Keith, RC,
Vertex and others will tell you that you are adding some DO2 to your water by the interaction with the bubbles, but the main reason you use bottom diffused aeration as I understand it is to move the bottom water to the surface where it will off gas the hydrogen sulfides and pickup some O2. The stirring action moves O2 holding water to the bottom with the circulation and opens new areas to your fish. This O2 will be consumed and must be replaces ( the reason for one turnover per day). The aerobic bacteria will use the O2 to break down the biological material on the bottom with the provided O2, helping to keep your pond bottom cleaner.
If you want to add a lot of O2, surface aeration is the best way. Moving dead water to the surface for natural cleansing, that is the job of bottom aeration as I understand it.
