand bacterioplanktonbacterioplankton
instead consume organic material sinking frominstead consume organic material sinking from
more productive surface waters above.more productive surface waters above.
This flux of sinking material, so-calledThis flux of sinking material, so-called
marine snowmarine snow, can be especially high following, can be especially high following
the termination ofthe termination of spring bloomsspring blooms..
Phytoplankton are free-floating microscopicPhytoplankton are free-floating microscopic
plants that are mostly unicellular and produceplants that are mostly unicellular and produce
chemical energy from light. This process ischemical energy from light. This process is
calledcalled primary productionprimary production..
The plant-like community of plankton isThe plant-like community of plankton is
calledcalled phytoplanktonphytoplankton, and the animal-like, and the animal-like
community is known ascommunity is known as zooplanktonzooplankton..
This convenient division is not without fault,This convenient division is not without fault,
for strictly speaking, many planktonicfor strictly speaking, many planktonic
organisms are neither clearlyorganisms are neither clearly plantplant nornor animalanimal
but rather are better described asbut rather are better described as protistsprotists..
Phytoplankton have a critical role in primaryPhytoplankton have a critical role in primary
production, nutrient cycling, andproduction, nutrient cycling, and food websfood webs
and make up a significant proportion of theand make up a significant proportion of the
primary production in aquatic systems.primary production in aquatic systems.
In many coastal systems, primary productionIn many coastal systems, primary production
is almost entirely a function of theis almost entirely a function of the
Even in salt marsh estuaries, where vascularEven in salt marsh estuaries, where vascular
plantplant biomassbiomass can greatly exceed that ofcan greatly exceed that of algaealgae,,
phytoplankton can contribute substantially tophytoplankton can contribute substantially to
overall primary production.overall primary production.
Phytoplankton are classified as microalgae andPhytoplankton are classified as microalgae and
include species from the following divisions:include species from the following divisions:
Cyanobacteria (blue-green algae), ChlorophytaCyanobacteria (blue-green algae), Chlorophyta
(green algae), Prochlorophyta, Euglenophyta,(green algae), Prochlorophyta, Euglenophyta,
Pyrrhophyta (dinoflagellates ), CryptophytaPyrrhophyta (dinoflagellates ), Cryptophyta
(cryptomonads), Chrysophyta, and(cryptomonads), Chrysophyta, and
Bacillariophyta (includes diatoms).Bacillariophyta (includes diatoms).
Most phytoplankton are motile, however,Most phytoplankton are motile, however,
movement in the water column is mostlymovement in the water column is mostly
through transport by currents.through transport by currents.
Phytoplankton are usually grouped according to cellPhytoplankton are usually grouped according to cell
size. Picoplankton are the smallest and are identifiedsize. Picoplankton are the smallest and are identified
as phytoplankton <2 micrometers (µm) in diameter.as phytoplankton <2 micrometers (µm) in diameter.
Nanoplankton are intermediate sized microalgae andNanoplankton are intermediate sized microalgae and
range in size from 2-20 µm.range in size from 2-20 µm.
Microplankton are the largest phytoplankton andMicroplankton are the largest phytoplankton and
include those algae >20 µm in diameter.include those algae >20 µm in diameter.
Freshwater phytoplankton, usually rich in greenFreshwater phytoplankton, usually rich in green
algae, also includes diatoms, blue-green algae, andalgae, also includes diatoms, blue-green algae, and
true flagellates.true flagellates.
In the oceans, phytoplanktonIn the oceans, phytoplankton biomassbiomass rises and fallsrises and falls
according to multiyear cycles and appears to beaccording to multiyear cycles and appears to be
sensitive to changes in sea surface temperatures.sensitive to changes in sea surface temperatures.
Microplankton are made up mostly of diatoms andMicroplankton are made up mostly of diatoms and
Nanoplankton are dominated by phytoplankton withNanoplankton are dominated by phytoplankton with
flagellas (e.g. cryptophytes and chrysophytes, andflagellas (e.g. cryptophytes and chrysophytes, and
These plankters can account for 75% of the totalThese plankters can account for 75% of the total
primary productivity of a system.primary productivity of a system.
Picoplankton are mostly prokaryotes such asPicoplankton are mostly prokaryotes such as
cyanobacteria and prochlorophytes as well ascyanobacteria and prochlorophytes as well as
several eukaryotic alga species.several eukaryotic alga species.
These tiny phytoplankters can account for as much asThese tiny phytoplankters can account for as much as
50% of the primary production in oceanic waters.50% of the primary production in oceanic waters.
The zooplankton is divided into two groups.The zooplankton is divided into two groups.
