The river continuum concept identifies biological connections between the watershed, floodplain, and perennial stream systems, and offers an explanation of how biological communities develop and change along perennial stream or river corridors. The concept is only a hypothesis, yet it has served as a useful conceptual model for describing some of the important features of perennial streams and rivers.
The river continuum concept assumes that, because of forest shading in many first- to third-order headwater streams, the growth of algae, periphyton, and other aquatic plants is limited. Since energy cannot be created through photosynthesis (autotrophic production), aquatic organisms in these lower-order streams depend on materials coming from outside the channel such as leaves and twigs. Consequently, these headwater streams are considered 'heterotrophic' (i.e., dependent on the energy produced in the surrounding watershed). The relatively constant temperature regimes of these streams tend to limit biological species diversity.
Proceeding downstream to fourth-, fifth-, and sixth-order streams, the channel widens, which increases the amount of incident sunlight and average temperatures. Primary production increases as a response. This shifts many stream organisms to internal autotrophic production and a dependence on materials coming from inside the channel. Species richness of the invertebrate community increases due to the increase in the variety of habitats and food resources. Invertebrate functional groups, such as the grazers and collectors, increase as they adapt to both out-of-channel and in-channel sources of food.
Mid-ordered streams also experience increasing temperature fluctuations. This tends to further increase biotic diversity. Larger streams and rivers of seventh to twelfth order tend to increase in physical stability, but undergo significant changes in structure and biological function. Larger streams develop increased reliance on primary productivity by phytoplankton, but continue to receive heavy inputs of dissolved and fine organic particles from upstream.
Large streams frequently carry increased loads of clay and fine silt. These materials increase turbidity, decrease light penetration, and thus increase the significance of heterotrophic processes. The frequency and magnitude of temperature changes decrease as flows increase, and this in turn increases the overall physical stability of the stream as well as species competition and predation.
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