Plankton: Definition, Classification and Facts

The term plankton was first coined by oceanographer Victor Hensen in 1887. The word plankton comes from the Greek word planktos which means to be driven or drifted away.

According to him, plankton is diverse collection of small organisms and the individual organism makes up the plankton, known as plankters. Onec a that time they were considered to be found in the sea water. Now they live in all the water bodies and move in water through the process of waves and other movements. Actually, they can not swim against a current due to very feeble locomotory organs.

Plankton (sing. Plankter) is a diverse group of organisms that live in water and cannot swim against the water current. They are used as an important food source for a variety of aquatic organisms such as fish and whales. They are floating organism such as various animals, protists, archaea, algae or bacteria etc. and they live in the pelagic region of the sea, ocean or freshwater. Although many plankton species are microscopic in size, they include a variety of organisms, including large organisms such as jellyfish.

Plankton usually move with the help of currents. However, some plankton can move freely. They can even swim hundreds of meters vertically in a day. This type of behavior is called dial vertical orientation. Their horizontal position is mainly determined by the currents. Nectons, on the other hand, such as squid, fish and marine mammals, can swim against strong currents and control their position in the aquatic environment.

Within plankton, holoplankton spend the entire life cycle as plankton, as do most algae, copypods, salps, and some jellyfish. Meroplankton, on the other hand, (sea urchins, starfish, crustaceans, marine insects, and most fish) spend part of their life cycle (usually in the larval stage) as plankton, and subsequently live alternately nectonic or benthic life. The abundance and spread of plankton depends on the concentration of nutrients, the physical condition of the water and the abundance of other plankton.

LaMotte 1063 Plankton Net

Planktonic organisms: Plankton vary widely in quality in different types of freshwater reservoirs. This type of plankton is found in large quantities in freshwater.

(A) Plants

(1) Algae: Plankton is representative of all classes of algae. However, some classes are widespread in inland water bodies.(2) Bacteria: A large number of bacteria are considered as plankton. It is assumed that bacteria are found in all the water bodies in the earth.

(B) Protista

 All classes and subclasses of protozoa (except sporozoa) contain plankton. In most cases ,there are many genera and species.

(C) Animals

(1) Coelenterata: Hydra is an opportunistic plankton. Sometimes they can be seen freely in open water.(2) Rotatoria: These are the most important group of zooplankton.(3) Gastrotricha: Plankton contains a small number of gastrotrica representatives.(4) Bryozoa: They are considered as statoblast plankton. Moreover, larvae that sometimes swim freely in special conditions are also considered plankton.(5) Arthropoda: Crustacea: Antomostracha, especially Cladosera and Copypods, form more important plankton groups.Some insect larvae exist as common plankton in stagnant water.

Occassional Plankton

The above group of organisms exists in large numbers as plankton. There are some groups of creatures that behave like real plankton, either accidentally or under unusual circumstances. Organisms belonging to the following groups are considered plankton in special cases.

Flowering plants: The role of these plants in the formation of actual plankton is very small. Lemnaceae (Wolffia) have been found in several river depths, as an exceptional examples.

(2) Coelenterata: Lives as Medusa plankton of freshwater rhinoceros.

(3) Platyhelminthis: Turbellarians are of little importance as freshwater plankton. However, at certain times of the year, they are found in abundance in damp ponds and other similar ponds. It is thought that some of these species adapt to the limnatic habitat. They are considered real plankton. Sarcaria lives as plankton. They are seen especially during collection of samples in shallow ponds.

According to Wesenberg-Lund (1974) freshwater lakes are inhabited by pelagic sarcaria with real planktonic life.

(4) Insects: The nymphs of some May flies are seen temporarily in special conditions in the limnatic habitat. Nymphs and mature insects of the aquatic Hemiptera live planktonically, especially in river channels and ponds. Occasionally, some chironomid larvae temporarily live in the lymphatic region. Some other larvae are thought to live as plankton in special conditions. In some seasons a significant number of aquatic Diptera and Ephemarid eggs are considered as plankton.

(5) Arachnida: Sometimes aquatic mites are considered as plankton.

(6) Vertebrates: Vertebrates are rarely found as plankton. However, the juvenile phase of fish lives planktonic life.

