You probably already know that oxygen transfer only occurs at the surface. Creating turbulence at the surface increases that transfer. One way is to use a floating aerator. The pump floats just under the surface and is connected to AC current. The water fountains into the air and back onto the surface. Here is a SITE whose equipment has power cords from 75 to 150 feet (22.86 to 45.72 meters). I'm sure you can find something similar where you are.

Note that water sprayed into the air does increase evaporation. Not a problem in a rainy environment, but a potential problem in drought prone areas. In that case you might prefer a bog bio-filter. A pump pushes water to a (usually) raised area bordering the pond. The water rises through a 12 inch deep layer of pea-sized gravel. Bog plants are placed in this area, and utilize the waste in the water sent by the pump. The cleaned water then falls back into the pond. This is an excellent filter for ponds with fish and should be considered. Here is an ARTICLE that explains.