Plankton
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Plankton are drifting organisms that inhabit the water column of oceans, seas, and bodies of fresh water.
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[edit] Definitions
The name plankton is derived from the Greek word πλανκτος ("planktos"), meaning "wanderer" or "drifter" (Thurman, 1997). While some forms of plankton are capable of independent movement and can swim up to several hundreds of metres vertically in a single day (a behavior called diel vertical migration), their horizontal position is primarily determined by currents in the body of water they inhabit. By definition, organisms classified as "plankton" are unable to resist ocean currents. This is in contrast to nekton organisms that can swim against the ambient flow of the water environment and control their position (e.g. squid, fish, krill and marine mammals).
Within the plankton itself, holoplankton are those organisms that spend their entire life cycle as part of the plankton (e.g. most algae, copepods, salps, and jellyfish). By contrast, meroplankton are those organisms that are only planktonic for part of their lives (usually the larval stage), and then graduate to either the nekton or a benthic existence. Examples of meroplankton include the larvae of sea urchins, sea stars, crustaceans, marine worms, and most fish.
Plankton abundance and distribution are strongly dependent on factors such as ambient nutrients concentrations, the physical state of the water column, and the abundance of other plankton.
The study of plankton is termed planktology. Individual plankton are referred to as plankters.
[edit] Size groups
Plankton are also often described in terms of size. Usually the following divisions are used:
| Group | Size range (ESD) | Major organisms | |
| Megaplankton | 2×10-1→2×100 m | (20-200 cm) | metazoans ; e.g. jellyfish |
| Macroplankton | 2×10-2→2×10-1 m | (2-20 cm) | metazoans ; e.g. pteropods |
| Mesoplankton | 2×10-4→2×10-2 m | (0.2mm-2 cm) | metazoans ; e.g. copepods |
| Microplankton | 2×10-5→2×10-4 m | (20-200 µm) | large eukaryotic protists; juvenile/small metazoans |
| Nanoplankton | 2×10-6→2×10-5 m | (2-20 µm) | small eukaryotic protists |
| Picoplankton | 2×10-7→2×10-6 m | (0.2-2 µm) | prokaryotic bacteria |
| Femtoplankton | < 2×10-7 m | (< 0.2 µm) | marine viruses |
However, some of these terms may be used with very different boundaries, especially on the larger end of the scale. The existence and importance of nano- and even smaller plankton was only discovered during the 1980s, but they are thought to make up the largest proportion of all plankton in number and diversity.
[edit] Distribution
Plankton are found throughout the oceans, seas and lakes of Earth. However, the local abundance of plankton varies horizontally, vertically and seasonally. The primary source of this variability is the availability of light. All plankton ecosystems are driven by the input of solar energy (but see chemosynthesis), and this confines primary production to surface waters, and to geographical regions and seasons when light is abundant.
A secondary source of variability is that of nutrient availability. Although large areas of the tropical and sub-tropical oceans have abundant light, they experience relatively low primary production because of the poor availability of nutrients such as nitrate, phosphate and silicate. This is a product of large-scale ocean circulation and stratification of the water column. In such regions, primary production, still usually occurs at greater depth, although at a reduced level (because of reduced light).
Studies have shown that the mineral iron (but only in the proper amounts) can lead to increased blooms of many (though not all) kinds of phytoplankton (e.g. Boyd et al., 2000). Iron is primarily made available to oceanic phytoplankton through the deposition of atmospheric dust on the sea surface. Oceanic areas adjacent to arid parts of continents thus typically have abundant phytoplankton (e.g., the western Atlantic Ocean, where trade winds bring dust from the Sahara Desert in north Africa). It seems a paradox that arid land areas may actually contribute to increased plant life proliferation in the world's oceans. It has been theorized (but never actually attempted) that large-scale 'seeding' of the world's oceans with iron would generate such massive blooms of phytoplankton so as to draw enough carbon dioxide out of the atmosphere to counteract the Greenhouse Effect (or global warming) (Martin, 1990).
While plankton are found in the greatest abundance in surface waters, they occur throughout the water column. At depths where no primary production occurs, zooplankton and bacterioplankton instead make use of organic material sinking from the more productive surface waters above. This flux of sinking material can be especially high following the termination of spring blooms.
[edit] Biogeochemical significance
Aside from representing the bottom few levels of a food chain that leads up to commercially important fisheries, plankton ecosystems play a role in the biogeochemical cycles of many important elements. Of particular contemporary significance is their role in the ocean's carbon cycle.
As stated, phytoplankton fix carbon in sunlit surface waters via photosynthesis. Through (primarily) zooplankton grazing, this carbon enters the planktonic foodweb, where it is either respired to provide metabolic energy, or accumulates as biomass or detritus. As living or dead organic material is typically more dense than seawater it tends to sink, and in open ocean ecosystems away from the coasts this leads to the transport of carbon from surface waters to the deep. This process is known as the biological pump, and is one of the reasons that the oceans constitute the largest (active) pool of carbon on Earth.
Some researchers have even proposed that it might be possible to increase the ocean's uptake of carbon dioxide generated through human activities by increasing the production of plankton through fertilization, primarily with the micronutrient iron. However, it is debatable whether this technique is practical at a large scale, and some researchers have drawn attention to possible drawbacks such as ocean anoxia and resultant methanogenesis (caused by the excess production remineralising at depth) (Chisholm et al. 2001; Johnson et al. 2002).
[edit] Popular Culture
In the animated television series SpongeBob SquarePants, Sheldon J. Plankton is the name of one of the primary antagonists SpongeBob faces. His relationship to plankton is manifested in his size, as he is much smaller than the other characters, and also by his single, copepod-like, eye.
In an episode of the animated television series The Simpsons, the family chooses to go shopping at a 33-cent discount store which offers a variety of strange foods. Homer purchases and eats expired canned plankton, and consequently falls ill as a result of red tide poisoning.
The science fiction novels Timescape by Gregory Benford, and The Secret Of Life by Paul McAuley both invoke environmental disasters caused by changes in the logical behaviour of plankton.
In Electroplankton, a videogame for the Nintendo DS handheld console, the ayer uses a plankton-themed interface to create music.
[edit] References
- Boyd, P.W., et al. (2000). "A mesoscale pytoplankton bloom in the polar Southern Ocean stimulated by iron rtilization". Nature 407: 695-702.
- Omori, M., T. Ikeda (1992). Methods in Marine Zooplankton Ecology. Malabar, USA: Krieger Publishing Company.
- Thurman, H. V. (1997). Introductory Oceanography. New Jersey, USA: Prentice Hall College.
[edit] See also
ca:Plàncton cs:Plankton da:Plankton de:Plankton es:Plancton eo:Planktono fr:Plancton gd:Meanbh-bheò hr:Plankton ia:Plancton it:Plancton he:פלנקטון hu:Plankton nl:Plankton ja:プランクトン no:Plankton pl:Plankton pt:Plâncton ru:Планктон simple:Plankton fi:Plankton sv:Plankton vi:Sinh vật phù du tr:Plankton uk:Планктон zh:浮游生物界
