If you can create ripples on the surface, you greatly increase the surface area of the pond surface. The more surface area you have the more contact you have with the sunlight and more oxygen you create.
Bottom diffuser bubbles lift the oxygen depleted bottom water to the surface and in contact with the sunlight and is thus oxygenated. The smaller the bubbles a bottom diffuser can create, the more "lift" it has.
I should probably know this, but I don't, what the % rate of bubbles goes into the water as dissolved oxygen. It's not as high as you'd think though.
