Plankton

The term plankton is derived from the Greek word ‘Planktos’, meaning ‘errant’. It was coined by a scientist called Victor Henson in 1887. The study of plankton is known as planktology and a planktonic individual is referred to as a plankter. Plankton size varies widely, from 0.2 m to more than 20 cms, from microscopic bacteria to large organisms such as jellyfish. Let us learn about the planktons in detail, their importance, and types with examples.

Planktons

Planktons do not depend on other organisms for movement and they can swim for hundreds of kilometres per day. Plankton’s horizontal position depends on the water movement surrounding them and they also flow with the ocean currents. In contrast to the nekton organisms, such as fish, squid, and marine mammals, that can swim against the ambient flow and can control their position in the respective environment.

What is Plankton?

Planktons are the organisms that are found in water, they contain both marine and freshwater organisms. Algae, protozoans, bacteria, mollusks, and coelenterates are examples. In the oceans, they provide food to small fishes and whales. Plankton is usually thought of as inhabiting water, but there are also airborne versions, the aeroplankton, that live part of their lives drifting in the atmosphere. These include plant spores, pollen, and wind-scattered seeds, as well as microorganisms swept into the air from terrestrial dust storms and oceanic plankton swept into the air by sea spray.

Plankton Examples: 

• Marine plankton: Bacteria, protozoa, archaea, and algae. It also includes animals present in the saltwater of oceans and the brackish water of the estuaries. 

• Freshwater plankton: They are the same as marine planktons but present in freshwater. 

Types of Plankton

Planktons are divided into different types based on the lifecycle, their size and the trophic levels. They are as follows:

Based on the Lifecycle:

1. Holoplankton: Organisms that remain in the planktonic form for their entire lifetime are called holoplankton. E.g. algae, jellyfish, etc.

2. Meroplankton: Organisms that can live as planktons only for some stages in their entire lifecycle are called meroplankton. E.g. larvae of starfish, worms, sea urchins, fish, etc.

Based on the Size:

1. Megaplankton: They are the large planktons whose size is greater than 20cm. Eg: Jellyfish

2. Macroplankton: Planktons whose size vary from 2 to 20cm. 

3. Mesoplankton: Planktons whose size vary from 0.2 to 20mm. 

4. Microplankton: Planktons whose size vary from 20 to 200 m. Eg: Protozoans and large protists. 

5. Nanoplankton: Planktons whose size vary from 2 to 20 m. Eg: Protists, diatoms and algae. 

6. Picoplankton: Planktons whose size varies from 0.2 to 2 m. Eg: Bacteria, chrysophytes. 

7. Femtoplankton: Planktons includes marine viruses with size less than 0.2 m.

Based on the Trophic Level:

1. Phytoplankton: They are autotrophs or producers. Eg: Cyanobacteria, algae, diatoms, dinoflagellates, etc.

2. Zooplankton: These include primary consumers, that feed on the other planktons. Eg. Small protozoans, larvae of fish and other animals.

3. Mycoplankton: These planktons include fungi.

4. Bacterioplankton: These planktons include bacteria which are important for nutrient recycling.

5. Virioplankton: These planktons include viruses.

6. Mixotrophs: These mixotrophs include both producers and consumers according to their environmental conditions. In the presence of light and the mineral nutrients, they undergo photosynthesis and in the absence of the resources, they feed on the other planktons for food.

Types of Phytoplankton

1. Diatoms

2. Dinoflagellates

3. Desmids

Importance of Plankton

Planktons play an important role in the ecosystem.

• As producers, they are responsible for half of the total amount of oxygen evolved during photosynthesis. Fisheries are dependent on plankton for food.

• Phytoplanktons are used as a source of food in space travel and also for fixation of carbon dioxide. Chlorella is used as a protein supplement.

• Planktons play an important role in nutrient recycling. 

• The 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 can be consumed in the oceans by increasing the phytoplanktonic growth.

• Bioluminescent planktons are responsible for a blue flash of light in the sea. In the presence of nutrients, the unicellular dinoflagellates are known to multiply rapidly and they cause a bloom. They produce toxins and are harmful to fish and other organisms feeding on fishes. They are known to cause red tide.