In article , says...

does the bubbles in an airstone do any gas exchange when moving up
the water, or only when they burst at the top and ripple the water?


Gas exchange happens where ever water meets air, so yes, each bubble
causes some oxygenation. And yes, little bubbles have a greater
surface to volume ratio, meaning fine bubbles are more efficient at
it than big ones.
But I wonder just how much extra surface area is created by a typical
airstone/pump? I bet it's not much. (I also wonder about the timing:
does the GE happen instantly or over time? If the latter, do the bubbles
last long enough to do a good job?)

But it's not the entire story. In an otherwise still container,
a column of bubbles causes a current on the surface. That means
water from below is rising to the surface (and surface water is
flowing downward elsewhere) to come in contact with the air
where more GE happens.

I claim it doesn't matter which effect dominates: airstones help
oxygenate the water.

I think it's time for Donald (or some other quantitative type person)
to do a pseudo-definitive study with multiple tanks, dissolved oxygen
tests, clipboards, white lab coats....the whole bit. ;-)

Gunther