As the seasons change, so does your lake. In the winter, a lake in the Midwest is about 33 degrees Fahrenheit on top and about 39 degrees Fahrenheit on the bottom. If ice does not form, the wind creates currents in the lake that continually mix the surface water allowing oxygen to diffuse into the water from the atmosphere. The latter act is important for three reasons. First, oxygen is necessary for the survival of fish and other lake organisms. Second, oxygen is continually used as dead plant and animal tissue decomposes, and finally, oxygen prevents the release of plant nutrients from the bottom sediment. If ice forms, the lake can no longer circulate and oxygen content can drop to such low levels that some or all fish in a lake can die.
In the spring, increased sunlight warms the surface temperature to 39 degrees Fahrenheit and complete mixing of the lake can occur. Later, the water on the surface warms up and stays on top since it is less dense than the deeper, colder water.
During the summer months, when water is warm, oxygen can be consumed faster than it can be replenished. Lakes can become "stratified" - the warmer, oxygen-rich upper water laying on top of the cool, denser, lower-oxygen deeper water. The lake in the summer is not only temperature stratified, but also oxygen stratified to some extent because the surface water where oxygen is diffusing from the atmosphere is also where oxygen-producing plants live. At the same time, oxygen is being used up continuously by decomposing organic material on the bottom. Therefore, a summer lake usually has a depth below which fish cannot survive because of a lack of oxygen. Such conditions inhibit levels of beneficial bacteria and their breakdown of organics. Bottom muck accumulation increases and excessive nutrients are readily available and thus fertilize the lake allowing lush growth of algae and other plants. This thermal stratification also makes conditions favorable for the production of noxious ammonia and hydrogen sulfide gases.
In the fall, the surface water cools to the temperature of the lower water and is circulated by the wind causing the lake to mix. By late fall or early winter the lake is destratified, meaning that the temperature and oxygen are fairly uniform from top to bottom as in the spring. Soon the surface layers cool more than the deeper layers, but stay on top because water is heaviest at about 39 degrees Fahrenheit.
A bottom diffuser aeration system mimics the spring and fall conditions found in a lake. Bottom aeration systems create a vertical current using the rising force of millions of small bubbles to entrain the water column, "turning the lake over" and allowing oxygen to be absorbed at the lake's surface. Moving the lower-oxygen water up from the bottom and eliminating thermal stratification increases oxygen levels throughout the water column. With wide swings in oxygen stabilized, plant nutrients are bound up in the sediment and annoying plant and algae growth may be reduced significantly. Decomposition continues to break down organic matter and thus reduce the amount of muck on the bottom. Under these conditions, fish can use the entire lake not just the upper water layer expanding their territory into formerly oxygen-deprived portions of the lake.
