Lake ice freezes first at the surface starting at the edges or shoreline for two reasons. Water near the shore is typically shallower and contains less heat than deeper water so it can reach the freezing point faster than deeper water.
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Shallower lakes usually freeze before deeper lakes since shallower lakes contain less water that needs to be cooled down. And, lakes freeze from their perimeter towards the center since there is less water in the shallower areas that needs to be cooled.
If water were most dense as a solid, lakes would freeze from the bottom up, eventually freezing solid. In that case, little or nothing would survive in the lake. Most lakes and ponds don't completely freeze because the ice (and eventually snow) on the surface acts to insulate the water below.
Water freezes from the perimeter of the lake to the center. It happens this way because the water is shallower at the lake's edge so it cools off faster. Water is most dense at 39 degrees Fahrenheit, so when it gets colder than that, the cold, lighter water floats on top of the lake.
Melting of lake ice usually occurs first near the shorelines or near the mouths of streams. At these points of contact with inflowing warm water, the ice melts faster than it does at central lake locations, where most melting is caused by the transfer of heat from the atmosphere.
Thus, the ice layer at lower temperature floats over the water below it. Due to this, the freezing of water into ice occurs continuously from the top towards the bottom.
Do fish die in frozen lakes or in lakes that are partially frozen? Since fish are cold-blooded animals, they can survive because they are able to regulate their body temperature to match their environment. However, they could die if a body of water freezes over completely and remains frozen for an extended period.
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.
If water were most dense as a solid, lakes would freeze from the bottom up, eventually freezing solid. In that case, little or nothing would survive in the lake. Most lakes and ponds don't completely freeze because the ice (and eventually snow) on the surface acts to insulate the water below.
Some species, like koi and gobies, may burrow into soft sediments and go dormant like frogs and other amphibians, but most fish simply school in the deepest pools and take a winter rest. In this resting state, fishes' hearts slow down, their needs for food and oxygen decrease, and they move about very little.
Only the top layer of the lake or river freezes. Underneath the frozen upper layer, the water remains in its liquid form and does not freeze. Also, oxygen is trapped beneath the layer of ice. As a result, fish and other aquatic animals find it possible to live comfortably in the frozen lakes and ponds.
The high concentration of salt in ocean water lowers its freezing point from 32° F (0° C) to 28° F (-2° C). As a result, the ambient temperature must reach a lower point in order to freeze the ocean than to freeze freshwater lakes.
Ice formed over flowing water and currents is often dangerous. This is especially true near streams, bridges, and culverts. Also, the ice on outside river bends, and inlets and outlets of lakes and ponds is usually weaker due to the undermining effects of the faster current.
Since water is good at holding heat, the more water there is, the more heat it will hold. This is why large deep lakes take longer freeze and melt than small shallow lakes. Water freezes from the perimeter of the lake to the center.
If you can see the ground through the ice, then there's going to be warmer water under it. Out in the middle, the ice grows thicker over the cold dark depths. Also, ice always melts away from the shore first.