Temporary plankton consists of planktonicTemporary plankton consists of planktonic
eggs andeggs and larvaelarvae of members of theof members of the benthosbenthos andand
nekton; permanent plankton (nekton; permanent plankton (seesee
zooplankton videozooplankton video) includes all animals that) includes all animals that
live their completelive their complete lifelife cycles in a floatingcycles in a floating
TheThe temporary planktontemporary plankton, particularly abundant, particularly abundant
in coastal areas, is characteristically seasonalin coastal areas, is characteristically seasonal
in occurrence, though variations in spawningin occurrence, though variations in spawning
time of differenttime of different speciesspecies ensure its presence inensure its presence in
all seasons.all seasons.
Representatives from nearly every phylum ofRepresentatives from nearly every phylum of
the animal kingdom are found in thethe animal kingdom are found in the
permanent plankton.permanent plankton.
Among theAmong the protozoansprotozoans, planktonic, planktonic
foraminiferans and radiolarians are soforaminiferans and radiolarians are so
abundant and widespread that their skeletonsabundant and widespread that their skeletons
constitute the bulk of bottom sediments overconstitute the bulk of bottom sediments over
wide ocean areas.wide ocean areas.
They are absent in fresh water.They are absent in fresh water.
The ciliate protozoans are represented mainlyThe ciliate protozoans are represented mainly
by the tintinnids, which are between 20 andby the tintinnids, which are between 20 and
640 microns in size and sometimes occur in640 microns in size and sometimes occur in
vast numbers.vast numbers.
Among the planktonic coelenterates are theAmong the planktonic coelenterates are the
beautiful siphonophores (e.g.,beautiful siphonophores (e.g., PhysaliaPhysalia, the, the
Portuguese man-of-war) and thePortuguese man-of-war) and the jellyfishesjellyfishes..
Planktonic ctenophores, called comb jellies, orPlanktonic ctenophores, called comb jellies, or
sea walnuts, are also common.sea walnuts, are also common.
Freshwater rotifers may be present in planktonFreshwater rotifers may be present in plankton
in vast numbers during the warmer seasons.in vast numbers during the warmer seasons.
A group of organisms that can be found at allA group of organisms that can be found at all
latitudes, both in surface water and at greatlatitudes, both in surface water and at great
depths, are the marinedepths, are the marine arrowwormsarrowworms
(e.g.,(e.g.,SagittaSagitta), important planktonic predators.), important planktonic predators.
Oysters, mussels, other marineOysters, mussels, other marine bivalvesbivalves, and snails, and snails
begin life as planktonic larvae.begin life as planktonic larvae.
The wing snails (Pteropoda) spend their entire lifeThe wing snails (Pteropoda) spend their entire life
cycles as plankton.cycles as plankton.
CrustaceansCrustaceans are the most important members of theare the most important members of the
zooplankton. They are the marine counterparts ofzooplankton. They are the marine counterparts of
insects on land; on land, as in the sea, theinsects on land; on land, as in the sea, the arthropodsarthropods
are the most diverse and numerous of all animalare the most diverse and numerous of all animal
TheThe copepodcopepod Calanus finmarchicusCalanus finmarchicus is important asis important as
food for the herring, and the euphausiidfood for the herring, and the euphausiid EuphausiaEuphausia
superbasuperba, commonly known as, commonly known as krillkrill, is the main food, is the main food
source for blue and finsource for blue and finwhaleswhales in the Antarctic Ocean.in the Antarctic Ocean.
These whales, particularly blue and finbackThese whales, particularly blue and finback
whales, migrate to waters where spawning ofwhales, migrate to waters where spawning of
these crustaceans occurs; and the rapid growththese crustaceans occurs; and the rapid growth
of these large mammals, feeding entirely onof these large mammals, feeding entirely on
plankton, is impressive.plankton, is impressive.
There is a pronounced tendency forThere is a pronounced tendency for
zooplankton to perform diurnal verticalzooplankton to perform diurnal vertical
migrations in both lakes and the sea.migrations in both lakes and the sea.
This migratory behaviour varies with stages inThis migratory behaviour varies with stages in
the life cycle, seasons of the year, latitude,the life cycle, seasons of the year, latitude,
hydrographic structure, and meteorologicalhydrographic structure, and meteorological
Generally, the animals ascend toward theGenerally, the animals ascend toward the
surface at sunset from daytime depths.surface at sunset from daytime depths.
At midnight, if there is no optical stimulusAt midnight, if there is no optical stimulus
(e.g., moon, artificial light), some of the(e.g., moon, artificial light), some of the
animals return to the daytime depths, thenanimals return to the daytime depths, then
approach the surface once again just beforeapproach the surface once again just before
As the sun rises, all descend to their daytimeAs the sun rises, all descend to their daytime
BacteriaBacteria and and fungifungi
Bacteria and fungi found in water belong byBacteria and fungi found in water belong by
definition to plankton, but, because of specialdefinition to plankton, but, because of special
techniques required for sampling and identification,techniques required for sampling and identification,
they usually are considered separately.they usually are considered separately.