(7) Extraterrestrial / physical plankton material: Many littoral and benthic organisms, especially small organisms, suddenly and temporarily merge with plankton.

Due to the severe storms, the rise in water level due to the action of the waves has led to the proliferation of inanimate creatures in the water bodies flowing during the floods. Some of the randomly occurring terbellarians, trematode sarcaria, freshwater nemartinians, some independent organisms aquatic nemadodes, smaller aquatic oligocytes especially other species of Aeoleosomatidae and Naididae, some small plankton or, some non-plankton crustaceans are considered as plankton.

In fact, in most cases, any littoral and benthic organism can turn into ectopic plankton. Moreover, small, marginal and bottom organisms can be considered as invertebrate plankton.

United Scientific SCDSK1 Plastic Secchi Disk

Classification of Plankton

Plankton are classified from different points of view. Below are some important classifications:

1. Based on quality

(A) Phytoplankton

The word phytoplankton comes from the Greek phyton which means plant. So these are the plant plankton. They can be further divided into two parts.

(1) Phytoplankton Proper: These are chlorophyll containing plankton. They are autotrophic, stem cell or real cellular algae that live on the surface of water where there is sufficient light. They take part in the process of photosynthesis. Notable among these groups are diatoms, cyanobacteria, dinoflagellates & coccolithophores.

(2) Saproplankton:  Bacteria and fungi belong to this class. Such plankton plays an important role in the dissociation of organic matter deposited in the bottom of the reservoir.

image of Phytoplankton

Image Showing Different Types of Phtoplankton Species

image of Phytoplankton

Image Showing Different Types of Phtoplankton Species

B) Zooplankton

These are animal plankton. The word zooplankton is derived from the Greek word zoon which means animal. These are small protozoans and metazoans (crustaceans and other animals) that feed on other plankton. Some of the larger animals include eggs and larvae such as fish, crustaceans and annelids.

Since the biological productivity of any aquatic habitat depends on planktonic organisms, it is vital to gain knowledge about them. Plankton are free-floating plants and animals that do not have the ability to move. Although mobility exists, it is so weak that it has to move based on currents and waves (Welch 1952).

Plankton are small animals that can float freely in lakes and sea water columns. Their distribution is mainly controlled by water flow and mixing action. The zooplankton community in most reservoirs is smaller than a few microns (Protozoa) to 2 mm (macrozuoplankton).

There are four main groups of freshwater zooplankton. These are the two subclasses of protozoa, rotifers, and crustaceans, cladoscerans and copepods. Little is known about the productivity of planktonic protozoans. In some cases, however, flagellates, rhizopods, and ciliated protozoans form components of the zooplankton community.

Rotifer is also a major component of zooplankton. According to Ali and Chakraborty (1992), there are 100 species of planktonic rotifers in the water bodies of Bangladesh. Because zooplactons are important food for fish and invertebrates, they act as a driving force in aquatic food nets. Zooplankton are dependent on algae, bacteria, protozoa and other invertebrates.

The important community in the aquatic ecosystem is the zooplankton. They are involved in the ultimate biological production. In the edible food chain of aquatic ecosystems, zooplankton play an important role in the transfer of energy from fish to primary producers.

They play an important role in the natural food chain by forming important food components of omnivorous and carnivorous fish. Carp larvae feed mainly on zooplankton (Berdach et al.1972; Dewan et al.1977). They feed on zooplankton to provide the protein needed for rapid growth and development of various organs, especially fish gonads.

TickiT Telescopic Pond Net

Fish production largely depends on zooplankton. Catla catla contains 48.6% of food content, Rui fish (Labeo rohita) contains 8.36% of food content (Ali and Islam 1981), Koi fish contains 72% of food content (Ali and Islam 1981), Galda icha (Macrobrachium rosenbergii) ( Shaofi and Quddus 1975) and Rohtee cotio 23% (Ali et al.1984) of zeoplankton. Zooplankton serves as an eater at the primary and secondary trophic levels of the food chain (Qassim 1977).

In most reservoirs, the density and species of zooplacton vary vertically and horizontally. Moreover, the density of zooplacton and the number of species vary in sampling at different times in these reservoirs. Seasonal changes in the life cycles of different populations result in drastic changes in the density and species of zooplankton.

