Plankton consist of any drifting organisms (animals, plants, archaea, or bacteria) that inhabit the pelagic zone of oceans, seas, or bodies of fresh water.
Phytoplankton are the autotrophic component of the plankton community.
An autotroph is an organism that produces complex organic compounds from simple inorganic molecules using energy from light (by photosynthesis) or inorganic chemical reactions.
Plankton are drifting organisms that live in the surface layers of the ocean. They are not strong enough to swim against ocean currents. The term is in contrast to nekton, who can control their movements. There are three groups:
- Phytoplankton: algae that live at the surface of the ocean to make food from sunlight.
- Eukaryote algae: diatoms, coccolithophores. Some dinoflagellates.
- Bacteria: cyanobacteria.
- Zooplankton: small protozoans or metazoans (e.g. crustaceans and other animals) that feed on other plankton. Ctenophores; jellyfish; rotifers; foraminifera. Some eggs and larvae of larger animals such as fish, crustaceans, and annelids.
- Some groups fall into both categories. Dinoflagellates are either photosynthetic producers or heterotrophic consumers, but many species are mixotrophic depending upon their circumstances. It is also hard to fit viruses into this scheme; yet they are present in great numbers.
Plankton are important in the ocean's food chain. They are the main source of food for almost all fish larvae as they switch from their yolk sacs to catching prey. Basking sharks and blue whales feed on them directly; other large fish feed on them indirectly, by eating fish of smaller size, such as herrings.
The distribution of plankton is governed more by nutrients than by temperature. Large tracts of ocean are blue and sterile. The reason is that these areas lack one or more crucial nutrients for the photosynthetic plankton, upon whom all the others depend. Broadly speaking, areas near land masses get nutrients by rivers and wind. The key nutrient lacking in the Pacific ocean is iron, essential in molecules such as ferredoxins, iron-sulfur proteins which mediate electron transfer in a range of metabolic reactions.
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Plankton consist of any drifting organisms (animals, plants, archaea, or bacteria) that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their phylogenetic or taxonomic classification. They provide a crucial source of food to more familiar aquatic organisms such as fish.
Though many planktonic species are microscopic in size, the plankton includes organisms covering a wide range of sizes, including large organisms such as jellyfish.
The name plankton is derived from the Greek adjective planktos, meaning "errant", and by extension "wanderer" or "drifter". By definition, organisms classified as plankton are unable to resist ocean currents. While some forms are capable of independent movement and can swim hundreds of meters vertically in a single day (a behavior called diel vertical migration), their horizontal position is primarily determined by the surrounding currents. This is in contrast to nekton organisms that can swim against the ambient flow and control their position (e.g. squid, fish, and marine mammals).
Within the plankton, holoplankton spend their entire life cycle as plankton (e.g. most algae, copepods, salps, and some jellyfish). By contrast, meroplankton are only planktonic for part of their lives (usually the larval stage), and then graduate to either the nekton or a benthic (sea floor) existence. Examples of meroplankton include the larvae of sea urchins, starfish, 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 and individual plankton are referred to as plankters.
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, and annelids, are included here.
- 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).
Distribution: Plankton inhabit oceans, seas and lakes. Local abundance varies horizontally, vertically and seasonally. The primary cause of this variability is the availability of light. All plankton ecosystems are driven by the input of solar energy (but see chemosynthesis), confining primary production to surface waters, and to geographical regions and seasons having abundant light.
A secondary variable is nutrient availability. Although large areas of the tropical and sub-tropical oceans have abundant light, they experience relatively low primary production because they offer limited nutrients such as nitrate, phosphate and silicate. This results from large-scale ocean circulation and water column stratification. In such regions, primary production usually occurs at greater depth, although at a reduced level (because of reduced light).
Significance: Aside from representing the bottom few levels of a food chain that supports commercially important fisheries, plankton ecosystems play a role in the biogeochemical cycles of many important chemical elements, including 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 organic material is typically more dense than seawater it tends to sink, and in open ocean ecosystems away from the coasts this transports 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 carbon sink on Earth.
Importance to fish: 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.
The paradox of the plankton is the name given to the situation where a limited range of resources (light, nutrients) supports a much wider range of planktonic organisms. The paradox stems from a result of the competitive exclusion principle (sometimes referred to as Gause's Law), which suggests that when two species compete for the same resource, ultimately only one will persist and the other will be driven to extinction. The high diversity of phytoplankton at all phylogenetic levels stands in contrast to the limited range of resources for which they compete with one another (e.g. nitrate, phosphate, silicic acid, iron).
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words "phyton" = "plant", and "planktos" = "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye. However, when present in high enough numbers, they may appear as a green discoloration of the water due to the presence of chlorophyll within their cells (although the actual color may vary with the species of phytoplankton present due to varying levels of chlorophyll or the presence of accessory pigments such as phycobiliproteins, xanthophylls, etc.).
Ecology: Phytoplankton obtain energy through the process of photosynthesis and must therefore live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, lake, or other body of water. Phytoplankton account for half of all photosynthetic activity on Earth. Thus phytoplankton are responsible for much of the oxygen present in the Earth's atmosphere – half of the total amount produced by all plant life. Their cumulative energy fixation in carbon compounds (primary production) is the basis for the vast majority of oceanic and also many freshwater food webs (chemosynthesis is a notable exception). As a side note, one of the more remarkable food chains in the ocean – remarkable because of the small number of links – is that of phytoplankton fed on by krill (a type of shrimp) fed on by baleen whales.
Phytoplankton are a key food item in both aquaculture and mariculture. Both utilize phytoplankton for the feeding of the animals being farmed. In mariculture, the phytoplankton is naturally occurring and is introduced into enclosures with the normal circulation of seawater. In aquaculture, phytoplankton must be obtained and introduced directly. The plankton can either be collected from a body of water or cultured, though the former method is seldom used. Phytoplankton is used as a foodstock for the production of rotifers, which are in turn used to feed other organisms. Phytoplankton is also used to feed many varieties of aquacultured molluscs, including pearl oysters and giant clams.
The production of phytoplankton under artificial conditions is itself a form of aquaculture. Phytoplankton is cultured for a variety of purposes, including foodstock for other aquacultured organisms, a nutritional supplement for captive invertebrates in aquaria, and as a source of bio-diesel. Culture sizes range from small-scale laboratory cultures of less than 1L to several tens of thousands of liters for commercial aquaculture. Regardless of the size of the culture, certain conditions must be provided for efficient growth of plankton. The majority of cultured plankton is marine, and seawater of a specific gravity of 1.010 to 1.026 may be used as a culture medium. This water must be sterilized, usually by either high temperatures in an autoclave or by exposure to ultraviolet radiation, to prevent biological contamination of the culture. Various fertilizers are added to the culture medium to facilitate the growth of plankton. A culture must be aerated or agitated in some way to keep plankton suspended, as well as to provide dissolved carbon dioxide for photosynthesis. In addition to constant aeration, most cultures are manually mixed or stirred on a regular basis. Light must be provided for the growth of phytoplankton. The colour temperature of illumination should be approximately 6,500 K, but values from 4,000 K to upwards of 20,000 K have been used successfully. The duration of light exposure should be approximately 16 hours daily; this is the most efficient artificial day length.
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