“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.
- 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 nutrient
- 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.
Plankton differs from other related terms used for various aquatic species, e.g.
Pleuston – refers to floating organisms
Nekton – used for actively swimming organisms
Neuston – organisms that are present on the water surface
Benthos – organisms present on the sea bed that are sessile, burrowing, creeping, etc.
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:
- Holoplankton – Organisms, which remain in planktonic form for their entire lifetime, e.g. algae, jellyfish, etc.
- 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:
- Megaplankton – they are large in size > 20 cm, e.g. Jellyfish, tunicates pyrosomes, etc.
- Macroplankton – their size ranges from 2 to 20 cm
- Mesoplankton – includes organisms with size 0.2 to 20 mm
- Microplankton – their size varies from 20 to 200 𝝁m, e.g. most of the phytoplankton, protozoans and large protists
- Nanoplankton – size ranges from 2 to 20 𝝁m, e.g. protists, diatoms and algae
- Picoplankton – size ranges from 0.2 to 2 𝝁m, e.g. bacteria, chrysophytes
- Femtoplankton – includes marine viruses with size < 0.2 𝝁m
- On the basis of various trophic level and characteristics, planktons are classified as:
- Phytoplankton – They are autotrophs or producers, e.g. cyanobacteria, algae, diatoms, dinoflagellates, etc.
- Zooplankton – It includes primary consumers, who feed on other plankton, e.g. small protozoans, larvae of fish and other animals
- Mycoplankton – It includes fungi
- Bacterioplankton – It includes bacteria and important for nutrient recycling
- Virioplankton – includes viruses
- 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 or limited resources, they feed on other plankton to fulfil their nutrient requirements
Ecological Significance and Importance of Plankton
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.