Abstract
Instream nitrogen (N) processing consists of complex interacting and highly time-varying pathways. To understand the role of a large agricultural stream river reaches in processing N thoroughly, it is urgently needed to continuously quantify high temporal resolution N processing pathways, reflecting seasonal shifts and multi-annual overarching effects. To this end, the hydrodynamic and river water quality model WASP 7.5.2 was applied in the 27.4-km reach of the 6th order agricultural stream Lower Bode (central Germany) for 5 years (2014–2018). Paired high-frequency data (15-min interval) of discharge (Q), nitrate (NO3−), dissolved oxygen (DO), and Chlorophyll-a at upstream and downstream stations were used as model boundaries and for model constraints. The WASP model simulated 15-minute intervals of Q, NO3− and DO with Nash-Sutcliffe-Efficiency values higher than 0.9 for calibration and validation, enabling the calculation of gross and net dissolved inorganic N (DIN) uptake and pathway rates on a daily, seasonal and multi-annual scale. Results showed daily DIN net uptake rate ranged from −17.4 mg N m−2 d−1 to 553.9 mg N m−2 d−1. The highest daily net uptake could reach almost 30 % of total input loading, which occurred at extreme low flow in summer 2018. The growing season (spring and summer) accounted for 91 % of the average net annual DIN uptake in the measured period. In spring, both the DIN gross and net uptake were dominated by the phytoplankton uptake pathway. In summer, benthic algae assimilation dominated the gross DIN uptake. Conversely, the reach became a DIN net source with negative daily net uptake values in autumn and winter, mainly because the release from benthic algae surpassed uptake processes. Over the five years, average DIN gross and net uptake rates were 124.1 and 56.8 mg N m−2 d−1, which accounted for only 2.7 % and 1.2 % of the total loadings the study reach in the Lower Bode, respectively. 5-year average gross DIN uptake decreased from assimilation by benthic algae through assimilation by phytoplankton to denitrification. Our study highlights the value of combining river water quality modelling with high-frequency data in obtaining reliable instream DIN-budget, which facilitates our ability to manage N in aquatic systems. This study provides a methodology that can be applied to any large stream to quantify N processing pathway dynamics and complete our understanding of N cycling.
Highlights
The instream processing of dissolved inorganic nitrogen (DIN) consists of complex and multiple simultaneous pathways (Hensley and Cohen, 2020)
Average dissolved inorganic N (DIN) gross and net uptake rates were 124.1 and 56.8 mg N m-2 d-1, which accounted for only 2.7% and 1.2% of the total loadings the study reach in the Lower Bode, respectively. 5-year 25 average gross DIN uptake decreased from assimilation by benthic algae through assimilation by phytoplankton to denitrification
Our results showed that assimilatory DIN uptakes by phytoplankton and benthic algae both play an essential role in the annual DIN uptake budget in 315 the Lower Bode
Summary
The instream processing of dissolved inorganic nitrogen (DIN) consists of complex and multiple simultaneous pathways (Hensley and Cohen, 2020). The classic method for reach-scale DIN pathway quantification is the addition of DIN isotope tracers (Mulholland et al, 2008) Using this methodology, Mulholland et al (2008) quantified the shares of denitrification and assimilation on total nitrate (NO3−) in-stream uptake in the Lotic Intersite Nitrogen eXperiment (LINX) Project in a wide range of biomes. To achieve a continuous estimation of assimilatory uptake, Rode et al (2016) correlated daily assimilatory NO3− uptake with gross primary production (GPP) using high-frequency oxygen and NO3− data Both methods rely on a robust diel pattern of NO3− concentration fluctuations, which is possibly only obtained where external inputs are well constrained and difficult to obtain in large agricultural streams (Hensley and Cohen, 45 2020). Due to the lack of effective methods, quantification of DIN processing pathways in large agricultural streams remains poorly explored
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