For the amount of trees around the pond, you're better off planning on using a chainsaw. It looks like there are less than 10 from the aerial view that contribute the most to the leaf load in the pond. The pond looks to be roughly .41 surface acres. With the pond being over 20 years old, there's a lot of organic material built up in the pond. If it's deeper than 10', there won't be much active bacteria breaking down the leaf load during the summer after a thermocline sets up. To help with future organic build-up, look at installing an aeration system to keep O2 levels high near the bottom of the pond. That will allow aerobic bacteria to work on the leaves all year long.
With the size of those trees, I don't know if Sonar will kill them at 90 PPB dosage rates.
How deep is the pond?
Unfortuantely, like you alluded to, just planting the Pickerel Weed and Cattails won't fix the existing problem, which is too many nutrients in the pond. The only way to get rid of them is to drain (or pump out the water with a gasoline powered trash pump), take out the accumulated organic material and let it refill with water. Before you let it refill with water, even tho it's a small area, dig a settling pond to the East, fill that with cattails to filter out as much sediment and nutrients as possible before the water hits the pond. Keep the cattails in that settling pond, and out of the main pond. There are other plants that are more suited to ponds without the downsides of cattails.
With the pond being at the most 90' wide, once it's drained down a long stick excavator can reach the whole pond bottom by working it's way around the pond. Build up the East side a bit, create a settling pond, and set the settling pond overflow to drain into the main pond. It looks like you have the area to dispose of the organic material on-site, or talk to the farmer to the East about spreading it out in his field after the crops are harvested in the Fall.
No worries about "chemical soup" as the chemicals break down over time and pose no threats to fish, wildlife or humans.
By trying to utilize plants to remove nutrients, it will take many, many years (if it's possible at all in that situation due to the field run-off) to remove the excess nutrients. Depending on the amount of runoff, you could be at a stalemate every year with the amount of nutrients coming into the pond and utilized by the plants. You'll have a much faster result, and you will be assured of accomplishing your goals of a clean pond by going the mechanical sediment removal route.
As for the fish in the pond, with it being under 1/2 acre, it won't cost a lot to restock, even with advanced sizes of fish.
We looked at dredging, got bids on it, no way we could afford it. 4 years ago I put in aeration, since then the pond has increased in depth 2-1/2 feet. Our aeration is a bit aggressive.
This pond was dug in 1961, we had looked at increasing it's size and the soil and water people said no. In fact they said under current standards they would allow the one that is there.
Three of the cottonwood trees are 50 feet tall or so, three two others are about 30 feet tall and there are a few smaller. That cove has some maples and oaks in it and a whole bunch of willow that runs up to 6 inches in diameter.
On the west side are pines. Not that they add anything to the pond litter wise, but they block the wind.
There is no other water source than the run off from that field. The real problems started when he spread AG lime two years ago. Not that it is the full cause of the watermeal problem, but it sure adds to it.
On of the biggest issues in this club is that we try to run this by democratic process. Instead of assigning a manager or a committee they want every issue voted on by the body. So, you have to try and educate 70 or so people at a meeting of the issues when they do not have to ability to pay attention and don't really want to learn.
If they ever gave me the authority to manage the pond the very first thing I would do is clear cut around that pond. Now that one big issue is gone then we can get to work on the rest. I've flat told them, cut those stupid garbage trees down or find someone else to work on it.
