“In most lakes there is enough oxygen under the ice that they survive the winter.” It's like having a bank account, he says: The fish gradually draw down their banked oxygen and hope there's enough to get through to spring. If not, some of them die.
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In water that is around the freezing point, a person is likely to survive only 15 to 45 minutes with flotation and possibly up to an hour or so with flotation and protective gear before the brain and heart stop (Table 1). The surface temperature of Lake Superior in early to mid-summer is about 40 to 50 F.
Most lakes and ponds don't completely freeze because the ice (and eventually snow) on the surface acts to insulate the water below. Our winters aren't long or cold enough to completely freeze most local water bodies. This process of lakes turning over is crtically important to the life in the lake.
Surface Temperatures. —During the winter, from the latter part of December to the breaking up of the ice in the spring, the temperature of the water under the ice is 32 F. The water then warms at a uniform rate to 72 F.