Difference between revisions of "Plankton" - New World Encyclopedia
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[[Image:plankton.jpg|thumb|right|250px|Photomontage of plankton organisms]] | [[Image:plankton.jpg|thumb|right|250px|Photomontage of plankton organisms]] | ||
| − | '''Plankton''' is the collective name for certain organisms (mostly microscopic) which drift in the water of the oceans. (Plankton are also found in lakes, rivers, and other bodies of water but this article | + | '''Plankton''' is the collective name for certain organisms (mostly microscopic) which drift in the water of the oceans. (Plankton are also found in lakes, rivers, and other bodies of water but this article discusses ocean plankton.) Plankton have a tremendous importance in the web of life on earth. |
==Definitions== | ==Definitions== | ||
| − | [[Image: | + | [[Image:copepodkils.jpg|thumb|right|200px|A copepod]] |
The name '''plankton''' is derived from the Greek word ''planktos'', meaning "wanderer" or "drifter". While some forms of plankton can swim, they can not do so strongly enough to avoid being carried along by the water current. This is in contrast to '''nekton''' organisms that can swim against the current and control their position (for instance [[fish]], [[squid]], and marine [[mammal]]s). It is also in contrast to '''benthos''' organisms which live on the bottom (for instance coral, clams, and lobsters). | The name '''plankton''' is derived from the Greek word ''planktos'', meaning "wanderer" or "drifter". While some forms of plankton can swim, they can not do so strongly enough to avoid being carried along by the water current. This is in contrast to '''nekton''' organisms that can swim against the current and control their position (for instance [[fish]], [[squid]], and marine [[mammal]]s). It is also in contrast to '''benthos''' organisms which live on the bottom (for instance coral, clams, and lobsters). | ||
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Within the plankton itself, '''holoplankton''' are those organisms that spend their entire life cycle as part of the plankton. By contrast, '''meroplankton''' are those organisms that are only planktonic for part of their lives, usually the larval stage. | Within the plankton itself, '''holoplankton''' are those organisms that spend their entire life cycle as part of the plankton. By contrast, '''meroplankton''' are those organisms that are only planktonic for part of their lives, usually the larval stage. | ||
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==Functional groups== | ==Functional groups== | ||
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Plankton are primarily divided into broad functional (or [[trophic level]]) groups: | Plankton are primarily divided into broad functional (or [[trophic level]]) groups: | ||
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==Phytoplankton== | ==Phytoplankton== | ||
Phytoplankton, like plants, obtain energy through [[photosynthesis]], and so must live in the well-lit surface layer (the [[euphotic zone]]) of the ocean. They provide the energy for almost all life in the sea. | Phytoplankton, like plants, obtain energy through [[photosynthesis]], and so must live in the well-lit surface layer (the [[euphotic zone]]) of the ocean. They provide the energy for almost all life in the sea. | ||
| + | [[Image:Diatoms through the microscope.jpg|thumb|left|200px|Some marine [[diatom]]s - a key [[phytoplankton]] group]] | ||
===Cyanobacteria=== | ===Cyanobacteria=== | ||
| − | Cyanobacteria, also called "blue-green algae" are [[bacteria]]. They are thought to be the first organisms on earth which developed photosynthesis. They are one of the few organisms which can take nitrogen from the air and "fix" it into organic molecules so it can be used by plants and animals. | + | Cyanobacteria, also called "blue-green algae", are [[bacteria]]. They are thought to be the first organisms on earth which developed photosynthesis. They are one of the few organisms which can take nitrogen from the air and "fix" it into organic molecules so it can be used by plants and animals. |
===Diatoms=== | ===Diatoms=== | ||
Diatoms are [[protist]]s. They are single-celled algae with protective cell walls made of silica. In favorable conditions diatoms "bloom", reproduce rapidly so that they dominate an area. When conditions turn less favorable, for instance the coming of winter or a depletion of nutrients, they can turn domant and sink to lower levels until currents return them to the surface and a new bloom can take place. | Diatoms are [[protist]]s. They are single-celled algae with protective cell walls made of silica. In favorable conditions diatoms "bloom", reproduce rapidly so that they dominate an area. When conditions turn less favorable, for instance the coming of winter or a depletion of nutrients, they can turn domant and sink to lower levels until currents return them to the surface and a new bloom can take place. | ||
