Carbon and Nutrient Export Regimes from Headwater Catchments to Downstream Reaches

Biogeosciences, 14, 4391–4407, 2017 https://doi.org/10.5194/bg-14-4391-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Carbon and nutrient export regimes from headwater catchments to downstream reaches Rémi Dupas1,5, Andreas Musolff2, James W. Jawitz3, P. Suresh C. Rao4, Christoph G. Jäger1, Jan H. Fleckenstein2, Michael Rode1, and Dietrich Borchardt1 1Department of Aquatic Ecosystems Analysis and Management, Helmholtz Centre for Environmental Research UFZ, Magdeburg, Germany 2Department of Hydrogeology, Helmholtz Centre for Environmental Research UFZ, Leipzig, Germany 3Soil and Water Science Department, University of Florida, Gainesville, Florida, USA 4School of Civil Engineering and Department of Agronomy, Purdue University, West Lafayette, Indiana, USA 5INRA, UMR1069 SAS, 35000 Rennes, France Correspondence to: Rémi Dupas ([email protected]) Received: 14 March 2017 – Discussion started: 10 May 2017 Revised: 1 September 2017 – Accepted: 5 September 2017 – Published: 29 September 2017 Abstract. Excessive amounts of nutrients and dissolved or- mestic contributions in the downstream reaches. Monitoring ganic matter in freshwater bodies affect aquatic ecosystems. the river continuum from headwaters to downstream reaches In this study, the spatial and temporal variability in nitrate proved effective to jointly investigate land-to-stream and in- − (NO3 ), dissolved organic carbon (DOC) and soluble reactive stream transport, and transformation processes. phosphorus (SRP) was analyzed in the Selke (Germany) river continuum from three headwaters draining 1–3 km2 catchments to two downstream reaches representing spatially in- 1 Introduction tegrated signals from 184–456 km2 catchments. Three headwater catchments were selected as archetypes of the main Riverine exports are a key component in the global biogeo- landscape units (land use × lithology) present in the Selke chemical cycles of carbon (C), nitrogen (N) and phosphorus catchment. Export regimes in headwater catchments were in- − (P) (Beusen et al., 2016). River export regimes of their dom- terpreted in terms of NO3 , DOC and SRP land-to-stream inant soluble forms, dissolved organic C (DOC), nitrate-N transfer processes. Headwater signals were subtracted from − (NO3 ) and soluble reactive P (SRP), result from the interplay downstream signals, with the differences interpreted in terms of land-to-stream diffuse transfer processes and in-stream of in-stream processes and contributions from point sources. − transformations, and can be altered by point-source contri- The seasonal dynamics for NO3 were opposite those of DOC butions (Seitzinger et al., 2010). Excess delivery of DOC, and SRP in all three headwater catchments, and spatial dif- − − NO3 and SRP into sensitive water-bodies affects ecosystem ferences also showed NO3 contrasting with DOC and SRP. structure and functions, and elemental stoichiometric ratios These dynamics were interpreted as the result of the interplay have been shown to be of major importance (Sardans et al., of hydrological and biogeochemical processes, for which ri- 2012). Therefore, detailed knowledge of the catchment pro- parian zones were hypothesized to play a determining role. cesses controlling the spatial and temporal variability in the In the two downstream reaches, NO− was transported al- − 3 delivery of DOC, NO3 , SRP and stoichiometric ratios at rel- most conservatively, whereas DOC was consumed and pro- evant management scales, e.g., the European Union Water duced in the upper and lower river sections, respectively. Framework Directive (EC, 2000) water-bodies, is a prereq- The natural export regime of SRP in the three headwater uisite for designing effective water pollution mitigation pro- catchments mimicked a point-source signal (high SRP during grams (Wall et al., 2011). summer low flow), which may lead to overestimation of do- Published by Copernicus Publications on behalf of the European Geosciences Union. 4392 R. Dupas et al.: Carbon and nutrient export regimes Water-quality assessment programs performed by en- poral variability in solute concentrations could lead to vari- vironmental agencies typically focus on relatively large ability in N and P limitation in streams and rivers. (> 100 km2) catchments, to increase spatial coverage within The main objective of this paper, therefore, was to charac- − a given hydrographic basin or administrative unit, while re- terize the spatial and temporal variability in NO3 , DOC and ducing the density of monitoring stations and thus the cost SRP export regimes from archetypal headwater catchments (Bouraoui and Grizzetti, 2011; Dupas et al., 2015a). How- to downstream reaches, and to analyze the resulting nutri- ever, large catchments include