My friend Marilyn Buscher, the Pond Lady
(http://home.wi.rr.com/windyoaks/), has these big greenhouses over
shallow water for cultivating water plants. duckweed uses up the
nutrients AND shades the water and is not good for business. She
basically rigged up something a lot like a pond skimmer to remove the
duckweed. It works, it keeps the weed and azolla off her ponds.
Ingrid