bz :
With the dw cutting off the sunlight from the pond bottom some then that will result in less FA. In addition the dw will use some of the nutrients which means less for the FA. One problem with dw is how to deal with the seed bed (turions) with out systemic herbicide that stays in the water for a while.
http://www.rook.org/earl/bwca/nature/aquatics/spirodela.html In fall, budding pouches produce smaller, rootless, dark green or brownish daughter plants called turions. These dense, dormant, starch filled structures settle to the bottom to overwinter.
Plants may overwinter as turions, or as seeds, sinking and resting at the bottom of the pond until they germinate.
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