As one of components of a hydrological cycle, or water cycle in the nature, the river is extremely important and considerable. As the special ecological environment it is the house for a set of living organisms.
Plankton
The bottom, surface and coast of the rivers became the favorable habitat for great variety of organisms among which not only fish. Actually, the river – a peculiar small world for all its inhabitants, and in it teems life. Inhabitants of this reservoir can be divided into three big groups: plankton, benthos and nekton. Life of each of representatives of these groups depends on others, and without any link this chain would become torn.
Plankton, for example, is that trophic level (a link in a food chain) which is food for other inhabitants of the rivers. Thereby plankton is considered a basis of river life.
The name "plankton" from Greek means "wandering, wandering". Usually plankton is in the top water layers and is presented, in particular, by phytoplankton and zooplankton. Various seaweed belong to phytoplankton: green, blue-green, diatomic, protokokkovy. Also among phytoplankton there are cyanobacteria. Phytoplankton carries such name because all components of this group have ability to carry out photosynthesis. Phytoplankton, unlike zooplankton, is a producer, that is the producer of primary products which other food links including zooplankton eat. "Water blossoming", so habitual to an eye of the person, happens because of rough reproduction and growth of phytoplankton. Zooplankton, in turn, is presented by already animal organisms, but, unlike a nekton and a benthos, they cannot resist a current and float there where will want. Therefore they are forced to be transferred together with water masses on a current. Numerous small Crustacea, larvae of animals, fish roe, kolovratka treat zooplankton. Zooplankton – such part of a food chain which connects among themselves phytoplankton and larger representatives of the rivers: nektonny and bentosny.
Benthos
The benthos mainly lives in soil of the river or on its surface, that is, at the bottom. In translation from Greek "benthos" is meant by "depth". A benthos as well as plankton, is subdivided into a zoobenthos and a phytobenthos. Representatives of a benthos can have various sizes: they can be small, average or large. Larvae of insects, various hearts, crawfishes, mollusks and many others belong to a benthos. All of them are livelihood for the majority of fishes and other river inhabitants, and some are even eaten by the person.
Nekton
Nekton – group of river inhabitants, the closest and familiar to the person. The majority of species of fish (about 20,000), some large invertebrates, mammals and reptiles concern to it. Nekton perfectly is able to resist a current of water and actively moves in river waters to considerable distances. Nekton from Greek – "floating". Here only some species of fish living in the river environment: catfish, carp, pike perch, perch, pike, crucian, ruff, small fry, rudd, sterlet. As well as in other habitats, in waters of the river of fish breathe gills.