When the temperature reaches 0˚C (32˚F), the top layer of the lake freezes and turns into ice. In solid form, water molecules are less dense and thus ice floats on the top of the lake.
This solid barrier prevents any wind-induced mixing as well as protects the relatively warm water underneath. Furthermore, light is also blocked so that photosynthesis is inhibited during this period. As the lake water is separated from the atmosphere, no direct oxygen diffusion occurs.
All these factors together causes a more even distribution of nutrients and gases in a lake in winter.
