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Abstract

In recent decades temperate lakes have experienced notable changes in their thermal regimes, including extended durations of summer stratification and shorter ice-covered periods. In addition, recent studies have shown that the presence of low levels of salinity can have dramatic effects on the circulation of ice-covered lakes. For example, the exclusion of salt from ice as it forms can result in a relatively fresh surface layer immediately after ice melt. The density contrast between the ice-melt and the more saline water beneath it can be sufficient to hinder, or even prevent, turnover. These changes have far-reaching ecological consequences, influencing oxygen availability, nutrient cycling, and biological productivity. Base Mine Lake Demonstration (BML) in Alberta, Canada (57°1'N, 111°37'W) is the first full-scale demonstration of an end pit lake in the oil sands industry. BML was formed by the backfilling of a previously mined-out pit with fluid fine tailings, followed by the addition of fresh water from a nearby reservoir. The evolution of the resulting brackish lake has been actively monitored since its commissioning in 2012. In the present study we investigate the inhibition and dynamics of spring turnover, and the establishment of summer stratification, in BML. Field observations from spring and summer of 2016 and 2017 are analyzed using a conceptual model, and compared with simulations from the one-dimensional General Lake Model (GLM), to assess changes in the temperature stratification during notable wind events. Results indicate that although significant mixing occurs after ice-off, complete turnover is not achieved; instead, the lake experiences long periods of upwelling/downwelling and short periods (hours) of partial spring turnover. Additionally, the dynamics at BML are influenced by the expression of saline pore water from the settling of the underlying tailings. These findings highlight both the complex mixing dynamics within BML and the limitations of using a one-dimensional model to represent this system.