Once the object, leaf, grass, piece of wood, ect. is dead and saturated with water....it does not continue to CONSUME oxygen or emit hazardous elements into the water that harm the fishery.
If it did then every Fall would result in a massive fish kill.....and there'd be no fish left.
And you'd definitely not find a fish in lakes like Truman, or Mark Twain.
When an area, like a large bed of milfoil, begins dying.....the fish LEAVE the area. They don't just sit there and suffer from oxygen depletion until they croak. 🤣