Howdy Radwoem,
Hey nice place. Its good see it with the map. I take it the bright green around the edges is duckweed/watermeal is before it gets bad or are these pond weeds? First I would say is that the tree situation isn't as bad as thought it might have been. So the situation is that that pond has been around a while AND efforts have been made to make it more productive (past fertilization program for a number of years). I'll begin by restating something Bill said, the presence of duckweed is an indicator of an advanced stage eutrophication. Duckweed is picky, it needs lots of nitrogen and potassium and if these are present in large concentrations the first thing that happens is that heavy blooms shade submerged vegetation and kill it out. Once the Duckweed gets in, it has the advantage of being able to float above the blooms of phytoplankton and shade them. Probably, the most rapid growth of Duckweed occurs when there is sudden collapse of standing population of phytoplankton. This returns alot of nutrients to the water and Duckweed is ready to exploit it. Every year a progression like this probably occurs where first there is a very strong bloom of phytoplankton, collapse, and subsequent duckweed bloom which could easily last the Summer..
But it takes really rich water to give Duckweed an upper hand against its competition. Bill noted how this is an indicator of trophic status and that the presence indicates hyper-eutrophic water. He wisely said, "Expect periodic unpredictable fish kills." So with this knowledge I would just say be prepared to experience another. Just some thought to chew on:
1. For now manage the water without adding additional phosphorus or organic matter of any kind. This includes feed of any kind or any additional fertilizer. Do not increase alkalinity or add lime. If you were to build a pond below ... do not initially feed or fertilize it either. Allow the enriched water of this pond to flow into the lower pond and the naturally fertile water feed your fish below.
2. OK. So instead of removing duckweed, why don't you use it? 1500 lbs/acre of Tilapia can be grown in fertilized water. If you harvest the fish ... guess what? You harvest what they retained of the duckweed they ate. Guess what else? You harvest a lot of detritus from the bottom and phytoplankton too. TP are 25% dry matter so removing 1500 lbs of TP per acre is just like undoing ~400 lbs of formulated feed per acre. It is just that you have to remove them in order to get the nutrients out. TP are great from late spring to late fall ... but there is another organism that would grow fat in that pond that does well in late winter to mid spring ... crawfish. I would bet, depending on the overwinter fish community, that you could grow up to 800 lbs of crawfish. Again, you have to get them out in order to remove their sequestered nutrients. One thing I will tell you, these organisms have the ability to turn back the clock if harvesting them is something you would do. They have value and if you can use or market them they are almost cost free dredgers that can renovate your pond over time. Both organisms can sustain foul water conditions and will be the last to succumb in a fish kill.
3. So if harvest isn't a viable option, you can bite the bullet and remove the accumulated muck. The pond will still likely be very fertile and probably wouldn't require food or fertilizer unless you might be in a hurry to fill it up with muck again
The key to success of any plan is population management. Fertility never means bigger fish ... it only means more of them. Feeders and Feed aren't required ... they are an option than can increase fish standing weights until the water is too eutrophic (then the effect of increasing primary production becomes limiting fish standing weights AKA fish kills).
4. I mentioned that I liked the option of building a pond below the existing one. Looking at the map ... this seems to a possible option assuming there are funds and a desire for it. But the nice thing about doing that is that it has a really rich source of fertile water from the existing bow. So if I faced with same scenario and had the opportunity to build a pond below the existing I would probably kill out the existing pond and restock with GSH, FHM, and Gams. These would proliferate and could be trapped to boost the lower pond. I would stock 3000 to 6000 2" TP fingerlings annually (in the upper pond) and if I didn't want to harvest them ... I would find someone who did. The same goes for Crays, I'd do same using the winter and early spring season for them. A pond that size is going to produce more than you guys want to eat. So you'd need some help or need to find a market. Treating the existing pond as a forage pond for a few years while removing many TP and Crays could renew the pond and take many years off its existing trophic age over time. When you begin to see submerged weed growing in abundance and no more duckweed blooms, you know you've done it and that may be a good time to think about introducing predators and managing it for sport again. Give careful consideration to a plan of restricting/removing nutrients on a long term plan in order to keep it in that sweet spot longer..