These organisms are important in the transformationThese organisms are important in the transformation
of dead organic materials to inorganic plant nutrients.of dead organic materials to inorganic plant nutrients.
Some of these marine and freshwater microorganismsSome of these marine and freshwater microorganisms
(including blue-green algae) fix molecular(including blue-green algae) fix molecular nitrogennitrogen
from water containing dissolved air, formingfrom water containing dissolved air, forming
ammonia or related nutrients important forammonia or related nutrients important for
phytoplankton growth.phytoplankton growth.
Although little is known about the extent ofAlthough little is known about the extent of
nitrogen fixation, bacteria and fungi alwaysnitrogen fixation, bacteria and fungi always
are found in water samples.are found in water samples.
A peculiar situation exists in the Black Sea,A peculiar situation exists in the Black Sea,
where water below 130–180 metres (aboutwhere water below 130–180 metres (about
425–590 feet) contains hydrogen sulfide and425–590 feet) contains hydrogen sulfide and
no oxygen.no oxygen.
Under these conditions only bacteria areUnder these conditions only bacteria are
Plankton and biological productivityPlankton and biological productivity
The productivity of an area is dependent uponThe productivity of an area is dependent upon
the availability of nutrients and water-stabilitythe availability of nutrients and water-stability
Currents that flow near continents areCurrents that flow near continents are
important to plankton production in an area.important to plankton production in an area.
In the sea an adequate supply of nutrients,In the sea an adequate supply of nutrients,
including carbon dioxide, enablesincluding carbon dioxide, enables
phytoplankton and benthic algae to transformphytoplankton and benthic algae to transform
thethe lightlight energy of the Sun into energy-richenergy of the Sun into energy-rich
chemical components throughchemical components through photosynthesisphotosynthesis..
The bottom-dwelling algae are responsible forThe bottom-dwelling algae are responsible for
about 2 percent of the primary production inabout 2 percent of the primary production in
the ocean; the remaining 98 percent isthe ocean; the remaining 98 percent is
attributable to phytoplankton.attributable to phytoplankton.
Most of the phytoplankton serves as food forMost of the phytoplankton serves as food for
zooplankton, but some of it is carried belowzooplankton, but some of it is carried below
the light zone.the light zone.
After death, this phytoplankton undergoesAfter death, this phytoplankton undergoes
chemical mineralization, bacterial breakdown,chemical mineralization, bacterial breakdown,
or transformation into sediments.or transformation into sediments.
Phytoplankton production usually is greatestPhytoplankton production usually is greatest
from 5 to 10 metres (16 to 33 feet) below thefrom 5 to 10 metres (16 to 33 feet) below the
surface of the water.surface of the water.
High light intensity and the lack of nutrient inHigh light intensity and the lack of nutrient in
the regions above a depth of 5 metres may bethe regions above a depth of 5 metres may be
the causes for suboptimalthe causes for suboptimal photosynthesisphotosynthesis..
Although bacteria are found at all depths, theyAlthough bacteria are found at all depths, they
are most abundant either immediately beloware most abundant either immediately below
great phytoplankton populations or just abovegreat phytoplankton populations or just above
the bottom.the bottom.
Biogeochemical significanceBiogeochemical significance
Aside from representing the bottom few levelsAside from representing the bottom few levels
of aof a food chainfood chain that supportsthat supports commerciallycommercially
importantimportant fisheriesfisheries, plankton, plankton ecosystemsecosystems playplay
a role in thea role in the biogeochemical cyclesbiogeochemical cycles of manyof many
importantimportant chemical elementschemical elements, including the, including the
ocean'socean's carbon cyclecarbon cycle..
Primarily by grazing on phytoplankton,Primarily by grazing on phytoplankton,
zooplankton provide carbon to the plankticzooplankton provide carbon to the planktic
foodwebfoodweb, either, either respiringrespiring it to provideit to provide
metabolicmetabolic energy, or upon death asenergy, or upon death as biomassbiomass oror
Organic material tends to beOrganic material tends to be denserdenser thanthan
seawaterseawater, and as a result it sinks into open, and as a result it sinks into open
ocean ecosystems away from the coastlines,ocean ecosystems away from the coastlines,
transporting carbon along with it.transporting carbon along with it.
This process is known as theThis process is known as the biological pumpbiological pump,,
and it is one reason that oceans constitute theand it is one reason that oceans constitute the
largestlargest carbon sinkcarbon sink onon EarthEarth..
It might be possible to increase the ocean'sIt might be possible to increase the ocean's
uptake ofuptake of carbon dioxidecarbon dioxide generated throughgenerated through
human activitieshuman activities by increasing planktonby increasing plankton
production throughproduction through seedingseeding, primarily with the, primarily with the