What causes lake "turnover" to occur?

Lake turnover is a process that occurs primarily in temperate regions, particularly during the transition between seasons, such as in the fall and spring. The phenomenon is largely driven by changes in temperature, specifically when the surface water of a lake cools.

During the warmer months, the surface water of the lake is warmed by the sun, while deeper layers remain cooler. As the temperature of the surface water drops, particularly in the fall, it eventually reaches the same density as the water below it. When this occurs, the cooler and denser water tends to sink, while the warmer water rises to the surface. This movement facilitates the mixing of the water layers, redistributing nutrients, oxygen, and other solutes throughout the lake.

In contrast, increased evaporation, heavy rainfall, and wind action do not directly cause the denser water to sink; they may influence surface conditions but do not promote the same convective mixing associated with temperature changes that drive turnover. Ultimately, it is the cooling of the surface water that initiates this critical ecological process in lakes, explaining why the identified answer is valid.