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1 September 2007 Spiraling down the river continuum: stream ecology and the U-shaped curve
Jackson R. Webster
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The spiraling concept provides an explicit approach to modeling the longitudinal linkages within a river continuum. I developed a spiraling-based model for particulate organic C dynamics in the Little Tennessee River to synthesize existing data and to illustrate our current understanding of ecosystem processes in river ecosystems. The Little Tennessee River is a medium-sized river flowing ∼100 km through the southern Appalachian Mountains of northern Georgia and western North Carolina (USA). Across this distance, allochthonous inputs decrease and autochthonous production increases, resulting in a U-shaped curve of energy input. The model was set up as an advecting seston compartment interacting with 3 benthic compartments: coarse benthic organic matter, fine benthic organic matter, and autotrophs. Model-estimated ecosystem respiration was consistently lower than measured values, suggesting a need to evaluate our measurements of whole-stream metabolism. Also, model-predicted seston concentrations were generally lower than measured values, reflecting a need to consider additional sources of organic C in the model. For the whole river system, leaves accounted for 19% of inputs, primarily near the headwaters, and the remaining input was from instream primary production in the lower reaches of the river. Almost ½ of the input was respired, 28% by autotrophic respiration and 21% by heterotrophic respiration, and the remaining 51% was transported downstream. Ecosystem efficiency was ∼50% along the length of the river, and turnover length increased from several hundred meters at the headwaters to >100 km downstream. Based on various measures, the transition from heterotrophy to autotrophy ranged from 25 to >100 km downstream from the headwaters. As this model illustrates, a consequence of downstream transport is that much of the particulate C in streams is metabolized a considerable distance downstream from where it enters the stream. This longitudinal linkage is essential to our understanding of stream ecosystems.

Jackson R. Webster "Spiraling down the river continuum: stream ecology and the U-shaped curve," Journal of the North American Benthological Society 26(3), 375-389, (1 September 2007).
Received: 8 September 2006; Accepted: 1 February 2007; Published: 1 September 2007

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Little Tennessee River
river continuum
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