High Supply, High Demand: A Fertilizer Waste Release Impacts Nitrate Uptake and Metabolism in a Large River

Current understanding of the relationship between nitrate (NO3-) uptake and energy cycling in lotic environments comes from studies conducted in low-nutrient (NO3- < 1 mg-N L-1), small (discharge <1 m(3) s(-1)) systems. Recent advances in sensor technology have allowed for continuous estimates of whole-river NO3- uptake, allowing us to address how the relationship between nutrient uptake and metabolism changes over time and space in larger rivers. We used a six-month, controlled nitrogen (N) waste release into the eighth order Kansas River (USA) as an ecosystem level nutrient addition experiment. We deployed four NO3- and dissolved oxygen sensors along a 33 km study reach, from February to May 2018, to assess the spatiotemporal relationship between nutrient uptake and stream metabolism during the waste addition. Contrary to our prediction, we did not find evidence of uptake saturation despite an extreme increase in nutrient supply during winter, a period of generally lower biological activity. Although high uptake rates were observed across the study reach, they were uncorrelated to gross primary production. Overall, despite winter temperatures, NO3- uptake rates were high compared to small streams and rivers. We provide evidence that large rivers can be effective ecosystems for retaining and transforming nutrients, while showing that the fine-scale mechanisms that regulate nutrient retention in large rivers are still largely unknown.