Many fish and aquatic plants can survive a cold winter because the layer of
ice that forms at the top of the lake insulates the water below and prevents
the lake from freezing solid. what unique property of water contributes to
this effect?

Water expands on cooling below 4 degrees Celcius.

Explanation:

This means the water becomes less dense because the same mass of water occupies more volume. This is called the ‘water anomaly’ because other liquids reduce in volume with temperatures.  Ice will, therefore, float on very cold water. In cold regions this ice sheet formation in lakes and oceans is significant in  insulating the waters below from losing more heat. Consequently, the waters below the ice sheet remain liquid maintaining the aquatic life below the ice.

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