The importance of in-stream uptake for regulating stream concentrations and outputs of N and P from a forested watershed: evidence from long-term chemistry records for Walker Branch Watershed

摘要

Long-term, weekly measurements of streamwater nitrogen and phosphorus concentrations in the West Fork of Walker Branch, a 1st order forested stream in eastern Tennessee, were used to assess the importance of in-stream processes for controlling stream concentrations and watershed exports. Over the period from 1991 to 2002, there was a slight declining trend in watershed export of dissolved inorganic N via streamflow, despite relatively high and constant wet N deposition rates (5 kg/ha/y). The watershed retains >90% of N deposition inputs. Concentrations of NO3 − and soluble reactive phosphorus (SRP) showed distinct seasonal patterns with autumn and early spring minima and summer maxima. An end-member mixing analysis indicated that these seasonal concentration patterns were largely a result of seasonal variations in in-stream uptake processes, with net uptake of NO3 − and SRP having the greatest impact on streamwater concentrations in November (reductions of 29 μg N/l and 2.5 μg P/l, respectively). This was likely a result of high rates of uptake by microbes colonizing new inputs of leaf detritus. For NO3 − there was a secondary peak in net uptake in March and April (about 9 μg N/l) resulting from increased uptake by stream algae and bryophytes. Summer was a period of net release of NO3 − to streamwater (peaking at 9 μg N/l in July) and minimal net effects on SRP concentrations. On average, in-stream processes resulted in removal of about 20% of the NO3 − and 30% of the SRP entering the stream from the catchment annually. This study, as well as other recent work, suggests that in-stream processes are important buffers on stream nutrient concentrations and exports reducing the effects of changes in inputs and retention in terrestrial portions of watersheds.