Final answer:
In springtime, the temperature of the surface water becomes heavier and sinks to the bottom, causing the spring turnover. During the summer months, the lake water stratifies with the warmest water at the lake surface.
Step-by-step explanation:
In springtime, air temperatures increase and surface ice melts. When the temperature of the surface water begins to approach 4 °C, the water becomes heavier and sinks to the bottom. The water at the bottom of the lake is then displaced by the heavier and denser surface water and, thus, rises to the top. As that water rises to the top, the sediments and nutrients from the lake bottom are brought along with it. This is called the spring turnover. During the summer months, the lake water stratifies, or forms layers, with the warmest water at the lake surface.