FWIW Our 1/4 acre pond, prolly 25 years old, had NO vegetation for the first 2 years of our ownership-2009 and 2010. In 2011 we started seeing a short grass like growth, about 3" long, in 2 to 5 feet of water. Then in 2012 and since, Elodea runs rampant.
There were many many crawfish in the pond initially, even with LMB, YP, and 2 big (30") GC. Plants I introduced fared poorly. Terrestrial weeds were absent around the property, in flower beds and in walkways.
Soon, the Aquacide catalogs began arriving, addressed to the former owner. A partial jug of long acting glyphosate was also left behind.
I don't think I'll ever know for sure, but I'm guessing the former owners used plenty of herbicides in the pond and on the ground. A small garden bed which still mysteriously doesn't grow stuff like other ground had a suffering grapevine in it. Grapes are hard to kill around here. Even with "hospitalization", the grape died.
Crawfish and the GC could be responsible for the absence of vegetation in the pond, but all of it? I don't think so.
Anyway, given time and heavy irrigation, I believe the soil is recovering; we now grow excellent tomatoes, chilies, taters, corn, cukes, and others.
