River management and excessive nitrate loading influence nitrate uptake in a large transboundary oligotrophic river

Biologically available nitrogen from human activities have altered nutrient dynamics across landscapes and aquatic ecosystems. Small spatial changes in land use and river management, may contribute to altered nutrient dynamics and influence denitrification and assimilatory uptake in river systems. Human actions can influence the stoichiometry of rivers. Construction of Libby Dam and the creation of the transboundary Koocanusa Reservoir has resulted in sequestration of approximately 60%–80% of the phosphorus entering the reservoir. Recent and ongoing expansion of surficial mining operations in one tributary upstream of Koocanusa Reservoir, the Elk River, has increased nitrate loading tenfold or more to Koocanusa Reservoir and to the Kootenai River. The combination of excessive nitrate loading and decreased phosphorus availability has skewed the N:P ratio to greater than 200:1 in both the river and reservoir. To address how this altered stoichiometry influences nitrogen spiraling in a large river, we estimated nitrate uptake over 16 years in five reaches of the Kootenai River. Reaches spanned 224 river km and types were based on natural and anthropogenically-influenced geomorphology. Although we documented a decline in nitrate moving longitudinally downstream indicating that nitrate is being used by the biota, the magnitude and timing of areal nitrate uptake varies among the reaches. Areal nitrate uptake did not differ between the early years with lower nitrate concentrations and the later years with higher nitrate concentrations suggesting that the Kootenai River is nitrogen saturated. Phosphorus addition, used as a management tool to offset P sequestration in the reservoir, increased areal nitrate uptake and extended the period of higher areal nitrate uptake. Without increases to the ecosystem functions of nitrogen transformation and removal, the ecosystem becomes saturated and the entire load is being transported downstream.