River Continuum Concept
The River (or stream) Continuum Concept (Vannote and others 1980) relates changes in stream and river communities and processes to the downstream gradient of abiotic factors like stream width, shading by trees, stream gradient, and water velocity. The stream does more than just transport materials; it also processes materials as the biota take up, use, convert, and release matter.
Among the many changes that occur along the river continuum, the relative importance of autochthonous and allochthonous energy sources is particularly significant for streams in the southern Appalachians. Along the river continuum, two changes occur in the ratio of autotrophic (plant) production within the stream to heterotrophic (plant and animal) respiration (P/R). The ratio is <1 in headwater streams where production in the stream is less than the total needed for respiration of plants and animals (the stream is subsidized by input of material from the adjacent terrestrial ecosystem). The ratio switches to >1 where instream production is greater than the total needed for respiration (the excess energy is transported downstream). Eventually, the ratio switches back to <1 in large rivers. Another significant change is the relative importance of various functional feeding groups of macroinvertebrates (pie charts in the figure). These changes are loosely related to stream order and stream width (left axis of this figure).
Encyclopedia ID: p1495
