The word ‘plankton’ has been derived from a Gr. word ‘planktos’ means drifting. So, plankton are those microscopic aquatic organisms or drifters whose movement is mainly driven by water current, wave, tide, etc. Although, some of them might possess some kinds of locomotory organs but those are not sufficiently strong to overcome the water current.
- The word ‘Plankton’ was first coined by Victor Hensen in 1887 to mean all the microscopic organisms and non-living minute particles that are drifted by wave, current and other means of water movement.
So scientifically, plankton are those microscopic or sub-microscopic suspended organisms of water who do not have any locomotion of their own or if they would have any flagella or cilia attached to them that is not sufficiently strong enough to overcome the strength of water movement, current or wave actions. It means the movement of plankton is mainly regulated by the motion of water.
Some organisms are only classified as plankton when they are young, but they eventually grow large enough to swim against the currents.
What Does Plankton Include?
Planktons include Viruses, Bacteria, members of Protista (phytoplankton and zooplankton) and Fungi (saproplankton). Basically, scientists classify plankton in several ways, depending on size, type, and how long they spend drifting. But the most basic categories dividing plankton into two groups are phytoplankton (plant drifters) and zooplankton (animal drifters).
There are about 5000 marine phytoplankton consisting of diatoms, dinoflagellates, cyanobacteria and eustigmatophytes (in a decreasing order). Though they are microscopic in size, they are considered as the “Pasturage of the sea” because of their abundance and the huge role they play in marine food web.
They maintain O2 and N2 balance of the atmosphere (about 80% supplied by phytoplankton). They occasionally form bloom, release toxins, affecting marine lives.
Phytoplankton are the autotrophic components of the plankton community and a key part of oceans, seas, and freshwater ecosystems.
Phytoplanktons are classified depending on
- Life Cycle
- Ecology: Habitat & Zones
Based on size
- Macroplankton: > 500 µm, e.g. Oscillatoria sp.
- Microplankton or net plankton: ranging from 50 µm to 500 µm, e.g. Volvox sp.
- Nannoplankton: ranging from 10-50 µm e.g. Rhodomonas sp.
- Ultraplankton: ranging from 0.5-10 µm , e. g. Chlorella sp
- Picoplankton: raging form 0.2 to 2.0 µm, mainly unicellular e.g. Synechocystis sp.
Based on life cycle
- Holoplankton: free-floating throughout their life, e.g. Navicula sp.
- Meroplankton: attached but certain stage of its life cycle is free-floating, e.g. Melosira sp.
- Tychoplankton: attached forms but free-floating due to the accidental breaking of thallus e.g. Oedogonium sp.
Based on ecology
A) Based on the nature of the habitat
- Limnoplankton: lake plankters; e.g. Cryptomonas sp.
- Rheoplankton: running (River) water plankters; e.g. Aulacoseira granulata
- Heleoplankton: pond plankters; e. g. Scenedesmus sp.
- Haliplankton: salt water plankters; e.g. Chaetoceros sp.
- Hypalmiroplankton: brackish water plankters; e.g. Stephanodiscus sp.
B) Based on the zones where they occur
- Epiplankton: plankters occuring in the euphotic zone, e.g. Navicula sp.
- Mesoplankton: plankters occuring in the disphotic zone, e.g. Chaetoceros sp.
- Hypoplankton or Bathyplankton: plankters live in the near bottom aphotic zone; e.g. Ceratium sp.
- Neritic: plankters occuring near the shore (coastal zone), e.g. Coscinodiscus sp.
- Pelagic: plankters occuring away form the shore (oceanic zone). e,.g. Cyclotella sp.
Based on origin
- Autogenic-plankton: plankters produced locally; e.g. Stephanodiscus sp.
- Allogenic-plankton: plankters introduced from other locality; e.g. Spirogyra sp.
Adaptation of Phytoplankton
Among aquatic organisms, the size and shapes of phytoplankton are really amazing. The unique features of size and shapes of phytoplankton actually help them to remain suspended in water.
Phytoplankton obtain their energy through photosynthesis. This means phytoplankton must have light from the sun to produce food. So, they afloat in the well-lit surface layers (called euphotic zone) of oceans and lakes to carry out primary production.
Plankton are usually heavier than water. This is important because if a planktonic (phyto- or zoo-) organism just floated on the surface of the water, it might not be able to get food sources below it or it might get too warm or too much light from the sun (even phytoplankton can be “bleached” by the sun!).
So plankton will tend to sink in the water column. But phytoplankton do need to stay where sunlight penetrates. On the other hand, zooplankton feed on phytoplakton. So the zooplankton want to stay where the phytoplankton are in the water column. Therefore, planktonic organisms will have adaptations that prevent them from sinking too quickly.
- Since the floating capacity and sinking velocity of phytoplankton cells in water are a function of their density, the response of phytoplankton having different densities towards depth are also different.
- Depending on their individual density and locomotion capacity three different kinds of phytoplankton can be seen.
- Non motile, negatively buoyant – whose density is higher than water e.g., Asterionella formosa, Melosira and Fragilaria, because of their body weight they start sinking gradually which under some special condition increases.
- Positively buoyant – whose density is lower than water . e.g. Microcystis, Aphanizomenon, Gloeotrichia, Coelosphaerium and Gomphosphaeria.
- Neutrally buoyant and motile – whose density is equal to water, they may be motile having the ability to keep themselves at a definite place or non-motile.
In evolutionary processes, special adaptive structures developed in their body so that these organisms are better able to cope with the problems of their dynamic habitat.
Plankters (with its protoplasm and skeleton the specific gravity is > 2.00) are heavier than the water (1.00 specific gravity) and will sink if there is no means to work against gravitational force.
However, for floatation phytoplankton are variously modified as follows:
a. Small size: Typical plankters maintain their planktonic existence through their small size. e.g. Pyramimonas sp.
b. Appendages: Presence of special structures may help phytoplankters to keep them in proper position in the water and to retard sinking, e.g. the frustules of Chaetoceros produce long chaetae, Corethron, Golenkinia with bristles, Dictyocha with spines and Ceratium with horns.
c. Shape: Cell shape has been changed in course of evolution for planktonic existence. Discoid (Coscinodiscus), needle (Rhizosolonia) or ribbon (Skeletonema, Melosira) shaped structures have increased surface area which prevent sinking vertically.
d. Colonial life: Unicellular forms evolve into colonial forms (Gonium sp.), raft like (Diatoma), star like (Asterionella) which serve to increase resistance to sinking.
e. Oil droplets: Certain plankters deposit oil as reserved food. The oil reduces the specific gravity to such on extent that the organisms become lighter than the water and hence help to float at or near the surface, e.g. diatoms and dinoflagellates.
f. Gas vacuoles: Many filamentous cyanobacterial cells produce gas filled protein membrane structure called gas vesicles, which are stacked in to vacuoles that keep the organism floating.
g. Air bladder: Some algae produce air bladders for the planktonic existance, e.g. Sargassum fluitans, a tychoplankton.
(This article is completely based on the Lecture sheet provided by respected Dr. Mohammad Azmal Hossain Bhuiyan, Professor, Department of Botany, University of Dhaka.
Some pictures and info have been added by the author. Any mistake, error, misinformation and other related things found in this article is only author’s to blame)