Recent anecdotal evidence has suggested that Lake Diefenbaker, a large reservoir in southern Saskatchewan, Canada, has been experiencing an increased frequency and severity of algal blooms, suggesting significant alterations in water quality. Due to limited historical water quality monitoring, a paleolimnological investigation of Lake Diefenbaker sediments was conducted in order to interpret spatial and temporal trends in the physicochemistry of sediment cores collected from mid-channel locations along a spatial gradient. Total phosphorus and three sediment species of phosphorus (apatite inorganic phosphorus, non-apatite inorganic phosphorus (NAIP) and organic phosphorus (OP) were measured along the vertical profile of the cores to interpret nutrient loading trends. Trends in historical primary productivity were inferred based on total organic carbon, total nitrogen, δ15N values and the organic carbon to nitrogen (C:N) ratios. In addition, sub-fossil biological remains of diatoms and chironomid larvae were isolated from selected subsections of the sediment cores and identified to assess shifts in community compositions and to infer historical changes in reservoir water quality conditions. Up-reservoir sampling locations showed consistent concentrations of total phosphorus and the three species of phosphorus, organic carbon, nitrogen, δ15N values and C:N ratios in the vertical profile of collected sediment cores. This suggested relatively consistent nutrient loading and primary production in the up-reservoir regions for the period represented by the core profiles. Down-reservoir sediment cores showed an increasing trend in total phosphorus concentration, mainly in the more biologically available NAIP and OP fractions, and enrichment in organic carbon, nitrogen and 15N in more recently deposited sediments. This combined with a decreasing C:N ratio in more recent sediments suggests increased nutrient loading to the sediments and increasing primary productivity at down-reservoir sites. Strong correlations between sediment total organic carbon content and the more biologically available sediment phosphorus fractions (NAIP and OP) suggest that phosphorus deposition patterns are strongly influenced by primary productivity within the reservoir. The compositions of diatom and chironomid communities were consistent over time at up-reservoir locations, with Stephanodiscus parvus dominating the diatom communities and Procladius sp. dominating the chironomid communities. This suggests relatively stable and consistent trophic status and primary productivity in this region. Conversely, major shifts occurred in the diatom and benthic chironomid communities at down-reservoir locations. These shifts were consistent with typical reservoir ontogeny. A shift in dominance from Stephanodiscus parvus to Asterionella formosa, Tabellaria flocculosa and Stephanodiscus medius followed by a shift to Aulacosiera ambigua suggests an initial period of eutrophy, followed by a period of mesotrophy, and finally a transition into a more productive system in recent years. The increase in relative abundance of the chironomid tribe Tanytarsini suggests rising levels of organic matter sedimentation, likely due to increased autochthonous primary productivity. Low chironomid head capsule counts were observed at all locations within Lake Diefenbaker. Despite the low head capsule counts, a 10-day whole-sediment toxicity test using Chironomus dilutus, revealed that Lake Diefenbaker sediments, in proximity to two potential point sources of contamination (aquaculture and municipal discharge), as well as two reference locations, were not of toxicological concern. Despite the lack of toxicity associated with Lake Diefenbaker sediments, it was concluded that Lake Diefenbaker sediments are likely poor habitat for chironomids. This study suggests that primary productivity and nutrient status has remained relatively constant at up-reservoir locations for the period represented by the collected cores. This is likely due to up-reservoir locations being more riverine than down-reservoir regions, which results in increased turbidity causing phytoplankton populations to be light limited. Primary productivity appears to increase spatially with increased distance down reservoir, likely due to a gradual transition to more lacustrine environments, resulting in less turbidity and less light limitation. Temporally, down-reservoir locations appear to be showing increases in primary productivity and nutrient entrainment in sediments. This is likely a result of increased nutrient availability over time and entrainment of these nutrients in sediments at down-reservoir locations by primary producers.