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The first thing you need to know is that water is it's most dense at 39 degrees F. Water warmer than 39 and cooler than 39 is less dense.
Let's start the cycle in mid summer. In summer, the water at the top of the lake is likely in the 70's, and the water at the bottom of the lake is likely in the low 50's, because that's what the ground / groundwater temp is in northern Wisconsin. There is likely a 'thermocline' in the lake, which means a sharp break where the top sun warmed water and cold bottom water meet. Wind and wave action only mix the warm top layer, and the bottom cold layer is essentially lifeless, as the oxygen gets depleted and isn't renewed by wave action or weed growth. The reason the colder water is at the bottom is that it's far more dense than the warmer water above so it stays on the bottom.
In fall, as the top layer cools, it will cool to a point where it approaches and goes lower than the temperature of that bottom layer. When that happens, the colder more dense water will sink, therefore the top layer will cool and sink or mix with that previously lifeless cold bottom layer. That's called 'fall turnover' and occurs in northern Wisconsin sometime in late September to very early October when the surface water cools to a point in the low to mid 50's. Obviously smaller shallower lakes will cool faster than large deep lakes, so all don't 'turn over' at the same time. Following this 'turnover', the water temp in the entire water column is the same from top to bottom. Fish that couldn't venture into the deeper oxygen depleted water in the summer can (and do) now use the entire water column.
As fall progresses, the water will cool to 39 degrees and beyond. When this happens, the 39 degree water will remain on the bottom (remember 39 degrees is the most dense) and the cooler water will remain up at the surface, forming ice when it cools to 32 degrees. Therefore in winter you will also have a temperature differential, but it isn't as great as summer. Summer differential from top to bottom may be up to 30 degrees F difference. In winter that difference is 7 degrees from top to bottom of the water column.
Come spring, the ice melts and the sun warms the water to 39 degrees (usually that occurs the day of or day after ice off) and the entire water column once again mixes and is the same temperature throughout. That's called 'spring turnover'. Obviouisly with a much smaller temperature differential it isn't as noticable as fall turnover, nor does it have as much affect on the fish. Following spring turnover, the sun will continue to warm the surface water and the colder water (39 degrees) will remain on the bottom, where it slowly warms to the low 50's over the course of the summer based mainly on ground / groundwater temp. A thermocline (sharp break) will form as the surface warms into the 70's or beyond and the bottom water stays in the 40's to low 50's. Then we start the cycle all over again."
So what does this mean as far as catching fish? Fishing is generally tough during turnover but you can catch fish. Post turnover can be excelent for all game fish. I have done my best when the water temps are in the 47-51 degree range. usually late October.
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