Final answer:
During the first phase of fall turnover, the heavy cold water in freshwater lakes sinks and displaces the water at the bottom. Nutrients rise to the surface while oxygen-rich water moves to the bottom.
Step-by-step explanation:
During the first phase of fall turnover in freshwater lakes, the temperature of the lake water cools down to 4 °C. This causes the heavy cold water to sink and displace the water at the bottom. The oxygen-rich water at the surface then moves to the bottom, while the nutrients at the bottom rise to the surface. This turnover process helps to redistribute nutrients and oxygen within the lake.