jeff, FWIW, yer pond is amazing being at least 60 years old with no management and being in the nice condition that it is. i believe the granite substrate with the influx of that beautiful sierran spring water has been the fountain of youth for you. that said, however, ponds age. with no management, ponds eventually die. i've watched the downstream 5 ac ranch pond systematically increase in weed growth each year (FA, elodea, pondweed) to the point there is barely any open water in late summer. within a couple years i wouldnt be surprised to see a eutrophic disaster down there. it happened in my pond before renovation, killed all life and became a septic mess....this is not likely in yer case due to the spring but this is why i mentioned you should consider aeration....to help slow down the natural pond aging process.
as you know, if you chemically treat your increasing aquatic weed growth it will rapidly create a bunch of dead organic matter that will also rapidly consume the O2 needed to degrade (this is as opposed to the normal slow seasonal natural die-off).
a couple things you might research/consider are solar circulator/fountain type set-ups to keep the surfacewater broken up, and/or for bottom diffused air, size a system for the central part of pond, not necessarily in the deepest part. it will create convection cells that will affect the largest area and you might not need as strong a pump.
somewhere a couple years ago i came across solar circulators that werent that expensive....i.e. you could get a couple of them, i'll see if i can find that stuff, or maybe someone here has experience with those and will chime in.
my couple of cents.
