Unlike shallow lakes and ponds, the physical characteristics of a deep lake vary from top to bottom. Stratification creates three different zones, the epilimnion, metalimnion, and hypolimnion (see Figure 4-8).
Epilimnion. The warmest zone near the water surface. The surface water area circulates freely and has variable temperatures. Mixing of the surface water is caused by wind, water currents, and both heating and cooling. This is usually the area where the majority of organisms are found. It is the area of greatest productivity and light penetration; the oxygen concentration is also usually greatest in this zone.
Metalimnion. The middle layer is characterized by steep and rapid declines in temperature. The thermocline is found in this zone—an area of the most rapid temperature decrease at 1°C per each meter of depth. Warm water is much lighter than cold water, which is heavy and tends to sink to the bottom of the lake.
Hypolimnion. The third layer is deep and cold with little temperature change and low oxygen due to little or no production of oxygen by plants. This water does not come into contact with air, and there is no mixing. The quantity of oxygen decreases with depth, the farther the distance from the boundary between the air and water, and from mixing by the wind. Oxygen is further depleted by bottom-dwelling organisms and bacteria feeding on organic matter. Little or no light penetrates the hypolimnion.
The Aging Process
Lakes and ponds go through a natural aging process called succession. Marshes, swamps, and bogs are generally the mature stages of succession