Spatial and temporal patterns in physical properties and dissolved oxygen in Lake Diefenbaker, a large reservoir on the Canadian Prairies

Lake Diefenbaker (LD) is a multi-purpose deep storage reservoir with complex longitudinal zonation. Despite the importance of LD to Saskatchewan, the last comprehensive evaluation of this large reservoir was completed 30 years ago. Therefore, an assessment of key features (patterns in stratification, turbidity, and dissolved oxygen (DO)) was undertaken during the ice-free period of 2011-2013 to characterize conditions that may affect water quality, including climate change. In addition, these features were compared with the expectations of the longitudinal zonation concept (LZC) of Kimmel and Groeger (1984). The reservoir was dimictic, and summer mixing depths (10-20 m) were similar between a drought year (1984) and the current study. Sections of the hypolimnia became hypoxic, or were sub-optimal for cold and cool water fish (≤ 5 mg/L). Volumetric hypolimnetic oxygen depletion rates were 0.034 to 0.12 mg L/day and overlapped with other lakes and reservoirs. Sections of LD would be prone to hypolimnetic anoxia had thermal stratification been prolonged for another 2 to 4 weeks. LD experienced three years of above average peak flows. The peak flow of 2013 brought high turbidity (e.g., 200 NTU) and organic matter that resulted in a metalimnetic DO minimum. Spatial patterns in thermal stratification and turbidity (but not DO) followed the expectations of the LZC. Drought conditions of 1984 resulted in reduced hypolimnetic volumes, anoxic conditions and algal blooms. Therefore, as the Northern Plains continue to warm, monitoring of Lake Diefenbaker during drought periods is warranted, especially when droughts follow high flow events.