| + | [[Image:Coccolithophore bloom.jpg|right|thumb|200px|NASA satellite image of the large phytoplankton bloom in the Bering Sea in 1998.]] | ||
===Dinoflagellates=== | ===Dinoflagellates=== | ||
| − | Dinoflagellates are also protists. They differ from diatoms in having a whip-like tail which enables them to move in the water, mostly up and down in response to changing conditions. Dinoflagellates also bloom, which can create a "red tide" in which release toxins which can kill fish and or humans who eat shellfish in which the toxins have become concentrated. | + | Dinoflagellates are also protists. They differ from diatoms in having a whip-like tail which enables them to move in the water, mostly up and down in response to changing conditions. Dinoflagellates also bloom, which can create a "red tide" in which release toxins which can kill fish and or humans who eat shellfish in which the toxins have become concentrated. Dinoflagellates which do not use photosynthesis but eat other organisms are considered zooplankton. |
===Seaweeds=== | ===Seaweeds=== | ||
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==Zooplankton== | ==Zooplankton== | ||
| − | Zooplankton are animals or animal-like organisms | + | Zooplankton are animals or animal-like organisms. They mostly live in the surface waters since they feed on the phytoplankton and on other zooplankton. |
===Copepods=== | ===Copepods=== | ||
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Krill are also crustaceans. Related to shrimp they are larger than copepods, most being about 1-2 cm long. They are sometimes not considered true plankton since they can swim, although they are still carried along by the currents. Most species of krill come together in large "swarms" which contain billions of individuals. They are eaten by many other animals, including many species of [[whale]]s. There is some commercial fishing of krill, mostly to feed fish in aquariums and aquaculture but also for human consumption. | Krill are also crustaceans. Related to shrimp they are larger than copepods, most being about 1-2 cm long. They are sometimes not considered true plankton since they can swim, although they are still carried along by the currents. Most species of krill come together in large "swarms" which contain billions of individuals. They are eaten by many other animals, including many species of [[whale]]s. There is some commercial fishing of krill, mostly to feed fish in aquariums and aquaculture but also for human consumption. | ||
| − | == | + | ===Jellyfish=== |
| − | + | Jellyfish are among the largest planktonic animals. Most feed on zooplankton and small fish. Some jellyfish can grow to as large as 7 feet in diameter, with the tentacles much longer. It is suspected that the world's jellyfish population is increasing as their competitors the large fishes are removed by human fishing. | |
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| + | ===Larva=== | ||
| + | The [[larva]] of fish and other animals such as coral and marine worms are also included in the zooplankton. The larva of the common eel, ''Anguilla anguilla'', looks so different from the adult that is was thought to be a seperate species until the 1890s. | ||
| + | ==Bacterioplankton== | ||
| + | Bacterioplankton are [[bacteria]] which mostly live by decomposing the remains of other organisms, although some are parasites. Cyanobacteria are also bacteria but they are considered to be among the phytoplankton since they use photosynthesis. Bacterioplankton are found at all levels of the ocean, not just near the surface as are most other plankton. They serve an important purpose of breaking down the remains of other organisms so that their nutrients can be reused, and are also eaten by zooplankton. Some bacterioplankton live near volcanic vents on the ocean bottom where they feed off of chemicals the vent releases into the water. They support a community of other living things which is remarkable among the earth's [[ecosystem]]s for not depending on sunlight for energy. | ||
==Size groups== | ==Size groups== | ||
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[[Image:Ceratium hirundinella.jpg|thumb|right|200px|''Ceratium'', a [[dinoflagellate]] phytoplankter]] | [[Image:Ceratium hirundinella.jpg|thumb|right|200px|''Ceratium'', a [[dinoflagellate]] phytoplankter]] | ||
