Why do lakes undergo spring and fall overturns?

Lakes undergo spring and fall overturns (also known as vertical mixing or circulation) due to seasonal changes in water temperature and the resulting shifts in water density.12

In temperate regions, lakes often exhibit a dimictic pattern, meaning they circulate twice per year. This process is driven by the following dynamics:12

• Fall Overturn: During summer, the lake is stratified, with a warm, less dense layer (epilimnion) floating atop a colder, denser layer (hypolimnion). As autumn temperatures drop, the surface water cools and becomes denser. Eventually, the surface water reaches a density that allows it to sink and mix with the deeper layers, aided by wind action, until the entire water column reaches a uniform temperature.2
• Spring Overturn: After the lake has been covered by ice or cooled during winter, the surface water begins to warm in the spring. As it approaches the temperature of maximum density (approximately 4 °C or 39 °F), the water becomes denser and sinks, again facilitating complete circulation throughout the lake.12

These overturns are critical for the inland water ecosystem, as they replenish dissolved oxygen in deeper waters and redistribute nutrients throughout the lake.23