system
Given the following post, respond as if you're commenting on Reddit:
human
If deep water is colder than surface water, then why does the water at the bottom of a lake not freeze before the surface water?
gpt
A few key things to keep in mind:

    - Nearly all heat transfer to/from a lake occurs at the surface
    - It takes increasingly more energy to mix water masses of increasingly different temperatures
    - The maximum density of water occurs at 4º C
    - Less dense masses float on top of more dense masses

So, for a typical temperate, dimictic lake during warm summer months, the surface waters of the lake are much warmer and less dense than deep water.  The difference is usually so extreme that lake water becomes stratified such that the surface waters ("epilimnion") do not mix at all with the bottom water ("hypolimnion").  Moving into autumn, the epilimnion water begins to cool until its temperature equals that of the hypolimion.  When this happens, the lake water begins to fully mix and lake stratification disappears.  As atmospheric temperature continues to fall, moving into winter, the surface water of the lake continues to cool.  However, since water is most dense at 4º C, water at this temperature sinks to the bottom while *colder* water, which is less dense, floats to the top.  Lake stratification becomes reestablished with the coldest water in the epilimnion and warmer (4º C) water in the hypolimnion.  The surface of the lake eventually cools to below 0º C and freezes into a solid; however, water has a unique property in that its solid phase is less dense than its liquid phase.  Thus, ice floats on liquid water.  When the lake begins to warm throughout the spring, the epilimnion reaches 4º C and again mixes with the same-density water of the hypolimnion.  Upon continued warming, the surface waters -- which are now less dense than the 4º bottom water -- become once again isolated by lake stratification throughout the summer.

This is generally the case of most lakes, especially in temperate latitudes.  These lakes are named "dimictic" because they mix twice during the year.  There are many other mixing regimes for lakes and it all depends on their climatic setting and, sometimes, the chemical composition of their water.  Some lakes mix once ("monomictic"), some mix many times ("polymictic"), and some don't mix at all ("meromictic").