Abstract
Abstract. Nitrate (NO3-) excess in rivers harms aquatic ecosystems and can induce detrimental algae growths in coastal areas. Riverine NO3- uptake is a crucial element of the catchment-scale nitrogen balance and can be measured at small spatiotemporal scales, while at the scale of entire river networks, uptake measurements are rarely available. Concurrent, low-frequency NO3- concentration and streamflow (Q) observations at a basin outlet, however, are commonly monitored and can be analyzed in terms of concentration discharge (C–Q) relationships. Previous studies suggest that steeper positive log (C)–log (Q) slopes under low flow conditions (than under high flows) are linked to biological NO3- uptake, creating a bent rather than linear log (C)–log (Q) relationship. Here we explore if network-scale NO3- uptake creates bent log (C)–log (Q) relationships and when in turn uptake can be quantified from observed low-frequency C–Q data. To this end we apply a parsimonious mass-balance-based river network uptake model in 13 mesoscale German catchments (21–1450 km2) and explore the linkages between log (C)–log (Q) bending and different model parameter combinations. The modeling results show that uptake and transport in the river network can create bent log (C)–log (Q) relationships at the basin outlet from log–log linear C–Q relationships describing the NO3- land-to-stream transfer. We find that within the chosen parameter range the bending is mainly shaped by geomorphological parameters that control the channel reactive surface area rather than by the biological uptake velocity itself. Further we show that in this exploratory modeling environment, bending is positively correlated to percentage of NO3- load removed in the network (Lr.perc) but that network-wide flow velocities should be taken into account when interpreting log (C)–log (Q) bending. Classification trees, finally, can successfully predict classes of low (∼4 %), intermediate (∼32 %) and high (∼68 %) Lr.perc using information on water velocity and log (C)–log (Q) bending. These results can help to identify stream networks that efficiently attenuate NO3- loads based on low-frequency NO3- and Q observations and generally show the importance of the channel geomorphology on the emerging log (C)–log (Q) bending at network scales.
Highlights
Transport and transformation of nitrate (NO3−) in river networks are major controls of downstream exports to receiving lakes, reservoirs and coastal systems (Alexander et al, 2000; Billen et al, 1991; Peterson et al, 2001; Seitzinger et al, 2002; Seybold and McGlynn, 2018)
When on the other hand Kw exceeds 6.8 and ad is larger than 0.4 or when ad is smaller than 0.4 but Curvmax is smaller than −0.45 and v is very small (< 0.04 m s−1), it is most likely that Da is large. These findings demonstrate that log(C)–log(Q) bending at the catchment outlet, together with the median velocity and the response of the width and the depth of a channel to discharge can help to classify the in-stream daily percentage of load removed Lr.perc, the Damköhler number Da and to a certain extent the uptake velocity vf
We explore how low-frequency NO3− log(C)– log(Q) relationships, observed at a basin outlet, can display bending as a result of network-scale in-stream uptake processes
Summary
Transport and transformation of nitrate (NO3−) in river networks are major controls of downstream exports to receiving lakes, reservoirs and coastal systems (Alexander et al, 2000; Billen et al, 1991; Peterson et al, 2001; Seitzinger et al, 2002; Seybold and McGlynn, 2018). Increased NO3− concentrations in surface waters can induce detrimental algae growths (Beusen et al, 2016; Canfield et al, 2010; Galloway et al, 1995), compromise river ecosystem health and jeopardize drinking water supplies. Since the beginning of the 20th century, human activities such as agricultural expansion and fossil fuel burning have mobilized additional reactive nitrogen (N), initiating and later exacerbating this problem (Seitzinger et al, 2002). J. Dehaspe et al.: Bending of the concentration discharge relationship
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call