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plankton:see marine biologymarine biology,
study of ocean plants and animals and their ecological relationships. Marine organisms may be classified (according to their mode of life) as nektonic, planktonic, or benthic. Nektonic animals are those that swim and migrate freely, e.g.
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the aggregate of organisms that inhabit continental and marine bodies of water and that drift under the influence of currents. Plankton consists of both plants—phytoplankton (including bacterioplankton)—and animals—zooplankton. It is distinguished from organisms found on the bottom, or benthos, and from free-swimming organisms, or nekton. In contrast to the latter, planktonic organisms are incapable of independent movement or have limited mobility. Freshwater plankton includes limnoplankton (lake plankton) and potamoplankton (river plankton).
Because photosynthesizing planktonic plants need sunlight, they inhabit the upper layers of the water, principally to depths of 50 to 100 m. Bacteria and zooplankton populate all depths, even the deepest layers of water. Marine phytoplankton consists principally of diatoms, peridinians, and coccolithophores; in fresh waters, diatoms, blue-green algae, and some groups of green algae predominate. The most numerous freshwater zooplankton are copepods, cladocerans, and rotifers; in the seas, crustaceans (mainly copepods, mysids, euphausiids, shrimps) predominate. Protozoans are numerous (radiolarians, foraminiferans, infusori-ans, tintinnids), as are coelenterates (medusae, siphonophores, ctenophores), pteropod mollusks, tunicates (appendicularians, salpae, Doliolum, pyrosomes), fish roe and larvae, and the larvae of various invertebrates (including many benthic ones). Species composition is most diverse in tropical ocean waters.
Planktonic organisms range in size from a few microns to several meters. For this reason one usually distinguishes nanno-plankton (bacteria and the smallest unicellular algae); micro-plankton (protozoans, rotifers, most algae, and many larvae); mesoplankton (copepods, cladicerans, and other animals smaller than 1 cm); macroplankton (many mysids, shrimps, medusae, and other comparatively large animals); and megaloplankton (a few of the largest organisms). Megaloplankton includes the ctenophore Venus’s-girdle (up to 1.5 m long), the medusa Cyanea (diameter up to 2 m and tentacles up to 30 m long), and colonies of pyrosomes (up to 30 m long and more than 1 m in diameter). However, the boundaries of these groups are not generally accepted.
Many planktonic organisms have developed adaptations that facilitate soaring in the water. Some adaptations decrease the specific gravity of the body (gas and fat inclusions, saturation of tissues with water, gelatinous nature of tissues, delicacy and porosity of the skeleton) and increase its specific surface (complex and often multibranched processes, flattened body).
Planktonic plants are the principal producers of organic matter in the water. Most aquatic animals feed on such matter. In shallow littoral areas, organic matter is also produced by benthic plants, or phytobenthos. The abundance of phytoplankton in various parts of bodies of water depends on the quantity of nutrient matter available in surface layers. In this respect, the limiting substances are phosphates, nitrogen compounds, and, for some organisms (diatoms and silicoflagellates), silicon compounds.
During the ocean’s long history, these substances accumulated in large quantities in the deeper layers, mainly as a result of decomposition and mineralization of organic particles from the upper layers. Therefore, abundant development of phytoplankton occurs in regions where there is a rise of waters from the bottom, for example, in the region were the warm Gulf Stream meets cold northern currents, in the zone of the equatorial divergence of waters, and in regions where winds are driven together near coasts. Inasmuch as phytoplankton is food for small planktonic animals, which serve as food for larger ones, regions of greatest phytoplanktonic development are characterized also by the abundance of zooplankton and nekton. River inflow has a substantially lesser and only local significance in the enrichment of surface waters with nutrient matter.
The development of phytoplankton also depends on the intensity of illumination, which, in cold and temperate waters, causes seasonal fluctuations in the development of plankton. In the winter, despite the abundance of nutrient matter carried to surface layers as a result of winter agitation of the water, there is little phytoplankton, owing to lack of light. In the spring rapid development of phytoplankton begins, followed by development of zooplankton. As planktonic plants use up nutrient matter and are themselves eaten by animals, they diminish in number. In the tropics the composition and quantity of plankton remain more or less constant year-round. Abundant development of phytoplankton leads to water bloom, which changes the color of the water and decreases its transparency. When certain peridinians flower, toxic substances are secreted into the water, which may cause mass destruction of planktonic and nektonic animals.
The biomass of plankton varies in different bodies of water and different regions. It also varies according to season. In the surface layer of the ocean the biomass of phytoplankton usually varies from several milligrams to several grams per cu m; the biomass of zooplankton (mesoplankton) varies from several dozen milligrams to 1 gram per cu m or more. As depth increases, plankton becomes less diverse and its quantity rapidly decreases. Most areas of the World Ocean are characterized by a relatively small plankton community. The central tropical regions on both sides of the equator have the least abundant plankton; the richest plankton communities are found in coastal regions of temperate and subtropical latitudes. The annual production of phytoplankton in the World Ocean has been estimated at 550 billion tons by the Soviet oceanographer V. G. Bogorov. This exceeds by almost ten times the total production of the entire marine animal population.
Many planktonic organisms make regular vertical migrations of hundreds of meters (sometimes greater than 1 km), which promote the transfer of food resources from rich surface layers to the depths. Thus, food is provided for deep-water plankton. As a result of the ability to migrate, the vertical zonality of plankton is less clearly expressed than that of benthos (see MARINE FAUNA). Many planktonic organisms are marked by bio-luminescence. Some serve to indicate the degree of contamination of a body of water, since they are sensitive in varying degrees to contamination.
Plankton serves, either directly or through intermediate links in the food chain, as a source of food for many commercial animals, including squid, fish, and whales. A number of planktonic crustaceans, such as shrimps and mysids, are commercially valuable. In recent years, such antarctic crustaceans as euphau-sids (krill), which sometimes form immense aggregates (up to 15 kg/m3), have become commercially valuable. The collection and use of marine plankton look promising, since its reserves exceed by many times the reserves of all marine organisms thus far considered commercially valuable.
REFERENCESZenkevich, L. A. Fauna i biologicheskaia produktivnost’ moria, vols. 1–2. Moscow, 1947–51.
Zhizn’ presnykh vod SSSR, vols. 1–3. Moscow-Leningrad, 1940–50.
Bogorov, V. G. “Produktivnost’ okeana.” In Osnovnye problemy okeanologii. Moscow, 1968.
Biologiia Tikhogo okeana: Plankton. Moscow, 1967. (Tikhii okean, vol. 7, book 1.)
Vinogradov, M. E. Vertikal’noe raspredelenie okeanicheskogo zooplank-tona. Moscow, 1968.
Beklemishev, K. V. Ekologiia i biogeografiia pelagiali. Moscow, 1969.
Kiselev, I. A. Plankton morei i kontinental’nykh vodoemov, vol. 1 Leningrad, 1969.
G. M. BELIAEV