Assessing the sources and magnitude of diurnal nitrate variability in the San Joaquin River (California) with an in situ optical nitrate sensor and dual nitrate isotopes

Abstract

Summary 1. We investigated diurnal nitrate (NO3−) concentration variability in the San Joaquin River using an in situ optical NO3− sensor and discrete sampling during a 5‐day summer period characterized by high algal productivity. Dual NO3− isotopes (δ15NNO3 and δ18ONO3) and dissolved oxygen isotopes (δ18ODO) were measured over 2 days to assess NO3− sources and biogeochemical controls over diurnal time‐scales. 2. Concerted temporal patterns of dissolved oxygen (DO) concentrations and δ18ODO were consistent with photosynthesis, respiration and atmospheric O2 exchange, providing evidence of diurnal biological processes independent of river discharge. 3. Surface water NO3− concentrations varied by up to 22% over a single diurnal cycle and up to 31% over the 5‐day study, but did not reveal concerted diurnal patterns at a frequency comparable to DO concentrations. The decoupling of δ15NNO3 and δ18ONO3 isotopes suggests that algal assimilation and denitrification are not major processes controlling diurnal NO3− variability in the San Joaquin River during the study. The lack of a clear explanation for NO3− variability likely reflects a combination of riverine biological processes and time‐varying physical transport of NO3− from upstream agricultural drains to the mainstem San Joaquin River. 4. The application of an in situ optical NO3− sensor along with discrete samples provides a view into the fine temporal structure of hydrochemical data and may allow for greater accuracy in pollution assessment.

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]. Coupling these measurements with techniques for quantifying water sources and/or flow paths [Gilbert et al, 2013;Bowes et al, 2015;Duncan et al, 2015] provides further opportunity for understanding and managing the drivers of coastal eutrophication.…”

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

“…Modeling studies suggest N removal varies spatially within river networks (Alexander et al 2009), and varies temporally with climate (Donner et al 2004;Botter et al 2010) and discharge (Doyle 2005;Wollheim et al 2008;Basu et al 2011). This variability has been directly observed (Hall, Baker et al 2009;Pellerin et al 2012); however, further measurements to validate model predictions at reach-to-network spatial scales and diel-to-event-to-seasonal and even interannual temporal scales are needed.…”

“…The more familiar is the Eulerian frame where changes in fluxes are observed at a stationary location over time. Using an Eulerian approach, with passive high-resolution sensing of fluxes passing a single or multiple stations, has yielded new insights about catchment N dynamics (Pellerin et al 2009(Pellerin et al , 2012 and river ecosystem removal rates and processes (Roberts and Mulholland 2007;Heffernan and Cohen 2010). These methods can be applied in larger systems; however, the observed Eulerian signal is an aggregate of all upstream processes, thereby offering limited insight into spatial variation in removal within river reaches.…”

“…The principal reason for this constraint is methodological, specifically a strong reliance on two methods: 15 N isotopic tracer addition (Hall et al 1998;Dodds et al 2000;Peterson et al 2001) and nutrient enrichment (Mulholland et al 2002;Payn et al 2005), both of which create logistical and financial limitations when applied to larger rivers. 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.…”

“…Despite the need to understand whole-network removal, most inferences about the timing and magnitude of removal come from models (Alexander et al 2000;Seitzinger et al 2002), many of which estimate large-river contributions to network removal by extrapolating observations from small streams, and thus potentially oversimplifying the effects of removal variability as a function of discharge (Hall et al 2013) and channel morphology (Helton et al 2011). While small headwater streams have empirically been identified as ''hot spots'' (Alexander et al 2000;Peterson et al 2001;Bernot and Dodds 2005), other models suggest that higher order reaches can also substantially influence river network removal (Seitzinger et al 2002;Wollheim et al 2006;Mulholland et al 2008). 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 (.…”