both diffuse and domestic ent stoichiometric ratios. Export regimes in headwater catch- − or industrial point-source contributions and possibly vari- ments are used for interpretation of land-to-stream NO3 , ous landscape units. Thus, the water-quality signal mea- DOC and SRP transfer processes and compared with those in sured at the outlet integrates several transfer mechanisms downstream reaches to infer in-stream processes and point- contributing to emissions in the river network, as well as source contributions. in-stream processes (Grathwohl et al., 2013). Therefore, it is difficult to decipher the diffuse contributions of different landscape units from point-source contributions and in- 2 Materials and methods stream transformations (Bishop et al., 2008; Temnerud et al., 2016). In contrast to environmental agency monitoring pro- 2.1 Study area grams, scientific programs often focus on headwater catchments free of point-sources and with relatively homogeneous The Bode catchment (3300 km2) is located in the German landscape types (Fealy et al., 2010; McGonigle et al., 2014), part of the Elbe river basin (144 055 km2) (Zacharias et al., where in-stream processes are often considered to be min- 2011). The Bode catchment stretches from the Harz moun- imal (Salmon-Monviola et al., 2013). A comparison of ex- tains, a low mountain range in central Germany (maxi- port regimes in contrasting catchments representing different mum altitude 1142 ma:s:l:), to the central German lowlands, landscape types can be performed to investigate the effect of, a flat and fertile area dedicated to arable agriculture (alti- for example, contrasting dominant land use, dominant flow tude around 100 ma:s:l:). This topographic gradient coin- paths or climate (Outram et al., 2014; Dupas et al., 2017; cides with gradients of climate, geology, soil and land use Minaudo et al., 2017), sometimes aided by the use of models pressures (Wollschlager et al., 2016). Long-term mean an- (e.g., Dupas et al., 2016a; Hartmann et al., 2016). In headwa- nual (1951–2011) precipitation and temperature in the Bode ter catchments, several studies have highlighted the impor- catchment ranges from 1700 mm and 5 ◦C in the Harz moun- tant role of landscape heterogeneity within hillslopes (Hern- tains to 500 mm and 9.5 ◦C in the lowland area, with the don et al., 2015; Musolff et al., 2017), notably the crucial role lowest and highest temperatures in January and July, respec- of reactive zones such as riparian wetlands (Dick et al., 2015; tively, (Wollschlager et al., 2016). The Selke tributary catch- Pinay et al., 2015; Tiwari et al., 2017) in controlling solute ment (456 km2) was selected for study here as it encompasses export regimes. the different combinations of land use and lithology present To upscale headwater signals to downstream reaches, pre- in the Bode catchment and it has been intensively monitored vious landscape mixing models, i.e., “models mixing head- since 2010. Other varying environmental factors, such as soil water signals in proportion to their patch coverage” (Tiwari type, climate and dominant farming systems, coincide with et al., 2017), often lack consideration of temporal variability the topographic gradient reflected in the differentiation ac- in headwater signals and explicit consideration of in-stream cording to lithology. transformations (e.g., Laudon et al., 2011; Agren et al., The 456 km2 Selke catchment (Fig. 1) comprises upper 2014). Few opportunities exist to study the export regimes and lower portions. The upper Selke (184 km2), monitored of several headwater catchments representing “archetypes” at Meisdorf station (MEIS), is located in the Harz moun- of the main landscape units in a larger catchment, for mul- tains (209–595 ma:s:l:). The dominant soil type is Cam- tiple solutes and on different time scales, and to compare bisol overlaying impervious schist and claystone, resulting headwater export regimes to the integral signal measured in in a dominance of shallow flow pathways (Jiang et al., downstream reaches. Monitoring data from the Bode catch- 2014b). These shallow groundwater systems favor the de- ment (3300 km2), part of the hydrological Harz/Central Ger- velopment of hydromorphic riparian soils (periodically wa- man Lowland Observatory in the Helmholtz Association terlogged soils near the streams, delineated in this study ac- Terrestrial Environmental Observatories (TERENO) network cording to a soil map), representing 10 % of the surface area. (Zacharias et al., 2011), offers such opportunities and it was The land use is dominated by 73 % forest (including broad- this catchment that is taken as a case study in this paper. leaved, coniferous and mixed forest) followed by 25 % agri- In this paper we hypothesized

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