2. Based on Size: 

Plankton can be divided into the following groups based on size, viz.

(A) Megaplankton: more than 20 cm in size. Some metazoans such as jellyfish; ctenophores; salps and pyrosomes (Pelagic tunicata); Cephalopoda; Amphipoda etc.

(B) Macroplankton: They are up to 2-20 cm in size. Some metazoans such as Pteropods; Chaetognaths; Euphausiacea (krill); Medusae; ctenophores; salps, doliolids and pyrosomes (Pelagic tunicata); Cephalopoda; Janthinidae; Amphipoda etc.

(C) Mesoplankton: They are 0.2-20 mm in size. Is up to. Some metazoans such as copepods; Medusae; Cladocera; Ostracoda; Chaetognaths; Pteropods; Tunicata; Heteropoda etc.

(D) Microplankton: They are 20-200 µm in size. These are large actual cell protists, mostly phytoplankton, Protozoa, Foraminifera; tintinnids; ; Other ciliates; Rotifera; Juvenile metazoans, crustacea (copepod nauplii), etc.

(E) Nanoplankton: They are 2-20 µm in size. These are small real cell protists; Diatoms; Small flagellates; Pyrrophyta; Chrysophyta; Chlorophyta; Xanthophyta etc.

(F) Picoplankton: They are 0.2-2 µm in size. Small actual cell protists; bacteria; Chrysophyta etc.

(G) Femtoplankton: They are smaller than 0.2 µm in size. Such as marine viruses.

A symposium on the size of plankton was held in 1958 in Bergen, Norway. This symposium is divided into the following types of plankton based on the size of plankton:

Plankton TypeSize
Megaloplankton>1 cm
Macroplankton1 mm-1 cm
Mesoplankton0.5-1 mm
Microplankton>60 μ
Nanoplankton [sic]>5 μ
Ultraplankton< 5 μ

Moreover, Dussart proposed the following classification of plankton in 1985: 

Plankton GroupsPlankton NameSize
NetplanktonMegaloplankton>2000 micrometers
Mesoplankton200- 2000 micrometers
Microplankton20- 200 micrometers
NanoplanktonNanoplankton2-20 micrometers
Ultraplankton< 2 micrometer

In 1984, Omori and Ikeda proposed the following classification of plankton: 

Plankton TypeSizeGroup
Megaloplankton>20 millimeters
Microplankton20-200 millimetersNet Plankton
Macroplankton2- 20 millimetersNet Plankton
Mesoplankton200 micrometers- 2 millimetersNet Plankton
Microplankton20- 200 micrometersWater bottle plankton
Nanoplankton2- 20 micrometersWater bottle plankton
Ultraplankton< 2 micrometersWater bottle plankton

Table: Zooplankton of excavated Harda Baor and non-excavated Chandbil Bill Baor in Meherpur district of Bangladesh, Kabir and Naser (2009).  (Zooplankton Classification based on Gojdics 1953, Parker 1982 and Ward & Whipple 1959)

Reef Phytoplankton

Phylum: Protozoa

Plankton NamePlankton Name
Class: Euglenophyceae
Order: Euglenales
Family: Euglenaceae
Genus: Euglena Ehrenberg
Euglena acus  Ehrenberg
Euglena fusca (Klebs) Lemmermann
Euglena clavata Skuja
Euglena oxyuris Schmarda
Euglena oxyuris var. minor Defl.
Euglena geniculata Dujardin
Euglena  granulata (Klebs) Schmitz
Euglena proxima  Dangeard
Euglena sanguinea Ehrenberg
Euglena elastica
Euglena gosdicsae Prescott
Euglena spathirhyncha Skuja
Euglena tripteris (Dujardin)Klebs
Euglena mutabilis Schmitz
Euglena mainxii Deflandre
Euglena viridis Perty
Euglenaallorgei Deflandre
Euglena sp.
Genus: Phacus Dujardin
Phacus longicaudatus  (Ehrenberg) Dujardin
Phacus pleuronectus Miiller
Phacus similis Christen

Phacusacuminatus var. indica
Phacus ranula Pochmann
Phacus sp.
Family: Volvocidae
Genus Volvox Linnaeus
Volvox sp.
Class: Dinophyceae
Order: Peridiniales
Family: Peridiniaceae
Genus:Peridinium Ehrenberg
Peridinium sp.
Genus: Ceratium Schrank
Ceratium hirundinella Schrank
Class: Rhizopoda
Order: Testacealobosa
Family: Difflugidae
Genus: Difflugia Leclerc
Difflugia corone  Wallich
Difflugia sp.
Order: Testaceafilosa
Family: Euglephidae
Genus: Euglepha Dujardin
Euglepha sp.


