A stream's role in watershed nutrient export.

T he 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 (6). Stream ecologists, using budget approaches inspired by Bormann and Likens (7, 8) and more process-oriented studies of nutrient fluxes from water columns to sediments (9–11), have since shown that streams themselves can be important sites of processing and retaining nutrients. Despite the knowledge that streams can be biogeochemical hot spots, stream element processing has not been experimentally linked to watershed elemental export. In this issue of PNAS, Bernhardt et al. (12) have demonstrated, for the first time, how processes within streams affect interpretation of watershed exports of nitrate nitrogen. They show that, after a large disturbance to the forest, nitrate concentrations in stream water were attenuated downstream of the disturbance. This research links the understanding that streams are biogeochemically reactive systems with fast rates of nitrogen conversion (10) with the overall pattern of nutrient export from a forested watershed ecosystem. In the winter of 1998, an ice storm removed a portion of forest vegetation in watersheds within the Hubbard Brook Experimental Forest (HBEF) (Fig. 1). After this storm, Bernhardt et al. (12) sampled the streams draining these watersheds both at the gauging weir at the base of the watershed and longitudinally along the stream reach up the watershed. As was previously observed from experimental vegetation removal at HBEF, nitrate concentrations in the stream water increased after the ice storm from reduced uptake by terrestrial vegetation combined with increased nitrification in soils, i.e., the conversion of ammonium to nitrate by chemoautotrophic bacteria (5). However, unlike previous experiments at HBEF, the ice storm disturbed a discrete elevation band of vegetation within the watersheds; thus, it was possible to estimate the degree to which the high-nitrate signal attenuated from the damaged zone downstream to the gauging weir at the base of the watershed. They found that the stream nitrate concentration decreased quickly along a stream reach transect from the damaged zone to the weir, and that this decrease was caused by instream nitrate uptake by microbes and not just from dilution via the inputs of low-nitrate groundwater. Over an annual scale, exports of nitrate at the weir would have been 80–140% higher given no stream removal of nitrate. Indeed, after the ice storm, the stream took up more nitrate than the annual export from the base of the watershed. Bernhardt et al. (12) show that that the disturbance itself increased the streams’ capacity to retain more nitrate. The fraction of nitrate removal in the stream section below the damaged zone was much higher after the ice storm than before, suggesting increasing rates of N processing. Mechanisms behind this increase may include an opening of the forest canopy that stimulated primary production by algae, which has been strongly correlated with nitrate uptake in other streams (11). Increased wood in the channel may retain organic detritus and associated microbial de-