and cloudy weather conditions, these systems would need to rely on a backup power source, or not function. Unfortunately, these periods are when dissolved oxygen concentrations are typically lowest. Wind powered systems would not function during the hot, calm summer weather when the need for de-stratification is most critical (Hargreaves 2003). Cost and performance of solar and wind driven diffuser devices may limit their practical use.

There are a variety of pond aerators on the market. Aeration provides a zone of adequate dissolved oxygen where fish can seek refuge until dissolved oxygen concentrations are naturally restored. A few basic types of aeration devices are described.

A propeller-aspirator pump aerator consists of a surface mounted electric motor supported by a plastic pontoon. A hollow, rotating shaft is attached to the motor and may be adjusted to operate at an injection angle of 25 to 45 degrees in the water column. A propeller is attached to the lower end of the shaft and forces water past the end of the hollow shaft creating a vacuum which draws air from intake ports located above the water surface. Air is injected into the water and then diffused by the propeller below the surface. Air bubbles may be driven to a depth of 5 or 6 feet while moving water horizontally away from the aerator. These units are best suited to shallow ponds (less than 8 feet). Smaller ponds would use 1-3 hp. units, while large models are available up to 100 hp.

Vertical pump aerators are primarily used as emergency aeration devices in small, shallow ponds of less than 8 feet deep. Additionally, they may be used to prevent ice formation in small pond areas and in boat houses, when fitted with an appropriate propeller. A vertically submerged electric motor (1/2 to 2 hp) is suspended inside a float and drives a propeller located just below the surface of the water (Tucker 2005). These aerators provide high oxygen transfer efficiency to the nearby water. They will not de-stratify a deep pond since water is drawn to the surface from only a few feet in depth. Some units may be fitted with a pipe to draw water from greater depths of 5 to 7 feet, creating a better pond de-stratifying device. However, this will reduce its effectiveness as an aerator. Horizontal water circulation is minimal inasmuch as the propeller forces the water straight up in the air. Nevertheless, these aerators are well suited for small and particularly shallow ponds.