During severe winter in the low temperature zones of the world, the superficial parts of the lakes are frozen, leaving water below. The freezing at the bottom is prevented because : 

The concepts involved in this complicated phenomenon are low conductivity of ice and anomalous expansion of water (water density is highest at $4^0\mathrm{C}$).
When initially the top layer of ice is formed on account of high exposure to low temperature, this ice sheet cuts off heat transfer from water (high temperature) to air (low temperature) on account of low conductivity.