Emerging controls of in-stream uptake at the catchment scale

Nitrate (NO₃^-) and phosphate (PO₄^3-) inputs to rivers are high in Germany and Europe following energy and food production demands, which can cause harm to aquatic ecosystems and jeopardize drinking water supplies. It is known that permanent and non-permanent nutrient uptake can retain significant amounts of NO₃^- and PO₄^3- in river networks, however, there is little knowledge about the mechanistic processes involved and their controlling factors on catchment scales. In this work we apply a data driven analysis using the shape of stable, multi-annual, low frequency concentration-discharge (C-Q) relationships in about 500 German monitoring stations. More specifically, the bending of NO₃^- C-Q relationship was shown to encode uptake efficiency. We systematically address the effects of light and shading, stream ecological status, land-use, hydrological conditions, stream network configurations and chlorophyll a patterns as potential in-stream processing predictors. This assessment allows us to conclude on dominant controls of NO₃^- uptake efficiency across a wide range of landscape types.