Articles | Volume 14, issue 18
Research article
29 Sep 2017
Research article |  | 29 Sep 2017

Carbon and nutrient export regimes from headwater catchments to downstream reaches

Rémi Dupas, Andreas Musolff, James W. Jawitz, P. Suresh C. Rao, Christoph G. Jäger, Jan H. Fleckenstein, Michael Rode, and Dietrich Borchardt

Abstract. Excessive amounts of nutrients and dissolved organic matter in freshwater bodies affect aquatic ecosystems. In this study, the spatial and temporal variability in nitrate (NO3), dissolved organic carbon (DOC) and soluble reactive phosphorus (SRP) was analyzed in the Selke (Germany) river continuum from three headwaters draining 1–3 km2 catchments to two downstream reaches representing spatially integrated signals from 184–456 km2 catchments. Three headwater catchments were selected as archetypes of the main landscape units (land use  ×  lithology) present in the Selke catchment. Export regimes in headwater catchments were interpreted in terms of NO3, DOC and SRP land-to-stream transfer processes. Headwater signals were subtracted from downstream signals, with the differences interpreted in terms of in-stream processes and contributions from point sources. The seasonal dynamics for NO3 were opposite those of DOC and SRP in all three headwater catchments, and spatial differences also showed NO3 contrasting with DOC and SRP. These dynamics were interpreted as the result of the interplay of hydrological and biogeochemical processes, for which riparian zones were hypothesized to play a determining role. In the two downstream reaches, NO3 was transported almost conservatively, whereas DOC was consumed and produced in the upper and lower river sections, respectively. The natural export regime of SRP in the three headwater catchments mimicked a point-source signal (high SRP during summer low flow), which may lead to overestimation of domestic contributions in the downstream reaches. Monitoring the river continuum from headwaters to downstream reaches proved effective to jointly investigate land-to-stream and in-stream transport, and transformation processes.

Short summary
Carbon and nutrient export regimes were analyzed from archetypal headwater catchments to downstream reaches. In headwater catchments, land use and lithology determine land-to-stream C, N and P transfer processes. The crucial role of riparian zones in C, N and P coupling was investigated. In downstream reaches, point-source contributions and in-stream processes alter C, N and P export regimes.
Final-revised paper