Spatial and temporal trends in reservoir physicochemistry and phosphorus speciation within Lake Diefenbaker, a Great Plains reservoir, as inferred from depositional sediments

Previous research and observations by area residents suggested that the frequency of algal blooms in the Qu'Appelle arm of Lake Diefenbaker, Saskatchewan, Canada, has been increasing over time. However, limited historical data were available to assess long-term trends in the overall reservoir. Narrow river-valley reservoirs, such as Lake Diefenbaker, are known to display marked longitudinal physicochemical and biological gradients. To assess both spatial and temporal geochemical trends, sediment cores were collected from eight sites along the longitudinal axis of this reservoir and sectioned vertically for analysis of various parameters. Surficial sediments showed an increasing trend in organic carbon, total phosphorus concentrations, and δ15N values, and decreasing trends in δ13C values and in the carbon to nitrogen ratio, with distance down-reservoir. This was attributed to reduced deposition of allochthonous detritus and increased entrainment of nutrients as settled autochthonous materials, as the reservoir transitioned to less dynamic lacustrine conditions. A strong correlation was determined in the Qu'Appelle arm between chlorophyll a concentrations and the more biologically available forms of P (non-apatite inorganic P + organic P) in the sediment profile (r = 0.955, p < 0.001), suggesting an increase in autochthonous production in this region of the reservoir in the mid-1990s, which subsequently plateaued. The spatial and temporal trends illustrated in the sediment core profiles of this study demonstrate that paleolimnological investigations of narrow river-valley reservoirs need to consider geochemical and biological spatial gradients associated with reservoir zonation.