Increasing biologically available nitrogen in the biosphere alters nutrient dynamics across landscapes and aquatic systems and can influence ecosystem metabolism, denitrification, and assimilatory uptake in river systems. In the Kootenai river, small spatial changes in land-use from surficial mining practices increased nitrate loading by 10-fold or more. Phosphorus concentrations are near detection limits frequently in this oligotrophic river, thus the nitrate loading can skew the N:P ratio in excess of 200:1. We measured dissolved oxygen and nitrate concentrations at 2 locations separated by 17.5 km in the Kootenai River at a 10-minute timestep for 3 years during the summer season. River management in this section includes P addition immediately downstream of the upstream site. Subsequently we estimated daily ecosystem metabolism for both sites, and 10-minute nitrate uptake for the reach between the 2 sites. We assessed temporal differences at annual, seasonal, and daily timescales. Gross primary production (GPP) increased downstream of the P addition but not upstream. The timing of elevated GPP and nitrate uptake align with the start and end of P addition. Phosphorus addition increased GPP and nitrate uptake, and the duration of elevated GPP and nitrate uptake. Management practices can benefit ecosystem functions beyond what they are intended to support.