Congrats on your wood duck arrivals. I always enjoy your posts showing them thriving at your place!
As to the FA doing better over the winter this year, there are many, many possible causes.
If it was a new pond, I would expect that the age-related, inevitable increase in captured nutrients caused the change. However, for an old pond it is less likely that you finally, and permanently crossed some important threshold.
In my (non-expert) opinion, it was probably some random weather-related event that helped your FA survive through the winter. For example, perhaps you got heavy rains that occurred right after the fertilizer was applied to your adjacent ag lands, and you therefore received an extra "dose" of fertilizer.
Unfortunately, if no FA is ever physically removed from your watershed, then even after your spring treatments this year, those extra nutrients will still remain in your pond.
I would follow your regular spring protocols, and if you do get some heavy mats, maybe pull some out and deposit them outside of your watershed basin that feeds your pond.
Also, maybe add a few more FA combat protocols. Can you put in some tilapia if you can get a split load from a PB buddy in your area? Can you plant some attractive emergent plants that will steal some of the nutrients from the FA? My wife likes that one, and says she would do most of the care if I can ever get my ponds re-done. There are several other reasonably priced options in the FA threads.
(I don't know about cold-tolerant FA. Perhaps our plant experts will know. However, once established, I think your eradication efforts would probably be very similar to normal FA procedures?)
Good luck on your efforts to have a nice pond this warm season!
