This video from the USA says about itself:
Field Recording 3: Duckweed and Watermeal on the Mississippi Current
A field study of vegetation and current patterns on the Mississippi River. This footage offers two moments of experience. A Biologist from the U.S. Fish & Wildlife Service has helped me identify the vegetation present here. It is threefold: Large & Small Duckweed (Spirodela polyrhiza & Lemna minor) and Common Watermeal (Wolffia columbiana).
On an additional note, the second piece of footage (beginning at 2:00) presents the sound of the freight train about 150 feet from where I was sitting. The impact on the “soundscape” at midpoint is noteworthy, and it shifts back to nature’s quiet at the end of the piece. Ferryville, Wisconsin. October 10, 2013.
Translated from a report by Dutch botanists:
September 5, 2014 12:12
The rootless duckweed (Wolffia arrhiza) having a length of less than one and a half millimeters, is by far the smallest native vascular plant in the Netherlands. In July of this year, we found the first Wolffia australiana in two ditches in South Holland province. Wolffia australiana originally was limited to Australia and New Zealand. Further research into Wolffia in different provinces soon yielded the second new species for the Netherlands: Colombian Wolffia (W. columbiana).