Why is it warmer at the bottom of a reservoir in winter than on land?

In winter, the air temperature drops to minus forty degrees, sometimes more. This is a very cold temperature, in such conditions, living organisms do not survive. Small reservoirs freeze completely, and large ones do not have time. Therefore, large water bodies freeze only on the surface and to a certain depth. Further, the water remains in a liquid state. But water freezes at zero degrees Celsius, which means it has a temperature of about zero. She does not have time to freeze all at once, but slowly increases the volume of ice on the surface. It turns out that at the bottom of the reservoir is about zero degrees, and on land the temperature is lower.