What is Plankton?

Planktons

Planktons are microscopic entities constituting the sustaining base of food chains in the ocean. They live their lives drifting in the water column.

Table of Content:

• Plankton Meaning
• Types of Plankton
• Plankton – Ecological Significance
• Frequently Asked Questions

Plankton Meaning

“Plankton” term is used for all the organisms found in marine as well as freshwater, which are non-motile and cannot swim against the water current. They are drifted by water currents. Plankton is collectively used to refer to organisms such as bacteria, algae, crustaceans, protozoans, coelenterates and molluscs and members from other phylum as well.

• Planktons vary widely in size, from 0.2 𝝁m to more than 20 cm. From microscopic bacteria to large organisms such as jellyfish
• Their distribution varies with the availability of light and nutrients
• Large aquatic organisms are dependent on planktons and feed on them
• They have a significant role to play in the ecosystem. Phytoplankton account for 50% of the total oxygen (O₂) produced by photosynthesis.

Pleuston, Nekton, Neuston, Benthos

Plankton is different from nekton, that comprises animals that are strong swimmers, and benthos, that creep, and burrow entities on the seafloor. Huge floating seaweeds and different multicellular algae are pleustons not planktons. Pleustons are life forms thriving at the air and water’s interface. Neustons, on the other hand, are entities which rest or swim on the surface of water. An example is the Alga Ochromonas.

Plankton differs from other related terms used for various aquatic species, e.g. –

Pleuston Definition – It refers to floating organisms.

They thrive in the thin surface layer found at the interface of water and air of water bodies. They are macroscopic and float by buoyancy. Example of Pleuston – Few cyanobacteria, Salvinia etc.

Nekton Definition and Examples – It is used to refer to actively swimming organisms.

These aquatic entities can swim and independently move in water currents. The chordate Nekton are said to include different species of bony fishes, cartilaginous fishes like the sharks, different reptiles (snakes, crocodiles in saltwater) and even mammals like the seals, whales etc. The molluscs nektons are ocotopods and squids.

Neuston Meaning – organisms that are present on the water surface.

Neustons are entities living near water surface. Typically, they thrive in the surface films of oceans, lakes and slow moving streams. Hyponeuston (entities living immediately under the surface of water) and epineuston (entities living above surface of water but immersed in water) are two types of Neuston. There is a reduction in the density of these entities with an increase in the turbulence.

Benthos Definition – organisms present on the sea bed that are sessile, burrowing, creeping, etc.

These entities are found at the bottom of freshwater bodies (streams, rivers and lakes). Most of these entities do not show a backbone and are referred to as invertebrates. Some of the common benthic invertebrates are sponges, anemones, corals, sea stars, worms, crabs, sea urchins etc.

Benthic entities could be sessile or attached to some firm surface such as rocks or could be mobile, freely moving or in the bottom sediment.

Also Read: Difference between Phytoplankton and Zooplankton 

Types of Plankton

Planktons are categorised on the basis of lifecycle, size and trophic level.

• On the basis of the lifecycle, planktons are classified as:

1. Holoplankton – Organisms, which remain in planktonic form for their entire lifetime, e.g. algae, jellyfish, etc.
2. Meroplankton – Organisms, who live as plankton at only some stages of their lifecycle, e.g. larvae of starfish, worms, sea urchins, fish, etc.

• On the basis of size, planktons are classified as:

1. Megaplankton – they are large in size > 20 cm, e.g. Jellyfish, tunicates pyrosomes, etc.
2. Macroplankton – their size ranges from 2 to 20 cm
3. Mesoplankton – includes organisms with size 0.2 to 20 mm
4. Microplankton – their size varies from 20 to 200 𝝁m, e.g. most of the phytoplankton, protozoans and large protists
5. Nanoplankton – size ranges from 2 to 20 𝝁m, e.g. protists, diatoms and algae
6. Picoplankton – size ranges from 0.2 to 2 𝝁m, e.g. bacteria, chrysophytes
7. Femtoplankton – includes marine viruses with size < 0.2 𝝁m

• On the basis of various trophic levels and characteristics, planktons are classified as:

1. Phytoplankton – They are autotrophs or producers, e.g. cyanobacteria, algae, diatoms, dinoflagellates, etc.
2. Zooplankton – It includes primary consumers, who feed on other plankton, e.g. small protozoans, larvae of fish and other animals
3. Mycoplankton – It includes fungi
4. Bacterioplankton – It includes bacteria and is important for nutrient recycling
5. Virioplankton – includes viruses
6. Mixotrophs – They act as both producers and consumers according to environmental conditions. When there is abundant light and nutrients available they perform photosynthesis and in the absence of limited resources, they feed on other plankton to fulfil their nutrient requirements

Explore: MCQs on Dinoflagellates

Plankton – Ecological Significance

Planktons play an important role in the ecosystem.

• As producers, they account for half of the total amount of oxygen evolved during photosynthesis
• Fisheries are dependent on plankton for fish food
• Phytoplanktons are used as a source of food in space travel and also for carbon dioxide fixation. Chlorella is used as a protein supplement
• They play an important role in nutrient recycling. Dead and decaying organic matter can be transformed into inorganic nutrients for plants
• Some of the species can fix nitrogen of dissolved air to ammonia and nitrates, which are useful for the growth of phytoplankton
• Phytoplankton and zooplankton are required for maintaining carbon dioxide and oxygen balance. CO₂ uptake of an ocean can be increased by phytoplanktonic growth
• Bioluminescent planktons are responsible for a blue flash of light in the sea. When there is abundant nutrient present, unicellular dinoflagellates are known to multiply rapidly and cause a bloom. They produce toxins and are harmful for fish and other organisms feeding on fishes. They are known to cause red tide.

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