| − | 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. | + | 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. Almost all plankton ecosystems are driven by the input of solar energy 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 [[tropics|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 current|ocean circulation]] and | + | A secondary source of variability is that of nutrient availability. Although large areas of the [[tropics|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 current|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{{citation needed}} that the mineral iron (but only in the proper amounts) leads to increased blooms of many (though not all) kinds of phytoplankton. 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). | + | Studies have shown{{citation needed}} that the mineral iron (but only in the proper amounts) leads to increased blooms of many (though not all) kinds of phytoplankton. 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). |
| − | 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 | + | 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. |
| − | == | + | ==Importance of Plankton== |
| + | [[Image:Copepodkils.jpg|thumb|right|200px|A [[copepod]] (''Calanoida'' sp.) ca. [[1_E-3_m|1-2 mm]] long]] | ||
| + | Plankton form the basis of almost all life in the sea. They convert the sun's energy into chemical energy which is passed on up various [[food chain]]s to support the great variety of ocean animals, including the fish and shellfish that are harvested by humans. | ||
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Aside from representing the bottom few levels of a [[food chain]] that leads up to [[Commerce|commercially]] important [[Fishery|fisheries]], plankton [[ecosystem]]s play a role in the [[biogeochemical cycle]]s of many important [[Chemical element|elements]]. Of particular contemporary significance is their role in the ocean's [[carbon cycle]]. | Aside from representing the bottom few levels of a [[food chain]] that leads up to [[Commerce|commercially]] important [[Fishery|fisheries]], plankton [[ecosystem]]s play a role in the [[biogeochemical cycle]]s of many important [[Chemical element|elements]]. Of particular contemporary significance is their role in the ocean's [[carbon cycle]]. | ||
Revision as of 20:52, 15 June 2006
Plankton is the collective name for certain organisms (mostly microscopic) which drift in the water of the oceans. (Plankton are also found in lakes, rivers, and other bodies of water but this article discusses ocean plankton.) Plankton have a tremendous importance in the web of life on earth.
Definitions
The name plankton is derived from the Greek word planktos, meaning "wanderer" or "drifter". While some forms of plankton can swim, they can not do so strongly enough to avoid being carried along by the water current. This is in contrast to nekton organisms that can swim against the current and control their position (for instance fish, squid, and marine mammals). It is also in contrast to benthos organisms which live on the bottom (for instance coral, clams, and lobsters).
Within the plankton itself, holoplankton are those organisms that spend their entire life cycle as part of the plankton. By contrast, meroplankton are those organisms that are only planktonic for part of their lives, usually the larval stage.
The study of plankton is termed planktology. Individual plankton are referred to as plankters.
Functional groups
Plankton are primarily divided into broad functional (or trophic level) groups:
- Phytoplankton (from Greek phyton, or plant), use photosynthesis to convert sunlight into chemical energy for life.
- Zooplankton (from Greek zoon, or animal), feed on the phytoplankton or other zooplankton.
- Bacterioplankton, mainly decompose the remains of other organisms.
This scheme divides the plankton community into broad producer, consumer and recycler groups. In reality, even the trophic level of some plankton is not straightforward. For example, although most dinoflagellates are either photosynthetic producers or heterotrophic consumers, many species do both depending upon their circumstances.
Phytoplankton
Phytoplankton, like plants, obtain energy through photosynthesis, and so must live in the well-lit surface layer (the euphotic zone) of the ocean. They provide the energy for almost all life in the sea.
Cyanobacteria
Cyanobacteria, also called "blue-green algae", are bacteria. They are thought to be the first organisms on earth which developed photosynthesis. They are one of the few organisms which can take nitrogen from the air and "fix" it into organic molecules so it can be used by plants and animals.
Diatoms
Diatoms are protists. They are single-celled algae with protective cell walls made of silica. In favorable conditions diatoms "bloom", reproduce rapidly so that they dominate an area. When conditions turn less favorable, for instance the coming of winter or a depletion of nutrients, they can turn domant and sink to lower levels until currents return them to the surface and a new bloom can take place.
