I have a 4.5 acre pond with similar depths as yours that was completely covered with duckweed 5 weeks ago. Today I can report no living duckweed! I also had copious amounts of filamentous algae (almost all gone now). I topically sprayed with 37% diquat (20 oz in 15 gal sprayer) and Cutrine Plus ( 40 oz in 15 gal sprayer) plus 3 oz nonionic surfactant. Mix diquat in 15 gal water first, then add Cutrine, agitate then add surfactant, spray within a few hours of mixing. I initially used just diquat but adding the copper significantly increased the effectiveness. I would caution to only treat 1/3 to 1/2 of pond at a time to avoid fish kill. My brother has 7 acre pond and sprayed entire pond with some fish kill. I also have shallow water aeration diffusers running 24/7. Started the aeration one week before I started spraying. I repeat sprayed every three to seven days as soon as I saw new growth. Once I started down this path, I was a little afraid I might kill all my fish so I called in the state pond management expert for our area. He completely supported my course of action. I have sprayed 5 or 6 times using a 15 gallon battery powered sprayer on the back of my golf cart and use the boat for areas I can't reach with the cart. I also used pond dye, two applications.
1st picture is one week after 1st treatment on May 14. The other two were taken today.
