We are treating the top surface of the water as the barrier though..... when using an airstone, you are artificially creating new atmospheres, and creating barriers between those atmospheres and the water, which are turbulent in their own, creating agitation for each bubble, and therefor exchanging gas with liquid before ever reaching the top of the water column.
The rapids are areas where the rivers are often less deep, and therefor more % volume of water contact to air.
Also turbulence, which even moreso increases the agitation and breaks surface tension, resulting in a SUPER OXYGEN RICH environment. Interesting about those plecos TTA, I wasn't aware they existed in such a defined area as 1 bend of 1 river, very cool!!!
The rapids are areas where the rivers are often less deep, and therefor more % volume of water contact to air.
Also turbulence, which even moreso increases the agitation and breaks surface tension, resulting in a SUPER OXYGEN RICH environment. Interesting about those plecos TTA, I wasn't aware they existed in such a defined area as 1 bend of 1 river, very cool!!!