forming a lower layer called the hypolimnion. The surface and water near the
shore are warmed by the sun, making them less dense, so that they form a
surface layer called the epilimnion.
Figure 4.10 Thermal stratificationSeasonal Turnover
Seasonal turnover refers to the exchange of surface and bottom water in a lake or
pond that happens twice a year (spring and fall). During the summer, the sun
heats water near the surface of lakes, which results in a well-defined warm layer
of water occurring over a cooler one (stratification). As the summer progresses,
temperature differences increase between the layers, and a thin middle layer, or
thermocline, develops, where a rapid transition in temperature occurs. With the
arrival of fall and cooler air temperatures, water at the surface of lakes begins to
cool and becomes heavier. During this time, strong fall winds move the surface
water around, which promotes mixing with deeper water—a condition known as
fall turnover. As the mixing continues, lake water becomes more uniform in
temperature and oxygen level. As the winter approaches in areas where
subfreezing temperatures are common, the lake surface temperatures approach