Water-level fluctuation enhances sediment and trace metal mobility in lake littoral.

Water-level fluctuation (WLF) is a widespread management action in lakes and reservoirs whose impacts on contaminant fate have seldom been investigated. We used near shore hourly measurements (n = 2122) of turbidity (contaminant proxy) and water velocity (sediment resuspension proxy) to track high-frequency contaminant dynamics during a 0.6 m change in water level observed in autumn 2017 in a large French lake. Simultaneously, discrete trace metal measurements highlighted that trapped sediment was more contaminated and finer than surficial sediment supporting that suspended particles (measured by turbidity) were a preferential medium for contaminant mobility. General additive models involving tensor products revealed the enhancement of wind-speed and river discharge effects on turbidity with water draw down. The decrease of the explained deviances by the models over time-lags indicated short time-scale response of turbidity to external forcing. Three of the four major turbid events occurred at the lowest water-level and were concomitant of sediment resuspension as well as precipitation events and/or river flood suggesting a complex interplay among in-lake and watershed processes at controlling sediment mobility during the WLF. These results shed in light that WLF can affect lake littoral hydrodynamic cascading up to the enhancement of contaminant mobility. Sediment resuspension may be an overlooked feature of WLF increasing contamination risk and exposure for littoral organisms with widespread ecological consequences due to the large number of water-level regulated ecosystems.