Water level drawdown affects physical and biogeochemical properties of littoral sediments of a reservoir and a natural lake

To determine the influence of water level drawdown on Littoral areas, we compared the temporal and spatial changes in the water column and sediment in the littoral region of a drinking water reservoir and a natural lake. The reservoir (Sooke) experiences more than six meters of seasonal drawdown compared to a nearby, morphometrically and trophically similar lake (Shawnigan) that experiences less than one meter of drawdown. A greater drawdown in Sooke increased the Littoral area and resulted in more Littoral water column mixing, more solar warming, and higher PAR at a greater range of Littoral depths than in Shawnigan. Based on sediment physical and chemical characteristics, sites farthest from shore were most similar, whereas sites in the drawdown exposure zone of Sooke and the upper Littoral area of Shawnigan showed the largest differences. Low macrophyte abundance and loss of fine sediments, nutrients, and organic matter from the drawdown exposure zone in Sooke compared to the equivalent Littoral area in Shawnigan suggest that drawdown enhances sediment erosion and focusing. Element and stable isotope ratios of sediment carbon and nitrogen suggest organic matter in the drawdown zone in Sooke is more allochthonous in origin and is coupled more strongly with deeper sites than in Shawnigan. Organic matter source and distribution also suggests that the Littoral area extends out farther in Sooke than Shawnigan. This study demonstrates that drawdown has the potential to fundamentally change reservoir littoral sediment and biogeochemical characteristics. Understanding how Littoral zones in reservoirs respond to drawdown compared to natural lakes may help water managers make more ecologically informed decisions regarding drawdown impacts on the ecology of littoral zones and water quality.