The need for oxygen at the bottom of the pond

quoted from
http://midmichiganponds.com/management/ecology.php
(bold added)

Pond and Lake Ecology 101

Euthrophication is the natural aging process for lakes and ponds. As a lake or pond ages, it slowly starts to fill in with organic material such as decaying aquatic vegetation, dead fish, leaf litter, trees, etc. However, this slow process is greatly accelerated when we add additional sources of nutrients to the ecosystem. Goose manure, fertilizer runoff from lawns, agricultural runoff, failed septic systems and erosion problems are some of the main causes.

Dissolved oxygen (DO) is the form of oxygen that fish and other aquatic life need to stay alive. It is the single most important water quality parameter. Plants produce through photosynthesis about 75% of a pond's DO supply. The remaining 25% comes by absorption of oxygen from the atmosphere. Depending on the species, most fish need at least 4–ppm DO to survive.

Thermostratification is the process where lakes naturally stratify into three layers based on temperature. The top layer can sometimes by 10–15 degrees warmer than the bottom. The middle layer (called thermocline) is a thin zone where the temperature changes quickly. Once these layers become established, densitiy differences prevent the top and bottom layers from mixing and exchanging DO. All the DO producing is confined to the top layer, so as the summer progresses the bottom layers become stagnant and depleted of DO. Anaerobic bacteria start building up harmful levels of hydrogen sulfide (smells like rotten eggs), ammonia and methane.

Nature's Way of Pond Management

Inversion is the natural cleansing process of a pond or lake. It occurs in the spring and fall when the cooler water at the surface is heavier than the bottom water and sinks. This is nature's way of supplying oxygen rich water from the surface layer to the stagnant water at the bottom. The oxygen drives off toxic gases that build up in the bottom layer. It also revives the aerobic bacteria population so they can start decomposing the organic litter that has built up. Eventually the microbial activity will use up this fresh supply of DO and the bottom layer will again return to its stagnant condition until the next inversion occurs. Dissolved oxygen at the bottom of the pond is the single most important water quality parameter that drives nature's cleansing process. Unfortunately, this twice a year process is not enough to keep up with the build-up of organic material and when even more nutrients are added to the ecosystem the pond or lake starts filling in even faster.

Pond Management with Aeration

Aeration systems rely on the same cleansing principles of inversion, only the DO is circulated continuously. Instead of inverting a pond twice a year, a properly designed aeration system will invert a pond up to several times a day. By using various types of compressors, an aerator pumps atmospheric oxygen down to a weighted diffuser. The diffuser releases oxygen to the stagnant bottom layer and sediments where it is needed most. As the oxygen bubbles to the surface, it creates a slight current that lifts the stagnant water off the bottom and brings it to the surface where it gets cleansed with oxygen and healthy microbial activity. This current creates circulation (or inversion) constantly bring fresh water down to the bottom where it is needed, as well as throughout the entire pond. Maintaining high DO levels throughout the pond and especially the bottom is the key to a healthy ecosystem. Aeration is the long term approach to pond management because it targets the source of the problem and not just the symptoms.