During the summer months ponds have a tendency to develop algae blooms depending on the amount of nutrients that flow into the pond. These blooms (unicellular) can be beneficial in the sense that they shade out the sunlight and to a degree stop the development of undesirable plant growth. These blooms are alike to terrestrial plants in that they produce oxygen during the daytime and are oxygen consumers at night. The algae bloom (unicellular) seldom exceeds a depth of greater than 3.0 feet; however, they prevent the penetration of sunlight to greater depths. As a result, the upper surface of the pond is supersaturated with oxygen and the bottom is frequently devoid of oxygen. If a surface aerator is used, the supersaturated water is thrown into the air and the oxygen in excess of saturation is dissipated into the atmosphere. If an air diffuser system is installed, it will lift water with rising bubbles. If correctly designed this will provide a laminar flow wherein the bottom water is mixed with the surface water creating a uniform distribution of oxygen throughout the pond. During the night time the same diffuser system will contribute oxygen, thus offsetting in part the oxygen consumed by the algae.

The above is described by limnologists in terms that relate these conditions to temperatures. These terms are generally utilized when discussing deep lakes or ponds, but the same conditions can exist in ponds as shallow as three feet. The pond or lake is divided into three regions in accordance with temperature and depth. The upper region is called the epilimnion and under normal conditions contains suitable amounts of oxygen and is the site of algae growth. The next is the thermocline which is the point where light penetration is minimal or nonexistent. A severe oxygen and temperature gradient exists between these two layers. The next region is called the hypolimnion. This is the bottom layer of the pond or lake. The amount of oxygen is dependant upon the characteristics of the lake. During the summer months these three areas are generally well defined. Because of these conditions fish are restricted to the upper regions in order to obtain sufficient oxygen to survive. Lake trout and other deep water fish are killed off by the low levels of oxygen. They are unable to feed on the bottom which if oxygenated would provide benthic organisms that in-turn would provide additional nutrition resulting in better growth and survival.

During the latter part of the fall, the water in the epilimnion becomes colder. The resulting increase in density causes it to sink to the bottom, bringing oxygen to the hypolimnion. However, if oxygen levels in the epilimnon have been low all summer then all three levels of the lake may contain very low levels of oxygen. Many species of fish like brook, brown, and lake trout spawn in the fall. Oxygen depletion is most likely at this time therefore little or no reproduction may result. More detail regarding limnology could be incorporated into this brochure; however, each area of the country or the world has somewhat different conditions and species of fish which react differently to varying conditions. It is suggested you contact your local Fish and Game authorities to determine the suitable times to provide the needed mixing and aeration.