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 similar to 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 similar to 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.