Plankton

Plankton (singular plankter) are a diverse group of organisms that live in the water column and cannot swim against a current. They provide a crucial source of food to many large aquatic organisms, such as fish and whales.

These organisms include drifting animals, protists, archaea, algae, or bacteria that inhabit the pelagic zone of oceans, seas, or bodies of fresh water; that is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification.

(Pelagic Zone:- Any water in a sea or lake that is neither close to the bottom nor near the shore can be said to be in the pelagic zone.

Phylogenetics:- In biology, phylogenetics is the study of evolutionary relationships among groups of organisms (e.g.species, populations), which are discovered through molecular sequencing data and morphological data matrices.

Taxonomy:- is the science of defining groups of biological organisms on the basis of shared characteristics and giving names to those groups.The Swedish botanist Carolus Linnaeus is regarded as the father of taxonomy, as he developed a system known as Linnaean classification for categorization of organisms and binomial nomenclature for naming organisms.)

Though many planktic species are microscopic in size, plankton includes organisms covering a wide range of sizes, including large organisms such as jellyfish.

The study of plankton is termed planktology and a planktonic individual is referred to as a plankter.

Plankton are primarily divided into broad functional (or trophic level) groups:

• Phytoplankton (from Greek phyton, or plant), autotrophic, prokaryotic or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthesis. Among the more important groups are the diatoms,cyanobacteria, dinoflagellates and coccolithophores.
• Zooplankton (from Greek zoon, or animal), small protozoans or metazoans (e.g. crustaceans and other animals) that feed on other plankton and telonemia. Some of the eggs and larvae of larger animals, such as fish, crustaceans, andannelids, are included here. Zooplankton are the initial prey item for almost all fish larvae as they switch from their yolk sacs to external feeding. Fish rely on the density and distribution of zooplankton to match that of new larvae, which can otherwise starve. Natural factors (e.g., current variations) and man-made factors (e.g. river dams) can strongly affect zooplankton, which can in turn strongly affect larval survival, and therefore breeding success.
• Bacterioplankton, bacteria and archaea, which play an important role in remineralising organic material down the water column (note that the prokaryotic phytoplankton are also bacterioplankton).

This scheme divides the plankton community into broad producer, consumer and recycler groups. However, determining the trophic level of some plankton is not straightforward. For example, although most dinoflagellates are either photosynthetic producers or heterotrophic consumers, many species are mixotrophic depending upon circumstances.

(Trophic Level:- The trophic level of an organism is the position it occupies in a food chain. The word trophic derives from the Greek (trophē) referring to food or feeding. A food chain represents a succession of organisms that eat another organism and are, in turn, eaten themselves. The number of steps an organism is from the start of the chain is a measure of its trophic level. 

Food chains start at trophic level 1 with primary producers such as plants, move to herbivores at level 2, predators at level 3 and typically finish with carnivores or apex predators at level 4 or 5. The path along the chain can form either a one-way flow or a food "web". Ecological communities with higher biodiversity form more complex trophic paths.)