Plankton

PLANKTON, a term originally introduced by Hensen (Hensen, 1887) to denote the living population of the seas. At present, plankton is called the aggregate of organisms inhabiting the water of any water body and conducting the entire biological cycle (euryplankton) or at least a part of it, for example the larval stage (meroplankton), without necessary connection with the substrate. Plankton passively moves in the water, carried by currents, since the intensity of active movement of the mobile forms comprising Plankton is insufficient for it to become the main cause of movement, as is the case with strongly mobile forms distinguished into a special group of aquatic organisms-nekton (chiefly fish). Representatives of Plankton belonging to completely different systematic groups nevertheless often possess similar structural features, which are very characteristic adaptations for "floating" in the water, for example long, radially divergent bristles on many algae and corresponding radiate forms of the skeleton of radiolarians, drops of fat in the protoplasm of planktonic protozoa and in the body of crustaceans. Plankton of any water body is a very characteristic complex of organisms, n 32S S24 sharply and characteristically differing in water bodies of different types. The qualitative and quantitative development of Plankton in each given water body depends on the conditions of the water body and first of all on the composition of mineral substances dissolved in the water serving as food material for the plant Plankton (phytoplankton), on which the development of the animal Plankton (zooplankton) proceeds. Plankton in turn, in the majority of water bodies, is the main source of accumulation of organic substance serving for the nutrition of the bottom fauna and fish and finally depositing on the bottom of the water body in the form of powerful layers of silt. The bottom fauna, determining the productivity of the water body in the fishery respect, is in close connection with the amount of Plankton formed in it. "Haliplankton" (marine Plankton) sharply differs in general character and species composition from freshwater (limnoplankton). It is incomparably richer than freshwater and includes in its composition besides diverse algae (diatoms, peridineae, green and blue-green algae) and protozoa (rhizopods, radiolarians, infusoria and flagellates) representatives of almost all main groups of the animal world. By size, marine plankton is divided into macroplankton - the largest representatives of Plankton (for example jellyfish), meso- and microplankton - smaller organisms entirely retained by the special silk net used for collecting Plankton (see Biological analysis) (crustaceans, large algae and protozoa belong here), and finally nanoplankton and ultraplankton - the smallest representatives of Plankton, freely passing through the holes of the cells of the plankton net (for example small algae, flagellates and bacteria - see Bacterioplankton). Marine Plankton of the coastal area (neritic) is characteristically distinguished by the presence of meroplanktonic forms, for example larvae of bottom animals, sea urchins and others, merogonic jellyfish, etc. The greatest amount of plankton is contained in the surface layers of oceanic water (up to 100 m), in which photosynthesis is possible, i.e. development of phytoplankton. Below the amount of Plankton decreases rapidly, however zooplanktons are present in insignificant quantity and at great depths. Besides the significance of Plankton as the primary source of formation of organic substance, its significance in geochemical processes of sedimentation of calcareous and siliceous bottom deposits, formed to a considerable extent and sometimes entirely at the expense of the skeletons of dying planktonic organisms, is very great. Limnoplankton is incomparably poorer in qualitative composition than marine. Among animal organisms, in freshwater Plankton representatives of only three groups - protozoa, rotifers and crustaceans - occur in significant quantity. Macroplankton is completely absent. Due to different conditions in rivers, lakes and ponds, Plankton of these water bodies is distinguished in a characteristic way. In the most typical form, Plankton is presented in lakes, where it reaches the greatest qualitative diversity. River Plankton (potamoplankton) is poor and includes a significant number of transported and meroplanktonic forms. Pond plankton (geloplancton) is characterized by abundant development of algae (water blooming), predominance of rotifers in summer and absence of Plankton in winter. The composition of limnoplankton during the year, both qualitatively and quantitatively, is subject to very strong and regular changes. Plankton of water bodies of the temperate zone reaches greatest development in summer in the period of greatest heating of the water bodies, when thermophilic blue-green and green algae develop. The spring and autumn periods are characterized by predominance of diatoms. In winter the amount of zooplankton decreases to a lesser extent than the amount of phytoplankton. Seasonal fluctuations, as well as the composition and amount of Plankton and the character of its distribution over the water body, stand in close dependence on the physical-chemical properties of water, determined in turn by the conditions of occurrence of the water body and the geological structure of the terrain. At present, three main types of freshwater water bodies are distinguished, each of which has a characteristic complex of planktonic forms. 1) Eutrophic water bodies - Water is rich in dissolved salts, in particular nitrogen and phosphorus. Plankton is characterized by great development of blue-green and green algae (water blooming), sharp distribution of Plankton by depths and sharp seasonal fluctuations in the composition of Plankton and intensive silt formation. 2) Oligotrophic water bodies - water is poor in dissolved salts. Plankton is poor, no blooming. 3) Distrophic (lakes on peat bogs) - are characterized by the presence in water of a large amount of humus substances giving it a yellow color and relative predominance of zooplankton. The amount of Plankton varies. Natural pollution of the water body during swamping and especially artificial pollution by man sharply changes the composition of Plankton. Therefore, the study of Plankton is necessary in the assessment of the sanitary state of the water body and in the study of the process of self-purification of water (see Biological analysis).