The air temperature continues to decline with the onset of winter, and the surface layer of the lake freezes. The water just below the ice can become supercooled, decreasing to below 4°C. This water is less dense than the water in deeper layers and once again a thermocline forms. With spring warming, the ice melts as the temperature rises and the thermocline is disrupted. The mixing brings nutrients locked in the lower “barrel” to the surface. This allows nutrients to enter various food webs and leads to the development of algal and microorganism blooms.