The limnetic, or open water zone, is occupied by many microscopic plants, many small crustaceans, and many fish. The deep, or profundal zone, consists of bacteria, fungi, clams, worms, annelids, and other small animals capable of surviving in a region of little light and low oxygen.

As compared to ponds, where the litterol zone is large, the water usually shallow, and temperature stratification in the shallow water is absent, lakes have large limnetic and profundal zones as well as a marked thermal stratification and a seasonal cycle of heat and oxygen distribution. In the summertime the water surface of the lake, called the epilimnion, becomes heated, while the hipilimnion, which is below, remains cold. There is no circulatory exchange between upper and lower layers, with the result that the lower layers frequently become oxygen deprived. Between the two is a region of steep temperature decline. This region is called the thermocline. The temperature in the epilimnion is between 25 and 24 degrees Centigrade. The temperature in the thermocline varies from, let's say 20 degrees all the way down to 13 degrees Centrigrade, which is a rather steep temperature decline.

As the cooler weather of fall approaches, however, the surface water cools down. The temperature is equal at all levels. The water of the whole lake begins to circulate and the deep water is again oxygenated by coming up to the surface or turning over. Hence, the term fall overturn. Why does cooler water at the top circulate?

Match the correct freshwater lake zones listed on the right to the correct description on the left.