I grew up on lakes and always knew the basics of "lake turnover." It causes a slight smell in the fall and spring. I wanted to know more so I checked out "A Quick Lesson on Lakes" on the Lakes Environmental Association website. It says:
Winter is a slow time for lakes because the ice cover cuts off the sunlight and wind and most of the inhabitants are enjoying a sluggish lifestyle in a stagnant environment. If it were not for the fact that ice is lighter than water, lakes would freeze from the bottom up and most of the lake creatures would be in serious trouble.
Spring brings ice melt, warming of the surface waters, winds, and a complete mixing – a kind of rebirth. As summer arrives, lakes deeper than 30 feet or so will gradually separate or stratify into a warm top layer and a cold bottom layer. Much of the important data is developed during the late summer when we see how lakes act under their worst conditions.
Following summer stratification, cool fall winds chill the warm upper waters until the temperature differential is weak. Then the stratification breaks down and the “fall turnover” occurs. This is a full mixing of the lake waters, preparing the way for winter.
When we checked the water temperature below the boat a few days ago it was 63 degrees. With air temperatures dropping down close to freezing a night, the surface temperature will soon be colder than the lower regions of the lake and the turnover will occur as the lake settles down for a long winter's nap.
For great information on lakes join the Lakes Environmental Association.