What unique property of water allows organisms living in lakes to survive in below zero winter conditions ?

Answer 1

This anomalous expansion plays an important role by only freezing the upper layer in lakes and rivers.

During winter months in colder countries the outside or atmospheric temperature is very low - it drops to below freezing - and the upper layers of water in the lakes and ponds start cooling. When the temperature of the surface layers falls to 4 °C, the water body acquires maximum density and sinks down. The water that sinks down displaces water below, and the lower layers of water simultaneously rise up. This also gets cooled to 4 °C and again sinks down.

When the temperature of the water body finally goes below 4 °C, the density or heaviness of water decreases and as a result water does not sink down. The surface water finally freezes at 0 °C while the lower part still remains at 4 °C. The light frozen layer of ice floats on top.