No, I wouldn't say that is, "Normal". DO levels will drop by about 10% every three feet due to hydrostatic pressure, but your drop is way more, not to mention your pH.
From what I know about your BOW, I would guess your contrasting pH and DO levels are the result of muddy water. All your photosynthesis is happening within the first foot of water, since there is no sunlight past that point. The plants are using the carbon dioxide, which acts like carbonic acid in water. CO2 removal reduces the acidity of the water so pH increases. Decomposition at the bottom of your water has the opposite effect and is decreasing pH and lowering DO.
I would "think" more aeration but I would like to see more tests prior to suggesting anything.