Charlie,
Forgive me if you've already posted this but when a thread gets this long I get lazy. :-)
Anyway a question:
Do you have an algae bloom (greenish or brownish water)? If so what is your water clarity? And if so, has it increased since you treated your weed problem with a herbicide?After a macrophyte die off it's very common for the phytoplankton or filamentous algae to take over to use nutrients -- especially those nutrients that are being mixed into the water column from the decomposing vegetation via the diffusers.
Saturation rates that exceed 100 percent during the day are not unusual with an algae bloom. This is know as "supersaturation." However just a few feet below that you will see a quick drop in saturation which is what I think I've seen in your first post.
One potential problem with an algae bloom that is so intense it causes supersaturation, is it can reverse at night. Supersaturation of oxygen during that day rarely is a problem for fish though and they can go a little deeper if it's uncomfortable.
My small 1/10th acre yellow perch fry production pond is prone to an intense algae bloom. Once an algae bloom gets established it often goes extreme. No problem during the day but if I didn't run the Kasco surface aerator during the night I would have hundreds of fry piping on the surface just before daybreak.
One of the primary factors which algae blooms influence is the dissolved oxygen content of the water. During the day, the masses of algae produce so much oxygen that the water becomes supersaturated. Then, at night, the algae actually use up oxygen in the water and can cause such extremely low dissolved oxygen levels that fish kills may result. http://water.me.vccs.edu/exam_prep/algae.htm 
