Bill, actually the water is mostly reoxygenated by exposure to the air at the surface, not neccessarily by phytoplankton near the surface. The more efficient the diffuser and the more water that is moved to the surface,the greater the lifting rate and turn over rate. The water is lifted from the bottom to the surface where it absorbs oxygen and releases gases and it continuosly layers out until the bottom water is back on the bottom again. The smaller the bubble the faster it moves and the more water it entranes to the surface. Bottom diffused aeration really is all about the lifting rate and circulation and the concept that dissolved oxygen is brought down to the bottom of the pond and through out the water column. It prevents stratification, eliminates the peaks and valleys and the bad kind of turn over that can kill your fish. Sometimes there are occurances that are out of everyone's control and oxygen is depleted faster than can be replaced by any type of aeration.
