River network alteration of C-N-P dynamics in a mesoscale agricultural catchment

Abstract

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Flowpath controls on high‐spatial‐resolution water‐chemistry profiles in headwater streams

“…We exclude in-stream transformation processes as a dominant control of longitudinal nitrate profiles because (1) both increasing and decreasing profiles exist and (2) the nitrate profile was significantly correlated with that of Cl À , a conservative tracer in river networks. The latter interpretation differs from previous spatial sampling studies in mesoscale catchments where in-stream processes played a major role (Dent andGrimm 1999, McGuire et al 2014) but is consistent with recent observation in the study region that in-stream processes produce more visible effects in high-order rivers than in headwaters (Casquin et al 2020). Although most variations in water-chemistry profiles were smooth, abrupt changes also existed at confluences, a farm point-source and a localized groundwater inflow point (Konrad 2006, Hensley et al 2020).…”

“…In four of the six catchments, profiles of nitrate concentration and agricultural land use did not show a significant positive correlation, although this relationship is commonly found in larger catchments in the study region (Casquin et al 2020, Frei et al 2020, Guillemot et al 2021) and elsewhere (Nielsen et al 2012). One possible reason for this lack of positive correlation is that land use alone is not sufficient to capture the spatial variability in N loading, since farming F I G U R E 5 Focus on abrupt variations in water chemistry as influenced by a point source in the main river of Douarnenez 1 (1260 m from the outlet) and by a groundwater inflow point in one tributary of Lieue de Grève 1 (1000 m from the outlet) practices vary spatially (Durand et al 2015).…”

Archetypes and Controls of Riverine Nutrient Export Across German Catchments

“…The impact of point sources is only visible for low P loads (<15 kg ha −1 y −1 ), suggesting possible threshold behavior. The lack of an expected clear relationship between point sources and slope b fits to the above interpretation that in-stream P cycling can significantly reshape P concentration dynamics (Casquin et al, 2020;Jarvie et al, 2012). The still prevailing dilution patterns of PO 4 -P could thus be related to biogeochemically induced P release (see above, Dupas et al, 2018;Smolders et al, 2017) besides the climatic controls.…”

Spatial Persistence of Water Chemistry Patterns Across Flow Conditions in a Mesoscale Agricultural Catchment

“…Second, the increasing importance of riparian and/or in‐stream biogeochemical processes during the low‐flow season could contribute to spatial instability in water chemistry patterns (Moatar et al., 2017). Decreased discharge increases residence time of solutes in different components of the stream network (e.g., riparian wetlands and hyporheic zones), while simultaneously decreases mass flux, thus increasing the exposure time to biogeochemical transformations and capacity for the in‐stream and hyporheic community to modify concentrations and fluxes (Casquin et al., 2020; Kolbe et al., 2019; Pinay et al., 2015; Wollheim et al., 2018). A recent study on the concentration‐discharge relationship in stream water supported this hypothesis, finding a dominant biogeochemical control on the concentration‐discharge slope when the discharge is below the median discharge (Moatar et al., 2017).…”