Subfossil Simuliidae track past river flow into an industrially contaminated lake

Stoco Lake (Tweed, Ontario, Canada) has a history of industrial contamination and is heavily influenced by inflow from the Moira River. Stoco Lake is frequently affected by nuisance algal blooms (including cyanobacteria), which have largely been attributed to cultural eutrophication. To further our understanding of the environmental dynamics of Stoco Lake, we applied paleolimnological techniques to examine long-term trends in subfossil invertebrate assemblages, geochemistry, and inferred trends in whole-lake primary production from a sediment core representing ~ 70 years of environmental history. We examined past trends in Simuliidae abundance, geochemical variables, and historical river flow records to examine past hydrological conditions in Stoco Lake. Inferred whole-lake primary production and sediment core organic content increased between the late 1950s and early 1970s, likely reflecting increased eutrophication (e.g. from agricultural intensification and increased catchment activities), with some stabilization in the post-1970s sediments coinciding with the installation of a sewage treatment plant. The geochemical composition of the core highlighted the decrease in catchment-derived sediment inputs and an increase in percentage of organic content towards the top of the core. In addition, declines in the proportion of As and Ni coincided with the closure of the upstream Deloro Mine Site in the 1960s. Chironomidae and Chaoboridae (Diptera) assemblages indicate expansion of littoral habitat after the 1960s and a general stability of low-oxygen conditions in the hypolimnion over the past ~ 70 years. Several Simuliidae (Diptera: Nematocera) remains, indicators of fluvial conditions rarely discussed in paleolimnological studies, were present at their highest abundances in the pre- ~ 1975 sediments of the core, but disappeared after the early 1990s. Our findings suggest that changes in river flow (discharge, velocity, or seasonality) may have contributed to recent algal blooms. Overall, this study highlights the potential of Simuliidae in paleohydrological studies to help understand the role of river flow on lake conditions.