Are Rivers Just Big Streams? A Pulse Method to Quantify Nitrogen Demand in a Large River

Abstract

Given recent focus on large rivers as conduits for excess nutrients to coastal zones, their role in processing and retaining nutrients has been overlooked and understudied. Empirical measurements of nutrient uptake in large rivers are lacking, despite a substantial body of knowledge on nutrient transport and removal in smaller streams. Researchers interested in nutrient transport by rivers (discharge >10000 L/s) are left to extrapolate riverine nutrient demand using a modeling framework or a mass balance approach. To begin to fill this knowledge gap, we present data using a pulse method to measure inorganic nitrogen. (N) transport and removal in the Upper Snake River, Wyoming, USA (seventh order, discharge 12000 L/s). We found that the Upper Snake had surprisingly high biotic demand relative to smaller streams in the same river network for both ammonium (NH4+) and nitrate (NO3-). Placed in the context of a meta-analysis of previously published nutrient uptake studies, these data suggest that large rivers may have similar biotic demand for N as smaller tributaries. We also found that demand for different forms of inorganic N (NH4+ vs. NO3-) scaled differently with stream size. Data from rivers like the Upper Snake and larger are essential for effective water quality management at the scale of river networks. Empirical measurements of solute dynamics in large rivers are needed to understand the role of whole river networks (as opposed to stream reaches) in patterns of nutrient export at regional and continental scales.

Inferring nitrogen removal in large rivers from high‐resolution longitudinal profiling

“…Despite high uptake rates, our estimates of S w are generally very long. This incongruity could be the result of river size relative to most previous studies, but Tank et al (2008) report U and S w values for the Snake River (Q 5 12 m 3 s 21 ) of 53 mg m 2 d 21 and 2.5 km, respectively, both which are far smaller than what we observed across sites. Notably, our v f values are comparable to other streams, which strongly suggests that the large values of U and S w observed in these spring-fed rivers are a result of high NO Across study sites, average S w values within sites were strongly correlated with specific discharge, but with a scaling exponent (S W 5 [Q/w] n ) of 2.10 6 0.31, larger than previously reported (Hall et al 2009(Hall et al , 2013.…”

“…In comparison with other rivers-Comparing our estimates to those from rivers of different sizes (Ensign and Doyle 2006;Tank et al 2008;Hall et al 2009) suggests that our values of U T are generally larger than those reported in the literature (Table 5), though Ensign and Doyle (2006) report comparable values in some larger rivers. Despite high uptake rates, our estimates of S w are generally very long.…”

“…Quantifying the role of higher order rivers in watershed N dynamics remains constrained, however, in part because direct measurements of N processing in larger streams and rivers (. 0.2 m 3 s 21 ) are lacking (Ensign and Doyle 2006;Tank et al 2008).…”

“…Isotopic enrichment, which is desirable because it retains ambient concentrations and yields pathway-specific removal information, is particularly impractical in large rivers because of the costs of isotopically labeled solutes. Unlabeled nutrient enrichment additions, despite not discriminating removal pathways, are more feasible at higher flows and longer residence times (Tank et al 2008), but may enhance removal rates (Mulholland et al 2002), requiring repeated dosing to varying plateau concentrations to backextrapolate removal at ambient conditions (Payn et al 2005). Pulse injection methods (Covino et al 2010) to estimate both ambient concentration removal rates and removal kinetics with increased concentration obviate constraints of repeated plateau additions, but retain substantial logistical constraints in large rivers, and in river networks.…”

Quantifying watershed‐scale groundwater loading and in‐stream fate of nitrate using high‐frequency water quality data

“…While field-based studies [Burns, 1998;Peterson et al, 2001;Duff et al, 2008;Mulholland et al, 2008Mulholland et al, , 2009Tank et al, 2008;Hall et al, 2009;Mulholland and Webster, 2010] and modeling approaches [Jaworski et al, 1992;Boynton et al, 1995;Alexander et al, 2000Alexander et al, , 2009Seitzinger et al, 2002;Boyer et al, 2006;Runkel, 2007;Ator and Denver, 2012] have provided much needed information on reach and watershed-scale nitrate dynamics, the limited spatial extent and/or low temporal resolution of discrete data collection continues to be a challenge for quantifying loads and interpreting drivers of change in watersheds. Recent studies have demonstrated that the collection and interpretation of high-frequency nitrate data collected using water quality sensors can be used to better quantify nitrate loads to sensitive stream and coastal environments [Ferrant et al, 2013;Bieroza et al, 2014;Pellerin et al, 2014], and provide insights into temporal nitrate dynamics that would otherwise be difficult to obtain using traditional field-based mass balance, solute injection, and/or isotopic tracer studies [Pellerin et al, 2009[Pellerin et al, , 2012Heffernan and Cohen, 2010;Sandford et al, 2013;Carey et al, 2014;Hensley et al, 2014Hensley et al, , 2015Outram et al, 2014;Crawford et al, 2015].…”