Dynamic modeling of nitrogen losses in river networks unravels the coupled effects of hydrological and biogeochemical processes

Abstract

The importance of lotic systems as sinks for nitrogen inputs is well recognized. A fraction of nitrogen in streamflow is removed to the atmosphere via denitrification with the remainder exported in streamflow as nitrogen loads. At the watershed scale, there is a keen interest in understanding the factors that control the fate of nitrogen throughout the stream channel network, with particular attention to the processes that deliver large nitrogen loads to sensitive coastal ecosystems. We use a dynamic stream transport model to assess biogeochemical (nitrate loadings, concentration, temperature) and hydrological (discharge, depth, velocity) effects on reach-scale denitrification and nitrate removal in the river networks of two watersheds having widely differing levels of nitrate enrichment but nearly identical discharges. Stream denitrification is estimated by regression as a nonlinear function of nitrate concentration, streamflow, and temperature, using more than 300 published measurements from a variety of US streams. These relations are used in the stream transport model to characterize nitrate dynamics related to denitrification at a monthly time scale in the stream reaches of the two watersheds. Results indicate that the nitrate removal efficiency of streams, as measured by the percentage of the stream nitrate flux removed via denitrification per unit length of channel, is appreciably reduced during months with high discharge and nitrate flux and increases during months of low-discharge and flux. Biogeochemical factors, including land use, nitrate inputs, and stream concentrations, are a major control on reach-scale denitrification, evidenced by the disproportionately lower nitrate removal efficiency in streams of the highly nitrate-enriched watershed as compared with that in similarly sized streams in the less nitrate-enriched watershed. Sensitivity analyses reveal that these important biogeochemical factors and physical hydrological factors contribute nearly equally Biogeochemistry (2009) 93:91-116 DOI 10.1007 to seasonal and stream-size related variations in the percentage of the stream nitrate flux removed in each watershed.

Nitrous oxide fluxes and dissolved N gases (N2 and N2O) within riparian zones along the agriculturally impacted San Joaquin River

“…- (Alexander et al 2009). N 2 concentrations were slightly under-saturated at times during the study period.…”

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

“…Streams and rivers are the principal flowpaths linking terrestrial N sources to these downstream locations, and river networks exhibit a substantial capacity to attenuate N fluxes (Galloway et al 2003;Seitzinger et al 2006;Mullholland et al 2008). 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).…”

“…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.…”

Nitrogen transformations in a through‐flow wetland revealed using whole‐ecosystem pulsed 15N additions

“…This excess N contributes to eutrophication and impairment of inland and coastal surface waters (Rabalais 2002). Aquatic ecosystems along the flow path from terrestrial to marine environments, such as streams, lakes, rivers, and reservoirs, can have a large influence on N transport to coastal zones (Seitzinger et al 2006;Alexander et al 2009;Harrison et al 2009). As water makes its way from landscapes into stream and river networks, often it passes through relatively lentic water bodies ranging from headwater wetlands to small impounded areas (e.g., beaver dams, mill ponds) to larger lakes and reservoirs.…”