Water source, climate, and water chemistry combine to influence DOC concentration and DOM quality in Buffalo Pound Lake, Saskatchewan

Freshwater lakes and reservoirs are key components of the global carbon cycle. Dissolved organic matter (DOM) is an important water quality characteristic that regulates physical, chemical, and biological functions in these systems. Elevated DOM quantity, measured as dissolved organic carbon (DOC) concentration, and changes in DOM source and composition (DOM quality), create challenges for water managers already facing deteriorating sourcewater quality due to cultural eutrophication, climate-related uncertainty, and water scarcity. High DOC and variable DOM quality are a concern to drinking water treatment plants owing to their effects on disinfection byproduct formation, added costs for removal, and risks of bacterial regrowth in water distribution systems. In highly managed drinking water reservoirs like Buffalo Pound Lake, Canada, in the Great Plains of North America, understanding the effects of water source, climate, and in-lake water chemistry on DOM quantity and quality is of particular concern. Inflows to this reservoir are dominated by water releases from a large upstream reservoir (Lake Diefenbaker) with episodic influxes of runoff from the local catchment. Sourcewater variability to Buffalo Pound Lake depends on local hydroclimate, which fluctuates through periods of extreme wet and dry conditions. Long-term analyses demonstrated large fluctuations (> 10 mg/L) in monthly DOC concentrations over a 30-year period, and revealed the importance of flows from Lake Diefenbaker and the local catchment, and in-lake nutrient (total phosphorus and ammonium) and solute (sulfate) chemistry, on driving DOC in Buffalo Pound Lake. On a shorter timescale, measurements of DOM quantity and quality along the length of the lake, and across four open-water seasons when Lake Diefenbaker was the primary water source, clearly illustrated the role of internal production on altering DOM quantity and quality from lake inflow to outflow. We observed increases in DOC of up to 1–2 mg/L from the Buffalo Pound Lake inflow to outflow in all years, and several DOM quality metrics suggested a shift toward autochthonous DOM production as water transited through the reservoir. In dry years with greater water residence times, these patterns suggest that long, narrow Buffalo Pound Lake may act similar to a slow-moving river with respect to internal DOM production and processing. This work advances efforts to disentangle long-term drivers of DOC and understand DOM quality dynamics in this shallow eutrophic reservoir and across freshwater systems globally. Our results provide a foundation for DOM quantity and quality forecasting in Buffalo Pound Lake and will inform the design of an ongoing $325M upgrade to the Buffalo Pound Water Treatment Plant.

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