Reconstructing diatom and chironomid assemblages to infer environmental spatiotemporal trends within Lake Diefenbaker, a narrow river-valley reservoir on the Canadian Prairies

Lake Diefenbaker (Saskatchewan, Canada) is a large reservoir in the southern Canadian Prairies. However, limited monitoring data were available to assess long-term trends in environmental quality. Therefore, a paleolimnological study was conducted to investigate temporal trends in the trophic status of the reservoir. Sediment cores were collected along the length of Lake Diefenbaker, and sediment subsections were analyzed for sub-fossil remains of diatoms and chironomid larvae. The compositions of diatom and chironomid communities were consistent over approximately the previous decade at up-reservoir locations, with a meso-eutrophic species, Stephanodiscus parvus, dominating the diatom communities. This taxon also dominated the early down-reservoir diatom communities along the main flow of the reservoir. However, during the 1980s, dominance of the diatom assemblage shifted from S. parvus to a combination of Aulacoseira ambigua and Tabellaria flocculosa in the majority of down-reservoir regions. Based on total phosphorus concentration optima for these species, this shift suggests a reduction in total phosphorus both spatially and temporally. However, these observed shifts could also have been due to increased water clarity or changes to thermal stratification regime both spatially (from up- to down-reservoir) and over time down-reservoir. An increase in relative abundance of the chironomid tribe Tanytarsini, both with distance down-reservoir and over time, was likely due to spatial and temporal changes in benthic habitat (i.e., sediment character). Results highlight the need to interpret temporal trends within the context of spatial gradients in physical processes, chemistry, and biology along the longitudinal axis of narrow river valley reservoirs.