I don't want to understate possible effects from any number of possible causes; but I don't see the O2 levels as a major problem in this case.
We just moved here from Cleveland, OH; and the walleye fishery there is one of the best. But read this piece (there are lots like it):
http://www.cleveland.com/outdoors/in...are_growi.html .
This has been going on for years. It's a pain and should be controlled, but fish (walleyes in particular) are very resilient.
Most fish kills of any significant proportion that I've ever seen have been a direct result of some kind of poison. Either a bleach spill or other biotoxin that kills via bloodstream (gills) ingestion; not O2 deprivation. Usually, some fool illegally dumps a tanker full of liquid hazmat into a stream at 2:00AM; bingo: you have your poison.
Don't forget, Lake Erie was once considered a dead lake (the Cuyahoga is the "river that burned"). It took a while, but it did bounce back. We're much smarter about controlling stupid things now (I'd argue, we're often now too cautious with some regulations.).
In any case, I'll be the first to eat my words if I'm wrong since the boat is docked on CH.