Plankton NamePlankton Name
Class: Crustacea
Order : Copepoda
Suborder: Calanoida
Family: Diaptomidae
Genus:Diaptomus  Forbes
Diaptomus gracilis Sars
Diaptomus sp.
Genus: Skistodiaptomus Light
Skistodiaptomus pygmeus Forbes
Genus:Neodiaptomus Kiefer
Neodiaptomus sp.
Suborder: Cyclopoida
Family: Cyclopidae
Genus:Cyclops Muller
Cyclops sp.
Genus: Ectocyclops  Brady
Ectocyclops  sp.
Genus: Microcyclops  Muller
Microcyclops varicans Lillijeborg
Microcyclops rubellus Lillijeborg
Microcyclops sp.
Genus: Mesocyclops Sars
Mesocyclops leukerti Claus
Mesocyclops edax  Forbes
Mesocyclops sp.
Genus:Macrocyclops  Claus
Macrocyclops sp.
Genus:Orthocyclops  Forbes
Orthocyclops modestus Herrick
Genus:Eucyclops  Claus
Eucyclops agilis Koch
Order: Cladocera
Family: Chydoridae
Genus:Alona Baird
Alona sp.
Genus:Chydorus Leach
Cydorus sp.
Family: Sididae
Genus:Diaphanosoma Fischer
Diaphanosoma brachyurum Lieven
Diaphanosoma sp.
Family: Daphnidae
Genus:Moina Baird
Moina brachiata Jurine
Moina sp.
Family: Macrothricidae
Genus:Macrothrix Baird
Macrothrix sp.
Order: Ostracoda
Family: Cypridae
Genus:Cypris Muller
Cypris sp.
Suborder: Harpacticoida
Family: Bryocamptidae
Genus:Bryocamptus Chappuis
Brycamptus sp.

