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instead of posting the info twice...I'll post the link in here. 
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Common Duckweed, also known as Lesser Duckweed, is a common floating plant found in ponds, marshes, lakes, and quiet streams.
It is one of the smallest flowering plants in the world. Common Duckweed is usually seen in thick mats, covering the surface of the water, often along with other species of duckweeds.
Duckweed does not have stems or leaves. It has a round, slightly oval-shaped body, called a "frond." A small root-like structure, known as a "rootlet," hangs down from underneath. Fronds tend to grab each other, forming the thick mats.
Common Duckweed fronds are usually pale green, but can be a reddish-purple, and are no bigger than 10 millimeters.
http/www.fcps.k12.va.us/StratfordLanding...es/duckweed.htm
Bioremediation
Fueling the rapid growth of duckweeds requires substantial amounts of nutrients. Thus duckweeds have evolved the ability to rapidly remove minerals necessary for their growth from the water on which they float. When present, duckweeds also can remove many organic nutrients. These mineral and organic nutrients are converted into the substance of the plants, that is, their biomass. Research has shown that duckweeds are especially adept at removal of phosphates and nitrogen, particularly ammonia.
http/www.mobot.org/jwcross/duckweed/duckweed.htm