During the winter months, the snow cover on a lake reflects sunlight, which insulates the lake. The snow cover maintains a certain heat in the water; the lake neither loses nor gains heat during this time.
Sediment on the bottom of the lake helps heat the lake a little bit during the winter months because the sediment has stored heat from the summer.
When the weather warms up, the ice starts to melt, which enables the sunlight to penetrate the ice. The water underneath the ice begins to warm, and the ice melts from the bottom up, explains Climate.umn.edu.
As the ice erodes because of the heat of the Sun, the ice changes into vertical crystals, referred to as candles, which conduct light even more efficiently. The ice will look black because it is not reflecting nearly as much sunlight as it was before changing into the candle structure.