Carbon and nutrient export regimes from headwater catchments to downstream reaches

Abstract

<p><strong>Abstract.</strong> Excessive amounts of nutrients and dissolved organic matter in freshwater bodies affect aquatic ecosystems. In this study, the spatial and temporal variability in nitrate (NO<sub>3</sub>), dissolved organic carbon (DOC) and soluble reactive phosphorus (SRP) was analyzed in the Selke (Germany) river continuum from headwaters draining 1–3 km<sup>2</sup> catchments to downstream reaches representing spatially integrated signals from 184–456 km<sup>2</sup> catchments. Three headwater catchments were selected as archetypes of the main landscape units (land use x lithology) present in the Selke catchment. Export regimes in headwater catchments were interpreted in terms of NO<sub>3</sub>, DOC and SRP land-to-stream transfer processes. Headwater signals were subtracted from downstream signals, with the differences interpreted in terms of in-stream processes and contribution of point-source emissions. The seasonal dynamics for NO<sub>3</sub> were opposite those of DOC and SRP in all three headwater catchments, and spatial differences also showed NO<sub>3</sub> contrasting with DOC and SRP. These dynamics were interpreted as the result of the interplay of hydrological and biogeochemical processes, for which riparian zones were hypothesized to play a determining role. In the two downstream reaches, NO<sub>3</sub> was transported almost conservatively, whereas DOC was consumed and produced in the upper and lower river sections, respectively. The natural export regime of SRP in the three headwater catchments mimicked a point-source signal (high SRP during summer low flow), which may lead to overestimation of domestic contributions in the downstream reaches. Monitoring the river continuum from headwaters to downstream reaches proved effective to investigate jointly land-to-stream and in-stream transport and transformation processes.</p>

Seasonal dynamics and exports of elements from a first‐order stream to a large inland lake in Michigan

“…In contrast, PO 4 3− variability was high (CV C 50–213%) and the CV C /CV Q ratio was chemodynamic at the mid flume and did not show a chemostatic trend at the upper flume. Higher variability in phosphorus (TP, PO 4 3− ) has also been observed by Thompson et al (), Musolff et al (), Dupas et al (), and Moatar et al (), and this variability could be due to threshold driven variability of sediment‐bound P, biological mediation, or to high rates of reactivity in streams being stronger controlling factors on concentration than discharge.…”

“…Recently, C‐Q relationships have been analysed across a wide range of watersheds with differing catchment areas, climate conditions, lithologies, land covers, and land management methods (e.g., Basu et al, ; Burns, Boyer, Elliott, & Kendall, ; Dupas et al, ; Godsey et al, ; Moatar et al, ; Musolff et al, ; Thomas et al, ). The C‐Q relationships for many solutes varied across these studies, with Godsey et al (), Basu et al (), and Thomas et al () concluding that chemostatic patterns dominated parent material weathering elements such as Si, Na + , Mg 2+ , and Ca 2+ (Godsey et al, ), and vegetation‐limiting nutrients, such as NO 3 − and PO 4 3− , in agricultural watersheds (Basu et al, ; Thomas et al, ).…”

A New Fully Distributed Model of Nitrate Transport and Removal at Catchment Scale

“…Seasonal variability in concentrations at the two upper stations represents mainly the nitrate behavior in natural forest‐dominated areas, which has high interflow concentrations and extremely low baseflow concentrations. Due to the impermeable geological property, a shallow and flashier flow pathway is developed in the upper Selke (Dupas et al, ). The fertilizers and plant residues added to the upper arable lands and forests, respectively, increased concentrations of soil moisture, but not of deeper groundwater.…”

“…The mHM‐Nitrate model provided detailed spatial information (e.g., spatially resolved nitrate terrestrial concentrations and fluxes) that is within reasonable ranges. Therefore, it offers promising opportunities for further evaluation of nutrient transport and removal processes spatiotemporally, for instance, to support future studies that target spatial agricultural mitigation measures (Hashemi et al, )and interactions between terrestrial and in‐stream processes (Dupas et al, ). Further validation of the new model needs to be done by cross validating for catchments that differ in natural conditions and scales.…”

Multidecadal Trajectory of Riverine Nitrogen and Phosphorus Dynamics in Rural Catchments

“…The long‐term evolution of a catchment's water quality under changing external forcings, termed hereafter trajectory, does affect not only mean annual concentrations and fluxes but also intraannual variations. Seasonal variations in nutrient concentrations are often interpreted in terms of (i) dominant point versus diffuse sources (Jarvie et al, ; Withers et al, ); (ii) spatial distribution of nutrient sources in catchment, vertically (Abbott et al, ; Dupas et al, ; Musolff et al, ) or laterally (Aubert et al, ; Musolff et al, ); and (iii) temperature‐dependent release and retention mechanisms, within soils (Aubert et al, ; Dupas, Musolff, et al, ) or the river network (Mulholland et al, ; Rode et al, ). Worrall et al () observed that different components of the Thames nitrate time series (e.g., annual maxima and minima) had different responses to land use and climate drivers, and that the amplitude and phase of seasonal variations varied over a 130‐year period.…”