At 4 degrees C or 39 degrees F, water is at its most dense. Ice will float in cold water. This leads to temperate lake oxygenation in deep lakes. As the lake water cools on the surface, when it reaches 4 degrees, it sinks to the bottom. The lake water on top is oxygenated. Warmer water comes to the top, cools and sinks. This continues until the entire lake has cooled to 4 degrees. Then the surface starts to freeze. The ice stays on the surface.
More shallow lakes may mix throughout the year, but in cold climates, they can also freeze right through. This has consequences for which species can survive the winter.
This lake turnover is in contrast to warmer areas. If the lake temperature never drops to 4 degrees C, the lake will not turn over. An oxygenated zone sits at the surface, but the lower levels of the lake use up the oxygen, and will be anaerobic. There are very different populations in temperate lakes that turn over and more tropical ones that don't.
In the spring, at least in the northern hemisphere, the surface ice will start to melt on temperate lakes. Again it will want to sink when it reaches four degrees. The surface of the lake will keep warming and the lake will turn over and oxygenate again.
https://www.cleanlakesalliance.org/lake-turnover/
Iron 2023: 4