A stream's role in watershed nutrient export

The small watershed concept developed by Bormann and Likens (1) is a powerful means to understand how watershed ecosystems function (2-4). The approach requires estimating both element inputs to a watershed (e.g., atmospheric deposition, weathering) and outputs, usually via the stream that drains the watershed. Outputs are calculated as the product of stream water element concentration and stream discharge at a gauging weir at the base of the watershed. Differences between inputs and outputs are caused by physical, chemical, or biological processes within the watershed. By experimentally manipulating a watershed, e.g., harvesting the trees, it is possible to estimate the biotic contribution to net element retention (5). One of the central findings from this type of research is that element export can dramatically increase after forest removal; e.g., nitrate-nitrogen concentrations can increase dramatically. If the small watershed approach is used to interpret only terrestrial processes, then a central assumption is that the stream is solely a transport mechanism out of the watershed, and any modifications to element export by the stream itself are minimal relative to changes caused by the terrestrial component of the ecosystem. As originally conceived, the small watershed approach included the stream as part of an integrated watershed ecosystem (1), but elevated element export after experimental forest removal showed that the terrestrial component of the ecosystem exerted much, if not most, of the control over element export and retention. Thus, ecologists applied this approach broadly to address terrestrial ecosystem function, for example, hypotheses of forest ecosystem nutrient retention during succession (2, 4), the role of riparian zones on controlling nutrient export from agricultural watersheds (3), and potential impacts from atmospheric deposition …