A transient but important part of Earth’s cryosphere is lake ice. As the upper layer of water is cooled by cold air temperatures, it becomes more dense than the water below and sinks to the bottom of the lake, pushing warmer, less dense water to the surface. The turnover process happens a number of times until the surface water is cool enough to freeze. This layer of ice then insulates the lake, keeping the deeper water warm enough to remain liquid. 

The ice (and then snow) on top of lakes reflect a lot of sunlight and keep it from penetrating into the water, which slowly kills aquatic plants and lowers the oxygen levels over winter. If the lake stays frozen for too long, oxygen levels can become dangerously low for fish and other aquatic life. On the other hand, if an ice season is short and a lake warms earlier than usual, it can alter the ecological balance of the water and lead to problems such as harmful algal blooms. For this reason, the timing of lake ice formation and melting are considered important indicators of climate change.