Apparently you had a fall turnover in October (windmills aren't the most dependable for mixing the water column as they only work part of the time, and the windmill and adjoining diffuser may have been undersized for a 3/4 acre pond).
Since you didn't have a fall turnover kill previously, I'm sure it's no coincidence that too rapid of a weed kill, and subsequent decomposition, exacerbated the anoxic layer that came up during the turnover. Chara is very dense and produces alot of decomposed matter when it dies -- especially the amount you described.
Occam's razor: "It is a principle urging one to select among competing hypotheses that which makes the fewest assumptions and thereby offers the simplest explanation of the effect."
http://en.wikipedia.org/wiki/Occam%27s_razor
Last edited by Cecil Baird1; 03/01/12 08:56 PM.
