Similar pattern of diurnal nitrate retention in different stream orders: seasonal to sub-daily insights from high-frequency data

High-frequency nitrate-N () data are increasingly available, while accurate assessments of the importance of in-stream retention processes is deviating stream orders are still unclear. In this presentation we hypothesize that similar diurnal nitrate uptake pattern exists in different stream orders and that these patterns can reveal insights into the dominance of uptake processes across stream scales. To test this assumption continuous 15-min estimates of  retention was derived in a 1^st stream and  a 6^th-order reach of the lower Bode River network (27.4 km, central Germany) using a one station method for the 1^st order agricultural headwater stream and a two-station approach for the 6^th order stream applying a data fusion framework capturing river hydraulics and their impacts on solute signal propagation through river hydrodynamic modelling (Yang et al. 2023). This methodological setting was used for long-term sensor monitoring data from 2015-2023 capturing highly deviating hydrological (normal and drought) and stream morphological conditions. The unique  retention estimates revealed very similar characteristic diurnal variation of  retention pattern. Three very similar clusters of diel uptake patterns were identified in both streams, potentially representing changes in dominant autotrophic and heterotrophic   retention processes. While the dominating N-uptake clusters were similar in both systems, their seasonal occurrence showed significant differences between the two streams. For example, clusters reflecting assimilatory N-uptake dominated in the 1^st order stream in all years and seasons. In the 6^th order reach autotrophy-characterized clusters mostly occurred during early seasons, which are then followed by a shift to heterotrophic-dominated uptake pattern during summer- autumn low-flow periods. In addition, dominance of autotrophic   retention extended more widely across seasons during the drought years. In contrast, the 1^st order stream showed relevance of both autotrophic and heterotrophic uptake even in the winter month due to the stimulation by elevated spring water temperature. The analysis of characteristic uptake clusters and the suggested framework can be flexibly transferred across sites and scales, thereby complementing high-frequency monitoring to identify in-stream uptake processes and to inform river management.

Reference

Yang, X., Zhang, X., Graeber, D., Hensley, R., Jarvie, H., Lorke, A., Borchardt, D., Lif, Q., Rode, M. (2023) Large-stream nitrate retention patterns shift during droughts: Seasonal to sub-daily insights from high-frequency data-model fusion. Water Research, 243, 120347.