Seasonal Temperature and Circulation Patterns in a Hybrid Polar Lake, Great Bear Lake, Canada

Great Bear Lake (GBL) is the largest lake entirely within Canada and the largest polar‐type lake in the world. It holds cultural and sustenance value to the Délı˛ne Got'ine. However, its baseline physical limnology and how this may be altered by climate warming and anthropogenic stressors have received little attention. To explore the roles that surface heat exchange, wind, seasonal ice cover, and thermodynamic constraints play in the seasonal progression of ventilation and stratification of GBL, we report data from two 2008‐09 moorings, satellite‐derived lake surface temperatures, and observations made in 1964. Three spatially constrained processes regulate seasonal patterns of ventilation and stratification. Mid‐lake temperatures remain below the temperature of maximum density (TMDsurf = 3.98°C) throughout the year. In this area, solar radiation drives vertical convection while cooling develops stratification. Waters along the perimeter of the lake and within its five major arms do rise above TMDsurf in summer and stratify. It follows that mixing between the inner and outer domains form water at TMDsurf to create a convergent sinking zone or thermal bar. Because TMD decreases with increasing pressure, ventilation in the deepest region of the lake (McTavish Arm, Zmax = 446 m) requires wind‐aided downwelling to force cold surface water to a depth where it lies closer to the local TMD, triggering thermobaric instability, which then drives full‐depth ventilation. These patterns of ventilation and stratification constrain the availability of light and nutrients, therefore setting rates of biogeochemical processes, and regulating the lake's overall response to climate change. Plain Language Summary Great Bear Lake (GBL), crossed by the Arctic Circle, is the world's largest polar‐type lake. It has three unique areas: the middle, where temperatures stay below 3.98°C, with dense water sinking in summer and stratification occurring in winter; the five protected arms, where temperatures exceed 3.98°C, creating stratification in both summer and winter, and interactions between the cooler main body and warmer arms form vertical barriers; and McTavish Arm, where depths reach 446 m show extra layering due to water compressibility, requiring wind convection for mixing. Our study examines these processes through comprehensive temperature data from 2008, 2009, and 1964, alongside satellite images, to understand the lake's ecology and response to climate change. Key Points