NUTRIENT DYNAMICS AND WATER QUALITY OF A LARGE RESERVOIR IN THE CONTEXT OF FLOW VARIABILITY

Abstract

Lake Diefenbaker is the largest reservoir of the South Saskatchewan River, located in Saskatchewan, Canada. The reservoir formed in 1967 by the construction of two earth-filled dams, the Gardiner and Qu’Appelle dams. As a major source of water for hydropower generation, irrigation, recreation and with flood control functionality, it is the most economically important waterbody to the province of Saskatchewan. The reservoir is long (181 km) and relatively narrow (6 km at widest) with the maximum and mean depth of 58 m and 22 m, respectively. At its full supply level (556.8 meters above sea level), the reservoir can store up to 9.4 km3 of water. Studies of nutrient mass balance and long-term evaluation of nutrient transport are very important for understanding nutrient dynamics within watersheds and water quality management of receiving waterbodies. Using a multi-method approach, I developed comprehensive nutrient budgets for the 2011–2014 period for Lake Diefenbaker. I also conducted a multi-decadal (1970–2014) examination of nutrient transport patterns from upstream tributaries to the reservoir and nutrient/sediment export patterns from the reservoir. My four-year, comprehensive nutrient budget revealed that the retention of nitrogen (N) and phosphorus (P) were relatively different in this reservoir. The four-year mean retentions of total nitrogen (TN) and total phosphorus (TP) were 10% and 87%, respectively. While inter-annual variation of P retention was minor, N retention ranged widely between 2011 and 2014. My results show that both TN and TP loading into the reservoir increased during 1970–2014. Although TN concentrations (flow-normalized and non-flow-normalized) and TP concentrations (non-flow-normalized) of the inflow increased, the flow-normalized TP concentrations of inflow decreased. Increasing flows and more precipitation as rainfall during the early summer and late spring are likely responsible for increasing nutrient loads and increasing trend of non-flow-normalized TP concentrations. Long-term nutrient imports and exports are indicative of significant increases in TN:TP ratios during the past two decades. I believe that this is most likely due to greater transport and input of anthropogenically sourced N within the river basin compared to anthropogenically sourced P, shorter water residence time in recent years caused by the occurrence of extreme flows and rainfall, and lower efficiency of nitrogen removal in the upstream areas due to changes in the watershed (e.g., loss of wetlands).