Sources of Energy for Streams

Streams and rivers receive energy indirectly from the sun through photosynthesis in the stream or in nearby terrestrial ecosystems. Rates of primary production (photosynthesis) in southern Appalachian streams and rivers vary with stream size, season, amount of shading, and nutrient availability. In forested areas, rates may range from 1.3 to 386 g C/m²/year (Wallace and others 1992).

When energy (and carbon) originates from outside the stream, it is called allochthonous, and when energy (and carbon) comes from photosynthesis within the stream, it is called autochthonous.  Most headwater streams in the southern Appalachians are allochthonous, and some larger streams and rivers are autochthonous. Many streams depend on both allochthonous and autochthonous energy.

In small, low-order streams (1-3), streamside vegetation dominates stream processes by shading the narrow channel and contributing organic matter in the form of leaves and wood. This allochthonous energy forms the basis for the food web. Thus, small streams are usually heterotrophic, which means the ratio of gross photosynthesis to the total respiration of the biological community is low (P/R < 1.0). More energy is consumed by respiration than is produced by photosynthesis in the stream.

In larger streams (order 4-6), the canopy opens up and sunlight contributes to autochthonous energy production by photosynthesizing diatoms, algae, and plants. If photosynthesis exceeds respiration in the community (P/R >1.0), the stream is termed autotrophic (Cole 1983).

Energy from various sources in southern Appalachian streams is not quite this predictable because leaf input of allochthonous energy and shading of the riparian forest is seasonal. Prior to leaf-out in early spring, in-stream photosynthesis peaks and the community may be autotrophic. During summer, a shaded stream becomes heterotrophic as adequate sunlight no longer reaches stream plants. Shading also varies along the length of the stream due to gaps in the riparian canopy, so that even in summer, patches of photosynthetic plants may be found. Finally, respiration may far exceed production in the autumn when fallen leaves serve as new energy sources (Cole 1983).

These allochthonous and autochthonous sources of energy form the basis of stream food webs.