Dinoflagellates
Dinoflagellates are also protists. They differ from diatoms in having a whip-like tail which enables them to move in the water, mostly up and down in response to changing conditions. Dinoflagellates also bloom, which can create a "red tide" in which release toxins which can kill fish and or humans who eat shellfish in which the toxins have become concentrated. Dinoflagellates which do not use photosynthesis but eat other organisms are considered zooplankton.
Seaweeds
Although the word "plankton" is most often used for microscopic, or at least very small, organisms; seaweeds, which are multi-cellular algae, can also be considered planktonic if they drift in the open ocean rather than are anchored to the bottom. Sargassum weed forms huge floating islands in the western Atlantic Ocean which provide a home for many unique animals.
Zooplankton
Zooplankton are animals or animal-like organisms. They mostly live in the surface waters since they feed on the phytoplankton and on other zooplankton.
Copepods
Copepods are small crustaceans, most about 1-2 mm long with some larger. They are unusual among animals in only having one eye. Most species feed on the single-celled phytoplankton and are in turn eaten by fish and other larger animals.
Krill
Krill are also crustaceans. Related to shrimp they are larger than copepods, most being about 1-2 cm long. They are sometimes not considered true plankton since they can swim, although they are still carried along by the currents. Most species of krill come together in large "swarms" which contain billions of individuals. They are eaten by many other animals, including many species of whales. There is some commercial fishing of krill, mostly to feed fish in aquariums and aquaculture but also for human consumption.
Jellyfish
Jellyfish are among the largest planktonic animals. Most feed on zooplankton and small fish. Some jellyfish can grow to as large as 7 feet in diameter, with the tentacles much longer. It is suspected that the world's jellyfish population is increasing as their competitors the large fishes are removed by human fishing.
Larva
The larva of fish and other animals such as coral and marine worms are also included in the zooplankton. The larva of the common eel, Anguilla anguilla, looks so different from the adult that is was thought to be a seperate species until the 1890s.
Bacterioplankton
Bacterioplankton are bacteria which mostly live by decomposing the remains of other organisms, although some are parasites. Cyanobacteria are also bacteria but they are considered to be among the phytoplankton since they use photosynthesis. Bacterioplankton are found at all levels of the ocean, not just near the surface as are most other plankton. They serve an important purpose of breaking down the remains of other organisms so that their nutrients can be reused, and are also eaten by zooplankton. Some bacterioplankton live near volcanic vents on the ocean bottom where they feed off of chemicals the vent releases into the water. They support a community of other living things which is remarkable among the earth's ecosystems for not depending on sunlight for energy.
Size groups
Plankton are also often described in terms of size. Usually the following divisions are used:
- Megaplankton, 2×10-1→2×100 m (20-200 cm)
- Macroplankton, 2×10-2→2×10-1 m (2-20 cm)
- Mesoplankton, 2×10-4→2×10-2 m (0.2 mm-2 cm)
- Microplankton, 2×10-5→2×10-4 m (20-200 µm)
- Nanoplankton, 2×10-6→2×10-5 m (2-20 µm)
- Picoplankton, 2×10-7→2×10-6 m (0.2-2 µm), mostly bacteria
- Femtoplankton, < 2×10-7 m, (< 0.2 µm), consisting of 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.
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. Almost all plankton ecosystems are driven by the input of solar energy 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[citation needed] that the mineral iron (but only in the proper amounts) leads to increased blooms of many (though not all) kinds of phytoplankton. 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).
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.
Importance of Plankton
Plankton form the basis of almost all life in the sea. They convert the sun's energy into chemical energy which is passed on up various food chains to support the great variety of ocean animals, including the fish and shellfish that are harvested by humans.
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).[citation needed]
ReferencesISBN links support NWE through referral fees
- Omori, M. and Ikeda, T. (1992). Methods in Marine Zooplankton Ecology, Krieger Publishing Company, Malabar, USA.
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