Phylum Rotifera

Plankton NamePlankton Name
Class: Bdelloidea
Order: Bdelloida
Family: Philodinidae
Genus: Rotaria Scopoli
Rotaria neptunia Ehrenberg
Class: Monogononta
Order: Ploima
Family: Brachionidae
Genus: Anuraeopsis Lauterborn
Anuraeopsis fissa Gosse
Genus: Brachionus Pallus
Brachionus angularis Gosse
Brachionus calyciflorus Pallus
Brachionus calyciflorus var. dorcas Gosse
Brachionus calyciflorus var. dorcus-spinosus
Brachionus calyciflorus f. amphiceros Ehrenberg
Brachionus calyciflorus f. borgerti Apstein
Brachionus calyciflorus f. anuraeopsis Brehm.
Brachionus  caudatus Barrois & Daday
Brachionus caudatus var. personatus Ahlstrom
Brachionus forficula Wierzeski
Brachionus forficula f. minor Virinkov
Brachionus  falcatus Zacharias
Brachionus falcatus var. hammatus
Brachionus plicatilis O.F. Muller
Brachionus nilsoni  Ahlstrom
Brachionus bidentata Anderson
Brachionus urceolaris Muller
Brachionus bennini Leissling
Brachionus rubens Ehrenberg
Brachionus  diversicornis Daday
Brachionus quadridentatus Hermann
Brachionus  zahnisheri Ahlstrom
Genus: Keratella Bory de St. Vincent
Keratella vulga Ehrenberg
Keratella cochlearis Gosse
Genus: Notholca Gosse
Notholca acuminata Ehrenberg
Notholca sp.
Genus: Platyias Herring
Platyias patulus Muller
Platyias quadricornis Ehrenberg
Platyias polycanthus
Genus: Epiphenes  Ehrenberg
Epiphenes sp.
Genus: Diplois Gosse
Diplois daviesiae Gosse
Diplois sp.
Family: Brachionidae
Genus: Euchlanis Ehrenberg
Euchlanis dilatata  Ehrenberg
Genus: Tripleuchlanis  Myers
Tripleuchlanis plicata  Levander
Genus: Colurella Bory de St. Vincent
Colurella bicuspidata Ehrenberg
Genus: Lepadella  Bory de St. Vincent
Lepadella sp.
Genus: Mytilina Bory de St. Vincent
Mytilina ventralis Ehrenberg
Family: Asplanchnidae
Asplanchna herricki  de Guerne
Asplanchna priodonta Gosse
Asplanchna sp.
Family: Lecanidae
Genus: Monostylla  Ehrenberg
Monostyllla closterocera Schmarda
Monostylla quadridentata Ehrenberg
Monostylla sp.
Genus: Lecane Nitsch
Lecane leontina Turner
Lecane curvicornis Murray
Lecane nodosai Hauer
Lecane sp.
Family: Trichoceridae
Genus: Trichocerca Lamarck
Trichocerca capucina Wierzeski
Trichocerca  porcellus Gosse
Trichocerca cylindrica Imhof
Trichocerca  longiseta Ehrenberg
Trichocerca similes Wierzejki
Trichocerca braziliensis Murray
Trichocerca rousseleti Voight
Trichocerca sp.
Family: Lindidae
Genus: Lindia Dujardin
Lindia sp.
Family: Gastropidae
Genus: Ascomorpha Perty
Ascomorpha sp.
Genus: Chromogaster Lauterborn
Chromogaster ovalis
Chromogaster sp.
Family: Notommatidae
Genus: Cephalodella Bory de St. Vincent
Cephalodella incila Wulfert
Cephalodella forficula Ehrenberg
Family: Synchaetidae
Genus: Polyarthra Ehrenberg
Polyarthra vulgaris Carlin
Polyarthra sp.
Order: Flosculariaceae
Family: Testudinellidae
Genus: Filinia Bory de St. Vincent
Filinia terminalis Plate
Filinia longiseta Ehrenberg
Filinia opoliensis Zacharias
Filinia camaseclaMyers
Genus: Pompholyx Gosse
Pompholyx sulcata Gosse
Genus: Horealla Donner
Horealla brehmi  Donner
Family: Flosculariidae
Genus: Fluscularia
Fluscularia sp.
Genus: BeauchampiellaRename
Beauchampiella sp.
Family: Hexarthridae
Genus: Hexarthra Schmarda
Hexarthra sp.
Pedalia fennica Levander
image of Protozoan-plankton-1

Diffreent types of Protozoan Zooplankton (Bar=50 μm, Photo: Dr N. Kabir)

Diffreent types of Protozoan Zooplankton (Bar=50 μm, Photo: Dr N. Kabir)

image of  some plankton (copepods)

Image Showing Some plankton (Copepods Species): Photo-Dr N Kabir

Image of Copepods

Image Showing Some Cladocerans Species: Photo-Dr N Kabir

image of some plankton (rotifers species)

Image Showing Some Plankton( Rotifers Species): Photo-Dr N Kabir

3. Plankton Classification Based on Local Environmental Expansion

There are five types of plankton based on local environmental expansion, viz.

  1. Limnoplankton: These are the plankton of the lake.
  2. Rioplankton: These are plankton of running water.
  3. Helioplankton: These are the plankton of the pond.
  4. Helioplankton: These are salt water plankton.
  5. Hypalmyroplankton: These are the plankton of the estuary region.

4. Plankton can be divided into the following two types based on their origin, viz-

  1. Autogenic plankton: Plankton produced locally is called autogenic plankton.
  2. Allogeneic plankton: Plankton from other places is actually called allogeneic plankton.

5. Plankton can be divided into the following two groups on the basis of structural elements, viz.

  1. Euplankton: These are the real plankton.
  2. Pseudoplankton: If rotten matter is mixed with plankton, it is called pseudoplankton.

6. Plankton can be divided into  the following two groups based on life history, viz-

  1. Holoplankton: They float for life.
  2. Metoplankton: They float freely at any stage of the life cycle.