Evaluation of controlling factors of nitrate removal process using high-frequency measurements in high order streams (central Germany)

Abstract:

Rivers are important conduits to transport nitrogen (N) from terrestrial area to the sea. Nitrate removal dynamics in rivers vary spatiotemporally and are affected by many factors. Recently, mass balance approaches from high-frequency measurements was demonstrated to be useful to quantify nitrate removal and investigate controlling factors in larger rivers. However, such in-situ measurements are limited to specific river types (e.g., spring-fed rivers). Here, we conducted multi-parameter high-frequency measurements, including two station time series and longitudinal profiling, to investigate controlling factors of reach-scale nitrate removal processes in three high order streams in central Germany (the Bode (6^th), the Weisse Elster (5^th) and the Mulde (7^th) river). Two station mass balance results showed the removal rates of nitrate were different between reaches with contrasting morphology. Relatively natural reaches exhibited higher removal rate of nitrate in summer, and showed net nitrate release in spring and autumn. While modified reaches usually showed lower removal rate of nitrate. During low-flow periods, nitrate removal process was highly related to metabolism processes and can be easily divided into assimilatory and dissimilatory uptake, with ratio nearly 2:1. Moreover, longitudinal profiling provided a straightforward picture of the existence and effects of hot spots (e.g., lateral tributary inflow) and was helpful to distinguish the mixing signal along the reach. In conclusion, nitrate removal process was sensitive to hydraulic (e.g., strong variation of discharge), biotic (e.g., dense perihyton in summer) and weather conditions (e.g., high temperature and enough radiation). Additional measurements are needed to compare different combinations of hydraulic, biotic and weather conditions. However, it is difficult to disentangle complex influential factors quantitatively due to unexpected meteorological variabilities during measurement campaigns. Hence, continues measurements are important to monitor nitrate removal process throughout the year, albeit it will be time- and labor- consuming. Furthermore, using in-stream hydraulic modelling can advance quantitative analysis.