High supply, high demand: A unique nutrient addition decouples nitrate uptake and metabolism in a large river

Abstract

Our current understanding of the relationship between nitrate (NO3-) uptake and energy cycling in rivers is primarily built on studies conducted in low-nutrient (NO3- < 1 mg-N L-1), small (discharge < 1 m3 s-1) systems. Recent advances in sensor technology have allowed for continuous measures of whole-river NO3- uptake, allowing us to address how the relationship between nutrient uptake and metabolism changes over time and space during a nutrient addition in a large river. We treated a six-month controlled nitrogen (N) waste release into the Kansas River (conducted by the City of Lawrence, KS) as an ecosystem-scale nutrient addition experiment. We deployed four NO3- and dissolved oxygen sensor arrays along a 33 km study reach from February to May 2018 to continuously monitor diel NO3--N and stream metabolism. We then evaluated NO3- uptake using the extrapolated diel method and modeled stream metabolism using the single station method. We found the highest uptake rates closest to the nutrient release point (866 g-N m-2 d1), despite high NO3- supply (4.36 mg-N L-1). Net ecosystem productivity was increasingly autotrophic with distance from the release, with the highest respiration rates observed closest to the release point (7.09 g-O2 m-2 d1). However, uptake was decoupled from metabolism metrics, likely due to fine-scale hydrologic and biotic factors. Overall, our work sheds light on the ability of large rivers to retain and transform nutrients, while demonstrating that the fine-scale mechanisms that regulate nutrient retention in large rivers are still largely unknown.