As surface water in a lake cools from 4 °C to 0 °C, the water expands and becomes less dense. Why is this change in the water beneficial to the organisms living in the lake? As the water cools to freezing, spaces form between water molecules, providing microhabitats for small organisms. Frozen surface water insulates the water below it, maintaining a habitat where organisms can overwinter. Cooler water is less viscous than warmer water, making it easier for organisms to move through the water. Less dense water holds more dissolved compounds, keeping water from freezing so organisms survive winter.