Scaling of biochemical and hydraulic controls on nitrate uptake and removal in streams

• Applicants: Professor Dr. Andreas Lorke; Professor Dr. Michael Rode
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 538042399

Nitrate pollution is a global threat to coastal marine and freshwater ecosystems. Current estimates suggest that about 13% of all land-based nitrate sources, 52 % of which are of anthropogenic origin, are removed by denitrification in rivers. Although the capacity of stream ecosystems to remove nutrients from the water column has been well documented, most existing field data are from small streams. Extrapolation to river networks and larger-scale estimates of nitrate removal rates are based on scaling relationships, which mechanistic basis is poorly understood and for which experimental validation is largely lacking. In this project, we plan to investigate the physical processes that control nitrate uptake in streams. We hypothesize that for the range of nitrate concentrations in anthropogenically impacted streams, the effective biogeochemical reaction rate for nitrate removal depends on flow depth, but also on the morphological complexity of the stream. Streambed and bank roughness promotes faster turbulent transport across the concentration boundary layer at the streambed, increases hyporheic exchange rates and eventually leads to higher denitrification rates in morphologically complex reaches, compared to uniform channels. We will validate this hypothesis for the first time in field measurements by combining measurements of nitrate fluxes and turnover in stream and in interstitial waters with detailed characterizations of channel morphology and turbulent flow fields. We will study reaches across three stream orders and with contrasting morphology (structural complexity). Application of newly available measurement tools and technology, including Hyporheic Flux Meters, autonomous sensors and laser-scanning facilitate a new quality of reach-scale assessments of nitrogen removal in streams and rivers. The novel process-based understanding and gap-filling data set on reach-scale nitrogen dynamics in larger streams will be used to establish mechanistic relationships between mass exchange and the removal rates in stream water and sediments as a function of bulk morphological parameters. The findings potentially contribute to improved management, restoration and mitigation strategies and to improved estimates of nitrate removal rates in river networks at larger scale.

• DFG Programme: Research Grants