The lower density of ice than liquid water at 0°C compels water in ponds and lakes to freeze from surface to the downward direction. Water attains the temperature of 4°C by the fall of temperature in the surrounding.
As the outer atmosphere becomes further cold, the water at the surface becomes less dense. This less dense water below 4°C stays on the top of slightly warm water underneath. A stage reaches when it freezes. This layer of ice insulates the water underneath for further heat loss. Fish and plants survive under this blanket of ice for months.