Nutrient Chemistry in the South Saskatchewan River Basin: Drivers of Spatial Variation and Long-term Change

The South Saskatchewan River Basin (SSRB - 172,900 km²) in Alberta, Canada encompasses the Bow River, Oldman River and Red Deer River watersheds. The SSRB transitions from mountainous headwaters in the Canadian Rocky Mountains through to semi-arid prairies in its lower reaches and is subject to a number of anthropogenic pressures. In this study, we examined long-term records (i.e., 1987-2017) of nitrogen (N) and phosphorus (P) concentrations, ratios and loads at 14 river stations within the SSRB. Nutrient concentrations and ratios in the SSRB exhibited substantial seasonality with distinct ice (Nov-Mar), open water (Jul-Oct) and freshet (Apr-Jun) chemistry. In addition to transitional changes in nutrient chemistry from upper to lower reaches, we observed pronounced spatial anomalies in N and P chemistry downstream of the city of Calgary and the Alberta badlands respectively. Elevated total N (TN), dissolved inorganic N to TN ratios (DIN:TN), and TN to total phosphorus ratios (TN:TP) downstream of Calgary were attributed to wastewater discharges to the Bow River. Variability in TP across the SSRB was driven largely by variability in suspended sediment concentrations with areas of higher total suspended solids (TSS) exhibiting higher particulate P and TP concentrations, and lower TN:TP and total dissolved P to TP ratios (TDP:TP). This was particularly prevalent at sites in the Red Deer River downstream of the Alberta badlands, a region of high sediment concentrations. Trend analysis identified widespread increases in nitrate concentrations across the SSRB and concurrent increases in TN:TP and DIN:TN ratios. Importantly, by examining nutrient chemistry over broad spatial and temporal scales, this study identifies a system undergoing widespread change and forms a basis for further study and effective management of the regions water resources.