Pond oxygen comes from algae. Notice the bubbles on the rock in the picture (click on it to enlarge). Oxygen is the result of photosynthesis where as long as the sun shines and algae grow, the pond will receive oxygen. Molecular oxygen is dimeric atomic oxygen and thus has an apparent molecular weight of 32 (16+16), heavier than water at 18. Further oxygen is more miscible in cooler water than warmer water; therefore if formed under the respiring water plants the oxygen will sink with the cool water produced by the photosynthesis of the plants.
Many of the macro-invertebrates such as crayfish, dragon fly larvae and microscopic crustaceans feed on algae and keep it in check. These macro-invertebrates feed fish and other invertebrates. Thus the algae form the base of the food chain and are extremely important in pond ecology. They require habitat; the best is river run rock as shown in the picture. The rock acts as a heat sink for the algae to enable growth. If you will notice, the algae actually are colonies of many species.
The microbiota of a pond is extremely complex but forms most of the living forms in the pond. They grow on algae and plant debris; and are responsible for most of the decomposition and respiration in the pond. Critical habitat for these microbes is dead woody debris. A deciduous sweet log rapidly forms this function.
A pond engine incorporating all of these components is the motor for the pond. You simply put a rock shelf at pond level into the pond; plant the top with slough sedge; and place a dead rotting log on it partially out of the water. Opening the log and planting seed often starts the rot.
Balancing the microbiota of a pond balances the entire ecology where fish will thrive along with all of the other inhabitants of a pond, including you. You continue to layer shade and bird habitats where a diverse healthy eco-system evolves. Your comfort is the ultimate habitat for your pond because you are there too.
PonDoc from the pond