FishinRod Has posted a very good & very likely senario.
About 25+ years ago our 3 acre retention pond was
full of weeds & we had a week of hot 90s, overcast, with very little wind type weather. We lost about 20 LMB & 20 grass carp all in the 2-3 lb & above range. The smaller fish seemed to tolerate the low O2. Called the local fish farm for advice & he said other ponds in the area had the same problem. I really do not think it is a lack of food problem. Lack of food would be a weeks or months problem to kill the fish. I now have a long term, 20 yrs, over population of LMB. Rws of 75 & below, they are not exactly health but they have adapted & survive.
Doing RWs of your LMB is the key to telling you if your LMB are getting enough to eat. If the Rws are low then there is probably to many LMB in the pond for the amount of forage fish.
That watershed is part of your ponds natural ecology. The fact that it consumes some of the nutrients is a balancing & stabilizing factor. With no littoral zone plants & all the nutrients available for the algae you may have tremendous boom-and-bust cycles & frequent times of extreme low O2 with large fish dying. One of the fish farms has a very good article as part of their web site about how the cycles develop.
https://americansportfish.com/fertile-ponds-and-plankton-blooms/The littoral zone is where the forage fish survive, hide, & grow. Where they get a chance to become 2-4" fish. Your 2-4lb LMB are thankful. The lack of a well developed littoral zone is one of my problems. The fry & fingerlings get eaten well before they get a chance to become good forage fish.
The pond is a balancing system: A change here will always produce a change somewhere else.
