Not sure where the graph came that you posted? I was referencing carbon dioxide acting as carbonic acid, which the article was basically about.
My guess, without the added aeration, you'll see greater stratification of your water. I.e. higher DO and pH at the surface and lower DO and pH at depths. Just a guess, but if correct, it signals an unhealthy pond bottom.
Any harbor in a storm, is the answer. It depends on the DO you think is poor Vs good. Anything less than 5ppm is detrimental to good fish health. In my opinion anything less than 6 will cause stress and is a reason for concern.