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.

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.

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.

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 Tyle



> 1 cm


1 mm - 1 cm


0.5 - 1 mm


> 60 μ

Nannoplankton [sic]

> 5 μ


< 5 μ

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

Plankton Groups

Plankton Name




> 2000 micrometers


200 - 2000 micrometers


20 - 200 micrometers



2 - 20 micrometer


< 2 micrometer

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

Plankton Types




> 20 millimeters

Characteristic 1


20 - 200 millimeters



2 - 20 millimeters


Characteristic 4


20 - 200 micrometers

water bottle


2 - 20 micrometers


< 2 micrometers

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)

                                                    Phylum: Protozoa

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

Euglena allorgei Deflandre

Euglena sp.

Genus: Phacus Dujardin

Phacus longicaudatus  (Ehrenberg) Dujardin

Phacus pleuronectus Miiller

Phacus similis Christen

Phacus acuminatus var. indica

Phacus ranula Pochmann

Phacus hameli

Phacus undulata

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.
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)

                                                   Phylum: Arthropoda

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.

imAGE OF copepods

Image Showing Some Copepods Species: Photo-Dr N Kabir

Image of Copepods

Image Showing Some Cladocerans Species: Photo-Dr N Kabir

                                             Phylum: Rotifera

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 camasecla Myers

Genus: Pompholyx Gosse

Pompholyx sulcata Gosse

Genus: Horealla Donner

Horealla brehmi  Donner

Family: Flosculariidae

Genus: Fluscularia

Fluscularia sp.

Genus: Beauchampiella Rename

Beauchampiella sp.

Family: Hexarthridae

Genus: Hexarthra Schmarda

Hexarthra sp.

Pedalia fennica Levander

image of rotifers

Image Showing Some 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.
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