Why does a northern lake turn over in the spring?

1. Water reaches its greatest density at 4 degrees Celsius, not at its freezing point. 2. Through winter the lake is capped with ice and the deepest water sits near 4 to 5 degrees. 3. In spring the air warms and the surface ice melts. 4. As the surface water warms toward 4 degrees it becomes denser than the water below it. 5. Being denser, it sinks, and the water it displaces rises to the top. 6. The rising water carries sediments and nutrients up from the lake bottom. 7. Saying the surface cools to 0 degrees describes winter, not the spring warming. 8. Bottom water does move, but it rises because it is displaced, not because it warms. _Source: OpenStax Biology 1e (CC BY 4.0), Chs 44 and 46, section 44.2 Biogeography_