Fall turnover in a lake happens when surface water cools to 4 degrees Celsius and then:

1. Turnover mixes the layers of a lake and redistributes oxygen and nutrients. 2. As air temperatures drop in the fall, the temperature of the lake water cools to 4 degrees Celsius. 3. This causes fall turnover as the heavy cold water sinks and displaces the water at the bottom. 4. The water at the bottom of the lake is then displaced by the heavier surface water and rises to the top. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 35 'Ecology and the Biosphere', sections 35.1-35.5_