Here's a case study on what under aerating a pond will do...
There's certainly a cautionary tale being told there, esshup, but as a scientist I must say that it's difficult to discern exactly what it is. Too little information about the before situation, and too many parts moving all at once preceding the after situation. I'll think more on it, though.
That and your follow-up post about balancing the air output between the diffusers brought up a few more questions, if you don't mind:
1) How is it decided whether one should use a single or more diffusers at a single spot? I've commonly seen singles and doubles, and sometimes quadruples, offered with no explanation of when to choose which, and in a photo in that case study I can see where they even used a quintuple!
It all depends upon the amount of water volume needed to be moved. Since weighted airline isn't inexpensive, the less runs that you can make to achieve proper aeration, the less costly the system will be.
2) How does one balance the air output between diffusers when there is more air being produced by the compressor than all of the diffusers hooked up to it can properly handle? For example, say I use the same compressor for three diffusers during the summer and only one diffuser during the winter, or start out by trying only one diffuser during the summer with the idea that I can add more in subsequent years if doing so seems justified. How do I keep diffusers from being overwhelmed? Is the excess air simply bled off at the compressor somehow?
Yes, you can have an extra air line hooked up to a sort of muffler to vent excess air (the muffler is to quiet the exhaust noise) or use more diffusers in the same location to handle the air being produced by the compressor. You need more air in summer than winter. You can also go with air stones during the winter, although they need more maintenance than a flexible rubber membrane disc. Sort of ties into the answer for question #1.3) In such a scenario as I described above, where less than all of the air produced is being released through the diffusers, how does one know how to optimize the diffusers? Is measuring O2 at depth really necessary, or is there some way one can just eyeball the roil at the surface of the water, or what?
The more hands on experience you have with aeration systems, the more "eyeballing" you can do. You can also take a temperature reading. If the bottom water temp is within a few degrees of the top water temp, you are good. 
