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~' , \;>'1 " , .J e 67 synthesis of the more recent literature, An attempt h,as been made to compare and contrast traditional ecological principles with recently evolved concepts such as the functional group concept, the River Continuum Concept, the nutrient spiraling concept, streambed retention concept, etc. The Ecosystem C01!!:,!.l2! Until recently, reluctance was express.!d to applying the ecosystem concept to streams (Rzoska 1978). This reluctance was partly due to the difficulty of defining the term ecosystem as it may apply to streams (Mann 1979) and partly due to the way streams hav,a or have n'Dt been visualized. Whereas most early stream studies concentrated on isolated and discrete stream reaches, recent studies have considered streams as longitudinally linked systems within drainage basins. In 'this way, streams can be visualized as continuums of changing environmental factors that extend from beginning headwaters to the mouth or estuary. The stream order system is proving to be a simplistic structural frame'Hork upon which and to which ecological concepts can be related to relative stream sizes. Thus traditional ecosystem and community concepts are discussed before describing and discussing their relationship to streams. ~ A stream ecosystem may be characterized and defined as a linear segment of a drainage system that includes all of the living organisms (biotic component) interacting with the physical environment (abiotic component) so that a flow of energy leads to a clearly defined trophic (feeding) structure, biotic diversity and material cycling (Odum 1971). Trophically, stream ecosystems (like other systems) ha~e two components separated in space and time. Plant life is characteristic of the autotrophic (self-nourishing) component that, utilizes simple inorganic substances such as carbon dioxide and water in the process of photosynthesis to produce organic matter (protein, fat, carbohydrate, nucleic acid, etc.). Animal life is character.istic of the heterotrophic (other-nourishing) component that utilizes (i.e., consumes), deco~poses (i.e., digests), and rearranges (i.e., synthesizes organic compounds) the products produced by the autotrophic componemt. ~ ,. , .' > ~ All stream ecosystems are "open" systems because their continued functioning depends upon a sustained input of energy across their boundaries. Energy may enter ecosystems as solar radiation or as organic matter largely produced by photosynthesis. ~len the organic matter input arises by photosynthesis from within a stre~un ecosystem it is termed autochthonous input. When the organic matter arises by photosynthesis from outside the strealll ecosystem, but becomes an 'input to the system,' it is termed allochthonous input. Autochthonous and allochthonous stream inputs vary widely in total amounts and compositioll. " , Structurally, stream ecosystems (like other ecosystems) include three major abiotic (nonliving) and three major bIotic (living) components. The abiotic components include (1) inorganic substances s",ch as carbon, hydrogen, nitrogen, oxygen, (2) organic compounds such as fats, proteins, '\ \..<,.