The molecules in ice form a very structured, open framework, so ice itself is less dense than water and, consequently, it floats. With this concept in mind, seasonal or thermal stratification within lakes can be explored Changes in the temperature profile with depth within a lake system is called thermal stratification. This profile changes from one season to the next and creates a cyclical pattern that is repeated from year to year. Let us begin with spring. After the ice melts on a lake, the lake water is generally the same temperature from the surface to the bottom.
Wind allows circulation and mixing of the lake water.
This circulation pattern is very important in that it allows relatively large amounts of oxygen to reach the bottom of the lake. Otherwise, oxygen would have to reach the bottom by the relatively slow process of diffusion. The mixing of the lake water at this time of year is called spring overturn. As air temperatures rise in late spring, heat from the sun begins to warm the lake. As the amount of solar radiation absorbed decreases with depth, the lake heats from the surface down.