Roland, welcome to the forum. Glad you decided to jump in! Another Virginian is always welcome...
Duckweed is very susceptible to fluridone. Creating a 45 ppb concentration over a 45+ day time period should eradicate it. The fact that you have minimal outflow will help keep the ppb up. Generally you want to add 50% in the initial dose, 25% 7-10 days later and the last 25% another 7-10 days later. Watermeal is also susceptible to fluridone, but is a bit hardier. A concentration of about 90 ppb would be needed. Obviously, this concentration will also kill the duckweed.
Diquat is a lot of work, requires multiple treatments and the plants often come back... Where as with the fluridone, particular in a pond like yours where there is minimal outflow, one treatment often last into the 2nd or 3rd year and keeps it from coming back.
Along with the fluridone, a copper-ethanolamine based treatment like citrine-plus can be used to eradicate the algae. Spot treating floating matts of the algae is best to prevent fish kills. Species like trout, channel cats and koi are especially sensitive. So keep that in mind if you have these species and value them...
Aeration will work to reduce the duckweed and watermeal. It will also help to evenly distribute the fluridone treatment. In my opinion, aeration if properly installed and ran is the best investment one can make in their pond... It can be used in conjunction with the bacteria. Hopefully someone with a bit more experience with bacteria can weigh in.
Has there been a change in the land usage around the pond recently? Increase run off from lawns, farm animals, etc? As ponds age, they also build up nutrients which makes them more susceptible to issues like duckweed and filamentous algae.
