BZ :

As soon as you can without causing a DO crash by spraying the duckweed in hot weather get it out of your pond. Duckweed thrives in still low O2 water with nutrients. It is very hard to get rid of and causes DO problems as follows. Because duckweed covers the surface it blocks all light from the plankton base in the water. The plankton is a much better source of O2{as well as food} for the pond than duckweed. In addition all the other beneficial functions that light causes in the pond are restricted by duckweed. Thick duckweed layers also inhibit the exchange of O2 to the pond surface waters and the exchange out of the pond waters of CO2 and methane gases from bottom decay.

One good point any active water movement by waves, aeration , sprayers etc will help reduce duckweed by the constant turning over of the plant so that its roots are exposed to air and light and are out of the water. ewest