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

Abstract

We present a method for estimating nitrogen (N) removal based on high-resolution longitudinal profiling, which facilitates repeated measurement in larger rivers. The Lagrangian reference frame allows removal to be spatially disaggregated, enabling identification of removal ''hot spots,'' and potentially passive assessment of reaction kinetics using ambient longitudinal concentration gradients. Applying the method in six spring-fed rivers in North Florida, we tested the hypothesis that removal is controlled by spatial variation in channel hydraulics and vegetation. Removal estimates obtained using this method were statistically robust, spatially consistent across replicate profiles, and comparable to measurements made in these and other systems of similar size using alternative methods. We observed significantly increased removal in reaches with lower specific discharge and in more heavily vegetated reaches. Assessing uptake kinetics directly from longitudinal profiles assumes negligible sampling effects from temporally variable removal, non-instantaneous sampling velocities, and the mixing of water along the flowpath. Results from a reactive transport model suggest these effects are significant, producing complex profile geometries. Relatively small differences between alternative kinetic models substantially limit inferences about reaction order when utilizing the small concentration gradients observed longitudinally within rivers. Across rivers, N removal follows either efficiency-loss (exponent 5 0.39) or Michaelis-Menten kinetics, but not zero-or first-order kinetics.

Diffusion and seepage-driven element fluxes from the hyporheic zone of a karst river

“…3G-K) and Cohen 2012) suggested a residence-time distribution with a mean of 6 h, with 98% of the dye leaving the river within 9 h, so we assumed longitudinal equilibrium after 12 h (2 × residence time). We collected longitudinal RWT concentration profiles along the river thalweg with a fluorometer (Turner Designs, Sunnyvale, California) suspended over the side of a canoe at a depth of 30 cm (Hensley et al 2014). We made measurements of RWT and spatial location with a handheld global positioning system (GPS) unit (Delorme, Yarmouth, Maine) every 10 s. We calculated river discharge at each location to the nearest 0.05 m 3 /s by dividing the RWT injection flux by the measured concentration.…”

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

“…Finally, it is not currently possible to identify the relative importance of the mechanism of nitrate retention/ loss (e.g., assimilatory biological uptake versus denitrification) using single-station data at the watershed scale. Applying methods to quantify time-variable assimilative nitrate uptake and denitrification using high-frequency water quality data [Heffernan and Cohen, 2010;Hensley et al, 2014] in larger river basins warrants further attention. To this end, combining the approach described here with estimates of stream metabolism from dissolved oxygen data is a potential avenue of future research.…”

“…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.…”

Nutrient Dynamics of the Delta: Effects on Primary Producers

“…Recent studies have also demonstrated how highfrequency sensors may be used to improve our understanding of environmental processes by mapping spatial variability in rivers, lakes and estuaries, often in conjunction with fixed-station measurements (Downing et al, forthcoming;Gilbert et al 2013;Hensley et al 2014;Wild-Allen and Rayner 2014;Crawford et al 2015). These examples include the Columbia River Estuary, where fixed station and mapping data allowed researchers to identify nutrient sources and transformations across a salinity gradient, and thus identify key transition zones (Gilbert et al 2013).…”

“…These examples include the Columbia River Estuary, where fixed station and mapping data allowed researchers to identify nutrient sources and transformations across a salinity gradient, and thus identify key transition zones (Gilbert et al 2013). In Florida, longitudinal profiling of several rivers permitted nutrient removal "hot spots" to be located (Hensley et al 2014). In the north Delta, Downing et al (forthcoming) mapped the spatial variation in water isotopes, from which they calculated water residence time (Figure 12), an important ecological parameter related to many biogeochemical processes-and one previously not possible to quantify from field measurements.…”