I should add that there are two other pits on the same property that had normal DO levels, one at 5.5ppm and the other at 6. The one that had 5.5 was treated about two months ago for watermeal, though it was never more than about 20% covered with the weed prior to treatment; the other pond never had watermeal. Both ponds have fewer trees around the banks than the two with DO problems.
