Spiraling down the river continuum: stream ecology and the U-shaped curve

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 1⁄2 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.